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.
Highlights
Carbon anodes used in the primary aluminum industry lead to high energy consumption, serious pollution, and other issues
The chemical compositions of the alloys were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) (Optima 4300DV, DuPont, Wilmington, DE., USA)
As the SEM/energy dispersive spectroscopy (EDS) images are very similar, here we only show results of 54.1Fe-37.3Ni-8.6Alalloy as an example
Summary
Carbon anodes used in the primary aluminum industry lead to high energy consumption, serious pollution, and other issues. At present, inert anodes are not applied industrially, mainly due to the lack of suitable materials with excellent resistance against oxidation and corrosion and good thermal and electrical conductivities under the conditions of cryolite-alumina molten salt electrolysis. The total reaction formula on the inert anode in aluminum electrolysis is Al2 O3 → 2Al + 3/2O2 , and the generated gas is not CO2 but O2. Ceramic anodes have excellent oxidation resistance and relatively good corrosion resistance, albeit with poor conductivity and bad machinability. Our research group tested Fe-Ni-Al2 O3 materials as inert anode, but found that they are difficult to use under the harsh conditions of Al electrolysis [4]. Alcoa has conducted industrial tests of Cu-FeNi2 O4 anodes, but those anodes still have the characteristics of ceramics and cannot be used industrially
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