Electrochemical polarisation and corrosion behaviour of cobalt and Co(W,C) alloys in 1 N sulphuric acid

The influence of carbon and tungsten additions to cobalt on the electrochemical properties has been studied. Carbon additions up to 0.15 wt% and tungsten additions up to 8 wt%, covering a range of compositions equal to that expected in the binder composition of WC–Co hardmetals, have been investigated using potentiodynamic and potentiostatic polarisation techniques in 1 N sulphuric acid. Pure cobalt does not passivate and remains active with increasing potential. A film forms on Co(W,C) alloys causing the current to become relatively independent of potential but remaining very high. This behaviour is termed here `pseudo-passivationʹ. The mechanism of film growth is determined. With increasing carbon and tungsten additions, the corrosion current density and critical current density are reduced. The corrosion potential shifts to more positive values with increasing additions. It is shown that carbon and tungsten additions influence the corrosion behaviour by a stabilisation of the fcc phase. Empirical equations are derived which relate quantitatively the volume fraction of the fcc phase to the composition of the alloy, i.e. the carbon and tungsten content in the binary and ternary alloys.