Microstructural origin of adhesion and corrosion properties of Ti-based conversion coatings on A6063 alloy

Abstract

Chemical conversion coatings are widely used as a surface treatment for aluminum alloys to improve the adhesion of organic coatings and corrosion resistance. Because hexavalent chromium in chromate conversion coatings is toxic and carcinogenic, much effort has been made to replace them with chromium-free coatings. In this study, we searched for conditions to obtain impact adhesion and salt spray corrosion resistance equivalent to those of chromate conversion coatings on extruded A6063 alloy, and observed the microstructure of titanium-based conversion coatings. Discontinuous fine particulate titanium-oxide formed on the aluminum matrix at the initial stage of immersion treatment. A continuous titanium-oxide layer of 50 nm thick formed on the matrix after 5 min of immersion, whereas the titanium-oxide above 1 µm in thickness formed on the Al–Fe–Si intermetallic particles. Controlling the thickness of titanium-oxide layer in sub-50-nm improves the adhesion, while the thick conversion coatings on the intermetallic particles did not affect the adhesion. However, the thin discontinuous conversion coatings have poor corrosion resistance due to insufficient barrier effect. Therefore, we succeeded in achieving impact adhesion and salt spray corrosion resistance equivalent to those of chromate conversion coatings by densely dispersing titanium-oxide particles 20–50 nm thick on the matrix.