TECHNICAL PAPERS - ELECTROCHEMICAL SCIENCE AND TECHNOLOGY The Secondary Passive Film for Type 304 Stainless Steel in 0.5 M H 2SO4 A. Atrens The Department of Mining, Minerals, and Materials Engineering, The University of Queensland, Brisbane, Queensland, Australia 4072 B. Baroux* and M. Mantel Ugine Research Centre, F-73400 Ugine, France ABSTRACT An examination has been carried out of the secondary passive film on Type 304 stainless steel in 0.5 M HSO4. The characterization techniques used were electrochemical (potentiodynamic, potentiostatic, and film reduction experiments) and surface analytical. A bilayer model for the secondary passive film is proposed. It appears that next to the metal, there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied potential. On top of this modified passive film, the experimental data are consistent with a "porous" corrosion-product film which adds to the total film thickness but has little influence on the electrochemical response. The composition of the secondary passive film corresponds most probably to a mixed Fe/Cr oxide/hydroxide enriched in Cr3 +, with a composition similar to a primary passive film. Introduction The aim of this study was to examine the secondary passive film on Type 304 stainless steel in 0.5 M HSO4 in the secondary passivity potential region, typically at potentials more positive than about 940 mVscE. The primary interest was in the characterization of the secondary passive film as it might be used in a technological application; that is after the film had been formed, dried, and exposed to air. Also of interest were the insights that could be gained concerning the secondary passive film by a study of film formation. The literature suggests three different possibilities for surface films in the secondary passive potential range: (i) a modified continuous passive film, (ii) a porous film, and (iii) a hydroxide gel. Possibility (i) derives from the passivity literature.'-" Possibilities (ii) and (iii) derive from the literature on colored stainless steels.35 43 The (primary) passive film on stainless steels has a complex structure'- 3 but can be thought of as a very thin (-1 to 3 nm) film composed of metal oxides/hydroxides and bound water. For stainless steels, the Cr enrichment in the bulk of the passive film is evidenced by a Cr°X/(FeOX + Cr °X) ratio significantly higher than the ratio Crm/(Fem + Crt) characteristic of the alloy 124'11 -13'16-22 There is further enrichment of Cr in the oxidized state, Cr° X, at the film/solution interface for films formed in acid conditions.l-.'3 The passive film grows on the metal surface by an electrochemical mechanism, covers the metal surface as a continuous layer, and inhibits corrosion by forming a barrier between the corroding metal and the environment. The film has a selfrepair property (under conditions such that the passive film is stable) because the film formation rate is highest on a film-free surface (as at a break in the film) and the film growth rate decreases rapidly with film thickness.4 '0 Thus, * Electrochemical Society Active Member. the film can be thought of as having a steady-state thickness, although attainment of the steady-state thickness can take up to 100 h in acid solutions.4' 0 Both Clayton and Olefjord2 and Macdonald 4 2 2- 6 propose a bilayer structure for the primary passive film consisting of an inner barrier layer (consisting mostly of Cr2 O, for stainless steels') and an outer precipitate layer. This composite structure has been postulated by Sato 28 '29(see also Ref. 30, 31) to have ion selective properties which can be important in the formation stages of the passive film. Furthermore, ions from the solution can be incorporated in the outer precipitate layer 22 23' and provide increased protection against passivity breakdown induced by chloride ions and consequent pitting corrosion. Sugimoto and Matsudal s analyzed passive and secondary passive films using ellipsometry for films formed on FeCr, alloys in NaSO at pH 2.0 and 6.0. For Fe-Cr,0,, the passive film thickness increased linearly over the whole potential region from 0.4 to 1.3 VSCE; this behavior is similar to that for pure Fe measured by Sato et al. 4 For FeCr. (x = 0.15 and 0.20) the primary passive films increased linearly in thickness with increasing potential in the range 200 to 800 mVsce (consistent with the other passivity literature), there was then a transition region in which the film thickness was approximately constant, and then the secondary passive films increased linearly in thickness with increasing potential in the range 1000 to 1400 mVsc. The secondary passive films with average film thicknesses of 2.5 to 3.0 nm were somewhat thicker than the primary passive films (with average thicknesses 1.5 to 2.0 nm),15 and they had somewhat different optical constants, indicating that there were differences between the passive and secondary passive films. Similar trends were measured by Song et al.3 using ellipsometry for type 304 stainless steel in 0.5 M H2SO 4 at ambient temperature. Song et al." using x-ray photoelectron spectroscopy (XPS), found high levels J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. 3697 3698 J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. of Cr 3+ and some Cr6+ in secondary passive films formed in 0.5 M Na2 SO4 . Kirchheim et al.4 used XPS and Ar sputtering to measure concentration profiles for passive and secondary passive films formed on Fe-1OCr in 0.5 M H2 SO4 . The profiles of Cr°X/(Fe° x + Crox) were quite similar for both types of films with the secondary passive film (2.0 nm) being slightly thicker than the primary passive film (1.8 nm). Both were enriched in CrOx in the bulk of the film -1 NIHq -... · t Tt -I ub- .0 N! I Experimental Type 304 stainless steel was used having the following chemical composition: 17.4% Cr, 8.4% Ni, 0.17% Mo, 1.4% Mn, 0.49% Si, 0.059% C, 0.15% Cu, 0.12% Co, 0.05% V, 0.013% Sn, 0.024% P, 0.0015% S, 0.05% N, 40 ppm 0. This was received from the mill in the bright annealed condition'(in-line continuous sheet anneal in a hydrogen atmosphere for about 5 min at 1000°C followed by rapid cooling in the same atmosphere) which ensures that all species are in solid solution and produce a bright stainless surface. The standard specimen preparation was a wet metallurgical polish to 1200 grit with silicon carbide, gentle air dry and exposure to laboratory air (of ambient humidity) for about a day (i.e., exposure to laboratory air for at least 8 h). The electrochemical apparatus' is shown in Fig. 1; this allowed the specimen to be introduced into the specimen cell and the specimen surface to be first deaerated by an equal mixture of H2 and N2, while this gas mixture was used to deaerate the electrolyte for 1 h in the deaeration a The electrochemical apparatus is one developed for studies of passivity and pitting corrosion. In studies of pitting corrosion, crevice corrosion at the specimen edges is always a concern and often a practical problem that masks the pitting corrosion that is being studied. This problem has been overcome4 in a number4 74of ways. One solution is that adopted by Baroux" and Atrens " where very careful attention is paid to deaeration of the sample surface and the deaeration of the test solution. This same approach was used in the present study. After the specimen is introduced into the specimen cell, the specimen is exposed to a flowing gas mixture of equal parts H and N2 . This gas flow removes oxygen from the specimen cell, removes (lightly bound) oxygen from the specimen surface and also removes oxygen from crevices at the specimen edges. Subsequently deaerated electrolyte is introduced into the specimen cell. For pitting studies, the procedure is useful in preventing crevice corrosion. If a less rigorous procedure is used, then there could be some oxygen trapped in crevices at the specimen edge and this could cause initiation of crevice corrosion. In our studies, removal of all oxygen was also a consideration to ensure low values of residual current in the reduction experiments. Our procedure was successful in producing low residual reduction values as indicated by the i values in Table III. ->N,fU, _ ,I.i Lab 'e aI i % ~1.i I-I .i to similar levels with Cr°X/(Fe ° x + CroX) - 0.4. The thick- ness of the CrOX region was somewhat greater in the secondary passive film. The primary passive film was further enriched in Crox at the very surface as discussed above. Coloring of stainless steel was developed 3 5 as a chemical anodization process. This was modeled 3 6-40 as due to the chemical precipitation of Fe(OH)3 and Cr2 (SO4 )3, 18H2 0 when their solubilities were exceeded locally at the metal surface. In contrast to the production of the passive film by electrochemical oxidation, these corrosion products were assumed to be produced by a chemical precipitation process. Porous corrosion products were assumed which did not influence the corrosion process, and the electrochemical response was modeled on the basis of the diffusion of ionic species in the liquid phase of the pores in the film. In this context it is worthwhile considering the properties of a hydroxide gel. Metal hydroxides can precipitate from solution to form a gel; there forms a skeletal threedimensional network of molecules throughout the medium giving the medium solidlike rigidity although these molecules (i.e., the solid) form a small fraction (-10 to 30%) of the total volume. Electrochemical corrosion is as for direct contact of the metal with liquid, as there remains a direct conduction path through a liquid phase of the gel. In the present study, secondary passive film formation was characterized by measuring potentiodynamic polarization curves and by carrying out potentiostatic experiments. Film characterization after film formation was carried out by electrochemical reduction and XPS. ?tfirWfoAg1A& swa [email protected] 4 W d(t) 1~- - ki J-I% l.- ,, ssamce ! ve fK Fig. 1. Electrochemical cell. cell. All solutions were made using analytical grade reactants and deionized water. The deaerated electrolyte was then introduced into the specimen cell without exposure to air, and electrochemical measurements were started a period of 2 to 4 min after electrolyte contact was established with the specimen. Prior experiments have determined that the oxygen concentration in the solution is around 10 ppb under these conditions. The counterelectrode was a platinum sheet. The reference electrode was Ag/AgCl. The potentials are reported with respect to a saturated calomel electrode to allow easier comparison with the literature. The temperature was maintained at 23°C by the circulation of the thermostated water through the outer water jacket of both the specimen chamber and the deaeration cell. This electrochemical apparatus was used for (i) characterization of film formation by measuring potentiodynamic polarization curves and by carrying out potentiostatic experiments, and (ii) film characterization after electrochemical film formation by electrochemical reduction. A rapid overview of the electrochemical behavior relevant to film formation was obtained by measuring the potentiodynamic polarization curve, in 0.5 M H2 SO4 . For comparison polarization curves were also measured in 5 M and 6 M H2 SO4 ; a borate buffer, 0.15 M sodium borate (Na2 B4 O, · 10 HO) + 0.15 M boric acid (H3BO3 ) at pH 8.4; and in a phosphate solution (4.7% Zn3(PO,) 2 + 9.3% H3PO 4 which had a pH of 1.47). All polarization curves were measured starting from a cathodic potential (-800 mVscE) at the rate of 10 mV/min. Details of the electrochemical behavior during film formation were obtained from potentiostatic experiments. In these experiments, a specimen was prepared in the standard manner. Its surface was deaerated. It was exposed for a few minutes to the deaerated 0.5 M H2SO 4 at the free corrosion potential (and passive film was formed on the specimen surface as shown by the potentiodynamic experiments). Subsequently, the potential was "instantaneously" applied at the value of interest, and the current recorded as a function of time. The secondary passive film after electrochemical film formation was characterized by electrochemical reduction at -350 mVscE in fresh deaerated 0.5 M H 2SO,. The reduction conditions were chosen to minimize contributions from other reduction reactions, including reduction of species in solution and also hydrogen evolution. b The potential value of -350 mVs,, was chosen because of the low current densities measured in the polarization curves in 0.5 M H2SO4 . For the reduction experiments reported in this paper a secondary passive film was first formed by holding a new specimen prepared in the standard manner at constant potential in deaerated 0.5 M H2SO4 while the current was recorded. The solution was drained from the specimen cell. The specimen, in the specimen holder, was removed from the specimen cell, washed in distilled water, dried in a jet of clean air, and reintroduced into the specimen cell where the surface of the specimen was deaerated b In a series of preliminary experiments, reduction was carried out immediately after film formation in the same solution. The experimental results were similar to those reported herein, although the background current densities were somewhat higher than those measured in the experiments reported in this paper. J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. by the N2 /H, gas mixture for about 1 h. During this hour, a new 0.5 M H2SO, solution was deaerated in the deaeration cell. After the new deaerated solution had been introduced into the specimen cell by means of the deaeration gas pressure, the potential of -350 mVscE was applied and the current recorded as a function of time. For each reduction experiment, there was a new specimen and a new deaerated solution. Some experiments were carried out to study the stability of the secondary passive film. These had the same procedure except that, after film formation, the specimen was removed from the specimen holder and subjected to an ultrasonic cleaning bath in an acetone/alcohol mixture. Subsequently, the specimen was washed in distilled water, reintroduced into the specimen cell, its surface was deaerated, and the surface film was characterized by reduction in a new deaerated 0.5 M HSO 2 4 solution. Film characterization after film formation was also carried out using XPS. XPS measurements were carried out to characterize films formed for (i) 200 s and (ii) 30 min at 1200 mVscE in deaerated 0.5 M HSO 4 (followed in each case by washing in distilled water and drying); these were compared with (iii) a passive film on a specimen after the standard specimen preparation (wet abrasion with 1200 grit, dried in air, and at least 8 h exposure to laboratory air), and (iv) the residual film on a specimen with secondary passive film formed for 30 min at 1200 mVsc in deaerated 0.5 M HSO,, reduced in a new deaerated 0.5 M HSO 2 4 solution at -350 mVscE, washed in distilled water, and dried in air. Because major emphasis was on gaining an understanding of the secondary passive film as it might be relevant in a technological application of the stainless steel, all specimens were exposed to the atmosphere during transfer to the XPS apparatus. This transfer arrangement might cause the loss of fragile surface species like OH and adsorbed water. Furthermore, the fine structure of a modified passive film may revert to that more characteristic of a passive film. Nevertheless, the XPS measurements should provide valid measurements of chemical species and film thickness. It is also worthwhile mentioning that the passive film has been shown to be quite robust and to change surprisingly little with air exposures. 1 XPS analysis was performed using a VG XR3E2 spectrometer employing a Mg K0 (1253.6 eV) monochromatic x-ray source operated at 15 kV with a power of 300 W. Typical operating pressures were -30 nPa (2 10 0 Torr). The spectrometer was calibrated using the following photopeaks: Au 4 f7/2 at 83.8 eV, Cu 3p at 75.2 eV, and Cu 2 p 2/3 at 932.7 eV All binding energies were referenced to the carbon C-H photopeak at 285.0 eV. The area analyzed was 10x 4 mm. Survey (wide) scan spectra were measured with a pass energy of 60 eV for all samples to determine what elements were present in the top 5 nm of the surface. Mutiplex (narrow) scan spectra were obtained with a pass energy of 30 eV for all major photopeaks. Angle dependent analysis at 30 and 90 was done by varying the take-off angle between the surface and the direction of electron detection. Detailed scans were carried out for Fe, Cr, 0, C, S, and Si with take-off angles of 300 and 900. Particular attention was devoted to the S measurements, because there was an expectation of sulfate incorporation in the secondary passive film from the literature on colored stainless steels 3-4 3 Potentiodynamic Polarization The polarization curves are presented in Fig. 2. Low currents were measured in the passive range (-200 to -950 mVsCE) in all cases. In 5 M H2S0 4 there was active corrosion followed by passivation. Specimens were selfpassivating in the borate buffer solution, in 0.5 M H2 SO, and in the phosphate solution. This is of significance for the interpretation of the subsequent potentiostatic experiments; these experiments show that there was a passive film on the specimen surface after the standard specimen preparation and exposure for a few minutes to 16.0- - 3699 I -a- 6MH2S04 -- 5M 142S04 borate buffer -- - 0.5MH2S04 -- phosphate 12.0 a r 8.0 0 b .0 4.4.0 W / .0~ [i Si-- /10 0.0 ;ry' _ 7R- .7 No7, -, ! 4 lr b I I I I '- -0.6 -0.2 0.2 0.6 1.0 1.4 I 1.8 potential, VsCE Fig. 2. Potentiodynamic polarization curves to characterize film formation conditions. the borate buffer-solution, to the phosphate solution, and to 0.5 M HSO4 . For 0.5 M HSO 2 4, within the secondary passivity range, the current increased to a plateau of about 4.6 mA/cm 2 at 1400 mV. In the more concentrated solutions, i.e., in5 and 6 M HSO the current densities were in general much 2 4 higher in the secondary passivity region. For the phosphate solution, the current density increased to a plateau of about 2 mA/cm 2 at 1250 mV The curve measured in the borate buffer was different from the other curves in that the current density increased nearly linearly with increasing potential; the "noise" on this curve of potentials above about 1175 mV might be indicative of oxygen evolution which is expected at these potentials in the higher pH values of the borate buffer. The potential E,, at which there was a significant current density increase indicative of the onset of secondary passivity, was similar for all the solutions used, Table I, with an average value of 940 mVscE, and soE, was approximately independent of pH over the range 0 to 8.4. Potentiostatic Experiments Figure 3 presents the current as a function of time measured at 500, 1000, and 1300 mVscE in the potentiostatic experiments. In the passive region (at 500 mVsc), the current decreased steadily with time, whereas in the secondary-passivity region, the current became independent of time after an initial transient; this is consistent with the literature.'," Figure 4 presents the initial shapes of the transients in the secondary-passivity potential range forpotentiostatic experiments at 1100, 1200, and 1300 mVscE. One curve at 1200 mVcE was recorded for 200 s, whereas all the other experiments were of 30 min duration, although Fig. 4 illustrates only the initial parts of the transient where there was significant change. The repeat experiments show excellent reproducibility. The transient at 1100 mVsCE displayed a single minimum similar to that at 1000 mVsCE (see Fig. 3). The transients at 1200 and 1300 mVscE showed a minimum followed by a maximum, which was more pronounced for the curve at 1300 mVscF. The steady-state value of the current, i, was very similar to the current density, i, measured in the potentiodynamic experiments. This is recorded Table . Potential, E, Solution marking the onset of secondary passivity in different solutions. Esp, mVsCE 6 M HSO4 5 M HSO 970 0.5 M H2 SO 4 Phosphate Borate 910 940 960 970 J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. 3700 Table II.Steady-state current density, i,, measured in potentiostatic experiments compared with the current density, id,measured in the potetiodynamic experiments. 10 - j B--- -`^I'----1 Potential E (mV) '.. Current density (mA/cm') id i C)4 [1 1 * 0.1 ©3 r:: ooW 00 I 00 a 001 "0 t) + 12 0,001 :- 10 i * 500 mV 0.0001 time, s Fig. 3. Potentiostatic curves to characterize film formation for type 304 stainless steel in deaerated 0.5 M H2SO 4 at 500, 1000, and 1300 mVscE. in Table II. The speed of the transient response indicated that there is a fairly rapid transformation of the passive film into the secondary passive film (which controls the electrochemical response) with most of the film formation completed within 400 s. Reduction Experiments Typical reduction curves are presented in Fig. 5 for secondary passive films formed for different periods at 1200 mVscE. For secondary passive films, formed under the normal conditions, the reduction curves could be interpreted as being composed of three reduction waves. Table III presents the charge associated with each reduction wave, Q1, Q2, and Q, as defined in Fig. 6. Table III also records values of the film thickness, d,, which has been estimated from" d = Q V,,ox/3F 1400 4.6 5.6 1300 1200 4.0 3.2, 3.3 1100 1000 1.8, 1.8 0.18, 0.16 4.0, 4.1 3.3, 3.4, 3.3, 3.3, 3.1, 3.3 2.0, 1.9 0.32 where 100% current efficiency is assumed during reduction, it is assumed that all the reduction charge Q is associated with M3+; VOX, the oxide molecular volume is taken as Vox = 10 cm3/mol which is the average value for iron oxides.' ° The charge associated with each reduction wave increased with increasing time of film formation, although there was considerable data scatter, for example, as shown by the differences in the two reduction curves for films formed for 30 min at 1200 mV. This data showed that these secondary passive films could withstand drying. However, there was very little charge (0.43 mC/cm2 ) associated with the reduction curve for the film subjected to the acetone/alcohol ultrasonic bath indicating that most of the secondary passive film had not survived this treatment. Low values of residual current, i3 (see Table III) indicated success in choosing reduction conditions that minimized reduction reactions other than film reduction. Measurement of the residual current, i3 due to other reduction reactions, allowed evaluation of the reduction charge associated only with film reduction. A most interesting observation is that the film thickness, as measured by reduction, continued to increase for film formation times greater than 400 s, Fig. 7. In this time period, there was little change in the current transients measured potentiostatically, see Fig. 4, indicating little change in the electrochemical response. It is worth stressing that the film thickness measurements using cathodic reduction are to be considered to underestimate the true film thickness for two reasons. First, it is widely believed that Cr,O, is not reduced under these conditions.' 44'45 This can place considerable limitations on this cathodic reduction technique as a means of measuring the film thickness. Second, the reduction experiments were carried out after the specimen had been washed in distilled water and dried. This specimen handling procedure could remove  o.- I -- -I ; -10.0 -I _- - 45 4.0 ' -20.0 3.5 - 30 min t -30.0 '-x- 3.0 2.5 = 2.i 05 2.0 -40.030 I 0.0 Is< "Gir,0.0 .0.1 U130 mV -+ 1200 mV 0.3 0.5 1100 mV 0.7 t (ks) Fig. 4. Potentiostatic curves to characterize film formation for type 304 stainless steel indeaerated 0.5 M HSO4 at 1100, 1200, and 1300 mVscE. Transients at 1200 mVs,,,, are for specimens held at 1200 mVscE for 200 s and 30 min. Transients at 1300 and 1100 mVscE are for repeated experiments. I 0.5 ,I 1.0 I 2.0 1.5 t(ks) 200 s -l 6min -- 30 min 2.5 3.0 3.5 Fig. 5. Film characterization by electrochemical reduction after formation of the secondary passive film at the stated times at 1200 mV in0.5 M H2SO4. Reduction was carried out at -350 mVscE in a fresh deaerated solution of 0.5 M H2SO 4 . All data are for the standard specimen preparation procedure (which included washing in distilled water and air exposure) except for data identified with a full circle which refer to specimens subjected to ultrasonic cleaning inacetone/alcohol, followed by washing indistilled water and air exposure. J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. 3701 Table III.Reduction charge, Q, Q2, and Q3, associated with each reduction wave as defined in Fig. 8. t t, (min) (s) i2 (gA/cm 2 ) Q5 t2 (mC/cm') (s) 3. 9 6 12 15 25 30 30 40 45 60 60 100 68 18 37 50 360 418 536 13.8 16.1 47 24.9 30 17.0 11.0 7.0 919 0.69 1.07 0.15 0.14 0.59 2.73 2.64 2.66 515 22 5.62 i2 (gzA/cm 2 ) 09 1 358 270 223 400 716 1300 1688 1054 1990 2704 Q2 (mC/cm2 ) 5.4 6.3 8.2 4.7 8.7 6.2 5.0 4.1 5.3 16 loosely adherent material from the surface but could not add material to the surface. XPS In all cases there was a significant amount of C present on the surface as is often observed with XPS analyses, and this has been attributed to contamination. The surface C is characterized in terms of the ratio C/O of carbon to oxygen peak heights, Table IV; the least carbon was observed for specimen (b) (i.e., the secondary passive film formed for 30 min at 1200 mVs,C), with C/O = 0.4 compared with C/O in the range 0.6 to 0.8 for the other samples. This is consistent with a clean surface being produced by the "corrosion" at 1200 mVscE. The major species present in the films were Fe, Cr, and 0; with only trace amounts of the other elements. This data is consistent with the secondary passive films being a mixed Fe/Cr oxide with significant hydroxide and is not consistent with significant incorporation of SO4 . The detailed oxygen spectra were also consistent with this interpretation. The Fe spectra are given in Fig. 8; this shows the largest signal for specimen (b) with similar signals for the other three specimens. The largest Fe signal for specimen (b) correlated with the lowest carbon contamination so a qualitative analysis of the peak heights leads to the conclusion of comparable amounts of Fe signal for all the specimens (a) to (d). In all cases the main signal was associated with an oxide with a binding energy of about 711 eV with some signal associated with the metallic state at a binding energy of 707 eV. The presence of a metallic Fe signal indicates that there are areas of the film that are very thin (in the range of a few nanometers), that is thinner than the escape depth of the photoelectrons associated t2 i3 (VA/cm 2) Q3 (mC/cm2 ) de (nm) 9 4611 99 2 11 2.4 2.4 4.1 2.9 5.9 12.4 9.0 5.0 6.5 10.6 1400 1802 1600 1106 2236 3127 5227 3121 7099 5396 2.8 3.2 4.6 2.5 2.5 2.3 3.0 1.4 3.7 8.9 4.9 5.2 7.5 4.5 15.9 22.9 18.4 11.9 15.1 43.2 1.7 1.8 2.6 1.6 5.5 7.9 6.4 4.1 5.2 14.9 with the metallic iron signal. This is consistent with the literature results of 4,15,32 which show that the secondary passive film is quite thin, in the range of a few nanometers, but is somewhat thicker than a primary passive film. The Cr spectra are given in Fig. 9. This also shows the largest signal for specimen (b) with decreasingly smaller signals for specimens (a), (d), and (c). The binding energy 3+ in all cases corresponds to Cr , with the presence of no Cr6 +. Cr3 + is expected to be oxidized to Cr6 + is the potential region of secondary passivity, and so a significant Cr6 + signal might be expected. Significant Cr 6 + signals were in32 + deed measured by Song et al. However, Cr is extremely 6 soluble and soluble Cr' on the film surface would have been removed by the water rinsing procedure used in our experiments. Furthermore, prior work characterizing Cr2 O, oxide on stainless steels after oxygen/plasma treatment, has shown that Cr6 + is easily removed by rinsing the sample with water. Another explanation could have been the well-known effect of beam reduction of Cr6+ or Cr3 + . However, there was no residual Cr6+ signal. Although beam-induced structural damage has been found to result in chemical alteration of metal oxides, we have not experienced any Cr6 + reduction in other comparable work. 2 ' The Cr enrichment in the secondary passive film can be analyzed from the ratios of the Cr and Fe signals with 90' 45 (N E U 40 35 30 time tI t, (s) t3 c 25 . i. g 20 . (A il U a) a' . 4- 15 4) 0 11 0 0 20 40 60 time, min Fig. 6. Schematic illustration of the three reduction waves and the definition of the charges Q,, Q2, and Q3. Fig. 7. Total reduction charge, Q3, (from Table III), increased as a function of film formation time at 1200 mV in 0.5 M H2S04. J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. 3702 Table IV.XPS peak height ratios. Specimen C/O Crg0/Feg0 Cr30/Fe0, a 0.66 0.38 0.55 0.83 0.94 0.82 0.31 0.75 0.67 0.71 0.21 0.5 b c d and 30 ° take-off angles: Cr,,/Fe,,0 and Cr30 /Fe30, see Table IV. The 30 ° take-off angle provided data with a greater contribution from the outer surface layers of the passive film, whereas the 90 ° take-off data had a greater contribution from a greater depth into the passive film. These data are consistent with the secondary passive film having a higher Cr enrichment than the primary passive film consistent with the results of Song et al.32 This observation of Cr enrichment is also consistent with the work of Kirchheim et al.4 who measured similar Cr°X/(Fe ° x + Cro X) concentration profiles for passive and secondary passive films both having Cr°X/(Fe ° x + Cro° ) - 0.4 with the thickness of the Cro° region being somewhat greater in the secondary passive film. There was a higher Cr enrichment in the film (d) i.e., after reduction, consistent with incomplete reduction of Cr2,O,3 . Despite the original concerns about specimen preparation and the transfer arrangements to the XPS apparatus, the consistency of the present results with literature results lends comfort to the reliability of the main conclusions to the drawn from the present results: the secondary passive film is thin (in the range of a few nanometers) and has a composition comparable with that of the passive film. The thinness of the secondary passive film in these experiments is to be contrasted with the very much thicker film (micrometers in thickness) formed during the coloring of stainless steels. 5 43 Proposed Model The present work taken together with the literature4 ' 53 2 has led us to propose a bilayer model for the secondary passive film. The general case is illustrated in Fig. 10, whereas Fig. 11 illustrates the specific case for the secondary passive film in 0.5 M H2SO4 after washing in distilled water and drying. It appears that, next to the metal, there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied Binding eerV (eV) Fig. 9. The Cr XPS spectra. 900 take-off angle. Specimen preparation as for Fig. 8. potential. On top of this modified passive film, the experimental data are consistent with a "porous" corrosion product film which adds to the total film thickness but has little influence on the electrochemical response. The corrosion product film could be in -the form of mounds separated by areas where the modified passive film is in direct contact with the solution. The overall composition of the secondary passive film is most probably of a mixed Fe/Cr oxide/hydroxide enriched in Cr3+ , with a composition quite similar to a primary passive film. The two layer aspect of the secondary passive film was proposed to explain the measured increase in the film thickness, as measured by reduction, for film formation times greater than 400 s, Fig. 7. In this time period, there was little change in the current transients measured potentiostatically, see, e.g., Fig. 4, indicating little change in the electrochemical response. The two layers were proposed to have different functions. It was proposed that the modified passive film controls the electrochemical response; i.e., governs the current for any applied potential. In contrast, it was proposed that the "porous" corrosion product film adds to the total film thickness but has little influence on the electrochemical response. corrosion product film - / ' / / / / / // /// /modified passive film/ metal u indi energy (ev) Fig. 8. The Fe XPS spectra. 90 ° take-off angle. Air transfer to XPS apparatus after following specimen preparation: (a)the secondary passive films formed for 200 s at 1200 mVscE in deaerated 0.5 M H2SO4 followed by washing in distilled water and drying; (b) the secondary passive films formed for 30 min at 1200 mVc in deaerated 0.5 M H2SO4 followed by washing in distilled water and drying; (c)passive film on a specimen after the standard specimen preparation (wet abrasion with 1200 grit, dried in air, and at least 8 h exposure to laboratory air); and (d)the residual film on a specimen with secondary passive film formed for 30 min at 1200 mVscE in deaerated 0.5 M H2SO4, reduced in a new deaerated 0.5 M H2SO4 solution at -350 mVsc,, washed indistilled water, and dried inair. Formed electrochemically, continuous, self repairing of uniform thickness. Formed by precipitation irregular, probably porous gel-like. Barrier film, ie controls current. No influence on current. Fig. 10. Model proposes a bilayer film for the general case of the secondary passive film. Next to the metal there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied potential. On top, there is a precipitate film which adds to the total film thickness but has little influence on the electrochemical response. J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. Fig. 11. For the specific case for the secondary passive film formed in0.5 M H2S0 4 after washing indistilled water and drying, it is proposed that the corrosion-product film is in the form of mounds between which the barrier film is in direct contact with the solution. There was a lot of scatter in the data of Fig. 7. This is consistent with a precipitate film, because precipitation is a stochastic process and the conditions of precipitation are expected to be very greatly influenced by such experimental variables as solution velocity at the specimen surface over which there was little control in the present apparatus. This would also explain the present measurements which imply secondary passive films somewhat thicker than those reported in Ref. 4, 15, and 32. The XPS results indicating that there are areas of the secondary passive film that are very thin (in the range of a few nanometers) are to be contrasted with the data from the reduction experiments that indicated that existence of a somewhat thicker film that continued to increase in thickness with increasing time at the secondary passivity potential, see Table III and Fig. 7. These two contrasting sets of data can be reconciled with the model given in Fig. 11 for the secondary passive film. Figure 11 shows the corrosion product film in the form of mounds separated by areas where there is no corrosion product film. The areas between the mounds would give rise to the XPS metallic iron signal indicative of very thin areas. The mounds of corrosion product film could continue to grow in thickness with increasing time at potential to give rise to higher reduction charges. Both the electrochemical reduction experiments and the XPS measurements were carried out for secondary passive films prepared in similar ways including electrochemical film formation, washing in distilled water, and drying. Thus these two data sets are directly comparable and lead to the model presented in Fig. 11 for the case of the secondary passive film formed in 0.5 M H2SO4 after washing in distilled water and drying. The next question is the relation between the secondary passive film after washing and drying (as characterized by the reduction experiments and the XPS measurements) and the secondary passive film during its formation (as characterized by the polarization curves and the potentiostatic experiments). It is worth repeating the argument that the reduction experiments underestimate the true film thickness. Therefore, a two-layer model is indeed required to reconcile the different trends for film formation times greater than 400 s identified in the reduction experiments and the current transients. The form of the corrosion product film could however depend on the influence on the washing and drying steps. Two possibilities can be identified: (i) Fig. 11 represents the secondary passive film during film formation and also during film characterization after washing and drying. This 3703 model is indeed consistent with all the data as argued above; and (ii) Fig. 10 represents the situation during formation of the secondary passive film. This would require the modified passive film to control the electrochemical response and the corrosion product film to the "porous" and to have negligible influence on charge-transfer. The subsequent washing and drying procedure changes the form of the corrosion product film into the form illustrated in Fig. 11: that is into a discontinuous film as required to be consistent with the experimental measurements. It is possible that the washing and drying steps changed the morphology of the as-formed secondary passive film into a film as illustrated in Fig. 11. Observations were carried out with optical microscopy and scanning electron microscopy up to a magnification of 30,000 times. No morphological features could be observed pertaining to the secondary passive film. This was not surprising considering the scale of Fig. 11. This means that a choice between the two above possibilities requires detailed observations of the morphology of the secondary passive film, both in situ and after drying, using for example a scanning tunneling microscope. This model illustrated in Fig. 10 and 11 gives rise to some interesting possibilities of interpretation for the general behavior of the current with increasing potential throughout the whole passivity potential region for which we can take the behavior in the phosphate solution as a model, see Fig. 2. Here it should be noted that the measured stationary current densities were in good agreement with the current densities measured with the potentiodynamic method, Table II, indicating that, in the secondary passivity region, the potentiodynamic method gives a measured current density which is a good approximation to the stationary current density. For the phosphate, the current density increased rapidly with potential at the start of the secondary passivity region, the current density peaked and then was approximately constant for increasing potentials up to about 1500 mVscE (see Fig. 2). The transport of current through the passive film is usually assumed to be by a field assisted migration of cations given by 9'" i i exp (BE) with B = aZFs/RT and i = kCv,  where the factor i is proportional to the concentration of moving species (as given by the vacancy concentration Cv and a proportionality constant k), ZF is the charge of the moving ions, s their jump distance, the change-transfer coefficient, and E is the electric field within the film. The increase in current density in the initial region of secondary passivity is consistent with an exponential increase in current with potential as governed by Eq. 2 if the structure of the film and the film thickness remains constant. The present work is consistent with the secondary passive film having a similar structure to the primary passive film, and Sugimoto and Matsuda"5 measured a constant thickness for the film thickness on stainless steels between the potential regions of primary and secondary passivity. The subsequent plateau in the stationary current density is explicable in the same manner as the constant stationary current density in the primary passivity potential region; viz. the high field migration model together with a linear increase of the secondary film thickness with increasing potential as was observed by Sugimoto and Matsuda."5 Kirchheim has interpreted the compositional changes within the (primary) passive film as being caused by the faster diffusion of Fe in the passive film. It would be tempting to use a similar explanation for the further compositional changes as the passive film transforms to the secondary passive film, particularly as our XPS measurements indicate that these further compositional changes appear to be associated with a further enrichment in Cr° X in the secondary passive film. For 0.5 M H,2SO4 there was a similar shape to the plot of stationary current density as a function of potential, see Fig. 2. Thus the interpretation should be essentially 3704 J. Electrochem. Soc., Vol. 144, No. 11, November 1997 © The Electrochemical Society, Inc. as for the phosphate solution. For 5 and 6 M H2 SO4 the only part of the curve recorded was that corresponding to the initial rapid increase in current density. For the borate buffer the oxygen evolution reaction seems to have influenced the measurements, and this makes the interpretation very difficult. The present work did not detect SO, in the secondary passive film in contrast to the expectations from the literature on colored stainless steels. 5 45 As the colored stainless steel films are known to be quite soft, it is possible that the corrosion-product films were lost between the electrochemical cell and the XPS apparatus. However, this does appear unlikely as our XPS results were in agreement with the literature. A more plausible explanation is that the details of corrosion-product film are very dependent on the specific conditions at the specimen surface. The scatter in our reduction data point also in this direction. Precipitation is a stochastic process, which is very dependent of the local flow conditions at the specimen surface; these flow conditions were not controlled in the present experimental apparatus. In the present work, the surface of the stainless steel specimen has been maintained by a potentiostat in the region of secondary passivity. The use of a potentiostat ensures that only the anodic reaction occurs at the specimen surface, with the cathodic reaction occurring at some distance away at the counter electrode. In contrast, coloring of stainless steels was developed as a chemical process where a reduction reaction polarizes the stainless steel surface into the region of secondary passivity. The presence of the reduction reaction at the specimen surface can cause significant local pH changes and also introduces additional chemical species which can then be incorporated into the corrosion product film. Conclusions The secondary passivity potential region occurs for potentials greater than about 940 mV.sc independent of pH (O < pH < 8.4). When a passive film was instantaneously polarized in 0.5 M H2SO4 to a potential within the region of secondary passivity, the duration of the transient response was -400 s in 0.5 M H2SO4 . This indicates a fairly rapid transformation of the passive film into the secondary passive film. The secondary passive film thickness in 0.5 M H2 SO4 , as measured by reduction, continued to change for film formation times greater than 400 s. In this time period, there was little change in the electrochemical response. This leads directly to the conclusion that the coulometric reduction experiments have a large contribution from the outer layers of the corrosion product film. This is the basis for the bilayer model for the secondary passive film. The secondary passive film in 0.5 M H2 SO4 is capable of withstanding drying but could not withstand an acetone/alcohol ultrasonic bath. The secondary passive film is most probably a mixed Fe/Cr oxide/hydroxide enriched in Cr 3+, with a composition quite similar to a primary passive film. 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