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The electronic properties of the passive film formed on pure chromium in an acidic sulfate medium at 30 °C was studied by using the multi-frequency Mott–Schottky approach and the point defect model. Conversely to common Mott–Schottky experiments, the relevance of the multi-frequency Mott–Schottky analysis relies on the determination of the space charge capacitance from a large frequency range impedance diagram. To analyze the Constant-Phase-Element behavior commonly observed for the impedance diagrams which characterize the passive film, the power-law model was used and allowed to determine the space charge capacitance. The consistency of the results showed that it was relevant to combine the power-law model to the predictive point defect model. The aim of this study is to extract the semi-conductive parameters of a passive film from impedance measurements using a sensitive analytical method. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

The effect of dry grinding on 304L stainless steel's passive behavior is compared to two other surface finishing (mechanical polishing down to 2400 with SiC emery paper and 1 µm with diamond paste, respectively). The characterization of the surface state was performed using scanning electron microscopy, transmission electron microscopy, 3D optical profilometer, and X-ray diffraction. Results indicate that each surface treatment leads to different surface states. The ground specimens present an ultrafine grain layer and a strong plastic deformation underneath the surface, while an ultrafine grain layer characterizes the subsurface of the polished specimens. Grinding induces high residual compressive stresses and high roughness compared to polishing. The characterization of the passive films was performed by electrochemical impedance spectroscopy and Mott–Schottky analysis. The study shows that the semiconductor properties and the thickness of the passive films are dependent on the surface state of the 304L stainless steel. [ABSTRACT FROM AUTHOR]