Study On Microstructure And Corrosion Properties Of Zn-xAl-yMg Alloys

In this paper,the microstructure and the corrosion properties of Zn-xAl-yMg（x=4,6,9,11;y=0,1.5,2.5,3.5）alloys were investigated.The microstructure and phase of ZnAlMg alloys were studied by metallographic microscopy,scanning electron microscopy,X-ray energy dispersive spectroscopy and electron energy spectroscopy.The polarization curves and AC impedance electrochemical characteristics of ZnAlMg alloys with different content of Mg in 3.5 wt.%NaCl solution were tested by CHI604E electrochemical workstation.The corrosion behavior was determined by full immersion corrosion method and electrochemical test method.The self-corrosion law of different Mg content on Zn-xAl（x=9,11）alloy and the effect of sacrificial anode protection cathode were investigated through the full immersion uniform corrosion experiment and the sacrificial anode protection cathode experiment.The composition and morphology of corrosion products with different time of Zn-xAl-yMg（x=9,11）alloys were studied by X-ray energy dispersive spectroscopy,scanning electron microscopy and electron energy spectroscopy.The corrosion mechanism of ZnAlMg alloy was discussed based on this analysis.The results show that the Zn-xAl-yMg alloy is mainly composed by Zn-rich phase,Al-rich phase and Zn-Al-MgZn₂/Mg₂Zn₁₁ ternary eutectic structure,but the microstructure of different ZnAlMg alloys has different changes with the increase of Mg content.Among them,Zn-4Al-Mg and Zn-6Al-Mg alloys have similar law,that is,as the Mg content increases,the coarse Zn-rich phase gradually refines and the amount is decreased,and the amount of ternary eutectic structure gradually is increased.when the content of Mg is 3.5%,the Al-rich phase is suppressed.The law of Zn-9Al-Mg and Zn-11Al-Mg is similar,that is,the coarse Zn-rich phase gradually decreases,and the Al-rich phase increases as the Mg content increases.When the content of Mg is 3.5%,the coarse MgZn₂ and Mg₂Zn₁₁1 appear and the number of ternary eutectic structures is reduced,and the grains are coarsened.The electrochemical polarization curves and AC impedance tests show that the corrosion rate of ZnAlMg alloys is lower than that of ZnAl alloys and pure Zn.The corrosion current density I_corr of Zn-4Al-Mg alloys and Zn-6Al-Mg alloys decreases with the increase of Mg content.The corrosion current densities I_corr of the Zn-4Al-3.5Mg and Zn-6Al-3.5Mg alloys are 1.586μA·cm^-2 and 1.139μA·cm^-2,respectively.The corrosion current density I_corr of the Zn-9Al-Mg and the Zn-11Al-Mg alloys increases with the increase of the Mg content.I_corr of the Zn-9Al-1.5Mg and Zn-11Al-1.5Mg alloys are 1.011μA·cm^-2 and 0.521μA·cm^-2,respectively.The charge transfer resistance R₁ in the different Zn-Al-Mg alloys decreases substantially as the Mg content increases.The results of sacrificial anode electrochemical performance test show that the sacrificial anode current efficiency of Zn-11Al-3.5Mg alloy is 95.14%,and the anode consumption rate is 8.29 Kg/A·a.The full immersion uniform corrosion test shows that the average corrosion rate of Zn-4Al-Mg and Zn-6Al-Mg alloys decreased with the increase of Mg content;the average corrosion rate of Zn-9Al-Mg and Zn-11Al-Mg alloys increased with the increase of Mg content,which is basically consistent with the results of polarization curve test.By observing the surface corrosion morphology and analyzing corrosion products of pure Zn and Zn-9Al alloys under different corrosion times,the layered double hydroxide Zn₆Al₂（OH）₁₆CO₃ and Mg₆Al₂（OH）₁₆CO₃·4H₂O were rapidly formed in the inner layer of the ZnAlMg alloy at the initial stage of corrosion,the outer layer is mainly Zn₅（OH）₈Cl₂ with good adhesion and poor conductivity.These corrosion products can effectively shield the external corrosion.During the corrosion process,the element of Mg can inhibit the transformation of Zn₅（OH）₈Cl₂ to ZnO and Zn₅（OH）₆（CO₃）₂,which have poor corrosion protection,thus it reduces the corrosion rate of the alloy.In the Zn-9Al-Mg alloy,the 3.5%content of Mg leads to the formation of single-phase coarse MgZn₂ and Mg₂Zn₁₁ in the microstructure,resulting in uneven distribution of the surface potential of the alloy.A lot of micro batteries will be formed and accelerate corrosion of alloy.