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 [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 -1.217 and -0.617 V vs standard hydrogen electrode were found for Cr and Alloy C22, respectively.