Stifling of Crevice Corrosion and Repassivation: Cathode Area Versus Controlled Potential Decreases Assessed with a Coupled Multi-Electrode Array

The effects of limited remote cathode area on crevice corrosion propagation, repassivation, and anode site reorganization were investigated on AISI 316 stainless steel (UNS S31600) in 0.6 M NaCl at 50°C. Both multiple crevice assemblies and rescaled coupled multi-electrode arrays were utilized. Potentiostatically activated crevices were subjected to conditions that limited propagation by either: (i) stepping the potential downward below the repassivation potential, (ii) performing a downward scan of the potential from high potential, or (iii) decreasing the area of a galvanically coupled platinum cathode situated outside the crevice. In the first two cases, repassivation occurred when the potential reached the lower statistical limit of the repassivation potential measured on creviced specimens in a downward scan. During the galvanically coupled test, repassivation occurred when the galvanic cathodic current supplied became lower than the current required to maintain a couple potential equal to the repassivation potential at the mouth of the crevice. The remaining crevice anode sites first reorganized into smaller active areas deeper in the crevice. Application of Galvele's product for crevice stabilization suggested that such negative potentials lowered the chemical stability product i · a or i · (a + d) below a critical level, with d being the crevice depth and a being the depth of crevice corrosion at individual crevice sites.