Relation between the semiconducting properties of passive films and electrochemical and corrosion properties

Aqueous corrosion is a common materials degradation mechanism and is thought to be a likely failure mode of high level nuclear waste canisters. Corrosion damage is often mitigated by a protective oxide film that forms naturally on many metals. The semiconducting properties of these films often have a large impact on the electrochemical and corrosion behavior of metal-passive film systems.;In this study the flatband potential and charge carrier density of the films that form on Alloy C22, Cr, Ti, Fe, Ni, Mo and mild steel were evaluated by Mott-Schottky analysis. All films with the exception of Ni oxide were n-type at all measured frequencies. The calculation of the flatband potentials and charge carrier densities was complicated by measured capacitances that were frequency dependent. A new method was proposed to fit the frequency dispersion with a Constant Phase Element (CPE) and then expressions developed by Brug et al. [J. Electroanal. Chem., 176, 275 (1984)] or Hsu and Mansfeld [Corrosion, 57, 747 (2001)] were used calculate effective capacitances as a function of potential. The success of this method was evaluated using cathodic polarization tests. Flatband potentials calculated using Brug et al.'s expression were in excellent agreement with the onset potential of reduction current growth in polarization scans.;It was found that films with more negative flatband potentials were less susceptible to localized attack due to the slower reduction kinetics associated with low flatband potentials. Alloy C22 has a very negative flatband potential and is highly resistant to localized corrosion at room temperature but improved reduction kinetics at elevated temperature contributes to Alloy C22's susceptibility to metastable pitting at high temperatures.