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 solu tions was studied, and the influence of the solution temperature on th e solubility rate was estimated. It was shown that the solubility proc ess of amorphous API membranes begins at pH<5.0 and pH>8.2. If the tim e of contact is long enough, such membranes can be completely dissolve d at pH<4.2 and pH>9.9. The solubility of normal AA membranes is a fea ture of amorphous AA and does not depend on the temperature of the pre liminary annealing if below the temperature at which crystallisation b egins. It was established in this study that polycrystalline PLA membr anes, formed by calcination at temperature, 850 degrees C, have an exc eptionally high resistance to acid and base, similar to crystalline al uminas of the boehmite series gamma-, delta-, theta- and alpha-Al2O3. The influence of the time of interaction between alumina and acid or b ase (up to 2200 h), the temperature of annealing (up to 1200 degrees C ), and other factors affecting the solubility of polycrystalline alumi na were studied. It is shown that the attack of acids and bases on pol ycrystalline 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.