抄録
The optical constant for a “film-free” surface of 18-8 stainless steel was determined as \barn3=2.67−4.03i (at 5461 Å) by ellipsometric measurements in dehydrated methanol immediately after the “film-removing” treatment in a 0.5% bromine-methanol solution.
Using this optical constant of the “film-free” surface, the ellisometric parameters, Δ and Ψ, for the steel measured in 1M-Na2SO4 solutions of vavious pH under potentiostatic control were analyzed in order to obtain the optical constants and the thickness of the films formed in these conditions. The optical constants of passive and transpassive films were dependent on the pH of solutions used, and summarized as follows;
for the passive film as \barn2=2.5−0.5i (pH=6.45), \barn2=2.5−0.6i (pH=4.0), \barn2=2.0−0.2i (pH=2.0∼3.0), \barn2=2.0−0.1i (pH=0.0), and for the transpassive film as \barn2=3.0−0.6i (pH=6.45), \barn2=3.0−0.3i (pH=4.0), \barn2=3.0−0.4i (pH=3.0).
The difference in optical constants between the passive and the transpassive films could be attributed to the difference in chemical compositions of these films, which were analyzed by means of ESCA.
The thickness of the passive film was a function of the electrode potential and of the pH of solutions used. The growth rate of the film thickness with potential increased with increasing pH of the solutions, and the maximum thickness of the passive films formed in neutral solutions was larger than those of the films formed in acidic solutions.
