Corrosion behavior of nitinol wires in body fluid environments

In this study, breakdown potentials were measured for unpolished and mechanically polished nitinol wires in simulated body fluids. These wires are similar to those used in the manufacture of stents. Considerable scatter was observed in the results indicating a variable surface state. After appropriate heat treatments, the measured breakdown values were lower but more reproducible for the mechanically polished samples. Significantly higher breakdown potentials were observed for cross-section wire samples. Some wires were tested in human blood and the breakdown values were higher than in Ringer and 0.9% NaCl solutions. Energy dispersive X-ray analysis of the surface layers indicated that oxide thickening occurred after heat treatments. Dynamic secondary ion mass spectroscopy also revealed thickened surface oxides on the wires. The oxide was predominantly made up of TiO(2) with a very thin layer of NiO at the outer surface. Galvanic corrosion tests were performed on nitinol wires coupled with gold, elgiloy/phynox, and stainless steel. Nitinol was found to be anodic in all cases yet the currents measured were small. In tests in which nitinol-gold couples were immersed in 0.9% NaCl for periods up to 12 months, only very small amounts of nickel (in the part per billion range) were released into solution and scanning electron microscopy examination revealed no corrosion.