Experimental Investigation and Thermodynamic Calculation of the Co-Si-Zn System
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Zinc and Zn alloy coatings are widely used to improve the corrosion resistance of steels. However, the presence of silicon in the steels to be galvanized will deteriorate the coating quality, causing, for example, uncontrollable coating thickness, poor corrosion resistance, and adsorptivity. The abnormal increase in coating thickness associated with Si bearing steels was first observed by Sandelin.[1] This phenomenon is referred to as Si reactivity.[2] There are two main methods that have been adopted to inhibit Si reactivity. One method is to raise the temperature to avoid the formation of the f (FeZn13) phase.[3] The alternative method is to add some alloying elements such as Ni, Bi, V, and Ti into the zinc bath.[4–7] Li et al.[8] and Zhao et al.[9] investigated the effect of Co in the zinc bath on the microstructures and growth dynamics of the hot-dip galvanizing coating. Co dissolving into the zinc bath can increase the silicon solubility in the FeZn13 phase and inhibit the silicon reactivity. In addition, Co-based superalloys were used in the galvanizing industry, such as in sink roll and stabilizer roll, due to their reasonable performance and low cost.[10] Therefore, the information on the phase relationship of the Co-Fe-Si-Zn quaternary system is important in understanding the effect of Co on the Fe-Zn reaction during hot-dip galvanizing of Si-containing steels and the design of new hot-dip zinc
WEILIN WANG, Master Student, FUCHENG YIN, MANXIU ZHAO, and ZHI LI, Professors, and YU WU, Lecturer, are with the Key Laboratory of Materials Design and Preparation Technology, Xiangtan University, Xiangtan, 411105, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys for the galvanizing industry. The quaternary system has four ternary systems: Co-Fe-Zn, Co-Si-Zn, Fe-Si-Zn, and Co-Fe-Si. Su et al.[11] and Sha et al.[12] experimentally investigated and thermodynamically assessed the Fe-Si-Zn system. The Co-Fe-Si system was assessed by Raynor and Rivlin.[13] There are some experimental data about the phase relations of the Co-Si-Zn[14] and Co-Fe-Zn[15] systems, but no calculation information in the literature. In the present work, the phase relations of the Co-Si-Zn ternary system at higher temperature were experimentally determined and thermodynamically calculated using the CALPHAD technique over the entire temperature range. The Co-Si experimental phase diagram was reviewed primarily by Ishida et al.[16] Kaufman was the first to calculate it,[17] and then Ishida et al.[16] evaluated the system with a thermodynamic calculation based on the experimental data. Most recently, it was reassessed by Zhang et al.[18] This system consists of five intermetallic compounds (aCo3Si, aCo2Si, bCo2Si, CoSi, and CoSi2) and three terminal solid solutions (high-temperature aCo, low-temperature eCo, and (Si)). The calculated Co-Si phase diagram is shown in Figure 1(a). Much of the experimental data on the Co-Zn phase diagram was summarized by Massalski et a
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