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

Abstract The oxidation behaviour of homogeneous Ni$$₂₉$$Fe$$₅₆$$Cu$$₁₅$$ wt.%, Ni$$₄₂$$Fe$$₃₈$$Cu$$₂₀$$ wt.% and Ni$$₆₀$$Fe$$₃₀$$Cu$$₁₀$$ wt.% is investigated under 1 atm flux of high-purity O$$_2$$ gas—using thermogravimetry—with respect to application as oxygen-evolving anodes for aluminium electrolysis. Stable cubic oxidation behaviour was observed in Ni$$₄₂$$Fe$$₃₈$$Cu$$₂₀$$ wt.% and Ni$$₆₀$$Fe$$₃₀$$Cu$$₁₀$$ 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$$_2$$O$$_4$$ by thickness, as measured from EDX line scans. NiO dominated the scale of Ni$$₆₀$$Fe$$₃₀$$Cu$$₁₀$$ 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$$₄₂$$Fe$$₃₈$$Cu$$₂₀$$ wt.%, evolved to become (Cu/Ni, Fe)O spinel oxides over time.