Thermal Analysis On Liquid Properties Of Zinc-based Alloys In Hot-dip Galvanizing

In hot-dip galvanizing, the pretreatment metal workpiece,are immerged into molten zinc. Fe-Zn alloy layer forms on the surface of workpiece through the reaction-diffusion between iron and zinc, this alloy layer can play the physical protection and chemical protective role. At present, with flexibility, high maneuverability, less pollution and excellent corrosion resistance performance, hot-dip galvanizing become a extensive way of steel corrosion resistance process.Recently, the studies mostly dedicated to the organizational structure, corrosion resistant performance and appearance quality of the coating. The research on the physical properties of liquid alloy, organization structure and the characteristics of crystallization of liquid alloy, are still insufficiently thorough specific. Following on the physical properties of liquid alloy, this paper further investigations of the organization and crystallizing properties of liquid alloy. Except to find the relationship between the coating performance and the organization structure of liquid alloy, or crystallization properties, so as to make predictions on coating performance.In this subject, cooling curve and differential curve of five different composition alloy of Zn-Al and Zn-Al-RE two series were measured by thermal analysis, under 450℃,460℃and 530℃, and found out the characteristics parameters of the alloy crystallization. The proportion of different organizations of the alloy crystallization was obtained by using the quantitative metallographic. The properties of the alloy coating were studied by Electrochemical workstation, Uniform corrosion immersion testing and related equipment.The following conclusions are obtained after analyzing and comparing the experiment data:when the percentage of dendritic area below 0.14246, at 450℃, the larger the undercooling(ΔT)of alloy crystallization, the smaller the percentage of dendritic of alloy solidification structure, palisade tissue or flake azn zinc phase and columnar of coating organization will be significantly refined or less, corrosion resistance performance of the coating are improved obviously; at 530℃, the smaller the cooling rate of alloy crystallization, the smaller the percentage of dendritic of alloy solidification structure, Fe2Al5 alloy layer become more dense, and columnar of surface tissue can significantly reduce, corrosion resistance performance of the coating are improved obviously; the larger the cooling rate of alloy crystallization, the larger the percentage of dendritic of alloy solidification structure, it will appear a lot of palisade tissue in coating structure, and corrosion resistance of coating will become poor; therefore, undercooling(ΔT)and cooling rate of alloy crystallization can be used in anticipation of the corrosion resistance performance of coating. when the percentage of dendritic area above 0.14246, the corrosion resistant performance of coating will increases as the decrease of the percentage of dendritic area, the corrosion resistant performance of coating will be worse.the percentage of secondary phase will decreases as the increases of the temperature and content of Al and RE, but it not clearly of the relationship between the undercooling, cooling rate and corrosion rate.