2823
ANODICFILMGROWTH ON IRON IN NEUTRAL SOLUTION
Ellipsometric Investigations of Anodic Film Growth. on Iron in Neutral Solution. The Prepassive Film
by H. Wroblowa, V. Brusic, and J. O’M. Bockris* T h e Electrochemistry Laboratory, The University of Pennsyleania, Philadelphia, Pennsylvania (Received December 31, 1070)
1.9104
Publication costs assisted by the Ofice of Saline Water, Department of the Interior
Transient ellipsometry has been applied to examine the mechanism of the electrolytic growth of the prepassive film on iron (borate buffer, neutral solution) prior to its passivation. The metal was clamped a t a series of potentials in the so-called active region and the film growth observed a t constant potential. Correspondingly, a series of constant currents has been suddenly applied to the electrode and the film growth observed under these conditions. In a region in which no change of film constitution was indicated, the ellipsometric parameter A is proportional to the thickness of the film. Thickness a t constant potential is linearly proportional to time of growth up to about 0.3 of a monolayer; thereafter it is proportional t o log t . Film growth commences less than 0.01 sec after switching on the current. Stirring does not affect the rate of growth. At constant time the growth rate is independent of the potential. Comparison of the form of the galvanostatic transients in the prepassive region with that in the passive range indicates that the prepassive film is ferrous; it is not a n adsorbed ionic layer; it is Fe(OH)2. At the passivation peak, it is 1-2 monolayers in thickness. The suggested mechanism is: discrete centers grow two-dimensionally (rotation of screw dislocation) up to 30% coverage. Coverage (“thickness”) increases proportionally with time a t constant potential. At higher coverages, the growth involves a rapid place exchange step with a rate-determining Temkin discharge of OHonto sites in which the Fe is already attached to an OH, displaced into the first layer beneath the surface. These mechanisms are unique in yielding the observed growth laws. Quantitative consistency is fair.
I. Introduction Examination of the mechanism of passive-layer formation has been radically changed by the introduction of the transient ellipsometry, applied in the region above the current peak. However, such information, even when supplemented by electrochemical and other means (e.g., Mossbauer spectra) would lead to little appreciation of the film growing before the peak of the current. Is it a two-dimensional phase oxide, or does it consist of an increasing adsorption of 02-or OHions or other anions? By what growth laws does the coverage of the electrode increase? What mechanism is consistent with such laws? Answers t o these questions are almost the answer to the question: What is the mechanism of passivation? Earlier, the information was difficult to obtain for the submonolayer quantities involved. Sensitive (and transient) ellipsometry allows a reasonably sure analysis.
11. Experimental Section Experimental technique has been discussed elsewhere,l-e and also is described in the proceedings of two symposiums on e l l i p s ~ m e t r y . ~ * ~ Ellipsometer and Ellipsometry. The instrument (Rudolph-Sons Inst.) was modified: A, the phase retardation, and $, the amplitude diminuation, could be determined (a) by manual rotation of polarizer and analyzer (steady state) and (b) by a continuous re-
cording of the intensity transients for 20.01 sec, as described elsewhere3 (transients). The experimental A and ir/ values were compared with A and ir/ calculated for the known X (5461 8))n,,1(1.338), angle of incidence (68.05), T L F ~and kF0 and different values of the film thickness, L, nF, and k ~ these ; last three were assumed to vary within reasonable limits in a way similar to the manner described in ref 5 . Such a comparison leads to (a) evaluation of the minimal possible thickness at each potential, (b) regions of possible values for TLF, kF, and LF,and (c) if one of the unknown parameters, e.g., thickness, was determined separately ( i e . , by coulometry, see below), values of nF and k~ could be evaluated. Electrochemical Cell and Electrodes. The three-elec(1) V. Brusio, Thesis, University of Pennsylvania, 1971. (2) J. O’M. Bockris, M. A. Genshaw, V. Brusic, and H. Wroblowa, Symposium on Passivation, Cambridge, England, 1970. (3) V. Brusic, M. A. Genshaw, and B. D. Cahan, J . A p p l . Opt., 9, 1634 (1970). (4) A. K . N. Reddy, M. A. Genshaw, and J. O’M. Bockris, J . Chem. Phys., 48, 671 (1968). ( 5 ) J. O’M. Bockris, A. K. N. Reddy, and B. Rao, J . Electrochem. Soc., 113, 1133 (1966).
(6) W. Paik, M. A. Genshaw, and J. O’M. Bockris, J . Phys. Chem., 74, 4266 (1970). (7) E. Passaglia, R. Stromberg, and J. Kruger, iVat. Bur. Std. (U.8.) Spec. Publ., No. 256 (1964).
(8) “Proceedings of the Symposium on Recent Developments in Ellipsometry,” N. M. Bashara, A. B. Buckman, and A. C. Hall, Ed., Korth-Holland Publishing Co., Amsterdam, 1969.
The Journal of Physical Chemistry, Vol. 76, No. 18, 1071
H. WROBLOWA, V. BRUSIC,AKD J. O’M. BOCKRIS
2824 trode cell was air-tight, with Teflon and Kel-F body, quartz windows, and removable top and bottom. A Pd bead saturated with hydrogen in the same solution, and Pt or Pd foil, separated by a frit, were used as the reference and counter electrode, respectively (all of the potentials are referred to nh scale). The working electrode, a polycrystalline iron rod, 99.998yG,was embedded into the bottom of the cell, mechanically polished, washed, assembled into the rest of the cell, and positioned at the ellipsometric table. Solution. Boric acid and borax were used to prepare solutions of pH 8.5 and 7.6, respectively; the solution was prepared in a closed system, under nitrogen atmosphere, preelectrolyzed for 17-20 hr prior to experiment, and introduced into the cell via air-tight connections. Optical References State. This was obtained by potentiostatic reduction at -740 mV, where A and $ were determined. Potentiostatic Oxidation and/or Reduction. This was carried out by a fast, single-step application of a preset desired potential; A (or #) and Q (mC/cmz) were continuously recorded. After a “steady state” was reached, A, #, and the current were determined and the electrode was reduced. GalvanostaticOxidation and/or Reduction. By means of a mercury relay switch, the potentiostat was disconnected and a preset current applied. The oxidation current was varied from 8 pA t o 4 mA/cm2. The reduction current was 30 pA/cm2. The A (or #) values and potential were continuously recorded. The Number of Millicoulombs Spent in Oxidation and Reduction. Q was continuously recorded in potentiostatic work. It was calculated from the A-time or potential-time curves in the case of galvanostatic oxidation and reduction.
-do0 -200
A
A00 400
LO
LOO
I
LOO
Pot8nliaI rnV.vs NHE
-2
+
Figure 1. Variation of A and with potential in steady-state (three experiments marked A, +; 0; and 0 ) and transient ( X , 0.8; 0, 0.32; and V, 0.08 mA/cm2) oxidation; i-V relationship is obtained in steady-state oxidation.
/ Solution was not stirred
2I
I
i
-600 -400 -200 0
I
I
200 400
I
I
600 800
I
,
1000
Potentiol m\( vs.N.H E .
Figure 2 . Q (mC/cmz) as a function of oxidation potential, obtained during oxidation (0),reduction ( X , V), and corrected for Hz (V).l
111. Results Initial reduction of the iron electrode leads to nFe = 3.24 and JCFe = 3.98, close to the values obtained on hydrogen reduced iron. 9,10 Potentiostatic Oxidation (Steady State). A and # measured as a function of the potential are given in Figure 1. The electrode was reduced (potentiostatically or galvanostatically) and only if d(A,#),,x = d(A,#),,d was it assumed that the change of optical parameters is a function of the formation and removal of a film. (Roughening is discussed elsewhere.’l) The millicoulombs obtained in the reduction are given in Figure 2. It was found that A (and, also the quantity, Q) increases linearly with change of the potential in the active region and at constant current density. It begins to change from the value for the reference state at -580 mV. The # value does not start to change until the potential of the peak of the current has been reached (see Figure 1). Correspondingly, a plot of the parameThe Journal of Physical Chemistry, V o l . 76, N o s 18, 1971
ter A, against the corresponding # for a series of potentials (Figure 3), shows three regions, with respect to slope. It is reasonable to assume that constancy of slope and the linear increase of Q mean constancy of n and k , i.e., the presence, during the potential interval in which the slope is constant, of a single species. Then, in such a yegion, the change of A will be proportional only to the change of filmthickness. On this basis, the continuously recorded plots of A values as a function of time are equivalent to thickness-time relations, and it is on this basis that the present approach to film growth is based. (See Figures 4-6.) Figures 5 and 6 give the following important relations (9) A. B. Winterbottom, “Optical Studies of Metal Surfaces,” Kgl. Nor. Vidensk. Selsk. Skr., 1 (1955). (10) H.T.Yolken and J. Kruger, J . O p t . Soc. A m e r . , 55, 842 (1965). (11) V . Brusic, M . A. Genshaw, and J. O’M. Bockris, unpublished work.
ANODICFILMGROWTHON IRON IN NEUTRAL SOLUTION
2825
L, = At (0.01 < t < 0.1 sec) with A = 8
L,
=
B
+ C log t
1.0
(0.05 < t
< 1200 sec)
for the potential of -540 and -490 mV with C = 1.35 8, independent of potential. Galvanostatic Oxidation Transients. Continuous recording of A (or +) and potential at constant current density for a variety of values of this parameter results in a typical curve as given in Figure 7 or 8. Several important features should be noted: (a) the influence of stirring is negligible (Figure 7 ); (b) the change of A starts immediately, iae*1within Oaol set> after the current has been switched On; ('1 when the A and relations as a function of time at constant current density are replotted as A and vs. potential (Figure l), one obtains similar relations as those observed in potentiostatic steady state. There is, however, a noticeable tendency, particularly clearly indicated in the
+
L (A)
15 4 2
002
004 a06
012
008 0 1 (secl
014 016
018
time
Figure 3. Variation of 1, ( X ), i (mA/cm2, O), and Q (mC/cm2, 0 ) during the initial of film growth at -490 m~ and variation of I, ( 0 )at -540 mV, nhe in unstirred solution.
+
8 7
-2
0 log t bet)
-I
I
2
Figure 6. L-log t variation during potentiostatic oxidation at ( 0 )- 540 and ( X ) -490 mV in unstirred solution and at - 540 mV in stirred solution (V).
\
F.
Figure 3. A+ relationship with increasing potential (three different experiments). 4
I
I
I
I
2
I
I
I
3
4
S
I I
lime (see)
Figure 7. Variation of A, IC., and potential with time of galvanostatic oxidation in unstirred (solid lines) arid stirred (dashed lines) solutions; i = 0.8 mA/cm2.
third region, of shifting the A line towards higher A (without change in slope) as the i decreases, Le., the time of oxidation increases. The latter observation will be discussed separately.12
! I 100
200 500
400 500 600 703
800
time (sec)
Figure 4. Increase of film thickness with time of potentiostatic oxidation at -490 mV, nhe.
900
1000
(12) H. Wroblowa, V. Brusic, and J. O'M. Bockris, unpublished work. The Journal of Phusical Chemistry, Vol. 7.5, IYO.18, 1971
H. WROBLOWA, V. BRUSIC, AND J. O’M. BOCKRIS
2826
Figure 8. Example of intensity (A) and potential change during the anodic oxidation with 0.16 mA/cm* in the solution of pH 7.6.
I
I15
117
I
I
1
IB
dAk)
I
121
l
l
,
123
Figure 10. Variation of the possible film thickness, n, and k of the film with potential (steady state, no stirring). The thick line refers to the minimum possible thickness and corresponding n and k.
I
125
+
Figure 9. Variation of A with during the galvariostatic oxidation (0)and subsequent potentiostatic reduction ( X ).
Also, if the galvanostatic oxidation is followed by potentiostatic reduction, basically the same A-+ curve is retraced (Figure 9).
I
-600 -400 -200 0 ,
I
200 400 600
Potentiol mV vs N H E
Figure 11. Variation of L, n, and k during the passivation of iron (steady state, no stirring): thick line, L (n and k ) obtained with &oath, corrected for Hzl; thin line, L (n and k) were obtained with &oath, average of &eath total, and &oath, corrected for Hz.
IV. Discussion 1. Film Composition. Ellipsometrically, a minimum possible film thickness was determined, as well as the regions of possible values for L, n, and k of the film (Figure 10). Using the coulometric results (Figure 2) n, k , and L of the film were determined (Figure 11) as a function of potential. Figure 11 gives the probable composition of the film which was deduced as follows. (a) The slope of the A+ (varying potential) and A-time curves (constant current) (Figures 1, 3, and 7) indicate that for film formation in the passive region, there are two characteristic regions of the ellipsometric plots [A+ at a series of potentials and current densities (Figures 1 and 3), and A-t at constant current (Figure 7)]. Hence in the passive region, film formation from the metal passes through more than one phase, e.g., through a ferrous on the way to a ferric state. The Journal of Physical Chemistry, Vol. 76, N o . 18, 1971
(b) I n A-t and Q-t relations, obtained during reduction at constant current density of the passive film, there are always two regions.*r2 When one calculates the coulombs in these regions, it is found that the ratio of Q / Q 2 is close to 1:2. This is regarded as evidence which supports that the passive film is a ferric film, and its reduction passes through the stages: Fea+ Fe2+ -+ Fe. (0) I n the prepassive region (below the peak) the galvanostatic transients do not show the two regions, but only one. Calculation of the relation of Q1 to Qz in the transition region around and above the peak shows QL/QZ is 1:x where x > 2 indicating that the film here is compared with both ferric and ferrous compounds. (d) The film cannot be an adsorbed layer, for the A values obtained during its formation (in the region be-+
ANODICFILMGROWTH ON IRON IN NEUTRAL SOLUTION low the peak) are substantially greater than those found by Genshaw and Chiu,13 or by Paik, et al.,6 in their study of adsorbed ionic layers. The prepassive film is hence a ferrous phase oxide. (e) A ferrous phase oxide suggests Fe(OH)2 or FeO. If FeO is assumed, the expected film thickness from coulometry is below the minimal film thickness from ellipsometry. (Assuming that a reasonable range in values of n and k were utilized to interpret the ellipsometric data independently of coulometry.) Hence, before the current peak in the passivation of Fe, a phase oxide, Fe(OH)2,is formed. 2. The Thickness of the Phase Oxide at the Current Peak. (a) From Ellipsometry. Analysis of purely ellipsometric results indicates the region of possible values of L of 4 to 16 A (Figure 10). (b) From Cathodic Coulometry. About 0.7 mC/cm2 after correction for the corresponding deposition of hydrogen.' The film thickness was calculated with the assumptions shown in Table I. The variation of film thickness Table I : Evaluation of the Film Thickness a t E = E , ----Experimentally
ness factor
Moles Fez +/cmz (calcd from experimental Q)
determined 0.7-1.2 mC/cmn--N o layers of HOFeOH Layers of formed, if Fe(0H)z N o layers of each Fe formed HOFeOH formed in metal ( p = 3.41) (ionlc radii) reacts
1 1.2 1.4
3.63-6.2 3.25-5.1 2.6-4.5
2.1-3.6 1.7-2.9 1.5-2.5
Rough-
1.6-2.8 1.2-1.9 0.9-1.5
1.3-2.26 1-1.6 0.7-1.2
with potential is linear if it is assumed that the Fe(OH)2 has a bulk density. One finds, for RF (roughness factor) = 1 and using a lower limit for Q (Figure 6, 11) dL/dV = 4 8 i / V ;
dL/dlogt
=
1.35 A
3. The Growth Mechanism. The A and potentialtime relationships obtained during the galvanostatic transients do not indicate an induction time or an influence of stirring. Hence, direct formation by discharge, not dissolution-precipitation, is indicated. Tbe linear increase of the thickness in the region of 0-1 A may be a result of (i) increasing electrode coverage by OH-, Le., increasing adsorption; or (ii) formation of discrete centers (nuclei). Initial Stages. Increasing Coverage with OH-. Assuming that buildup of coverage proceeds according to
+
+
Fe OH-,,l +FeOH,d, e(1) the current can be divided into the part spent in dissolution (il)and the part spent in film formation (iz). The total current (is) will depend on the fraction, e, covered Le.
2827 (2) Then de - = iz(idt
e)
(3)
A thickness of 0.5 to 1 8 corresponds to an OH- coverage of 17 to 35.8% (Lmonolayer = 2 r ( 0 2 - = o H - ) =2 x 1.4 8). According to eq 3, the reaction rate (de/dt = dL/dt) should decrease as coverage increases, which is contrary to observation (Figure 5 ) . Growth of Discrete Centers. Experimental results show a linearity between the thickness and time, i.e., dL/dt (proportional to it) is constant, independent of time. Qualitative similarity between experiment and theory can thus be obtained only if the increase of thickness is due to one-dimensional growth of the centers formed by simultaneous nucleation as expected from
it
=
2FkStNo
(4)
where N o is the number of the growth sites at t = O D , IC is the rate constant for the growth of the nuclei, and St is the area of growing nucleus. ( a ) One-Dimensional Growth, Outward f r o m the Electrode. The increase of the average thickness with time can be due to instantaneous formation of a small number of nuclei (LtE0.01= 0) and their growth outward from the electrode. Such nuclei would have to grow to a relatively large height to account for the observed variation of thickness. Physically the model would have to break down because it cannot give a consistent answer to the questions: (i) What is the origin of the Fez+ for the outer layers of the nuclei? (ii) Why does the mechanism of film formation change when a certain (average) thickness is reached? ( b ) One-Dimensional Growth on the Electrode Surface. Suppose that a number of small crystallites one monolayer thick have been formed on the electrode surface, along steps or screw dislocations. Then, onedimensional growth of the nucleus could be followed by the moving of the edge of the step along the surface and/or around the point where screw dislocation emerges. The observed increase of thickness would be proportional to the size of the original screw dislocation and the total number of these active sites. After the Burgers vector has rotated once around the dislocation, the surface on which a further rotation could give rise to a second layer is in fact now an oxide and thus cannot give a supply of Fez+ for further surface diffusion to the growth sites on the crystal edge. Hence, no further growth by this mechanism can occur. The expected concentration of dislocations on a mechanically polished surface is 10" to 10l2 sites cm-2. (13) y. Ch. Chiu and M. A. Genshaw, J . Phyu. Chem., 7 3 , 3571 (1969). The Journal of Physical Chemistry, Vol. 76, N o . 18,1971
H. WROBLOWA, V. BRUSIC, AND J. O’M. BOCKRIS
2828 Let a small portion,14 say 1%, of these be active. At the end of the linear growth law the average thickness is 1 8, and the radius of a screw dislocation is assumed t o be cm. Thus, the area per patch is T ( ~ O - ~ ) and the total volume 109hn(10-5)2 = l o M xcm3, with h = 3.3 A, a reasonable magnitude based on the unit cell of Fe(OH)2. The surface area covered with film will be ( N r 2 n ) , 3001,. (If nucleation starts on, say lo8actiye sites and if the height of the nucleus is, say, 4.47 A (c axis in Fe(OH)z), r becomes 2.6 X cm, and the electrode is 22% covered.) Thus, 2 2 4 0 % is the maximal coverag,e of the model, which gives the average thickness of 1 A, and a minimal height of a nucleus as 3.3-4.47 8. T h i s mechanism gives a reaction rate independent of time, as it is based on one-dimensional growth of the nuclei, and also allows for the change in the film growth mechanism at longer times, after a further rotation could give rise to a second layer which would be in fact now an oxide. Later Xtate of Prepassive F i l m Growth, 0.06 (0.1) < t < 700 (1,900) Sec. Starting at film thicknesses below monomolecular, L-t becomes a logarithmic (Figure 6) i.e.
L, = B l n t
+c
(5)
Is the further increase of thickness due to twodimensional or three-dimensional growth? The steady-state i-V curve for iron dissolution was calculated and the current compared with the observed one: io for iron dissolution was estimated from the work of Bockris, Drazic, and Despic16 to be 1.65 X lo-’ at a p H of 8.5 and taking CW+ as mol/l. It was assumed that CYanodic equals 3/2, i.e., that the same dissolution mechanism applies as observed in p H range 1-516 and similar to that postulated for alkaline solutions by Mabanov, Burstein, and Frumkin.I6 Thus, comparison of the current for iron dissolution and that experimentally observed indicated a decrease of the free surface area to a small percentage of the value from B = 0.3 during film growth, which is, therefore, twodimensional. Similarly, the coulombs recorded in the first 100 sec is 0.7 mC/cm2, equivalent to 5.1 A of the initial film (or about a monolayer). The mechanism for film formation must give the observed relationship between L, t, and potential, Le. L,
=
F(V)
+ k In t
(6)
+ f(t)
(7)
i-lr curve as given in Figure 1 is preferentially determined by i d i a w l u t i o n . ] Several mechanisms are consistent with an L-log t ~ relation. (a) A Rate-Determining Chemisorption Step. I n Temkin kinetics this gives an expression for the reaction rate in agreement with experiment. However, the ellipsometry indicates a phase oxide. ( b ) Place Exchange Mechanism. Lanyon and Trapnell” suggested that the initial oxidation of a metal may be determined by interchange of adsorbed oxygen with underlying metal atoms. Bare metal atoms are exposed to the surface, made available for adsorption. I n the beginning, (8 = 0) interchange will take place readily; toward the end (8 --t 1) interchange involves the parallel alignment of dipoles and will take place with difficulty, i.e., the activation energy increases with the amount of material reacted. It is this feature which gives rise to a logarithmic growth law. However, the growth of the film is lateral, hence may occur via a vacancy migration along the surface. If so, the increase of the energy of activation .Ir-ithincreasing coverage, expected in the place exchange theory, cannot be understood. (c) A Consistent Mechanism f o r the Prepassive F i l m Growth, at t > 0.1 Sec. The following mechanism is suggested
+ OH- I_ FeOH,d, + e(or Fe + HzO 2 FeOH,d, + e- + H+) Fe
(8)
(8a)
with the fast place exchange FeOH,d,
If HOFe
(9)
rds
HOFe
+ OH- +HOFeOH,d, + e-
(10)
rds
(or HOFe
+ HzO + HOFeOH,d,
+ e- + H+)
(loa)
followed by HOFeOH,d,
Fe(OH)z
(11)
Assuming that the heat of adsorption of OH on iron decreases linearly with coverage, the rate equation for the rate-determining step (rds) under Temkin conditions is
and L , = AV
Thus, we are looking for the mechanism which will give the reaction rate d L / d t , , , constant, i.e., the steadystate current of f i l m formation will not depend on potential. [From (dL/dt)t=,OOaec= 0.001 @set (Figure 4) if steady state equals A/cm2; shov;ing that the The Journal of Physical Chenzistry, Vol. 75, N o . 18, 1971
(14) H. Kita, M . Enyo, and J. O’M. Bockris, Can. J . Chem., 39, 1670 (1961). (15) J. O’M. Bockris, D . Drazic, and A . R. Despic, Electrochim. Acta, 4, 326 (1961). (16) B. N. Kabanov, R. Burstein, and A. Frumkin, Discuss. Faraday Soc., 1, 259 (1947). (17) A. H. Lanyon and B. M. W. Trapnell, Proc. Roy. Soc., Ser. A , 227, 387 (1955).
ANODICFILMGROWTH ON IRON IN NEUTRAL SOLUTION
2829 and one expects
From (9) OHOFe
Jr
(13)
kZoFeOH
(2.3)(2)RT 3r
and from (8) 6FeOH
= klcOH-(l
-
0T)e
- f (BT)/RTeF V / R T
(14)
and
It was assumed that the first electron transfer is not a rate-determining step. The latter is in agreement with kinetics of iron dissolution observed in acid15 and suggested for alkali,16which may have similar first reaction in agreement with experiment. In order to compare steps as the initial film f o r m a t i ~ n .Once ~ ~ ~the ~ ~ ~ these ~ ~ predictions of the model with experiment, the first step has occurred, and being exothermic, it liberates variations of 6Fe(OH)* = f?T are expressed as a function of energy, utilized in affecting place exchange; the latter film thickness, which are experimentally known. step may be easy as incorporation may be helped by If RF = 1, @Fe(OH)g = 1 when L = 4.5 8. Hence existing defects at the surface. Fast place exchange dL would offer a new reaction site, i e . , new iron ions available for further slow adsorption with a second electron transfer being assumed to be a rate-determining (4.5) (2.3)(2)RT step. Combining (12-14) gives (22) 3r = ]clkzk3CoH-2(1 - &)e- W ~ ( B T ) / R T ~ V F V / R T (15) dt
5
If 0.2 < f?T < 0.8, one can neglect the preexponential (1 - 6,) term. Also, OT = O F ~ ( O H ) ~ OFeOH. Since FeOH is consumed not only in the film formation, but also in a parallel reaction of iron dissolution
+
FeOH,d,
--f
FeOH+
+ e-
(16)
and Fe(OH)2 is accumulating at the surface, it is assumed that 6 F e O H
