| Magnesium alloys have been widely concerned by military project,automobile industry,electronic 3C and various other fields because of their lightweight,high strength/weight ratios and excellent electromagnetic shielding.Especially,Mg-Zn-Al-Sn(ZAT)alloys have become a hot research area due to their low cost,good heat resistance and excellent mechanical properties.However,ZAT alloys possess high chemical activity and poor corrosion resistance,which seriously restrict its commercial application as engineering materials.In this work,Sn element was added into the Mg-5Zn-4Al alloy and surface coating technology was used to improve the corrosion property of ZAT alloys.The effects of Sn element on the microstructure and corrosion resistance of extruded Mg-5Zn-4Al alloy were systematically investigated by OM,SEM,TEM,XRD and electrochemical test technology.Then Al coating was prepared on the optimized Mg-5Zn-4Al-1Sn alloy surface by supersonic plasma spraying technology with orthogonal test design and variance analysis.Subsequently,Nano-Ti polymer coating was applied on the surface of Al coating.Microstructure and anti-corrosion properties of Al coating and composite coating were systematically studied,and the corrosion protection mechanisms were also explained in detail.The conclusions were as follows:(1)The extruded Mg-5Zn-4Al-xSn(ZAT54x)alloys were mainly composed of?-Mg,Mg32(Al,Zn)49 and Mg2Sn phase.Grain size of the extruded alloys was refined with the increase of Sn content.ZAT541 alloy with 1 wt.%Sn exhibited the best anti-corrosion properties through hydrogen evolution test and electrochemical test,which possessed the corrosion potential(Ecorr)and corrosion current density(Icorr)of-1.331 V and 6.707×10-6A·cm-2,respectively.The corrosion product was mainly composed of Mg(OH)2,Mg CO3,Mg Cl2·6H2O and a small amount of Al2O3,Al(OH)3,Zn O and Sn O2.(2)Supersonic plasma spraying was applied to prepare Al coating on extruded ZAT541alloy and orthogonal test results showed the order of influence factors on coating quality was spraying voltage,spraying distance and spraying current.When the spraying voltage,spraying distance and spraying current was 120 V,100 mm and 360 A respectively,the optimal Al coating with low porosity(1.27%),high bonding strength(30.53 MPa)and microhardness(40.8 HV)was obtained.Meanwhile,there was obvious grain refinement in Al coating and typical mechanical and local metallurgical bonding appeared at the coating interface.(3)The surface of composite coating was smooth with Ra about 32 nm,dense with the porosity about 0.065%and bonding strength of about 10 MPa.Corrosion resistance tests demonstrated that the anti-corrosion properties of Al coating and composite coating were better than that of ZAT541 alloy,and composite coating had the best protective effect with its Ecorr,Icorr and|Z|0.01Hz(30d)of about-0.95 V,3.53×10-9 A·cm-2 and 1.59×106?·cm2,respectively.Even after 1000h accelerated corrosion by neutral salt spray experiment,no blisters,pulverization and macro-defects on the composite coating surface were observed,which was mainly due to the combination effect of polymer coating?s nano-sized effect and passivation effect of the dense Al coating. |
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