Determination of the Crevice Corrosion Stabilization and Repassivation Potentials of a Corrosion-Resistant Alloy

Ni-Cr-Mo alloys are highly resistant to general and localized corrosion, but they may suffer crevice corrosion in aggressive environmental conditions, such as high-chloride concentrations, applied potentials, and temperatures. It is assumed that localized corrosion will only occur when the corrosion potential (E-CORR) is equal or higher than the crevice corrosion repassivation potential (E-R,E-CREV). The latter is measured by different electrochemical techniques using artificially creviced specimens. These techniques include cyclic potentiodynamic polarization (CPP) curves, the Tsujikawa-Hisamatsu electrochemical (THE) method, or other non-standard methods. Recently, as a variation of the THE method, the potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) technique was introduced. The goal of the present work was to determine reliable critical or protection potentials for the crevice corrosion stabilization and repassivation of Alloy 22 (UNS N06022) in chloride solutions at 90 C. Conservative methodologies (which include extended potentiostatic steps) were applied for determining protection potentials below which crevice corrosion cannot stabilize and propagate. These values were compared to repassivation potentials obtained from the PD-GS-PD technique to assess its reliability. The CPP and THE methods were also considered for comparison. The repassivation potentials from the PD-GS-PD technique were conservative and reproducible, and they did not depend on the amount of previous crevice corrosion propagation in the studied conditions.