The Effect Of Mg Addition On Hot-Dipped Zn-Al Alloy Coatings

The hot-dip Zinc-Aluminum-Magnesium alloy coating is a type ofenvironmentally friendly coatings which has numerous commercial applications inindustry, because the Zn-Al-Mg alloy coating with excellent corrosion resistance, itcan adapt to different corrosive areas and environment. In recent years, a serious ofinvestigation have been reported by domestic and foreign researchers on corrosionresistance, they did not do further study, and no recordation about inhibition layerexistence of Zn-Al-Mg alloy coatings in open papers. In this paper, SEM and EDSwere applied to investigate the microstructures, composition and growth of theintermetallics layer and solidification layer. Corrosion resistance of different coatingswas evaluated by means of immersing corrosion test, electrochemical technology andneutral salt-spray test.The quality of hot-dip galvanizing varies according to different flux and fluxprocess parameters choosing. The experiment demonstrated when Al and Mg wereadded into the bath, the fluidity and brightness of molten bath have a greatimprovement, and the Zn-6%Al-3%Mg alloy coating is smooth and bright, withoutsurface defect. However, owe to excessive Mg addition, the surface quality was hardto control, even bare spots emerged. The Flux with the composition of ZnCl2：107g/L,NH4Cl：100g/L, SnCl2：80g/L has best effect on Zn-Al-Mg alloy coating. The bestprocess temperature is70℃, and optimum process time is120s.The study found that the intermetallics layer in the hot-dip Zn-6%Al-3%Mgalloy coating consisted of the Fe-Al-Zn intermetallics compound, the Fe-Al-Znintermetallics layer inhibits formation of the brittleness Fe-Zn compound layer. Theaddition of Mg into the Zn-6%Al bath promotes the interface reaction and growth ofthe intermetallics layer and makes it dense. Meanwhile, the intermetallics layerbecomes too stable to be eroded by the liquid zinc. At the beginning of theintermetallics layer formation, the diffusion path crosses the two-phase region of Znbath and relevant Fe-Al compound following a tie-line. A corresponding denselamellar Fe-Al compound forms firstly on the interface. Without the Mg addition, the diffusion path will move gradually to the line connecting with two diffusioncomponent points with prolonging of dipping time, diffusion path cross tie-line of theliquid/intermetallics two-phase region, liquid channel occur in the intermetallics layerand intermetallics layer will be less dense. With the Mg addition the movement of thediffusion path to the line connecting with two diffusion component points is beinhibited. The thickness of the coating increases with prolonging of dipping time. Thegrowth of the coating in Zn-6%Al-3%Mg bath is controlled by diffusion.When Mg addition lower than3%, the solidification of alloy coating takes place,at first the Al-fcc phase forms as primary phase, then the Zn-hcp as the second phasenucleates in the coating. Both the Zn-fcc and Al-hcp phase grow further until theeutectic transformation and then then the liquid reach the eutectic composition andZn/Al/MgZn2forms. MgZn2becomes the first phase instead of Zn-hcp if the Mg%is3%-4%. Once the4%Mg adds into the Zn bath, the first phase to form is MgZn2,follows by the Al-fcc, after eutectic transformation the Zn/Al/MgZn2forms.Based on the results of immersing corrosion test, electrochemical test and neutralsalt-spray test, the corrosion resistance of the Zn coating improved greatly with theaddition of Al, the corrosion resistance of the Zn-Al alloy coating enhance with theincreasing the amount of Mg in coating. The Zn-6%Al-3%Mg has excellent corrosionresistance in all samples. As the content of Mg is more than4%in the alloy, thecorrosion resistance and the surface quality decreases. The results of XRD show thatcorrosion product of the Zn-6%Al-3%Mg alloy coating is mainly Zn5(OH)8Cl2H2O.The characteristic of Zn5(OH)8Cl2H2O is dense, insoluble and short of electricconductivity. Since the addition of Mg promotes formation of Zn5(OH)8Cl2H2O, Mgin the coating will improve the corrosion resistance of the Zn-Al alloy coating.