The Effect of Various Thermally Grown Oxides on the Corrosion Performance of Nitinol

Corrosion behavior of nitinol is strongly dependent upon the nature of the surface oxide, which is developed during processing.  Studies have suggested that thicker oxides can have poor corrosion performance. It is hypothesized that corrosion performance of nitinol with thermally produced oxide is related not only to oxide thickness, but also to its structure and composition.  To investigate this, nitinol wire samples were processed to form surface oxides with different thicknesses and potentially different underlying structures.  Samples were processed using various initial surface conditions, furnaces (air furnace, fluidized bath), temperatures and thermal exposure times.  The resultant oxide thicknesses were determined using Auger electron spectroscopy (AES).  Nickel elution testing was conducted in phosphate buffered saline for at least 60 days to determine nickel release rate curves and cumulative nickel release.  ASTM F2129 (cyclic potentiodynamic polarization) testing was performed to determine breakdown potential, which assesses resistance to pitting corrosion.  TAFEL extrapolations were performed on the electrochemical curves to calculate an initial corrosion rate to determine whether the short-term TAFEL method can predict nickel elution behavior.  Results from these evaluations demonstrate that oxide thickness alone does not determine corrosion performance and thinner oxides do not necessarily guarantee superior nickel elution results.