Preparation and characteristics of well-aligned macroporous films on aluminum by high voltage anodization in mixed acid

Well-aligned macroporous films on aluminum were prepared by high voltage anodization (HVA) in mixed acid electrolyte consisting of oxalic acid, sodium tungstate, phosphoric acid and hypophosphorous acid. The total concentration (about 10 g l− 1) of the electrolyte is much lower than that of normal anodization electrolyte. This anodizing process is characterized by a “saddle” shape change of the current density and short anodization period (around 15 min), comparing to conventional anodizing process. The aligned cell arrangement was markedly affected by the peak current density, which was optimized as 6–10 A dm− 2. The effects of the concentrations of phosphoric acid, oxalic acid and hypophosphorous acid on the morphologies of the films were discussed in detail. The results showed that the pore diameter ranged from 250 to 500 nm, and increased with increasing of the concentration of phosphoric acid, oxalic acid and hypophosphorous acid. The pore geometry could change from round to hexagonal, which was related to the electrolyte components and their concentrations.