Effect of Micro- and Nano-Structure of Anodic Aluminum Oxide on the Formation Behavior of Cracks Induced by Heat Treatment

Anodizing aluminum is widely used for various industrial applications such as corrosion protection, hardness enhancement, and coloring. However, when the anodized aluminum is exposed to high temperatures, many cracks form in the anodic oxide layer. Therefore, a deep understanding of crack formation behavior is essential for using anodized aluminum at higher temperatures. In the present investigation, the effect of the micro- and nano-structures of porous anodic aluminum oxide on the formation behavior of cracks in its oxide due to heat treatment at 523 K was investigated. High-purity aluminum plates and A6016-T4 aluminum alloys were anodized in sulfuric and oxalic acid solution to form a porous oxide film. Heat-treatment of the porous oxide with a hydrated layer formed by pore sealing leads to the formation of many cracks. Whereas the film thickness and sealing time have a significant effect on the crack formation behavior during heating at 523 K, the nanostructures of the porous oxide, such as the barrier layer thickness and pore size hardly contribute to the formation of cracks. These cracks formed in the porous oxide during heating may result from the difference in the amount of the hydrated oxide at the outermost surface and pore bottom of the oxide, and thus result uneven elongation during thermal expansion. The porous oxide with higher heat and corrosion resistance could be successfully fabricated by optimizing the pore sealing time for 15 min and temperature at 373 K.