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Using tungsten oxide, cobalt oxide and carbon black as raw materials, the ultrafine WC-η composite powder was rapidly synthesized by the novel technique of in situ reactions. Under the optimized process conditions, the as-synthesized composite powder has a mean particle size of 260 nm, containing only WC and η phases. Then the composite powder was agglomerated into the thermal spray powder with high density and good flowability. Using this powder as the feedstock, the ultrafine-structured WC-based coating was fabricated by high-velocity oxy-fuel spraying technique. The fabricated coating has an average hardness as high as (1406±34) HV_0.3 and only a porosity of 0.3%. The results indicated that the ultrafine-structured WC-based coating exhibited excellent corrosion resistance to molten zinc. The corrosion mechanism can be explained by the slow dissolving of the coating matrix from the surface layer to the inner area caused by the diffusion of molten zinc along micro-cracks, which was then followed by the formation of penetrative cracks and subsequent massive exfoliation of coating materials.