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.

Effect of ZnAl‐LDH on corrosion resistance of Zn–55Al alloy in NaCl solution

“…At present, many different preparation methods of LDH have been reported, such as in situ growth, [28,[40][41][42] co-deposition [34,43], and anion exchange methods. [44] LDH prepared by the in situ growth method has attracted a lot of attention from researchers due to its simple preparation process, strong coating adhesion, and remarkable corrosion resistance.…”

“…[17,18,[25][26][27] Therefore, it is speculated that ZnAl-LDH may be the main factor responsible for the improved corrosion resistance of Zn-Al coating. Furthermore, Sima et al [28] reported that the ZnAl-LDH can improve the corrosion resistance of galvanized steel. The dense corrosion product film containing LDH can effectively inhibit the diffusion of corrosive solution into the galvanized coating and prevent it from being eroded.…”

“…[44] LDH prepared by the in situ growth method has attracted a lot of attention from researchers due to its simple preparation process, strong coating adhesion, and remarkable corrosion resistance. [28,[40][41][42]45] LDH film has been successfully prepared on Zn-Al alloys to improve the corrosion resistance of the alloy. [28,46] To simulate the actual growth of LDH, LDH was prepared on the Zn-55Al alloy by the in situ growth method in this work.…”

“…[28,[40][41][42]45] LDH film has been successfully prepared on Zn-Al alloys to improve the corrosion resistance of the alloy. [28,46] To simulate the actual growth of LDH, LDH was prepared on the Zn-55Al alloy by the in situ growth method in this work.…”