The development of a stable citrate electrolyte for the electrodeposition of copper-nickel alloys

The stability of citrate electrolytes for the electrodeposition of copper-nickel alloys and multilayers has been investigated. It was found that electrolytes operating at pH 4 are unstable due to the formation of an insoluble citrate complex. This complex had previously been thought to be copper citrate dihydrate, Cu2C6H4O7 . 2H(2)O, but elemental analysis of the precipitate revealed it to be a heteronuclear copper nickel citrate complex. Calculations of the distribution of citrate species in a typical plating solution was undertaken to identify experimental conditions under which the insoluble citrate complex is not formed. These studies indicate that increasing the pH of the citrate solution from 4 to 6 should produce a stable electrolyte. Experimental studies show that the electrolytes at pH 6 are stable for periods of several weeks, In agreement with the predictions of the speciation calculations. The stable citrate electrolytes were used to deposit Cu-Ni alloys. It was found that alloys could be deposited from these electrolytes with almost 100% current efficiency and with a morphology and composition comparable to those obtained from the unstable citrate electrolytes.