Anodization of aluminum in a sealed container

Abstract

Sealed anodization was studied for the first time in this paper. With the other conditions remaining the same, anodization under normal pressure was also carried out for comparative purposes. It was found that the pressure of anodization had a great effect on the morphology of the anodic alumina formed. Under normal pressure, we obtained alumina nanotubes with nanograss on the top. However, under sealed conditions, porous anodic alumina covered with a relatively dense film was obtained, with a length about 2.3 times that of the nanotubes obtained under normal pressure. These experimental observations are contrary to the traditional field-assisted dissolution theory, but can be explained by the oxygen bubble mould theory. It is assumed that the oxygen gas plays a key role in the anodizing process. With the generation of hydrogen gas at the cathode, the pressure in the sealed container gradually increases, with the result that the oxygen bubbles cannot easily rupture the anion-contaminated layer. The volume expansion of the oxygen bubbles in the channels promotes the rapid upward growth of the anodic oxide. These interesting findings contribute to an understanding of the formation mechanism of various anodic oxides and suggest more approaches to the anodization of metals in the future.