Effect Of The Elements On Interface Reaction During The Hot-dip Galvanizing And Related Phase Equilibrium

Hot dip galvanizing used for steel protection has a history of over100years.However, Si reactivity and equipment corrosion have been troubling the galvanizingindustry. Galvanizing the steels in alloyed baths is one of the effective ways used tocontrol silicon reactivity. Nickel is the most used element added to zinc bath. However,The expensive nickel in zinc bath can only restrain the Si reactivity of the steel with lessthan0.2wt.%Si, and a mass of zinc dross would form as nickel element is added to zincbath. Therefore, it is necessary to develop a zinc-based alloy with lower cost to suppressthe Si reactivity of the steel with higher content of Si. All zinc baths contain differentlelves of aluminum for different purpose, such as suppressing Fe-Zn interface reaction,improving surface quality, or obtaining coating with better performance. Cobalt-basedsuper-alloys materials are used for both sink roll and stabilizer roll in the galvanizingindustry due to their relatively low cost and reasonably good performance. However, theservice life of Cobalt-based super-alloys are shorten as Al concentration in the molten zincbath increased. Therefore, it is necessary to understand the interaction of the Co-basedalloy and Zn-Al bath. To satisfy the demands for better corrosion-resistance andoxidation-resistance at elevated temperatures, higher levels of aluminum are added to Znbath. Hot dip55%Al-Zn is a very promising technique used for steel protection. However,the higher levels of aluminum would result in the strong and rapid exothermic reactionbetween the steel matrix and the A1-Zn bath. The brittle Fe-Al compounds whichseriously undermine the performance of the iron and steel products form quickly. Therelated researches are carried out about the above problems in the present work. The phaseequilibria of the Zn-Al-Co, Zn-Co-Ni and Zn-Fe-Co-Si systems have been determinedusing equilibrated alloys with the aid of a diffusion couple approach. The specimens wereinvestigated by means of scanning electron microscopy equipped with energy dispersiveX-ray spectroscopy, electron probe microanalysis and X-ray diffraction. The effect of Co,Ni, Si on interface reaction during the hot-dip galvanizing have been studied by hot-dipgalvanizing and hot-dip55%Al-Zn.To understand interaction between cobalt-based alloys and the molten Al-Zn and theinfluence of contemporary use of Co and Ni in zinc bath on Fe/Zn interface reaction, thephase relations of the Zn-Al-Co system at450,600, and800°C and the Zn-Co-Ni systemat450and600°C have been investigated. Ten three-phase regions exist in the isothermalsection at450°C, nine at600°C and seven at800°C. The liquid phase is in equilibrium with all Al-Co compounds except Al3Co. AlCo phase can coexist with all Co-Zn binaryphases. The Zn solubility in AlCo and Al13Co4increases with temperature increases, andthat in Al5Co2decreases. The maximum solubility of Zn in AlCo, Al5Co2, Al9Co2is12.0,18.1and1.6at.%, respectively. The Al solubility in the Co-Zn compounds is extremelylimited, no more than0.1at.%,0.3at.%,0.9at.%and1.4at.%The X-ray powderdiffraction patterns and crystal structures of the CoZn and Co5Zn21have been obtainedexperimentally. Three three-phase regions have been confirmed in the isothermal sectionsof the Zn-Co-Ni ternary system at450°C and600°C, respectively. Three continuous solidsolutions regions, i.e., α-Co/α-Ni, γ-Co5Zn21/γ-Ni4Zn22and γ2-CoZn13/δ-NiZn8, exist in thesections The maximum solubility of Ni in β1(CoZn) and γ1(CoZn9) is11.8and8.1at.%,and that of Co in β1’(NiZn) is16.5at.%,. No true ternary compound was found inZn-Co-Ni ternary systems.To understand the mechanism of action of Co on Fe/Zn interface reaction and providethe theory foundation for development of zinc-based alloys, the phase relations of theZn-Co-Si ternary system at450and600°C and the450°C isothermal section ofZn–Fe–Co–Si quaternary system with the Zn content being fixed at93at.%have beeninvestigated. Nine three-phase regions exist in the isothermal section of Zn-Co-Si systemat450°C, and eight at600°C. The CoSi phase can coexist with all compounds in Zn-Cobinary system except the β1-CoZn phase. The Zn solubility in Co-Si binary compoundsincreases with temperature increases. The maximum solubility of Zn in CoSi2, CoSi andCo2Si is about2.0at.%,4.3at.%and2.7at.%at450°C, and2.0at.%,6.2at.%, and5.4at.%at600°C, respectively. It was found that2four-phase regions exist in the isothermalsection of Zn–Fe–Co–Si quaternary system at450°C. No true ternary or quaternarycompound was found in the present study. The ζ-FeZn13and ζ-CoZn13form a continuoussolid solution. The Fe solubility in CoSi2is no more than1.21at.%, whereas the Cosolubility in FeSi2is high up to7.8at.%. The maximum solubility of Zn in (Fe,Co)Si,FeSi2, and CoSi2is1.9,1.5,2.9at.%, respectively. The Si solubility in binary ζ-FeZn13phase is rather limited, with Co dissolving in this compound, the solubility of Si is up to0.8at.%.To develop a low cost zinc-based alloy used for suppressing the Si reactivity of thesteel with higher content of Si, the interface reactions between the iron-silicon alloys anddifferent baths containing Co or Co and Ni have been studied. An appropriate amount ofCo in Zinc bath make the ζ phase be columnar, and avoid the formation of the diffused-△phase and outbreak microstructure. When Co element was added into zinc bath, thegrowth of the intermetallic layers was diffusion-controlled, in which its thickness increased parabolically with immersion time. The suppressing effect of Co in the zinc bathon silicon reactivity of iron–silicon alloys with no more than0.4wt.%Si is better, and thebest effet is achieved when Co content increased to0.1wt.%. Adding0.05～0.2wt.%Coor0.05Co+0.05Ni wt.%to zinc bath can effectively restrain Fe/Zn interface reaction ofhigher reactivity steel with no more than0.5wt.%Si when immersion time is on morethan180s. The mechanism of action of Co on Fe/Zn interface reaction is explained usingthe phase relations of the Zn-Fe-Co-Si quaternary system. The Si solubility in the ζ-FeZn13phase is up to0.8at.%with Co dissolving in this compound, which avoids the enrichingof silicon at the boundary of ζ-FeZn13phase and the forming of the liquid channel. Thecompact ζ-FeZn13layer could form, and the silicon reactivity during the hot-dipgalvanizing of the Si-containing steel is restrained.To supress Fe/Al interface reaction, the effect of silicon in the55%Al-Zn bath on themicrostructure and the growth kinetics of the coating has been investigated. The typiesand forming sequence of intermetallic compounds in the55%Al-Zn coating were changedas silicon is added to the bath. The containing-Si Fe2Al5tends to stable. The interfacereaction can be effectively holded back by Fe2Al5with Si. The activation energy of Fe2Al5was evaluated to be207,274,275,260, and237kJ/mol, when the content of silicon in thebath is0.6,1.6,2.6,3.0, and3.6(wt.%) respectively. The growth rate constant of Fe2Al5was obtained under different conditions. The forming sequence of intermetalliccompounds and the evolving regulars of the microstructure in the coating have beeninvestigated. The results obtained by present work could help to optimize technologicalparameters, control zinc dross forming and obtain high performance coating.