AN ELECTROCHEMICAL STUDY OF MAGNETRON-SPUTTERED TI-COATED AND TIN-COATED STEEL

In this work the microstructure as well as the electrochemical behavio ur of coated AISI 4340 steel substrates are reported. Titanium, reacti ve titanium nitride (TiN) and TiN films, obtained from a titanium film nitrided at 900 degrees C, were deposited on steel substrates by magn etron sputtering. A solid titanium sample was also nitrided at 900 deg rees C for the purpose of comparison. The study of corrosion resistanc e of the samples, processed in a solution of 3% NaCl, was evaluated by using the potentiodynamic polarisation technique. The samples were al so examined by scanning electron microscopy to determine the quality o f the coated surface. The results from the potentiodynamic analysis in dicated that reactive TiN coatings and nitrided titanium films were ch aracterised by low porosity and pinhole concentration. On the other ha nd, the relatively low corrosion resistance of reactive TiN films indi cated that they did not coat the metal substrate entirely, leaving lar ge and deep pinholes. Titanium coatings significantly improved the cor rosion performance of the steel; however, they were prone to corrosive attack as a consequence of the presence of microstructural defects su ch as surface roughness and, mainly, porosity. Galvanic corrosion betw een the coating and substrate resulted in significant attack of the me tal, allowed by the penetration of small pinpoints into the substrate. (C) 1997 Elsevier Science Ltd.