Synthesis of Artificial Passivation Films by MOCVD

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A series of Fe_2O_3-Cr_2O_3-NiO composite oxide thin films were formed on Pt by a MOCVD method. The corrosion resistance of the films was examined in 5M-HCI by measuring the film thickness and the chemical analysis of the test solution. The decrease rates of film thickness as a function of potential were also examined in 1M-H_2SO_4 using insitu ellipsmetry under potentiostatic control. The dissolution rate of Fe_2O_3-Cr_2O_3 and NiO-Cr_2O_3 composite films decreased exponentially with an increase in the cationic mass fraction of Cr^<3+> ions, X_<Cr>, of the films. The reduction dissolution of Fe_2 O_3 and the oxidation dissolution of Cr_2O_3 proceeded in the cathodic and anodic polarization ranges, respectively. The intrinsic passivity range where no dissolution of Fe_2O_3-Cr_2O_3 films occurred was found between the above two dissolution ranges. The addition of Ta_2O_5 to Fe_2O_3-Cr_2O_3 films provided a beneficial effect to suppress the oxidation dissolution of the films. The activation time of Fe coated with Fe_2O_3-Cr_2O_3 (X_<Cr>>0.5) and Fe_2O_3-Cr_2O_3-Ta_2O_5 (X_<Cr>>0.3, X_<Ta>>0.3) films in H_2SO_4, HCI and NaOH solutions increased with increasing film thickness, and reached a maximum in the thickness range of 70-100 nm.