On the Mechanisms of Chromium Electrodeposition

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Abstract
Steady‐state polarization measurements were made on Pt bead cathodes in solutions of , , and chromic acid. Three main regions can be distinguished: oxide reduction, evolution, and Cr(VI) reduction to Cr(III) or to black Cr or both. To obtain the black Cr deposit, a polychromate must be present and a low pH must be maintained. Steady‐state polarization measurements were made on Pt bead cathodes in solutions of and to which had been added and of chromic acid to which or had been added. These data indicate that the active catalyst is ion which catalyzes only the reduction of the chromous products of the reduced cathode film. Bisulfate ion also acts as a blocking agent which inhibits electron transfer to chromium ions. A detailed mechanism for the reduction of Cr(VI) solutions to Cr metal is proposed. To prevent the formation of stable complexes, a cathodic film of chromic dichromate is formed with one end blocked by complexing with ions. Decomposition of the film by reaction with ions produces chromous hydroxide, the reduction of which to metallic Cr is catalyzed by ions. By the concept of a distribution function of trichromatebisulfate complexes, the narrow range of ion concentrations which must be controlled to maintain successful Cr deposition is made understandable.

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