In Vitro Corrosion Studies of Surface Modified NiTi Alloy for Biomedical Applications

Electropolishing was conducted on NiTi alloy of composition 49.1 Ti-50.9 Ni at.% under potentiostatic regime at ambient temperature using perchloric acid based electrolyte for 30 sec followed by passivation treatment in an inorganic electrolyte. The corrosion resistance and biocompatibility of the electropolished and passivated alloys were evaluated and compared with mechanically polished alloy. Various characterization techniques like scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were employed to analyze the properties of surface modified and mechanically polished alloys. Water contact angle measurements made on the passivated alloy after electropolishing showed a contact angle of 35.6°, which was about 58% lower compared to mechanically polished sample, implying more hydrophilicity. The electrochemical impedance studies showed that, for the passivated alloy, threefold increase in the barrier layer resistance was obtained when compared to electropolished alloy due to the formation of compact titanium oxide. The oxide layer thickness of the passivated samples was almost 18 times higher than electropolished samples. After 14 days immersion in Hanks’ solution, the amount of nickel released was 315 ppb which was nearly half of that obtained for mechanically polished NiTi alloy, confirming better stability of the passive layer.