Using In Situ Atomic Force Microscopy to Investigate the Kinetics of Corrosion of WC–Co–Cr Cermet Coatings Applied by High-Velocity Oxy-Fuel

Most of the early applications of thermal spray coatings were focused toward providing a remedy to excessive wear degradation. However, as the introduction of such coatings into wider industrial sections increases there is also exposure to other potential degradation processes—aqueous corrosion is one such process. The complex microstructures in cermet coatings have been shown to translate to complex modes of corrosion attack. In this paper an electrochemical test methodology to probe the local/microaspects of corrosion initiation and propagation will be described. A new electrochemical cell has been devised in which the corrosion can be followed “live” and in “real-time.” The surface is subjected to in situ imaging by atomic force microscopy which shows that not only the binder (Co, Cr) corrodes in high-velocity oxy-fuel thermal spray coatings but also the hard phase, with oxidation and dissolution of $WC∕W2C$ taking place. Also potentiostatic tests indicated that the corrosion of WC-based coatings follows an Arrhenius relationship enabling the determination of activation energy $(Ea)$ for the corrosion of WC and demonstrating that the oxidation and dissolution of WC are temperature, particle size, potential, and pH related