Durability of WC/Co Sprayed Coatings in Molten Pure Zinc.

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Abstract
In order to develop protective coatings for sink rolls used in a continuous hot-dip galvanizing, two kinds of the WC/Co sprayed coating were formed on mild steels by High Velocity Flame Spraying system using commercially produced WC-12 mass%Co powders and their durability in molten pure zinc (703-783 K) was studied on the basis of the constitutional changes. It was found that the durability of WC/Co sprayed coating in molten pure zinc depends on the binding phase, which varies according to its powder-preparation process. Namely, the binding phase in the WC/Co sprayed coating (SD-coating) made of the powder prepared by the Spray-dried process mainly consists of Co phase, but another WC/Co sprayed coating (SC-coating) made of the powder prepared by the Sintered and Crushed process mainly consists of η-phase (Co3W3C and Co6W6C). In case of SD-coating, the binding phase (Co phase) dissolved into molten pure zinc so that the thickness decreased in a short time. In case of SC-coating, its thickness and microstructure did not change for a long time, though a Zn-rich and Co-poor layer was formed just below the surface where the hardness dropped to Hv 500-1000. The apparent activation energy for the growth of the Zn-diffusion layer in SC-coating was 170 kJ/mol, which was nearly equal to that for dissolution of Co phase. It is considered that Zn diffusion path in SC-coating is Co phase and its excellent durability is caused due to much longer diffusion path than that in SD-coating.

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