Characterization And Investigation Of LDH Composite Coating And Corrosion Inhibition On Mg Alloy

Mg alloys are capable of allowing the design of lightweight-engineered systems,including encouraging implications to curtail energy consumption.Furthermore,Mg alloys are also evolving available biodegradable materials and battery electrodes.Magnesium alloys have attracted growing attention overthe last decades as lightweight materials formany applications,i.e.automobile,aerospace,sports equipment,biological implants,and aviation.However,several intrinsic drawbacks,including susceptibility to corrosion,poorformability,and low creep strength,limit the extensive use of Mg alloys.Applying the different coatings with higherelectrochemical stability,such as micro-arc oxidation（MAO）,polymercoatings,and chemical conversion coatings is one of the promising methods to overcome the poorcorrosion performance of the magnesium alloys.Among the improvement techniques,conversion coatings are a noteworthy surface treatment method that can improve the anti-corrosion performances of magnesium alloys.Layered double hydroxides（LDH）structure,which can be formed by the conversion process,is one of the promising coatings formagnesium alloys.Also,LDH structures can be composite with the 2D materials,i.e.rGO,forapplying directly to the Mg alloys and/orMAO pretreated samples.As well,the performance of the LDH coating is influenced by the preparing parameters,divalent and trivalent/tetravalent cations,p H of the precursorsolution,and the mole ratios of the cations.In this regard,an efficient and environmentally-friendly coating,layered double hydroxides（LDH）,with and without the mixture of rGO（reduced graphene oxide）nanosheets,are applied on the MAO-pretreated AZ31 to retard the corrosion rate via sealing the micro-cracks and the pores.However,the MAO pre-treatment process can be done at high voltages and preparing an appropriate MAO coating on an industrial scale,needs approximately expensive and complex facilities.Also,the sealing process is possible by doing more preparation steps which are time-consuming.So,enhancement of the anti-corrosion performances of the LDH coating by controlling the key parameters that affect the growing process,i.e.using rare earth elements as cations,p H of the preparation solution,and mole ratios of the cations,leads to omitting the MAO pre-treatment and applying the LDH coating directly on the Mg alloys.Accordingly,in the second part of this study,the dense LDH coatings comprising Mg²⁺and Ce³⁺as di-and trivalent cation,are applied on the AZ31 substrate by the combined co-precipitation and hydrothermal processing technique forthe first time.To study the influence of cerium ions on the deposition of the appropriate coating on AZ31 as well as the protective performances,the LDHs with different concentrations of Ce ions are applied.Also,in the third part of the study,by controlling the key parameters,the various mole-ratios of the Ce and Zn cations,and the p H values are employed to prepare the precursorsolution.Consequently,environmentally friendly and smart LDH coating containing Zn²⁺and Ce^3+/4+cations are successfully deposited on the AZ31 magnesium alloy via combined co-precipitation and hydrothermal method.Accordingly,the TEM,SEM,EDS,XRD,XPS,and FT-IR techniques are conducted to investigate the composition,morphologies,structures,and hydrophobicity of the coatings comprehensively.Besides,the corrosion resistance properties of the coatings and self-healing property of the coating are investigated in the 3.5 wt.%Na Cl solutions.According to the results,it is figured out that the combination of the rGO sheets with Zn-Al LDH flakes can efficiently improve the hydrophilicity,and anti-corrosion performances(i _Corr=5 n A/cm²in 3.5 wt.%Na Cl solution),as well as the long-term degradation of the MAO-pretreated AZ31magnesium alloys.Also,it’s proved that a dense coating with the two different types of structures（according to the concentration of cerium）can be successfully produced directly on the AZ31 magnesium alloys.As well,the cerium containing LDH-coating with the molarratio of Mg to Ce 3:1 is highly suitable forthe improvement of the corrosion performance of the AZ31 magnesium alloy.The corrosion current density forthe optimized coated sample is 5.12×10^-8A/cm²in 3.5 wt.%Na Cl solution.Moreover,controlling the key parameters of the growing LDH coating on the magnesium alloy,reveals that the enhanced LDH coating containing cerium cation can nearly protect the magnesium alloys from degradations and corrosion as well as the modified MAO coatings.The results demonstrate that the suitable LDH coating with appropriate morphology,dense structures,hydrophobicity,and corrosion current density(i_corr=1.29×10^-8A/cm²)is deposited on the AZ31 magnesium alloy when the Zn/Ce ratio is 2,and the p H value of the precursorsolution is approximately 11.Consequently,the prepared LDH coatings,as a cost-effective coating,in the optimum condition can be an appropriate alternative forthe MAO and toxic coatings.