Dynamic aspects of the electroreduction of chromic acid solutions

Potentiodynamic and impedance spectroscopy measurements were performed on A RMCO iron, steels (C30, C40, C50), cast iron and graphite electrodes in a b ath of industrial composition: CrO3 250 g dm(-3), H2SO4 2.5 g dm(-3). Atten tion focused on the electrochemical reactions occurring before the start of the chromium deposition. The electrochemical evidence is markedly affected by the chemical nature of the cathode. This is rationalized by proposing a reaction mechanism involving the formation, depending on the chemical natu re of the substrate, of a passive adsorbed film consisting of mixed chromiu m and iron oxides. Moreover, the proposed overall scheme accounts for the p resence of periodic current oscillations observed both in potentiodynamic a nd potentiostatic curves. In close agreement, impedance spectra show negati ve loops when measured at the corresponding potentials.