Dissolution and passivation processes in the corrosion of copper and nickel in KF.2HF at 85 degrees C

The process of electrolytic production of fluorine is conducted in a melt of KF.2HF at about 85 degrees C. The metallic cell materials, copper and copper-nickel alloys (Monel), and the steel electrodes that are used as the cathode, are subject to substantial corrosion in the liquid electrolyte. Comparative studies are reported on the relative extent of corrosion of copper and nickel in the above melt, and the respective passivation behaviours of these two metals. The formation of fluoride films on the two metals is examined by means of cyclic voltammetry and complementarily through recording of open-circuit potential-decay transients, following passivation. Flade behaviour is observed and the role of diffusion-controlled processes in depassivation is quantitatively evaluated. Weight-change measurements also support the electrochemically derived results. The passive film at Ni is much less easily reducible than at Cu.