Electrochemical and corrosion properties of Fe2O3-Cr2O3-MoO2 artificial passivation films in HCl solutions

A series of Fe2O3-Cr2O3-MoO2 artificial passivation films that analogize with passive films on Fe-Cr-Mo alloys were prepared on Pt substrates by ion beam sputter deposition. Thinning rates of the films were measured in 1 M HCl by in situ ellipsometry under potentiostatic polarization. The addition of MoO2 to Fe2O3-Cr2O3 films suppressed the reductive dissolution of the Fe2O3 component of the films under cathodic polarization, but accelerated the oxidation dissolution of the films due to the transpassive dissolution of MoO2. The transpassive dissolution of MoO2 was suppressed by increasing Cr3O2 content of the films. An Fe2O3-Cr2O3-MoO2 film with the same cationic fractions as those of passive film on an Fe-18Cr-10Mo alloy hardly dissolved in the potential range of -0.3 to 1.0 V. This potential range covers the stable passivity region of Fe-18Cr-10Mo alloy in the same solution. Fitting potentials of Fe-Cr-Mo alloys containing Cr and/or Mo less than the Fe-18Cr-10Mo alloy exist in the reduction dissolution region of the Fe2O3 component of Fe2O3-Cr2O3-MoO2 films. This suggests that the reductive dissolution of the Fe2O3 component in the passive films on the alloys may participate in pitting on the alloys. (C) 2000 The Electrochemical Society. All rights reserved.