Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in hydrochloric acid solutions

The corrosion properties of TiN layers obtained by glow discharge on Ti-6Al-4V samples were investigated by polarisation curves, cyclic voltammetry and loss weight tests, in different HCl solutions. The results were compared with those obtained on the untreated alloy tested in the same conditions. The corrosion resistance of nitrided samples depends on the quality of the film and increases as the thickness increases. At potential lower than 1000 mV (SSE) nitrided samples show passive behaviour even in concentrated HCl solution (4M). If the potential is higher than a threshold, which depends on the acid concentration, it is observed a high anodic current related to the TiN oxidation. This enhanced oxide growth promotes cracking of the oxide layer and the exposure of fresh TiN surface.