Critical Factors in the Initiation and Repassivation of SS 316L Crevice Corrosion

RIP2025-00089: Results are presented from investigations of SS 316L crevice corrosion initiation and repassivation. Real-time fluorescence measurements of the chloride concentration ([Cl-]) inside SS 316L remote crevice assemblies as a function of bulk [Cl-] and potential found that crevice corrosion initiates at a location on the crevice wall when the [Cl-] at that location exceeds the value necessary to initiate pitting for the wall potential. That is, the wall potential is equal to the pitting potential for a threshold chloride concentration. Conversely, when the [Cl-] within the crevice remained below the threshold, initiation did not occur even when the [Cl-] reached 2 M. In other measurements, no change in [Cl-] with time was measured prior to initiation indicating the necessary threshold was attained by a spontaneous event, such as a metastable pit. Typically, this only occurred at applied high potential (e.g. close to the pitting potential). Thus, the pitting potential can be thought of as a threshold for SS 316 crevice corrosion initiation. With respect to repassivation, the onset occurs with salt film dissolution at the so called transition potential. Below this potential is a region of IR controlled propagation and the crevice fully repassivates when the current falls to zero at the crossover potential, when the wall potential and applied potential are equal. Implications of this finding on the Tsujikawa-Hisamatsu electrochemical method (THE) for crevice repassivation as described in the latest ASTM standard are discussed.