The corrosion resistance of nitinol alloys is highly dependent on the surface condition. Materials with as-drawn and heat-treated surfaces are more susceptible to pitting corrosion due to the presence of heavy oxide and processing contamination.
Materials with a passive oxide layer, such as mechanically polished or electropolished and then passivated parts, are highly corrosion resistant and have the ability to repassivate in the event of a small local destruction of the passive film.
Effect of dissimilar materials on the corrosion resistance and biocompatibility of nitinol
Nitinol’s corrosion resistance and biocompatibility is highly dependent on the coupling material. Materials such as stainless steels, Ti, and Ta have weak galvanic effects with nitinol and are safer to use as compared to precious metals such as Au and Pt that have strong galvanic effects.
Biocompatibility of nitinol
Nitinol is generally a safe implant material as the FDA has approved many devices for long-term implant applications. A large amount of data is available in various publications. A very good comprehensive study may be found here: http://herkules.oulu.fi/isbn9514252217/.
According to an in-vitro study of passivated nitinol in Hanks solution, the Ni release rate was the highest of 14.5 x 10-7 mg/cm-2sec-1 in the first day but decreased quickly to an undetectable level in 10 days. In-vivo studies of nitinol implants in soft tissues indicated that the overall inflammatory response to nitinol was very similar to that of stainless steels and Ti-6V-4Al alloy. Studies on nitinol vascular stents showed a mild inflammatory response, minor atrophy of vessel media, acceptable fibro cellular tissue growth and endotheliazation, indicating that the biocompatibility of nitinol stents is equal to or better than that of stainless steel stents. A comparative in-vivo study of nitinol and stainless steel intramedullary rods on osteotomy healing indicated more healed bone unions and closer bone contact for nitinol when compared to the stainless steel group. The callus size and the mineral density were similar between the two groups. Studies on the use of nitinol bone implants in humans generally reported good clinical results. The existing data suggest that nitinol with proper surface finish is a safe biomaterial for vascular, soft tissue and orthopedic applications.