The role of formic acid and methanol on speciation rate and quality in the electrodeposition of chromium from trivalent electrolytes

A successful enhancement of both the quality and the deposition rate has been obtained by the addition of some quantities of formic acid and methanol. The results were obtained by using very conventional conditions such as 25°C, no pH adjustment, low current density and very gentle agitation. An excellent quality of deposit with a deposition rate of 50-100 μm/hr was obtained. And sustained deposition up to 200 μm was achieved with prolonged electrolysis. The change in the chemical speciation of the electrolyte was studied by using Ion-exchange chromatography. A separation and identification of the various species, formed as the bath was aged with time, was obtained. The chromium concentration of each band was measured using the ICP spectrometer, monitoring the change in the concentration of the species with ageing. U.V. visible spectrometry was also used to confirm identification of the species. A clear correlation has been established between loss of deposition rate and the formation of higher oligomeric chromium (III) species. The role of methanol and formic acid in inhibiting this effect is discussed.