Effect Of Indium On Microstructure Of Hot Dip Galvanized Coating On Q235 Steel And Correlation Phase Equilibrium

Hot dip galvanizing is a kind of anticorrosive coating technology for iron and steel materials,which is widely used in the world.Silicon,as a common impurity or alloying element,exists in iron and steel materials.However,silicon in steel will lead to the explosion and growth of Fe-Zn compounds in galvanized layer,and the coating is too thick and dark and rough,which is called the Sandellin effect.Alloy elements are often used to improve the coating in order to suppress the Holy-delin effect.The low melting point of In can improve the fluidity of Zn liquid and the surface quality of hot-dip galvanized workpiece.At the same time,the atomic radius of In（1.66A）is larger than that of Zn（1.39a）and Fe（1.27A）.When In atom is dissolved into Fe-Zn compound,the barrier needed to be covered by atomic diffusion is increased,the reaction rate of Fe-Zn is slowed down,and the coating thickness is reduced.In order to explore the mechanism of effect of In In zinc pool on hot dip galvanizing of silicon steel,this work focuses on the effect of In on galvanized layer structure and related phase equilibrium.The results are of great theoretical significance to the development and design of zinc-based alloys and to enrich the thermodynamic database of zinc-based alloys.Phase equilibria of Fe-Zn-in ternary system at 450℃and 600℃were studied by equilibrium alloy method and scanning electron microscopy（SEM）and energy dispersive spectrometer（EDS）and X-ray diffraction（XRD）.The results show that Fe-Zn-In ternary system has 5 three-phase regions at 450℃,3 three-phase regions were measured experimentally:αFe+Γ+（In）,Γ₁+δ+（In）andδ+（In）+（Zn）.The solubility of In inαFe,Γ,Γ₁,δandζwas measured to be 0.1 at.%,7.7 at.%,4.1 at.%,2.4 at.%and 0.4 at.%,respectively;The Zn solubility inαFe was 3.5 at.%;The Fe and Zn solubility in liq.（In）are 1.0 at.%and 8.3 at.%.The Fe and In solubility in liq.（Zn）are 0.8 at.%and 0.3 at.%.At 600℃,the Fe-Zn-In ternary system has 3three-phase regions,2 three-phase regions were measured experimentally:αFe+Γ+（In）andδ+（In）+（Zn）.The solubility of In inαFe,Γandδwas 0.6 at.%,7.6 at%and 1.5at.%,respectively;The Zn solubility inαFe was 6.8 at.%;The solubility of Fe and Zn in liq.（In）phase are 0.9 at.%and 13.9 at.%.The solubility of Fe and In in liq.（Zn）phase are 0.5 at.%and 1.1 at.%.In this study,no ternary compound was discovered at 450 and 600℃.There is a new phase relationship between the Zn-In binary boundary.According to the experimental results,the boundary of the binary system at this temperature is modified.After the Q235 steel plates were dipped in the pure zinc bath,the compound layer of coating were composed of looseζphase and discontinuousδphase,and theζphase grew explosively.With the addition of In,The denseΓphase layer formed in the coating except theζphase andδphase.As the increase of the In content,theδphase layer grew continuous and dense.The denseδphase layer andΓphase layer hindered the interdiffusion between elements Fe and Zn,and restrained theζphase’s explosive growth.The calculation results of coating growth kinetics shows that the thickness of intermetallic compound increases with the prolongation of the immersion time.When In is added to the zinc bath,the growth of the compound layer is controlled by diffusion.The maximum solubility of In inζphase is 0.39 at.%,and the diffusion coefficient of Fe atoms in theζphase layer decreases significantly with the increase of the In content inζphase.Thus,the growth ofζphase is restrained.The electrochemical test results show that the corrosion resistance of the In-containing coating is better than that of the pure Zn coating.Compared with the pure Zn coating,the self-corrosion current density of the Zn-0.4 In at.%coating decreased by9.585μA/cm²,and radius of capacitive reactance increases.