Corrosion resistance of NiTi shape memory alloy after nitriding and oxynitriding processes under glow discharge conditions for medical applications

abstractEN: NiTi alloy is being increasingly used in medicine due to its unique properties, i.e. shape memory and superelasticity. As a self-passivating material it is characterized by relatively high biocompatibility, however its use for long-term medical implants is questionable due to the nickel content of ≥ 50%. Therefore, the investigations on the surface modification of NiTi alloy are carried out to improve its corrosion resistance and thus reduce the metalosis effect, i.e. the migration of the alloy constituents, especially nickel, into the surrounding tissue. In this paper, the surface topography and corrosion resistance of NiTi alloy (50,8%Ni) both before and after low temperature nitriding and oxynitriding processes under glow discharge conditions, are presented.The study of surface topography showed a slight increase in roughness parameters after nitriding process and a significant increase in these parameters after the oxynitriding process. A similar trend was observed in the study of corrosion resistance. Both processes increase the corrosion resistance of NiTi alloy, as shown by both the impedance spectroscopy results, the values of corrosion potential (-65 mV for the alloy in the initial state, - 45 mV for the alloy with the nitride layer, + 18 mV for the alloy with the oxynitrided layer) and the values corrosion current (respectively 0.047 μA/cm2, 0.043 μA/cm2, 0.015 μA/cm2). These comparative studies present an improvement of corrosion resistance of NiTi after the processes under glow discharge. The best results were obtained for the oxynitrided layer.score: 8collation: 92-9