Plasma Electrolytic Oxidation (PEO) coatings on aluminum alloy 2024: A review of mechanisms, processes, and corrosion resistance enhancement

AA2024, containing around 4% Cu and 1.5% Mg, has outstanding mechanical properties among the aluminum alloys, but poor corrosion resistance. Because of its substantial tensile strength, fatigue and wear resistance, it finds broad application in the aeronautical field. Its use, however, requires the application of a coating for ensuring adequate durability to the components. Plasma Electrolytic Oxidation (PEO) is a plasma-based coating process which has been object of growing interest in recent years because of its ability at producing thick and protective coatings by using simple, water-based electrolytes, thus reducing the environmental and safety risks presented by traditional anodizing processes. This is a promising technique for AA2024 coating, although it involves many different parameters, often interacting with each other. Thus, when optimizing it for this specific material, different effects should be considered, among which the main ones are the electrical input applied to the workpiece, the composition of the electrolytic solution and the presence of pre- or post-treatment. In the present review the main features of the PEO coating of AA2024 are treated. The main process parameters are discussed and a complete description of the discharge mechanism and of the coating growth is given. Then, the properties of the PEO coatings grown upon AA2024 are treated, mainly from the point of view of corrosion resistance enhancement. Finally, some insights into the upscaling of PEO for its industrial application are given.