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A detailed photoelectrochemical study was carried out in order to identify the nature and the properties of the passive layers on chromium under different experimental conditions. Different compositions of the films, related to various hydration degrees of the Cr(III) oxides, have been inferred on the basis of the bandgap values measured at changing pH, electrode potential, and polarization time. By comparison with air‐grown films, it has been concluded that anhydrous is never formed in acidic solution. The water content in the passive film increases at decreasing potentials and for long polarization times. In some cases, especially at neutral pH values, passive films possess a duplex structure, consisting of two layers with different hydration degrees. The photoelectrochemical study suggests an insulating or a slightly p‐type behavior of the passive layers.
Sunseri et al. (Wed,) studied this question.