Formation of hard anodic films on the 7075-T6 aluminum alloy by anodization in sulfuric acid and ethylene glycol

The 7075-T6 aluminum alloy was anodized in 1.5 mol·dm−3 sulfuric acid alone for 60 min, and small horizontal pores, along with the typical straight pores, were formed inside the pore wall, attributed to the chemical dissolution of the anodic film during the anodization. Conversely, after adding 50 vol% ethylene glycol to the sulfuric acid, the porous film almost maintained its initial structure with a low porosity even with the same anodization period of 60 min. The coating ratio and Martens hardness of the film formed in the mixture of sulfuric acid and ethylene glycol increased by 1.35-fold and 3.4-fold, respectively, compared with the reference values of the film formed in sulfuric acid alone. The properties of the film strongly reflect the dissolution behavior of not only the alloying elements in the anodic film but also the alumina, which constitutes the cell structure.