Impact of HVOF-Sprayed Ni–Cr and (Co,Ni)O Coatings on the Corrosion Behavior of Cu–Ni–Fe Anodes for Green Aluminum Production

Cu–Ni–Fe alloys are promising O2-evolving anode materials for green Al production but their corrosion-resistance needs to be improved. In the present work, (Co,Ni)O top-coats and Ni–Cr bond-coats are deposited by high-velocity oxy-fuel (HVOF) process on two Cu–Ni–Fe alloys (Cu-rich and Ni-rich). It is firstly shown that (Co,Ni)O materials have a low solubility in the alumina-saturated cryolitic bath at 1000 °C. Then, the impact of the coatings and substrate composition on their oxidation behavior at 1000 °C in air and during Al electrolysis is studied. Although [Ni–Cr/(Co,Ni)O]-coating significantly increases the dry oxidation resistance of Cu–Ni–Fe alloys, the corrosion-resistance of the [Ni–Cr/(Co,Ni)O]-coated Cu–Ni–Fe anodes is low due to the coating detachment during aluminum electrolysis. However, coating Cu–20Ni–15Fe anodes with a single (Co,Ni)O layer increases its corrosion-resistance, prevents it from fluorination, and leads to the rapid formation of a coherent protective NiFe2O4 layer beneath the (Co,Ni)O top-coat. Under the same conditions, the protective NiFe2O4 layer is not formed on the Ni–25Fe–10Cu anode.