Effect of deposition sequence of MgAl-LDH and SiO2@PDMS layers on the corrosion resistance of robust superhydrophobic/self-healing multifunctional coatings on magnesium alloy

A multi-functional anti-corrosion coating integrating wear-resistant superhydrophobic(SH) and self-healing is expected to provide effective protection for magnesium alloys. In this work, the SiO2@PDMS (SP) coating and cysteine-intercalated MgAl-LDH (CL) are respectively chosen as the wear-resistant SH and the self-healing anti-corrosion coating, then two kinds of bilayer coatings with different deposition sequences: SP/CL and CL/SP were obtained accordingly. The morphologies, compositions, structures, superhydrophobic properties, wear-resistance, corrosion resistance and self-healing properties of these two coatings were investigated. SEM results show small SiO2 particles and large SiO2 particles intermingle to form a micro-nano structure on the SP/CL coating surface. While in the CL/SP coating, MgAl-Cys-LDH nanosheets are dispersed and grown in the gaps between SiO2 particles and on the surface of the magnesium substrate. Besides, their wear resistance is excellent, with the contact angles remaining at a high level (149.9° and 149.8°) after 50 cycles of sandpaper abrasion. Electrochemical tests indicate the corrosion resistance of CL/SP coating is better than that of SP/CL coating. It is mainly ascribed to the much denser CL/SP coating and the better adhesion between the SP and CL layers. A large number of CL nanosheets growing in the SP gaps can trap corrosion ions in time and effectively, significantly reducing the concentration of corrosion ions in the vicinity of the Mg surface. And the corrosion current density of CL/SP coating (Icorr=4.28×10−11 A/cm2) is 3 orders of magnitude lower than that of most of the related reports. Furthermore, the CL/SP coating shows a strong self-healing ability.