Effect of WC grain size on the corrosion behavior of WC–Co based hardmetals in alkaline solutions

The corrosion behavior of hardmetals with different grain sizes was investigated in alkaline solutions by electrochemical methods. A clear tendency was observed that the smaller the grain size, the higher the corrosion resistance. Surface analytical characterization of the samples was carried out to obtain insight information on the origin of this behavior. Laterally resolved Auger Electron Spectroscopy revealed that significant amounts of W and C diffuse into the Co binder matrix during the sintering process, and that the W and C concentration in the Co phase increases with decreasing grain size. Moreover, the distribution of W and C in the binder phase is not homogeneous, instead a seam with a higher concentration is observed near the interface to WC. W and C dissolved in Co stabilize the fcc crystal structure of the Co binder, which is otherwise not thermodynamically stable at the room temperature. X-ray diffraction experiments demonstrate that the small-grained hardmetals showed a higher fraction of fcc Co in the structure. The higher corrosion resistance of the small-grained hardmetals may therefore be ascribed to the higher amount of fcc Co, which has a better corrosion behavior than hcp Co due to the higher thermodynamic stability. As Cr3C2 and VC additions were used for grain-refining, possible effects of the chemical modifications of the hardmetals on the corrosion behavior are shortly discussed.