The purpose of this work was to reveal the effect of phosphating pre-treatment on the protective performance of water-based epoxy varnish coating on surface of aluminum alloy(AA).A phosphating film was firstly prepared on the surface of 2024 aluminum alloy by chemical conversion method for phosphating pre-treatment, and then the surface of aluminum alloy without and after phosphating pre-treatment was coated with water-based epoxy varnish.After the varnish was completely cured into a coating, the protective effect of water-based epoxy coating on aluminum alloy without and after phosphating pre-treatment was evaluated by electrochemical impedance spectroscopy(EIS).Finally, the effect of phosphating pre-treatment on the protective performance of water-based epoxy varnish coating and its mechanism were revealed.Results showed that the phosphating coating on surface of aluminum alloy had a uniform and dense columnar crystal structure, which was mainly composed of Zn3(PO4)2·4H2O.In 3.5%(mass fraction)NaCl solution, both the protective layer resistance(Rc)and the impedance module at low frequency(|Z|0.01)of the epoxy/AA(E/A)system and the epoxy/phosphating coating/AA(E/P/A)system decreased gradually with the extension of immersion time; however, both Rc and|Z|0.01 of the E/P/A system were always greater than those of the E/A system.On the other hand, due to the phosphating pre-treatment, the average adhesive force(Fa)of water-based epoxy coating on the surface of AA increased from 10.47 MPa to 15.58 MPa.Compared with the direct coating of water-based epoxy varnish without phosphating pretreatment on the surface of AA, the coating with phosphating pre-treatment before water-based epoxy varnish treatment changed the whole protective layer on the surface of AA from a single epoxy layer to a composite epoxy layer/phosphating coating, resulting in a significant increase in Rc and|Z|0.01.More importantly, the existence of phosphating film improved the adhesion of water-based epoxy coating on the surface of AA, and even fundamentally changed the fracture form of the whole protective layer.The combined effects of the above two aspects improved the protective effect of water-based epoxy varnish coating on AA substrate.Also, the mechanism of phosphating pre-treatment on improving the protective performance of water-based epoxy varnish coating was revealed.
