Analysis on the passive films of Fe, Cr and Fe-25Cr alloy by photo-electrochemical methods

Abstract

Passive films on Fe, Cr and Fe-25Cr formed in pH 8.5 buffer solution at potentials 0 to 900 mV vs. SCE were examined by photo-electrochemical methods that use monochromatic light source generated by Xe lamp with a scanning digital monochromator automatically controlled by a stepping motor. The passive film formed on Fe was structurally similar to gamma-Fe2O3. Band gap energy for the passive film exhibiting an amorphous semi-conductor was newly interpreted by considering the contribution of localized states to the photo current. Photo current spectra of passive films on Fe were analyzed in terms of gamma-Fe2O3 with a spinel structure. The p-d and d-d excitation processes were involved in the photo current spectra of passive film on Fe. The structure of the passive film formed on Fe and Fe-25Cr alloy were discussed by comparing the influences of passivation potential on the band gaps measured at the same potential. Effects of addition of Cr to Fe on the electronic structure of the passive film were studied by comparing the photo current spectra and band gaps for the passive film on Fe-25Cr with those on Fe and Cr. A significant reduction in the photo current for passive film on Fe-25Cr compared with that on Fe was associated with a decrease in the space charge layer thickness by an increase in the density of the localized energy state..