A CORROSION STUDY OF TIN (PHYSICAL VAPOR-DEPOSITION) HARD COATINGS DEPOSITED ON VARIOUS SUBSTRATES

Titanium nitride (TiN) coatings 2.8 mum thick, reactively deposited on various substrates by physical vapour deposition, were studied using electrochemical methods in combination with scanning electron microscopy and Auger electron spectroscopy. Three different substrates were used, namely stainless steel AISI 304, high speed steel ASP 30 and stabilized iron CK 45. Multisweep cyclic voltammetry experiments in acetate buffer solution were performed in order to test the microporosity of the coatings. An Auger depth analysis performed on TiN coatings polarized at potentials more positive than 1.3 V in 0.5 M H2SO4 proves the formation of an oxidized layer, indicating the partial transformation of titanium nitride to a titanium oxynitride layer. A change in the corrosion behaviour associated with the presence of the TiN coating was observed in 0.5 M NaCl. Whereas the uncoated iron undergoes general corrosion, localized corrosion occurs on the TiN-coated iron. This localized corrosion is a consequence of the microdefects present in the coating.