Passivation Model of 316L Stainless Steel by Point Defects Model

In this study, the electrochemical behavior of passive films anodically formed on 316L stainless steel in 0.05 M sulfuric acid has been examined using electrochemical impedance spectroscopy (EIS). For this purpose, passive films were formed potentiostatically at selected potentials for 1 h and then EIS measurements were done. For EIS measurements, an excitation voltage of 10 mV and an applied frequency ranging from 100 kHz to 10 mHz have been used. Results showed that the interfacial impedance and the polarization resistance (Rpol) initially increase with applied potential, within the low potential passive. However, at a sufficiently high potential passive (E > 0.5 V), the interfacial impedance and the polarization resistance decrease with increasing potential. Also, an electrical equivalent circuit based on the point defect model (PDM) was suitable to the actual passivation process. In this electrical equivalent circuit, the total impedance was taken to be the sum of impedance of steel/passive film interface, passive film and passive film/solution interface.