Abstract: A colored oxide film on the surface of aluminium or an alloy thereof can be formed by anodizing at a D.C. voltage in an electrolyte based on sulfuric acid, followed by alternating current electrolysis at an A.C. voltage which is lower than the D.C. voltage.In order to increase the degree of coloring of the colored oxide film, after the alternating current electrolysis the A.C. voltage is raised to a higher level but lower than the D.C. voltage, followed by dropping the voltage down to a level of the A.C. voltage or thereabout.

Abstract: A paint film based on a thermo-setting resin which is applied on an aluminum article may be improved in its adhesion properties and resistance to chemical and mechanical attacks as well as in its resistance to weathering, if the surface of the aluminum article is previously anodized to form the oxide surface layer or is treated in boiling water to form the boehmite surface layer and if the oxide surface layer, including the boehmite layer, is pre-treated with a silane compound prior to the application of the coating composition. The application of the paint composition is conducted in this invention by means of a known coating technique such as dipping, spraying, showering, brushing or roller-coating, other than the electro-deposition technique.