In order to gain a better understanding of the change in the chemical composition of surface films on stainless steels as a function of the alloy composition or electrode potential, surface analyses by modulation spectroscopy weree performed on a series of Fe-Cr alloys (5 to 70% Cr), 20Cr-25Ni stainless steel and pure metals constituting these alloys (Fe, Cr and Ni) at various anodic potentials in 1M-Na2SO4 solution (pH 2.0). On the basis of photon energies and intensities of characteristic

more »

... s in the modulated reflection spectra, the kinds and quantities of constituent species of the films were examined. It was found that a sharp and substantial increase in Cr3+ ions in the film of Fe-Cr alloy occurs at potentials in the passivity region with increasing Cr content of the alloy up to 20%. The film composition continuously changes with electrode potential, e. g., the film formed on Fe-20Cr alloy at a potential in the passivity region is mainly composed of chromic oxides and in the secondary passivity region is of ferric oxides. The passive film on 20Cr-25Ni stainless steel has nearly the same composition as that on Fe-20Cr alloy except for containing a small amount of nickel oxides.