Abstract: Aluminum alloy has been widely applied in aviation, transportation, construction and other fields with a wealth of merits such as low cost, high strength-to-mass ratio, good processability, etc. The corrosion protection, however, is necessary for applications as the pinhole corrosion and intergranular corrosion are prone to emerge on its surface. Cerium is one of the most advantageous rare-earth elements in terms of the preparation of conversion films on the substrate surface of aluminum alloys due to its unique electronic layer structure and physical and chemical properties, and the cerium-based conversion film technology developed is considered to be the most promising alternative to the chromate passivation. Until now the technologies reported of cerium-based conversion films include chemical immersion, sol-gel method, electrochemical process, magnetron sputtering et al. Among them, chemical immersion is the simplest for the preparation process, but both conversion rate and deposition rate of cerium ions are difficult to control, and therefore there are many micron cracks present in the film layer. The sol-gel method endows the film with high bonding strength with hypokeimenon and excellent corrosion resistance, but requires substantial contents of cerium salts while producing a vast amount of waste acid or alkali solution. While the electrochemical process can achieve controllable cerium-based conversion films even at low temperature at a low price, the film causes a lot of intergranular cracks, loose structures as well as poor quality. The coating prepared by magnetron sputtering is uniform with controllable composition, but due to low utilization rate of the target this method fails to achieve low temperature and high speed sputtering for the strong magnetic materials. Overall, these methods still face various issues, such as poor stability of the solution system, the prepared conversion film layer being unstable and easy cracking, and high cost, which make it difficult to meet requirements of industrial application. The formation mechanism of cerium-based conversion film is principally believed to be the cathode formation film theory, that is, the redox microcell is formed on the surface of aluminum alloy. The dissolved oxygen in the solution and added H2O2 can be used as the donor of hydroxyl group, and the local pH value, components, electron and charge concentration of the solution can be changed, benefitting the oxidation reaction of cerium elements and the deposition on the substrate surface. The corrosion resistance mechanism of the cerium-based conversion film is the cathodic inhibition, that is, the film formed on the surface of aluminum alloys prevents the oxygen transmission and electron transfer, consequently, preventing the reduction reaction on the cathode micro-zone and the corrosion and dissolution of the aluminum alloy. However, this mechanism only emphasizes the cathode inhibition, while ignoring the anode inhibition. Furthermore, these cerium-based conversion films demonstrate a self-healing ability since electrochemical kinetic inhibition takes place. In this paper, the technological progress on the cerium-based conversion film for aluminum alloys at home and abroad was reviewed, and the advantages and disadvantages of technologies were also described. Moreover, the mainstream formation and anticorrosion theories were expounded. Finally, we predict the direction of process improvement and theory research in future.
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