The Study of the Passivation Films on Fe-Cr Alloys by Modulation Spectroscopy

In order to gain a better understanding of the change in the film, composition as a function of the Cr content of an alloy or the electrode potential, surface analyses by modulation spectroscopy were performed on a series of Fe-Cr alloys (5-70 weight percent Cr), pure Cr, and pure Fe at various anodic potentials in an acid 1M Na2SO4 solution (pH 2,0). On the basis of photon energies and intensities of characteristic peaks in the measured 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 the Fe-Cr alloy occurs at potentials in the passivity region with increasing Cr content of alloy up to 20% Cr. The film composition continuously changes with changing electrode potential, e.g. the films formed on Fe-20% Cr alloy at potentials in the passivity region are composed of Cr(III) oxyhydroxides containing small amounts of Fe2+ ions and those formed in the secondary passivity region are of Fe(III) oxides, in which small amounts of Cr3+ and Cr6+ ions are contained.