Electronic and transport properties of passive films grown on different Ni-Cr binary alloys in relation to the pitting susceptibility

In the presence of chloride ions, passive films grown on Nickel-Chromium alloys may be sensitive to chemically induced breakdown. As chloride ion adsorption on the passive film/electrolyte interface and their penetration through the film are the main reasons for passivity breakdown, their mechanisms need to be assessed and understood. In the present study, the passivity of pure Ni, pure Cr and different Ni-Cr alloys (with 16, 20, 24 and 28 wt% Cr) was characterized for two film growth conditions by electrochemical impedance spectroscopy (EIS) and the Mott-Schottky analysis. EIS spectra, obtained after the immersions, were analyzed from an Equivalent Electrical Circuit taking into account migration phenomena through the passive film (transport resistance, apparent diffusivity). Mott-Schottky analyses were performed to obtain the semiconductor properties of the passive films, such as semiconducting type, charge carrier density and flat band potential. All of these results were compared as a function of substrate composition and film-forming conditions. The relation between chloride anions adsorption or penetration and the electronic and transport properties of the passive films is then discussed. Particular attention is paid to the migration parameters, the flat band potential and the charge carrier density.