Effect of Inclusion Size and Distribution On the Corrosion Behaviour of Medical Device Grade Nitinol Tubing

Friday, May 24, 2013: 13:45
Congress Hall 1 (OREA Pryamida Hotel)
Dr. Markus Wohlschlögel , Admedes Schuessler GmbH, Pforzheim, Germany
Dr. Rainer Steegmüller , Admedes Schuessler GmbH, Pforzheim, Germany
Dr. Andreas Schüßler , Admedes Schuessler GmbH, Pforzheim, Germany
In the past years an increasing interest in nonmetallic inclusions in Nitinol with regard to the fatigue performance can be recognized. These inclusions – containing oxygen and carbon – are brittle in mechanical nature and are already introduced during the melting process, i.e. during ingot production. During hot and cold work processes following the vacuum melting these inclusions are fractionalized due to the mechanical deformation imposed. While their effect on the mechanical properties especially the fatigue performance has been a topic for materials scientists in recent years, results on the effect of those inclusions on the electrochemical behaviour of Nitinol are rare.

In the present study the variation the corrosion behaviour of Nitinol material of different suppliers in different stages of the manufacturing process was investigated: i) for the hollow (rod-shaped ingot with centric hole), ii) after hot drawing and iii) after the final drawing step. The final tubing dimensions were: outer diameter 0.3 mm and wall thickness 0.1 mm. By taking samples from different stages of the manufacturing process various inclusion sizes and distributions could be analyzed. Metallographic and optical microscopy techniques were applied to evaluate inclusion sizes and distributions. Electropolished samples prepared from each of the above-mentioned tube drawing steps 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[1].

Corrosion test results were correlated with the size and distribution of inclusions in the material. Results are presented and discussed in the light of an increased demand for Nitinol material quality in the production of vascular implants.


[1] ASTM F2129-08: Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices

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