NH₄ ⁺ generation: The role of NO₃ ⁻ in the crevice corrosion repassivation of type 316L stainless steel

The role of NO₃ ⁻ in the repassivation of crevice corrosion of Type 316L stainless steel was investigated. In crevice corrosion tests, the solution was changed from 1 M NaCl to NaCl- NaNO₃. NO₃ ⁻ led to complete repassivation, and after potentiostatic polarization in 2 M KNO₃, NH₄ ⁺ was detected in the crevice solution by ion chromatography analysis. Repassivation of the crevice corrosion was found to take place in two steps. In the first step, the estimated current density inside the crevice gradually decreased from ca. 5 mA cm⁻² to ca. 5 μA cm⁻². After that, the current density suddenly decreased to less than 0.1 μA cm⁻². From the potentiodynamic polarization in acidic solutions simulated inside the crevice (pH 0.2) and in situ observations of the crevice corrosion morphology, the first step was thought to be generated by the suppression of active dissolution by NO₃ ⁻. It would appear that the generation of NH₄ ⁺ results in a pH increase and the further suppression of active dissolution, and then repassivation occurs.