Research On The Silicon Reactivity And The Influence Of Alloy Elements On The Hot-Dip Galvanizing

Hot-dip galvanizing is a widespread used industrial technology in protecting steel from corrosion.But when the content of Si in steel is higher than 0.07wt%,the coating become thicker with poor appearance, and is easily flaked from the base steel in batch galvanizing.This is commonly known as silicon reactivity.Phosphorus-containing steel behaves in an analogous manner with silicon-containing steel when galvanizing.Up to now,there is still lack of convincing knowledge about the mechanism about silicon reactivity;no methods were completely effective in avoiding silicon reactivity.Therefore,there are a lot of fundamental work to do in galvanizing steels contained silicon or phosphorous.The researches are carried out about the basic problems occurred during hot-dip galvanizing.Diffusion experiments of solid Zn/Fe,Zn/Fe-Si diffusion couples were carried,and the effect of silicon on the formation and the growth kinetics of diffusion layer were analyzed.The thickening kinetics of total layer for each couple were seen to be parabolic,but growth of the phase was hindered heavily,the interface between theζphase and the phase became irregular,δlayer grew intoζlayer in island shape in Zn/Fe-Si couples because of the adding of silicon.The amount of silicon had little influence to the structure and the thickness of total layer.The zinc corner of 450℃isothermal section of the Zn-Fe-P and Zn-Al-P system had been determined experimentally,and the phase relation of these systems at the temperature related galvanizing were obtained.Experimental results indicated that,there were eight three-phase fields with no indication of a ternary compound in the portion of the Zn-Fe-P phase diagram lying below 50at.%P;there were three three-phase fields in the portion of the Zn-Al-P phase diagram lying below 50 at.%P;P solubility in liquid zinc,α-Fe and all four Zn-Fe compounds,includingζ,δ,Γ₁,andΓ,is limited at 450℃.According the phase diagram,the effect of phosphorus on the microstructure of coating was analyzed.Silicon reactivity arose in the phosphorus-containing steel while galvanizing.In order to understand the effect of Bi to the Zn-Fe reaction,the 450℃isothermal section of the Zn-Fe-Bi ternary phase diagram was determined experimentally.There were five three-phase fields and six two-phase fields with no indication of a ternary compound in this section. Bi solubility inα-Fe and four Zn-Fe compounds and Fe solubility in the Bi-rich phase were limited at this temperature.The effect of Bi in the zinc bath on the structure of coating was analyzed according to the information of ternary phase equilibria.Bi had no effect on controlling silicon reactivity.The effect of bath temperature,the content of Si in steel and the synergy of Si and P on the structure of coatings were studied systematically by hot-dip galvanizing pure iron and silicon-containing steel,and the rule of growth kinetics of the coating were analyzed.The rule of coating thickness changed with bath temperature in silicon-containing steel is similar to that in pure iron,but the growth kinetics of two steels was different.When pure iron was hot-dip galvanized in the zinc bath at 450℃,the growth of the coating was diffusion-controlled,in which the coating thickness increased parabolically with immersion time,but the kinetics of the coating in silicon-containing steel seemed to be linear suggesting interfacial reaction control.Silicon hindered the growth of the 5 phase and provoked the growth of theζphase.When the steel contained Si and P synchronously, and the effective Si reached the amount of 0.07 wt%,the coating became thicker,and the microstructure had the characteristic of silicon reactivity.The model of silicon reactivity in hot-dip galvanizing was proposed on the basis of diffusion path in the 450℃isothermal section of Zn-Fe-Si ternary system.When the steel with lower silicon was hot-dip galvanized, normal intermetallic layers formed at first.Since Si in coating trended to gather in the grain boundaries and phase boundaries,the diffusion path in coating overstepped theζphase,cut the tie-line ofζand liquid phase equilibrium.The liquid phase appeared at theζphase boundaries,in which liquid channels formed.Therefore,the liquid phase eroded the substrate through theδphase,the growth of the coating was controlled by the reaction speed between the liquid phase and theδphase,the growth speed was invariable,and the thickness of coatings increased linearly.It can be concluded that there was an incubation period in forming the reactive microstructure.When the content of Si in the steel was higher, the diffusion path overstepped theζphase from the very beginning,and cut the tie-line ofζand liquid phase equilibrium.Therefore it needn't incubation period in forming the reactive microstructure.