INTERACTION OF O2 WITH THE FE0.84CR0.16(001) SURFACE STUDIED BY PHOTOELECTRON-SPECTROSCOPY

Soft X-ray photoelectron spectroscopy (SXPS; hv = 130-600 eV) employing synchrotron radiation and X-ray photoelectron spectroscopy (XPS; hv = 1486.6 eV) employing AlKalpha X-rays were used to study the effect of molecular oxygen adsorption (1 to 10(4) L) on the (001) surface of the steel-like alloy Fe0.84Cr0.16 at room temperature. The use of multiple photon energies allowed a qualitative determination of the distribution of species in the oxide film perpendicular to the surface. Prior to oxidation, Fe0.84Cr0.16 samples were annealed to different temperatures to produce varying concentrations of Cr in the top few monolayers of the (001) surface. The differences between the oxide films produced on these different surfaces were determined from SXPS primarily of the Fe3p, Cr3p, and valence levels. Small amounts of carbidic carbon (less-than-or-equal-to 1 ML) that had segregated to the (001) surface of the Fe0.84Cr0.16 alloy during initial sputter/anneal treatments were removed during the first approximately 5 L O2 exposure. In general, the resistance of the Fe0.84Cr0.16(001) surface to oxidation was directly related to the initial Cr concentration at the surface. The oxide films were richer in Cr compared to the bulk concentration for the lowest O2 exposures, and became increasingly enriched in Fe for increasing exposures, in agreement with studies by other groups. The resulting mixed Fe-Cr-O film contained Cr that was in the form of Cr3+ for all exposures. The Fe was in the form of Fe2+ and Fe3+, with an increasing Fe3+/Fe2+ ratio as the O2 exposure is increased. The results indicate that for O2 exposures < 10(4) L, the oxide films do not exhibit sharp boundaries between the Cr-rich and Fe-rich layers, in contrast to previously reported results for much larger (i.e., atmospheric) exposures. The observation of a mixed film, along with thermodynamic analysis, low-energy electron diffraction results, and comparison with other studies is consistent with an oxide film consisting of appreciable amounts of an amorphous, Fe1+xCr2-xO4-like species. The concentration of Fe1+xCr2-xO4 at the films' surfaces decreased in favor of Fe2O3 as the O2 exposure level increased.