Pulse Plating of Hard Chromium from Trivalent Baths

摘要

A trivalent chromium bath is relatively non-toxic compared with a hexavalent chromium bath. It is difficult, however, to obtain thick chromium electrodeposits from a trivalent chromium bath with direct-current plating. This paper describes a study using pulsed current to plate thick chromium electrodeposits for hard chromium applications from a trivalent chromium bath, using either ammonium formate or sodium hypophosphite as the complexing agent. It was found that the chromium plating rate using pulsed current was higher than that for direct current. Pulsed current improved the maximum coating thickness that could be obtained from the trivalent chromium baths. In addition, pulsed current decreased the internal stress of a trivalent chromium deposit by 25 to 75 percent. The internal stress decreased with decreasing current density, duty cycle and pulse frequency. When sodium hypophosphite was used as the complexing agent in the trivalent bath, a significant quantity (15 to 30 percent) of phosphorus co-deposited on the chromium coating. The phosphorus content increased with decreasing duty cycle and with increasing pulse frequency. The microhardness of a trivalent chromium deposit obtained with pulsed current was in the range of 610 to 850 on Knoop's hardness scale. These values were comparable to that obtained with direct current. Increasing pulse frequency in the range of 10-1000 Hz increased the microhardness of the chromium deposit.