Influence of the Composition of Electrolyte for Hard Anodizing of Aluminum on the Characteristics of Oxide Layer

The procedure of hard anodizing is carried out at a temperature of (– 4)–0°C for 60 min. A 20% aqueous solution of H2SO4 is used as the base electrolyte. In the course of anodizing, the current density is equal to 5 A/dm2. To determine the influence of strong oxidants on the characteristics of anode layers (oxides), different amounts (30, 50, 70, and 100 g/liter) of hydrogen peroxide (H2O2) were added to the electrolyte. In some cases, it was blown off with an ozone-air mixture at a rate of 5 mg∙min/liter of ozone. It was discovered that the oxide layer (Al2O3∙H2O) is formed in the course of hard anodizing on aluminum alloys not only by oxygen ions appearing as a result of decomposition of water but also by neutral oxygen atoms formed as a result of decomposition of hydrogen peroxide and ozone. It is shown that both hydrogen peroxide and the procedure of blowing of the electrolyte with an air-ozone mixture increase the thickness and microhardness of the anodized layer by 50% due to the twofold reduction of the number of water molecules in aluminum oxide. Hydrogen peroxide and ozone clearly also decrease the thickness of the barrier layer of the coating through which oxygen and aluminum ions may penetrate and combine to form the oxide layer. It is demonstrated that the maximum microhardness of the anodized layer can be attained as a result of heat treatment, namely, as a result of heating of the anodized samples to 300°C.