Corrosion behavior of titanium nitride coated Ni–Ti shape memory surgical alloy

Nickel–titanium (NiTi, nitinol) shape memory alloy was nitrided using an original powder immersion reaction assisted coating (PIRAC) method in order to modify its surface properties. PIRAC nitriding method is based on annealing the samples in the atmosphere of highly reactive nitrogen supplied by decomposition of unstable nitride powders or, alternatively, by selective diffusion of the atmospheric nitrogen to the sample surface. Being a non-line-of-sight process, PIRAC nitriding allows uniform treatment of complex shape surgical implants. Hard two-layer titanium nitride (TiN)/Ti2Ni coatings were obtained on NiTi surface after PIRAC anneals at 900 and 1000°C. PIRAC coating procedure was found to considerably improve the corrosion behavior of NiTi alloy in Ringerʹs solution. In contrast to untreated nitinol, no pitting was observed in the samples PIRAC nitrided at 1000°C, 1 h up to 1.1 V. The coated samples were also characterized by very low anodic currents in the passive region and by an exceedingly low metal ion release rate. The research results suggest that PIRAC nitriding procedure could improve the in vivo performance of NiTi alloys implanted into the human body.