Effect of fluoride complexation by aluminium ions on the corrosion behaviour of aluminium cathodes used in zinc electrowinning

Fluoride (F−) impurity at 20 mg L−1 in the zinc sulphate electrolyte increased by 73% and 98% the corrosion rate of Al-1050 and Al-1070 alloys, respectively, which are used as cathode in zinc electrowinning process. The addition of aluminium ions, Al3+, to the electrolyte reduced the corrosion rate by about 21% for the Al-1050 alloy and about 28% for the Al-1070 alloy. The effect was ascribed to the formation of AlF2+ and AlF2+ complexes, thus decreasing fluoride activity. Electrochemical impedance spectroscopy demonstrated that the corrosion resistance of Al alloys is increased in the presence of Al3+. Scanning electron microscopy indicated uniform corrosion of the Al surface, the degree of which depends on fluoride concentration. The higher corrosion resistance of Al-1050 was related to its coarser microstructure, which is less susceptible to corrosion in a non-passivating media. The selection of effective complexing agent for fluoride ion and the consequent reduction in the corrosion rate of aluminum cathodes are relevant in scenario of increasing co-processing of secondary zinc sources.