Modulated reflection spectra of passive films have been measured in 1 kmol·m-3 Na2SO4 (pH 2.0 and 6.0), 0.15 kmol·m-3 H3BO3+0.0375 kmol·m-3 Na2B4O7 (pH 8.45) and 0.1 kmol·m-3LiOH (ph 12.8) solutions, respectively. Quantitative relations between the characteristic of spectra and the composition of the films have been derived from the spectra for thin composite films of NiO-Cr2O3-Fe2O3 prepared on Pt by MO-CVD. From these relations, cationic mass fractions of Ni2+, Cr3 and Fe3+ ions, XNi, XCr and XFe of passive films were determined. The cationic mass fractions changed with potential; for the films formed at pH 2.0, the value of XCr decreased from 0.7 at 0.0V to 0.1 at 1.0V and at the same time, the value of XFe increased from 0.05 to 0.3 with increasing potential. This indicates that Cr3+ ions are substantially enriched in the passive films and Fe3+ ions are in the transpassive films. The value of XCr for the passive films gradually decreased with increasing pH, while that of XFe was almost independent of pH.
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