Corrosion Protection Of Mg-li Alloy Pretreatedby Silanes

In this paper, the anti-corrosion bahavior of novel pre-treaments based on modifiedsilane on Mg-Li substrates were investigated. Then the influence of silane pre-treatments onthe corrosion resistance of an epoxy coated Mg-Li alloy has been studied. The film structure,surface morphology, properties and corrosion resistance mechanism were evaluated in detailduring immersion in3.5%NaCl solution by the methods of diffuse reflection infraredspectrum （DRIFT）, scanning electron microscope （SEM）, electrochemical impedance spectra（EIS） and potentiodynamic polarization （Tafel）. The main research contents are listed asfollowing:1、The vinyltriethoxysilane （KH-151） silane films on the substrates of Mg-Li alloys wereprepared by dipping coating method. The effects of the silane concentration and hydrolysistime on the performance of silane coatings were investigated. In addition, the influence of theconcentration of ZrO₂, CeO₂or Ce（NO₃）₃on the performance of KH-151silane coatingswerealsostudied. The electrochemical results showed that the modified pre-treatmentprovidesbettercorrosion protection to the substratescomparatively to the use of undopedKH-151pre-treatments. Among them, the silane doped with500ppmof ZrO₂,150ppm ofCeO₂or500ppm of Ce（NO₃）₃can significantlyimprove the corrosion resistance, respectively.The substrates pre-treated with the silane solutions doped with the Ce（NO₃）₃was moreuniform, density.2、The bis（3-（triethoxysilyl）propyl）tetrasulfide （KH-69） silane films on the substrates ofMg-Li alloys were prepared by dipping coating and the effects of curing temperatureon theperformance of silane coatings were investigated. In addition, the influence of theconcentration of ZrO₂, CeO₂and Ce（NO₃）₃on the performance of KH-69silane coatingswerealsostudied. The substrates pre-treated with the silane solutions doped with the ZrO₂orCe（NO₃）₃were more uniform density. The electrochemical results showed that the silanepre-treaments doped with ZrO₂, CeO₂or Ce（NO₃）₃with the same concentration of150ppmcan considerably improve the protective behaviour of the silane film, respectively. Thisimprovement is most likely due to the additional active corrosion protection originated fromthe inhibiting action of the inhibitor impregnated in the silane matrix.3、The influence of KH-151silane pre-treatments on the performance of the epoxy coated Mg-Li alloy with coating method has been studied. For the protective system with theuntreated sample deposited from3.5%NaCl solution the corrosion process was initiated after15days of exposure, on the other hand, the sample pre-treatment with KH-151began to occurlocal pitting because of a small amount of solution penetrate the coating/substrate interfacewhen exposure after38days. It showed that the Mg-Lialloys pretreatment by KH-151filmimproved barrier properties of epoxy coating. The composition of the deposited coatingsinvestigatedby DRIFT enabled the clarification of the bonding mechanism.This conclusionwas understood as proved by the equivalent circuit fitting.