Relationship between the Resistivity Profiles Obtained from the Power Law Model and the Physico-Chemical Properties of Passive Films

The constant-phase-element behavior measured for passive films was analysed using the power law model. It allows to obtain the variation of the resistivity through the passive film thickness. To avoid many unknown parameters required for the model, a methodology to analyze the impedance diagrams has been described. It is illustrated with several examples. First, the resistivity profiles were obtained for a Ni-20Cr alloy immersed in solutions with different pH values to validate the proposed methodology. The dielectric properties of the passive films were discussed from the duplex character of these passive layers. It was shown that the shape of the resistivity profile could be attributed to the contribution and the thickness of the inner dense oxide and the outer hydroxide layers, respectively. Then, the second example provided information on the evolution of the behavior of the passive film formed on 316L stainless steel under irradiation.