Passivity breakdown on Type 316L stainless steel in the presence of C1¯ and NO¯3 species has been studied and the data are interpreted in terms of the Point Defect Model. Linear relationships between the breakdown potential (Vc) and log ([C1¯]) and log ([C1¯]/[NO¯3]), as predicted by the PDM are found experimentally. The interfacial reaction, in which a cation is ejected from the barrier layer to form the surface vacancy pair, is found to be a charge transfer process with the transition state being located close to the final state on the reaction coordinate. The dependence of Vc on potential scan rate yields the critical, areal concentration of condensed cation vacancies (ξ) at the metal/barrier layer interface that is in good agreement with that calculated theoretically based on the structure of Cr2O3, of which the barrier layer is postulated to comprise.
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