Multistep anodization of 7075 – T6 aluminum alloy

Abstract

Abstract This study successfully demonstrated the overall advantages of multistep anodization of heat-treated wrought aluminum alloy AA7075-T6 that is a widely used in aerospace, automotive and fracture-critical applications. The coating properties and morphology are studied in detail for four anodization regimes: a conventional R1 with a constant electric current and R2, R3, R4 with raising the current in two, four and eight steps, respectively. Whereas processes R1 and R2 form coatings with the atomic Al/O ratio of 0.53 that is smaller than 0.67 for oxide Al2O3, R3 and R4 create coatings with the Al/O ratio of 0.83. Due to a higher level of infused oxygen, coatings built in R1 and R2 have burns and powdery appearance, while coatings formed in processes R3 and R4 form exhibit smooth solid-like surfaces. Compared to R1 and R2, R3 and R4 increase the overall growth rate of oxides by 23.4% and 25.6%, respectively, reduce the pore size by 94% and 45% respectively and decrease by 8.4% the amount of a transferred electric charge per one micrometer of the layer thickness. Process R4 creates coatings that are 74.0% more resistant to abrasion, 14.6% harder and 25.4% thicker than coatings formed in R1. As no specialized equipment is required, presented regimes of multistep anodization are well suited for large-scale manufacturing.