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.