Crevice corrosion stabilization and repassivation behavior of alloy 625 and alloy 22

The crevice corrosion resistance of Alloy 625 (UNS N06625) and Alloy 22 (UNS N06022) was compared based on critical potentials for stabilization and repassivation as well as crevice stabilization rates derived from critical potential data. The effects of temperature and anion composition in 5 M lithium chloride (LiCl) brine-type electrolytes were examined. Repassivation potentials measured at 95degreesC were similar for both alloys and were not greatly influenced by bulk electrolyte composition, pH, or accumulated anodic charge associated with crevice attack. However, a decrease in temperature increased repassivation potentials to a greater extent for Alloy 22 in comparison to Alloy 625. Crevice corrosion stabilization properties of Alloy 625 and Alloy 22 were influenced by temperature and electrolyte composition but not bulk solution pH ranging from 2.75 to 7.75. Crevice stabilization occurred at more active critical potentials in 5 M LiCl electrolytes with a molar ratio of chloride ions to total oxyanions of 100:1 compared to 10:1 or 1:1 (e.g., 1 M LiCl, 1 M sodium sulfate [Na2SO4] + sodium nitrate [NaNO3]) electrolytes, Moreover, a greater potential driving force was required to maintain the same potential-dependent crevice corrosion generation rate, lambda(E), for the 10: 1 ratio electrolyte (e.g., 5 M LiCl:0.5 M Na2SO4 + NaNO3) compared to the 100:1 ratio electrolyte (e.g., 5 M LiCl:0.05 M Na2SO4 + NaNO3). Calculated critical potentials at 1:1 molar electrolyte ratios, obtained from a multiple regression model of critical potential vs electrolyte ratio and temperature, were compared to long-term open-circuit potential data in simulated acidified concentrated waters with molar ratios close to 1:1 chloride:(Na2SO4 + NaNO3) at near 1 M Cl- ion concentrations. The observed lack of stable crevice corrosion propagation on Alloy 22 specimens exposed for 2 years at open-circuit potential (E-ocp) in these environments was successfully predicted from comparison of lambda(E) to E-ocp.