| Hot-dip galvanizing is one of the most popular and versatile technique used toprotect steels against corrosion, because an alloyed zinc coating will be formed inhot-dip galvanizing which separates the steel from the corrosion environment. Duringthe batch galvanizing, due to the effect of silicon, the growth of alloy layers ofsilicon-containing steels will be aggravated with poor adhesion and brittleness. Thisphenomenon is commonly referred to as silicon reactivity. By numerous research,Technigalva alloy has been developed to control the silicon reactivity of the steelswhich contained Si less than0.25%, but the steels with higher Si are out of control.Therefore, two kinds of high Si-containing steels Q235and Q345which are widelyused in the infrastructure projects are chosen as research materials in the present work.The dissolution of steel with Si in the Ti alloyed bath involved Zn-Fe-Si-Ti system.Therefore, a detailed knowledge of the phase equilibrium in the Zn-Fe-Si-Ti system at450℃would be useful in elucidating the mechanism of Ti additions in controlling Sireactivity. Therefore, the phase relation of this system was investigatedexperimentally at the galvanizing temperature.Both Q235and Q345were immersed for0.5to8min in Zn baths with fivelevels of Ti, ranging from0to0.2wt.%. Scanning electron microscopic coupled withenergy dispersive spectrometric （SEM-EDS） were used to analyse the microstructureand the composition of compound layer. Base on the available information from theexperiment, the effect of Ti adding to the zinc bath on the growth kinetics of Fe-Znphase layer was investigated. The main conclusions obtained in the present study areas flowing: Firstly, Ti added to the zinc bath can decrease the incubation time of δ andΓ, promote the growth of δ and accelerate the disappearance of ζ. But the growth-ratetime constant is still less than0.5which indicatives of parabolic diffusion controlledgrowth. Secondly, adding0.05wt.%Ti can control silicon reactivity of highSi-containing steels. Thirdly, the ternary phase T formed at the interface of ζ andliquid, which can absorb Si accumulated at the boundary of ζ.The450℃isothermal section of the Zn-Fe-Si-Ti quaternary system with Zncomposition being fixed at93at.%was determined experimentally by means ofSEM-EDS, and X-ray power diffraction. Twenty four phases regions are existent inthe Zn rich corner of this system, and fourteen four-phase regions have beenconfirmed directly. The liquid phase is in equilibrium with all compounds which exist in the system. Ti solubility in the Fe-Si binary phases is limited, while Fe solubility insome of the Ti-Si binary compounds is somewhat higher, for instance, the maximumsolubility of Fe in Ti5Si3is2.1at.%. The solubility of Zn in τ2is3.0at.%, but limitedin another ternary phases which exist in Fe-Ti-Si system. Si has a maximumsolubility of0.4at.%in T phase, but limited in ζ. |
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