Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al–F–H₂O, Zn–F–H₂O and Fe–F–H₂O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al–Fe alloy accelerates the corrosion of aluminium in H₂SO₄ and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.