One-step hydrothermal synthesis of 2,5-PDCA-containing MgAl-LDHs three-layer composite coating with high corrosion resistance on AZ31

The addition of 2,5-pyridinedicarboxylic acid (2,5-PDCA) to the Mg-Al LDH coating, which was prepared by one-step hydrothermal synthesis, had extremely enhanced the corrosion protection of AZ31 Mg alloy, although the 2,5-PDCA could not be intercalated into the interlayer spacing. The corrosion current density of 0.05 mol L−1 2,5-PDCA LDH containing LDH coating is 3.18 nA cm−2, reduced by two orders of magnitude compared to the LDH coating without inhibitor, and the corrosion inhibition efficiency of the coating is 98.05%. The coating formed on the surface of AZ31 was peeled off from the substrate by using a mechanical method and SEM observation of the cross-section showed that the coating consisted of three different layers. The innermost layer is a thick layer that consists of Mg(OH)2 and the intermediate layer is LDH, which is vertical to the substrate and the outmost layer is a thin but very dense deposit layer of LDH agglomerates with complexes of 2,5-PDCA and Mg. This kind of sediment/LDH/Mg(OH)2 three-layer composite structure was accountable for the increase in the corrosion resistance of AZ31 Mg alloy.