Corrosion protection of aluminum alloy substrate with nano-silica reinforced organic–inorganic hybrid coatings

Organic–inorganic hybrid (OIH) thin films derived from the sol–gel process have emerged as sustainable metal pretreatment alternatives to toxic heavy metal-based systems. In recent years, such OIH systems based on Si, Zr, and Ti have been successfully developed and commercialized for pretreatment of aluminum alloys, galvanized steel, cold-rolled steel, and many other metals and alloys, for improving adhesion and corrosion resistance performance. A variety of approaches are being used to further enhance performance of such OIH systems to match or surpass that obtained from chromate-based systems. In the present study, a novel bis-silane compound has been synthesized and used as a primary sol–gel precursor for OIH coatings. In order to further improve their mechanical and corrosion resistance performance, colloidal nanoparticles have been incorporated. The microstructure of the deposited films as a function of their composition and formation of Si–O–Si structural network has been studied by Confocal Raman spectroscopic technique. The chemical structure of the OIH films has been characterized by FTIR analysis. Electrochemical impedance spectroscopy, DC polarization measurements, and accelerated neutral salt-fog test (ASTM B117) have been used to evaluate corrosion resistance performance of coatings on industrial aluminum alloy AA 3003 H14. Nano-indentation tests of these OIH films have been performed to study the effect of colloidal nanoparticles on coating micro/nano structure and their mechanical properties. The study reveals that colloidal nanoparticles improve the corrosion resistance of OIH coatings by formation of a protective barrier to diffusion of corrosive species to the metal surface. The optimum content of colloidal nanoparticles that can provide best corrosion protection has been determined. Electrochemical study provides useful insight into the significance of interaction between the sol–gel hybrid and silica particles in the corrosion protection mechanism.