The corrosion behaviour of sputtered zirconium boride and zirconium boron nitride coatings on carbon steel substrates was investigated. Coatings were produced employing non-reactive as well as reactive d.c. magnetron sputtering deposition using zirconium diboride targets. The copper decoration technique was used to make visible the types of coating defects and their distribution. To quantify the corrosion behaviour, potentiodynamic polarization measurements and electrochemical impedance spectroscopy were employed. It was found that the corrosion protection depends strongly on the porosity and adherence of the coatings. Zirconium boron nitride films are more promising in this aspect owing to their lower number of coating defects.