TiN coating improves the corrosion behavior of superelastic NiTi surgical alloy

Abstract

The key to the biocompatibility of NiTi surgical devices resides in the improvement of the material's corrosion resistance. To protect the surface of NiTi from corrosion, an original PIRAC nitriding method was proposed. In the present work, the corrosion and electrochemical properties of NiTi samples annealed under a low pressure of highly reactive nitrogen at 900 and 1000°C were studied. The microstructure of PIRAC coatings was characterized employing XRD and SEM/EPMA and was found to consist of a hard TiN outer layer followed by a Ti 2Ni layer. Electrochemical tests were performed in Ringer's solution, both upon immersion in deaerated solution and after long term exposure (400 h) under aeration at open circuit potential (OCP). PIRAC nitriding treatment was found to significantly improve the corrosion resistance of NiTi alloy. Samples nitrided at 900°C, 6 h and at 1000°C, 0.5–5 h were passive in a wide range of potentials with no signs of activation up to 1.1 V. A very low anodic current in the passive region and an exceedingly low metal ion release rate were measured on PIRAC coated samples. The relationship between the nitriding parameters and protective properties of PIRAC coatings has been established and the role of the outer and inner surface layers in the improvement of NiTi corrosion resistance is discussed.