Langmuir 1987, 3, 860-866
860
Films Formed on Stainless Steel Single-Crystal Surfaces in Aqueous Solutions: Studies of the (100) Plane by LEED, Auger Spectroscopy, and Electrochemistry? Douglas G. Frank, Victor K. F. Chia, Mark R. Schneider, and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 Received June 6, 1986. I n Final Form: September 11, 1986 Reported here are studies by Auger spectroscopy, thermal desorption mass spectroscopy, LEED, and electrochemistry (voltammetry) of the (100) plane of a face-centered-cubicFe-Cr-Ni alloy single crystal of composition (70 atom % Fe, 18 atom % Cr, 12 atom % Ni) resembling that of type 304 stainless steel. When the crystal was cleaned by argon ion bombardment and annealed, reconstruction of the surface to a Fe-Cr-Ni(lOO)(ZxZ) structure resulted, with a composition similar to that of the bulk. Exposure of the clean surface to water vapor or immersion into liquid water or aqueous electrolytes formed an amorphous, hydrated film. Annealing this film at 800 "C resulted in the formation of an ordered CrO film, a hexagonal lattice of chromium and oxide ions in rotational alignment with the substrate, which was reconstructed in the presence of the film. Immersion of the annealed oxide layer into 0.1 M HC1 disordered the film, in contrast to results found for the (111)-oriented stainless steel surface on which a Cr203(001)film was stable. The annealed film formed on the (100) surface contained significantly less Cr than the film formed on the (111)-oriented stainless steel surface.
Introduction There have been numerous studies of the oxygenous film on stainless steel, using electrochemical a n d surface analysis techniques.' Most of t h e work has dealt with polycrystalline samples or involved procedures in which the surface became contaminated (with air, abrasives, etc.) during preparation a n d transfer operations. In work from this laboratory, the surface was characterized without contamination during transfer by using an electrochemistry/ultrahigh vacuum apparatus specially designed for the p ~ r p o s e .W ~ e have previously reported studies of (111)-oriented surfaces of t h e Fe-Cr-Ni alloy of composition (70 a t o m % Fe, 18 a t o m % Cr, 12 a t o m % Ni) resembling type 304 stainless steel.2 T h e (111) surface was characterized by LEED, Auger spectroscopy, thermal desorption mass spectroscopy, and related techniques prior to and following electrochemical t r e a t m e n t in sulfate, Those borate, perchlorate, or chloride electrolyte^.^ studies involved a range of acidities a n d potentials in t h e prepassive, passive, and transpassive regions in order t o study t h e growth, structure, a n d stability of the films o n stainless steel and their breakdown b y agressive anions such as chloride. In the present article we report analogous studies of t h e (100) plane of t h e same material which demonstrate t h e influence of crystallographic orientation on stainless steel surface behavior.
Experimental Section The surface electrochemistryinstrumentation used in this work has been de~cribed.~.~ A face-centered-cubicsingle crystal composed of 70 atom % Fe, 18 atom % Cr, 12 atom % Ni, and less than 3 ppm C, supplied by Role Nationale Superieure des Mines, was oriented (by Lau6 X-ray reflection photography), cut, and polished to within *0.25" of the (100)plane. Two support wires (Nichrome, 0.025-in. diameter) spot-welded to the crystal served for electrical heating. At a constant electric current, the crystal
* To whom correspondence should be addressed.
Presented at the symposium on Corrosion, 191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986.
0743-7463187/2403-0860$0l .SO/0
reached steady-state temperature after approximately 2 min. Temperature was measured by means of a pyrometer. The cleaning procedure was ion bombardment (Ar+,640 eV, 3 WA/cm2) at room temperature and annealing (800 "C), followed by Auger spectroscopy. The cleaning cycle was repeated until the Auger spectrum indicated that the surface was clean, typically after about 60 min. The clean crystal was translated to a small antechamber brought to atmospheric pressure with Ar, and immersed in electrolyte presparged with N2. A conventional three-electrode circuit was used for the electrolysis, with a Ag/AgCl reference electrode. Potentials are reported with respect to this reference. Following electrolysis,the crystal was rinsed with pure water. All apparatus contacting the liquid was constructed of Pyrex glass and Teflon. Solutions were prepared from water pyrolytically distilled in oxygen. After the solution was drained away, a sequence of sorption, cryogenic and ionization pumping brought the crystal to ultrahigh vacuum for surface characterization by LEED,Auger spectroscopy, and thermal desorption mass spectroscopy (TDMS). Theoretical Auger yield parameters of the elements of interest were calculated as described previously>e except that intensities were not corrected for self-scattering. The peak-to-peak height of one peak for each element was divided by the yield parameter (1)(a) Passiuity of Metals; Frankenthal, R. P., Kruger, J., Eds.; The Electrochemical Society: Manchester, NH, 1978. (b) Passivity of Metals and Semiconductors; Froment, M., Ed.; Elsevier: New York, 1983. (c) Sato, N.; Okamoto, G. In Comprehensive Treatise 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, I , 12.
(2) (a) Garwood, G. A., Jr.; Hubbard, A. T.; Lumsden, J. B. Surf. Sci. 1982,121,L524. (b) Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Schardt, B. C.; Salaita, G. N.; Hubbard, A. T. Corros. Sci. 1985,25,849. (c) Harrington, D.A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Lumsden, J. B. Proceedings of the Pourbaix Symposium, The Electrochemical Society; Pennington, NJ, 1984. (d) Harrington, D. A.; Wieckowski, A.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1986, I , 232. (3)Hubbard, A. T.; Stickney, J. L.; Soriaga, M. P.; Chia, V. K. F.; Rosasco, S. D.; Schardt, B. C.; Solomun, T.; Song, D.; White, J. H.; Wieckowski, A. Electroanal. Chem. Interfacial Electrochem. 1984,168, 43.
(4)(a) Hubbard, A. T. Acc. Chem. Res. 1980,13, 177. (b) Hubbard,
A. T. J. Vac. Sci. Technol. 1480,17, 49. (5) Schoeffel, J. A.; Hubbard, A. T. Anal. Chem. 1977,49,2330. ( 6 ) Adolphi, B.;Mussig, H.-J. Krist. Tech. 1978,13 ( 3 ) ,317. 0 1987 American Chemical Society
Films Formed on Stainless Steel Single Crystals
Langmuir, Vol. 3, No. 6, 1987 861
KINETIC ENERGY (eV)
of that element and corrected for the transmission of the cylindrical mirror analyzer to give a semiquantitative measure of the amount of each element in the surface region. The composition of the surface was then calculated by expressing each amount as a percentage of the total for all elements (Table I). Quantitation of Cr required spectral subtraction to separate overlapping signals due to oxygen (510 eV) and Cr (529 eV), as follows: an oxygen spectrum obtained at a Pt surface was scaled-and-subtracted iteratively until the remaining stainless steel Cr spectrum had the morphology of an oxygen-free Cr peak (as for the clean surface);the resulting oxygen and Cr peaks were employed for composition calculations as described above.
Results and Discussion Auger spectra of the clean Fe-Cr-Ni(100) surface (Figure 1A) indicated that the surface composition of the alloy was similar to that of the bulk (Table I), with carbon and nitrogen present at low levels (1-3 atom %). This is in contrast to the (111)surface, at which Cr enrichment was observed.2 LEED patterns for the clean surface (Figure 2A) revealed that a surface reconstruction had taken place to a (2x2) structure. This structure consists of atomic rows spaced v'2 apart at 45" to the unit mesh vector of the (100) plane (Figure 3A). This structure results in near extinction of the (1/2,0)and equivalent LEED beams because atoms in the oblique mesh are nearly equivalent. Based upon the LEED patterns, the minimum metal-metal distance in this structure is 0.24 nm, compared with the average bulk value of 0.25 nm. 1. Immersion in H20.After the clean surface was immersed for 2 min into liquid water at open circuit, the Auger spectra indicated the presence of an oxide film having Fe/Cr/Ni proportions similar to the bulk (Figure 1B and Table I). As noted in ref 2d, the low-energy Auger peaks (below 100 eV) for Fe and Cr were shifted relative to the metallic peaks, indicating that Cr and Fe were present in oxidized form.' The LEED pattern was totally diffuse, indicating that no long-range order was present in the surface region within the sampling depth of LEED (about 1 nm). Thermal desorption mass spectrometric (TDMS) data for this hydrated film consisted of two water peaks: a peak at 300 "C and a peak at 800 "C (Figure 6A). This result is similar to that for the (111)face of the same alloy.2 Only desorption peaks traceable to water were found. When the surface was heated at 800 "C following treatment with liquid water, the surface film composition varied with time as shown in Figure 4A. After 3 min at 800 "C, a LEED pattern consisting primarily of a ring of 12 spots (Figure 2B) began to emerge, reached maximum clarity at 10 min heating time, and began to fade noticeably a t 12 min. On the basis of the Auger spectrum at maximum brightness of the LEED pattern (Figure lB), chromium enrichment of the oxide film had occurred (Table I). Annealing converted the low-energy Auger doublet of oxidized Fe to a singlet, while the Auger intensities for Fe and Ni at higher energies were in a ratio of about 5:l as for the alloy, suggesting that their presence in the spectrum at this stage was due primarily to Auger electrons emitted from the substrate. The rhombic unit mesh of this pattern corresponds to 0.28 nm. There are two domains, 90" apart, in rotational alignment with the substrate. A model structure consistent with these findings is shown in Figure 3B. This structure consists of a CrO layer in which the
(7) (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.
c
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.
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.
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.
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.
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KINETIC ENERGY
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Figure 1. Auger electron spectra: (A) clean surface, Fe-CrNi(100)(2X2);(B) after immersion into H20 and 10-min anneal, Fe-Cr-Ni(lOO)-CrO; (C) after cyclic scan in 0.05 M H2S04;(D) after immersion into 0.15 M borate; (E) after positive scan to +0.4V in 0.1 M HC1. Conditions: primary beam, A at 2000 eV; normal incidence, 2 V (P-P) modulation amplitude (CMA).
oxygen atoms are arranged hexagonally with an 0-0 interatomic distance of 0.28 nm, equal to the van der Waals distance of oxygen (0.28 nm) and identical with the 0-0 distance in corundum (0.28 nm). Although the oxide layer is rotationally aligned with the substrate, its lattice constant is not a simple multiple of that of the substrate. This zinc blende (111) layer is related to the corundum structure by addition of metal ions to the corundum tetrahedral vacancies. Similarly to the CrO subphase formed on the
862 Langmuir, Vol. 3,No.6,1987
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Fkure 2. LEED patterns: (A) clean surface, FeCrNi(100)(2XZ), 101 eV; (B)after immersion into H,O and annealing, 52eV. FeNi-Cr(lW)-hemgonal CrO: (C) after open-circuit immersion into HfiO, and annealing, 88 eV, F e C r N i ( I W ) - h e x a g o d CrO, (D) after w a n to +0.75 V in H$O, and annealing, 66 eV, FeCrNi(100) ( \ 2X6\/2)R45CrO (El after open-circuit immersion into borate and annealing, 56 eV, FeCrNi(100)khexagonaI CrO with streaks; (F) after open-circuit immersion into HCI and annealing, 61 eV, FeCr-Ni(l00)(7~'2xx21R45-CrO.
stainless steel (111) plane (Figure 2A), thii layer was unstable in acidic electrolytes, particularly HCl (Table 9 as described below. 2. Exposure to H,O Vapor. The clean surface was exposed to water vapor in an atmosphere of argon for 3 min at 296 K. The subsequent LEED pattem was diffuse,
as for liquid H20, and the composition of the amorphous film was similar to that after exposure to liquid water yielded water (Table I). Thermal desorption from this fh similarly to that following treatment with liquid water (Figure4B). The LEED patterns were qualitatively similar to those for liquid water but displayed less clarity.
Langmuir, Vol. 3, No. 6, 1987 863
Films Formed on Stainless Steel Single Crystals
.Metal
B
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ion
00 Oxygen
m
Substrate
Oxygen O M e t a l ion
Figure 3. Surface structures: (A) clean surface, Fe-Cr-Ni(100)(2X2);(B)CrO on Fe-Cr-Ni(100); (C) Fe-Cr-Ni( 100)(d2X6d2)R45°; (D) Fe-Cr-Ni( 100)(7~'2X7.\/2)R45~.
3. Electrolysis in Sulfuric Acid. The open-circuit potential in 0.05 M H2S04was in the range from -0.35 to -0.38 vs. NHE. This range was between the open-circuit potential ranges found for HC1 and borate solutions and just preceded the dissolution/oxidation peak. LEED patterns following immersion into sulfuric acid were diffuse regardless of the electrode potential, The film formed at open circuit contained no detectable chromium. Thermal desorption from the film displayed water desorption rate peaks similar to those found for films formed in pure water (Figure 5A), as well as desorption rate peaks characteristic of sulfate fragmentation products (S', SO+, SO2+,SO3+) (Figure 5B-F). The sulfur peak (32 amu at 825 "C) was observed only after immersion at open circuit (Figure 5C). The surface sulfate was removed by rinsing with pure water; thermal desorption released the sulfate located deeper in the oxide (Table I). Annealing following immersion into H2S04at open circuit yielded a LEED pattern (Figure 2C) containing the pattern of 12 spots characteristic of the hexagonal CrO film (Figure 3B) and half-index beams assignable to the reconstructed substrate (Figure 3A). This result could be due either to a thin oxide layer atop the reconstructed substrate or to bare spots on the surface. In contrast, the (111)surface yielded mixtures of CrO and Cr203under these same conditions (Figure 2D). A cyclic voltammogram for 0.05 M H2S04is shown in Figure 6B. The main features are hydrogen evolution near -0.4 V, a multiplet for anodic dissolution/oxidation near
0.0 V corresponding to the activepassive transition: and oxygen evolution at 1.2 V. Immersion at negative potentials resulted in a higher proportion of Cr in the film than a t open circuit. However, LEED patterns formed after immersion at negative potentials and annealing at 800 "C were the same as found for immersion at open circuit and annealing. Immersion at positive potentials resulted in films with more Cr than either open circuit or negative polarization; similar results were obtained with the (111) plane (Figure 2D). Evidently, dissolution of Cr is favored at negative potentials while dissolution of Fe predominates at positive potentials. Separate voltammetric peaks were observed for these processes at the (111)surface (Figure 5C in ref 2d). Immersion at potentials greater than about 0.75 V and annealing resulted in the emergence of beams characteristic of a (2/2X62/2)R45" pattern and beams due to the fcc substrate and attenuation of the beams of the pattern of 12 (Figure 2D). The oxygen Auger signal of this surface (43%) was lower than for other pretreatments (55-65%) which combined with the LEED results suggests the presence of a thin, rectangular mesh, as shown in Figure 3C: In this model the oxygens are arranged nearly octahedrally with a minimum oxygen-oxygen distance of
(8) (a) Corrosion; Shier, L. L., Ed.; Newnes-Butterworths: London, 1976; Vol. 1. (b) Sedriks, A.J. Corrosion of Stainless Steel; Wiley: New York, 1979.
864 Langmuir, Vol. 3,No. 6, 1987
Frank et al.
TEMPERATURE ?C) 900
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0
5
0.30 nm, comparable to the van der Waals distance of oxygen (0.28nm). The metal ions are coplanar with oxygen, as in the zinc blende (100) plane. 4. Electrolysis in Borate Electrolyte. The opencircuit potential in 0.15 M borate buffer (pH 10.4)was in the range from -0.38 to -0.52 V vs. NHE and was more negative than for HC104, H2S04,or HCl electrolytes, indicating the least degree of spontaneous surface diesolution or oxidation. LEED patterns following open-circuit immersion into borate solution were diffuse, as found for water and sulfuric acid. No detectable Cr was present prior to annealing in films produced at open circuit or at positive potentials (Table I). Film composition following annealing showed restoration of Cr. LEED beams characteristic of the reconstructed substrate surface remained invisible in the presence of the hexagonal annealed oxide film when formed in borate solutions. Streaks were present in the pattern indicating variability in the oxide structure in directions parallel to the substrate mesh (Figure 2E). A cyclic voltammogram in borate electrolyte i s shown in Figure 6A. The primary features are hydrogen evolution
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TIME (sec.) Figure 5. Thermal desorption mass spectroscopy: (A) H20 immersion, 18 amu (H,O+); (B)H#04, negative scan to -0.75 V, 32 amu (S+);(C) H2S0,, open-circuit immersion, 32 amu (S); (D)H2S04,negative scan to -0.75 V, 48 amu (SO'); (E) H2S04, negative scan to -0.75 V, 64 amu (SO2+)(F)H#04, negative scan to -0.75 V, 80 amu (S08'); (G)borate, positive scan to +0.65 V, 11amu (B+); (H) borate, positive scan to +0.65 V, 27 amu (BO+); (I) borate, positive scan to +0.65, V, 43 amu (BOz+);(J) HC1, open-circuit immersion, 36 amu (HCP).
at -1.1 V and oxygen evolution a t +1.1V. In contrast to sulfuric acid, the borate (pH 10.4)electrolyte led to passivation of the surface with substantially less net charge
Films Formed on Stainless Steel Single Crystals
Langmuir, Vol. 3, No. 6. 1987 865
ELECTRODE POTENTIAL, V vs NHE -12
-0.8 -0.4
0.0 0.4
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Figure 6. Cyclic v o l t a ” ~(A) 0.15 M KHaO,, pH 10.4, dashed h e II) cathodic scan f”rest potential to hydrogen bulk evolution; (B)0.05 M H&30,; (C) 0.1 M HCI. Experimental conditions: scan rate, 5 mVJs; tempertawe, 23 1°C.
*
flow (compare parts A and B of Figure 6). Thermal desorption from films formed in,borate electrolyte displayed two water desorption rate peaks, similar to those for immersion into pure water (Figure 5A). In addition, desorption peaks characteristic of borate were observed (B+, BO+, BO,+)(Figure 5 G I ) . These borate inclusions were not strongly bound to the oxide: rinsing with pure water removed the surface borate as judged by Auger spectroscopy (Table I), while borate located deeper in the oxide film was released by heating and seen by TDMS. 5. Electrolysis in Hydrochloric Acid. The opencircuit potential in 0.1 M HCI was in the range from -0.15 to -0.16 V vs. NHE. This was the most positive result
Figure 7. Nomaraki microscope photugrsphn of Fe-Cr-Ni surlam follnanng voltammetric scans m aqueouq HCI solutiom (A) Fe-Cr-NitlM));(RJ Fe-Cr-NitlIlJ
among the three electrolytes studied, indicating the greatest degree of spontaneousdissolution. The primary features of the voltammogram in 0.1 M HCl were hydrcgen evolution at 4 . 4 V,surface oxidation near 0.0 V,and rapid diasolution above 0.6 V (Figure 6C). Similar voltammetric behavior was observed for the (111)plane of the same alloy (Figure 51 of ref 2). Thermal desorption following open-circuit immersion into 0.1 M HCI yielded the usual two peaks for water; in addition species related to HC1 were observed 0’.HCl+, CrCP, NiCl+) (Figure 55). Again based upon the Auger spectra, the C1 species were removed from the surface by rinsing with pure water, but the C1 species observed by TDMS were located in the body of the film prior to heating.
866 Langmuir, Vol. 3, No. 6, 1987
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Table 1. Surface ComDositions Determined b s Auger SDectroscoDs' -surface treatment! %Fe %Cr %Ni -
pure H20 Fe-Cr-Ni( 100) OC, immersion OC immersion, 10-min anneal vapor exposure vapor exposure, 14-min anneal 0.05 M HzSO? OC immersion OC immersion, 6-min anneal neg scan to -0.5 V neg scan to -0.5 V, 3-min anneal pos scan to +.75 V pos scan to +.75 V, 3-min anneal post scan to 1.1 v 0.15 M KHzBOB OC immersion OC immersion, 3-min anneal pos scan to +0.8 V cyclic scan, emerse +0.8 V, 10-min anneal pos scan to +LO5 V neg scan to -1.09 v 0.1 M HC1 OC immersion OC immersion, 5-min anneal OC immersion, 5-min anneal, pos scan to +0.96 V, OC immersion, 5-min anneal, pos scan to +0.96 V, 4-min anneal neg scan to -0.38 V neg scan to -0.38 V, 3-min anneal pos scan to +0.28 V pos scan to +0.28 V, 3-min anneal pos scan to +0.93 V pos scan to +0.93 V, 3-min anneal HC1, H20 OC immersion in H,O oc immersion in H;O, anneal (form structure) above and OC HCl immersion above and 3-min anneal
67 26 26 30 27 29 31 29 43 14 29 14 33 46 30 26 30 33 28 43 13 33 26 45 20 38 20 23 26 40 23 42
I
-
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66 52 62 52 68 52 54 32 64 43 66 59 35 66 62 68 49 53 32 65 34 53 29 62 38 61 47 60 36 53 35
aAuger electron yields: Fe(647) 1.88, Cr(526), 3.12, Ni(849) 1.14, O(516) 1.85, S(149) 27.54, K(250) 11.14, B(168) 11.24, Cl(181) 20.49. *All voltages are vs. NHE; OC = open circuit.
Following open-circuitimmersion into HC1 solutions,the oxide film was disordered, as usual. Annealing yielded the pattern of 12 and integral index and new beams characteristic of (7V'2X7d2)R45O symmetry (Figure 2B). The oxygen signal from this structure (32%) was relatively low. These results suggest a thin film having a rectangular mesh such as shown in Figure 3D. Experiments were performed in which a Fe-Cr-Ni(100) surface containing the hexagonal CrO film (prepared by immersion into pure water or 0.1 M HC1) was immersed at open circuit into 0.1 M HC1. As a result of immersion into HCl solution, the surface layer became disordered as judged by LEED. The annealed, oxide-filmed surface displayed a small but noticeable surface oxidation/dissolution peak in 0.1 M HC1 electrolyte. These results are interesting in comparison with those for the (111)surface of the same alloy where a Cr203(001)subphase was stable but the CrO component was unstable. That is, the CrO film which predominates at the (100) surface was not as stable (and therefore not as effective at passivating the surface) as the Cr203film formed on the (111) surface. 6. Nomarski Microscopy. Following completion of the experiments described above, the Fe-Cr-Ni(100) surface was examined by optical microscopy (Nomarski differential interference contrast) and compared to the
Fe-Cr-Ni(ll1) surface previously studied (Figure 7). There are very noticeable differences in the texture of the two surfaces, although both surfaces had been subjected to a similar series of treatments. In particular, the (111) surface displayed very prominent islands about 1pm wide, while the (100) had a comparatively fine texture, suggesting that the etching rate of the (111)plane was lower than along various other directions, leading to establishment of a pattern of grooves with boundaries falling in directions other than (111).Also, examination of the entire surface area of the crystals revealed that the scratch marks from initial mechanical polishing were relatively completely removed from the (100) surface, indicating greater dissolution of the (100) face than of (111). This very noticeable difference in dissolution rate and finish between (100) and (111)orientations invites further investigation of singlecrystal FeCrNi alloy surfaces for practical as well as fundamental purposes.
Acknowledgment. Acknowledgment is made to the National Science Foundation for support of this research. We are thankful to Bruce C. Schardt, Kathleen Werner, and Arunabha Datta for helpful discussions and assistance. Registry No. H20, 7732-18-5; Cr 18, Fe 70, Ni 12 (atomic), 94944-54-4; stainless steel, 12597-68-1.
