SURFACE MORPHOLOGY OF NITINOL AND ITS EVOLUTION DURING CORROSION IN HANK SOLUTION AT ROOM TEMPERATURE

One of the most frequently used materials for the production of vascular stents and surgery implants is nitinol. Among its attractive properties are superelasticity and shape-memory.¹

The knowledge of the changes occurring on the surface of nitinol immersed in blood or another physiological liquid is critical for many considerations concerning the use of nitinol for implantation into a human body.

In this study nitinol plates were ground by diamond paper or polished electrochemically. A comparison of morphologies of plates treated by each method to that of ‘pristine’ plates was made by the use of a digital optic microscope. Corrosion behavior of the three types of nitinol plates in Hank’s solution was investigated by Electrochemical Impedance Spectroscopy and two different polarization methods.

The electrochemical polish produces significantly smoother surfaces than the mechanical treatment. Small hollows present on pristine samples become less deep although do not disappear completely after the electrochemical polish. Samples first polished electrochemically and then immersed into Hank’s solution demonstrate the smoothest surface, apparently due to the formation of continuous and smooth surface layer.

Corrosion potentials and current densities are not impacted too much by the treatments. It may be concluded that the mechanical treatment results in more localized corrosion process, while the electrochemical treatment may have no impact or it may cause strong pitting if too high current densities were used during the electropolish.

1. T. Duerig, a Pelton, and D. Stöckel, Mater. Sci. Eng. A, 273–275, 149–160 (1999).