25 - Chemical and Electrochemical Stability of Copper in Molten KF-2HF

Abstract

Fluorine is produced from the electrolysis of KF/HF mixtures at around 95°C. In the cell configuration, carbon anodes are screwed onto a copper busbar. Much attention has been paid to the stability of copper since the corrosion and redeposition of this metal on the cathodes is one of the main factors (side reaction) that limit the production yield of fluorine gas, the lifetime of the cells, and the development of new electrolyzers. Therefore, in the frame of this study, various experiments were carried out to determine the corrosion rate of copper for a wide range of HF ratios and temperatures. A statistical approach to the electrochemical data allowed us to predict the corrosion rate for many of the operating conditions. At open circuit voltage (OCV), copper shows good corrosion resistance even for high HF ratios. However, under 6-V anodic polarization, copper corrosion rate increases drastically with an increase of the temperature and/or the HF ratio. Characterization techniques have shown that only a thin copper fluoride layer has been detected on copper at OCV. By contrast, two types of copper fluorides were evidenced at the electrode surface (CuF2 and KCuF3) when a potential was applied to Cu. Under anodic polarization, a thin CuF2 layer is formed at the copper surface, whereas KCuF3 is detected on the electrode surface resulting from the precipitation of Cu2+. For a better interpretation of the results, erosion–corrosion phenomenon emanating from the fluorine bubbles' impacts and electrolyte movements have been highlighted by weight losses of copper pieces. The breakdown of the passivation layer on copper and the exposition of the surface to the corrosive medium imply a quicker degradation of the metallic pieces.