Behaviour of inert anodes for aluminium electrolysis in a low temperature electrolyte, Part II

A low temperature bath (BaF 2 -NaF-AlF 3 -Al 2 O 3 ) saturated with alumina was used to test inert anodes (LAs) based on NiO-Fe 2 O 3 -Cu cermets with various doping agents. Only the multiphase materials Ni-Fe-Cu-O were chosen for long-term tests after preliminary testing, because of their stable behaviour under electrolysis at the operating temperature of about 800°C. Samples with a composition developed by Alcoa (NiFe 2 O 4 -18NiO-17Cu) were prepared and the bulk density, electrical conductivity, compressive strength and thermal expansion were determined. This material was used as a reference material. Long-term tests (up to 134 hours) at 800°C and an anode current density of 0.2 A/cm 2 showed that corrosion rates corresponding to less than 1.10 -3 g/cm 2 .h could be achieved. The corrosion rate of the same material was more than 8.10 -3 g/cm 2 .h in a cryolite melt at the operating temperature of 950°C. Larger scale tests showed higher' corrosion rates. The tests demonstrated the importance of ensuring proper anode pre-heating and uniform current distribution.