MECHANISMS OF CHROMIUM ELECTRODEPOSITION

Steady-state polarization measurements were made on Pt bead cathodes in N2-stirred solutions of Na2CrO4, Na2Cr2O7, and chromic acid. Three main regions can be distinguished: oxide reduction, H2 evolution, and Cr(VI) reduction to Cr(IIl) 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 N2-stirred solutions of Na2CrO4 and Na2Cr2O7 to which Na2SO4 had been added and of chromic acid to which Na2SO4 or H2SO4 had been added. These data indicate that the active catalyst is HSO4- 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 Cr(H20)6+3 complexes, a cathodic film of chromic dichromate is formed with one end blocked by complexing with HSO4- ions. Decomposition of the film by reaction with H3O+ ions produces chromous hydroxide, the reduction of which to metallic Cr is catalyzed by HSO4- ions. By the concept of a distribution function of trichromate-bisulfate complexes, the narrow range of HSO4-ion concentrations which must be controlled to maintain successful Cr deposition is made understandable. © 1979, The Electrochemical Society, Inc. All rights reserved.