© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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CORROSION UNDERSTANDING THE BASICS

Edited by J.R. Davis Davis & Associates

ASM International® Materials Park, Ohio 44073-0002

© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Copyright Ó 2000 by ASM International® All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner. First printing, January 2000 Great care is taken in the compilation and production of this book, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publication is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM’s control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended. Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trademark, and nothing contained in this book shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. ASM International staff who worked on this project included Scott Henry, Assistant Director, Reference Publications; Bonnie Sanders, Manager of Copy Editing; Grace Davidson, Manager of Book Production; Nancy Hrivnak and Carol Terman, Copy Editors; Candace Mullet and Jill Kinson, Book Production Coordinators. Library of Congress Cataloging-in-Publication Data Corrosion: understanding the basics / edited by J.R. Davis. p. cm. Includes bibliographical references and index. 1. Corrosion and anti-corrosives. I. Davis, J.R. (Joseph R.) TA462.C668 2000 620.1’1223—dc21 99-057146 ISBN: 0-87170-641-5 SAN: 204-7586 ASM International® Materials Park, OH 44073-0002 http://www.asm-intl.org Printed in the United States of America

© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

Contents Preface ...................................................................................ix CHAPTER 1: The Effects and Economic Impact of Corrosion .....1 The Definition of Corrosion.............................................................2 The Effects of Corrosion ..................................................................3 The Many Forms of Corrosion.........................................................4 Methods to Control Corrosion .........................................................6 Material Selection...........................................................................6 Coatings...........................................................................................7 Inhibitors .........................................................................................8 Cathodic Protection ........................................................................8 Design..............................................................................................8 Opportunities in Corrosion Control.................................................9 The Economic Impact of Corrosion ..............................................10 Sources of Information ...................................................................14 Appendix: Addresses of Trade Associations and Technical Societies Involved with Corrosion ..........................17 CHAPTER 2: Basic Concepts Important to Corrosion .........21 Behavior of a Metal in an Environment ........................................21 The Four Requirements of a Corrosion Cell.................................23 Metal Characteristics Important to Corrosion ..............................25 Metallurgical Characteristics .......................................................25 Inherent Reactivity .......................................................................35 Formation of Corrosion Products ................................................37 Important Solution Characteristics ................................................38 Corrosion Rate Expressions and Allowances ...............................45 CHAPTER 3: Principles of Aqueous Corrosion ....................49 The Thermodynamics of Aqueous Corrosion ...............................50 iii

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© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Corrosion Reactions and Free-Energy Change...........................50 Free Energy and Electrochemical Potential................................53 Tendency for Metals to Corrode..................................................55 Effect of Ionic Concentration on Electrode Potential ................56 Electromotive Force Series ..........................................................59 Galvanic Series .............................................................................60 Standard Electrode Potentials for Other Reactions ....................62 Potential-pH Diagrams: General Aspects ...................................62 Potential-pH Diagrams for Specific Metals................................67 Strategies for Corrosion Control from E-pH Diagrams .............74 Limitations of E-pH Diagrams ....................................................76 The Kinetics of Aqueous Corrosion ..............................................77 Electrochemical Reactions...........................................................77 Mixed-Potential Theory ...............................................................79 Types of Polarization ...................................................................82 Applications of Mixed-Potential Theory Diagrams ...................88 Exchange Currents........................................................................95 CHAPTER 4: Forms of Corrosion: Recognition and Prevention.....99 Uniform Corrosion .......................................................................100 Pitting Corrosion...........................................................................102 Crevice Corrosion.........................................................................107 Tuberculation ..............................................................................114 Deposit Corrosion.......................................................................118 Filiform Corrosion......................................................................122 Poultice Corrosion ......................................................................125 Galvanic Corrosion.......................................................................125 General Description....................................................................125 Galvanic Series ...........................................................................126 Polarization .................................................................................129 Factors Influencing Galvanic Corrosion Behavior...................129 Situations That Promote Galvanic Attack ................................130 Prevention of Galvanic Corrosion .............................................133 Erosion-Corrosion ........................................................................134 General Description....................................................................134 Critical Factors Influencing Erosion-Corrosion .......................137 Prevention of Erosion-Corrosion...............................................144 Cavitation ....................................................................................146 Fretting Corrosion ......................................................................149 Intergranular Corrosion ................................................................151 General Description....................................................................151 Intergranular Corrosion of Austenitic Stainless Steels ............152 Intergranular Corrosion of Other Alloy Systems .....................155 Exfoliation ..................................................................................157 Dealloying Corrosion ...................................................................158 Dezincification............................................................................158 Graphitic Corrosion....................................................................162 Stress-Corrosion Cracking ...........................................................164 Corrosion Fatigue .........................................................................175

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© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

Hydrogen Damage ........................................................................180 Hydrogen Embrittlement............................................................180 Hydrogen-Induced Blistering ....................................................184 Cracking from Precipitation of Internal Hydrogen ..................185 Hydrogen Attack.........................................................................186 Hydride Formation .....................................................................187 Prevention of Hydrogen Damage ..............................................188 Liquid-Metal Embrittlement ........................................................189 CHAPTER 5: Types of Corrosive Environments .................193 Characteristics of Corrosive Environments ................................194 Biologically Influenced Corrosion ..............................................199 Industries and Organisms Involved...........................................200 Tuberculation ..............................................................................203 Prevention of MIC ......................................................................204 Atmospheric Corrosion ................................................................205 Underground/Soil Corrosion........................................................211 Factors Affecting Underground/Soil Corrosion .......................211 Types of Underground/Soil Corrosion......................................213 Corrosion Control .......................................................................215 Natural and Treated Waters .........................................................216 Understanding Corrosion in Acids ..............................................217 Corrosion by Sulfuric Acid ..........................................................220 Materials Selection Guidelines for Sulfuric Acid ....................220 Use of Steel in Sulfuric Acid .....................................................221 Use of Cast Irons in Sulfuric Acid ............................................223 Use of Stainless Steels in Sulfuric Acid ...................................223 Use of Nickel Alloys in Sulfuric Acid ......................................224 Other Metals Used in Sulfuric Acid ..........................................225 Nonmetallic Materials Used in Sulfuric Acid ..........................225 Corrosion by Nitric Acid..............................................................226 Materials Selection Guidelines for Nitric Acid ........................227 Corrosion by Hydrochloric Acid .................................................227 Materials Selection Guidelines for Hydrochloric Acid ...........228 Corrosion by Hydrogen Fluoride and Hydrofluoric Acid..........228 Materials Selection Guidelines for Hydrofluoric Acid ............229 Corrosion by Phosphoric Acid.....................................................230 Materials Selection Guidelines for Phosphoric Acid ...............231 Corrosion by Organic Acids ........................................................231 Acetic Acid .................................................................................232 Other Organic Acids...................................................................234 Corrosion by Alkalis ....................................................................234 Materials Selection Guidelines for Alkalis...............................234 CHAPTER 6: Corrosion Characteristics of Structural Materials.....................................................237 Carbon Steels ................................................................................238 Corrosive Service .......................................................................238 Protection of Steel from Corrosion ...........................................239 v

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© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Weathering Steels .........................................................................242 Alloy Steels ...................................................................................244 Cast Irons ......................................................................................244 Commercially Available Cast Irons ..........................................245 Graphitic Corrosion....................................................................246 Stainless Steels..............................................................................247 Stainless Steel Families..............................................................247 Mechanism of Corrosion Resistance.........................................252 Forms of Corrosion of Stainless Steels .....................................253 Corrosion in Various Applications............................................256 Nickel and Nickel-Base Alloys ...................................................259 Effects of Major Alloying Elements .........................................260 Chemical-Processing Applications............................................262 Seawater Applications................................................................263 Applications in Pulp and Paper Mills .......................................264 Flue Gas Desulfurization Applications .....................................265 Sour Gas Applications................................................................265 High-Temperature Applications ................................................265 Copper and Copper-Base Alloys .................................................266 Effects of Alloy Composition....................................................267 Types of Attack ..........................................................................269 Applications of Copper-Base Alloys.........................................269 Aluminum and Aluminum-Base Alloys ......................................270 Effects of Alloy Composition....................................................271 Modes of Corrosion That Attack Aluminum ............................272 Corrosion Protection of Aluminum ...........................................275 Applications of Aluminum-Base Alloys...................................277 Titanium and Titanium-Base Alloys ...........................................278 Mechanism of Corrosion Resistance.........................................279 Modes of Corrosion That Attack Titanium...............................280 Corrosion Protection of Titanium..............................................281 Applications of Titanium-Base Alloys .....................................281 Zinc and Zinc-Base Alloys ..........................................................282 Magnesium and Magnesium-Base Alloys...................................282 Lead and Lead Alloys...................................................................284 Tin and Tin-Base Alloys ..............................................................286 Zirconium and Zirconium-Base Alloys.......................................287 Tantalum........................................................................................287 Niobium and Niobium-Base Alloys ............................................288 Cobalt-Base Alloys.......................................................................289 Polymers........................................................................................289 Types of Polymers ......................................................................290 Properties of Polymers ...............................................................290 Environmental Degradation of Polymers..................................291 Ceramics........................................................................................295 Other Nonmetallic Materials........................................................297 Rubber .........................................................................................297 Carbon and Graphite ..................................................................299 Woods..........................................................................................299

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© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

CHAPTER 7: Corrosion Control by Proper Design ............301 Design as a Process ......................................................................302 The Design Team........................................................................302 Steps in the Design Process .......................................................303 General Considerations in Corrosion-Control Design ...............303 Design Details that Accelerate Corrosion...................................308 Design Solutions for Specific Forms of Corrosion ....................320 Corrosion Allowance....................................................................324 Design Considerations for Using Weathering Steels .................325 Failures Involving Corrosion of Structural Steel .....................326 CHAPTER 8: Corrosion Control by Materials Selection ....331 Elements of the Materials Selection Process ..............................333 Materials Considerations..............................................................341 Selecting Materials to Avoid or Minimize Corrosion ................349 General Corrosion ......................................................................353 Localized Corrosion ...................................................................358 CHAPTER 9: Corrosion Control by Protective Coatings and Inhibitors ..............................................................363 Organic Coatings and Linings .....................................................364 Design and Selection of a Coating System ...............................365 Surface Preparation ....................................................................367 Inspection and Quality Assurance .............................................369 Coating and Lining Materials ....................................................371 Environmental, Health, and Safety Considerations .................379 Metallic Coatings..........................................................................382 Electroplated Coatings ...............................................................382 Electroless Nickel Plating .........................................................386 Hot-Dip Coatings........................................................................387 Thermal Spray Coatings.............................................................391 Clad Metals .................................................................................392 Pack Cementation .......................................................................394 Vapor-Deposited Coatings.........................................................395 Surface Modification..................................................................395 Nonmetallic Inorganic Coatings ..................................................396 Concrete and Cementatious Coatings and Linings...................397 Porcelain Enamels ......................................................................398 Conversion Coatings ..................................................................399 Aluminum Anodizing.................................................................401 Inhibitors .......................................................................................401 Types of Inhibitors .....................................................................402 Biocides.......................................................................................404 Application of Inhibitors............................................................405 CHAPTER 10: Corrosion Control by Cathodic and Anodic Protection ................................................407 Cathodic Protection ......................................................................407 How Cathodic Protection Works ...............................................408 vii

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© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Types of Cathodic Protection ....................................................410 Anode Materials .........................................................................411 Criteria for Cathodic Protection ................................................414 Problems with Cathodic Protection...........................................415 Applications of Cathodic Protection .........................................417 Anodic Protection .........................................................................422 The Concept of Anodic Protection ............................................422 Equipment Required for Anodic Protection .............................423 Applications of Anodic Protection ............................................425 CHAPTER 11: Corrosion Testing and Monitoring..............427 Classification of Corrosion Testing.............................................427 Purposes of Corrosion Tests ........................................................429 Steps in a Corrosion Test Program ..............................................430 Preparation and Cleaning of Test Specimens .............................432 Specific Types of Laboratory Tests.............................................433 Simulated Atmosphere Tests .....................................................434 Salt-Spray Testing ......................................................................435 Immersion Tests .........................................................................438 Field Tests .....................................................................................441 Atmospheric Tests ......................................................................442 Electrochemical Tests...................................................................448 Electrochemical Test Classification ..........................................448 Reference Electrode ...................................................................449 Types of Electrochemical Measurements .................................451 Applications of Electrochemical Tests .....................................456 Corrosion Monitoring...................................................................467 Selecting a Corrosion-Monitoring Method...............................470 Strategies in Corrosion Monitoring...........................................472 CHAPTER 12: Techniques for Diagnosis of Corrosion Failures .......................................................475 Factors That Influence Corrosion Failures .................................475 Analysis of Corrosion Failures ....................................................481 Collection of Background Data .................................................482 On-Site Examination ..................................................................483 On-Site Sampling .......................................................................483 Preliminary Laboratory Examination........................................484 Microscopic Examination ..........................................................485 Chemical Analysis......................................................................486 Bulk Material Analysis ..............................................................488 Nondestructive Evaluation.........................................................489 Corrosion Testing .......................................................................490 Mechanical Testing ....................................................................491 Analyzing the Evidence, Formulating Conclusions, and Writing the Report...........................................................492 APPENDIX 1: Glossary of Corrosion-Related Terms .........497 Index ...................................................................................517 viii

© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Preface Most people are familiar with corrosion in some form or another. Whether it is a rusty nail in a backyard fence, corroded fenders and/or mufflers on our automobiles, or a perforated underground water pipe, it is safe to say that corrosion is all around us. It is costly to prevent or repair, and it is generally not pleasing to look at. In the industrial workplace, corrosion is certainly one of the most common causes of failure of engineered components and structures. The complexities of corrosion phenomena challenge corrosion scientists, chemists, mechanical, civil, and metallurgical engineers, coating specialists, and maintenance and operating personnel. In order to better understand corrosion, it is important to first examine the basic concepts that influence the corrosion process; hence, the title of this publication—Corrosion: Understanding the Basics. Included in these 12 chapters are practical discussions on the following: · Thermodynamic and electrochemical principles of corrosion · Recognition and prevention of various forms of corrosion · Types of corrosive environments commonly encountered and environmental variables that can increase or decrease corrosion rates · Corrosion characteristics of metals and alloys and nonmetallic materials · Methods of corrosion prevention, including design considerations, materials selection, coatings, inhibitors, and cathodic and anodic protection · Corrosion testing and monitoring · Techniques for diagnosing corrosion failures

ix

© 2000 ASM International. All Rights Reserved. Corrosion: Understanding the Basics (#06691G)

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Although the book is primarily intended for professionals who are not corrosion experts, it should also serve as a quick and useful corrosioncontrol guide for corrosion engineers. Assisting in the preparation of this book was Larry Korb from Rockwell International. Larry, who is a Fellow of ASM International and longtime member and former chairman of the ASM Handbook Committee, meticulously reviewed each chapter. I have long been in awe of my friend’s exhaustive knowledge of materials and their failure mechanisms (including corrosion), and his keen insight into the editorial process. It is always an honor and a privilege to work with Mr. Korb. I also wish to acknowledge the contributions of Nalco Chemical Company (Naperville, IL). Many of the photographs illustrating the different modes of corrosion were supplied by Nalco. These originally appeared in two excellent books on failure analysis authored by Nalco engineers Harvey M. Herro (an ASM member) and Robert D. Port. I am indebted to Ms. Connie Szewczyk, a Communications Specialists with Nalco, for supplying these photographs. Thanks are also extended to Kenneth B. Tator and Alison B. Kaelin from KTA-Tator Inc. (Pittsburgh, PA). Ken supplied an extensive table that reviewed the advantages and limitations of organic coating resins. Alison prepared material on environmental, health, and safety considerations for the coatings industry. Their contributions appear in Chapter 9. The efforts of the ASM staff are also duly noted. In particular, I would like to thank Scott Henry and Bonnie Sanders from the Publications Department and Eleanor Baldwin and her coworkers from the ASM Library for the help and support throughout the project Last, I would be remiss in not acknowledging the fact that several chapters in the book were adapted from the ASM Materials Engineering Institute (MEI) course on corrosion that was prepared by Dr. Joe H. Payer from Case Western Reserve University (Cleveland, OH). Chapters 2 and 3, as well as the description of electrochemical test methods in Chapter 11, were based on Dr. Payer’s work. Joseph R. Davis Davis & Associates Chagrin Falls, Ohio

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