Reaction of WC–Co coating with molten zinc in a zinc pot of a continuous galvanizing line

In a continuous galvanizing line (CGL), three so-called pot rolls are usually used to guide steel strips in a zinc pot. Generally zinc pot rolls are coated with WC–Co to protect the pot roll surface from severe corrosion by molten zinc. The degradation process of the WC–Co coating was evaluated through analyses of coated pot rolls used in a CGL zinc pot for 33 and 56 days. The rolls were coated by high velocity oxygen fuel spraying with WC–12% Co powders that were produced through sintering and crushing processes. On the surface of the WC–Co coated rolls, several types of deposits were observed including top dross (Fe2Al5 inter-metallic compound) that might induce dross defect on the surface of galvanized steel. The diffusion depth of zinc into the WC–Co coating used for 33 days was only 10 μm, but some areas were severely attacked along cracks within the coating layer because of a preferential reaction with aluminum. Through SEM study, we observed that not only zinc but also aluminum diffused into the WC–Co coating after being in the zinc pot for 56 days. In some cases, Al–Fe–Zn layers were observed on the surface of the spray coating. Those layers were analyzed to be Fe2Al5 as their chemical compositions are very similar to those of Fe2Al5 top dross. However, they are not the Fe2Al5 dross, but the reaction products between molten zinc and the coating layer according to their morphologies.