The non-linear fitting method to analyze the measured M–S plots of bipolar passive films

Mott–Schottky (M–S) analysis is an effective approach to investigate the electronic property of passive films of metals, and it is well suitable for the passive film with single space charge capacitance. But there is no proper method to analyze the Csc−2 vs. Vm plots of passive films with several space charge capacitances in series connection, such as bipolar passive films. In this paper, the relationship between the space charge capacitance of the bipolar passive film and the applied potential was deduced and the features of corresponding plots were given out simultaneously. Accordingly, a non-linear fitting method was presented to analyze the Csc−2 vs. Vm plots of bipolar passive films. Then the method was used to study the semiconductor characteristics of bipolar passive films formed on the surface of Nickel base alloy after being corroded in the environments with high temperatures and high partial pressures of H2S/CO2. The fitting results indicate that the non-linear fitting of M–S plots can well help to understand the anti-corrosion mechanism of bipolar passive films.