Anodic oxide films have been prepared for use as gas separation membranes on aluminum, having ultrafine pores of a diameter considerably smaller than those referred to in previous studies. Anodic films formed at the same voltage were found to have pore diameters that depended on the electrolyte, increasing in the order sulphuric acid, oxalic acid, chromic acid and phosphoric acid. Since the rate of dissolution of anodic films in sulfuric acid is rather high, oxalic acid was chosen as the most suitable electrolyte for the preparation of membranes with ultrafine pores. By using the method of a sudden voltage drop during anodizing or changing electrolyte from barrier type to porous type duplex anodizing, it was possible to form pores having a diameter only one-half to one-third of that of pores formed under steady state anodizing. Pores formed at 5V and directly measured with an electron microscope was 5nm in the case of the first of these methods and 3.5nm for the second. It is also found that the rate of chemical dissolution of the films formed at voltage around 5V was extremely low.
It was also clarified that heterogeneously proceeded sealing during immersion in hot water, that is to say granular hydrates precipitated, and eventually filled up the pores.