Surface passivation of 5556 aluminum alloy in solutions based on cerium nitrate

A solution has been developed for applying protective-adhesive cerium-containing coatings to the surface of the 5556 aluminum alloy in order to replace the toxic chromating process in the automotive and other industries. The developed solution contains: 5-10 g/l Ce(NO3)3 center dot 6H(2)O, 30-40 ml/l H2O2 and 0.5-1.5 g/l gallic acid ester. Coatings with the best physicochemical characteristics are formed in a solution with pH=2-3 at a temperature of 18-25 degrees C and a process duration of 10-15 minutes. The optimum drying temperature is 120-160 degrees C. The coatings formed under these conditions consist of cerium oxides CeO2, Ce2O3 and aluminum oxide Al2O3. The addition of a gallic acid ester (in an amount of 0.5-1.5 g/l) to the working solution leads to a change in the chemical composition of the forming coatings, namely, to the exclusion of CeO2 compounds in their composition. This increases the protective ability of coatings. Corrosion tests in a salt-spray chamber have shown that the developed cerium-containing coatings are superior in protective ability to the currently widely used chromate coatings. The developed coatings have good adhesion properties and withstand exposure to high temperatures without degradation of their protective properties. Adhesion after 750 hours of salt-spray corrosion tests decreased by 13.0 and 13.8% for cerium-containing and chromate coatings, respectively. The thickness of cerium-containing coatings is 280-320 mu. The developed solution for the formation of protective adhesive coatings on aluminum is an alternative to toxic chromating solutions.