Study On Biomimetic Construction And Protection Mechanism Of Graphene-based Coating On Magnesium Alloy

As the lightest structural metallic material,Mg alloy has many excellent properties,especially its low density and high specific strength/stiff,which make it have great application prospects in the fields of aerospace,automobile and electronic products.However,Mg exhibits active chemical properties and low standard electrode potential,which makes it prone to hydrogen evolution corrosion in humid environments,salt solutions and acid solutions.To overcome the above problems,researchers have proposed a variety of methods to improve the corrosion resistance of Mg alloys,such as high-purity Mg alloys,structure control,alloy design,and surface treatment.At present,the surface modification as the most used method mainly involves the construction of anti-corrosion coating.The traditional coatings lack chemical bonding effect,which is easily damaged and peeled off.Consequently,it is necessary to fabricate a protection coating with integrated corrosion and wear resistance on Mg alloy.The integrated properties can not only improve the anti-corrosion property of the sample but also overcome the wear and peel-off problems of traditional coatings,leading to a prolonged service life of coating.Graphene has been widely applied in the surface protection field due to its outstanding impermeability,oxidation resistance,high strength and lubricity.In addition,its chemical derivative,graphene oxide(GO),has abundant oxygen-containing functional groups,which avoids the high chemical inert and agglomeration problems of graphene.The Nacre component of shell which live in the marine environment,displaying a“bricks and mortar”structure through the alternate arrangement of aragonite flakes and an organic matrix.It exhibits high mechanical strength and outstanding resistance to penetration,which can protect the mollusk against the attack from external force and complex chemical environment.Consequently,in this article,the anti-corrosion/wear integrated graphene-based protective coatings is constructed on the surface of Mg-6.0Zn-0.5Ca alloy by imitating the“bricks and mortar”of nacre.Meanwhile,the influence of different cross-linking degree,different bonding effect,insert of the cerium-based intermediate layer,pre-alkaline reduction of GO and post-heat reduction of sample on the coating structure and anti-corrosion/wear property are studied.The main research contents are summarized as follows:(1)The homogeneous reduced graphene oxide/poly(vinyl alcohol)(RGO/PVA)coating is successfully fabricated on the surface of Mg alloy in the current study,through a facile spin-assisted layer-by-layer assembly(SA-LBL)technology.The hybrid hydrogen/covalent bond networks form between PVA chains and RGO sheets,the RGO sheets and PVA chains are similar in function to“bricks”and“mortar”,respectively.The microstructure of hybrid coating is strongly influenced by the cross-linking effect of PVA molecule.when the content of PVA is suitable(50wt%),the Mg alloy substrate is covered by the densely stacked lamellar coating with excellent binding strength and reduced intrinsic defects,which can significantly improve the corrosion resistance of the Mg alloy.Besides,the high hardness and good lubricant effect of RGO sheets endows the hybrid coatings with outstanding wear-resistant properties.Therefore,the bio-inspired RGO coating with a certain amount of PVA has a superior barrier effect and lubrication,which can provide integrated protection for the corrosion and wear behaviors of Mg alloy.(2)The influence of the pre-alkali reduction for GO flakes on the construction and performance of the coating were discussed via the comparison analysis for the structure and performances of GO/PVA and RGO/PVA.GO/PVA coating(50 wt%)deteriorates the corrosion resistance of Mg alloy due to its loose microstructure and typical cathodic reaction.The removal of oxygen-containing groups by chemical reduction leads to a recovery of conjugated structure and reduces the intrinsic capillaries,and the corrosion rate of RGO/PVA film is 18 times lower than that of GO/PVA,through hindering the galvanic corrosion and weakening the cathodic reaction.Additionally,the wear rate of Mg alloys protected by GO/PVA and RGO/PVA coating remarkably decreases by 98.6%and 98.2%,respectively.(3)To avoid the galvanic corrosion phenomenon caused by the oxidation-reduction reaction between GO and magnesium alloy,a Ce(IV)intermediate layer was introduced between Mg alloy and GO/PVA coating.The intermediate layer is mainly composed of Ce O₂ and Ce(OH)₄,which forms a synergistic interaction of hydrogen bond/ionic bond with the GO/PVA biomimetic coating.The Ce(?)intermediate layer blocks the redox reaction caused by GO,inhibits the galvanic corrosion phenomenon during the corrosion process,and further promotes the orderly arrangement of the upper GO sheet.The GO/PVA coating also in turn compensates for the defects and gaps on the surface of the intermediate layer.Therefore,the combination of two coatings with poor corrosion resistance shows a coupling enhancement effect and exhibits improved barrier performance.In addition,the Ce(IV)/GO/PVA coating also showed excellent wear resistance,and its wear rate is reduced by two orders of magnitude compared with that of Mg alloy.(4)The covalent cross-linking effect within RGO/PVA coating is achieved by glutaraldehyde(GA),and following with a thermal reduction process under 170 ^oC and 200 ^oC,obtaining the RGO/PVA/GA-170(RPG-170)and RGO/PVA/GA-200(RPG-200)coating with a high degree of reduction finally.The influence of different bonding effects within the coating and the thermal reduction on the coating property was studied.The results show that suitable covalent crosslinking and thermal reduction can improve corrosion resistance to a certain extent.The weak hydrogen bond between PVA molecules in the coating is converted into a stronger and more stable covalent bond via the crosslinking effect of GA,which helps to keep the integrity of the bonding network within the coating and decreases the generation of pores after the thermal reduction.Meanwhile,the hydrophobicity and barrier effect of RGO sheets are enhanced after thermal reduction.In addition,after the covalent crosslinking and thermal reduction process,the wear resistance of the sample is slightly reduced compared with RGO/PVA,which is resulted from the enhanced binding strength of the coating and weakened relative sliding of RGO.However,it can still isolate the frictional contact between the steel ball and the Mg alloy substrate and reduce the wear.(5)In order to further enhance the long-term corrosion protection effect of the graphene-based coating.The reduced-polyvinyl alcohol/glutaraldehyde-graphene/glutaraldehyde(R-PG-GG)coating is fabricated via the method that the cross-linked layer and the barrier layer are alternately assembled.The improvement of biomimetic structure gives full play to the coupling enhancement advantages between covalent cross-linking and thermal reduction,resulting in an excellent long-term anti-corrosion performance.The layered arrangement of PVA/GA and GO/GA achieves effective covalent bonding within the coating and leads to high adhesion strength.After the thermal reduction treatment at 200 ^oC,a dense and continuous cross-linked network can still be maintained inside the coating,which helps to avoid holes and gaps.In addition,the RGO sheets display an increased barrier effect.Electrochemical experiments show that the coating still has high impedance value after being immersed in 3.5 wt% NaCl solution for 6 d.