The coupled kinetics of film growth and dissolution of stainless steel repassivating in acid solutions

Corrosion Science, Vol. 24, No. 5, pp. 449-462, 1984 Printed in Great Britain.

0010-938X/84 $3.00 + 0.00 (~ 1984Pergamon Press Ltd.

THE COUPLED KINETICS OF FILM GROWTH AND DISSOLUTION OF STAINLESS STEEL REPASSIVATING IN ACID SOLUTIONS G. T. BURSTEIN and P. I. MARSHALL Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Abstract--The kinetics of growth of passivating films on freshly generated 304L stainless steel in perchloric acid solutions is presented. The passive film grows initially by oxidation of all components of the alloy with the rate of film thickening being controlled by ion migration under high electric field. As the film thickens oxidised iron begins to dissolve from the passive film into the strongly acidic electrolytes; dissolution commences when the electric field across the growing film has relaxed to a particular value. The rate of dissolution is described quantitatively in terms of simple non-steady state diffusion of Fe 2+ through the film, and composition profiles of the film are deduced. This form of dissolution is constructive in that it provides repassivation by virtue of a chromium enriched oxide. No direct dissolution from the bare metal surface can be detected.

INTRODUCTION

THE EXISTENCEof a passivating oxide film on stainless steel in aqueous environments was first established by Tronstad ~using reflected polarised light. Successful isolation of the film was, however, only achieved some time later. 2 These workers provided chemical analysis of a stripped film and showed it to be enriched in chromium when compared with the composition of the underlying steel; in some cases this enrichment occurred to the extent of 90%. The work was supported by Rodin 3 who used X-ray and colourimetric methods to show that the passive film was depleted in iron when compared with the bulk alloy. More recently the use of electrochemical radioactive tracer methods has confirmed that preferential dissolution of iron from the oxide film does o c c u r . 4"5 In order for the film to be enriched in chromium either dissolution of iron from the existing film occurs or chromium is preferentially oxidised during formation of the film. In the former case iron may dissolve from the film alone or it may continue to dissolve through the film from the substrate, resulting in chromium enrichment in the substrate as well as in the film itself. If chromium is preferentially oxidised in the first place the metal side of the substrate-film interface must become enriched in iron. With the advent of surface analytical techniques such as Auger electron spectroscopy and X-ray photo-electron spectroscopy many studies of the passive film composition have been made .6-9These studies confirm that chromium enrichment of the film does indeed occur, that it develops with time 7 and that the extent of enrichment is potential dependent. 7'9 None of this work 6-9 has, however, provided any evidence of enrichment of the underlying substrate in iron or chromium. While Manuscript received 2 July 1983. 449

450

G.T. BURSTEINand P. I. MARSHALL

this does not eliminate the possibility of such enrichment, it does support the idea that chromium enrichment in the film arises from dissolution of the iron component of the film. The concentration of chromium in the outer surface of the film has also been shown to depend on the chromium content of the alloy. 7 It is well known that a minimum chromium content of 12% in the alloy is necessary to achieve lasting passivity in neutral aqueous environments. It has been suggested 1° that in alloys containing less than 12% chromium, although rapid chromium enrichment of the film occurs initially, the low mobility of atomic chromium causes its depletion in the substrate. The chromium content of the film necessary to maintain passivity cannot consequently be upheld for long periods. In such cases the chromium-rich oxide is replaced by a less stable hydrated iron oxide. Other workers 11'~2have shown that the films formed on alloys of less than 12% Cr differ considerably from those formed on alloys of greater than 12% Cr. The important differences are not only chemical but physical in nature, in that chromium-rich passivating films tend to be thinner 12 and probably more resistant to mechanical disruption as well as being more stable chemically. When considering mechanisms of localised failure of stainless steels, such as stress corrosion cracking and cavitation, not only the stability of the oxide film, but also the way in which it reforms after rupture, is important. During film nucleation and growth the chromium enrichment process must occur for a stable passive film to be reformed. Observation of this process has to date, only been conducted on specimens already carrying the passive film. This work describes the repassivation of freshly generated stainless steel surfaces in acid solutions and provides the kinetics of chromium enrichment. The work follows from previous studies of the repassivation of stainless steel in near-neutral to strongly alkaline solutions,~3'14where it was shown that the rate of film growth on the newly generated metal surface can be related to the field strength across that amount of film already reformed by

i(t)=aexp[B~t)]

(1)

where i(t) is the rate of reaction of the surface at time t after it has been created, V is the voltage drop across a film which has grown to thickness h(t) in time t. A and B are constants related to the dimensions of the energy barrier associated with movement of ions across either the metal--oxide interface or the film itself. This is the classic mechanism of ion conduction under high electric field by Verwey 15and Cabrera and MottJ 6 It was shown 13"x4that the anodic charge flowing from the newly generated metal surface goes exclusively into film formation and no significant dissolution of the metal occurs. The charge density, q(t) flowing from the surface can therefore be related to the film thickness at time t by

q(t) = zEta h(t)

(2)

where M is the molecular weight of the film, p is its density and z is the charge number on the current-carrying ion. Rearranging equations (1) and (2) gives

Film growth and dissolution of stainless steel

451

log i(t) = log A + BVzFp

(3)

2.3Mq(t)

showing that log fit) is inversely related to q(t). EXPERIMENTAL METHOD The alloy used in this investigation was a low carbon austenitic steel (304L), identical to that used for previous studies, t3.14Its composition in wt % was C 0.03, Mn 1.6, Cr 18.3, Ni 10.6, Mo 0.05, Fe bal. It was used in the solution annealed condition. Electrodes were prepared as circular discs, area 0.5 cm 2, ground to a 1200 grit finish and ultrasonically cleaned in doubly distilled water. Four acidic electrolytes were used: these were 0.25, 0.5, 1.0 and 2.5 M HCIO4, giving pH values of 0.60, 0.30, 0.00 and - 0 . 4 0 respectively, where the activity coefficients have been ignored. These were made from Aristar grade HCIO4 and doubly distilled water. The ionic strength of the electrolytes was not maintained constant, but the conductivity of even the most dilute solution was adequate to render the ohmic resistance insignificant. 17.18 The potentiostatically controlled scratched rotating disc electrode was used to examine the repassivation of the freshly generated stainless steel surfaces in the above electrolytes. The experimental arrangement has been described elsewhere. 13"14'19'2°The apparatus allows a bare metal surface to be created by mechanical perturbation of originally steady-state metal whilst the electrode remains under potentiostatic control in solution. For this purpose a diamond stylus is allowed to impinge on the rotating disc surface for a duration of - 1 ms. The scratch generated is in the form of an arc of length --0.2 cm and width --20 # m (for the electrode rotating at 100 Hz), giving a surface area of - 4 × 10- 4 c m2. The rapid reaction of the newly generated metal surface was observed as a current transient recorded by a series of transient recorders and an x - t recorder, all connected in parallel. This allowed the current decay to be followed accurately from the creation of the scratch to a level approaching the steady state current density. A typical current transient due to creation of a scratch on stainless steel followed by its repassivation was presented earlier. 13 In order to examine the repassivation kinetics two parameters were measured from the current transients, both as a function of time, t, after scratching. The current density flowing from the scratch at time t is given by i(t)

-

1

- 2zrro~yt c

[l(t) - Ib]

(4)

where to is the rotation rate, y is the scratch width, r is its distance from the centre of rotation, te is the contact time, 1 (t) is the total current flowing from the whole electrode, including the scratch, at time t and lb is the base current flowing from the whole electrode before scratching. As the scratch current density approaches the base current density (ib) equation (4) must be modified to give i(t)

-

1 21rrooytc

[l(t) - Ib] + ib.

(5)

Also measured from the current transients was the charge density that has flowed from the scratch at time t. This was derived by integration of the current transients to give

1 I' q(t)

-

2¢rro~Y'~-'--~c

,~o

[l(t) - lb] dt

(6)

Experiments were conducted at ambient temperature of 18 + 2°C in electrolytes which were purged with nitrogen. Potentials were measured with respect to a saturated calomel reference electrode separated from the cell by an electrolyte bridge. All quoted potentials are on the normal hydrogen electrode scale (NHE). EXPERIMENTAL

RESULTS

The kinetics of decay of current density on the scratch can be represented by plotting log i(t) as a function of log t. Such plots have been shown to be linear, with slope 0 log i(t)/O log t --~ - 1 , for stainless steel repassivating in neutral or alkaline solutions. 13'14 Figure 1 shows typical log i(t) versus log t plots for repassivation of 304L stainless steel in solutions of pH 4.7 and 0.0. As with the previous work 13'14the graphs are linear of slope - - - 1. However as the current from the scratch decays for

452

G . T . BURSTE1N and P, I. MARSHALL

I 0 -I

'EI.I •I-.

1(~2

10-3 _

I

I

I0"3

I

I0"2

I

ICr-I

I

t (s)

FIG. 1.

Decay of current density with time after scratching 304L stainless steel in acid solutions at E = +645 mV(NHE). (o) pH 0.0, (x) pH 4.7.14

~

i 0 -I

fx

/ ~7

10-2

E

,o

.p, 10-3 _

I t

10"4_

I"

50

/ /

I

I00 CI (t.)-I

I

150

I

200

( c m 2 C-I)

FIG. 2. Repassivation of scratched 304L stainless steel at pH 0.0 according to equation (3). (x) E: = +845 m V ( N H E ) ; (o) E = +945 m V ( N H E ) . Broken lines show extrapolation of the second film growth regime.

Film growth and dissolution of stainless steel

453

the more acidic electrolyte, deviation to longer times occurs and repassivation evidently becomes slower. Plots of this kind are not readily amenable to mechanistic interpretationla'm4; however, clearly they show that beyond the point of deviation a greater charge density is required to reduce the current density than would be expected from the simple linear plots found for neutral and alkaline solutions (cf. pH 4.7, Fig. 1). The data have consequently been examined by plotting log i(t) as a function of 1/q(t) in accord with equation (3). Figure 2 illustrates such kinetic plots for data obtained at pH 0.0. At each potential the current decay shows two short regions of linearity, in which log i(t) oc 1/q(t), the earlier region [high i(t), low q(t)] being at lower slope than the later region. These data are in both qualitative and quantitative agreement with those obtained in the higher pH electrolytes. 13'14 Similar data are shown in Fig. 3 as a function of pH, all prepared at constant potential. From these data it is possible to deduce the values of A and B (equations 1 and 3) in a similar fashion to that described earlier 13"14and these are given in Table 1. They are in full agreement with the values described earlier. |3,14 If the potential drop across the film, V = E - Eg, where E is the applied electrode potential and Eg is the minimum potential required to produce film growth by these kinetics (equation 3) (i.e. E - Eg is the overpotential for film growth) then E. can also be evaluated from the data and these values for pH 0.0 were given earlier.l~

io-f

/

/

/

/ X

ic~ 2

.i-,

10-3

10-4

- -

I

50

I

I00

I

150

I

200

q l t T I (cmz C-ll

FIG. 3. Repassivationof scratched 304L stainless steel at E = + 1045 mV(NHE) by film growth under high electric field (equation 3). (o) pH 0.0; (x) pH 0.3; (A) pH 0.6.

G. T. BURSTEIN and P. I. MARSHALL

454 TABLE 1.

PARAMETERS RELATING FILM GROWTH KINETICS DURING REPASSIVATION OF 304L STAINLESSSTEELTO THE ELECTRICFIELDACCORDINGTO EQUATION(3) p H 0.0

p H 0.3

p H 0.6

logA (A cm -2) (high current regime)

- 3 . 4 + 0.55

- 3 . 4 + 0.45

- 3 . 3 + 0.47

logA ( A c m -2) (low current regime)

- 6 . 3 + 1.01

- 6 . 5 + 0.80

- 6 . 1 _+0.75

106B'cmV-t (high current regime)

2.7+0.4

4.9+0.5

3.8+_1.1

106 B, cm V -t (low current regime)

7.4 + 2.0

09.0 +- 0.7

8.4 + 0.9

As the current decays through the second kinetic regime the graphs show deviation from this high field film growth behaviour, as depicted in Figs. 2 and 3. The deviation is towards higher values of q(t) [lower values of 1/q(t)], consistent with the time plots shown in Fig. 1. To illustrate this phenomenon the second current decay regime has been extrapolated in Fig. 2 (broken lines). This apparent deviation from film growth under high electric field is peculiar to acidic solutions and indicates that an excess charge density flows during films growth over and above that required to produce the film. After the excess charge has passed the current decay becomes rapid once again. This phenomenon was observed at all potentials greater than Eg in all the acidic solutions mentioned above. The current density at which deviation from equation (3) occurs was found to be independent of electrode potential. This is equivalent to a constant electric field criterion required for the onset of the process (see equation 3), and is described by plotting for the charge density at which deviation occurs, qd, against potential. These plots are given in Fig. 4 for the electrolytes of pH 0.0, 0.3 and 0.6 and they are linear. The slopes of these lines, aE/aqo, give the electric field in V cm 2 C -~ across the film at which deviation occurs and this is independent of E. By allowing an estimated 500/xC cm -2 of anodic charge to form a monolayer of film, and for a monolayer thickness of 0.5 nm, the electric field OE/Ohdin V m -1 is obtained, where hd is the film thickness at which deviation occurs. The electric field dE/Oqd (or OE/Ohd) is plotted as a function of electrolyte pH in Fig. 5. Included in Fig. 5 is the datum point for the pH - 0 . 4 electrolyte; for this pH however, only those data obtained at low potentials gave an accurately defined point of deviation. At higher potentials the point of deviation occurred so early in the current decay that its identity was obscured. Figure 5 shows linear behaviour between the field at the onset of deviation and the electrolyte pH. The relationship is defined empirically by

aE (V cm 2 C -1) = 7 9 - 28 pH.

Oqd

(7)

Extrapolation of this plot to zero electric field (representing either zero overpotential across existing film or infinite film thickness) gives a pH of 2.8. Thus for solutions of pH >2.8 such deviation from high field film growth would not be expected to occur.

Film growth and dissolution of stainless steel

455

X X 002

Q~

C

o.o,

I

0

I

500

I

1000

E [mV (NHE)] FIG. 4. Charge density at which deviation from high field film growth kinetics (equation 3) occurs as a function of electrode potential for 304L stainless steel repassivating in solutions of pH 0.0 (o); 0.3 (x) and 0.6 (A).

IOO

-

-

Io

f,

~ao~ 6o-

~

-a i>

-6

40--

--4

20--

--2

I

0

I

I

-

~O

2

pH FIG. 5. The electric field, OE/Oqd, at which deviation from compliance with equation 3 occurs as a function of electrolyte pH. I-1 is a single datum pont from the electrolyte of pH - 0 . 4 . The ordinate is also calibrated in terms of the film thickness, ha, by OE/Ohd.

DISCUSSION

The charge density that flows during repassivation of 304L stainless steel in excess of that required to produce film growth clearly does not contribute to the passive film p e r se. T h e excess charge is therefore lost and the process by which it occurs is that of dissolution into the strongly acidic electroytes. The dissolution must be transient in nature since the current density flowing from the scratch does finally fall to its low steady state value when the scratch is repassivated. Dissolution therefore occurs concomitantly with film growth. We propose that it is the iron component of the oxide film that is dissolving, leaving behind a chromium enriched passivating oxide

456

G.T. BURSTEINand P. I. MARSHALL

film since this hypothesis is in agreement with all previous studies of the composition of the oxide film which show it to be chromium enriched. 3-~2 The observation that current decay on the scratch complies with equation 3 (and therefore equation 1) up to the specific electric field at which dissolution commences implies that the field must decay to a given value (at a given pH) before dissolution can commence, and while the field is greater than this value no dissolution can occur. Moreover, inspection of Figs. 2 and 3 shows that the film thickness is considerably greater than that expected for fine monolayer of film ( - 5 0 0 / z C cm-2), 13 even if a generous monolayer charge density is considered. Thus dissolution occurs only from thefilmed surface. Indeed, it seems from the present results in perchloric acid that a film must be present in order for the dissolution of iron to occur. This means that until the time that dissolution commences the passive film in acidic electrolytes is similar in composition to that achieved in neutral and alkaline electrolytes, and this is consistent with the similar values of A and B for all pH. 14It is also consistent with the continuous change in Eg that occurs with p H through the range 0 ~< p H ~< 14.14 On these grounds the initial film probably contains the metallic elements in the same ratio as occurs in the substrate alloy. Surface analysis by X-ray photo-electron spectroscopy 1° has confirmed this observation. This is in direct contrast to the destructive dissolution processes involved in the breakdown of passive films during pitting in environments containing chloride ions. 21 Since the ultimate state of the surface is that of passivity by a chromium enriched film, this dissolution is constructive: it depletes the surface of elements which dissolve rapidly. The fact that it is indeed iron which dissolves from the film is supported to a first approximation by the initial current densities at which dissolution takes place. Thus dissolution commences at current densities of the order of 10 m A cm -2 (see Figs. 2 and 3) and this figure would be expected to be a little lower than the steady state dissolution rate of pure iron under similar conditions; the steady state dissolution rate of iron at E = +745 m V ( N H E ) and pH 0.0 was measured as ~20 mA cm -2. The observation that the dissolution rate from stainless steel is a little lower reflects the presence of chromium (and nickel) in the film even in the initial stages of growth. As the p H of the electrolyte is reduced the dissolution process commences at higher electric fields (Fig. 5), dictated by thinner films for constant potential experiments, and therefore at higher current densities. This too, is the expected prediction from the pH dependence of the dissolution of iron itself. In 2.5 M HCIO4, p H - 0 . 4 , dissolution commences at a sufficiently high current density for its delineation to be obscured. However, using the known values of A and B, and the predicted value of Eg 14 it is possible to construct the kinetics of film growth. Such a graph is shown in Fig. 6, where the solid line gives the experimental data and the broken line shows the predicted film growth rate for this pH. The initial dissolution rate from the oxide film, represented by the intersection of the solid and broken lines, is very high, and the second film growth regime cannot be observed in the experimental data. The kinetics of dissolution of iron from the growing oxide film can be elucidated from the data described above provided assumptions are made concerning the kinetics of concomitant film growth. For this purpose we consider that the film itself grows in a kinetically similar fashion to that at which it would grow if no dissolution

Film growth and dissolution of stainless steel

/

io-t

'E ,..,

1(52

x

f

457

X

X

I I

iG 3 I I I I I

I

I

5O

i oo

q ( t ) - t (cm 2 C-I)

FIG. 6. Decay of current density on scratched 304L stainless steel at pH - 0 . 4 , E = + 1245 m V ( N H E ) . The broken line shows the predicted kinetics of film growth under high electric field (equation 3) using A = 3.5 × 10 -4 and 1.4 × 10 -6 A cm -2, B = 0.06 and 0.10 cm V -I, and Eg = +400 m V ( N H E ) , t4

were to occur. In other words, we consider that A, B and Eg (equation 3) remain the same at all stages of repassivation, irrespective of the dissolution process. This means that the extrapolated film growth lines given in Fig. 2, and the predicted film growth line given in Fig. 6, actually do represent the kinetics of film growth. The validity of this assumption is, of course, suspect since as dissolution of the iron component of the film proceeds the film composition changes, and thus values of A, B and Eg may also change. Nevertheless, it is at this stage the only reasonable assumption and small variations in A and B probably make very little difference to the deduced kinetics of dissolution. We can now consider the excess charge density, Aq(t) = q(t)--qf(t)

(8)

as a function of t, where qf(t) is the charge density that has gone into film formation alone at time t. The excess charge density thus gives the total amount of dissolution that has occurred at time t after scratching. Since dissolution commences only at time to after scratching the time scale is better considered as (t - to), providing kinetics of dissolution as a function of the time during which dissolution has occurred. We further consider that the rate at which iron ions are dissolved from the film-electrolyte interface is fast compared with the rate of diffusion of iron ions to that interface from the interior of the film. The diffusion of iron ions to the film--electrolyte interface thus becomes rate controlling. Provided the diffusion rate is not affected by electromigration then the boundary conditions to Fick's second law are

C=Coat(t-to)=O C=

Cs= 0atx

= 0

and and

0
458

G.T. BURSTEINand P. I. MARSHALL

where C is the concentration of iron ions in the film at co-ordinates (x, t), Co is the iron concentration in the film before dissolution, Cs is the surface concentration of iron ions after dissolution has commenced (considered to be zero) and x is the distance into the film from the metal-film interface. Co can be estimated assuming the metal oxides in the film to be in the same ratio as they are in the bulk alloy. The solution to this equation is given by: 22

C_ Co

2 Iir2o'%-to) ''~ exp (__A2) d,~

,B.I/2

(9)

where D is the diffusion coefficient of iron ions in the oxide film and A is the integration variable. From equation (9) the total amount of iron ions which have passed from the film at time (t - to) is 2~r-1/2D 1/2(t - to)1/2Co• Application of Faraday's law provides a solution for Aq(t): Aq(t) =

2zFcr-ltZDl/2(t - to)l/2Co

(10)

where z is the charge number on the dissolving ion. Thus plots of Aq(t) against (t - to) 1/2 should be linear and of slope

OAq(t) = O(t - to) 1/2

2zF'Ir-I/2Dll2Co

.

(11)

This relationship is shown to be obeyed in Fig. 7 for a range of potentials and over the small p H range investigated. If the assumptions above are accurate the slope given in equation (11) should be independent of E and pH. Computed values of the slope are shown in Fig. 8. Within the scatter of the data they are indeed independent of E and pH although the scatter is large. The mean value is

OAq(t) = (2.9 + 1.3) x 10-3Ccm-2s-1/2. O(t- to) 1/2 By assuming values for the other constants in equation (11) the data permit evaluation of the diffusion coefficient, D, of iron ions in the parent oxide. For this purpose we have z = 2 (Fe 2÷) and Co = 0.059 mol cm -3. This gives D = 5 x 10 -14 cm 2 s -l. The value is reasonable for diffusion of ions through a solid oxide; it is far too low for diffusion of ions in an aqueous solution, demonstrating that the latter process is not rate-determining. Figure 8 also shows the ordinate calibrated in terms of D ~r2, with the above values of z and Co. Any potential dependence which does exist in Fig. 8 is masked by the scatter in the data; it must, however, be small. It is worth bearing in mind that because of the scatter in the data other forms of relationship between Aq(t) and (t - to) can also be applied; nevertheless the simple non-steady state diffusion equations fit the data satisfactorily. Using these parameters it is possible to construct composition profiles of iron (assumed as Fe 2÷) in the film for different times after the commencement of dissolution. Published values of the normalised probability integral for equation (9) were used 22 after definition of x and (t - to), and a typical set of concentration profiles, C versus x, as a function of (t - to) is shown in Fig. 9. The data can be used to establish the iron composition profile (as Fe 2÷) within the

Film growth and dissolution of stainless steel

459

of 4-'

I

3

2 (t-tj

4

~2 (s ''2)

FIG. 7. Kinetics of dissolution of iron ions as controlled by non-steady state diffusion through the growingoxide film (equation 11). (A) pH 0.6, E = + 645 mV(NHE); (x) pH 0.3, E = +845 mV(NHE); (o) pH 0.0, E = +1045 mV(NHE).

A

,-~

5

u (J

4 0

E

A

x[

0

3

o x

o A

2

A

--4

X

cr

C N

%

x

I

I

I

I

600

800

1000

1200

E

[mV(NHE)]

FXG. 8. Diffusion gradient (equation 11) and computed diffusion coefficient (as D Iy2, assuming z = 2, Co = 0.059 mol cm-3) for movement of iron ions (Fe2÷) through the oxide film growing on 304L stainless steel as a function of E. (o) pH 0.0; (x) pH 0.3; (A) pH 0.6. film when the steady state has been fully re-established on the scratched surface. The film growth kinetics shown in Fig. 2 must now be extrapolated to the passive current density to produce the steady state film thickness (assumed to be 0.5 nm per m o n o l a y e r and 0.5 m C cm -2 per monolayer). The time required to reach the steady state in practice was estimated by plotting the charge in the film, qf(t), versus the dissolution charge, Aq(t), and extrapolating to the steady state film charge, qf(oo), to produce Aq, the total dissolved charge at the m o m e n t the steady state is reached. By imposing the consequent value of Aq into the dissolution kinetics (Fig. 7) the time to achieve the steady state is established. The Aq(t) versus qf(t) plots are shown in Fig. 10 and they are linear. The time to achieve steady state is given in Table 2; the values show considerable scatter, caused primarily by the long extrapolation involved. H o w e v e r it is clear that the times are

460

G . T . BURSTEIN and P. I. MARSHALL

0 06

Co

-

-- -

x

T

-

-

-

-

x

--O- . . . .

~

0.02

I

I

2

I

4

I

6

8

x (nm)

FIG. 9. Non-steady state concentration profiles for iron ions (assumed as Fe 2÷) in the oxide film growing on 304L stainless steel at E = + 1045 m V ( N H E ) in electrolyte of p H 0.0. Co is the Fe 2+ concentration in the film before dissolution and x is the distance into the film from the film-electrolyte interface. Times after c o m m e n c e m e n t of dissolution: (A) 0.01 s; (x) 0.2 s; (o) 1.0 s. T h e arrow indicates the computed film thickness, which is approximately constant for the three profiles. I

I

I

P

,j

6

2

I

I0

I

20 q,(t)

1

6

I

30

I

40

(nC crn "z)

FIG. 10. Dissolution charge density [Aq(t)] as a function of the film charge density [qf(t)] during repassivation of 304L stainless steel at pH 0.6. (o) E = +845 mV(NHE); (x) E = +1145 mV(NHE); (A) E = +1245 mV(NHE). (See equation g.)

long compared with those for measurement of scratch current density. Using these times and the steady state film thickness the composition profile at steady state was generated and is shown in Fig. 11. The profiles show that there is a continuous variation in the F e E+ content of the film with depth in the steady state. The composition of the film at the metal-film interface shows a significant depletion in iron compared with the parent alloy. Such profiles can in principle be determined for

Film growth and dissolution of stainless steel TABLE 2.

TIME(S)

461

TO R E - E S T A B L I S H S T E A D Y STA T E FILM T H I C K N E S S AFTER

G E N E R A T I O N OF A F R E S H

E [mV(NHE)]

304L S T A I N L E S S S T E E L

S U R F A C E IN ACID S O L U T I O N S

Time to achievesteady state(s) pn 0.0 pH 0.3 pH 0.6

+ 845 +945 +1045 +1145 + 1245

1764 81 104

25 72 42 71

27 81 94 100

i

I I I--" I I

0.06 C0

ov

,I

5

r0

15

20

25

x (nm) FIG. 11. Typical concentration profiles or iron (assumed as Fe 2+) for the steady state oxide film on 304L stainless steel after complete repassivation in electrolytes pH 0.0 (x) and 0.3 (o) at E = +1045 mV(NHE). The position x = 0 represents the film-electrolyte interface; the

position x = 25 nm represents the metal-film interface. all p H , and bear out qualitatively the composition profiles measured by surface analytical techniques. 7 These profiles (Fig. 11) are in principle quantitatively accurate. H o w e v e r determination of the time taken to reach the steady state requires long extrapolation and the composition profiles are subject to this error. In the analysis given above it is assumed that only Fe E+ (or Fe 3+) dissolves from the film, with both the chromium and nickel components remaining in the film. That the chromium does not dissolve at a significant rate until the steady state is re-established is clear; the fate of Ni 2+ in the film is, as yet, obscure. We have also assumed that the oxide film remaining after dissolution retains the same dimensions; this is unlikely since the defect density arising from the loss of Fe 2+ (or Fe 3+) to the electrolyte would be inordinately high. We would thus expect the iron-depleted film to condense somewhat but this would not be expected to alter the kinetics of dissolution too greatly. CONCLUSIONS 1. Repassivation of 304L stainless steel in perchloric acid solutions occurs by two consecutive rate laws, each governed by ion migration under high electric field.

462

G.T. BURSTEI~and P. I. MARSHALL

2. A s the p a s s i v a t i n g film g r o w s a n d t h e e l e c t r i c field across it r e l a x e s t h e iron c o m p o n e n t o f t h e film b e g i n s to dissolve l e a v i n g t h e film e n r i c h e d in c h r o m i u m . T h e o n s e t o f this d i s s o l u t i o n o c c u r s at a p a r t i c u l a r electric field which d e p e n d s on pH. 3. T h e kinetics o f d i s s o l u t i o n o f t h e i r o n c o m p o n e n t o f the g r o w i n g o x i d e film follows e q u a t i o n s o f n o n - s t e a d y s t a t e diffusion o f i r o n ions t h r o u g h t h e film, with D = 5 x 10 -14 cm 2 s -1. 4. W h e n r e p a s s i v a t i o n is c o m p l e t e t h e o u t e r surface of t h e o x i d e film c o n t a i n s n o i r o n ions. C o m p u t e d c o m p o s i t i o n profiles s h o w t h a t t h e i r o n c o n c e n t r a t i o n o f the film i n c r e a s e s c o n t i n u o u s l y f r o m t h e f i l m - e l e c t r o l y t e i n t e r f a c e to t h e m e t a l - f i l m interface. 5. This t y p e o f d i s s o l u t i o n is c o n s t r u c t i v e in t h a t t h e resulting c h r o m i u m e n r i c h e d o x i d e film r e t a r d s f u r t h e r d i s s o l u t i o n t h e r e b y p r o d u c i n g passivity. 6. In the p H r a n g e - 0 . 4 < p H < 0.6 n o d i r e c t d i s s o l u t i o n f r o m t h e freshly b a r e d stainless steel surface can b e d e t e c t e d ; all d e t e c t a b l e d i s s o l u t i o n occurs f r o m t h e o x i d e film p h a s e . Acknowledgements--We are grateful to the S.E.R.C. and to the A. E. R.E. for financial support of P. I.M.

Provision of laboratory facilities by Prof. R. W. K. Honeycombe is gratefully acknowledged.

1. 2. 3. 4.

REFERENCES L. TRONSTAD, Trans. Faraday Soc. 29,502 (1933). W. H. J. VERNON,F. WORMWELLand T. J. NURSE,J.[.S.[. 150, 81 (1944). T. N. RODIN,Corrosion 12, 123t (1956). Y. M. KOLOTRYKIN,Electrochim. Acta 18, 593 (1973).

5. C. LEYGRAF,G. HULTQUIST,I. OLEFJORD, B. O. ELFSTR6M, V. M. KNYAZHEVA,A. V. PLASKEYEVand Y. M. KOLO3"tCRKIN,Corros. Sci. 19, 343 (1979). 6. H. OGAWA,H. OMATA,I. ITOHand H. OKADA,Corrosion 34, 2 (1978). 7. J. R. COHOONand R. BANDY,Corrosion 38, 299 (1982). 8. G. O. OKAMOTO,K. TACHIBANA,T. SHIBATAand K. HOSmNO,J. Jap. Inst. Met. 38, 117 (1974). 9. I. OLEFJORU,Mat. Sci. Engng. 42, 161 (1980). 10. I. OLEFJORDand H. FISeHME1S~R,Corros. Sci. 15, 697 (1975). 11. K. ASAM1,K. HASHIMOTOand S. SmMODAntA,Corros. Sci. 16, 387 (1976).

12. R. P. FRANKENTHAL,27th meeting of the International Society of Electrochemistry. Zurich, Switzerland (1976). 13. G. T. BURSTE1Nand P. I. MARSHALL,Corros. Sci. 23,125 (1983). 14. P. I. MARSHALLand G. T. BURSTErN,Corros. Sci. 24,463 (1984). 15. E. J. W. VERWEY,Physica 2, 1059 (1935). 16. N. CABRERAand N. F. MoRt,Rep. Prog. Phys. 12, 163 (1948). 17. J. NEWMAN,J. electrochem. Soc. 113,501 (1956). 18. H. J. PEARSON,G. T. BURSTEINand R. C. NEWMAN,J. electrochem. Soc. 128, 2297 (1981). 19. F. P. FORD,G. T. BURSTEINand T. P. HOAR, J. electrochem. Soc. 127, 1325 (1980). 20. G.T. BURSTEtNand D. H. Davies,Z electrochem. Soc. 128, 33 (1981). 21. Z. S. SMIALOWSr~,Corrosion 27,223 (1971). 22. L. S. DARKEN, R. W. GURRYand M. B. BEVERin Physical Chemistry of Metals, p. 440. McGraw-Hill,

New York (1953).