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 paramete r in the long-term prediction of pitting resistance of two Fe-Ni-Cr-Mo alloys that are candidate materials for high-level nuclear waste cont ainers. This potential was found to be independent of the extent of pr ior pit growth for alloy 825 (UNS N08825) and type 316L (UNS S31603) s tainless steel (SS). Repassivation potential decreased by an increase in backward scan rate after pits were grown under potentiostatic condi tions. This was related to the effect of potential on repassivation ti me. The corrosion potential (E(corr)) in simulated pit-crevice solutio ns 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 slightl y with an increase in temperature. The relationship between repassivat ion potential and E(corr) in simulated pit solutions was discussed. It was hypothesized that the repassivation potential for shallow pits wa s related to the time-dependent alteration of solution composition ins ide the pits and that the repassivation potential for deep pits was re lated to E(corr) of the depassivated metal inside the pit modified by the ohmic potential drop in the pit.