Potential modulation reflectance of passivated type 304 stainless steel in sulfuric acid solution

Potential modulation reflectance (PMR) was applied to passivated type 304 stainless steel covered by a passive oxide film in 0.1 mol dm−3 sulfuric acid solution. The ellipsometry measurements showed that the passive oxide film was 1.0 nm thick at the beginning of passivation at 0.1 V vs. Ag/AgCl and increased to 1.8 nm with potential at 0.9 V in the transpassive region. Under positive bias, the signal intensity of PMR was proportional to the capacitance of the space charge formed in the n-type semiconducting passive oxide. The Mott–Schottky type plot was applicable to PMR as well as capacitance. From the plots the oxide film was found to behave as an n-type semiconducting layer. A threshold photon energy of 2.4 eV in the PMR–wavelength relation may correspond to the optical absorption edge of the passive oxide.