The electrochemical study of a chromium plating bath. I. Reactions leading to solution-free species

Abstract

The electrode reactions occurring at C, Cr, Cu and Ni cathodes in the standard electroplating solution for chromium, 200 g dm−3 CrO3 and 2 g dm−3 H2SO4, have been reinvestigated. This paper emphasises the study of those reactions which lead to solution-free products; these occur at potentials positive to that necessary for film or metal deposition. For each cathode, theI-E curve shows a wave or peak for the reduction of chromium (VI) to chromium(III) and it is demonstrated that this process is mass transport controlled, apparently by the transport of sulphate ions to the electrode surface. At Cr, Ni and Cu, the potential of the chromium(VI)/chromium(III) couple is determined by the removal of an insulating oxide film which forms spontaneously when the metal is immersed in the bath. In certain conditions it is also possible to define a well formed wave for proton reduction.