Corrosion Prevention and Protection Practical Solutions

V. S. SASTRI Sai Ram Consultants, Ottawa, Ontario, Canada EDWARD GHALI Department of Metallurgical Engineering, Laval University, Quebec, Canada MIMOUN ELBOUJDAINI Materials Technology Laboratory, CANMET, Ottawa, Canada

Corrosion Prevention and Protection

John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (þ44) 1243 779777

Email (for orders and customer service enquiries): [emailprotected] Visit our Home Page on www.wileyeurope.com or www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London WIT 4LP, UK, without the permission in writing of the Publisher. Requests to the Publisher should be addressed to the Permissions Department, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, or emailed to [emailprotected], or faxed to (þ44) 1243 770620. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher and the Author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the Publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the Publisher nor the Author shall be liable for any damages arising herefrom. Other Wiley Editorial Offices John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany John Wiley & Sons Australia Ltd, 42 McDougall Street, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 John Wiley & Sons Canada Ltd, 6045 Freemont Blvd, Mississauga, ONT, L5R 4J3 Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Library of Congress Cataloging-in-Publication Data Sastri, V. S. Corrosion prevention and protection : practical solutions / V. S. Sastri, Edward Ghali, Mimoun Elboujdaini. p. cm. Includes index. ISBN-13: 978-0-470-02402-7 ISBN-10: 0-470-02402-X 1. Corrosion and anti-corrosives. I. Ghali, Edward. II. Elboujdaini, Mimoun. III. Title. TA462.S3185 2006 2006022728 620.10 1223–dc22 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 10: 0 470 02402 X (HB)

ISBN 13: 978 0 470 02402 7 (HB)

Typeset in 10/12 pt Times by Thomson Digital, India Printed and bound in Great Britain by Antony Rowe, Chippenham, Wiltshire This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production

I am grateful for the blessings of Sri Vighneswara, Sri Venkateswara, Sri Anjaneya, Sri Satya Sai Baba, my parents and teachers. I am also thankful for the support of my wife, Bonnie, and my children, Anjali and Martin Anil Kumar Sastri. We also recognise Anjali Sastri for her help with the figures, and to Gerry Burtenshaw for assistance in the preparation of the manuscript. V. S. Sastri

I cannot adequately explain my gratitude to my wife, Helen, and Doctors Rafik and Sonia Ghali, whose encouragement, patience and love played an indispensable part in my career as Professor and participant in this book. Edward Ghali

I would like to express my gratitude to my employer and my colleagues at CANMET Materials Technology Laboratory. I sincerely appreciate the exceptional understanding of my wife, Manon, along with my children Anaı¨s, Ismae¨ l, and Gabriel, who sometimes did not get the attention they deserved. Mimoun Elboujdaini

Contents Preface Acknowledgments PART I 1

Introduction and Principles of Corrosion 1.1 Impact of Corrosion 1.2 Preliminary Aspects of Thermodynamics and Kinetics 1.3 Nature of Corrosion Reactions 1.3.1 Electrochemical Cells 1.3.2 Standard Electrode Potentials 1.3.3 Pourbaix Diagrams 1.3.4 Dynamic Electrochemical Processes 1.3.5 Concentration Polarization 1.4 Oxidation and High-temperature Corrosion 1.4.1 Oxidation of Alloys 1.5 Corrosion Prevention 1.6 Design Factors 1.7 Life Prediction Analysis of Materials 1.8 Corrosion Protection 1.8.1 Corrosion Inhibitors 1.8.2 Protective Coatings 1.8.3 Cathodic Protection 1.8.4 Impressed Current Protection 1.8.5 Anodic Protection References

2 Corrosion Testing, Detection, Monitoring and Failure Analysis 2.1 Corrosion Testing 2.1.1 Testing for Environmentally Assisted Cracking (EAC) 2.1.2 Atmospheric Corrosion Testing 2.1.3 Galvanic Corrosion Testing 2.1.4 Testing of Polymeric Materials

xiii xv 1 3 12 18 20 22 23 28 33 46 54 59 63 67 75 80 80 90 100 105 106 106 109 109 111 117 119 120

viii

Contents

2.1.5 Corrosion Testing of Refractories and Ceramic Materials 2.1.6 Testing of Corrosion Inhibitors 2.2 Corrosion Detection and Monitoring 2.2.1 Visual Examination 2.2.2 Laser Methods 2.2.3 Replication Microscopy 2.2.4 Radiographic Methods 2.2.5 Liquid Penetrant Testing Method 2.2.6 Magnetic Particle Testing 2.2.7 Eddy Current Inspection Method 2.2.8 Ultrasonic Inspection Method 2.2.9 Acoustic Emission Technique 2.2.10 Other Nondestructive Methods 2.2.11 Thermal Methods of Inspection 2.3 Failure Analysis 2.3.1 Visual or Macroscopic Examination 2.3.2 Metallography 2.3.3 Microfractography 2.3.4 Fracture Mechanics in Failure Analysis 2.3.5 Determinaton of Residual Stress by X-ray Diffraction 2.3.6 Mechanical Properties 2.3.7 Corrosion and Wear-related Failures 2.3.8 Failure Analysis of Polymeric Materials 2.3.9 Failure Analysis of Ceramic Materials References

122 122 125 127 128 129 129 134 135 136 137 142 145 149 150 154 156 159 159 161 162 164 169 172 173

3 Regulations, Specifications and Safety 3.1 Regulations and Specifications 3.2 Safety Considerations 3.2.1 Safety in the Corrosion Laboratory 3.2.2 General Outline for a Model Chemical Hygiene Plan 3.2.3 Safety Guidelines for Radiation Sources 3.2.4 Nonionizing Radiation Sources 3.2.5 Safety at the Design Stage 3.2.6 Safety in Field Plant Inspection 3.2.7 Safety in Storage and Transport References

177 177 181 192

4 Materials: Metals, Alloys, Steels and Plastics 4.1 Cast Irons 4.2 Carbon and Low-alloy Steels 4.2.1 Corrosion of Carbon Steels in Fresh Waters 4.2.2 Corrosion of Carbon Steels in Seawater 4.2.3 Corrosion of Carbon Steels in Soils

201 201 202 204 207 210

193 194 196 197 198 198 199

Contents

4.3

Stainless Steels 4.3.1 Duplex Stainless Steels 4.3.2 Martensitic Stainless Steels 4.4 Aluminum and Aluminum Alloys 4.4.1 Corrosion Behavior of Aluminum and its Alloys 4.5 Copper and Copper Alloys 4.5.1 Atmospheric Corrosion 4.5.2 Soil Corrosion 4.5.3 General Corrosion in Aqueous Media 4.5.4 Pitting Corrosion 4.5.5 Dealloying 4.5.6 Flow-induced Corrosion 4.5.7 Behavior in Chemical Environments 4.5.8 Biofouling 4.5.9 Stress–Corrosion Cracking 4.5.10 Miscellaneous 4.6 Nickel and its Alloys 4.7 Titanium and its Alloys 4.7.1 Resistance to Waters 4.7.2 Resistance to Chemical Environments 4.7.3 Galvanic Corrosion 4.8 Cobalt Alloys 4.9 Lead and Lead Alloys 4.9.1 Lead Alloys and their Uses 4.10 Magnesium and Magnesium Alloys 4.10.1 Magnesium Alloys 4.10.2 Corrosion of Magnesium and its Alloys 4.11 Zinc and Zinc Alloys 4.11.1 Atmospheric Corrosion 4.11.2 Corrosion in Aqueous Media 4.11.3 Corrosion in Soils 4.11.4 Corrosion of Painted Materials 4.11.5 Corrosion in Concrete 4.11.6 Forms of Corrosion 4.12 Zirconium and its Alloys 4.12.1 Corrosion of Zirconium Alloys 4.12.2 Corrosion of Zirconium Alloys in Acids and Alkalis 4.13 Tin and Tin Plate 4.13.1 Aqueous Corrosion 4.13.2 Corrosion of Tin Plate 4.14 Refractories and Ceramics 4.14.1 Corrosion of Structural Ceramics 4.15 Polymeric Materials 4.15.1 Application of Polymers in Corrosion Control References

214 219 224 227 228 236 237 238 238 241 241 241 242 242 242 244 244 255 257 257 259 259 263 270 270 271 271 282 282 285 287 287 288 289 291 291 292 292 293 296 297 298 300 302 305

Contents

5 Corrosion Economics and Corrosion Management 5.1 Corrosion Economics 5.2 Corrosion Management 5.3 Computer Applications References

311 311 317 319 327

PART II

329

6 The Forms of Corrosion 6.1 Corrosion Reactions 6.2 Corrosion Media 6.2.1 Atmospheric Exposure 6.2.2 Aqueous Environments 6.2.3 Underground Media 6.2.4 Process Media 6.3 Active and Active–Passive Corrosion Behavior 6.4 Forms of Corrosion 6.5 Types and Modes of Corrosion 6.6 The Morphology of Corroded Materials 6.7 Published Corrosion Data 6.7.1 General Corrosion 6.7.2 Galvanic Corrosion 6.7.3 Localized Corrosion 6.7.4 Metallurgically Influenced Corrosion 6.7.5 Microbiologically Influenced Corrosion 6.7.6 Mechanically Assisted Corrosion 6.7.7 Environmentally Induced Cracking References Bibliography

331 331 332 332 332 332 332 333 336 337 338 339 340 344 355 370 384 393 423 453 459

7 Practical Solutions 7.1 Cathodic Protection of Water Mains 7.1.1 Ductile Iron Main 7.1.2 Cast-iron-lined Main Bibliography 7.2 Internal Corrosion of Aluminum Compressed Air Cylinders 7.2.1 Destructive Visual Inspection 7.2.2 Corrosion-induced Cracking 7.2.3 Corrosion Mechanism 7.2.4 Summary Bibliography 7.3 Some Common Failure Modes in Aircraft Structures 7.3.1 Example 1 7.3.2 Example 2 7.3.3 Example 3 7.4 Premature Failure of Tie Rods of a Suspension Bridge 7.4.1 Conclusions

461 461 461 462 465 465 465 466 468 469 469 469 470 471 472 473 476

Contents

Corrosion and Lead Leaching of Domestic Hot and Cold Water Loops in a Building 7.5.1 Hot Water System Corrosion 7.5.2 Conclusions References 7.6 Cathodic Protection of Steel in Concrete Bibliography 7.7 Corrosion of Aluminum Components in the Glass Curtain Wall of a Building 7.7.1 Introduction 7.7.2 Observations 7.7.3 Recommendations References 7.8 Corrosion in a Water Cooling System 7.9 Pitting Corrosion of 90/10 Cupronickel Chiller Tubes 7.9.1 Optical Examination 7.9.2 SEM and EDS Studies 7.9.3 Pitting Initiation and Propagation Mechanism 7.9.4 Conclusion Bibliography 7.10 Weld Metal Overlay: a Cost-effective Solution to High-temperature Corrosion and Wear Problems Bibliography 7.11 Equipment Cracking Failure Case Studies 7.11.1 Industrial Engine Crankshaft Failure 7.11.2 Electric Motor Drive Shaft Failure 7.12 Failure of a Conveyor Drive Shaft 7.12.1 Conclusions 7.13 Failure Analysis of Copper Pipe in a Sprinkler System 7.13.1 Observations 7.13.2 Conclusions 7.14 Failure of Rock Bolts 7.14.1 Corrosion Modes of Rock Bolts 7.14.2 Fracture and Failure References 7.15 Failure Analysis of 316L Stainless Steel Tubing of a High-pressure Still Condenser 7.15.1 Problem 7.15.2 Material 7.15.3 Results 7.15.4 SEM Examination 7.15.5 Conclusions 7.15.6 Prevention References 7.16 Failure of a Landing Gear Steel Pin Reference

7.5

476 476 478 478 478 480 480 481 481 483 483 483 486 486 486 487 489 489 489 492 492 492 495 499 500 501 501 504 504 505 505 509 509 509 510 510 511 512 514 514 515 516

xii

Contents

7.17 Hydrogen-induced Cracking 7.17.1 Extent of Problem: Failures due to Hydrogen-induced Cracking 7.17.2 HIC Development and Failures Occur Predominantly in Welded Pipe 7.17.3 Pipeline Failure 7.17.4 Mechanism of Hydrogen-induced Cracking References 7.18 Micromechanisms of Liquid and Solid Metal-induced Embrittlement 7.18.1 Liquid Metal-induced Embrittlement (LMIE) 7.18.2 Conclusion References 7.19 Nitrate SCC of Carbon Steel in the Heat Recovery Steam Generators of a Co-generation Plant 7.19.1 Materials 7.19.2 Conclusions References 7.20 Performance of Stainless Steel Rebar in Concrete Bibliography 7.21 Corrosion of an Oil Storage Tank 7.21.1 Sampling 7.21.2 Color 7.21.3 Corrosion 7.21.4 Pitting 7.21.5 Pitted Surface 7.21.6 Factors Affecting Pitting 7.21.7 Discussion 7.21.8 Conclusions References 7.22 Corrosion of a Carbon Steel Tank in a Phosphatizing Process 7.22.1 Galvanic Corrosion 7.22.2 Localized Corrosion 7.22.3 Overall Corrosion Scenario 7.23 Underground Corrosion of Water Pipes in Cities 7.23.1 Observations 7.24 Corrosion in Drilling and Well Stimulation 7.24.1 Materials 7.24.2 Corrosion Inhibition 7.24.3 Corrosion in Underbalanced Drilling Operations References Index

516 518 519 523 523 524 525 525 528 528 529 529 532 532 533 535 536 536 537 538 538 539 540 540 541 541 541 543 545 547 547 547 549 550 550 550 551 553

Preface There are many books and monographs on corrosion, such as Corrosion and Corrosion Control by H. H. Uhlig and R. W. Revie; Corrosion Engineering by M. G. Fontana and N. D. Greene; Principles and Prevention of Corrosion by D. A. Jones; An Introduction to Corrosion and Protection of Metals by G. Wranglen; and Corrosion for Science and Engineering by K. R. Trethewey and J. Chamberlain. The present title differs from existing books in more ways than one, such as the chapters dealing with practical solutions. The title was chosen to reflect the content of the subject matter, which is presented in two parts. The first chapter presents the historical development of corrosion concepts, such as the electrochemical theory of corrosion, the economic significance of corrosion and its impact, cathodic protection, Faraday’s laws, the role of oxygen, passivity, inhibitors and their classification, the role of thermodynamics, the historical development of the corrosion literature, and the establishment of scientific organizations dealing with corrosion, and centres and laboratories for studies on corrosion phenomena, progressive development of the scientific literature on corrosion, safety and its impact. The role of thermodynamics and kinetics in corrosion, electrochemical principles of corrosion, Pourbaix diagrams, the Helmholtz double layer and its significance in corrosion, electrochemical polarization, Tafel plots, activation polarization, hydrogen overvoltage, mixed potential theory of corrosion, AC impedance and potential noise in corrosion phenomena, high-temperature corrosion, corrosion prevention strategies such as design factors, corrosion-based life prediction analysis of materials, corrosion inhibitors and their role in corrosion prevention, coatings and their role in corrosion control, cathodic protection and impressed current protection are presented. The second chapter deals with corrosion testing for environmentally assisted cracking, atmospheric corrosion, galvanic corrosion, tests for degradation of polymeric materials, refractories and ceramic materials and corrosion inhibitors. This is followed by a discussion of corrosion detection and monitoring by methods such as visual examination, laser inspection, replication microscopy, radiographic inspection, neutron radiography, liquid penetrant method, eddy current method, ultrasonic testing, acoustic emission testing, finite element analysis, strain gage methods and thermal methods of inspection. Failure analysis consisting of various modes of failure by different damage mechanisms are presented, including microfractography, fracture mechanics, determination of

xiv

Preface

residual stress by X-ray diffraction, role of surface analysis techniques in failure analysis, including failure in polymeric and ceramic materials. The next chapter deals with regulations, specification and safety. The regulations and specifications of materials used in industry are briefly discussed. This is followed by safety considerations to be observed both in the corrosion laboratory and field inspection. Hazard identification techniques at different stages of the project, a checklist for process hazard analysis, safety procedures in the corrosion laboratory, including a model chemical hygiene plan, guidelines in using radiation sources including lasers and safety considerations in the design stage and field plant inspection are presented. Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. Corrosion economics and corrosion management forms the theme of the fifth chapter. Discounted cash flow calculations, depreciation, the declining balance method, double declining method, modified accelerated cost recovery system and present worth calculation procedures are given, together with examples. In the second part, corrosion management, including the people factor in corrosion failure is briefly presented. Some of the expert systems presently available in the literature are briefly discussed. The second part of the book consists of two chapters; namely the forms of corrosion and practical solutions. The chapter, ‘Forms of Corrosion’ consists of a discussion of corrosion reactions, corrosion media, active and active–passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. The last chapter is a collection of case histories or practical solutions the authors have provided to various clients. These solutions span a wide range of industrial problems in a variety of environments frequently encountered. It is the experience of the authors that the material in the first part of the book can be covered in one semester lasting 12 weeks. The second part of the book can be covered in a subsequent semester lasting 12 weeks. It is also possible that some laboratory work can be carried out by the students when the instructor is teaching the second part of the book. The authors have received their education in universities in North America and Europe and have a combined experience of approximately 100 years in corrosion and its mitigation. The present monograph is a product of this rich experience.

Acknowledgments I cannot adequately express my gratitude to my wife, Bonnie Sastri whose efforts and encouragement played the greatest role in sustaining me through the challenge of writing this book. Our children, Anjali and Martin Sastri, also deserve my appreciation for their help and understanding. Grateful appreciation is expressed to the American Chemical Society, the American Society of Metals, Plenum Publishing, EG&G Princeton Applied Research, and Longman Publishers, NACE International, Houston, Texas, John Wiley & Sons, N.Y., USA, McGraw-Hill, N.Y., Elsevier, Oxford, UK for their permission to reproduce figures or tables from the literature. V. S. Sastri E. Ghali M. Elboujdaini

Please note that figures 7.53 (Plate 2), 7.54 (Plate 3), 7.55 (Plate 4), 7.56 (Plate 5), 7.57 (Plate 6), 7.58 (Plate 7), 7.59, 7.60 and 7.61 are ß Crown in Right of Canada, sourced from Failure Investigation Report #0505007 (Report on Copper Pipe or Sprinkler Pipes Experiencing MIC), Mr. Nick Dawe, P. Eng, D/Chief Building Services - PSEPC.

Part I

1 Introduction and Principles of Corrosion The term corrosion has its origin in Latin. The Latin term rodere means ‘gnawing’ and corrodere means ‘gnawing to pieces’. It is rather interesting to examine the historical aspects of the developments of corrosion. Metallic corrosion has no doubt been a problem since common metals were first put to use. Most metals occur in nature as compounds, such as oxides, sulfides, silicates or carbonates (very few metals occur in native form). The obvious reason is the thermodynamic stability of the compounds as opposed to the metals. The process of extraction of a metal from the ore is reduction. 2 Fe2 O3 þ 3 C ! 4 Fe þ 3 CO2 In the extraction of iron, the oxide is reduced to metallic iron. On the other hand, the oxidation of iron to produce the brown iron oxide commonly known as rust is the opposite reaction to the production of the metal from the oxide. The extraction of iron from the oxide, must be conducted with utmost careful control of the conditions, such that the backward reaction is prevented. During the Gupta Dynasty (320–480 A.D.) the production of iron in India achieved a remarkable degree of sophistication as attested by the Dhar Pillar, a seven-tonne, onepiece iron column made in the fourth century A.D. This implies that the production of metallic iron from the ores was a well-established process, and the people involved at that time were aware of the reverse reaction involving the oxidation of iron to produce the oxide (the familiar rusting of iron). Other examples involve the use of copper nails coated with lead by the Greeks in the construction of lead-covered decks for ships.1 They probably realized that metallic couples of common metals are undesirable in seawater. Protection of iron by bitumen, tar, etc., was known and practiced by the Romans. The earliest published accounts of the causes of corrosion are the two publications by Robert Boyle (1627–1691) entitled ‘Of the Mechanical Origin of Corrosiveness’ and Corrosion Prevention and Protection: Practical Solutions # 2007 John Wiley & Sons, Ltd

V. S. Sastri, E. Ghali and M. Elboujdaini

Corrosion Prevention and Protection

‘Of the Mechanical Origin of Corrodibility’, which appeared in 1675 in London.2 It was not until the turn of the19th century3,4 that some of the basic principles were understood, soon after the discovery of the galvanic cell and Davy’s theory on the close relationship between electricity and chemical changes.5 The impetus for further developments was the recognition of the economic significance of corrosion phenomenon during the 19th century that led the British Association for the Advancement of Science to sponsor corrosion testing projects such as the corrosion of cast and wrought iron in river and seawater atmospheres in 1837. Early academic interest in corrosion phenomenon (up to the First World War) was followed by industrial interest due to the occurrence of equipment failures. An example of this is the corrosion-related failure of condenser tubes as reported by the Institute of Metals and the British Non-ferrous Metals Research Association in 1911. This initiative led to the development of new corrosion-resistant alloys, and the corrosion related failure of condenser tubes in the Second World War was an insignificant problem. Corrosion and its control mean the corrosion process and the measures taken to control or keep in check the corrosion process. Sometimes it is also referred to as corrosion, prevention and protection. Although the terms ‘prevention’ and ‘protection’ appear to be synonymous, prevention means measures taken to control corrosion to a limited extent while protection means extensive or more comprehensive measures taken to control the corrosion process. In more general terms preventive measures are knowledge-based while protection involves both known and unknown factors, such as natural disasters. The heart of corrosion science has been identified as electrochemical science coupled with the thermodynamic and kinetic values. Other limbs are oxidation and hightemperature oxidation of metals, protective coatings, passivity, inhibitors, microbialinduced corrosion, corrosion fatigue, hydrogen embrittlement and corrosion-resistant alloys. Having identified the limbs of corrosion science, it is instructive to examine how the various aspects came into existence over a period of time. The French chemist Louis Jacques Thenard first enunciated electrochemical nature of corrosion phenomenon explicitly in 1819. Some research activities that led to the firm electrochemical foundations of corrosion process are summarized below: Sir Humphry Davy Auguste de la Rive Michael Faraday

Svante Arrhenius W.R. Whitney A.S. Cushman 9 Walker > > = Cederholm Bent > > ; William Tilden

1824 1830 1834–1840

1901– 1903 1907 9 1907 > > = 1908

> > ;

Principle of cathodic protection Established best quality of zinc for galvanic batteries Provided relations between chemical action and generation of electric currents based on Faraday’s laws Postulated the formation of microcells Confirmed the theory of microcells Established role of oxygen in corrosion as a cathodic stimulator

Corey Finnegan Kay Thompson A: Thiel Luckmann

Introduction and Principles of Corrosion

1939

)

1940 1928

Heyn and Bauer

1908

Whitman and Russell U. Evans G.V. Akimov

9 1924 = 1928 ; 1935

Investigated attack of iron by oxygen-free water Investigated attack of iron by dilute alkali with liberation of hydrogen Corrosion studies of iron and steel, both alone and in contact with other metals, leading to the concept that iron in contact with nobler metal increased the corrosion rate, while in contact with a base metal resulted in partial or complete protection Observed increased corrosion rate when a small anode is connected to a large cathode

Other important and related phenomena in corrosion and their historical development are summarized below: John Stewart MacArthur

1887

P.F. Thompson

1947

Process of cyanide dissolution of gold (gold is not soluble in hot acids) Dissolution of gold in dilute cyanide solutions recognized as electrochemical process

Concept of passivity James Keir Christian Friedrich Scho¨ nbein W. Mu¨ ller (Konopicky and Willi Machu) Bengough (Stuart, Lee and Wormweil)

1790 1799–1868 1927 1927

Observed that iron in conc. nitric acid altered in its properties Suggested the state of iron in conc. HNO3 as passivity Posulated the mathematical basis of the mechanism of anodic passivation Systematic and carefully controlled experimental work on passivity

Role of oxygen 1900 1905

Hydrogen peroxide was detected during the corrosion of metals The view that acids are required for corrosion to occur was dispelled by the observation of rusting of iron in water and oxygen

Corrosion Prevention and Protection

Marianini Adie Warburg V.A. Kistiakowsky Aston

1830 1845 1889 1908 1916

McKay

1922

U.R. Evans

1923

Evans and co-workers

1931–1934

The research work of these scientists indicated the electric currents due to the variations in oxygen concentrations Role of local differences in oxygen concentration in the process of rusting of iron Currents due to a single metal of varying metal ion concentrations Differential aerations and their role in metallic corrosion Electric currents due to corrosion of metal in salt solutions were measured and a quantitative electrochemical basis of corrosion was propounded. The oxygen-rich region becomes cathodic and the metal is protected, and the lower oxygen region, being anodic, is attacked

Inhibitors Roman civilization

Murangoni Stephanelli Chyzewski

—

Protection of iron by bitumen, tar, extracts of glue, gelatin and bran to inhibit corrosion of iron in acid

1872 1938

—

John Samuel Frost Roetheli and Brown

1930

Friend

1920

Herzog

1936

Classified inhibitors as cathodic and anodic inhibitors Distinction between inhibitive paints and mechanically excluding paints was made based on laboratory, and field tests Development of paints containing zinc dust Protective property of coating varied and depended on the rate of supply of oxygen to the surface Colloidal solution of ferric hydroxide acts as an oxygen carrier, passing between ferrous and ferric states Posulated that iron, on long exposure to water, becomes being covered by a magnetite overlaid with ferric hydroxide. Magnetite layer acts as cathode and ferric hydroxide is cathodically converted to hydrated

Introduction and Principles of Corrosion

V.S. Sastri

1990

V.S. Sastri

1988

V.S. Sastri, J.R. Perumareddi and M. Elboujdaini V.S. Sastri, J.R. Perumareddi and M. Elboujdaini

1994

2005

magnetite. Hydrated magnetite may lose water and reinforce the pre-existing magnetite or absorb oxygen from air to give ferric hydroxide Modern classification of inhibitors as hard, soft and borderline inhibitors (11th International Corrosion Congress, V. 3, p. 55) Classification of corrosion inhibition mechanisms such as interface inhibition, interphase inhibition, intraphase inhibition and precipitation coating (Corrosion ‘88, Paper 155) Novel theoretical method of selection of inhibitors (Corrosion, 50, 432, 1994) Sastri equation relating the percent inhibition to the fractional electronic charge on the inhibitor. (Corrosion Eng. Sci. & Tech., 40, 270, 2005)

High-temperature oxidation Gustav Tammann

N.B. Pilling and R.E. Bedworth Leonard B. Pfeil

1920

1922 1923 1929

Portevin Pre´ tet Jolivet Carl Wagner

1934

Hoar, Price Mott, Cabrera

1938 1939, 1948

Enumerated ‘Parabolic Law’ (i.e., rate of oxidation of metal decreases as oxide layer thickness increases) Logarithmic law of oxidation of metals Distinction between porous and nonporous oxide layer Concept of movement of metal outward rather than oxygen inward into the oxide layer Extensive studies on the oxidation of iron and its alloys High-temperature oxidation involves passage of ions and electrons through the growing oxide layer. Postulated an equation relating oxidation rate with the electrical properties of the oxide layer Derivation of Wagner’s equation Oxide film growth controlled by ions jumping from site to site over intervening energy barriers

Corrosion Prevention and Protection

Karl Houffe Ilschner

—

Tammann

1920–1926

Constable

1927

Finch Quarrell

1933

Significant work on the oxidation of alloys. Also criticism of Mott’s theory Interference method of obtaining thickness of oxide films Spectroscopic method to obtain thickness of oxide film X-ray and electron diffraction methods to study oxide films

Microbiological corrosion R.H. Gaines

1910

Corrosion fatigue

1900

Stress–corrosion cracking

1900

Sulfate-reducing bacteria in soils produce H2S and cause corrosion Alternating stresses and chemical environment together cause corrosion fatigue Applied stress and chemical environment causing stress–corrosion cracking

Hydrogen embrittlement Haber–Bosch process for synthesis of ammonia

1916

Microcracks in the steel reactor were observed due to the reaction of hydrogen with carbon in the steel to produce methane. Mo and Cr were found to prevent hydrogen embrittlement

Role of thermodynamics —

Marcel Pourbaix

—

1940

Corrosion of metal obeys the laws of thermodynamics. This was recognized in the early development of corrosion science Pourbaix diagrams involving pH and potential give regions of corrosion, immunity and passivity

Kinetics Evans, Hoar

1932

F. Habashi

1965

Quantitative correlation of corrosion rates with measured electrochemical reaction rates Validity of a single kinetic law irrespective of the metal, composition of aqueous phase, and evolution of hydrogen when no insoluble products, scales or films are formed

Introduction and Principles of Corrosion

The number of published scientific papers through 1907–2003 illustrates development of the corrosion science in the form of published scientific literature as shown below: Title Corrosion Corrosion and protection Corrosion and prevention

1907

1950

2000

2003

35 3 3

922 122 320

10985 1162 1639

10655 1050 1358

The journals that came into existence are given below: Title

Year

Corrosion Corrosion Science British Corrosion Journal Werkstoffe und Korrosion Corrosion Prevention and Control Anti-corrosion Methods and Materials Materials Performance

1945 1961 1965 1950 1954 1962 1962

Some of the leading organizations championing corrosion science, which were founded, are detailed below. This list does not include academic institutions. American Society for Testing Materials (ASTM) American Society of Metals (ASM) Corrosion Division of the Electrochemical Society National Association of Corrosion Engineers Comite´ International de Thermodynamique et Cine´ tique E´ lectrochimique (CITCE) International Society of Electrochemistry (ISE) International Corrosion Council The Corrosion Group of the Society of Chemical Industry Belgium Center for Corrosion Study (CEBELCOR) Commission of Electrochemistry National Corrosion Centre (Australia) Australian Corrosion Association Chinese Society of Corrosion and Protection National Association of Corrosion Engineers (in Canada)

1898 1913 1942 1943 1949 1971 1961 1951 1951 1952 1980 1980 —

Research groups, which became active in the field of corrosion in the early stages, are shown below. It is prudent to state that the list is by no means exhaustive. Massachusetts Institute of Technology National Bureau of Standards

Corrosion Prevention and Protection

Ohio State University University of Texas University of California, Los Angeles National Research Council, Ottawa Cambridge University Technical University, Vienna Industrial laboratories such as U.S. Steel, International Nickel company and Aluminum Company of America, DuPont have also initiated their own corrosion research. The progress made in the scientific approach and the degree of sophistication attained over the years becomes evident from the following title papers: 1. A.S. Cushman, Corrosion of Iron as an Electrolytic Phenomenon, U.S. Bur. Agr., Electrochemical Metallurgy Industry, Vol. 5, No. 256, C.A., 1907, p. 2360. Hydrogen ions are the primary cause of rusting and oxygen the secondary cause. Iron passes into solution in the form of ferrous ions as the result of galvanic action; the ferrous ions are then oxidized by the oxygen of the air to ferric ions. Alkaline solutions prevent rusting because they contain no hydrogen ions. Chromic acid and its salts prevent rusting because an oxygen film is formed, and the iron becomes polarized in the sense of becoming an oxygen electrode.

2. R.H. Brown, G.C. English and R.D. Williams, The Role of Polarization in Electrochemical Corrosion, NACE Conference, St. Louis, Missiouri, USA, 4–7 April 1950. In its most practical aspects as well as in its fundamental mechanisms electrochemical corrosion is almost always associated with irreversible electrode phenomena. The multitude of factors involved in these phenomena may be defined as electrochemical polarization. Idealized schematic as well as actual polarization diagrams are discussed. Methods of correlating polarization with corrosion data such as weight loss are shown. A method for obtaining the contribution made by the polarization of each electrode reaction to the total polarization observed at an electrode is described along with the implications, thereof in the evaluation of the true over-voltage values. In addition, other factors, which may fall within a broad definition of polarization, are treated. The relationship of the so-called IR drop or true ohmic resistance at metal liquid interfaces to polarization diagrams, and to over voltage concept is discussed.

3. R. Balasubramanium, A.V. Ramesh Kumar, Corrosion Resistance of the Dhar Iron Pillar, Corrosion Science, 45, 2451–2465, 2003. The corrosion resistance of the 950 year old Dhar iron pillar has been addressed. The microstructure of a Dhar pillar iron sample exhibited characteristics typical of ancient Indian iron. Intergranular cracking indicated P segregation to the grain boundaries. The potentiodynamic polarization behaviour of the Dhar pillar iron and mild steel, evaluated in solutions of pH 1 and 7.6, indicate that the pillar iron is inferior to mild steel under complete immersion conditions. However, the excellent atmospheric corrosion resistance of the phosphoric Dhar pillar iron is due to the formation of a protective passive film on the surface. Rust analysis revealed the presence of crystalline magnetite (Fe3-xO4), a-Fe2O3 (hematite), goethite (a-FeOOH), lepidocrocite (g-FeOOH), akaganeite (b-FeOOH) and phosphates, and amorphous d-FeOOH phases. The rust cross-section revealed a layered structure at some locations.

Introduction and Principles of Corrosion

The experimental techniques used are optical and scanning electron microscopes, electron microprobe, potentiodynamic polarization, X-ray diffraction, Fourier transform infrared spectroscopy and transmission Mo¨ ssbauer spectroscopy. Some significant titles, which are worth noting are shown below: Gustav Tammann

Ulick R. Evans

Marcel Pourbaix

Lehrbuch der Metallkunde Die Aggregatzu¨ stande Lehrbuch der heterogenen Gleichgewichte The Corrosion of Metals Metallic Corrosion, Passivity and Protection An Introduction to Metallic Corrosion The Corrosion and Oxidation of Metals (first supplementary volume) (second supplementary volume) Thermodynamics of Dilute Aqueous Solutions Atlas of Electrochemical Equilibria in Aqueous solutions Atlas of Chemical and Electrochemical Equilibria in the presence of Gaseous Phase Lectures on Electrochemical Corrosion

Herbert H. Uhlig

H.H. Uhlig, R.W. Revie Mars Guy Fontana N.D. Greene J.I. Bregman V.S. Sastri I.L. Rozenfeld H. Van Droffelar J.T.N. Atkinson Kenneth R. Trethewey John Chamberlain G. Wranglen D.A. Jones P.R. Roberge P.R. Roberge K. Seymour Coburn L.S. Van Delinder A.R. Troiano

Uhlig’s Corrosion Handbook (2nd edn) Corrosion and Corrosion Control Corrosion and Corrosion Control (revised) Corrosion Corrosion Engineering Corrosion Inhibitors Corrosion Inhibitors Principles and Applications Corrosion Inhibitors Corrosion and its Control An introduction to the subject Corrosion for Science and Engineering An introduction to Corrosion and Protection of Metals Principles and Prevention of Corrosion Handbook of Corrosion Engineering Corrosion Doctor website on Internet Corrosion Corrosion Basics – an introduction Hydrogen Embrittlement and Stress Corrosion Cracking

1914 1922 1924 1924 1937 1948 1960 1968 1976

1948 2000 1963 1986 1957 1967 1986 1963 1998 1982 1995 1988, 1995 1972 1992 1999 1999 1984 1984 1984

Corrosion Prevention and Protection

G. Charles Munger W.H. Ailor J. Yahalom J.M. West E. Mattsson F. Hine J. Toucek P.A. Schweitzer G. Welsch J.B. Little Y.I. Kuznetsov P.A. Schweitzer L.L. Shreir R.S. Munn R.H. Jones A.J. McEvily G. Prentice A.S. Bradford R. Baboian

Corrosion Prevention by Protective Coatings Atmospheric Corrosion Stress Corrosion Cracking Basic Corrosion and Oxidation Basic Corrosion Technology for Scientists and Engineers Localized Corrosion Theoretical Aspects of the Localized Corrosion of Metals Encyclopedia of Corrosion Technology Oxidation and Corrosion of Intermetallic Alloys Microbiologically Influenced Corrosion Organic Inhibitors for Corrosion of Metals Encyclopedia of Corrosion Technology Corrosion Computer Modeling in Corrosion Stress–Corrosion Cracking Atlas of Stress–Corrosion and Corrosion Fatigue Curves Perspectives on Corrosion Corrosion Control NACE Corrosion Engineer’s Reference Book

1984, 1999 1982 1980 1980 1989 1988 1985 1998 1996 1997 1996 1998 1994 1992 1992 1990 1990 2001 2002

1.1 Impact of Corrosion There are three areas of concern when corrosion and its prevention are considered. The three major factors are economics, safety and environmental damage. Metallic corrosion, although seemingly innocuous, indeed affects many sectors of a nation’s economy. The National Bureau of Standards (NBS) in collaboration with Battelle Columbus Laboratory (BCL) studied the costs of corrosion in USA using the input/output model.7 Some elements of the costs of corrosion used in the model are shown below: Capital costs Replacement of equipment and buildings Excess capacity Redundant equipment Control costs Maintenance and repair Corrosion control

Introduction and Principles of Corrosion

Table 1.1 Corrosion costs in the United States (billions of dollars) Industry All industries Automotive Aircraft Others

1975 Total Avoidable Total Avoidable Total Avoidable Total Available

82.0 33.0 31.4 23.1 3.0 0.7 47.6 9.3

1995

2005

296 104 94.0 65.0 13.0 3.0 159.0 36.0

403 142 125.3 86.7 18.0 4.2 260 50

Design costs Materials of construction Corrosion allowance Special processing Associated costs

Loss of product Technical support Insurance Parts and equipment inventory

The data resulting from the calculations using I/O model are given in Table 1.1. The data given for the year 2005 are olny estimates. The corrosion costs in Canada8 as of 2003, along with the various sectors are given in Tables 1.2 and 1.3. The cost of corrosion in other countries in the world is given in Table 1.4. The staggering costs of corrosion affect the national economy significantly and it is meaningful and justified that the corrosion scientists involved should adopt corrosion control measures so that significant savings are achieved. Reference in this regard may be made to a report17 of the NACE Task Group T-3C-1 entitled, ‘Economics of Corrosion’ which deals with: (i) economic techniques that can be used by personnel as a decision making tool; (ii) facilitating communications between corrosion scientists and the management; and (iii) justifying the investments in corrosion preventive measures to achieve long-term benefits. Table 1.2 Corrosion costs in Canada Sector Utilities Transportation Infrastructure Government Production and manufacturing Total

$ billion 8.2 5.1 3.8 3.5 3.0 23.6

Corrosion Prevention and Protection Table 1.3 Total corrosion costs in Canada $ billion Total direct cost of corrosion Cost of corrosion (extrapolated to Canada economy) Estimated savings by corrosion control

23.6 46.4 14.00

It is evident from the data presented on the economics of corrosion that corrosion costs amount to about 2–4% of GNP, and about 25% of the costs are avoidable by adopting corrosion control measures. The measures taken to combat corrosion in UK, USA, Australia, China and Canada have been discussed.18 The following is a short summary of the activities in various parts of the world to combat corrosion. UK National Corrosion Service

1975

National Corrosion Coordination Centre

1980

United States

Australia

1982

UK National corrosion service Educating engineering undergraduates in corrosion awareness. Published and distributed 15 guides on corrosion, and 6 booklets on ‘controlling corrosion’. Assembled corrosion prevention directory of corrosion personnel Multiclient 50/50 cost-shared research on high-temperature corrosion, metal finishing, microbial corrosion and expert systems in corrosion engineering National Association of corrosion engineers in collaboration with National Bureau of Standards developed corrosion data program National Corrosion Centre. Established nationwide – referral service for corrosion problems

Table 1.4 Corrosion costs Country

Year

Corrosion costs ($)

Percent of GNP 3.5 3.0

UK West Germany Sweden

1969–1970 1968–1969 1964

3.2 billion 1.5 billion 58–77 million

Finland Russia Australia India Japan

1965 1969 1973 1960–1961 1976–1977

47–62 million 6.7 billion A $470 million $320 million 9.2 billion

2.0 1.5 1.8

Avoidable cost $ 0.73 billion 0.375 billion 15–19 million 20–27 million

Reference 9 10 11 12 13 14 15 16

Introduction and Principles of Corrosion

Peoples Republic of China

1980

Canada

Educating and training of personnel in corrosion. Organized 15 technical courses. Established 11 institutes of higher learning to give courses in corrosion So far no organization has been established to implement corrosion control methods to achieve savings of the order of several millions of dollars. Proposal to establish a national corrosion secretariat19–21 to educate industrial personnel, operate a referral service, assemble directory of corrosion experts, initiate site visits and show movies on corrosion and establish 50/50 cost-shared research projects was submitted in 1994

The most important factor of impact of corrosion is safety. This factor must be uppermost in the minds of personnel working in industry. Although corrosion of materials is as severe as cancer or AIDS in its economic and safety consequences to a nation, the issue has not received much attention from the governmental organizations and it continues to be conveniently ignored. It is now useful to turn our attention to some common industries, which have contributed to the so-called modern way of life of comfort and examine the role of corrosion, and its consequences in these industries.

Industry

Corrosion Problems

Aircraft

Pitting and crevice corrosion develop intergranular stress–corrosion cracking and corrosion fatigue Fatal crashes in early times due to fatigue

Automotive

General corrosion of the body of cars due mainly to deicing salts; car scrapped due to accidents, obsolescence and corrosion, correlation between injuries in accidents and age of the car

Remedy Damage tolerance standards introduced into the design; corrosion inhibiting primers and sealants are used. Titanium alloys are used. Fiber-reinforced plastics are used. Nondestructive inspection such as ultrasonic, eddy current, optical and radiography methods are used periodically Improvement in the body of the car and its resistance to corrosion. Rustproofing treatments were developed

(contd.)

Corrosion Prevention and Protection

(contd.) Industry Chemicals

Defense

Construction

Marine

Electronics

Medicine

Corrosion Problems Chemicals such as HF and hot NaOH among others, cause severe corrosion problems. Severe corrosion and stress– corrosion cracking. Accidents involving cyclohexene (Flixborough) and HCN (Bhopal) caused deaths of several thousands Failure of weapons due to corrosion. Examples can be torpedo or missile (generally made of lightweight alloys). An example is failure of ammunition Corrosion of steel bars in reinforced structures. Corrosion of suspension bridge over Severn River, Pelham bridge in Lincoln, bridges in New York state and other areas containing deicing salts. This is an expensive problem that needs immediate attention Corrosion of metal structures due to the most aggressive environment. Corrosion of iron hulls observed in 1800. Many accidents due to loss of ships resulting in loss of many lives Galvanic corrosion between Al and Au. Corrosion of power cables due to sheath damage caused by lightning, rodents, etc. Medical implants suffer corrosion. Fatigue failure of heart valve. Hip joints made of titanium and steel may fail in warm serum solutions

Remedy Corrosion-resistant materials must be chosen along with periodic inspection

Periodic tests to ensure the safe operation

Protective coatings

Selection of corrosion resistant materials and proper coatings.

Sheathing of coated Al and clad metals of copper adjacent to steels

Improved designs

Introduction and Principles of Corrosion

(contd.) Industry

Corrosion Problems

Remedy

Nuclear

Unique corrosion problems although Zr and its alloys are used. Loss in capacity of the reactors due to corrosion. Radiation hazards. Long-term storage of spent fuel causes serious problems Corrosion due to SO2 in coal-fired power plants H2S, CO2, NaCl are corrosive. Hydrogen induced cracking, pitting, sulphide-stress cracking, stress-corrosion cracking and SOHIC occur. Pipeline failures have occurred. The effect is pollution of environment

Progress has been made in long-term storage of spent fuel. Hydrogen-induced cracking continues to be a significant problem

Fossil fuels Energy, oil and gas

Flue gas scrubber system eliminates SO2 Inhibitors are used in existing pipelines, clean steels devoid of MnS inclusions are chosen for new pipelines

The foregoing discussion of the industries, with the associated corrosion-related accidents or failures, emphasizes the importance of safety to personnel involved in the industries as the most important factor that is reflected as the impact of corrosion. Corrosion has a tremendous effect on the environment in the sense corrosion-related failure of oil pipelines or gas pipelines or oil tankers can have very detrimental effect on the environment in the form of water and air pollution, leading to the demise of aquatic life. Corrosion-related accidents can in principle destroy natural fauna or flora since these are irreplaceable. Another aspect of concern is the limit of resources in the world. Some decades ago, the term recycling was almost unknown. At the present time recycling is a household term and recycling of metal products, paper and plastics has been recognized for the important role it plays in conserving our resources. We have reached a level of maturity to be able to recognize that our natural resources are limited and finite in our world, and that methods to conserve these resources by recycling and other methods have a prominent role to play. Corrosion prevention and protection arrests the degradation of metals/materials and contributes in a significant way to the conservation of resources with minimum damage to the ecosystems. Since materials are prone to corrosion it is useful to know the factors both direct and indirect, which affect the choice of materials and their related corrosion resistance of paramount importance in the design of an engineering structure.

Corrosion Prevention and Protection

Materials selection Direct factors

Indirect factors

Cost Appearance Ease of availability Ease of fabrication Application Environmental

Mechanical properties Corrosion resistance Metallurgical Safety

In the design of a new plant the first step involves selection of materials, which should have reasonable corrosion-resistance properties in the environment of chosen application. This requires the corrosion studies of the material. The corrosion framework encompasses electrochemistry, corrosion modes, environments, inhibitors, metallurgical factors, coatings, design considerations, corrosion detection, monitoring, and testing and failure analysis. Within the framework of materials, knowledge of economics, safety, specifications and corrosion management is essential. It is also necessary for the corrosion scientist to have knowledge of the various forms of corrosion encountered in some familiar environments along with the recommended solutions. To this end, efforts are made in the following sections to discuss as succinctly as possible the required material for both students and practicing corrosion scientists.

1.2 Preliminary Aspects of Thermodynamics and Kinetics Before embarking on the discussion of corrosion and the electrochemical nature of corrosion it is useful to examine corrosion process from the point of view of energetics. Thermodynamics deals with energy and its changes in reactions. Reactions are viewed in terms of changes in free energy. According to the first law of thermodynamics energy can be neither created nor destroyed. The second law states that the free energy is released from the system to surroundings in all spontaneous changes. Corrosion reactions are spontaneous and are governed by the laws of thermodynamics. Consider the reaction: kf

A þ BÐC þ D kb

where kf is the forward reaction rate constant and kb the rate constant for the backward reaction. From the point of view of transition state theory, A and B react to form an activated complex (A B), which leads to the products C and D, depending upon energetically favourable conditions. The energy profile of the reaction is represented in Figure 1.1. G is the energy difference between the reactants and products, and since the reaction is spontaneous G must have a negative value. Note that G6¼ is the energy barrier that A and B have to surmount to give products C and D. This arises from the internal energy of A and B. Once the activated complex (A B) is formed is can revert to A and B or give C and D, and the latter is energetically favoured because the reaction under

Introduction and Principles of Corrosion

Transition state

Free energy, G

Reactants A+B

Products C+D

reaction co-ordinate

Figure 1.1 Energy profile of a reaction

consideration is spontaneous, and the transition state with the complex (A B) reaches lower energy by giving C and D. Rate of forward reaction ¼ kf ½A ½B

Rate of reverse reaction ¼ kb ½C ½D

At equilibrium we have kf ½A ½B ¼ kb ½C ½D

K¼

kf ½C ½D

¼ kb ½A ½B

When kf kb we have a large value for K and the forward reaction is predominant and the reverse reaction is negligible. On the other hand when kf kb the reverse reaction is favoured and the value of K is very small. For the forward reaction the temperature dependence is given by the Arrhenius equation. kf ¼ Ae G

6¼

=RT

where A and R are constants, T the temperature and G6¼ the activation energy. The rate constant increases with increase in temperature. For the reverse reaction (i.e., conversion of C and D into A and B), the activation energy required will be greater than G6¼ , and the reverse reaction is not favored. From thermodynamics we have the relationship between free energy and equilibrium constant for a reaction: G ¼ RT ln K G ¼ 2:303 RT log K

Corrosion Prevention and Protection

G for the reaction can be calculated by using the values for reactants and products. G298 ¼ ½G298 ðCÞ þ G298 ðDÞ ½G298 ðAÞ þ G298 ðBÞ

When the equilibrium constant is known it is possible to obtain G . G can also be calculated from G values of products and reactants given in thermochemical tables. When G is negative the reaction is favoured in the forward direction. Thus for corrosion reactions to occur the G values have to be negative. For copper and gold the oxidation reactions are: Cu þ H2 O þ 1=2 O2 ! Cu ðOHÞ2 G ¼ 119 kJ mol 1 3 Au þ H2 O þ 3=4 O2 ! Au ðOHÞ3 G ¼ þ66 kJ mol 1 2 and G values of 119 and þ66 kJ mol 1 , show that corrosion of copper can occur whereas gold resists corrosion. These predictions are in agreement with the experimental observations (i.e., tarnishing of copper occurring and gold not tarnishing). It should be pointed out that these free energy values indicate whether a reaction as written will occur or not, but do not give any idea as to the rate at which it will occur. The rates of the corrosion are given by kinetic studies. It should also be pointed out that the environment of exposure has a decisive role in determining the occurrence of corrosion of the metal. This is exemplified by the fact that gold is resistant to atmospheric oxidation, and resistant to acid attack, but it is well known that gold dissolves in aerated cyanide solutions. In fact the cyanide process for extraction of gold and its assay is well known. Thus it is important to note that the environment also plays an important role in corrosion phenomena. Another example is the ease of rusting of iron by atmospheric attack, and the preservation of iron (free from corrosion) by keeping the metal in peat bogs.

1.3 Nature of Corrosion Reactions Consider the system in which metallic iron is immersed in a solution of copper sulfate. In course of time metallic copper begins to appear. This process is known as a cementation reaction. The species present initially and after a lapse of time are as follows: Initial

Final

Fe2þ Cu

Cu2þ

Then, we may write the reactions occurring as: Fe ! Fe2þ þ 2e Cu

2þ

þ 2e ! Cu

Fe þ Cu

! Fe

Oxidation ðanodicÞ Reduction ðcathodicÞ

þ Cu

Introduction and Principles of Corrosion

The equilibrium constant K and the free energy change in the overall cementation reaction may be written as: G ¼ G þ RT ln

½Fe2þ ½Cu

½Fe ½Cu2þ

Since the corrosion of iron in copper sulfate solution involves an oxidation and reduction reactions with exchange of electrons, the reaction must involve an electrochemical potential difference, related to the equilibrium constant. This relationship may be written as: G_ ¼ n FE and is known as the Faraday’s law. Here F (the Faraday) ¼ 96 494 coulombs, E is the potential difference, n, the number of electrons transferred. Under standard state conditions, G ¼ n FE Neglecting solids we may write, for the reaction of iron in copper sulfate solution

nFE ¼ nFE þ RT ln

½Fe2þ

½Cu2þ

Division by nF leads to E ¼ E

RT ½Fe2þ

ln nF ½Cu2þ

In general terms E ¼ E

RT ln nF

½Products

½Reactants

Converting to log and T ¼ 298, and inserting numerical F; R values E ¼ E

0:059 ½Products

log n ½Reactants

This equation is known as the Nernst equation, and is extensively used in electrochemical measurements. Under equilibrium conditions E ¼ E and the experimentally obtained values of E are tabulated in the literature. E values can be used to determine whether a reaction will occur or not. Having established the criterion for the reaction, such as oxidation of a metal, in terms of the oxidation potential value and its sign and magnitude, it is useful to learn as to how these potentials are experimentally determined. The oxidation potentials are obtained by measuring against a standard hydrogen electrode, consisting of a platinium electrode immersed in 1 M HCl with hydrogen gas at 1 atmosphere pressure passing through it. The standard hydrogen electrode is assigned a value of 0.0 for the reaction 2Hþ þ 2e ! H2

Corrosion Prevention and Protection

and the measured potential of the metal in question, say copper or zinc, is obtained with respect to the standard hydrogen electrode. In the following electrochemical cell operation and measurement of standard electrode potentials is briefly discussed. 1.3.1

Electrochemical Cells

Consider the redox reaction 2þ ZnðsÞ þ Cu2þ ðaqÞ ! ZnðaqÞ þ CuðsÞ

Zinc transfers two electrons to cupric ion. Thus zinc is the reducing agent. Cupric ion is the oxidizing agent. By separating the zinc (reducing agent) and the cupric ion (oxidizing agent) physically, the transfer of electrons can occur through an external conducting medium and as a result electricity is generated due to the progress of the redox reaction. An electrochemical cell is an experimental apparatus for generating electricity by 2þ using a redox reaction. The Daniel cell for the system ZnðsÞ þ Cu2þ ðaqÞ ! ZnðaqÞ þ CuðsÞ is shown in Figure 1.2. Zinc; anode; positive

ZnðsÞ ! Zn2þ ðaqÞ þ 2e

Copper; cathode; negative

Cu2þ ðaqÞ

Oxidation

þ 2e ! CuðsÞ Reduction

A salt bridge enables the movement of ions from one container to another and acts a conducting medium and completes the circuit. During the redox reaction electrons flow from the zinc anode through the wire and voltmeter to the copper cathode. In solution

cations Zn2þ, Cu2þ and Kþ move toward the copper cathode and anions SO2 4 ; Cl move toward the zinc anode. In the electrochemical cell an electric current flows from the zinc anode to the copper cathode. The difference in electrical potential between the anode and cathode is measured by the voltmeter, and is known as the cell voltage. The cell voltage is also

Figure 1.2 The Daniel electrochemical cell

Introduction and Principles of Corrosion

known as the cell potential or electromotive force (emf). The cell voltage also depends on the concentration of the ions. Electrochemical cells are represented as cell diagrams. For the Daniel cell, assuming the solutions are 1.0 M, we may write: ZnðsÞj Zn2þ ð1MÞk Cu2þ ð1MÞjCuðsÞ The single vertical line denotes a boundary between solid electrode and solution, and the double vertical lines signify the salt bridge. By convention the anode is on the left and the cathode is on the right of the salt bridge. 1.3.2

Standard Electrode Potentials

The measured voltage of the Daniel cell is 1.10 V. This is the overall voltage of the cell consisting of two half-reactions, namely the oxidation of zinc and the reduction of cupric ion. The individual potentials of the half-reactions cannot be measured. The potentials of the half-reactions can be obtained relative to a standard. The standard is a hydrogen electrode. This consists of a platinum electrode immersed in 1 M HCl with hydrogen gas bubbling through at 1 atmosphere pressure. The reaction is: 2Hþ ð1 MÞ þ 2e ! H2 ð1 atmÞE ¼ 0 V and the potential of this is zero. The hydrogen electrode is known as standard hydrogen electrode (SHE) with a reduction potential of 0. Now we connect the zinc electrode system to a standard hydrogen electrode system with a salt bridge, as shown in Figure 1.3. The cell diagram is as follows: ZnðsÞk Zn2þ ð1MÞ k Hþ ð1MÞ k H2 ð1 atmÞjPtðsÞ Zinc is the anode,

ZnðsÞ ! Zn2þ ðaqÞ þ 2e

Figure 1.3

oxidation

Ezn=Zn 2þ ¼ 0:76 V

An electrochemical cell consisting of zinc and hydrogen electrodes

Corrosion Prevention and Protection

At the Pt electrode

2Hþ ðaqÞ þ 2e ! H2ðgÞ

reduction

EH þ =H2 ¼ 0:0 V

When Zn2þ ¼ 1 M; Hþ ¼ 1 M; H2ðgÞ at 1 atm (i.e., standard state conditions) the emf of the cell ¼ 0:76 V at 25 C. ¼ Eox þ Ered Ecell Ecell ¼ Ezn=zn 2þ þ EHþ =H 2 0:76 ¼ Ezn=zn 2þ þ 0

The standard oxidation potential of zinc = 0.76 V. The overall reaction is the sum of oxidation and reduction potentials (see Figure 1.3). The standard electrode potential can be obtained by connecting a hydrogen electrode system to a copper electrode system through a U-bridge and a voltmeter, as shown in Figure 1.4. In this system copper is the cathode since reduction occurs.

Cu2þ ðaqÞ þ 2e ! CuðsÞ

For this system the cell diagram may be written as: PtðsÞ j H2 ð1 atmÞ j Hþ ð1MÞ k Cu2þ ð1MÞ j CuðsÞ The half-cell reactions are: Anode ðoxidationÞ

H2 ð1 atmÞ ! 2Hþ ð1 MÞ þ 2e

CathodeðreductionÞ Cu2þ ð1 MÞ þ 2e ! CuðsÞ Overall

EH 2 =Hþ ECu 2þ =Cu

H2 ð1 atmÞ þ Cu2þ ð1 MÞ ! 2Hþ ð1 MÞ þ CuðsÞ Ecell

The potential under standard conditions is 0.34 V at 25 C. Ecell ¼ EH 2 =Hþ þ ECu 2þ =Cu 0:34 V ¼ 0 þ ECu 2þ =Cu

Figure 1.4 An electrochemical cell consisting of copper and hydrogen electrodes

Introduction and Principles of Corrosion

Thus, the standard reduction potential of copper ECu is 0.34 V. Hence, the standard 2þ =Cu oxidation potential, ECu2þ =Cu ¼ 0:34 V.

Now we consider the Daniel cell: Anode ðoxidationÞ

ZnðsÞ ! Zn2þ ð1 MÞ þ 2 e

EZn=Zn 2þ

Cathode ðreductionÞ

Cu2þ ð1 MÞ þ 2 e ! CuðsÞ

ECu 2þ =Cu

Overall

2þ

ZnðsÞ þ Cu ð1 MÞ ! Zn ð1 MÞ þ CuðsÞ

Ecell

The emf of the cell Ecell ¼ EZn=Zn 2þ þ E Cu2þ =Cu

¼ 0:76 V þ 0:34 V ¼ 1:10 V The standard potential series obtained is given in Table 1.5. In the table, potential values given are reduction potentials and when the potential values are reversed in sign we have the oxidation potentials. The sign and magnitude of the potentials give a rough idea as to the ease of oxidation or reduction as the case may be. Table 1.5 Standard potential series Electrode Liþ , Li Kþ , K Ca2þ , Ca Naþ , Na Mg2þ , Mg Ti2þ , Ti Al3þ , Al Mn2þ , Mn Zn2þ , Zn Cr3þ , Cr Fe2þ , Fe Co2þ , Co Ni2þ , Ni Sn2þ , Sn Pb2þ , Pb Fe3þ , Fe Hþ , H2 Saturated calomel Cu2þ ,Cu Cuþ ,Cu Hg2þ 2 , Hg Agþ, Hg Pdþ , Pd Hgþ , Hg Pt2þ , Pt Au3þ, Au Auþ , Au

Reaction

Ered (V)

Liþ þ e ! Li Kþ þ e ! K Ca2þ þ 2 e ! Ca Naþ e ! Na Mg2þ þ 2 e ! Mg Ti2þ þ 2 e ! Ti Al3þ þ 3 e ! Al Mn2þ þ 2 e ! Mn Zn2þ þ 2 e ! Zn Cr3þ þ 3e ! Cr Fe2þ þ 2 e ! Fe Co2þ þ 2 e ! Co Ni2þ þ 2 e ! Ni Sn2þ þ 2 e ! Sn Pb2þ þ 2 e ! Pb Fe3þ þ 3e ! Fe 2 Hþ þ 2 e ! H2 Hg2 Cl2 þ 2 e ! 2 Hg þ 2 Cl (Sat. KCl) Cu2þ þ 2 e ! Cu Cuþ þ e ! Cu

Hg2þ 2 þ 2 e ! 2 Hg 2þ

Ag þ 2 e ! 2 Hg Pdþ þ 2 e ! Pd Hgþ þ e ! Hg Pt2þ þ 2 e ! Pt Au3þ þ 3e ! Au Auþ þ e ! Au

3.024

2.924

2.87

2,714

2.34

1.75

1.67

1.05

0.761

0.71

0.441

0.277

0.250

0.140

0.126

0.036

0,000 0.244 0.344 0.522 0.798 0.799 0.83 0.854 1.2 (ca) 1.42 1.68

Corrosion Prevention and Protection

From the values given for copper and gold for the reduction, we have: Cu2þ þ 2e ! Cu Au

3þ

þ 3e ! Au

0:344 V 1:42 V

and it is easy to note that gold has a higher value than copper, indicating that the reduction is more favored. In general, as one goes down the table we have increasing positive values, indicating the ease of reduction. By the same token the ease of oxidation is indicated by increasing negative values as one goes upwards in the table. It should be pointed out that the potential values given in the table are with respect to hydrogen electrode, which is difficult to use in routine measurements. A common standard electrode routinely used is the calomel electrode. Now let us recall the two examples of copper and gold, which were shown to be feasible, and not feasible, respectively, with respect to corrosion, based on the free energy values, calculated for the respective reactions. The same conclusion can be reached based on the electrochemical potentials for the reactions of copper and gold. 2 CuðsÞ þ 2H2 OðlÞ þ O2ðgÞ ! 2 CuðOHÞ2ðsÞ

E ¼ 0:63 V

4 AuðsÞ þ 3 02ðgÞ þ 6 H2 OðlÞ ! 4 AuðOHÞ3

E ¼ 0:23 V

Using the above values for the potentials and the relationship between the potential and free energy, G ¼ nFE we get 120 kJ/mole and 66.5 kJ/mole for the reactions of copper and gold, which are in good agreement with the value obtained earlier. It is well known that gold is extracted and also assayed by the familiar cyanide leaching. 8NaCN þ 4Au þ O2 þ 2H2 O ! 4Na AuðCNÞ2 þ 4NaOH The potential for the reaction is 1.0 V, and using this value one obtains a value

96:5 kJ/mole for the free energy of the reaction, showing the feasibility of this reaction as well as the important role that the environment plays in these reactions. Now that corrosion phenomenon has been firmly established as electrochemical in nature, we will examine the all too familiar rusting of iron due to corrosion. The mechanism of corrosion is complex and a simplistic mechanism is as follows. The iron metal surface consists of a region, which acts as an anode. Oxidation occurs at the anodic site. FeðsÞ ! Fe2þ ðaqÞ þ 2e

oxidation

Since there is oxygen in the atmosphere, reduction occurs at another region of the metal surface. O2ðgÞ þ 4Hþ þ 4e ! 2H2 OðlÞ

reduction

Introduction and Principles of Corrosion

Figure 1.5 The electrochemical process involved in rust formation

The source of Hþ can be dissolved CO2 or SO2 in industrial areas. The overall reaction is: 2þ 2FeðsÞ þ O2ðgÞ þ 4Hþ ðaqÞ ! 2FeðaqÞ þ 2H2 OðlÞ

The brown rust observed is hydrated iron oxide, Fe2 O3 nH2 O (Figure 1.5). The sodium chloride used in deicing roads accelerates the corrosion due to the fact chloride ion is aggressive and promotes the electrochemical process. The standard potential series can be used as only a rough guide with respect to the ability of a metal to resist corrosion. In most of the corrosion reactions, the potential values shown in the table are not applicable because of the presence of a film on the metal surface, and the change in potential because the activity of metal ions is less than unity. An alternate guide to the corrosion resistance of metals and alloys is the galvanic series. When two metals are coupled together and immersed in an electrolyte, an electrode potential difference is observed due to exchange of electrons and ions. Using this procedure galvanic series in seawater has been developed and is given in Figure 1.6. From the galvanic series shown in the figure it is possible to assess the behavior of metals and alloys with respect to corrosion in flowing seawater. It is clear from the figure that magnesium and zinc with negative potentials at the top portion of the figure with tendency to corrosion while platinium and graphite at the bottom of the figure with positive potentials and exhibiting greater resistance to corrosion. The potentials of various metals shown in Table 1.5 are with respect to the standard hydrogen electrode (SHE). The hydrogen electrode is not very convenient to use in practice as a reference electrode in the context of measuring electrode potentials. Some of the reference electrodes used in practice are detailed below (see Table 1.6). The measured electrode potential with respect to calomel electrode is added to the value of calomel electrode to obtain the value of the potential with respect to the standard hydrogen electrode. ESHE ¼ Emeas þ Ecal

Corrosion Prevention and Protection

Figure 1.6 The galvanic series in seawater

1.3.3

Pourbaix Diagrams

When an electrochemical reaction is perturbed from its equilibrium state, the relative stabilities of the species in the reaction are changed. The change due to the perturbation is reflected in the measured electrode potential, which differs from the equilibrium

Introduction and Principles of Corrosion

Table 1.6 Reference electrodes System at 25 C

Electrolyte

2Hg þ 2Cl ¼ Hg2 Cl2 þ 2 e Calomel Cu ¼ Cu2þ þ 2 e Copper – copper sulfate Ag þ Cl ¼ Ag Cl þ e

Platinized platinum Gold Zinc

0:01 M KCl 0:1 M KCl 1:0 M KCl Sat. KCl 0:1 M CuSO4 0:5 M CuSO4 Sat. CuSO4 0:001 M KCl 0:001 M KCl 0:1 M KCl 1:0 M KCl 0:1 M NaCl 0:1 M NaCl Seawater

Potential (V vs SHE) 0.389 0.333 0.280 0.241 0.284 0.294 0.298 0.400 0.343 0.288 0.234

0.12 (approx.)

0.25 (approx.)

0.79 (approx.)

electrode potential of the reaction. If the measured electrode potential is positive with respect to the equilibrium electrode potential, the reaction proceeds irreversibly from left to right. Reduced species ¼ oxidized species þ ne When the measured potential is negative with respect to the equilibrium value, the reaction favours the reduced form. Consider water, which is an electrochemically active species. The electrochemical reactions involving water are: 2H2 O ¼ 4Hþ þ O2 þ 4e H2 þ 2OH ¼ 2H2 O þ 2e For solutions in which the activity of water and the fugacities of oxygen and hydrogen gases are unity, the equilibrium electrode potentials for the above reactions at 25 C are: E0;H2 O=O2 ¼ þ1:23 0:059 pH E0;H2 =H2 O ¼ 0:059 pH The electrode potentials given are with respect to SHE. Pourbaix has shown that plotting electrode potentials of electrochemical reactions against pH of the solution is useful in identifying regions of stability of various chemical species in solution. These plots are commolny known as Pourbaix diagrams. These diagrams are extremely useful in corrosion science. The Pourbaix diagram for the system H2 O–H2 –O2 –Hþ –OH will be considered (Figure 1.7). The lines (1) and (2) represent plots of E0;H2 O=O2 and E0;H2 =H2 O vs pH respectively. Inspection of line (1) and the reaction 2H2 O ¼ 4Hþ þ O2 þ 4e

Corrosion Prevention and Protection

ELECTODE POTENTIAL (V vs. SHE)

2.0

1.0

H+ OH–

H 2O

0.0 2 H2 –1.0

Figure 1.7 Pourbaix diagram for the system: water–hydrogen–oxygen–hydrogen ion– hydroxyl ion. Activity of water is unity; fugacities of hydrogen and oxygen are unity. Temperature 25 C

shows that water is a stable entity at potentials below this line and conversely, unstable at potentials above this line. Similar analysis of line (2) and the reaction, H2 þ 2OH ¼ 2H2 O þ 2e shows that water is a stable entity at electrode potentials above line (2) and unstable below this line. The Pourbaix diagram for this system (Figure 1.7) is similar to a phase diagram. This can be considered as a form of electrochemical phase diagram. The region where a species is stable is identified with the chemical symbol of the species. The Pourbaix diagram is extremely useful in determining stable chemical species for metals in contact with aqueous solutions. When the Pourbaix diagram for water is superimposed on the metal of interest, two types of behavior are observed. This behavior classification of metals is based on the relationship between the metal–metal ion equilibrium reaction and the region of stability of water. In one class of metals, the metal–metal ion equilibrium potential falls within the region of stability of water. In these cases, it is possible to measure the equilibrium electrode potential of metal–metal ion reaction in aqueous solution and devise a method by which the kinetic properties of the reaction may be obtained with minimal kinetic complexity. In the second class of metals, the metal–metal ion equilibrium electrode potentials fall below the region of stability of water. These metals form mixed potential systems with solvent water. Therefore, the equilibrium electrode potential of the metal–metal ion reaction cannot be measured in aqueous solution, and the kinetics of the complete reaction cannot be determined with ease. Copper is an example of the first type and iron is an example of the second class of metals. Figure 1.8 is the Pourbaix diagram for copper and some of its ionic species and compounds in aqueous solution at 25 C. The equilibrium electrode potential for the copper–cupric ion reaction is located within the region of stability of water represented by dashed lines. Thus, the

Introduction and Principles of Corrosion

ELECTRODE POTENTIAL (V vs. SHE)

2.0

5 1.0

Cu2+

CuO

2 3

0.0

–1.0

Cu O 2

Figure 1.8 Pourbaix diagram for the system copper–cupric ion–cuprous oxide–water. Activity of cupric ion is .01. The dashed lines denote the stability range of water.The temperature is 25 C. The reactions considered: 1. copper going to cupric ion and two electrons; 2. cuprous oxide reacting with hydrogen ion to give cupric ion, water and two electrons. 3. cuprous oxide reacting with water to give cupric oxide, hydrogen ion and two electrons; 4. copper metal reacting with water to give cuprous oxide, hydrogen ion and electrons; 5. cupric ion reacting with water to give cupric oxide and hydrogen ion

measurement of the equilibrium electrode potential is possible and the kinetics of the copper–cupric ion system can be studied without interference from reactions involving solvent decomposition. All the reactions involved in this system are electrochemical except the reaction, Cu2þ þ H2 O CuO þ 2Hþ and can be studied by standard electrochemical techniques. If the measured electrode potential and pH are known, this figure may be used to determine the stable form of copper and its compounds under those conditions. Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25 C. The equilibrium potential of the reaction Fe ¼ Fe2þ þ 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. In the potential–pH region where iron metal is the stable species, corrosion cannot occur since the reactions are not thermodynamically favorable. Pourbaix calls these regions ‘immune’ to corrosion. However, in the broader sense of corrosion, iron may corrode in this region, such as the fracture of iron alloys in the presence of hydrogen. Hence, it is not sufficient for a metal to be ‘immune’ for it to be free of corrosion.

Corrosion Prevention and Protection ELECTRODE POTENTIAL (V vs. SHE)

Fe3+ 6 1.0 5

Fe2O3 4 Fe2+

0.0

3 1 –1.0

7 2

Fe3 O

Fe 0

Figure 1.9 Pourbaix diagram for the system iron–ferrous ion–ferric ion–haematite–ferric oxide. Activities of ferrous and ferric ions are 10 6 . The temperature is 25 C. The reactions considered: 1. iron metal giving ferrous ion and two electrons; 2. iron reacting with water to give haematite, hydrogen ion and electrons; 3. ferrous ion reacting with water to give haematite, hydrogen ion and electrons; 4. ferrous ion reacting with water to give ferric oxide, hydrogen ion and electrons; 5. ferrous ion giving rise to ferric ion and electron; 6. ferric ion reacting with water to give ferric oxide and hydrogen ion; 7. haematite reacting with water to give ferric oxide, hydrogen ion and electrons

Considering the Pourbaix diagram for iron, the region of stability of iron oxide shows that the film of iron oxide on the metal surface forms a barrier between the metal and the environment. This condition is called passivity and is characterized by the measured electrode potentials in the regions where the passive oxide film is stable. Figure 1.10 is a simplified Pourbaix diagram for iron. The diagram clearly distinguishes the regions of immunity, corrosion and passivity. Use of Pourbaix diagrams. Consider copper metal at þ0:150 V vs SCE in an aqueous solution of pH 2.5 and a cupric ion activity of 0.01 at 25 C. In order to use the Pourbaix diagram, the potential is converted to SHE scale. E2 ¼ þ0:150 þ 0:241 ¼ 0:391 V vs SHE Reference to the Pourbaix diagrams for water and copper shows the stable species to be Cu2þ ; Hþ and H2 O. Corrosion is possible under these conditions. Let us consider the case of iron metal at 0:750 V vs SCE in a solution of pH 5.0 and a ferrous ion activity of 10 6 at 25 C. The electrode potential with respect to SHE is 0:509 V and reference to Pourbaix diagrams for water and iron shows the stable species to be Fe2þ and H2 , and that corrosion is possible under these conditions. It is needless to emphasize that Pourbaix diagrams reveal information on stability of species and whether corrosion is likely to occur under a given set of conditions.

Introduction and Principles of Corrosion

ELECTRODE POTENTIAL (V vs. SHE)

2.0

1.0 PASSIVITY

0.0 CORROSION

–1.0

IMMUNITY 0

Figure 1.10

1.3.4

7 pH

Pourbaix diagram for iron in terms of corrosion, passivity and immunity

Dynamic Electrochemical Processes

The potential series and the Pourbaix diagrams involving equilibrium conditions discussed thus far led to determine the feasibility of the corrosion process based on thermodynamics. These concepts do not give any information on the rates of corrosion processes. In order to ascertain the corrosion rates it is imperative to understand the intimate dynamical processes occurring at the metal exposed to an electrolyte solution. Let us consider an electrode immersed in an electrolyte. A potential difference arises at the interface between the electrode and the surrounding electrolyte solution. The potential difference arises due to charge separation. If electrons leave the electrode and reduce the cations in solution, the electrode acquires a positive charge, the solution loses electroneutrality and anions would move closer to the positively charged electrode. The situation is depicted in Figure 1.11. Thus, we have a pair of positive and negative sheets, which is known as the electrical double layer. The process of contact adsorption of an ion consists of desolvation of anion, removal of solvent water molecule from the surface of the electrode and the desolvated anion taking the vacant position on the electrode as a result of departure of water molecule from the electrode surface. The steps of anion adsorption in terms of energy are: Eðelectrode

anionÞ > Eðwater on electrodeÞ þ Eðdesolvation of anionÞ The disposition of water molecules attached to positive ions and negative ions with metal–oxygen and anion–hydrogen interactions are shown in Figures 1.12 and 1.13, respectively. The first-row water molecules near the electrode surface of an electrode play an important role in the sense that they have to allow either an anion or a solvated cation to

Corrosion Prevention and Protection IHP OHP

OHP Electrode

Solution

Contact adsorption

Electrode

Contact adsorbed ion

These water molecules must be displaced

Solution First row water

Figure 1.11 Process of contact adsorption in which a negative ion can lose its hydration and displace first-row water. The locus of the centers of the ions defines the inner Helmholtz plane

come close to the electrode surface. The locus of the centre of the first-row molecules on the electrode surface defines the inner Helmholtz plane, and the distance between the ˚ . Similarly the locus of the center of the inner Helmholtz plane and the electrode is 3 A water molecules adjacent to the first row is the outer Helmholtz plane, and this plane is ˚ from the electrode surface. about 5–6 A The solvation of cations and anions by water molecules is possible due to the dipolar nature of water in the sense that oxygen and hydrogen have partial negative and positive charges respectively. δ– O δ+ H

Figure 1.12

δ+ H

Schematic of a positive ion surrounded by a sheet of apex-inward water dipoles

Introduction and Principles of Corrosion

Figure 1.13 Schematic of a negative ion surrounded by a sheet of apex-outward water dipoles

It should also be noted that the bonding between a cation and water dipole is stronger than an anion and a water dipole. Now we turn to the situation when the electrode is negatively charged as shown in Figure 1.14. It is important to note that the solvated cations lie at the outer Helmholtz plane, unlike the situation in which the anions adsorb on the positive electrode surface. In terms of interaction forces the cation–water interactions are stronger than the negatively charged electrode – water interactions. In terms of more intimate mechanism the water molecules attached to the cation do not exchange with water molecules adsorbed on the electrode surface. The solvated cation is situated at the outer Helmholtz plane.

Solvated ions

Electrode

Outer Helmholtz plane First row water

Figure 1.14 A layer of hydrated positive ions whose hydration sheath cannot be stripped, on the first layer of water molecules. The locus of the centers of these ions define the outer Helmholtz plane

Corrosion Prevention and Protection

Figure 1.15 Gouy–Chapman model of the double layer

The double layer model of Helmholtz assumes fixed layer of charges on the electrode and the outer Helmholtz plane. This model has been modified by Guoy–Chapman analysis, which assumes that the ions of charge opposite of the charge on the electrode distribute themselves in a diffuse manner as shown in Figure 1.15. The variation of potential with distance is exponential according to this model, while it is linear according to the Helmholtz model. Another modification of the double layer theory is due to Stern, who proposed a synthesis of the Helmholtz and Guoy–Chapman models in that both the fixed layer of charges and diffuse layer of charges are taken into consideration. Stern’s model, showing charge distribution and the variation of potential with distance, is shown in Figure 1.16. The figure also shows that by assuming the layer of charges as parallel plate capacitor, the relationship between the total differential capacity and the capacitances due to Helmholtz ðCH Þ and Guoy–Chapman ðCG Þ. The variation of potential with distance shows both linear and nonlinear portions, confirming that this model embraces both Helmholz, and Guoy–Chapman models. The variation of potential with distance when a test charge located at a distance from the electrode approaches the electrode surface passing through the electrolyte solution is shown in Figure 1.17. It should be borne in mind that this trend in potential variation is the predicted trend. It should also be noted that the potential difference between the metal and the solution is f and is given by: f ¼ fM fs where fM and fs are the potentials of metal and solution. The potential difference f (M, S) is the measured parameter when an electrode is immersed in an electrolyte.

Introduction and Principles of Corrosion

‘Stuck’ ions –

(a)

Scattered ions

–

– –

Positively charged electrode

– –

–

– – –

–

– –

– – –

– –

–

a X = –a

X=O Two regions of charge separation

(b) Linear variation

Potential

X = –a X=0

(c)

1 = 1 + 1 C CH CG

Figure 1.16 The Stern model. (a) A layer of ions stuck to the electrode and the remainder scattered in cloud fashion; (b) the potential variation according to this model; (c) the corresponding total differential capacity C is given by the Helmholtz and Gouy capacities in series

In corrosion the dynamic electrochemical processes are of importance and hence considerations of the consequences of perturbation of a system at equilibrium are considered. Let us consider the familiar Daniel cell consisting of copper metal in copper sulfate, and zinc metal in zinc sulfate solution. This, as depicted in Figure 1.18 gives an electromotive force of 1.1 V when there is no current flow. When a small current flows through the resistance R, the potential decreases below 1.1 V. On continued flow of current, the potential difference between the electrodes approaches a value near zero, and

Corrosion Prevention and Protection

Figure 1.17 Variation of potential with distance from an electrode

the potential difference continues to decrease as the current increases since the electrodes polarize. The effect of net current flow on the voltage of the Daniel cell can be represented by plotting the individual potentials of copper and zinc electrodes against the current, as shown in Figure 1.19. This is known as the polarization diagram. When there is no net current flow, we have fCu and fZn, the potential values of copper and zinc known as open-circuit potentials. The zinc electrode polarizes along a–b–c, and the copper electrode along d–e–f. At a current value of I1 the polarization of zinc is the potential at b minus fZn (or the open-circuit potential at a). Similarly in the case of copper we have f at e minus f at d or fCu. The potential difference of the polarized electrodes i.e., zinc at b and copper at e is equal to I1 ðRe þ Rm Þ where Re and Rm are electrolytic and metal resistance in series. At a point c the current is maximum and we have potential difference at a minimum and equal to Imax Re. The potential R

A V

CuSO4

ZnSO4

Figure 1.18

Polarized copper–zinc cell

Introduction and Principles of Corrosion

d Cu

Potential

e f (corros.)

I(max.) Re

a I1

I(max.) Current

Figure 1.19 Polarization diagram for copper–zinc cell

corresponding to Imax is known as the corrosion potential. Using the equivalent weight of zinc (65.32/2), the Faraday equal to 96 500 coulombs/equivalent, and the value of Imax, the amount of zinc corroding in unit time may be calculated. Similarly an equivalent amount of copper will be deposited during the cathodic reaction. The lines (a–b–c) and (d–e–f) are known as the anodic and cathodic branches of the polarization diagram. Note that Faraday’s law has been used to determine the rate of corrosion of zinc in the case of a polarized Daniel cell (i.e., Faraday’s law relates the charge Q created by ionization of M moles metal such as zinc). Q ¼ zFM where Z is the valence, F, the Faraday and M, the number of moles dQ dM ¼ zF dt dt Noting that dM=dt ¼ J the flux of the substance, I the current flowing through unit area of cross-section, i is the current density, we can write: i ¼ zFJ Thus the flux of the material is the corrosion rate and we arrive at the corrosion rate equated to the current density. Polarization is defined as a type of perturbation, which results in disturbing the equilibrium and producing a dynamic situation. The three types of polarization are concentration polarization, activation polarization, and IR drop. Let us consider metallic copper sample immersed in water and let us suppose the metallic copper in contact with water gives rise to cupric ions and the energy profile for

Corrosion Prevention and Protection

Figure 1.20 Energy profile for oxidation of copper. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

the system will be as shown in Figure 1.20. Cu ! Cu2þ þ 2e The environment must be capable of providing sufficient energy to aid metallic copper to corrode and give cupric ions, i.e., the metal must overcome the barrier G6¼ . For some time there will be corrosion and as a result cupric ion concentration increases. This tendency of copper to corrode will decrease as the current increases from zero, and the value of G decreases, along with potential in conformity with Faraday’s law. The thermodynamic energies of the metal atoms and the ions approach each other. After a lapse of time an equilibrium state is reached when the rate of metallic copper producing cupric ions will equal the rate of reduction of cupric ions to produce metallic copper. The energy profile for such an equilibrium state is depicted in Figure 1.21. We note that the energy of activation G6¼ and that for the case of copper we may write. ia

Cu Ð Cu2þ þ 2e ic

and in more general terms ia

M Ð M2þ þ 2e ic

Introduction and Principles of Corrosion

Figure 1.21 An energy profile for copper in equilibrium with a solution of its divalent ions; ia ¼ ic ¼ i0 (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

At equilibrium, ia ¼ ic and the measured current density imeas ¼ ia ic and no net current flows. There is current flow, but it is equal and opposite and cannot be measured. This quantity is known as exchange current denoted by I0 and when divided by area, it is i0. Thus we note that current density gives a measure of corrosion rate. Corrosion rates are generally expressed in practice as mpy (mils per year), ipy (inches per year), ipm (inches per month) and mdd (loss of weight in milligrams per square decimetre per day), and a nomograph for interconversion of one unit into another unit is depicted in Figure 1.22. Perturbation of a system at equilibrium in terms of potential is known as polarization. The polarization of the equilibrium is also termed as overpotential or overvoltage denoted by Z: consider the system. ia

M Ð M2þ þ 2e ic

for which the corrosion rate r may be written as:75 r ¼ kcorr ½reactants

where kcorr ¼ Ae G

6¼

=RT

and A is a constant r ¼ Ae G

6¼

=RT

½reactants

Corrosion Prevention and Protection

Figure 1.22

Nomograph for mpy, ipy, ipm, and mdd

At equilibrium kf ¼ kr or ia ¼ ic ¼ i0. Since solid metal is corroding, it may be taken as constant, ia ¼ i0 ¼ A0 e G=RT The energy profile for the system at equilibrium along with anodic polarization of the equilibrium state is shown Figure 1.23.

Introduction and Principles of Corrosion

Figure 1.23 An energy profile for an anode at equilibrium and a similar profile for anodic activation polarization (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

By denoting the anodic polarization as aZ and cathodic polarization as ð1 aZÞ, we can write:75 ia ¼ A0 e G

6¼

þaZF=RT

¼ A0 e G

6¼

=RT aZzF=RT

ia ¼ i0 e

aZzF=RT

ic ¼ i0 eð1 aÞZzF=RT Since bulk current flow, imeas ¼ ia ic, imeas ¼ i0 ½eaZzF=RT eð1 aÞZzF=RT

This equation is known as Butler–Volmer equation. By letting azF=RT ¼ A0 0

ia ¼ i0 eðA ZÞ By taking logarithms ln ia ¼ ln i0 þ A0 Z ln ia ¼ ln i0 þ A0 Z ia ¼ A0 Z ln i0 or Z¼

2:303 logðia =i0 Þ A0

Corrosion Prevention and Protection

If 2:303 2:303RT ¼ A0 azF Za ¼ ba logðia =i0 Þ b¼

In general terms Z ¼ c log i þ D. This is known as Tafel equation. For the anodic and cathodic processes we have: Za ¼ ba log ia ba log i0 Zc ¼ bc log ic bc log i0 where ba ¼

2:303RT azF

bc ¼

2:303RT ð1 azFÞ

In the Tafel equations ba and bc are known as the anodic and cathodic Tafel constants. Tafel plots are useful in obtaining corrosion rates. Consider a sample of metal polarized 300 mV anodically and 300 mV cathodically from the corrosion potential Ecorr . The potential scan rate may be 0.1–1.0 mV/s. The resulting current is plotted on a logarithmic scale. The plot is shown in Figure 1.24. The corrosion current icorr is obtained from the

ANODIC mV/ DECADE 1 DECADE

e·171

POTENTIAL RELATIVE TO E

+150

–150 CATHODIC

–300

100 nA LOG CURRENT

Figure 1.24 Experimentally measured Tafel plot

171 mV

+300

, mV

+20

POTENTIAL RELATIVE TO E

Introduction and Principles of Corrosion

+10

OXIDATION

0 = 3.57 OHMS –10 REDUCTION –20 6

CURRENT, mA

Figure 1.25 Experimentally measured polarization resistance

plot by extrapolation of the linear portions of the anodic and cathodic branches of the curve to the corrosion potential Ecorr . The corrosion current may then be used to calculate the corrosion rate using the following equation: Corrosion rate ðmpyÞ ¼

0:13 icorr ðEq wtÞ d

where icorr is corrosion current density, (A/cm2), d, the density of the corroding metal (g/cm3) and Eq wt is the equivalent weight of the corroding metal in grams. The linear polarization technique is rapid and gives corrosion rate data, which correlate reasonably well with weight loss method. The technique involves scanning through 25 mV above and below the corrosion potential and plotting the resulting current against potential as shown in Figure 1.25. The corrosion current icorr is related to the slope of the line. E bA þ bc ¼ 2:3ðicorr ÞðbA þ bc Þ i bA bc i icorr ¼ 2:3ðbA þ bc Þ E corrosion rate ðmpyÞ ¼

0:13icorr ðEq wtÞ d

Polarization experiments on a corrosion system are carried out by using a potentiostat. The experimental arrangement of the cell consists of a working electrode, reference electrode and a counter-electrode. The counter-electrode is used to apply a potential on the working electrode both in the anodic and the cathodic direction, and measure the resulting currents. The electrochemical cell is depicted in Figure 1.26.

Corrosion Prevention and Protection

Figure 1.26

1.3.5

Electrochemical cell used for potentiodynamic polarization studies

Concentration Polarization

Concentration polarization may be illustrated by considering copper cathode in copper sulfate solution as an example. Using the Nernst equation we have for the oxidation potential. E1 ¼ 0:337

0:059 logðCu2þ Þ 2

Upon current flow, copper is deposited, thereby reducing the amount of cupric ions to a new value ðCu2þ Þs , when, E2 ¼ 0:337

0:059 logðCu2þ Þs 2

Introduction and Principles of Corrosion

The difference in potential E2 E 1 ¼

0:059 ½Cu2þ

log 2 ½Cu2þ s

As the current flow increases, the smaller is the ðCuÞs value and the larger is the polarization; this is known as concentration polarization. When ½Cu s approaches zero, the current density is known as the limiting current density. When the limiting current density is iL for the cathodic reaction, as i approaches iL an expression is obtained of the form, E2 E1 ¼

RT iL ln iL i nF

The limiting current density is obtained from: iL ¼

DnF c 10 3 dt

where D is the diffusion coefficient, F, the Faraday, d the thickness of the electrode layer and n is the number of electrons. It is to be noted that the Helmholtz double layer plays a significant role in concentration polarization since the concentration of the ions on the electrode surface, and the diffusion of ions from the bulk of the solution into the Helmholtz plane are contributing factors to the limiting current density. This situation may be visualized as shown below: Electrode

Cu Cu Cu Helmholtz Plane

Activation polarization involves a slow step in the electrode reaction. The reduction of hydrogen at the cathode involves: Hþ þ e ! H; H þ H ! H2 and this is also known as the hydrogen overvoltage. Another example is: 2 OH ! ½ O2 þ H2 O þ 2e and this is known as the oxygen overvoltage.

Corrosion Prevention and Protection Table 1.7 Hydrogen reaction22 Metal

Exchange current density (A/cm2) 10 13 10 11 10 10 10 6 10 7 10 4 10 2

Pb, Hg Zn Sn, Al, Be Fe Ni, Ag, Cu and Cd Pd, Rh Pt

The smaller the value for the exchange current density, the more polarizeable the metal.

Activation polarization, Z in general terms may be given by: Z ¼ b log i=i0 where b and i0 are constants for a metal and a particular medium. The significance of the hydrogen overvoltage lies in the fact that acids attack most metals, and it is useful to know the exchange current densities and the hydrogen overpotentials of some common metals (see Table 1.7). The smaller the exchange current density the greater the polarizability. Some data on hydrogen overvoltage for some metals at current density of 1 mA/cm2 are given in Table 1.8. In the first instance the equilibrium oxidation potential determines whether corrosion is likely or not. Then the hydrogen overpotential determines the corrosion rate of the metal in question. Mixed potential theory can be illustrated by considering the two metals iron and zinc in an acid solution. The two factors to be considered are the anodic polarization line of the metal and the exchange current density for hydrogen evolution on the metal. Although zinc is expected to corrode according to its position in the galvanic series, it is the iron that corrodes in this system because the exchange current density for hydrogen evolution is higher on iron than on zinc. A mixed potential diagram for the iron, zinc system is shown in Figure 1.27. The lines a and b, refer to zinc alone and a00 and b00 are those of iron corroding in an isolated condition. The lines (a þ a0 ) and (b þ b0 ) represent the mixed electrode system of iron and zinc.75 Table 1.8 Overvoltage data23 Metal Pt Pd Ni Fe Cu Al Sn Zn Pb

i0 (A/cm2) 10 3 2 10 4 8 10 7 10 7 2 10 7 10 10 10 8 1:6 10 11 2:0 10 13

Z, V (1 mA/cm2) 0.00 0.02 0.31 0.40 0.44 0.70 0.75 0.94 1.16

Introduction and Principles of Corrosion

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

The example of the iron/zinc couple refers to an acid solution, if a neutral or alkaline (basic) solution is considered the cathodic reaction would be: 2 H2 O þ O2 þ 4e ! 4 OH and the rate of diffusion of oxygen to the surface of the metal has an important effect. The electrochemical technique that is popular in corrosion studies is potentiodynamic polarization. This technique consists of using the sample specimen as the working electrode, with a reference electrode such as calomel electrode and a platinum counter- (auxiliary) electrode. A Luggin capillary is placed as close to the working electrode as possible to avoid or minimize the effect due to IR drop. The electrode assembly is immersed in the corrosive medium and the corrosion potential recorded. The potential is applied in the positive direction at a suitable rate (0.1 mV/s) and the resulting current recorded. Then the applied potential is in the negative direction to the corrosion potential and the resulting current noted. Thus, the potentiodynamic polarization gives the curves shown in Figure 1.28. The corrosion potential, the corrosion current obtained by drawing the anodic and the cathodic Tafel slopes, which interest at ic and the various regions corresponding to activation polarization, concentration polarization and resistance polarization are labeled. The experimental arrangement for potentiodynamic polarization experiment is shown in Figure 1.26. The experiment is done using the software, and polarization curves (both anodic and cathodic branches of polarization) are recorded at a suitable scan rate. The software performs the calculations and gives the data for corrosion potential and corrosion current density for the system on hand.

Corrosion Prevention and Protection CONCENTRATION OVERPOTENTIAL

ANODIC

LIMITING ia

ECrev 2H

EMV

e H

Ecorr +

LIMITING ic

Earev

CONCENTRATION OVERPOTENTIAL CATHODE

icorr i CURRENT

Figure 1.28 Polarization diagram illustrating various parameters

With the rapid developments of electrochemical techniques and the required instrumentation electrochemical impendance and electrochemical potential noise and current noise techniques are gaining prominence in corrosion studies. Electrochemical impedance data give information on the kinetics and mechanism of the corroding system. Alternating current techniques have some advantages over DC techniques: (i) AC techniques use very small excitation amplitudes in the range of 5–10 mV peak-to-peak; (ii) data on electrode capacitance and charge transfer kinetics provide mechanistic information; (iii) AC techniques can be applied to low-conductivity solutions while DC techniques are subject to serious potential errors in these media. Consider the application of a small sinusoidal potential ðE sin otÞ on a corroding sample, which results in a signal along with the current flow of harmonics 2o, 3o, etc. Then the impedance I sinðot þ fÞ is the relation between E=I and phase f. In the case of corrosion studies, the sample is made part of a system known as equivalent circuit,24 which consists of the solution resistance Rs , charge transfer resistance RCT and the capacitance of the double layer Cdl. The measured impedance plot appears in the form of

Introduction and Principles of Corrosion

Rs RCT

–Z′′

RCT = 2|Z| tan

max

w max =

1 C

Z RCT/2

Figure 1.29

Rs + RCT

Z′

Representation of equivalent circuit24

semicircule (Nyquist plot). Both the equivalent circuit and the impedance plot are shown in Figure 1.29. The electrochemical experimental arrangement consisting of AC impedance analyzer, the electrochemical cell, and the computer to acquire the data over a period of time is depicted in Figure 1.30. It is useful to note that the polarization resistance data obtained can be used to calculate the corrosion rate of the corroding sample.

Figure 1.30

Experimental set-up for AC impedance

Corrosion Prevention and Protection Process Water - Impedance Polarization (kOhm) 250 % Inhibition A = 84 % B = 84 % 200 83 %

150

* * *

79 %

* *

* * *

* * * * * * * *

100

Time (day) No Inhibitor

Figure 1.31

Polarization resistance as a function of time

Polarization resistance of the corroding sample may also be monitored over an extended duration. Thus, AC impedance may be used for online monitoring of a corrosion system such as on-line determination of corrosion inhibitor performance, as depicted in Figure 1.31.

Figure 1.32 Electrochemical cell24

Introduction and Principles of Corrosion Multimeter

HP86 Computer Filter

1 ohm Sample

Sample

Reference Electrode

Figure 1.33 Apparatus used for potential noise measurements24

Electrochemical potential noise or current noise is gaining importance in monitoring corrosion processes, especially when localized corrosion such as pitting corrosion is involved. The electrochemical cell and the experimental arrangement for potential noise measurements are depicted in Figure 1.32 and 1.33, respectively. The figures clearly show the simplicity of the technique. This technique is particularly suited for on-line monitoring of corrosion processes for long durations and typical data obtained in the evaluation of the performance of corrosion inhibitors in a field study are shown in Figure 1.34. The AC impedance data given in Figure 1.31 and the potential noise data given in Figure 1.34 refer to the same system and shows an inhibition efficiency of 79–84%.

Process Water - Noise Amplitude Noise Amplitude (uV) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0

* 0.2

0.4

* 0.6

0.8

* 1.2

1.4

* 1.6

1.8

Time (day) No Inhibitor

Figure 1.34 Potential noise amplitude vs time24

z 2.2

2.4

Corrosion Prevention and Protection

1.4 Oxidation and High-Temperature Corrosion In the early part of this chapter winning metals by the reduction of oxides with reducing agent was cited: 2 Fe2 O3 þ 3C ! 4 Fe þ 3 CO2 and the reverse reaction as oxidation of the metal, 4 Fe þ 3 O2 ! 2 Fe2 O3 otherwise known as corrosion of iron. In the oxidation of metal, we have: M ! Mnþ þ ne ½ O2 ¼ 2e ! O2 As a result of oxidation the product metal oxide is formed. In one of the earliest studies of oxide formation, Pilling and Bedworth25 correlated oxidation resistance of metals with the volume ratio of oxide to metal per gram atom of metal. The oxide to metal volume ratios are given in Table 1.9. In a qualitative sense the ratio values of less than 1.0 and those between 2 to 3 are likely to be nonprotective oxides. It should be noted that this ratio is olny empirical and indicates protective nature; other factors such as good adherence, high melting point, low vapor pressure, good hightemperature plasticity to fracture, low electrical conductivity or low diffusion coefficients for metal ions and oxygen define the protection nature. Oxidation of metals occurs at the metal–scale interface and oxygen reduction at the scale–gas interface. This results in the formation of metal oxide scale on the surface. This is analogous to aqueous galvanic corrosion of metals. The oxide layer formed serves the

Table 1.9 Values for the ratio volume of oxide produced/volume of metal consumed in producing the oxide26 Metal Li Ca Mg Al Ni Zr Cu Ti Fe U Cr Mo W Md ¼ volume of oxide; md ¼ volume of metal

Md =md 0.57 0.64 0.81 1.28 1.52 1.56 1.68 1.77 1.77 1.94 1.99 3.24 3.35

Introduction and Principles of Corrosion

M+

O–2

Metal

Oxide

Oxygen

Figure 1.35 Schematic illustration of electrochemical processes occurring during gaseous oxidation

purpose of an ionic conductor (electrolyte), an electronic conductor, as an electrode at which oxygen is reduced and as a diffusion barrier through which electrons pass, and ions must migrate over defect lattice sites, indicated as VM and Vo in Figure 1.35. Kinetics of oxidation of metals forming oxides obeys different rate laws. The four different rate laws obeyed by metals during oxidation are illustrated in Figure 1.36. The oxidation of metals which form volatile oxides show a linear weight loss and these oxides are not protective. Oxidation of metals showing gain in weight may obey linear, parabolic and logarithmic kinetics. The oxidation of metal which results in weight gain and follows a parabolic kinetics is typified by an oxide film which remains intact on the metal surface, offering a uniform barrier to the diffusion of metal or oxide ions through the film. The parabolic rate law for the oxidation when the oxide thickness is x; t the time and c is constant.75 dx ¼ c1 =x dt x2 ¼ c1 t Examples of this category are oxides of cobalt, copper, nickel and tungsten. The oxidation of metal at high temperature such as iron at 1000 C and magnesium at 500 C obey rectilinear rate law. dx ¼C dt x ¼ ctðc is a constantÞ In oxidation of this type the oxide is unable to offer a barrier to oxygen coming to the metal surface. If the oxide formed cracks or spalls due to internal stresses a few parabolic weight gain steps might occur which might appear linear overall.

Corrosion Prevention and Protection Weight gain Rectilinear

Parabolic

Logarithmic

Time

Volatile oxide formation

Weight loss

Figure 1.36 Oxidation rate loss showing weight gain and weight loss with different kinetic forms. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

In the case of some metals such as magnesium below a temperature of 200 C, a thin oxide layer is formed, which resists diffusion of oxygen and as a result an initial formation of oxide is followed by practically zero growth of the oxide. The rate law governing this type of oxide growth is logarithmic.75 x ¼ C0 logðC1 t þ C2 Þ where C0 , C1 and C2 are constants and x the thickness of the oxide layer. Another familiar example obeying the logarithmic rate law of oxide growth is aluminum below 50 C. In some cases of oxidation of metals such as zirconium, the growth rate of oxide film has multiple stages of growth, as illustrated in Figure 1.37. The figure shows two points of change in curve A and one point of change in curve B where the oxide growth patterns change suddenly. These points are known as breakaway points, and are referred to as breakaway corrosion. Curve A shows two breakaway stages while curve B shows one breakaway point. An example of such behavior is the oxidation of zirconium. Tin is added to zirconium to minimize breakaway corrosion. Some examples of the phenomenon of breakaway corrosion are encountered in nuclear reactors used for power generation. In some reactors carbon dioxide is used as a coolant. Mild steel is chosen for use in gas-cooled reactors at 400 C, steel containing chrominum

Introduction and Principles of Corrosion Weight gain

B y = c2t A y2 = c1t Breakaway

Time

Figure 1.37 Typical breakaway corrosion curves. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

and molybdenum for temperatures at 400–500 C and austenitic stailness steel for temperatures greater than 550 C. In the nuclear reactors failure of bolts and nuts was observed due to breakaway corrosion. The mechanism of breakaway oxidation is complex and a simplistic explanation is as follows: The protective oxide on ferritic steel consists of two layers, which are porous to carbon dioxide. The inner layer consists of crystallites of Cr and Si and the outer layer Fe3 O4 (magnetite) formed by the reaction of metallic iron and CO2 and the product CO giving elemental carbon. 3 Fe þ 4 CO2 ! Fe3 O4 þ 4 CO 2 CO ! CO2 þ C The elemental carbon on reaching a level of 10% by weight in the inner oxide layer results in loss of protective action and initiation of breakaway corrosion. We have so far dealt with the rate laws that govern the oxidation of metals and the empirical nature of the protective properties of the oxides. It has been pointed out earlier that the oxidation of metals at high temperatures is analogous to galvanic corrosion phenomena. Metal oxides, the products of oxidation of metals are ionic compounds with the metal ions and oxide ions arranged in arrays in the crystal lattices. When the metal oxide contains excess metal ions in interstitial positions, they are known as n-type (or negative carrier type) oxides. When the metal oxides contain vacant sites (deficient in metal ions) in the lattice the oxides are known as p-type (positive carrier type). n-type p-type

ZnO Cu2 O

CdO NiO

A12 O3 Cr2 O3

In the formation of metal oxides, both diffusion of oxygen inwards through the film on metal as well as diffusion of metal outwards may occur. Diffusion of oxide inwards to the metal/oxide interface is typified by titanium–oxygen and zirconium–oxygen systems.

Corrosion Prevention and Protection

Figure 1.38 Schematic diagram of the mechanism of oxidation of copper: (a) Diffusion of Cuþ ions from the metal to the air/oxide interface via cation vacancies; (b) reaction of oxygen molecules with Cuþ ions at the air/oxide interface; (c) diffusion of positive charge inwards to neutralize the excess of electrons in the metal. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)

Diffusion of metal outwards from the metal/oxide interface occurs in the copper–oxygen system. The mechanism of oxidation of copper to produce cuprous oxide may be visualized as shown in Figure 1.38. Cuprous oxide is a p-type material and in Figure 1.38(a), the diffusion of cuprous ions to the metal to air/oxide through cation vacancies is shown with the path indicated by arrows. The oxygen molecules react with cuprous ions at the

Introduction and Principles of Corrosion

Figure 1.39 Idealized lattice structure of zirconium oxide, an n-type semiconductor: (a) pure ZrO; (b) effect of Ca2þ addition; (c) effect of Ta5þ addition

air/oxide interface as shown in Figure 1.38(b). The diffusion of positive charge inwards and electrons outwards to maintain electroneutrality occurs in the last step. One of the requirements of this type of mechanism is that the metal under consideration should have dual oxidation states. Examples of such metals are iron (II, III), Ni (II, III) and Cu (I, II). In the case of NiO which is p-type metal-deficient material we have a structure with Ni2þ, O2 and Ni3þ in nickel vacancy sites. Addition of lithium ion impurity to NiO makes it a p-type conductor and oxide growth rate is reduced compared with the Ni–O2 system alone. On the other hand addition of Cr to Ni results in greater ease of oxidation than nickel alone due to higher cation diffusivity. Zirconium oxide, an n-type oxide, has a monoclinic lattice and has excess metal. The vacant sites are compensated by electrons to maintain electroneutrality. In this case electron current is carried by the electrons and ion transport by the oxide ion. The addition of Ca2þ ions to the zirconium system results in a disposition such that Ca2þ ions fit into the monoclinic lattice, which is schematically shown in Figure 1.39. In the case of calcium, anion vacancies are formed and charge compensation occurs. Substitution by addition of tantalum results in considerable changes in ionic diffusivity and electrical conductivity. 1.4.1

Oxidation of Alloys

Detailed systematic studies on the oxidation of alloys by Hauffe, Wagner et al.27–29 led to some generalizations with respect to the role of minor elements on the oxide growth. In the case of Zn(n-type) addition of a small amount of Al results in considerable increase in oxidation rate of zinc. Addition of chrominum to nickel (which forms a p-type oxide)

Corrosion Prevention and Protection

Table 1.10 The effect of alloying upon the rate of oxidation75 Oxide type

Valency of alloying element compared with parent metal

Effect

Diffusion-controlled oxidation rate

p-type Cu2O NiO FeO Cr2O3 CoO Ag2O MnO SnO

Higher valency

Increases number of vacancies Decreases number of parent metal ions with a higher valency

Increases

Lower valency

Decreases

n-type ZnO CdO Al2O3 TiO2 V2O5

Higher valency

Decreases number of vacancies Increases number of parent metal ions with a higher valency Decreases concentration of interstitial metal ions Increases number of free electrons Increases concentration of interstitial metal ions Decreases number of free electrons

Increases

Lower valency

Decreases

results in increase in oxidation rate. On the other hand addition of lithium to nickel oxide results in reduced diffusion of nickel. Some of the general rules deduced by Hauffe and Wagner with respect to oxidation of alloys, and the role of alloying elements on the oxidation rates are summarized in Table 1.10. Over the years many alloys have been developed for applications in which the alloys resist oxidation and give good performance without failure. Table 1.11 gives the nominal compositions of some alloys for use in industrial environments. The performance of alloys in various industrial environments is summarized in Table 1.12. Miscellaneous metal–gas reactions30 other than metal–oxygen reactions may be referenced as follows. Decarburization and hydrogen attack. This is encountered when hydrogen removes the carbon from the steel resulting in loss of strength of the steel. Hydrogen combines with carbon to produce methane and causes decarburization. The reaction with hydrogen may also cause cracking of steel (HIC). In the petroleum industry carbon monoxide and carbon dioxide reactions with steel are encountered which may also cause decarburization or carburization, depending upon the direction of the equilibrium. C þ CO2 Ð 2CO In the context of fossil fuels, sulfur compounds and their interaction with metals at high temperatures are important and this is termed sulfidation. The details of sulfidation attack are discussed in the literature31. The alloys which perform well in these atmospheres have already been cited (see Table 1.12).

22 75 57 65 20 37 37 71 67 47

Material

Haynes 188 214 230 242 556 HR-120 HR-160 Hastelloy N Hastelloy S Hastelloy X

800 A >1000

A, B, C as in Table 4.93

Some important features of structural ceramics with respect to corrosion are given below: Alumina

Zirconia Thoria Silicon carbide

Silicon nitride

Boron nitride

Versatile ceramic resistant to acids, alkalis, molten metals, molten salts, oxidizing and reducing gases at high temperatures; widely used in high-temperature laboratory and pilot-scale operations Generally used in conjunction with CaO, MgO or Y2O3 as stabilizers; vulnerable to acid and alkali attack Highly stable with a melting point of about 3300 C; used in nuclear industry Resistant to acids, including HF and molten metals; not resistant to alkalis, molten sodium sulfate; SiC produced by sintering is prone to oxidation and corrosion while single phase SiC is resistant; oxidation results in formation of SiO2; SiC is more sensitive to hot corrosion than Si3N4 Oxidation produces SiO2 and has been studied in detail;l12 exhibits high corrosion resistance at 1400 C in presence of 105% Na, V, 0.5% S, but corrodes at 900 C in presence 0.005% Na, 0.005% V and 3% S; resistant to alkali chloride melts and alkali solutions; attacked by alkali metal sulfates and vanadium Attacked by acids and not by alkalis; oxidizes at 700 C; can be used in reducing conditions; resistant to wetting by metals and alloys

4.15 Polymeric Materials The best known polymeric materials are natural rubber and synthetic rubber. These materials have low modulus of elasticity. The flexibility of these materials enables their application in tubing, belting and automotive tires as encountered in everyday usage. Resistance to chemicals, abrasive attack and insulating property can be advantageous in corrosion control applications. Naturally occurring rubber consists of a long-chain polymer with isoprene as the basic building unit. The rubber has high elasticity and a temperature limit of 160 F. Vulcanization consists of addition of elemental sulfur to rubber, followed by heating. The

Materials: Metals, Alloys, Steels and Plastics

301

resulting product becomes hard. The semi-hard and hard products are used in tires and tank liners. The general trend is increase in corrosion resistance with increase in hardness. The modulus of elasticity is in the range of 500–500 000 psi for soft and hard rubber, respectively. Synthetic rubbers of many types are available in the market. Neoprene and nitrile rubber have resistance to oils and gasoline and are used as gasoline hoses. Neoprene– lined vessels are used for storing strong sodium hydroxide solutions. The impermeability of butyl rubber to gases has been advantageously used in innertubes and process equipment such as seals for floating-top storage tanks. Butyl rubber is resistant to oxidizing agents such as dilute nitric acid. Compounding rubbers results in products of different hardness and mechanical properties, as for example tensile strength range of 900–4500 psi and elongation in the range of 0–1000 %. Soft rubbers can be used for applications such as in erosion conditions. Elastomers are also classified as semirigid or nonrigid plastics with modulus of 10 000–100 000 psi for semirigid and 100 000 psi. Some common elastomers with salient features are as follows:113 Chlorosulfonated, polyethylene (Hypalon) Ethylene propylene (Nordel) Epichlorhydrin (Hydrin) Fluoroelastomers (Viton)

Urethane (Adiprene)

Close resemblance to neoprene with greater resistance to heat, ozone and some chemicals Similar to butyl rubber; good resistance to ozone, heat, water, and sunlight Resistance to swelling in fuels and oils, ozone, good heat-aging properties Good resistance to many corrosives and hydrocarbons, sunlight, ozone and heat-aging, useful at high temperatures Good tensile strength, wear resistance, resistance to oxidation, oils and ozone

Physical properties such as adhesion to metals tear resistance, abrasion resistance, resistance to diffusion of gas as well as resistance to dilute and concentrated acids, aliphatic and aromatic hydrocarbons, ketones, oil and gasoline, water absorption, oxidation, ozone, sunlight, heat aging, low temperature and flame of the common elastomers are documented in the literature.114 Rating of elastomers with respect to resistance to the factors cited above are in terms of outstanding, excellent, very good, good, fair and poor. Plastics production and utilization in terms of various forms such as pumps, billiard balls, valves, pipes, fans, nose cones, airplane canopies, hosiery, cabinets, pot handles, heat valves and other implants make one to realize the role played by them in everyday life. Plastics are produced by casting, molding, and extrusion and calendaring; solid parts, lining, coatings, foams, fibers and films are also readily produced and available. The definition of plastics according to the American Society of Testing Materials is: ‘A plastic is a material that contains as an essential ingredient an organic substance of large

302

Corrosion Prevention and Protection

molecular weight, is solid in its finished state, and at some stage in its manufacture or in its processing into finished articles can be shaped by flow’. Plastics are weaker, softer, more resistant to chloride and HCl; less resistant to HNO3 and organic solvents compared with metals and their alloys. The two types of plastics are: (i) thermoplastics; and (ii) thermosetters. The main features of the two types are: Thermoplastics

Thermosetters

Can be remelted and reprocessed; can be welded; soluble in some solvents; can be either amorphous or semicrystalline; contain linearchain or branched-chain polymer; acrylics, fluoropolymers, vinyls, polystyrenes, polyphenelyneoxide, polysulfones, polypropylenes and polybutylenes are some examples of thermoplastic polymers Cannot be remelted and reprocessed; decompose on heating to high temperatures; cross-linked structure gives rise to thermosetting properties; swelling occurs in some solvents; epoxies, melamines, phenolics, polyesters and rigid urethanes are some examples of thermosetters

Some of the polymers used in corrosion control along with their trade names are: Polyvinyl fluoride Polyvinylidene difluoride Polyethylene Polypropylene (Poly) ethylene chlorotrifluoroethylene (ECTFE) Polytetrafluoroethylene (PTFE) Polyvinyl chloride (Poly) ethylene tetrafluoroethylene (ETFE) (Poly) Fluorinated ethylenepropylene (FEP) Perfluoroalkoxy

Tedlar Kynar Polythene Halar Teflon PTEE Tefzel Teflon FEP Teflon PFA

Composite materials consist of a combination of two generically dissimilar materials brought together for synergy where one phase, the matrix, is continuous (thermoplastic or thermosetting), the other phase being a reinforcement which is discontinuous. Reinforcements can be particulates, fibers or cloth. A familiar example of a composite material is fiber-reinforced plastic (FRP) used in making tanks, piping, filled fluoroplymer gaskets, scrim-filled elastomers for gaskets and impoundment basin liners. 4.15.1

Application of Polymers in Corrosion Control

The polymeric materials are viscoelastic (creep) and this property is very important in corrosion applications. The temperature limit is 260 C and most of the applications are below 150 C. The tanks and piping are used below 100 C. The mechanical properties of

Materials: Metals, Alloys, Steels and Plastics

303

Figure 4.8 Overview of polymer materials for corrosion control

polymeric materials change over time since they absorb liquids and are permeable, unlike metals. The polymeric materials are used in: (i) barrier applications; (ii) self-supporting structures (tanks, piping, valves and pumps); (ii) column internals, seals, gaskets, adhesives and caulkings. An overview of polymeric materials used in corrosion control applications is given in Figure 4.8. The American Society for Testing and Materials (ASTM) provides standards, compatibility testing, and tests for mechanical properties, recommended practices and procedures as well as codes for polymeric materials. The various industry codes and standards are summarized in Table 4.95.

304

Corrosion Prevention and Protection

Table 4.95 Industry codes and standards Product ASTM ASTM ASTM ASTM ASTM ASTM

D2310 D2517 D2996 D2997 D4024 D3299

ASTM D4097 ASTM F1545 ASTM F492 ASTM D1784 ASTM D1785 ASTM D3350 ASTM D1248 ASTM D4020 ASTM F1123 ISO 3994 ASTM D2564 ASTM D1998 ASTM F118 ASTM F104 ASTM F104 ASTM D1330

Standard classification of machine-made reinforced thermosetting resin pipe Standard specification for reinforced thermosetting (epoxy) resin pipe Standard specification for filament wound reinforced thermosetting pipea Standard specification for centrifugally cast reinforced thermosetting pipe Standard specification for reinforced thermosetting resin flanges Standard specification for filament-wound glass-fiber-reinforced thermoset resin chemical tanks Standard specification for contact-molded FRP chemical-resistant tanks Standard specification for PTFE; PFA; FEP- and ETFE-lined piping Standard specification for PE and polypropylene (PP) plastic-lined ferrous metal pipe and fittings Rigid poly(vinyl chloride) (PVC) compounds and chlorinated poly(vinyl chloride) (CPVC) compounds Standard specification for poly(vinyl chloride) (PVC) plastic pipe, schedules 40, 80 and 120 Standard specification for polyethylene pipe and fitting materials Polyethylene molding and extrusion material Ultra-high-molecular-weight polyethylene material Nonmetallic (rubber) expansion joints Chemical transfer hose Solvent cements for PVC (CPVC) joints Polyethylene upright storage tanks Definition of Gasket terms Nonmetallic (rubber) expansion joints Nonmetallic gasket materials Standard specification for rubber sheet gaskets Compatibility testing

ASTM ASTM ASTM ASTM

D543 D471 C868 C581

ASTM C3491 ISO 175 ISO 4599 ISO 4600 ISO 6252 ISO 8308 ASTM F363 ASTM F146 ASTM D3615 ASTM D3681 (Not Standardized)

Test methods for determining chemical resistance of plastics Effects of liquids on rubbers, test method Test method for chemical resistance of protective linings Test method for determining chemical resistance of fiberglass-reinforced thermosetting resin Chemical resistance test for rubber linings by Atlas blind flange test Determination of effects of liquid chemicals on plastics Determination of environmental stress cracking (ESC) by the bent strip method Determination of environmental stress cracking (ESC) by the ball or pin impression method Determination of environmental stress cracking (ESC) by the constant tensile test method Liquid transmission and permeation through hose and tube Corrosion testing for gaskets Fluid resistance of gasket materials Chemical resistance of thermoset-molded compounds used in the manufacture of molded fittings Chemical resistance of reinforced thermosetting resin pipe in a deflected condition Roberts cell blind flange test for linings exposed to high temperatures and pressures (contd.)

Materials: Metals, Alloys, Steels and Plastics

305

Table 4.95 (contd.) Testing for mechanical properties ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM

D638 D412 D1415 D2240 D2538 F38 D395 F152 F36 F37 F586 D2290 D2412 D695 D429 D1781 D903 D790 D2584 D4541 D4060

Standard testing for tensile properties of plastics Standard testing for tensile properties of rubbers Test method for indentation hardness of rubbers, international hardness Test for Durometer hardness of rubbers Indentation hardness by Barcol hardness tester for rigid plastics Creep relaxation of gasket materials Compression set of rubbers at ambient and elevated temperatures Tension testing of gasket materials Short-term compressibility and sealability of gasket materials Sealability of gasket material Determination of Y and M values of gaskets Apparent tensile strength of tubular (pipe) products by the split disk method External loading characteristics of plastic pipe by parallel plate loading Compressive properties of plastics Adhesion of rubbers to metallic substrates (Procedure E1) Climbing drum peel test Peel strength of adhesives on metals Flexural properties of reinforced and unreinforced plastics Ignition loss of cured reinforced resin Method of pull-off strength of coatings using portable tester Test method for abrasion resistance of organic coatings Recommended practices and procedures

SSPC SP-5 SPI FD118 MTI Project 84 NACE RPO 188 ASTM D4787 ASTM D4417 ASTM D4258 ASTM D3486

Steel Structure Paint Council white metal blast surface preparation Proposed test method for pinhole detection by high-voltage spark testing by DC Spark testing practices for linings Tinker Rasor wet sponge testing for thin linings Practice for continuity testing for linings on concrete Method for measuring surface profile of steel surfaces for coating Cleaning of concrete Installation of vulcanizable rubber tank linings Codes

RTP-1 Section X ASME B31.3

Reinforced Thermoset Plastic Corrosion Resistant Equipment, ASME, NY ASME Boiler and Pressure Vessel Code (for fiber-reinforced plastic pressure vessels) Chemical Process, Petroleum Refinery Piping Code

There is no industry standard for contact-molded FRP piping

References 1. K. Horikawa, S. Takiguchi, Y. Ishizu, M. Kanazashi and Boshoku Gijitu, Corr. Eng., 16, 153 (1967). 2. Rikujo Tekkotsu Kozobutsu Boshoku Kenkyu Kai, Boshoku Gijutu, Corr. Eng., 22, 106 (1973). 3. Task Force on the Calibration of Atmospheric Corrosivity, ASTM STP 435, ASTM, Philadelphia, PA, 1968, p. 360.

306

Corrosion Prevention and Protection

4. I. Matsushima, Uhlig’s Corrosion Handbook, R.W. Revie (ed.), John Wiley & Sons, Inc., 2000, p. 515. 5. I. Matsushima, Corrosion and Protection, The Society of Materials Science, Japan, Vol. 19, 1980, p. 94. 6. K. Masamura and I. Matsushima, 23rd Annual Symposium on Corrosion and Protection, Japan Society of Corrosion Engineering, Tokyo, Nov. 1978, p. 104. 7. I. Matsushima, Dr. Eng. Thesis, A Study on Localized Corrosion of Steel, University of Tokyo, 1982. 8. S.C. Dexter, Metals Handbook, 9th edn, American Society of Metals, Metals Park, OH, Vol. 1, 1978, p. 893. 9. S.C. Dexter and C.H. Culberson, Mater. Performance, 19(9), 16 (1980). 10. C.R. Southwell and A.L. Alexander, Mater. Prot., 9(1), 179, 1970. 11. F.L. Laque, Corrosion Handbook, H.H. Uhlig (ed.), John Wiley & Sons, Inc., NY, 1948, p. 383. 12. ‘‘Deterioration of Structures in Sea water’’, 18th Report of the Committee of the Institution of Civil Engineers, London, 1938. 13. W. Stum and J.J. Morgan, Aquatic Chemistry, 2nd edn, Wiley-Interscience, Toronto, Ontario, 1981. 14. J.E. Zajic, Microbiological Biogeochemistry, Elsevier, NY, 1969. 15. G.H. Booth, A.W. Copper and P.M. Cooper, Br. Corr. J., 2, 114 (1967). 16. C.L. Durr and J.A. Beavers, Corrosion 1998, Paper 667, NACE International, 1998. 17. J.D. Palmer, Mat. Perf., 13(1), 41–46. 18. R.A. King, TRRL Supplementary Report 316, TRRL, Crowthorne, 1977. 19. R.G. Worthingham, T.R. Jack and V. Ward, Biologically Induced Corrosion, NACE, 1986, p. 335. 20. R.A. King and J.D.A. Miller, Nature, 233, 491–492 (1972). 21. I. Matsushima, Chijin Shokan, Tokyo, Japan, 1995, p. 21. 22. A.G. Larkin, Metallurgia, April 1966, p. 165. 23. M.A. Streicher, Corrosion, 29, 337 (1993). 24. M.A. Streicher, Uhlig’s Corrosion Handbook, R.W. Revie (ed.), John Wiley & Sons, 2000. 25. J.E. Truman, U.K, Corrosion 1987, Brighton, UK, October 26–28, 1987, Institution of Corrosion Science and Technology, Birmingham, UK, 1988, pp. 111–129. 26. Allegheny Ludlum Technical Data Blue Sheet Martensitic Stainless Steel Types, 410, 420, 425 Mod and 440A, Allegheny Ludlum Corporation, 1998. 27. E.H. Hollingsworth, H.Y. Hunsicker and P.A. Schweitzer (eds.), Corrosion and Corrosion Protection Handbook, Marcel Dekker, NY, 1983, pp. 111–145. 28. R.B. Mears, Corrosion Handbook, H.H. Uhlig (ed.), John Wiley & Sons, Inc., 1976, pp. 39–56. 29. R.B. Mears and J.R. Akers, Proc. Amer. Soc. Brewing Chem., St Paul, Minnesota, Annual Meeting, 1942. 30. D.J. De Renzo, Corrosion Resistant Materials Handbook, Noyes Data Corporation, Park Ridge, New Jersey, USA, 1985, p. 621. 31. E.H. Hollingsworth and H.Y. Hunsicker, Metals Handbook, 9th edn, Vol. 13, Corrosion, ASM International, Metals Park, OH, 1987, pp. 583–609. 32. H. Reboul and R. Canon, Corrosion galvanique de l’aluminium, mesures de protection, Revue de l’Aluminium, 1922, 403–406 (1984). 33. P.M. Aziz, Corrosion, 12, 35 (1956). 34. M.C. Reboul, T.J. Warner, H. Maye and B. Baroux, Materials Science Forum, Transtech Publications, Switzerland, Vols. 217–222, 1996, pp. 1553–1558. 35. S.C. Dexter and J. Ocean, Sci. Eng., 8(1), 109 (1981). 36. I.L. Rosenfeld, Localized Corrosion, NACE International, Houston, TX, 1974, pp. 386–389 (localized corrosion). 37. J.E. Hatch, Aluminum, Properties and Physical Metallurgy, ASTM International, Metals Park, OH, 1984, pp. 248–319, 263, 307. 38. S.C. Dexter, Localized Corrosion of Metals Handbook, ASM International, Corrosion, Vol. 13, 1987, pp. 103–122.

Materials: Metals, Alloys, Steels and Plastics

307

39. J.J. Elpick, Microbial Corrosion in Aircraft Fuel Systems in Microbial Aspects of Metallurgy, J.D.A. Miller (ed.), Elsevier, NY, 1970, pp. 157–172. 40. V.A. Marichev, Werkst Korr, 34, 300 (1983). 41. M.O. Spiedel, Hydrogen in Metals, I.M. Bernstein and A.W. Thompson (eds.), ASM International, Metals Park, OH, 1974, p. 249. 42. D.O. Sprowls and E.H. Spuhler, Greenletter, Alcoa, Avoiding SCC in High Strength Aluminum Alloy Structures, ASM International Corrosion, Vol. 13, Metals Park, OH, Jan. 1982. 43. L.J. Korb, Corroson in Aerospace Industry, Metal Handbook, 9th edn, Corrosion, ASM International, Vol. 13, Metals Park, OH, 1987, p. 1082. 44. D.M. Aylor and P.J. Moran, CORROSION/86, NACE International, Paper No. 202, Houston, Texas. 45. V.S. Sastri, Corrosion Inhibitors, John Wiley & Sons, Ltd, 1998. 46. C. Vargel, Corrosion de l’aluminium, Dunod, Paris, 1999, pp. 154–163. 47. H.P. Godard, The Corrosion of Light Metals, John Wiley & Sons, Inc., NY, 1967, pp. 46, 70–73. 48. R.E. Lobnig, R.P. Frankenthal, D.J. Siconolfi and J.D. Sinclair, J. Electrochem. Soc., 140, 1902 (1993); 141, 2935 (1994). 49. T.E. Graedel, J.P. Franey and G.W. Kammlott, Corrosion Science, 23, 1141 (1983); 27, 639 (1987). 50. V. Kucera, D. Knotkova, J. Gullman and P. Holler, International Congress on Metallic Corrosion, Madras, India, Vol. 1, CERI, 1987, p. 167. 51. W.W. Kirk and H.H. Lawson (eds.), Atmospheric Corrosion, ASTM-STP 1239 Philadelphia, 1991. 52. M. Elboujdaini, V.S. Sastri and M. Sahoo, American Foundrymen Society Transactions, Chicago, 29–36, 1993, ibid: 1994. 53. H.S. Campbell, J. Inst. Met., 77, 345 (1950). 54. V.F. Lucey, Br. Corr. J., 1, 53 (1965). 55. H.S. Campbell, Localized Corrosion, R.W. Staehle (ed.), NACE, Houston, Texas, 1974. 56. A.M. Beccaria and J. Crousier, Br. Corr. J., 26, 215 (1991). 57. J.G.N. Thomas and A.K. Tiller, Br. Corr. J., 7, 256 (1972). 58. E. Mattson, Br. Corr. J., 15, 6 (1980). 59. H.A. Videla, Biofouling and Biocorrosion in Industrial Water Systems, G.G. Geesey, Z. Lewandowski and H.C. Flemming (eds.), Lewis Publishers, Boca Raton, Florida, 1994, pp. 231–241. 60. C.A.C. Sequiera, Uhlig’s Corrosion Handbook, 2nd edn, John Wiley & Sons, Inc., NY, 2000. 61. Y. Yanish, Proc. 2nd International Conf. on Heat Resistant Materials, Gatlinberg, TN, Sept. 11–14, 1995, ASM, Metals Park, OH, pp. 655–656. 62. W.Z. Friend, Corrosion of Nickel and Nickel Base Alloys, John Wiley & Sons, NY, 1980. 63. V. Heubner, Nickel Alloys and High Alloy Special Stainless Steels, Krupp YDM, GmbH, P.O. Box 1820, Werdoh, Germany, 1987, 1st edn, 2nd edn. 64. G. Lai, High Temperature Corrosion of Engineering Alloys, ASM International, Materials Park, OH, 1990. 65. J.S. Grauman and J.J. McKetta (eds.), Encyclopedia of Chemical Processing Design, Marcel Dekker, NY, Vol. 58, 1998, pp. 123–147. 66. J.A. Mountford and J.S. Grauman, Titanium for Marine Applications, 2nd Corrosion Control Workshop, Colorado School of Mines, New Orleans, LA, February 1997. 67. M.J. Blackburn, W.H. Smyrl, J.A. Feeney and B.F. Brown (eds.), Stress–Corrosion Cracking on High Strength Steel and in Titanium and Aluminum Alloys, Naval Research Laboratory, Washington, DC, 1972, pp. 245–363. 68. D.J. Simbi and J. Scully, Corr. Sci., 37, 1325 (1995). 69. R.J.H. Wanhill, Br. Corr. J., 10, 69 (1975). 70. R.L. Fowler and J.A. Luzietti, Corrosion 1980, Paper No. 18, NACE, Houston, TX, 1980. 71. R. Boyer, G. Welsch and E.W. Collins (eds.), Materials Properties Handbook, Titanium Alloys, ASM, Materials Park, OH, 1994. 72. R. Murakami and W.G. Ferguson, Fatigue Fract. Eng., Mater. Struct., 16, 255 (1993).

308

Corrosion Prevention and Protection

73. P. Crook, Mater. Perform, 30, 64 (1991). 74. Lead Industries Association, Lead for Corrosion Resistant Applications – A Guide, NY, 1974. 75. F.W. Fink and W.K. Boyd, The Corrosion of Metals in Marine Environments, Defence Metals Information Centre, Battelle Institute, Columbus, OH, DMIC Report 245. 76. H.H. Uhlig (ed.), Corrosion Handbook, John Wiley & Sons, Inc., NY, 1948. 77. I.J. Polmear, Mater. Sci. and Technol., 10, 1 (1994). 78. A. Froats, T. Kr. Aune, D. Hawke, W. Unsworth and G. Hillis, Corrosion of Magnesium and Magnesium Alloys, Metals Handbook, 9th edn, Vol. 13, Corrosion, ASM International, Materials Park, OH, 1987, pp. 740–754. 79. D.L. Hawke, J.E. Hillis and W. Unsworth, Preventive Practice for Controlling the Galvanic Corrosion of Magnesium Alloys, Technical Committee, International Magnesium Association, Mclean, VA, 1988. 80. A.I. Aspahani and W.L. Silence, Pitting Corrosion, Corrosion, ASM, Metals Park, OH, Vol. 13, 9th edn, 1987, p. 113. 81. W.E. Mercer and J.E. Hillis, The Critical Contaminant Limit and Salt Water Corrosion Performance of Magnesium AE 42 Alloy, SAE Technical Paper No. 920073, 1992. 82. O. Lunder, K. Nisancioglu and R.S. Hansen, Corrosion of Die Cast Mg-Al Alloy, Corrosion and Exposition, Paper No. 930 755 SAE, Detroit, MI, 26 February–2 March 1993, pp. 117–126. 83. G.L. Makar and J. Kruger, J. Electrochem. Soc., 13(2), 414 (1990). 84. C. Kirby, Corr. Sci., 27(6), 567 (1987). 85. S.C. Dexter, Localized Corrosion, Metals Handbook, 9th edn, Vol. 13, Corrosion, ASM International, 1987, p. 106. 86. M.O. Spiedel, Metall. Trans A 6A, 631 (April 1975). 87. D.K. Priest, F.H. Beck and M.G. Fontana, ASM Trans., 47, 473 (1955). 88. E. Groshart, Magnesium Part I – The Metal, Metal Finishing, Vol. 83, 10, October 1985. 89. M.A. Timonova, Corrosion Cracking of Mg Alloys and Methods of Protection in Intercrystalline Corrosion and Corrosion Metals under Stress, I.A. Levin (ed.), Translated from Russian, Consultant Bureau in New York, 1962, pp. 263–282. 90. V.A. Serdyuk and N.M. Grinberg, Int. J. Fatigue, 5(2), 79 (1983). 91. Corrosiveness of Various Atmospheric Test Sites as Measured by Specimens of Steel and Zinc, Metal Corrosion in the Atmosphere, STP 435, ASTM, Philadelphia, PA, 1968, pp. 360–391. 92. X.G. Zhang, Corrosion and Electrochemistry of Zinc, Plenum, NY, 1996. 93. H. Guttman, Effect of Atmospheric Factors on the Corrosion of Rolled Zinc, Metal Corrosion in the Atmosphere, STP 435, ASTM, Philadelphia, PA, 1968, pp. 223–239. 94. F.H. Haynie and J.P. Upham, Materials Protection and Performance, 9(8), 35–40 (1970). 95. W.H. Ailor, Atmospheric Corrosion, John Wiley & Sons, Inc., 1982, pp. 913–921. 96. R.J. Neville, Automotive Corrosion by Deicing Salt, R. Baboiam (ed.), NACE, Houston, Texas, 1981, pp. 182–218. 97. G.L. Cox, Ind. and Eng. Chem., (8), 902–904 (1931). 98. M. Romanoff, Underground Corrosion, US National Bureau of Standards Circular 579, Washington, DC, 1957. 99. H.E. Townsend, NACE Corrosion’91 Conference, Cincinnati, OH, 11–15 March, 1991, Paper No. 416. 100. K.W.J. Treadway, B.L. Brown and R.N. Cox, Corrosion of Steel in Concrete, STP 713, ASTM, Philadelphia, PA, 1980, pp. 102–131. 101. L. Kenworthy, J. Inst. Metals, 69, 67 (1943). 102. ASTM Metals Handbook, Vol. 5, Materials Park, OH, USA, pp. 339–348, 360–371, 349–359, 1994. 103. B.S. Frechem, J.G. Morrison and R.T. Webster, STP 728, ASTM, Philadelphia, PA, 1981, pp. 85–108. 104. IAEA-TECDOC-996 (1998), Waterside Corrosion of Zirconium Alloys in Nuclear Power Plants, International Atomic Energy Agency, Vienna. 105. B. Cox and Y.M. Wong, J. Nucl. Mater., 245, 34 (1997). 106. B. Cox, J. Nucl. Mater., 170, 1 (1990).

Materials: Metals, Alloys, Steels and Plastics

309

107. S.C. Britton, Tin vs. Corrosion, ITRI London, UK, 1975. 108. V.S. Sastri, Water Research, 29, 1827 (1995). 109. H. Leidheiser, The Corrosion of Tin, Copper and their Alloys, John Wiley & Sons, Inc., NY, 1971. 110. M. Rigaud, Uhlig’s Corrosion Handbook, 2nd edn, R.W. Revie (ed.), John Wiley & Sons, Ltd, 2000, p. 395. 111. R.A. McCauley, Corrosion of Ceramics, Marcel Dekker, NY, 1995. 112. Y.G. Gogotsi and V.A. Lavrenko, Corrosion of High-Performance Ceramics, Springer-Verlag, Berlin, 1992. 113. Dupont Elastomers Notebook, E.I. Dupont de Nemours, Wilmington, Delaware, April 1976, p. 189. 114. M.G. Fontana, Corrosion Engineering, McGraw-Hill, Inc., NY, 1986.

5 Corrosion Economics and Corrosion Management Corrosion economics is important when materials for construction of plant or equipment are chosen. The corrosion costs may be significant during the lifetime of plant. Corrosion mitigation becomes important which adds to the costs of the plant operation. The other alternative is to choose corrosion-resistant materials, which may be expensive, resulting in high initial costs. It is also possible to use coatings or cathodic protection, resulting in additional costs. The economic considerations and factors such as plant life dictate the choice of materials and the corrosion prevention strategies used.

5.1 Corrosion Economics The definitions of the terms encountered in discounted cash-flow calculations are taken from the literature.1–3 Discounted cash flow is concerned with the analysis of the value of money as a function of time. The money available at a future date has less value at the present and should be discounted by the interest to present worth (PW). P is the notation for the present sum of money, which should be used in such a manner that it produces a future income more than other forms of investment, irrespective of whether it is used for new equipment or the corrosion mitigation process. Let F be the amount of money available in the future when an investment of P is made. The relation between P and F may be written as: F ¼ Pð1 þ iÞn where i is the interest rate and n is the number of years. The rearrangement leads to F=P ¼ ð1 þ iÞn

Corrosion Prevention and Protection: Practical Solutions # 2007 John Wiley & Sons, Ltd

V. S. Sastri, E. Ghali and M. Elboujdaini

312

Corrosion Prevention and Protection

where F/P is known as the single-payment compound – amount factor. The future amount of money F may be discounted to the present value P by the reciprocal multiplication factor P/F known as the single-payment present-worth factor. The annual cash flow denoted by A and the present value of A discounted by interest rate i is given by: ð1 þ iÞn 1 P¼A ið1 þ iÞn The P/A ratio is known as the uniform-series present-worth factor. The annual cash flow may be discounted to present worth by multiplication by P/A. Multiplication of present value by A/P gives the amount of annual cash flow. The factors P/F, P/A and A/P for a period of 30 years at interest rates of 6, 8, 10, 12, 15 and 20% for a period of 1–30 years are given in Table 5.1. Depreciation D is an annual tax allowance for the

Table 5.1 Interest factors in DCF calculations 6% interest factors

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

10% interest factors

15% interest factors

P=F

P=A

A=P

P=F

P=A

A=P

P=F

P=A

A=P

0.9434 0.8900 0.8396 0.7921 0.7473 0.7050 0.6651 0.6274 0.5919 0.5584 0.5268 0.4970 0.4688 0.4423 0.4173 0.3936 0.3714 0.3503 0.3305 0.3118 0.2942 0.2775 0.2618 0.2470 0.2330 0.2198 0.2074 0.1956 0.1846 0.1741

0.9434 1.8334 2.6730 3.4651 4.2124 4.9173 5.5824 6.2098 6.8017 7.3601 7.8869 8.3838 8.8527 9.2950 9.7122 10.1059 10.4773 10.8276 11.1581 11.4699 11.7641 12.0416 12.3034 12.5504 12.7834 13.0032 13.2105 13.4062 13.5907 13.7648

1.0600 0.5454 0.3741 0.2886 0.2374 0.2034 0.1791 0.1610 0.1470 0.1359 0.1268 0.1193 0.1130 0.1076 0.1030 0.0990 0.0954 0.0924 0.0896 0.0872 0.0850 0.0830 0.0813 0.0797 0.0782 0.0769 0.0757 0.0746 0.0736 0.0726

0.9091 0.8264 0.7513 0.6830 0.6209 0.5645 0.5132 0.4665 0.4241 0.3855 0.3505 0.3186 0.2897 0.2633 0.2394 0.2176 0.1978 0.1799 0.1635 0.1486 0.1351 0.1228 0.1117 0.1015 0.0923 0.0839 0.0763 0.0693 0.0630 0.0573

0.9091 1.7355 2.4869 3.1699 3.7908 4.3553 4.8684 5.3349 5.7590 6.1446 6.4951 6.8137 7.1034 7.3667 7.6061 7.8237 8.0216 8.2014 8.3649 8.5136 8.6487 8.7715 8.8832 8.9847 9.0770 9.1609 9.2372 9.3066 9.3696 9.4269

1.1000 0.5762 0.4021 0.3155 0.2638 0.2296 0.2054 0.1874 0.1736 0.1627 0.1540 0.1468 0.1408 0.1357 0.1315 0.1278 0.1247 0.1219 0.1195 0.1175 0.1156 0.1140 0.1126 0.1113 0.1102 0.1092 0.1083 0.1075 0.1067 0.1061

0.8696 0.7561 0.6575 0.5718 0.4972 0.4323 0.3759 0.3269 0.2843 0.2472 0.2149 0.1869 0.1625 0.1413 0.1229 0.1069 0.0929 0.0808 0.0703 0.0611 0.0531 0.0462 0.0402 0.0349 0.0304 0.0264 0.0230 0.0200 0.0174 0.0151

0.8696 1.6257 2.2832 2.8550 3.3522 3.7845 4.1604 4.4873 4.7716 5.0188 5.2337 5.4206 5.5831 5.7245 5.8474 5.9542 6.0472 6.1280 6.1982 6.2593 6.3125 6.3587 6.3988 6.4338 6.4641 6.4906 6.5135 6.5335 6.5509 6.5660

1.1500 0.6151 0.4380 0.3503 0.2983 0.2642 0.2404 0.2229 0.2096 0.1993 0.1911 0.1845 0.1791 0.1747 0.1710 0.1679 0.1654 0.1632 0.1613 0.1598 0.1584 0.1573 0.1563 0.1554 0.1547 0.1541 0.1535 0.1531 0.1527 0.1528

Corrosion Economics and Corrosion Management

313

Table 5.1 ðcontd:Þ 8% interest factors

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

12% interest factors

20% interest factors

P=F

P=A

A=P

P=F

P=A

A=P

P=F

P=A

A=P

0.9259 0.8573 0.7938 0.7350 0.6806 0.6302 0.5835 0.5403 0.5002 0.4632 0.4289 0.3971 0.3677 0.3405 0.3152 0.2919 0.2703 0.2502 0.2317 0.2145 0.1987 0.1839 0.1703 0.1577 0.1460 0.1352 0.1252 0.1159 0.1073 0.0994

0.9259 1.7833 2.5771 3.3121 3.9927 4.6229 5.2064 5.7466 6.2469 6.7101 7.1390 7.5361 7.9038 8.2442 8.5595 8.8514 9.1216 9.3719 9.6036 9.8182 10.0168 10.2007 10.3711 10.5288 10.6748 10.8100 10.9352 11.0511 11.1584 11.2578

1.0800 0.5608 0.3880 0.3019 0.2505 0.2163 0.1921 0.1740 0.1601 0.1490 0.1401 0.1327 0.1265 0.1213 0.1168 0.1130 0.1096 0.1067 0.1041 0.1019 0.0998 0.0980 0.0964 0.0950 0.0937 0.0925 0.0914 0.0905 0.0896 0.0888

0.8929 0.7972 0.7118 0.6355 0.5674 0.5066 0.4523 0.4039 0.3606 0.3220 0.2875 0.2567 0.2292 0.2046 0.1827 0.1631 0.1456 0.1300 0.1161 0.1037 0.0926 0.0826 0.0738 0.0659 0.0588 0.0525 0.0469 0.0419 0.0374 0.0334

0.8929 1.6901 2.4018 3.0373 3.6048 4.1114 4.5638 4.9676 5.3282 5.6502 5.9377 6.1944 6.4235 6.6282 6.8109 6.9740 7.1196 7.2497 7.3658 7.4694 7.5620 7.6446 7.7184 7.7843 7.8431 7.8957 7.9426 7.9844 8.0218 8.0552

1.1200 0.5917 0.4163 0.3292 0.2774 0.2432 0.2191 0.2013 0.1877 0.1770 0.1684 0.1614 0.1557 0.1509 0.1468 0.1434 0.1405 0.1379 0.1358 0.1339 0.1322 0.1308 0.1296 0.1285 0.1275 0.1267 0.1259 0.1252 0.1247 0.1241

0.8333 0.6944 0.5787 0.4823 0.4019 0.3349 0.2791 0.2326 0.1938 0.1615 0.1346 0.1112 0.0935 0.0779 0.0649 0.0541 0.0451 0.0376 0.0313 0.0261 0.0217 0.0181 0.0151 0.0126 0.0105 0.0087 0.0073 0.0061 0.0051 0.0042

0.8333 1.5278 2.1065 2.5887 2.9906 3.3255 3.6046 3.8372 4.0310 4.1925 4.3271 4.4392 4.5327 4.6106 4.6755 4.7296 4.7746 4.8122 4.8435 4.8696 4.8913 4.9094 4.9245 4.9371 4.9476 4.9563 4.9636 4.9697 4.9747 4.9789

1.2000 0.6545 0.4747 0.3863 0.3344 0.3007 0.2774 0.2606 0.2481 0.2385 0.2311 0.2253 0.2206 0.2169 0.2139 0.2114 0.2094 0.2078 0.2065 0.2054 0.2044 0.2037 0.2031 0.2025 0.2021 0.2018 0.2015 0.2012 0.2010 0.2008

From D.A. Jones, Principles and Prevention of Corrosion, Macmillan Publishing Co., New York, 1992. Reprinted by permission, Prentice-Hall, Upper Saddle River, NJ

deterioration of equipment, plant or property and is given by the equation: D ¼ fðP SÞ where P is the present cost, S is the salvage cost after depreciation over N yr and f is the fraction allowed in a given year. When straight-line depreciation is involved, we have f ¼ 1=N, f and D are the same each year over N yr. The product P=A f gives the discounted value of fractional depreciation. The depreciation is governed by tax laws, which define depreciation period and the method for various properties and equipment. The two methods of depreciation are: (i) the sum of years digits (SOYD); and (ii) declining balance (DB). In SOYD for a period of 5 yr, we have 5 þ 4 þ 3 þ 2 þ 1 ¼ 15 and the f values for the five years are 5/15, 4/15, 3/15 2/15 and 1/15, respectively.

314

Corrosion Prevention and Protection

In the declining-balance (DB) method, the depreciation in n yr is given by: Dn ¼ fðP Dn Þ where n has values from 1 to N, f is a given percentage, P is the initial cost and Dn is the total depreciation to the nth year. When the double declining method is considered, f ¼ 2=N, the annual depreciation is twice that of straight-line depreciation and the amount depreciated decreases with increase in the value of n. In the case of double declining balance depreciation method, the book value PDn in the nth year is given by4: P Dn ¼ P½1 2=Nn The depreciation is allowed up to salvage value, S by US tax laws. The various products are classified into 3-, 5-, 7-, 10-, 15- and 20-year groups in the modified accelerated cost recovery system (MACRs) in the US in 1986. The annual depreciation prescribed by the MACR system is given in Table 5.2. Some typical products belonging to various classes are noted in Table 5.3. Verink’s equation for determining the present worth (PW) for different economic design situations using straight-line depreciation were written as: PW ¼ P ¼ tðP SÞP=A Xð1 tÞP=A þ SðP=FÞ Table 5.2 Annual depreciation prescribed by modified accelerated cost recovery Recovery year 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

3-yr class 5-yr class (200% DB) (200% DB) 33.00 45.00 15.00* 7.00

20.00 32.00 19.20 11.52 11.52 5.76

7-yr class (200% DB)

10-yr class (200% DB)

14.28 24.49 17.49 12.49 8.93* 8.93 8.93 4.46

10.00 18.00 14.40 11.52 9.22 7.37 6.55* 6.55 6.55 6.55 3.29

15-yr class 20-yr class (150% DB) (150% DB) 5.00 9.50 8.55 7.69 6.93 6.23 5.90* 5.90 5.90 5.90 5.90 5.90 5.90 5.90 5.90 3.00

3.75 7.22 6.68 6.18 5.71 5.28 4.89 4.52 4.46* 4.46 4.46 4.46 4.46 4.46 4.46 4.46 4.46 4.46 4.46 4.46 2.25

*Year of switch to straight-line depreciation to maximize deduction From DA Jones, Principles and Prevention of Corrosion, Macmillan Publishing Co., New York, 1992. Reprinted by permission, Prentice-Hall, Upper Saddle River, NJ

Corrosion Economics and Corrosion Management

315

Table 5.3 Typical properties belonging to different classes Number of years 3 5 7 10 15

Examples Rubber products, finished plastic products, fabricated metal products Computers, copiers, typewriters, trucks, trailers, cars, property used in research, cargo containers Office furniture, railroad track, agricultural equipment Vessels, assets used in petroleum refining or tobacco products and food products Sewage-treatment plants, telephone distribution plants

where P is the present value of the money, t is the tax rate, S is the salvage value, and X is the sum of the expenses excluding depreciation. The values of P/A and P/F given in Table 5.1 are functions of the interest rate. P is the present value of the money and is negative since it is expenditure. The second term represents tax savings or incomes and has a positive sign. X and Xt, are negative and positive respectively because they represent expenditure and tax savings. The final term represents salvage value and is positive. The relationship for the present worth (PW) can be converted to annual cost at interest rate i over n yr and is written as: A ¼ ðPWÞðA=PÞ The relationship of present worth as applied in the context of modified accelerated cost recovery method of depreciation may be written as: PW ¼ P þ tðPÞ½n ðP=FÞn Xð1 tÞðP=AÞ where the term containing S is omitted. The Canadian Income Tax Act IT-285R2 dated March 31, 1994 gives some general definitions and remarks on depreciation of property and capital cost allowance. Schedule II lists the types of property divided into classes, percentage depreciation and description of the property. Some examples are given in Table 5.4. Some examples illustrative of discounted cash-flow calculations taken from the literature are as follows: Example 1: A new heat exchanger is required and the lifetime of a carbon steel exchanger is 5 yr with a cost of $9500. The 316 stainless steel heat exchanger with a lifetime of 15 yr costs $26500, which can be written off in 11 yr. Assuming the minimum acceptable rate of 10%, tax rate of 48% and a straight-line depreciation, determine which unit is more economical based on annual costs. Using the equations PW ¼ P þ ftPðP=AÞ10% n A ¼ PWðA=PÞ10% ; n and the given data, the annual costs of the carbon steel and stainless steel heat exchangers are $1594 and $2497, respectively. In the case of the carbon steel heat

316

Corrosion Prevention and Protection

Table 5.4 Depreciation of property Class Depreciation (%) 1

4 5

6 10

100

Property or material Bridge, culvert, dam, road, parking area, storage area, railway track, railway traffic control equipment, subway or tunnel, electrical generating equipment, pipeline other than gas and oil building Electrical generating equipment, generating or distributing equipment and plant, manufacturing plant, water distribution equipment, property for producing gas Building or structure, including heating equipment, elevators, dock, windmill, wharf and jetty Railway system, tramway or trolley bus system Chemical pulp mill, wood pulp mill including building and machinery Building of frame, log, stucco, galvanized iron not included in another class, fence, greenhouse, oil or water storage tank, railway locomotive Canoe or rowboat, scow, vessel, furniture, spare engine, marine railway Manufacturing or processing machinery not included in class 2, 7, 9, 11 or 12, building such as kiln, tank or vat; building for storing fresh fruit or vegetables, gas well, mine, oil well, tramway track, rapid transit car and greenhouse. Electrical generating equipment, radar equipment, radio transmission and receiving equipment and aircraft Automotive equipment, sleigh or wagon, trailers, electronic data processing equipment including software, telecommunication spacecraft, refinery, contractor’s movable equipment, including buildings, gas or oil well equipment, airport, dam, dock, fire hall, hospital, natural gas pipeline, sewage disposal plant, water pipeline, equipment for cutting timber and logging, crude oil processing and passenger vehicle Property not included in any other class and which is used for rental purpose Property not included in any other class, books, china, cutlery, kitchenware, die, jip, pattern, mould, medical and dental equipment, mine shaft, linen, cutting or shaping part of a machine, video tapes, movies, computer, software and videotape cassette

exchanger when the maintenance cost is $300 per year, the annual cost works out to be $3012 which is higher than the value of $2497 obtained for the stainless steel exchanger. Example 2: A paint system costing $0.38 per sq. ft failed after 4 yr. (a) If the paint system is renewed at the same cost for a total life of 12 yrs. Calculate the annual cost, assuming the first application is capitalized and those in the 4th and 8th years are expenses, interest rate of 10%, tax rate of 48% and straight-line depreciation. Using the equations for PW and A, the annual cost of $0.068 sq. ft is obtained.

Corrosion Economics and Corrosion Management

317

(b) The maximum that can be spent on preventive maintenance such as biennial touchup may be calculated using the relationship. X 0 ¼ X½ðP=FÞ10% ; 2y þðP=FÞ10% ; 4Y þðP=FÞ10% ; 6y þ ðP=FÞ10% ; 8y þðP=FÞ10% ð10 yrÞ ðA=PÞ10% ð12 yrÞ The value of X ¼ $0.1492 sq. ft is obtained. Example 3: A plant storage tank requires either: (i) replacement to last 25 yr with no maintenance at a cost of $690 000; or (ii) a coating system with maintenance cost of 10% of installation costs, tax rate of 48%, interest rate of 10% and straight-line depreciation. Using the relationship PW ¼ P þ tPð1=nÞðP=AÞ10% ; 25 y a value of $569 700 for replacement is obtained. For coating system the value of $513 700 is obtained for P. Thus coating system is preferred to the replacement of the tank.

5.2 Corrosion Management According to Trethewey and Chamberlain5 corrosion management is defined to include people along with corrosion control which concerns with the materials used in a particular environment. The corrosion related failure may be due to: (i) primary corrosion mechanism; and/or (ii) secondary corrosion mechanism as shown below: Corrosion failure Fracture Cracking or pitting High weight loss Primary corrosion mechanism Any one mode of corrosion out of the eight forms of corroison Secondary corrosion mechanism Personnel contributions such as producers, designers, manufacturers, users, monitors; unforeseen changes in environmental conditions such as pH, oxygen, temperature, flow-rate, biological content

The secondary corrosion mechanism involves the contributions of people in different ways and the people factor should be rated as high in corrosion management. The various

318

Corrosion Prevention and Protection

functions of personnel in different stages of a corrosion performance of systems may be identified as follows: Procurer Specifications of the system Intersystem compatibility, control of budget

Designer

Manufacturer and supplier

Shape and structure, choice of materials operational conditions; use of coatings

Correct production of design; use of specified materials Correct heat treatment Correct fabrication methods Correct application of coatings

User/monitor Correct operation Use correct replacement parts Monitoring environment Checking protective coatings for integrity

In the analysis of failures, it is possible to identify the failure when the failure occurs due to a simple corrosion mechanism such as galvanic corrosion, localized corrosion such as pitting or crevice corrosion, stress–corrosion cracking, erosion–corrosion or severe general corrosion. In all these cases the factors involved are materials; environment and flow rate of the medium. These types of failure are avoidable when proper choice of materials, design and the environment are considered. In spite of good available knowledge of various factors contributing to corrosion failures, we do encounter corrosion-related failures at remarkable frequency. In quite a few instances of corrosion-related failures such as galvanic corrosion-related failure, squarely the culprit is the improper choice of materials selected and designed by the engineer. This clearly shows the people factor involved in improper choice of materials and its use in the particular environment. Proper corrosion management ensures that simple mistakes such as improper selection of materials, or errors in design or improper selection of inhibitors are avoided by the corrosion personnel. In some cases unusual factors may be involved in the case of a corrosion failure. An example is a failure due to stray current corrosion. The identification of such a failure by a conventional approach might be time consuming, in the worstcase scenario it may be impossible to identify the root cause of the failure. The conventional approach can be time consuming as well as leading to the wrong conclusions. On the other hand, an expert systems approach with the focus on the system as a whole can lead one, with a series of questions, in a logical stepwise manner. The answers to the questions lead one to a logical cause of the failure with great reliability in a very short time compared with the conventional, less reliable non-expert approach with singular focus on materials. As seen from the foregoing discussion the contribution of people to the failure of system is important and can be introduced into the total probability of corrosion failure as: Probability of corrosion failure ¼ Probability factor due to materials probability factor due to environment probability factor due to personnel:

Corrosion Economics and Corrosion Management

319

Corrosion failure incorporating the people factor may also be analyzed in terms of fault-tree analysis as given below:

Corrosion failure

People factor

Materials

Environment

The people factor is difficult to quantify in any case of failure. It is needless to emphasize that extensive multidisciplinary knowledge in a variety of subjects such as chemistry, electrochemistry, physics, statistics, mechanical engineering, metallurgy and management is desirable in order to reduce the people contribution factor to a minimum (0 < people factor < 1). An example where reliability and safety has been very high is provided by the aerospace industry. The fault-tree structure for corrosion failure can be programmed into a computer and used to analyze the risk of system failure. This methodology has been successfully used by Nova Oil Corp. to analyze pipeline failures caused by stress corrosion cracking.6 The accounting of decisions made by people is based upon a host of disciplines such as fuzzy logic7,8, Bayesian logic9, influence diagrams10,11 and approximate reasoning principle12 in the course of research in artificial intelligence.

5.3 Computer Applications Knowledge–based systems, also known as expert systems, have been developed to reduce erratic human influence. The computer simulation of human knowledge can be classified as: (i) skill-based knowledge and action which lends itself to automation and replaces human operators by microprocessor devices and human knowledge by software; (ii) rule-based knowledge and action is goal-driven and applicable to predefined situations; and (iii) formal reason-based knowledge and action applicable to unfamiliar situations in which human experts often invoke heuristic and other high-level intelligent responses. The computer-based corrosion problem-solving systems have been categorized as: (i) systems for modeling corrosion/cracking processes; (ii) material selection and equipment specification programs; (iii) computer-based corrosion monitoring systems; (iv) computer-based systems for control of corrosion testing equipment; (v) databases and hypertext systems; and (vi) internet-based databases and software programs.

320

Corrosion Prevention and Protection

Table 5.5 Early sofware (expert) systems in corrosion Name

Application

Achilles

Diagnosis and prediction of localized corrosion Prediction of localized corrosion in austenitic steels Prediction of corrosion of austenitic stainless steels Materials selection for hazardous chemical service Evaluation of cracking in stainless steels Materials selection Materials selection for chemical process industry Selection of CRAs for oil field service Evaluation of SSC in steels

Aurora Auscor ChemCor DIASCC KISS Prime Socrates Suscept

Country of development

Shell used

Spices

Finland

Level 5

Savior

USA

KES/Level 5

Japan

OPS83

Germany Belgium

Nexpert Object KEE

USA

PCPLUS

USA

PCPLUS

Many of the early computer programs are known as expert systems since the programs were based on human expertise in corrosion13 and these programs were based on research-based development efforts and lacking rigorous software engineering foundations necessary for commercial distribution. Most of these programs used software platforms known as shells that allowed implementation of rules of thumb and common reasoning concepts. Some of the programs developed in late 1980s and early 1990s14–19 are given in Table 5.5. The most popular application of computers involves programs developed for modeling corrosion and cracking as well as programs for diagnosis, failure analysis and prediction/ analysis. The systems developed are: Lipucorr21 Cormed22 Predict23 Coris24 Socrates25 Selmatel26 Chem Corr13 CORSUR27

Prediction of general corrosion Expert system database that integrates corrosion problem-solving expertise with a comprehensive database in corrosion, materials and thermodynamic properties of corrosive media Automated basis for materials selection for oil and gas production Selection of materials exposed to high temperatures in refinery furnace tubes Evaluation of materials applicable to different segments in chemical processing industries Large database application on corrosion behavior of metals and nonmetals in over 700 chemical environments

The types of computer programs which have been applied in corrosion engineering and science fall into categories such as: (i) conventional software systems; (ii) artificial intelligence and expert systems applications; (iii) object-oriented software systems;

Corrosion Economics and Corrosion Management

321

Knowledge Acquisition Subsystem Knowledge Base

Knowledge Engineer

Inference Engine Domain Expert

User Interface

End User

Explanation Subsystem

Figure 5.1 Structural components in an expert system

(iv) neural networks; and (v) hybrid systems that utilize one or more previously mentioned methods. Early computer programs developed during 1970s and 1980s used Basic, Fortran, Pascal languages which provided front end for data modeling and analysis. Expert systems or knowledge-based systems differ from the early computer programs in the sense that the knowledge is usually separated from the reasoning process, also known as the inference engine. The typical structural components in an export system are shown in Figure 5.1. The knowledge base obviously contains the expertise and the inference engine controls the logical path used by the expert system to access the information in the knowledge base to make decisions. Some applications of expert systems are in different aspects of corrosion,6 cathodic protection,28 assessment of cracking in light water nuclear reactors29 and prediction of localized corrosion of stainless steels.30 Object-oriented systems use the concept of reusable entities that contain both the data and procedures relevant to the object, and thus eliminating the separation of knowledge and reasoning found in expert systems. In object-oriented systems computation is behavior simulation of real-life systems. Once certain classes of objects are created, they can be reused to create other objects and properties with interface and behavior. The self-contained character of objects is known as encapsulation. Inheritance allows derivation of new objects from existent ones, and encapsulation defines the limits of services an object can provide to other objects. An example of this system is provided by GENERA,31 and a schematic representation is given in Figure 5.2. The materials and corrosion are represented as objects whose states are defined in terms of critical parameters along with inerrelationships between the parameters. The neural networks evolving from neurobiological concepts have been used in corrosion data modeling and prediction,32 corrosion data reduction33 and analysis of electrochemical impedance data.34 The neural networks are built from a large number of very simple processing elements that deal individually with pieces of a big problem. A processing element (PE) multiplies the input by a set of weights and the result is transformed by a nonlinear function into an output value. The strength of neural computation depends upon the massive interconnection among the processing elements which carry tasks of overall processing and the adaption of the parameters (weights) that interconnect the processing elements.

322

Corrosion Prevention and Protection

Figure 5.2 Structural framework of the generic development system

A neural network consists of several layers of processing elements. The most basic and common neural network is the multilayered perceptron (MLP). A typical MLP consists of an input layer, a middle or hidden layer and the output layer of elements and connecting lines represent a scaling factor or weighted connections between processing elements. The MLP calculation is based on the desired output (signal) and error criterion. The output is compared with the desired response to produce an error. The calculation is repeated with different weights until the error of the desired signal is minimized. A simple form of an MLP is depicted in Figure 5.3.

INPUT LAYER HIDDEN LAYER (there may be several hidden layers)

OUTPUT LAYER

Figure 5.3 Simple multilayered perceptron of an artificial neural network

Corrosion Economics and Corrosion Management

323

Currently, computer programs for corrosion assessment and control use a combination of expert systems and an object-oriented program such as evaluation of hydrogeninduced cracking of steels,35 or a combination of expert systems and neural networks in corrosion data modeling and prediction.32 The developments in computer tools for corrosion assessment and control have been quite extensive, as evidenced by 57 expert systems and many hundreds of computer programs, including databases. A representative sample of current software programs for corrosion and their control is shown in Table 5.6. Some specific examples of computer systems for different applications are: (i) (ii) (iii) (iv) (v)

Programs for selection of materials for oil and gas application; Programs for online corrosion monitoring and control; Database systems for evaluation of metals and non-metals; Use of neural networks in corrosion data reduction; Computer-aided packages for teaching corrosion

Table 5.6 Representative sample of current day software programs for corrosion Application

Type of system

CORSUR, corrosion data for metals and nonmetals in over 700 chemical environments CP Diagnosic, trouble-shooting and diagnosis of sacrificial anode and impressed current cathodic protection systems Damage predictor, evaluation of material performance for stress–corrosion cracking in boiling water reactors and determination of crack growth rates ECORR, computer-aided learning package designed to assist in training students in corrosion concepts EIS data extrapolation, uses neural networks to train on electrochemical impedance spectroscopy data for extrapolation Filter debris analysis (FDA) expert system, condition monitoring of aircrafts GENERA, generic problem-solving framework for characterizing corrosion and materials problems LipuCor, prediction of corrosion in oil and gas systems Predict, prediction of corrosion in oil and gas production and transmission environments Socrates, selection of materials for oil and gas production service

Relational database with a user friendly front-end Shell-based expert system with a database on CP data

Strategy, programs for evaluation of cracking in steels used in pipelines and refineries USL Corrosion Model, program for prediction of corrosion in gas condensate wells

FORTRAN program modules that calculate radiolytic species concentration and assess crack growth rates Uses an authoring package to provide multimedia information Artificial neural network (ANN) application Visual basic-based interface and knowledge base Object oriented system implemented in Cþþ Implemented as a conventional structured program Object-oriented program using lab data and numerical relationships Object-oriented expert system implemented in Cþþ interfacing with a database on materials and compositions Implemented in Cþþ and integrated with databases in Paradox Implemented as an expert system in visual basic

324

Corrosion Prevention and Protection

The CORMED program predicts the probability of corrosion in oil wells using detailed analysis of field experience on CO2 corrosion, and other data in the literature, involving factors such as CO2, in situ pH and bicarbonate in the wells.22 The corrosion damage is also predicted by computer models such as Predict23, Lipucorr21 and USL, based on a larger number of factors.36 The object-oriented Socrates system is concerned with material selection such as corrosion-resistant alloys (CRA) in a chosen environment. The system uses published data in the literature involving both laboratory and field data in the oil and gas environments. The flow diagram depicted in Figure 5.4 shows an example of the sequential steps. Since the system is object-oriented it is not necessary to use the data in a sequential manner. The initial set of applicable materials can be obtained by the chosen application.

Application Type Strength Hardness Mechanical Condition

H2S CO2 Bicarbonates Temperature Chlorides Sulfur Gas to oil Ratio Water to Gas Ratio

Application rules Application Requirements

Environment Characterization

Application Specific CRAs from Database Rules for Env. Severity Evaluation of Steels and Inhibitors

Evaluation for Stress Corrosion Cracking

SCC rules

Alloys Database Max. Temp Env. Severity

Evaluation for Localized Corrosion

Pitting rules

SSC and Special Requirements Cost Analysis

Literature Database search Recommended Set of CRAs

Figure 5.4 Flow of data in the Socrates system

Corrosion Economics and Corrosion Management

325

The system defines the severity of the environment with respect to various forms of corrosion, using the environmental conditions and the metallurgical parameters. One of the most important applications of neural network methodology is in the extrapolation of electrochemical impedance data obtained in corrosion studies.34 Electrochemical impedance spectroscopy (EIS) can be used to obtain instantaneous corrosion rates. The validation of extension of EIS data frequency range, which is conventionally difficult, can be done using a neural network system. In addition to extension of impedance data frequency range, the neural network identifies problems such as the inherent variability of corrosion data and provides solutions to the problems. Furthermore, noisy or poor-quality data are dealt with by neural works through the output of the parameters variance and confidence.33 ECORR or engineering corrosion37 is a computer-aided learning (CAL) system, representing application of computing to corrosion education. This program is very valuable to students in learning about corrosion. The student is presented with a series of problems to be solved by reference to supporting information. The system consists of objects such as text, photographs, video or sound objects. The system contains a theory base, case studies, a glossary, and a control center to coordinate navigation and information flow between the student and different modules. The basic corrosion information provided can be used in studying the particular case study. There are twelve case study modules at basic/Level 1 and Advanced/Level 2 in the program. Computers and programs are also important in corrosion monitoring systems, such as an automated constant rate extension system (A Cert) shown schematically in Figure 5.5. This system may be used for conducting corrosion and cracking evaluation tests. The system is capable of initiation and conducting tests as well as data acquisition and analysis. There is very little human intervention and the system is capable of corrosion evaluation in environments which are difficult to simulate. This system is a

Control PC

Control Instrumentation

CERT Software

Motor Indexer

Data Acquisition Hardware

Data Acquisition Panel

Stepper Motor

End User Drive

Parametric Inputs (Load, Strain, T, P)

Figure 5.5 Schematic representation of an automated constant extension rate tester

326

Corrosion Prevention and Protection

computer-controlled, closed-loop feedback system, capable of operation over a wide range of speeds. This automated system can be programmed to conduct constant extension rate tests, cyclic slow-strain-rate tests, and crack growth rate tests and fracture mechanics tests. The role of internet in corrosion science and engineering, as in any other field, has been increasingly useful in the past decade. The internet is playing a significant role in sharing and exchanging information. The resources available to the corrosion engineer/ scientist are: (i) e-mail-based; (ii) newsgroups and discussion threads; and (iii) technical resource websites. Electronic mail is a convenient mechanism for exchanging information of interest among scientists. This may also involve a mailing list of people interested in the technical topic and an example of this is the CORROS-L list38 operated by the corrosion and Protection Centre at the University of Manchester, email: [emailprotected]. Newsgroups are analogous to worldwide bulletin boards where questions, messages, comments, discussions and answers are given. Currently there exist over 6000 Usenet newsgroups covering a wide range of corrosion related topics, such as metallurgy, materials, electrochemistry, chemistry, engineering and nondestructive testing. Discussion threads are conceptually similar to newsgroups, but can be accessed on the web. The two most popular threads relevant to corrosion and materials are www. intercorr.com/discuss/x where x can be coatings, inhibitor, cathodic protection, multiphase flow, fracture mechanics, etc. The web-based resources for corrosion may be classified as: (i) resources for solving corrosion problems; (ii) resources for sharing information with the aid of virtual libraries and online databases; and (iii) resources for sharing information within the organization (intranets). Some of the relevant websites for corrosion and materials are: www.Metalogic.be www.intercorr.com www.nist.gov/srd/material.htm www.oilonline.com http://www.xs4all.nl/-edewaard/

Overview of corrosive environments A number of important papers, data and software on corrosion and materials NIST materials databases Online news, rig counts, product and literature on oil industry Overview of corrosion processes

Websites are also available dealing with electrochemistry, corrosion mechanisms, industrial corrosion and materials. The internet also provides significant technical resources online and some examples are: (i) different types of technical databases such as data on general corrosion, localized corrosion, galvanic, intergranular, erosioncorrosion, SCC, SSC, HEC: materials data on steels, plastics and composites; (ii) corrosion and materials problem-solving software; (iii) archival laboratory testing data and reports with respect to oil and gas, power generation and nuclear industries; (iv) conference proceedings and journals available online; and (v) virtual conferences such as the first global internet corrosion conference, Intercorr/96. It is conceivable that most of the tasks performed by humans may be replaced by computers in the future, but the role of people in corrosion management will not be diminished in spite of the technical advances being made.

Corrosion Economics and Corrosion Management

327

References 1. E.D. Verink, Metals Handbook, Vol. 13, Corrosion, 9th edn, Metals Park, OH, ASM International, p. 389. 2. E.D. Verink, Corrosion/86, Paper No. 383, NACE, Houston, Texas. 3. Engineering Economy, Z94-5, American National Standards Institute, Institute of Industrial Engineers. 4. D.G. Newman, Engineering Economic Analysis, San Jose, CA, Engineering Press, 1975, p. 195. 5. K.R. Trethewey and J. Chamberlain, Corrosion for Science and Engineering, Longman, 1995. 6. P. Roberge, Modelling Aqueous Corrosion, K.R. Trethewey and P.R. Roberge, ed. Kluwer Academic Academic, Dordrecht, The Netherlands, 1994, pp. 319–416. 7. B. Kosko, Neural Networks and Fuzzy Systems, Prentice Hall, Englewood Cliffs, NY, 1992. 8. J. Durkin, Fuzzy Logic, MacMillan, NY, 1994, pp. 363–403. 9. J. Durkin, Bayesian Logic, MacMillan, NY, 1994, pp. 305–331. 10. R.A. Howard, J.E. Matheson, Influence Diagrams in the Principles and Applications of Decision Analysis, Editors R.A. Howard, J.E. Matheson, Strategic Decisions Group, Menlo Park, CA, Vol. II, pp. 720–762. 11. R.D. Schacter, Probabilistic Influence and Influence Diagrams, Operations Research, Vol. 36, 1988, pp. 589–604. 12. R.K. Bhatnagar, L.N. Kanal, Handling Uncertain Information: A Review of Numeric and NonNumeric Information, in Uncertainty in Artificial Intelligence, Editors L.N. Kanal, J.F. Lemmer, North Holland, Amsterdam, Vol. 4, 1986, pp. 3–26. 13. P.R. Roberge, CORROSION/94, Paper No. 368, NACE, Houston, Texas, 1994. 14. The Achilles Club Project, Expert System on Corrosion and Corrosion Control, NPL, Teddington, Middlesex, UK, 1986. 15. J.G. Hines, A. Basden, British Corrosion Journal, 26(3), 151 (1986). 16. A. Jadot, L. Lancus, ‘Escort’, Expert Software for Corrosion Technology, PhD Thesis, K.V. Leuven University, Belgium, 1985. 17. G.M. Ugiansky, A.C. Vanorden, D.E. Clausen, ‘‘The NACE-NES Corrosion Data Program in Computers Corrosion Control’’, J. Fu, R. Heidersbach and R. Erbar, Editors, NACE, Houston, Texas, 1986, pp. 15–20. 18. C.S. Fang, J.D. Garber, P. Perkins, J.R. Reinhardt, ‘‘Computer Model of a Gas Condensate Well Containing Carbon Dioxide’’, CORROSION/89’’, Paper No. 465, NACE, Houston, Texas, 1989. 19. S. Srinivasan, R.D. Kane, ‘‘Expert Systems for Material Selection in Corrosive Environments’’, Paper No. 564, CORROSION/90, NACE, Houston, TX, 1990. 20. C.P. Sturrock, W.F. Bogaerts, A.S. Krisher, Paper No. 375, CORROSION/94, NACE, Houston, TX, 1994. 21. Y.M. Gunaltan, Paper No. 27, CORROSION/96, NACE, Houston, TX, 1996. 22. J.L. Crolet, M.R. Bonis, ‘‘Prediction of the Risks of CO2 Corrosion in Oil and Gas Wells’’, SPE Production Engineering, 6(4), 449 1991. 23. S. Srinivasan, V.R. Jangama, R.D. Kane, ‘‘Prediction of Corrosivity of CO2/H2S Systems’’, EuroCorr/97, The European Corrosion Congress, Trondheim, Norway, Sept. 22–25, 1997, Publ. by Norwegian University of Science and Technology, Trondheim, Norway, Vol. 1, 1997, pp. 33–40. 24. A. Hatzinosios, ‘‘CORIS’’, CORROSION/94, Paper No. 373, NACE, Houston, TX, 1994. 25. S. Srinivasan, ‘‘Role of Expert System in Technology Transfer of Materials for Petroleum Applications’’, Proceedings of the 12th International Corrosion Congress, 19–24, September 1993, NACE International, Houston, TX, 1993. 26. A. Valdes, A. Gonzalez, R. Techorrewski, B. Hopkinson, ‘‘An Expert System for the Selection of Materials Exposed to Elevated Temperatures’’, Paper No. 275, CORROSION/92, NACE, Houston, TX, 1992. 27. B. Mashayekhi, C.P. Sturrock, D. Flanigan, ‘‘Corrosion Data Survey, The Next Generation’’, Paper No. 604, CORROSION/97, NACE International, Houston, TX, 1997.

328

Corrosion Prevention and Protection

28. V.L. Van Blaricum, A. Kumar, Y.T. Park, Paper No. 384, CORROSION/94, NACE, Houston, TX, 1994. 29. M. Urqidi-McDonald, Paper No. 340, CORROSION/90, NACE, 1990. 30. T.J. Hakkarainen, CORROSION/96, Paper No. 363, NACE, Houston, TX, 1996. 31. S. Srinivasan, CORROSION/96, Paper No. 368, NACE, Houston, TX, 1996. 32. D.C. Silverman, E.M. Rosen, CORROSION/92, Paper No. 264, NACE, Houston, TX, 1996. 33. R.A. Cotis, I. Helliwell, M. Turega, CORROSION/96, Paper No. 379, NACE, Houston, TX, 1992. 34. M. Urquidi-MacDonald, P.C. Egan, Corrosion Reviews: Special Issue on Application of Computers in Corrosion, P.R. Roberge, ed., 15(1–2), 1997. 35. R.D. Kane and S. Srinivasan, Serviceability of Petroleum, Process and power Equipment, D. Bagnoli, M. Prager, D.M. Schlader, Editors, PVP Vol. 239, ASME, NY, 1992. 36. P.R. Roberge, Corrosion Reviews: Special Issue on Applications of Computers in Corrosion, P.R. Roberge, Editor, 15(1–2), 1997. 37. R.A. Cotis, M. Fay, S. Faidi, CORROSION/96, Paper No. 377, NACE, Houston, TX, 1996. 38. R.A. Cotis, S.B. Lyon, CORROSION/96, Paper No. 376, NACE, Houston, TX, 1996. 39. S. Srinivasan, R.D. Kane, Materials and Corrosion Resources on the Internet, 2nd NACE Latin American Corrosion Congress, Rio de Janeiro, Brazil, September 1996.

Part II

6 The Forms of Corrosion Note: Some reference citations in this chapter refer to Handbooks containing several individual references. These are indicated in the text as (ASTM G 52004)4, for example, and the addition of RO4 in the reference list stands for revised 2004.

6.1 Corrosion Reactions Corrosion of materials is a direct chemical reaction of a metal with its environment or a flow of electricity in an electrochemical reaction in an aggressive medium such as natural media (atmospheric, water or underground) or process media. Local cells (shortcircuited) electrochemical cells of the same active metal or between an active metallic surface and that of another more noble conducting material can give rise to corrosion. The following general reaction may be written as: aA þ bB ! c C þ d D where a moles of solid substance A (metal for example react with b moles of substance B (environment) to form c moles of substance C (oxidized metal or material) and d moles of D (reduced environment). A ‘wet’ reaction of iron in an oxygenated aqueous medium can be represented by the equation: Fe þ 0:5 H2 O þ 0:75 O2 ! 0:5 Fe2 O3 H2 O A ‘dry’ reaction of corrosion of iron can be written as: Fe þ 0:75 O2 ! 0:5 Fe2 O3 Oxidation of metals includes all reactions in which the charge is transported through a film of reaction product on the metal surface. Parabolic, logarithmic, asymptotic, rates involve the presence of a rate-determining film, while linear growth rates correspond to the absence of such films. These reactions are generally considered as dry reactions. Corrosion Prevention and Protection: Practical Solutions # 2007 John Wiley & Sons, Ltd

V. S. Sastri, E. Ghali and M. Elboujdaini

332

Corrosion Prevention and Protection

Exceptions can exist since the corrosion in a wet solution of the interior boiler drum (steel) with dilute caustic soda at high temperature and high pressure and the reaction of high temperature water with aluminum and zirconium have been found to be best interpreted in terms of a dry corrosion mechanism.1 Corrosion can also occur by a direct chemical reaction of a metal with its environment such as the formation of a volatile oxide or compounds, the dissolution of metals in fused metal halides. The reaction of molybdenum with oxygen and the reaction of iron or aluminum with chlorine are typical examples of metal/gas chemical reactions. In these reactions, the metal surface stays film-free and there is no transport of electrical charge.1 Fontana and Staehle2 have stated that corrosion should include the reaction of metals, glasses, ionic solids, polymeric solids and composites with environments that embrace liquid metals, gases, aqueous and other nonaqueous solutions.

6.2 Corrosion Media The majority of corrosion reactions fall into three categories: (1) aqueous corrosion; (2) nonaqueous corrosion; and (3) high-temperature corrosion. The corrosion media consist of four types: 6.2.1

Atmospheric Exposure

The natural environments—rural, marine and industrial—and some combination of these are of primary concern. Some contaminated atmospheres, such as those containing hydrogen sulfide, ammonia, sulfur dioxide, etc., should be considered rigorously. Relative humidity and its cycling are of major importance for corrosion kinetics and a material’s resistance to corrosion. Temperature and pressure are major factors to consider, together with the chemical composition of the medium. 6.2.2

Aqueous Environments

These include natural or industrial waters, as well as extremely dilute inorganic or organic chemical media. 6.2.3

Underground Media

This category includes the underground installations of piping and vessels and the underside of structures such as tank bottoms. 6.2.4

Process Media

This includes organic and inorganic syntheses and other processing environments such as liquids or gases that affect materials.3 The nature of the corrosion product plays an important part in deciding if the reaction occurs through a film or film-free reaction. This can distinguish electrochemical from chemical reactions. If a film is formed due to the oxidation of the metal, the properties of this film, such as coverage and protection capacity, partial pressure, resistivity, porosity,

The Forms of Corrosion

333

toughness, hardness as well as resistance to different chemicals and gases, are important. This is true for almost all electrochemical reactions in aqueous solutions as well as for high-temperature oxidation. Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2O3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the film. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin film of oxide is present on the metal surface. These reactions cannot be considered film-free although the film is not a rate-determining one.1

6.3 Active and Active–Passive Corrosion Behavior The active state of a metal corresponds to the dissolution or the attack of the metal in a certain environment since this metal is not stable naturally or thermodynamically in its new material/media conditions. There is a marked tendency of the metal to go back to its thermodynamic stable state or to another stable one. This electrochemical active state can be described by electrochemical polarization tests. In these tests, using a potentiostat, the applied potential is varied over a wide range, covering the cathodic and anodic regions of the metal/solution system, and the corresponding current at every potential is recorded. Similarly, an infrequently changing current can be imposed on the metal, recording the corresponding potential. (ASTM G5)4 Plots of Eappl versus log I are called Tafel plots (Figure 6.1). The corrosion current density icorr, can be determined using two equations by extrapolation of the Tafel lines: Eappl ¼ Ecorr þ ba logði=icorr Þ Eappl ¼ Ecorr bc logði=icorr Þ where ba and bc are positive constants. From icorr the corrosion rate can be calculated using Faraday’s law. (ASTM G102)4 In measurement of the polarization resistance, a small electrochemical signal of the order of 10 mV is applied to ensure that the system under consideration stays linear. This requirement does not perturb the system and can be used for monitoring. The polarization resistance Rp is defined as Rp ¼ ðdE=diÞEcorr and is determined as the slope of the polarization curve when I ¼ 0 or ia ¼ ic in absolute values. (ASTM G59)4 The icorr ¼ B=Rp and B ¼ ba bc =2:3ðba þ bc Þ, where Rp is a constituent of the total resistance in the circuit (Rp and Rs ). Elimination of Rs is highly recommended, as discussed before. Also, if the accurate values of ba and bc are determined from Tafel slopes, this method reflects corrosion rates because of minimum perturbation of the state of equilibrium. Figure 6.2 gives a standard polarization curve for type 430 stainless steel in 1.0 NH2SO4, purged with hydrogen, nitrogen or argon at 30 C. Experimental curves should lie between curves 1 and 3, otherwise experimental errors occur.4,5

334

Corrosion Prevention and Protection

Figure 6.1 Theoretical Tafel plots describing the tafel slopes and illustrating a graphical method of icorr determination5,18, (Mansfeld)5

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1–4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active–passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6 Some inhibitors produce films on the anode and hence stifle the corrosion reaction (iron in chromate or nitrite solutions). Several authors consider the presence of a thick barrier of corrosion products, relatively protective, on the metallic surface as passivation. Inhibitors may enhance the formation of passive films on top of the substrate, such as benzotriazole on copper or benzoate on iron, or they may form monomolecular

The Forms of Corrosion

335

Noble(+)

6 2

5 4

= 3.47 mA/cm2 (Curve 2) = 1.77 mA/cm2 (Curve 3)

1 3

2 1 0 –1 –2

Active (–)

–3 –4 –5 –6 –30

–20

Cathodic

–10

Anodic

Figure 6.2 Standard potentiodynamic polarization curves in the vicinity of Ecorr for type 430 stainless steel; in 1 N H2SO4 at 30 C (Mansfeld)5

adsorption layers and prevent the dissolution of the substrate and the reduction of oxygen by changing the potential drop across the interface and/or the reaction mechanism.7–9 Starting with an oxide-free surface of iron for example, the current increases with increase in potential, reaches a maximum and then decreases again. There is a region of over a volt where iron does not dissolve or more precisely corrodes at a very slow rate, corresponding to the passive current for every potential, and then the current rises again, due to oxygen evolution and/or transpassive metal dissolution. In the passive region, iron is covered by a thin film of cubic oxide (gFe2O3/Fe3O4) formed. The film thickness increases with anodic potential until a limiting value approaches 5 nm (Figure 6.3). This is the same type of film that is formed by the reaction of clean iron with oxygen or dry air. The dissolution occurs because of the imperfections of the film since there is always some existing porosity. Also, the rate of dissolution can increase due to a film breakdown and the kinetics depends in this case on film breakdown and repair.10,11 It is now well accepted that the passive film is not a single layer, but rather has a stratified structure. The inner layer plays the role of a barrier layer against corrosion and the outer layer plays the role of an exchange layer. The chemical composition is a

336

Corrosion Prevention and Protection

Figure 6.3 Schematic of anodic polarization curve of iron,10 showing active–passive behavior of iron in sodium borate–boric acid buffer solution at pH 8.4

function of the microstructure of the metal, the pH of the electrolyte and the level of the anodic potential. Film growth is generally a direct growth due to the reaction between the metal and the aqueous solution. However, it could be also the result of a dissolution/ precipitation (dissolution of metal ions and subsequent precipitation of an oxide, oxyhydroxide, or hydroxide) and anodic deposition process that consists of anodic oxidation of metal ions in the solution and deposition on the surface. Aluminum and other metals (titanium, zirconium, hafnium, tantalum, and niobium), show that thicker anodic oxide films can be grown because these oxides are insulators. Thick films of 100 mm are generally porous whereas the barrier film is < 1 mm. These oxide films formed on valve metals exhibit dielectric breakdown at high voltages.12–13

6.4 Forms of Corrosion Bruce Craig and Steven L. Pohlman14 presented five forms of corrosion that have been considered by Covino and Cramer,5 and added the microbiologically influenced corrosion as a separate corrosion form. This approach is based on the mechanism of attack involved rather than the proposed eight forms of corrosion by Fontana.15 Uniform corrosion, localized corrosion, metallurgically influenced corrosion and microbiologically influenced corrosion fit under the classification of corrosion that is not influenced by an external influence while mechanically assisted degradation and environmentally induced cracking involve corrosion that is influenced by another process. Six forms of corrosion can be identified based on the apparent morphology of corrosion, the basic factor influencing the mechanism of corrosion in every form. The six forms are given in Table 6.1.

The Forms of Corrosion

337

Table 6.1 The six forms of corrosion including some types or categories of every form (Craig and Pohlman)14, (Covino and Cramer)5 1 General Corrosion 2 Localized Corrosion 3 Metallurgically influenced corrosion 4 Microbiologically influenced corrosion 5 Mechanically assited corrosion 6 Environmentally induced cracking

Uniform, quasi uniform and non uniform corrosion; galvanic corrosion Pitting corrosion; crevice corrosion; filiform corrosion Intergranular corrosion; sensitization; exfoliation; dealloying

Wear corrosion; erosion–corrosion; corrosion fatigue, etc. Stress–corrosion cracking; hydrogen damage; embrittlement; hydrogen-induced blistering; high-temperature hydrogen attack; hot cracking; hybride formation; liquid metal embrittlement; solid metal-induced embrittlement

The following observations explain the significance of these forms of corrosion: 1. In many cases, one form of corrosion can lead to another or may mitigate another, and a single case of corrosion may provide more than one form of corrosion. 2. Galvanic corrosion can cause general or localized corrosion, depending on the relative surface areas of anode to cathode, the geometry of the corrosion cell, agitation, the nature of the material, the conductivity and the composition of the corrosive media. Natural corrosion media such as atmospheric, marine, underground corrosion, etc., may give general and localized corrosion. 3. Stray-current corrosion leads to general corrosion, but creates localized corrosion areas in many circ*mstances, such as that on a pipe tube at the departure of the vagabond current from the pipeline, however it is treated as general corrosion form. Parting is treated as a type of metallurgically influenced corrosion, and it can give general or localized corrosion. 4. High-temperature corrosion frequently shows general corrosion, as in oxidation, sulfidation, carburization, hot corrosion and hydrogen effects, etc. It should be noted that subsurface corrosion or internal corrosion at high-temperature corrosion is a highly localized corrosion phenomenon. 5. There is an overlap between the different forms of corrosion, for example between general and/or localized corrosion and stress–corrosion cracking. Dealloying or selective leaching may be a precursor to stress–corrosion cracking. The transition from uniform corrosion to highly localized attack is not clearly understood. In certain circ*mstances pitting or crevice corrosion is observed while in other conditions environmentally induced cracking can occur.3,14 These six forms are explained in more detail in Sections 6.7.1–6.7.6.

6.5 Types and Modes of Corrosion Types of Corrosion. It is suggested that a corrosion case should be described at least by its form, the corrosion reactions and corrosion products. This can also include their

338

Corrosion Prevention and Protection

occurrence, mechanism, kinetics and the description of galvanic cells that intervene. When well described, this could be considered as a type or subtype of corrosion. A type of corrosion is expected to characterize, and describe more profoundly the circ*mstances leading to a corrosion or a failure of a certain material and complement the more vast divisions of the six forms of corrosion. Modes of Corrosion. Fontana defined eight forms of corrosion, as general corrosion, pitting corrosion, intergranular corrosion, parting, galvanic corrosion, crevice corrosion, stress–corrosion cracking and erosion–corrosion.15,16 In refining these eight forms, there are really two broad categories of modes of corrosion: Intrinsic and extrinsic. The intrinsic modes of corrosion which occur independent of design are:

General corrosion Pitting Intergranular corrosion Parting Stress–corrosion cracking

The extrinsic modes affected by the design include:

Crevice or under-deposit corrosion Galvanic corrosion Erosion–corrosion Fretting corrosion Corrosion fatigue

To further define the differences: within a crevice it is possible for any of the intrinsic modes to occur. For example, within a crevice it is possible to have general corrosion, pitting, intergranular corrosion while none of these is occurring on the free surface. Thus, there is no unique crevice corrosion; rather the intrinsic modes may occur within a crevice. From the point of view of design, the designer may elect to remove or minimize the crevice and thereby prevent these modes from occurring. The option to prevent the various intrinsic modes by reconsidering the extrinsic or design-dominated modes is the responsibility of the designer.17,18

6.6 The Morphology of Corroded Materials In reality, the identification of one or more forms of corrosion requires visual observation, nondestructive inspection methods, optical microscopic examination, and sometimes electron scanning microscopy, etc. The first study of the corrosion appearance of a case should divide corrosion into uniform and localized corrosion. Localized corrosion can be further identified as macroscopic or microscopic local corrosion. Microscopic attack refers to a minute amount of dissolved metal, accompanied by considerable damage, before the phenomenon becomes visible to the naked eye. Macroscopic forms of corrosion affect greater areas of corroded metal and are generally observable with the naked eye or can be viewed with the aid of a low-power magnifying device. Macroscopic examination can identify the following forms: galvanic, erosion–corrosion, crevice or pitting, exfoliation, and dealloying. Microscopic

The Forms of Corrosion

339

examination can identify: intergranular corrosion, stress–corrosion cracking, corrosion fatigue, and subsurface corrosion (frequently observed at high temperature).19 Microscopy, in conjunction with other techniques, may be used to study different types of corrosion. For example, a scanning electron microscope is a valuable tool to differentiate intergranular stress corrosion cracking from hot-short cracking, and transgranular stress corrosion cracking from corrosion fatigue. Identification of the influence of the media needs more care. The effects of particulate matter in a stream may require microscopy to assess its influence combined with the flow pattern of water in erosion– corrosion. A biochemical analysis is required to identify the organism in bacterial corrosion. Subsurface corrosion in case of high-temperature corrosion needs special attention since observation by the naked eye of the surface could lead to faulty conclusion.3

6.7 Published Corrosion Data The most desirable data are those obtained for the material of interest in the intended conditions of exposure. Such data are not readily available in the literature. Published data on atmospheric corrosion should be used with caution since atmospheric conditions are changing with time, as for example acid rain as a variable factor. Accelerated testing, including electrochemical tests, should have a good link with the natural and practical conditions. Published data should be consulted because they are generally useful. Some published data are mentioned here as examples since they are useful in selecting materials or discussion of case histories: However, the performance data of alloys can vary from the reported or published data based on accelerated testing, for some reasons such as: a. Impurities in the alloys b. Impurities in the environment (natural or industrial); e.g., small amounts of copper in solution can deposit as metallic copper on aluminum or steel surfaces and accelerate corrosion c. Suspended solids can give rise to erosion–corrosion, or corrosion–erosion, i.e., the intervention of other phenomenon associated with corrosion d. Aeration and oxidants such as ferric and cupric ions; however, the most frequent cell is the oxygen differential aeration cell e. Agitation and interfacial properties as solution pH at the interface and concentration in electrochemical and nonactive ions f. Microorganisms in action which is frequently an important factor that has been overlooked by investigators or designers. Examples of corrosion resistance of many metals and alloys in a variety of media and conditions that are available in the literature and given in the following books mentioned in the Bibliography: [NACE Graver ed., 1985; ASM International, 1995; Schweitzer, 1986; De Renzo, 1985*; McNaughton, 1980; ASM International, 1990; Climax Molybdenum Company, 1961]. *De Renzo (ed). Corrosion Resistant Materials (1985)

340

6.7.1

Corrosion Prevention and Protection

General Corrosion

Uniform and Quasi-Uniform Corrosion. Definition and Characteristics. The first and most common form of corrosion is general corrosion. This can be uniform (even) or quasi-uniform (near-uniform corrosion) or uneven general corrosion. General corrosion accounts for the greatest loss of metal or material. However, it is predictable and the designer can avoid catastrophic accidental corrosion problems. Generally, the galvanic abundant cell in corrosion is complex and corresponds to the dissolution of the active metal and oxygen reduction or hydrogen evolution on the cathode surface. Electrochemical general corrosion in aqueous media can include galvanic or bimetallic corrosion, atmospheric corrosion, stray-current dissolution, and biological corrosion. Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments); exposure in fresh, brackish, and salt waters; or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white rust on zinc on atmospheric exposure are due to uniform corrosion.14 In such reactions as the tarnishing of silver in air, the oxidation of aluminum in air, or attack of lead in sulfate-containing environments, thin, tightly adherent protective films are formed, and the metal surface remains smooth. It should be mentioned that underground corrosion is frequently observed as localized corrosion. Oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion can be considered as types of general corrosion.20 Liquid metals and molten salts at high temperature lead very frequently to general corrosion.1 Micro-electrochemical cells result in uniform general corrosion. Dissolution of metals in acids is due to the presence of indistinguishable anodic and cathodic sites. Uniform general corrosion can be observed during chemical, electrochemical polishing, and passivity where anodic and cathodic sites are physically inseparable. A polished surface of a pure active metal immersed in a natural medium (atmosphere) can suffer from galvanic cells. Most of the time, the asperities act as anodes and the cavities as cathodes. If these anodic and cathodic sites are mobile and change in a continuous dynamic way, uniform or quasi-uniform corrosion is observed. If some anodic sites persist and are not covered by protective corrosion products or do not passivate, localized corrosion is observed.1 Some macro-electrochemical cells can cause a uniform or near-uniform general attack of certain regions. General uneven corrosion or quasi-uniform corrosion is observed in natural environments and is much more common. For some metals or alloys, uniform corrosion produces a somewhat rough surface by removal of a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. As an example, following a careful removal of the rust after general atmospheric corrosion of steel, the surface reveals an undulated surface, indicating nonuniform attack of different areas (Figure 6.4.).1 In natural atmospheres, the general corrosion of metals can be localized. The conductivity, ionic species, temperature of the electrolyte, alloy composition, phases and hom*ogeneity in microstructure of the alloy, differential oxygenation cell, etc. can influence the

The Forms of Corrosion

341

Figure 6.4 Even and uneven general corrosion, and high-temperature attack (scaling)1,3

corrosion morphology. Figure 6.4 also shows high-temperature attack, normally uniform. However, subsurface corrosion films within the matrix of the alloy can be observed by microscopic examination due to film formation at the interface of certain microstrucrures in several alloys at high temperature.3 Key Factors 1. The agitation of the medium or the level of agitation of the electrolyte has a great influence on the corrosion performance of most of the metallic alloys since agitation causes acceleration of diffusion of aggressive species or destruction of the passive layer mechanically. 2. Acid pH accelerates corrosion for most of the alloys, since, for an active metal such as iron or zinc, the cathodic reaction controls the rate of the reaction according to E ¼ E 0 0:0592 pH. Figure 6.5 shows the Evans diagrams that can be obtained by extrapolation of the Tafel slopes for the cathodic and anodic polarisation curves (shown in Figure 6.1). In general, cathodic Tafel slopes are more reproducible and more reliable to evaluate corrosion rates since they represent the almost noncorroded or original state of the surface. It can be observed from Figure 6.5a that there is a marked increase in corrosion current for a more acidic solution. It should be noted that the influence of pH depends also on the composition of the alloy as shown in Figure 6.5b. In the case of zinc curve B, amalgamation with a more noble metal such as mercury decreases the corrosion rate in addition to the slower hydrogen evolution reaction which requires high overpotentials (curve A). Platinum gives high corrosion rates because it provides effective cathodic sites for hydrogen evolution (curve C). Another factor is the stability of the passive film of the system metal/solution in acid, neutral or alkaline pH. Magnesium fluoride is for example protective for magnesium in alkaline medium. Aluminum oxide is amphoteric and is stable1 at pH ( 4–8). 3. A difference in temperature in the case of copper tube at different temperatures can create a corrosion cell. Generally, the increase in temperature accelerates corrosion. For temperatures between 15 and 70 C, the rate of corrosion of steel in dilute acidic solutions can be doubled for every increase of 10 C. Above this range of

342

See Also

2024 July InMaricopa Magazine

Corrosion Prevention and Protection

Figure 6.5 Schematics of corrosion of zinc in deoxygenated solution as a function of: (a) pH and (b) composition of the alloy where A, B, and C correspond to the alloys Zn–Hg, Zn and Zn–Pt respectively25

temperatures, the solubility of oxygen in water is low and the rate of corrosion cannot be doubled as before since oxygen has an accelerating effect on the cathodic reaction. 4. Protective passive films similar to that of stainless steel, for example, result in uniform corrosion because of the mobility of active sites that passivate readily. Corrosion products and/or passive films are characteristic of numerous electrochemical corrosion of alloys. A film is protective depending on coverage capacity, conductivity,

The Forms of Corrosion

343

partial pressure, porosity, toughness, hardness and resistance to chemicals and gases. Rust (Fe) and white rust (Zn) are generally not protective, while patina (Cu), Al2O3, MgO and Cr2O3 are protective in certain environments. Corrosion is generally controlled by diffusion of active species through the film. The film free reaction is generally considered a chemical reaction. (Craig and Pohlman)14 Prevention of General Corrosion. Proper selection of materials. In design, a metal or alloy that forms a stable passive film should be recommended. A surface pretreatment in oxidized solutions has been adopted for stainless steels and is recommended in many circ*mstances. The most popular process ‘a 300 min immersion in a 20 vol% nitric acid at 50 C’ is recommended for some types of stainless steels.21 The environment can be modified in the bulk and should be effective at the interface in adding oxidizing agents, such as nitrite or strong nitric acid, that maintain the passive state on some metals and alloys.8 Proper design and thickness considerations. Proper design is based on knowledge (Figure 6.6). Landrum22 suggested the following thickness of a tank wall (steel) based on 10-yr useful life based on the predicted general corrosion rate: 1. Specify wall thickness for mechanical considerations 2. Expected life (10 yr) and uniform corrosion rate 0.4 mm/yr 3. Safety considerations (100% of the total corrosion rate) Recommended thickness

5 mm 4 mm 4 mm

) 13 mm

Corrosion control. Generally corrosion inhibitors, cathodic protection, anodic protection, and coatings are used for this purpose or combination of them. However, cathodic protection is the only method that avoids corrosion completely if the system is not sensitive to hydrogen embrittlement or alkaline medium. Anodic protection is a recent approach when the metal can be passivated in the corrosive solution. In this technique, a current can be applied using a potentiostat, which can set and control the potential at a value greater than the passive potential Ep or below the pitting potential Epit for environments containing corrosive species such as chlorides, bromides, etc.

Figure 6.6 Comparison between a good and bad design of a boiler tank173

344

6.7.2

Corrosion Prevention and Protection

Galvanic Corrosion

Formation of a Galvanic Cell. When a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte, a galvanic cell is created. The electromotive force and current of the galvanic cell depend on the properties of the electrolyte and polarization characteristics of anodic and cathodic reactions. The term galvanic corrosion has been employed to identify the corrosion caused by the contact between two metals or conductors with different potentials. It is also called dissimilar metallic corrosion or bimetallic corrosion where metal is the conductor material. Galvanic corrosion can lead to general corrosion, localized corrosion and sometimes both. Although the dissolution of active metals in acids is due to the presence of numerous galvanic cells on the same metallic surface, it is generally referred to as general corrosion. However, in less aggressive media, such as some natural media (dissimilar electrode cells), galvanic corrosion can start as a general corrosion that can be lead to localized corrosion because of different microstructures or impurities in several cases. Local galvanic attack depends on the distribution and morphology of metallic phases, solution properties, agitation, temperature etc. Localized galvanic corrosion case generally results in perforation or failure of the structure. The following are some factors that lead to general or localized corrosion: Dissimilar metals. Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact in the same solution [Figures 6.7 and 6.8]. In both cases, the corrosion of iron (steel) is exothermic and the cathodic reaction is controlling the corrosion rate. The more noble metal, copper increases the corrosion through cathodic reaction of hydrogen ion reduction and hydrogen evolution A passive oxide film on stainless steel for example can accelerate hydrogen reduction reaction. In engineering design, a junction of two different metals is seldom recommended. Sometimes, alloys with close values of potentials in a certain medium can be used. However, mechanical shaping, bending, or lamination of a part of the metal, thermal treatment of a part of a metallic structure, welding, cooling coils in vessels, and heat exchangers can create galvanic cells in the same metal. Sometimes, these electrochemical cells are called macrogalvanic cells to distinguish them from the microgalvanic cells which are present on even pure metals in a corrosive medium.22 (Baboian)5

H2O 2+

H+

e–

Cu Fe

Figure 6.7 A schematic presentation of galvanic corrosion of a mild steel elbow fixed to a copper pipe22

The Forms of Corrosion

345

Figure 6.8 Glavanic corrosion of painted steel auto body panel in contact with stainless steel wheel opening molding (Baboian)5

Generally, the galvanic cell is influenced by: (i) the difference of potential between the two materials; (ii) the nature of the environment; (iii) the polarization of every metal; and (iv) the geometry of the anodic and cathodic sites (shape, relative surface areas, distance, etc.). For example, it is not desirable to have a structure containing a small anode connected to a large cathode since the anodic dissolution will be localized and enormously accelerated. Figure 6.9a shows rivets of copper on a steel plate (on the left) and steel rivets on a copper plate (on the right). After 15 months immersion in sea water, the steel plate was covered by corrosion products while the rivets in Figure 6.9b

Figure 6.9 Area effect of the galvanic corrosion cell on (a) steel and (b) copper plates16

346

Corrosion Prevention and Protection

corroded and disappeared completely. The copper is more noble and accelerated the hydrogen reduction reaction for the oxidation of the steel plate. In the case of copper plate, severe corrosion of the steel rivets was observed because of the relative important cathodic surface of copper. The same reasoning is valid for the corrosion of the limited noncoated parts in the auto body panel in contact with the important large surface of stainless steel in Figure 6.8. Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heatexchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe3O4), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5 Metallic coatings and sacrificial anodes. Sacrificial metal coatings provide cathodic protection for base metal, such as galvanized steel or Alclad aluminum. If the metal coating is more noble than a base metal, such as nickel on steel, pitting of the base metal can occur at pores, damage sites, and edges (Figure 6.10). Care should be taken to keep the coat free of pores, scratches or any penetrating chemical attack or deterioration of the coat (a paint for example). Sacrificial anodes, such as magnesium, zinc, and aluminum are used extensively for cathodic protection, especially in some urban locations where impressed-current systems are forbidden because of stray currents. Polarity inversion. The properties of the electrolyte (pH, potential, temperature, fluid flow, concentration of different ions, dissolved gases and conductivity) can change with time and influence polarization, the properties of the interface and the galvanic potential of every component. For example the change of the ion activity of one metal can reverse the polarity of Fe–Sn couple for iron-plated tin in food media. (Hack)5. This shows the importance of the medium in galvanic corrosion where the canned food boxes are made of iron and coated inside with a protective layer of tin. The tin sometimes reacts with some constituents of food and forms a soluble complex. The concentration of tin could correspond then to some ppm and, applying the Nernst equation, the tin potential becomes more active or anodic to that of iron and corrodes intensively. Also, iron is protected by a zinc coating as a sacrificial anode (galvanization) because of the formation of loose flocculent Zn(OH)2 (white rust), however at temperatures above >60 C a hard compact ZnO layer is formed, which is cathodic to

Figure 6.10 Perforation in the coating gives rise to localized attack of steel (left) or consumption of the sacrificial Zn (right)16

The Forms of Corrosion

347

iron and this layer is able to reverse the polarity sign of the couple Fe–Zn. This is observed for other couples such as copper on aluminum or silver on copper.7,16,20 Deposition corrosion. Dissimilar metallic corrosion can occur due to the cementation of a more noble metal. If soft acid water (containing carbonic acid) flows through copper pipes and into a galvanized tank, any dissolved copper ions can be deposited in the tank according to the equation Cu2þ þ Zn ! Cu þ Zn2þ and this causes additional galvanic corrosion of zinc.20 Severe corrosion may occur for active aluminum or magnesium alloys, for example in neutral solutions of salts of heavy metals, such as copper, iron, and nickel. Such corrosion occurs when the heavy metal, the heavy metal basic salts, or both, plate out to form active cathodes on the anodic magnesium surface. This is a type of galvanic corrosion that leads typically to localized pitting corrosion. Hydrogen cracking or damage. Self-tapping of martensitic stainless steel screws has been observed due to a spontaneous quick cracking of the screws after being attached to an aluminum roof in a seacoast atmosphere (anodic surface). Similarly, hardened martensitic stainless steel propellers coupled to the steel hull of a ship have failed by cracking soon after being placed in service. Tantalum is readily embrittled by hydrogen at room temperature when the metal is cathodically polarized, or when coupled in an electrolyte to a more active metal in the galvanic series.7 High-temperature galvanic corrosion. Galvanic coupling at high temperature affects the corrosion rate. The reaction of silver with gaseous iodine at 174 C in 1 atm oxygen, for example, is accelerated by contact of silver with tantalum, platinum, or graphite.7 Factors Involved in Galvanic Corrosion. Emf series and ‘practical nobility’ of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of ‘solution potentials’ since it allows to classification of the metals in order of ‘nobility’ according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/l). This ‘thermodynamic nobility’ can differ from ‘practical nobility’ due to the formation of a passive layer and electrochemical kinetics. Ignoring the kinetic factors and assuming as a first approximation that the passivating films are perfectly protective, practical nobility depends on the immunity and passivation domains and the stability domain of water. In the case of identical surfaces for some metals, it is accepted that this ‘practical nobility’ is greater the more the immunity and passivation domains extend below and above the stability domain of water, and the more these domains overlap the section of the diagrams corresponding to pH values between 4 and 10, which are most common in practice. Table 6.2 shows the classification of 43 elements on one hand according to ‘thermodynamic nobility’ and on the other, according to ‘practical nobility’. This table shows the ennobling effect which passivation has on niobium, tantalum, titanium, gallium, zirconium, hafnium, beryllium, aluminium, indium and chromium. This must be regarded as guide since the electrochemical equilibrium diagrams are themselves approximations and in some cases necessitate drastic alteration (e.g., those for nickel and cobalt, and because corrosion and/or passivation reactions are sometimes strongly irreversible).23

348

Corrosion Prevention and Protection

Table 6.2 Emf series of some elements and classification of metals and metalloids in order of nobility23

The Forms of Corrosion

349

The galvanic series and corrosion. The practical change of the potential of every component of a galvanic couple as a function of time is of major importance. If the potential difference between the two metals is sufficient to create a sustained galvanic cell, the potential of every material or electrode can be subjected to certain changes because of the active–passive behavior, the properties of the passive or corrosion barriers and the change in the ion concentrations etc. The galvanic series is a list of corrosion potentials, each of which is formed by polarization of two or more half-cell reactions to a common mixed potential Ecorr , measured with respect to a reference electrode such as calomel electrode. Figure 6.11 shows the galvanic series of some metals and alloys in seawater. The material with the most negative potential has a tendency to corrode when connected to a material with a more positive or noble potential. Some alloys in this medium can have active and passive potentials and sometimes a potential between these two extremes. (Hack)5 The galvanic series thus gives qualitative indications of the likelihood of galvanic corrosion in a given medium under certain environmental conditions. The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7 The potential difference developed between aluminium and stainless steel is about the same as that developed between aluminium and copper. The cathodic reaction is easier on copper oxide than that on the highly protective passive oxide of stainless steels. Then, it is not the difference of potential between anode and cathode which counts, but the facility and rate of every reaction. A bare metal is generally a much better cathode than one covered with an oxide. Aluminium is more active than zinc in the electrochemical series. Practically, zinc protects aluminium which becomes covered with an oxide film.20 All more noble metals accelerate corrosion similarly, except when a surface film (e.g., on lead) acts as a barrier to diffusion of oxygen or when the metal is a poor catalyst for reduction of oxygen. Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7 The resistance overpotential of the cell IR is mainly a function of the solution conductivity of the electrolyte and the distance between the electrodes since the electrolytic resistivity is far more important than the electric resistance of metals.

350

Corrosion Prevention and Protection Volts versus saturated calomel reference electrode

(Active) –1.8

(Noble) –1.4

–1.2

–1.0

–0.8

–0.6

–0.4

–0.2

0 Graphite Platinum

Ni-Cr-Mo alloy C Titanium Ni-Cr-Mo-Cu-Si alloy G Nickel-iron-chromium alloy 825 Alloy 20 stainless steels, cast and wrought Stainless steel–types 316, 317 Nickel-copper alloys 400, K-500 Stainless steel–types 302, 304, 321, 347 Silver Nickel 200 Silver-bronze alloys Nickel-chromium alloy 600 Nickel-aluminum bronze 70-30 copper nickel Lead Stainless steel–type 430 80-20 copper-nickel 90-10 copper-nickel Nickel silver Stainless steel–type 410, 416 Tin bronzes (G & M) Silicon bronze Manganese bronze Admiralty brass, aluminum brass 50Pb-50Sn solder Copper Tin Naval brass, yellow brass, red brass Aluminum bronze Austenitic nickel cast iron Low-alloy steel Low-carbon steel, cast iron Cadmium Aluminum alloys Beryllium Zinc Magnesium

Figure 6.11 Galvanic series for some metals in seawater (Hack)5

0.2

The Forms of Corrosion

351

Figure 6.12 Effect of oxygen concentration on corrosion of mild steel7 in slowly moving distilled water, 48 h test, 25 C

Thus, if dissimilar pipes are butt-welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the active metal. The current of the galvanic cell takes the path of least resistance and this affects corrosion in that current does not readily flow around corners. In soft water, the critical distance between copper and iron may be 5 mm; in seawater it may be several decimeters. The critical distance is greater the larger the potential difference between anode and cathode. Then, the geometry of the circuit affects galvanic corrosion and this is observed in the case of stray current corrosion.7 (Baboian)5 The relative area of the anodic to cathodic sites is critical for general and/or localized attack. This parameter, together with the conductivity of the electrolyte can control the corrosion rate. If the surface area of the anode is relatively small compared with that of the cathode, and the electrolyte has a low conductivity, uniform corrosion can be change to quasi-uniform corrosion, to severe pitting or to other types of localized corrosion of the alloy. Since the diffusion of oxygen is frequently the rate-determining factor in aqueous corrosion, large cathode/anode area ratios will frequently result in intense galvanic attack.20 For the case where diffusion of the corrosive ions is the rate controlling reaction, it has been found that P ¼ p0 ð1 þ Ac =Aa Þ where p is the penetration that is proportional to the corrosion rate and p0 is the corrosion rate of the less noble uncoupled metal Ac and Aa are the areas of the more noble and active metal respectively (Uhlig and Revie, pp. 101–103).7 If a galvanic cell is not avoidable, a large anode and a limited size of cathode are recommended. Stagnant conditions and weak electrolytes may lead to pitting in spite of the large area of the exposed active metal. The following example illustrates the effect of the relative areas of anodic to cathodic surfaces. Reservoirs made of steel covered with a phenolic paint and the bottom clad with stainless steel were intended to handle mild corrosive solutions for steel. A few months later, perforation of the steel wall started at approximately 5 cm away from the weld of steel wall close to the stainless steel bottom (Figure 6.13).16 Large cathodes and small anodic surfaces should be avoided since there is no perfect coating, and the paint on steel

352

Corrosion Prevention and Protection

Figure 6.13 Perforation of coated steel due to pores and large cathodic surface at the bottom clad steel16

has always some defects and the pores act as small anodic surfaces close to the stainless steel bottom (big cathode). The corrosion current (microampere/cm2) can be multiplied by a factor of 10–20, leading to a quick penetration of the anodic sites. In other words, the cathodic control which limits the corrosion current in several aqueous solutions is not operational. This failure also shows the effect of the distance since the perforation was observed near the welding junction, especially in low-conducting solutions. Prevention of Galvanic Corrosion. Galvanic corrosion can be prevented or reduced by: 1. Avoid contact between metals with different potentials. Complete isolation of one from the other should be made and inspected periodically. To predict or avoid galvanic corrosion, the potential of every metal or conductor should be evaluated as a function of time in the intended medium. The variation of the properties at the interface should be carefully evaluated as a function of time. 2. Protection by metallic, nonmetallic, nonorganic, organic (paints, lacquer, etc.) coatings is recommended. If the metal coating is more noble than the base metal, care should be taken to have a coating free of pores. Applying a sacrificial coating such as Al, Zn, Mg or Cd to an active metal is recommended. Also, adding a paint layer to sacrificial zinc coating, for example, can extend the life by a factor of ten. Applying nonmetallic coatings as anodic films should be accompanied by sealing to avoid the small bare surfaces. 3. Large cathodes and small anodic surfaces should be avoided. The current practice considers coating of the more noble surface first, and preferably both cathodic and anodic surfaces. Such effects of large cathodes and small anodes are most likely to occur at joints of structures joined together by a different metal. They can be avoided by plating, e.g., with zinc or any other metal, provided that the new couple is safer.20

The Forms of Corrosion

353

4. Electrochemical testing and determination of polarization characteristics of every component are recommended. If one of the metals has active–passive behavior, the state of the contact material should be considered for the expected active and passive states. Both Pourbaix pH diagrams and the potential of the passive metal or alloy can be helpful for this purpose. Bacterial corrosion in case of intended media and conditions should be investigated. 5. The use of corrosion inhibitors in appropriate concentrations could improve the corrosion resistance through different mechanisms of adsorption, barrier film, etc. 6. Prediction of the anodic and cathodic components of the galvanic cell and inversion of polarity of the cell should be considered. 7. Cathodic protection offers the only complete protection of the metallic surface, however, the metal should not be sensitive to fragility or attack in alkaline medium. Testing of Galvanic Corrosion. Corrosion testing for galvanic corrosion can be predicted specifically by ASTM standards in the form of potential measurements. In general, the corrosion potential difference between anode and cathode becomes the driving force for galvanic corrosion. Galvanic corrosion tests between different metals or microstructures can be accomplished in acid or neutral chloride solutions containing hundreds of ppm Cl up to 5% Cl ion at ambient laboratory temperature or at the desired temperature to examine the galvanic effect of different materials. The galvanic currents are measured between two dissimilar metals using a zero resistance ammeter (ZRA) for an appropriate duration. The ratio of the anode to the cathode areas of the specimen is generally 1:1 or adjusted to the projected use. Agitation or circulation of the electrolyte should be as in the conditions of the projected use. Stray-current Corrosion. Stray currents in the past have resulted from DC-powered trolled systems, which have become obsolete. An electric welding machine on board a ship with a grounded DC line located on shore will cause accelerated attack of the ship’s hull as the stray currents generated at the welding electrodes pass out of the ship’s hull through the water back to the shore. Houses in close proximity can dramatically corrode at the waterline. The pipes in one house can be completely corroded, while those in the neighboring house may be intact. The major stray-current corrosion problems now result in cathodic protection systems. Current from an impressed-current cathodic protection system will pass through the metal of a neighboring pipeline at some distance before it returns to the protected surface. Increased anodic corrosion is frequently localized on the pipe at the zone where the current leaves the pipe back to the protected steel tank. Stray current flowing along a pipeline very frequently will not cause damage inside the pipe, because of the high conductivity of the electric path compared with the electrolytic one. The damage occurs when the current re-enters the electrolyte and will be localized on the outside surface of the metal. If the pipe has insulated joints and the stray current enters the internal fluid, the corrosion will be localized on the internal side of the pipe. The best solution is the electrical bonding of nearby structure and adding as required additional anodes and possibly increasing rectifier capacity.24 (Craig)5 Stray currents follow paths other than their intended circuit. They leave their principal path because of poor electrical connections or poor insulation around the intended conductive material. The escaped current then will pass through the soil, water or any

354

Corrosion Prevention and Protection

other suitable electrolyte to find a low buried path such as a metallic pipe. Stray currents cause accelerated corrosion when they leave the metal structure and enter the surrounding electrolyte. These sites can be hundreds of meters away or more. At points where the current enters the structure, the site will become cathodic in nature because of changes in potential, while the area where the current leaves the metal will become anodic. Electric railways, cathodic protection, electrical welding machines, and grounded DC electrical sources are subject to stray current corrosion. (Craig)5 Although the damage of stray current is always localized in a part of the system, stray current may lead to uniform corrosion of this part of the system, and so this is considered as a general form of corrosion although, the attack by stray current is generally more localized, causing in certain circ*mstances a concentration of pits. Stray-current corrosion can show penetration along the grain boundaries or a selective attack of the ferrite within the matrix of gray cast iron. Aluminum and zinc (amphoteric metals) can show signs of corrosion at the cathodic portion of the metal because of the localized alkalinity. Buried power lines can give rise to AC stray currents. Basically, alternating current causes less damage than DC, the stray current corrosion decreasing with increase in the frequency. However, damage to passive alloys such as stainless steel and aluminum alloys is important because of the alternating reduction and oxidation of the passive or barrier layer on the surface and which may lead to porous and non protective passive layers. (Craig)5 Prevention of stray-current corrosion. Measurement of the current before it enters the soil as around the electrolyte can monitor escaping current. The current leakage from the metallic structure to the ground should be stopped by good electrical connections and insulation. Bonding consists of connecting the stray-current conductor with the source ground via a separate conductor and thus eliminating the need for the current to leave the metal and enter the soil. Sacrificial anodes can be used to prevent stray-current corrosion. Insulation should be applied with care and should be sufficient to make the current sufficiently small. Coatings are not suitable since cracks, pinholes or pores will accelerate localized corrosion. However coatings on cathodically protected structures are beneficial and usually make the problems of stray current less severe and more easily controlled. Figure 6.14 illustrates two close underground structures designed to avoid stray-current corrosion. (Hanson)5

Ground level

DC source −

Steel tank Corrosion

Electric current

Anode

Electric current Anode

Induced anode

−

Ground level

Pipeline

(a) Original design

Induced cathode

Pipeline

Steel tank

DC source Electric current

Anode

Insulated buss connection

(b) Improved design

Figure 6.14 Localized corrosion of (a) unprotected buried pipeline and (b) an improved design including cathodic protection of both tank and pipeline(Hanson)5

The Forms of Corrosion

355

Figure 6.15 Some morphologies and types of localized corrosion3

6.7.3

Localized Corrosion

This is the most insidious corrosion because it is by far less predictable than general corrosion and can have serious consequences such as total failure. All forms of general corrosion that result in a nonuniform surface can be considered as localized corrosion. Figure 6.15 shows the various types of localized corrosion. The two major types of localized corrosion discussed are pitting corrosion, and crevice corrosion including filiform corrosion. In spite of the different morphological appearance of these two types of corrosion (Figure 6.15), the electrochemical basis of these two types are almost the same. The difference may rise from different causes in the initiation step of pitting or crevice corrosion.25 Pitting Corrosion. Pitting corrosion may be caused by a number of factors such as mill scale, which is a very common form of pitting corrosion of steel. Areas where a brass valve is incorporated into steel or galvanized pipeline serve as good examples. The junction between the two areas is often pitted, and if the pipe is threaded, the thread in close contact with the brass valve rapidly pits, causing a leak. This occurs frequently in industry, as well as in homes and farms. The deep pitting of tankers on the horizontal surfaces of cargo ballast tanks is a particularly aggravated type of pitting (i.e., pits are deep and frequent). In this case, pits are caused by frequent changes of cargo and salt water which perpetrate the oxidation reduction corrosion cycle.26 Pitting can also occur under atmospheric conditions. The corrosion starts at the break and continues to undercut the coating, forming a rather heavy tubercle of hard rust or scale with the pit underneath the original metal. The corrosion products help to isolate the aggressive medium inside the pit. These are common in marine environments as well as various industries where strong corrosive conditions exist.26 Also, pits with their mouth open (uncovered) exist and are responsible for loss of thickness and can act as stress raisers. Pitting is often a concern in applications involving passivating metals and alloys in aggressive environments. It can also occur in nonpassive alloys with protective coatings or in certain heterogeneous corrosive media. (Sprowls)14 Although in appearance pitting corrosion may seem unimportant, the depth of the pit and pit propagation rate are extremely dangerous and one of most serious types of corrosion. When the active state

356

Corrosion Prevention and Protection

Figure 6.16 Frequent variations of the cross-sectional shape of the pits (ASTM G46-94)4

within pits and crevices is maintained over an extended period of time, rapid metal dissolution usually occurs. The resulting pit and crevice geometries as well as the surface state within the pits vary markedly from open and polished hemispherical pits on free surfaces to etched crack-like shapes within crevices, depending largely on the type of rate-controlling reactions during the growth stage.27 Stress–corrosion cracking, and fatigue corrosion may result from pitting. Figure 6.16 shows the influence of three shapes of pits and the extent of undercutting or subsurface corrosion as well as the influence of the microstructure on the orientation of the grain attack. (ASTM G46)4 Mechanism of formation. The initiation of pitting starts at the defects in the passive film such as thinning, rupture, scratch, pore, etc. These sites are anodic in relation to the remainder of the surface that leads to the dissolution of metal. Once the process of dissolution starts, the dissolution does not need to be stimulated anymore because the process is generally autocatalytic. Although, in certain cases the propagation can be mitigated, in a temporary or permanent way, if some impervious products precipitate on the active sites or due to some other factors such as the rates of pit growth compared with that of repassivation, or a difference in the microstructure of the alloy, etc. The anodic zone limits itself to a specific area, on a point and leads to deep perforation. Pits usually grow gravitationally. The mechanism of pitting is depicted in Fig. 6.17. It is self-initiating and selfpropagating. The initiation of the pit can be caused by a discontinuity in the film, an impurity, different phase or a scratch, etc. For example, active metal, immersed in an oxygenated solution of NaCl, dissolves itself in the pit and the oxygen goes close to the pit. The positive charge of dissolved cations attracts chloride ions from the solution inside the pit. There is metal chloride formation and hydrolysis of these chlorides that leads to form Hþ and Cl, accelerating the metal dissolution: Mþ þ Cl ! MCl MCl þ H2 O ! MOH þ HCl HCl ! Hþ þ Cl

The Forms of Corrosion

Figure 6.17

357

Mechanism of pitting corrosion (Frankel)5

The pH changes during pitting corrosion can arise from two different reduction reactions; reduction of dissolved oxygen and that of hydrogen ions. If the metal hydroxide precipitates at the mouth or the sides of the pit, this can help the autocatalytic nature of penetration of the pit. The rust tubercules on cast iron indicate that pitting is in progress, and the environment inside the rust bubble has been found lower in pH and higher in chlorides than outside the bubble. Figure 6.18 shows the corrosion of iron. It represents a section of the pit and a growing pit inside the metal. The pitting factor ¼ P=d considers the deepest pit compared with the uniform corrosion loss (Figure 6.19). However to characterise the pitting phenomenon statistically, it is recommended to take the average of the deepest 10 pits as recommended in ASTM G48.4 There are two distinct processes before stable pit formation occurs: pit nucleation and growth of the metastable pit; and the pit precursors or metastable pits cannot grow until a pitting potential is reached.28 There are many examples of pitting in practice as follows: Underground structures and pitting. The bottom of a metallic pipe or hose buried in the earth, with a relatively limited surface of metal poorly aerated, has the tendency to

358

Corrosion Prevention and Protection

Figure 6.18 Mechanism of iron corrosion16

become anodic in relation to the large aerated surface of the rest of the metal. During the periodic verification of underground tanks or pipes, etc., the bottom of the buried metallic pipe in soil should be checked since it is vulnerable to the oxygen differential cell in its outermost performance.

Figure 6.19

The deepest pit in relation with the penetration in metal and the pitting factor7

The Forms of Corrosion

359

Mill scale (rust) and pitting. The three layers of iron oxide scale formed on steel during rolling vary with the operation performed and the rolling temperature. When mill scale is placed in an electrolyte, any defect in the mill scale surface becomes the anode; the remainder of the mill scale, which is usually many times larger than the defect, becomes a very strong cathode. An electric current can easily be produced between the steel and the mill scale. This electrochemical action will corrode the steel without affecting the mill scale. The thickness of the mill scale depends upon the final rolling conditions and can be 0.051–0.51 mm. The main composition is FeO and Fe2O3 having Fe3O4 as intermediate (magnetic). FeO can be mixed with the surface crystalline structure, unstable and oxidizes to ferric accompanied by increase in volume. Also, the black magnetite Fe3O4 can be converted to Fe2O3 (rust). This becomes loose and creates a galvanic cell having an electromotive force of 200–300 mV, similar to copper/steel couple.29 The initial stages of pitting and the e.m.f. can be slightly different, but during the propagation state, reactions are comparable: Anodic reaction

Fe ! Fe2þ þ 2 e

at 25 C with traces of ferrous iron. Application of the Nernst equation to calculate the e.m.f. of the cell, considering that the activity of Fe2þ is equal to 10-6 M give RT aproducts 0:0592 ln log 106 þ Za ¼ E 0Fe=Fe2þ zF areactants 2 Cathodic reaction 2 Hþ þ ½ O2 þ 2 e ! H2 O at 25 C E ¼ E0

E ¼ 1:23 0:0592 pH Zc where Za and Zc are the respective anodic and cathodic overpotentials. Oxygen comes from the mill scale or diffuses from the air to the steel surface. The coating breakdown occurs because of movement of the electrolyte, gases (oxygen) and moisture through the film to the pinholes and micropores. Water and gases passing through the mill scale film dissolve ionic material and cause osmotic pressure. Water diffusion and visual blistering can be observed. Permeation is at its best at (65–96 C) accelerated by osmotic pressure, electroendosmotic pressure, thermal agitation and vibration of the coating film molecules. The electroendosmotic gradient is created between the corroding area and the protected areas in electrical contact. Poultice Corrosion. Poultice corrosion is a special case of localized corrosion due to differential aeration, which usually takes the form of pitting. It occurs when an absorptive material such as paper, wood, asbestos, sacking, cloth, etc., is in contact with a metallic surface that becomes wetted periodically. During the drying periods adjacent wet and drying regions develop. Near the edges of wet zones and because of limited quantities of dissolved oxygen, differential aeration cell develops and this leads to pitting. As example is the extensive damage observed for the aluminum surface of fuel tanks in aircraft because of the accumulation of organic materials inside tanks due to bacterial and fungal growths in jet aviation fuel. Poultice corrosion can be avoided by avoiding the contact of absorptive materials with a metallic surface, by design or by painting for example.30

360

Corrosion Prevention and Protection

Crevice Corrosion. This type of corrosion is due to the presence of a corrosive solution that is stagnant in the neighborhood of a hole, under a deposit, or any geometric shape that can form a crevice. It is also called cavernous corrosion or corrosion under deposit. It results from a concentration cell formed between the electrolyte within the crevice, which is generally oxygen starved, and the electrolyte outside the crevice, where oxygen is more plentiful. The material within the crevice acts as the anode, and the exterior material becomes the cathode. This difference in aeration produces a different equilibrium potential, given by the Nernst equation applied to the equation: ½ O2 þ 2 Hþ þ 2 e ! H2 O. Using the activities of the dissolved species in water and considering the activity of water equal to 1, we get: E ¼ E0 þ 0:0592=2 logðO2 Þ1=2 ðHþ Þ2 =H2 O ¼ 1:23 0:0592 pH in volts at 25 C. The difference in this potential due to oxygen concentration leads to a cell sufficient for localized corrosion. The more aerated surfaces act as the cathode because of their more noble potential. In other situations difference in metal ion concentration can cause localized corrosion where the crevice rich in ions can play the role of the cathode (Figure 6.20). Crevices may be produced by design or accident. Crevices caused by design occur at gaskets, flanges, rubber O-rings, washers, bolt holes, rolled tube ends, threaded joints, riveted seams, overlapping screen wires, lap joints, beneath coatings (filiform corrosion) or insulation (poultice corrosion described in pitting), and anywhere close-fitting surfaces are present.31

Figure 6.20 Two types of crevice corrosion; oxygen concentration cell and metal ion concentration cell174

The Forms of Corrosion

Figure 6.21

361

Crevice corrosion16

Oxygen differential cells could be established between the oxygenated seawater outside or at the opening of the crevice surfaces, for example, and inside crevice anodic areas. This crevice must be sufficiently large to permit the entry of the corrosive solution, but narrow enough to form a stagnant state and hold a solution with the desired characteristics. The opening is generally of the order of 50–200 mm. The narrow space present between two metals or between a metal and a nonmetal is favorable site for crevice corrosion (Figure 6.21). Hydrolysis reactions within crevices could produce changes in pH and chloride concentration in the crevice environment. The space between the two materials is less aired, has a weak surface and contains a solution often rich in salt. (Kain)14 It is very probable that crevice corrosion can be initiated in special cases such as in magnesium and magnesium alloys due to the hydrolysis reaction and acid formation only, in certain conditions where it is believed that oxygen does not play a major role in corrosion mechanism.5 The crevice corrosion mechanism is very similar to that of pitting with respect to the autocatalytic propagation. However, the causes of initiation, the morphology and the penetration of pitting are quite different from that of crevice corrosion (Figure 6.21).16 Filiform Corrosion. This consists of the filamentary corrosion occuring on metallic surfaces and is a special type of crevice corrosion, sometimes called underfilm corrosion. It is frequently observed under the painted body of some used cars. It appears as a blister

362

Corrosion Prevention and Protection

Figure 6.22

Cross-section of a filament on painted steel16

under the paint. The filament propagation underfilm can appear, split or can join together, since, they propagate in direct lines, some of them reflecting because of obstacles such as adhesive parts of the organic film to the substrate and become trapped in a very narrow place (final stage).16 Figure 6.22 gives a schematic view of a filament on painted iron, and illustrates the role of the oxygen differential cell. The filament occurs on metals covered by an organic film and because of a certain discontinuity in the film, air and water penetrate through the coating and reach the underlying metal. This adjacent humid layer becomes saturated or rich in corrosive ions from soluble salts, and forms the zone called the active head of the filament. The dissolution of metal decreases as the solubility of the oxygen increases. The metallic ions oxidize and form compounds or corrosion products. These zones are called tails. Figure 6.22 illustrates a section and explains the mechanism of initiation and propagation of the filament. Filiform corrosion of AZ 91 magnesium alloy has been studied in detail and the mechanism different from the conventional mechanism has been postulated. In this case dissolved oxygen is not necessary and the filiform propagation is fueled by hydrogen evolution at the filament head and is controlled by mass transfer due to the salt film on the tip of the filament.32,33 Breakdown of Passivation. Pitting and crevice corrosion are usually associated with the breakdown of passivity. During pitting corrosion of passive metals and alloys, local metal dissolution leads to the formation of cavities within a passivated surface area. In practice, pitting corrosion of passive metals is commonly observed in the presence of chlorides or other halides. However, pitting may also occur in pure water as in the case of carbon steel in high-purity water at elevated temperature or aluminum in nitrate solutions at high potentials. In all these forms of localized corrosion, active and passive surface states are simultaneously stable on the same metal surface over an extended period of time, so that local pits can grow to macroscopic size.27 (Sprowls)14

The Forms of Corrosion

363

According to Hoar,6 it is necessary to exceed the critical anodic potential Ebd for the electrochemical breakdown of passivation by pitting and consisting of the following general steps: 1. Presence of damaging species such as chlorides or higher atomic weight halides at the interface 2. Induction time for initiation of the breakdown process and ending with localized conditions that could raise the localized corrosion current density 3. The local sites became immobile and localized at certain sites resulting in favourable environmental conditions inside the pit for propagation Electrochemical breakdown of oxides of some metals is possible since they are readily reduced cathodically to the metal in many solutions such as copper, tin and lead while ferric oxide is readily reduced to ferrous ions in aqueous solutions. Other metal oxides, such as those of zinc and aluminum are not cathodically reducible at the potentials obtainable in aqueous media; hydrogen is reduced instead. However, the vigorous evolution of hydrogen gas assisted by the electron-conducting zinc oxide can accelerate the breakdown of passivity. In the case of aluminum, the very feebly electron-conducting aluminum oxide can produce hydrogen gas that can push the film and followed by protons entering the film without discharge and render it conducting. Under high-field conditions, aluminum ions may even transfer from film to metal since the metal/film interface is nonaqueous.1,25 Among metals, there are differences in the composition and stoichiometry of these films that influence the stability and growth of these oxides. In aqueous solutions, anions such as halide and non halide types, can play a major role in passive film growth and breakdown. Borates, for example, appear to have a beneficial effect. It is necessary to consider the nature of the oxide film, the solution in which the film is formed, and the electrochemical conditions of formation of the film to evaluate the characteristics of the passive layer. Halide ions such as Cl can give rise to severe localized corrosion (e.g., pitting) Pitting is associated with particular combination of film thickness and halide concentration. Chloride is more aggressive than Br–. It is agreed that well-developed pits have high chloride concentration and low pH. Pitting can be random and amenable to stochastic (statistical) theory, and can be considered as deterministic, but very sensitive to experimental parameters such as induction time and electrochemical properties which are difficult to reproduce. Electrochemical noise may clarify the initial conditions for pit initiation.34 The potency of halides to complex with metal cations is very important in understanding the stabilization of a corrosion pit by prevention of the repassivation of a defect site within the passive layer. Enhancement of the transfer of metal cations from the oxide to the electrolyte by halides, especially the strongly complexing fluoride, is valid for many metals. The slow dissolution kinetics of the Cr (III) salts can explain the resistance of chromium to localized corrosion (Strehblow, pp. 201–237).13 Detrimental effects of sulfur species has been encountered in a large number of service conditions. However, the effects of chloride ions on passivity have been studied extensively. Recently the connection between the effects of atomic-scale surface reactions of sulfur and passivity has been studied in detail.12,35 Chemical dissolution of passivating oxide films is in general an exothermic reaction. Higher-valent oxides are usually the best passivating

364

Corrosion Prevention and Protection

films because of their slow rate of dissolution. Increase in temperature accelerates dissolution rate of oxides such as Fe2O3, Cr2O3 and Al2O3.25 Breakdown of passivity is the first stage in pitting corrosion. Pit growth and repassivation phenomena are characteristic of every corrosion passivation system. Metals show different patterns of passivation. Al and Cu are not passive in strongly acidic electrolytes, while Fe, Ni, and steels are passive, even in strongly acidic electrolytes, in disagreement with the predictions of Pourbaix diagrams. Localized acidification by the hydrolysis of corrosion products may serve as a stabilizing factor for pitting in certain metals.13 Alloying additions such as chromium and molybdenum to steel can substantially change the structure and composition of the passive oxide film and improve the process of passivation. For austenitic stainless steels, the barrier oxide layers, the salt deposit layers, and the alloy surface layers play an important role in the process of passivity and breakdown of passivity. Modeling passivation processes can provide new insights into alloy performance, and new alloy design concepts.36,37 There are two forms of pitting that follow breakdown of a passive metal or alloy surface, and they can be distinguished as pitting at low and high potentials. Pitting at low potential is influenced by cathodic or self activation and leads to merging etch pits that can eventually lead to general corrosion with etching. High-potential pitting takes the form of hemispherical pits corresponding to anodic dissolution in the electrobrightening mode. This requires a random dissolution due to the presence of film-breakdown factors and is independent of the crystal structure of the metal. This takes place through a random defective solid film that is none the less a very good ion conductor. Pits of either kind lead to occluded corrosion.1,25 This is also in agreement with the definition of two types of passivation depending on the level of potential of the passive metal (noble or active potential.8) Recently, statistical and stochastic approaches involving Gaussian and Poisson distributions to localized corrosion have been reviewed by.38 The Poisson distribution was found to be a better approach for pit generation. The results indicate that different pit generation rates can be observed as a function of time. The models consider either pit generation events alone or pit generation and subsequent repassivation processes. Theoretical models that describe the initiation process leading to passive film breakdown may be grouped into three classes: (1) adsorption and adsorption-induced mechanisms, where the adsorption of aggressive ions such as Cl is of major importance; (2) ion migration and penetration models; and (3) mechanical film breakdown model.27 Coatings and localized corrosion. This type of corrosion can occur where protective coatings are applied over metal and where there is a break in the coating so that the large coated area acts as a cathode, and the small defective area as the anode.26 Electrochemical Studies of Localized Corrosion. There are several electrochemical methods to determine the electrochemical conditions of pitting and crevice corrosion. However, emphasis is placed here on pitting as an example: Cyclic potentiodynamic polarization method. Electrochemical studies of pitting corrosion usually indicate that pitting occurs only within or above a critical potential or potential range. Therefore the susceptibility of passive metals to pitting corrosion is often investigated by electrochemical methods such as potentiodynamic or potentiostatic

The Forms of Corrosion

365

Figure 6.23 Schematic of a polarization curve showing Ep (pitting potential) ER (repassivation potential) relative to Ecorr , critical potentials and metastable region (Frankel)5

methods. The parameters are: (1) the critical current density icrit characterizing the active–passive transition; (2) the pitting potential where stable pits start to grow; and (3) the repassivation or protection potential (after reversal of the potential sweep direction) below which the already growing pits are repassivated and the growth stops.27,28 Cyclic potentiodynamic polarization, the conventional method to determine pitting potential consists of scanning the potential to more anodic and protection potentials during the forward scan and return scan and compare the behavior of different alloys in the same conditions of scan rate, solution and experimental conditions [Figure 6.23]. The polarization curve of an alloy (with or without coating) showing active-passive behavior can be then obtained in a certain medium as a function of increasing chloride concentration Eb , Ebd , Epit , or Ep (pitting potential) are symbols used frequently for the breakdown potential while Eprot , Eprotection (protection), ER (repassivation) or Erep or Epp are used frequently for the potential at which the reverse scan curve re-crosses the passive current measured on the forward scan. Scan rates of 0.05–0.2 mV/s can be used to obtain the standard active–passive curve and argon or nitrogen should be bubbled to deaerate the solution. The breakdown potential corresponding to the considerable increase of the anodic current at a certain scan rate gives the susceptible condition for the initation of localized attack. The more noble is this breakdown potential, the more resistant will be the alloy to pitting and crevice corrosion. The potential at which the hysteresis loop is completed upon reverse polarization scan determines the potential below which there is no localized attack for this scan rate. (ASTM G5)4 (ASTM G61)4 However, the absolute values of pitting or breakdown potential and the protection potential are dependent on the scan rate and do not reflect the induction time required for pitting. Also, allowing too much pitting propagation to occur along with the accompanying chemistry changes can influence the reversal in the scan rate. (Scully)14

366

Corrosion Prevention and Protection

Recent experimental work suggests that Epit and Eprot would converge to a unique pitting potential.39 Thompson and Syrett40 introduced a unique pitting potential which corresponds to both the most active value of Ep determined after a long incubation time and the most noble value of Er measured following only minimal pit growth. Some authors suggest that the stationary pitting potential corresponds to a value between that of the pitting and protection potentials. A critical pitting temperature (CPT) has been defined, below which a steel in an aggressive Cl-containing solution, usually a FeCl3 solution, would not pit, regardless of potential and exposure time.27 Also, a good measure of pitting susceptibility is the difference between protection and pitting potentials. Alloys that are susceptible to pitting corrosion exhibit a large hysteresis. This range of potentials can correspond to metastable pitting, a term used to express the region where pits initiate and grow for a limited time before repassivation. Large pits can stop growing for different reasons, but metastable pits are typically considered to be those of micrometer size, at most, with a lifetime of the order of seconds or less but under certain conditions, they continue to grow to form large pits.39,41 (Frankel)5 Increasing temperature usually also increases the pitting tendency of metals and alloys. At low temperature, high pitting potentials are observed. Temperature dependence of pitting susceptibility of stainless steels has been used in ranking of these steels with respect to their pitting resistance. Galvanostatic methods for localized corrosion. At constant chosen currents, the evolution of potential as a function of time is recorded until the rate of change in potential with time approaches zero. This technique is under development for aluminum alloys in ASTM G14 as a test method for application to aluminum alloys. (Scully)14 Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. The critical pitting potential Ecpr lies between the breakdown potential and the protection potential and can be determined by the scratch repassivation method. In the scratch repassivation method for localized corrosion, the alloy surface is scratched and exposed to a constant potential. The current change is monitored as a function of time and this will show the influence of potential on the induction time and the repassivation time. A careful choice of the level of potential between the breakdown potential and the critical pitting potential can give the critical pitting potential for a chosen material in given conditions.42 (Scully)14 Prevention of Localized Corrosion. Available data on the various physical and chemical aspects of passivity, including the composition, thickness, structure, growth, and properties of passive layers should be used in the studies of localized corrosion. A good understanding of the surface reactions involved in the formation and composition of passive films, passivation/repassivation, is necessary for the development of highly

The Forms of Corrosion

367

corrosion-resistant alloys. A Good level of understanding of the metallurgical factors and the rational use of alloyed elements can help to control general and localized corrosion such as pitting. The mechanism of failure or passivity breakdown of passive films in pitting corrosion is essential for safety purposes.25 Design-to-prevent is the best approach to avoid pitting and crevice corrosion. Some examples of surface treatments and coatings are: sacrificial coatings such as zinc (galvanization or plating) for steel Anodizing and sealing are effective methods for natural media, accompanied by appropriate painting for more aggressive environments steel phosphating before painting, or using corrosion inhibitors as a pigment in paintings (chromate, phosphate or molybdate etc.) or zinc-rich primers (70–75 mm thick) light barrier protective paint coating 100 mm, or heavy 6–13-mm thick to acid proof brick lining a thin organic coating for sacrificial zinc can extend useful life of zinc by a factor of 10. A compatible organic coating with an accompanying cathodic protection is the best solution It is recommended to examine the strained portions of metal including the welded areas that tend to be anodic with respect to unstrained portions (cathodic areas) and the bottom of underground structures such as steel tanks in aggressive environments, etc. Testing and Evaluating of Localized Corrosion. Basic electrochemical testing. The susceptibility of alloys to localized corrosion can be evaluated, such as that of an alloy having active–passive behavior in certain media by cyclic voltammetric, potentiodynamic, galvanostatic, scratch potentiostatic, triboellipsometric methods, pit-propagation rate curves, impedance spectroscopic studies and electrochemical noise measurements. Time-to-perforation data can be obtained by designing a specimen that is pressurized with air. This pressure is monitored over a period of time until failure is indicated by a decrease in pressure. (Scully)14 Some selected procedures or standards. Measurement of mass loss, along with visual comparison of pitted surfaces, may be sufficient to rank the relative resistance of alloys in laboratory tests. Visual examination of pits should determine the size, shape, density of pits as far as possible. Metallographic examination is very important to show the correlation with the microstructure and to distinguish pits between pitting, intergranular corrosion or dealloying4. ASTM G464 gives the standard rating chart for pits and the different methods of examination of pits and also describes nondestructive inspection of pitting that include radiographic, electromagnetic, ultrasonic, and dye penetration inspection. The statistical approach is briefly described in ASTM G464 but is covered in detail in ASTM G16.4 It is useful to compare the resistance to pitting as a function of the critical concentration of chloride which causes pit initiation on different alloys. The appearance, morphology, distribution and depth of pits (average penetration of the 10 deepest pits and the deepest one) should be determined in parallel with pitting potential determinations. The statistical distribution of pits and morphology of the pit should be examined by different microscopic techniques. The pit depth can be

368

Corrosion Prevention and Protection

measured by several methods, including metallographic examination, machining, use of a micrometer or a depth gage, and the microscope. The metal penetration can be expressed as a pitting factor, which is the ratio of the deepest metal penetration to the average metal penetration. Determining the pitting and crevice corrosion resistance of stainless steel and related alloys in 6% ferric chloride solution at 22 or 50 C is described in ASTM G48.4 This can be applied to other alloys. ASTM G784 is an important guide for crevice corrosion testing of iron and nickel base alloys in seawater and shows the importance of the crevice geometry and specimen preparation on the corrosion results. The Materials Technology Institute of the Chemical Process Industry (MTI); has identified five corrosion tests for iron- and nickel-based alloys, out of which two concern the resistance to crevice corrosion. The method MTI-2, originating from ASTM G48, involves the use of 6% ferric chloride solution for determining the relative resistance of alloys to crevice corrosion in oxidizing chloride environment. The method MTI-4 uses an increase in neutral bulk Cl concentration at eight levels, ranging from 0.1 to 3% NaCl, to establish the minimum critical Cl concentration that produces crevice corrosion at room temperature (20–24 C).43,44 The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14 The corrosion potential of the working electrode (specimen) is generally recorded for one hour in the 9 g/L NaCl solution. The initial and final potentials E1 as well as pH should be noted. The current is recorded at þ0.8 vs SCE for a period that depends upon the reaction. If localized corrosion is not simulated in the initial 20 s, the polarizing currents will remain very small or decrease rapidly with time. If localized corrosion is not stimulated in 15 min, the test is terminated and the material is considered to have a very high resistance to localized corrosion. Stimulation of localized corrosion is indicated by increasing polarization current with time or by current densities that exceed 500 mA/cm2. In these two cases, the potential is then returned quickly to the final corrosion potential E1 (after 1 h immersion), to determine if the specimen will repassivate or if localized corrosion will continue to propagate. If the pitted or local regions do not repassivate then the critical voltage is this value. The test consists of alternating between stimulation at 0.8 vs SCE and returning to a preselected potential (E1 þ a jump of þ50 mV in the noble direction) until continuous increases or large fluctuations in current during the 15-min observation period are observed. Evidence of pitting and crevice corrosion should be noted in ASTM F746.4 Loss in mechanical properties. The change of mechanical property can be used to identify or quantify the degree of pitting in the case of relatively intensive pitting corrosion. However, replicates for exposed and reference samples are recommended. Consideration should be given to such factors as edge effects, direction of rolling, and surface conditions [ASM Sprowls, 1987].14

The Forms of Corrosion

369

Some Electrochemical Techniques of Evaluation. Electrochemical noise technologies. New and accurate technologies are required to investigate the rather fast kinetics of localized corrosion. Electrochemical noise is an interesting tool to monitor metastable pitting by recording the galvanic current between nominally identical electrodes at the corrosion potential of a single electrode.28,45 The spatial separation of the anodic processes in the pit and the cathodic processes on the surrounding surfaces necessitates the passage of current that gives rise to the measured noise signals. Since localized corrosion sites are typically very small, of the order of 100 mm in diameter or less, the current densities inside these cavities can be of the order of 1 A/cm2. Electrochemical noise studies can be performed under open-circuit potential, and close to the natural conditions of pitting. Also, electrochemical noise fluctuations in an anodic current below the pitting potential, under potentiostatic control or fluctuation of the potential under applied anodic current have been carried out by several researchers in chloride environments. It has been suggested that current transients below the pitting potential may be attributed to the formation of pit embryos.28 Noise measurements are extensively used in the studies of metastable pits. Pistorius37 discussed several factors that can influence the proper interpretation of electrochemical noise measurements (ENM). These factors can be: probe size, sampling rate, and system noise. The current measurements seem to give clearer information on the corroding system than that of the potential.28,46 Noise electrochemistry (NE) gives instantaneous data that can show the relative corrosion resistance of different alloys in the same medium over long periods. However, agitation or convection can mask the noise signals, making it difficult to simulate operational conditions in certain situations. The zero resistance ammeter (ZRA) is an electronic instrument designed to measure the current flowing in a circuit without introducing the additional voltage drop associated with standard ammeter. As with the potentiostat, the main functional component is an operational electronic amplifier that supplies current necessary at its output to maintain zero potential difference between the two input potentials so that no current flows into or out of its input terminals.39 Noise analysis obtained from microelectrochemical investigations of stainless steels under potentiostatic conditions revealed that the current noise, expressed as standard deviation si of the passive current, increases linearly with the size of the exposed area, whereas the pitting potential decreases.47 However, to complete the electrochemical studies and distinguish between repassivating superficial pits and penetrating ones, microscopic studies are highly desirable. The scanning reference electrode technique (SRET) should be an appropriate complementary tool.28 EIS and localized corrosion. Electrochemical impedance spectroscopy for localised corrosion is still in progress for localized corrosion studies involving pitting. The statistical variation of pit nucleation and the absence of steady states prevent long-time measurements in the low-frequency region. In addition, in the pitting region a complicated Nyquist plot is obtained and difficult to interpret. However, Mansfeld et al.48 demonstrated that characteristic changes have been discovered in the low-frequency region. It should be noted that the impedance spectra for pits in stainless steels and magnesium are different from those of aluminum.27,28 The scanning reference electrode technique (SRET). The SRET has enabled the measurement of localized corrosion current densities in the vicinity of pits in stainless

370

Corrosion Prevention and Protection

steel in natural water. Novel potentiodynamic pitting scans have been obtained for localized areas immediately adjacent to accurately defined regions of the electrode surface. Isaacs has designed the scanning reference electrode technique to study the pitting and intergranular corrosion in stainless steels. He is also associated with the scanning vibrating electrode technique (SVET), in which the probe is mounted on a biomorph piezoelectric reed which vibrates the tip normal to the electrode at a characteristic frequency.49 Another variation of the technique has been used in localized measurement of electrochemical impedance spectra (LEIS).50 The description of the electrochemistry of the method is provided by Eden.45 Recently, a new microelectrochemical technique applying microcapillaries as electrochemical cells has been developed. Only small surface areas, a few micrometers or even nanometers in diameter, are exposed to the electrolyte. This leads to current resolution, down to picoamperes. Microelectrochemical techniques, combined with statistical evaluation of the experimental results may give greater insight into the mechanism of these processes.27 6.7.4

Metallurgically Influenced Corrosion

Very pure single crystals have defects that can effect corrosion, but impurities and alloying elements, grain boundaries, second phases, and inclusions often have serious effects. Welded structures invariably corrode first at the welds because of metallurgical heterogeneities that exist in and near welds. The most susceptible site or defect in a metal will be the first to be attacked on exposure to a corrosive environment. Sometimes such attack simply results in innocuous removal of the susceptible material, leaving a surface with improved corrosion resistance. (Frankel)5 Metallurgically influenced corrosion is mainly composed of the corrosion due to chemical composition (alloying elements, metalloids and impurities), metallurgical properties (metallic phases, grain joints) and fabrication procedures (thermal treatments, lamination and welding). Figure 6.24 shows weld zone, dealloying, exfoliation and internal modes of attack.

Figure 6.24 Heat-affected zone and some morphlogies of metallurgically influenced corrosion3

The Forms of Corrosion

371

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. Influence of Metallurgical Properties in Aqueous Media. Chemical composition and microstructure. When an alloy composed of various elements corrodes, usually one or more elements dissolve preferentially, leaving a surface enriched with other elements. This dealloying depends strongly on factors such as environment and potential. However, the dealloyed microstructure is vastly altered, often resulting in a reduction of strength and other properties. (Frankel)5 The corrosion resistance of stainless steels and nickelbased alloys varies markedly, depending on the alloying elements and processing conditions. When a solutionized low-sulfur stainless steel is exposed to boiling nitric acid, the attack is not localized, but rather is dominated by the orientation of the grains so that a stepped structure develops with the most susceptible orientations corroding faster. (Frankel)5 In amorphous form, glassy metals have been formed by very rapid cooling, of the order of 106 K/s during the solidification of a melt. This ‘freezes’ the atoms almost instantly in nearly the same positions they occupied in the liquid state. The resultant material is thus chemically and structurally hom*ogeneous and free form one- and two-dimensional defects, secondary phases, and grain boundaries. As a result of the extensive disorder, glassy metals often have physical, chemical, and mechanical properties that differ greatly from those of crystalline alloys of the same elemental compositions. They can be much more corrosion resistant than the compositionally equivalent crystalline alloys, in large part due to the absence of multiple phases, grain boundaries, and other defects. (Noe¨ l)5 The various classes of metallic phases that may be encountered in crystalline alloys include substantially pure elements, solid solutions of one element in another and intermetallic compounds. In crystalline form, alloys are subject to the same type of defects as pure metals. Crystalline alloys may consist of a solid solution of one or more elements (solutes) in the major (base) component, or they may contain more than one phase. That is, adjacent grains may have slightly or extremely different compositions and be of identical or disparate crystallographic types. Often, there is one predominant phase, known as the matrix, and other secondary phases, called precipitates. The presence of these kinds of inhom*ogeneities often results in the alloy having radically different mechanical properties and chemical reactivities from the pure constituent elements. (Noe¨ l)5 Grain boundaries. Usually, the spatial orientations of different grains (as defined by their intrinsic crystallographic planes) are random with respect to each other. This means that there must be a zone of transition over which the crystallographic orientation changes from that found in one grain to that found in its neighbors. This transitional zone is known as a grain boundary. This disorder makes grain boundaries energetically favorable sites of residence for impurities and other imperfections. Impurities tend to find the grain boundary regions to be energetically more favorable sites of residence than sites within the crystal lattice, especially if the impurities are present in concentrations beyond their solubility limits. The grain boundaries in alloys and impure metals are preferred sites for accumulation (segregation) of solute atoms. Likewise, solute atoms and impurities tend to congregate at defects within the grains as well. Grain boundaries

372

Corrosion Prevention and Protection

are also the favored precipitation sites for secondary phases formed during solidstate transformations. Consequently, the grain boundaries are often more resistant to mechanical deformation and have different chemical reactivities from the grains themselves.51, (Noe¨ l)5 Defects and inclusions. Point defects (zero-dimensional) include interstitial atoms, which are atoms present in the spaces between the lattice positions, vacancies (the absence of one or more atoms from the crystal lattice), and foreign atoms in lattice positions. Line defects (one-dimensional) are of two types: edge dislocations and screw dislocations. An edge dislocation is the region of imperfection that lies along the internal edge of an incomplete plane of atoms within a crystal. In a screw dislocation, a portion of the crystal is displaced, comprised of a single, continuous ribbon-like structure. Plane defects – stacking faults (two-dimensional) are imperfect regions of a crystal, resulting from errors in the positioning of entire atomic layers. Stacking faults arise during crystal growth or as a consequence of plastic deformation, but can occur only along the closepacked planes within a crystal. (Noe¨ l)5 Inclusions are three-dimensional defects consisting of soluble particles of foreign material in the metal and may be either intentionally or accidentally incorporated. Voids (another type of three-dimensional defect) are empty or gas-filled spaces within the metal. Particles composed of various oxides, sulfides, and silicates are common inclusions in metals and alloys; for example, manganese sulfide in stainless steels, well known as sites for pitting initiation. For example, cold working of a metal, introduces defects amounting to 100kJ/kg of stored energy that can create only about 3 mV of potential difference. This potential shift corresponds to a corrosion rate change of about 10%.51, (Noe¨ l)5 Passivation. For a passivating metal, the corrosion rate is less dependent on potential, and hence intergranular corrosion driven by free energy differences arising from disorder in the metal are even less likely. However, on surfaces protected by a passive film, grain boundaries and defects can sometimes promote preferential attack by disrupting the formation of a continuous protective layer. (Noe¨ l)5 Passivation and repassivation tend to be enhanced on densely packed crystallographic planes because there are fewer steps and kinks. Hence texture can have an impact on the passivation characteristics. The susceptibility to stress– corrosion cracking can also be impacted by material texture, because the bias in grain orientation will favor alignment of stacking faults in different grains along preferred directions, with consequent effects on slip processes and, ultimately, stress-corrosion cracking, depending on the direction of tensile stresses. (Noe¨ l)5 Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27 Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium

The Forms of Corrosion

373

single-phase conditions can be attained by techniques, such as rapidly quenching or physical vapor deposition. The resulting microstructure is either nanocrystalline or amorphous. Sputter-deposited aluminum alloys containing only a few atom percent of metal solute such as Cr, Ta, Nb, W, Mo, or Ti, exhibit an increase of Ep of 0.2–1 V. The increase in pitting resistance was explained by the reduced pit initiation tendency as well as by a more protective passive film, favoring rapid repassivation.47 Pitting potential increased with increase in chromium contents 20 wt%, and molybdenum of 2–6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5 Internal or subsurface attack (oxidation). High-temperature corrosion can be identified by simple visual observation of the surface. However, subsurface phenomena within the matrix of the alloy, as well as obscured relations at the interface of the alloy with the surface films formed in many high temperature exposures can be seen in Figure 6.24. Electrochemical corrosion at high temperature at the interface also involves the diffusion of the aggressive gas phase to the vulnerable phase in the subsurface, leading to corrosion most of the time. Dealloying or Selective Dissolution. Dealloying is a corrosion process involving selective dissolution of one or more elements, leaving behind a porous residue of the remaining element(s). Dealloying, also referred to as selective leaching or parting corrosion, is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a porous weak deposit of the more noble metal. For example, the preferential leaching of zinc from brass is called dezincification. In the case of gray iron, dealloying is called graphitic corrosion.31 Dealloying can occur in nearly any system in which a large difference in equilibrium potential between the alloying components and the fraction of the less noble constituent (s) exits and is significantly high. (Corcoran)5 Dezincification. Copper–Zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Uniform dealloying in admiralty brass is shown in Figure 6.25.5,7,53,54 Brass is only one strong phase of the copper and zinc dissolved. In certain circ*mstances one notes a preferential dissolution of brass however. This dezincification (Figure 6.24) can be localized (plug dezincification) or more uniformly distributed (layer dezincification).7 Figure 6.26 shows the selective attack of the rich phase of the alloy. Dezincification of a-brass can be minimized by adding 1% Sn, as in admiralty brass (71 Cu–28Zn–1Sn, and further inhibited by adding less than 0.1% of arsenic),55 antimony, or phosphorus. Where dezincification is a problem, red brass, commercial bronze, inhibited admiralty metal, and inhibited brass can be successfully used. Graphitic corrosion has been observed on buried pipelines after many years of service. Gray cast iron has a continuous graphite network in its microstructure that is cathodic to iron and remains behind as a weak, porous network as the iron is selectively removed

374

Corrosion Prevention and Protection

Figure 6.25

Selective corrosion by layer type (dezincification) of a bolt in brass7

from the alloy. (Corcoran)5 Attack by graphitic corrosion is reduced by alloy substitution (e.g., use of a ductile or alloyed iron rather than gray iron), altering the environment (raising the water pH to neutral or slightly alkaline levels), the use of inhibitors, and avoiding stagnant conditions and/or cathodic protection can be used to prevent graphitization.31 Dealuminification. Recent investigations have shown the importance of the dealloying of S-phase (Al2CuMg) particles on the corrosion of aluminum aircraft alloys, specifically aluminum alloy 2024-T3. In 2024-T3, the S-phase particles represent approximately 60% of the particle population. These particles are of the order of 1 mm diameter, with a separation of the order of 5 mm representing an surface area fraction of 3%.56 The selective removal of aluminum and magnesium from these particles leaves behind a porous copper particle that becomes the preferential site for oxygen reduction.57, (Corcoran)5 Dealloying has also been observed with Ag–Au, Cu–Au, Cu–Pt, Al–Pt, Al–Cu, Cu–Zn–Al, Cu–Ni and Mn–Cu alloys. (Corcoran)5 Other Systems. Evidence for dealloying has been reported in austenitic stainless steel and iron–nickel alloys in acidified chloride-containing solutions, reduction of titanium dioxide in molten calcium chloride, and copper-zinc-aluminum alloy pellets in NaOH solutions to produce Raney metal particles. (Corcoran)5 Mechanisms and Models of dealloying. In the dealloying process, typically one of two mechanisms occurs: alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness, and ductility.31 For brass, for example, it is generally accepted that the alloy attack occurs, giving rise to both dissolution of copper and zinc ions and the subsequent redeposition of copper during the corrosion of

The Forms of Corrosion

375

Figure 6.26 Dezincification of a brass of two phases175 (preferential attack of the phase b(top) rich in zinc as compared to the phase a(bottom) 133

copper–zinc alloys in many electrolytic solutions. However, this is not the only mechanism for dezincification to occur, although it enhances the rate. In case of the gold–silver system, only selective removal of silver occurs in 0.1 M HCIO4. (Corcoran)5 The dissolution process is maintained beyond the first few monolayers by volume diffusion of both elements in the solid phase. The inherent problem with this mechanism is that, at room temperature, the rate of transport of the less noble element to the surface is not sufficient to support the dealloying current densities greater than 10 mA/cm2 observed experimentally. (Corcoran)5 Two suggested ways to explain the continuous rapid supply of the more active or less noble metal give rise to two models of dissolution. One of them proposes that the less noble element is preferentially dissolved. The remaining more noble element is now in a highly disordered state and begins to reorder by surface diffusion and nucleation of islands of almost pure noble metal. The coalescence of these islands continues to expose fresh alloy surface where further dissolution will occur, leading to the formation of tunnels and pits. (Corcoran)5

376

Corrosion Prevention and Protection

The second model extends the surface diffusion model to include the importance of the atomic placement of atoms in the randomly packed alloy. The model considers that a continuous connected cluster of the less noble atoms must exist to maintain the selective dissolution process for more than just the few monolayers of the alloy. This percolating cluster of atoms provides a continuous active pathway for the corrosion process as well as a pathway for the electrolyte to penetrate the solid. This is expected to depend on a sharp critical composition of the less noble element, below which dealloying does not occur.54, (Corcoran)5 Intergranular Corrosion and Exfoliation. Intergranular corrosion (IGC) is preferential attack of either grain boundaries or areas immediately adjacent to grain boundaries in a material exposed to a corrosive environment, but with little corrosion of the grains themselves. (Phull)5 This dissolution is caused by potential differences between the grain boundary region and any precipitates, intermetallic phases, or impurities that form at the grain boundaries. Susceptibility to intergranular attack depends on the corrosive solution and on the extent of intergranular precipitation, which is a function of alloy composition, fabrication, and heat-treatment factors. (Phull)5 Steel phases have an influence on the rate of corrosion. Ferrite has a weak resistance to pitting. The presence of martensite can increase the hydrogen fragilization of steel. Intermetallic phases as Fe2Mo in high Ni content alloys can influence the corrosion resistance. The precipitate CuAl2 in aluminum alloys the series 2000 is more noble than the matrix, with corrosion around the precipitate. The majority of case histories reported in the literature have involved austenitic stainless steels, aluminum alloys, and to a lesser degree, some ferritic stainless steels and nickel-based alloys.31 Impurities that segregate at grain boundaries may promote galvanic action in a corrosive environment by acting either as anodic or cathodic sites. For example, in 2000-series (2xxx) aluminum alloys, the copper-depleted (anodic) band on either side of the grain boundary is dissolved while the grain boundary is cathodic due to the CuAl2 precipitates. Conversely, in the 5000-series (5xxx) aluminum alloys, intermetallic precipitates such as Mg2Al3 (anodic) are attacked when they form a continuous phase in the grain boundary. During exposures to chloride solutions, the galvanic couples formed between these precipitates and the alloy matrix can lead to severe intergranular attack. Actual susceptibility to intergranular attack and degree of corrosion depends on the corrosive environment and on the extent of intergranular precipitation, which is a function of alloy composition, fabrication, and heat-treatment parameters.31, (Phull)5 Precipitates that form as a result of the exposure of metals at elevated temperatures (for example, during production, fabrication, and welding) often nucleate and grow preferentially at grain boundaries. If these precipitates are rich in alloying elements that are essential for corrosion resistance, the regions adjacent to the grain boundary are depleted of these elements. The metal is thus sensitized and is susceptible to intergranular attack in a corrosive environment. An example is in austenitic stainless steels such as AISI type 304; the cause of intergranular attack is the precipitation of chromium (Davis) p. 25.31 Another example of grain boundary segregation is s-phase formation in Cr-Mo alloys; s-phase is usually more difficult to resolve visually in the microstructure than Cr-carbides. (Phull)5

The Forms of Corrosion

377

At temperatures above approximately 1035 C, chromium carbides are completely dissolved in austenitic stainless steels. However, when these steels are slowly cooled from these high temperatures or reheated into the range of 425–815 C, chromium carbides are precipitated at the grain boundaries. These carbides contain more chromium than the matrix. The precipitation of the carbides depletes the matrix of chromium adjacent to the grain boundary. This sensitization occurs because the depleted zones have higher corrosion rates than the matrix in many environments. (Fritz)5 Most serious cases of intergranular attack arise from compositional dissimilarities rather than structural defects. One form of corrosion in which structural defects do seem to play a role is stress–corrosion cracking. The presence of a large number of stacking faults makes it easier for grains to slip (i.e., facilitates shear displacement of one part of the grain with respect to another). The material then has a greater tendency to creep rather than crack to relieve tensile stresses. In some materials, decreasing the free energy stored in stacking faults can change the nature of stress–corrosion cracking from intergranular to transgranular. (Noe¨ l)5, (Phull)5 Intergranular corrosion at elevated temperatures is a serious problem in sulfidation of nickel alloys. Deep penetration can occur rapidly, until it becomes complete through the thickness of the alloy. The techniques for evaluating this type of IGC include: (a) X-ray mapping during examination in a scanning electron microscope (equipped with an energy-dispersive X-ray detector); and (b) transmission electron microscopy. (Phull)5 Intergranular corrosion in aluminum alloys is controlled by material selection and by proper selection of thermal (tempering) treatments that can affect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to intergranular corrosion is obtained by the use of heat treatments that cause precipitation to be more general throughout the grain structure. Guidelines for selecting proper heat treatments for these alloys are available.57 Exfoliation is a form of macroscopic intergranular corrosion that primarily affects aluminum alloys in industrial or marine environments. Corrosion proceeds laterally from initiation sites on the surface and generally proceeds intergranularly along planes parallel to the surface. The corrosion products that form in the grain boundaries force metal away from the underlying base material, resulting in a layered or flake-like appearance.31 In certain materials, corrosion progressing laterally along planes parallel to rolled surfaces is known as exfoliation, and it generally occurs along grain boundaries – hence, intergranular corrosion. A layered appearance is a common manifestation of exfoliation (also referred to as layer corrosion), resulting from voluminous corrosion products prying open the material, for example, in aluminum alloys. (Phull)5 The most susceptible alloys are the high-strength heat-treatable 2xxx and 7xxx alloys. Exfoliation corrosion of Al 6xxx in salt medium has been observed. Exfoliation corrosion in these alloys is usually confined to relatively thin sections of highly worked products.57 Exfoliation corrosion is observed in unalloyed magnesium above a critical chloride concentration, but this morphology was not seen in magnesium alloys, in which individual grains were preferentially attacked along certain crystallographic planes. The early stages of this form of attack caused swelling at points on the surface due to apparent delamination of the magnesium crystals with interspersed corrosion products, but, as attack proceeded, whole grains or parts of grains disintegrated and dropped out, leaving the equivalent of large irregularly shaped pits.58

378

Corrosion Prevention and Protection

Testing of intergranular attack necessitates metallographic examination. The standard practice ASTM G110,4 1992 evaluates intergranular corrosion resistance of heat-treatable aluminum alloys by immersion in sodium chloride and hydrogen peroxide solution. For example, ASTM A2624 contains six practices for detecting susceptibility to IGC in austenitic stainless steels using different oxidizing reagents at different temperatures, and kinetics is examined by microscopic examination of the etched microstructure for Cr23C6 sensitization and weight loss measurements. (Phull)5 Similarly, tests for detecting susceptibility to IGC in ferritic stainless steels have been incorporated into ASTM A7634 and for wrought Ni-rich, Cr-bearing alloys, into ASTM G28.4, (Phull)5 ASTM G1084 describes a laboratory procedure for conducting a nondestructive electrochemical reactivation (EPR) test on Types 304 and 304L stainless steel to quantify the degree of sensitization. The metallographically mounted and highly polished test specimen is potentiodynamically polarized from the normally passive condition, in 0.5 M H2SO4 þ 0.01 M KSCN solution at 30 1 C, to active potentials–a process known as reactivation. The amount of charge passed is related to the degree of IGC associated with Cr23C6 precipitation, which occurs predominately at the grain boundaries. After the single-loop EPR test, the microstructure is examined. (Phull)5 Certain media have been commonly used for evaluating the susceptibility to IGC of magnesium, copper, lead, and zinc alloys. (Phull)5 Weldment Corrosion. The factors that can initiate or propagate different forms of corrosion of welded regions are numerous, interrelated and hard to define, however a systematic approach should consider: weldment design, fabrication technique, welding practice, welding sequence, moisture contamination, organic or inorganic chemical species, oxide film and scale, weld slag and spatter, incomplete weld penetration or fusion, porosity, Cracks (crevices), high residual stresses improper choice of filler metal, and final surface finish. Consequently, the corrosion resistance of welds may be inferior to that of the properly annealed base metal because of: microsegregation, precipitation of secondary phases, formation of unmixed zones, recrystallization and grain growth in the weld heat-affected zone (HAZ), volatilization of alloying elements from the molten weld pool, contamination of the solidifying weld pool.5 [ASM, Frankel, 2003] Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5 Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heataffected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design.

The Forms of Corrosion

379

Additionally, specific environmental conditions can induce localized corrosion such as temperature, conductivity of the corrosive fluid, or thickness of the liquid corrosive film in contact with the metal. In some cases, both metallurgical and geometric factors will influence behavior, such as in stress–corrosion cracking. Preferential weldment corrosion of carbon steels has been investigated since the 1950s, commencing with the problems on icebreakers, but the problem continues today in different applications. (Bond)5 There is clearly a microstructural dependence, and studies on HAZs show corrosion to be appreciably more severe when the material composition and welding parameters are such that hardened structures are formed. It has been known for many years that hardened steel may corrode more rapidly in acid conditions than fully tempered material, apparently because local microcathodes on the hardened surface stimulate the cathodic hydrogen evolution reaction. (Bond)5 Grooving corrosion. There is a particular case of preferential weldment corrosion worth highlighting in respect to electric-resistance-welded/high-frequency-inductionwelded (ERW/HFI) pipe, where attack of the seam weld HAZ/fusion line can occur in aqueous environments or when exposed to the water phase in a mixed-phase system due to flow conditions or water dropout at low points. This grooving corrosion has been attributed to inclusions within the pipe material being exposed at the pipe surface and modified by the weld thermal cycle.59 This type of corrosion is probably caused by the redistribution of sulfide inclusions along the welding. It has been suggested that MnS is concentrated by the movement of the liquid metal during welding. The elevated temperature can dissociate the sulfide of manganese to sulfur and can form iron sulfide.60 A normalizing heat treatment can reduce or prevent the occurrence. The remedial action is the selection of a cleaner alloyed steel. Corrosion is due to electrochemical potential differences (galvanic corrosion) between the HAZ/fusion line and the parent material, attributed to the unstable MnS inclusions produced during the welding cycle. It was observed that enhanced corrosion of the weld metal was due to electrochemical potential differences between the weld metal and the base metal, such that the weld metal is anodic in the galvanic couple. The potential difference may only be of the order of perhaps 30–70 mV, but the low surface area ratio of anode to cathode results in high corrosion rates (1–10 mm). (Bond)5 Stress-corrosion cracking. This failure may occur by both active path and hydrogen embrittlement mechanisms, and, in the latter case, failure maybe especially likely at lowheat-input welds because of the enhanced susceptibility of the hardened structures inevitably formed. Most SCC studies of welds in carbon and carbon–manganese steels have evaluated resistance to hydrogen-induced SCC, especially under sour (H2S) conditions prevalent in the oil and gas industry, which is commonly referred to as sulfide stress cracking (SSC). Although full definition of the effect of specific microstructural types has not been obtained, an overriding influence of hardness is evident. On this basis, it is probable that soft, transformed microstructures around welds are preferable. In order that hard spots in the HAZ are not overlooked in weld procedure qualification, many standards now require the use of the Vickers hardness testing for welds. SCC in Oil Refineries. (Bond)5 Failures in refineries have shown cracks parallel or normal to welds, depending upon the orientation of principal stresses. Both transgranular and intergranular cracks have been observed.

380

Corrosion Prevention and Protection

Equipment found to suffer from cracking included tanks, absorbers, carbon treater drums, skimming drums, and piping. All welds of deaerator vessels in carbon steel should be post-weld stress relieved to minimize cracking and pitting. (Bond)5 Stainless steels are iron-based alloys that contain a minimum of approximately 11% Cr, the amount needed to prevent rusting. Few stainless steels contain more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms and heals itself in the presence of oxygen. (Krysiak)14 The conditions created by arc-welding operations produce a scale composed of elements that have been selectively oxidized from the base metal. The region near the surface of an oxidized stainless steel is depleted in one or more of the elements that have reacted with the surrounding atmosphere to form the scale. (Wahid)61 Backing rings are sometimes used when welding pipe. In corrosion applications, it is important that the backing ring insert be consumed during the welding process to avoid a crevice. (Krysiak)14, (Wahid)61 The welding filler metal must at least match the contents of the base metal in terms of specific alloying elements, such as chromium, nickel, and molybdenum. The cycle of heating and cooling that occurs during the welding process affects the microstructure and surface composition of welds and adjacent base metal. Consequently, the corrosion resistance of autogenous welds (welds made without the use of filler metals) and welds made with matching filler metal may be inferior to that of properly annealed base metal because of: microsegregation, precipitation of secondary phases, formation of unmixed zones, recrystallization and grain growth in the weld heat-affected zone (HAZ), and volatilization of alloying elements from the molten weld pool, contamination of the solidifying weld pool. (Krysiak)14, (Wahid)61 Unmixed zones. All methods of welding stainless steel with a filler metal produce a weld fusion boundary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. An unmixed zone has the composition of base metal, but the microstructure of an autogenous weld. The microsegregation and precipitation phenomena characteristic of autogenous weldments decrease the corrosion resistance of an unmixed zone relative to the parent metal. Unmixed zones bordering welds made from overalloyed filler metals can be preferentially attacked when exposed on the weldment surface.14,61 Sensitization. Welding is the common cause of the sensitization of stainless steels to intergranular corrosion. In austenitic stainless steels, the principal weld metal precipitates are d-ferrite, s-phase, and M23C6 carbides. Small amounts of M6C carbide may also be present. Although the cooling rates in the weld itself and the base metal immediately adjacent to it are sufficiently high to avoid carbide precipitation, the weld thermal cycle brings part of the heat-affected zone (HAZ) into the precipitation temperature range. Carbides can precipitate, and a zone somewhat removed from the weld becomes susceptible to intergranular corrosion (Figure 6.27). Once the precipitation has occurred, it can be removed by reheating the alloy to above 1035 C and cooling it rapidly. This practice is commonly termed solution anneal.5 The best-known weld-related corrosion problem in stainless steels is weld decay (sensitization) caused by carbide precipitation in the weld HAZ. This sensitized

The Forms of Corrosion

381

Figure 6.27 Schematic diagram of different microstructures (sensitization) in an austenitic stainless steel weldment (Fritz)5

microstructure is much less corrosion resistant, because the chromium-depleted layer and the precipitate can be subject to preferential attack (Figure 6.28). Pitting and stress–corrosion cracking. Under moderately oxidizing conditions, such as in pulp and paper bleach plants, weld metal austenite may suffer preferential pitting in alloy-depleted regions. This attack is independent of any weld metal precipitation and is a consequence of microsegregation or coring in weld metal dendrites. (Wahid)61, (Krysiak)14 In the case of austenitic steels (18-8), ruptures of SCC are intergranular when steel is subjected to non suitable thermal treatment such as in the zone of carbide precipitation, between 400 and 800 C. With a steel subjected to ‘hyperquenching’ ruptures are almost always transgranular. Austenitic stainless steels that are susceptible to intergranular corrosion are also subject to intergranular SCC. The problem of the intergranular SCC of sensitized austenitic

Figure 6.28 Schema of a grain boundary (on the left), and a concentration profile near the grain boundary (on the right) of a sensitized stainless steel type 30416

382

Corrosion Prevention and Protection

stainless steels in boiling high-purity water containing oxygen has been well studied. Cracking of sensitized stainless steels in many boiling water nuclear reactors has been observed. Sensitized stainless alloys of all types crack very rapidly in the polythionic acid that forms during the shutdown of desulfurization units in petroleum refineries. (Fritz)5 Intergranular corrosion of 430, 434 and 446 steels during welding has been observed. (Wahid)61, (Krysiak)14 Figure 6.27 shows a schematic diagram of weld decay (sensitization) in an austenitic stainless steel weldment. (Fritz)5 The grain joints become impoverished in chromium because of the formation of carbides of chromium (Figure 6.28). Knife-line attack. Stabilized austenitic stainless steels may become susceptible to a localized form of intergranular corrosion known as knife-line attack or knife-line corrosion. During welding, the base metal immediately adjacent to the fusion line is heated to temperatures high enough to dissolve the stabilizing carbides, but the cooling rate is rapid enough to prevent carbide precipitation. If weldments in stabilized grades are then heated into the sensitizing temperature range of 425–815 C, for example, during stress-relieving treatments, high-temperature service, or subsequent weld passes, chromium carbide can precipitate. The precipitation of chromium carbide leaves the narrow band adjacent to the fusion line susceptible to intergranular corrosion. Knife-line attack can be avoided by the proper choice of welding variables and by the use of stabilizing heat treatments.5 Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active–passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14

Figure 6.29 Three favorable conditions of penetration of bacteria through a pinhole, causing a corroded subsurface cavity in stainless austenitic steels31

The Forms of Corrosion

383

Prevention. In North America, susceptibility to intergranular corrosion and sensitization can be avoided generally by the use of low-carbon grades such as type 316L (0.03% C maximum) in place of sensitization-susceptible type 316 (0.08% C maximum). In Europe, it is more common to use 0.05% C (maximum) steels, which are still reasonably resistant to sensitization, particularly if they contain molybdenum and nitrogen; these elements appear to raise the tolerable level of carbon and/or heat input. (Wahid)61, (Krysiak)14 However, this method is not effective for eliminating sensitization that would result from long-term service exposure at 425–815 C. At temperatures above 815 C, titanium and niobium form more stable carbides than chromium and are added to stainless steels to form these stable carbides, which remove carbon from solid solution and prevent precipitation of chromium carbides. (Fritz)5 In the wrought condition, duplex stainless steels have microstructures consisting of a fairly even balance of austenite and ferrite. The new generations of duplex alloys, which have a composition centered around Fe-26Cr-6.5Ni-3.0Mo, are now being produced with low carbon and a nitrogen addition. These alloys are useful because of their good resistance to chloride SCC, pitting corrosion, and intergranular corrosion in the as-welded condition. (Wahid)61, (Krysiak)14 Choosing the proper welding parameters, balancing alloy compositions to inhibit certain precipitation reactions, shielding molten and hot metal surfaces from reactive gases in the weld environment, removing chromium-enriched oxides, and chromiumdepleted base metal from thermally discolored (heat tinted) surfaces. (Wahid)61, (Krysiak)14 Corrosion resistance of aluminum alloys. Researchers have shown that aluminum alloys, both welded and unwelded, have good resistance to uninhibited HNO3 (both red and white) up to 50 C. Above this temperature, most aluminum alloys exhibit knife-line attack (a very thin region of corrosion) adjacent to the welds. In inhibited fuming HNO3 containing at least 0.1% hydrofluoric acid (HF), no knife-line attack was observed for any commercial aluminum alloy or weldment even at 70 C (160 F). Weldments of non-heat-treatable alloys have good resistance to corrosion. In the case of heat-treatable alloys, corrosion is selective in the weld or in the heat-affected zone. Welding can crack because of mercury–zinc amalgam and residual stresses, e.g., welded crack of Al 7xxx alloys by Hg–Zn amalgam. Stress–corrosion cracking cases in weldments due to residual stresses introduced during welding are rare. Brazed joints in aluminum alloys have good resistance to corrosion, but excessive flux should be removed. Soldered joints are good in milder environment, but not in more aggressive ones.62 Corrosion Resistance of Nickel-based Alloys. Nickel-based alloys are solid solutions based on nickel. Nickel-based alloys used for low-temperature aqueous or condensed systems are generally known as corrosion-resistant alloys (CRA), and nickel alloys used for high-temperature applications are known as heat-resistant alloys (HRA), hightemperature alloys (HTA), or superalloys. The corrosion performance could change due to the presence of second phase or a weld seam. (Rebak)5 The most common failures are associated with oxidation, carburization and metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a

384

Corrosion Prevention and Protection

chromia-forming alloy. Attack by other elements, such as chlorine and sulfur, depends strongly on the partial pressure of oxygen in the environment. (Rebak)5 In general, industrial environments can be divided into two broad categories: reducing and oxidizing. These terms refer to the range of electrode potential that the alloy experiences, and it is controlled by the cathodic reaction in the system. Uniform corrosion can occur under reducing conditions in the active region of potentials and also under oxidizing conditions in the form of a slow, passive corrosion. Localized corrosion, such as pitting and crevice corrosion, generally occurs under oxidizing conditions. Stress–corrosion cracking (SCC) or environmentally assisted cracking (EAC) can occur at any electrochemical potential range. (Rebak)5 Based on the chemical composition, corrosion-resistant nickel-based alloys consist of commercially pure nickel. Ni–Cu alloys, Ni–Mo alloys, Ni–Cr–Mo alloys, and Ni–Cr–Fe alloys.63 The cast versions of the nickel-based alloys do not have the same corrosion resistance as the corresponding wrought products, mainly due to the higher carbon and silicon contents and the anisotropic microstructure of the cast products. (Rebak)5 The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5 6.7.5

Microbiologically Influenced Corrosion

Growth and Metabolism. The most important requirement for microbial growth is the existence and availability of water. Microorganisms take up substances that are dissolved in water (nutrients) and produce cell material.64 Under favorable conditions, some bacteria can double in number every 20 min or less. Thus, a single bacterium can produce a mass of over one million microorganisms in less than 7 h The bacteria as a group can survive from 10 to >100 C, pH 0–10.5, dissolved oxygen (0 to saturation), pressure (vacuum to >31 MPa) and salinity (parts par billion to about 30%). Most bacteria that have been implicated in corrosion grow best at temperatures 15–45 C and pH 6–8. (Dexter)14 A large percentage of these microorganisms can form extracellular polymeric materials termed simply polymer, or slime. The slime helps glue the organisms to the surface, helps trap, concentrate nutrients as food for microbes and shields the organisms from biocides. This slime can change the concentrations of different elements and pH at the electrochemical interface by acting as a diffusion barrier. (Dexter)14 Environments. A great variety of microscopic organisms (microorganisms) are present in virtually all natural aqueous environments, such as bays, estuaries, harbors, coastal and open ocean seawaters, as well as rivers, streams, lakes, ponds, aqueous industrial fluids and waste waters. Larger, macroscopic organisms, such as the well-known barnacles and mussels, are also present in many environments. In natural conditions, sulfate reducing bacteria (SRB) grow in association with other microorganisms and use a range of

The Forms of Corrosion

385

carboxylic acids and fatty acids, which are common by-products of other microorganisms. Biological slimes are commonly found in the water phases of industrial process plants. A wide range of common bacteria (e.g., Pseudomonas and Flavobacterium) can secrete large amounts of organic material under both aerobic and oxygen-free (anaerobic) conditions. (Dexter)14 Biological corrosion in freshwater environments. In fresh water environments, there are bacteria and algae (yeasts and moulds). The organisms attach themselves to and grow on the surface of structural materials, resulting in the formation of a biofilm. The film itself can range from a microbiological slime film on fresh water heat-transfer surfaces to a heavy encrustation of hard-shelled fouling organisms on structures in coastal seawater. It is important to note, that the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)14 Microbial films will affect the general corrosion rate only when the film is continuous. However, this is not frequently the case since micro-organisms form in discrete deposits or colonies, and the resulting corrosion is likely to be localized. A uniform slime film formation on the piping of potable water-handling systems and on the heat-transfer surfaces of low-temperature heat exchangers is inconsequential unless it leads to obstruction of the flow, leading to a health hazard by growth of the organisms or localized corrosion. (Dexter)14 Biological corrosion in marine environments. A heavy fouling of macro-organisms (barnacles, mussels, etc.) decreases the amount of dissolved oxygen at the interface and acts as a barrier on structural steel in the splash zone and shields the metal from the damaging effect of wave action. Also, a continuous film of bacteria, algae, and slime (microorganisms) can have the same beneficial effect as that of the macro-organisms. However, in most cases, these films are not continuous and an oxygen preferential cell is created. Microbial films are suspected of being capable of inducing pit initiation on stainless steels and copper alloys in marine environments. Natural seawater is more corrosive than artificial solutions because of the living organisms. (Dexter)14 Figure 6.30 shows an incomplete coverage by barnacles that is more likely to initiate pitting and crevice corrosion. Effectively, barnacles were attached to the periphery of a high-strength steel rudder, which had originally been coated with an antifouling paint. During use, the paint around the edges had been removed by mechanical action, thus allowing the attachment of barnacles. On the other hand, a continuous film of bacteria, algae, and slime can occur, sometimes providing a barrier film and limiting corrosion; however, it is rare that a film of microorganisms in the marine environment is continuous over larger areas of exposed surface. (Dexter)14 Industries affected. The various industries that have been affected by microbiological corrosion problems are numerous and problems in every industry depend on the material and the characteristics of the medium. Industries affected by microbiologically influenced corrosion include: Chemical processing, energy generation systems, pulp and paper, hydraulic systems, fire protection, water treatment, sewage handling and treatment, highway maintenance, buildings and stoneworks, aviation, underground pipeline and onshore and offshore oil and gas equipment. (Dexter)5 Influence of Some Microbiological Species on Corrosion.64 Some bacteria are involved directly in the oxidation or reduction of metal ions, particularly iron and manganese.

386

Corrosion Prevention and Protection

Figure 6.30 (Dexter)14

An incomplete recovery of high-strength steel rudder by barnacles in ocean water

Some microbes can produce organic acids, such as formic and succinic, or mineral acids such as sulfuric acid. Some bacteria can oxidize sulfur or sulfide to sulfate or reduce sulfates, very often to hydrogen sulfide as end product. (Dexter)14 Sulfate-reducing bacteria are anaerobic (oxygen-free) bacteria that obtain their required carbon from organic nutrients and their energy from the reduction of sulfate ions to sulfide. Sulfate is abundant in fresh waters, seawater, and soils. Sulfide appears as H2S (dissolved or gaseous), HS ions, S2 ions, metal sulfides, or a combination of these, according to the conditions. Sulfides are highly corrosive. (Stott)5 SRB are anaerobic bacteria and facilitate the cathodic reaction which controls the corrosion rate in these media. Considering a buried steel in a soil containing a near-neutral pH solution, the presence of sulfate-reducing bacteria accelerates the electrochemical reaction of corrosion according to the following equations. (Dexter)14 4 Fe ! 4 Fe2þ þ 8 e þ

8H þ 8e ! 8H SO2 4 2þ

Fe Fe

2þ þ

þ SRB

ðcathodicÞ 2

þ 8 H ! S þS

! FeS

ðanodicÞ þ 4 H2 O ðcorrosion productÞ

þ 2 ðOHÞ ! FeðOHÞ2 2

2H þ S

! H2 S

ðcorrosion productÞ

ðpossible gas productÞ

The ferrous sulfide film is not continuous and the base iron can corrode. Hydrogen sulfide can also be produced. The SRB have been identified to contribute to the corrosion of stainless steels, copper and aluminum alloys. (Dexter)14

The Forms of Corrosion

Figure 6.31

387

Association of anaerobic and aerobic bacteria65

Most sulfate-reducing bacteria are obligate anaerobes, yet they are known to accelerate corrosion in aerated environments. This is possible when aerobic organisms form a film or colony and then, through their metabolism, create a microenvironment favorable for anaerobic bacteria. Aerobic organisms near the outer surface of the film consume oxygen and create a suitable habitat for the sulfate-reducing bacteria (SRB) at the metal surface (Figure 6.31) The accompanying flora delivers the nutrients SRB needs, e.g., acetic acid, butyric acid, etc., and consumes the oxygen which is toxic for the SRB.64,65, (Dexter)5, (Dexter)14 Anaerobic corrosion. Desulfovibrio, Desulfotomaculum, and Desulfomonas in anaerobic microenvironments can exist under biodeposits of aerobic organisms, in crevices built into the structure, and at flaws in various types of coating systems. The most commonly encountered SRB type is known as Desulfovibrio. The most corrosive environments are often those in which alternate aerobic–anaerobic conditions exist because of the action of variable-flow hydrodynamics or periodic mechanical action. Conditions at the base of even thin slimes (biofilms) can be ideal for the growth of SRB, with high organic nutrient status, without oxygen, low redox potential, and protection from biocidal agents. (Stott)5 Figure 6.32 explains the corrosion of iron and steel showing the action of sulfate-reducing bacteria (SRB) in removing hydrogen from the surface to form FeS and H2S. (Dexter)5 SRB-induced corrosion is encountered in oil and gas industry.64 Attack by organisms other than SRB. Ammonia and amines are produced by microbial decomposition of organic matter under both aerobic and anaerobic conditions (ammonification). (Stott)5 These compounds are oxidized to nitrite by aerobic bacteria such as Nitrosomonas or Nitrobacter species. Nitrobacter is very efficient at destroying the corrosion-inhibition properties, of nitrate-based corrosion inhibitors by oxidation, unless a biocidal agent is included in the formulation. The release of ammonia at the surfaces of heat-exchanger tubes has a detrimental effect. (Stott)5 Bacteria of the genus Thiobacillus, obtain energy not by oxidation of organic compounds, but by oxidation of inorganic sulfur compounds (including sulfides) to sulfuric acid. The following reactions are performed by mixed cultures of Thiobacilli

388

Corrosion Prevention and Protection

Figure 6.32 Schematic of the corrosion of iron and steel in presence of anaerobic reducing bacteria (SRB) (Dexter)14

acting on elemental sulfur or sulfides. Certain of the Thiobacilli will also leach metal sulfide ores according to the following reaction. 4 FeS2 þ 15 O2 þ 2 H2 O ! 2 Fe2 ðSO4 Þ3 þ 2 H2 SO4 The important point to note is that these bacteria require oxygen and a source of reduced sulfur. The end product is sulfuric acid. The problem of septic sewage systems in hot climates starts with growth of anaerobic SRB in the sewage, producing H2S. This gas migrates to the air space at the top of the line, where it is oxidized into sulfuric acid in the water droplets at the crown of the pipe by Thiobacillus. The corrosion problem is due to the combination of the bacterial action that results in dissolution of the alkaline mortar by the acid, followed by corrosion of the ductile iron. (Stott)5 Corrosion Mechanisms. The influence of microbiological organisms can be the initiation of either general or localized corrosion. This influence derives from the ability of the organisms to change variables such as pH, oxidizing power, velocity of flow, and concentration of chemical species at the metal/solution interface. (Dexter)5 Corrosion can be influenced by microorganisms in the following ways: Production of differential aeration cell: A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5

The Forms of Corrosion

389

Considering pit formation on the surface of iron, the anodic and cathodic reactions are: 2 Fe ! 2 Fe2þ þ 4 e

O2 þ 2 H2 O þ 4 e ! 4 OH

ðanodicÞ

ðcathodicÞ

The insoluble corrosion product Fe(OH)2 can help bacterial film to control the diffusion of oxygen to the anodic sites in the pit. This forms a typical tubercle. If chlorides are present in the aqueous solution, the pH of the solution trapped in the tubercle can become very acid due to the autocatalytic propagation mechanism of localized corrosion due to deposit formation and generation of hydrochloric acid. FeCl2 þ 2 H2 O ! FeðOHÞ2 þ 2 HCl Production of biofilms. The bacteria implicated in corrosion may begin their lives on a metal surface as a scatter of individual cells. As the biofilm matures, however, the organisms are usually found as individuals or in colonies embedded in the matrix of a semicontinuous and highly heterogeneous biofilm (Figure 6.33). (Dexter)5 Microorganisms start on the surface from scattered individual bacteria to thick, semicontinuous films or colonies (slime or polymer) which can influence corrosion. Depending on the velocity of fluid flow, the thickness varies from 10 to 100 mm, and it may cover from less than 20% to more than 90% of the metal surface. Biofilms or macrofouling in seawater can cause redox reactions that initiate or accelerate corrosion. Biofilms accumulate ions, manganese and iron, in concentrations far above those in the surrounding bulk water. They can also act as a diffusion barrier. Finally, some bacteria are capable of being directly involved in the oxidation or reduction of metal ions, particularly iron and manganese. Such bacteria can shift the chemical equilibrium between Fe, Fe2þ, and Fe3þ, which often influences the corrosion rate. (Dexter)5

Figure 6.33

Bacteria cells in a colony 2700 (Dexter)14

390

Corrosion Prevention and Protection

Production of sulfides. This may involve the production of FeS, Fe (OH)2 etc. and an aggressive chemical agent such as hydrogen sulfide (H2S) or acidity. Micro-organisms may also consume chemical species that are important in corrosion reactions (e.g., oxygen or nitrite inhibitors). Alternatively, their physical presence may form a slime or poultice, which leads to differential aeration cell attack or crevice corrosion. They may also break down the desirable physical properties of lubricating oils or protective coatings. (Stott)5 Production of organic and inorganic acids. The sulfur oxidizing bacteria can produce up to about 10% H2SO4. This low pH is highly corrosive to many metals, coatings, ceramics and concrete. Other bacteria can produce organic acids such as formic and succinic acids which are also harmful, and especially to some organic coatings. (Dexter)5 Gases and corrosion processes. Organisms that have a fermentative type of metabolism produce carbon dioxide (CO2) and hydrogen (H2) while others can utilize these gases. Other microbes can use CO2 and H2 as sources of carbon and energy, respectively. Numerous species of bacteria and algae either produce or use oxygen. One series of bacteria can reduce nitrates to nitrogen gas, others can convert nitrates to nitrogen dioxide, or vice versa, or they can break it to ammonia. Some of these gases can cause corrosion. (Dexter)5 MIC of Materials. Many cases have been documented of the biodeterioration by bacteria and/or fungi of architectural building materials, stonework, fiber-reinforced composites, polymeric coatings, and concrete.66 Biodeterioration then proceeds by the processes of staining, patina formation, pitting, etching, disaggregation, and exfoliation. (Dexter)5 Wood and, polymers. Natural materials as well as materials manufactured from plant or animal origin, such as wood, cotton, paper products, wool, and leather, etc., are fully biodegradable under aerobic conditions. (Dexter)5 Plastics are materials that consist mainly of highly polymeric, organic compounds. Also, nondegradable polymer may become degradable by a combined chemical, physical, and biological attack.64,67 Hydrocarbons. This generic term includes crude oils, distillation and cracking products (coal tar), as well as emulsions of these substances. Principally, all these substances are microbially degradable. Since hydrocarbons offer microorganisms good living conditions, microbial growth often causes damage to surrounding materials, e.g., fuel tanks or pipelines.64 Concrete can be damaged by the acids, sulfates, ammona, and other species produced by micro-organisms. Steel reinforcing bars in concrete corrode readily in such environments. Even in the absence of acid attack on the concrete, it has been shown that H2S production by SRB can cause corrosion of the rebar in reinforced-concrete structures. (Dexter)5 Concrete deterioration is caused by bacteria of the genus Thiobacillus. These are bacteria that transform sulfur and/or reduced sulfur compounds by producing and excreting sulfuric acid. The acid reacts with the [Ca (OH)2, CaCO3] to form gypsum (CaSO4), which can be washed out. In this way, sewage pipelines made of concrete are destroyed.64 Forms of corrosion of metals and alloys. It should be noted that organisms are more likely to cause localized than general corrosion because of the differential oxygen cell. In each case, the localized attack was found beneath macrofouling layers. Corrosion of copper, steel, and aluminum anodes was significantly higher when connected to cathodes on which the biofilm was allowed to grow naturally Unexpectedly rapid localized

The Forms of Corrosion

391

corrosion of steel bulkheads in marine harbor environments and of ship hull plating of several tankers has been documented. (Dexter)14 The increase in cathodic kinetics due to the action of biofilms on passive alloy surfaces can also increase the propagation rate of galvanic corrosion. Potentiodynamic polarization studies show that cathodic kinetics are increased during biofilm formation on passive alloy surfaces. Tests on crevice corrosion samples of passive alloys S30400 and S31600 revealed that crevice initiation times were reduced when natural marine biofilms were allowed to form on the exposed external cathode surface. (Dexter)5 Pitting corrosion of integral wing aluminum fuel tanks in aircraft that use kerosenebased fuels has been a problem since the 1950s.The fuel becomes contaminated with water by vapor condensation during variable-temperature flight conditions. Attack occurs under microbial deposits in the water phase and at the fuel–water interface. Cladosporium resinae is usually the principal organism involved: it produces a variety of organic acids (pH 3–4 or lower) and metabolizes certain fuel constituents. These organisms may also act in concert with the slime-forming Pseudomonas to produce oxygen concentration cells under the deposit. Active SRB have sometimes been identified at the base of such deposits.68, (Dexter)5 Hormoconis resinae is a continuing problem in fuel storage tanks and in aluminum integral fuel tanks of aircraft. Brown, slimy mats of Hormoconis resinae may cover large areas of aluminum alloy, causing pitting, exfoliation, and intergranular attack due to organic acids produced by the microbes and the differential aeration cells. The problem of fungal growth in the fuel tanks of jet aircraft has generally diminished as the design of fuel tanks has improved to facilitate better drainage of condensed water and as biocides such as organoboranes have gained acceptance as fuel additives.5 Only those organisms having a high tolerance for copper are likely to have a substantial effect. Thiobacillus thiooxidans, for example, can withstand copper concentrations as high as 2%. Localized corrosion of copper alloys by SRB in estuarine environments has been observed. (Dexter)5 Copper–nickel tubes from the fan coolers in a nuclear power plant were found to show pitting corrosion under bacterial deposits. Slime-forming bacteria acting in concert with iron- and manganese-oxidizing bacteria were responsible for the deposits.5 Monel 400 (66.5% Ni, 31.5% Cu, and 1.25% Fe) tubing was severely pitted after exposure to marine and estuarine waters containing SRB. (Dexter)5 It has been shown that welds provide unique environments for the colonization of SRB with the subsequent production of sulfides that affect the weld seam surface of the heat-affected zone. Exposure of sulfide-derived surfaces to fresh, aerated seawater resulted in rapid spalling on the downstream side of weld seams. The bared surfaces became anodic to the sulfide-coated weld root, initiating and accelerating localized corrosion. (Dexter)5 Microbiological Impacts and Testing. Evidence of harmful impact. Microorganisms, including the corrosion-inducing microorganisms, are present in soils, fresh water, seawater, and air. In most cases, the organisms influence corrosion. However, in a few cases the organisms can reproduce the attack when introduced into a sterile system. (Stott)5 Hydrotesting of fabricated stainless steel structures has often been done with untreated fresh well waters. Such waters may contain micro-organisms, such as gallionella, which cause corrosion.

392

Corrosion Prevention and Protection

Four types of evidence – metallurgical, microbiological, chemical, and electrochemical – are generally used to determine if the influence of the organisms is important enough to invoke a corrosion control program. (Dexter)5 Metallographic evidence. Some types of MIC are recognizable, in part, by the pattern of corrosion products on the surface. Microbiological evidence. The data must be gathered while the corrosion site is still wet. It is important to photograph the initial appearance of the corrosion site soon, while the organisms are still alive. Analysis of biological materials and corrosion products must be done. Chemical evidence. Detailed chemical analysis should be done for the corrosion products and any biological mounds present at or near the corrosion site. Evaluation of the chemistry of the liquid phase and its variability, both spatially and with time in relation to the observed corrosive attack is necessary. Details should include the color, texture, odor, and distribution of the materials as well as their organic and inorganic chemistries. (Dexter)5 The color change of corrosion deposits from black to brown is, in itself, a good indication of sulfide corrosion product. (Stott)5 Tests for sulfate, total organic carbon (TOC), pH, sulfide and oxygen concentration are also useful indicators of the potential for SRB growth. (Stott)5 Electrochemical evidence. Conventional techniques such as variation of corrosion potential and corrosion rates at different periods. Noise electrochemical techniques should be very fruitful for monitoring corrosion evolution. (Little)5 Monitoring schemes. An effective monitoring scheme for controlling both biofouling and biocorrosion should include the generation of as many of the following types of data as possible.69,70, (Dexter)5 Sessile bacterial counts of the organisms in the biofilm on the metal surface done by either conventional biological techniques or optical microscopy.71, (Stott)5 Direct observation of the community structure of the biofilm. This can be done on metal coupons made from the same alloy used for the system Electrochemical corrosion measurements using electrical resistance or polarizationresistance-types probes. Electrochemical noise measurements should give an interesting dimension, but this is not a routine technique. Water quality and oxidation–reduction potential measurements. Identification of the microorganisms found in both the process water and on the metal surface. Tests for nitrite-utilizing bacteria or sulfur oxidizers are more complex. (Stott)5 Identification and analysis of corrosion products and biofilms. Evaluation of the morphology, form and type of corrosion after removal of biological and corrosion product deposits. A group of researchers at Shell Petroleum Company have developed an approach to risk assessment of carbon steel pipelines, based on the details of water chemistry and operation parameters.72, (Stott)5 Prevention. The general approaches to maintaining a system free of biocorrosion problems vary with the materials of construction, environment, economics, and duty cycle of the equipment. The most common approaches involve the use of sterilization,

The Forms of Corrosion

393

coatings, cathodic protection and appropriate selection of materials. The most important step in prevention is to start with a clean system and to keep it clean. Sterilization by physical methods such as irradiation (gamma or UV) for disinfection of materials and environments.64 Sterilization by chemical methods. Biocidal action has been widely used for many years to control biofilm formation in closed systems, such as heat exchangers, cooling towers, and storage tanks.64, (Dexter)5 6.7.6

Mechanically Assisted Corrosion

Mechanically assisted degradation can consist of the following types of corrosion: erosion–corrosion, water drop impingement erosion, cavitation erosion, erosive and corrosive wear, fretting corrosion, and corrosion fatigue (Figure 6.34). Erosion–corrosion phenomena are generally corrosion processes enhanced by erosion or wear. Erosive and corrosive wear are strong combinations of these phenomena of degradation because of suspended solid particles in the flow. Fretting corrosion is a wear process enhanced by corrosion. Corrosion fatigue appears in the presence of combined actions of a fluctuating or cyclic stress and a corrosive environment. Corrosion and Wear. The progressive deterioration, due to corrosion and wear, of metallic surfaces in use in major industrial plants ultimately leads to loss of plant efficiency and in the worst case to shutdown. For example, corrosion and wear damage to materials, both directly and indirectly, cost the United States economy almost $300 billion per year at current prices. Similar studies on wear failure have shown that the wear of materials costs the U.S. economy about $20 billion per year

Figure 6.34

Some corrosion types of mechanically assisted degradation3

394

Corrosion Prevention and Protection

(in 1978 dollars) compared with about $80 billion annually for corrosion during the same period.31,73 The combination of wear or abrasion and corrosion results in more severe attack than with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosion corrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, devoid of mechanical action. Although corrosion can often occur in the absence of mechanical wear, the opposite is rarely true. Corrosion accompanies the wear process to some extent in all environments, except in vacuum and inert atmospheres. Corrosion and wear often combine to cause extensive damage in a number of industries, such as mining, mineral processing, chemical processing, pulp and paper production, and energy production. Wear debris and corrosion products that are formed during crushing and grinding of mineral ores (comminution) affect product quality and can adversely affect subsequent beneficiation by altering the chemical and electrochemical properties of the mineral.31 Exposure to the environment (gases and humidity) affects mechanical properties, friction, and wear of polymers. Most of the time, synergistic effects between abrasion, wear and corrosion are created and that amplifies the damage.74,75 Dunn76 has summarized the dominant and synergistic influence of every factor as follows. Abrasion: removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents, forming sites for impact hydraulic splitting. Plastic deformation by high-stress metalmineral contact causes strain hardening and susceptibility to chemical attack. Wear impact: plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phases changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. Corrosion influence: produces pits that induce microcracking. Microcracks at pits invite hydraulic splitting during impact. Roughens surface, reducing energy needed to abrade metal. May produce hydrogen with subsequent absorption and cracking in steel. Selectively attacks grain boundaries and less noble phases of multiphase microstructures, weakening adjacent metal. Wear damage mechanisms. Wear is the surface damage or removal of material from one or both of two solid surfaces in a sliding, rolling or impact motion relative to one another. Wear damage precedes actual loss of material, and it may also occur independently. Strictly, wear, as in the case of friction, is not an inherent material property; it depends on the operating conditions and surface conditions. Wear rate does not necessarily relate to friction. Wear occurs by mechanical and/or chemical means and is generally accelerated by frictional heating. Various regimes of mechanical (plastically

The Forms of Corrosion

395

dominated) and chemical (oxidation) wear for a particular sliding material pair are observed on a single-wear-regime mode.75,77,78 Principal wear mechanisms include: (1) adhesive, (2) abrasive, (3) fatigue, (4) impact by erosion and percussion, (5) chemical, and (6) electrical-arc-induced wear. Other, not distinct, mechanisms are fretting, fretting corrosion and fretting corrosion fatigue, a combination of adhesive, corrosive, and abrasive forms of wear. Wear by all mechanisms, except by the fatigue mechanism, occurs by gradual removal of material. Of the aforementioned wear mechanisms, one or more may be operating in any particular machine. In many cases, wear is initiated by one mechanism and it may proceed by other wear mechanisms, thereby complicating failure analysis.75 Adhesive wear. Adhesive wear occurs because of adhesion at asperity contacts at the interface. These contacts are sheared by sliding which may result in detachment of a fragment from one surface to another surface. Some are fractured by a fatigue process during repeated loading and unloading, resulting in formation of loose particles. During sliding, surface asperities undergo plastic deformation and/or fracture. The subsurface, up to several micrometers in thickness, also undergoes plastic deformation and strain hardening with microhardness, by as much as a factor of two or higher, than the bulk hardness.75 Abrasive wear occurs when asperities of a rough, hard surface or hard particles slide on a softer surface, and damage the interface by plastic deformation or fracture in the case of ductile and brittle materials, respectively. In many cases, the wear mechanism at the start is adhesive, which generates wear particles that get trapped at the interface, resulting in three-body abrasive wear. In most abrasive wear situations, scratching is observed with a series of grooves parallel to the direction of sliding.75 Fatigue wear. Subsurface and surface fatigue are observed during repeated rolling (negligible friction) and sliding (coefficient of friction 0.3), respectively. The repeated loading and unloading cycles to which the materials are exposed may induce the formation of subsurface and surface cracks, which eventually, after a critical number of cycles, result in the breakup of the surface with the formation of large fragments, leaving large pits on the surface. In this mode, negligible wear takes place, since wear does not require direct physical contact between two surfaces. Mating surfaces experience large stresses, transmitted through the lubricating film during the rolling motion, such as in well-designed rolling element bearings. The failure time in wear fatigue is statistical in nature. Chemically induced crack growth (most common in ceramics) is commonly referred to as static fatigue. In the presence of tensile stresses and water vapor at the crack tip in many ceramics, a chemically induced rupture of the crack-tip bonds occurs rapidly, which increases the crack velocity. Chemically enhanced deformation and fracture results in an increased wear of surface layers in static and dynamic (rolling and sliding) conditions.75 Prevention of wear fatigue corrosion involves inherent physical properties of the alloy, for example, a gear must be tough and fatigue resistant yet have a surface that resists wear. For applications requiring only a moderate degree of impact strength, fatigue resistance and wear resistance, higher-carbon through-hardening steel may be sufficient. For more severe conditions, however, surface-hardened steel may be used.31 Impact wear includes erosive and percussive wear. Erosion can occur by jets, liquid droplets, and implosion of bubbles formed in the fluid and streams of solid particle. Solid

396

Corrosion Prevention and Protection

particle erosion occurs by impingement when discrete solid particles strike a surface, and the contact stress arises from the kinetic energy of particles flowing in an air or liquid stream as it encounters a surface. Wear debris formed in erosion occurs as a result of repeated impacts.75 Neighboring particles may exert contact forces, and a flowing fluid, if present, will cause drag. Under some conditions, gravity may be important. As in the case of abrasion, erosive wear mechanisms can involve both plastic deformation and brittle fracture. The particle velocity and impact angle combined with the size of the abrasive particle gives a measure of the kinetic energy of the impinging particles, that is proportional to the square of the velocity. Ductile materials will undergo wear by a process of plastic deformation in which the material is removed by the displacing or cutting action of the eroded particle. Some brittle materials undergo wear predominantly either by flow or by fracture, depending on the impact conditions. Wearrate dependence on the impact angle for ductile and brittle materials is different.75,79 Slug flow is the dominant flow regime in multiphase systems. Flow visualization has shown that bubbles distort and elongate in the vicinity of the pipe wall in a manner similar to collapsing bubbles. The corrosion rate increases due to a thinning of the mass transfer and corrosion product layers, as well as due to localized damage of the corrosion product film. Multiphase environments can also occur in several industries. Change in pressure and temperature in process equipment and the mixing of various streams can force the mixture into two- or three-phase situation. The associated corrosion is of concern in nuclear and thermal power plants, chemical process industries, and in waste management. Slug flow can occur sooner in smaller diameter pipe, where the coalescence of bubbles can cause a transition from bubble flow to slug flow. Slug flow effects corrosion in two ways. First, a dramatic increase in the turbulent intensity that can increase the mass transport of corrosive species by 1000 times. In addition, extensive damage to the corrosion product layer occurs and exposes the surface to more attack.80 Percussion is a repetitive solid body impact, such as experienced by print hammers in high-speed electromechanical applications and high asperities of the surfaces in a gas bearing. Repeated impacts result in progressive loss of solid material. Percussive wear occurs by hybrid wear mechanisms, which combine several of the following mechanisms: adhesive, abrasive, surface fatigue, fracture, and tribochemical wear.75 Chemical or corrosive wear. Corrosive wear occurs when sliding takes place in a corrosive environment. In air, the most dominant corrosive medium is oxygen. Therefore, chemical wear in air is generally called oxidative wear. In the absence of sliding, the chemical products of the corrosion (e.g., oxides) would form a film typically less than a micrometer thick on the surfaces, which would tend to slow down or even arrest the corrosion, but the sliding action wears the chemical film away, so that the chemical attack can continue. Thus, chemical wear requires both chemical reaction (corrosion) and rubbing. Corrosion occurs in a highly corrosive environment and in high-temperature and high-humidity environments. Corrosive fluids may provide a conductive medium necessary for electrochemical corrosion to occur on the rubbing surfaces. The most common liquid environments are aqueous, and here small amounts of dissolved gases, commonly oxygen or carbon dioxide, influence corrosion.75 Chemical wear is important in a number of industries, such as mining mineral processing, chemical processing, and slurry handling. A typical example of a corroded roller subsequent to running in a bearing is shown in Figure 6.35. The corrosion left a

The Forms of Corrosion

397

Figure 6.35 SEM micrograph of 52 100 quenched and tempered roller bearing after corrosive wear75

multitude of dark-bottomed pits, the surroundings of which are polished by running. The condition subsequently creates extensive, surface-originated spallings from a multitude of initiated points.75 Friction modifies the kinetics of chemical reactions of sliding bodies with each other, and with the gaseous or liquid environment, to the extent that reactions which occur at high temperatures occur at moderate, even ambient, temperatures during sliding. Chemistry dealing with modification of chemical reaction by friction or mechanical energy is referred to as tribochemistry, and the wear controlled by this reaction is referred to as tribochemical wear.81 The mechanisms by which friction increases the rate of chemical reaction (tribochemistry) are frictional heat, removal of product scale resulting in fresh surfaces, accelerated diffusion and direct mechanochemical excitation of surface bonds. The tribochemical reactions result in oxidative wear of metals and tribochemical wear of ceramics.75 Oxidative wear. Interface temperatures produced at asperity contacts during sliding of metallic pairs under nominally unlubricated conditions result in thermal oxidation, which produces oxide films several micrometers thick. The oxidation is generally a beneficial form of corrosion. A thick oxide film reduces the shear strength of the interface, which suppresses the wear as a result of plastic deformation.81 In many cases, tribological oxidation can reduce the wear rate of metallic pairs by as much as two orders of magnitude, compared with that of the same pair under an inert atmosphere. Tribological oxidation can also occur under conditions of boundary lubrication when the oil film thickness is less than the combined surface roughness of the interface. The oxidation can prevent severe wear. In oxidation wear, debris is generated from the oxide film.75 At low ambient temperatures, oxidation occurs at asperity contacts due to frictional heating. At higher ambient temperatures, general oxidation of the entire surface occurs and affects wear. In the case of steels, the predominant oxide present in the debris

398

Corrosion Prevention and Protection

depends on the sliding conditions. At low speeds and ambient temperatures, the predominant oxide is a-Fe2O3, at intermediate conditions it is Fe3O4, and at high speeds and temperatures it is FeO.81 Oxygen and other molecules are adsorbed on clean metals and ceramic surfaces, and form strong chemical bonds with them. Since this reaction is controlled by the diffusion of the reacting species through the layers of corrosion product, oxidation of iron and many metals follows a parabolic law, with the oxide film thickness increasing with the square root of time.75 h ¼ Ct1=2

ð6:1Þ

where h is the oxide film thickness, t is the average growth time, and C is the parabolic rate constant at elevated temperatures. Since diffusion is thermally activated, the growth rate in oxide film thickness during sliding as a function of temperature, similar to thermal oxidation under static conditions, follows an Arrhenius type of relationship K ¼ AeðQ=RTÞ

ð6:2Þ

where K is the parabolic rate constant for the growth of the oxide film, A is the parabolic Arrhenius constant (kg2/m4s) for the reaction, Q is the parabolic activation energy associated with oxide (kJ/mol), R is the universal gas constant and T is the absolute temperature of the surface. It has been reported that the Arrhenius constant for sliding is several orders of magnitude larger than that for static conditions. The oxidation rate during sliding may result from increased diffusion rates of ions through a growing oxide film, which generally involves a high defect density due to mechanical perturbations.75 Electric-arc-induced wear. When a high potential is present over a thin air film in a sliding process, a dielectric breakdown results, leading to arcing. During arcing, a relatively high-power density occurs over a very short period. The heat-affected zone is usually very shallow (of the order of 50 mm) and the heating results in considerable melting and subsequent resolidification, corrosion, hardness changes, and other phase changes, and even in the direct ablation of material. Arcing causes large craters, and any sliding or oscillation after an arc either shears or fractures the lips, leading to three-body abrasion, corrosion, surface fatigue and fretting.75,82,83 Erosion–corrosion. Generally, all types of corrosive media can cause erosion–corrosion, including aqueous solutions, organic media, gases, and liquid metals. The corrodent can be a bulk fluid, a film, droplets or a substance adsorbed on or absorbed on another substance. For example, hot gases may oxidize a metal at high velocity and blow off an otherwise protective scale. Solid suspensions in liquids (slurries) are particularly destructive from the standpoint of erosion corrosion.16,31 All types of equipment, exposed to fluids in motion, are subject to the erosion– corrosion phenomena. This can include pipeline networks (particularly curves, elbows and T-squares), floodgates, pumps, centrifugal fans, helixes, wheels of turbine, tubes of intersections of heat exchangers and measuring devices. In many cases, failures due to erosion–corrosion occur in a relatively short time.16 Most metals are susceptible to erosion–corrosion in the liquid phase under specific conditions. Resistance of metals depends on the physical and chemical properties of the corrosion product and/or the

The Forms of Corrosion

399

Figure 6.36 Erosion corrosion of a pump propeller in Cu–10Sn–2Zn alloy1

passive layers and their adhesion to the metallic surface. Passivating metals such as stainless steels and titanium are relatively immune to erosion–corrosion in many oxidizing environments. A stainless steel pump impeller with an expected lifetime of two years failed in three weeks in a reducing solution. Metals that are soft are readily damaged or worn mechanically; examples are copper and lead. Even the noble or precious metals, such as silver, gold, platinum, are subject to erosion–corrosion.16,31 Impingement is a type of erosion–corrosion in which the liquid is in turbulent flow, containing bubbles of air and suspended particles that can hit the metallic surface strongly and destroy its protective film. The shock of bubbles of water or air against metal provokes wear of the surface. It takes the shape of a directional progress of the attack. The bottom portion becomes anodic in relation to the adjacent outer surfaces. The attack can also occur in this way when the solution does not contain bubbles of air or particles in suspension. Figure 6.36 shows a typical attack by impingement.1,25 Erosion– corrosion is characterized by the appearance of grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern. The Figure 6.37 gives a section of

Figure 6.37

Schematic of erosion–corrosion of a condenser tube wall31

400

Corrosion Prevention and Protection

the metallic surface, showing the resulting attack where the protective film on the tube was disrupted.31 Agitation or circulation of the electrolyte leads to an increase in corrosion because of: Destruction or prevention of the formation of a protective film. (Froats)14 Increase in the rate of diffusion of aggressive ions due to the speed of the fluid decreases the cathodic polarization and increases the corrosion current density as in the case of steels in the presence of oxygen, carbon dioxide or bisulphate. However, the speed of the fluid can decrease the attack in some systems through: Improving the efficiency of inhibitors, while bringing them more quickly to the metal– solution interface. Forming the passive layer for an active metal and that can reduce corrosion rate. An increase in the flow speed permits to supply oxygen at the interface and attain the critical passive current for alloy in a given medium. Sweeping away corrosive ionic species. The attack of stainless steel decreases as the solution flow speed increases. Stainless steel in nitric acid without agitation, is attacked autocatalytically because of the formation of nitrous acid, as a cathodic reaction product. Agitation and circulation at certain speeds of the medium can avoid pitting and crevice corrosion.16 Turbulence. When the rate of the flow of a fluid becomes very large, another form of attack occurs by the combined action of erosion and corrosion. In general, the greater the speed, the greater the abrasive action of the flow. The speed strongly influences mechanisms intervening in corrosion. In the majority of cases, erosion–corrosion is due to the turbulence of the flow and this influences the chemical and mechanical stability of surface films. The type of flow (laminar or turbulent) depends on the rate and the quantity of transported fluid and also on the geometry or design of the equipment. Flow in a straight line is less damaging. Turbulence results in a more intimate contact between the environment and metal. Other factors such as edges, cracks, deposits, abrupt changes of section, and other obstacles disrupting the laminar flow contribute to the turbulence and the attack by impingement.16 Figure 6.38 describes schematically how turbulent eddies probably thin the protective film locally to account for downstream undercutting.84 Elevated speeds have a marked effect on wear, and this is more pronounced if the solution contains some solid particles in suspension. Aluminum forms films of aluminum nitrate or oxide in fuming nitric acid. At low flow rates there is no attack whereas for speeds greater than 1.22 m s1, the protective layer is removed and erosion–corrosion occurs more readily.16 Galvanic effect. The galvanic cell between two different metals can have serious effects in a flowing system. For example, the galvanic cell was not present between stainless steel type 316 and lead in 10% sulfuric acid under static conditions, but when the flow rate increased to 11.89 m s1, the rate of erosion–corrosion increased enormously

The Forms of Corrosion

(a)

(b)

(c)

(d)

401

Figure 6.38 Erosive and corrosive wear by turbulent eddy mechanism for downstream undercutting pits84

because of the destruction of the passive film by the combined effect of galvanic corrosion and erosion corrosion.16 Water droplet impingement erosion. In liquid impingement erosion, with small drops of liquid striking the surface of a solid at high speeds (as low as 300 m s1), very high pressures are experienced, exceeding the yield strength of most materials. Thus, plastic deformation or fracture can result from a single impact, and repeated impact leads to pitting and erosive wear.75 Water droplet impingement causes pitting and may cause cavitation damage. Damage may appear to be somewhat different from cavitation damage in ductile materials. The cavities in the surface show a directionality that is related to the angle of the attack of the drop, as in erosion–corrosion. Two areas most vulnerable for this type of corrosion are steam turbines and water rotor blades. In turbines, condensation of steam produces droplets that are carried into the rotor blades and causes surface damage. Rain drop erosion on helicopter blades can generate tensile stresses just below the surface and leads to cracking. (Glaeser and Wright)14 Prevention. The following measures can be taken to reduce or prevent erosion– corrosion: 1. A proper geometric design of the system in order to get a laminar flow with minimum turbulence, as in pipelines of large diameters and avoid abrupt changes or streamline bends. 2. The composition of a metal largely determines its resistance to corrosion. A noble metal or alloy has an inherent high resistance to corrosion, while for a more reactive material its resistance to corrosion depends mainly on its ability to form and to maintain a protective film.16 The protective nature and properties of the film on some metals and alloys are very important for the resistance to erosion–corrosion. The chemical composition has a great influence on the metal properties. The addition of a third element to the alloy often increases the resistance to erosion–corrosion through the formation of a strong passive or protective film. For example, an addition of 2% Al to brass to form a brass–aluminum alloy and an addition of 1.2% Fe to cupronickels: in the two cases, the addition of an element to the alloy produces a marked increase of the resistance to impingement. It has been shown that the high performance of

402

6. 7.

Corrosion Prevention and Protection

condensation tubes in brass–aluminum against the impingement due to the presence of iron in the protective film, coming from products of the corrosion in water.1 Also, the addition of Mo to 18-8 stainless steel to form steel type 316 is widely known for its resistance. Toughness can influence the performance of materials under conditions of erosion– corrosion. The soft metals are often more susceptible to erosion corrosion because they are more susceptible to mechanical wear. The toughness is a good criterion for the resistance to the mechanical erosion or abrasion, but this is not necessarily a good criterion to predict the resistance to the erosion–corrosion. Stellite (Co–Cr–W–Fe–C alloy), which has better toughness than 18-8 stainless steel, showed better resistance to cavitation erosion on a water brake.25 There are several ways to harden alloys. A certain procedure to increase the resistance to the erosion corrosion is the hardening by solid solution. One adds an element to another to produce a solid solution that is resistant to the corrosion by hardening the metal. The thermal treatment is also a method to harden a metal or alloy, but it changes the microstructure and can induce a greater susceptibility to corrosion. Hardening by cold work is also an important procedure and it is the reason for using stainless steel to resist cavitation erosion. This material, initially hard, attains an even harder surface by cold work and becomes more resistant to attack and erosion. Modification of the environment. Deoxygenation and the addition of inhibitors are useful methods to decrease the aggressiveness of a flowing liquid environment; they are not economical, even though they minimize erosion corrosion. One can also filter out the particles in suspension. The temperature should be lowered when possible. Applying hard, tough protective coatings with resilient materials such as rubber and some plastics is recommended.31 Cathodic protection is efficient in order to reduce the electrochemical attack.

Cavitation is a form of corrosion that is caused by the repeated nucleation, growth, and violent collapse of vapor bubbles in a liquid against a metal surface. Cavitation erosion arises when a solid and fluid are in relative motion, and bubbles formed in the fluid become unstable and implode against the surface of the solid. Cavitation erosion is similar to surface wear fatigue.75 A metal having undergone a cavitation erosion has an appearance similar to a pitted metal.31 Figure 6.39 shows an example of damage caused by cavitation erosion on a cylinder of diesel motor. The appearance of cavitation is similar to pitting, except that surfaces in the pits are usually much rougher. The affected region, if observed immediately, is free of deposits and accumulated corrosion products. One uses the term cavitation corrosion when the mechanical effect of the cavitation is limited to destruction of the oxide layer formed by corrosion. On the other hand, if the mechanical effect damages metal appreciably, it is rather cavitation erosion. Damage by these processes is found in components such as ship propellers, runners of hydraulic turbines, centrifugal pumps, pump impellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. Cavitation erosion. In certain conditions, a thin layer of liquid, nearly static at the metal–liquid interface can prevent impingement of the surface by the turbulent flow of the liquid. However, due to the turbulence of water, bubbles of air or gas of size larger

The Forms of Corrosion

403

Figure 6.39 Cavitation erosion damage of a cylinder liner of a diesel engine7

than the thickness of the layer can interrupt, break the border layer and cause a continuous rupture of the protective film. A film of semiconductor oxide on the surface as Cu2O, surrounding the damaged areas, produces a big cathode for reduction of the dissolved oxygen on the surface and causes pitting. Impacting bubbles. When bubbles collapse that are in contact with or very close to a solid surface, they collapse asymmetrically. Consider a spherical bubble impacting a

404

Corrosion Prevention and Protection

Pressurized

Metal (c) Compression Collapse of vapor bubbles

Metal oxide (d)

Approaching microjet torpedo

Destruction of metal oxide on impact

Repair of metal oxide at expense of metal

Figure 6.40 Schematic representation of the disintegration of protective corrosion product by impacting microjet (torpedo)31

plane solid surface. The bubble becomes elongated with a tail, and then collapses. The jet from the bubbles is believed to cause cavitation erosion on a solid wall.85 As shown in Figure 6.40, implosion of a vapor bubble creates a microjet of liquid or microscopic ‘torpedo’ of water that is ejected from the collapsing bubble at velocities that may range from 100 to 500 m s1. When the torpedo impacts on the metal surface, it dislodges protective surface films and/or locally deforms the metal itself. The solid material will absorb the impact energy, resulting in elastic or plastic deformation or even fracture. The latter two processes may cause localized deformation and/or erosion of the solid surface. Douglas et al.86 suggest that the bubble grows and collapses, giving rise to high pressure for a few milliseconds. The local pressures observed during those few milliseconds may be up to 4000 atmospheres, with a temperature increase up to 800 C. This accelerates the corrosion rate. Fresh surfaces are exposed to corrosion, followed by reformation of protective films, which is followed by more cavitation. This cycle repeats itself.31,75 Single-phase flow may have a definite pattern, whereas several flow patterns exist for multiphase flows. Multiphase flow involves the simultaneous flow of more than one phase within a pipe as for example: oil/water or three-phase oil/water/gas flows in carbon steel pipelines have an important influence on the form and kinetics of corrosion. Slug flow is the dominant flow regime in multiphase systems. It involves unique flow mechanisms with pulses of gas bubbles being released into a turbulent mixing zone due to a mixing vortex behind the slug front. These bubbles impact and collapse on the pipe wall, causing severe localized, cavitation type corrosion, which can be up to 1000 times the values normally seen in other flow regimes.80,85,86

The Forms of Corrosion

405

Prevention. The following preventive measures can be used in addition to the measures given under erosion–corrosion: 1. Cavitation can be minimized by a proper design to minimize hydrodynamic pressure differences and specifying a smooth finish on all critical metal surfaces. One can use higher pressure or lower temperature to reduce the formation of the damaging steam bubbles causing cavitation. Proper operation of pumps and equipment is necessary, for example, a pump should not be operated on plugged or impaired flow lines.84 2. Cathodic protection is sometimes beneficial to avoid cavitation, not because of the reduced rate of corrosion but because of the cushioning effect of hydrogen evolved on the surface. Removal of dissolved air is often beneficial because dissolved gases more easily nucleate cavitating bubbles at low pressures. 3. The hard, tough metals or elastomeric polymers may be used to resist the cavitation erosion. The physical property intervening here is the resilience, which is the capacity to dispose the energy of an impact without absorbing some. Another property of the rubber that can also have an influence on the resistance to cavitation erosion is its resistance to abrasion.25 Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations (often, the relative movement is barely discernible ranging from a few tens of nanometers to a few tens of micrometers). Fretting occurs where low-amplitude oscillatory motion in the tangential direction takes place between contacting surfaces, which are nominally at rest.87,89, (Waterhouse)88 It is necessary that the load be sufficient to produce a distortion of surfaces. This is a common occurrence, since most machinery is subjected to vibration, both in transit and in operation. Figure 6.41 shows a typical example. Pressed-on wheels can often fret at the shaft/wheel hole interface.31 Oxidation is the most common factor in the fretting process. In oxidizing systems, fine metal particles removed by adhesive wear are oxidized and trapped between the fretting surfaces. The oxides act like an abrasive (such as lapping red) and increase the rate of

Figure 6.41 Example of typical fretting corrosion location16

406

Corrosion Prevention and Protection

material removal. The red material, easily lost from between the contacting surfaces, is an example of this type of fretting in ferrous alloys. Examples of vulnerable components are shrink-fits, bolted parts, and splines. The contacts between hubs, shrink- and press-fits, and bearing housings on loaded rotating shafts or axles and many parts of vibrating machinery are particularly prone to fretting damage. Flexible couplings and splines, particularly where they form a connection between two shafts and are designed to accommodate some misalignment, can suffer fretting wear.75 Fretting corrosion is frequently observed between the crown of a ball bearing and its axle, or the head of a screw and the metallic surface, and in jewel bearings, elements of machines in movement, suspension springs, electric relay contacts, king pins of auto steering mechanisms.7,90 Fretting corrosion is characterized by discoloration and takes the form of local surface dislocations and deep pits. It is at such pits that fatigue cracks eventually nucleate. These occur in regions where slight relative movements have occurred between mating, highly loaded surfaces.31 With time, fretting corrosion can cause a tarnished appearance of the metallic surface and a variation of piece sizes. Products of corrosion can also cause blockages in machines in movement. As examples, these products are (NiO þ a little Ni) for Ni, (Cu2O þ lesser amounts of CuO and Cu) for Cu; and (brown Fe2O3 þ a small quantity of iron powder) for steel, (particles of black alumina with a core of metallic aluminium) for Al.7,90 Figure 6.42 shows the scanning electron micrograph of a 303 stainless steel shaft after it underwent fretting corrosion. Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to Fe2O3, and the

Figure 6.42 corrosion75

Scanning electron micrograph of 303 stainless steel shaft surface after fretting

The Forms of Corrosion

407

Surface Oxide Bare Metal Metal and Oxide Debris

Figure 6.43

Schematic of the fretting process showing the final effects of wear and oxidation19

characteristic fine reddish-brown powder is produced, known as ‘cocoa’. These oxide particles are abrasive. Because of the close fit of the surfaces and the oscillatory smallamplitude motion (of the order of a few tens of micrometers), the surfaces are never brought out of contact, and therefore there is little opportunity for the products of the action to escape. Further oscillatory motion causes abrasive wear and oxidation, and so on. Therefore, the amount of wear per unit sliding distance due to fretting may be larger than that from adhesive and abrasive wear.75 Two approaches are considered, depending on what phenomenon initiates and propagates the damage: wear-oxidation and oxidation-wear.91 In wear-oxidation two surfaces are in contact; and they are never perfectly smooth. They are in contact through their asperities and the relative displacement of the two surfaces entails the wear of their crests.92 It occurs as a type of cold welding or fusion at the interface of metal surfaces under pressure. During displacements, these points of contact break and fragments of metal are produced. These fragments are very small and immediately oxidize because of the heat generated due to friction. This phenomenon repeats itself and residues accumulate (Figure 6.43). Subsequent oxidation of the damaged material has a secondary effect. Wear by fretting without debris of oxides has been observed for noble metals, mica, glass, etc.16 In oxidation-wear, most the metallic active surfaces are first protected from atmospheric oxidation by a thin layer of adherent oxide. When metals are in contact (under load) and subjected to repeated weak movements, the layer of oxide is broken at the level of asperities and it removes some oxide particles. The fresh exposed surfaces oxidize all over again and the process repeats itself. The basis of this approach is that the accelerated oxidation is considered to be due to the effects of friction.90 A more plausible theory could be a combination of the two previous approaches with the relative importance of one or the other depending on the particular system, and is therefore a function of the medium, surface finishing and the nature of materials in contact. However, in both approaches the presence of oxygen accelerates the corrosion by fretting, especially for the ferrous alloys.16 Evidently, fretting is worse in air than in an inert atmosphere.25 There is less damage in a humid atmosphere than in dry air since humidity can have a lubricant action, and the hydrated oxides are less hard than the dry oxides.25,90,93 Surfaces subjected to fretting wear have a characteristic appearance with red-brown patches on ferrous metals and adjacent areas that are highly polished because of the lapping quality of the hard iron oxide debris. There appears to be no critical measurable amplitude below which fretting does not occur. However, if the deflection caused is only elastic, it is not likely that fretting

408

Corrosion Prevention and Protection

damage can occur. A rapid increase in wear rate occurs with slip amplitude over a certain amplitude range. For a given slip amplitude, the amount of wear per unit sliding distance per unit applied normal load increases linearly with a number of oscillating cycles up to an amplitude of about 100 mm. Above this amplitude, wear rate per unit sliding distance becomes constant.75 The fretting wear rate is directly proportional to the normal load for a given slip amplitude. However, in the total-slip situation, the frequency has little effect. (Waterhouse)88 In a partial-slip situation, the frequency of oscillation has little effect on wear rate per unit distance in the low-frequency range, whereas the increase in the strain rate at high frequencies leads to increased fatigue damage and increased corrosion due to a rise in temperature.75 However, the effect of temperature on fretting depends on the oxidation characteristics of the metals. If increased temperature results in the growth of a protective, tough oxide layer that prevents metal-to-metal contact, the fretting rate is lower. An incubation period is observed when fretting wear is negligible, followed by a steady rate. (Glaeser and Wright)5 A rapid increase in wear rate with slip amplitude occurs over a range of amplitude. Wear debris is generally classified as plate-shaped, ribbon-shaped, spherical and irregularly shaped, based on morphology. Wear of a material is dependent on the mating material (or material pair), surface preparation and operating conditions. Clean metals and alloys exhibit high adhesion, and consequently high friction and wear. Any contamination mitigates contact, and chemically produced films, which reduce adhesion, result in reduction in friction and wear. In dry sliding, identical metals, particularly iron on iron, are metallurgically compatible and exhibit high friction and wear and they must be avoided. Soft and ductile metals such as In, Pb, and Sn exhibit high friction and wear. Hexagonal metals such as Co and Mg as well as some nonhexagonal metals such as Mo and Cr exhibit low friction and wear. Lead-based white metals (babbitts), brass, bronze, and gray cast iron generally exhibit relatively low friction and wear, and are commonly used in dry and lubricated bearing and sea applications. For high-temperature applications, cobalt-based alloys are used which exhibit good galling resistance. Galling or welding resistance is a measure of normal stress at which two materials loaded against each other. Nickel-based alloys are poor in unlubricated sliding because of generally catastrophic galling.75 Modeling fretting corrosion. An equation has been used for steel to evaluate the loss of weight W caused by fretting corrosion based on a model that combines the chemical and mechanical effect of the corrosion by fretting. The chemical factor concerns the oxidation that occurs at the time of wear, corresponding to adsorption of oxygen to form the oxide. The mechanical factor concerns the loss of particles, at the asperities on the opposite surface. C ð6:3Þ W ¼ ðK0 L1=2 K1 LÞ þ K2 lLC f chemical factor

mechanical factor

where L ¼ charge between surfaces, C ¼ number of cycles, f ¼ frequency of movements, l ¼ slip and K0 ; K1 ; K2 are constants. The chemical factor decreases with increase in frequency of movement because there is less available time for the chemical reaction. The mechanical factor is a function of the slip

The Forms of Corrosion

409

and the load. In an atmosphere of nitrogen only the mechanical factor is important for the wear of steel debris on the metallic iron powder, and W is independent of the frequency.7 Fretting corrosion fatigue. Fatigue failures traceable to fretting corrosion are found in many aircraft engine parts, such as connecting rods, knuckle pins, splined shafts, clamped and bolted flanges, couplings, and many others. Such failures also occur in railway axle shafts at the wheel seats and in automobile axle shafts, suspension springs, steering knuckles, etc.25,90,91 The oscillatory movement is usually the result of external vibration, but in many cases it is the consequence of one of the members of the contact being subjected to a cyclic stress (i.e., fatigue), which results in early initiation of fatigue cracks and usually results in a more damaging aspect of fretting, known as fretting fatigue.75 Fretting also causes the rupture of adhesive ties because of the strengths of oscillations, and this action can generate the fine cracks that in certain conditions propagate to form a major fracture of the sample. It is found in some systems that the quantity of metal lost by fretting is directly related to the reduction of the resistance to fatigue. The frequent oscillations cause the formation of pits that initiate cracks of fatigue and increase the susceptibility to fatigue fractures. Fretting corrosion causes loss of material in highly loaded bolted assemblies. This loosens bolts and studs, increasing their liability to fatigue failure. This is very serious when the bolt or stud is very short as in aircraft engine cylinder hold-down studs.91 It has been shown that the initiation of the fretting fatigue crack is located at the boundary of the fretting scar at the fretted zone. The fatigue crack then propagates into the surface at an angle to the surface, as shown in Figure 6.44. (Glaeser and Wright)5

Figure 6.44 Section showing fretting damage and fatigue crack initiation in 0.2% C steel (Waterhouse)5

410

Corrosion Prevention and Protection

Prevention. Various design changes can minimize fretting wear. The machinery should be designed to reduce oscillatory movement, reduce stresses or eliminate two-piece design altogether.75 Some examples of possible approaches to consider are: 1. Lubrication of the faying surfaces with low-viscosity, high-tenacity oils and greases to exclude direct contact with air. This is an adequate but not complete remedy. Phosphate conversion coatings of steels, for example, are used in conjunction with lubricants since they are porous and provide oil reservoirs.16 2. Use of gaskets to absorb vibration and limit oxygen at bearing surfaces. 3. Restricting the degree of the movement; shot peening offers the double advantage of inducing residual surface compression stress to retard corrosion and roughening the surface to increase friction.91 4. Increase the hardness of one or both of the contacting surfaces. Surface hardening such as carburizing and nitriding or applying suitable protective coatings by electrodeposition, plasma spraying, or vapor deposition, anodizing of aluminum alloys. Hard materials are more resistant than soft materials. At sufficiently large loads, soft metals serve to exclude air at the interface; furthermore, a soft surface can yield by shearing instead of sliding at the interface.7 5. Increase load to reduce slip between mating surfaces.16,31, (Waterhouse)88 6. Considerable improvement in wear resistance can be achieved when dissimilar metals are coupled, and this is especially true for steels coupled with silicon bronze and Stellite alloys. The wear data further suggest that improvement in wear resistance can be achieved by altering the surface characteristics, such as by surface treatment or by adding a coating.75 Testing. The measurement of corrosion, wear, and corrosion–wear interactions as well as erosion–corrosion interactions is a multistep process. Each component of the interaction must be measured separately. The results may then be combined to identify the synergistic effects and create a complete picture of the damage process. Measurement of the interaction between corrosion and wear modes or damage is more difficult. The standard (ASTM, G119)4 applies to systems in liquid solutions or slurries and some aspects of it can be adapted to dry corrosion and wear interactions as well. (Tylczak and Adler)5 Measurement of wear and corrosion. Jet and whirling arm tests are currently used in the field of erosion.4 In case of the whirling-arm test, the impact velocity is well known, and an entire face of the specimen can be eroded, producing a more uniform surface. The machining test is the most commonly used in high-temperature abrasive tests, because the process of machining produces an elevated temperature. For slow abrasive elevatedtemperature tests one of the most commonly used is a high-temperature ring-on-disk test. (Tylczak and Adler)5 (Madsen)88 Galling stress. Wear galling is a good measure of wear resistance of a given material pair. Galling appears as a groove, or score mark, terminating in a mound of metal. Galling data are indicative and show for example that identical metal couples usually do poorly in terms of galling compared with dissimilar metal couples. When stainless steels are coupled with each other, with the exception of some Nitronic steels, they exhibit worse galling resistance than all other steels by a factor of 2 or more.75

The Forms of Corrosion

411

Stress Amplitude (MPa)

1000 Air 0.6M NaCl, (OCP) –1250mV (SCE)

800

600 In-Air Fatigue Limit

400

200 103

104

105

106

107

Number of Cycles to Failure

Figure 6.45 Fatigue life data, S–N curves, for a high-strength steel under different environmental conditions. Stress ratio R ¼ 1. Loading frequency 1 Hz for tests in 0.6 M NaCl solution. Horizontal arrows indicate failure condition not attained. OCP ¼ open-circuit potential102

Corrosion Fatigue. Corrosion fatigue (CF) is a term that is used to describe the phenomenon of cracking in materials under the combined actions of a fluctuating or cyclic stress and a corrosive environment. The damage due to corrosion fatigue is usually greater than the sum of the damage by corrosion and fatigue acting separately. Figure 6.45 shows an example of the reduction of fatigue life and the elimination of the fatigue limit of high-strength steel in a sodium chloride solution.94 It also shows that cathodic polarization restores the fatigue properties of the steel. As an example, the shaft of a ship propeller, slightly above the waterline, can normally function until a leak occurs, allowing the water to impinge on the shaft in the area of maximum alternating stress. Another example is failure of the shaft in a period of days. Tubes of pumping made of steel for oil well have a limited life span because of the corrosion fatigue that results from exposures to oil well brines. Also, pipes carrying steam or hot liquids of variable temperature may fail because of periodic expansion and contraction (thermal cycling).7 Morphology of CF ruptures. Corrosion fatigue produces fine-to-broad cracks with little or no branching and this is different from SCC, which often exhibits considerable branching. The cracks may occur singly, but commonly appear as families or parallel cracks. It rarely appears as a single important crack. They are typically filled with dense corrosion product. They are frequently associated with pits, grooves, or some other forms of stress concentrator. Transgranular fracture paths, frequently ramified or branched, are more common than intergranular fractures, except for lead and zinc (Figures 6.46 and 6.47, respectively). Some systems show the combination of both paths.7,31 When the surface of a ruptured material by fatigue is examined, two zones of variable importance can be distinguished: a smooth, silken aspect, not having undergone any plastic distortion, and resulting from the propagation of the fatigue crack through the metal. The cyclic constraint has the tendency to smooth the surface of the rupture by wearing. A final rupture zone, rough and damaged due to plastic deformation, is noted. The crack propagates until the area of the transverse section of metal is reduced,

412

Corrosion Prevention and Protection

Figure 6.46 Corrosion fatigue crack through mild steel sheet, resulting from fluttering of the sheet in a flue gas condensate7

sufficient so that the ultimate stress limit is reached and then a fragile and brutal rupture occurs. Key factors of corrosion fatigue. Corrosion fatigue is not specific since most materials suffer degradation due to their fatigue properties in aqueous media. Steel for example undergoes corrosion fatigue in fresh waters, seawater, products of combustion condensates, general chemical environments, etc.7 For a given material, the fatigue strength (or fatigue life at a given maximum stress value) generally decreases in the presence of an aggressive environment. The effect varies widely, depending primarily on the particular metal–environment combination. The environment may affect the probability of fatigue crack initiation, the fatigue crack growth rate, or both.95 Corrosion fatigue depends strongly on the combined interactions of the mechanical (loading), metallurgical, and environmental variables.31 Stresses. The main mechanical properties to consider are: maximum stress or stress intensity factor, smax or Kmax, cyclic stress or stress-intensity range, s or K, stress ratio R, cyclic loading frequency, cyclic load waveform (constant-amplitude loading), load interactions in variable-amplitude loading, state of stress, residual stress, and crack size and shape, and their relation to component size geometry.31 The greater the applied stress at each cycle, the shorter the time to failure. Mechanical damage is more important when load and frequency are high, while corrosion damage

The Forms of Corrosion

413

Figure 6.47 Example of an intergranular corrosion fatigue failure in a Ti-6Al-4V alloy (Glaeser and Wright)5

becomes dominant at an intermediate load and/or frequency. In such conditions, cracking may be transgranular or intergranular, and the morphology may become very similar to SCC crack morphology. Cyclic stress has a negligible influence on the resistance to fatigue while the resistance to corrosion fatigue is influenced distinctly by the frequency of the cyclic stress. The corrosion influence in fatigue corrosion is more pronounced at low frequencies because contact between the corrosive species and metal is longer. Materials that are extremely sensitive to the environment, such as ultra-high-strength steel in distilled water, are characterized by high growth rates that depend on stress intensity range K to a reduced power. Time-dependent corrosion fatigue crack growth occurs mainly above the threshold stress intensity for static load cracking and is modeled through linear superposition of SCC and inert environment fatigue rates. (Glaeser and Wright)5 The description of corrosion fatigue behavior should consider the level of mechanical loading, the frequency and the shape of the cycles. It is then recommended to express corrosion fatigue as a function of crack growth rate da=dt, rather than the most frequently used cycle-dependent crack growth rate.95 Cyclic load frequency is the most important factor that influences corrosion fatigue for most material environment and stress intensity conditions. The dominance of frequency is related directly to the time dependence of the mass transport and chemical reaction steps involved for brittle cracking. Stress ratio. Rates of corrosion fatigue crack propagation generally are enhanced by increased stress ratio R, which is the ratio of the minimum stress to the maximum stress.

414

Corrosion Prevention and Protection

Some other factors, including the metallurgical condition of the material (such as composition and heat treatment) and the loading mode (such as uniaxial), affect corrosion fatigue crack propagation. (Glaeser and Wright)5 Environmental factors. In general, environmental properties to consider are: types of environments (gaseous, liquid, liquid metal, etc.), temperature, partial pressure of damaging species in gaseous environments, concentration of damaging species in aqueous or other liquid media, corrosion potential, pH, conductivity, halogen or sulfide ion content, viscosity of the environment, oxygen concentration, solution composition, inhibitors, coatings, etc.31 Corrosion products are identified on or within fracture surfaces. Corrosion fatigue cracking of high-strength steel exposed to a hydrogenproducing gas, such as water vapor, may be difficult to differentiate from some other forms of hydrogen damage. At sufficiently high frequencies, the fracture surface features produced by corrosion fatigue crack initiation and propagation do not differ significantly from those produced by fatigue in nonaggressive environments. (Glaeser and Wright)5 The usual test of fatigue of metals in air is influenced by oxygen or humidity and always represents a measure of corrosion fatigue. Early tests showed that the endurance limit for copper in a partial vacuum was found to be 14% higher than that in air. The main effect has been assigned to oxygen, which has little influence on initiation, but has a considerable influence on crack propagation.7 The natural waters and particularly brackish waters have a greater effect on corrosion fatigue of steel than of copper. Controlled changes in the potential of a specimen can result in either the complete elimination or the dramatic increase in brittle fatigue cracking. (Glaeser and Wright)5 Material factors. The main metallurgical properties of importance are: alloy composition, distribution of alloying elements and impurities, microstructure and crystal structure, heat treatment, mechanical working, preferred orientation of grains and grain boundaries (texture), mechanical properties (strength, fracture toughness, etc.).31 Tests for fatigue consist in subjecting a metal to alternate cyclic stresses, compression– tension of different values, and measuring the time (number of cycles N) before rupture. A short characteristic of the fatigue test is called the C–N curve, giving the number of cycles N to rupture. The value of the maximal stress for which an infinite number of cycles can be supported without rupture is called the endurance limit or fatigue limit. This exists for steels, but not necessarily for other metals, and is roughly equal to half the tensile strength. Nonferrous metals, such as the aluminum and magnesium, alloys of copper, do not possess a fatigue limit. In these cases, one refers to fatigue strength or resistance to a certain arbitrary number of cycles, e.g., 108 cycles.90 Growth rates of cracks are influenced by metallurgical variables, including impurities in composition, microstructure, and the cyclic deformation mode. In carbon steel, cracks often originate at hemispherical corrosion pits and often contain significant amounts of corrosion products. The cracks are often transgranular and may exhibit a slight amount of branching. Surface pitting is not a prerequisite for corrosion fatigue cracking of carbon steels nor is the transgranular fracture path; corrosion fatigue cracks sometimes occur in the absence of pits and follow boundaries or prior-austenite grain boundaries. (Glaeser and Wright)5 In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation

The Forms of Corrosion

415

is normal to the axis of principal stress. This is contrary to the behavior of fatigue cracks initiated in dry air, where initial growth follows crystallographic planes. Initial corrosion fatigue cracking normal to the axis of principal stress also occurs in aluminum alloys exposed to humid air, but pitting is not a requisite for crack initiation. (Glaeser and Wright)5 Corrosion fatigue cracks in copper and various copper alloys initiate and propagate intergranularly. Copper–zinc and copper–aluminum alloys, however, exhibit a marked reduction in fatigue resistance, particularly in aqueous chloride solutions. This type of failure is difficult to distinguish from stress–corrosion cracking (SCC), except that it may occur in environments that normally do not cause failures under static stress, such as sodium chloride or sodium sulfate solutions. (Glaeser and Wright)5 The corrosion fatigue of low alloy steels in water at high temperature is an example where modification of the crack tip chemistry has been identified as the main cause of an environmentally assisted cracking (EAC) phenomenon. In some instances, hot water may cause a drastic increase of the fatigue crack propagation rate in low-alloy steels.96 However, this effect occurs only if the sulfur content of the wrought steel exceeds a critical value of 100–150 ppm.96,97 The cause of EAC has been identified as the accumulation of sulfide ions in the crack tip environment, originating from the dissolution of MnS inclusions bared by the crack advance.98,99 In aerated environments, the potential gradient inside the crack tends to inhibit outward diffusion of the sulfide and this leads to EAC over a larger range of loading conditions, particularly at lower frequencies.100,101 Mechanisms of corrosion fatigue. The mechanism of the fatigue in air proceeds by localized slip within grains of the metal caused by stress. Adsorption of air on the fresh surface exposed at slip steps prevents rewelding on the reverse stress cycle. Continued slip produces displaced clusters of slip bands, which protrude above the metal surface (extrusions); corresponding incipient cracks form elsewhere (intrusions). The corrosion process may remove barriers to plastic deformation (e.g., dislocations), induce plastic deformation by reducing surface energy and favors slip step formation, or by injecting dislocations along slip planes (Figure 6.48).7 After or during the initiation of microcracks, it is likely that the propagation results, in part, due to the adsorption of oxygen or water or different ionic species along partitions of the crack. This adsorption reduces the energy of surface and prevents the welding of the metallic surface during the inverse constraint cycle. The formation of differential aeration cells due to different concentrations of oxygen in the localized sites can play a role on the dissolution of metal at the bottom of the crack (anode) and contribute to the propagation of the crack. A corrosive environment eliminates the fatigue limit or shortens the life above the fatigue limit. Wang102 classified fatigue damage into four stages: 1. Precrack cyclic deformation that includes the formation of persistent slip bands, formation of extrusions and intrusions. 2. Crack initiation and stage one growth that deepens the intrusions within the plane of high shear stress. 3. Stage two crack propagation of well-defined cracks on the planes of high tensile stress in the direction normal to the maximum tensile stress. 4. Ductile fracture propagation.

416

Corrosion Prevention and Protection

Figure 6.48 Extrusions and intrusions in the copper7 after 6 105 cycles in air. Sample covered with silver and mounted at an angle to magnify surface protuberances 20

Crack initiation. Corrosion fatigue cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. (Glaeser and Wright)5 Crack initiation takes place independently of the fatigue limit in air since it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting strongly favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of corrosion fatigue are anodic slip dissolution and hydrogen embrittlement, as schematically shown in Figure 6.49.103 As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve: diffusion of water molecules or hydrogen ions to the crack tip; reduction to hydrogen atoms at the crack tip; surface diffusion of adsorbed atoms to preferential surface locations; absorption and diffusion to critical locations in the

The Forms of Corrosion

417

Rupture of oxide film

Metal Dissolution

Passivation e–

H3O+

(1)

(3) HADS (4) (5)

(2) HABS (6)

Rupture of Oxide Film

(a)

(b)

Figure 6.49 Schematic presentation of (a) the slip dissolution and (b) the hydrogen embrittlement models102

microstructure (e.g., grain boundaries, the regions of high triaxiality ahead of a crack tip, or void). Under cyclic loading, fretting contact between the mating crack faces, pumping of the aqueous environments to the crack tip by the crack walls, and continual blunting and resharpening of the crack tip by the reversing load influencing the rate of dissolution.102 Fatigue crack initiation in commercial alloys occurs on the surface or the subsurface and is usually associated with surface defects or discontinuities such as nonmetallic inclusions, notches and pits. For low-stress, high-cycle fatigue, crack initiation spans a large portion of the total lifetime. For high-strength steel exposed to sodium chloride, it was found that sulfide inclusions contribute sites for corrosion pits and subsequent fatigue crack initiation. Corrosion pits were developed by selective dissolution of MnS inclusions. Cathodic polarization suppresses the dissolution rate and prevents pit formation, but hydrogen effects can increase crack growth rates of well-defined

418

Corrosion Prevention and Protection

cracks.102 Duquette104 classified materials and corrosive environments into three groups on the basis of surface corrosion conditions:102 1. Active dissolution conditions 2. Electrochemically passive conditions 3. Bulk surface films, such as three-dimensional oxides In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102 Crack propagation. The effect of the environment on crack propagation in corrosion fatigue can lead to increase in the crack growth rate. Three types of behavior have been reported.95,105 Figure 6.50a schematically illustrates the sigmoidal variation of fatigue crack growth as a function of stress intensity factor range on a log–log scale under purely mechanical loading conditions. The typical variation in crack velocity da=dt as a function of the applied stress factor K is plotted on a log–log scale in Figure 6.50b for growth of cracks in metallic materials under sustained loading in the presence of an environment. It can be seen that the environment has no effect on fracture behavior below a static intensity factor KISCC, the threshold intensity stress factor for the growth of stress corrosion cracks in tensile opening mode. Above KISCC the crack velocity increases precipitously with increasing stress intensity factor K (region I). This region is followed by a region II in which da=dt is independent of K. There is then a steep increase in crack velocity as the maximum intensity factor approaches the fracture toughness of the material (KIC region III). Figure 6.50c illustrates type A true corrosion fatigue growth behavior, in which the synergistic interaction between cyclic plastic deformation and environment produces cycle- and time-dependent crack growth rates. True corrosion fatigue influences cyclic fracture, even at maximum stress intensity factor Kmax in fatigue less than KISCC . The cyclic load form is important.102 The crack growth rate increases and the fatigue threshold decreases compared with that in air. The crack growth rate obeys a Paris law with an increase crack growth rate and a decreased fatigue threshold compared with the behavior in air.95 Figure 6.50d shows the stress–corrosion fatigue process, type B, purely time-dependent corrosion fatigue crack propagation that is a simple superposition of mechanical fatigue, (Figure 6.50a) and SCC (Figure 6.50b). Stress–corrosion fatigue occurs only when Kmax > KISCC . In this model, the cyclic character of loading is not important. The combination of true corrosion fatigue and stress–corrosion fatigue results in type C, the most general form of corrosion fatigue crack propagation behavior.102 This behavior is characterized by a plateau region, which prevails above a definite threshold Kth . It is often referred to as stress–corrosion fatigue because SCC systems usually exhibit this behavior, and the most common theory assumes that the crack growth rate results from the addition of SCC, and pure fatigue crack advance. This is a type of

Log da/dt

Log da/dN

The Forms of Corrosion

419

III II I

Log ∆K (a)

Kiscc

inert KIgcc

Log da/dN

agressive

inert KIscc

Log da/dN

agressive

Kmax

Log da/dN

agressive

Log ∆K (b)

Log ∆K (c)

Log ∆K (d)

Kmax

inert

Log ∆K (e)

Figure 6.50 Schematic representations of the combinations of the mechanical fatigue and environmentally assisted crack growth. (a) Fatigue crack growth behavior in inert environments; (b) stress corrosion crack growth under sustained loads; (c) type A corrosion fatigue crack growth, true corrosion fatigue, arising from synergistic effects of cyclic loads and aggressive environment; (d) stress-corrosion fatigue behavior obtained from a superposition of mechanical fatigue (a) and stress corrosion cracking (b); (e) mixed corrosion fatigue behavior obtained from a combination of (c) and (d)102

synergistic effect that can be observed in systems that are not sensitive to SCC, for example, ferritic stainless steels in seawater under cathodic polarization.106 It is often associated with hydrogen embrittlement. It is likely that the plateau behavior corresponds to control of the crack growth rate by nonmechanical processes, as for example, transport processes.95 Figure 6.50e shows cyclic time-dependent acceleration in da=dN below KISCC, combined with time-dependent cracking (SCC) above the threshold.102 This combines the environment effects of behaviors c and d. Examples of types A and C corrosion fatigue has been observed in similar steels under different potential conditions, type C behavior being clearly associated with the cathodic potential and thus with hydrogen embrittlement.95 This is a mix of true fatigue and stress–corrosion fatigue where one can dominate the other in its influence on crack growth, depending on the properties of the interface.107

420

Corrosion Prevention and Protection

Prevention of corrosion fatigue. One or a combination of the following procedures is recommended to prevent corrosion fatigue: 1. Redesigning to reduce or eliminate both temporary and permanent cyclic stresses. Compared with reducing the maximum stress level, it is often more beneficial and more cost effective to reduce the magnitude of the stress fluctuation.102 2. Selecting a material or heat treatment with higher corrosion fatigue strengths. 3. Use of corrosion inhibitors, reduction of oxidizers or pH increase, depending on the system and the environment, can delay the initiation of corrosion fatigue cracks. For example, in the case of mild steel, the deoxygenation of a saline solution brings back the limit of corrosion fatigue to that measured in air;108 an addition of 200 ppm of Na2Cr2O7 to the city water supply reduces the corrosion fatigue of normalized 0.35% C steel to a lower level than that measured in the air. 4. Cathodic protection, provided that the material is not susceptible to embrittlement. Sacrificial zinc (galvanized) coatings. 5. Application of surface treatments such as shot peening, nitriding of steels and sandblasting of the surface of metal or others producing constraints of compression are beneficial.1 6. Organic coatings can successfully impede corrosion fatigue. Organic coatings can be applied if they contain some inhibitory pigments in the primary layer. The relatively low corrosion fatigue strength of carbon steel is reduced still further when local breaks in a coating occur.31,84 7. Noble metal coatings (e.g., nickel) can be effective, but only if they remain unbroken and are of sufficient density and thickness. Electrolytic deposits of tin, lead, copper or silver on steel are also considered as protectors and probably do not modify the normal features of fatigue because they suppress contact with the surrounding environment. Observations made on the use of deposits of nickel or chromium are contradictory.7 Testing of corrosion fatigue. The following factors must be considered in corrosion fatigue testing: Stresses: stress intensity range, load frequency and stress ratio Environmental: electrode potential in aqueous environment and intended environment composition. Metallurgical: alloy composition, microstructure, and yield strength. (Phull)5 The scientific basis for reliable estimate of fatigue life for all conceivable load and environmental combinations remains elusive, despite the estimated billions of dollars spent combating fatigue. Tests of fatigue consist in submitting the material to a certain frequency of alternate cyclic stresses (compression–tension) of different values and plotting the C–N curve. The parameters mentioned earlier should be considered. The presence of rust or other products of corrosion does not necessarily mean that the fatigue has decreased. It is necessary to carry out fatigue tests and show that the fatigue resistance has decreased.16 Types of tests. Laboratory corrosion fatigue tests can be classified as either cycles-tofailure (complete fracture) or crack propagation (crack growth) test. In cycles-to-failure testing, specimens or parts are subjected to a sufficient number of stress cycles to initiate and propagate cracks until complete fracture occurs. Such data are usually obtained by

The Forms of Corrosion

421

testing smooth or notched specimens. However, it is difficult to distinguish between CFC initiation and CFC propagation life with this type of testing. In crack propagation testing, preexisting cracks or sharp defects in a material reduce or eliminate the crack initiation portion of the fatigue life of the component. Both types of testing are important.109 In the United States, the ASTM International issues voluntary standards and recommended practices. F 1801 concerns the Corrosion Fatigue testing of metallic implant materials. (ASTM F1160)4, (Phull)5 A typical fatigue test specimen has three areas: the test section and the two grip ends. The design and type of specimen depend on the fatigue testing machine and the objective of the fatigue study. The test section in the specimen is reduced in cross-section to prevent failure in the grips ends. Round specimens for axial fatigue machines may be threaded, buttonhead, or constant-diameter types for clamping in V-wedge pressure grips. For rotating-beam machines, short, tapered grip ends with internal threads are used, and the specimen is pulled into the grip by a draw bar. Torsional fatigue specimens are generally cylindrical. Flat specimens for either axial or bending fatigue tests are generally reduced in width in the rest of the section, but may also have thickness reductions. (Phull)5 Fracture mechanics approach. Fracture mechanics provides the basis for many modern fatigue crack-growth studies. K is the stress intensity range (Kmax –Kmin ) where K is the magnitude of the mathematically ideal crack-tip stress field in a hom*ogeneous linearelastic body and is a function of applied load and crack geometry. pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ K ¼ a paðsmax smin Þ

ð6:4Þ

In Equation (6.4) a is a function of the geometry of the rupture and the test sample, and s is the stress. The growth or extension of a fatigue crack under cyclic loading is principally controlled by the maximum load and stress ratio (minimum/maximum stress). However, as in crack initiation, there are a number of additional factors that may exert a strong influence, especially in the presence of an aggressive environment. Most CFC growth-rate investigations attempt to follow the general provisions of standard test method ASTM E647.4 In this constant-load-amplitude method, crack length is measured visually or by an equivalent method as a function of elapsed cycles, and these data are subjected to numerical analysis to establish the rate of crack growth. Crack growth rates are then expressed as a function of crack tip intensity range K, which is calculated from expressions based on linear-elastic stress analysis. (Phull)5 Expressing the crack growth rate da=dN (where a is crack length and N is number of cycles) as a function of K provides results that are independent of specimen geometry, and this enables the comparison of data obtained from a variety of specimen configurations and loading conditions. (Phull)5 Results of fatigue crack growth rate tests for many metallic structural materials have shown that complete da=dN versus K curves have three distinct regions of behavior, Figure 6.51. In an inert (or benign) environment, the rate of crack growth depends strongly on K at K levels approaching Klc (plane-strain fracture toughness) at the high end (region III) and at levels approaching an apparent threshold Kth , at the lower end (region I), with an intermediate region II that depends105 on some power of

422

Corrosion Prevention and Protection

Figure 6.51 Corrosion fatigue crack propagation rate (da/dN) as a function of the cyclic crack tip stress intensity range (K) (Phull)5

K or K of the order of 2–10. This is described by the power-law relationship. (Phull)5 da ¼ CðKÞn dN

ð6:5Þ

where C and n are constants for a given material and stress ratio. In an aggressive environment, the CFC growth curve can be quite different from the pure fatigue curve, depending on the sensitivity of the material to the given environment and the occurrence of various static stress fracture mechanisms. The environmental effects are quite strong above some threshold for SCC (KISCC ) and may be negligible below this level (KISCC is the stress intensity threshold for plane-strain environment-assisted cracking). In addition,

The Forms of Corrosion

Figure 6.52

423

Schema of CF test sample and test apparatus110,112

certain loading factors, such as frequency, stress ratio, and stress waveform, can have marked effects on the crack growth curves in aggressive environments. (Phull)5 Apparatus for electrochemical measurements during corrosion fatigue. CF tests can be done using an apparatus designed by the Continental Oil Company, as shown in Figure 6.52.110,111 The polarization potential and current can be controlled for the four samples tests at the same time. The apparatus consists of a Monel tank in which four specimens are subjected to cyclic bending. The preliminary step in the experiment is to determine the displacement caused by the desired applied load. The exact stresses are then determined with the use of strain gages. The frequency of the applied cyclic bending is equal to 60 cycles/min and the stress amplitude R ¼ 1. The electrolyte is generally an aqueous solution of 3% sodium chloride utilized at ambient temperature and deoxygenated by the bubbling of argon for 1 h before and during the test. Samples can be polished, sandblasted, shot peened, etc. The behavior of the alloy in CF can be studied at the free corrosion potential under different percentages of stress amplitude of the elastic limit. From potentiokinetic curves I ¼ f ðEÞ, the protection or the pitting potential can be applied and maintained for the entire duration of the stress tests. Each test can include up to four samples for each material at the same time. The average difference between the results obtained for similar tests depends on the particular system, but generally does not exceed 15%.112 6.7.7

Environmentally Induced Cracking

In some environments and under certain conditions, a microscopically brittle fracture of materials can occur at levels of mechanical stress that may be far below those required for general yielding or those that could lead to significant damage in the absence of an environment. This susceptibility also depends on the chemical composition and

424

Corrosion Prevention and Protection

microstructure of the alloy. This form of corrosion necessitates an interaction between the electrochemical dissolution of the metal, hydrogen absorption, and the mechanical loading conditions (stress, strain, and strain rate).95 The nature of these fracture modes varies from one class of material to another. However, all given fracture modes are largely similar to each other. Environmentally assisted cracking (EAC) is not limited to metals, and it also occurs in glasses (plexiglass), ceramics, and polymers. Structural failures due to EAC are often sudden and unpredictable, occurring after a few hours of exposure, or after months or even years of satisfactory service.113,114 Environmentally assisted cracking can be divided to mechanically assisted cracking (MAC) and environmentally induced cracking (EIC) which is the main topic of this section. The total cost of material fracture amounts to 4% of gross domestic product in the USA as well as in Europe.115,116 Fracture modes included in these studies were stressinduced failures (tension, compression, flexure, and shear), overload, deformation, delamination, and time-dependent modes, such as fatigue, creep, SCC, and embrittlement. The EAC problem is directly related to maintenance of the safety and reliability of potentially dangerous engineering systems, such as nuclear power plants, fossil fuel power plants, oil and gas pipelines, oil production platforms, aircraft and aerospace technologies, chemical plants, etc. Losses resulting from the EAC of materials annually exceed many billions of dollars and are increasing throughout the world.114 Stress–Corrosion Cracking. The conditions for SCC to occur are: (i) a crack-promoting environment; (ii) the susceptibility of material to SCC; (iii) tensile stresses must exceed the threshold value. SCC is distinguished by the fact that the stress corrosion faces suffer very low corrosion, even in solutions that cause some damage to the free surfaces. As an example is the SCC of stainless steel at 200 C in a caustic solution or in aerated chloride solution where almost no traces of dissolution are visible on the crack faces (Figure 6.53).84,95 For example, SCC of metals has been by far the most prevalent

Figure 6.53 Schematic presentation of the simultaneous three conditions of SCC84

The Forms of Corrosion

425

cause of the failure of steam generator components in pressurized water reactors (PWRs) (69% of all cases), piping in boiling water reactors (59.7%) and PWRs (23.7%). More than 60% of inspected steam turbines in nuclear power plants have stress corrosion cracked disks.117 Two classic examples of the SCC are seasonal cracking of brass and the caustic embrittlement of steel. Seasonal cracking refers to SCC of brass cartridge cases. Cracks were observed during the period of heavy rainfall during hot weather in the tropics. This intergranular SCC was attributed to the effect of internal stresses, in ammonia solution that resulted from the decomposition of organic matter in presence of O2 and humidity. Many explosions of riveted boilers occurred in the early steamdriven boilers at the tubes of riveted furnaces due to the fact that some areas were cold worked during riveting operations. A carbon steel subjected to a stress close to the elastic limit and exposed to the hot concentrated alkali solutions, or nitrate solutions is susceptible to SCC. SCC was observed in rivets used in water boilers although the furnace water was treated normally with alkalies to minimize corrosion. Crevices between rivets and the boiler plate of the furnace allowed boiler water to concentrate until the content of alkali in the crevice is sufficient to attend the required pH to induce cracking.16 Morphology. Failed specimens appear macroscopically brittle and exhibit highly branched cracks that propagate transgranularly and/or intergranularly, depending on the metal–environment combination. Transgranular stress corrosion crack propagation is often discontinuous on the microscopic scale and occurs by periodic jumps of the order of a micrometer (Figure 6.54) while intergranular cracks are believed to propagate continuously or discontinuously, depending on the system (Figure 6.55).16,95 Intergranular and transgranular cracking often occur simultaneously in the same alloy. Such transitions in crack modes are observed in alloys with high amounts of nickel, iron chromium and brasses. In corrosion under tension, ruptures are fragile and are sometimes characterized by the presence of cleavages, notably in the case of hydrogen embrittlement.16 Cleavage is a brittle fracture that occurs along specific crystallographic planes. Cleavage has a well-defined crystallographic orientation and it is easy to recognize its occurrence by optical microscopy as it exhibits brilliant and flat fracture facets that are related to the dimension of the grain size of the material under study. Under the scanning electron microscope, flat fracture facets exhibit cleavage steps and river patterns that are caused by the crack moving through the crystal along a number of parallel planes which form a series plateaus and connecting ledges. Key factors of SCC. The stress applied on a metal is nominally static or slowly increasing tensile stress. The stresses can be applied externally, but residual stresses often cause SCC failures. Internal stresses in a metal can be due to cold work or a heat treatment. In fact, all manufacturing processes create some internal stresses. Stresses introduced by cold work arise from processes such as lamination, bending, machining, rectification, drawing, drift, and riveting. Stresses introduced by thermal treatments are due to the dilation and contraction of metal or indirectly by the modification of the microstructure of the material. Welded steels contain residual stresses near the yield point. Corrosion products have been shown to be another source of stress and can cause a wedging action.

426

Corrosion Prevention and Protection

Figure 6.54

Transgranular SCC of stainless steel grade 304ð 500Þ16

Stage 1 corresponds to a rapid increase of the crack propagation with the stress intensity factor (Figure 6.56). No crack is observed below some threshold stress intensity. The stress corrosion cracks initiate when the stress exceeds a threshold value s1 and propagate when the stress intensity factor is in excess of a threshold KISCC . This threshold stress-intensity level, KISCC is determined by the alloy composition, microstructure, and the environment (composition and temperature). The values lie typically between 10 and 25 MPa m2 and s1 is usually of the order of 60–100% of the yield stress, but much lower values can be observed, such as for 304 stainless steel exposed to boiling magnesium chloride at 154 C. In certain cases, stresses as low as 10% of the elastic limit have been observed. It is necessary to use this level with prudence since the environmental conditions of a system can change at the metal/solution interface during service and accidental presence of a slash or a corrosion pit can increase stresses locally and reach the necessary level of KISCC .16 At intermediate stress intensity levels (stage 2) the crack propagation rate shows a plateau velocity Vplateau that is virtually independent of the mechanical stress, but depends on the alloy/environment interface and the the rate-limiting environmental processes such as mass transport of the aggressive species to the crack tip. The plateau in a quenched and tempered low-alloy steel of 1700 MPa yield strength in deaerated water at 100 C

The Forms of Corrosion

Figure 6.55

427

Intergranular SCC of brass16

3 2 1

Figure 6.56 Schematic of the regions of crack propagation as a function of the crack tip stressp intensity magnitude factor K expressed in MPa m (Jones)5

428

Corrosion Prevention and Protection

p (e.g., 104 m/s at stress intensity of 30–80 MPa m) can be higher than that in a similar steel of 760 MPa yield strength ( 1011 m/s) by 7 orders of magnitude.114,117,118 Stage 3 corresponds to the critical stress intensity level for mechanical fracture in an inert environment. The crack growth rate depends on the strength of the metal, in almost all aggressive steel (from 800 to 1600 environments. Doubling the yield strength sYS of martensitic p MPa) is accompanied by a tenfold (from 70 to 7 MPa m) decrease of the threshold stress intensity KISCC, corresponding to the onset stress corrosion crack growth in alloys in chloride solutions at ambient temperature.113,119 Stress sources likely to promote cracking are weldments and inserts. Welded structures of these alloys require stress-relief annealing. In some media, stress–corrosion cracking can occur above a certain temperature. Also, increasing the temperature generally lowers the threshold for cracking (s1 and KISCC ) and increases the growth rate of propagation. An example consists of SCC of stainless steel near neutral solutions above 40 and 80 C as opposed to SCC at pH–1 and room temperature.95 Hydrogen absorption can favor local plasticity, very near the crack tip region, due to enhanced dislocation velocities with hydrogen. Hydrogen penetration can be accelerated very near the crack tip region by stress-assisted diffusion and dislocation transport.120 Material parameters in SCC. The susceptibility to SCC is affected by the chemical composition, the preferential orientation of grains, the composition and the distribution of the precipitates (particularly intergranular), the interaction of dislocations, the progression of the phase transformations and cold work.16 The carbon content and its distribution in the steel matrix are the main metallurgical factors controlling the SCC. The threshold stress for cracking was found to be a function of carbon content of the steel.1 The effect of carbon on the mechanical properties of steel can play a favorable role, but the influence of carbon particles on the microstructure is important, and depends on the distribution of the different phases. For example, identification of carbon particles at the ferrite grain boundaries has been observed in the case of intergranular cracking for carbon steels with >0.1% C.1 The other alloy elements have a role beneficial or harmful to SCC resistance, depending on whether they favor the segregation of the carbon particles (cementite) at grain joints. These elements sometimes tend to segregate at the grain joints, but their influence is much weaker than that of C because of the low concentrations. Increasing the zinc content in brasses increases the sensitivity to cracking in ammoniacontaining solutions while low amounts of tin, lead, or arsenic improve their resistance.121 The addition of molybdenum to austenitic steels improves their resistance to cracking in chloride environments and decreases it in caustic media. This susceptibility also depends also on the nature of the environment. For example, the presence of a semicontinuous network of intergranular, coherent, chromium-rich carbide precipitates increases the resistance to intergranular cracking in hot caustic solutions, but the related chromium depletion of grain boundaries (sensitization) promotes intergranular cracking in hot, aerated ‘pure’ water or in polythionates.95 Potential–pH diagram and SCC. Critical potentials for SCC of a system metal/solution can be related to its E–pH diagram, because these diagrams describe the conditions at

The Forms of Corrosion

429

Figure 6.57 E–pH diagram showing the zones of severe susceptibility of carbon steel in various environments and the stability regions for solid and dissolved species (Jones and Ricker)24

which film formation and metal oxidation occur. An example of an E–pH diagram of Fe/H2O in relation to carbon steel is given in Figure 6.57. SCC is associated with potentials and pHs at which phosphate, carbonate, or magnetite films are stable while the species Fe2þ, HFeO2– are metastable. A comparable diagram exists for a 70Cu–30Zn brass in a variety of solutions. (Jones and Ricker)24 The effect of factors such as pH, oxygen concentration, and temperature can be related to their effect on the E–pH diagram. Considering the iron E–pH diagram, it can be seen that a shift of pH and/or potential from a region of stability due to the formation of an oxide film, to a region of active general corrosion or a zone of severe cracking susceptibility for a specific ion. Increasing oxygen concentration for example shifts the potential to more noble or positive potentials and temperature variation has an influence on the regions of stability. Hydrogen evolution or reduction of iron in water at 25 C is shown by the dashed line a in Figure 6.57 and this indicates that hydrogen evolution becomes less endothermic with a shift of pH to more acidic values and this is important in SCC–hydrogen-induced subcritical crack growth mechanisms. The E–pH diagram of the cracking metal/solution interface is a basic tool to evaluate and understand the mechanism of the SCC, however there are certain difficulties for a

430

Corrosion Prevention and Protection

Figure 6.58 Potentiodynamic polarization curve and electrode potential values at which stress–corrosion cracking appears (Jones)5

precise evaluation. The E–pH of the crack tip–solution interface is substantially different from that of the bulk solution.122–124, (Jones and Ricker)24 Active–passive behavior and susceptible zone of potentials. An example of active– passive behavior of a metal/environment interface such as stainless steel in 1 M sulfuric acid solution is shown in Figure 6.58. (Jones)5 It is interesting to define two ranges of potentials that can show transgranular SCC. Intergranular SCC can occur in a wider ranges of potentials. Zones 1 and 2 correspond to the active–passive and passive–active state transitions. The active state describes that of the crack tip and the passive state or film formation corresponds to that of the crack walls. Zone 2 is frequently above the pitting potential of the system indicating the possibility of pit initiation and propagation which is important in SCC. Identification of critical zones of potentials susceptible to SCC can be achieved by determining the potential–current curves at different speed rates. An example for carbon steel is given in Figure 6.59. Potentiodynamic polarization curves consist of scanning a suitable region of potentials with a fast sweep rate (1 V min1) for a filmfree surface and recording the corresponding current. This simulates the state of the crack tip where there is very thin film or no film at all. In order to simulate the state of the walls of the crack, one makes a slow sweep rate of the order of 10 mV min1 so that the allocated time permits the formation of the oxide or passive film. The intermediate anodic region between these two curves is the region where SCC is expected to occur. This technique correctly anticipates the SCC of carbon steel in a number of different media. These measurements are applicable to the systems where the formation of airformed oxide films can be reductively dissolved so that a film-free surface can be prepared before the sweeps, otherwise, straining or scraping electrodes are used to study the bare surface. The polarization curve also shows the active zone of pitting and the stable passive zone before and after the expected zone of SCC susceptibility, respectively.

The Forms of Corrosion

431

Figure 6.59 Potentiodynamic polarization curves showing especially the domain of susceptiblity to SCC1

Electrode Potential and crack growth. In SCC systems, slight variations of material composition or microstructure, or slight modification of the environmental composition or redox potential, may drastically change the sensitivity to cracking. The crack growth rate is determined frequently by the electrode potential, whereby an insignificant change, with all other conditions (specimen, solution, pH, temperature, frequency, stress ratio, etc.) being constant, can lead to the acceleration of fatigue crack growth by a dozen times. This can cause the rapid growth of the crack that was ‘static’ at open-circuit potential (when K < KISCC ), retard, or stop the crack growth at K > KISCC .113,125,126 Inhibitors. The SCC of an alloy/solution system is generally associated with the presence of a specific corrosive environment that, most of the time, would attack the alloy only very superficially in the absence of mechanical stress. It is necessary to note that nitrates and hydroxides, that generally act as anodic inhibitors for the corrosion of carbon steels, also cause the cracking of mild steels. Similarly, the failure of storage steel reservoirs containing anhydrous NH3 generally occurs only in the presence of air containing CO2 that forms the well-known anodic inhibitor (NH4)2CO3 which causes cracking. Figure 6.59 illustrates the current density differences slow in polarization curves at various potentials between fast and slow sweep rates for mild steel immersed in hydroxide, carbonate–bicarbonate and nitrate solutions. Figure 6.60 shows the results from controlled potential slow-strain-rate tests involving the same species, and it is interesting to note that the potential ranges in which cracking occurred are those predicted for each of the three solutions. Differential aeration galvanic cell. Distilled water is an important medium since it is commonly used. Evans’ differential aeration cell (a galvanic cell created by a difference in oxygen concentration) for pitting has been shown to be important or essential for cracking in distilled water. (Miller)24

432

Corrosion Prevention and Protection

Figure 6.60 75 C1

Effects of applied potential upon cracking of mild steel in 2 N (NH4)2CO3 at

Alloy/liquid interface. Table 6.3 lists some of the alloy–environment combinations that result in SCC. This table, as well as others published in the literature, should be used only as a guide for screening candidate materials prior to further in-depth investigation, testing, and evaluation.31 SCC often occurs in hot gaseous atmospheres on materials under creep or fatigue conditions. A few cases of susceptibility have been reported for certain titanium alloys in gases, including moist chlorine, dry HCl, and dry hydrogen. In moist chlorine gas at 288 C Ti–8Al–1Mo–1V alloy exhibits cracking, for example. Various binary titanium alloys are also known to experience cracking in this gas at 427 C. (Schutz)24 Table 6.3 Some environment–alloy combinations known to result in SCC31 Alloy

Environment

Aluminum alloys Carbon steels

Aqueous chloride; cyanide; high-purity hot water Aqueous amines; anhydrous ammonia; aqueous carbonate; CO2; aqueous hydroxides; nitrates Aqueous amines; aqueous ammonia; hydrofluoric acid; aqueous nitrates; aqueous nitrites; steam Aqueous chlorides; concentrated chlorides; boiling chlorides; aqueous fluorides; concentrated hydroxides; polythionic acids; high-purity hot water Aqueous chlorides; concentrated chlorides; aqueous hydroxides; concentrated hydroxides; polythionic acids; sulfides plus chlorides; sulfurous acid Aqueous chlorides; concentrated chlorides; aqueous hydroxides; concentrated hydroxides; sulfides plus chlorides Aqueous hydroxides; concentrated hydroxides; aqueous nitrates; sulfides plus chlorides Dry hot chlorides; hydrochloric acid; methanol plus halides fuming nitric acid; nitrogen tetroxide Aqueous bromine; aqueous chloride; chlorinated solvents; methanol plus halides; concentrated nitric acid

Copper alloys Nickel alloys Austenitic stainless steels Duplex stainless steels Martensitic stainless steels Titanium alloys Zirconium alloys

The Forms of Corrosion

433

Hot salt SCC and hot gaseous SCC of titanium alloys have been reported. (Schutz)24 Irradiation-assisted SCC has also been reported in nuclear reactors. (Andersen)24 Hydrogen embrittlement and hydrogen stress cracking. The interaction between hydrogen and metals can result in the formation of solid solutions of hydrogen in metals, molecular hydrogen, gaseous products that are formed by reactions between hydrogen and elements constituting the alloy, and hydrides. Depending on the type of hydrogen/ metal interaction, hydrogen damage of metal manifests itself in one of the following several ways.31,117 Hydrogen embrittlement (HE) is a loss of ductility of materials containing hydrogen and occurs most often in high-strength steels, primarily quenched-and-tempered and precipitation-hardened steels, with tensile strengths greater than about 1034 MPa. (Craig)5 There are two types of hydrogen embrittlement. Hydrogen environment embrittlement occurs during the plastic deformation of alloys in contact with hydrogen-bearing gases or a corrosion reaction and is therefore strain rate dependent. Explicit examples are the degradation of the mechanical properties of ferritic steels, nickel-based alloys, titanium alloys, and metastable austenitic stainless steel when the strain rate is low and the hydrogen pressure and the purity are high. Hydrogen stress cracking or hydrogen-induced cracking (HIC) is characterized by a brittle fracture under sustained load in the presence of hydrogen. This cracking mechanism depends on the hydrogen fugacity, strength of the material, heat treatment/ microstructure, applied stress, and temperature. For many, steels, a threshold stress exists below which hydrogen stress cracking does not occur, but this cannot be considered as a material property since it depends on the strength of the steel and the specific hydrogenbearing environment. This is sometimes called stepwise cracking (SWC). All modes of cracking have been observed in most commercial alloy systems; however, hydrogen stress cracking usually produces sharp, singular cracks in contrast to the extensive branching observed for SCC. Experimental evidence supporting a hydrogen embrittlement mechanism is that immersion in a cracking solution before stress application produces a fracture similar to a SCC fracture. The effect of preimmersion in a cracking solution is reversed by vacuum annealing. Testing in gaseous hydrogen results in the same crack characteristics produced in aqueous solution tests. SCC occurs at crack velocities at which only when adsorbed hydrogen is present at the crack tip. A critical minimum stress exists, below which delayed cracking will not take place. The critical stress decreases with increase in hydrogen concentration. These effects are shown in Figure 6.61 for SAE 4340 steel (0.4% C) charged with hydrogen by cathodic polarization in sulfuric acid, then cadmium plated to help retain hydrogen, and finally subjected to a static stress.7, (Craig)5 Formation of metallic Hydrides. The precipitation of a brittle metal hydride at the crack tip results in significant loss in strength and large loss in ductility and toughness of some metals, such as magnesium, tantalum, niobium, vanadaium, thorium, uranium, zirconium, titanium, and their alloys in hydrogen environments.31 Alloy systems that form hydrides fracture by ductile fracture. Nickel and aluminum alloys may also form a highly unstable hydride that contributes to hydrogen damage of these alloys. However some of these alloys are susceptible to failure in hydrogen by other mechanisms. (Craig)5

434

Corrosion Prevention and Protection 210,0 Uncharged

Applied stress (kg.mm–2)

192,5 175,0

Bake 24 h

157,5

Bake 18 h

140,0 Bake 12 h

122,5 105,0 87,5

Bake 7 h

70,0

Bake 3 h

52,5

Bake 0,5 h

35,0 0,01

0,1

100

1000

Fracture time,h

Figure 6.61 Delayed fracture times and minimum stress for cracking of 0.4% C steel for various hydrogen concentrations obtained by different baking times at 150 C of cathodically charged specimens (Craig)5

Accelerating ions. Chemical species that have been reported to accelerate hydrogen damage include hydrogen sulfide (H2S), carbon dioxide (CO2), chloride (Cl), cyanide (CN), and ammonium ion (NHþ 4 ). Some of these ions help to produce severe hydrogen charging of steel equipment and may lead to HIC and stress-oriented hydrogen-induced cracking (SOHIC), either of which can cause failure. It is essential to characterize the cracking severity of environments, so that either aggressive environment can be modified and/or materials can be selected with adequate resistance to cracking. Cracking requires the production of nascent hydrogen atoms at the steel surface for example, usually by a corrosion reaction in an H2S-containing, aqueous solution. H O

H2 S þ Fe2þ 2! FeS þ 2H The hydrogen atoms produced at the steel surface may combine to form innocuous hydrogen gas molecules (H2); however, in the presence of sulfide or cyanide, the hydrogen recombination reaction is poisoned, so that the nascent hydrogen atoms diffuse into the steel rather than recombining on the metal surface to form hydrogen gas. Hydrogen atoms that enter the metallic lattice and permeate through the metal can cause embrittlement and failure of structures in service environments. It is generally observed that, if large amounts of hydrogen are absorbed, there may be a general loss in ductility. Internal blisters may occur if large amounts of hydrogen collect in localized areas.127,128 Small amounts of dissolved hydrogen may also react with microstructural features of alloys to produce failures at applied stress far below the yield strength. All these phenomena are referred to as hydrogen embrittlement. Hydrogen-induced cracking may propagate in a straight or stepwise manner. Straight growth occurs in steels having ferrite pearlite structures if there are high levels of Mn and

The Forms of Corrosion

435

P segregation or if there are martensitic or bainitic transformation structures. The Mn level around linear cracks may be twice that in the matrix, whereas the P level may be elevated by a factor of 10.129 Sulfide stress cracking (SSC) is an important cracking process in the embrittling environments containing hydrogen sulfide and is considered as a special case of hydrogen-induced cracking (HIC). Natural environments (rainwater, seawater, and atmospheric moisture) contaminated with hydrogen sulphide is particularly serious. The presence of H2S in high concentrations in salt water associated with certain deep oil wells (sour wells) places an upper limit of 620 MPa on the yield strength that can be tolerated in stressed steel in such environments. (Phull)5 Sulfide stress cracking is a form of hydrogen embrittlement that occurs in highstrength steels130 and in localized hard zones in weldments131 of susceptible materials. In the heat-affected zones adjacent to welds, there are often very narrow hard zones combined with regions of high residual tensile stress that may become embrittled to such an extent by dissolved atomic hydrogen that they crack. Sulfide stress cracking is directly related to the amount of atomic hydrogen dissolved in the metal lattice and usually occurs at temperatures below 90 C (194 F).132 Sulfide stress cracking also depends on the composition, microstructure, strength, and the total stress (residual stress plus applied stress) levels of the steel.133 Sulfide stress cracking was first recognized in oil industry failures of tubular steels and well head equipment134 constructed from steels with hardness values greater than HRC 22. Heat treatment of steels to hardness levels less than HRC 22 eliminates this damage. Although the base metal of steel pipe normally has hardness levels well below this value, service failures have occurred in regions of high hardness in the weld heat-affected zone. It is, therefore, common to apply the HRC 22 limit (equivalent to Vickers hardness HV 248) to the weld/HAZ areas in pipeline steels. There are, however, a number of well-established test procedures for SSC, of which the most widely used is the NACE test.130,135 The important features of HIC and SSC are given in Table 6.4. Hydrogen-induced blistering, and precipitation of internal hydrogen. Hydrogeninduced blistering is a cracking process due to absorbed hydrogen atoms. Also commonly referred to as hydrogen-induced cracking (HIC), it occurs in lower-strength (unhardened) steels, typically with tensile strengths less than about 550 MPa (80 ksi). Pipeline steels used in sour gas environments are susceptible to HIC.19,31 Table 6.4 Comparison of features of HIC and SSC135 Hydrogen-induced cracking

Sulfide stress cracking

Crack direction Applied stress Material strength Location Microstructure

Parallel to applied stress No effect Primarily in low-strength steel Ingot core Trivial effect

Environment

Highly corrosive conditions, appreciable hydrogen uptake

Perpendicular to stress Affects critically Primarily in high strength steel Anywhere Critical effect, Q and T treatment enhances SSC resistance Can occur even in mildly corrosive media

436

Corrosion Prevention and Protection

Figure 6.62 Cross-section of carbon steel plate showing a large hydrogen blister removed after 2 yr exposure from a petroleum process stream16

Another damage also caused by the penetration of hydrogen into metal is the formation of blisters. This phenomenon also entails a mechanical property deterioration. An example is illustrated in Figure 6.62 and the mechanism of blister formation is illustrated Figure 6.63. The inside of the reservoir contains an acidic electrolyte and the outside is exposed to the atmosphere. There is evolution of hydrogen on the internal surface, resulting from corrosion reaction or cathodic protection. A part of atomic hydrogen diffuses through the steel and most of it evolves on the exterior surface, but if some atoms of hydrogen diffuses into a void, they combine into molecular hydrogen. Molecules of hydrogen do not diffuse and the pressure of the hydrogen gas within the void increases. The pressure of the molecular hydrogen in contact with the atomic hydrogen is several hundred thousand atmospheres, sufficient to cause the rupture of any known engineering material.16 Filiform corrosion can form blisters and hydrogen evolution under paint can show a swelling, but the mechanisms of formation of these similar defects in appearance are completely different from that of hydrogen blistering. Hydrogen blistering occurs as a result of nascent hydrogen atoms diffusing through the steel and accumulating at hydrogen traps, typically voids around inclusions. When hydrogen atoms meet in a trap and combine, they form hydrogen gas (H2) molecules in the trap. As more gas molecules form, the pressure increases, causing HIC and blister

Electrolyte H+ H

H+ H2

e–

e– H

H H2 H Void

H Air

Figure 6.63 Schematic showing the mechanisn of hydrogen blistering16

The Forms of Corrosion

437

Figure 6.64 Hydrogen-induced cracking: (a) centreline cracks; and (b) blister crack111

formation. Blisters occur primarily in low-strength steels (0.2%) is also effective. It is claimed that Ca and rare earth metals inhibit HIC susceptibility by modifying the morphology of inclusions.158 Calcium and rare earth metals, such as La and Ce, spheroidize nonmetallic inclusions, raising Cth , (i.e., the resistance); these additions are often used in making steels for severe environments.159 Also, Co, Ccopper and cobalt, copper and tungsten, and Ni are stated to be effective. Hot strip mill products are more susceptible than plate mill products. If S is reduced from 0.002 to 0.005%, the number of nonmetallic inclusions in steel, such as MnS, may be reduced and HIC inhibited. One can prevent the formation of hydrogen blisters by using a ‘clean’ steel without voids such as killed steels instead of rimmed ones. Manufacturing processes and

The Forms of Corrosion

451

treatments affect the MnS morphology and influence sensitivity; for example, rimmed and Si-killed steels have relatively low susceptibility.160,161 If the MnS inclusion content is sufficiently low, even the adverse effects of low-temperature controlled rolling are reduced.162 Both quenching and tempering treatments can reduce the susceptibility substantially.163 Tempering is also effective in eliminating localized Mn and P segregation if the Mn level is more than 1%; tempering reduces hardness around inclusions, and thus, the HIC susceptibility.135 Reduce hydrogen embrittlement during welding by using dry conditions, without humidity since water and the steam are the major sources of hydrogen. Low-hydrogen welding rods should be used.16 One can add inhibitors to reduce the corrosion reaction rate which is a source of hydrogen. Post-processing bake-out treatments can also be used. Baking of electroplated high-strength steel parts reduces the possibility of hydrogen embrittlement. Careful inhibitor additions during pickling can avoid vigorous hydrogen evolution and cause a subsequent decrease in hydrogen pickup. Proper choice of plating baths can reduce the hydrogen pickup during plating. A common way of removing hydrogen from steels for example is by baking at 93–148 C and this is highly recommended for electroplated galvanized steels. The pH can be raised to reduce HIC.164 Changing the environment can be very efficient. For example, blistering rarely occurs in pure acid corrosives without hydrogen–evolution poisons such as sulfides, arsenic compounds, cyanides, and phosphorus-containing ions.31 Coatings are recommended for hydrogen damage as for SCC, but they should be impervious to hydrogen penetration as well as resistant to the corrosive medium. Metallic, inorganic and organic coatings are often used to prevent the formation of HIC and blisters of hydrogen.135,165–167

Corrosion Testing. Typical objectives of SCC testing programs include: Determination of the risk of SCC for a given application and comparison of alloys and mill products Examination of the influence of chemical composition, metallurgical processing, and fabrication practices for structural components Evaluation of protective systems and prediction of service life Development of new alloys that can be less expensive, and offer longer, safes and more efficient service for certain environments Evaluation of claims for SCC performance of improved mill products Predictions of the corrosion performance should be obtained from published data and through testing.The essential requirements of accelerated testing are that the acceleration should produce the same mode of failure and reflect at least a known order of resistance of some alloys in service media.168 The most common approaches employed to achieve testing objectives in SCC are the use of high stresses, slow continuous straining, precracked specimens, higher concentration of species in the test environment than in the service environment, increased temperature, and electrochemical stimulation.169 For electrochemical corrosion, the properties of the medium at the interface should be considered in accelerated tests.

452

Corrosion Prevention and Protection

Media considerations. SCC tests can be divided into those conducted in natural environments, such as atmospheric exposure tests and seawater immersion tests, and those which are conducted under laboratory conditions or other fabricating operations. The principal disadvantage of atmospheric exposure tests is the comparatively long time required for their completion; however, they are reliable since they can reflect the projected use. There is a standard practice for evaluating stress–corrosion cracking resistance of metals and alloys by alternate immersion in a solution of NaCl 3.5%, pH 6.5. For spray testing, ASTM B-117, 2003 states the relevant conditions for conducting the test. (ASTM G44)4 Test specimens. The specimens commonly used for tests under elastic-range stress are bend-beam specimens, C-ring specimens, O-ring specimens, tension specimens and tuning fork specimens. Plastic strain specimens and residual stress specimens are also used for certain conditions. Static loading of precracked specimens as well as slow-strain-rate testing should be considered. Stressed O-rings have also been used to evaluate protective treatmens for SCC prevention. The specimens can be subjected to various loading conditions involving constant load, constant strain, or monotonically increasing strain to total failure in some of the slow-strain-rate tests. Other tests include cyclic loading as well as slow straining over a limited stress range.170, (Sprowls)24 Stressors. Corrosion acceleration for testing alloys is achieved through the use of various ‘stressors’ such as cold work of the material, higher concentration of the aggressive ion, lower pH, higher temperature, higher stress, etc. Externally applied loads are easier to evaluate and to control, but residual stresses are those which normally are responsible for stress–corrosion failures under service conditions. It is a good practice to employ both methods of stressing. Although laboratory tests are useful in encouraging conservative design of the alloy structures, results of long-term atmospheric tests of tensile-loaded specimens are considered to be more reliable. Constant-load SCC tests have been shown to be more severe than constant-deflection tests. Under a constant load, stress increases as the cross-section is reduced by cracking or corrosion. However, this condition produces decreasing stress when deflection is fixed. It has been suggested that SCC threshold stress is associated with the onset of plastic deformation, that is, the elastic limit of the alloy. The elastic limit is difficult to measure unambiguously, however, the stress at which 0.2% plastic deformation occurs is generally used. Slow strain test. The strain rate chosen frequently for the tests, based on several studies, indicates important susceptibility to cracking at about 2 106 s1 for steels, aluminum and magnesium alloys. However, the tests refer to open-circuit conditions and the strain rate sensitivity of cracking is dependent upon potential as well as solution composition. Where necessary the potential of the specimens can be controlled using a potentiostat during slow-strain-rate tensile testing.171 The reduction of area is a simple and appropriate way to quantify the susceptibility to SCC. Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172

The Forms of Corrosion

453

References 1. Shreir, L.L., Jarman, R.A., Burnstein, G.T., (eds.), ‘‘Corrosion’’, Vol. 1, Butterworth Heinemann, Principles of Corrosion and Oxydation, pp. 1:1–303; Environments, pp. 2:120–142, Effect of Mechanical Factors on Corrosion, pp. 8: 1–244, 1994. 2. Fontana, M.G., Staehle, R.W., Advances in Corrosion Science and Technology, Plenum Press, NY, preface to volume 1, p. 2, 1990. 3. Dillon, C.P., Introduction, in Forms of Corrosion - Recognition and Prevention, NACE Handbook, Vol. 1, C.P. Dillon (ed.), NACE International, Houston, TX, USA, pp. 1–4; Verink, E.D., pp. 5–18, 1982. 4. Annual Book of ASTM Standards, Vol. 01.03, A 262-02, A 763-93 (2004); Vol. 03.01, E 647-00; Vol. 03.02, B 117-03; G 1-90; G 5-94 R04; G 16-95; G 28-02; G 44-99 R05; G 46-94; G 48-92; G 59-97 R03; G 61-86; G 76-04; G 78-95; G 102-89 R04; G 108-94 R04; G 110-92 R03; G 119-04; Vol. 13.01, F 746-04, and F 1160-05, 2005. 5. ASM, Corrosion, Vol. 13A, ASM Committee: 301–321, S.D. Cramer, B.S. Covino (eds.), ASM International, Ohio, USA, Mansfeld: pp. 446–462; Baboian: 210–213, 83–87; Baloun: 207– 211; Bond: 294–300; Corcoran: 287–293; Craig: 367–380; Dexter: 398–416; Frankel: 236–241, 257; Fritz: 266–274; Glaeser and Wright: 322–330; Hack: 562–567; Hanson: 214–215; Jones: 346–366; Kane: 228–235; Kolman: 381–397; Little: 478–486; Noe¨ l: 258– 265; Phull: 568–616, 625–638; Rebak: 279–286; Stott: 644–649; Tylczak and Adler: 338–344; Waterhouse R.B., Fig. 17, p. 237, 2003. 6. Hoar, T.P., Corrosion Science, 7, 355 (1967). 7. Uhlig, H.H., Revie, R.W., Corrosion and Corrosion Control, 3rd edn, John Wiley & Sons, NY, pp. 327–340; 28–30; 90–164; 198–199; 320; 386; 6-15; 123–164; 60–89; 405–414, 1985. 8. Kruger, J., Passivity, in Uhlig’s Corrosion Handbook, R.W. Revie (ed.), John Wiley & Sons, Inc., NY, pp. 165–170, 2000. 9. Sastri, V.S., An Overview of Corrosion Inhibition, in Corrosion Inhibitors, Principles and Applications, John Wiley & Sons, Ltd, pp. 33–49, 1998. 10. Macdougall, B., Graham, M.J., Growth and stability of passive films, in Corrosion mechanisms in theory and practice, P. Marcus and J. Oudar (eds.), Marcel Dekker, Inc., pp. 143–173, 1995. 11. Nagayama, M., Cohen, M., Journal Electrochemical Society, 109, 781, 110, 670 (1962 and 1963). 12. Marcus, P., Maurice, V., Passivity of Metals and Alloys, Corrosion and Environmental Degradation, M. Schu¨ tze (ed.), Wiley-VCH, Weinheim, Germany, pp. 131–169, 2000. 13. Strehblow, H.H., Mechanisms of Pitting Corrosion, in Corrosion Mechanisms in Theory and Practice, P. Marcus, J. Oudar (eds.), Marcel Dekker, NY, pp. 201–237, 1995. 14. ASM Metals Handbook, Corrosion, Vol. 13, 9th edn, L.J. Korb, D.L. Olson (eds.), ASM International, Ohio, USA, Craig and Pohlman: pp. 77–189; Dexter: 87–88; Froats: 745; Glaeser and Wright: 136–144; Kain: 303–310; Kamdar: 171–189; Scully: 212–220; Sprowls: 231–233; Krysiak (ASM Committee chairman): 344–368, 1987. 15. Fontana, M.G., The Eight Forms of Corrosion, Process Industries Corrosion, NACE International, Houston, TX, USA, pp. 1–39, 1975. 16. Fontana, M.G., Greene, N.D., The Eight Forms of Corrosion, Corrosion Engineering, McGraw-Hill, New York, pp. 1–115; Fig. 3-24, 1978. 17. Staehle, R.W., Environmental Definition, 30th Annual Conference of Metallurgists of CIM, Proc. of the Inter. Symp. on Materials Performance Maintenance, Ottawa, Ontario, Canada, August 18–21, Pergamon Press, NY, pp. 3–46, 1991. 18. Staehle, R.W., Lifetime Prediction of Materials in Environments, in Uhlig’s Corrosion Handbook, R.W. Revie (ed.), pp. 27–84, 2000. 19. ASM, Forms of Corrosion: Recognition and Prevention, in Corrosion: Understanding, the Basics, J.R. Davis (ed.), ASM International, Ohio, USA, pp. 99–192; 10–15, 2000. 20. Scully, J.C., The Fundamentals of Corrosion, Pergamon Press, pp. 90–92, 1966. 21. Peckner, D., Bernstein, I.M., Handbook of Stainless Steels, McGraw-Hill, NY, p. 24, 1977.

454

Corrosion Prevention and Protection

22. (a) Landrum, R.J., Fundamentals of Designing for Corrosion Control, NACE International, Houston, TX, USA, pp. 1–24, 1992. (b) Piron, D.L., Corrosion Failures of Metals, in The Electrochemistry of Corrosion, NACE International, Houston, TX, pp. 164–174; 151–181, 1991. 23. Pourbaix, M., Corrosion, in Atlas of Electrochemical Equilibrium in Aqueous Solutions, NACE International, CEbelcor, pp. 70–85, 1974. 24. ASM, in Stress-Corrosion Cracking, R.H. Jones, R.E. Ricker (eds.), ASM International, Metals Park, Ohio, USA, Jones and Ricker: pp. 1–40; Andersen: 181–210; Miller: 251–263; Schutz: 256–297; Sprowls: 336–415, 1992. 25. Shreir, L.L., Corrosion, Metal/Environment Reactions, Vol. 1, L.L. Shreir (ed.), NewnesButterworths, London, 2nd edn, pp. 1:169–174; 8:1–129, 1976. 26. Bogaerts, W.F., Agena, K.S., Active Library on Corrosion, Elsevier, Amsterdam, The Netherlands in conjunction with NACE, Houston, TX, USA, 1996. 27. Bo¨ hni, H., Localized corrosion of passive metals, in Uhlig’s Corrosion Handbook, 2nd edn, R.W. Revie (ed.), John Wiley & Sons, Inc., NY, pp. 173–190, 2000. 28. Szklarska-Smialowska, Z., Pitting and Crevice corrosion, NACE International, Houston, TX, USA, pp. 5–43, 2005. 29. Berger, D.M., Electrochemical and galvanic corrosion of coated steel surfaces, in Corrosion Source Book, NACE, Houston, TX, USA, pp. 11–14, 1984. 30. Godard, H.P., Localized Corrosion, in NACE basic corrosion course, NACE, Houston, TX, pp. 8:1–15, 1970. 31. ASM, Surface Engineering for Corrosion and Wear Resistance, J.R. Davis (ed.), ASM International, Ohio, USA, pp. 16–27; 1–81, 2001. 32. Lunder, O., Lein, J.E., Hesjevik, S.M., Aune, T.K., Nisancioglu, K., Werkstoffe und Korrosion, 45, 331–340 (1994). 33. Yamamoto, A., Watanabe, A., Sugahara, K., f*ckumoto, S., Tsubakino, H., ESCCD, Applying a Vapor Deposition Technique to Improve Corrosion Resistance in Magnesium Alloys, Proc. of the 2nd Int. Conf. on Environment Sensitive Cracking and Corrosion Damage, M. Matsumura, H. Nagano, K. Nakasa, Y. Isomoto (eds.), Nishiki Printing, Hiroshima, Japan, pp. 160–167, 2001. 34. Macdougall, B., Graham, M.J., Growth and Stability of Passive Films, in Corrosion Mechanisms in Theory and Practice, P. Marcus, J. Oudar (eds.), Marcel Dekker, NY, pp. 143–173, 1995. 35. Marcus, P., Sulfur Assisted Corrosion Mechanisms and the Role of Alloyed Elements, Corrosion Mechanisms in Theory and Practice, P. Marcus, J. Oudar (eds.), Marcel Dekker, NY, pp. 239–263, 1995. 36. Clayton, C.R., Olefjord, I., in Corrosion Mechanisms in Theory and Practice, P. Marcus, J. Oudar (eds.), Marcel Dekker, New York, pp. 175–199, 1995. 37. Pistorius, P.C., Corrosion, 53, 273 (1997). 38. Shibata, T., Corrosion, 52, 813 (1996). 39. Thompson, N.G., Payer, J.H., DC Electrochemical Test Methods, in Corrosion Testing Made Easy, B.C. Syrett (ed.), NACE International, Houston, TX, USA, pp. 72–77, 1998. 40. Thompson, N.G., Syrett, B.C., Corrosion, 48, 649 (1992). 41. Frankel, G.S., J. Electrochemical Society, 145(6), 2186–2198 (1998). 42. Sato, N., Okamoto, G., Electrochemical Passivation of Metals, in Comprehensive Treatise of Electrochemistry, J.O.M. Bockris, B.E. Conway, E. Yeager, R.E. White (eds.), Plenum Press, NY and London, pp. 193–245, 1981. 43. Kain, R.M., Corrosion, 40(6), 313–321 (1984). 44. Treseder, R.S., Kachick, E.A., MTI Corrosion Tests for Iron and Nickel–Base Corrosion Resistant Alloys, in Laboratory Corrosion Tests and Standards, STP 866, ASTM, pp. 373–399, 1985. 45. Eden, D.A., Electrochemical Noise, in Uhlig’s Corrosion Handbook, R.W. Revie (ed.), pp. 1227–1238, 2000. 46. Pistorius, P.C., Burstein, G.T., Phil. Trans. R. Soc. Lond., A 341, 531 (1992). 47. Suter, T., Bohni, H., Electrochim. Acta, 43, 2843 (1998).

The Forms of Corrosion

455

48. Mansfeld, F., Wang, Y., Shih, H., in Critical Factors in Localized Corrosion, G.S. Frankel, R.C. Newman (eds.), Electrochemical Society, Proceedings, Pennington, NJ, USA, p. 497, 1999. 49. Jaffe, L.F., Nuticelli, R., The Journal of Cell Biology, 63, 614–628 (1974). 50. Isaacs, H.S., Kendig, M.W., Corrosion, 36, 269 (1980). 51. Samuels, L.E., Metals Engineering: A Technical Guide, ASM International, pp. 5; 6; 238, 1988. 52. Muller, L., Galvele, J.R., Corrosion Science, 17, 1201 (1977). 53. Moore, H., Beckinsale, S., Mallinson, C.E., Journal of Institute of Metals, 25, 35–153 (1921). 54. Sieradzki, K., Kim, J.S., Cole, A.T., Newman, R.C., J. Electrochemical Society, 134, 1635–1639 (1987). 55. Pryor, M.J., Kar-Kheng, G., J. Electrochemical Society, 129, 2157–2163 (1982). 56. Buchheit, R.G., Grant, P.P., Hlava, P.F., McKenzie, B., Zender, G.L., Journal of the Electrochemical Society, 144, 2621–2628 (1997). 57. (a) Dimitrov, N., Mann, J.A., Vukmirovic, M., Sieradzki, K., Journal of the Electrochemical Society, 147, 3282–3285 (2000). (b) ASM, Corrosion of Aluminum and Aluminum Alloys, J.R. Davis (ed.), ASM International, Materials Park, Ohio, USA, p. 25, 1999. 58. Mitrovic-Scepanovic, V., Brigham, R.J., Corrosion, 48(9), 780–784 (1992). 59. Robinson, J.L., Preferential Corrosion of Welds, The Welding Institute Research Bulletin, Vol. 20, Jan. and Feb. 1979. 60. Matsushima, I., Carbon Steel-Corrosion in Fresh Water, Uhlig’s Corrosion Handbook, 2nd edn, R.W. Revie (ed.), John Wiley & Sons, Inc., NY, pp. 529–544, 2000. 61. Wahid, A., Olson, D.L., Matlock, D.K., Cross, C.E., ‘‘Corrosion of Weldments’’, in Welding, Brazing, and Soldering, ASM Metals Handbook, Vol. 6, ASM International, Ohio, USA, pp. 1065–1069, 1993. 62. Ghali, E., Aluminum and Aluminum Alloys, in Uhlig’s Corrosion Handbook, 2nd edn, R.W. Revie (ed.), John Wiley & Sons, Inc., NY, pp. 677–716, 2000. 63. Rebak, R.B., Corrosion of Non-Ferrous Alloys, Part I: Nickel-, Cobalt-, Copper-, Zirconiumand Titanium-Base Alloys, Corrosion and Environmental Degradation, Vol. II, Wiley-VCH, Weinheim, p. 69, 2000. 64. Sand, W., Microbial Corrosion, in Corrosion and Environmental Degradation, M. Schutze (ed.), Wiley-VCH, Weinheim, pp. 172–202, 2000. 65. Hamilton, W.A., Maxwell, S., Biological and Corrosion Activity of SRB in Natural Biofilms, Proceedings of Biologically Induced Corrosion, NACE International, pp. 131–136, 1986. 66. Perry, T.D., Breuker, M., Mitchell, R., Biodeterioration of Concrete and Stone, Proceedings of the Corrosion 2002 Research Topical Symposium: Microbiologically Influenced Corrosion, NACE International, pp. 113–121, 2002. 67. Holmes, P.E., Journal of Applied Environmental Microbiology, 52, 1391 (1986). 68. Elpjick, J.J., Microbial Corrosion in Aircraft Fuel Systems, Microbial Aspects of Metallurgy, J.D.A. Miller (ed.), American Elsevier, pp. 157–172, 1970. 69. Videla, H.A., Manual of Biocorrosion, CRC Press, pp. 13–45, 121–133, 160–173, 179–185, 228–237, 232, 1996. 70. Zintel, T.P., Licina, G.J., Jack, T.R., Techniques for MIC Monitoring, A Practical Manual on Microbiologically Influenced Corrosion, Vol. 2, J.G. Stoecker (ed.), NACE International, p. 10.1, 2001. 71. NACE Standard TMO-194-94, Field Monitoring of Bacterial Growth in Oilfield Systems, NACE International, Houston, TX, 1994. 72. John, R.C., Rippon, I.J., Maarten, J.J., Thomas, S., Kapusta, S.D., Girgis, M.M., Whitham, T., B.F.M. Pots, Paper 02235, NACE International, Houston, TX, USA, 2002. 73. Battelle Columbus Laboratories and the National Institute of Standards and Technology Economic, Effects of Metallic Corrosion in the United States, 1978 and 1995. 74. Schubert, R., ASME J. Lub. Tech., 93, 216–223 (1971). 75. Bhushan, B., Introduction to Tribology, John Wiley & Sons, Inc., NY, pp. 331–420, 2002. 76. Dunn, D.J., Metal Removal Mechanisms Comprising Wear in Mineral Processing, Wear of Materials, K.C. Ludema (ed.), American Society of Mechanical Engineers, pp. 501–508, 1985. 77. Lim, S.C., Ashby, M.F., Brunton, J.H., Acta Metallurgica, 35, 1343–1348 (1987b). 78. Lim, S.C., Ashby, M.F., Acta Metallurgica, 35, 1–24 (1987a).

456

Corrosion Prevention and Protection

79. Hutchings, I.M., Tribology: Friction and Wear of Engineering Materials, CRC Press, Boca Raton, Florida, pp. 171–197, 1992. 80. Gopal, M., Jepson, W.P., Effect of Multiphase Flow on Corrosion, in Corrosion and Environmental Degradation, Wiley-VCH, Vol. 1, M. Schu¨ tze (ed.), pp. 265–284, 2000. 81. (a) Fischer, T.E., Anderson, M.P., Jahanmir, S., Salher, R., Wear, 124, 133–148 (1988). (b) Quinn, T.F.J., Review of Oxidational Wear-Part I and II, Tribol. Inter., 16, 257–271; 305–315 (1983). 82. Guile, A.E., Juttner, B., Basic Erosion Process of Oxidized and Clean Metal Cathodes by Electric Arcs, IEEE Trans. Components, Hybrids, Manuf. Technol., PS-8, pp. 259–269, 1980. 83. Bhushan, B., Davis, R.E., Thin Solid Film, 108(2), 135–156 (1983). 84. Jones, D.A., Principles and Prevention of Corrosion, Prentice-Hall, Upper Saddle River, N.J., USA, 2nd edn, pp. 235–291; 343–356, 1996. 85. Van Dyke, M., An Album of Fluid Motion, The Parabolic Press, p. 107, 1982. 86. Douglas, J.F., Gasiorek, J.M., Swaffield, J.A., Fluid Mechanics, Pitman, p. 648, 1979. 87. Hurricks, P.L., The Mechanism of Fretting-A Review, Wear, 15, 389–409 (1970). 88. ASM b, in Friction, Lubrication and Wear Technology, Vol. 18, P.J. Blau (ed.), ASM International, Ohio, USA, Waterhouse: pp. 242–256; Madsen: pp. 271–279, 1992. 89. Waterhouse,R.B.,Fretting Wear,Proc.Int.Conf.onWearofMaterials,ASME,NY,pp.17–22,1981. 90. Uhlig, H.H., Corrosion et protection, Dunod, Paris, France, pp. 98–108, 136–143; CF pp. 148–157, 1970. 91. Almen, J.O., Fretting Corrosion, in The Corrosion Handbook, The Electrochemical Society, H.H. Uhlig (ed.), John Wiley & Sons, Inc., NY, pp. 590–597, 1948. 92. Dorlot, J.M., Baı¨llon, J.P., Des Mate´ riaux, 2e e´ dition, E´ cole Polytechnique de Montre´ al, pp. 201–239, 1986. 93. Wranglen, G., An Introduction to Corrosion and Protection of Metals, Chapman and Hall, London, England, pp. 118–119, 1984. 94. Wang, Y.Z., Akid, R., Miller, K.J., Fatigue Fracture Eng. Mater. Structures, 18, 293, Publ. Blackwell Science Ltd., Oxford, UK, Fig. 1, p. 295, 1995. 95. Magnin, T., Combrade, P., Environment Sensitive Fracture, in Materials Science and Technology, Corrosion and Environmental Degradation, Vol. 1, M. Schutze (ed.), Wiley-VCH, pp. 207–263; 216–318, 2000. 96. Bamford, W., Ins. Mech. Eng. Conf. Publ., 4, 51 (1977). 97. Amzallag, C., Bernard, J.L., Slama, G., On Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Proc. Int. Symp. Myrtle Beach, South CA, NACE, p. 727, 1984. 98. Atkinson, J., Tice, D., Scott, P.M., in Proc. 2nd IAEA Specialist’s Meeting on Subcritical Crack Growth, Sendaı¨, W.H. Cullen (ed.), NUREG/CP-0067, Vol. 1, p. 251, 1985. 99. Combrade, P., Foucault, M., Slama, G., in Proc. 2nd IAEA Specialist’s Meeting on Subcritical Crack Growth, Sendaı¨, W.H. Cullen (ed.), NUREG/CP-0067, Vol. 2, p. 201, 1985. 100. Van Der Sluys, W.A., in Proc. 4th Int. Symp. On Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Traverse City, MI, p. 277, 1988. 101. Young, L.M., Andresen, P.L., in: Proc. 7th Int. Symp. On Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, NACE, Vol. 2, p. 1193, 1995. 102. Wang, Y.Z., Corrosion Fatigue, in Uhlig Handbook, R.W. Revie (ed.), pp. 221–232, 2000. 103. Suresh, S., Fatigue of Materials, Cambridge Solid State Science Series, Cambridge University Press, Cambridge, UK, pp. 363–368, 1991. 104. Duquette, D.J., Corrosion Fatigue Crack Initiation Processes: A State-of-The-Art Review, in Environment-Induced Cracking of Metals, R.P. Gangloff, M.B. Yves (eds.), NACE-10, Houston, TX, p. 45, 1990. 105. McEvily, A.J., Wei, R.P., Fracture Mechanics and Corrosion Fatigue: Chemistry, Mechanics and Microstructure, O. Devereux, A.J. McEvily, R.W. Staehle (eds.), NACE, Houston, TX, pp. 381–395, 1973. 106. Amzallag, C., Mayonobe, C., Rabbe, P., in Electrochemical Corrosion Testing, ASTM STP 727, F. Mansfeld and U. Bertocci (eds.), ASTM, Philadelphia, USA, p. 69, 1981. 107. Vosikovski, O., Trans. ASME, 97(4), 298 (1975).

The Forms of Corrosion

457

108. Duquette, D., Uhlig, H.H., Trans. Am. Soc. Metals, 61, 449 (1968). 109. Westwood, H.J., Lee, W.K., Corrosion-Fatigue Cracking in Fossil-Fueled Boilers, Corrosion Cracking, Proc. in the International Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, ASM, Salt Lake City, UT, pp. 23–34, 1986. 110. Mehdizadeh, P., Mcglasson, R.L., Landers, J.E., Corrosion, 22(12), 325–335 (1966). 111. Elboujdaini, M., Shehata, M.T., A Review on the Initiation of Environmentally Assisted Cracking in Line Pipe Steel, Proceedings of the Egyptian Corrosion Society, pp. 1–13, December 2004. 112. Elboujdaini, M., Shehata, M.T., Ghali, E., Stress Corrosion Cracking and Corrosion Fatigue of 5083 and 6061 Aluminum Alloys, Microstructural Science, D.E. Alman, J.A. Hawk, J.W. Simmons (eds.), IMS and ASM International, Ohio, USA, Vol. 25, pp. 41–49, 1997. 113. Shipilov, S.A., Technology, Law & Insurance, 1(3), 131–142 (1996a). 114. Shipilov, S.A., Fundamentals of Physico-Chemical Mechanics of Fracture: Purposes and Contents of the New Education Course, Teaching and Education in Fracture and Fatigue, H.P. Rossmanith (ed.), E & FN Spon, London, pp. 293–299, 1996b. 115. Reed, R.P., Smith, J.H., Christ, B.W., The Economic Effects of Fracture in the United States: Final Report, NBS Special Publication, pp. 647–1; 19, 1983. 116. Faria, L., The Economic Effects of Fracture in Europe: Final Report, Study Contract No. 320105, Commission of the European Communities, pp. 1–57, 1991. 117. Shipilov, S.A., Catastrophic Failures Due to Environment-Assisted Cracking of Metals: Case Histories, in Proceedings of the International Symposium on Environmental Degradation of Materials and Corrosion Control in Metals, M. Elboujdaini, E. Ghali (eds.), The Conference of Metallurgists, COM, METSOC, pp. 225–242, 1999. 118. Speidel, M.O., Stress Corrosion Cracking and Corrosion Fatigue - Fracture Mechanics, in Corrosion in Power Generating Equipment, M.O. Speidel, A. Atrens (eds.), Plenum Press, NY, pp. 85–132, 1984. 119. Peterson, M.H., Brown, B.F., Newbegin, R.L., Groover, R.E., Corrosion, 23(5), 142–148 (1967). 120. Van Leeuwen, H.P., Engineering Fracture Mechanical, 6, 141 (1974). 121. Mazille, E.H., Uhlig, H.H., Corrosion, 28, 427 (1972). 122. Turnbull, A., Progress in the Understanding of the Electrochemistry in Cracks, in Embrittlement By the Local Crack Environment, R.P. Gangloff (ed.), The Metallurgical Society, p. 3, 1984. 123. Danielson, M.J., Oster, C., Jones, R.H., Journal of Corrosion Science, 32(1), 1 (1991). 124. Parkins, R.N., Prevention of Environment Sensitive Fracture by Inhibition, in Embrittlement by the Local Crack Environment, R.P. Gangloff (ed.), The Metallurgical Society, p. 385, 1984. 125. Vosikovsky, O., Journal of Engineering Materials and Technology, 97H(4), 298–304 (1975). 126. Tsai, W.T., MoccariI, A., Szklarska-Smialovska, Z., Macdonald, D.D., Corrosion, 40(11), 573–583 (1984). 127. McHenry, I., Read, D.T., Shives, T.R., Mater. Perform., 26, 18 (1987). 128. Birnbaum, H.K., Environment Sensitive Fracture of Engineering Materials, Z.A. Foroulis (ed.), American Institute of Mining, Metallurgical, and Petroleum Engineers, Warrendale, PA, pp. 326–357, 1979. 129. Taira, T., Kobayachi, Y., Seki, N., Tsukada, K., Tanimura, M., Inagaki, H., Technol. Rep. NKK (Nipon Kokan) (67), 421 (1980). 130. NACE Standard MR175-90, Standard Materials Requirements-Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment, NACE International, Houston, TX, 1990. 131. Kane, R.D., Greer, J.B., Journal of Petrochemical Technology, 1483 (1977). 132. Gutzeit, J., Mat. Perform., 29, 54 (1990). 133. Buchheim, G.M., Oil and Gas Journal, 92 (1990). 134. Parades, F., Mize, W.W., Oil Gas Journal, 52, 99 (1954). 135. Elboujdaini, M., Hydrogen Induced Cracking and Sulfide Stress Cracking, CANMET Materials Technology Laboratory, Ottawa, Canada, pp. 1–13, 2005. 136. Elboujdaini, M., Shehata, M.T., Revie, W., Ramsingh, R.R., Corrosion 98, Proceedings of the 53rd annual conference at San Diego California, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 748, 1998. 137. Bartz, M.H., Rawlings, C.E., Corrosion, 4, 187 (1948).

458

Corrosion Prevention and Protection

138. Moore, E.M., McIntyre, D.R., Materials Performance, 37, 77 (1998). 139. Merrick, R.D., Corrosion 87, Proceedings of the annual conference at San Francisco California, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 190, 1987. 140. Merrick, R.D., Bullen, M.L., Corrosion 89, Proceedings of the annual conference at New Orleans Louisiane, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 269, 1989. 141. Lynch, S.P., Trevena, P., Corrosion, 44(2), 113–124 (1988). 142. Ford, F.P., in Corrosion Processes, R.N. Parkins (ed.), London: Applied Science, p. 271, 1982. 143. Pessall, N., Corrosion Science, 20, 225 (1980). 144. Le, H.H., Ghali, E., Applied Electrochemistry, 19, 368–376 (1989). 145. Cragnolino, G., Lin, L.F., Szlarska-Smialowska, Z., Corrosion, 37(6), 312–320 (1981). 146. Parkins, R.N., An Overview- Prevention and Control of Stress Corrosion Cracking, Materials Performance, 24, 9–20 (1995). 147. Latanision, R.M., Metallurgical Transactions, Scientific and Technical Book Service, Vol. 5, p. 483, 1974. 148. Szlarska-Smialowska, Z., Gust, J., Corrosion Science, 19, 753 (1979). 149. Szklarska-Smialowska, Z., Hydrogen Embrittlement and Stress Corrosion Cracking, R. Gibala, R.F. Hehemann (eds.), ASM International, pp. 99; 207–230, 1995. 150. Bhatt, H.J., Phelps, E.H., Corrosion, 17, 430t–434t (1961). 151. Magnin, T., Chieragatti, R., Oltra, R., Acta Metallurgica, 38, 1313 (1990). 152. Magnin, T., Chambreuil, A., Bayle, B., Acta Metallurgica, 44(4), 1457 (1996). 153. Revie, R.H., Uhlig, H.H., Acta Metallurgica, 22, 619 (1974). 154. Forty, A.J., Humble, P., Philos. Mag., 8, 247 (1963). 155. Beavers, J.A., Rosenberg, I.C., Pugh, E.N., in Proceedings of the 1972 Tri-Service Conference on Corrosion, MCIC-73-19, Metals and Ceramics Information Center, p. 57, 1972. 156. Uhlig, H.H., Physical Metallurgy of Stress Corrosion Fracture, T.N. Rhodin (ed.), Interscience, p. 1, 1959. 157. Scully, J.C., in SCC of Magnesium, L.L. Shreir, R.A. Jarman, G.T. Burstein (eds.), 3rd edn, Butterworth Heinemann, pp. 8; 127–129, 1994. 158. Pe´ rez, T.E., Quintanilla, H., Rey, E., Corrosion 98, Proceedings of the annual conference at San Diego California, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 121, 1998. 159. Kane, R.D., Cayard, M.S., Roles of H2 and H2S in Behaviour of Engineering Alloys in Petroleum Applications, Proceedings Materials for Resource Recovery and Transport, L. Collins (ed.), The Metallurgical Society of CIM, Calgary, pp. 3–49, August 1998. 160. Moore, E.M., Warga, J.J., Materials Performance, 15, 17 (1976). 161. Schuyler, R.L., Materials Performance, 18, 9 (1979). 162. Miyoshi, E., Tanaka, T., Terazaki, F., Ikeda, A., J. Eng. Ind., 98, 221 (1976). 163. Taira, T., Wet H2S Cracking of Carbon Steels and Weldments, R.D. Kane, R.J. Horvarth, M.C. Cayard (eds.), NACE International, pp. 471–478, 1996. 164. Perumareddi, J.R., Elboujdaini, M., Sastri, V.S., Inhibition of Hydrogen Entry Into Steel, Proceedings Materials for Resource Recovery and Transport, L. Collins (ed.), The Metallurgical Society of CIM, Calgary, Canada, pp. 117–187, August 1998. 165. Miyasaka, A., Yamaguchi, Y., Miyagawa, T., Nakamura, A., Corrosion 94, Proceedings of the annual conference at Baltimore Maryland, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 83, 1994. 166. Masamura, K., Takeuchi, Y., Tamaki, K., Miyagawa, T., Nakamura, A., Corrosion 94, Proceedings of the annual conference at Baltimore Maryland, ed., Houston, Texas, Paper No. 84, 1994. 167. Tamaki, K., Nakamura, A., Miyagawa, T., Ogasawara, M., Corrosion 94, Proceedings of the annual conference at Baltimore Maryland, National Association of Corrosion Engineers, ed., Houston, Texas, Paper No. 85, 1994. 168. Lifka, B.W., Aluminum (and Alloys), Corrosion Testing and Standards, R. Baboian (ed.), ASTM, West Conshohocken, PA, pp. 447–457, 1995.

The Forms of Corrosion

459

169. Hillis, J.E., Magnesium, in Corrosion Testing and Standards: Application and Interpretation, R. Baboian (ed.), ASTM, Philadelphia, PA, USA, pp. 438–446, 1995. 170. Parkins, R.N., Suzuki, Y., Corrosion Science, 23, 577 (1983). 171. Ebtehaj, K., Hardie, D., Parkins, R.N., Corrosion Science, 28(8), 811–829 (1988). 172. Dietzel, W., Schwalbe, K.H., Monitoring Stable Crack Growth Using a Combined AC/DC Potential Drop Technique, Material Prufing, Band 28, pp. 368–372, 1986. 173. Coburn, S.K., Corrosion Source Book, ASM, Metals Park, Ohio NACE International, pp. 95– 101, pp. 99, 100, 1984. 174. Landrum, R.J., Fundamentals of Designing for Corrosion Control- A Corrosion Aid for the Designer, NACE International, Houston, TX, pp. 49–67; 51, 1989. 175. Gilbert, P.T., Copper and Copper Alloys, Corrosion, L.L. Shreir, pp. 4:33–67, 1976. 176. Hay, M., COM’, Shell Canada, Short Courses at the 42nd Annual Conference of Metallurgists of CIM, August 24–27, Vancouver, B.C., Canada, 2003.

Bibliography ASM, 1990, Atlas of Stress-Corrosion Cracking and corrosion fatigue curves, A.J. McEvily (ed.), ASM International, Ohio, USA. ASM, 1995, Handbook of Corrosion DATA, B.D. Craig, D.S. Anderson (eds.), 2nd edn, ASM International, Ohio, USA. Climax Molybdenum Company, a division of American Metal Climax, Inc., A guide to corrosion resistance 304, 316, 317, 20 and NI-O-NEL, N.Y., U.S.A., 1961. Corrosion and environmental degradation, VCH Germany, Vols. 1 and 2, M. Shutze (ed.), Wiley, VCH, Weinheim, Germany, 2000. Corrosion mechanisms in theory and practice, P. Marcus and J. Oudar (eds.), Marcel Dekker, Inc., New York, 1995. Corrosion Resistant Materials Handbook, De Renzo (ed.), 4th edn, Noyes Data Corporation, N.J., U.S.A., 1985. Corrosion, Metal/environment reactions, 3rd edn, Vols. 1 and 2, L.L. Shreir, R.A. Jarman and G.T. Burstein, Butterworth Heinemann, London, 1995. Evans, U.R., The Corrosion and Oxidation of Metals, Edward Arnold LTD., London, 1960. Fontana, M.G. and Greene, N.D., Corrosion Engineering, McGraw-Hill Book Co., N.Y., U.S.A., 1978. Landrum, R.J., Fundamentals of Designing for Corrosion Control, NACE International, Houston, Texas, U.S.A., 1989. Metals Handbook, Vols. 13A and 13B, Corrosion: Fundamentals, Testing, and Protection, and Corrosion: Materials, S.D. Cramer and B.S. Covino (eds.), ASM International, Metals Park, Ohio, U.S.A., 2003. McNaughton, K.J., Materials Engineering 1, Selecting Materials For Process Equipment, Chemical Engineering Magazine, McGraw-Hill Publications Co., NY, USA, 1980. NACE, Corrosion Data Survey, 6th edn, D.L. Graver (ed.), TX, USA, 1985. National Association of Corrosion Engineers, Corrosion Testing made easy series, Houston, Texas, U.S.A., 1985. Pourbaix, M., Atlas of electrochemical equilibrium in aqueous solutions, NACE International, CEBELCOR, Houston, Texas, USA, 1974. Piron, D.L., Corrosion failures of metals in The electrochemistry of corrosion, 1991. Roberge, P.R., Handbook of Corrosion Engineering, Mc-Graw-Hill, N.Y., U.S.A., 1999. Sastri, V.S., Corrosion inhibitors, Principles and applications, John Wiley, London, 1998. Schweitzer, A.S., Corrosion Resistant tables, metals, plastics, nonmetallics, and rubbers, 2nd edn, Marcel Dekker, 1986. Uhlig, H.H. and Revie, R.W., Corrosion and Corrosion Control, 3rd edn, John Wiley & Sons, Inc., NY, 1985. Uhlig’s Corrosion Handbook, 2nd edn, R.W. Revie (ed.), John Wiley & Sons, Inc., NY, U.S.A., 2000.

7 Practical Solutions 7.1 Cathodic Protection of Water Mains Two examples of cathodic protection are discussed: protection of a ductile iron main; protection of a cast-iron-lined main. 7.1.1

Ductile Iron Main

In this example, the valve, buried in 1995, was in soil area of approximate resistivity 800–900 cm, thus providing active conditions. The 8-inch valve was epoxy-coated with an anode (zinc) attached to the top body by an exothermic weld. This type of valve is a two-part casting and is fabricated with a sealing ring and 5/8-inch diameter bolts around the flange. The larger bolts create a continuous electrical circuit. This valve failed as a result of a combination of ½-inch diameter bolts holding the top body instead of 5/8-inch bolts and corrosion affecting the undersized ½-inch bolts until they failed. The corrosion took place on the lower section of the valve. As the valve was epoxy-coated, the corrosion occurred mainly on the 12-inch coupling bolts and on the undersized ½-inch bolts that held the valve together. These bolts were carbon steel and not coated. The undersized ½-inch bolts did not provide electrical continuity between the top body and the valve as with the larger bolts, resulting on the lower part of the valve being unprotected (Figure 7.1). In the valve system, bolts are generally the first to be affected by corrosion. In chambers, it is sometimes very clear as to the cause of the corrosion, and in the early stages corrective action can be taken. These chambers are generally filled with water and soil that run into the chambers from the road runoff. In addition water from the ditches and along the water main enters in openings where the main enters the manhole. Leaks can be reduced, but with the loading of traffic and shifting of the chamber in the frost

Corrosion Prevention and Protection: Practical Solutions # 2007 John Wiley & Sons, Ltd

V. S. Sastri, E. Ghali and M. Elboujdaini

462

Corrosion Prevention and Protection

Figure 7.1 12-inch bolt from coupling. (Reprinted with permission of Dave Raymond, City of Ottawa, Public Works and Services)

cycles, it becomes very difficult to completely eliminate the inflow of water. Chambers in these areas are found with bolt heads completely corroded. Bolts are also found with the stem reduced to pencil lead thickness. Rungs of aluminum ladders for access to these chambers have completely disappeared. These are some of the problems encountered during rehabilitation of these valve chambers that, in some cases, were installed in the mid 1980s. Corrective action consists of replacing mild steel bolts with 304 stainless steel bolts. The bolts were coated with a wax-based primer to reduce the corrosion of the bolts in flooded chambers. In the chambers crevice corrosion and galvanic corrosion along with general corrosion of the aluminum ladder were observed (Figure 7.2). Crevice corrosion was also observed on couplings and bolts with valves. The aluminum steps in some valve chambers were attacked by chlorides/phosphates. 7.1.2

Cast-iron-lined Main

A chamber in service for a 10-yr period in which rebuilt valves were present was found to be saturated with chlorides and phosphates. The chloride caused the corrosion of bolts to such an extent that the chamber was leaking. The chamber housed 24-inch and 16-inch valves along with air drain-out. All the bolts on the valves required replacement. Furthermore, the aluminum ladder rungs were completely corroded. The corrosion prevention strategy consisted of wrapping the valve with wax tape, along with use of magnesium anodes. The chamber was thoroughly cleaned and valves wrapped in wax tape, as shown in Figure 7.3. Since the chamber was entered for limited number of times the corroded ladder was left intact and enter with the restraining device and/or portable ladder.

Practical Solutions

463

Figure 7.2 Aluminum ladder rungs. (Reprinted with permission of Dave Raymond, City of Ottawa, Public Works and Services)

The use of wax taping and magnesium anodes resulted in very little corrosion, requiring only normal operating maintenance and inspection of the chamber for valve integrity. Corrosion of bolts in areas other than valves was observed, as shown in Figures 7.4 and 7.5. The economics of cathodic protection compared with replacement of the water main are illustrated in Table 7.1.

Figure 7.3 Tape-wrapped air valve. (Reprinted with permission of Dave Raymond, City of Ottawa, Public Works and Services)

464

Corrosion Prevention and Protection

Figure 7.4 4 inch T-bolt from 8-inch valve. (Reprinted with permission of Dave Raymond, City of Ottawa, Public Works and Services)

Figure 7.5 Hydrant bolt from base flange. (Reprinted with permission of Dave Raymond, City of Ottawa, Public Works and Services) Table 7.1 Cost of replacement and of cathodic protection, showing savings obtainable 1 Km

Replacement ($)

Cathodic protection ($)

Savings ($)

% cost

6-inch water main 12-inch water main

$800,000.00

$20 000.00 3 cycles

$740 000.00

7.5

$1100 000.00

$37 500.00 3 cycles

$987 500.00

10.25

Practical Solutions

465

Bibliography ASTM B 843, Type M1-C for chemical composition ASTM G 97, Standard Current Efficiency Test for Magnesium RPO169-96, NACE Book of Standards, Standard Recommended Practices J.T.N. Atkinson and H. Van Droffelaar, Corrosion and Its Control, An introduction to the Subject, 2nd Edition, NACE International Houston, 1995, pp. 270, 280, 281.

7.2 Internal Corrosion of Aluminum Compressed Air Cylinders Aluminum compressed air cylinder safety has been an important issue in recent years. Safety procedures regarding the use, care and maintenance of compressed air cylinders have been in placed to ensure the public safety. Many attentions have been paid especially so-called sustained-load cracking (SLC) failures. SLC is a metallurgical anomaly that occasionally develops in high-pressure cylinders made generally from aluminum alloys. The objective of investigation is to clarify the mechanisms of corrosion-induced pitting and cracking of the aluminum compressed air cylinder for the purpose of determining key factors. 7.2.1

Destructive Visual Inspection

The aluminum cylinder was made of 6161 aluminum alloy in 1989. It was taken out of service after 5 yr because it failed a safety test due to high moisture content (>200 ppm). An optical probe inspection (with mirror and a light source) revealed internal corrosion. Figure 7.6 is a photo of the aluminum cylinder before it was opened. The cylinder was cut open (Figure 7.7). The internal surface of cylinder was completely corroded. A corrosion pattern that appeared to be caused by condensation was observed.

Figure 7.6 Out-of-service compressed air cylinder. (Copyright of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2004, 2006)

466

Corrosion Prevention and Protection

Figure 7.7 Overall view of internal surface. (Copyright of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2004, 2006)

Figure 7.8 is a view of the bottom of the cylinder. It appears that corrosion pattern is uniformly distributed inside the cylinder. Figure 7.9 is a close-up of the corrosion surface. The white spots appear to be corrosion pits. 7.2.2

Corrosion-induced Cracking

In the visual inspection, the white spot is some kind of corrosion pit. A cross-section sample was subjected to SEM investigation. The pitting corrosion is more like corrosioninduced cracking. As shown in Figure 7.10, the pitting grows intergranularly, e.g., the pit deepens along the boundary of the aluminum grains. Figure 7.11 provides a closer look at the corrosion and cracking pattern.

Figure 7.8 Close-up of the internal surface of the bottom of the cylinder. (Copyright of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2004, 2006)

Practical Solutions

467

Figure 7.9 Close-up of the internal surface of the opened cylinder. (Copyright of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2004, 2006)

Figure 7.10 SEM photo of the cross-section of a corrosion ‘pit’. The intergranular cracks shown penetrate about 12% of the cylinder wall thickness (2.72 mm thick). (Copyright of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2004, 2006)

Figure 7.11 SEM photo of an enlarged cross-section of a corrosion ‘pit’

468

Corrosion Prevention and Protection Table 7.2 O/Al ratios determined at various locations in the cracks Location

O (atom %)

Al (atom %)

O/Al ratio

57.9 59.2 66.4 65.1 66.9 68 59.9 63.6

42.1 40.8 33.6 34.9 33.1 32 40.1 36.4

1.375 1.45 1.97 1.865 2.021 2.125 1.494 1.747

1 2 3 4 5 6 7 8

Oxygen/aluminum ratios were also determined at various locations in the cracks as indicated in Figure 7.11 and results are given in Table 7.2. The oxygen/aluminum ratio increases from 1.5 to 2.0 as the crack location moves towards the crack tip. The ratios of 1.5 and 2.0 likely represent Al2O3 and AlO2, respectively. This observation tends to indicate that corrosion induced cracking took place. 7.2.3

Corrosion Mechanism

High-purity aluminum has excellent corrosion resistance. Information collected over the years from manufacturers and users has shown that aluminum structures will provide reliable service for periods in excess of 30 yr. The factor that assures the long life of aluminum is a self-forming microscopically thin surface layer of aluminum oxide. The air-formed film on new aluminum surfaces is about 2.5 nm thick, while the film on aluminum that is several years old may be 10 nm thick or more. The film is composed of two parts: a thin, inner barrier layer; a much thicker bulk outer layer, which is more permeable than the inner barrier layer. Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. In the case of compressed air cylinder corrosion, there are additional factors that should be taken into account: sustained pressure (normally at 2200 psi); moisture content and condensation; carbon dioxide content.

Practical Solutions

469

The fact that the aluminum cylinder is constantly under pressure is believed to be a contributing factor for stress corrosion. The moisture content is very critical since condensation occurs when pressure changes rapidly. Therefore, moisture content and slow refilling procedure are strictly regulated. Carbon dioxide content is also critical since it increases its dissolubility as pressure increases. As a result, it could bring the pH value of condensed water below the level where aluminum oxide is no longer stable. 7.2.4

Summary

The internal surface of the cylinder has been entirely covered by white spots appear to be corrosion pits. All cross-sections of a corrosion pit exhibits intergranular corrosion caused by selective attack of grain boundaries. The metallographic cross-section of a pit area shows that penetration is about 10–15% of the cylinder wall thickness. The aluminum cylinder is constantly under pressure and this is believed to be a contributing factor for stress corrosion cracking (SCC). The moisture content is very critical since condensation occurs when pressure changes rapidly. Carbon dioxide content is also critical since it increases its dissolubility as pressure increases. If the oxide film of aluminum is broken by chemical action, and in the presence of moisture, corrosion proceeds rapidly. The effect can be severe when stress (pressure) is present.

Bibliography J. Steinbachs, Protective gear fails firefighters, Ottawa Sun, 28 April 2003. H. Lake, Firefighter fix cost $400 000, Ottawa Sun, 28 April 2003. C. Barriere, Respirateurs contamines, Le Droit, 29 April 2003. W.L. High, 6351 Alloy tanks go bust – Analysis of cracking and rupture of SCBA and SCUBA aluminum cylinders, http://www.cdnn.info/article/high/high.html B. High, Cracking and ruptures of SCBA and SCUBA aluminum cylinders, http://www.psicylinders .com/library/cracking.htm

7.3 Some Common Failure Modes in Aircraft Structures It is a well-known fact that failure of an aircraft component can have catastrophic consequences such as loss of precious life and aircraft. It is obvious from Table 7.3 that failures due to fatigue are predominant in aircraft components. When the component is no longer able to withstand the imposed stress, failure will occur. Thus failures are associated with stress concentrations which can occur due to: (i) design defects presence of holes, notches, tight fillet radii; (ii) presence of voids, inclusions in microstructure; (iii) corrosion such as pitting can cause a local stress concentration.

470

Corrosion Prevention and Protection

Table 7.3 Failure mode (in 2002)1 Failures (%)

Corrosion Fatigue Overload High temp. corrosion SCC/Corrosion Fatigue/Hydrogen Embrittlement Wear/abrasion/erosion Brittle Fracture Creep

7.3.1

Engineering components

Aircraft components

29 25 11 7 6

16 55 14 2 7

3 16 3

Example 1

A nose undercarriage turning tube failed on landing after only 1300 flight cycles, well below the expected service life. Scanning electron microscopy (SEM) examination of the sample tube showed the fracture surface to have ductile appearance consistent with a ductile overload. Figure 7.12 shows fatigue striations indicating the fracture mode to be fatigue resulting in a fast fracture (overload) upon reaching a critical crack length. The striations on the fracture surface consisted of distinct bands of repetitive units of striation spacing as seen in the figure. Measurement of striation spacing and band spacing at various points along the crack from the origin to the end enabled the determination of the crack growth rate graphically. Since the striations and bands are related to load cycles, a comparison of the load cycles with the anticipated load spectrum makes it possible to determine the load cycles required to propagate the fatigue crack to the point of failure. The number of cycles that caused the failure was in the range that the component was in service which indicated that the fatigue crack initiated very close to the beginning of the component’s service life.

Figure 7.12

Fatigue striations observed on the fracture surface

Practical Solutions

Figure 7.13 catches1

471

Outer surface of wing panel and door after paint stripping and removal of

The origin of the crack was located at a notch on the surface of the tube. In all likelihood the notch produced the required stress concentration in the surface of the tube, thereby reducing the initiation time of the crack. In all probability the notch in the surface of the tube occurred during manufacture. 7.3.2

Example 2

During service inspection corrosion of an upper surface wing panel containing an access door was observed. The panel and door were made from an alloy plate to which aluminum catches were attached for securing the door in the closed position. Stainless shims were fitted between the catches and the aluminum plate. The outer surface of the panel and door after stripping point and removing the catches is shown in Figure 7.13. There appears to be no damage to the plate and the door. The examination of the inner surface showed extensive exfoliation corrosion on the panel and the door in the catch positions (Figure 7.14). Cracking originating from the catch position

Figure 7.14 location1

Exfoliation corrosion on the inner surface of the panel and door around a catch

472

Corrosion Prevention and Protection

Figure 7.15 Cross-section through the panel, showing the exfoliation corrosion1

was observed in the stiffening ribs. A cross-section of plate containing extensively corroded area is shown in Figure 7.15. Exfoliation corrosion occurs when the attack occurs along grain boundaries, particularly when they are elongated and form thin platelets. The voluminous corrosion product causes splitting of layers uncorroded material. The material conformed to the specification and the aluminum alloy is known to be subject to exfoliation corrosion. The corrosion was extensive at the catch positions and attributed to the stainless steel shims fitted below the catches. The paint between aluminum and stainless steel shims deteriorated, resulting in galvanic corrosion with the stainless steel acting as the noble metal. 7.3.3

Example 3

A bolt from an aircraft flap control unit fractured in the threaded region of the shank near the shoulder with the head upon installation after a major service. The bolt was made from cadmium-plated high-strength steel. The bolt conformed to the specifications and had ultimate tensile strength of 1400 MPa.

Figure 7.16

Ductile fracture surface at the center of the bolt1

Practical Solutions

Figure 7.17 the bolt1

473

Intergranular region of the fracture surface around the outer circumference of

SEM examination showed ductile features (Figure 7.16) on the center of the bolt and intergranular features on the outer circumference (Figure 7.17) both modes of cracking were caused by static overload failure with the ductile features at the center present throughout. The intergranular appearance around the edge is suggestive of embrittlement leading to premature failure at loads below those expected. The embrittlement is attributed to cadmium plating on the bolts applied to protect them from corrosion. During plating with cadmium hydrogen is absorbed by steel and cadmium acts as a barrier for the escape of hydrogen. In high-strength steels (>1100 MPa) hydrogen embrittlement (HE) occurs, but can be minimized by baking at 175 C for 24 h or by treatment with 1:1:1 mixture of nitric, acetic and phosphoric acids.

Reference 1. S.J. Findlay and N.D. Harrison – ‘‘Why aircraft fail’’, Materialstoday, November 2002, pp. 18–25.

7.4 Premature Failure of Tie Rods of a Suspension Bridge Tie rods of a newly built suspension bridge in service for only six winter months at temperatures of 20 C failed due to the cleavage and lack of low-temperature toughness of the steel. Fatigue crack propagation due to cyclic and uneven loading was also a contributing factor. The use of steels of higher toughness with intrinsic weathering resistance has been advanced as a remedial measure. The bridge is 150 m in length with three sections. The middle section is 70 m in length sandwiched between four columns that support tie rods. The other two sections are 40 m in length. There are 32 tie bars which are 90 mm in diameter. The bars attached to several columns, 4 overhead columns that rise above and 8 columns that drop below to the base of the bridge. Figure 7.18 shows a view of the column with a missing tie rode. Figure 7.19 shows a close-up view of the exposed tie rods and corrosion of the threaded

474

Corrosion Prevention and Protection

Figure 7.18 General view of the bridge1

portions is evident. Figure 7.20 is another view of the tie rods and corroded threads are obvious. Figure 7.21 shows the fractured surface of the failed tie rod. Metallographic examination showed that the cracks initiated at the first thread, immediately adjacent to the nut. Beach marks were present which indicate the presence of fatigue. The final area of the fracture showed the presence of shiny granular appearance, indicating a sudden brittle (cleavage) fracture and this is also confirmed by the SEM photographs showing cleavage steps and river pattern. Fractured tie rods were sectioned and examined with an optical microscope and no evidence was found for microcracks at the root of the threaded notch and along the fracture path.

Figure 7.19 Close-up view of the exposed tie rods; corrosion of the unpainted exposed areas may be noted1

Figure 7.20 Close-up view of the upright column with a missing tie rod1

Figure 7.21 Close-up view of tie rods showing corroded threads, nuts and washer1

476

7.4.1

Corrosion Prevention and Protection

Conclusions

1. The presence of beach marks shows that fatigue is a major factor in the failure. Relatively small areas of crack propagation zone indicate short service life. 2. Steel 350 W is unable to withstand 20 C temperature. 3. Fatigue cracks initiated next to nut on the washer side due to stress concentration when the nut is tightened. 4. Lack of microcracks shows the absence of machining defects. 5. Use of steels (350 WT, 350 AT, A710) is recommended.

Reference 1. Edward Ghali and Madhavarao Krishnadev,‘‘Physical and mechanical metallurgy of premature failure of tie rods of a cable stayed bridge’’, Engineering failure analysis, Volume 13, Issue 1, January 2006, pages 117–126.

7.5 Corrosion and Lead Leaching of Domestic Hot and Cold Water Loops in a Building 7.5.1

Hot Water System Corrosion

A schematic diagram of the domestic hot water system is given in Figure 7.22. During the visual inspection, the following observations were made: (i) Different metals such as copper, galvanized steel and cast iron pipes, and brass joints, etc., are used in the hot water distribution line. (ii) A noticeable temperature difference is observed in the heat exchanger loop where the water in the hot water tank is circulated to a heater. No mechanical pump is used in this loop, the water flow is by convection. Slow water circulation results in a larger temperature difference between the upper and lower pipe sections. A temperature difference is also seen in the heat circulation loop where circulated hot water is mixed with incoming cold water. Severe corrosion was observed in these areas. (iii) In some sections of the pipe, a large copper pipe area in contact with a relatively small area of galvanized iron and steel pipe was observed. Water samples taken from various locations (denoted in the National Printing Bureau Domestic Water Systems Preliminary Investigation Report),1 indicated the high iron content in samples taken from locations where the cold and hot water are mixed, e.g., the heat exchange loop and hot water recirculation pump (Figure 7.22). Visual inspection and the results of metal ion content in water samples, shows internal pipe corrosion in various locations in the hot water distribution system that compromises the quality of hot water. The cause of the internal pipe corrosion can be attributed to the following factors or a combination of the factors. Galvanic Cell Formation. When dissimilar metals are in contact and with the presence of electrolyte, a so-called galvanic cell will be established due to the electrochemical potential difference.2 The result of the galvanic cell is corrosion of the metal that is less noble. For instance, when copper is in contact with iron in the presence of electrolyte

Practical Solutions

477

Figure 7.22 Schematic of the hot water system

(water); iron (the anode) will corrode to generate Fe2þ ions, which further react with oxygen to form iron oxide, a brown product, while oxygen reduction reaction takes place on the copper (the cathode). The reactions are as follows. Anodic reaction: Fe2e ¼ Fe2þ Fe2þ þ 2H2 O ¼ FeðOHÞ2 þ 2Hþ 1 2FeðOHÞ2 þ O2 ¼ Fe2 O3 þ 2H2 O 2 Cathodic reaction: 1 O2 þ H2 O þ 2e ¼ 2OH 2 It should be noted that there is a reduction of pH in the anode area, which further accelerates the iron dissolution (corrosion). Temperature Difference. The severe corrosion that takes place in the heat exchange and heat circulation pump locations is explained by the temperature effect. Thermodynamically, a potential difference can result in a temperature difference, even within the same

478

Corrosion Prevention and Protection

material. For example, if there is a pipe with hot water at one end and with cold water at the other end, such a temperature difference can result in a potential difference. A simple estimation of 60 C difference could contribute to >20 mV electrochemical potential difference, which may be enough to accelerate iron pipe corrosion in the area of lower temperature. As there is no mechanical pumping in the heat exchange system, the circulation of water depends solely on convection, which creates a relatively slow flow and causes a larger temperature difference. All this will contribute to the severe corrosion of iron components nearby. Large Cathode and Small Anode. Such a situation is known to lead to an acceleration of corrosion of the anode. If a small section of iron or steel pipe is in contact with a large piece of copper pipe, the area effect will accelerate the corrosion of the iron pipe that acts as an anode of a galvanic cell. 7.5.2

Conclusions

The cause of corrosion in the domestic hot water system in the building is attributed to the improper use of dissimilar metal pipes and associated components. The temperature difference in the heat exchanger and heat circulation locations makes the corrosion more severe. An area effect is also a contributor to the corrosion. The remedial measures should be aimed at reducing the existing galvanic cell, to minimize the temperature and area effects. The high contents of Al and Zn reported may possibly explained by the use of galvanized pipes and a sacrificial anode cathodic protection system in the hot water tank.

References 1. National Printing Bureau Domestic Water Systems Preliminary Investigation report, Mansour Keenan and Associates Limited, January 2003. 2. Corrosion Engineering, M. G. Fontana, 1986, p. 43.

7.6 Cathodic Protection of Steel in Concrete More than 5 billion dollars are spent every year in repairs to concrete structures such as bridges, buildings, parking garages and other structures. Carbon dioxide enters the concrete and reacts with the lime, lowering the pH by forming carbonic acid. Further, the chloride in deicing salts, along with oxygen and water, creates an aggressive corrosive environment. An electrochemical corrosion cell is formed and delamination occurs. The rebar corrodes and the resulting rust is voluminous, leading to cracking, spalling and delamination of the concrete. Figure 7.23 illustrates the corrosion process. Cathodic protection is one of the methods to mitigate the corrosion of steel in concrete Figure 7.24. Some factors to be considered in this connection are: remaining service life of the structure should be more than 10 yr; delamination and spalls should be less than 50% by weight of concrete; half-cell potential should be less than 200 mV (indicating breakdown of passive film); the structure should be sound; the reinforcing bars should be electrically continuous; AC power should be available.

Practical Solutions

479

Contamination ( Water+ oxygen+ salt )

Hair cracks and or porosity

Steel rebar

Figure 7.23 Atmospheric Contamination

The two types of cathodic protection are: (i) sacrificial anode; and (ii) impressed current systems. The sacrificial anode system typically uses magnesium, zinc or aluminum and their alloys Figure 7.25. These metals or alloys act as anodes when coupled with steel and its

ZINC MESH ANODE

MO RT AR

xx xx xxx xx xxx xx xx xx xx xx xxx xx xx xx xx xxxx xx xx xxx x xxxxx x x x xx x x x x xx x x xx xxxxx xx xx xx xx x xxxxxxxxxx x x xx x x x xx x x x xx x x x x x xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx xxxxxxxx OV ER LA Y

PRIMARY ANODE

I STEEL REBAR i

ELECTRICAL BOND

REINFORCED CONCRETE

FL AM E Z I N OR C AR AN C O D SP E RA Y

Figure 7.24 Galvanic anode cathodic protection

PRIMARY ANODE

STEEL REBAR i

ELECTRICAL BOND

REINFORCED CONCRETE

Figure 7.25

Arc-sprayed galvanic layer anode

480

Corrosion Prevention and Protection +

xxxxxxxxxx xxxxxxxxxxx xxxxxxxxxxxx xxxxxxxxxxx xx xxxxxxxANODE xxxxMESH xxxxxxxxx xxxxxxxxxxxxx xxxxxxxxx xx xxxxxxxxxxxxx xx xx xx xx xx xx xx xx xx xxx xxxxxxxxxxxx xx xx xx xx xx xx xx

I I

STEEL REBAR CATHODE

I I Figure 7.26

I Impressed current system line diagram

alloys. These metals or alloys act as anodes when coupled with steel and preferentially corrode. Magnesium is often used in fresh water media while zinc and aluminum are used in seawater and brackish water media. Impressed current cathodic protection requires: (i) (ii) (iii) (iv)

DC power supply (rectifier); an inert anode such as catalyzed titanium anode mesh; a Wiring conduit; an embedded silver/silver chloride reference electrode.

A schematic of an impressed current cathodic system is depicted in Figure 7.26. By an impressed current the potential of the steel is shifted to greater than 850 mV, thus making the steel bar cathodic and prevent the corrosion.

Bibliography M.Y. El-Shazly, Cathodic Protection of Steel in Concrete, 24th Annual Conference on Corrosion Problems in Industry, Egyptian Corrosion Society, 5–8 December 2005, Les Rois Hotel, Egypt.

7.7 Corrosion of Aluminum Components in the Glass Curtain Wall of a Building Corrosion was observed on the aluminum pressure plates and dress-caps that hold the glazing in place on a curtain-walled building. The dress-caps (which have L-shaped cross-sections) were externally clad with thin copper sheet.

Practical Solutions

481

Figure 7.27 A pressure plate (bottom) and a dress-cap (top). The dress cap is clad with copper sheet. The pressure plate is fitted with two rubber inserts. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

7.7.1

Introduction

In order to hide the aluminum extrusions, rubber strips and screws (dress-caps) were fitted over the top of the pressure plates. They were also aluminum extrusions with L-shaped cross-sections. For aesthetic reasons, the original building design required the dress-caps to be externally clad with thin copper sheet. The copper sheet was mechanically attached by rolling over the two longitudinal edges onto the internal surfaces of the dress-cap. Figure 7.27 shows a pressure plate and one of the dress-caps. Several years after the building was constructed, it was noticed that some of the copper cladding was separating from the dress caps, particularly near the base of the building. 7.7.2

Observations

Pressure Plates. The aluminum extrusions were partially covered with loose, white-grey corrosion products. Water washing and scrubbing removed these products and revealed general corrosion and pitting of the aluminum surfaces. Dress-caps. The aluminum extrusions showed extensive general corrosion and pitting on their inner (non-clad) surfaces (Figure 7.28). The copper cladding sheet was bulged at a few locations along the dress-cap edges. Cross-sectional cuts were made through some of these bulges. A typical cross-section is shown in Figure 7.29. It can be seen that the bulge was caused by the formation of a white corrosion product on the outer surface of the aluminum extrusion. This voluminous material, trapped between the copper and the aluminum, exerted pressure on the relatively soft copper sheet, causing it to deform. The cross-section shown in Figure 7.29 reveals that the aluminum has been badly corroded at one edge and has cracked away. This has resulted in the copper sheet

482

Corrosion Prevention and Protection

Figure 7.28 Aluminum corrosion products on the interior surface of a dress-cap. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

becoming partially detached at this location. All of the copper cladding, which was a commercially pure architectural grade, was found to be in excellent condition with no significant metal loss. The major damage suffered by the dress-caps was clearly caused by galvanic corrosion between the copper cladding and the aluminum extrusion. In addition, if a coppercontaining solution runs over aluminum, electrochemical reactions will cause the aluminum to corrode and metallic copper to be deposited. These reactions establish local galvanic cells that can cause rapid deterioration of the aluminum. Copper has a very low corrosion rate in most atmospheric environments, thus enabling it to perform well for decades in architectural applications. The fact that copper is very slowly corroding can be judged by the blue-green run-off that is seen coming from the copper roofs of many public buildings.

Figure 7.29 Cross-section through a bulge in the copper cladding on a dress-cap; note the white aluminum corrosion product and the thinned and cracked edge (lower right). (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

Practical Solutions

7.7.3

483

Recommendations

The new dress-cap substrate material should be either fiber-reinforced plastic (FRP) or Type 316 stainless steel. Being a nonmetallic insulator, FRP would eliminate any possibility of galvanic corrosion. Type 316 stainless steel demonstrates good passivity in urban atmospheres and has been successfully used in contact with copper.1 Contact between aluminum and metallic copper and/or copper-containing solutions must be avoided irrespective of the corrosivity of the environment. Although corrosion engineers are well aware of the mechanism and the detrimental effects of galvanic (dissimilar metal) corrosion, it would seem necessary to make some designers, architects and engineers in other disciplines aware of this very damaging form of corrosion.

References 1. H.P. Godard, W.B. Jepson, M.R. Bothwell and R.L. Kane, The Corrosion of Light Metals, Chap. 3, John Wiley & Sons Inc., New York 1967. 2. W. Seale, Copper Development Association, private communication, 28 October 2004.

7.8 Corrosion in a Cooling Water System Cooling water tubes are used in extensively industry. Corrosion of cooling water tubes or pipes is a common phenomenon. The heat exchanger is opened to examine the extent of corrosion. Heavy deposits are revealed inside the tubes (Figure 7.30). The outer surface of the carbon steel tubes are in good condition and free from pitting attack (Figure 7.31). Some tubes were split open and subsequent examination showed the presence of very hard deposits on the surface (Figure 7.32). Isolated, but deep pits were present under the hard deposits (Figure 7.33). The measured thickness of the tube, mechanical tests, chemical analysis and etching showed the tubes to conform to the properties specified for SA 179 tubes. Examination of cooling water side tubing showed mild pitting and a hard, sticky, uniform deposit inside the tube (Figure 7.34). At some points the deposit was in the form

Figure 7.30

Hard deposits inside GE3B tubes1

484

Corrosion Prevention and Protection

Figure 7.31

GE3B tube outside surface shows no corrosion or pitting1

Figure 7.32 GE3B tube filled with deposits1

Figure 7.33

Deep pitting inside GE3B tube1

Practical Solutions

485

Figure 7.34 Hard deposit inside GE16 tube1

of lumps. Deep pits were also observed below the solid deposits, and perforation below the deposit in one case (Figure 7.35). Measurement of the thickness of the tubes showed no general thinning. Flow measurements showed flow rates lower than expected and backwashing was observed. The backwash contained brownish siliceous matter and other solid impurities. All three heat exchangers had silt and dirt deposits and partially plugged tubes. The tube ends were corroded and thinned down. Perforation of the tubes originated from inside (cooling water side). Pitting shows it to be underdeposit corrosion. A commercial cooling water program during start-up and stabilization of the cooling tower after major repairs is likely to avoid the adverse results. The water quality should be maintained.

Reference 1. R.J. Sampathkumar and A.N. Screeram – International Congress, Gloccorr 2002, National Corrosion Council of India.

Figure 7.35 Perforated GE 16 tube1

486

Corrosion Prevention and Protection

7.9 Pitting Corrosion of 90/10 Cupronickel Chiller Tubes The in-service 90/10 cupro-nickel tubes in a water-chiller system suffered from severe corrosion damage during their relatively short service life causing unexpected down time. Figure 7.36 shows the top section of the condenser in question where large numbers of tubes are undergoing internal corrosion. Several tube sections were opened and subjected to SEM and energy dispersive X-ray (EDS) to investigate the corrosion morphology and to identify the chemical composition and elements involved in the corrosion process. 7.9.1

Optical Examination

Various sections of the opened tubes were examined by optical techniques. Figure 7.37a shows one of many pits where there are several pinhole-sized pits surrounding a large pit that appears to have burst open, and most of its corrosion products were washed away, although areas of different colors are clearly seen. The light-blue product consists of copper salts, including CuO/Cu(OH)2,CuCl2, etc.; the red is a layer of redeposited Cu/Cu2O, and the golden color is the brass substrate. 7.9.2

SEM and EDS Studies

Microscopic examination revealed the pit initiation. Figure 7.37b shows a typical small pit (the size of a pinhole) observed on the inside surface of the tube. It is clear that a layer of deposit can be seen. EDS analysis was carried out in three of the representative locations as shown in Figure 7.37b: (i) on the surface of the deposit layer, (ii) inside a pit, and (iii) immediately outside the pit. Higher magnification SEM images revealing details of the bottom the pit are shown in Figure 7.37c and d, respectively. EDS study of location A showed O, Al, Si, Fe and Cl peaks. It is suspected that the deposit layer consists of iron

Figure 7.36 (Plate 1) Top section of a condenser. White indicates tubes plugged with extensive internal corrosion; blue indicates tubes identified with internal corrosion to be monitored; and orange indicates tubes previously replaced. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

Practical Solutions

487

Figure 7.37 (a) A large pit that was opened and washed (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM); (b) SEM photo of a tiny pit outside the large pit; (c) a close-up of a pit; (d) a close-up of the inside of as it

oxide and fine sand deposit. The Cl peak appears strong. Many other locations were also examined with similar results. Sulfur peaks were seen. EDS study at location B, at the bottom of a pit showed that location was mainly composed of Cu and Ni with a small amount of Fe, which could be attributed to contamination since Fe was not detected in some other pits. The EDS result indicates no ‘denickelification’ inside the pit since both Ni and Cu were found, and the crystals appear to be compact with no evidence of any copper crystal deposit or selective nickel dissolution leaving a porous structure. It should also be noted that there was no corrosion product at the bottom of the pit, and there was clear evidence of copper redeposit at the edge of the pit, as indicated in EDS of the copper and oxygen peaks. 7.9.3

Pitting Initiation and Propagation Mechanism

The main mode of attack is pitting of Cu-Ni 90/10 tubes. Pit-like attack in condenser tubes is often caused by impingement that occurs at sites where large air bubbles impinge on the tube surface and break into small bubbles on impact, causing local destruction of

488

Corrosion Prevention and Protection

Figure 7.38 (a) Copper oxide ringlets observed around newly initial pits; (b) copper oxide ringlets observed around larger pits. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

the protective film. However, pits produced in this way have a characteristic shape; they are elongated in the direction of the flow and are usually clean and free of corrosion products. The pitting corrosion observed in this study did not show any of these characteristics. In contrast, copper oxide ringlets were observed around the pits, indicating that the corrosion occurred in an environment with appropriate concentrations of corrosive ions, moisture and oxygen, as indicated in Figure 7.38. This observation suggests that the condenser might not have been kept full (with water) for a period of time. Pitting probably occurs due to the reactions: Low pH (anodic): Cu 2e ¼¼ Cu2þ

Ni 2e ¼¼ Ni

2þ

1ðaÞ 1ðbÞ

Neutral solution (pH close to 7) (anodic): Cu 2e þ H2 O ¼¼ CuðOHÞ2 þ 2Hþ

2ðaÞ

Ni 2e þ H2 O ¼¼ NiðOHÞ2 þ 2Hþ

2ðbÞ

Cathodic reactions: ½ O2 þ H2 O þ 2e ¼¼ 2 OH Cu

2þ

þ 2e ¼¼ Cu ðredepositÞ

3ðaÞ 3ðbÞ

Anodic reactions Equation (1a,b) lead to dissolution of metal (copper–nickel 90/10). At the beginning of pit initiation, the reactions of Equation (2a,b) might take place,

Practical Solutions

489

resulting in a local pH decrease. Cathodic reaction (Equation 3a) also likely occurred during pit initiation due to the abundance of available oxygen. However, pit propagation can take place without oxygen as the copper ions redeposit (Equation 3b). This reaction is thermodynamically driven by the low pH and high copper ion concentration inside a pit and supported by observation of copper redeposit around a pit. It appears that the denickelification is a result of pit propagation. It is possible that the copper redeposit reaction accelerates the pit propagation, thus reducing the lifetime of the condenser tubes. 7.9.4

Conclusion

The results of this study indicate that pit initiation takes place in areas where the oxide film is broken or damaged under a stagnant environment in the presence of sufficient moisture and oxygen. It is possible that pit propagation could occur without oxygen, and that it is accelerated by the copper redeposit reaction. In such a case, preventing pit initiation becomes very important.

Bibliography 1. W.S. Janssen, Corrosion Control in a Refinery Sea Water Cooling System, Materials Protection, 1(10), 42–53 (1962). 2. W. Matthewman and G.J. Evans, Power Station Condensers, Corrosion Technology, 1964, 15–17. 3. I.G. Slater, L.Kenworth and R.May, Corrosion and Related Problems in Sea-water Cooling and Pipe Systems in H.M. Ships, Corrosion, 8(12), 417–429 (1952). 4. H.A.Todhunter, Material Selection for Condenser Tubes, Corrosion, 11, 39–44 (1955). 5. CTL Report, Forensic Analysis of Condenser Tubes from York Chiller in Ottawa, April 2003. 6. Rt Worthington, Copper–Nickel Tubes, their Advantages for Steam Condensers, Metal Progress, 24(1), 20–24 (1933). 7. V.D. Baan, Experience with Condenser Tubes at a Major Oil Refinery, Corrosion, 6(1), 14–18 (1950). 8. N.V. Nowlan, Influence of Water Movement on Corrosion, Corrosion Technology, 397–399 (1960). 9. R.C. Mifflin and D.B. Bird, Performance of tube Alloys Cooled by Brackish Delaware River Water, Materials Protection, 8(9), 72–76 (1969).

7.10 Weld Metal Overlay: a Cost-effective Solution to High-temperature Corrosion and Wear Problems High-temperature corrosion and wear is encountered in various industries such as waste incineration, fossil energy, pulp and paper, petroleum refining, chemical and petrochemical, mining and smelting operations. One of the methods to combat corrosion and wear and its control is to select suitable material, i.e., an alloy, for the plant design and maintenance. The selection of proper material for plant design and fabrication is followed

490

Corrosion Prevention and Protection

Table 7.4 Applications of overlay technology Application

Corrosives

Number of Boilers using the overlay

Waste-to-energy boilers

Municipal waste containing chloride, sulfur, alkali metals, zinc and lead Sulfidation attack; boiler tube wastage (50–60 mpy) Thiosulfate and polysulfides Sulfate, thiosulfate Chloride

59 (alloy 625 overlay weld metal used)

Coal-fired Boilers Pulp and paper digesters Kraft recovery boilers

8 (alloy 625 and 309 SS) 21 (overlaid with 309 SS) 11 (309 L on lower furnace sidewalls; 625 on floor tubes and smelt opening)

by a weld metal overlay of the plant equipment so that failures due to corrosion and wear may be avoided. Welding Services Inc. has developed Unifuse overlay technology based on pulse spray gas metal arc welding (PSGMAW). The technology has made significant improvements in welding metallurgy to achieve low heat input, fast deposition rates covering large areas, sound metallurgical bonds with minimal lack of fusion and other defects, low dilution for the overlay chemistry and minimal distortion for the part being overlaid. Some of the industrial applications of uniform overlay technology are noted in Table 7.4. Failed tube 310 stainless steel in a waste-to-energy boiler is depicted in Figure 7.39. Figure 7.40 shows the appearance of uniform composite tubes with alloy 625 overlay on Cr, Mo steel. Figure 7.41 shows the 309 overlay on Cr, Mo boiler tube in service in a coal-fired boiler devoid of corrosion and cracking for 7 yr.

Figure 7.39 A Type 310 SS tube failed after less than 2 yr service in a waste-to-energy boiler (Reproduced from COM’ 1997 with permission from the metallurgy Society of CIM)

Figure 7.40 Surface appearance of the Unifuse composite tubes with alloy 625 weld overlay (Reproduced from COM’ 1997 with permission from the metallurgy Society of CIM)

Figure 7.41 309 Unifuse overlay on 1-1/4Cr-1/2Mo boiler tube in the wall blower region after service for about 7 yr in a supercritical unit of the coal-fired boiler: (a) surface appearance of the weld overlay, weld beads are still clearly visible; (b) cross-section of the weld overlay showing no corrosion and no cracking on the weld overlay at the fusion line (Reproduced from COM’ 1997 with permission from the metallurgy Society of CIM)

492

Corrosion Prevention and Protection

Bibliography G. Lai, M. Jirinec, P. Hulsizer and F. Novac, Proceedings of International Symposium on Corrosion and wear of Metals: Metallurgical Society of Canadian Institute of Mining, 36th Annual Conference, Sudbury, Ontario, Canada, August 1997.

7.11 Equipment Cracking Failure Case Studies One factor, which should always be considered when performing materials failure investigations, is the influence of human error. This section provides three examples of equipment failures due to cracking for which the root cause of failure was found to be associated with human error. 1. The first involves the misapplication of chromium electroplating to an engine crankshaft during a rebuild procedure, which resulted in a fatigue failure shortly after the unit was put back into service. 2. The second failure involves an error made during the weld repair of an electric motor drive shaft, which again resulted in a fatigue failure. 3. The third example describes mistakes made during the design, procurement and construction of a high-pressure steam piping system, resulting in a catastrophic failure of a pipe connector, due to chloride stress corrosion cracking. Material failures can be caused by a number of factors, either individually, or in combination, including: substandard materials, inappropriate materials selection, poor design, equipment abuse, unexpected stresses or environmental conditions, and poor maintenance practices and/or neglect. Many failures, in one way or another, involve human error to some extent. The failure investigations described in this paper illustrate two situations where human error during common maintenance and repair activities ultimately played a role in the failure of industrial machinery. The third failure investigation revealed how, even when the potential for failure is known (i.e., SCC of 304 SS), human error during the procurement and/or installation phase of the piping clamps resulted in an SCC-susceptible material being installed and ultimately failing in a catastrophic manner. 7.11.1

Industrial Engine Crankshaft Failure

This failure involved the crankshaft from an eight cylinder 2400 horsepower naturalgas-fired engine, operating at a speed of approximately 900 rpm, and used to drive a gas compressor. The engine had operated virtually trouble-free for approximately 50 000 h (’ 5 ½ yr), having been subjected to the normal oil changes and other regular maintenance practices. After 50 000 h, the engine was torn down for a major overhaul, as is common practice for industrial engines of this type. The crankshaft was removed for inspection and polishing of the bearing journal surfaces at an experienced engine maintenance shop. No cracking or abnormal wear was observed during this overhaul; however, measurements of the critical crankshaft dimensions after polishing revealed that the journals had been over-polished, resulting in the journal diameters being approximately 0.03 mm under tolerance. At this time, it was agreed between the

Practical Solutions

493

Figure 7.42 Appearance of large crankshaft crack. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

maintenance shop and the engine owner that the journals would be lightly machined and then built back up with the application of a chromium electroplating layer. The crankshaft journal dimensions were back within tolerance after the machining, electroplating and honing procedures. The engine was reassembled, including all new crankshaft bearings and piston liners. After recommissioning, the engine operated for approximately 2000 h (’83 days), at which time the crankshaft failure was discovered. It was noted by the operating staff that the engine exhibited abnormal operating characteristics in the days leading up to the discovery of the failure. The crankshaft was removed from the engine and was found to be cracked through the web between the last rod journal and the last main journal, next to the flywheel, as shown in Figure 7.42. Failure Investigation. The crack shown in Figure 7.42 had propagated through almost the entire cross-sectional area of the crankshaft web. The remaining ligament was broken open to expose the crack faces where, as is shown in Figure 7.43, it became readily

Figure 7.43 Fatigue fracture face, showing crack growth direction. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

494

Corrosion Prevention and Protection

Figure 7.44 Prepared metallographic specimen: F ¼ fatigue fracture; W ¼ web; J ¼ journal. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

apparent that the failure was associated with a high-cycle, low-stress fatigue crack, which initiated in the machined fillet at the edge of the main journal and had propagated through the web until it reached the fillet of the adjacent rod journal. The fracture faces exhibited the characteristic ratchet marks, beach marks and smooth texture, typically associated with high-cycle fatigue. Wet fluorescent magnetic particle inspection of the remaining journal fillets on this crankshaft revealed the presence of a number of secondary cracks in the same orientation and position as the failure initiation site. A metallographic specimen, including the matching halves of the fracture from the initiation site in the fillet of main journal, was cut, mounted, polished and etched in 2% nital, in preparation for examination with an optical microscope. The polished and etched specimen is shown in Figure 7.44. Metallographic examination of the prepared specimen revealed the presence of a very thin chromium layer on the journal side of the fracture. As can be seen in the photomicrograph in Figure 7.45, the fracture initiated at the tip of this tapered chromium layer. The underlying crankshaft microstructure was a relatively coarse (ASTM E112 grain size ’7 : m) mixture of ferrite and pearlite, with a measured Rockwell hardness of 90–95 HRB. There was no evidence of prior shot peening in this fillet region. The fracture profile was nonbranching and transgranular, consistent with high-cycle fatigue. A secondary crack, exhibiting a mixed intergranular/transgranular morphology, is also evident on the left side of the photomicrograph. Due to the observed presence of the chromium layer adjacent to the fatigue initiation, Canspec performed a macroetching procedure, using 15% nital, on several of the other journals in the fillet regions. Since the nitric acid etch will darken a machined steel surface, but will not affect chromium, this etching technique was capable of delineating the edges of the chromium electroplated layer on the journals. From this etching examination, a number of journal fillets were found to have chromium electroplating present. Conclusions. This case study reveals how a common and normally acceptable engine overhaul procedure can lead to premature failure. There are a number of industry

Practical Solutions

495

Figure 7.45 Chromium layer (arrow) at fatigue initiation site (F). (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

specifications, including ASTM, Military (MIL) and U.S. Federal Specifications (QQ), that warn of the dangers of fatigue cracking when using chromium electroplated steels in dynamically loaded service environments. These specifications go on to describe the recommended cleaning, shot peening, electroplating, heat treating and quality control procedures that should be employed for the use of industrial hard chromium coatings. In particular, for dynamically loaded steel components, there are guidelines for shot peening of steel surfaces to be coated, as well as advice with respect to masking high-stress areas (e.g., journal fillets) from the electroplating process. It is apparent that, for the crankshaft in this case, the fillet regions were not shot peened and the chromium electroplating was inadvertently applied to many of the journal fillets, where masking should have been used. 7.11.2

Electric Motor Drive Shaft Failure

A 350 horsepower electric motor drive shaft broke adjacent to the coupling, which was connected to a three-stage compressor. The motor operated at a speed of 1200 rpm and had run for a period of approximately 5 yr. Approximately 3.5 yr after the initial start-up, the motor shaft had been severely damaged by wear, due to a lack of lubrication on the bearings, as a result of poor maintenance practices. A repair was made by machining the shaft down to sound metal and building up the shaft surface to its original diameter with a weld overlay. After approximately 1.5 yr of service, the shaft broke in the region of this weld repair. The original shaft material was reported to be a medium-carbon, low-alloy Ni–Cr–Mo steel. Failure Investigation. The shaft broke at a 45 angle and the macroscopic fracture morphology was consistent with high-cycle fatigue (i.e., macroscopically smooth, ratchet marks at the origin and concentric beach marks radiating from the origin). The 45 fracture orientation was consistent with torsional fatigue. The fatigue crack had propagated through almost the entire 4-inch diameter shaft, indicating that the nominal stresses on the shaft were relatively low, in comparison to the shaft strength. As shown in Figure 7.46, a foreign material was embedded in the shaft just below the surface and

496

Corrosion Prevention and Protection

Figure 7.46 Drive shaft fatigue fracture face, showing initiation point (arrow) and foreign keyway material (K)

coincident with the fatigue initiation point. It was subsequently determined that it is common practice to insert a ‘dummy’ material into an old shaft keyway when applying a weld overlay to repair a worn shaft. The use of a ‘dummy’ key saves machining and welding time, thus minimizing the repair cost. Metallographic examinations of a cross-section through the ‘dummy’ key at the fatigue origin revealed evidence of slag and nonfusion associated with the half-inch-thick weld overlay. There was also a distinct difference observed in the microstructural appearance between the overlay and the original underlying shaft material. A photograph of the polished and etched cross section is shown in Figure 7.47. The weld overlay was analyzed and found to be a plain carbon steel, with a hardness of Rockwell 80 HRB, consistent with a low-strength welding consumable (e.g., E7018). The

Figure 7.47 Polished and etched section, showing fracture profile (F), weld overlay (W), foreign key material (K) and original shaft material (S)

Practical Solutions

497

original shaft material was found to be a normalized plain carbon steel (AISI 1040), with a hardness of 90 HRB. It was noted that the shaft material did not comply with the specified Ni–Cr–Mo low alloy steel composition, and was significantly softer than this specified shaft alloy. Conclusions. Most of the information associated with the cause of the failure could be acquired from the visual examination of the fracture face. The fracture morphology and orientation indicated that this was a torsional fatigue failure. The presence of a foreign (nonfused) material embedded in the former keyway and coincident with the fatigue initiation point suggests that this subsurface anomaly acted as the stress raiser responsible for the fatigue initiation. The fact that the fatigue crack had propagated through almost the entire shaft thickness without the shaft breaking indicated that the applied stresses on this shaft were relatively low. In addition to the nonfusion defects and other flaws in the weld overlay repair, the relatively low strength of the weld overlay material (80 HRB) undoubtedly exacerbated the problem. This low-strength weld deposit would be expected to have relatively low fatigue resistance. In addition, it was determined that the original shaft material did not comply with the compositional and hardness requirements specified by the manufacturer. Again, the lower than expected strength of the shaft core material would also have a detrimental effect on the fatigue strength of the shaft. 7.11.3

Pipe Clamp Joint Connector Failure

A clamp-type pipe joint connector failed a short time after it was put into service. This clamp, shown in Figure 7.48, had been used to join two ends of NPS 8 carbon steel steam piping, operating at a pressure of approximately 11 MPa (1600 psi). The boiler feedwater used in this process had been softened by cation exchange (Ca $ Na) and was fully deaerated. The feedwater was recycled water from an oilfield operation and contained several thousand ppm dissolved chlorides. The steam was 80% quality and at a temperature of approximately 330 C.

Figure 7.48 Fractured pipe clamp (arrows) and fasteners. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

498

Corrosion Prevention and Protection

As can be seen in Figure 7.48, the pipe clamp consisted of two U-shaped half-shells, with two 1 ½-inch threaded studs at each end, used to tighten the two half-shells together around the nubbin ends of the pipe. The failure involved the complete fracture of the bottom half-shell, as it was positioned on the pipe. All four of the threaded fasteners were still intact, with no evidence of cracking, although the studs had been bent as a result of the failure. The failure was preceded by a noticeable steam leak at this piping joint. Attempts were made to tighten to clamp, but this failed to stop the leak. The ensuing clamp fracture and pipe rupture caused an extensive amount of damage within the process building. Fortunately, no one was present in the building at the time of the failure and there were no injuries. Failure Investigation. The clamp was nonmagnetic and the stamped identification on the side of the U-shaped shells indicated that they were fabricated from forged 304 austenitic stainless steel. Visual examination of the fracture surfaces revealed they were entirely brittle and exhibited a very coarse fracture morphology. Liquid penetrant inspection revealed the presence of additional cracks in the fractured half of the clamp. There was no cracking present in the high-strength steel fasteners. Chemical analysis of scale deposits present on the surface of the failed clamp by X-ray diffraction revealed the presence of predominantly sodium iron oxide, sodium carbonate sodium chloride (10%), iron oxide and iron sulfide. The scale composition was consistent with the evaporated residue from the 80% quality steam, which had been leaking from the joint prior to the failure. The high sodium concentration in the scale was attributed to the zeolite ion exchange system used to soften the boiler feedwater, while the chlorides and sulfides were naturally present in the feedwater. Metallographic examinations of a cross-section through the failed clamp material confirmed it was an austenitic stainless steel and that extensive branched transgranular cracks were present throughout the material, as illustrated in Figure 7.49. Conclusions. Examination of the failed pipe clamp revealed the presence of brittle fracture surfaces as well as an extensive array of branched transgranular cracks. The identification of the clamp material indicated it was a forged 304 stainless steel component

Figure 7.49

Photomicrograph of transgranular cracking

Practical Solutions

499

and metallographic examination confirmed this. The high-pressure 80% quality steam being handled by the piping was produced from treated recycled water from oil production fluids and contained several thousand parts per million of dissolved chlorides. The pipe joint had been leaking steam and water prior to the failure, and chemical analysis of the scale deposits on the clamp surface after the failure confirmed the presence of a number of sodium-based mineral compounds from the leaking steam, including approximately 10% sodium chloride. The presence of high concentrations of moist, hot chloride salts on the highly stressed austenitic stainless steel surface, particularly with concurrent exposure to atmospheric oxygen, created an ideal chloride stress–corrosion cracking (SCC) environment. It could also be argued that this failure would not have occurred if the steam piping had been joined by welding, instead of using high-pressure clamps. However, these types of pipe joint connectors are reportedly commonly used in many process piping installations, including drilling rigs, power plants and petrochemical plants, without incident. The root cause of this failure was the human error associated with the installation of stainless steel in this service. It was reported that the engineering personnel involved in the design and construction of this steam plant were aware of the potential for SCC of stainless steels in this hot saline process. For this reason, all of the piping and clamp materials were specified to be carbon steel during the design phase. A review of the remaining pipe clamps in the process after the failure revealed the presence of one additional stainless steel clamp, while the rest were forged carbon steel, as specified. It was then determined that, due to a shortage of carbon steel clamps of the appropriate pressure rating for NPS 8 piping, these two stainless steel clamps were substituted as an ‘upgraded’ replacement material. Unfortunately, the potential catastrophic consequences of this materials ‘upgrade’ were not appreciated by those involved in the procurement and installation of these clamps who were aware of the material substitution.

7.12 Failure of a Conveyor Drive Shaft A steel coal-conveyor drive shaft failed in service due to the occurrence of a transverse crack that passed through the right keyway near the centre of the keyway as shown in Figure 7.50. The shaft was cut so that the crack could be opened to expose the corroded mating fracture surfaces. After cleaning with hydrochloric acid, examination of the fracture surfaces showed them to be relatively smooth textured, flat and perpendicular to the axis of the shaft and the keyway. Two sets of crack arrest marks was concentric to

Figure 7.50 Coal-conveyor drive shaft that cracked in service. (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

500

Corrosion Prevention and Protection

Figure 7.51 The fracture surfaces were relatively smooth-textured and flat; two sets of concentric crack arrest marks were visible on the fracture surface. (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

each corner of the keyway, as shown in Figure 7.51. Transverse cross-sections were found to have secondary cracks (Figure 7.52). The chemical analysis, mechanical properties, hardness and impact properties were determined and found to conform to the specifications of the German Grade 30 NiMoCr8 alloy steel suitable for use as a shaft. 7.12.1

Conclusions

The conveyor drive shaft failed as a results of corrosion fatigue in bending. Failure was initiated at both corners of the keyway. Numerous secondary fatigue cracks were observed, indicating the presence of a large number of stress concentrator sites that caused by presence of corrosion pitting on the surface. Fatigue cracks initiate at locations of maximum local stress and/or minimum local strength. It is concluded that the conveyor drive shaft failed as a result of corrosion fatigue in bending. The use of protective coatings or some other means of shielding the shaft from the corrosive medium could prolong the life of the drive shaft.

Figure 7.52 Secondary cracks. (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

Practical Solutions

501

7.13 Failure Analysis of Copper Pipe in a Sprinkler System Two type of copper piping, namely straight sections and T-sections, were examined for the forms of corrosion present in the samples. The water samples and the corrosion products were tested for microbiological activity. 7.13.1

Observations

The straight section of the pipe showed dark red and greenish spots on the outer surface, (Figure 7.53); through-wall pits were present in the red spots. The T-sections did not have reddish spots on the external surface (Figure 7.54), but one sample had a crack (Figure 7.55). This crack may be due to poor fabrication technique. The appearance of the joint is suggestive of lack of shielding and overheating which might allow penetration of oxygen, leading to cracking. Internal examination of pipe sections showed thick black adherent scale, (Figure 7.56). Green deposits were also present (Figure 7.57). Some areas in the pipe showed the

Figure 7.53 (Plate 2)

Figure 7.54 (Plate 3)

Dark red and greenish spots on the outside surface

Sample did not display the reddish spots on the exterior surface

502

Corrosion Prevention and Protection

Figure 7.55 (Plate 4) Sample did not display the reddish spots on the exterior surface, even though the surface contained a visible crack

Figure 7.56 (Plate 5) scale

Internal surface of pipe is coated with a thick, black, tightly adherent

Figure 7.57 (Plate 6)

Internal surface shows green deposits

Practical Solutions

Figure 7.58 (Plate 7)

503

Bright shiny copper can be seen with greenish deposits

presence of both green deposits and shiny copper (Figure 7.58). Pits were present in areas where copper is surrounded by green deposits (Figure 7.59). The green deposits indicate significant corrosion. The black scale was carefully removed to identify the morphology of pits (Figure 7.60). The pits were located at the bottom of a hole (Figure 7.61). The morphology of the pits is characteristic of microbiologically influenced corrosion (MIC).

Figure 7.59

Well-developed pits, with pits impinging upon one another

Figure 7.60 Pits impinging upon one another

504

Corrosion Prevention and Protection

Figure 7.61 Many pits were elongated, but retained a smooth, scooped-out appearance characteristic of MIC

The T-sections were free from pitting and the inside surface was clean. The water samples and the corrosion products were analyzed for microbiological activity. The water contained significant amount of iron-reducing biological activity. 7.13.2

Conclusions

Significant microbiologically induced corrosion due to the presence of bacteria in the water is evidenced by saucer-shaped pits, smooth sided pits, bright shiny copper to matte red clean areas. The black deposits, corrosion products from carbon steel, may cause underdeposit corrosion and may cause the failure. Treatment of the water with biocide may minimize microbiologically induced corrosion.

7.14 Failure of Rock Bolts The properties and design of rock bolts are available in the literature.1 A Swellex rock bolt is shown in Figure 7.62. The modes of support for rock bolts can be: (i) (ii) (iii) (iv)

beam building; suspension; pressure arch; support of discrete bolts.

Figure 7.62 Swellex bolt, after Atlas Copco2

Practical Solutions

7.14.1

505

Corrosion Modes of Rock Bolts

The corrosion modes are: (i) uniform corrosion; (ii) localized corrosion pitting or crevice corrosion; (iii) galvanic corrosion. Factors involved in corrosion are: low-pH acid mine water due to oxidation of iron sulfides or bacterial oxidation such as SRB; carbon dioxide; chloride and sulfate (10 ppm Cl, 25 ppm sulfate; 1080 ppm Cl; 300 ppm sulfate). Mine air contains SO2 and NO2. 7.14.2

Fracture and Failure

A bolt fails when it can no longer provide the support it is designed for. Fracture is the separation of a solid body into two or more parts under the action of stress. It consists of crack initiation and propagation. Ductile fracture is characterized by considerable plastic deformation prior to and during propagation of the crack. An important amount of gross deformation is usually present at the fracture surfaces. Figure 7.63 is an example of a ductile fracture of a Swellex bolt. Note the failure plane at 45 and localized reduction of the surface. Brittle fracture in metals is characterized by a rapid rate of crack propagation, with no gross deformation and very little microdeformation. This is demonstrated by cleavage. Brittle fracture can occur without warning. Cleavage fracture exhibits little or no plastic deformation and occurs along well-defined crystallographic planes. A ‘dimple’ is a concave depression on the fracture surface resulting from microvoid growth in coalescence. As a result of the state of stress during fracture, the dimple may be elongated, oval or equiaxed. The average composition of Swellex bolts is given in Table 7.5. Visual appearance of a fracture is shown in Figure 7.63.

Figure 7.63 Visual examination of a fracture

506

Corrosion Prevention and Protection

Table 7.5 Swellex bolt composition, SAE J403 grade 1010 C 0.099

0.377

0.017

0.005

0.010

0.003

0.041

0.038

0.022

0.034

Figure 7.64 shows a failed bolt where the failure surface is investigated by SEM at 1000 magnification. The observed dimples are relatively shallow with respect to the surface and oblique. The presence of inclusions as in (A), characteristic of this type of steel that initiate dimples. The shape and orientation of dimples can be used to indicate the direction and application of load as well as the degree of ductility of the samples. In this case, as the dimples are parallel to the direction of fracture, this was classified as a shear fracture. Figures 7.64 and 7.65 provide a view of the fracture surface of a bolt recovered following a fall of ground from a hardrock mine. This is at magnification 10. A closeup of dimpled areas is presented in Figure 7.66, at a magnification 1500. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasicleavage, is observed (absence of ‘river’ pattern that would be associated with brittle fracture). In the upper right part of the photo dimpled areas are masked by oxidation. Metallographic analysis allows the determination of the surface reduction along the fracture and defines the fragile or ductile nature of the fracture. By employing a longitudinal section, it was possible to evaluate the magnitude of corrosion close to the surface of the fracture. Figure 7.67 shows an extremely irregular attack of the external surface of a longitudinal section of rock bolt. The presence of the oxide layer in Figure 7.67 and the dispersion of a metallic piece in Figure 7.68 support the analysis that, for this particular rock bolt, corrosion attack was rigorous and probably contributed in the failure of the rock bolt. It is useful to conduct metallographic analysis of the transverse sections, far away from the fracture surface as a reference. Figure 7.69 is a micrograph which shows typical equiaxed grains, free from plastic deformation. However, oblique section metallography through the fracture surface shows the presence of elongated grains developed during shearing at failure (Figure 6.70).

Figure 7.64 Fractography of a Swellex bolt

Practical Solutions

507

Figure 7.65 Fractography of failure surface, 10

Figure 7.66 Close-up of the failure surface of Figure 7.64, 1500

Figure 7.67 Longitudinal section showing extreme localized attack and fragmentation of the metallic surface

508

Corrosion Prevention and Protection

Figure 7.68 Longitudinal section displaying extreme localized attack and fragmentation of the metallic surface

Figure 7.69

Micrograph away from the fracture surface, showing equiaxed grains

Figure 7.70

Oblique section metallograph showing elongated grains

Practical Solutions

509

References 1. J. Hadjigeorgiou and F. Charette, Rock Bolting for underground Excavations, Chap. 63 of Underground Mining, Society of Mining Engineers, Hustrulid & Bullock, eds, pp 547–554, 2001. 2. V.S. Sastri, G.R. Hoey and R.W. Revie, CIM Bulletin, p. 87–99, 1994.

7.15 Failure Analysis of 316L Stainless Steel Tubing of a High-pressure Still Condenser 7.15.1

Problem

Corrosion problems were observed with stainless steel tubing of a high-pressure still condenser employed for ammonia recovery. The feed solution in the high-pressure still is typically 135 g/L ammonia and 4.7 g/L CO2. Figure 7.71 shows a simplified view of the high-pressure still (HPS) and condenser. The HPS condenser is made of 316L seamless tubes with a nominal outer diameter of 25 mm, a nominal wall thickness of 2.2 mm and a length of 6.4 m. The tube ends are joined to the tube sheet (100 mm thick) to form a bundle 1.1 m in diameter that is welded to the shell to make up the shell-and-tube condenser. The tubes are weld sealed at the top of the tube sheet as shown in Figure 7.72. The condenser is installed vertically with severe operating conditions1 at the inlet of the top tube sheet as the temperature reaches 150 C and pressure 250 psi. Corrosion has resulted in leak failures of many tubes and led to several shutdowns. The corrosion attack was evident in the top 100 mm where the tube was expanded (rolled) to

to evaporator and cooling water systems

High Pressure Still

Condenser

Preheaters low strength NH3

Reboiler

to cooling tower

solutions from vent gas recovery scrubbers and

steam

aqua storage spheres

distillate storage and reuse

still bottoms cooling water

Figure 7.71 A simplified view of the high-pressure still (HPS) and condenser. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

510

Corrosion Prevention and Protection Seal weld

316L stainless steel tube

Rolled joint ~100 mm Tubesheet

Condensate

Vapor phase (Dead space)

Vapor Cooling water Process fluid

Figure 7.72 Schematic, showing a section of the top portion of the HPS condenser (corrosion end). (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

join the top tube sheet, but the rest of the tube was still intact and in its original condition. The rolled end of the tube is referred to as the corrosion end. Repairing the condenser by plugging the leaking tubes was unsuccessful, as evidenced by severe corrosion of the plugs, top tube sheet and seal welds. Plugging the tubes affected flow and caused turbulence which eroded the passive film on the steel plugs and the top plate severe corrosion of the plugs, top tube sheet and seal welds was evident.2

7.15.2

Material

Table 7.6 shows the chemical composition of SA213 316L stainless steel. The microstructure was determined to be an equiaxed structure with average grain size of 50 mm, measured by the mean intercept method.

7.15.3

Results

A portion of the tube about 3 cm long containing the cracks was cut and examined by SEM. The inner surfaces of the three tubes were covered with a mixed black/brown scale that extended for about 100 mm over the length of the tube-to-tubesheet joint. The

Table 7.6 Nominal chemical composition (wt %) of the 316L stainless steel Cr 16.2

12.1

0.02

1.6

0.0293

0.39

0.2

1.6

Bal

Practical Solutions

511

Figure 7.73 (Plate 8) Section of tube showing the inner surface covered with a mixed black/brown scale that extended over the rolled portion of the tube for 100 mm. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

remaining part of the tubes’ inner surfaces appears to be unaffected, and the original protective oxide layer was still intact. A section of tube A is shown in Figure 7.73 where the inner surface was covered with a mixed black/brown scale that extended over the whole length of the tube joint. 7.15.4

SEM Examination

SEM examination revealed several major circumferential cracks, ranging from 3 to 4 mm long. Table 7.7 lists the number of major cracks seen on each tube. Figure 7.74 is an example of a circumferential crack seen in tube B. Circumferential cracks appeared to grow in a zigzag fashion with their tips close together. Figure 7.75 is an optical photo of a typical chloride SCC propagated in the axial direction. Some smaller cracks showed a combination of axial and circumferential growth. Microcracks were also observed as well as signs of crevice corrosion in the rough area adjacent to the cracks shown in Figure 7.74. Because oxygen diffusion into the crevice is restricted, a differential aeration cell is formed between the crevice and the bulk environment. Differential cells can lead to the creation of highly corrosive conditions, even in a benign bulk environment. Longitudinal sectioning of the tube wall revealed multiple crack initiation that took place on the outer surface of tube B as shown in Figure 7.76. Note the direction of the crack growth shown in Figure 7.76 is from top (outer surface) to bottom (inner surface).

Table 7.7 Number of major cracks and their orientations observed for each tube

Tube A Tube B Tube C

Longitudinal cracks

Circumferential cracks

Mixed orientation

0 0 1

0 4 6

0 1 1

512

Corrosion Prevention and Protection

Figure 7.74 SEM photo of one of the major circumferential cracks observed in tube B. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

The cracks showed significant branching. In the case of tube B, none of the cracks penetrated the wall thickness; they appeared to extend through only about one third of the wall thickness. Cracks propagated in both axial and circumferential directions to form semi-elliptical cracks. The cracks were fine in the early stages of growth, and they widened by secondary corrosion at a later stage due to the aggressive crevice conditions. More cracks were evident in tube C with significant secondary corrosion leading to wider cracks. In some cases, wide cracks appeared to develop into deep pits. Figure 7.77 shows one of the cracks that penetrated the whole wall thickness and resulted in a leak failure accompanied with significant plastic deformation. Tube A was the least attacked, and it showed much smaller cracks. 7.15.5

Conclusions

Failure analysis revealed that the tubes had suffered both internal and external corrosion attack; the corrosion attack was confined to the region within the joint between the tube

Figure 7.75 Optical photo of typical chloride SCC of austenitic stainless steel observed in the longitudinal direction in tube C. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

Practical Solutions

513

Figure 7.76 Longitudinal section of tube B wall, showing branching and multiple initiation of SCC at the external surface of the tube. (Reproduced from COM’1999 and 2005 with permission from the Metallurgy Society of CIM)

Figure 7.77 A circumferential crack that penetrated the whole thickness of tube C, resulting in a leak failure

514

Corrosion Prevention and Protection

and tubesheet. Inlet erosion corrosion was observed inside the tube throughout the length of the joint due to the process fluid that contained ammonia, CO2 and ammonium carbonate solution. Stress–corrosion cracking and crevice corrosion was evident on the external surface of the tube at the rolled end. It was concluded that SCC occurred due to the chloride buildup in the shielded area at the joint in the absence of proper venting. Some of the cracks grew throughout the whole thickness and ultimately led to leakage failure. Most of the SCC attacks were oriented in the circumferential direction, suggesting that the primary source of stress was the tensile residual stress in the axial direction that was created due to tube rolling of the joint. Results suggest that poor venting and tube end over-rolling appear to play a major a role in the degradation of the tube in such a short time. 7.15.6

Prevention

1. Greater care should be taken and proper procedures followed by the condenser manufacturer when rolling the tube ends to avoid excessive wall reduction and tube extrusion. To prevent crevice corrosion in the tube-to-tubesheet joint, the tube end should be rolled for the full thickness of the tube sheet.12 2. Proper venting of the condenser to eliminate the dead space below the top tubesheet is extremely important. This is to ensure that there is no vapor space and that the tubes will be wet all the time, thus avoiding wetting and drying cycles.10 About one-third of SCC failures were caused by the existence of a vapor phase.12,13 The proper method of venting vertical water-cooled condensers, is given in the MTI Manual.4 3. Development of an effective cooling water treatment that minimize chloride and oxygen contents14 would be beneficial. 4. Stress-relief by heat treatment or shot peening is recommended subsequent to rolling of the tube ends to reduce residual stress to a level at which SCC is less likely to occur. 5. Using thin-walled metallic tube shields over the inlet end to combat erosion/corrosion of tube inner surface5, is recommended.

References 1. T.M. Ahmed and A. Alfantazi, High Temperature High Pressure Corrosion in Ammonia Carbamate Environments: Literature Review, Report submitted to Sherritt International Corporation, November 2004. 2. S. Yakemchuk and D. Roth, Equipment Inspection Report, Sherritt Final Report, S.O. 343371, November 23, 2003. 3. C.P. Dillon & Associates, Guidelines for Control of Stress Corrosion Cracking of Nickelbearing Stainless Steels and Nickel Alloys, MTI Manual No. 1, The Materials Technology Institute of the Chemical Process Industries Inc., 1979. 4. P.M. Tallman, Restore Corroded Heat Exchanger Tubes, Chemical Engineering Progress, 96(8), 47–50 (2000). 5. M. Allam, A. Chaaban and A. Bazergui, Estimation of Residual Stresses in Hydraulically Expanded Tube-to-Tubesheet Joints, Journal of Pressure Vessel Technology, 120, 129–137 (1998). 6. D.R. McIntyre, Evaluation of the Cost of Corrosion-Control Methods, Chemical Engineering, April 1982, 127–132.

Practical Solutions

515

7. X. Hu, X. Renb, X. Chen, W. Han and F. Zou, A Study on the Corrosion of 316L Stainless Steel in the Media of High-Pressure Carbamate Condenser, Proceedings of the Asian-Pacific Corrosion Control Conference, Bangkok, December 6–11 1993, 237–244. 8. S. Yokell, Expanded and Welded-and-Expanded Tube-to-Tubesheet Joints, Journal of Pressure Vessel Technology, 114, 157–165 (1992). 9. S. Haruyama, Stress Corrosion Cracking by Cooling Water of Stainless-Steel Shell and Tube Heat Exchangers, Materials Performance, March 1982, 14–19. 10. C.R. Ascolese, D.I. Bain, Take Advantage of Effective Cooling Water Treatment Programs, Chemical Engineering Progress, 94(3), 49–54 (1998).

7.16 Failure of a Landing Gear Steel Pin The landing gear pin is shown in Figure 7.78 and the arrow indicates the location of the failure. The failure occurred in 1.23 inch diameter cylindrical part of the pin which was corroded on the outer surface. The fractured surface is shown in Figure 7.79. Convergent chevron patterns are seen at the initiation site. The fracture surface is covered by the dark corrosion product. A set of coarse low-cycle fatigue crack arrest marks were present at the ends of two discolored crack propagation fronts. The exterior surface of the cylindrical part of the pin had corrosion pits. Pitting corrosion was also seen at the fracture initiation site.

Figure 7.78 Landing gear pin failure.1 (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

Figure 7.79 Fracture surface shows convergent chevron patterns as the fracture initiation site.1 (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

516

Corrosion Prevention and Protection

Figure 7.80 Fracture surface profile shows the presence of a circumferential ridge and depression.1 (Reprinted with the permission of M. Zamanzadeh, ATCO Associates, Pittsburgh, USA)

The fracture surface profile is shown in Figure 7.80. The presence of a circumferential ridge and depression in the cylindrical surface is to be noted in the case of both the broken and reference pins. By comparing the cylindrical parts of the broken pin with the reference pin it was concluded that the fracture of the broken pin initiated at the circumferential depression. Macroetching with 50% hydrochloric acid for 30 s enabled the identification of fracture initiation site in Figure 7.80. A transverse cross-section through the fracture initiation site was examined by metallography. The fracture surface profile was found to be relatively flat and there was no crack branching. The microstructure showed dark-etching-tempered martensite. Further no plastic deformation was observed at the fracture initiation site. It is concluded that the pin failed due to fatigue initiated at the outside cylindrical surface where wear and pitting corrosion occurred. The fracture initiated at a shallow circumferential groove and corrosion of the fracture occurred after the rupture. There was no evidence of stress–corrosion cracking.

Reference 1. M. Zamanzadeh, E. Larkin and D.Gibbon, A Re-Examination of Failure Analysis and Root Cause Determination, Matco Associates, Pittsburgh, Pennsylvania, December 2004.

7.17 Hydrogen-induced Cracking Hydrogen-induced cracking (HIC) is one of several related mechanisms whereby absorbed hydrogen atoms can compromise the integrity of components manufactured from lowstrength steels. HIC is a term applied to phenomena which occur at low temperatures (typically less than about 90 C), and must not be confused with high-temperature hydrogen attack of low-strength carbon-manganese and low-alloy steel materials exposed to hot hydrogen-gas-containing environments. Several different names have been used to identify the types of hydrogen-related damage which have been observed in low strength steels: Hydrogen-induced cracking (HIC): The development of internal cracks in low strength steels as a consequence of the trapping of absorbed hydrogen atoms (H atoms) as gas

Practical Solutions

517

Figure 7.81 Hydrogen-induced cracking in plate steel exposed to sour gas. (Courtesy of Malcolm Hay)

molecules (H2 molecules) at inhom*ogeneities is termed HIC. The cracks most often (but not always) lie parallel to the rolling plane and the surfaces of the steel component. Residual or applied tensile stress is not needed for HIC development (Figure 7.81). Hydrogen blisters or hydrogen-induced blister cracking (HIBC): Blisters are a form of HIC in which the build-up of hydrogen gas pressure at the initiated cracks or preexisting (mill) laminations results in localized deformation and bulging of the steel to the closest surface (or to both surfaces, if mid-wall). Hydrogen gas pressures as high as 2700 psi (18.6 MPa) have been measured inside blisters. Blisters often occur when the hydrogen-induced crack is unable to propagate further parallel to the surface, and is unable to link up with HIC on adjacent planes in the steel. This may be because the hydrogen-atom-trapping inhom*ogeneity which caused the blister to form has a finite length, or there are no other hydrogen-induced cracks in close proximity (same or adjacent planes, Figure 7.82). Stepwise cracking (SWC): SWC is a form of HIC in which adjacent cracks on different planes in the steel link-up in a stepwise fashion. This may lead to throughwall cracking and the loss of vessel integrity. Applied or residual stress is not necessary for SWC development. The hydrogen gas pressure inside HIC cracks or blisters can increase to hundreds of atmospheres. This pressure generates internal stress at the tips of the HIC cracks, resulting in localized plastic deformation of the

Figure 7.82

Hydrogen blister in NPS 6 sour gas pipeline (Courtesy of Malcolm Hay)

518

Corrosion Prevention and Protection

Figure 7.83

Stepwise cracking in plate steel exposed to sour gas (Courtesy of Malcolm Hay)

steel. The cold-worked steel is susceptible to hydrogen embrittlement cracking (HEC), which is the mechanism of link-up (Figure 7.83). Stress-oriented hydrogen-induced cracking (SOHIC): The presence of tensile stress in the component may cause individual ligaments of HIC to form in a stacked, throughthickness array. This is a necessary precursor to what is called SOHIC. This array is oriented perpendicular to the principal applied stress. The HIC may subsequently completely link up to cause through-wall cracking and loss of vessel integrity, i.e., SOHIC. SOHIC, like SWC, is probably a combination of HIC and either hydrogen embrittlement cracking (HEC) or sulfide stress cracking (SSC). HEC and SSC are possible in low-strength steels if the material is highly stressed, especially if it is plastically deformed and there is a stress-concentrating mechanism, and there is a high enough hydrogen atom charging rate. SOHIC is most common in the heat-affected zones (HAZs) of welds in low-strength steels, though can form in the base material (Figure 7.84). 7.17.1

Extent of Problem: Failures due to Hydrogen-induced Cracking

HIC has especially been reported in low-strength steels exposed to sour oil and gas environments. HIC has also been reported in other industries where hydrogen

Figure 7.84 Stress-oriented hydrogen-induced cracking in linepipe ( 50) (Courtesy of Malcolm Hay)

Practical Solutions

519

atom charging is possible, for example in vessels in anhydrous hydrofluoric acid service, in the cathodes of electrochemical cells, and where over-cathodic protection of pipelines has occurred. HIC has been recognized as being a problem in the production and processing of sour hydrocarbons (oil, condensate and gas) for over 50 years. The equipment most commonly affected in the oil and gas industry has been gathering system pipelines, and gas plant and refinery pressure vessels (Table 7.8). Coiled tubing used in sour oil and gas wells is also potentially vulnerable to hydrogen damage mechanisms including HIC and SOHIC. Failures of sweet gas transmission pipelines have been ascribed to overcathodic protection or other mechanisms in which the hydrogen atom charging is from the outside surface rather than the internal surface. 7.17.2

HIC Development and Failures Occur Predominantly in Welded Pipe

Failures have occurred in both single and double submerged arc-welded (SAW and DSAW) (Figures 7.85, 7.86). Failures have also occurred in electric-resistance-welded

Figure 7.85

SOHIC failure of NPS 16 spiral-welded linepipe (Courtesy of Malcolm Hay)

Figure 7.86 Cross-section through SOHIC in spiral-welded linepipe (Courtesy of Malcolm Hay)

16 16

24 8

359

290

414

290

359

API 5L-X60 SAW

API 5L-X42 spiral SAW Sour gas pipeline API 5L-X52 ERW

16 24

359

290

414

CSA Z245.3 ERW

API 5L-X60 SAW

386

CSA Z245.5 spiral DSAW CSA Z245.5 spiral DSAW API 5L-X52 SAW

16 and 24

359

API 5L-X52 DSAW API 5L-X52 spiral DSAW API 5L-X42 ERW

Size (NPS)

Grade (MPa)

Specification and type

Material

12.70

3.96

12.70

9.53

4.78

6.35

8.76

9.53

6.88

8.6

1.6

3.0

15.0

2.2

1.5 6.0

Sour oil 3.4

1.0

2.2

1.0

Wall Thickness H2S (mm) (mol %)

7.8

5.7

8.5

4.3

4.0

8.9

8.8

8.7

4.0

15.0

CO2 (mol %)

Temperature ( C)

2.8

7.0

9.8

7.5

7.6

0.7

2.4

4.5

6.9

Pressing (MPa)

Uninhibited; several ruptures at blisters Inhibited; failed in weld HAZ within 48 h Blistering, SWC and rupture after a few months Failed by SWC after a few months Three ruptures near spiral weld; HIC over Cracked after 5 yrs Inhibited; ruptured at midwall blister Rupture/SOHIC in weld HAZ after 30 h Rupture/SOHIC in weld HAZ after 40 h Extensive HIC within months Rupture along HIC-damaged during ERW Crude oil þ 140 ppm H2S; leaked at blister

Comments

Service and performance

Table 7.8 HIC-related damage and failures of linepipes (Courtesy of Malcolm Hay)

7 3

6, 7

1, 2

Reference

10 42 6

241

483

359

241

4 12

290

API 5L-B seamless

8 34 34

290 414 414

API 5L-X42 ERW API 5L-X60 SAW API 5L-X60 spiral SAW CSA Z245 ERW ASTM A106 seamless API 5L-B seamless CSA Z245.2 DSAW CSA Z245 ERW

290

API 5L-X42 ERW

4.00

12.00

9.27

5.99

7.20 9.25 7.80

7.11 8.5 4.0

8.8

‘High’

2.0

3.2

4.5

6.5

9.0 turned sour

23.0 10 ppm 0

3.7

5.5

8.3

0.8

8.3

8.9 4.8 3.6

1.7

Leaked at blister/SOHIC after 41 yr Rupture at midwall HIC/SWC after 12 yrs Rupture at midwall blisters after 5 yrs Rupture due to HIC/SOHIC at pits after 1 yr

Blistering at pre-existing (mill) laminations SOHIC and rupture after 7.4 yrs Ruptures at blisters after 12 yrs Crude oil; over-cathodic protection Rupture after 9 yrs Rupture at SWC after 24 yrs

16,17

10 15

12 13 14

522

Corrosion Prevention and Protection

Figure 7.87 NPS 3 linepipe ruptured during hydrotest due to HIC at ERW (Courtesy of Malcolm Hay)

(ERW) pipe (Figures 7.87, 7.88). HIC may develop in the base material or in the weldment. The chemical composition and processing that occurs during the manufacturing of the skelp (flat-rolled plate) used to make welded pipe encourages the formation of the inhom*ogeneities responsible for trapping diffusing hydrogen atoms, leading to HIC damage. The manufacturing route followed does not result in the formation of hydrogen damage initiation sites, or at least does not result in a high density of these inhom*ogeneities compared with welded pipe. However, a failure of a seamless linepipe (Figures 7.89, 7.90) has been reported by Malcolm Hay (lecture delivered at the Conference of the Metallurgical Society CIM 2003, Vancouver, BC, Canada).

Figure 7.88 Cleaned pipe: HIC-damaged and inhibitor-protected surfaces (Courtesy of Malcolm Hay)

Practical Solutions

523

Figure 7.89 Leak location at blister in seamless linepipe (Courtesy of Malcolm Hay)

7.17.3

Pipeline Failure

Table 7.8 was constructed from literature sources.1–20 The table demonstrates the great variety of materials and conditions under which HIC failures have occurred. Although the impetus to understand the HIC phenomenon and develop HIC-resistant linepipe and other materials (e.g., pressure vessel plate) started after several failures in the Middle East in the early 1970s, failures had been reported at least as early as 1954. Despite the availability of adequately HIC-resistant linepipe since the late 1970s, failures are still occurring in pipelines constructed since that time. 7.17.4

Mechanism of Hydrogen-induced Cracking

HIC occurs when H atoms diffusing through a linepipe steel become trapped as H2 molecules at inhom*ogeneities in the steel. A planar, gas-filled defect is created, which grows parallel with the vessel surfaces as it traps more diffusing H atoms. If the

Figure 7.90 Cross-section through leak location in seamless linepipe (Courtesy of Malcolm Hay)

524

Corrosion Prevention and Protection

defect grows sufficiently large, it may cause blister formation. HIC failure occurs if a mechanism exists for linkage of one or (normally) more nearby defects or blisters with the internal and external vessel surfaces. Possible linkage mechanisms are SWC and SOHIC. The H atom source is normally the cathodic reaction of an acid corrosion mechanism occurring at the internal vessel surface, i.e., the reduction of hydrogen ions, Hþ: Anodic reaction: Cathodic reaction:

Fe ! Fe2þ þ 2e 2Hþ þ 2e ! 2Hads

The H atoms produced by the cathodic reaction are adsorbed onto the steel surface. They may combine to form a H2 gas molecule, which enters the internal vessel environment. Alternatively, they may become absorbed into the steel: H2 gas formation: H atom absorption:

Hads þ Hads ! H2 Hads ! Habs

(H2 evolved from steel surface) (H atom enters steel)

The presence of specific chemical species in the corrosive environment ‘poisons’ or retards the rate of the Hads atom combination reaction, thereby permitting a higher fraction of the H atoms generated by corrosion to become absorbed by (enter into) the steel. Bisulfide ions (HS), formed when H2S molecules are dissolved in water, are very effective H atom combination poisons. Other effective H atom combination poisons are cyanide ions (CN) and arsenic ions (As3þ).

References 1. F. Paredes and W. W. Mize. Hydrogen Blisters - Their Causes and Methods of Prevention. Gas 40(12) 89 (1954). 2. F. Paredes and W. W. Mize. Unusual Pipeline Failures Traced to Hydrogen Blisters. Oil and Gas Journal 52(12) 99 (1954). 3. D. N. Williams. Correlation of the Results of Stepwise Cracking Tests with Service Experience in Sour Gas. Battelle report to NG-18 Line Pipe Research Supervisory Committee of the American Gas Association, October 1983. NG-18 Report 138. 4. I. Class. Report on Investigation of Sulfide Stress Corrosion Cracking of Steels, Particularly of Steels of Comparatively Low Tensile Strength. Second International Conference on Metallic Corrosion, New York, U.S.A., 1963-03-11/15, pp 342–355 (NACE 1966). 5. J. Leslie, J. A. King. Sour Gas Pipelines. Paper OTC 5741, Volume 2, pp 499, 20th Annual Offshore Technology Conference, Houston, Texas, U.S.A., 1988-05-02/05. 6. Nippon Steel Corporation. The Investigation of the Cause of Gas Pipe Failure. NSC Report 1975-10-20. 7. E. M. Moore, D. A. Hansen. Specifying Linepipe Suitable for Safe Operation in Sour, Wet Service. Transactions ASME, Journal of Energy Resources Technology 104, 134, (1982), and ASME 81-PET-1. 8. E. M. Moore, J. J. Warga. Factors Influencing the Hydrogen Cracking Sensitivity of Pipeline Steels. NACE Corrosion/76 Paper 144, Houston, Texas, U.S.A., 1976-03-22/26, and Materials Performance 15(6), 17 (1976). 9. I. M. El-Jundi. Qatar NGL-2 Pipeline Problems. Society of Petroleum Engineers Production Engineering 1(6), 478 (1986). 10. Alberta, Canada, Energy Resources Conservation Board data, including Pipeline Integrity Workshop, 1992-10-08.

Practical Solutions

525

11. E. M. Moore. Hydrocarbon-Induced Damage in Sour, Wet Crude Pipelines. SPE 11514, Middle East Oil Technical Conference of the Society of Petroleum Engineers, Manama, Bahrain, 198303-14/17. 12. W. Bruckhoff, O. Geier, K. Hofbauer, G. Schmitt, D. Steinmetz. Rupture of a Sour Gas Line Due to Stress Oriented Hydrogen Induced Cracking - Failure Analyses, Experimental Results and Corrosion Prevention. NACE Corrosion/85, Paper 389, Boston, USA, 1985-03-25/29. 13. A. M. Kurdi, M. S. Abougfeefa, A. K. Denney. Rehab Permits Desert Line to Run at Original Pressures. Oil and Gas Journal, 91(30), 76 (1993). (Fourth European and Middle Eastern Pipeline Rehabilitation Seminar, Abu Dhabi, United Arab Emirates, 1993-04-24/27.) 14. A. Punter, A. T. Fikkers, G. Vanstaen. Hydrogen-Induced Stress Corrosion Cracking on a Pipeline. Materials Performance 31(6), 24 (1992). (Proceedings of the Pipe Protection Conference, Cannes, France, 1991.) 15. S. Y. Gajam, A. El-Amari. Failure of a Gas/Condensate Line. Materials Performance 31(10), 55 (1992). 16. M. G. Hay, M. D. Stead. The Hydrogen-Induced Cracking Failure of a Seamless Sour Gas Pipeline. NACE Canadian Region Western Conference, Calgary, Canada, 1994-02-07/10. 17. M. G. Hay, D. W. Rider. Integrity Management of a HIC-Damaged Pipeline and Refinery Pressure Vessel Through Hydrogen Permeation Measurements. NACE Corrosion/98 Paper 395, San Diego, California, U.S.A., 1998-03-22/27. 18. Commodity Pipeline Occurrence Report - Natural Gas Pipeline Rupture. Transportation Safety Board of Canada Report Number P94H0003, 1995-08-23. 19. P. Jones, T. Hetu. A Case Study of Hydrogen Induced Cracking Failure - Investigation, Remedial Action and Maintenance. NACE International, Calgary Section, One Day Seminar on Failure Mechanisms, Calgary, Alberta, Canada, 1998-04-07. 20. B. D. Craig, T. V. Bruno, W. M. Buehler. Complexities of Failure Analyses in Sour Systems. Canadian Welding Society/American Society for Metals conference ‘Materials and Welding for Sour Service and Low Temperature Service’. Calgary, Alberta, Canada, 1997-11-20/21.

7.18 Micromechanisms of Liquid and Solid Metal-induced Embrittlement The importance of this phenomenon is in the development of safe accelerator-driven nuclear reactors/incinerators; heavy liquid metals are used spallation targets, as well as coolants; solid metals are used as protective coatings. Liquid metal-induced embrittlement (LMIE), particularly solid metal-induced failure result in accelerated brittle failure on normally ductile metals under applied or residual stresses when in contact with liquid or solid low-melting point metal. SMIE was first noted as the delayed failure of steels in solid Cd environments. 7.18.1

Liquid Metal-induced Embrittlement (LMIE)

Typical experimental data on the crack growth rate vs stress intensity factor in LMIE conditions are shown in Figure 7.91. Large cracks are at the top portion (>2 mm) and small cracks (

Corrosion Prevention and Protection Practical Solutions - PDF Free Download (2024)

References