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.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

The presence of fluoride impurity at 20 mg/L in zinc sulfate electrolyte significantly increased the corrosion rate of Al-1050 and Al-1070 alloys used as cathodes in zinc electrowinning. The addition of Al3+ ions to the electrolyte reduced the corrosion rate by about 21% for Al-1050 alloy and about 28% for Al-1070 alloy. This effect was attributed to the formation of AlF2+ and AlF2+ complexes, which decreased the activity of fluoride. Electrochemical impedance spectroscopy confirmed the increased corrosion resistance of Al alloys in the presence of Al3+. Scanning electron microscopy revealed uniform corrosion on the surface of Al, with the degree depending on fluoride concentration. The coarser microstructure of Al-1050 contributed to its higher corrosion resistance in a non-passivating media. The selection of effective complexing agent for fluoride and the subsequent reduction in the corrosion rate of aluminum cathodes are important in the context of increasing co-processing of secondary zinc sources.