Corrosion Behavior of Pre-oxidized 56Cu-22Ni-12Fe-8Al-2La Anode Alloy in High-Temperature Molten Electrolyte for Aluminum Electrolysis

Abstract:

The corrosion behavior of unoxidized and pre-oxidized 56Cu-22Ni-12Fe-8Al-2La anode alloys was investigated in a Na₃AlF₆-K₃AlF₆-AlF₃-Al₂O₃ electrolyte for aluminum electrolysis at 800 °C. Electrochemical tests of the metal anodes in molten salt for aluminum electrolysis were conducted, and the corrosion mechanism of the alloys was investigated. Results show that the oxide layer formed on the pre-oxidized alloy at 800 °C in an O₂ atmosphere mainly consists of oxides of aluminum, nickel, iron, copper, and lanthanum. After aluminum electrolysis for 3 h, the corrosion products formed on the unoxidized anode alloy demonstrate a tri-layered structure, and the discontinuous and porous oxide layer provides a connected channel during electrolysis, allowing the electrolyte to directly penetrate the metal matrix to a depth of approximately 320 μm. The corrosion oxide layer formed on the pre-oxidized anode alloy is about 190 μm in thickness. The outer and inner oxide layers include continuous Ni-/Fe-/Al-/Cu-rich oxides (mainly NiFe₂O₄ and CuAlO₂). The middle layer, with a thickness of about 15 μm, contains a dense and continuous NiFe₂O₄ layer. A small amount of electrolyte appears in the oxide layer and penetrates into the metal matrix. The pre-oxidized alloy exhibits better corrosion resistance and stability than the unoxidized alloy during aluminum electrolysis.