Oxyanion Inhibition of Passivity Breakdown and the Nucleation of Pits on Type 316L Stainless Steel

Abstract Passivity breakdown of Type 316L SS (UNS S31603) in the presence of aggressive Cl− and inhibitive NO3− anions has been experimentally studied and the results have been interpreted in terms of the Point Defect Model (PDM). By expanding the PDM to include competitive adsorption of Cl− and NO3− into surface oxygen vacancies at the passive film/solution interface, the critical breakdown potential (Vc) has been predicted to vary linearly with log[Cl−] and with log([Cl−]/[NO3−]), which is found experimentally. The slope of Vc vs. log[Cl−] is found to be unaffected by NO3−, thereby yielding the same values for the polarizability of the film/solution interface, regardless of the nitrate concentration. The critical breakdown potential increases weakly with increasing nitrate concentration at low ⌊NO3−⌋ but, at a concentration of 0.06 M, Vc increases sharply and pitting attack is no longer observed. The viability of the PDM for accounting for passivity breakdown on Type 316 SS is explored by measuring the voltage scan rate dependence of the critical breakdown potential, from which the critical areal (two dimensional) concentration of condensed vacancies at the metal/barrier layer interface can be derived. Good agreement between the value obtained from experiment and those calculated from structural arguments demonstrate the validity of the PDM.