Theoretical and experimental studies of the pitting of type 316L stainless steel in borate buffer solution containing nitrate ion

Passivity breakdown on Type 316L stainless steel (UNS S31603) in the presence of aggressive (Cl−) and inhibitive (NO3−) species has been studied and the experimental data are interpreted in terms of the Point Defect Model (PDM). Combining the PDM with competitive adsorption of Cl− and NO3− into surface oxygen vacancies at the passive film/solution interface predicts that the critical breakdown potential (Vc) will vary linearly with log[Cl−] or log([Cl−]/[NO3−]) for systems containing only chloride ion or chloride plus nitrate ion, respectively. The experimental measurements confirm these linear relationships and fundamental parameters contained within the PDM have been determined. The transfer coefficient, α0, of the reaction in which a cation is ejected from the barrier layer to form the surface vacancy pair, VMVO(2−χ), is about 1.00, indicating that the transition state resembles the products. Furthermore, the dependence of Vc on potential scan rate has been measured, and the critical, areal concentration of condensed cation vacancies (ζ) at the metal/barrier layer interface been calculated. That value is in good agreement with the theoretical value calculated from the structure of Cr2O3, of which the barrier layer is postulated to comprise, demonstrating that the PDM successfully accounts for passivity breakdown on UNS S31603 in aqueous solutions containing both aggressive and inhibitive species.