Mechanistic insights into pitting potential, incubation time and critical temperature: An extension of the Point Defect Model

A refined mechanistic framework was proposed to extend the Point Defect Model (PDM) by incorporating quantitative descriptions of Cl – adsorption on passive film surface, its reaction order, the dependence of critical pitting potential on key factors, and the effect of point of zero charge on pitting susceptibility. Cl⁻ adsorption and insertion into oxygen vacancy promote film thinning and the generation of metal cation vacancies. The model uniquely predicts that surface concentration of adsorbed Cl⁻ depends on both its aqueous concentration and the applied potential, while film thickness decreases with increasing Cl⁻. The reaction order with respect to Cl⁻ was determined as 0.88 for iron (Fe) in a pH 8.60 borate buffer solution. The framework quantitatively correlates pitting potential, pitting incubation time, and critical pitting potential with environmental conditions, and bridges macroscopic pitting behavior with atomic-scale descriptors, including Cl⁻ adsorption, insertion and dissolution activation energies from first-principles calculations. It has been validated for Fe and extended to Fe-based alloys, with the potential to provide a predictive approach for evaluating pitting susceptibility. The model preliminarily incorporates several physically motivated quantities to extend the PDM, while the refinement of boundary conditions and broader validation under practical service conditions are expected to be addressed in future work.