The relationship between electrochemical behaviour and in-service corrosion of WC based cemented carbides

In long-life applications, the corrosion properties of cemented carbides can have a large influence on overall performance. Cemented carbides with improved corrosion resistance have been developed and are now commercially available. The understanding of the corrosion behaviour has been mostly empirical and satisfying explanations of the relationship between the electromechanical behaviour and in-service corrosion have been lacking. In this paper, the electromechanical behaviour of WC-Co is modelled using the behaviour of pure WC and Co(W,C) alloys, according to a linear rule of mixtures. By comparing WC-Co with WC-Ni(Cr,Mo) in both normal sulphuric acid and a synthetic mine water, it is shown that the behaviour of the two grades is inherently different. WC-Co exhibits a 'pseudo-passivity' during electromechanical tests but corrodes actively in industrial applications. In contrast, WC-Ni(Cr,Mo) passivates and the rate of corrosion can be several orders of magnitude lower than that of WC-Co. (C) 1997 Elsevier Science Limited.