Study on the Zinc Corrosion Resistance and Failure Mechanism of Cobalt-Based Carbide Coatings

To study the protective effect of thermal spraying cobalt-based carbide coatings on hot-dip galvanized sinker rolls, a WC-12Co coating was prepared on a 316 L stainless steel substrate using the supersonic flame spraying method. The coating was corroded in a molten liquid zinc at 430 °C to analyze the coating service life and failure process. The results show that the coating provided good protection of the base metal. The process of coating failure was that Zn first diffused into the coating and reacted chemically with the Co between the carbide particles, forming a Co-Zn compound. Then, some transverse cracks appeared through the Co-Zn compound layer, resulting in the coating microstructure falling off in blocks and into the liquid zinc, causing a gradual thinning of the coating thickness, which eventually failed and loosened its protective effect. The types of Co-Zn compounds in the coating were analyzed by energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD), and the formation process of Co-Zn compounds was further investigated by placing pure Co rods into the liquid zinc at the same temperature. It was found that there were two types of Co-Zn compounds formed, Co 5 Zn 21 and CoZn 13 , which were produced by forming Co 5 Zn 21 at first and then transformed into CoZn 13 with further diffusion and aggregation of Zn, which was the main cause of the cracking of coatings.