Sol-gel enhanced polyurethane coating for corrosion protection of 55% Al-Zn alloy-coated steel

In this study, silica-based sol-gel systems were prepared through hydrolysis and condensation reactions of tetraethyl orthosilicate (TEOS) and (3-Glycidyloxypropyl) trimethoxysilane (GLYMO). The sol-gel systems, combined with a nonionic surfactant, silica dioxide (SiO2) and a waterborne polyurethane (WPU) emulsion, could form RoHS compliant corrosion protection coatings for 55% Al-Zn alloy-coated steel and were developed as alternatives to conventional chromate-based coatings. Gel content, indentation hardness, and salt spray resistance of the polyurethane (PU) coating were greatly enhanced by the addition of the sol-gel systems and the additives. Impact of TEOS to GLYMO ratio, morphology, elemental composition of the coated surface, nanomechanical properties and corrosion resistance of the sol-gel enhanced PU coating were examined. The results indicated that the coatings which combined the advantages of the sol-gel matrix and the PU emulsion outperformed a commercial chromate conversion coating and a commercial chromium-free coating, and could be considered as alternatives for eco-friendly applications.