Electrochemical oxidation of binary copper–nickel alloys in cryolite melts

Anodic oxidation of copper, nickel and two copper–nickel alloys was studied in cryolite melts at 1000 °C. In an oxide-free melt, anodic dissolution of each material was observed, and the dissolution potential increases with the content of copper. SEM characterization of a Cu55–Ni45 alloy showed that nickel is selectively dissolved according to a de-alloying process. In an alumina-containing melt, a partial passivation occurs at the copper-containing electrodes, at potentials below the oxygen evolution potential. A passive film forms on the copper electrode, while on the nickel electrode no dense oxide layer develops. Copper–nickel alloys were found to form a mixed oxide layer. At higher potentials, the formation of oxygen bubbles on the electrodes results in a degradation of the passive films and a strong corrosion.