An XPS study of the fluorination of carbon anodes in molten NaF–AlF3–CaF2

In the Hall–Heroult process for aluminum production electrolysis takes place in molten cryolite (NaF–AlF3–CaF2) with carbon electrodes. Dewetting of the anode leads to operational instability. A surface energy change due to surface fluorocarbon formation during electrolysis is suggested as a contributing factor to diminishing the wettability of the anode. In this work the surface composition of graphite anodes after electrolysis in molten NaF–AlF3–CaF2 is investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) for evidence of fluorination. Fluorocarbon is identified on an electropolished region of an anode surface resulting from anode effect. The discovery of surface fluorination provides an insight into high temperature electrochemical reactions of carbon with molten fluoride salts and informs aluminum smelter cell operation.