Although natural diamond is a complete chemically-inert material for a wide range of aggressive environments, its comparative scarcity and problems for coating design have hampered its utility as a corrosion protective coating. The recent discovery and development of chemical vapour deposition methods for growing diamond crystals and polycrystalline diamond films has opened up a wide range of applications thanks to their excellent tribological, electronic and optical properties. Various applications are in progress for corrosion and combined wear and corrosion protection. This paper presents the first study of the corrosion behaviour of continuous polycrystalline diamond films using electrochemical impedance electroscopy. Diamond films have been deposited on molybdenum substrates by means of the acetylene flame combustion method (FCVD). Electrochemical behaviour has been studied in a 0.6 M NaCl solution, it being seen that despite the inert character and apparent continuity of the film, there are areas of the base material which are exposed to the electrolyte. This behaviour has been modelled by means of an equivalent circuit which allows for the corroboration of the proposed mechanism.