Investigation On Subsequent Anodization Behavior Of Aluminium Covered With Pre-existed Oxide Film And Menbrane Structure

Anodization of aluminum is one of the main methods of preparing nano-porous alumina membrane. In this paper, aluminum was pre-anodized in sulfuric acid and the subsequent anodization of the sample in oxalic acid was discussed. The multi-anodization of aluminum was positively and usefully explored, which was beneficial to enrich the theory of aluminum anodization and prepare multilayer alumina film with nano-pore structure. The main results are as follows:1. The subsequent oxidation behavior of aluminum covered with sulfuric acid oxidation film was studied in oxalic acid. The effect of the pre-existed oxide film on aluminum oxidation in oxalic acid was systematically investigated. The multilayer clear, channel ordered alumina film was prepared for the first time by continuous oxidation in two different electrolytes. The pre-existed oxide film on aluminum significantly changed the subsequent oxidation characteristic curve in oxalic acid, the peak voltage of oxide film increased obviously, the pore size and particle size of alumina film also increased, but the hole density decreased.2. The effect of the modification structure of pre-existed oxide film on the subsequent oxidation of aluminum in oxalic acid was systematically studied. The methods of chemical dipping widening pore and electrochemical thinning were used to treat the porous layer and barrier layer, respectively. The effects of the structure of porous layer and barrier layer of pre-existed oxide film on the subsequent anodization of aluminum in oxalic acid were investigated. The results show that: at the same constant oxidation current, the thicker the pre-existed oxide film, the higher the subsequent oxidation voltage. The longer the time of widening pore, the lower the oxidation voltage. However, too long time of widening pore will destroy the structure of pre-existed oxide film and hence inhibit the subsequent oxidation. Under the same oxidation condition, the subsequent oxidation voltage was significantly reduced by thinning the barrier layer thickness of pre-existed oxide film. And the subsequent oxidation voltage lowered with the decrease of the barrier layer thickness.3. The effects of the parameters of subsequent oxidation on the anodization behavior were systematically studied in oxalic acid. The effects of current density, voltage of subsequent oxidation on grain size, degree of order, growth rate, induction time of nanoporous alumina film formed in oxalic acid were discussed. The results illustrate that: for oxidation of the sample with pre-existed oxide film, the current density is a critical parameter, which has a linear relationship with grain size distribution and also the thickness of alumina film. Moreover, the pre-existed oxide film affected the growth of oxide film in oxalic acid. The degree of grain order of nanoporous alumina film increased with prolonging the oxidation time, which is a dynamic process from disorder to order.