Metal matrix composites (MMCs), similar to those generated in an industrialhard-facing process, have been prepared in the laboratory by melting high carbon tool steel in anargon arc with particles of fused tungsten carbide. Cobalt will improve shock resistance but itseffect on the corrosion of the MMC is unknown. MMCs with different cobalt concentrationswere therefore prepared and their microstructures examined before and after corrosion usingscanning electron microscopy (SEM). Energy dispersive X-ray spectrometry (EDX) was alsoemployed for chemical analysis and X-ray diffraction (XRD) for phase identification. Specimenswere examined after exposure to both aerated sulphuric acid and to aerated, alkaline sodiumchloride. The nature of the anodic and cathodic corrosion reactions was studied bypotentiodynamic polarisation. SEM showed that Co dissolves into the grains of the base steel and Fe3W3C grain boundary deposits are formed as for the base MMC. Cobalt additions act to reducethe corrosion rate of the base MMC in acid. However, in alkaline chloride, and in contrast to thebehaviour of the base MMC, cobalt markedly diminishes the ability of the iron to form aprotective passive film.