Abstract:

In this paper, the space charge capacitance of a bipolar semiconductor passive filmwas considered to be equivalent to a series connection of a capacitance at the passive film/solution interface and an np-junction capacitance at the outer layer film/inner layer film interface. Based on a uniform expression established in our previous work, a non-linear method was used to fit the Mott-Schottky (M-S) plots of bipolar semiconductor passive films. We then investigated the semiconductor characteristics of passive films formed on the surface of the nickel base alloy G3 after environmental corrosion at high temperatures and high partial pressures of H2S/CO2 using this method. Fitting results of measured M-S plots indicated that the majority carrier density of the p type semiconductor outer layer of the bipolar passive film obviously increased as the temperature increased while the majority carrier density of the n type semiconductor inner layer was almost unchanged. The fitting results show that the non-linear fitting method can produce multiple semiconductor characteristic parameters of the inner/outer layer of bipolar passive films. From these fitting results a mechanism for the formation and breakdown of passive films was established. A variable mechanism for the passive film structure and the role of the np-junction in the corrosion inhibition process were discussed by considering X-ray photoelectron spectroscopy (XPS) results as well.

Key words: Bipolar, Passive film, Mott-Schottky plot, Non-linear fitting