In situ analysis of passive films on alloy 600 by modulated UV-visible reflection spectroscopy

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, X_Ni, X_Cr and X_Fe of passive films were determined. The cationic mass fractions changed with potential; for the films formed at pH 2.0, the value of X_Cr decreased from 0.7 at 0.0V to 0.1 at 1.0V and at the same time, the value of X_Fe 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 X_Cr for the passive films gradually decreased with increasing pH, while that of X_Fe was almost independent of pH.