Inert anodes for aluminium production can be made of ceramics, metals, or a mixture of those (cermets). Regardless of the type of anode, the surface will be an oxide. At high enough anode potential, the surface oxide will be decomposed upon formation of the corresponding fluoride, eventually leading to a catastrophic defect. The decomposition voltage was calculated for materials based on Ni, Fe, Cu, Co, and Cr in terms of the activities of alumina and aluminium fluoride at 960 °C and at 800 °C. The risk of failure was higher at low temperature, partly because low-melting baths have high activity of aluminium fluoride, and partly because it will be challenging to maintain high enough activity of alumina, even in a "slurry cell" where the bath is a suspension of tiny solid alumina particles. It was estimated that the alumina particles in the slurry cannot be larger than about 10 microns.