Correlation between predictive and descriptive models to characterize the passive film - Study of pure chromium by electrochemical impedance spectroscopy

Passive metals play a key role in many fields. The numerous phenomena affecting the behavior of the film are linked to the main issue that help us to understand the processes of passivation. The description of Electrochemical Impedance Spectroscopy (EIS) data allows us to evaluate kinetic parameters, but there are many models offered and it is difficult to select one to fit EIS data. Mass transport is a significant factor when characterizing the passive film and it is a good way of identifying models. This study proposes a methodology allowing us to choose the most appropriate model and to determine objectively if the mass transport has to be taken into account. The mass transport coefficient of the point defect through oxide is determined through the Point Defect Model. Knowing the thickness of the oxide, through X-ray Photoelectron Spectroscopy (XPS) analysis, the time constant of the transport is determined. Based on this value, a descriptive model is selected avoiding the over-parameterization of the EIS data. This methodology is used in acidic solutions at 30 °C and 80 °C on pure chromium considered as model material and demonstrates that the mass transport has to be taken into account at 80 °C.