Uniform formation of Ti-based conversion coatings on aluminum alloy surfaces by alkaline-acid etch treatment to improve organic coating properties

Environmentally friendly chromium-free Ti-/Zr-based conversion coatings are used as a pretreatment for organic coatings applied to aluminum alloys to replace conventional chromate conversion coatings. Chemical cleaning of alloy surfaces with alkalis and acids prior to conversion treatment is known to improve the corrosion resistance and adhesion of organic coatings. In recent years, there have been many attempts to optimize the acid treatment, for example, by adding HF to the acid to improve the etching performance. In this study, the microstructure formed during acid treatment with and without HF was analyzed in detail to investigate its effect on the adhesion property of organic coatings. Ti-based conversion coatings were formed on A3003 alloy after chemical cleaning with NaOH followed by H 2 SO 4 or HF-containing H 2 SO 4 . Cleaning with HF-containing H 2 SO 4 resulted in the formation of an F-rich layer on the alloy surface and the reprecipitation of dealloyed Cu. Subsequent Ti-based conversion treatment enriched the F-rich layer and formed Cu-containing Ti-oxide aggregates, which reduced the adhesion of organic coatings. Impact adhesion and salt spray corrosion resistance were comparable to those of chromate conversion coatings when Ti-based conversion coatings were formed uniformly under optimum conditions after acid treatment without HF.