APPLICABILITY OF REPASSIVATION POTENTIAL FOR LONG-TERM PREDICTION OF LOCALIZED CORROSION OF ALLOY-825 AND TYPE-316L STAINLESS-STEEL

Repassivation potential (E(rp)) was investigated for use as a parameter in the long-term prediction of pitting resistance of two Fe-Ni-Cr-Mo alloys that are candidate materials for high-level nuclear waste containers. This potential was found to be independent of the extent of prior pit growth for alloy 825 (UNS N08825) and type 316L (UNS S31603) stainless steel (SS). Repassivation potential decreased by an increase in backward scan rate after pits were grown under potentiostatic conditions. This was related to the effect of potential on repassivation time. The corrosion potential (E(corr)) in simulated pit-crevice solutions increased with a decrease in pH and was independent of chloride (Cl-) concentration even to the point of depassivation. The depassivation pH (pH(D)) was independent of Cl- concentration and increased slightly with an increase in temperature. The relationship between repassivation potential and E(corr) in simulated pit solutions was discussed. It was hypothesized that the repassivation potential for shallow pits was related to the time-dependent alteration of solution composition inside the pits and that the repassivation potential for deep pits was related to E(corr) of the depassivated metal inside the pit modified by the ohmic potential drop in the pit.