Preparation and Corrosion Behaviour of Cerium Based Sol-gel Composite Coatings on AA2024-T4 Aluminum Alloy

A serial of composite coatings produced by combination of cerium sol-gel coatings and cerium conversion layers has been developed in this work: (1) sol-gel coating sealed by conversion coating; (2) sol-gel coating sealed by low-density sol; (3) conversion coating thickened by sol-gel coating. For both sol-gel coatings and conversion coatings, cerium chloride heptahydrate (CeCl3·7H2O) was used as cerium source. In order to evaluate the anti-corrosion properties of these composite coatings, Electrochemical Impedance Spectroscopy (EIS) and salt spray test (SST) were employed as the assessing method. Furthermore, the surface morphology of these composite coatings was characterized by SEM observation. The EIS and SST results thus obtained showed that the anti-corrosion properties of the composite coating (1) had been remarkably improved compared to the any single layer of sol-gel coating or conversion coating. Furthermore, the SEM observation revealed that the morphology of that composite coating was uniform and homogeneous, which may explicate the superior corrosion-resistant performance obtained by the composite coating (1). By contrast, other composite coatings including composite coating (2) and (3) were not as good as composite coating (1) in corrosion resistance, which could be attributed to cracks and heterogeneous microstructure existing on the surface of these composite coatings.