Anodic polarization and corrosion of cemented carbides with cobalt and nickel binders

Abstract

The polarization of pure cobalt, nickel and tungsten, and hardmetals based on WC/Co and WC/Ni, in a solution of 0.01 M H2SO4 + 0.99 M Na2SO4 (pH 2.55), has been investigated using potentiodynamic and metallographic techniques. Pure cobalt dissolved rapidly, nickel had an excellent passivity, while tungsten had a high degree of passivity over the whole potential range up to 2.O V (SCE). Extensive dissolution of WC was observed metallographically at potentials in the range 0.6 to 1.4V (SCE). The polarization of WC/Co hardmetals reflected mainly the behaviour of the binder phase modified by the presence of tungsten in the binder and passivating film. The effects of the carbon in the binder are discussed in relation to the tungsten in the binder. Tungsten carbide grain size, and the presence of TaC and TiC, had no noticeable effects. Hardmetals with nickel binders had a superior performance compared with cobalt binders. Chloride in the solution had a relatively small effect.