Nitinol: Tubing Versus Sputtered Foil – Microcleanliness and Corrosion Behavior

Recently, novel ingot qualities exhibiting reduced number and size of nonmetallic inclusions entered the market for medical-device grade Nitinol tubing. Depending on ingot supplier, various microcleanliness characteristics are found. Utilizing these new material qualities can help improve the fatigue life of new types of Nitinol implants such as neurovascular stents or transcatheter heart valve frames. Besides fatigue life, Nitinol corrosion susceptibility was also found to be influenced by microcleanliness. While the microcleanliness of Nitinol tubing is mainly influenced by the ingot melting process, sputtered Nitinol – due to the cleaner production process – typically exhibits no detectable impurities such as nonmetallic inclusions.

In this study, the corrosion behavior of Nitinol tubing with different microcleanliness characteristics was compared to that of sputtered Nitinol. Electropolished half-shell samples were prepared from Nitinol tubing (outer diameter 7 mm, wall thickness 0.5 mm) and three different material qualities – a standard one and two different enhanced microcleanliness qualities – were investigated. Sputtered Nitinol samples were prepared from a 50 µm thin foil. Inclusion size distributions were assessed employing quantitative metallography. Per sample type and tubing material quality 20 specimens with equivalent tested surface areas were subjected to potentiodynamic polarization testing in phosphate-buffered saline at body temperature. Corrosion susceptibility was evaluated by determining the breakdown potentials according to ASTM F2129-08.

Corrosion test results are correlated to the various material qualities and sample types and discussed in the context of microcleanliness aspects.