Localized Corrosion of Alloy 22 in Sodium Chloride Solutions at Elevated Temperature

The localized corrosion behavior of Alloy 22 (UNS N06022) in acidified (pH = 3) sodium chloride (NaCl) brines at elevated temperature (80°C) and under potentiostatic and open-circuit conditions has been investigated. Both pitting corrosion and intergranular attack are observed on specimens polarized into the transpassive dissolution region. Under open-circuit conditions, the corrosion potential is observed to shift in the positive direction with time, with a steady state being achieved after about four months of exposure. The steady-state corrosion potential is found to be more positive (0.25 V vs. standard hydrogen electrode [SHE]) in aerated, saturated NaCl at 80°C (6.2 m) than in the deaerated environment (0.15 VSHE). After an exposure period to pH = 3 for 180 days, saturated NaCl brine at 80°C, small pits were observed on the surfaces, particularly under aerated conditions. This observation is consistent with the detection of metastable pitting in potentiostatic transients (chronoamperograms) measured at a higher potential (0.49 VSHE vs. 0.25 VSHE) and at a slightly higher temperature (95°C vs. 80°C). Damage functions constructed for pits that nucleate and grow under high potential conditions (948 mVSHE or 998 mVSHE, within the transpassive dissolution region) indicate that Alloy 22 is a strongly repassivating system, which limits the depths to which pits will grow into the surface. Damage functions calculated from damage function analysis confirm that the penetration of pits into the substrate under prototypical repository conditions should be severely limited by the strong tendency toward delayed repassivation. The available evidence suggests that pitting corrosion will not pose a significant threat to canister integrity in a Yucca Mountain-like repository, but this conclusion needs to be confirmed with further experimental and theoretical analysis. The high resistance of Alloy 22 to passivity breakdown and stable pitting is attributed to a low metastable pit nucleation rate, a low survival probability, and a low stable pit penetration rate, in addition to the strong tendency for pit repassivation, as noted above.