Corrosion and Electrochemistry of Zinc Corrosion and Electrochemistry of Zinc

Xiaoge Gregory Zhang Cominco Ltd. Product Technology Centre Mississauga, Ontario, Canada

Springer Science+ Business Media, LLC Library of Congress Cataloging-in-Pub1ication Data

Zhang, Xiaoge Gregory. Corrosion and electrochemistry of zinc / Xiaoge Gregory Zhang, p. cm. Includes bibliographical references and index. ISBN 978-1-4757-9879-1 1. Zinc—Corrosion, 2. Electrochemistry. I. Title. TA480.Z6Z45 1996 620. 1'84223—dc20 96-32551 CIP

ISBN 978-1-4757-9879-1 ISBN 978-1-4757-9877-7 (eBook) DOI 10.1007/978-1-4757-9877-7

10 9 8 7 6 5 4 3 2 1

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, microfilming, recording, or otherwise, without written permission from the Publisher To my mother Youliu Zhang and father Hongtao Zhang Foreword

Humankind's use of zinc stretches back to antiquity, and it was a component in some of the earliest known alloy systems. Even though metallic zinc was not "discovered" in Europe until 1746 (by Marggral), zinc ores were used for making brass in biblical times, and an 87% zinc alloy was found in prehistoric ruins in Transylvania. Also, zinc (the metal) was produced in quantity in India as far back as the thirteenth century, well before it was recognized as being a separate element. The uses of zinc are manifold, ranging from galvanizing to die castings to electronics. It is a preferred anode material in high-energy-density batteries (e.g., Ni/Zn, Ag/Zn, ZnJair), so that its electrochemistry, particularly in alkaline media, has been extensively explored. In the passive state, zinc is photoelectrochemically active, with the passive film displaying n-type characteristics. For the same reason that zinc is considered to be an excellent battery anode, it has found extensive use as a sacrificial anode for the protection of ships and pipelines from corrosion. Indeed, aside from zinc's well-known attributes as an alloying element, its widespread use is principally due to its electrochemical properties, which include a well-placed position in the galvanic series for protecting iron and steel in natural aqueous environments and its reversible dissolution behavior in alkaline solutions. Dr. Zhang has undertaken the monumental task of describing the corrosion properties and electrochemistry of zinc in a single book. The reason why this task is "monumental" is that the literature on this important metal is highly fragmented, no doubt reflecting zinc's diversity of use. Furthermore, the literature stretches from the very fundamental to the very applied, with some of the reports on the properties of the metal and its compounds being anecdotal in nature. The task of assembling all of the relevant information into a single monograph, in a manner that is logical and easy to read. is the task that Dr. Zhang undertook. He has succeeded admirably, and this book will surely become an authoritative source of information on the electrochemistry of this technologically important metal.

Digby D. Macdonald The Pennsylvania State University University Park, Pennsylvania

vii Preface

Zinc is one of the most widely used metals. Its most important commercial application is corrosion protection of steel. In the past decades, a tremendous amount of research work has been done on the various aspects of zinc corrosion and electrochemistry. This book provides a systematic review of the enormous volume of technical results generated from these investigations. It is hoped that it will not only be useful to those interested in specific information on this subject but also will stimulate those currently working in the field to carry out further research. This book attempts to combine fundamental information on the electrochemistry of zinc with practical corrosion data for zinc and its alloys and to connect the academic and industrial realms of interest, which are often detached from each other. In general, books on corrosion written from an academic perspective usually treat individual metals or alloys either as examples or in a rather general fashion, while those issuing from the metals industries cover little fundamental information. However, from the viewpoint of a metals user or researcher, it is most beneficial that all the relevant corrosion and electrochemistry information, theoretical and practical, on one metal be systematically organized in one single source. Additionally, only a dozen or so metals, including zinc, are used in massive quantities in today's society, and a compilation of all the corrosion, electrochemistry, and related information in a single book for each of them would be very useful for more effective application of these metals in the future. This book focuses on corrosion and does not cover other applied aspects of zinc electrochemistry. However, as it contains a large collection of electrochemical informa• tion on zinc, it can also serve as a source of reference for electrochemical processes such as electroplating, electrowinning, and batteries. Much of the electrochemical information presented in this book is related to the elemental reactions such as dissolution, hydrogen evolution, oxygen reduction, and passivation, which are also important in electrochemical processes other than corrosion. Two general approaches have been taken in the selection and treatment of the information presented in this book. The first is to emphasize the properties pertaining to zinc as a material, rather than those pertaining to specific products (such as coatings, wires, plates, cast alloys, etc.). The behavior of the various zinc products is taken into account in the consideration of the effect of specific physical or chemical factors, such as alloying elements, physical dimensions, temperature, solution composition, and pH. The

ix x PREFACE second is to emphasize the specificity of corrosion data in relation to corrosion environ• ments. The environmental conditions pertaining to each corrosion situation are specified, together with detailed data. Generalizations are provided when a consensus exists in the data. Also, because the environment is as important as the material in a corrosion process, information is provided at the beginning of each chapter to describe the corrosion environments and to define the various factors involved. The book consists of 15 chapters. The first chapter of the book presents the basic physical and chemical properties of zinc. The remainder of the first half of the book is concerned with the electrochemistry of zinc. More specifically, Chapter 2 on Electro• chemical Thermodynamics and Kinetics, Chapter 3 on Passivation and Surface Film Formation, and Chapter 4 on Electrochemistry of Zinc Oxide deal with the fundamental electrochemistry of zinc, and Chapter 5 on Corrosion Potential and Corrosion Current, Chapter 6 on Corrosion Products, and Chapter 7 on Corrosion Forms. These last three chapters, in different aspects, connect the fundamental electrochemistry with the practical corrosion behavior of zinc. The remaining chapters deal with corrosion performance in various environments.

ACKNOWLEDGMENTS I am profoundly grateful to the management of Cominco Ltd., particularly to Dr. E. M. Valeriote and Mr. S. R. Wilkinson, for their support of my undertaking of such a time-consuming task. In addition, I would like to personally thank Dr. Valeriote, who, as the manager of the Co minco Product Technology Centre, was not only instrumental in initiating this project but was also always keen to assist by providing advice and resources. A very special thanks is due to Professor D. D. Macdonald of The Pennsylvania State University, who suggested that I write this book and kindly helped at various stages during the process. Also, I am deeply indebted to Professor M. Pourbaix of I'Universite Libre de Bruxelles, who has had a great influence on my career and whose work and spirit have inspired me in my writing of this book. It would not have been possible for the book to arrive at its present form without the constructive suggestions and criticisms of many people. I sincerely thank the following people, who have helped in reviewing the various chapters of the manuscript: Dr. T. D. Burleigh, University of Pittsburgh, United States Dr. T. G. Chang, Cominco Ltd, Canada Dr. B. R. Conard, INCO Ltd., Canada Dr. F. E. Goodwin, International Lead and Zinc Research Organization, United States Dr. J. A. Gonzalez, Cominco Ltd., Canada Dr. T. E. Graedel, AT&T Bell Laboratories, United States Professor T. M. Harris, University of Tulsa, United States Professor M. B. Ives, McMaster University, Canada Professor D. W. Kirk, University of Toronto, Canada Mr. G. P. Lewis, Lewis Consulting, Canada Professor C. Leygraf, Royal Institute of Technology, Sweden Dr. J. H. Lindsay, General Motors Research Laboratories, United States Professor D. D. Macdonald, The Pennsylvania State University, United States PREFACE xi

Dr. 1. Odnevall, Royal Institute of Technology, Sweden Professor P. Searson, Johns Hopkins University, United States Dr. H. E. Townsend, Bethlehem Steel Co., United States Dr. K. Tomantschger, Cominco Ltd., Canada Dr. E. M. Valeriote, Cominco Ltd., Canada Professor R. Wiart, Universite Pierre et Marie Curie, France In particular, the critical reading of the manuscript and the very helpful suggestions made by Mr. G. P. Lewis of Lewis Consulting and Dr. H. E. Townsend of Bethlehem Steel Co. are greatly appreciated. Many people at the Product Technology Centre of Cominco have helped at different stages in the development of the manuscript. To them I am most grateful. In particular, I would like to thank Mr. J. E. Valeriote, who helped in the preparation of the figures, Mr. J. Hwang, who provided feedback by reading the first draft of the manuscript, Mrs. V. Rodic and Mrs. P. L. Doyle, who helped in obtaining and organizing the references, Mrs. H. Laur for handling correspondence and mailing, and Ms. M. F. Haughton, who assisted in preparing the tables and making manuscript corrections. Also, I would like to acknow• ledge The Second Cup at 292 Dundas West in Toronto, where I read the literature and revised the manuscript during the morning hours of numerous weekends. The work of Amelia McNamara, Arun Das, Kenneth Howell, and Jacqueline Sedman at Plenum Press is greatly appreciated. Finally, I wish to express my deeply felt thanks to my wife, Li, for her understanding and moral support during the long period required for the writing of this manuscript.

Xiaoge Gregory Zhang Mississauga, Ontario, Canada Contents

LIST OF SYMBOLS ...... XIX

CHAPTER 1. Properties, Products, and Processes

1.1. Introduction ...... 1.2. Basic Properties ...... 1.2.1. Physical Properties .. 1.2.2. Mechanical Properties 2 1.2.3. Alloying Properties .. 3 1.3. Main Products and Applications 3 1.3.1. Zinc Coatings . 3 1.3.2. Cast Products .... . 7 1.3.3. Rolled Zinc ...... 7 1.3.4. Zinc Dust and Powder 7 1.4. Coating Processes . . . . . 7 1.4.1. Hot-Dip Galvanizing 7 1.4.2. Electroplating. 13 1.3. Phosphating 15 1.4. Chromating . . . . . 16

CHAPTER 2. Electrochemical Thermodynamics and Kinetics 2.1. Introduction ...... 19 2.2. Thermodynamic Stability 19 2.3. Ionic Properties ..... 25 2.4. Double-Layer Properties 27 2.5. Kinetics of Elemental Reactions 29 2.5.1. Dissolution .... . 29 2.5.2. Deposition .... . 36 2.5.3. Hydrogen Evolution 39 2.5.4. Oxygen Reduction . 48

xiii xiv CONTENTS

2.6. Corrosion Processes ...... 54 2.6.1. General Considerations ...... 54 2.6.2. Impedance of Corroding Electrodes 54

CHAPTER 3. Passivation and Surface Film Formation 3.1. Introduction ...... 65 3.2. Characteristics and Conditions. 65 3.3. Alkaline Solutions .. . 68 3.3.1. i-VCurves .. . 68 3.3.2. Passivation Time 70 3.3.3. Characteristics .. 73 3.3.4. Mechanisms of Formation of Passive Films 75 3.4. Other Solutions...... 77 3.4.1. Slightly Alkaline and Carbonate Solutions 77 3.4.2. Phosphate Solutions. . . 80 3.4.3. Miscellaneous Solutions 84 3.5. Anodization ...... 85 3.6. Stability of Passivation ... . 87 3.6.1. Type of Passivation .. 87 3.6.2. Passivation Breakdown 89

CHAPTER 4. Electrochemistry of Zinc Oxide 4.1. Introduction ...... 93 4.2. Basic Properties ...... 93 4.2.1. Physical Properties . 93 4.2.2. Electronic Properties 95 4.3. Semiconductor Electrochemical Behavior. 97 4.3.1. Basic Theories .. . 97 4.3.2. Flatband Potential ...... 100 4.3.3. Band Structure ...... 103 4.3.4. Electrode Kinetics in the Dark 105 4.3.5. Photoelectrochemical Kinetics 109 4.3.6. Electroluminescence 114 4.4. Thin ZnO Films ...... 115 4.5. Stability ...... 119 4.5.1. Conditions of Stability and Decomposition Reactions 119 4.5.2. Rate of Decomposition ...... 121

CHAPTER 5. Corrosion Potential and Corrosion Current 5.1. Introduction ...... 125 5.2. Relation between Corrosion Potential and Corrosion Current 125 5.2.1. Polarization Resistance and Corrosion Current. 127 5.2.2. Conversion Factors ...... 129 CONTENTS xv

5.3. Corrosion Potential and Reaction Kinetics 130 5.4. Ecorr and icorr under Various Conditions 133 5.4.l. Effect of Zinc Ions ...... 133 5.4.2. Effect of Anions and Cations. 135 5.4.3. Effect of pH ...... 137 5.4.4. Effect of Temperature ... . 140 5.4.5. Effect of Aeration and Convection. 141 5.4.6. Effect of Surface Condition 143 5.5. Zinc Alloys ...... 144 5.6. Effect of Time ...... 149 5.7. Correlation between Corrosion Current and Weight Loss Rate 153

CHAPTER 6. Corrosion Products

6.l. Introduction ...... 157 6.2. In Atmospheric Environments .. 158 6.2.1. Composition and Structure . 158 6.2.2. Quantity and Morphology 163 6.2.3. Formation Processes 165 6.3. In Waters ...... 168 6.3.l. Fresh Waters 168 6.3.2. Seawater .. 170 6.4. In Solutions . . . . 171 6.4.l. Effect of pH . 171 6.4.2. Formation Processes 173 6.4.3. Zinc Alloys ..... 176 6.5. In Other Environments .. 176 6.6. Effect of Corrosion Products on Zinc Corrosion 178

CHAPTER 7. Corrosion Forms

7.1. Introduction . . . . · 183 7.2. Galvanic Corrosion 183 7.2.1. Introduction. 183 7.2.2. Theoretical Aspects . 185 7.2.3. Practical Factors .. 196 7.2.4. Polarity Reversal .. .203 7.2.5. Galvanic Corrosion in Natural Environments .208 7.2.6. Galvanic Protection of Steel by Zinc. · 213 7.3. Pitting Corrosion ...... 217 7.3.l. Introduction...... · 217 7.3.2. Occurrence of Pitting. · 217 7.3.3. Pitting Potential · 221 7.3.4. Morphology .224 7.3.5. Mechanisms .. .225 xvi CONTENTS

7 A. Intergranular Corrosion 227 704.1. Introduction . . 227 704.2. Occurrence. . . 227 704.3. Metallurgical Effects. 229 70404. Effect of Environmental Factors . 232 704.5. Effect on Mechanical Properties. 234 704.6. Mechanisms ...... 235 7.5. Wet Storage Stain ...... 236 7.6. Hydrogen Embrittlement and Corrosion Cracking 238

CHAPTER 8. Atmospheric Corrosion

8.1. Introduction ...... 241 8.2. Atmospheric Factors . . 241 8.2.1. Type of Wetting 241 8.2.2. Air Pollutants . 243 8.3. Corrosion in Outdoor Environments. 245 8.3.1. Typical Corrosion Rates . 245 8.3.2. Effect of Time of Wetness . . 248 8.3.3. Effect of Pollutants ..... 249 8.3.4. Effect of Elevation and Distance from Seawater 252 8.3.5. Effect of Initial Weather Conditions. 252 8.3.6. Effect of Climate ...... 254 8.3.7. Effect of Sample Configuration 254 8.3.8. Effect of Sheltering 255 8.3.9. Galvanized Steel ...... 256 8.3.10. Effect of Alloying .... . 258 8.3.11. Effect of Surface Treatment 260 8.3.12. Effect of Corrosion Products 261 8.3.13. Forms of Corrosion . . . . 261 8.3.14. Highway Environment .... 262 804. Corrosion in Indoor Environments . 264 8.5. Corrosion in Simulated Environments. 266 8.5.1. Humidity Chamber Exposure 267 8.5.2. Water and Salt Spray .. 270 8.5.3. Cyclic Test ...... 272 8.504. Thin-Layer Electrolytes 274 8.6. Corrosion Mechanisms .... 278

CHAPTER 9. Corrosion in Waters and Aqueous Solutions 9.1. Introduction ...... 283 9.2. Characteristics of Waters . 283 9.2.1. Fresh Waters . . . 283 9.2.2. Seawater ...... 284 9.3. Corrosion in Pure Water . 286 9.4. Corrosion in Natural Waters 288 CONTENTS xvii

9.4.1. Cold Fresh Water. .288 9.4.2. Hot Fresh Water . .289 9.4.3. Seawater ..... · 291 9.5. Corrosion in Aqueous Solutions .296 9.5.1. Effect of Dissolved Species .296 9.5.2. Effect of pH ...... 298 9.5.3. Effect of Immersion Conditions · 301 9.5.4. Effect of Surface Treatments .. .302 9.5.5. Effect of Metallurgical Factors. .302

CHAPTER 10. Corrosion in Soil 10.1. Introduction ...... 305 10.2. Characteristics of Soil .305 10.3. Corrosion Rates . . . .308 10.3.1. Effect of Soil Factors .308 10.3.2. Galvanic Corrosion . · 312 10.4. Electrochemical Measurements · 312

CHAPTER 11. Under-Paint Corrosion

11.1. Introduction ...... · 315 11.2. Basic Characteristics of Paint · 315 11.2.1. Components in Paint . · 316 11.2.2. Barrier Properties of Paint . · 316 11.3. Corrosion Tests...... · 317 11.4. Corrosion Behavior ...... · 319 11.4.1. Characterization of Corrosion . .319 11.4.2. Effect of Coating Type. . .321 11.4.3. Effect of Test Conditions .325 11.4.4. Effect of Paint System . .328 11.4.5. Galvanic Action .332 11.5. Corrosion Mechanisms .... · 333

CHAPTER 12. Zinc-Rich Coatings 12.1. Introduction ...... 337 12.2. Coating Characteristics .337 12.3. Protection Mechanism. .339 12.4. Performance ...... · 341 12.4.1. Effect of Zinc Content . · 341 12.4.2. Effect of Zinc Particle Size .343 12.4.3. Effect of Binders ...... 344 12.4.4. Effect of Coating Thickness . .345 12.4.5. Effect of Additives ...... 345 12.4.6. Effect of Surface Condition .347 12.4.7. Other Factors ...... 348 xviii CONTENTS

CHAPTER 13. Corrosion in Concrete

13.1. Introduction ...... 351 13.2. Concrete Environment . . . . 352 13.2.1. Formation of Concrete 352 13.2.2. Characteristics .... 353 l3.3. Corrosion of Steel Reinforcement in Concrete. 358 13.3.1. Effect of Corrosion . 358 l3.3.2. Protection Methods ...... 359 13.3.3. Galvanized Coatings ...... 359 13.4. Corrosion of Galvanized Steel in Concrete 360 13.4.1. Testing Methods ...... 360 13.4.2. Field Test Results ...... 360 l3.4.3. Results from Simulated Tests 365

CHAPTER 14. Corrosion in Batteries 14.1. Introduction ...... 373 14.2. Zinc Cells and Batteries 373 14.2.1. Leclanche Cell . 373 14.2.2. Zinc Chloride Cell 375 14.2.3. Zinc Alkaline Cell 375 14.2.4. Zinc-Air Battery . 376 14.2.5. Zinc-Nickel Battery 377 14.3. Corrosion ...... 377 14.3.1. Effect of Testing Time 379 14.3.2. Effect of Electrolyte . 379 14.3.3. Effect of Chemical Agents 382 14.3.4. Zinc Electrode ... . 385 14.3.5. Operating Conditions .. . 390

CHAPTER 15. Corrosion in Other Environments 15.1. Introduction ..... 393 15.2. Organic Solvents .. 393 15.2.1. Classification 393 15.2.2. Corrosion .. 395 15.3. Gaseous Environments 399 15.4. Zinc Anodes ...... 403 15.4.1. Sacrificial Anodes 403 15.4.2. Anodes for Impressed Current Cathodic Protection 407

REFERENCES ...... 409

INDEX ...... 463 List of Symbols

Symbol Definition Section A Surface area 5.2 aH+' aOH- Activity of hydrogen and hydroxyl ions 2.5 B Stern-Geary constant 5.2 b Tafel slope 2.5 C Capacitance 2.4 Cd Capacitance of double layer 2.6 C,c Capacitance of space charge layer 4.3 CH Capacitance of Helmholtz double layer 4.3 Co Bulk concentration 3.3 C, Surface concentration 3.3 Ccrit Critical concentration for passivation 3.3 D Diffusion coefficient 3.3 d Distance between anode and cathode 7.2 E Electrode potential 2.5 Eo Standard potential 2.2 EII2 Half-wave potential 2.5 E" Potential of anode 7.2 E" Breakdown potential 3.2 Ec Lower edge of conduction band 4.2 Potential of cathode 7.2 Ecorr Corrosion potential 5.2 EF Fermi level 4.2 ED Decomposition potential 4.5 Etb Flatband potential 4.3 E~ Width of band gap 4.2 Egc Potential of a galvanic couple 7.2 E" Passivation potential 3.2 E" Top edge of valence band 4.2 E, Solution potential 4.3 e-, p+ Electron and hole 4.3 e Electron charge 4.3 F Faraday constant 2.5 hv Photon 4.5 fa Anodic current 7.2

xix xx LIST OF SYMBOLS

Symbol Definition Section Ie Cathodic current 7.2 Ig Galvanic current 7.2 ) Current density 2.5 ~ --7 i, I Oxidation and reduction current density of a redox reaction 5.2 io Exchange c'urrent density 2.5 iOa' iOe Exchange current densities for anodic and cathodic reactions 7.2 ia, ie Anodic and cathodic current density 3.4 icorr Corrosion current density 5.2 ig Galvanic current density 7.2 igt Gravimetric corrosion rate 5.7 im Limiting current density 2.5 i" Passivation current 3.2 iphoto Photocurrent density 4.3 Kf Rate constant for forward reaction 2.5 L Polarization parameter 7.2 L, Width of space charge layer 4.3 L" Hole diffusion length 4.3 M Molar concentration per liter 2.5 Molar mass weight 5.2 Effective density of states in conduction band 4.3 Dopant concentration 4.3 Bulk electron and hole density 4.3 Charge per atom 5.2 nox, nred Number of oxidizing and reducing species 4.3 n" Ps Surface electron and hole density 4.3 PH"PO, Partial pressure of hydrogen and oxygen 2.5 Q; - Immobile charge of space charge layer 4.3 R Resistance 7.2 Gas constant 2.5 Corrosion rate 8.3 Charge-transfer resistance 2.6 Electrolyte resistance 2.6 Capacitance of corrosion product film 2.6 Metallic resistance 7.2 Polarization resistance 5.2 Capacitance of charge transfer 2.6 Potential scanning rate 3.4 Passivation time 3.3 Temperature 2.5 Thickness loss 5.2 Time 3.3 t+, L Transport number 2.3 VH Potential drop across Helmholtz double layer 4.3 Vrd Rest dark potential 4.3 V, Potential drop across space charge layer 4.3 VSCE Potential versus saturated calomel electrode V SHE Potential versus standard hydrogen electrode LIST OF SYMBOLS xxi

S~mbol Definition Section

VCSE Potential versus copper sulfate electrode AVmAVe Potential drops across anode and cathode 7.2 AVd Potential drop between anode and cathode 7.2 W Weight loss 5.2 Width of steel cathode 7.2 X Galvanic protection width 7.2 Z Electrochemical impedance 2.6 Zw Warburgimpedance 2.6 a Charge-transfer coefficient 2.5 Photo absorption coefficient 4.3 fJ Tafel slope 5.2 e Relative permittivity 4.3 eo Permittivity of vacuum 4.3 'I Overpotential 2.5 'la' 'Ie Anodic and cathodic overpotential 7.2 'II' Passivation overpotential 3.2 () Area factor 7.2 K Rate constant 4.3 A+"L Equivalent conductance 2.3 v Stoichiometric coefficient 2.2 p Density 5.2 Specific resistivity 7.2 (J Cross section 4.3 ¢ Potential difference between electrode and solution 2.5 w Electrode rotation speed 2.5 Frequency of AC signal 2.6 AC Alternating current 2.6 CCT Cyclic corrosion test 11.3 ohp Outer Helmholtz double layer 4.3 mpy Mils per year PZC Potential of zero charge 2.4 RH Relative humidity 8.3 r.d.s. Rate-determining step 2.5 rpm Number of rotations per minute 2.5 SCE Potential of saturated calomel electrode 2.5 SHE Standard potential of hydrogen electrode 2.5 SST Salt spray test 11.3