Modulated reflection spectra of passive films on a series of Ni-Cr alloys (5 to 28 mass% Cr) have been measured in 1 kmol multiplied by (times) m** minus **3 Na//2SO//4 of pH 6. 0. Quantitaive relations between the characteristics of spectra and the composition of the films have been derived from spectra for thin composite films of NiO-Cr//2O//3 artificially prepared on Pt by MO-CVD technique. From these relations, cationic mass fractions of Cr**3** plus ions, X//C//r, of passive films on the alloys were determined. It was found that passive films on the alloys were composed of Ni(II) and Cr(III) oxides, and that their cationic fractions of Cr**3** plus ions changed remarkably with the Cr content of the alloy. That is, the values of X//C//r for the films formed at 0. 10 v increased from 0. 10 for Ni-5 mass% Cr alloy to 0. 88 for Ni-28 mass% Cr alloy with increasing Cr content of the alloy. This indicates that Cr**3** plus ions were substantially enriched in the films on the alloys containing more than 15 mass% Cr. The value of X//C//r for each alloy was almost independent of the potential in the passivity region below 0. 5 v.
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