Corrosion Behavior of Fe-Ni-Al Alloy Inert Anode in Cryolite Melts

Abstract

Fe-Ni-based alloys are promising materials of inert anodes for use in aluminum electrolysis and adding Al can further improve the corrosion resistance. Fe-Ni-Al alloys with 1.4–8.6 wt.% Al were prepared by vacuum melting, and their corrosion as anodes during the production of pure Al (98.14–99.68%) by electrolysis was studied in a melt of NaF-AlF3-NaCl-CaF2-Al2O3 at 850 ◦C. The corrosion layer on the anode contains fluorine salt that corrodes the oxide film, and the inner layer is Ni-enriched while the outer layer is enriched with Fe and O due to the preferential oxidation of Fe. The electrolytically deposited oxide films on Fe-Ni-Al alloys with different compositions contains Fe2O3, Fe3O4, NiO, Al2O3, FeAl2O4, NiFe2O4, and other protective oxides, making the alloys very corrosion-resistant. The linear voltammetric curves can be divided into three parts: active dissolution, passivation transition, and over-passivation zones. The alloy with 3.9 wt.% Al (57.9Fe-38.2Ni-3.9Al) has a relatively negative passivation potential, and therefore, is easier to become passivated. According to the Tafel curve, this alloy shows a relatively positive corrosion potential as anode (1.20 V vs. Al/AlF3), and thus can form a protective film.