Corrosion resistance of 2524 Al alloy anodized in tartaric-sulphuric acid at different voltages and protected with a TEOS-GPTMS hybrid sol-gel coating

Corrosion is a major problem for high strength aluminium alloys. Thickening of the naturally formed oxide layer through anodizing is one of the main approaches to improve the corrosion resistance of these materials. Chromate anodizing is extremely efficient to produce anodized layers with good corrosion resistance and painting adhesion, however chromate based surface treatments must be banished from industrial use. The corrosion resistance of aluminium alloy 2524 (AA2524) anodized in tartaric/sulphuric acid (TSA) bath and protected with a hybrid sol-gel coating was evaluated by means of electrochemical impedance spectroscopy (EIS) and salt-spray tests. The morphologies of the obtained layers were characterized by SEM-FEG while the chemical in depth distribution of the hybrid layers was evaluated by means of Rf-GDOES. The effect of anodizing voltage on the sol-gel impregnation and the protection afforded by the layers was evaluated. Electrical equivalent circuit fitting of the EIS data has shown that the anodized layer thickness plays an important role in the protection mechanism of the sol-gel layer. Salt-spray tests highlighted the significant contribution of the sol-gel distribution in the anodized layer.