In this work the microstructure as well as the electrochemical behaviour of coated AISI 4340 steel substrates are reported. Titanium, reactive titanium nitride (TiN) and TiN films, obtained from a titanium film nitrided at 900 °C, were deposited on steel substrates by magnetron sputtering. A solid titanium sample was also nitrided at 900 °C for the purpose of comparison. The study of corrosion resistance of the samples, processed in a solution of 3% NaCl, was evaluated by using the potentiodynamic polarisation technique. The samples were also examined by scanning electron microscopy to determine the quality of the coated surface. The results from the potentiodynamic analysis indicated that reactive TiN coatings and nitrided titanium films were characterised by low porosity and pinhole concentration. On the other hand, the relatively low corrosion resistance of reactive TiN films indicated that they did not coat the metal substrate entirely, leaving large and deep pinholes. Titanium coatings significantly improved the corrosion performance of the steel; however, they were prone to corrosive attack as a consequence of the presence of microstructural defects such as surface roughness and, mainly, porosity. Galvanic corrosion between the coating and substrate resulted in significant attack of the metal, allowed by the penetration of small pinpoints into the substrate.