The Role of an In-Situ Grown Zn-Al Layered Double Hydroxide Conversion Coating in the Protective Properties of Epoxy Coating on Galvanized Steel

Abstract

A Zn–Al layered double hydroxide (LDH) conversion coating was investigated as a surface treatment for galvanized steel. Zn–Al LDH crystals were grown in situ on the galvanized steel surface in room temperature at different periods. Grazing Incidence Xray Diffraction verified the formation of Zn–Al LDH flakes on the surface. Scanning Electron Microscopy showed that Zn–Al LDH flakes were evenly distributed as a continuous coating on the surface of galvanized steel. Roughness and surface free energy of the coatings were evaluated by Atomic Force Microscopy and contact angle measurements. Corrosion propensity of the LDH coatings was analyzed utilizing Electrochemical Impedance Spectroscopy (EIS) and polarization. A 6 times increase in impedance at 0.01 Hz and 10 times decrease in corrosion current density was achieved for galvanized steel by LDH conversion coating in best practice. To assess the role of the LDH conversion coatings on adhesion strength and corrosion resistance of overlying organic coatings, epoxy coating was applied on LDH conversion coated surfaces and tested by pull-off, salt spray and EIS. Results signified 23 times increase in impedance at 0.01 Hz and 80% decrease in adhesion loss for organically coated galvanized steel after LDH treatment at optimal condition.