In order to develop protective contings for sink rolls used in a coatinuous hot-dip galvanizing, sprayed cermet coatings were formed on a mild steel by High Velocity Flame Spraying system using two commercially produced WC-12 mass% Co powders and their durability in molten pure zinc(703783 K) have been studied by SEM, XRD, EDX and hardness tester.
It was found that the durability of WC/Co sprayed coatings in molten pure zinc depended on the binding phases, which varied according to a powder-preparation process. Namely, the binding phase in WC/Co sprayed coatings made of powders prepared by a Spray-dried process ( SD-coating) mainly consisted of β-Co phase, but that prepared by a Sintered & Crushed process (SC-coating) mainly consisted of η-phase (Co3W3C and Co6W6C). In case of SD-coating, the binding phase, β-Co, dissolved into molten pure zinc so that the thickness has decreased in a short time. Though, in case of SC-coating, a Zn-rich and Co-poor layer was build up under the surface and the hardness dropped to HV 5001000 just below the surface, no changes of the thickness and microstructures have been observed for a long time.
The appearant activation energy for the growth of Zn-invaded layer in SC-coating was 170 kJ/mol, which was nearly equal to that for dissolution of β-Co. It was suggested that a small amount of β-Co was also contained in SC-coating and such a excellent durability must have been obtained due to the different distribution manner of β-Co.
