Preparation And Properties Of Silane Composite Anticorrosion Coatings On Mg-Li Alloy

Magnesium-lithium?Mg-Li?alloy,a kind of super lightweight metal materials,has attracted considerable interest in automobiles,aerospace,military and nuclear industries because of their lower density,high strength,high ductility and good formability.However,the high chemical activity of lithium means the Mg-Li alloys are susceptible to corrode in applied environment and difficult to protect,which limits their widespread practical application.Therefore,it is important to develop efficiently anticorrosion technologies for Mg-Li alloys.Among the many corrosion protective methods,surface coatings are simple and practicable which have various applications in anticorrosion.In this dissertation,silane composite anticorrosion coatings were prepared on the surface of Mg-Li alloys via composite with rare earth,polyaniline?PANI?doped with phytic acid?PhA??PANI-PhA?,PANI grafting SiO₂?SiO₂@PANI?based on silane conversion coatings.The chemical structure,compose and morphologies of composite component,anticorrosion coatings and the surface of Mg-Li alloys were evaluated by scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?and X-ray diffraction?XRD?.The anticorrosion properties of silane composite anticorrosion coatings for Mg-Li alloy were tested by electrochemical impedance spectroscopy?EIS?,potentiodynamic polarization measurements and open circuit potential.The conditions of coatings processing were discussed and the possible anticorrosion mechanism was also proposed based on procedure analysis.Silane conversion coatings were prepared on Mg-Li alloy by sol-gel and chemical soaking methods using vinyl trimethoxysilane?VTMS?and tetraethy orthosilicate?TEOS?as precursor.Formation conditions of silane conversion coatings?ratio of ethanol and water,m?TEOS?:m?VTMS?,dosage of silane,etc.?has been discussed.The prepared silane conversion coating has good anticorrosion performance to Mg-Li alloy,the corrosion current density reduced two orders of magnitude and electrochemical impedance value enhanced one order of magnitude compared with Mg-Li alloy.Dense cerium conversion coatings were prepared on the surface of Mg-9.95Li alloy by immersing Mg-9.95Li alloy in cerium nitrate solution which added H₂O₂ and citric acid.Ce³⁺could transfer to Ce⁴⁺by the oxidation of H₂O₂.The conversion coating mainly composed of CeO₂ and Ce?OH?₄ cross-linked by citric acid,then make the conversion coating more dense and uniform.The dense cerium conversion coatings have better anticorrosion properties to Mg-9.95Li alloy due to the effect of H₂O₂ and citric acid.Cerium/silane composite conversion coatings were prepared by combining cerium and silane.The composite conversion coatings have better corrosion protect properties compared with cerium and silane conversion coatings.The corrosion current density is lower and the electrochemical impedance is higher.A phosphorized PANI doped with PhA?PANI-PhA?was synthesized by the chemical oxide method with PhA as a dopant.The results of FTIR,UV-Vis-NIR,XPS,XRD and SEM showed that the as-synthesized phosphorized PANI exists in doped emeraldine salt state with net-like structures crosslinked by phosphate carboxyl groups.The conductivity and thermostability of PANI-PhA were better than those of PANI doped with phosphoric acid?PANI-H₃PO₄?and undoped PANI.The PANI-PhA was applied to improve the anticorrosion properties of Mg-Li alloys after blending with silane sol.The results of electrochemical measurements showed that the anticorrosion ability of PANI-PhA is the best among the three PANI samples.The dosage of ammonium persulfate?APS?,PhA and PANI-PhA were obvious effect to the properties of PANI-PhA and PANI-PhA/silane composite coatings.The electrochemical impedance is up to 10⁸?·cm²,and the corrosion current is merely 10^-10 A·cm^-2.The analysis of mechanism showed that PANI-PhA/Silane have good anticorrosion properties to Mg-Li alloy due to the synergistic effect of PhA,PANI and silane.SiO₂@PANI sol with core-shell structure through chemical bonding were synthesized with SiO₂ which modified by?3-glycidoxypropyl?-trimethoxylsilane?GPTMS?as core and PANI as shell.The SiO₂@PANI sol has good dispersion stability in water.Further,SiO₂@PANI/VTMS sol were prepared by in situ embedding SiO₂@PANI particles with VTMS,and the sol could form coatings by coat it on the surface of Mg-Li alloy directly without epoxy.The results showed the SiO₂@PANI/VTMS coatings have a high hydrophobic angle which reached 145.5°when m?An?:m?TEOS?=7:100 and m?VTMS?:m?TEOS?=4:4.The composite coatings have excellent corrosion resistance performance to Mg-Li alloy which the electrochemical impedance value is 7.5×10⁴?·cm²and the corrosion current density is 4.47×10^-8 A/cm².The SiO₂@PANI/VTMS composite coating exhibits high corrosion protection ability for the Mg-Li alloy due to the synergistic effect of surface hydrophobicity,barrier and passivation.