Dissolution of Iron - ACS Symposium Series (ACS Publications)

5 Dissolution of Iron MORRIS C O H E N

Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on September 6, 2014 | http://pubs.acs.org Publication Date: January 31, 1979 | doi: 10.1021/bk-1979-0089.ch005

National Research Council of Canada, Division of Chemistry, Ottawa, Canada

In this lecture I w i l l deal with the mechanisms involved in first, the dissolution or corrosion of iron and second, in the inhibition of corrosion by the formation of various types of oxide films. A few definitions w i l l help to focus attention on the specific nature of my subject. Mechanism 1. A sequence of steps in a chemical reaction. 2. The fundamental physical processes involved in or responsible for a reaction. Dissolution The act or process of dissolving or breaking up. Corrosion Destruction of a metal by chemical or electrochemical reaction with i t s environment. Inhibitor A chemical substance which, when added to the environment, usually in small concentrations effectively decreases corrosion, after concentrations. Passivity A metal is passive if it substantially resists corrosion in an environment where thermodynamically there is a large free energy decrease associated with i t s passage from the metallic state to appropriate corrosion products. Corrosion and i t s inhibition or passivity both involve reactions between the metal surface and the solution. In the case

0-8412-0471-3/79/47-089-126$06.75/0 © 1979 A m e r i c a n C h e m i c a l Society

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

5.

COHEN

Dissolution

of

Iron

127

of corrosion the reaction products are either soluble or form solids which are either loose or porous and do not protect the metal. In the f i r s t part of this presentation I w i l l deal with conditions in which corrosion of iron continues while in the last part I w i l l outline some of the mechanisms for the formation of solid protective films which can lead to both anodic passivity and inhibition.


Surface of Metal Metals normally exist as polycrystalline solids in which the atoms are arranged in a manner characteristic of the metal. The atoms in iron are arranged in a cubic array, (i.e.) the metal is composed of groups of cubic cells in which there are iron atoms at each corner of the cube and one in the middle. Large groups of these form crystals which join with some misfit at grain boundaries to make up the body of the metal. Aside from these grain boundaries other imperfections in the solid can be subgrain boundaries, dislocations, vacancies and either segregated or soluble impurities. Of course, a l l of these imperfections show up at the surface and affect the manner in which the iron w i l l react. Some of these imperfections are shown in Figure 1 . In this figure one can see ledges, impurity atoms, vacancies where an atom is missing and a kink step which results from the emergence at the surface of a dislocation. These imperfections w i l l a l l have different reactivities and at near equilibrium conditions, w i l l react at quite different rates. Under conditions far from equilibrium, such as in electropolishing the imperfections w i l l have very l i t t l e influence on dissolution rate and smoothing takes place. Under etching conditions both the effects of imperfections and crystal structure can be observed. This is shown in Figure 2 for iron electrolytically dissolved in perchloric-acetic acid in the etching region. On different grains one can observe either triangular or tetragonal pits, depending on the orientation of the grain. Aside from basic structural imperfections the dissolution rate and mechanism can be markedly affected by both bulk and surface impurities. Some surface impurities may act as preferred sites for hydrogen atom recombination and hydrogen evolution and hence increase the rate of dissolution. Others may act to poison the hydrogen re-combination reaction and lead to hydrogen dissolution followed by embrittlement and cracking. Some bulk impurities act as nucleating points for hydrogen atom recombination and lead to blistering. Carbon in iron is a hardener and makes iron more susceptible to stress-corrosion cracking. These effects make i t necessary to study both the surface and bulk composition of the iron in order to predict i t s corrosion behavior .



128

CORROSION C H E M I S T R Y

Figure 1.

Imperfections on solid sur faces

Figure 2.

Geometric pitting and grain boundary etching in anodic dissolution, original mag. χ 30,000


5.

COHEN

Dissolution

of

129

Iron

Solution Composition The second reactant in the dissolution or corrosion system is the solution. In most cases this is basically water contain ing dissolved substances which ionize more or less to cations and anions. The solution may also contain dissolved gases such as °2> H , or C0 . These constituents of the aqueous system can affect the corrosion rate in a variety of ways. 0 in small amounts may act to increase the rate of corrosion and in s u f f i ciently high concentrations to inhibit i t . In the presence of chloride ions, pits tend to go acid with a consequent increased rate of dissolution of the iron. Some metal ions such as copper, plate out on the iron surface by an exchange reaction and increase the corrosion rate by acting as hydrogen depolarizers. Complexing agents increase the reaction by lowering the effective concentration of the dissolved ferrous ions. Other constituents, such as organic amines may adsorb on the surface and slow down the rate of solution. Others, oxidizing agents, such as chromate or molybdate, help to form protective oxide films and inhibit the dissolution reaction. Hence a knowledge of the effects of various possible constituents of solutions is necessary before predicting the corrosion behavior of iron. 2

2


2

Application of Thermodynamics With sufficient thermodynamic and equilibrium data i t is possible to predict whether or not a dissolution reaction w i l l take place. Also with a knowledge of the free energy change for a reaction one can calculate the potential Ε at which the reaction w i l l occur from the relationships AG = nFE and Ε = E

Q

Q

nF + — log Concentration

where AG is the free energy change for the reaction, η is the number of electrons involved, F is the Faraday and E is the Stan dard Potential. Using these considerations, M. Pourbaix and co-workers at CEBELCOR constructed "equilibrium" potential-pH diagrams showing some of the stable phases for iron in aqueous solutions.Q) By further assuming that the formation of a stable solid oxide would lead to s t i f l i n g of corrosion (passivity), he simplified the dia gram for iron to that shown in Fig. 3. In this diagram areas of pH and potential are designated by the terms Immunity, Corrosion and Passivation. In the area labeled Immunity any ferrous ions in solution would be plated back on the metal (i.e.) iron is thermodynamically stable in water in this region. In the area labelled corrosion the corrosion product is either ferrous or f e r r i c ion in solution. In the area labelled passivation the iron Q


CORROSION

130

is supposed to be covered by a s o l i d f i l m of F e 0 or FeaO^ which stops c o r r o s i o n completely. Dashed l i n e (b) is the e q u i l i b r i u m p o t e n t i a l f o r 0 and (d) f o r H . The Pourbaix diagram, w i t h i n i t s assumptions gives a good general p i c t u r e of i r o n c o r r o s i o n behavior. However i t cannot be a p p l i e d to s p e c i f i c s i t u a t i o n s f o r a number of reasons. Iron can form a l a r g e number of s o l i d c o r r o s i o n products with water inc l u d i n g hydrated and anhydrous oxides as w e l l as i l l - d e f i n e d gels and amorphous products. Some of these may form p r o t e c t i v e coatings w h i l e others w i l l not. The s o l i d products formed may be porous and lead to p i t t i n g , cracking, e t c . We must always remember that in a l l regions other than that l a b e l l e d immunity the i r o n is thermodynamically unstable and in the long run nature w i l l have i t s way and the metal w i l l corrode. The a d d i t i o n of other ions to the s o l u t i o n , while probably having a small e f f e c t on the o v e r a l l thermodynamics of the s i t u a t i o n can have a very l a r g e e f f e c t on the k i n e t i c s and hence on the k i n e t i c s of d i s s o l u t i o n or c o r r o s i o n . They may, as in the case of c h l o r i d e s , lead to l o c a l i z e d attack by breakdown of prot e c t i v e f i l m s . In t h i s way although the most of the metal surface follows the behavior p r e d i c t e d by the Pourbaix diagram, d i s s o l u t i o n at confined areas leads to f a i l u r e by p i t t i n g or cracking. The Pourbaix diagrams a l s o t e l l one very l i t t l e about mechanism of r e a c t i o n , k i n e t i c s of r e a c t i o n or i n h i b i t i o n of r e a c t i o n . These aspects w i l l be d e a l t with in the r e s t of t h i s chapter. 2

2


CHEMISTRY

Electrochemistry

3

2

and

Corrosion

M a t e r i a l s can d i s s o l v e by d i r e c t d i s s o l u t i o n or by separate steps. An example of the former is the d i s s o l u t i o n of sugar in water and of the l a t t e r the c o r r o s i o n of i r o n in aqueous solutions,. D i r e c t d i s s o l u t i o n or i r o n may occur in some organic solvents and in the presence of some c o n s t i t u e n t s in aqueous s o l u t i o n s . In most aqueous c o r r o s i o n systems i r o n (and most other metals) goes i n t o s o l u t i o n v i a an e l e c t r o c h e m i c a l process in which there are separate anodes and cathodes. In a de-aerated s o l u t i o n the separate r e a c t i o n s are 2+ Fe *Fe 2H + 2e

+ 2e ->H 2

(anode) (cathode)

In the presence of oxygen the o v e r a l l cathodic 0

2

+ 2H 0 + 4e 2

• 40H~

reaction

is

(cathode)

I f we measure the p o t e n t i a l of the d i s s o l v i n g iron,we w i l l not o b t a i n an e q u i l i b r i u m p o t e n t i a l (as in the Pourbaix diagram) but a value somewhere between the anodic and cathodic p o t e n t i a l s . T h i s number w i l l depend on such f a c t o r s as how the p o t e n t i a l s



5.

COHEN

Dissolution

of

Iron

131

change with c u r r e n t , ( i . e ) the slopes of the p o l a r i z a t i o n curves, the various r e s i s t a n c e s in the system, and d i f f u s i o n of reactants and r e a c t i o n products. With i r o n the slope of the anodic p o l a r i z a t i o n curve is considered to be much l e s s than that of the cathodic curves. With these considerations in mind U.R. Evans suggested what are now known as the Evans* diagrams to describe the e l e c t r o c h e m i c a l mechanism of c o r r o s i o n . T y p i c a l curves f o r i r o n are shown in Figure 4, (2). In Figure 4 the top curves with the downward slopes are f o r cathodic r e a c t i o n s . In 4(a) are shown curves f o r anodic and cathodic r e a c t i o n which are only c o n t r o l l e d by a c t i v a t i o n p o l a r i z a t i o n . A t y p i c a l example is a w e l l s t i r r e d a c i d s o l u t i o n . The top cathodic curve is f o r oxygen and the lower one f o r hydrogen. Obviously the p o s s i b l e c o r r o s i o n r a t e in the presence of oxygen is higher than that in the presence of hydrogen only. Because of t h i s oxygen is sometimes c a l l e d a good cathodic d e p o l a r i z e r . In 4(b) and 4(c) are shown the e f f e c t s of p o l a r i z a t i o n due to d i f f u s i o n c o n t r o l . In (b) excess cathodic p o l a r i z a t i o n may be caused by a low concentration of oxygen from the bulk of the s o l u t i o n . In (c) high anodic p o l a r i z a t i o n may be caused by slow d i f f u s i o n of ferrous ions away from the anode. With both (b) and ( c ) , the p o l a r i z a t i o n curves and hence the d i s s o l u t i o n r a t e w i l l be h i g h l y dependent on concentration and s t i r r i n g . With (c) i r o n i o n complexing agent w i l l a l s o markedly a f f e c t the r a t e of d i s s o l u t i o n by i n c r e a s i n g the d i f f u s i o n r a t e by i n c r e a s i n g the concentration gradient. The p o l a r i z a t i o n curves shown in 4(d) are t y p i c a l of systems in which anodes and cathodes are separated by c o r r o s i o n products and/or s o l u t i o n s of some r e s i s t a n c e . The c o r r o s i o n r a t e (current density) depends on the average r e s i s t a n c e between the anodes and cathodes and the p o t e n t i a l which is measured depends on the p o s i t i o n of the probe e l e c t r o d e in r e l a t i o n to the r e s i s t a n c e separating the anode and cathode. In a corroding system of t h i s type there is a current flowing through the s o l u t i o n which can be c a l c u l a t e d by measuring the p o t e n t i a l d i f f e r e n c e s between two spaced e l e c t r o d e s in the s o l u t i o n and the r e s i s t a n c e of the s o l u t i o n . Evans and h i s co-workers d i d t h i s with i r o n in b i c a r bonate s o l u t i o n s and showed that a l l the c o r r o s i o n could be accounted f o r by the currents flowing between anodes and cathodes, (3) . T h i s confirmed the e l e c t r o c h e m i c a l mechanism of c o r r o s i o n . Mechanisms of Reactions Although the o v e r a l l anodic and cathodic r e a c t i o n s are those given in Equations 1-3,it is u s u a l l y considered that the r e a c t i o n s take p l a c e in a s e r i e s of steps, one of which is the Rate Determining Step. The most widely accepted anodic scheme is that proposed by K e l l y (4J and is p a r t i c u l a r l y a p p l i c a b l e in a c i d s o l u t i o n s . The r e a c t i o n steps are as f o l l o w s .



132


0

2

Figure S.

4

6

pH

8

10

12

14

16

Pourbaix diagram for iron in water

Figure 4. Evans-type polarization curves: (a) anodic curve intersecting two pos sible cathode reactions; (b) diffusion control of cathodic reaction; (c) diffusion control of anodic reaction; and (d) resistance pohrization, measure Ε will depend on where probe is in relation to anode and cathode


COHEN

5.

Dissolution

Fe + H 0

133

Iron

^Fe(H 0) adsorbed

£

2

Fe(H 0)ads

i

^Fe(OH~)ads +

(FeOH)ads + e

2

Fe(0H~)ads

+

(FeOH) + H

H

+

4

Fe(OH)ads


of

KFeOH) * + e +

±z

=^

44

Fe " +

RDS

H0 2

This mechanism postulates that the electron transfer step takes place in two stages with the oxidation of (FeOH)ads being the rate determining step(RDS). In the presence of halides there is a change in mechanism and Lorenz (jï) has suggested the following steps. Fe + H 0

Fe(H 0)ads

2

2

Fe(H 0)ads + X~ * ; FeX~ + H0 2 ads I FeX~ + 0H~ •FeOH* + X" + 2e ads Fe0H+ + H ^=5- Fe " + H 0 o

o

+

RDS

44

2

Here the oxidation involves an adsorbed halide iron-surface metal atom complex. In both cases oxidation is to ferrous ion in solution. The f i n a l concentration of FeOff* and Fe""" is strongly pH dependent. The two ions w i l l only be stable in acid or oxygenfree systems. In the presence of oxidants the Fe""* w i l l be oxidized to various insoluble ferric compounds such as FeaO^. Fe20 and the hydrated oxides, a, 3 and γ FeO(OH). The two main cathodic reactions involved in dissolution (and corrosion) reactions are the reduction offf*"to H and the reduction of dissolved 0 to OH". The following reaction schemes are the ones which have been postulated. -

4 1

1

3

2

2

H H

+

+e aq or H 0 + e 2

Η + Η H

+

+

Reduction Η , ads Η . + OH ads aq

H

+ Η ads + e

aq H0 Ι Η - + e 2 ads -

H

2

2

1(a) 2 3

+ 0H_. 3(a) aq (1) and (3) w i l l predominate over 1(a) and 3(a) at low ph s. The rate determining step depends in part on the catalytic properties of the surface. This is particularly true for the recombination reactions (2) and 3 or 3(a). In some cases the recombination re action is more d i f f i c u l t than dissolution of atomic hydrogen into the metal and an alternative reaction o

H

(1)

2

T


134


H

H metal

ads

(4)

becomes a predominant r e a c t i o n . I f c a t a l y t i c s i t e s f o r hydrogen atom recombination e x i s t w i t h i n the metal, such as i n c l u s i o n s , then hydrogen gas is formed in the metal and b l i s t e r i n g and rup ture can occur. 0^ Reduction


0 0

+ e

2

+ H

2

0 ads. 2

+

2

H0

2

+ e"

H0

H0

2

+ H~*

H

H

2°2ads OH + e

0

H0 ads.

+

6

H 0 .

2 ° 2 aq

2

ΈΓ

K)H +

H 0 2

2

+ 2H

2

+ 2e

OH" 2H 0.

0

2H 0.

+ 4H

+ 2e

2

OH" + 4H+ + 4e

2

+ 2H

2

+ 4e~

2

^2H 0. 2

2

H2O2 has been i d e n t i f i e d as a t r a n s i e n t species. These r e a c t i o n s are a l l q u i t e r a p i d in comparison to the hydrogen e v o l u t i o n r e a c t i o n and hence the oxygen is a b e t t e r cathodic d e p o l a r i z e r than H+ i o n . K i n e t i c s of D i s s o l u t i o n The r a t e s of both chemical and e l e c t r o c h e m i c a l r e a c t i o n s a r e a f f e c t e d by the a v a i l a b i l i t y o f the reactants and products, and hence t h e i r concentrations and rates of d i f f u s i o n , and by a term known as the a c t i v a t i o n energy f o r the r e a c t i o n . The r a t e o f an e l e c t r o c h e m i c a l r e a c t i o n can a l s o be a f f e c t e d by the various r e s i s t a n c e s in the system. I f we can s e t up a system in which the r e a c t i o n is not con t r o l l e d by e i t h e r d i f f u s i o n or r e s i s t a n c e , the r a t e o f the r e a c t i o n w i l l be determined by the a c t i v a t i o n o v e r - p o t e n t i a l . This follows from the Arrhenius equation which states that k = A exp (-AG*/RT) where k is the r e a c t i o n r a t e , A is a constant, AG* is the a c t i v a t i o n energy, R the gas constant and Τ the absolute temperature. As pointed out e a r l i e r AG* can be r e l a t e d to p o t e n t i a l by the r e relation AG* = nFAE*


5.

COHEN

Dissolution

135

of Iron

where ΔΕ* is the A c t i v a t i o n o v e r p o t e n t i a l . The a p p l i c a t i o n o f p o t e n t i a l to the e l e c t r o d e w i l l t h e r e f o r e increase the r a t e o f e i t h e r the forward or reverse r e a c t i o n . A schematic a c t i v a t i o n energy diagram f o r a r e a c t i o n is shown in F i g . 5. The dotted l i n e represents the e q u i l i b r i u m s t a t e and the s o l i d l i n e a p o l a r i z e d s t a t e due to an a p p l i e d potential. In the e q u i l i b r i u m s t a t e _AG*


*a

=

A

a ^RT* _AG*

where s u b s c r i p t s a and c a r e anodic and cathodic and i - where i is the exchange current. When a p o t e n t i a l Ε is a p p l i e d in an anodic d i r e c t i o n then

= i ^= i

a

a· , (AGa-nFE) then Δ ι = + A e ( — -) a a RT and A i = A e -(AGc (l-a)nFE) C C RT ' l

x

(

+

where a is the p r o p o r t i o n of the a p p l i e d p o t e n t i a l which is e f f e c t i v e at the anode. As a r u l e α = 0.5. The e f f e c t of the a p p l i e d anodic p o t e n t i a l is to increase i and decrease i . For the t o t a l current we must add i . However at s u f f i c i e n t l y high o v e r p o t e n t i a l s both i and i become small in r e l a t i o n to ± and a

c

0

c

Ε = a + b logi .

Q

(Tafel

a

equation).

Some o f these r e l a t i o n s h i p s a r e i l l u s t r a t e d g r a p h i c a l l y in Figure 6. E is the e q u i l i b r i u m p o t e n t i a l . At s u f f i c i e n t l y high o v e r p o t e n t i a l s a s t r a i g h t l i n e T a f e l region is observed. At higher a p p l i e d p o t e n t i a l s current may become independent of p o t e n t i a l because of d i f f u s i o n c o n t r o l or may f a l l o f f the s t r a i g h t l i n e due to r e s i s t a n c e p o l a r i z a t i o n . In the anodic r e a c t i o n t h i s is u s u a l l y due to the formation o f f i l m s on the sur face of the metal. These concepts can be a p p l i e d q u i t e d i r e c t l y to the c o r r o s i o n behavior of i r o n . The e f f e c t of d i f f u s i o n c o n t r o l on the c o r r o s i o n r a t e was shown in the Evans Diagrams (b) and (c) of F i g u r e 4. In the cathodic case the current becomes constant over a wide range of p o t e n t i a l s because of c o n t r o l by the r a t e o f d i f f u s i o n o f the reactant oxygen gas to the cathode. The anodic r e a c t i o n can be dependent on the r a t e of d i f f u s i o n of ¥e++ from the anode. The r a t e s in both cases can be increased by s t i r r i n g . The r a t e of Fe** i o n removal from the anode can a l s o be in creased by the presence of complexing agents in the s o l u t i o n . This e f f e c t w i l l be i l l u s t r a t e d l a t e r on with i n h i b i t o r s . Q



136


Figure 5. Activation energy diagrams for metal dissolution reaction. rium state ( ); shift attributable to application of potential Ε (

RESISTANCE POLARIZATION CONCENTRATION POLARIZATION

(DIFFUSION CONTROL)

Log i

EXCHANGE CURRENT

CATHODIC Figure 6.

ANODIC Polarization curves for anodic reactions


Equilib ).


5.

COHEN

Dissolution

of Iron

137

The effect of the formation of films under some anodic conditions has been known and studied for many years. Millier (j6) observed that at high applied potentials salt films could form at the anode which would reduce the current density. When these films diffused away,the current rose again. This is a case of intermittent resistance polarization. In some solutions i t is possible to stop iron from dissolving under some conditions. Two of these sets of conditions are illustrated in Fig. 7. In Fig. 7(a) the current density is raised in steps. The potential cur rent relationship starts out in the normal way but at a s u f f i ciently high current density a sudden discontinuity occurs in the log i-E curve with a sharp rise of potential. At the same time the metal stops dissolving and a second anodic reaction such as oxygen evolution takes over. This type of behavior was f i r s t shown by Edeleanu (7) who also suggested the use of anodic protection to prevent corrosion. In Fig. 7(b) the potential is raised in steps. Again, at some potential the current drops sharply and iron stops dissolving and remains "passive" over quite a wide range of potential. This type of behavior is characteris t i c of iron over a wide range of pH conditions and has been studied extensively. It is of interest not only for anodic pro tection but also for understanding the action of anodic inhibi tors. This passive condition is not something that would be predicted from kinetic considerations and is probably due to the formation of a solid protective phase over the iron which slows down the reaction. Passivation of Iron (a) Electropolishing. The deviation from the theoretical anodic dissolution behavior can be used to advantage in several ways. One of these is for the electropolishing of iron. An anodic polarization curve for iron in a perchloric-acetic acid solution is shown in the Fig. 8. It can be seen that there are three regions of the curve. In the f i r s t region, A, the log i-E curve is the expected one and the iron dissolved in a general way with macro etching. In Region Β the current changes very l i t t l e with potential and the specimen smooths. This is probably due to the formation of film which is resistant to cation movement from the metal. At higher potentials, RegionC.,this film breaks down locally and the specimen dissolves by a combination of smoothing and pitting. Region Β can be considered as a region of passivation in that the dissolution is less than would be ex pected from the overpotential. Under some conditions this anodic dissolution can be made very small and the specimen is considered to be anodically passivated. (b) Anodic Passivation. Anodic passivation can be obtained over a wide range of pH s and in a variety of electrolytes. In this section, however, I shall deal mainly with neutral solutions f



138


I

ι

(a) Figure 7.

(b) Anodic passivation curves

6.0

20

Figure 8.

40 60 80 CELL POTENTIAL-volts

100

120

Anodic dissolution in perchloric-acetic

acid



5.

COHEN

Dissolution

of

Iron

139

because, f i r s t , i t is the medium in which I have done most of my own work and second, because r e s u l t s in t h i s range are most a p p l i c a b l e to l a t e r c o n s i d e r a t i o n s of p a s s i v i t y in i n o r g a n i c in h i b i t o r systems. In F i g . 9 three p o l a r i z a t i o n curves f o r i r o n in b u f f e r e d sodium borate s o l u t i o n s are shown. In A the s o l u t i o n was sodium b o r a t e - b o r i c a c i d at a pH of 8.4 (8). The curve was obtained by p o l a r i z i n g i r o n at a constant p o t e n t i a l and measuring the current a f t e r 20 minutes. S t a r t i n g with an o x i d e - f r e e specimen the current f i r s t increases as the p o t e n t i a l is increased, reaches a maximum and then decreases again. There is a r e g i o n of over a v o l t in which i r o n does not d i s s o l v e , or is p a s s i v e , and then the current r i s e s again, due mainly to oxygen e v o l u t i o n . In the passive region the i r o n is covered by a t h i n f i l m of cubic oxide of the y-Fe203-Fe30i* type which is probably formed by a r e a c t i o n such as 2Fe + 3H 0 -> F e 0 2

2

3

+ 6H

+

+

6e.

T h i s is the same type of f i l m as is formed by the r e a c t i o n of clean i r o n with dry a i r or oxygen. For Curve Β the s o l u t i o n was sodium c h l o r i d e - b o r i c a c i d b u f f e r e d to pH 7.8. The p o l a r i z a t i o n c o n d i t i o n s were the same. However in the presence of the c h l o r i d e the maximum current be f o r e the onset of p a s s i v i t y was increased and the p o t e n t i a l range for p a s s i v i t y was decreased. The high currents observed in the "passive r e g i o n " were mainly due to l o c a l i z e d a t t a c k which leads to p i t t i n g . The major part of the s u r f a c e is s t i l l covered by the same type of i r o n oxide f i l m as that found with the pure borate b u f f e r . A t h i r d type of anodic p o l a r i z a t i o n curve, C., is obtained i f the borate b u f f e r s o l u t i o n contains Fe** i o n in s o l u t i o n . This may be present as an added Fe** s a l t or due to some d i s s o l u t i o n of the specimen during p a s s i v a t i o n . In t h i s case the passive current is again higher than in the borate b u f f e r i t s e l f . This is due to the anodic o x i d a t i o n of the d i s s o l v e d Fe** at the e l e c t r o d e and the consequent d e p o s i t i o n of yFeOOH on the i r o n s u r f a c e . The anodic r e a c t i o n is Fe(OH)* + H 0 2

+ FeO (OH)

+ 2H

+

+

e.

T h i s deposit is formed over the oxide f i l m as w e l l as at pores w i t h i n the oxide f i l m . The d i s s o l u t i o n of i r o n by c o r r o s i o n processes almost always takes p l a c e because of f i l m breakdown and the r a t e of c o r r o s i o n depends not only on the thermodynamic and k i n e t i c c o n s i d e r a t i o n s mentioned in the f i r s t part of t h i s chapter but a l s o on the f a c t o r s leading to f i l m breakdown and r e p a i r . This is the subject of the l a s t part of t h i s chapter.


CORROSION

140 F i l m Breakdown and

Repair

When f i l m f r e e i r o n is exposed to dry a i r or oxygen at room temperature, i t w i l l form a p r o t e c t i v e f i l m of a cubic oxide of the FeaOtt-yFeaOa type which grows l o g a r i t h m i c a l l y to about 15-20 A° t h i c k . This is the same type of f i l m as is formed by anodic p a s s i v a t i o n and by the a c t i o n of c e r t a i n oxidants in s o l u t i o n . Under other c o n d i t i o n s the o v e r a l l composition of t h i s cubic f i l m can be a l t e r e d by the a d d i t i o n of hydrated oxides or other i n s o l u b l e oxides, such as C r 0 , which can change the thickness, topo graphy and s t a b i l i t y of the o v e r a l l f i l m . In some cases these i m p u r i t i e s are present as separate i n c l u s i o n s w i t h i n the f i l m l e a d i n g to weakened areas at which l o c a l i z e d breakdown may occur or they may be on top of the cubic oxide f i l m , such as a depo s i t e d γ-FeOOH and enhance the p r o t e c t i v e n e s s of the f i l m . These various cases are discussed in t h i s s e c t i o n . 2


CHEMISTRY

3

Structure of the Oxide F i l m The major p o r t i o n of the p r o t e c t i v e oxide f i l m on i r o n is s t r u c t u r a l l y the same whether i t is grown by exposure to a i r , i n h i b i t o r s o l u t i o n s or formed by anodic o x i d a t i o n Ç9,1Q). I t is cubic in nature and grows in an e p i t a x i a l manner with the underl y i n g metal. Although the oxide formed on s i n g l e c r y s t a l s is h i g h l y oriented, there is some mismatch between i n d i v i d u a l c r y s t a l l i t e s and the p a r t i c l e s i z e is 15-30A . The thickness of the air-formed f i l m at room temperature is 15-20 A ° . The f i l m s formed a n o d i c a l l y in n e u t r a l s o l u t i o n can be up to 40 A° in thickness, the thickness i n c r e a s i n g with higher p o l a r i z a t i o n p o t e n t i a l s . An e l e c t r o n d i f f r a c t i o n p a t t e r n of the oxide on s i n g l e c r y s t a l s of i r o n is shown in F i g . 10. T h i s shows the very ordered s t r u c t u r e as w e l l as the broadened spots due to small p a r t i c l e s i z e . The p a t t e r n is c h a r a c t e r i s t i c of the cubic FeaO^Y F e 0 system. This system has a constant face-centered cubic l a t t i c e of oxygen with varying amounts of i r o n in both o c t a h e d r a l and t e t r a h e d r a l p o s i t i o n s . The oxide probably v a r i e s in composition from the metal-oxide i n t e r f a c e to the oxide-gas or s o l u t i o n i n t e r f a c e with the yFe203 at the o u t s i d e . Under some c o n d i t i o n s the f i l m w i l l contain i n c l u s i o n s of other i r o n compounds, such as hydroxides or phosphates, and as mentioned e a r l i e r may a l s o have an overlay of p r e c i p i t a t e d or dep o s i t e d compounds. 0

2

3

Breakdown of

Films

In general, oxide f i l m s can be removed as p r o t e c t i v e l a y e r s by chemical d i s s o l u t i o n , undermining, or cathodic r e d u c t i o n . Anhydrous oxide f i l m s are u s u a l l y q u i t e d i f f i c u l t to d i s s o l v e . The r a t e of s o l u t i o n increases with lower pH and higher temperature. In n e u t r a l s o l u t i o n the r a t e of d i s s o l u t i o n of the


COHEN

Dissolution

of

Iron


- CHLORIDE-BORATE SOLUTION

Lu or or

ο 10 f-

-800

-400

0

20

400

800

1200

POTENTIAL vs. S.C.E. Figure 9.

Figure 10.

Anodic polarization curves in borate buffer solutions

Electron diffraction pattern of thin cubic oxide film on iron



142


passive f i l m is so slow that i t is probably not a f a c t o r in the breakdown of oxide f i l m s on i r o n . The hydrated oxides are more e a s i l y d i s s o l v e d , e s p e c i a l l y in s l i g h t l y a c i d s o l u t i o n s and t h e i r removable by d i s s o l u t i o n could be a d e f i n i t e f a c t o r in f i l m break down. I f the f i l m has pores in i t which expose the metal to the c o r r o s i v e s o l u t i o n , t h e metal can d i s s o l v e both at the bottom of the pore and between the f i l m and the metal. T h i s l a t t e r e f f e c t loosens the oxide by undermining and e v e n t u a l l y leads to i t s removal. U.R. Evans took advantage of t h i s phenomenon to s t r i p oxides from i r o n f o r chemical and p h y s i c a l examination ( 1 1 ) . To prevent cathodic r e d u c t i o n of the oxide he made the system anodic. The major cause of removal of oxide f i l m s on i r o n is the r e d u c t i v e d i s s o l u t i o n of the oxides. The anodic current f o r the process may be s u p p l i e d by an o u t s i d e source or by areas of the metal at which i r o n is d i s s o l v e d . In both cases the f e r r i c oxide is reduced to Fe ". The s o l u b i l i t y of Fe(0H) is much higher than e i t h e r F e 0 , FeaO^ or the FeOOH's and i t d i s s o l v e s . The magne t i t e component o f the oxide is reduced in part to Fe** and in part to Fe metal. The cathodic r e d u c t i o n of an a n o d i c a l l y formed oxide f i l m is shown in F i g u r e 11. The f i l m was formed by anodic o x i d a t i o n in a n e u t r a l sodium borate s o l u t i o n . I t was c a t h o d i c a l l y reduced in the same s o l u t i o n , in the absence of oxygen, at ΙΟμΑ/cm . Two major waves a r e observed. F i r s t the F e 0 is reduced with almost 100% current e f f i c i e n c y to form d i s s o l v e d Fe** i o n . The second wave corresponds to an i n e f f i c i e n t r e d u c t i o n of Fe30i* to form both s o l u b l e Fe** i o n and m e t a l l i c i r o n . The equations f o r these pro cesses are 44

2

2

3

2

2

1st A r r e s t Y F e ^ + 3H 0 + 2e 2

• 2Fe

+ +

3

+ 60H~

(A)

2nd A r r e s t F e 0 , + 4H 0 + 8e • Fe + 80H~ (B) 3 4 2 F e 0 . + 4H 0 + 2e • 3Fe** + 80H~ (C) 3 4 2 During the second a r r e s t there is a l s o c o n s i d e r a b l e hydrogen evol u t i o n form the a l t e r n a t e cathodic r e a c t i o n 2H

+

o

o

o

o

+ 2e

• H . 2

I f the r e d u c t i o n process is c a r r i e d on f o r a long time,there is a l s o some e l e c t r o d e p o s i t i o n of i r o n from Fe** in the s o l u t i o n . The major source of e l e c t r o n s f o r the cathodic r e d u c t i o n pro cess on open c i r c u i t is the c o r r o s i o n of the i r o n i t s e l f . These are s u p p l i e d by the r e a c t i o n Fe

44

• F e " + 2e

The p o t e n t i a l f o r t h i s r e a c t i o n is more negative than that f o r r e a c t i o n (A) and hence the e l e c t r o n s are a v a i l a b l e f o r the r e d u c t i v e d i s s o l u t i o n of y F e 0 3 . On open c i r c u i t the oxide can be removed 2


5.

COHEN

Dissolution

of

143

Iron

by a combination of r e d u c t i v e d i s s o l u t i o n to open up pores and undermining to remove the remaining Fe O and unreduced f i l m . The rates of d i s s o l u t i o n of Fe O and the hydrated oxides both i n c r e a s e q u i t e q u i c k l y w i t h decrease in pH. Hence breakdown of f i l m s is more r a p i d in a c i d s o l u t i o n s . I t is a l s o more r a p i d in s o l u t i o n s c o n t a i n i n g ions which l e a d to a c i d formation in the anodic pores. C h l o r i d e i o n is the most damaging of t h i s type of i o n although sulphates are a l s o d e s t r u c t i v e . The i r o n c h l o r i d e s a l t s which form during c o r r o s i o n in the pores hydrolyse to give p r e c i p i t a t e d i r o n s a l t s and a c i d . C h l o r i d e may a l s o act somewhat l i k e a complexing agent to i n c r e a s e the c o r r o s i o n r a t e . The e f f e c t of various ions on f i l m breakdown was r e c e n t l y d i s cussed at a symposium on p a s s i v i t y (12). 3


3

I n h i b i t i o n of

tt

it

Corrosion

I n h i b i t o r s can slow down the c o r r o s i o n r a t e by i n t e r f e r i n g with e i t h e r the anodic or cathodic r e a c t i o n s . The cathodic inh i b i t o r s can be e f f e c t i v e by e i t h e r poisoning the surface f o r hydrogen e v o l u t i o n and/or oxygen r e d u c t i o n or by i n c r e a s i n g e l e c t r o n i c r e s i s t a n c e . Some examples are z i n c s a l t s which form prec i p i t a t e s , adsorbing organic substances and p o s s i b l y the p o l y phosphates under some c o n d i t i o n s . The d i s c u s s i o n in t h i s l e c t u r e w i l l deal mainly with anodic i n h i b i t o r s . These act by i n t e r f e r i n g with the anodic d i s s o l u t i o n - p r o c e s s by a s s i s t i n g in the formation and p r e s e r v a t i o n of an oxide f i l m , s i m i l a r to that produced during anodic o x i d a t i o n of i r o n . Anodic I n h i b i t o r s The anodic i n h i b i t o r s are u s u a l l y i n o r g a n i c s a l t s which, above a minimum c o n c e n t r a t i o n decrease c o r r o s i o n to a n e g l i g i b l e amount. A t y p i c a l c o n c e n t r a t i o n versus weight-loss curve is shown f o r sodium n i t r i t e in F i g u r e 12. In the absence of i n h i b i t o r or very low concentrations the c o r r o s i o n is l o c a l i z e d and takes the form of p i t t i n g . At s u f f i c i e n t l y high concentration the i r o n is completely p r o t e c t e d . T h i s phenomenon of p i t t i n g at concentrations j u s t below that r e q u i r e d f o r i n h i b i t i o n is important and must be taken i n t o account when anodic i n h i b i t o r s are used. The conc e n t r a t i o n of i n h i b i t o r r e q u i r e d to stop c o r r o s i o n is dependent on the composition of the s o l u t i o n and is u s u a l l y decreased by the presence of oxygen and increased by a d d i t i o n s of c h l o r i d e or complexing agents. The e f f e c t of a strong complexing agent, v e r sene, is shown in Figure 13. The time required to achieve i n h i b i t i o n , as measured by the attainment of a passive p o t e n t i a l is increased as the concentration of complexing agent is increased. The complexing agent keeps the i r o n in s o l u t i o n and prevents the r e p a i r of pores by d e p o s i t i o n of f e r r i c s a l t s . The c l o s e connection between p o t e n t i a l and c o r r o s i o n is shown by the two s e t s of graphs in F i g u r e 14. Here one can see, that f o r i r o n in phosphate s o l u t i o n in the presence of oxygen, the time



-400 CATHODIC REDUCTION CURVE FOR 1 hr ANODIZED SPECIMEN AT + 6 0 0 mV


• 500 > -600 Ε -J C -700 -

_o

E„

',

CATHODIC CURRENT 10μ A/cm BORIC ACID-BORATE

2

\

g - 800

-900HI

-1000

Figure 11.

I

I

!

20 30 40 QUANTITY OF ELECTRICITY mC

50

Cathodic reduction of anodically formed oxide film on iron

1200

-GENERAL CORROSION

lOOppmKCI IN AERATED DISTILLED WATER

, 800

PITTING

X Ο

400 1

INHIBITION

10

20

30

40

50

60

70

80

X

ppm SODIUM NITRITE Figure 12.

Inhibition of corrosion by sodium nitrite


90


COHEN

Dissolution

of Iron

TIME-HOURS Figure 13. Effect of complexing agent, versene, on passivation by sodium nitrite: (A) etched iron in NaN0 solution; (B) same as in A plus versene 2



146


Journal of the Electrochemical Society Figure 14.

Rehtionship between corrosion and potential in neutral phosphate solutions: (A) deaerated solution; (B) air saturated (10)


COHEN


5.

Dissolution

of

147

Iron

at which the p o t e n t i a l r i s e s to the passive r e g i o n is a l s o the time at which c o r r o s i o n ceases. In the absence of oxygen c o r r o s i o n continues and the p o t e n t i a l remains in the a c t i v e r e g i o n . The n e c e s s i t y f o r the presence of oxygen f o r i n h i b i t i o n is common to a l a r g e number of i n o r g a n i c substances. In general these are n o n - o x i d i z i n g s a l t s which have some b u f f e r i n g c a p a c i t y in the n e u t r a l r e g i o n . There is a l s o a group of anodic i n h i b i t o r s which are o x i d i z i n g agents and i n h i b i t in both the presence and absence of oxygen. The o x i d i z i n g agents i n h i b i t at a much lower concentration than the n o n - o x i d i z i n g agents in the presence of oxygen (13). This is shown in F i g u r e 15. P o t e n t i a l time curves f o r the same s e r i e s of i n h i b i t o r s are shown in t h i s F i g u r e . The p o t e n t i a l s f o r the n o n - o x i d i z i n g i n h i b i t o r s in the absence of oxygen reach a steady value f o r i r o n in a saturated s o l u t i o n of Fe""*" at a concentration c h a r a c t e r i s t i c of the pH of the s a l t s o l u t i o n . The p o t e n t i a l of the o x i d i z i n g i n h i b i t o r s is somewhat higher and with s u f f i c i e n t time goes i n t o the p a s s i v e r e g i o n c h a r a c t e r i s t i c of oxide-covered metal. Sodium chromate is not shown in t h i s F i g u r e , but the p o t e n t i a l of i r o n in de-aerated s o l u t i o n goes i n t o the p a s s i v e r e g i o n very r a p i d l y . U n t i l r e c e n t l y chromate was the p r e f e r r e d i n h i b i t o r f o r many a p p l i c a t i o n s . However chromate is undesirable from an environmental standpoint and there is renewed research and development in the use of n i t r i t e , molybdate and small concentrations of phosphate. There is another group of i n h i b i t o r s which act by adsorption onto e i t h e r the metal or the oxide. These are u s u a l l y organic m a t e r i a l s and the most e f f e c t i v e are e i t h e r a l c o h o l s or amines. They are mainly used in s p e c i a l i z e d a p p l i c a t i o n s such as i n h i b i t i o n of a c i d c o r r o s i o n during p i c k l i n g or in m i t i g a t i o n of c o r r o s i o n in a c i d o i l w e l l s . The exact a c t i o n of these i n h i b i t o r s is beyond the scope of t h i s chapter but they are discussed by Hackerman and others (14). Some b u f f e r i n g i n h i b i t o r s , such as sodium benzoate may a l s o act by adsorption on the s u r f a c e . 1

Localized

Corrosion

In the d i s c u s s i o n to t h i s point we have noted three cases where the c o r r o s i o n occurred in a l o c a l i z e d manner v i a p i t t i n g . These were in an e l e c t r o p o l i s h i n g s o l u t i o n at a current d e n s i t y (and p o t e n t i a l ) higher than in the p o l i s h i n g r e g i o n , in a borate b u f f e r s o l u t i o n c o n t a i n i n g c h l o r i d e at a p o t e n t i a l a few hundred m i l l i v o l t s higher than the p a s s i v a t i o n p o t e n t i a l , and in i n h i b i t o r s o l u t i o n s at j u s t below the concentration of i n h i b i t o r r e q u i r e d to prevent c o r r o s i o n . In a l l of these examples the major p o r t i o n of the specimen is covered with the usual FesOt* - y F e 0 type of prot e c t i v e f i l m and c o r r o s i o n is concentrated at pores w i t h i n t h i s film. The c o r r o s i o n at the anodic pores is a c c e l e r a t e d by the l a r g e area covered by p r o t e c t i v e f i l m a c t i n g as a cathode. T h i s breakdown of the p r o t e c t i v e f i l m at i s o l a t e d areas is an important f a c t o r in two of the most damaging types of c o r r o s i o n f a i l u r e , 2

3

American Chemical

Society Library 1155

16th

St.

Washington, D. C.

N. W, 20038



Figure 15.

Inhibition in both oxidizing and nonoxidizing inhibitors. Weight-loss data in aerated solutions. Electrode potential curves in absence of oxygen (13).



5.

COHEN

Dissolution

of

Iron

149

namely the p e r f o r a t i o n of metal by p i t t i n g and s t r e s s c o r r o s i o n cracking. Examples of p i t t i n g in phosphate s o l u t i o n s w i t h d i f f e r e n t c h l o r i d e contents are shown in Figure 16. Two things are evident. F i r s t , the l a r g e r the c h l o r i d e concentration the l a r g e r the p i t s . Second, the number of p i t s is dependent on the g r a i n s t r u c t u r e . This l a t t e r f a c t would i n d i c a t e that the p e r f e c t i o n of the oxide is p a r t i a l l y dependent on the metal o r i e n t a t i o n . The dependence of p i t behavior on s t i r r i n g of the s o l u t i o n is shown in F i g u r e 17. Here one can see that s t i r r i n g causes an increase in the p o t e n t i a l of i r o n in n i t r i t e s o l u t i o n s . This is probably due to the lowering of cathodic p o l a r i z a t i o n by i n c r e a s i n g the r a t e of oxygen and n i t r i t e a r r i v a l at the cathode. This increase in cathodic curent appears to a i d the r e p a i r of the pore. In contrast to t h i s behavior, s t i r r i n g decreases the p o t e n t i a l of the p i t t e d s p e c i men in phosphate s o l u t i o n . T h i s i n d i c a t e s that s t i r r i n g leads to d e p o l a r i z a t i o n of the anodic areas, p o s s i b l y by removing c o r r o s i o n products at the p i t s . Recovery with phosphate is slow. In general, the p o t e n t i a l s with phosphate are lower than with n i t r i t e , i n d i c a t i n g a l a r g e p r o p o r t i o n of a c t i v e areas and hence a mixed p o t e n t i a l which is c l o s e r to that of the anode. Stress c o r r o s i o n cracking a l s o i n v o l v e s l o c a l i z e d breakdown of the p r o t e c t i v e f i l m . The c o r r o s i o n is narrowly confined within the metal due to s t r e s s f a c t o r s which may a r i s e from e i t h e r r e s i d u a l i n t e r n a l s t r e s s or a p p l i e d e x t e r n a l s t r e s s . In some cases the s t r e s s f a i l u r e can be a c c e l e r a t e d by chemical f a c t o r s , such as surface adsorption or hydrogen d i s s o l u t i o n from cathodic hydrogen l e a d i n g to embrittlement. L o c a l i z e d f i l m breakdown can r e s u l t from a number of causes. Chemical and p h y s i c a l inhomogeneities w i t h i n the f i l m are the usual reasons although breakdown can a l s o occur by s t r e s s e s set up w i t h i n the f i l m e i t h e r by poor e p i t a x i a l f i t of the oxide to the metal, or as Sato (15) has suggested by equivalent condenser pressure during anodic growth of the f i l m . Vermilyea (16) has shown current increases due to the cracking and r e p a i r of f i l m s during s t r e t c h i n g of passivated i r o n wires. The r o l e of s t r e s s is s t i l l a subject of a c t i v e research. Some of the r e a c t i o n s which can take place at imperfections in f i l m s and which can lead to e i t h e r d i s s o l u t i o n or f i l m r e p a i r are shown in the f i n a l f i g u r e , Figure 18. Here a specimen covered with an oxide f i l m with pores or imperfections is shown. A number of d i f f e r e n t r e a c t i o n s can take place both at the pores and on the oxide s u r f a c e . In general the r e a c t i o n s at the pores r e present e i t h e r anodic or d i s s o l u t i o n processes, while those on the oxide surface are cathodic in nature. Reaction 1 coupled with r e a c t i o n s (6) or (7) represents the standard d i s s o l u t i o n process. This can be a c c e l e r a t e d by a number of f a c t o r s such as lowered pH, presence of c h l o r i d e and complexing agents. Reaction (1) coupled with r e a c t i o n (8) gives both d i s s o l u t i o n and removal of the p r o t e c t i v e f i l m , which w i l l then lead to a c c e l e r a t e d d i s -



150


Corrosion Figure 16.

Pitting in phosphate-chloride solutions, 1000 ppm Na HPOj ; ppm CI', (b) 10 ppm CÏ (17) 2

t


(a) 100

5.

COHEN

Dissolution

151

of Iron


O.I2i

TIME, hr Corrosion Figure 17. Effect of stirring on potential of iron in chloride and inhibitor solution: (A) started bubbling; (B) stopped bubbling; ® 1000 ppm NaN0 + 100 ppm NaCl; © 1000 ppm Na HPO^ + 25 ppm NaCl) (17) 2

2

H^O

0

N0

2

ATA

Fe

Fe

FeOOH + 3H* + e

(3)

2Fe 0

(4)

2

4Fe + 3 0

2H

+

+2e

Fe 0 2

2

3

+ H 0 +e 2

+2e+ 3 H 0 2

Figure 18.

2

+

3

2

^2Fe 0 2

H

0 + 2 H 0 + 4e 2

2

+ 6H

(I)

Fe +2H 0

4Fe + N 0 i + 3 H

N0

+ 2e

+ +

Fe 0

+ +

2

OH"

+

2Fe + 3 H 0 2

AT C

H

2

+

+ 6e

(2)

3

3

+ NH +N (5) 3

2

(6)

2

^40H"

(7)

NH

(8)

3

*~2Fe

(etc) + +

+ 60H

(9)

Reaction on film with pore


CORROSION

152

CHEMISTRY

s o l u t i o n . The pores can be r e p a i r e d e i t h e r e l e c t r o c h e m i c a l l y , by anodic o x i d a t i o n as in (2) and by anodic d e p o s i t i o n as in (3). I t is a l s o p o s s i b l e f o r the f i l m to be r e p a i r e d chemically by the a c t i o n of i n h i b i t o r s as depicted in r e a c t i o n s (4) and (5). This type of approach lends i t s e l f to a systematic treatment of the problems of d i s s o l u t i o n and i t s i n h i b i t i o n . Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16.

17.

Pourbaix, M. " A t l a s of E l e c t r o c h e m i c a l E q u i l i b r i u m in Aqueous S o l u t i o n s " , Pergamon Press, 1966. Evans, U.R. "An I n t r o d u c t i o n to M e t a l l i c C o r r o s i o n " , Edmund A r n o l d Co. London 1948, Pages 71-3. T h o r n h i l l , R.S. and Evans, U.R. J. Chem. Soc. (1938) 614. K e l l y , E . J . J. Electrochem. Soc. (1965) 112, 124. Lorenz, W.J. C o r r o s i o n Science, (1965) 5, 121. M u l l e r , W.J. Trans. Faraday Soc. (1931), 27, 737. Edeleanu, C. and Gibson, G. Chem. Ind. (1961), 301. Nagayama, M. and Cohen, M. J. Electrochem. Soc. (1962), 109, 781. Nagayama, M. and Cohen M. J. Electrochem. Soc. (1963), 110, 670. Cohen, M. J. Electrochem. Soc. (1974), 121, 191C. Evans, U.R. and Stockdale, J. J. Chem. Soc. (1929), 2651. Staehle, R.W. and Okode, H. E d i t o r s . P a s s i v i t y and I t s Breakdown on Iron and non-Base A l l o y s , U.S.A.-Japan Seminar. NACE, Houston, Texas. 1976. Pryor, M.J. and Cohen, M. J. Electrochem. Soc. (1953), 100, 203. Hackerman, N. and Cook, E . I . J. Phys. Chem. (1952), 56, 524. Sato, N. E l e c t r o c h i m i c a A c t a . (1971), 16, 1683. Bubor, S.F. and Vermilyea, D.A. J. Electrochem. Soc. (1966), 112, 882. Cohen, Μ., C o r r o s i o n (1976) 32,

461.

R E C E I V E D September 1, 1 9 7 8 .


Dissolution of Iron - ACS Symposium Series (ACS Publications)

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