The electrochemical behaviour of W-M-N (M = Ni, Ti, Al) thin-film hard coatings on high speed steel has been studied. Films of 4 mu m thickness of W-Ni, W-Ti and W-Ti-Al with and without nitrogen were produced by DC diode sputtering. Electrochemical measurements were carried out in aqueous potassium chloride solution. Open circuit potential measurements, Tafel plots and electrochemical impedance all show the importance of the influence of nitrogen on the coating behaviour, and some differences between coating types due to the combination of metallic elements are evident. Corrosion is localised at specific points in the coating, which appear in the first hour after immersion. They exhibit the same general features in all cases, involving corrosion beneath the coating followed by film rupture. Electrochemical observations are supported by scanning electron microscopy observations, X-ray diffraction and microindentation measurements before and after corrosion has taken place. A model describing the corrosion process is developed and discussed.