Electronic structure of passive films formed on molybdenum-containing ferritic stainless steels

The effect of molybdenum on the electronic structure of the passive films formed on ferritic (Fe-Cr and Fe-Cr-Mo) stainless steels is examined by capacitance and photoelectrochemical measurements. The capacitance study is support ed by a mathematical analysis of the Schottky barrier developed at the semiconductor-electrolyte interlace in the case of a semiconductor with multiple bulk electronic states in the bandgap. The numerical simulations, based on the more general Mott-Schottky relation proposed, are in good agreement with the experimental results. It can be concluded that the capacitance behavior of the passive films is related to the contributions of a shallow donor level very close to the conduction band and a deep donor level at about 0.4 eV below the conduction band. The addition of molybdenum decreases the donor density oi the deep level. Photoeffects observed for subbandgap photon energies reveal that this deep donor level behaves like a trapping electronic state situated in the space-charge zone of the passive film. On the basis of the results obtained, the crystallographic factors determining the nature of the donor species are discussed.