Abstract
In order to develop protective coatings for sink rolls used in continuous hot-dip galvanizing, the sprayed WC/Co cermet coating was formed on a mild steel by the High Vilocity Oxygen Fuel Spraying process and its durability in the molten zinc bath (753 K) containing 0-3 mass%Al has been investigated on the basis of the constitutional change measured by SEM and EDS. The following results were obtained.
(1) During immersion periods, Al was enriched in the Zn-Al solution close to the sprayed coating surface and Al-rich phase was deposited on the sprayed coating surface.
(2) Under the sprayed coating surface, the diffusion layer, where Zn was rich and Co was poor, was built up. As Al concentration in the molten zinc bath was increased, the thickness of the diffusion layer was decreased and the durability of the sprayed coating could be kept for longer times.
(3) It is presumably a reason for such improvement of the durability that the Al-rich phase acts as a diffusion barrier against Zn and Co at the interface of molten zinc/sprayed coating and consequently suppresses the growth of the diffusion layer.
