To increase production in copper electrorefining operations higher current densities are desirable. Unfortunately, too high current densities may cause anode passivation i.e. stoppage of the electrorefining process altogether. The presence of nickel and oxygen in the anode and in the electrolyte has long been suspected as contributing to copper anode passivation.

A 3-electrode arrangement cell was used to investigate the effect on copper anode passivation of the presence of nickel in the anode and in the electrolyte. Anodes with 0, 1, 2, 3, 4, 5, and 6 w% nickel and known oxygen content were prepared in an induction furnace, under argon/ 7% vol H₂ atmosphere in cylindrical boron nitride crucibles. The anodes were then homogenized for 10 hours at a temperature of 950 °C, under an argon/ 7% vol H₂ atmosphere and were tested using linear sweep voltammetry (LSV), chronopotentiometry (CP) and cyclic voltammetry (CV) in a cell approved by ASTM using an EG&G model 273A potentiostat controlled by Labview' software. Electrochemical impedance spectroscopy (EIS) was conducted in a specifically designed three-electrode flat cell using the potentiostat and a Frequency Response Analyzer (FRA).

To assess the effect of nickel and oxygen on the passivation of synthetically prepared copper anodes as well as unrefined copper anodes tests were conducted using synthetic electrolyte compositions of 160 g.L⁻¹ H₂SO₄, 40 g.L⁻¹ Cu²⁺ and 0, 10, 20, 30, or 40 g.L⁻¹ Ni²⁺ on anodes containing nickel 1, 2, 3, 4, 5 and 6 w% under bubbling argon gas at a bath temperature of 60 ± 2 °C. Unrefined commercial copper anodes were also tested. Additional tests were performed on the pure copper and anodes containing 4 w% nickel, using bubbling oxygen gas to compare with the experiments under argon at a bath temperature of 60 ± 2 °C.

The oxygen content in the electrolyte for all the experiments was monitored using an oxygen meter.

The formation of an oxide layer was investigated by examining physical properties, structural properties (study by XRD, SEM, EPMA), and the chemical composition.

The results indicate that an increase in the nickel content of the copper anode caused the samples to passivate in shorter times. It was also found that as the nickel ion concentration in the electrolyte increases from 10, 20, 30, and 40 g/L, the time to passivate the sample is decreased.

Cyclic Voltammetry studies showed that the passivation of the samples was affected by the presence of nickel in the electrolyte as well as the anode. In conjunction with XRD it was shown that as the nickel concentration in the electrolyte increased the passive layer changed in nature. Cu2O was observed on the inside layer and CuSO₄.5H₂O and NiSO₄.6H₂O were observed on the outer layer when the nickel concentration was at 10 and 20 g/L. Conversely, mostly CuO on the inside layer and CuSO₄.5H₂O and NiSO₄.6H₂O on the outer layer were found when the nickel concentration was at 30 and 40 g/L.

The EIS data indicated that nickel in the anode as well as the electrolyte had an effect on the electrical properties of the passivation layer. Nickel in the electrolyte decreased the capacitance of passivation layer, whereas nickel in the anode caused changes in the capacitance of the Passivation layer. The findings are important in determining for the first time the implications that known amounts of nickel in copper anodes as well as in the electrolyte have on the passivation of copper/nickel alloys. In addition the mechanism of anode passivation is better understood.