Effect of alloying elements on the morphology of anodic films formed on 7075 aluminum alloys tempered under different conditions via anodization in sulfuric acid

The anodization of aluminum is widely used to form an anodic film that endows aluminum with various surface properties beneficial for practical applications. However, the effects of alloying elements on the structure of the resulting film remain unknown. Additionally, although tempering is widely used to improve the properties of aluminum alloys, its effect on anodization is not fully understood. Thus, to expand our understanding of aluminum alloy anodization, herein, we investigated the anodization in sulfuric acid (1.5 mol dm⁻³) of 7075 aluminum alloys (AA7075) tempered under different conditions. The anodization behavior of AA7075–T6 (standard sample) was compared with those of AA7075–O, AA7075–T4, and pure aluminum (99.99 %). The voltage–time curves during constant-current anodization differed depending on the material. The barrier layer formed on AA7075 showed higher porosity than that on high-purity aluminum in the initial stages of anodization. The distribution of the alloying elements (e.g., Zn and Cu) in the aluminum matrix strongly affected the structure and properties of the film. Particularly, in AA7075–T6, the formation efficiency and hardness of the anodic oxide film were notably reduced because the film was highly porous and contained numerous horizontal pores. Thus, although tempering improves the mechanical properties of aluminum alloys through the formation of fine precipitates within the aluminum matrix, it can have detrimental effects on the properties of anodic oxide films by causing unexpected excessive chemical dissolution of the anodic film.