Improvement of the corrosion performance of AA2024 alloy by a duplex PEO/clay modified sol-gel nanocomposite coating

Due to the intrinsic porosity of the layers formed by the plasma electrolytic oxidation (PEO) process, the application of silane-based coatings as an eco-friendly layer is a promising way to diminish penetration of the corrosive species into the PEO coating by pores sealing. In this study, to enhance the corrosion protection of AA2024, the performance of the duplex system achieved by adding different concentrations in sodium montmorillonite (Na-MMT) dispersed into the silane coating was assessed. In this study, a hybrid sol-gel layer (30% V/V) obtained from tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) sol-gel solution was applied on AA2024 previously covered by an optimized PEO layer using a solution containing sodium silicate and potassium hydroxide solution as electrolyte. Electrochemical impedance spectroscopy (EIS) revealed the significant impact of the sol-gel/clay nanocomposite layer on the corrosion protection performance of the PEO layer on the AA2024 substrate in a 0.1 M NaCl solution. Regarding, the low-frequency impedance of different coating systems upon five weeks exposure to the aggressive solution reported the sealing ability of the sol-gel coatings in which the silane coating modified with 1000 ppm of clay nanoparticles had the maximum corrosion resistance among them (i.e., higher than 10⁷ after 5 weeks immersion). The flake-like structure of sodium montmorillonite not only enhanced the barrier performance, but also FT-IR outcomes reflected the reticulation of the silane network through the interaction of nanoparticles with SiOH groups of the silane layer.