Dependence of corrosion resistance of Fe₂O₃-Cr₂O₃ artificial passivation films on crystal structure and chemical state of constituent elements of the films

Fe₂O₃-Cr₂O₃ artificial passivation films were formed on Pt substrates by low pressure metallorganic chemical vapor deposition technique using iron(III) acetylacetonate [Fe(O₂C₅H₇)₃] and chromium(III) acetylacetonate [Cr(O₂C₅H₇)₃] as precursors at substrate temperatures of 150-350 °C. Relationships between the crystal structure, and also the chemical state of constituent elements, and the corrosion resistance were examined on the films in acid solutions. The films deposited above 300 °C had crystalline structures, and those deposited below 250 °C had amorphous structures. The films deposited above 250 °C had a high amount of M-O-type chemical bonds, and those deposited below 200 °C had a high amount of M-OH-type chemical bonds. The films deposited above 300 °C hardly dissolved in 1.0 M HCl and those deposited below 250 °C, however, easily dissolved in the solution. The dissolution rate of the films in the solution increased with decreasing substrate temperature. Passive and transpassive current densities of the films in 1.0 M H₂SO₄ were dependent on the substrate temperature and increased with decreasing the temperature. When polarized cathodically in 1.0 M H₂SO₄ and 0.5 M HCl, the films deposited below 250 °C dissolved due to the reduction of the Fe₂O₃ component in the films. The reduction of Fe₂O₃ component was, however, suppressed on the films deposited above 300 °C. Therefore, with increasing crystallinity and amount of M-O-type chemical bonds, the corrosion resistance of the films increases in HCl and H₂SO₄ solutions.