Preparation And Corrosion Resistance Of Environment-friendly Phosphating Film On LZ91 Alloy

Light Mg-Li alloy is a highly regarded structural metal material,which has been widely used in aerospace,automotive and consumer electronics industries due to its high specific stiffness,low density,attractive vibration absorption and excellent electromagnetic shielding properties.Mg-Li alloy is the most commonly used magnesium alloy in engineering structures.The corrosion current density of LZ91 and LAZ931 alloy in 3.5%Na Cl solution is much higher than that of AZ31 alloy.Therefore,it is necessary to improve the corrosion resistance of Mg-Li alloy by surface modification.Among various surface modification methods,chemical conversion film can not only improve the corrosion resistance of the base alloy,but also have better bonding ability to the top layer coating.In this paper,the surface treatment of LZ91 and LAZ931 alloy by phosphate conversion method is adopted to explore the optimal process parameters of phosphate conversion,aiming at studying the corrosion behavior of environmentally friendly phosphate conversion film on the surface of Mg-Li alloy.In order to explore the most suitable experimental conditions for the preparation of phosphate conversion film,scanning electron microscope（SEM）,energy dispersive X-ray spectrometer（EDS）,atomic force microscope（AFM）and electrochemical workstation were used to characterize the morphology and electrochemical behavior of the phosphating films prepared by different phosphating reaction time,temperature and PH.The results show that when the phosphating reaction time is 20min,the phosphating solution temperature is 40℃,the phosphating solution PH is 3,the substrate surface is covered with dense vane-like phosphating film,the corresponding corrosion current density is lower,the surface morphology and corrosion resistance are the best.LZ91 and LAZ931 alloys were phosphating under the optimum technological parameters,and the microstructure,composition,electrochemical properties and film forming mechanism of phosphating films were further studied.The results show that the microstructure of the phosphating film is irregular overlapping and dense,and the thickness of the film is～25μm.X-ray diffraction results show that the phosphating film is mainly composed of calcium hydrogen phosphate dihydrate（Ca HPO₄·2H₂O）and magnesium phosphate（Mg₃（PO₄）₂）.The electrochemical results show that the corrosion protection efficiency of phosphating film on LZ91 and LAZ931 Mg-Li alloys is 98.04%and 95.89%,respectively.The phosphating film is adsorbed on the surface of the alloy to form a dense lamellar protective film,which almost covers the anode（β-Li）and cathode（α-Mg）.The microgalvanic corrosion of the alloy in Na Cl solution is inhibited,and the corrosion resistance of the alloy is effectively improved.