Corrosion And Air Battery Discharge Behavior Of Mg-Li-Al-Zn And Mg-Y Alloys

The development of low-carbon economy and sustainable development around the world have greatly promoted the development of fuel battery.Magnesium-air(Mg-air)battery is similar to fuel battery,which uses Mg alloys as"fuel"to generate electric energy.The discharge performance of Mg-air batteries is largely affected by the microstructure of Mg alloys,such as the texture,second phase,grain size and twins.In addition,the discharge products formed on the surface of Mg alloys also affect the discharge performance of Mg-air batteries.In this paper,Mg-Li-Al-Zn and Mg-Y alloys were used as the research objects,and the discharge performance of Mg-air batteries based on the above alloys was tested.The effects of texture,second phase,grain size,twins and discharge products on the discharge performance of Mg-Li-Al-Zn and Mg-Y alloys were studied by controlling the microstructure of the above alloys.After fully understanding the effects of the above influence factors on the discharge performance of Mg alloys,the microstructure of Mg alloys was purposefully regulated and novel Mg alloys were developed and designed to improve the discharge performance of Mg-air batteries.Moreover,there were some arguments on the effect of grain size on the corrosion properties of Mg alloys.In this paper,the influence law of grain size on the corrosion properties of Mg alloys was summarized by studying the influences of grain size on the corrosion properties of the above alloys and combining with the relevant literature reports of grain size on the corrosion properties of Mg alloys.The main conclusions were summarized as follows:(1)The discharge performance of Mg-air batteries based on the four AZ31 alloys with different texture characteristics was affected by the texture and discharge products.The prismatic oriented(10-10)and(11-20)grains had higher discharge activities than the basal oriented(0002)grains.The dense discharge products formed on the ET0°alloy(mainly consisted of the basal oriented(0002)grains)surface largely restricted the contact between the electrolyte and the unreacted matrix,whereas the loose discharge products with coarse cracks formed on the ET90°alloy(mainly consisted of the prismatic oriented(10-10)and(11-20)grains)surface promoted the contact between the electrolyte and the unreacted matrix.Therefore,compared with other AZ31 alloys,the ET90°alloy possessed the highest and steadiest discharge voltage and the highest anodic efficiency.(2)The grain boundaries dissolved preferentially than the grain interiors,and the(10-10)and(11-20)orientated grains dissolved preferentially than the(0002)orientated grains on the corrosion and discharge processes of Mg-Li-Al-Zn alloys with low Li content.Moreover,the discharge products formed on the LAZ531 alloy surface could not effectively restrict its discharge process.Therefore,compared with the LAZ131 alloy,the LAZ531 alloy possessed higher and steadier discharge voltage and higher anodic efficiency.(3)The twins dissolved preferentially than the grain interiors on the discharge process of LAZ531 alloy.The uniform distribution of twins led to the uniform dissolution of LAZ531-20%alloy.Therefore,the introduction of twins was helpful to improve the discharge performance of Mg-Li-Al-Zn alloys with low Li content.(4)Mg-1Y(E)and Mg-1Y(H)alloys with significantly different grain sizes were prepared by annealing treatment.The results showed that the grain boundaries were the preferential sites for the attack during the discharge process and the discharge products formed on the Mg-1Y(E)alloy surface with a fine uniform grain size were more easily to fall off than those formed on the Mg-1Y(H)alloy surface with a large uneven grain size.Therefore,the Mg-1Y(E)alloy exhibited higher and steadier discharge voltage.(5)The additions of trace Sn(<0.7 wt.%)were helpful to improve the discharge performance of Mg-Y alloy.Of those,the Mg-0.7Sn-1.4Y had the most excellent discharge performance,which was related to its unique microstructure characteristics:fine grain size(3.8?m),uniform distribution of fine second phases(0.6?m),and a high content of(11-20)/(10-10)orientated grains.The Sn₃Y₅ and Mg Sn Y phases were effective cathodes causing micro-galvanic corrosion which promoted the dissolution of their surrounding Mg matrix.The uniform distributions of Sn₃Y₅ and Mg Sn Y phases led to the uniform dissolution and greatly inhibited the chunk effect of Mg-0.7Sn-1.4Y alloy.(6)The additions of appropriate Ca contents(0.5 wt.%and 1.0 wt.%)significantly improved the discharge performance of Mg-Y alloy,which was mainly attributed to the grain refinement and the introduction of Mg₂Ca phase.However,when the Ca content reached 1.5 wt.%,a dense discharge products film formed the alloy surface,which largely prevented the contact between the electrolyte and the unreacted matrix and therefore resulted in a decayed discharge voltage of Mg-1.5Ca-1Y alloy.(7)The suitable microstructure characteristics of Mg alloys for the anodes of Mg-air batteries were summarized as follows:fine grain size,uniform distribution of fine second phases,introduction of twins and a high content of(11-20)/(10-10)orientated grains.In addition,because some elements(such as Ca and Zn)could form a highly protective discharge products film on the surface of Mg alloys after discharge,excessive addition could increase the thickness of discharge product s film and thus hindered the contact between the electrolyte and the unr eacted matrix,so an appropriate additive amount of these elements was needed.(8)Based on the experiments in this paper and previous literature reports about the influences of grain size on the corrosion properties of Mg alloys,the conclusions were summarized as follows:(i)in terms of the influences of grain size on the corrosion properties of Mg alloys,it should be clear that the grain boundaries dissolved preferentially and therefore accelerated the corrosion rates of Mg alloys,and then it should be considered that whether the corrosion products film formed on these preferred corroded grain boundaries had an enough effective protection;(ii)if there was an enough effective protection of the corrosion products film,the corrosion rates of Mg alloys decreased,otherwise,the corrosion rates of Mg alloys increased.