Study On Surface Coating And Corrosion Characteristic Of Mg-Li Alloys

Mg-Li alloy is one of the lightest metal structural materials,which should bewidely used in automotive,electronic,weapon and aeronautical industries owingto its excellent properties,such as high strength/weight ratio,high impacttoughness,bending strength and deforming machining easily.However,it is thepoor corrosion resistance that limits its extensive applications.In this thesis,the effects of concentration and pH value of corrosive solutionson corrosion behavior of Mg-Li alloy were studied.The corrosion behavior wasdescribed through corrosion rate,morphology of the corroded surface,corrosionproducts and electrochemical analysis.In NaCl solution,the main corrosionmechanism was the selective pitting due to the special characteristics of chlorideion.The large corrosion pits were formed because the corrosion pits expandedtowards the matrix inner and enlarged all around.The corrosion products of thealloy in alkaline NaCl solution were the most,and the size of the corrosion pits inacidic NaCl solution was the largest.The main components of the corrosionproducts in alkaline solution were Mg(OH)₂,Al₂O₃ and Li₂O₂,which were allcrystals.But in acidic and neuter solution the products were Mg(OH)2 and Al₂O₃.The corrosion of Mg-Li alloy developed more seriously with the increase in Cl^-concentration,because the corrosion rate and corrosion current increased.Thecorrosion potential gradually increased with the increase of alkalescence,whichindicated that the passivation of the surface film was improved and the corrosionvelocity was restrained provisionally.When the acidity increased,the corrosioncurrent gradually increased and the linear resistance decreased,this means that thecorrosion reaction accelerated.The technical parameters of conversion coatings on the Mg-Li alloy wereoptimized by orthogonal experiments.The optimum processing parameters of Ce-based conversion coating were confirmed as follows:solution concentration is0.05 mol/L,treating temperature is 35℃and treating time is 10 rain.Theoptimum process parameters ofphytic acid conversion coating were confirmed asfollows:pH value of the solution is 6,solution concentration is 20 g/L,treatingtime and treating temperature is 10 min and 35℃,respectively.The corrosionresistance ability was evaluated by hydrogen evolution experiment andelectrochemical analysis.The results showed that the conversion coatingsimproved the corrosion resistance of the alloy.Analysis by SEM indicated that theCe-based conversion coating was a grey uniform coating with a fiber-likemorphology,its thickness was 12μm.The phytic acid conversion coating was adark grey coating with white flower-like deposit and some micro cracks.Thecomponents of coatings were investigated by EDS,XRD,XPS and FT-IR.Themajor component of the Ce-based conversion coating was a mixture of oxide andhydroxide of Ce(â…¢) and Ce (â…£).The phytic acid conversion coating mainlyconsisted of Mg,Al,O,P and C,the dark grey area was mainly magnesic chelatecompounds and white area was mainly aluminous chelate compounds.Theforming processes of the conversion coatings were discussed through the OCPevolution and components of coatings.Nano-SiO₂ precursor was synthesized through sol-gel technique,then it wasmodifed in situ byγ-epoxypropoxytrimethoxysilane.The modified nano-SiO₂precursor was characterized by XRD and FT-IR.The epoxy/nano-SiO₂ hybridcoating was prepared by using the modified nano-SiO2 precursor.The analysis bySEM showed that coating was two-phase structure,and nano-scale SiO₂ particleswere dispersed in epoxy matrix homogeneously.The corrosion resistance andhardness of hybrid coating were better than epoxy coating.The analyses ofelectrochemical potentiodynamic polarization and impedance spectroscopyindicated that the protection was best when mass fraction of nano-SiO₂ was 3%. The epoxy/nano-SiO₂ hybrid coating was prepared on the conversioncoatings of Mg-Li alloy,this is called composite coatings or composite protectiontechnique.The results of hydrogen evolution and salt spray corrosion experimentsindicated that the composite coatings had better corrosion resistance than themono-coatings.The corrosion resistance of phytic acid-epoxy/nano-SiO₂composite coating was the best.The protection process and predominance ofphytic acid- epoxy/nano-SiO₂ composite coating was analyzed by diffusion theorythrough electrochemical impedance spectroscopy.The hardness and adhesion ofcomposite coatings were better than epoxy coating and hybrid coating,which canprovide more effective protection for Mg-Li alloy.