232
Langmuir 1985, 1, 232-239
electrical potential was generated directly by the odorant molecules; previous models could not explain how nervous excitation was induced in the olfactory organ. It is also noteworthy that in our system no external force, such as pressure, electrical current, or voltage, was applied. A concentration difference of sodium and potassium ions between the two aqueous phases was essential to induce the observed electrical potential. This seems analogous to the fact that a concentration difference between sodium and potassium ions is essential for the excitability of biological membra ne^.^^' There are many reportssvs on interfacial instability (6) Hodgikin, A. L.; Huxley, A. F. J.Physiol. (London)1952,116,449, 473,497;11 7,500. (7)Ishii, T.; Kuroda, Y.; Yoshikawa, K.; Sakabe, K.; Matsubara, Y.; Iriyama, K.Biochem. Biophys. Res. Commun. 1984,123,792.
(Marangoni effect) between gas and liquid phases and between liquid and liquid phases in systems that are far from equilibrium. Thus similar oscillatory phenomena to those observed in this study may occur in a wide variety of systems that have an interface. The present preliminary study suggests the possibility of developing an artificial olfactory system.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research to K.Y. from the Ministry of Education, Science and Culture of Japan. Registry No. Oleic acid, 112-80-1;1-propanol,71-23-8; tetraphenylphosphonium chloride, 2001-45-8. (8)Sortensen, T. S., Ed. ‘Dynamics and Instability of Fluid Interfaces”; Springer-Verlag: Berlin, 1979. (9)Zierep, J., Oertel, H., Jr., Eds. “Convective Transport and Instability Phenomena”; G.Braun: Karlsruhe, 1982.
Films Formed on Well-Defined Stainless Steel Single-Crystal Surfaces in Borate, Sulfate, Perchlorate, and Chloride Solutions: Studies of the (111) Plane by LEED, Auger Spectroscopy, and Electrochemistry David A. Harrington, Andrzej Wieckowski, Stephen D. Rosasco, Ghaleb N. Salaita,t and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 Received September 18, 1984. I n Final Form: January 2, 1985 Reported here are studies by LEED, Auger spectroscopy,electrochemistry, and thermal desorption mass spectroscopy of the (111)plane of a face-centered cubic Fe-Cr-Ni alloy single crystal of composition (70 atom % Fe, 18 atom % Cr, 12 atom % Ni) resembling that of type 304 stainless steel. Surface films resulting from treatment in borate solution, HzS04, HC104,KC1, and HC1 were amorphous and hydrated. Films formed in acidic media contained up to 20% Cr, due to selective dissolution of Fe and Ni. Annealing at 800 “C led to ordered, Cr-enriched oxide films, consisting primarily of Crz03(001),a hexagonal lattice of chromium and oxide ions. A square CrO mesh was also observed for annealed films formed in acidic media. Significant amounts of C1 were incorporated into the film formed in HCl, but only traces of C1 were detected after pitting breakdown of annealed films in HCl.
Introduction There have been numerous studies of the oxygenous f i i on stainless steel, using electrochemical and surface science techniques.’ However, most of that work dealt with polycrystalline samples or involved procedures in which the surface became contaminated (with air, abrasives, electrolytes, etc.) during preparation and transfer operations. We have previously reported studies of (111)-oriented, well-characterized surfaces of the Fe-Cr-Ni alloy single crystal of composition (70 atom % Fe, 18 atom % Cr, 12 atom % Ni) following treatment with water vapor2 or immersion into liquid water,14 and in the present article we report studies of the same alloy surface after immersion and electrolysis in various electrolytes. In these studies the surface was examined by LEED, Auger spectroscopy, XPS, and thermal desorption mass spectrometry before and after immersion, without contamination during transfer, using an electrochemistryultrahigh vacuum (uhv) apparatus specially designed for the p ~ r p o s e . ~The present studies involved several anion types (BO3-,S042-, C1042-,Cl-), a range of acidities (pH 0-8.5), and potentials Fulbright Scholar. Permanent address: University of Jordan, Amman, Jordan.
0743-7463/85/2401-0232$01.50/0
in the prepassive, passive, and transpassive regions, in order to study the growth, structure, and stability of the (1) (a) Frankenthal, R. P., Kruger, J., Eds. ‘Passivity of Metals”; The Electrochemical Society: Manchester, NH, 1978. (b) Froment, M., Ed. “passivity of Metals and Semiconductors”; Elsevier: New York, 1983. (c) Sato, N.; Okamoto, G. In “ComprehensiveTreatise of Electrochemistry”; Bockris, J. O’M., Conway, B. E., Yeager, E., White, R. E., Eds.; Plenum: New York, 1981;Vol. 4. (d) Adams, R. 0. J. Vac. Sci. Technol.,A 1983, 1, 12. (2)Garwood, G. A., Jr.; Hubbard, A. T.; Lumsden, J. B. Surf. Sci.
1982,121,L524. (3) Harrington, D.A.; Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Salaita, G. N.; Hubbard, A. T. Corros. Sci., in press. (4)Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Lumsden, 3. B., Proceedings of the Pourbaix Symposium, The Electrochemical Society, Pennington, NJ, 1984. (5)(a)Felter, T. E.; Hubbard, A. T. J.Electroanal. Chem. 1979,100, 473. Ib) Katekaru. J. Y.: Hershbereer. J.: Garwood. G. A.. Jr.: Hubbard, A. T. surf. Sci. 1982, 121, 396. -(c) Hubbard, A. T.; Young, M. A.; Schoeffel, J. A. J.Electroanal. Chem. 1980,114,273. (d) Garwood, G. A., Jr.; Hubbard, A. T. Surf,Sci. 1982,112,281;118,223. (e) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. J.Electrochem. SOC.1984,131,260. (f) Stickney, J. L.; Roaasco, S. D.; Schardt, B. C.; Hubbard, A. T. J. Phys. Chem. 1984,88,251. (9) Wieckowski, A,; Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984,23,565.(h) Solomun, T.;Wieckowski, A.; Rosasco, S. D.; Hubbard, A. T. Surf.Sci. 1984,147, 241. (i) Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Bent, B.; Zaera, F.; Somorjai, G. A. J. Am. Chem. SOC.,in press. (j) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985,1, 66.
0 1985 American Chemical Society
Films Formed on Stainless Steel Surfaces
Langmuir, Vol. I, No. 2, 1985 233
Table I. Surface Compositions Determined by Auger Spectroscopy" surface treatment %Fe %Cr %Ni 62 27 8 clean Fe-Cr-Ni(ll1) surface immersion in 0.15 M borate, pH 8.4, one rinse 1 13 0 immersion in 0.15 M borate, pH 8.4, three rinses 0 1 18 positive-going scan to 1.2-2.8 V in borate 0 1 25 positive and negative to -1.2 V in borate 0 0 25 immersion in borate of borate-formed annealed Crz03(001) surface 3 26 0 3 0 positive-going scan in borate on borate-formed annealed Crz03(001)surface 16 26 0 0 after potential step from rest potential to +1.1 V in borate any borate treatment annealing a t 700-800 OC 6 34 0 1 28 0 immersed in 0.05 M HzSO4, no rinse positive scan to +0.1 V in 0.05 M HzS04 24 5 0 21 10 positive scan to 0.62 or 1.22 V in 0.05 M H$04 0 18 after multiple (4-6) scans in 0.05 M H2S04 0 16 any HzS04treatment + annealing at 700-900 OC 24 38 0 21 14 1 immersion in 0.1 M HC104 positive-going scan to +0.88 V in 0.1 M HC104 21 0 10 11 2 immersion in 0.1 M HC104 of HC104-formed annealed oxide 28 8 0 positive-going scan to 0.88 V in 0.1 M HC104 on HC104-formed annealed oxide 30 1 16 any HC104 treatment + annealing a t 700-900 OC 38 immersion in 0.1 M HCl 2 24 10 12 4 positive-going scan to +0.93 V in 0.1 M HC1 16 any 0.1 M HC1 treatment + annealing a t 700-900 "C 2 39 19 immersion in 1.0 M HC1 11 3 22 positive-going scan to 0.56 or 0.6 V in 1.0 M HC1 2 17 19 37 3 any 1.0 M HC1 treatment + annealing a t 700-900 "C 16 positive-going scan to 1.5 V in 0.1 M KCl 2 0 25 11 38 any 0.1 M KC1 treatment + annealing a t 700-900 OC 0 11 immersion in 0.1 M HCl of HC104-formed annealed oxide 27 0 19 positive-going scan to +0.88 V in 0.1 M HCl on HC104-formed annealed oxide 0 40 positive-going scan to 0.48 V in 0.15 M KHzB03/0.1 M KCl 21 5 0 positive-going scan to 0.84 V in 0.15 M KHzBOs/O.l M KC1 1 1 23
%O
% others
~~
+
0 51 57 70 73 60 76 72 56 70 71 68 63 30 60 68 58 61 43 62 67 39 56 56 42 66 49 61
3 (C,N,S,Cl)
40 68 69
"Calculated as described in the text, using the following yield parameters (in unita of cm2): Fe(703 eV), 2.36, Cr(529 eV) 4.31, Ni(848 eV) 2.25, O(503 eV) 4.99, C(272 eV) 22.6, N(379 eV) 11.9, S(152 eV) 176.6, Cl(l81 eV) 125.1, B(179 eV) 46.0, K(252 eV) 20.7.
f i i on stainless steel and their breakdown by aggressive anions such as chloride. Experimental Section The combined uhv-electrochemistry instrumentation used in this work has been described6 and reviewed.' A crystal composed of 70 atom 70Fe, 18 atom % Cr, 12 atom % Ni, and C < 3 ppm wt, supplied by Ecole Nationale Superieure des Mines, with composition similar to that of type 304 stainlesa steel was oriented, cut, and polished parallel to the (111) planes. Two support wires (Nichrome, 0.025-in. diameter) spot welded to the crystal served for electrical heating. A third wire (Alumel, 0.019-in. diameter) together with one of the support wires acted as a thermocouple whose temperature was calibrated against a pyrometer. At a constant electric current, the crystal reached steady-state temperature after approximately 2 min. The standard cleaning procedure was ion bombardment (Ar+, 640 eV, 3 &/an2) at elevated temperature, followed by annealing (800 "C). Clean surfaces were readily obtained in this way. The crystal was then transferred t o a n antechamber, brought t o atmospheric pressure with Ar, and immersed in the electrolyte presparged with Nz. A conventional three-electrode circuit was used for electrolysis, with a Ag/AgCl (0.1M NaC1) reference electrode, but electrode potentials are reported with respect t o the normal hydrogen electrode. Following electrolysis, the crystal was rinsed with three aliquots of the electrolyte diluted 1OOO-fold. Rinsing at open circuit followed electrolysis a t potentials positive t o t h e rest potential, or a t the electrolysis potential otherwise. After the electrolyte was drained away, a 15-min sequence of (6) (a) Garwood,G. A., Jr.; Hubbard, A. T. Surf. Sci. 1980,92,617. (b) Ishikawa, R. M.; Hubbard, A. T. J. Electroanal. Chem. 1976,69,317. (c) Iahikawa, R. M.; Katekaru, J. Y.;Hubbard, A. T. J. Electroanal. Chem. Interfacial Electrochem. 1978,86, 271. (7) (a) Hubbard, A. T. Acc. Chem. Res. 1980,13,177. (b) Hubbard, A. T. J. Vac. Sci. Technol. 1980, 17, 49. (c) Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Roeasco, S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. J. Electroanal. Chem. Interfacial Electrochem. 1984, 168, 43.
sorption, cryogenic, and ionization pumping brought the crystal to uhv for surface characterization by LEED, Auger spectroscopy, and thermal desorption maas spectroscopy (TDMS). All apparatus contacting the liquid was constructed of Pyrex glass and Teflon. Solutions were prepared from water pyrolytically distilled in oxygen.8 Theoretical Auger yield parameters of the elements of interest were calculated as described previously: except that intensities were not corrected for self-scattering. Correction for scattering would have required assumptions as t o the spatial distribution of each element in the interfacial region. The peak-to-peak height of one peak for each element was divided by the yield parameter and corrected for t h e transmission of the CMA t o give a semiquantitative measure of the amount of each element in the surface layer. T h e composition of the surface was then calculated by expressing each amount as a percentage of the total for all elements (Table I). Values for Cr were less reliable when a large oxygen peak (510 eV) overlapped the Cr peak (529 eV).
Results Auger spectra of the clean Fe-Cr-Ni(ll1) surface (Figure 1A) indicated the presence of Fe, Cr, and Ni, with somewhat more Cr than would be expected from the bulk composition (Table I). Some variation in the composition with annealing conditions was noted, as observed by others.lo C and N were also present, but usually below 2%. LEED patterns for the clean surface showed the hexagonal (1x1) pattern seen previously24 (Figure 2A), the spots of (8) Conway, B. E.; Angerstein-Kozlowska,H.; Sharp,W. B. A.; Criddle, E. E. Anal. Chem. 1973,45, 1331. (9) (a) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D. J. Electroanal. Chem.Interfacial Electrochem. 1983,150,165. (b) Stickney, J. L.; Rosasco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Surf. Sci. 1983, 130, 326. (c) Schoeffel, J. A,; Hubbard, A. T. Anal. Chem. 1977,49,2330. (10) (a) Blasik, G.; Weihart, M. Surf. Sci. 1979,82,215. (b) Betz, G.; Wehner, G. K.; Toth, L.; Joshi, A. J. Appl. Phys. 1974,45, 5312.
234 Langmuir, Vol. I, No. 2, 1985
Harrington et al. KINETIC ENERGY (eV) 200
400
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800
KINETIC ENERGY (eV) 200
400
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Figure 1. Auger electron spectra: (A) Fe-Cr-Ni(ll1) clean surface, (B)after open-circuit immersion into 0.15 M borate buffer (pH 8.4) (one rinse), (C) after open-circuit immersion into borate
buffer and annealing at 800 "C, (D)after immereion of annealed Cr203(001)-filmed surface in borate buffer, (E) after immersion of annealed Cr203(OOl)-filmedsurface in borate buffer and pitive scan to 1.25 V. (F) after positive-going scan to +0.62 V ( W E ) in 0.05 M HC104,(G)after positive-goingscan to +O.W V (NHE) in 0.1 M HC104,(H) after immersion of HC104-formedannealed oxide in 0.1 M HC104,(I) after open-circuit immersion in 0.1 M HCI, (J) after open-circuit immersion in 1.0 M HC1. Conditions: primary beam 10 FA at 2000 eV; angle of incidence 7 9 O ; modulation amplitude, 1 V (p-p) (cylindrical mirror analyzer). Lowenergy regions recorded at 5-fold lower sensitivity. type 1in Figure 3; that is, the Fe-Cr-Ni alloy behaved like a pure element in LEED. 1. Electrolysis in Borate Electrolyte. During immersion of the clean surface into a 0.15 M borate buffer (KH2B03,adjusted to pH 8.4) the open-circuit electrode potential was in the range -0.22 to -0.12 V vs. NHE. The subsequent LEED pattern was totally diffuse, indicating that the surface film was lacking in long-range order to a depth greater than the sampling depth of LEED, that is, more than a few monolayers. Auger spectroscopy showed that B and K were present in the surface layer, Table I and Figure 2B, in amounts dependent on the number of rinses, indicating that some potassium borate salt remained on the surface after removal to UHV. The oxygenous film was mainly an oxidation product of Fe (Fe:O = ca. 1:3) with very little Cr or Ni, as observed for water immersi~n.~ The low-energy Auger peaks for Cr and Fe were shifted with respect to the metallic peaks, indicating that they were present in oxidized form.'lJ2 Comparison of the shape of the low-energy Auger doublet of Fe with spectra (11) (a) Allen, S. C.; Wild, R. K. J. Electron Spectrosc. Relat. Phenomen. 5 , 1974,409. (b)Ekelund, S.;Leygraf, C . Surf. Sci. 1973,40,179. (12) (a) Seo, M.; Lumsden,J. B.; Staehle, R. W. Surf. Sci. 1971,50, 541. (b) Mussig, H.-J.; Arabczyk, W. Krist. Tech. 1980, 15, 1091.
of iron oxides12indicated that the Fe was present in an environment resembling FepO3, at least in the outermost part of the film. Annealing the filmed surface at 800 "C for 10 min caused a LEED pattern to appear. The pattern consisted of beams of an hexagonal mesh (type 4, Figure 3) with lattice constant 4.9 A, rotated 30" with respect to the mesh of the clean alloy surface. As reported previously, these are the correct dimensions, within experimental error, for the unit mesh of the (001) plane of Cr2O3, oriented with the 0-0 vectors parallel to the interatomic vectors of the alloy3 (Figure 4A). The integral-index beams were not seen, indicating that the film was thicker than a few monolayers. Auger spectra for the annealed surface showed substantial enrichment of Cr at the expense of Fe and a Cr:O ratio appropriate to Cr203(Figure IC). The low-energy Auger peak for Cr was shifted with respect to that for metallic Cr and had a shoulder, indicating the presence of two transitions. The principal low-energy Fe Auger peak corresponded to metallic Fe, but the existence of a small satellite peak indicated the presence of traces of oxidized Fe. Evidently, heating the oxygenous layer led to oxidation of Cr to Cr2O3 and reduction of Fe to the metallic state. Heating removed the K from the surface, but the B remained in the film. A linear scan voltammogram from the open-circuit potential to 1.3 V (NHE) is shown in Figure 5A. The currents were small and the surface became passivated without the large active-passive peak observed in more acidic media. The current increase at potentials above 1.1 V is attributable to oxygen evolution. When the cathodic scan was extended to more negative potentials, cathodic current due to hydrogen evolution occurred, but there was no distinct cathodic peak corresponding to fiim reduction. Removal of the crystal from solution at positive (1.2-2.8 V) or negative (-1.2 V) potentials yielded results similar to those for immersion at the open-circuit potential: diffuse LEED patterns and O/Fe ratios ranging from 2.5 to 3.0; heating caused the hexagonal LEED pattern to appear (Figure 2B) and led to Cr enrichment (O/Cr = 1.5-1.9). Prior to annealing, low-energy Auger spectra contained a doublet characteristic of oxidized Fe, while mainly metallic Fe was present before annealing, as before. The low-energy Cr Auger peaks were similar before and after heating except that after removal at the cathodic potential, the peak shape before annealing was more like ~ that observed for exposure to water at open ~ i r c u i t .The same results were obtained for samples stepped to positive potentials as with potential scans. Thermal desorption from the oxygenous film into vacuum yielded primarily H 2 0 in a sharp peak at 300 OC and a broad peak at 800 "C, as observed previously for immersion into pure water.2 Thermal desorption of K occurred at about 900 OC, consistent with its loss in the Auger spectra. When the Cr203(OOl)film, formed by electrolysis and annealing, was reimmersed into borate solution at open circuit, only slight changes occurred: the original LEED pattem was returned, the Auger spectrum revealed a slight increase in O/Cr ratio, and no significant change was observed in Fe content (Figure 1D). Heating after reimmersion restored the surface to its preimmersed state. Application of a positive-going scan to the Cr203(001)-filmed surface led to formation of an Fe-rich disordered oxide film on top of the Cr203 film. The voltammetric curve is shown in Figure 5B. A prominent new oxidative peak was present at 1.2 V. Removal of the surface from solution at 1.26 V gave a LEED pattern containing weakened Cr203(OOl)beams and substantial
F i g u r e 2. LEED patterns: (A) Fe-Cr-Ni(ll1) clean surface, 65 eV, ( B ) after positive-going scan to 2.9 V in 0.15 M burate buffer (pH 8.4) and annealing at 900 "C, 100 eV, (C) after positive-going scan to +0.11 V in 0.05 M H2S0, and annealing a t 800 "C, 55 eV, (D)after positive-going scan to +0.88 V in 0.1 M HCIO, and annealing at 800 "C, 110 eV, (00) beam below center, (E) after positive-going scan t o +0.88 V in 0.1 M HCI and annealing at 800 "C, 100 eV, (F) after positive-going scan to C0.61 V in 1 M HCI and annealing at 800 "C. 98 eV.
236 Langmuir, Vol. 1, No. 2, 1985
Figure 3. Composite LEED schematic for surfaces exposed to electrolytes and annealed.
diffuse intensity. The Auger spectrum indicated about 16% Fe, up from 3% prior to electrolysis. Evidently, the Cr2O3(001)-filmedsurface was passive at open circuit but was susceptible to further oxidation above about 1.1 V. 2. Electrolysis in 0.05 M Sulfuric Acid. The opencircuit potential in 0.05 M sulfuric acid was in the range -0.2 to -0.1 V (NHE). A linear scan voltammogram from the open-circuit potential to +1.2 V is shown in Figure 5C. A small peak at +0.05 V with a peak current density of 280 d / c m 2 was followed by a larger peak at +0.38 V with a peak current density which varied from 0.63 to 2.8 mA/cm2. The larger peak is assignable to the activepassive transition,13J4while the smaller peak has not yet been assigned. The principal reaction early in the larger peak may be assumed to be metal dissolution, but later a passive, oxygenous layer forms on the surface and inhibits further reaction. Negative sweeps from potentials positive of the active-passive peak showed no reduction peaks before hydrogen evolution occurred. The second anodic peak persisted on multiple scans, although it was progressively diminished in height. Removal of the crystal from solution at any potential gave totally diffuse LEED patterns, as observed before. The composition of the oxygenous layer was more variable and Cr content was greater from sulfuric acid than from borate or water, based upon Auger spectra (Figure 1F and Table I). The extra variability was probably due to the irreproducible nature of the rapid, selective dissolution process. Immersion into sulfuric acid at open circuit led to a lower Cr content than for films produced at potentials more positive than the active-passive transition. Subsequent scanning into the hydrogen evolution region led to little further change in surface composition (Table I). Cr enrichment in the passive region is attributable to selective dissolution of iron over chromium at prepassive potentials, although iron remained a major constituent of the film (1530%). Thermal desorption from the oxygenous film from sulfuric media yielded primarily water in two peaks (Figure 6) as observed for borate. Smaller amounts of O2 and fragmentation products of sulfate (SO+,SO2+,SO3+)were (13) Shreir, L. L., Ed. “Corrosion”; Newnes-Butterworths: London, 1976;Vol. 1. (14)Sedriks, A. J. ”Corrosion of Stainless Steels”; Wiley: New York, 1979.
Harrington et al.
observed above 400 OC, suggesting that the sulfur species present on the surface after removal from solution were sulfate salts. Annealing at 700-900 “C led to Cr enrichment, as before. The final Cr concentration varied widely depending on the time and temperature of annealing. A Cr:O ratio between 1:l and 1.3:l was usually found when the annealing time was less than 15 min. This is close to the ratio observed after immersion in water: but it twice that observed with borate. The Fe concentration was also variable, although Fe was a significant constituent of the film (15-30%). On the basis of low-energy Auger peak positions, Fe was present mainly in metallic form, as before. The amount of sulfur in the surface layer increased to 5 4 % with annealing, despite the loss of sulfur species observed in the TDMS. Annealing an immersed, unrinsed surface increased the sulfur content from 3% to about 20%; apparently the surface region was relatively depleted of sulfate until heated. Extended annealing removed much of the oxygen, but not the sulfur, from the surface and produced a surface rich in Cr. Annealing sulfate-treated surfaces did not produce a single, consistent type of LEED pattern, unlike the borate-treated surfaces. The Cr203(001)pattern sometimes occurred, while on other occasions the pattern-of-twelve spots (type 2, Figure 3) assigned earlier to a square mesh of CrO (3), was observed (Figures 2C and 4B). Faint other beams were present, but could not be positively identified. Formation of ordered layers from these surfaces required longer annealing times than those formed in borate. The Cr:O ratio (131)observed after sulfuric acid treatment was the same as that after immersion into pure water, and in both cases the pattern-of-twelve spots was observed. The Cr:O ratio was 2:3 from borate electrolyte, and the pattern-of-twelve spots was not seen. Extended annealing produced the clean-surfaceLEED pattern, consistent with the removal of oxide noted in the Auger spectra, although residual sulfur (5-6%) and oxygen (less than 25%) remained. 3. Electrolysis in 0.1 M Perchloric Acid. The electrochemical behavior of Fe-Cr-Ni(ll1) in perchloric acid was very similar to that in sulfuric acid: the opencircuit potential was not significantly different and the linear scan voltammogram again showed a small peak followed by a larger active-passive peak, as before. Open-circuit immersion or electrolysis in perchloric acid led to amorphous surfaces with similar composition to those produced in sulfuric acid (Figure 1G). Up to 3% of C1 was detected on the surface. Annealing at 800 OC led to ordered, Cr-enriched surfaces, as before, and removed the C1 from the surface. The Cr:O ratio for annealed surfaces was between 0.6:l and 1.51, similar to that for surfaces treated with sulfuric acid and water but slightly higher than that observed for boratetreated surfaces (0.51 to 0.7:l). LEED patterns (Figure 2E) showed the Cr203(001)beams (type 4,Figure 3) and, after longer annealing, the pattern-of-twelve spots (type 2, Figue 3) and weak integral-index beams (type 1, Figure 3). Apparently, long annealing causes some regions of the film to become thinner than the sampling depth of LEED, causing the integral-index beams to reappear. Less annealing time was required to give ordered surfaces after perchloric acid treatment than after sulfuric acid treatment. This suggests that the relative amount of disorder seen in the sulfuric acid case is due to the presence of significant amounts of sulfur in the film; in the case of perchloric acid, there was no significant participation of the anion in film formation.
Langmuir, Vol. 1, No. 2, 1985 237
Films Formed on Stainless Steel Surfaces 003'
B
00 02-
Oxygen !03
Chromium
tal lattice
Figure 4. Surface structures: (A) (001) plane of the Cr203oxide structure, located on the alloy substrate as experimentally determined. For clarity, only a few layers of the oxide are shown. (B) Square CrO structure on the alloy substrate. (C)Chemisorption structure. Unit cell (11x11). (D) Chemisorption structure. Unit cell (11Xd3) rectangular.
Experiments were performed in which the annealed oxide surface was reimmersed in perchloric acid to determine the degree of passivity of the surface (Figure lH, Table I). Reimmersion led to formation of a thin, ironcontaining film on top of the ordered chromium oxide layer, as shown by the presence of a peak in the low-energy Auger spectrum, assignable to oxidized Fe, a large oxygen uptake, and the existence of an ordered LEED pattern. Reimmersion into perchloric acid caused the Cr content of the film to decrease from the previous value of 36-40% down to 28-30%. The LEED pattern after reimmersion showed only the Cr203(001)beams and not the squre-CrO (type 2) or integral-index beams, although these latter beams reappeared when the surface was reannealed. That is, immersion of the annealed film caused a thin film rich in hydrated iron oxides to form preferentially on regions of initially square-Cr0 (type 2) structure. The changes in structure and composition upon reimmersion were the same whether reimmersion was at open circuit or was followed by positive- (Figure 5E) or negative-going scans. This indicates that the ordered oxide film was passive toward perchloric acid at all potentials studied (-1.5 to +0.9 V). this is in sharp constrat to the ordered film formed in borate solutions, which was passive at open circuit but not at more positive potentials. 4. Electrolysis in Chloride Media. The characteristic feature of electrolysis of Fe-Cr-Ni(ll1) in 0.1 M KC1,O.l M HC1, and 1M HC1 was a large anodic dissolution current during the positive potential scan (Figure 5G,I). No passivation process was observed, in contrast to borate, sulfate, or perchlorate media. A peak was observed in the early stages of dissolution in 1 M HC1 but was absent in the other chloride media. The decrease of dissolution rate following the maximum may have been due to a local
increase in pH, arising from rapid consumption of H+ by the accompanying hydrogen-evolution reaction. Scan reversal in chloride media led to anodic currents larger thgn on the positive scan; this characteristic has been associated with localized pitting of a passive film.14 Some noise was observed in the current at potentials more positive than about +0.4 V, suggesting that random pitting was occurring.15 Evidently a film was present and dissolution occurred in film-free regions of the surface. Open-circuit potentials in HC1 were in the range -0.18 to -0.12 V, comparable to those in the other acidic media, but values in 0.1 M KC1 were more positive (-0.10 to -0.06 V). Electrolysis in HC1 led to 10-20% Cr in the film, similarly to HCIOl and H2S04,indicating selective dissolution of Fe and Ni during formation of the oxygenous layer. Films formed in KC1 solution contained only 2-3% Cr (Table I, Figure 5G). Low-energy Cr Auger spectra following immersion or electrolysis in 0.1 M HC1 displayed the same prominent Cr Auger peak at 31 eV observed for the other acidic media. However, the low-energy Auger spectra from 1 M HCl were more like those observed for nonacidic media (including 0.1 M KC1) (Figure 11,J). Evidently the state of Cr in the outer region of the film (responsible for the low-energy Auger emissions) resulting from the very aggressive 1M HC1 electrolyte differed from that for the other acids and more dilute HC1; the highenergy Cr Auger signals were the same for all acidic electrolytes, however, indicating similarities in Cr content deeper in the film. Surfaces treated in chloride media contained significant amounts of C1 (2-8%, depending upon HC1 concentration). Examination by LEED showed (15) Herbsleb, G.; Schwenk, W.Corros. Sci. 1973,13, 739.
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Harrington et al.
POTENTIAL, VOLT VS NHE -02
02
-02
10
06
I -04
00
04
08
12
POTENTIAL, VOLT
02
, 00
VS
,
06
, 04
,
IO
,
,
OP
NHE
Figure 5. Linear scan voltammograms. (A) clean surface in 0.15
M borate buffer (pH 8.4),(B) Annealed Crz03(Wl)-filmed surface in 0.15 M borate buffer (pH 8.4),(C) clean surface in 0.05 M HzS04, (D) clean surface in 0.1 M HC104, (E) HC104-formed, annealed oxide in 0.1 M HC104,(F) HC104-formed,annealed oxide in 0.1 M HC1, (G) clean surface in 0.1 M KCl, (H) clean surface in 0.1 M HCl, (I) clean surface in 1.0 M HC1, (J) clean surface in 0.15 M KHZBO3/0.1M KCl, pH 8.4, Scan rate 10 mV/s, crystal area 0.710 cm2. TEMPERATURE
0
100
200
PC)
300 400 T I M E (SI
500
600
Figure 6. Thermal desorption of water (18 amu) from Fe-CrNi(ll1) after anodic scan to +1.22 V (NHE) in 0.05 M H2S04.
that the chloride-treated surfaces were amorphous, as before. Thermal desorption from the unrinsed 1 M HC1 immersed surface yielded fragmentation products of FeC1, and NiC12 (Fe+, FeCl+, Ni+, NiCl+, NiC12+)but not of chromium chloride, confirming that the chromium oxides were resistant to HC1. TDMS following electrolysis in sulfate media detected SO3+,SOz+, and SO+ but no metal salts, although the film composition observed by Auger spectroscopy was indicative of selective dissolution in this case as well. TDMS following rinsing yielded mainly water and small amounts of C1 and HC1, but no metal salts. After annealing chloride-treated surfaces at 800 "C, the surface composition was similar to that observed in other
electrolytes. In took longer to remove C1 from HC104treated surfaces, suggesting that it was incorporated into the film rather than present as a salt on the surface. LEED patterns showed the integral-index beams, the beams for Cr203(OOl)(type 4,Figure 3), square CrO (type 2, Figure 2), and a pattern of six pairs of split spots previously observed for water-vapor-dosed surfaces and assigned to antiphase domains of chemisorbed oxygen on the alloy surface3 (Figure 4, part A or B). This chemisorption structure was not seen for the other electrolytes and may be due to the removal of regions of the oxygenous film by HC1, allowing chemisorption onto the exposed alloy surfaces. Since, in practical situations, the stainless steel surface already contains an oxide film prior to exposure to chloride media, we undertook experiments in which surfaces containing an annealed oxide structure were immersed or electrolyzed in 0.1 M HC1. In either case, the LEED patterns indicated loss of the beams due to the substrate and the square-Cr0 structure, similar to Figure 2B for borate. Low-energy Auger features due to oxidized Fe appeared along with significant increase in oxygen signal, as in Figure 1H for perchlorate. LEED beams for Crz03(001) domains of the film remained. Evidently, HC1 attacked the ordered oxide film at the comparatively thin CrO regions, either replacing or covering these regions with a thin, hydrated amorphous iron oxide layer. These results resemble those described above for immersion or electrolysis of annealed oxide films in 0.1 M HC104. However, the voltammetric scans in HC1 and HC104differed (Figure 5E,F) in that strong dissolution without passivation was observed in HC1. Only small amounts of C1(
