Corrosion behaviour and chemical stability of transparent hybrid sol-gel coatings deposited on aluminium in acidic and alkaline solutions

The aim of this study was to prepare transparent hybrid sol-gel coatings to serve as a barrier for corrosion protection of aluminium. The coatings were synthesised using silicon precursors tetraethyl orthosilicate (TEOS) and organically modified silicon precursor 3-methacryloxypropyltrimethoxysilane (MAPTMS) which were then mixed with different amounts of zirconium(IV) propoxide (ZTP) chelated using methacrylic acid (MAA). The synthesis process was followed using in-situ Fourier transform infrared spectroscopy and liquid-state ¹H, ¹³C and ²⁹Si nuclear magnetic resonance spectroscopy. The coating properties, such as transparency, topography, chemical composition, morphology and wettability were evaluated. Corrosion properties were analysed using electrochemical methods in ∼0.1 N NaCl in a pH range between 3 and 10. The chemical stability was evaluated in strongly alkaline solution. Homogeneous composition of the coatings was achieved through condensation reactions between Si- and Zr-based precursors. Obtained coatings were highly transparent, increased the contact water angle for polar and non-polar liquids compared to bare substrate, and improved the cleaning properties. The electrochemical measurements confirmed remarkable barrier protection in NaCl solutions of various pHs, especially for coatings with high Zr/Si ratios. In harsh media such as 0.1 N NaOH, the coating with low Zr/Si ratio was more stable. The results were interpreted in terms of different degrees of polycondensation of Si species as a function of Zr content in the coating. The importance of chemical speciation of the sol during synthesis were emphasised as this affects the subsequent coating properties.