Corrosion and Wear Resistance of Thick Chromium Deposits from Accelerated Cr(III) Electrolytes

Summary

A study of a number of chromium(III) electrolytes has indicated that the unacceptably low deposition rate from such electrolytes is due to the formation of μ-hydroxo bridged oligomers, which adsorb and poison the crystal growth sites. Methanol and formic acid have been shown to be particularly effective in delaying oligomer formation. An environmentally acceptable trivalent chromium electrolyte with optimized additions of methanol and formic acid has been developed. The hydroxo-bridged oligomers in the nonmethanolic bath produce a fine dispersion and this together with a high residual stress gives an exceptionally hard, brittle coating with a high wear rate. A minimum in wear rate is obtained at 20% methanol in the bath, at which the wear is slightly less than that of conventional hexavalent chromium plating. Corrosion initiates at cracks in the coating and their high residual stress levels at low methanol concentrations, in the electrolyte, generate wide through-thickness cracks and high corrosion rates. Methanol reduces hydrogen evolution during deposition, which diminishes the residual stress and through-thickness cracking. The corrosion rate falls to that of conventional hexavalent chromium in coatings from 20–30% methanol baths. An environmentally acceptable process has been developed that provides coatings up to 250 μm in thickness at 50 μm/hour of similar quality to conventional hexavalent chromium and is expected to be suitable for practical plating shop operations.