Preparation And Self-healing Properties Of LDHs Films On Magnesium Alloy Surface

As the lightest structural material,magnesium alloys have been used in various industries,such as computers,communications,aerospace and automobiles.Due to the poor corrosion resistance of magnesium alloys,its application is limited.Therefore,many surface modification methods have been proposed to improve its surface corrosion resistance,such as chemical conversion coatings,polymer coatings,micro-arc oxidation and composite coatings.Among protective coating methods,layered double hydroxides(LDHs)have excellent corrosion resistance due to the unique layered structure and the exchangeability of interlayer anions.Therefore,the protective effect of LDHs on magnesium alloys has received more and more attention.In this paper,five kinds of Mg Al-LDHs coatings were prepared by one-step hydrothermal method on AZ31 magnesium alloy by adjusting the p H of solution.In the solution with p H=12.5,Mg Al-LDHs-NO₃,Mg Al-LDHs-WO₄and Mg Al-LDHs-Cys were prepared on the surface of AZ31 magnesium alloy by hydrothermal method and anion-exchange method.X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),scanning electron microscope(SEM)and energy spectrometer(EDS)were used to characterize the composition and morphology of the film layer.Electrochemical test and scratch immersion experiments were used to prove the protective effect and the self-healing ability of the film layer.And the self-healing mechanism was proposed.(1)Mg Al-LDHs-NO₃membranes were prepared by hydrothermal treatment at 120?and 24h in solutions with different p H values.Electro-chemical experiments were carried out in 3.5 wt.%Na Cl solution with 5 kinds of p H on film layers and bare substrates.The results showed that all film layers have higher corrosion resistance than the substrate.And when p H=12.5,the corrosion resistance of the film was better.(2)The p H of the solution was adjusted to 12.5,and Mg Al-LDHs-NO₃film layer cross-linked by bamboo leaf-like nanosheets was obtained by hydr-othermal treatment.Ion exchange was carried out at a constant temperature of60?for 24 h.While adding WO₄^2-and Cys anions to the Mg Al-LDHs layer,Mg Al-LDHs-WO₄and Mg Al-LDHs-Cys were prepared.After anion exchange,the coating basically keeped the microscopic morphology of LHDs unchanged.WO₄^2-and Cys anions were embedded between LDHs in a horizontal orientation and single layer arrangement.(3)The values of the corrosion potential E_corrand the corrosion current density I_corrof the three films were LDHs-WO₄>LDHs-Cys>LDHs-NO₃in order.This was because LDHs-WO₄and LDHs-Cys not only have the property of LDHs-NO₃to form a barrier between the corrosive medium and the substrate,but also can release the corrosion inhibitor during the corrosion process,which improved the corrosion resistance of the film.Through the immersion experiment,the corrosion resistance mechanism of the film layer was proposed.(4)The self-healing properties of the three film layers were studied through scratch immersion experiments.The results showed that three membrane layers have certain self-healing ability,and the three film layers self-heal,in 3.5wt.%Na Cl solution.The ability of self-healing was LDHs-WO₄>LDHs-Cys>LDHs-NO₃.(5)The self-healing mechanism of the film layers was proposed.The reaction during the self-healing process was a gradual change process.At the initial stage of immersion,the compound formed by the scratch products Mg(OH)₂and Mg²⁺and the anion of the corrosion inhibitor forms an insoluble precipitate.During the transition period,the corrosion inhibitor compound and Mg Al-LDHs protected the matrix together.Moreover,the LDHs in the transition period had not nucleated and grown.As the immersion time prolonged,the scratches were covered with dissolved and recrystallized Mg Al-LDHs to complete self-healing.