Microstructure control of Ti-based conversion coatings for optimizing organic coating properties in A6063 and A3003 alloys

Abstract

Ti-/Zr-based conversion coatings are environmentally friendly alternatives to conventional chromate conversion coatings. A prerequisite for their widespread use on aluminum alloys is an efficient treatment process applicable to multiple alloys. In this study, Ti-based conversion coatings with an ideal microstructure were successfully formed on both A6063 and A3003 under the same conversion bath conditions. These coatings exhibited satisfactory impact adhesion with organic coatings and salt spray corrosion resistance for practical applications. In-depth analyses of the deposition process of Ti-based conversion coatings by scanning and transmission electron microscopy showed that Ti-oxide nanoparticles were initially deposited on the matrix, which grew into a continuous layer on A6063, whereas a continuous Ti-oxide layer was formed on A3003 from the initial immersion stage. Lowering the pH of the conversion bath prevented the continuous growth of the Ti-oxide layer on A6063, thereby reducing the adhesion and corrosion resistance. Increasing the bath concentration accelerated the growth rate of Ti oxides in A3003 and A6063, and the formation of Ti oxides with excessive thickness reduced the adhesion. By optimizing the concentration and pH, a continuous Ti-oxide layer of 20–40 nm thickness was formed with a reasonable growth rate on both A6063 and A3003, resulting in satisfactory impact adhesion and salt spray corrosion resistance.