Corrosion characteristics of Fe₂O₃Cr₂O₃ artificial passivation films under potentiostatic control

Fe₂O₃Cr₂O₃ composite oxide thin films, that is, artificial passivation films, with various Cr₂O₃ content were formed on Pt substrates by a MOCVD method. The rate of change of film thickness as a function of potential was examined in 1 kmol m^-3 H₂SO₄ using in-situ ellipsometry under potentiostatic control. The rate of thinning can be divided into four characteristic ranges according to potential; i.e. the range of the reduction dissolution of Fe₂O₃ component, the range of no dissolution, the range of the field-assisted dissolution of Fe₂O₃ component, and the range of the oxidation dissolution of Cr₂O₃ component. The range of no dissolution of Fe₂O₃Cr₂O₃ composite films was named the intrinsic passivity. The anodic polarization curve for an Fe18Cr alloy in passive and transpassive potentials was also divided into four characteristic ranges which correspond to the ranges for the artificial passivation films. The dissolution behavior of passive and transpassive films can be explained based on the origin of the four ranges. It was presumed that passive films on FeCr alloys exhibit their intrinsic nature in the potential range of the intrinsic passivity.