Effect Of Si And Zr On The Al-Zn Coating's Microstructure And Correlated Phase Equilibria

Corrosion of steel not only caused great losses to the national economy,but also seriously endangered people's lives and property.Hot-dip galvanizing is one of the most direct and effective protection technologies for steel.Galvalume coating is widely used in various fields because of its good corrosion resistance,high temperature oxidation resistance and appearance.However,Galvalume coating have involved some serious drawbacks in production process,such as thick intermetallic compound layer,too much dross in the bath,uneven spangle on the coating surface.This study intended to inhibit the growth of Fe-Al intermetallic compound layer and reduce the thickness of 55% Al-Zn layer by adding different content of Si and Zr into the baths.So the Fe-Al-Zr ternary system was studied by equilibrium alloy method,and the influences of adding elements Si,Zr and hot-dipping temperature on the microstructure and growth kinetics of the coating were investigated by hot-dipping experiment.The research results have a strong guiding and application value for the development of new aluminum-zinc alloy coating.The isothermal section of Fe-Al-Zr system at 600°C was determined experimentally using equilibrium alloy method.The microstructures and phases constituents in alloys were analyzed by means of the optical microscope,scanning electron microscope-energy spectrometer and X-ray diffractometer.The results show that 19 three-phase regions are found in the isothermal section of Fe-Al-Zr ternary system at 600°C,which are Zr3Fe+?-Zr+?2,Zr3Fe+ZrFe2+?2,Zr3Al+?-Zr+?2,Zr3Al+Zr2Al+?2,Zr3Al2+Zr2Al+?2,Zr3Al2+?1+?2,FeAl+?1+?1,FeAl+Fe Al2+?1,FeAl2+Fe2Al5+?1,Fe2Al5+FeAl3+?1,?2+?1+?3,?2+ZrAl3+?3,?1+ZrAl3+?3,ZrAl3+FeAl3+?1,ZrAl3+FeAl3+?-Al,ZrFe2+FeAl+?-Fe,Zr3Al2+Zr4Al3+?2,Zr2Al3+ZrAl+?2 and Zr2Al3+ZrAl2+?2.And the other six three-phase regions can be inferred,which are ZrFe2+FeAl+?1,ZrFe2+?2+?1,?2+?1+?1,Zr3Al2+?1+?2,ZrAl2+ZrAl3+?3 and ZrAl+Zr4Al3+?2,respectively.Zr is almost insoluble in Fe-Al intermetallic compounds.The maximum solubility of Fe in ZrAl2,Zr3Al2 and Zr Al3 are 10.54,1.76 and 0.93 at%,respectively,whereas other Zr-Al intermetallic compounds show negligible solubility of Fe.The solubility of Al in Zr3 Fe is 2.09 at.%.Five ternary compounds are found,which are ?1,?2,?1,?2 and ?3.During the process of hot-dip 55% Al-Zn on the surface of Q235 steel,theeffects of Si,Zr,and different temperatures on the Al-Zn coating microstructure and growth dynamics were investigated by using drying solvent method of the hot-dipping coupled with scanning electron microscopy and energy dispersive spectroscopy.The results show that Zr does not directly inhibit the Fe-Al reaction when only Zr was added to the 55 wt.% Al-Zn bath.However,when both Si and Zr were added to the bath,Zr can promote the formation of ?5 phase,leading to a retarded reaction of Fe-Al and a thin intermetallic compound layer.With the addition increasing of Si to 55 wt.%Al-Zn-0.1 wt.% Zr bath,the formation sequence of the intermetallic compounds was changed.When the Si content is 0.6-1.6 wt.%,the formation sequence is FeAl3??5?Fe2Al5;when the Si content is 2.6 wt.%,the sequence turns to?5?FeAl3?Fe2Al5;when the content of Si is 3.6 wt.%,the sequence is ?5?Fe2Al5.The higher the Si content,the thinner the intermetallic compound layer.When adding1.6 wt.% Si and 0.1 wt.% Zr in the 55 wt.% Al-Zn bath,the growth rate of the coating shows the minimal value of 0.5879,and the growth of the intermetallic compound layer is mainly controlled by the diffusion reaction.The higher the temperature of the hot-dipping,the thicker the coating of Fe-Al intermetallic compounds layer.Therefore,the optimum hot-dip temperature is 600°C.