Corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Based on research on cermet inert anodes for aluminium production, it has been suggested that nickel ferrite spinel might be suitable for use as a sidewall refractory in Hall-Héroult cells. A corrosion resistant sidewall would allow elimination of the frozen bath ledge, and has potentially huge benefits in terms of energy savings and increased productivity. However, little work has been done to assess nickel ferrite's suitability as a refractory. Dense nickel ferrite samples were prepared and characterized, and corrosion tests in cryolite based baths were conducted. Results confirm that the spinel does have good corrosion resistance. The corrosion mechanism is complex, involving grain boundary attack and formation of a Ni–Fe alloy. This alloy could pose a risk in terms of contamination of the aluminium. The use of additives to restrict penetration of grain boundaries may be the key to development of a successful spinel based refractory.