Generation of a surface pattern having conical surface features by anodic polarization of aluminum

This work explores anodic polarization process to generate a surface pattern composed of conical features on an aluminum surface in hot acid mixtures composed of sulfuric and phosphoric acids. It is shown that the composition of the electrolyte with respect to its phosphoric acid and water content and the surface roughness of the aluminum substrate are the critical parameters for the formation of the patterns. The best surface pattern was generated in a hot acidic mixture (60°C) containing 150 mL sulfuric and 350 mL phosphoric acids on a ground aluminum surface. For the formation of conical structures, a mechanism based on the competitive etching action of sulfuric acid and the formation of phosphate precipitates is proposed. Additionally, the possibility of porous anodization of these surfaces in an oxalic acid based electrolyte has been also demonstrated.