Semiconductive behavior of passive films formed on Fe-Cr alloy

Semiconductive properties of passive films formed on Fe-18Cr alloy in a borate buffer solution were investigated using photoelectrochemical response and electrochemical impedance spectroscopy. Band gap energy of the passive films, E-g, was estimated from a photoelectrochemical spectrum that could consist of two components. E(g)s were recognized as approximately 2.4 eV and 3.4 eV, which were mainly derived from Cr hydroxide layer and Cr oxide layer, respectively. The values were almost constant independent of passivation time, film formation potential and additive element. Positive photocurrents generated from the outer part hydroxide and the inner oxide in the passive film increased as applied potential increased, which indicates that the passive films behaved as an n-type semiconductor. Mott-Schottky plot of capacitance showed positive slope. This means that the passive films behaved as an n-type semiconductor. Donor densities of the passive films estimated by the Mott-Schottky plots depended on passivation time, film formation potential and additives. Corrosion resistance of Fe-18Cr alloy was discussed in terms of the semiconductive properties.