Electrochemical Oxidation of Fe–Ni Alloys in Cryolite–Alumina Molten Salts at High Temperature

Abstract

The anodic oxidation process of Fe–Ni alloys containing various components as anodes in 48.2NaF–43.8AlF3–8.0Al2O3 (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 Fe2O3, 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.