The superalloys Cu-Ni-Al, Cu-Ni-Fe, and Cu-Ni-Cr were studied as anodes for aluminum electrolysis. The alloys were tested for corrosion in acidic electrolyte molten salt and for oxidation in both air and oxygen. The results showed that the Cu-Ni-Al anodes possess excellent resistance to oxidation and corrosion, and the oxidation rates of Cu-Ni-Fe and Cu-Ni-Al anodes were slower than those of pure copper or nickel. During electrolysis, the cell voltage of the Cu-Ni-Al anode was affected most by the concentration of alumina in cryolite molten salt. The Cu-Ni-Fe anode exhibited corrosion resistance in electrolyte molten salt. Comparatively, the Cu-Ni-Cr anode showed poor resistance to oxidation and corrosion. The testing found that further study is warranted on the use of Cu-Ni-Al and Cu-Ni-Fe as inert alloy anodes.
The anodic oxidation process of Fe-Ni alloys containing various components as anodes in 48.2NaF-43.8AlF 3 -8.0Al 2 O 3 (wt%) molten salts at 980°C was characterized using linear sweep voltammetry. The anodic polarization curves of all of the Fe-Ni alloys, which displayed a typical active-passive-transpassive feature, are similar to that of Fe. Based on the dissolution-precipitation mechanism, an initial passivation film comprised of FeO was formed at a passivation potential. Subsequently, FeO was oxidized to Fe 2 O 3 , resulting in the formation of an outer film overlying the FeO film, which corresponded with the scanning electron microscopy and energy-dispersive X-ray spectrometry results, showing that the main components of the passivation film were iron oxides. Because of the chemical dissolution of oxides, oxides are continuously formed to resist chemical dissolution and protect the alloy matrix. The passivation film was broken down during oxygen evolution because of a lack of alumina. No passive behavior was observed on Ni. In addition, the preoxidized alloys possessed superior corrosion resistance to that of the nonoxidized alloys. The preoxidized 43Fe-57Ni alloy was observed to be stable during constant-current electrolysis based on a chronopotentiometric study, indicating that the preoxidized 43Fe-57Ni alloy had a good corrosion resistance to the cryolite-alumina molten salts.
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