On the Mechanisms of Chromium Electrodeposition

Abstract

Steady-statepolarization measurements were made on Pt bead cathodesin N2-stirred solutionsof Na2CrO4, Na2Cr2O7, and chromic acid. Three mainregions can bedistinguished: oxide reduction, H2 evolution, and Cr(VI) reduction to Cr(III)or to black Cr or both. To obtain the blackCr deposit, apolychromate must be present and a low pHmust be maintained. Steady-statepolarization measurements were made on Pt beadcathodes in N2-stirred solutions of Na2CrO4 and Na2Cr2O7 to whichNa2SO4 had been added and ofchromic acid to which Na2SO4or H2SO4 had been added. These data indicatethat the activecatalyst is HSO4- ion which catalyzes only the reduction ofthechromous products of the reduced cathode film. Bisulfate ion alsoactsas a blocking agent which inhibits electron transfer to chromiumions. A detailed mechanism for the reduction of Cr(VI) solutionsto Cr metal is proposed. To prevent the formation ofstable Cr(H2O)6+3 complexes, a cathodicfilm of chromic dichromate is formedwith one end blocked by complexingwith HSO4- ions. Decomposition ofthe film by reaction with H3O+ ions produces chromous hydroxide,the reduction of which to metallic Cr is catalyzedby HSO4- ions. By the concept of a distribution function of trichromatebisulfatecomplexes, the narrow range of HSO4- ion concentrations whichmust becontrolled to maintain successful Cr deposition is made understandable.