The Phase Relations Of The Co-Zn-Zr Ternary System

Hot-dip galvanizing is a mature and popular process used for steel components toprovide effective corrosion protection. However, the Zn-Al melt excessively corrodesthe equipment used in zinc baths, which will reduce the service lifetime and result inthe stratch on the surface of the coating and the formation of dross in the bath. Thus,the inhibition of the corrosion is the key factor which improve the quality of thecoatings and the effectiveness of the galvanizing line. So far, cobalt-based super-alloys are used for both stabilizer roll sink roll and stabilizer roll, because of theirrelatively reasonably good performance and low cost, such as tribaloy alloys(Co-Mo-Cr-Si) and stellite alloys(Co-Cr-W-C). Short and Mackowiak demonstrated that Zr inthe Zn bath effectively suppressed the excessive growth of Fe-Zn compounds byforming a ternary compound at the growth front of the ζ(FeZn13) layer in the coatings.When Zr is added to the Zn bath, the bath becomes a Co-Zn-Zr ternary system. Tounderstand the effect of Zr on the corrosion resistance of Co-based alloy, it is essentialto determine the phase relationship in the Co-Zn-Zr ternary system at the galvanizingtemperature. Nevertheless, no information about the isothermal section of Co-Zn-Zrternary system was reported.In the present work, the isothermal section in the Co-Zn-Zr system at450℃and600℃were determined experimentally by using X-ray diffraction(XRD), scanningelectron microscopy coupled with energy dispersive spedtroscopy(SEM-EDS) andoptical microscope.Fifteen three-phase regions were found in the isothermal section at450°C, T1+γ1+γ、T1+β1+γ、T1+β1+Co2Zr、ε-Co+β1+Co2Zr、T1+T3+Co2Zr、T1+T2+T3、T1+T2+ZrZn3、T1+Zr5Zn39+ZrZn3、T1+Zr5Zn39+L、T4+ZrZn2+ZrZn、T2+T3+T4、CoZr+T3+T4、ZrZn2+ZrZn3+T2、T2+T4+ZrZn2、CoZr2+CoZr+T4.Ten three-phase regions were found in the isothermal section at600°C, T1+γ+γ1、T1+β1+γ、ε-Co+β1+Co2Zr、T1+T2+T3、T1+Zr5Zn39+ZrZn3、T1+Zr5Zn39+ZrZn22、ZrZn22+L+T1、L+γ2+T1、γ1+γ2+T1、ZrZn2+ZrZn3+T2.Co was hardly detected in ZrZn3, Zr5Zn39, ZrZn2, ZrZn and ZrZn22. And little Zrwas dissolved in all Co-Zn intermetallic compounds. The solubility of Zn in α-Co isup to20.32at.%at600℃, and24.95at.%at450℃. However, Zr was hardlydissolved in α-Co.We also found four ternary compounds in the system. They were named as T1、 T2、T3and T4. Only the composition range of T1can be determined.It has a narrowcomposition range,80.94~86.40at.%Zn、5.10~6.21at.%Zr、7.94~13.25at.%Co. Inthe work, I made successfully the single phase alloy of T1and T2. The phaserelationship for Co-Zn-Zr system at600℃is similar to it at450℃.