Repassivation of stainless steels: A unifying quantitative framework

This study presents a unifying quantitative framework for understanding the repassivation process, connecting the values of E_rp to key parameters such as pH, (i⋅x)_crit, anodic and cathodic kinetics, i_c/i_a, f, and electrode-electrolyte interfacial chemistry. The framework integrates potentiostatic, fast potentiodynamic, and galvanodynamic experiments with thermodynamic modeling using a mixed solvent thermodynamic database. Using SS316L and SS304 in 0.6 M NaCl as exemplars, the study demonstrates that repassivation potential decreases with pit depth until it plateaus, with SS316L showing a potential of −0.15 to −0.165 V vs. SCE and SS304 at −0.18 to −0.2 V vs. SCE. The differences in repassivation potential between the alloys are attributed to two roles the Mo in SS316L plays, namely suppressing anodic kinetics and accelerating cathodic kinetics. The relative rate of local cathodic kinetics within the pit are crucial for repassivation via CrOOH precipitation, with experimental i_c/i_a values of 2–4.5 % for SS316L and $\le$1 % for SS304 and up to 9.5–10.5 % required theoretically at high fractions of saturation. The (i⋅x)_crit was found to be similar for both alloys, approximately 0.32 A/m², with f ranging between 0.35 and 0.4. Anodic and cathodic kinetics were found to be independent of the pit depth at critical repassivation conditions.