Theoretical Analysis of the Effects of Alloying Elements on Distribution Functions of Passivity Breakdown

Work over the past decade has shown that the critical voltage and induction time for the breakdown of passive films in aqueous systems containing aggressive anions follow statistical frequency distributions. Recently, we have demonstrated that the point defect model (PDM) can be used to explain the frequency distributions in and as a function of voltage ( only), chloride concentration, and pH. Furthermore, we developed the solute/vacancy interaction model (SVIM) to account for the effects of minor alloying elements (e.g., Mo) on the breakdown parameters for a single site on a film. This model assumes that highly charged solutes, when substitutionally present in the film, electrostatically interact with mobile cation vacancies resulting in a shift in the breakdown voltage to more positive values and in the induction times to longer times. In this paper, we combine the models to account for the effects of minor alloying elements on the distributions in the breakdown voltage and induction time for the nucleation of pits on heterogeneous metal surfaces. This is done by estimating solute/vacancy equilibrium constants using ion‐pairing theory with an appropriate Debye‐Huckel correction for screening. The model is shown to account for the shift in for ferritic stainless steels containing molybdenum (Fe‐17Cr, Fe‐18Cr) in sodium chloride solutions, assuming that the molybdenum exist in the +6 state. The model also predicts that solute/vacancy complexing will shift the distribution in to longer times, thereby further enhancing the resistance of the film to environmentally induced breakdown.