An experimental study of the influence of surface oxide layer properties on the open-circuit potential of superelastic nitinol wires

Characterization of pitting corrosion susceptibility of Nitinol medical devices includes determination of the open-circuit potential (OCP) prior to cyclic potentiodynamic polarization measurement. Typically, the OCP is considered to be the value that is obtained after a duration of one hour of immersion in an electrolyte, e.g., phosphate buffered saline (PBS). However, it has been observed that the OCP does not always stabilize during such short measurement duration compared to longer open-circuit measurements. Possible factors influencing the development of the OCP over time are likely related to the surface properties of Nitinol devices, such as oxide layer thickness and homogeneity, crystallographic structure and Nickel content.

In this study, superelastic Nitinol wires (50.9 at. % Ni) with different surface oxide layers were subjected to short (1 hour) and long (48 hours) duration OCP measurements to investigate the correlation of the above-mentioned surface properties. The oxide layers of the Nitinol wires were formed by thermal oxidation or electrochemical treatment. TEM was used to assess details of the oxide layer, e.g., thickness and structure. It is shown that value and progression of the OCP differs significantly comparing the different conditions. In conclusion, the experimental results of this study can help to test the reproducibility of long-time OCP measurements and further to understand the possible surface reactions and modifications that take place during the immersion of Nitinol devices in the electrolyte before determining their corrosion susceptibility.