During the degreasing of aluminum alloys, a dark smear consisting of the products of solution of the alloying constituents is formed on their surface. To remove this smudge film, the parts are treated in a solution containing 100 kg. chromic acid, 18.4 kg. 98 percent sulfuric acid and 1 Klt water. This solution is used at 15-25 oC and the time of treatment is 2-5 minutes. The smudge can also be removed in a 30-55 percent solution of nitric acid. After the removal of smudge, the parts are washed with water and are then ready for anodizing.