Ellipsometric Measurement of Optical Constants and Thickness of Passive Films Formed on 18–8 Stainless Steel

The optical constant for the film-free surface of 18–8 stainless steel was determined by the ellipsometric measurement in dehydrated methanol immediately after the film removal treatment in a 0.5% bromine-methanol solution.Using this optical constant of the film-free surface, the ellipsometric parameters, Δ and Ψ, measured for the steel in 1M–Na2SO4 of varying pH under potentiostatic control were analyzed and the optical constants and thickness of passive and transpassive films were determined. The optical constants of these films were dependent on the pH of solutions and closely related to the difference in film composition. The thickness of passive film and its increase rate with potential also depended on pH and became larger with increasing pH.