Anodic dissolution of metals in oxide-free cryolite melts

The anodic behavior of metals in molten cryolite-alumina melts has been investigated mostly for use as inert anodes for the Hall-Héroult process. In the present work, gold, platinum, palladium, copper, tungsten, nickel, cobalt and iron metal electrodes were anodically polarized in an oxide-free cryolite melt (11%wt. excess AlF3 ; 5%wt. CaF2) at 1273 K. The aim of the experiments was to characterize the oxidation reactions of the metals occurring without the effect of oxygen-containing dissolved species. The anodic dissolution of each metal was demonstrated, and electrochemical reactions were assigned using reversible potential calculation. The relative stability of metals as well as the possibility of generating pure fluorine is discussed

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