Corrosion: The Most General Problem of Materials Science - ACS

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Corrosion: The Most General Problem of Materials Science

Downloaded by UNIV OF DAYTON on August 12, 2014 | http://pubs.acs.org Publication Date: January 31, 1979 | doi: 10.1021/bk-1979-0089.ch001

DONALD TUOMI Solid State Physics, Roy C. Ingersoll Research Center, Borg-Warner Corp., Des Plaines, IL 60016 The following chapters systematically address varied aspects of the field known as corrosion. In this introductory overview, an effort w i l l be made to touch only lightly the breadth and depth of corrosion - the most general problem of materials science. For the f i r s t step to developing an overview, a meaning to the word corrosion needs expression. To my wife, corrosion means that there is a dirty brown splotch on the side of the car which means it is worn out and time to trade in is here. They don't make them as well as before ! This brings our attention to the classic meaning of corrosion as being a loss in value for some solid object which has been suffering from chemical attack at the surface. I suspect that for many this feeling of decreased value through corrosion leads to a negative feeling towards the topic. Corrosion, in general, is used to describe degradation in value of useful objects. An alternative perspective is present in corrosion engineer anecdotes. "Nope, we don't have any corrosion problems in this plant. The pumps wear out every six weeks and we replace them." Or alternatively, "We just dump the scrap here and as soon as we can fill a railroad car we haul it to a hole a hundred miles away and bury it. Some day when the corrosion finishes the junk off, we'll build condominiums on the land." Or yet another one - "Hey, would you believe that with six months' data they want me to warrant that tank for 50 years of radioactive waste storage?" Yes, these do represent some famous last words. Enough for the anecdotal description; we are not gathered here to be entertained. Corrosion is a serious, costly materials science problem. In the years ahead as our resources are increasingly utilized to the limits of practicality, 1

0-8412-0471-3/79/47-089-001$08.50/0 © 1979 American Chemical Society

In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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and c o n s e r v a t i o n and r e c y c l i n g a r e watchwords, n o t c a t c h w o r d s ; t h e c o r r o s i o n e n g i n e e r w i l l be a t t h e forefront. May I emphasize a g a i n t h e t i t l e , " C o r r o s i o n : The Most G e n e r a l P r o b l e m o f M a t e r i a l s S c i e n c e . " So many I e n c o u n t e r speak i n a d i s p a r a g i n g v o i c e about c o r r o s i o n problems - t h a t i t * s j u s t a b l a c k a r t - w i t h o u t s u p p o r t ive science. J u s t go i n t h e r e and do y o u r t h i n g d o n ' t b o t h e r me w i t h t h e f a c t s . As a m a t t e r o f f a c t , however, t h e r e i s so much s c i e n c e i n t h i s show t h a t a c o n s t i p a t i o n o f words r e a d i l y s e t s i n b e f o r e t h e c o n s t i p a t i o n o f i d e a s i s even approached. For t h e n e x t 90 m i n u t e s o r so l e t us e x p l o r e t h i s f i e l d - CORROSION. We s h a l l t r a v e l i n two d i s t i n c t ways. One i s t o b r i n g y o u r a t t e n t i o n t o a r r a y s o f k e y words and c o n c e p t s . The o t h e r i s t o f o r m u l a t e a p a t t e r n o f s o l i d s t a t e s u r f a c e s t r u c t u r a l c h e m i s t r y w h i c h may be u s e f u l a s a framework f o r remembering complex o p t i o n s and f o r e x p l o r i n g new o p t i o n s . The l a t t e r w i l l be w i t h i n two a r e a s : (1) exchange c u r r e n t d e s c r i p t i o n s o f c o r r o s i o n p o t e n t i a l s and (2) t h e n a t u r e o f c o r r o d i n g surfaces. The e m p i r i c i s m t h a t a p p e a r s t o f o l l o w t h e c o r r o s i o n e n g i n e e r a r i s e s from t h e m u l t i - f a c t o r , m u l t i component s i t u a t i o n s w h i c h a r e p r e s e n t w i t h i n a l l common a r t i c l e s o f commerce, be t h e y made from i r o n , aluminum, o r more e x o t i c m a t e r i a l c o m b i n a t i o n s . The variables are continually interacting i n increasingly complex ways so we can become e n g r o s s e d f o r even a l i f e t i m e w i t h j u s t one p r o b l e m i n p e r f o r m a n c e such as a i r f o i l stress corrosion c r a c k i n g , b o i l e r feed water s t a b i l i z a t i o n s , o r c o r r o s i o n i n h i b i t i n g p a i n t s , and many more. B u t does t h i s mean t h e work l a c k s f o r glamour and e x c i t e m e n t ? W e l l , t h e m a t e r i a l s engineer i s f r e q u e n t l y caught with orders t o s e l e c t materials combinations i n a pract i c a l s i t u a t i o n g i v i n g desired property balances with adequate p e r f o r m a n c e d e s i g n e d t o f i r s t o r t o l i f e c o s t s . A l t e r n a t i v e l y , he s e l e c t s model Mark I I m a t e r i a l s t o c o v e r model Mark I m i s t a k e s . More i m p o r t a n t t o d a y , m a t e r i a l s e l e c t i o n s must a v o i d w a r r a n t y p a y o u t s c a p a b l e o f c o r p o r a t e b a n k r u p t c y as w e l l a s extended p e r s o n a l liability. How i n d e e d a r e R & D d a t a p r e d i c t i n g f i e l d r e l i a b i l i t y ? What i s a v a l i d t e s t ? The c o r r o s i o n e n g i n e e r i s f r e q u e n t l y t h e pragmat i s t - so what i s t h e p r o b l e m - g e t r i d o f t h e o f f e n d ing material. A n o t h e r approach i s "Hey, what c a n we add t o , i n t r o d u c e t o , o r o t h e r w i s e c o a t t h e s y s t e m w i t h so t h e p r o b l e m w i l l d i s a p p e a r l o n g enough t o keep t h e c u s t o m e r s a d e q u a t e l y happy? Or a t l e a s t n o t



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s e r i o u s l y d i s a p p o i n t e d i n t h e v a l u e r e c e i v e d ? We c a n ' t s e l l many g o l d - p l a t e d C a d i l l a c s . " Or t h e customer asks how can I c h e a p l y save money i n t h e l o n g r u n w i t h o u t paying the f a c t o r y ? Ziebart i t ! W e l l , enough c o m m e r c i a l s - who i s t h i s c o r r o s i o n e n g i n e e r and what i s he l i k e ? How many y e a r s has he spent i n t h e i v o r y t o w e r s ? Or has he e v e r v i s i t e d them? W e l l , i f I had my d r u t h e r s , I would see t o i t t h a t t h e c o r r o s i o n e n g i n e e r had a b i t o f f l a v o r i n g from m a t e r i a l s s c i e n c e as shown i n F i g u r e 1. I n t h e s e days of s p e c i a l i z a t i o n , i t would be n i c e t o t r a i n some generalists. As an e x p e r t r e s p o n s i b l e f o r t h e f u t u r e b e h a v i o r s o f most o f o u r s o l i d m a t e r i a l s , s u r e l y a mastery o f t h e f u l l range c o v e r e d by c h e m i s t r y , p h y s i c s m e t a l l u r g y , and j u s t a few o t h e r i n c i d e n t a l f i e l d s would be c o n s i s t e n t w i t h a s s i g n e d r e s p o n s i b i l i t i e s . I f he f e e l s a b i t s h o r t , c l e a r l y r e c o u r s e w i l l be t a k e n t o t h e c o n s u l t a n t s and t h e c o l l e a g u e s f o r m i n g h i s s u p p o r t ive t e c h n i c a l s o c i e t y . What, you t h i n k t h e s u r v e y i s f a c e t i o u s and o u t o f p l a c e ? L e t ' s t a k e a l o o k a t a c o r r o s i o n smorgasbord o f t e r m i n o l o g y i n F i g u r e 2. The e x p e r i e n c e s o f a c o r r o s i o n e n g i n e e r a r e r e p r e s e n t e d by t h e t e r m a r r a y . Sampl ing them a t random, i t seems t o be a j u m b l e . However, i t i s an o v e r v i e w . Who, t h e n , i s t h e c o r r o s i o n e n g i n e e r ? F o r many h e r e , he has an e x c i t i n g , r e s p o n s i b l e way o f d o i n g complex m a t e r i a l s s c i e n c e i n s e r v i c e t o our s o c i e t y . T h i s i n d i v i d u a l may n e v e r have d e l i b e r a t e l y p l a n n e d t h e career which developed. R a t h e r as a c i v i l e n g i n e e r , a m e c h a n i c a l e n g i n e e r , a c h e m i s t , a p h y s i c i s t , an a n a l y s t , one day s u d d e n l y he has a r e s p o n s i b i l i t y t h r u s t on him r e l a t i n g to materials r e l i a b i l i t y , to materials perfor mance o r j u s t t o s o l v e t h a t p r o b l e m b e f o r e we a r e b r o k e . Thus, many r o u t e s l e a d t o a c o r r o s i o n e n g i n e e r ' s r e s p o n s i b i l i t y as i s r e f l e c t e d i n F i g u r e 3 . He c o n t r i b u t e s t o A P r i o r i P r o b l e m S o l v i n g b u t economics o f f i r s t c o s t s f r e q u e n t l y l e a d t o P o s t - F a c t u m Problem S o l v i n g . For, too o f t e n , c o r r o s i o n i s t i e d d i r e c t l y t o systems' weakest l i n k v a r i a b l e s . Much o f t h e needed knowledge i s absorbed a l m o s t by an osmosis p r o c e s s because t h e r e j u s t a r e n ' t s i m p l e v a l i d d e s c r i p t i o n s o f g e n e r a l use. A l l q u i c k l y become s p e c i f i c . L a d i e s and gentlemen - i f you were t o examine t h e agenda f o r t h e n e x t few weeks, q u i c k l y a r e c o g n i t i o n w o u l d d e v e l o p t h a t t h e key-words have t o u c h e d on t h e program e x p o s u r e s each w i l l e x p e r i e n c e - can you see t h e beauty o f t h e t o u g h e s t m a t e r i a l s s c i e n c e problems emerging t h r o u g h t h e program?


In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. Chemistry

Chemistry

Physics

Consultants

Metallurgy

Ceramics

ENGINEER

CORROSION

Polymer

Figure 1.

Corrosion engineer

Thermo Analysis

Design

Colleagues

Experimental

Instrumentation

Instrumental

Irreversible

Quantum Mechanics

Mechanics

Chemistry

Statistical

Electronics

S t a t i s t i c a l A n a l y s i s o f Experiments

Semiconductor P h y s i c s

D i f f u s i o n Transport

Hydrodynamics = L-G-S

Civil Mechanical ) ' Engineering Electrical '

Chemistry

Thermodynamics

Analytical

SURFACE AND SOLID STATE MATERIALS SCIENCE

S o l i d State

Electrochemistry

Physical

INORGANIC AND ORGANIC CHEMISTRY



I

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d

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Soil

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Attack

- Cracking

Cruds

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Stress Corrosion

Stress

Fatigue

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Inter Trans

Oxidation

Dissolution

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Corrosion term smorgasbord

CORROSION SMORGASBORD

Electrochemical

Figure 2.

Gases

Steam

Brines

F r e s h Water

Aerated

inorganic Organic

Pipelines

Attack Attack Pitting Crevicing Intergranular Intercrystalline Grooving

General Local

Inhibition

Corrosion Potentials Equilibrium Potentials Mixed P o t e n t i a l s Confused P o t e n t i a l s A c t i v e Surface Passive Surface Activation Passivation


CORROSION

C H E M I S T R Y


THE CORROSION ENGINEER'S RESPONSIBILITY

Aesthetics

Environments

Economics

Performance

P r a c t i c a l Engineering P r o p e r t y Balance

THE PROBLEM A P r i o r i Materials Selection

C o r r o s i o n and Weakest L i n k Figure 3.

Variables

Corrosion engineers responsibility



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L e t us l o o k a t a n o t h e r p a i r o f smorgasbord f i g u r e s on c o r r o s i o n . The f i r s t , F i g u r e 4, f o c u s s e s l a r g e l y on , s u r f a c e phenomena from a p h y s i c a l c h e m i s t - e l e c t r o c h e m i s t o r s u r f a c e c h e m i s t c o n t e x t . Do t h e words have a f a m i l i a r meaning? S h o u l d I d e f i n e a few o r perhaps we should look at another, Figure 5 , which o u t l i n e s a n o t h e r s e t o f , p r o b a b l y l e s s f a m i l i a r t o most, key words. These are r e l a t e d t o t h e d e s c r i p t i o n o f s o l i d s t a t e s t r u c t u r e s w h i c h a r e e n c o u n t e r e d on t h e s o l i d side of the e l e c t r i c a l double l a y e r . I t i s the d i s t u r b i n g phenomena p r o c e e d i n g w i t h i n t h e s o l i d s u r f a c e s i d e t h a t g i v e s r i s e t o the d i v e r s e experiences of the c o r r o s i o n engineer. F o r an o v e r v i e w we c l e a r l y c o u l d spend much t i m e on each word, l e a r n i n g what meaning was e x p e r i e n c e d by each o f us. Each would be s u r p r i s e d what a range c o u l d be r e v e a l e d by t h e d i a l o g u e . T h i s i s why so much o f t h e management end o f t h e t a l e n t s p e c t r u m i s i n c l i n e d t o say, please p r a c t i c e your a r t w i t h o u t f u l l y r e c o g n i z i n g o r e x p e r i e n c i n g t h e complex b e a u t y present. F o r i t i s o n l y a r t t o t h e b e h o l d e r - l e t us continue s k e t c h i n g the canvas. D i d you e v e r s t o p t o c o n t r a s t t h e c o n c e p t u a l i z a t i o n o f c h e m i s t s common models, o r f o r t h a t m a t t e r , most d e s c r i p t i o n s o f s o l i d m a t e r i a l b e h a v i o r s ? In F i g u r e 6 we are e x a m i n i n g the p h e n o m e n o l o g i c a l s i t u a t i o n o v e r w h i c h we spend so much t i m e p u z z l i n g . The m e n t a l images u s u a l l y do n o t a s s i g n d e t a i l e d s t r u c t u r e t o t h e s o l i d phase. We can c l e a r l y see i t b e f o r e us - i t i s so o b v i o u s . The model we a r e u s u a l l y s p e a k i n g about i s shown i n t h e lower l e f t c o r n e r - a s t r u c t u r e l e s s s o l i d b e h a v i n g i n an e n v i r o n m e n t o f l i q u i d , o r gas, more r a r e l y an i m p e r f e c t , impure vacuum. The l a t t e r e n v i ronment has t h e s p a t i a l e x t e n t . More t r u l y as i n t h e l o w e r r i g h t c o r n e r t h e s o l i d i s d e f i n e d by x-y-z c o o r d i n a t e s w h i c h d e f i n e what i s a c t u a l l y happening where as a f u n c t i o n o f time i n an experiment. S t i l l more p r e c i s e l y , t h e s o l i d i n t h e e x p e r i m e n t i s b e i n g space averaged t o produce the d a t a i n t h e f i g u r e s above. The s o l i d i s b e s t d e s c r i b e d g e n e r a l l y f o r t h e c o r r o s i o n e n g i n e e r ' s p u r p o s e s as a " p o l y p h a s e inhomogeneous s o l i d c o n t a i n i n g h e t e r o g e n e i t y homogeneously d i s p e r s e d . " The d e s c r i p t i o n i s r a t h e r p r e c i s e a c c o r d i n g t o Webster s U n a b r i d g e d D i c t i o n a r y . T h i s r e f e r e n c e can be c i t e d i f the statement r e q u i r e s s u b s t a n t i a t i o n . Goodness, w i t h t h i s c o m p l e x i t y where do we go from 1

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Ionic

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Open-Circuit Potentials Short-Circuit Potentials Cell Potentials Over P o t e n t i a l s Anodic-Cathodic P o t e n t i a l s Mixed P o t e n t i a l s

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Polarization Potentials Activation Polarization Concentration P o l a r i z a t i o n

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Oxidation-Reduction

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Corrosion smorgasbord (continued)

Exchange C u r r e n t s

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Figure 4.

Polarizable

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Faradaic Processes

Constant

Constant C u r r e n t

Potential Transients

Compact Double Layer

Diffuse

The Helmholtz

E l e c t r i c Dipole F i e l d s

Atomic

Phase)

Model Components

(Liquid


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oo

In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. Defects

Phase

Solutions

Bands

Figure 5.

Impurity

Potential

Bands

Bands

Dislocations

Fermi L e v e l s

Work F u n c t i o n s

Surface P o t e n t i a l s

Conduction

Corrosion smorgasbord (continued)

States

Defects

E l e c t r o n s and Holes

Surface

Surface

Charge T r a n s f e r Catalysis

-Metal

Insulator-Semiconductor

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Band S t r u c t u r e

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Imperfections

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Zeolite to Perfect C r y s t a l -

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Mass T r a n s p o r t



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Figure 6.

CHEMISTRY

Phenomenological view of a one-dimensional solid contrasts with the three-dimensional reality of most solid systems.


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Science


1

h e r e ? W e l l , l e t s j u s t acknowledge t h a t t h i s has a l w a y s been t h e s t a g e on w h i c h most o f us a r e a c t i n g out our p r o f e s s i o n a l l i v e s . Thus, i n F i g u r e 7, S o l i d s a r e d e s c r i b e d as b e i n g i d e a l l y p e r f e c t v e r s u s i d e a l l y i m p e r f e c t w i t h most being i n the l a t t e r category. A sharp consequence was t a u g h t t o me many y e a r s ago by L e r o y Dunham o f t h e E d i s o n P r i m a r y B a t t e r y . When I s m a r t l y d e s c r i b e d a s t a t i s t i c a l quantum-mechanics model o f c h a r g e t r a n s f e r c a t a l y s i s o f oxygen r e d u c t i o n on an e l e c t r o d e s u r f a c e , he s m i l e d as i f he was e n j o y i n g h i m s e l f . He had been f o r c e d t o l e a v e s c h o o l a t t h e e i g h t h grade l e v e l . , L a t e r i n t h e day as he and I s t r o l l e d t h r o u g h the p r o d u c t i o n p l a n t making h i s e l e c t r o d e s , he q u i e t l y s a i d , "Don, I e n j o y e d y o u r comments so v e r y much. Back i n t h e '30*s when I was d o i n g the development work many c o n f u s i n g r e s u l t s came from e x p e r i m e n t s . In the e v e n i n g s I have e n j o y e d r e a d i n g t h e E n c y c l o p e d i a B r i t a n n i c a . One day i t dawned on me t h a t the d i s t r i b u t i o n of people i n the United S t a t e s versus other a r e a s r e s e m b l e d t h e charge t r a n s f e r b e h a v i o r . To change t h i n g s t h e p e o p l e must move a r o u n d , get i n t o and o u t o f c a r s , t r a i n s , p l a n e s . What a memorable moment - t h e t h e o r y d i d not have t o be a b s o l u t e l y r i g h t t o be u s e f u l i n g u i d i n g e x p e r iments. Why have I t a k e n such a c i r c u m s p e c t p a t h t o r e a c h t h i s point? W e l l , I wish to present a c o n c e p t u a l i z a t i o n o f t h e p r o b l e m s w h i c h has been u s e f u l t o me. It has h e l p e d t o b r i d g e t h e chasm s e p a r a t i n g d i f f e r e n t experimental s i t u a t i o n s . The f o l l o w i n g F i g u r e 8 i n d i c a t e s t h a t the l e c t u r e s c o u l d be p l a c e d i n a v a r i e t y o f c a t e g o r i e s . F i r s t , the f a m i l i a r m e t a l - e l e c t r o l y t e system w i t h gas e v o l u t i o n p o t e n t i a l l y present. Second, t h e systems i n v o l v i n g m e t a l s s e p a r a t e d from the l i q u i d by a c o v e r i n g f i l m s t r u c t u r e , and t h i r d t h e m e t a l s e p a r a t e d from a gas by an i n t e r m e d i a t e phase l a y e r . There i s some a m b i g u i t y e v i d e n t i n t h i s d e s c r i p t i o n ; however, i n t h e n e x t few m i n u t e s p e r h a p s some new p e r s p e c t i v e s f o r t h o u g h t o r g a n i z a t i o n can emerge. I n a more g e n e r a l sense s u r f a c e c o r r o s i o n p r o c e s s e s can be d e s c r i b e d i n terms o f the phases p r e s e n t as shown i n F i g u r e 9. Here t h e s o l i d i s n o t i d e n t i f i e d s i m p l y as a m e t a l but a more g e n e r a l term S i s b e i n g used w i t h L r e p r e s e n t i n g l i q u i d phase and G gas phase. The systems l o c a l l y p r e s e n t can be r e p r e s e n t e d by t h e n o t a t i o n s S - L , S - L - G , S-G or, i f a c o v e r i n g c h e m i c a l l y a l t e r e d l a y e r e x i s t s on t h e s u r f a c e , then S i - S i s present w i t h S i the s t a r t i n g m a t e r i a l . 2



12

CORROSION

CHEMISTRY

Solids Ideally Perfect

R e a l i t y + I d e a l l y Imperfect THEORY :

i f i t leads t o experiments does n o t need t o be RIGHT t o be v e r y v a l u a b l e . Figure 7.

Modeling



TUOMi

Corrosion:

Problem

of Materials

Science

Metal-Electrolyte-Gas

Metal-Solid-Liquid

Corrosion E l e c t r o chemistry Iron D i s s o l u t i o n Mechanisms Corrosion I n h i b i t i o n and I n h i b i t o r s Stress Corrosion Cracking

Solid Electrolyte-Ionic E l e c t r o n i c Transport Iron Passivation Valve Metals D i e l e c t r i c Layers I n d u s t r i a l Problem: C o o l i n g Waters and Treatments

LECTURE SERIES CORROSION

Metal-Solid-Gas High-Temperature Corrosion Low-Temperature Atmospheric Corrosion C o r r o s i o n Phenomena N o v e l Energy Conversion Processes Iron P a s s i v a t i o n Figure 8.

Corrosion lecture classifications


14

CORROSION

CHEMISTRY

The p e r s p e c t i v e o f s i m p l e phases and phase e q u i l i b r i a i s adequate f o r many c o r r o s i o n p r o b l e m s . T h i s i s p a r t i c u l a r l y t r u e f o r the c a s e s where thermodynamic f r e e energy d a t a r e a l l y d e s c r i b e what phases are p r e s e n t . L e t us examine t h i s f o r a few m i n u t e s from the viewpoint of everyday experience. We w i l l u t i l i z e the key d e v i c e w h i c h was r e s p o n s i b l e f o r our a r r i v a l a t the l e c t u r e - t h e b l a c k box c a l l e d


THE

LEAD-ACID STORAGE BATTERY.

T h i s system p e r m i t s b r i e f d e s c r i p t i o n s o f some key c o n c e p t s e n c o u n t e r e d i n c o r r o s i o n phenomena: electrode p o t e n t i a l s , exchange c u r r e n t s , mixed p o t e n t i a l s , c o r r o s i o n p o t e n t i a l s , p a s s i v e f i l m s , as w e l l as l e a d i n g t o thermodynamic d e s c r i p t i o n s o f s y s t e m s . — The r e d o x systems o f t h e b a t t e r y a r e s i m p l y p r e sented as: Negative : Anodic

=

Pb,

ν + SO*

-

{ S )

PbS0iw ν

+ 2e

e

+ 0.335

( S }

Cathodic

Positive: Pb0 , x + 4H 2

Pb

( s )

c

+Pb0

+

_ + SO * + 2e

2 ( s )

+ 2Η 50ι> 2

Cathodic ±? PbSO* + 2 H 0 Anodic 2

-

PbSO^

( g )

+ 2H 0 2

+

1.685

^ 2 ν

At the n e g a t i v e lead i s o x i d i z e d t o lead s u l f a t e d u r i n g d i s c h a r g e w h i l e at the p o s i t i v e l e a d d i o x i d e i s r e d u c e d t o l e a d s u l f a t e . The f u n c t i o n i n g o f the b a t t e r y f o r many c y c l e s depends upon m a i n t a i n i n g the reactions i s o l a t e d to t h e i r respective electrodes, to h a v i n g the e l e c t r o n s f l o w i n g t h r o u g h an e x t e r n a l c i r c u i t , and o n l y i o n i c c u r r e n t s f l o w i n g i n t h e i n t e r n a l electrolyte circuit. I f the p o t e n t i a l - c u r r e n t (E-i) c h a r a c t e r i s t i c s of t h e i n d i v i d u a l r e a c t i o n s were measured, the r e a c t i o n s c o u l d be r e a d i l y modeled as e l e c t r o c h e m i c a l r e a c t i o n s w i t h t h e b a t t e r y a t open c i r c u i t as i n d i c a t e d by the p r o c e s s e s i n F i g u r e 10. I f dynamic e l e c t r o d e p o t e n t i a l c u r r e n t r e l a t i o n s h i p s were d e t e r m i n e d , t h e e l e c t r o d e i s e x p e c t e d t o show t h e c l a s s i c T a f e l s l o p e b e h a v i o r s as the exchange c u r r e n t o f the a n o d i c - c a t h o d i c e q u i l i b r i u m i s s h i f t e d i n t o e i t h e r d i r e c t i o n . From t h e T a f e l c u r v e s a v a l u e f o r t h e E and i o f the e l e c t r o d e c o u l d be d e f i n e d . Q

0


1.

τυοΜι

Corrosion:

Problem

of Materials

15

Science

LOCAL REGION SOLID SURFACE TRANSFORMATIONS


Solid-Liquid S-L

Solid 1-Solid 2-L

Solid-Liquid-Gas S-L-G

S o l i d Gas SG

S o l i d 1-Solid 2-L-G S o l i d 1-Solid 2-G

S i ~ S 2 """ L

S i — S2~ L — G

Figure 9.

Sι

—

S 2 "" G

Corrosion system

Pb0

+ 2H + H SO„ + 2e

2 ( s )

2

Positive Pb0

+

2 ( s )

+2H +H SO 2

PbSOu

Potential E

{s)

PbSOu

Anodic

v

l(

+2e -

+ 2 H 0 2

(s)

+ 2e - P b

( s )

+ SO,

^

Negative Pb

( s )

+

S 0 ; - PbSO,

(s)

log i Figure 10.

Model battery anodic-cathodic

reactions


+ 2e

16

CORROSION

CHEMISTRY

I f the two e l e c t r o d e s a r e s h o r t - c i r c u i t e d t o g e t h e r , t h e c a t h o d i c p r o c e s s o f t h e p o s i t i v e combines w i t h t h e a n o d i c p r o c e s s o f the n e g a t i v e as shown i n F i g u r e 11. The b a t t e r y now has a s i n g u l a r p o t e n t i a l - t h e s h o r t circuited potential. T h i s c l e a r l y i s a mixed p o t e n t i a l o r c o u l d be v i e w e d as a c o r r o s i o n p o t e n t i a l o f the system. L o o k i n g a t F i g u r e 12, a s k e t c h o f a s i n g l e c e l l shows t h a t t h e p o s i t i v e i s a p a s t e o f P b 0 i n a l e a d grid. T h i s e l e c t r o d e i n t h e e l e c t r o l y t e c l e a r l y must have a mixed p o t e n t i a l s i n c e m e t a l l i c l e a d i n t h e p r e s e n c e o f l e a d d i o x i d e and s u l f u r i c a c i d i s thermodynamically unstable. At the n e g a t i v e the l e a d g r i d contains pasted l e a d ! This i s a l s o unstable w i t h respect to charging the b a t t e r y . Both e l e c t r o d e s represent a c o r r o s i o n c o n t r o l problem! R e a l l y , t h i s i s a l i t t l e t o o s i m p l e , because i f t h e e l e c t r o d e s a r e f u l l y c h a r g e d , t h e v o l t a g e may be i n c r e a s e d so oxygen and hydrogen a r e e v o l v e d a t t h e p o s i t i v e and n e g a t i v e , r e s p e c t i v e l y . T h i s i s i n d i c a t e d by t h e p o t e n t i a l - l o g c u r r e n t l i n e s i n F i g u r e 13. These r e a c t i o n s c o n v e r t t h e b a t t e r y i n t o a p o t e n t i a l e x p l o s i v e by t h e H2-O2 r e c o m b i n a t i o n t o form w a t e r i f a match i s used t o a i d i n s e e i n g how much w a t e r needs t o be added. I f t h e 0 r e d u c t i o n c o u l d be made t o p r o c e e d on t h e n e g a t i v e s u r f a c e as a l o c a l c o r r o s i o n c u r r e n t , and i f t h e H c o u l d be made t o undergo o x i d a t i o n on t h e p o s i t i v e s u r f a c e , a h e r m e t i c a l l y s e a l e d c e l l c o u l d be made. These e l e c t r o c h e m i c a l r e a c t i o n s , however, r e p r e s e n t forms o f l o c a l c e l l r e a c t i o n s f a m i l i a r to c o r r o d i n g systems. As any o f t h e s e r e a c t i o n s p r o c e e d a t t h e e l e c t r o d e surface, the surface chemistry i s continuously changing. The c o m p o s i t i o n o f t h e l o c a l e l e c t r o l y t e i n c o n t a c t w i t h t h e s o l i d s i s c h a n g i n g . Thus, t h e s i m p l e chem i s t r y i m p l i e d by t h e r e a c t i o n


2

2

2

Pb + P b 0

2

+ 2 H S0i> + PbSOi* + 2 Ηι»0 2

r e a l l y does n o t e x i s t . There a r e numerous changes o f a complex form p r o c e e d i n g on t h e m e t a l s u r f a c e s , on t h e m e t a l l i c h e a v i l y - d o p e d o x i d e P b 0 , on t h e PbSOi+ c r y s t a l l i t e s , on t h e Pb p a r t i c u l a t e s i n t h e n e g a t i v e g r i d , as w e l l as i n systems o f g r a i n b o u n d a r i e s w h i c h are present. S u d d e n l y , as we p e e r beyond t h e c a s u a l s o l u t i o n c h e m i s t r y e q u a t i o n , r a t h e r complex s o l i d s t a t e c h e m i s t r y i s i n t e r a c t i n g w i t h s o l u t i o n and gas phases processes. 2


Corrosion:


τυοΜι

Figure 11.

Problem

of Materials

Science

Short circuit of battery terminals creates characteristic short circuit potential (E ) and short circuit current (i ). 8C

8C


18

CORROSION

CHEMISTRY

LOAD

Pasted Pb

Pasted Pb0

|charge y-

2


dischargèj MME Q = i t

+ H0

Pb Grids

2

Figure 12.

Simple cell in a lead-acid battery

H

2

V r

H 0

2 ( g )

h

°Mg)

V)

+ 2 H

+2H + 2e+H l

+

0

2e

U

Positive - Ρ

.

*

H

Mg) ^

H

(t)

+

e

Negative - Ν Η H2

α Γ * (g) log i Figure 13.

Hydrogen and oxygen evolution reactions at the battery electrode


)

1.

τυοΜι

Corrosion:

Problem

of Materials

Science

19

C l e a r l y t h e system has e l e c t r o c h e m i c a l - c h e m i c a l exchange c u r r e n t s a t t h e p o s i t i v e s and n e g a t i v e s w h i c h d e f i n e t h e p o t e n t i a l s o b s e r v e d . The m e t a l g r i d must show c o m b i n a t i o n s o f PbSOt* f i l m , P b 0 f i l m , as w e l l as h y d r i d e f i l m phenomena. I s n ' t c o r r o s i o n c h e m i s t r y e x c i t i n g i n t h a t b l a c k box under t h e hood? The b a t t e r y e l e c t r o d e mixed p o t e n t i a l b r i n g s a t t e n t i o n t o the c o r r o s i o n engineer's problem o f c o n t r o l l i n g e i t h e r the cathodic or anodic process t o minimize the c o r r o s i o n c u r r e n t . The problems o f s u r f a c e p a s s i v a t i o n , the question of i d e n t i f y i n g the d i s t i n c t i v e s p a t i a l l o c a t i o n s of the r e a c t i o n processes are f r e q u e n t l y p r e s e n t i n p r a c t i c a l s i t u a t i o n s - what i s c a t h o d i c t o an a n o d i c r e g i o n o r v i c e v e r s a , what a r e u s e f u l ways t o m o d i f y t h e s u r f a c e p r o c e s s e s ? Thus, when a t t e n t i o n i s g i v e n t o d e t a i l , what appeared as a s i m p l e p r o b l e m i n i t i a l l y has w i t h a l i t t l e t h o u g h t become complex. F o r a few moments l e t ' s d r o p t h e key words and turn t o c r e a t i n g a skeleton f o r the c o r r o s i o n engi neer's interphase t r a n s p o r t processes. We need a l l t h e tools possible t o bring the materials science r e s o u r c e s t o o u r a i d . How can we e x p r e s s q u e s t i o n s so a i d can come from o t h e r e x p e r t s ?


2

Part I I T h i s phase o f t h e d i s c u s s i o n i s c o n c e r n e d w i t h concepts o f s o l i d s t a t e chemistry r a t h e r than a d e t a i l e d a n a l y s i s o f a p a r t i c u l a r case. The o b j e c t i v e i s t o b r i n g a t t e n t i o n t o a v a r i e t y o f s u r f a c e chemis t r y p e r s p e c t i v e s . T h i s v a r i e t y c a n be h e l p f u l because t h e number o f ways we can scavenge i n f o r m a t i o n from r e l a t e d m a t e r i a l s s c i e n c e a r e a s becomes expanded. F u r t h e r , a v a r i e t y o f s e e m i n g l y u n c o n n e c t e d phenomena can be b r o u g h t i n t o r e l a t e d b a l l g a m e s . The m u l t i d i s c i p l i n a r y character of r e a l material science i si t s real richness. E a r l i e r a t t e n t i o n was b r o u g h t t o t h e c o r r o s i o n p r o c e s s e s o f l i q u i d - s o l i d systems. L e t ' s s t a r t w i t h a m e t a l c o n t a c t i n g an e l e c t r o l y t e as do a l l e l e c t r o chemistry t e x t s . The s i m p l e s t model o f t h e e l e c t r i c a l d o u b l e l a y e r between a m e t a l and an e l e c t r o l y t e i s t h e s i m p l e c a p a c i t o r v i s u a l i z e d by Helmholtz- as shown i n F i g u r e 14. The d i f f u s e i o n d i s t r i b u t i o n i n t h e l i q u i d phase was r e c o g n i z e d by Gouy and Chapman^-' - t o form a space charge r e g i o n a d j a c e n t t o t h e e l e c t r o d e s u r f a c e . S t e r n ^ i n 1924 combined t h e s t r u c t u r e s t o form a compact d o u b l e l a y e r a t t h e e l e c t r o d e s u r f a c e w i t h a 1

5


20

CORROSION


HELMHOLTZ 1879

I

Θ Θ Θ Θ fe©e Θ ΙΘ ΘΘ © © © Θ Θ

I

f i'

Θ

\Helmholtz Plane

y

Ο. STERN (1924)

i l '

Ι© ϋ „

STERN-GRAHAME

imD

©

Θ © © Θ Θ +4 Θ© Θ ©Θ Θ ^ ΙΊ Θ

Helmholtz Plane

Figure 14.

GOUY CHAPMAN 1910-1913

:*Outer Helmholtz Plane Inner Helmholtz Plane

Modeh for the electrical double hyer at a metal surface


CHEMISTRY

1.

Corrosion:

τυοΜι

Problem

of

Materials

21

Science


2

d i f f u s e space c h a r g e . I n 1947 D a v i d Grahame added t o t h i s t h e s p e c i f i c a d s o r p t i o n o f i o n s f o r m i n g an i n n e r and o u t e r H e l m h o l t z p l a n e w i t h h y d r a t e d c a t i o n s not a p p r o a c h i n g as c l o s e l y as t h e a n i o n s a t t h e i d e a l metal substrate surface. In t h i s i d e a l p o l a r i z a b l e e l e c t r o d e model p e r s p e c t i v e , no charge t r a n s f e r o c c u r s between m e t a l and l i q u i d phase - i . e . , no F a r a d a i c processes are present. I t i s important t o v i s u a l i z e t h i s s u r f a c e r e g i o n as formed o f atoms, m o l e c u l e s , and ions having s i g n i f i c a n t thermal v i b r a t i o n a l , r o t a t i o n a l and t r a n s l a t i o n a l e n e r g y . The s k e t c h e d s t r u c t u r e s a r e f o r o n l y a moment i n t i m e b u t a r e a v e r a g i n g o v e r space a d j a c e n t t o t h e e l e c t r o d e s u r f a c e when measurements are b e i n g made. A subsequent d e s c r i p t i o n by B o c k r i s and a s s o c i ates- - drew a t t e n t i o n t o f u r t h e r c o m p l e x i t i e s as shown i n F i g u r e 15. The m e t a l s u r f a c e now i s c o v e r e d by combinations of o r i e n t e d s t r u c t u r e d water d i p o l e s , s p e c i f i c a l l y adsorbed a n i o n s , f o l l o w e d by s e c o n d a r y water d i p o l e s along w i t h the hydrated c a t i o n s t r u c t u r e s . T h i s model s e r v e s t o b r i n g a t t e n t i o n t o t h e dynamic s i t u a t i o n i n w h i c h changes i n p o t e n t i a l i n v o l v e s e q u e n t i a l as w e l l as s i m u l t a n e o u s r e s p o n s e s o f molec u l a r and a t o m i c systems a t and near an e l e c t r o d e s u r f a c e . Changes i n p o t e n t i a l d i s t r i b u t i o n i n v o l v e i n t e r a c t i o n s e x t e n d i n g from atom p o l a r i z a b i l i t y , t h r o u g h d i p o l e o r i e n t a t i o n , t o i o n movements. The e l e c t r i c a l f i e l d e f f e c t s a r e complex i n t h i s i d e a l p o l a r i z e d e l e c t r o d e model. The models c l e a r l y have n o t a s s i g n e d any a t o m i c s t r u c t u r e t o the metal s i d e . With a m e t a l l i c s u b s t r a t e R i c e , i n 1 9 2 8 , s h o w e d the e l e c t r i c f i e l d p e n e t r a t i o n was i n d e e d s l i g h t . C o n s e q u e n t l y , t h i s model was ade quate f o r t h e i d e a l p o l a r i z a b l e e l e c t r o d e w i t h o u t F a r a d a i c charge t r a n s f e r . A f u r t h e r c o m p l i c a t i o n i s i n t r o d u c e d i n F i g u r e 16 where t h e p r e s e n c e o f s u r f a c e adatoms i s i n d i c a t e d as w e l l as m e t a l l a t t i c e v a c a n c i e s i n t h e s u b s t r a t e s u r f a c e . W i t h i n t h e system a t t e n t i o n now can be drawn to F a r a d a i c processes i n v o l v i n g the s u b s t r a t e s t r u c t u r e . The t r a n s f e r o f an atom t o t h e s u r f a c e can be e x p r e s s e d by the e q u a t i o n 2

M/D

+ D

m

s

î

M

/ n

s

+

n

M

where M/Qs/ the s u r f a c e adatom, i s f u r t h e r p o t e n t i a l l y i n v o l v e d i n t h e exchange p r o c e s s M

/Q

+ s

n

H

2°

+

M +

(H 0) 2

n

+ e +

Q

s


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Figure 16.

Electrical double layer with structure introduced on the metal side



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T h i s s u g g e s t s t h a t a t t e n t i o n needs t o be g i v e n t o i n t e r p h a s e exchange c u r r e n t s w h i c h i n v o l v e n o t o n l y e l e c t r o n t r a n s f e r from t h e m e t a l s u r f a c e , b u t may i n v o l v e atom exchanges from t h e b u l k t o t h e s u r f a c e r e g i o n . The p l a n e m e t a l s u r f a c e a s s o c i a t e d w i t h s t u d y i n g t h e model i d e a l p o l a r i z e d e l e c t r o d e b e h a v i o r now becomes p a r t o f an i n t e r p h a s e system s e p a r a t i n g m e t a l from an e l e c t r o l y t e o r a gas phase system. The e v o l u t i o n o f s e m i c o n d u c t o r e l e c t r o n i c s depen ded upon d e v e l o p i n g a d e t a i l e d m a t e r i a l s s c i e n c e , f i r s t f o r germanium and t h e n f o r s i l i c o n . This con fronted the electrochemist w i t h a f u r t h e r refinement o f t h e e l e c t r i c a l d o u b l e l a y e r systems model. A t t h e surface of a s i l i c o n semiconductor c r y s t a l the e l e c t r o n i c p r o c e s s now i s no l o n g e r s i m p l y an e l e c t r o n t r a n s f e r from a m e t a l b u t i n v o l v e s two d i s t i n c t r e a c t a n t s , e l e c t r o n s o r h o l e s . Thus, t h e f o l l o w i n g two r e a c t i o n s are not equivalent at the surface of a semiconductor, S : 1Si

S + OH"

•>

S0H + e

S + OH" + ρ •> S0H A f u r t h e r c o m p l i c a t i o n a r i s e s when a t t e n t i o n i s f o c u s s e d on t h e e l e c t r o n d e n s i t y d i s t r i b u t i o n w i t h i n the semiconductor s o l i d . T h i s , i n c o n t r a s t t o the m e t a l c a s e , now i s a b l e t o v a r y from a l o w t o a h i g h c o n c e n t r a t i o n l e v e l as e l e c t r o n s i n a c o n d u c t i o n band o r as h o l e s i n a v a l e n c e band. The e l e c t r i c f i e l d on t h e s o l i d s i d e o f t h e e l e c t r i c a l d o u b l e l a y e r now has s p a t i a l extent - a d i f f u s e double l a y e r character exists w i t h i n the s o l i d . The c o n v e n t i o n a l e l e c t r i c f i e l d e f f e c t s p r e v i o u s l y a s s o c i a t e d w i t h i o n motion and i o n d i s t r i b u t i o n s i n t h e e l e c t r o l y t e have a c o u n t e r p a r t w i t h i n t h e s o l i d phase. A f u r t h e r c o m p l i c a t i o n i s brought t o the f o r e f r o n t by s i l i c o n - e l e c t r o l y t e e l e c t r o c h e m i c a l s t u d i e s . T h i s i s t h e phenomena o f s u r f a c e s t a t e s a t t h e s o l i d e l e c t r o l y t e i n t e r f a c e . These become c r i t i c a l l y i n v o l v e d i n c h a r g e t r a n s f e r r e a c t i o n s i n v o l v i n g com p l e x r e a c t a n t s a t t h e s u r f a c e . Thus s i m p l e e l e c t r i c f i e l d d i s t r i b u t i o n s are not present except i n p a r t i c u l a r model c a s e s . On t h e o t h e r hand, r a t h e r t h a n h a v i n g a semicon d u c t o r r e p l a c i n g t h e m e t a l , t h e a c t u a l s i t u a t i o n may i n v o l v e growth o f s e m i c o n d u c t o r l a y e r s on a m e t a l s u r f a c e , o r a d i e l e c t r i c f i l m , o r more g e n e r a l l y a compound MX s e p a r a t i n g t h e base m e t a l from t h e c o r r o s i o n media. A new phase s e p a r a t e s t h e c o r r o d i n g base from the r e a c t a n t s .


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The e x a m i n a t i o n o f t h e m a t e r i a l MX as a model twod i m e n s i o n a l l a t t i c e s o l i d phase, AB, i n F i g u r e 17 i s r e v e a l i n g . The m a t e r i a l can have a v a r i e t y o f l a t t i c e imperfection s t r u c t u r e s i n c l u d i n g 1 1


i n t e r s t i t i a l A and Β atoms Α/Δ, Β/Δ i m p r o p e r s i t e A and Β atoms A/G3, B/CB l a t t i c e v a c a n c i e s f o r A and Β s i t e s Γ+1 > FT a s s o c i a t e d A and Β v a c a n c i e s 1+1~1 as r e a d i l y d e s c r i b e d by a s i m p l i f i e d symbolism. In r e a l systems a t t e n t i o n must be g i v e n t o d e f i n i n g a t l e a s t c o n c e p t u a l l y t h e n a t u r e o f t h e phases p r e s e n t . F r e q u e n t l y , under o r d i n a r y c o n d i t i o n s the s o l i d does not f i t any s i m p l e s t o i c h i o m e t r y o r e l e c t r i c a l n e u t r a l i t y compound model but i s a s t r u c t u r e u n i q u e t o the a c t u a l system under s t u d y . Thus, u s e f u l r e s u l t s may come from u n o r t h o d o x approaches t o r e a l systems. I f a t t e n t i o n i s given to the combination of a m e t a l M becoming c o v e r e d w i t h MX the model as shown i n F i g u r e 18, s e v e r a l o b s e r v a t i o n s can be made as t o i n t e r p h a s e systems and t h e growth o f t h e f i l m l a y e r . At the e x t e r i o r s u r f a c e a p a r t i c u l a r combination of i n t e r p h a s e exchange c u r r e n t s f o r s o l i d phase growth can be f o r m u l a t e d depending on t h e d e t a i l e d c h a r a c t e r of the s o l i d phase, the s u r f a c e s t a t e s , the allowed e l e c t r o n i c processes. S i m i l a r l y , a set of interphase exchange c u r r e n t s can be f o r m u l a t e d a t t h e boundary o f the m e t a l and t h e MX l a y e r a g a i n s u b j e c t t o c o n s t r a i n t s d e f i n e d by t h e mass t r a n s p o r t p r o c e s s e s p e r m i s s i b l e i n t h e c o v e r i n g l a y e r and i n t h e m e t a l . The s i t u a t i o n i s i l l u s t r a t e d i n g r e a t e r d e t a i l i n t h e f o l l o w i n g model s i t u a t i o n s where a t t e n t i o n i s g i v e n t o t h e i n t e r p h a s e boundary exchange c u r r e n t s i n j e c t i n g the l a t t i c e i m p e r f e c t i o n s which are r e s p o n s i b l e f o r atom t r a n s p o r t t h r o u g h t h e compound MX A** Thus i n F i g u r e 19 l a t t i c e v a c a n c y w i t h a t r a p p e d h o l e i s i n j e c t e d a t t h e compound e l e c t r o l y t e i n t e r f a c e , t h e i m p e r f e c t i o n a t t h e metal-compound i n t e r f a c e r e a c t s t o r e l e a s e a v a c a n c y i n t o the m e t a l . Alternatively, i n F i g u r e 20 t h e exchange p r o c e s s i n j e c t s a l a t t i c e v a c a n c y on t h e c a t i o n l a t t i c e w h i c h a p p e a r s a t t h e m e t a l compound i n t e r p h a s e t o r e l e a s e an e l e c t r o n and t r a n s f e r a metal i o n i n t o the s u r f a c e r e g i o n . T r a n s p o r t models such as t h e s e have been c r e a t e d t o b r i n g a t t e n t i o n t o t h e p o s s i b i l i t y o f t h e boundary exchange c u r r e n t s i n j e c t i n g i m p e r f e c t i o n i n t o a c r y s t a l l i n e phase d u r i n g an a n o d i c p r o c e s s . In f a c t , t h e y may d e t e r m i n e t h e s o l i d phase s t r u c t u r e s formed. F u r t h e r m o r e , t h e m e t a l w i t h a s o l i d phase c o v e r i n g f i l m



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B/n~ Journal of the Electrochemical Society Figure 17.

A model two-dimensional AB compound lattice illustrating simple imperfection structures expected as a systems variable (12)

ANION VACANCY METAL ATOM VACANCY

CATION VACANCY

• CATION SURFACE SITE

•Ï AO-ATOMSr*y* AD-IONS M

METAL

)

ANION SURFACE SITE

SURFACE

Journal of the Electrochemical Society Figure 18. Model situation where metal, M, is separated from corroding medium by covering-layer compound (MX) containing imperfection structures (12)


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COMPOUND MX-ELECTROLYTE INTERFACE

®®|©


®® ®®© ®® Journal of the Electrochemical Society Figure 19. Model boundary interphase exchange currents involving the injec tion of a lattice vacancy with trapped hole at compound-electrolyte interface and vacancy release into the metal with a hole and metal ion in the compound (12)

METAL M

®® ®j L

. ® ® | @ _ ® ® ®fe> ρ

® /οΐΙ® ®p/ j!g D

® ®| COMPOUND MX

®®| METAL COMPOUND M MX

(g) ® I © φ

©g®'®®

® ® j ® ®__®?® I ® ® ®

® ! P/OÎ®^"® D ; @ ® N

® ® | ® @

M

® ®

® ®

COMPOUND MX- ELECTROLYTE INTERFACE

® ®

® ® ® ®r

®®^g)

® ® _ ® ® @ _ ® ® ® j ® ® 0~® D Î ® * ~ ® D î | ® ®®i Journal of the Electrochemical Society Figure 20. Model boundary interphase exchange currents involving httice vacancy injection at the compounds-electrolyte interface and vacancy exchange into the metal releasing an electron and transferring a metal ion into the compound hyer (12)

METAL M

®®i COMPOUND MX

® ® |

METAL COMPOUND MX M

® ® ®® ®® ® ® ®

®

(M)

®

®

®

® ®_jgfc®!® ® •ΐ ® ^ ® •„ ® ® ® ® ® ® ® ®



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can be i n r e v e r s i b l e e q u i l i b r i u m w i t h an e l e c t r o l y t e w i t h o u t h a v i n g exposed m e t a l n e c e s s a r i l y i n c o n t a c t w i t h an e l e c t r o l y t e . I n such a model, an i n c r e a s e i n t h e d r i v i n g c u r r e n t s may i n c r e a s e the mass t r a n s p o r t r a t e s ( a l t e r s o l i d phase s t r u c t u r e s ) w i t h i n l i m i t s so t h e r a p i d phase t r a n s f o r m a t i o n s (M t o MX) can o c c u r w i t h o u t n e c e s s a r i l y i n v o l v i n g i o n - s o l u t i o n and p r e c i p i t a t i o n r e a c t i o n s i n t h e o r d i n a r y sense. T h i s d i s c u s s i o n has b r o u g h t a t t e n t i o n t o t h e p o t e n t i a l p r e s e n c e o f two c l a s s e s o f i n t e r p h a s e t r a n s p o r t c o n d i t i o n s i n s o l i d s t a t e systems. L e t us examine t h e s e f o r a moment f r o m t h e p e r s p e c t i v e o f f o r m i n g s o l i d phases under model c o n d i t i o n s . F o r t h e s i n g l e i n t e r p h a s e exchange p r o c e s s o f s o l i d phase P2 i n t e r a c t i o n s w i t h P i shown i n F i g u r e 21, a s e r i e s o f model c o n d i t i o n s a r e i l l u s t r a t e d . F o r each a s e t o f i n t e r p h a s e atom exchange r e a c t i o n s can be f o r m u l a t e d w h i c h have b o t h thermodynamic and k i n e t i c i n t e r p r e t a t i o n s . The f i r s t i s the l o c a l d e p o s i t i o n o f m e t a l l i c t i t a n i u m by t h e t h e r m a l d e c o m p o s i t i o n o f T i C l i * gas on a hot w i r e . T h i s v a p o r p l a t i n g t e c h n i q u e can be p e r f o r m e d t o grow a v a r i e d c r y s t a l l i t e s t r u c t u r e o f a r e l a t i v e l y pure t i t a n i u m on t h e h o t f i l a m e n t . More g e n e r a l l y , v a p o r p l a t i n g t e c h n i q u e s o f many complex forms a r e used t o grow e p i t a x i a l s i l i c o n l a y e r s o f c o n t r o l l e d impurity content onto s i l i c o n s u b s t r a t e s . The c a r e f u l c o n t r o l o f c o n d i t i o n s r e s u l t s i n an amaz i n g l y homogeneous f i l m growth o f h i g h s e m i c o n d u c t o r q u a l i t y . The i m p e r f e c t i o n s t r u c t u r e i n a c o m p o s i t i o n and s t r u c t u r a l sense depends on t e c h n i q u e d e t a i l s . The second example i s e l e c t r o l y t i c p l a t i n g o f copper f i l m s . C o n t r a r y t o some e x p e c t a t i o n s growth c l o s e t o e q u i l i b r i u m p o t e n t i a l c o n d i t i o n s does n o t r e s u l t i n the h i g h e s t q u a l i t y d e p o s i t . I n g e n e r a l , the c o m p o s i t i o n and s t r u c t u r e o f the d e p o s i t depends on t h e d e t a i l e d c o m b i n a t i o n o f t r a n s p o r t p r o c e s s e s towards and away from t h e e l e c t r o d e s u r f a c e . A d d i n g t h e l o c a l hydrodynamic v a r i a b l e s p r o v i d e s t h e system w i t h an e x t r e m e l y b r o a d s t r u c t u r a l c h e m i c a l range. The t h i r d example i n v o l v e s growth o f a c r y s t a l l i n e s a l t phase from t h e s a t u r a t e d s o l u t i o n . The thermo dynamic d e s c r i p t i o n a g a i n i s i n a d e q u a t e f o r d e s c r i b i n g the d e t a i l e d s t a t e s of the s o l i d . The r e l a t i v e l y anhydrous c h l o r i d e i o n r e a d i l y d e p o s i t s i n t o the s u r f a c e l a t t i c e but t h e sodium i o n must be d e h y d r a t e d t o form N a C l . T h i s means t h a t w a t e r must be d i f f u s e d away from t h e s u r f a c e d u r i n g s o l i d i f i c a t i o n . The w h i t e c a s t o f s a l t , termed v e i l i n g , i n v o l v e s s o l u t i o n i n c o r p o r a t i o n d u r i n g growth w i t h a subsequent d i f f u s i o n o f t h e s o l v e n t o u t o f the c r y s t a l .


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IP

1

î

f

Substrate

Pi


Ρ2 Filament

=

S2

Pi

Ti

Vapor Plating

gas

+ +

Fe

Cu

Seed Crystal

Na CI

Seed Crystal

Ge

\

Cu Plating Solution

Saturated NaCl

Electrolytic Plating

Solution Precipitation

Molten Ge

Solidification of Melt \

Figure 21.

Single interphase exchange current model for solid-phase formation (P ) through interphase reactions with second phase (Pi) 2



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The f o u r t h example i s t h e c o n t r o l l e d s o l i d i f i c a t i o n o f germanium (or s i l i c o n ) t o produce s e m i c o n d u c t o r grade m a t e r i a l s . The s o l i d phase s t r u c t u r e and compo s i t i o n depend s t r o n g l y upon the i n t e r p h a s e exchange p r o c e s s e s w h i c h can enhance i n c o r p o r a t i o n i n t o the s o l i d o r i n t o t h e l i q u i d d e p e n d i n g upon ??? This example i m p l i c i t l y i n c l u d e s t h e p r o d u c t i o n o f most o f the m e t a l l i c s t r u c t u r a l m a t e r i a l s . The m e t a l a l l o y systems g e n e r a l l y have b o t h c o n t r o l l e d and u n c o n t r o l l e d i m p u r i t i e s b e i n g d e p o s i t e d and r e d i s t r i b u t e d w i t h i n the s o l i d as a r e s u l t o f complex l i q u i d hydrodynamic i n t e r p h a s e r e g i o n p r o c e s s e s as w e l l as s e c o n d a r y s o l i d s t a t e d i f f u s i o n and t r a n s f o r m a t i o n processes. This materials science i s f a m i l i a r to a l l c o r r o s i o n e n g i n e e r s who become i n v o l v e d w i t h c o m m e r c i a l systems. The second i n t e r p h a s e t r a n s p o r t model i n c o r p o r a t e s t h e dynamic i n t e r a c t i o n o f t h r e e phases w i t h two i n t e r phase exchange c u r r e n t s a t t h e s e p a r a t e d b o u n d a r i e s as i n d i c a t e d i n F i g u r e 22 where a s o l i d phase P i s formed by i n t e r a c t i o n s i n v o l v i n g the s e p a r a t e d phases P and Pi. The c l a s s i c o x i d a t i o n o f aluminum t o form a p a s s i v e s u r f a c e i s the f i r s t i l l u s t r a t i o n . While the second model i s t h e a n o d i c o x i d a t i o n o f aluminum, the i n t e r p h a s e t r a n s p o r t phenomena h e r e can be d i s t r i b u t e d w i t h i n more complex c o n t e x t s as a r e s u l t o f the boundary l a y e r c o m p o s i t i o n changes i n the e l e c t r o l y t e . T h i s model i s a p a r t i c u l a r l y f a s c i n a t i n g one because s e v e r a l decades ago i t was c l e a r t h a t t h e l a r g e n e g a t i v e l y c h a r g e d oxygen a n i o n c o u l d n o t m i g r a t e i n such o x i d a t i o n p r o c e s s e s - the m e t a l i o n +3 aluminum was s m a l l and o f c o u r s e had t o t r a n s p o r t a l l t h e current through the f i l m . Famous l a s t words t h a t c r e a t e d a c r i s i s when good t r a n s p o r t number e x p e r i m e n t s were p e r f o r m e d w h i c h showed b o t h atoms move i n the film-forming process The s i l v e r t a r n i s h i n g r e a c t i o n i n v o l v i n g hydrogen s u l f i d e i s a c l a s s i c of s o l i d s t a t e m a t e r i a l s science l i t e r a t u r e , and the z i n c o x i d a t i o n i s y e t a n o t h e r example o f a more complex p r o t e c t i v e l a y e r c o r r o s i o n p r o b l e m f o r w h i c h wide r a n g e s o f d a t a e x i s t r e l a t i n g to p u r i t y , to k i n e t i c c o n d i t i o n s , etc. T h i s i n t e r p h a s e exchange c u r r e n t m o d e l i n g o f the s o l i d phase f o r m a t i o n p r o c e s s s e r v e s t o emphasize the v a r i e d p e r s p e c t i v e s from w h i c h u s e f u l i n f o r m a t i o n can be drawn t o a i d i n d e s c r i b i n g and u n d e r s t a n d i n g c o r r o s i o n p r o c e s s e s i n v a r i e d systems. R a r e l y are the r e a l systems s i m p l e and many p i e c e s o f d a t a a r e examined b e f o r e t h e models do i n c o r p o r a t e t h e f u l l ranges of 2

3


CORROSION

IP

CHEMISTRY

IP

2-3

1-2


4

P

2

- S:

Al

AI2O3

Al

lAnodic Oxide

Ag

Ag S

Passivation I

gas

j

2

Anodizing Electrolyte }

H2S

Tarnishing

\

\

i

1 Zn

1

ZnO

Alkaline Electrolyte

- — -

Figure 22.

Passivation II (capacitors)

Battery Reaction

i

Double interphase exchange current model for solid-phase formation 2 s and P2-Pi boundaries

(P ) through interphase reactions (IP) at P -P 2



1.

τυοΜι

Corrosion:

Problem

of

Materials

31

Science

variables present. The c o r r o s i o n e n g i n e e r ' s e x p e r i e n c e s e x i s t i n a complex s y n e r g i s t i c r e l a t i o n s h i p t o t h e v a r i e d i n t e r phase exchange c u r r e n t r e l a t i o n s h i p s p r e s e n t w i t h i n systems o f p r a c t i c a l c o n c e r n . I t i s i n d e e d r a r e t h a t h i s problems a r e e x p r e s s e d w i t h i n any model m a t e r i a l s science contexts. He must n e c e s s a r i l y work w i t h the e c o n o m i c a l l y f e a s i b l e m a t e r i a l s f o r the p r a c t i c a l a p p l i c a t i o n s . As t h e p r o d u c t i o n e n g i n e e r s work w i t h s t e a d i l y i n c r e a s i n g l a b o r c o s t s i n a s s e m b l y , the m a t e r i a l s e n g i n e e r - c o r r o s i o n e n g i n e e r - wear e n g i n e e r become i n c r e a s i n g l y on t h e s p o t t o e x t r a c t t h e r e q u i r e d v a l u e s from cheaper m a t e r i a l s . I t i s no j o k e t h a t the c o n t r a c t went t o t h e l o w e s t b i d d e r . What does t h i s mean t o a l l o f us assembled h e r e ? F i r s t , a r e c o g n i t i o n needs t o e x i s t t h a t i n a l l p r a c t i c a l s i t u a t i o n s m a t e r i a l s s c i e n c e i s a tough b a l l game - i t cannot r e m a i n a p h e n o m e n o l o g i c a l a r t i f we are e x p e c t e d t o w a r r a n t y t h e p e r f o r m a n c e . Second, a r e c o g n i t i o n needs t o e x i s t t h a t , though the problems a r e complex, many new t o o l s and s k i l l s a r e i n c r e a s i n g l y a v a i l a b l e t o c l a r i f y d i r e c t l y the c h a r a c t e r o f t h e problems p r e s e n t . The a r t i c l e s w h i c h follow bring attention to t h i s . T h i r d , a r e c o g n i t i o n needs t o e x i s t t h a t o r g a n i z e d knowledge on s o l i d s t a t e c h e m i s t r y i s becoming i n c r e a s i n g l y a v a i l a b l e t o us. T h i s i s i l l u s t r a t e d by Ν. Β. Hannay's m u l t i - v o l u m e T r e a t i s e on S o l i d S t a t e C h e m i s t r y . - ^ As n o t e d by Hannay i n h i s f o r e w o r d , "... Yet even though the r o l e o f c h e m i s t r y i n t h e s o l i d s t a t e s c i e n c e s has been a v i t a l one and t h e s o l i d s t a t e s c i e n c e s have, i n t u r n , made enormous c o n t r i b u t i o n s t o chemi c a l t h o u g h t , s o l i d s t a t e c h e m i s t r y has not been r e c o g n i z e d by t h e g e n e r a l body o f c h e m i s t s as a major s u b f i e l d o f c h e m i s t r y ... S o l i d s t a t e c h e m i s t r y has many f a c e t s , and one o f t h e p u r p o s e s o f t h i s t r e a t i s e i s t o help d e f i n e the f i e l d . " I f c h e m i s t s are t o be t h e a t o m i c - m o l e c u l a r domain custodians of s o l i d s t a t e m a t e r i a l s science, a serious c o n c e r n w i l l need t o e x i s t f o r a c q u i r i n g u s e f u l b a c k grounds i n t h i s a r e a . A s i g n i f i c a n t related publica t i o n i s t h e r e c e n t appearance o f F. A. K r o g e r s second e d i t i o n of the Chemistry of Imperfect C r y s t a l s - i n t h r e e volumes. The d e f e c t c h e m i s t r y c o n c e p t s from t h i s a r e a need t o be i n c o r p o r a t e d more g e n e r a l l y i n t o the new s u r f a c e s c i e n c e w h i c h r e l a t e s t o t h e e n v i r o n m e n t a l 1

1 5



32

CORROSION

CHEMISTRY

s t a b i l i t y of materials. F o u r t h and l a s t , b u t n o t l e a s t , a g r o w i n g r e c o g n i t i o n needs t o e x i s t t h a t the a p p l i c a t i o n o f the new complex i n s t r u m e n t a l t e c h n i q u e s can c l a r i f y the s u r face chemistry s p e c u l a t i o n s p r e v i o u s l y necessary. So f r e q u e n t l y our c o n c e p t u a l i z a t i o n s o f the r e a l p r o b l e m a r e wrong. I p e r s o n a l l y a c c e p t t h e h y p o t h e s i s t h a t t h e f i r s t s i x models when t e s t e d i n d e t a i l w i l l p r o v e wrong. However, as t h e r e a l model emerges i n i t s complex beauty, t h e number o f p a t h s t o o p t i m i z a t i o n s have m u l t i p l i e d and s c i e n c e i s no l o n g e r dead ended. The complex i n s t r u m e n t a t i o n s i n c l u d e LEED, low energy e l e c t r o n d i f f r a c t i o n , w h i c h i s r e v e a l i n g a complex model f o r s u r f a c e s t r u c t u r e when a p p a r e n t m u l t i l a y e r a d s o r p t i o n o f oxygen p r o c e e d s on c l e a n m e t a l surfaces. I t a l s o i n c l u d e s ESCA, e l e c t r o n s p e c t r o s c o p y f o r chemical a n a l y s i s , or x-ray photoelectron spectroscopy t h a t can p r o v e t h a t t h e p o s t u l a t e d c o v e r i n g f i l m , by g o l l y , i s n o t c o v e r i n g , and what's the s u r f a c e compos i t i o n ? E v e r t r y t o d e a l w i t h n i n e s u r f a c e components a t once? W e l l , you can s e r i o u s l y e x p l o r e t h a t d i s t r i b u t i o n f o r e l e c t r o l e s s n i c k e l f i l m s b e i n g d e p o s i t e d on a catalyzed substrate. I t a l s o i n c l u d e s Auger e l e c t r o n s p e c t r o s c o p y microprobe i d e n t i f i c a t i o n of the surface crud i n the p i t , or d e f i n i t i o n of the surface composition g r a d i e n t p r e v i o u s l y o m i t t e d from t h e s p e c u l a t i o n on atom transport processes. The s c a n n i n g e l e c t r o n m i c r o s c o p e p r o v i d e s us neophytes w i t h a r e a l i s t i c l o o k a t s u r f a c e s t r u c t u r e as e n c o u n t e r e d i n t h e system. When teamed w i t h Auger o r ESCA a c o n f r o n t a t i o n can be c r e a t e d w i t h s e l f , t r y i n g t o r a t i o n a l i z e the o l d c o m f o r t a b l e models t h a t d i d n o t have t o acknowledge t h a t s u r f a c e c h e m i c a l s t r u c t u r e r e a l l y e x i s t e d on t h e " p o l y p h a s e inhomogeneous s o l i d c o n t a i n i n g h e t e r o g e n e i t y homogeneously dispersed." C o r r o s i o n i s h e r e t o s t a y . The wedding t o m a t e r i a l s s c i e n c e i s i m p l i c i t l y r e v i e w e d i n the f i r s t five figures. I t i s a c l e a r l y i m p o r t a n t complex m a t e r i a l s s c i e n c e d r a w i n g on a l l t h e o t h e r d i s c i p l i n e s . As t h e s u r f a c e s c i e n c e o f i n t e r p h a s e mass and charge t r a n s p o r t phenomena on s o l i d s c o n t i n u e s t o e v o l v e t h e n more c l e a r l y , d e f i n e d new r o u t e s f o r p r o d u c t o p t i m i z a t i o n s w i l l be e v i d e n t . Would you agree t h a t c o r r o s i o n i s t h e most g e n e r a l problem o f m a t e r i a l s s c i e n c e ? May a l l have f u n e x p l o r i n g t h e new v i s t a s o f our p h y s i c a l world through the f o l l o w i n g c h a p t e r s . For


01. τυοΜι

Corrosion: Problem of Materials Science

33

indeed we are a l l stewards of the u t i l i z a t i o n of mater i a l s on t h i s scene i n s e r v i c e to a l l . May the mastery of c o r r o s i o n extend the resources a v a i l a b l e to a l l those who w i l l c e l e b r a t e the t r i c e n t e n n i a l , for few of us w i l l have t h i s p r i v i l e g e .


Acknowledgements I would l i k e to s p e c i f i c a l l y acknowledge the c h a l lenge provided by Dr. Rudolph Hausler when he i n d i c a t e d that preparing the overview would be an easy t a s k . It wasn't, but i t d i d provide an opportunity to survey c o r r o s i o n l i t e r a t u r e and to recognize more f u l l y the t i t l e of t h i s a r t i c l e : Corrosion (is) the Most General Problem of M a t e r i a l s Science. General

References

1.

Tedmon, C r a i g S . , e d i t o r , "Corrosion Problems i n Energy Conversion and Generation," Corrosion D i v ision, E l e c t r o c h e m i c a l Society (1974). 2. Evans, U. R . , "Corrosion and Oxidation of M e t a l s : Scientific P r i n c i p l e s and P r a c t i c a l A p p l i c a t i o n s , " A r n o l d , London (1960). 3. Fontana, M. G., Greene, N . D . , "Corrosion Engin e e r i n g , " M c G r a w - H i l l , New York (1967). 4. Hauffe, Κ., "Oxidation of M e t a l s , " Plenum Press, New York (1965). 5. Hamner, Ν. E., compiler, "Corrosion Data Survey: Metals and Non-Metals," Fifth E d i t i o n , N a t i o n a l A s s o c i a t i o n of Corrosion Engineers, Houston, Texas (1974). 6. Nathan, C. C . , e d i t o r , "Corrosion I n h i b i t o r s , " N a t i o n a l A s s o c i a t i o n of Corrosion Engineers, Houston, Texas (1973). 7. Pourbaix, Μ., "Lectures on Electrochemical C o r r o s i o n , " Plenum Press, New York (1973). 8. U h l i g , Η. Η . , "Corrosion and Corrosion C o n t r o l : An I n t r o d u c t i o n to Corrosion Science and Engin e e r i n g , " John Wiley, New York (1963). 9. U h l i g , H. H., et. al., "Corrosion Handbook," W i l e y , New York, (1948). 10. " C o r r o s i o n , the Journal of Science and Engineering," published monthly by the N a t i o n a l A s s o c i a t i o n of Corrosion Engineers, Houston, Texas.


CORROSION CHEMISTRY

34 Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15.

General References 1-10. Stern, M . , et. al., J . Electrochem. Soc. (1957) 104 pp. 56-63, 559-63, 645-50. Helmholtz, H . , Wiss Abhandlphysik. Tech. Reichenstalt I (1879) p. 925. Gouy, G . , J. Phys. (1910) 9 p. 457. Chapman, D. L., P h i l . Mag. (1913) 25 (6) p. 475. Stern, Ο., z. Electrochem. (1924) 30 p. 508. Grahame, D. C . , Chem. Rev. (1947) 41 p. 441. Bockris, J . O'M., et. al., Proc. Roy. Soc. (1963) A274 pp. 55-79. Rice, Ο. K . , Phys. Rev. (1928) 31 p. 1051, MacDonald, J . R., J. Appl. Phys. (1964) 35 (10) p. 3053. See, for example H. Gerischer, 177-204, H. Gatos, 381-4 08, "The Surface Chemistry of Metals and Semiconductors," H. C. Gatos, editor, John Wiley, New York (1960). Rees, A. L. G . , "Chemistry of the Defect Solid State," John Wiley, New York (1954). Croft, G. T., and Tuomi, D., J. Electrochem. Soc. (1961) 108 p. 915. Davies, J. Α . , et. al., J. Electrochem. Soc. (1965) 112 p. 675. Hannay, Ν. Β . , editor, "Treatise on Solid State Chemistry," Volumes 1-7, Plenum Press, New York (1974). Kroger, F. Α . , "Chemistry of Imperfect Crystals Volume 1, Preparation, Purification, Crystal Growth, and Phase Theory; Volume 2, Imperfection Chemistry of Crystalline Solids; Volume 3, Applications of Imperfection Chemistry: Solid State Reactions and Electrochemistry," North Holland-American Elsevier, New York (1974).

RECEIVED

September 1, 1978.


Corrosion: The Most General Problem of Materials Science - ACS

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