Effect Of Alloying Elements On Microstructure And Properties Of Hot Dip Zn-5Al-xMg Alloy

Hot dip plating is an economical and effective method for surface treatment of steel,which can improve the corrosion resistance and prolong the service life of steel.And hot dip plating has the characteristics of simple production process and low cost,so it is widely used in construction,automobile and home appliances and other industries.With the rapid development of modern industry,the traditional hot dip Zn coating has been unable to meet the needs of steel anticorrosion.Many research results showed that the corrosion resistance and mechanical properties of zinc coating can be improved by alloying elements such as Al and Mg.A variety of Zn-Al and Zn-Al-Mg alloy coatings have been successfully commercialized,such as Galvalume（Zn-55Al-1.6Si）,Galfan（Zn-5Al-0.1RE）,ZAM（Zn-6Al-3Mg）,Superzinc（Zn-4.5Al-0.1Mg）and so on.Hot dip Zn-Al-Mg alloy coating has become a research hotspot because for its excellent corrosion resistance.In this paper,the effects of alloying elements on the microstructure and properties of hot-dipped Zn-5Al-x Mg alloy were systematically studied by XRD,SEM,EDS,EIS test,electrochemical polarization curve test,total impregnation test and XPS test.The alloy with the best corrosion resistance and Zn-5Al-x Mg alloy were selected for hot dip plating experiment,and the microstructure and properties of the coatings were characterized.The microstructure of Zn-5Al-2Mg alloy is mainly composed of Zn-rich phase,dendritic Al-rich phase and lamellar Zn/Al/Mg Zn₂ ternary eutectic structure.Si-rich phase,Ti(Al_1-xSi_x)₃ phase,Al₃Zr phase and Al₃（Ti Zr）phase appear in the alloy after the addition of Si and Ti,Si and Zr as well as Ti and Zr elements in equal proportion,which can be used as nucleation substrate to refine the grain size of the alloy.The addition of Ti and Zr elements has the best refining effect on the microstructure.With the addition of alloying elements,the hardness of the alloy increases with the improvement of microstructure refinement and uniformity.The corrosion resistance of Zn-5Al-2Mg alloy was improved by the addition of Si and Zr,Ti and Zr elements.This is because the grains are refined with the addition of elements,which inhibits the occurrence of intergranular corrosion and makes the alloy more prone to uniform corrosion.At the same time,during the corrosion process,the formation of dense Zn₅（CO₃）₂（OH）₆corrosion products is promoted,and the corrosion product layer is more dense,effectively blocking the corrosion of solution ions and improving the corrosion resistance of the alloy.The cross-section morphology of the coating shows that the Fe-Al intermediate alloy layer of Zn-5Al-2Mg coating is thicker and the coating grain is coarse.However,there is no intermediate alloy layer in the Zn-5Al-2Mg-0.2（Ti Zr）coating,which strengthens the bonding force between the steel substrate and the coating,reduces the grain size and improves the corrosion resistance of the coating.When the Mg content is 3wt.%,the microstructure of Zn-5Al-3Mg alloy changes obviously,the Zn-rich phase disappears,and the double hexagonal Mg Zn₂ phase appears,and the distribution range of lamellar Zn/Al/Mg Zn₂ ternary eutectic structure increases.At this Mg content,the addition of Si element mainly exists in the form of Mg₂Si phase.When Si and Ti,Si and Zr and Ti and Zr elements are added in equal proportion,the nucleation substrate increases and the grain is refined.The addition of alloying elements increases the hardness of Zn-5Al-3Mg alloy.The electrochemical test results show that the corrosion process of Zn-5Al-3Mg alloy with alloying elements is mainly controlled by charge transfer,and the resistance of charge transfer increases.At the same time,the addition of alloying elements in the corrosion process inhibits the formation of loose and porous Zn O,promotes the formation of dense Zn₅（CO₃）₂（OH）₆,makes the corrosion product layer more dense,delays the corrosion process,and improves the corrosion resistance of the alloy.The results of morphology analysis show that the intermediate alloy layer of Zn-5Al-3Mg coating is thick and the coating grain is coarse.However,the intermediate alloy layer in the Zn-5Al-3Mg-0.2（Ti Zr）coating becomes thinner obviously,which enhances the bonding force between the steel plate and the coating,and the corrosion resistance of the coating is greatly improved.