INSITU ANALYSIS OF PASSIVE FILMS ON FE-CR-NI ALLOY BY POTENTIAL-MODULATED UV-VISIBLE REFLECTION SPECTROSCOPY

Modulated reflection spectra of passive films on Fe-20Cr-25Ni alloy have been measured in 1 kmol.m-3 H2SO4 (pH 0.1), 1 kmol.m-3 Na2SO4 (pH 2.0, 6.0, and 9.5), and 0.1 kmol.m-3 NaOH (pH 13.1) 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 Fe2O3-Cr2O3-NiO with known composition, which were prepared on Pt by metallorganic chemical vapor deposition. From these relations, cationic mass fractions of Fe3+, Cr3+, and Ni2+, X(Fe), X(Cr), X(Ni), of passive films on the alloy were determined. It was found that the cationic mass fraction of passive films on the alloy changed remarkably with potential; for films formed at pH 0.1, the value of X(Cr) decreased from 0.70 at 0.0 V to 0.10 at 1.1 V and, at the same time, the value of X(Fe) increased from 0.25 to 0.78 with increasing potential, while that of X(Ni) remained almost constant at ca. 0.10. This indicates that Cr3+ ions are substantially enriched in the passive films and Fe3+ ions are in the transpassive films, whereas Ni2+ ions are depleted in both the films. The value of X(Cr) for the passive films gradually decreased with increasing pH and that of X(Fe) increased corresponding, while that of X(Ni) was almost independent of pH.