A hybrid organic-inorganic sol-gel coating for protecting aluminium alloy 7075-T6 against corrosion in Harrison's solution

A two-step procedure for synthesizing a new type of hybrid coating is presented. Coatings were synthesized from tetraethyl orthosilicate, 3-(trimethoxysilyl)propyl methacrylate and zirconium(IV) propoxide (ZTP). Zirconium(IV) propoxide was chelated with methacrylic acid. The synthesis, which proceeded at room temperature, was optimized using in situ Fourier transform infrared spectroscopy (FTIR). The effects of ZTP content, ageing and Zr/Si ratio were investigated. The viscosity of the sols increased with the ratio of Zr to Si. The terminal stability of the hybrid coatings was determined by thermogravimetric analysis and their composition and morphology analyzed by FTIR and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. Sols were deposited on the aluminium AA7075-T6 substrate by spin-coating. The corrosion properties in dilute Harrison's solution of samples coated by hybrid coatings were determined by electrochemical polarization methods. The coatings substantially improved the corrosion resistance of the substrate, as shown by a broad range of passivity extending up to high electrode potentials. The degree of protection was dependent on the content of zirconium and the time of ageing. The strong corrosion protection was ascribed to the bonding between silicon, oxygen and zirconium (Si-O-Si, Si-O-Zr), as proved by the FTIR analysis.