| Mg–Li alloy has been attracting much interest from researchers in a wide range of fieldsfor its excellent properties. Especially, it has potential applications in the fields of aerospaceand automotive industries. However, the high chemical activity of Mg–Li alloy limits theirpractical application environment. Therefore, it is substantial that perform suitable surfacetreatment to improve the corrosion resistance of Mg–Li alloy. Recently, the common methodis covering coatings on Mg–Li alloy to prevent erosive ions reaching the substrate.In this paper, the methods of silane treatment, organic coatings, rare–earth conversioncoating and hot–pressing were used to assemble different zeolites coatings on Mg–Li alloy.The microstructure, morphology, chemical composition, properties and formation mechanismwere discussed in detail by the methods of XRD, SEM, potentiodynamic polarization curvesand electrochemical impedance spectra (EIS).Mg–Li alloy was covered with film using bis–[triethoxysilylpropyl]tetrasulfide silanecoupling agent. The alloy was immersed in silane solution for different time. The morphology,chemical composition and corrosion resistance of the silane film were discussed. The resultsindicated that the silane film can improve the corrosion resistant of Mg–Li alloy. The film wasuniform and covered on alloy completely when it was treated for5min. The corrosion currentdensity decreased about two orders of magnitude.The NaY zeolite was synthesized by hydrothermal synthesis. The NaY/epoxy resincoating was assembled on Mg–Li alloy. The XRD patterns indicated that the compositecoating showed the peaks of NaY zeolite and amorphous peak. In the composite coating, theNaY zeolite particles were dispersed into epoxy resin uniformly and formed enwrappedstructure. The corrosion current density of the coating decreased about two orders ofmagnitude and the corrosion resistant of Mg–Li alloy was improved. The mechanism ofcorrosion resistance of the composite coating was studied. The result showed that the NaYzeolite particles were enwrapped by the epoxy resin blocking the channels of resin and thepores of that were sealed by the viscous resin. The synergetic effect of the organic andinorganic coating can improve the corrosion resistant of Mg–Li alloy. NaY zeolite layer wasassembled on Mg–Li alloy by the method of hot–pressing, usingbis–[triethoxysilylpropyl]tetrasulfide silane coupling agent as bond. The effects of different hot–pressing conditions to the layer were discussed. The results indicated that the NaY zeolitelayer can be formed at80oC for15min. The layer has good adhesion to substrate and hasbetter corrosion resistant.Hexagonal ZSM–5zeolite with MFI structure was prepared by hydrothermalcrystallization method using n–butylamine (NBA) as template. ZSM–5zeolite layer wasassembled on Mg–Li alloy by the method of hot–pressing, usingbis–[triethoxysilylpropyl]tetrasulfide silane coupling agent as bond. The combinedmechanism between ZSM–5layer and substrate was discussed. The corrosion resistant ofZSM–5layer was also studied and the corrosion current density of the layer decreased abouttwo orders of magnitude. ZSM–5zeolite was prepared by hydrothermal method usingtetrapropylammonium bromide (TPABr) and tetrapropylammonium hydroxide (TPAOH) astemplates. ZSM–5zeolites showed an ellipsoid shape stacking of crisscross sheet layers. TheZSM–5zeolite coating was assembled on Mg–Li alloy by the hot–pressing method. Thecoating was consistent of the ellipsoid–lamellar structure, three–dimensional structure ofZSM–5zeolite partials and the network of the silane coupling agent. The multi–layerstructure improved the corrosion resistance of Mg–Li alloy effectly. The stereo–hindranceeffect of template to corrosion resistant was also discussed. The ZSM–5(with NBA, TPABr,TPAOH as templates)/epoxy resin coating was assembled on Mg–Li alloy. The structure,morphology and corrosion resistant of the coatings were studied. The corrosion resistant ofMg–Li alloy is related to the stereo–hindrance effect of templates. The template with smallerrelative molecular mass can bring greater stereo–hindrance and better corrosion resistant.Lanthanum–based conversion coating on Mg–Li alloy was prepared by themicrowave–assisted method. The composition of the coating is the intermetallic compoundsof lanthanum (Al2La0.15Mg0.85, La2Al24.4O39.6and LiLaO2). The coating is formed by theaggregation of floccular filament. Compared with sample treated at room temperature, themicrowave–assisted can shorten time to form coating and make the aggregative filament moredispersed. And then, the coating formed flaky structure covering on Mg–Li alloy. Fromcross–sectional SEM, it can be seen that the coating obtained by microwave–assisted is morecompact. The corrosion current density decreased about two orders of magnitude comparedwith that of substrate. The corrosion resistant is improved.Cerium–based conversion coating was prepared by microwave–assisted method. With the cerium–based coating as transition layer, ZSM–5zeolite coating was assembled on Mg–Lialloy by hot–pressing method using bis–[triethoxysilylpropyl]tetrasulfide silane couplingagent as bond. The intermetallic compounds of cerium (CeMg12and LiCeO2) were formed onMg–Li alloy. And the MFI structure of ZSM–5zeolite was also detected in the coating.Hexagonal ZSM–5zeolite was shown on the surface of Mg–Li alloy. From thecross–sectional SEM, it can be seen that the coating was consisted of two layers. The result ofEDS analysis indicated that the outer layer was ZSM–5coating and the inter layer wascerium–based conversion coating. And some ZSM–5zeolite partials infiltrate into thetransition layer. The silane coupling agent can form the groups of CeOSi, AlOSi and MgOSiby hydrolysis and curing. And it played an intermediary in connecting Mg–Li alloy and thecoating by chemical bond. The corrosion current density of the coating decreased about fourorders of magnitude... |
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