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

Zinc and zinc-based alloys are mainly used in hot-dipping galvanizing. More than half of zinc is used for this purpose. As a de-oxidant agent or strengthen element, alloying element Si is introduced into steels. Therefore, some Si will inevitably be residual in steels. And when the content of Si reaches to a certain value, dark and poor adhesion of coating appearance will reduce the quality of the steel which was called the Sandlin effect or Si reactivity. The alloying elements added in the zinc bath play a key role in changing the structure and properties of coating. Generally, alloys elements such as Al, Bi, Ni, Mg, Ti, Sn are added in zinc bath. Researchers found that element Co added in the zinc bath can also partially inhibit the Si-reactivity, improve the quality and the corrosion resistance of the coatings. In order to study the reaction mechanism of Co and optimize the zinc alloy of Co, the 450℃and 600℃isothermal sections of the Zn-Co-Si system were determined using equilibrated alloys, and the thermodynamic calculation of Zn-Co-Si system was conducted by CALPHAD technique.The 450℃and 600℃isothermal sections of the Zn-Co-Si system was experimentally determined in this work using optical microscopy, scanning electronic microscopy coupled with energy dispersive X-ray spectroscopy(SEM-EDS) and X-ray diffractometry(XRD). Experimental results show that the 450℃isothermal section of the Zn-Co-Si system consists of seven ternary phse regions: L (Zn) + CoSi₂ + (Si), L (Zn) + CoSi₂ + CoSi, L (Zn) +γ₂ + CoSi,γ₂ +γ₁ + CoSi,γ₁ +γ+ CoSi,γ+ CoSi + Co₂Si andβ₁ +γ+ Co₂Si.The CoSi phase can coexist with all compounds in Zn-Co binary system exceptβ₁ phase. The solubility of Zn in the Co-Si alloy phases such as CoSi₂,CoSi,Co₂Si is high.Theγ₂ phase disappears in the 600℃isothermal section of the Zn-Co-Si system, thus it just consists of six ternary phse regions: L (Zn) + CoSi₂ + (Si), L (Zn) + CoSi₂ + CoSi, L (Zn) +γ₂ + CoSi,γ₁ +γ+ CoSi,γ+ CoSi + Co₂Si andβ₁ +γ+ Co₂Si.Based on the experimental results of this work and related data of binary system, the Zn-Co-Si ternary system was extrapolated by CALPHAD technique. Compared with the experiment, the three-phase region (hcp-Co+γ+Co₂Si) appeared in the calculated 723K isothermal section, which is inconsistent with the experiment. The 873K isothermal section calculated was agree well with experimental data of cobalt-rich corner, at which is a (hcp-Co+β₁+Co₂Si) three-phase region. The reason for accurate reaction temperature is that the hcp-Co phase of the Co-Si binary system is so stable that result the peritectoid-eutectoid reaction Co₂Si+β₁→hcp-Co+γoccurred at 805K. Based on the experiment, peritectoid-eutectoid reaction should occur below 723K, therefore, the assessment of hcp-Co optimization needs more works.