Thermal Modification of Porous Oxide Films Obtained by Anodizing of Aluminum–Magnesium Alloy

Anodizing of aluminum in acidic electrolytes leads to the formation of porous oxide films on the metal surface. Heat treatment is one of possible ways to control the functional properties of this material. In this work, anodizing of aluminum alloy A5005 in a 0.3 M solution of sulfuric acid was carried out in the kinetic mode. A multi-stage heat treatment protocol has been proposed that allows controlled two-stage crystallization of as-prepared amorphous anodic alumina with preservation of the porous structure. At the first stage, anodic alumina crystallizes into a mixture of low-temperature Al2O3 polymorphs, accompanied by the removal of electrolyte impurities from its structure and an increase in the specific surface area to 42 m2/g due to the formation of a mesoporous structure. Subsequent heat treatment at 1200°C leads to the formation of α-Al2O3 films with an average grain size of 4 μm, with preservation of a porous structure with an average pore diameter of 26 nm. The crystallization of as-prepared amorphous anodic alumina results in an increase in its chemical stability by several orders of magnitude, which makes it possible to use the developed methods for creating membranes capable of functioning in aggressive media and catalyst carriers.