| Intercity railway,subway,light rail are booming development with the rapid economic development in recent years,as well as the continuous improvement of people’s living standards,the rapid development of rail transit.So in the next few years it has become the key areas of infrastructure construction investment in China.There is no doubt that lightening is the most important means to realize the acceleration of high-speed railway."Replacing steel with aluminum" is one of the key technologies to realize the lightening of high-speed railway.However,the electrode potential of 7050 aluminum alloy commonly used in high-speed railway carriages is relatively low.Especially when it comes into contact with other metals,it is more likely to be used as an anode to accelerate corrosion.Also its wear resistance is poor.Therefore,the surface of aluminum alloy is often sprayed with one or more coatings for protection.With the development of economy and society and the enhancement of people’s environmental protection consciousness,the choice of its surface coating has been developing towards the water-based coating with low VOC emission,and the water-based coating has been widely used in high-speed railway cars.However,there are still some problems in waterborne coatings,such as the existence of hydrophilic groups in waterborne polyurethane coatings,which leads to poor corrosion resistance in practical applications.The corrosion resistance of aluminum alloy and aluminum alloy sprayed with waterborne polyurethane is poor.In order to meet the requirements of environmental protection and corrosion resistance of high-speed railway,this paper adopts silicate electrolytic liquid system to conduct micro-arc oxidation treatment on 7050 aluminum alloy,so as to prepare ceramic layer with uniform surface,high corrosion resistance and excellent binding strength with waterborne polyurethane.And use the roughness meter,thickness meter,SEM,XRD,contact Angle measurement,drawing experiment,electrochemical characterizing the polar curve measurement methods,to explore the different electrical parameters in the process of micro-arc oxidation treatment,electrolyte composition and concentration,oxidation time and so on conditions on the micro arc oxidation ceramic layer thickness,roughness,and the aqueous polyurethane coating bond strength and corrosion resistance.The thickness and roughness of the 7050 micro-arc oxide ceramic layer for high-strength aerospace aluminum alloy were consistent,which increased with the increase of reaction time,current density,duty ratio and electrolyte concentration,and then decreased with the first increase of power pulse rating,and reached the maximum at 250 Hz.In this reaction condition,the main phase of the film layer is-Al2O3,there are pores and volcanic protrusions on the surface of the micro-arc oxide ceramic layer,which are uniformly distributed on the surface of the film layer.The optimal technological conditions are as follows:pulse frequency is 250 Hz,current density is 12 g/L of sodium silicate,6 g/L of sodium fluoride,and 4 g/L of sodium hydroxide.Under these conditions,the MAO film was homogeneous in growth with a thickness of approximately 11.2 μm.The roughness is approximately 0.786 μm.The 7050 aluminum alloy treated by micro-arc oxidation has a smaller affinity with water after modified by silane coupling agent.Compared with the unmodified version,the bonding strength between the modified MAO film layer and the waterborne polyurethane coating increased,and the corrosion current after spraying the waterborne polyurethane was 2.462×10-8 A/cm2,which decreased by three orders of magnitude compared with the matrix and two orders of magnitude compared with the bare-coated waterborne polyurethane. |
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