Research On New Hot-dipped Zinc-aluminum Coatings And Processes

Hot dip zinc-aluminum alloy coating is a widely used metal coating in industry.Further optimization of alloy composition, the development of new zinc-aluminumalloy coating is necessary. In this paper, the application of electrochemistry corrosiontechnology, neutral salt spray test, scanning electron microscope, energy dispersivespectroscopy, metallographic microscope and X ray diffraction method were used tostudy the effect of alloying elements and cooling method on coating microstructureand corrosion resistance. An excellent corrosion resistance alloy coating (Zn-30%Al-1.1%Si-1.0%Mg-0.3%La) with low aluminium was developed. Through the experi-mental study and analysis of the research, the following findings were obtained:(1) When the silicon content in the range of0.5%~1.5%, with the increase of sili-con content, Al-Si eutectic in Zn-40%Al-Si alloy solidification structure increased,the grains of the coating were refined, the corrosion resistance of the coating was con-tinuously reinforced. When the silicon content reached2%, plating leakage defectappeared. When the silicon content increased to3%, the blocky primary siliconformed in the solidification coating. X ray diffraction analysis confirmed the maincorrosion products of the coating is Zn5(OH)8Cl.H2O.(2) When the iron plate was immersed in the bath of Zn-30%Al-1.1%Si andZn-40%Al-1.1%Si at550℃, as the hot dip time was180s, the kinds of the reactionphases no longer changed, the order of the formed phases was as follows: ma-trix/(Fe2Al5+τ1)/FeAl3/τ5C.With the increasing of the reaction time, the τ5Cphasebroke, so that the liquid began to enter FeAl3phase, The liquid consumed the iron inFeAl3, Fe2Al5and the substrate, promoted the growth of FeAl3and τ5C, which accel-erated the growth of intermetallic compound layer. As a result of silicon-rich τ1phaseenriched in the interface of Fe2Al5and FeAl3phase, and the role of dissolved siliconin Fe2Al5phase, the aluminum diffusion was inhibited, so growth of Fe2Al5phase isslow.(3) For the hot dip coating of Zn-30%Al-1.1%Si-1.0%Mg, the optimum fluxingtemperature is90℃, the drying time is20s; The optimal fluxing agent is: ZnCl2(23g),NH4Cl (27g), SnCl2(5g), K2ZrF6(1~2g) in100ml water; K2ZrF6can effectively de-stroy the surface oxide in zinc-aluminum bath, make the coating surface more smoothand shorten the drying time, effectively eliminate the plating leakage defect; The cor- rosion resistance of Zn-30%Al-1.1%Si-1.0%Mg-0.3%La coating is better than that ofGalvalume coating.(4) When the aluminum content was lower than23%, with the increase of the con-tent of aluminum, the corrosion resistance of zinc-aluminum alloy decreased. Whenthe aluminum content was at about23%, the corrosion resistance of the coating is theworst. When the aluminum content was greater than23%, with the increasing of thecontent of aluminum, the corrosion resistance of zinc-aluminum alloy increased. Theeffect of cooling mode on the corrosion resistance of zinc-aluminum alloy with17%~29%Al is obvious, mainly because of the big difference of the compositionbetween the primary phase and eutectic phase. For the zinc-aluminum alloy with thealuminum less than17%or greater than29%, the cooling method has less effect onthe corrosion resistace of the coating.