Enhanced Corrosion Protection of Bio-inspired Textured AA2024 via Optimized Plasma Electrolytic Oxidation Coatings

This study investigates the corrosion resistance and durability of Plasma Electrolytic Oxidation (PEO) coatings on flat and biomimetic-textured (riblet) AA2024 aluminum alloy substrates. Two silicate-based electrolytes were compared: a reference alkaline bath and a modified formulation containing 10 g/L of acetic acid designed to regulate plasma discharge kinetics. To further enhance barrier properties, a hydrothermal post-treatment sealing ZnAl-based was applied. Morphological analysis showed that the addition of acetic acid reduced the average coating thickness from 19 ± 1 µm to 14 ± 1 µm while promoting a significantly denser oxide structure. Electrochemical Impedance Spectroscopy in freshwater revealed that the sealed hybrid system achieved a low-frequency impedance modulus (|Z|0.01 Hz) of 2.9·107 Ω·cm² on flat substrates and 1.7·106 Ω·cm² on textured riblets, representing an increase of up to three orders of magnitude compared to the bare alloy (103 Ω·cm²). Potentiodynamic polarization corroborated these findings, demonstrating that the sealed systems achieve corrosion current densities two orders of magnitude lower than the untreated alloy in simulated freshwater. In aggressive simulated seawater, while the higher chloride concentration triggers a general increase in current density, the sealed coatings maintain a significant performance advantage, outperforming unsealed PEO formulations by at least one order of magnitude. Accelerated corrosion testing confirmed exceptional durability, with the hybrid coatings remaining intact after 2000 h of continuous Neutral Salt Spray exposure. Conversely, cyclic Prohesion testing triggered localized failure after approximately 500 h, highlighting the role of salt crystallization in promoting coating perforation. These results validate the hybrid PEO-LDH system as a robust protection strategy that preserves the integrity of functional surface textures for aeronautical and marine applications.