Correlation between the structure and the anticorrosion barrier properties of hybrid sol–gel coatings: application to the protection of AA2024-T3 alloys

Hybrid sol–gel materials have been extensively studied as viable alternatives to toxic chromate (VI)-based coatings for the corrosion protection of AA2024-T3 in the aerospace industry, due to the wide range of available chemistries they offer and the tremendous development potential of innovative functional coatings. However, so far, little work has been performed in identifying the effect of the employed chemistries on the structure and anticorrosion properties of the coatings. This work proposes to contribute to a better understanding of the relationship existing between the structure, morphology and anticorrosion properties of hybrid sol–gel coatings deposited on AA2024-T3 aluminium surfaces, the most widely used alloy in the aerospace industry. The sol–gels are prepared employing two hybrid precursors; an organosilane, 3-trimethoxysilylpropylmethacrylate, and a zirconium complex prepared from the chelation of zirconium n-propoxide, and methacrylic acid. The structure of the hybrid sol–gel formulation is modified by altering the concentration of the transition metal complex. The structure and morphology of the coatings are characterised by dynamic light scattering, fourier transform infrared spectroscopy, silicon nuclear magnetic resonance spectroscopy, differential scanning calorimetry, scanning electron microscopy, atomic-force microscopy and the anticorrosion barrier properties characterised by electrochemical impedance spectroscopy and neutral salt-spray. It is found that the transition metal concentration affected the morphology and structure, as well as the anticorrosion performances of the hybrid sol–gel coatings. A direct correlation between the morphology of the coatings and their final anticorrosion barrier properties is demonstrated, and the optimum material amongst this series is determined to be comprised of a concentration of between 20 and 30% of transition metal.