Effect Of Graphene Oxide Doping On Corrosion Resistance Of Lanthanum Salt Conversion Film For Magnesium Alloy

As a kind of engineering material,magnesium alloy has been widely used in automotive,electronics,transportation,aerospace and other fields due to its high specific strength,toughness and excellent cutting performance.However,the poor corrosion resistance of magnesium alloy severely limits its application scope.Rare earth salt conversion film is a green surface treatment technology with good corrosion resistance,which has been widely studied in recent years.Graphene oxide is a two-dimensional material with great application potential in the direction of metal coating.In this paper,the optimal film forming process of rare earth lanthanum salt conversion film on magnesium alloy surface was investigated by single factor experiment,and then graphene oxide was doped into lanthanum salt conversion film by one-step method and self-assembly multi-step method.Experiment using spot testing,hydrogen evolution,immersion test and electrochemical method（EIS,Tafel）on the corrosion behavior of membrane layer in 3.5%Na Cl solution,using scanning electron microscopy（SEM）,energy spectrometer（EDS）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）to explore the microstructure and composition of membrane layer.The main conclusions are as follows:（1）Lanthanum salt conversion film can greatly improve the corrosion resistance of magnesium alloy.The surface of the film is"cracked",and the main component is La（OH）₃.The corrosion resistance of magnesium alloy samples is the best when the film forming time is 30min,the transformation temperature is 20℃,and the mechanical stirring is 50 r/min.Compared with the bare magnesium alloy matrix,the self-corrosion current density of the sample is decreased by 4 orders of magnitude,and the self-corrosion potential is obviously positive.（2）A GO/La composite film was prepared on the surface of AZ31B magnesium alloy matrix by doping graphene oxide in the lanthanum salt conversion solution in a one-step method.The surface morphology of the composite film is similar to that of the single lanthanum salt conversion film,and there are turtle cracks on the surface.Doped GO can improve the corrosion resistance of lanthanum salt conversion film.When the doping amount of GO is 50 mg/L,the corrosion resistance is the best.Compared with the undoped GO sample,the self-corrosion current density of the film decreases by about one order of magnitude.In addition,the self-etching current density of the samples decreased by an order of magnitude when the mechanical agitation was slight（50 r/min）,compared with that of the composite films without agitation.（3）La/Go/GLYMO composite coatings with excellent corrosion resistance were prepared by self-assembly.When the concentration of GO solution was 0.5 g/L,its self-corrosion current density was as low as 1.024×10^-10 A·cm^-2,and the corrosion resistance of the coating was the best,the corrosion current density decreased by an order of magnitude compared with that of the Go/La composite films prepared by one-step method,indicating that the corrosion resistance of the substrate could be significantly improved by the spin coating GO.The hydrogen evolution experiment and the brine immersion experiment were consistent with the conclusions of the electrochemical experiment.