Potential-controlled PEO coatings on biomimetic textured AA2024: treatment optimization and corrosion resistance

AA2024 alloy (4 % Cu, 1.5 % Mg) is one of the most used in aeronautical applications because of its outstanding mechanical properties due to the presence of intermetallic precipitates. These, however, act as cathodes coupled to the aluminum matrix undermining the corrosion resistance. Therefore, AA2024 always needs a coating for being serviced. Plasma electrolytic oxidation (PEO) is a quite recent anodic coating technology exploiting plasma discharges in aqueous solutions for growing a protective oxide layer upon valve metals such as AA2024. At the same time, sharkskin biomimetic texturing is an emerging solution for the passive reduction of fluid dynamic drag. A pattern made of adjacent crests, if realized with proper dimensions, can reach a significant fuel saving upon vehicles both in air and in water. This study aims at optimizing a short potentiostatic PEO coating treatment for the application upon a biomimetic textured AA2024 surface, with the double aim of preserving the pattern parameters and enhancing corrosion resistance. The relation between electrolyte composition, spark features and coating properties has been studied by considering as main variables the cationic composition of the electrolyte (K+ and Na+) and its alkalinity. SEM observations and corrosion resistance tests allowed to establish that a reduced spark intensity can reduce the defectiveness of the oxide layer, at the price of a lower coating thickness. In particular, K+ substitution with Na+ cations and alkalinity reduction appeared to be effective in doing this, showing the best corrosion resistance and the most accurate texture reproduction.