Corrosion vol 1 metal environment reactions 3ed 1994 shreir, jarman burstein

Shreir, OBE Preface to the third edition Preface to the first edition Effects of Metallurgical Structure on Corrosion Corrosion in Aqueous Solutions Passivity and Localised Corrosion Lo

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Corrosionledited by L L Shreir, R A Jarman G T Burstein

p cm

Includes bibliographical references and index

Contents: v I MetaVenvironmental reactions - v 2 Corrosion control

I Corrosion and anti-corrosives 1 Shreir, L L

11 Jarman, R A 111 Burstein, G T

TA462.C6513 1993

ISBN 0 7506 1077 8 (for both volumes) CIP

Printed and bound in Great Britain

FOR E V E N T m B THAT WE PUBLISH, B ~ O R T H N E W M A N N

CONTENTS

L L Shreir, OBE

Preface to the third edition

Preface to the first edition

Effects of Metallurgical Structure on Corrosion

Corrosion in Aqueous Solutions

Passivity and Localised Corrosion

Localised Corrosion

Bimetallic Corrosion

Lattice Defects in Metal Oxides

Continuous Oxide Films

Discontinuous Oxide Films

Erosion Corrosion

2 Environments

Soil in the Corrosion Process

Chemicals

Corrosion by Foodstuffs

Corrosion in Fused Salts

Corrosion Prevention in Lubricant Systems Corrosion in the Oral Cavity

High-nickel Cast Irons

High-chromium Cast Irons

Silicon-Iron Alloys

Amorphous (Ferrous and Non-Ferrous) Alloys

CONTENTS

7.2

7.3

The Oxidation Resistance of Low-alloy Steels

vii

8 Effect of Mechanical Factors on Corrosion

Thermodynamics and Kinetics of Gas-Metal Systems

Mechanisms of Stress-corrosion Cracking

Stress-corrosion Cracking of Ferritic Steels

Stress-corrosion Cracking of Stainless Steels

Stress-corrosion Cracking of High-tensile Steels

Stress-corrosion Cracking of Titanium, Magnesium and Aluminium Alloys

Corrosion Fatigue

Fretting Corrosion

Cavitation Damage

Stress-corrosion Test Methods

Volume 2 Corrosion Control

Corrosion Control in Chemical and Petrochemical Plant

Design in Marine and Offshore Engineering

Design in Relation to Welding and Joining

10 Cathodic and Anodic Protection

Pretreatment Prior to Applying Coatings

12 Methods of Applying Metallic Coatings

13 Protection by Metallic Coatings

Tin and Tin Alloy Coatings

Copper and Copper Alloy Coatings

Nickel Coatings

Chromium Coatings

Noble Metal Coatings

14 Protection by Paint Coatings

14.1 Paint Application Methods

Synthetic Resins

Glossary of Paint Terms

15 Chemical Conversion Coatings

17.4 Boiler and Feed-water Treatment

Conditioning the Atmosphere to Reduce Corrosion

Corrosion Inhibition: Principles and Practice

xi

Plastics and Reinforced Plastics

Rubber and Synthetic Elastomers

Corrosion of Metals by Plastics

Wood

The Corrosion of Metals by Wood

19 Corrosion Testing, Monitoring and Inspection

20 Electrochemistry and Metallurgy Relevant to Corrosion

20.3

20.4

The Potential Difference at a Metal/Solution Interface

Outline of Structural Metallurgy Relevant to Corrosion

1.1 SHREIR, OBE 1914-1992

devoted to the science and technology of corrosion and education His

which time he was employed by the Mond Nickel Company, Baker Platinum Ltd and Plessey Ltd At the same time he continued his higher education on

subsequently renamed the University of Surrey, eventually attaining the

of the Sir John Cass College (now London Guildhall University), a post he enjoyed by greatly expanding the Department, its research and general

Lionel’s contribution to corrosion was outstanding In addition to his

sion Science for many years He was engaged as a consultant to a number

covered many fields, including hydrogen in metals, anodic oxidation and

1978 and was awarded the OBE in 1982 in recognition of his services to

corrosion In this context, one of his most notable activities was to advise

Award was awarded for the first time by the Institute of Corrosion Science and Technology

man He will be affectionately remembered for his boundless energy and infectious enthusiasm by his peers, colleagues, friends and the countless past students privileged to have made his acquaintance during what was a

xv

The huge success of the first two editions of Corrosion has inevitably created the demand for a third edition Corrosion science and technology, like most

of the physical sciences, has progressed and advanced significantly in the seventeen years since the second edition was published Such knowledge requires transferral from the laboratory and the journal literature to the wider audience: the student, the teacher, the engineer, the metallurgist and workers in other fields who require knowledge and understanding of the

the fruits of Lionel Shreir’s hard labours, have fulfilled this multiple role admirably and the new editors hope that this new edition will continue to do

did not live to see its publication, is a personal and deeply poignant sorrow

respected him as scientist and friend

The ever-increasing research into corrosion, and the knowledge that this

in seeking a detailed understanding of the intricacies of the interfacial pro- cesses driving corrosion and passivation Such a self-fulfilling drive cannot

of itself however, be indefinitely sustainable, despite the fascination that this science engenders, since research is costly Such advances are led primarily

engineering imperative Corrosion science, multidisciplinary in itself, is pro- bably unique in crossing the borders of almost all the technologies: environmental stability of all components of those technologies remains a prime requirement for their success New technologies, new engineering prac-

The eighties and nineties, and beyond, see a further need to underpin research

issues Most materials and components made from them require large energy resources to produce; clearly the quest for longevity and reliability of struc- tures is a significant and worthy contribution towards conserving energy and

xvi

PREFACE TO THE THIRD EDITION xvii

to the general format and structure of their antecedents Some sections have been completely rewritten to bring them up to date, while others have been altered and extended New sections have been included to cover areas not previously treated The incorporation of new authors to carry out such revi- sions and additions is the inevitable consequence of the fact that thirty years

hopes this is beneficial to the work in providing a broader cross-section of

user to judge One hopes too, that the third edition remains a tribute to the

GTB

Cambridge

The enormous scope of the subject of corrosion follows from the definition which has been adopted in the present work Corrosion will include all reac-

of batteries, electrorefining, chemical machining, chemical and electro- chemical polishing, etc

The fact remains, nonetheless, that the environmental reaction of a metal used as a construction material is the most important type of corrosion reac- tion, and the one of most concern to the engineer The technological and

appreciated, and figures have been published which show the enormous finan- cial losses, both to the individual organisation and to the economy of the country as a whole, resulting from the deterioration of metals The need for

and the ‘corrosion consciousness’ which prevails today is largely due to their efforts

The effective control of corrosion reactions must be based on an under-

knowledge to practical problems The work, regarded as a whole, represents

corrosion science and corrosion engineering Thus in the planning of the

tially practical

xviii

PREFACE TO THE FIRST EDITION xix

value to the student, the corrosion worker, and the engineer in the field of corrosion

Corrosion represents the joint effort of over 100 authors, all of whom have

been free, within the necessary limitations of length, to express their own views Grateful acknowledgements are made to the individual authors from Great Britain, the United States, and Canada for their valuable and enthu- siastic co-operation

The task of the editor in finding suitable authors for various topics was

Chemical Industry or to the Institute of Metal Finishing, and acknowledge-

respectively of the I.M.F.) The editor wishes to express his appreciation of

Bailey of INCO (Mond) in providing authors from this organisation The editor also acknowledges with pleasure the encouragement and assist-

lurgy) and Dr D M A Leggett (Principal) of the Battersea College of

been discussed with various workers in the field, and the editor would like to

criticism

and advice in connection with the present work

Finally, grateful acknowledgements are made by the editor to Mr T F

for their kind co-operation at all stages of the work

L.L.S

1963

CONTRIBUTORS

K G Adamson", AMCST, LIM

Development Oficer,

Magnesium Elektron Ltd, Manchester

J C B Alcock*, ARCS, DSc, PhD, CChem,

FRIC

Professor and Chairman,

Dept of Metallurgy and Materials Science,

M D Allen, CEng, MIM, MICorrST

Section Head, Corrosion and Protection,

Defence Research Agency, Poole, Dorset

J E Antill*, PhD, BSc

Head, Chemical Metallurgy Group,

Materials Development Division, UKAEA,

Harwell

V Ashworth

Global Corrosion Consultants Ltd

Shifnal, Shropshire

D J Astley, BSc, ARCS, PhD, PGCE

Formerly Senior Technical Oficer

IMI Research and Development, Birmingham

*Contributor to earlier editions

T A Banfield*, PhD, DIC, ARCS, CChem,

FRIC, FICorr, FTSC

Deputy Manager, Group Research Laboratory, Berger Jenson and ivicholson Ltd

P J Barnes, BSc, MRSC, CChem, ATSC

Consultant

E W Beale*, A R K

Senior Scientific Ofleer, Materials Quality Assurance Directorate Ministry of Defence

J Bentley", BSc, DipChemEng, CEng,

MIChemE

Principal Chemist, Wastes Division Directorate General, Water Engineering, Dept of the Environment

W Betteridge", DSc, FInstP, FIM

Consultant,

P J Boden*, PhD, CEng, CChem, FRSC,

MIM, FlCorr

Materials Science, Nottingham University

C J L Booker', BSc, PhD,

A R K , FICorr

Formerly Senior Lecturer in Corrosion Science,

City of London Polytechnic

J W L F Brand*, MITE, TEng(CEI),

MICorr

Divisional Manager, Corrosion Control Division, Corrosion and Welding Engineering Ltd

xx

CONTRIBUTORS xxi

C F Britton*, LRIC, FICorr, FInstPet

Corrosion Consultant

Formerly vfAEA Technology and

Rohrback Instruments Lid

S C Britton*, MA, CChem, FRIC, FIM,

IMF, FICorr

Tin Research Institute (Retired)

J A Brydson*, FPRI, ANCRT

V E Carter*, FICorr, FIMF

Corrosion and metal finishing consultant,

J E Castle', BSc, PhD, CChern, FRSC

FICorr

University of Surrey, Guildford

K A Chandler*, BSc, ARSM, FICorr

Head, Corrosion Advice Bureau, BSC

A R L Chivers', MA

Senior Technical Oflcer,

Zinc Development Association,

London

M Clarke*, BSc, PhD, DSc, CChem, FRIC,

FIM, FICorr, FIMF

Consultant, formerly Principal Lecturer,

Dept of Metallurgy and Materials,

R J Clarke*, MA, CEng, FIChemE, FIFST

Won Visiting Lecturer in Food

Engineering,

Queen Elizabeth College, London

H G Cole', BSc, FIMF, FICorr

Principal Scientific Oflcer, Ministry of

Defence (Procurement Executive)

H H Collins*, BSc, CChem, FRIC

Superintendent, Chemistry Research,

J Congleton, BSc, PhD, FIM, CEng

Senior Lecturer

Department of Mechanical, Materials and

Newcastle upon Tyne

J B Cotton*, CChem, AMCT, ARIC,

R N Cox, BSc, CEng, MIM

Building Research Establishment, Garston, Watford

G W Currer, CEng, MIEE, MICorrST

P D Donovan*, MSc, ARIC, FIM

Principal Scientific Oflcer,

Ministry of Defence

C W Drane*, BSc, CChem, FRIC

Technical Manager, Water Specialities and Services,

Industrial Chemicals Division, Albright and Wilson Ltd

F G Dunkley*, FICorr

Consultant,

Formerly of British Rail, Derby

E J Easterhrook*, BSc(Eng), ARSM,

AMIMM, MIM

Formerly Principal Lecturer,

Dept of Metallurgy and Materials, City of London Polytechnic

J Edwards, BSc, PhD

Consultant

Metals Research Association and International Nickel Limited

T E Evans', BSc, ARIC, FICorr

Principal Technologist, International Nickel Ltd., Birmingham

P C Frost,

Senior Research Scientist, Cookson Group plc Yarnton, Oxfordshire

Institute of Polymer Technology and

Materials Engineering, Loughborough

P J Gay", BSc, FTSC, FICorr

Consultant

J S Gerrard* AMIEE

Metal and Pipeline Endurance Ltd

G N J Gilbert

P T Gilbert', BSc, PhD, CChem, FRIC,

FIM, FIMarE, FICorr, CEng

Metallurgical Consultant

Formerly BCIRA

T B Grimley*, BSc, PhD

Reader in Theoretical Chemistry,

Dept of Inorganic, Physical and

Kansas State University

S J Harris, MSc, PhD, CEng, FIM, FIMF

Tohoku University Sendai Japan

M H a * , CChem FRIC, FIMF, FTSC

Member of the Association of Consulting Scientists

Consulting Chemist and Paint Technologist

R A E Hooper, BMech, FIM, CEng, FICorr

Group Technical Manager,

H Howarth*, AMICorr, AMet

Investigator, Production Metallurgy Section, Special Steels Division, BSC

J C Hudson*, DSc, DIC, ARCS, FIM

Dept of Metallurgy and Materials Science

Imperial College, London

R A Jarman*, MSc, PhD, CEng, MIEE,

FWeldI, FIM

Consultant, formerly School of Engineering, University of Greenwich

L Kenworthy*, MSc, ARCS, CChem, FRIC,

FIM FICorrT

Consultant, Formerly Navy Dept (Ministry of Defence)

B T Kelly, MSc, ChP InstP

Consultant Physicist

CEng, MInstE, FICorr

Dept of Materials Engineering and

Materials Deign University of Nottingham

D Kirkwood, PhD

Senior Lecturer

Engineering

The Robert Gordon University, Aberdeen

F LaQue', BSc, LLD Past President, Nat

Assoc Corrosion Eng., Am Soc Test and

Mat., Electrochemical Society

Senior Lecturer, Scripps Institution of

Oceanography, University of California

D N Layton*, PhD, MSc, ARCS, DIC,

MInstP, FIMF

Managing Director,

Fredk Mountford (Birmingham) Ltd

EurIng M F Leclerc, BSc, PhD, MIM,

m n g

Technical Executive

Biome? Ltd

D A Lewis*, BSc(Eng) FICorr

Consulting Engineers

E L Littauer', BSc, PhD, MIM, AMIMM

Manager, Electrochemistry and

Environmental Sciences,

Lockheed Missiles and Space Co.,

California, USA

C 0 Lloyd*, BSc

Principal Scientific Oflcer,

Division of Materials Applications

National Physical Laboratory

N A Lockington*, MA, PhD, A R K , FIM

Metallurgist Director,

The Chrome-Alloying Co Ltd

C L Long*, PhD, CChem, FRIC

Principal Scienti@c Oflcer,

Energy Technology Support Unit

UKAEA, Harwell

W A Lure, BMetEng

Retired

P Lydon

Roxby Engineering International Ltd, Kent

J Mackowiak, BSc, PhD, CEng, MIM

Surrey, Guildford Consultant in high temperature corrosion

R J Oliphant, BA, MSc, PhD, AWIEM

Technical Specialist WRc plc, Swindon

D S Oliver*, BSc, PhD, FIM, FInstP

Group Director of Research and

Development, Pilkington Bros Ltd

M W O'Reilly* Dip Tech, LRIC

Decorative Paints Market Team Leader, ICIPaints Division

S Orman*, BSc, PhD, FICorr, CChem, FRIC

Senior Principal Scientific Oficer,

F C Porter*, MA, FIM, FICorr, FIMF

Zinc Development Association, London

ICI Paints, Slough

B A Proctor, DSe, FIP

Formerly Manager, Fibres and Glass,

Pilkington Group Research

R P M Procter*, MA, PhD, CEng, FIM,

FICorr

Vice Principal

The University of Manchester Institute of

Science and Technology UMIST

Loughborough University of Technology

R G Robson', BSc(Eng) MIEE

Chartered Engineer

D van Rooyen*, BSc, PhD

Advisory Scientist, Westinghouse Bettis

M Roper

EurIng C E D Rowe BSc, CEng, MIM

Manager, Technical Services/Quality Assurance, Climax Special Metals Fabrications Ltd, Brentwood

I R Scholes, BSc, CChem, FRSC, FICorr

Formerly Manager, IMI Research and Development Wilton, Birmingham

B A Scott*, ARCS, BSc, PhD, CChem,

FRIC

Deputy Information Oficer, Group Technical Information Service, British Aluminium Co Ltd

P M Scott, BSc, PhD

Framatome, Paris

J C Scully*, MA, PhD, CEng, FIM, FICorr

Senior Teaching Fellow,

School of Materials,

University of Lee&

H J Sharp* PhD, MSc CChem? FIM, FPRI

R E Shaw*, BSc, FIM, FIMF

Former Head, Dept of Metallurgv and

CONTRIBUTORS XXV

H Silman*, BSc, CEng, CChem FRIC

FIChemE, FIM FIMF

Consultant

E W Skerrey', BSc, CChem FRIC,

FICorrT, AIM

Assistant Manager, Application Technology

Department, Research Division, British

DRA Swynnerton, Defence Research

Agency, Nr Stone, Staffs

W H Tatton', ARIC, FIMF, FTSC

Technical Oflcer,

British Standards Institution

D S Tawil, BSc

Technical Marketing Manager,

Magnesium Elektron Inc,

Lakehurst New Jersey

J G N Thomas*, BSc, PhD, ARCS, DIC

Jessop Saville Ltd, Sheffield

S Turgoose, MA, PhD, MICorrST

Engineering, UMlST

G P A Turner, MA

Formerly Industrial Paints Research

Manager, IC1 Paints, Slough

R Walker., BSc DipEd MSc, MSc(Eng),

PhD

LeGurer, Metallurgy and Materials Technology Dept University of Surrey, Guildford

G W Walkiden', BSc, CChem, FRIC, MIM

Consultant, Ever Ready Central Laboratories

J R Walters*

R B Waterhouse', MA, PhD, FIM, FICorr

Reader in Metallurgy, Dept of Metallurgy and Materials Science University of Nottingham

K 0 Watkias' FIM FlCorr

Corrosion Advice Bureau, BSC

S A Watson', BSc PhD CChem, FRIC, FlMF

Senior Development Oflcer, Internatiorral Nickel Ltd

H C Wesson* MA, BSc, CChem, FRIC

Formerly Technical Manager, Lead Development Association (Retired)

E E White*, CChem, FRIC, FIM, CEng,

MIMM, FCS, MIInfSc, FICorrT, FIMF

Consultant Inter-Services Laboratory, BSC

N R Whitehouse, BSc PhD

The Paint Research Association

Teddington, Middlesex

C Wilson

Escol Products Ltd Huntingdon

R W Wilson*, MA, PhD, CEng FICorr, FIM

1 PRINCIPLES OF CORROSION

AND OXIDATION

Corrosion in Aqueous Solutions

Passivity and Localised Corrosion

Localised Corrosion

Bimetallic Corrosion

Lattice Defects in Metal Oxides

Continuous Oxide Films

Discontinuous Oxide Films

1.1 Basic Concepts of Corrosion

Modern technology has at its disposal a wide range of constructional materials -metals and alloys, plastics, rubber, ceramics, composites, wood,

the important responsibility of the design engineer No general rules govern

availability, relative costs, etc of a variety of materials; frequently the ultimate decision is determined by economics rather than by properties, and ideally the material selected should be the cheapest possible that has ade- quate properties to fuIfil the specific function

Where metals are involved, mechanical, physical and chemical properties must be considered, and in this connection it should be observed that whereas mechanical and physical properties can be expressed in terms of

the precise environmental conditions prevailing during service The relative

lines elasticity, tensile strength, hardness and abrasion resistance will be of major importance, whereas electrical conductivity will be of primary signi-

thermal conductivity is necessary, but this may be outweighed in certain environments by chemical properties in relation to the aggressiveness of the

superior to that of aluminium brass or the cupronickels, the alloys are pre- ferred when high velocity sea water is used as the coolant, since copper has very poor chemical properties under these conditions

While a metal or alIoy may be selected largely on the basis of its mech- anical or physical properties, the fact remains that there are very few applica- tions where the effect of the interaction of a metal with its environment can

be completely ignored, although the importance of this interaction will be

sleepers) is of far less importance than the rapid perforation of a buried steel

hydroxide solution

1:3

1 : 4 BASIC CONCEPTS OF CORROSION

The effect of the metal/environment interaction on the environment itself

Section 2.7) For instance, lead pipes cannot be used for conveying plumbo- solvent waters, since a level of lead > 0.1 p.p.m is toxic; similarly, gal- vanised steel may not be used for certain foodstuffs owing to the toxicity of

environment by traces of metallic impurities that would affect colour or taste

of products or catalyse undesirable reactions; thus copper and copper alloys cannot be used in soap manufacture, since traces of copper ions result in coloration and rancidification of the soap In these circumstances it will be essential to use unreactive and relatively expensive metals, even though the

such as mild steel A further possibility is that contamination of the environ-

ment by metals’ ions due to the corrosion of one metal can result in the enhanced corrosion of another when the two are in contact with the same

domestic water may not be particularly deleterious to copper plumbing, but

it can result in the rapid pitting and consequent perforation of galvanised steel and aluminium that subsequently comes into contact with the copper-

Finally, it is necessary to point out that for a number of applications metals are selected in preference to other materials because of their visual appearance, and for this reason it is essential that brightness and reflectivity are retained during exposure to the atmosphere; stainless steel is now widely used for architectural purposes, and for outdoor exposure the surface must remain bright and rust-free without periodic cleaning (Section 3.3) On the other hand, the slow-weathering steels, which react with the constituents of the atmosphere to form an adherent uniform coating of rust, are now being

face is usually regarded as aesthetically unpleasant

The interaction of a metal or alloy (or a non-metallic material) with its environment is clearly of vital importance in the performance of materials

of construction, and the fact that the present work is largely confined to a detailed consideration of such interactions could create the impression that this was the sole factor of importance in materials selection This, of course,

is not the case although it is probably true to say that this factor is the one that is the most neglected by the design engineer

Definitions of Corrosion

beneficial or adventitious and deleterious Thus this definition of corrosion,

BASIC CONCEPTS OF CORROSION 1 : 5

will include, for example, the deliberate anodic dissolution of zinc in cathodic protection and electroplating as well as the spontaneous gradual wastage of zinc roofing sheet resulting from atmospheric exposure

environment that adversely affects those properties of the metal that are to

crete, etc and embodies the concept that corrosion is always deleterious

of a metal results in anomalies which will become apparent from a considera-

Steel, when exposed to an industrial atmosphere, reacts to form the reac-

from the environment; the reaction thus proceeds at an approximately linear rate until the metal is completely consumed Copper, on the other hand forms an adherent green patina, corresponding approximately with bron-

ing satisfactorily, and it is apparent that the formation of bronchantite is not deleterious to the function of copper as roofing material-indeed, in this particular application it is considered to enhance the appearance of the

aesthetically objectionable

H, SO, with the formation of Ti4+ aquo cations conforms with both defini-

tion) a thin adherent protective film of anatase, TiO,, is formed, which

decreased The formation of this very thin oxide film on titanium, like that

of the relatively thick bronchantite film on copper, clearly conforms with the

transformation definition of corrosion, but not with the deterioration defini-

significantly detrimental to the metal concerned Again, magnesium, zinc or aluminium is deliberately sacrificed when these metals are used for the cathodic protection of steel structures, but as these metals are clearly not required to be maintained as such, their consumption in this particular

Furthermore, corrosion reactions are used to advantage in technological

processes such as pickling, etching, chemical and electrochemical polishing

and machining, etc

The examples already discussed lead to the conclusion that any reaction

of a metal with its environment must be regarded as a corrosion process

subsequent stages of the reaction It is not illogical, therefore, to regard

passivity, in which the reaction product forms a very thin protective film that

H,SO, and the slow dissolution of passive titanium in that acid must be

1:6 BASIC CONCEPTS OF CORROSION

regarded as corrosion processes, even though the latter will not be detrimen- tal to the metal during the anticipated life of the vessel

It follows that in deciding whether the corrosion reaction is detrimental to

of the corrosion reaction must all be taken into account In addition, due consideration must be given to the effect of the corrosion reaction on the environment itself Thus corrosion reactions are not always detrimental, and our ability to use highly reactive metals such as aluminium, titanium, etc in aggressive environments is due to a limited initial corrosion reaction, which results in the formation of a rate-controlling corrosion product Expressions such as ‘preventing corrosion’, ‘combating corrosion’ or even ‘fighting cor- rosion’ are misleading; with the majority of metals corrosion cannot be avoided and ‘corrosion control’ rather than ‘prevention’ is the desired goal

the extent, nor the rate of the corrosion reaction must be detrimental to the

certain applications the corrosion reaction must not result in contamination

it must involve a consideration of materials, availability, fabrication, protec- tive methods and economics in relation to the specific function of the metal

without any protective system, at the other the environmental conditions prevailing may necessitate the use of platinum

The scope of the term ‘corrosion’ is continually being extended, and Fon-

metals, glasses, ionic solids, polymeric solids and composites with environ- ments that embrace liquid metals, gases, non-aqueous electrolytes and other non-aqueous solutions’

Vermilyea, who has defined corrosion as a process in which atoms or

into vacuum should come within the scope of the term, since atomically it

and non-metals, as short-circuited electrochemical cells, or as the demolition

of the crystal structure of a metal

These considerations lead to the conclusion that there is probably a need for two definitions of corrosion, which depend upon the approach adopted:

of a solid with its environment

reaction of an engineering constructional metal (material) with its environment with a consequent deterioration in properties of the metal (material)

BASIC CONCEPTS OF CORROSION 1:7 Methods of Approach to Corrosion Phenomena

last, as pointed out earlier, cannot be divorced from the environmental con- ditions prevailing Any fundamental approach to the phenomena of corro-

metal, the nature of the environment and the reactions that occur at the metal/environment interface The more important factors involved may be summarised as follows:

macroscopic heterogeneities, stress (tensile, compressive, cyclic), etc

2 Environment - chemical nature, concentrations of reactive species and deleterious impurities, pressure, temperature, velocity, impingement, etc

3 Metd/environment interface - kinetics of metal oxidation and dissolu-

corrosion products; film growth and film dissolution, etc

From these considerations it is evident that the detailed mechanism of metallic corrosion is highly complex and that an understanding of the various phenomena will involve many branches of the pure and applied sciences, e.g metal physics, physical metallurgy, the various branches of chemistry, bacteriology, etc although the emphasis may vary with the par-

ticular system under consideration Thus in stress-corrosion cracking (see

Section 8.1) emphasis may be placed on the detailed metallurgical structure

in relation to crack propagation resulting from the conjoint action of corro- sion at localised areas and mechanical tearing, while in underground corro-

Although the mechanism of corrosion is highly complex the actual control

relatively simple concepts Indeed, the Committee on Corrosion and Protection6 concluded that ‘better dissemination of existing knowledge’ was the most important single factor that would be instrumental in decreasing the

Corrosion as a Chemical Reaction at a MetaVEnvironment lntertace

As a first approach to the principles which govern the behaviour of metals

in specific environments it is preferable for simplicity to disregard the detailed structure of the metal and to consider corrosion as a heterogeneous chemical reaction which occurs at a metalhon-metal interface and which involves the metal itself as one of the reactants (cf catalysis) Corrosion can

be expressed, therefore, by the simple chemical reaction:

aA + bB = CC + dD

where A is the metal and B the non-metal reactant (or reactants) and C and

( l l )