12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Hot-dip Zn-Al alloy coated steel sheets such as 55%Al-Zn-1.6%Si and Zn-5%Al coatings are widely used in the construction industry because of their higher corrosion resistance than zinc coatings. It is well known that the corrosion resistance of hot dip Zn-Al alloy coatings depends on the microstructure formed through solidification process1). Thus, controlling the solidification microstructure is important to control the corrosion resistance. When we consider the microstructural evolution process, suitable phase diagram is necessary. While, there are two kinds of Zn-Al binary phase diagrams2, 3). One includes paritectic reaction but another doesn’t. In most of the former studies, the phase diagrams without the peritectic reaction were used to consider the microstructure of Zn-Al alloy coatings. Meanwhile, according to the Zn-Al binary phase diagram with peritectic reaction, the peritectic structure should form when the Al content is higher than 13%. Therefore, the present study investigated the microstructure in hot-dip Zn-(11, 22)%Al coatings in order to clarify which Zn-Al binary phase diagram is more suitable for understanding the solidification microstructure evolution of hot-dip Zn-Al alloy coatings. Zn-(11, 22)%Al coatings on mild steel substrates were prepared by using a hot dip simulator machine. The temperatures of the molten Zn-(11, 22)%Al bathes were set at 480, 500 ℃, respectively. The dipping time was 3 seconds and the average cooling rate was controlled at 10 ℃/s from the bath temperature to 150 ℃ by using Nitrogen gas flow. The morphology and compositions of microstructure in coating samples were examined by means of X ray diffraction (XRD), scanning electron microscopy (SEM). As a result of microstructural investigations, the peritectic structure formed inside of dendrites in the Zn-22%Al coating, but not in the Zn-11%Al coating. This indicates that the peritectic reaction occurred in the course of solidification process in the Zn-22%Al coating. Therefore, the Zn-Al binary phase diagram including peritectic reaction is considered to be suitable for understanding the solidification microstructure evolution of hot-dip Zn-Al alloy coating steel sheets. Reference 1) T. Kiyasu, ISIJ, Vol. 72, No. 8, 1986 2) T. B. Massalski, ASTM International, p.239, 1990 3) A. Presnyyakov, Y. A. Goltban and V. C. Cherptyyakova Russ. J. Phys. Chem. 35, 1961