Influence of Passive Film's Semiconductor Properties on Metastable/Stable Pitting Corrosion

We are investigating the influence of the electronic properties of passive films on the alloys' resistance to pitting corrosion. In particular, we seek to determine if the amount of metastable pitting and the transition from metastable pitting to stable pitting correlates with the passive film's carrier concentration and/or flat band potential. To this end, three types of tests have been conducted on iron in borate buffer and dilute aqueous chloride solutions, and on Alloy C22 and chromium in mildly alkaline and acidic chloride solutions, at room temperature. First, the occurrence of metastable pitting and the transition from metastable pitting to stable pitting were determined from measurements of the alloy's free corrosion potential. Second, electrochemical impedance spectra (EIS) were obtained on samples polarized in the passive region. Third, Mott-Schottky (M-S) tests were conducted immediately following the EIS in order to determine the carrier identity, the carrier concentration and the flatband potential of the passive film. M-S tests conducted at several different frequencies indicated the capacitance of the passive films was dependent on frequency. The capacitance of the film was extracted from the M-S measurements through an analysis developed by Brug et al. to correct for frequency dispersion of the capacitance in EIS tests. The results indicate that for n-type passive films, the alloy's susceptibility to stable pitting corrosion correlates with the difference between the alloy's corrosion potential and its passive film's flatband potential. This correlation suggests that the pitting susceptibility is related, in part, to the rate of electrochemical reduction reactions occurring outside the pit. Electrochemical reduction of oxygen outside the pit enhances pit acidification and drives the local oxidation reaction. The susceptibility to metastable pitting did not correlate with the film's defect concentration, as measured by the slope of the Mott-Schottky plot.