Microstructural characterization and corrosion resistance evaluation of chromate-phosphate/water-soluble resin composite conversion coating on Al surfaces

Chromate-phosphate/water-soluble resin composite conversion coatings (CP/RC) were prepared on the aluminum surface through a non-washing, dip coating-baking chemical conversion treatment method. Chromate-phosphate conversion film (CP) and water-soluble resin conversion film (RC) were also prepared for comparison studies. The CP/RC surface was found to be the most homogeneous, with the strongest protection against aluminum, and was not corroded during a 20-day neutral salt spray (NSS) test and a 3.5 wt% NaCl immersion test. The analysis shows that CP/RC is mainly composed of three parts: the outermost covering layer with the resin as the main body (polymer, AlPO₄, CrO₃ and Cr₂O₃); the internal composite conversion layer composed of resin and inorganic skeleton (Al₂O₃, Cr₂O₃, CrO₃, AlPO₄, CrPO₄, Al, AlF and polymer); the surface layer of aluminum substrate (Al, Al₂O₃ and AlPO₄). XPS also shows that Cr⁶⁺ reacts with resin to form related compounds, which proves that the resin component and inorganic component in the coating are compounded.