CORROSION AND WEAR-RESISTANCE OF THICK CHROMIUM DEPOSITS FROM ACCELERATED CR(III) ELECTROLYTES

A study of a number of chromium(III) electrolytes has indicated that t he unacceptably low deposition rate from such electrolytes is due to t he formation of mu-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 environmen tally acceptable trivalent chromium electrolyte with optimized additio ns of methanol and formic acid has been developed. The hydroxo-bridged oligomers in the nonmethanolic bath produce a fine dispersion and thi s together with a high residual stress gives an exceptionally hard bri ttle coating with a high wear rate. A minimum in wear rate ir 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-thi ckness cracks and high corrosion rates. Methanol reduces hydrogen evol ution during deposition, which diminishes the residual stress and thro ugh-thickness cracking. The corrosion rate falls to that of convention al hexavalent chromium in coatings from 20-30% methanol baths. An envi ronmentally acceptable process has been developed that provides coatin gs up to 250 mu m in thickness at 50 mu m/hour of similar quality to c onventional hexavalent chromium and is expected to be suitable for pra ctical plating shop operations.