Study of pulse plating and reaction mechanism of trivalent chromium deposition process

Trivalent chromium bath is relatively non-toxic compared with hexavalent chromium bath. However, it is difficult to obtain thick chromium deposits from trivalent chromium bath with direct current plating. Pulse current plating of thick chromium deposits was studied in a trivalent chromium bath containing either ammonium formate or sodium hypophosphite as the complexing agents. It was found that the plating rate of chromium deposit using pulse current was higher than that of direct current. Pulse current improved the maximal deposit thickness that could be obtained from the trivalent chromium baths. In addition, pulse current decreased the internal stress of trivalent chromium deposit by 25% to 75%. The microhardness of trivalent chromium deposit obtained with pulse current was comparable to that of chromium deposit obtained with direct current.; In a trivalent chromium bath containing ammonium formate and sodium acetate as the complexing agents, the reduction reaction of trivalent chromium electrodeposition was studied. It was found that without the presence of complexing agents chromium deposit could not be obtained. Chromium electrodeposition reaction occurred only in a narrow range of applied potentials, and the dominant reaction was hydrogen evolution reaction. The measured cathodic Tafel slope for chromium electrodeposition was -0.15 V/decade. Rotating disk experiments showed that the chromium electrodeposition reaction was diffusion controlled. AC impedance experiments did not reveal any existence of a cathode film.