Abstract
Cathodic current-potential curves were measured for iron of 99.94% in purity in chromic acid solutions containing various amounts of sulfates. The curves were characterized by the presence of two current waves, III and IV, which appeared in the range of 0.40-0.95V and under -0.95V (vs. SCE), respectively. The wave II, observed for the platinum electrode in the potential region above -0.45V, was not observed for the iron electrode. The partial currents for electrode reactions and the amounts of Cr and Fe ions incorporated into the chromic-chromate films were measured as functions of the cathode potential. The effect of the composition of solution was also examined. It was found that a group of reactions [Cr(VI)→Cr(III)aq., H+→H2, and Cr(VI)→Cr(O)*] proceeded in the waves, II and IV. However, the formation of metallic chromium marked with an asterisk was not observed before the potential was lowered to under 0.8V. The chromium content or the thickness of chromium-chromate film was found to increase in a stepwise fashion with the lowering of potential. In any cathode potential, the thickness of the film decreased with the increase of sulfate concentration; and the concentration of chromic acid had little effects on the film thickness. The Fe ion content of the film was considerably high at above -0.45V, but dropped to a small value when the potential was lowered to under -0.7V. The characteristic features of current-potential curves and the mechanism of cathodic reduction of chromic acid on the iron electrode were discussed on the basis of these facts.
