Abstract
In the present study, the surface of AZ31 Mg alloy was pretreated by etching in nitric acid and plasma electrolytic oxidation (PEO). Then Mg-Al layered double hydroxide (Mg-Al/LDH) was synthesized on the pretreated surface of substrates via hydrothermal treatment to increase the corrosion resistance. The effect of surface pretreatment (etched substrate and PEO layer) on LDH formation, structure, and corrosion resistance was investigated. Benzoxazine resin (thermosets), has been used for coatings, however, the high curing temperature limits their applications. For this purpose, a new bio-based benzoxazine resin with a lower curing temperature (160 °C) was developed to be applicable for Mg alloys. Phenol (Phloretic acid), diol (Dodecandiol), and amine (monoethanolamine) were used to synthesize innovative benzoxazine with exchangeable ester functions and self-healing ability. This resin was further used for post-treatment of LDH samples. The morphology, chemical composition, and crystalline structure of LDH samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The corrosion resistance of the coatings was investigated by salt spray test as well as electrochemical impedance spectroscopy (EIS). The LDH layer on the etched substrates showed better corrosion resistance than the LDH on PEO layer due to having a thicker inner layer (11.8 ± 0.2 μm and 7.0 ± 0.6 μm for LDH on the etched substrate and PEO layer, respectively). Post-treatment of samples with benzoxazine resin showed the effective capability of this polymeric coating for enhancing the corrosion resistance of Mg alloy substrate. This benzoxazine coating increased the corrosion resistance of the coating/substrate system around 104 times in comparison to the bare substrate. Moreover, it was observed that the LDH layer can increase the adhesion and compatibility of the substrate with benzoxazine resin. These results were confirmed by EIS and cross-cut adhesion tests experimentally as well as theoretical calculations of the adsorption energy of benzoxazine molecule to the LDH layer using Density Functional Theory (DFT).
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