Relation Between the Semiconducting Properties of a Passive Film and Reduction Reaction Rates

The films forming on pure chromium and Alloy C22 in aqueous borate buffer were investigated with potential polarization scans, electrochemical impedance spectroscopy, and Mott–Schottky analysis. The capacitances of both films were frequency dependent, and a constant phase element (CPE) best described the frequency dispersion. Cathodic polarization scans were used to assess the accuracy of flatband potential values calculated from Mott–Schottky tests using different models to extract an effective capacitance from a CPE. It was found that using effective capacitances calculated with an expression developed by Brug et al. [ J. Electroanal. Chem. , 176 , 275 (1984)] led to flatband potentials that closely agreed with cathodic polarization scans. Both films were found to be n-type, and flatband potentials of and vs standard hydrogen electrode were found for Cr and Alloy C22, respectively.