Stifling of Crevice Corrosion And Repassivation: Cathode Area Versus Controlled Potential Decreases

ABSTRACT The effects of limited remote cathode area on crevice corrosion propagation, repassivation and reorganization were investigated on 316 stainless steel (UNS S31600) in 0.6 M NaCl at 50°C. Both multiple crevice assemblies (MCA) and rescaled crevices enabling testing of coupled multi-electrode arrays (CMEA) were utilized. Potentiostatically activated crevices were subjected to situations which limited propagation by either (i) stepping the potential from levels required to initiate crevice corrosion to those below the repassivation potential, (ii) performing a downward or negative scan of the potential from high potential, or (iii) progressively decreasing the area of a galvanically coupled platinum cathode situated outside the crevice after periods of crevice corrosion propagation. 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. Thecrevice anode sites simultaneously reorganized into smaller active areas active deeper in the crevice. Application of Galvele’s product for crevice stabilization suggested that such low potentials lowered the chemical stability product i·a or i·(a+d) where d is the crevice depth and a is the depth of crevice corrosion at individual crevice sites below a critical level. INTRODUCTION One of the earliest stages of crevice corrosion initiation requires the separation of the anode and the cathode, as described by Oldfield and Sutton.¹ This occurs after the oxygen present in the crevice is completely depleted by the local cathodic reactions where the rate of cathodic reactions exceeds the transport rate of oxygen into crevices. The major cathode is then repositioned to the oxygen rich environment outside the crevice and will support the anodic reactions in the crevice.