The passivity of AISI 321 stainless steel in 0.5 MH2SO4 solution, in the steady-state condition, has been explored using electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. Based on the Mott–Schottky analysis in conjunction with the point defect model (PDM), it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. The thickness of the passive film was increased linearly with the formation potential. These observations were consistent with the predictions of the PDM, noting that the point defects within the passive film are metal interstitials, oxygen vacancies, or both.
