On the long-term oxidation behaviour of homogeneous Ni–Fe–Cu alloys for CO2-free aluminium electrowinning applications at 800 °C

The oxidation behaviour of homogeneous Ni $$_{29}$$ 29 Fe $$_{56}$$ 56 Cu $$_{15}$$ 15 wt.%, Ni $$_{42}$$ 42 Fe $$_{38}$$ 38 Cu $$_{20}$$ 20 wt.% and Ni $$_{60}$$ 60 Fe $$_{30}$$ 30 Cu $$_{10}$$ 10 wt.% is investigated under 1 atm flux of high-purity O $$_2$$ 2 gas—using thermogravimetry—with respect to application as oxygen-evolving anodes for aluminium electrolysis. Stable cubic oxidation behaviour was observed in Ni $$_{42}$$ 42 Fe $$_{38}$$ 38 Cu $$_{20}$$ 20 wt.% and Ni $$_{60}$$ 60 Fe $$_{30}$$ 30 Cu $$_{10}$$ 10 wt.% after 30 h and 264 h, respectively, with the former showing particular promise for electrowinning applications due to a tendency to form a well-adhered scale consisting of approximately 70% NiFe $$_2$$ 2 O $$_4$$ 4 by thickness, as measured from EDX line scans. NiO dominated the scale of Ni $$_{60}$$ 60 Fe $$_{30}$$ 30 Cu $$_{10}$$ 10 wt.% up to 272 h, showing worse scale adhesion and prominent intergranular oxidation of iron. Partially blistered regions of the scale locally increased NiO/CuO formation in Ni $$_{42}$$ 42 Fe $$_{38}$$ 38 Cu $$_{20}$$ 20 wt.%, evolved to become (Cu/Ni, Fe)O spinel oxides over time.