A study on the preparation of passivating surface using bi-layer of nanostructured ZnO and silane functionalized polymer: an alternate option to chromate passivating coating

A novel bilayer coating was developed over flat steel surface using a simple solution dipping method. Zinc oxide nanostructure (ZnO-NS) on the surfaces of flat steel was grown by chemical treatment with zinc nitrate hexahydrate [Zn(NO₃)·6H₂O] and hexamethylenetetramine (HMTA, C₆H₁₂N₄) in aqueous solution, followed by a thin-layer deposition of amine-terminated poly-dimethylsiloxane (PDMS-NH₂). We noticed surface passivation, surface hydrophobicity, and anti-corrosion properties by this robust bilayer concept where both chemical and physical effects were present. The surface properties were measured by contact angle and scanning electron microscope. This nanostructured grown mild steel, i.e., cold-rolled closed annealed steel, was compared with nanostructure grown over zinc-coated steel, i.e., galvannealed and galvanized iron. Different flat-coated specimens were characterized by Fourier transform infrared spectroscopy. The formation of ZnO-NS on steel substrate was confirmed by XRD analysis. The corrosion behavior of the different specimens was also studied through DC polarization and electrochemical impedance spectroscopy. An excellent intact bilayer coating was proposed in the interest of structural and automotive applications.