NEW AND MODIFIED ANODIC ALUMINA MEMBRANES .2. COMPARISON OF SOLUBILITY OF AMORPHOUS (NORMAL) AND POLYCRYSTALLINE ANODIC ALUMINA MEMBRANES

The solubility of amorphous anodic alumina (AA) membranes (annealed at temperatures up to 820 degrees C) in HCl, H3PO4, H2C2O4 and NaOH solutions was studied, and the influence of the solution temperature on the solubility rate was estimated. It was shown that the solubility process of amorphous API membranes begins at pH<5.0 and pH>8.2. If the time of contact is long enough, such membranes can be completely dissolved at pH<4.2 and pH>9.9. The solubility of normal AA membranes is a feature of amorphous AA and does not depend on the temperature of the preliminary annealing if below the temperature at which crystallisation begins. It was established in this study that polycrystalline PLA membranes, formed by calcination at temperature, 850 degrees C, have an exceptionally high resistance to acid and base, similar to crystalline aluminas of the boehmite series gamma-, delta-, theta- and alpha-Al2O3. The influence of the time of interaction between alumina and acid or base (up to 2200 h), the temperature of annealing (up to 1200 degrees C), and other factors affecting the solubility of polycrystalline alumina were studied. It is shown that the attack of acids and bases on polycrystalline alumina is primarily a surface etching process and takes place at pH<4.7 and pH>9.0 and that the rate of the process depends on the surface condition of the oxide, and generally on the state of its hydroxyl cover. The potential use of polycrystalline AA as membranes is therefore enhanced.