Electrochemical behaviour of tungsten-carbide hardmetals

Accurate measurements of the polarisation curves of tungsten carbide-cobalt composites as well as those of the individual components (tungsten carbide, pure cobalt and a cobalt-tungsten-carbon alloy representing the composition of the binder phase of the liquid-phase sintered composites) in normal sulphuric acid were carried out. The linear rule of mixtures first proposed by Stern for heterogeneous alloys was re-examined. The corrosion behaviour of the hardmetal composites cannot be predicted from that of tungsten carbide and pure cobalt. Two models were tested, one in which the carbide crystals are separated from the binder alloy by a thin layer of cobalt low in both carbon and tungsten, and one where this layer is not considered. The second model agrees very well with the experimental results while the first one shows significant deviations. ations of the relative removal rates show that the binder corrodes faster than the carbide and is leached out in spite of exhibiting a pseudopassive behaviour owing to the formation of a porous corrosion layer. The influence of the carbide grain size and of the cobalt content on the corrosion kinetics is shown to be too small as to be of any practical importance.