Effect of Potential on Crevice Corrosion Kinetics of Alloy 22 Available to Purchase

Alloy 22 (UNS N06022) is a Ni-Cr-Mo alloy, highly resistant to localized corrosion. It has a wide range of industrial applications. Because of its versatility, Alloy 22 has been proposed as a corrosion-resistant barrier for high-level nuclear waste containers. Crevice corrosion is expected to occur in hot chloride solutions if the corrosion potential is equal to or greater than the crevice corrosion repassivation potential. For less-aggressive environments, transpassive dissolution limits the alloy performance at high potentials. The goal of the present work was to establish the effect of potential on the kinetics of chloride-induced crevice corrosion of Alloy 22 at 90°C. Electrochemical tests carried out included cyclic potentiodynamic polarization (CPP) curves, polarizations at constant applied potentials followed by repassivation, and electrochemical impedance spectroscopy (EIS) measurements at anodic applied potentials. The attack profiles were measured using a roughness checker. Crevice corrosion susceptibility of Alloy 22 was independent of bulk solution pH, but it was a strong function of potential. Likelihood of transpassive dissolution was found to increase for high pH solutions. Two different regions were distinguished in polarization tests at a constant potential. A first region was associated with crevice corrosion incubation and a second one was associated with crevice corrosion initiation and propagation. The crevice corrosion current density was estimated as a function of potential. It increased with potential up to a maximum value of approximately 20 mA/cm². Absence of pitting corrosion was explained in terms of such a small current density, which was insufficient to satisfy a critical value of the product x·i, necessary for pitting corrosion to occur. A diagram of crevice corrosion susceptibility and transpassive dissolution for Alloy 22 in 1 M chloride solutions at 90°C was presented. If the corrosion potential exceeds the crevice corrosion repassivation potential, crevice corrosion will initiate, provided that the diffusion path associated with the crevice is long enough.