Formation of High Corrosion Resistant Artificial Passivation Films by LP-MOCVD

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Single oxide films of Al_2O_3, TiO_2, Ta_2O_5, ZrO_2 and SiO_2, and composite oxide Films of Fe_2O_3-Cr_2O_3-NiO and Ta_2O_5-ZrO_2 have been formed on Pt substrates by metal organic chemical vapor deposition (MOCVD) technique. The corrosion resistance of these films were examined in 5M-HCl and 5M-NaOH by the measurement of film thickness using ellipsometry and the chemical analysis of test solutions using inductively-coupled plasma-emission spectroscopy.It was found that the corrosion resistance of the single oxide films increased in the following order: Al_2O_3<TiO_2<SiO_2<ZrO_2<Ta_2O_5 in 5M-HCl, and Al_2O_3<SiO_2<TiO_2<Ta_2O_5<ZrO_2 in 5M-NaOH. Composite oxide films of Ta_2O_5-ZrO_2 having cationic mass fraction of Ta^<5+> ions between 0.3 and 0.5 showed the highest corrosion resistance against both the HCl and NaOH solutions. The dissolution rate of Fe_2O_3-Cr_2O_3 composite films in 5M-HCl decreased exponentially with an increase in the cationic mass fraction of Cr^<3+> ions, X_<cr>, of the films. The same type of changes in the dissolution rate with X_<cr> was also observed for the NiO-Cr_2O_3 composite films. On the other hand, the addition of NiO to Fe_2O_3-Cr_2O_3 composite films containing adequate amount of Cr_2O_3 did not bring an effective improvement in corrosion resistance. The corrosion resistance of SiO_2-coated SUS304 stainless steels increased with increasing thickness and formation temperature of the SiO_2 films.