Etude electrochimique de la corrosion du cuivre et du magnésium dans les mélanges fondus HF-KF: Influence de leurs Teneurs en HF

Abstract

Industrial scale fluorine electrowinning is performed from HF-KF mixtures in the temperature range 85–105°C. Depending on the technique, carbon anodes are connected to copper or magnesium busbars. The corrosion of these metals at the busbar-anode interface is one of the limiting factors of the lifetime of industrial electrolytic cells. The corrosiveness of these mixtures depends on the HF content and the temperature. In order to specify the process involved, we have investigated the chemical and electrochemical behaviour of these metals, both from a thermodynamic and a kinetic point of view. The potential-acidity diagrams summarizing the properties of these metals have been set up from thermochemical data in the literature. In particular, their analysis showed that the only stable oxidation states of copper were 0 and II, and that magnesium was chemically corroded into MgF 2 by HF-KF mixtures. A voltammetric study has shown that for low amounts of HF, copper undergoes a passivation reaction under anodic polarisation due to formation of Cu(II) complex fluoride. Chemical corrosion of magnesium leads to the formation of a passivating layer of magnesium fluoride (nil voltammogram response). The corrosion rates of copper and magnesium have been deduced from the steady-state corrosion current measured as a function of acidity. Whatever the temperature, copper busbars have to be preferred when low acidic mixtures are used. On the other hand, when HF amounts are higher and temperatures lower, magnesium is a more corrosion-resistant material. In addition, the joint use of impedance spectroscopy and chronoamperometry allows us to suggest reaction pathways for the corrosion of copper and magnesium.