Reaction of Co phase in the WC-Co coatings with molten zinc

The main objective of this study is to investigate the detailed reaction mechanism of Co phase in the WC-Co Coating with molten zinc. Specimens of pure cobalt were immersion tested in a molten pure zinc bath at 460°C, 490°C, and 520°C. For the understanding of degradation processes, the specimens were analyzed with a scanning electron microscope (SEM) and energy dispersive spectrum (EDS), and electrochemical stripping method. After immersion of Co in a molten zinc bath at 460°C to 520°C four kinds of Co-Zn intermetallic compound layers, β1, γ, γ1, and γ2 were formed on the Co matrix, which was confirmed by both microstructural observation and electrochemical stripping method. According to the Co-Zn binary phase diagram, they were all the possible intermetallic compounds at the temperatures of experimental conditions. The relationship between weight loss after immersion tests and immersion time followed typical parabolic rate law. The rate controlling step for this reaction was analyzed as the mutual diffusion process of Co & Zn atoms through β1 compound layer and the activation energy for the process was calculated to be 214.9 kJ/mole.