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Abstract
The development of an inert anode for the aluminium electrolysis process is one of the key challenges facing the primary aluminium industry. Recent research has demonstrated that nickel-iron-based anodes offer promising performance. It has been claimed that the wear resistance of such anodes can be reduced by applying a protective metal oxide scale. Unfortunately, the behaviour of the scale during electrolysis is not well understood. In the present study, the behaviour of nickel-iron-based anodes having thermally grown protective oxide scales was investigated.
Oxidised binary NixFey (x = 50-80 wt%) and ternary NixFeyCoz anodes (x/y(wt) = 1.85; z = 10, 30 and 50 wt%) were subjected to short-term electrolysis at 0.8 A/cm2 for up to 12 hours in a laboratory-scale cell. The presence of the thermally grown oxide scale was found to significantly improve the wear resistance and stability of the anodes. Unfortunately, the oxide scale was severely damaged during electrolysis. Iron and cobalt-rich anodes exhibited high wear rates, associated with selective formation and dissolution of Fe2O3 and Co3O4. Nickel-rich anodes (≥ 80 wt%) exhibited a tendency to passivate, probably associated with formation of nickel fluoride.
Failure of the anodes was observed during extended electrolysis, largely due to spalling of the protective scale. The anode geometry was found to be of critical importance, influencing the local current density, level of oxide strain and oxygen bubble flow. Under very high current densities, direct dissolution of the metal was observed.
The oxidation behaviour of a range of binary and ternary Ni-Fe-based alloys was studied in air, with a view to identifying the optimum alloy composition and conditions of thermal pre-oxidation. Alloys containing 60-68 wt% Ni, 25-31 wt% Fe and 3-8 wt% Co were found to offer low oxidation rates, associated with the suppression of Fe2O3 and Co3O4 formation. Preferential formation of the quaternary (Ni,Co)xFe3-xO4 spinel was observed. To achieve a compromise between scale thickness, scale adhesion and ferrite spinel formation, oxidation temperatures of 800-900°C and times of 24-48 h were found to be optimal.