In this study, effect of immersion time on the electrochemical behaviour of AISI 321 stainless steel (AISI 321) in 0.1 M H2SO4 solution under open circuit potential (OCP) conditions was evaluated by potentiodynamic polarization, Mott–Schottky analysis and electrochemical impedance spectroscopy (EIS). Mott–Schottky analysis revealed that the passive films behave as n-type and p-type semiconductors at potentials below and above the flat band potential, respectively. Also, Mott–Schottky analysis indicated that the donor and acceptor densities are in the range 1021 cm-3 and increased with the immersion time. EIS results showed that the best equivalent circuit presents two time constants: The high-medium frequencies time constant can be correlated with the charge transfer process and the low frequencies time constant has been associated with the redox processes taking place in the surface film. According to this equivalent circuit, the polarization resistance (interfacial impedance) initially increases with the immersion time (1 to 12 h), and then it is observed to decreases. This variation is fully accordance with potentiodynamic polarization results.
