A Study of Recycling Process of Cobalt-based Conversion Coating Formed on Aluminium Alloys

Abstract

In this study, the influence of different cyclic treatment and reuse process on the performance of the conversion film was systematically studied by electrochemical test, surface characterization and determination of cobalt ions in the conversion solution, and new technology of chemical conversion of cobalt salts in aluminum alloys were determined. Mode A indicated the practice of cyclical processing of the test object in the original conversion solution, in which components of the circulation liquid were consumed continuously. After the first cycle, the Ecorr and Rp were – 691.8 V and 63.5 kΩ. In the later time, Rp was just about 13.0 kΩ, and the corrosion resistance of conversion film degraded progressively. Mode B represented the practice of adding to the original solution before cyclical processing of the test object, so that volume of the solution remains unchanged, and components of the circulation liquid kept increasing. RP increased from 9.3 kΩ to the 69.9 kΩ at the beginning of the cycle, followed by a slow descent, and content of cobalt in the film was stable, also higher than that under model A. This demonstrates that mode A has high utilization of the cobalt in the conversion solution in the early recycling, whereas mode B can prolong the cyclic service life of the conversion solution in circulation. After adding oxidants, as NaClO3, NaBrO3, H2O2, NaClO, and KMnO4, to the circulation liquid before recycling, the reuse and recycle ratio of cobalt reaches up to 41.3 %, and Rp can be promoted to123.2 kΩ at most. The life of the recycling was prolonged, while the costs of chemical conversion of cobalt were reduced.