Phase Equilibria Of Ce-Co-Zn System And The Effect Of Co And Ce On The Microstructure And Property Of Galvanized Si-containing Steels

Hot-dip galvanizing have been widely used in industy as an effective steel protection technology.However,when the silicon-containing steel is hot-dip galvanized,the coating become thicker with dark surface,and adhesion between the coating and substrate become wore.This is commonly known as silicon reactivity.Adding alloy elements to the zinc bath is an effective way to avoid silicon reactivity.The addition of Co to the zinc bath can control the silicon reactivity,and addition of the Ce element can refine the structure,but the synergistic effects of Co and Ce on the hot-dip galvanizing has not been reported.This work focused on the phase relationship of Ce-Co-Zn ternary system at hot-dip galvanizing temperature and the synergistic effect of Co and Ce on the microstructure and properties of hot-dip galvanizing.The research results have a strong guiding significance and certain application value in enriching zinc-based database and developing the new galvanizing alloy with Ce-Co-Zn system.The equilibrated alloys approach and diffusion couples were adopted to determine the phase relations of the Ce-Co-Zn ternary system.The specimens were investigated by means of optical microscopy,scanning electron microscopy/energy-dispersive spectrometry and x-ray diffraction.13 three-phase regions are found in the isothermal section at 450℃,which are L-Zn + γ2 + CeZn11,γ2 + γ1 + CeZn11,γ + γ1+ CeZn11,γ + Ce3Zn22 + Ce2Zni7,γ + Ce3Zn22 + β1 Ce2Co17+Ce3Zn22 + β1,CeCo2 + Ce3Zn22 + CeZns,CeCo2 + Ce13Znss + CeZns,Ce3Zn11 +CeZn3 + CeCo2,CeZn3 + CeZn2+ CeCo2,CeZn+ CeZn2+ CeCo2,Ce2Co7 + Ce5Co19+ Ce3Zn22 and Ce2Co7 + CeCo3 + Ce3Zn22.And the other 7 three-phase regions can be inferred,which are γ + CeZnu + Ce2Zn17,α-Co + Ce2Co17 + β1,Ce2Co17 +Ce3Zn22 + Ce5Co19,CeCo2 + Ce3Zn22 + CeCo3,CeCo2 + Ce13Zn58 + Ce3Zn11,L +γ-Ce + CeCo2 and L + γ-Ce + CeZn.The maximum solubility of Ce in γ2,γ and β1 are 5.2,1.5 and 2.8 at.%,respectively.The maximum solubility of Co in Ce2Zn17,Ce3Zn22,CeZns,CeZn2 and CeZn are 6.5,13.3,6.4,8.2 and 10.3 at.%,respectively.And that of Zn in CeCo2 is up to 4.0 at.%at 450 ℃.11 three-phase regions are found in the isothermal section at 600℃,which are L-Zn + γ1+ CeZn11,γ+γ1 +CeZn11,β1+ Ce3Zn22 + Ce2Zn17,γ+ Ce2Zn17 +β1,β1+ Ce3Zn22 + Ce2Co17,CeCo2 +Ce3Zn22 + CeZn5,Ce3Zn11 + Ce3Zn58 + CeCo2,CeZn3 + CeZn2 + CeCo2,CeZn +CeZn2 + CeCo2,Ce2Co7 + CesCo19 + Ce3Zn22 and Ce2Co7 + CeCo3 + Ce3Zn22.And the other 7 three-phase regions can be inferred,which are γ CeZn11 + Ce2Zn17,α-Co + Ce2CO17 + β1,Ce2Co17 + Ce3Zn22 + Ce5C19,CeCo2 + Ce3Zn22 + CeCo3,CeCo2 + Ce13Zn58 + CeZn5,Ce3Zn11 + CeZn3 + CeCo2 and L + CeZn + CeCo2.The solubility of Ce in γ1 is up to 3.0 at.%,the maximum solubility of Co in Ce2Zn17,Ce3Zn22,CeZn5 and CeZn2 are 10.1,11.1,4.3 and 3.8 at.%,respectively.And that of Zn in CeCo2 is 3.1 at.%at 600 ℃。And there is no new ternary compound at this system in the present work.The synergistic effect of Ce and Co on the microstructure and properties were investigated using the drying flux method during hot-dip galvanized silicon-containing steels.The microstructures and phase compositions of the coatings were analyzed by means of SEM-EDS.The results show that the addition of Co and Ce in the zinc bath can improve the appearance quality,and the formation sequence of the intermetallic compounds,i.eΓ,δ,ξ,η-Zn,was not changed.With 0.04%-0.10%Ce and 0.05%-0.10%Co adding into the zinc bath,the reactivity of silicon in hot-dip galvanizing of Q235 steel is restrained in some extent,the thickness of the alloy coating is significantly reduced,and the interface between theξ and the 8 layer becomes clear.When 0.04%Ce and 0.10%Co were added to the zinc bath,the coating is the thinnest,the growth rate time index of the coating is 0.54,and the growth of the intermetallic compound layer was controlled by diffusion.Neutral salt spray test shows that the corrosion rate of the Zn-0.10%Co-0.04%Ce alloy coating decreases 1/3 compared with the pure zinc coating,its corrosion product also has a certain protective effect on the coating;and electrochemical tests show that the corrosion current of the Zn-0.10%Co-0.04%Ce alloy coating is minimal,and has a good corrosion resistance.