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, , 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.