The Use of ASTM F3044-14 to Assess the Galvanic Corrosion Behavior of Nitinol

Nitinol is the material of choice in a variety of implantable medical devices due to its unique mechanical and shape-memory properties. However, the continued development of smaller and more complex devices has led to challenges in x-ray visibility and the addition of dissimilar metals. In order to improve the radiopacity of a device, markers with Tantalum, Gold, Platinum, Platinum-Iridium or Tungsten are often used. These marker materials tend to be more noble than Nitinol. The resulting differences in potential are the driving force for galvanic corrosion and may influence the corrosion rate of the device by several orders of magnitude. Previous investigations of galvanic corrosion of Nitinol were based on ASTM G71. The present study uses ASTM F3044-14 to assess the galvanic corrosion behavior of Nitinol wire coupled with the most commonly used implant materials. The effects of different Nitinol surface finish treatments, including thermal oxide, mechanically polished and electropolished, are also evaluated with respect to galvanic current. Whereas mechanically polished surfaces may show pitting at the open circuit potential, with concomitant increases in current, electropolishing decreases the susceptibility to galvanic corrosion significantly. Also considered are the experimental setup, including effects of anode-to-cathode surface area ratios, anode-to-cathode fixturing distance and exposure duration. Furthermore, to interpret the galvanic data, results from like materials are compared to establish acceptance criteria.