Fabrication of high quality anodic aluminum oxide (AAO) on low purity aluminum—A comparative study with the AAO produced on high purity aluminum

Abstract

In this work the quality, arrangement, composition, and regularity of nanoporous AAO formed on the low-purity (AA1050) and high-purity aluminum during two-step anodization in a mixture of sulfuric acid solution (0.3M), water and glycol (3:2, v/v), at various voltages (15, 20, 25, 30, 35V) and at temperature of −1°C, are investigated. The electrochemical conditions have allowed to obtain pores with the size ranging from 30 to 50nm, which are much larger than those usually obtained by anodization in a pure sulfuric acid solution (<20nm). The mechanism of the AAO growth is discussed. It was found that with the increase of applied anodizing voltage a number of incorporated sulfate ions in the aluminum oxide matrix increases, which was connected with the appearance of an unusual area in the current vs. time curves. On the surface of anodizing low- and high-purity aluminum, the formation of hillocks was observed, which was associated with the sulfate ions incorporation. The sulfate ions are replacing the oxygen atom/atoms in the AAO amorphous crystal structure and, consequently, the AAO template swells, the oxide cracks and uplifts causing the formation of hillocks. The same mechanism occurs for both low- and high-purity aluminum. Nanoporous AAO characterized by a very high regularity, not registered previously for low purity aluminum, was obtained. Furthermore, no significant difference in the regularity ratio between the AAO obtained on low- and high-purity aluminum, was observed. The electrochemical conditions applied in this study can be, thus, used for the fabrication of high quality AAO on inexpensive, technical AA1050 aluminum.