Efeito do íon fluoreto na corrosão de ligas de alumínio utilizadas como catodos na eletrorrecuperação de zinco e avaliação de alternativas para inibição do processo corrosivo

Abstract

The present work evaluates alternatives to minimize the impacts of fluoride ions on the corrosion rate of aluminium alloys used as cathodes in zinc electrowinning and, consequently, to extend their useful life. Currently, the fluoride content in the electrolyte is 10 to 20mg/L. This value may increase due to the processing of secondary sources of zinc, such as electric arc furnace dust, an industrial waste with a significant content of this element. Fluoride ions cause aluminium corrosion by the dissolution of the passive layer. Its presence in the electrolyte was also associated with increased adhesion of the zinc deposit to the cathode. Electrochemical and immersion tests were performed using two Al alloys (Al-1050 and Al-1070) and a synthetic electrolyte containing 55g/L Zn2+, 8g/L Mn2+, 160g/L H2SO4 to characterize the rate and mechanism of corrosion at 36°C and 42°C, and at three levels of fluoride concentration: 0, 20 and 60mg/L. At 20mg/L of fluoride, an increase of 73% and 98% in the corrosion rate of the alloys, Al-1050 and Al-1070, respectively, was observed. At 60mg/L, the observed increase was 212% and 216%. Aluminium ions, Al3+, were added to the electrolyte at 20mg/L fluoride concentration as a complexing agent for F-. The addition reduced the corrosion rate by 21% and 28% for the Al-1050 and Al-1070 alloys, respectively. This effect was attributed to the formation of AlF2+ and AlF2+ complexes, reducing fluoride activity in the solution. Electrochemical tests demonstrate increased corrosion resistance due to the presence of Al3+. The pre-treatment of cathodes by chemical conditioning did not show consistent results in reducing the corrosion rate or increasing the corrosion resistance. The higher corrosion resistance of the Al-1050 alloy was correlated with its coarser microstructure, which is less susceptible to corrosion in non-passivating aqueous media. Images obtained by electron microscopy suggest uniform corrosion of the aluminium surface with a homogeneous distribution of concavities, an effect that depends on the fluoride concentration in the electrolyte. Measures that reduce aluminium cathode corrosion are relevant in the scenario of increased use of secondary zinc sources and in minimizing operating costs.