抄録
WC-Co thermal spray coating is widely applied for high wear resistance coatings. But it is also known that the coating has poor corrosion resistance in aqueous solution. In this study, two kinds of powder, WC-12Co and WC-10Co-4Cr, were used to form the coating by high velocity oxygen-fuel flame spraying on mild steel. Corrosion behavior in Na_2SO_4 solution was investigated under anodic polarization, impedance response and distribution of potential on the coating surface. The current density of WC-12Co thermal spray coating in passive regions changed with the pH of the aqueous solution, becoming high in neutral or alkaline solutions, and low in acid. Therefore, WC-12Co thermal spray coating shows poor corrosion resistance in neutral and alkaline solutions. However, since the current density on WC-10Co-4Cr thermal spray coating, in which Cr is added, to the WC-Co system, is lower than that in WC-12Co thermal spray coating for each pH, the former has higher corrosion resistance. On the other hand, it was found that changes in impedance diagram at constant potential corresponded to the changes in the polarization curve. Therefore, it was suggested that the reaction on the corrosion interface could be estimated from the changes in the diagram. The corrosion reaction rate for WC-10Co-4Cr thermal spray coating is better than that of WC-12Co thermal spray coating. Thus, it is thought that control of the dissolution of the coating and substrate components is owing to the early formation and recovery of the passive state of the coatings.
