Pulse plating of hard chromium from trivalent baths

A trivalent chromium bath is relatively non-toxic compared with a hexavalen t chromium bath. It is difficult, however, to obtain thick chromium electro deposits from a trivalent chromium bath with direct-current plating. This p aper describes a study using pulsed current to plate thick chromium electro deposits for hard chromium applications from a trivalent chromium bath, usi ng either ammonium formate or sodium hypophosphite as the complexing agent. It was found that the chromium plating rate using pulsed current was highe r than that for direct current, Pulsed current improved the maximum coating thickness that could be obtained from the trivalent chromium baths, In add ition, 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 quan tity (15 to 30%) of phosphorus co-deposited on the chromium coating. The ph osphorus content increased with decreasing duty cycle and with increasing p ulse frequency. The microhardness of a trivalent chromium deposit obtained with pulsed current was in the range of 610 to 850 on Knoop's hardness scal e. These values were comparable to that obtained with direct current, Incre asing pulse frequency in the range of 10-1000 Hz increased the microhardnes s of the chromium deposit.