Passivity and Protection of Metals Against Corrosion

Passivity and Protection of Metals Against Corrosion

Nikon D. Tomashov and Galina P. Chernova Institute of Physical Chemistry Academy of Sciences of the USSR

Translated from Russian by Boris H. Tytell Consulting Engineer Greenwood, Massachusetts

Translation Editor Herbert H. Uhlig Department of Metallurgy Massachusetts Institute of Technology Cambridge, Massachusetts

With a Foreword by Edward C. Greco Research Department United Gas Corporation Shreveport, Louisiana

~ PLENUM PRESS· NEW YORK· 1967

Nikon Danilovich Tomashov, Scientific Superintendent of the Alloy Corro­sion Laboratory of the Institute of Physical Chemistry of the Academy of Sciences of the USSR, has been active for thirty years in research on cor­rosion and the protection of metals. His work on the theory of corrosion processes and the principles of corrosion-resistant alloying and his de­velopment of new methods for the protection of metals have been given wide recognition and have found important practical applications. Profes­sor Tomashov's fundamental work Corrosion Theory and the Protection of Metals has been translated into English and published in the USA in 1966 by the Macmillan Company. In 1966, he was awarded the W. R. Whitney medal by the American National Association of Corrosion Engineers.

Galina Prokof'evna Chernova, Senior Scientific Worker in the Corrosion Department of the Institute of Physical Chemistry of the Academy of Sciences of the USSR, completed her· studies at the Moscow Institute of Steel and Alloys in 1947. Her research concerns fundamental work in the

field of the passivity of alloys.

The Russian text, originally published for the Institute of Physical Chemistry, Academy of Sciences of the USSR, by Nauka Press, Moscow, 1965, has been revised by the authors

for the American edition.

Huxon J(anuJto8U1f, TOMaUlo8, raJtuna llpoxo¢be8na 'CJepn08a

IIaCCHBHOCTb H 3am;HTa ~leTaJIJIOB OT ROPP03HH

PASSIVNOST'I ZASHCHITA METALLOV OT KORROZII

PASSIVITY AND PROTECTION OF METALS AGAINST CORROSION

Library Congress Catalog Card Number 66-19933

lSBN-13: 978-1-4684-1730~2 e-lSBN-13: 978-1-4684-1728-9 DOl: 10.1007/978-1-4684-1728-9

© 1967 Plenum Press Soficover reprint of the hardcover 1st edition 1967

A Division of Plenum Publishing Corporation 227 West 17 Street, New York, N. Y. 10011

All rights reserved

No part of this publication may be reproduced in any form without written permission from the publisher

Foreword

Considerable progress has been made in the past 20 years toward understanding the basic mechanisms of corrosion, and the application of this knowledge to its control. From the very beginning, educational institutions and industrial research laboratories have contributed greatly toward determining and elucidating the fundamental principles of corrosion reactions. Some of the basic principles involved in cor­rosion of metals can be credited to early investigators. Michael Faraday in 1830-1840 studied the relationship between the quantity of a metal dissolved and the electric current which was produced by this reaction. He also proposed that the passivation of iron was through the formation of a film and that the dissolution of a metal was electro­chemical in nature. Sir Humphrey Davy in 1824 worked out the funda­mentals of galvanic corrosion of ships' hulls and applied sacrificial zinc anodes to protect them from sea water corrosion. Richard Arlie in 1847 demonstrated that corrosion produced by oxygen at the surface of iron in a flowing stream generated a current. With the fundamental knowledge available to him from these early investigators, Willis Rodney Whitney developed and expressed, in its most useful form, one of the basic scientific principles which provides modern corrosion specialists with a fundamental basis of corrosion control. Dr. Whitney concluded that corrosion of iron is electrochemical, and that the rate is simply a function of the electromotive force and resistance of the circuit.

It is indeed fitting that I had the privilege during the Third Inter­national Congress on Metallic Corrosion in Moscow, May 1966, of presenting to the senior author of this volume, Professor Tomashov, the NACE Willis Rodney Whitney Award, in recognition of his notable and extensive contributions to the science of corrosion.

Scientists and engineers of many countries have contributed to the rapid development of corrosion science, and prominent among these

v

vi FOREWORD

are the corrosion specialists of the Soviet Union. Professor Tomashov and Galina P. Chernova are among the respected contributors and leaders of corrosion research in the USSR. Their research has re­received world-wide acclaim. Professor Tomashovhas spent 35 years in scientific research and is presently director of the laboratory of alloy corrosion at the Institute of Physical Chemistry in Moscow. His work on the theory of electrochemistry as applied to the investigation of passivity of metals, electrochemical and corrosion behavior of stainless and copper steels, titanium and its alloys, and new ways of increasing the corrosion resistance of alloys by alloying them with more noble elements, is well known and widely acclaimed. Professor Tomashov is the author of 200 papers and 6 books on corrosion tech­nology.

Throughout this volume he and Galina Chernova have presented the subject of passivity in a very clear and skillful manner. The book includes 6 chapters: the phenomenon of passivity in metals, passive systems, basic principles for increasing the corrosion resistance of alloys by increasing their passivity, anodic protection, passivation of metals by contact with cathodes, and passivation of metals by the introduction of oxidizers into the corrosive medium.

In Chapter 4, the authors do a commendable job of discussing the basic concepts and application of anodic protection, one of the more recent methods employed for protection of metals against corrosion. They cover in a concise manner the effectiveness of anodic protec­tion, the effect of various factors on the parameters of anodic protec­tion, including the radius of effective protection, anodic protection against intergranular corrosion, the effect of anodic polarization on corrosion cracking, the effect of halide ions, and the anodic protection of carbon steels and titanium. Corrosion specialists and students will find this excellent and informative volume indispensible; they are indebted to Professor Tomashov and Galina Chernova for this fine contribution to the literature on corrosion technology. They are also indebted to Mr. Tytell for his painstaking and first-rate translation, and to Professor Uhlig for his work of editing the translation.

I wish to add my thanks for the privilege of writing the Foreword to Passivity and Protection of Metals Against Corrosion and extend my gratitude to Professor Tomashov and Galina P. Chernova for making this useful monograph available to corrosion students and specialists in the English -speaking countries.

Shreveport, Louisiana April 24, 1967

Edward C. Greco

Preface

Various new methods for upgrading the corrosion resistance of metals and alloys have become available within the past decade. Some of the most interesting and important of these deal with the phenomenon of passivity and with the so-called passive metals. This category of metals encompasses, among others of the transition group, titanium, chromium, zirconium, and nickel, plus their alloys including the stain­less steels. Major improvements in these metals have been paralleled by the introduction of anodic protection using applied electric currents. This has been established as an effective new technique for corrosion control, particularly in the chemical industry.

Professor Tomashov and his associates occupy a primary role in these world-wide developments. They have through their own re­searches advanced the forefront of corrosion science, and, at the same time, have shown how these advances can be interpreted in terms of practical alloys and corrosion inhibitors. The principles they care­fully present, many of which they developed themselves, are in accord with advanced concepts of the electrochemical theory of passivation, and are essential to an understanding of the remarkable alloys to which these principles lead. The basic theory, in this connection, is also necessary to an understanding of how the improved alloys are best used in practice.

In addition, the book contains detailed practical data on the cor­rosion and electrochemistry of passive metals and alloys which have not heretofore been made available in English. This feature plus a complete description of underlying theory and practice insure that the book will be welcomed by corrosion engineers, chemical engineers, materials scientists, and by a broad segment of corrosion students who wish to keep abreast of developments in a rapidly changing scientific and engineering discipline.

Cambridge, Massachusetts May 8. 1967

vii

H. H. Uhlig

Preface to the American Edition

The authors of the book were gratified to learn that their work was to be translated into English, and would thus become more accessible to a wide circle of scientific and technical specialists in the United States, where considerable attention is given to the study of the cor­rosion of metals-and particularly to means of combating it.

With the awareness both of the requirements and the pressing questions of new technology, development of the science of the corro­sion and protection of metals has been given renewed impetus in recent decades. In the past, the attention of scientists has been directed mainly to research on the passivity of metals-and this is understandable-since the passivity of metals is not merely an interesting natural phenomenon, attracting the attention of inquiring minds, but, at the same time, it is the principal cause of the relative durability of the majority of industrial metals and alloys, which by their nature are thermodynamically unstable.

Research on the mechanism of the passivity of metals and the principal factors which determine their transition to the passive state and, conversely, their activation, as well as on the development of new methods of investigating passivity, is currently proceeding on a wide front, and often along similar lines, in a number of industrially developed countries.

In recent years, considerable attention has been devoted to the development of methods of application of available scientific achieve­ments in the field of passivity toward a solution of the practical prob­lem of increasing the effectiveness of anticorrosion protection, and to the development of new, more corrosion-resistant alloys.

In this connection, there has been definite progress not only in improving already known methods of prqtection-for example, pro­duction of more effective anodic inhibitors, improvements in the passivating surface treatment of metals, production of alloys for

ix

x PREF ACE TO AMERICAN EDITION

direct inhibition of the anodic process, and so forth-but also in the discovery and practical application of fundamentally new methods of protection, unknown 10 or 20 years ago, such as cathodic alloying of alloys, application of anodic electrochemical protection, and others.

The discovery of these methods has not been an accident, but the logical outcome of lengthy and systematic research in the study of the kinetics of corrosion processes in passivating systems.

These methods, first proposed in the USSR, have been further developed in the USA, where the maximum achievements are currently being obtained in th.'eir practical application. Examples are the adop­tion in the USA of the industrial production of cathodically alloyed metals (e.g., titanium base) and the application ofthe method of anodic protection in industry. The practical experience acquired in the USA in this field will undoubtedly be useful in the development of these new methods of protection.

Since the translation of our book follows fairly soon after its first publication in the USSR, there is as yet no need to undertake any radical revision of the book for the American edition. Slight amend­ments and additions have been made by the authors at the request of the American publisher.

The authors express their sincere gratitude to Professor H. H. Uhlig, who has undertaken the task of the scientific editing of the translation of our book, and to the American publisher for the careful preparation of the translation.

N. D. Tomashov and G. P. Chernova

Preface to the Russian Edition

With the development of the science dealing with corrosion and protection of metals, various anticorrosion approaches and methods for preserving metallic structures are continually being refined and improved. New methods which make it possible to use metals and alloys under more and more complex technological conditions are being developed. There can be no doubt that progress in the develop­ment of corrosion protection can be achieved only by the simultaneous, thorough development of the scientific study of corrosion phenomena and by the establishment of quantitative relationships between the kinetics of corrosion processes and the various factors promoting them, on the part of both the metal and the corrosive medium. The technology of corrosion protection and the scientific methods for investigating corrosion processes under a variety of conditions are being studied intensively at present.

One of the basic problems of corrosion theory is, without doubt, the problem of passivity. It presently attracts the attention of many scientists. The authors of this book devoted many years to the investigation of passivity. Studies were carried out to determine the essence of the phenomenon and the mechanism involved in establishing the passive state in metals and alloys, and to develop methods for investigating this complex phenomenon of passivity for the purpose of enhancement of corrosion resistance in practical metallic systems.

An attempt has been made to summarize briefly the results of our past investigations as well as our most recently published data on im­proving the corrosion resistance of metals and alloys by increasing their passivity and maintaining their stable passive state.

The results of investigations on these problems are scattered throughout many journals, and they do not always include ways and means of utilizing these phenomena for corrosion protection. Methods for increasing the corrosion resistance of metals by increasing their

xi

xii PREF ACE TO RUSSIAN EDITION

passivity (alloying with cathodic elements, anodic protection, use of cathodic couples) are, for the most part, accomplishments of Soviet science. However, their utilization, unfortunately, lags behind experimental applications abroad.

We hope that the systematic presentation in this work of problems in the field of passivation and corrosion resistance will promote a much wider application of the new methods and a more extensive use of established methods of protection based on increased stability of the passive state. We hope, too, that the present volume will be use­ful in the solution of specific problems on protection corrosion arising with the expansion of the chemical industry.

A few words are in order concerning the content and arrange­ment of the book. In the Introduction, we discuss briefly the importance of passivity as one of the methods available in the battle against metal corrosion. In Chapters I and II, the modern theory of passivity, the kinetics of passivation processes, and the methods used for investi­gating passivity and the structure of passive films are presented.

In Chapters III, IV, and V, we discuss the more original material dealing with the basic principles involved in increasing the corrosion resistance of alloys by increasing their passivity, the new principle of anodic protection of metallic structures, and the use of cathodic couples. The authors' investigations throughout many years on problems of passivity in metals and alloys are summarized in these sections. We also deemed it necessary to dwell briefly on some insufficiently investigated aspects of anodic protection, such as the introduction into the solution of metallic cations of electropositive metals and the effect of electrochemical protection on cathodic coatings.

Chapter VI is devoted to the practically important and scientifically interesting problem of increasing corrosion resistance by introducing passivators (oxidizing inhibitors) into the corrosive medium.

Problems on the protection of metals by processing in passivating solutions or by anodic oxidation-that is, methods of protection associated with the formation on the metal surface of protective films resulting from· prior surface treatment-are not discussed in this book, although they too present an opportunity to apply directly the phenomenon of passivity to practical corrosion protection. These methods have been widely used for some time and are fully described in special books and articles on the subject.

The authors humbly invite any criticism and comments by the readers of this volume.

N.D. Tomashov and G. P. Chernova

Contents

A Note on Soviet Alloy Designations • • • • • • • • • • • . • . • . • •• xvi Introduction . . • . • . • • • • • . . . • . • . • . • . • . • . • . . . • . • . • 1

Chapter I The Phenomenon of Passivity in Metals. . . . . . . . . . . . . . . . . 9

Definition of Passivity. . . . . . . . . . . . • . . . • . . . . . • . • • 9 Mechanism and Theory of Passivity. . . . . . . . . . . . . . • . . 12 Electrochemical Methods for Investigating Passivity . . . • . 14 Anodic Potentiostatic Charge Curves. . . . • . . . . . . . . . . . 15 Anodic Galvanostatic Charge Cu:::-ves . . . . . . . . . . . . . . .. 17 Cathodic Reduction Curves. . . . . . . . • . • . . . • . . . . . • . • 18 Study of Potential Drop Curves. • . . . . . . . . . • . . . . . . .. 23 Measurement of the Double-Layer Capacity. . . . . . . . . . . 25 Study of the Kinetics of Electrode Processes Under the Ac-

tion of Alternating Currents . . . . . . . . . . . . . . . . • • . 28 Investigation of the Nature and Structure of Passive Films. 32

Optical Method . . . . • . . . . . • . • . • . . . • . . . . . . . . • • 32 Electron Diffraction. . . . . • . • . • • • • . . . . • . . . . . • . • 34 Microchemical Method for Determining Film CompOSition 36

Kinetics of the Anodic Processes . . • . . . . . . . . . . . . • . • 38 Anodic Processes on a Passive Electrode . • . . . . . . . .. 38 Method of Plotting Potentiostatic Curves . . . . • . • . • . •. 43

Chapter II Passive Systems. . . . . . . . . . • . • . . . • . • . . . . • • . . . . . . • 55

Active State. . . . . . • . . . . . • . . . • . . . . . . . • . • . . • • . . 55 Passive-Active State. . . • . • . . . . . . . . . . . . . • • . . . . • . 56 Spontaneously Stable Passive State . . . . • • . . • . . . . . • . . 58 Pitting Formation. . . . . • . • . . . . • • . . . . . . . . . . . • . • . 60

xiii

xiv CONTENTS

The Transpassive State and Secondary Passivity. . . . . . • • 60 Construction of Corrosion Rate-Potential Curves. • . . . 61

Chapter III Basic Principles for Increasing the Corrosion Resistance of

Alloys by Increasing Their Passivity. . . . . . . . . 67 Inhibiting the Anodic Process. . . . . . . . . . . . . • . • . . . .. 67

Mechanism of Increasing Passivity by Alloying. . . . . . . . 67 Factors Determining Anodic Passivation of Alloys. . . . . . 68 Effect of the Anodic Process on Passivity of Alloys. . . . . 79

Reduction of Corrosion in Passive Systems by Increasing the Effectiveness of the Cathodic Process . . . . . . . . . . 81

Effect of the Cathodic Process on Passivity of Alloys. • . • 82 Mechanism of the Effect of Cathodic Alloying Elements . • 83 Significance of Anodic Passivation in Cathodic Alloying . . 86 Increase in Corrosion Resistance of Stainless Steels, Ti-

tanium, Zirconium, and Chromium by Alloying with Ca-thodic Additives . • . . . . . • . . . • . . . . . . . . . . . • . . . 91

Chapter IV Anodic Protection .

Anodic Protection of Stainless Steels ... Effectiveness of Anodic Protection ............... . Effect of Various Factors on the Parameters of Anodic

Protection .....•...............•.•....... Radius of Effective Anodic Protection ........•...•. Anodic Protection and Intergranular Corrosion .. Anodic Polarization and Corrosion Cracking ......... . Effect of Halide Ions on Anodic Protection ..........•

Anodic Protection of Carbon Steels ................ . Anodic Protection of Titanium .................•••

Anodic Protection of Titanium in Sulfuric Acid •....... Anodic Protection of Titanium in Hydrochloric and Other

Acids ........•......................•.• Practical Application of Anodic Protection •.•......... Prospects for the Development of Anodic Protection.

Chapter V Passivation of Metals by Contact with Cathodes ........•.•

Use of Cathodic Protectors ••.............•.•.... Protection with Metallic Cathodes .•.....•.•...•... Protection with Oxide Cathodes ..•.....•..•..•.•.

107 107 107

110 114 117 125 126 129 135 136

138 145 149

151 151 152 156

CON1ENTS

Protection Against Hydrogen Embrittlement and Corrosion Cracking •.......•.•...•...........•.....

Cathodic Coatings ........•.....•.•...........• Introduction of Cations of Electropositive Metals into the

Corrosive Medium ...........•...••.•..••...

Chapter VI Passivation of Metals by the Introduction of Oxidizers into the

Corrosion Medium .•.........•...•.....•.... Mechanism of the Protective Effect of Oxidizing Inhibitors .

Inhibitors Which Retard the Anodic Process. . . . . . . . . . Oxidizers Which Increase the Effectiveness of the Ca-

xv

163 163

167

181 181 183

thodic Process. . . . . . . . . . . . . . . . . . . . . . . . . . .• 185 Nature of the Oxidizing Inhibitor. . . . . . . . . . . . . . . . . .• 189 Effect of the Metal and Aggressiveness of the Medium . . .. 195

References. . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . • •. 199

fudex • • • • • . • . • . • . • • • . • • . . • . . . . • • . • • . • . • • • . .• 205

A Note on Soviet Alloy Designations

The alloy designations in this book have been transliterated from the Russian rather than translated to avoid confusion with Western alloys that are similar but not identical. The explanation of these designations that follows is based on the one contained in the Handbook of Soviet Alloy Compositions (U. S. Department of Commerce. OTS. Washington 25. D.C .• PB 171331) and the British Iron and Steel Industry Translation Service Publi­cation No. BISI 2000. A complete description of steel and alloy compositions is given by Soviet steel standards. GOST 4543-57 and 4632-51.

Constructional carbon steels of "ordinary" quality. which are specified according to their mechanical properties. are StO. St!. St2. St3. St4. St5. St6. and St7. The higher the figure. the greater the carbon content. Depending on the method of manufacture. the prefix B or M may be added to describe a Bessemer or open-hearth steel. respectively.

Constructional "quality" carbon steels have numbers between 05 and 70. which indicate the average carbon content in hundredths of one percent. These numbers may carry suffixes kp. sp. and ps. denoting rimmed. killed. and semikilled grades. respec­tively.

"Quality" carbon tool steels have deSignations U7. UB. etc •• up to U12. the number indicating the average carbon content in tenths of one percent.

A B D F

In alloy steel designations. the following symbols represent the alloying elements:

Nitrogen Columbium Copper Vanadium

G K Kh M

Manganese Cobalt Chromium Molybdenum

N P R S

Nickel Phosphorus Boron Silicon

T Titanium Ts Zirconium V Tungsten Ya Aluminum

If the percentage of the element is not greater than about 1%. the letter for the element is not followed by a figure. If the amount of the element is greater than 1%. a figure representing the content is placed to the right of the letter. e.g •• 4% Ni is represented by N4.

The average carbon content is shown to the left of the letters as hundredths of one percent. In the case of a very low carbon content Oess than about 0.08%). the numeral 0 is placed before the letters. Occasionally. the carbon figure is omitted altogether.

The letter A appended to these deSignations denotes "high quality" (narrower com­position limits and lower sulfur and phosphorus contents). This must not be confused with the symbol A representing nitrogen. Frequently. the letter L is used as a suffix to denote a cast steel.

Several groups of steel designations carry a prefixed letter which indicates a par­ticular purpose or characteristic of the steel. e.g •• A free-machining. E magnetic. Zh straight chromium stainless. R high-speed. Shball-bearing. E electrical. and Ya chrome­nickel stainless. The numbers following these letters are not normally indicative of the actual composition.

xvi