One of the many challenges faced by medical-device manufacturers and researchers alike is using short-term in-vitro data to evaluate long-term in-vivo corrosion performance of implants. A common measure of nitinol's susceptibility to pitting corrosion is given by the breakdown potential (Eb) relative to the rest potential (Er), as determined by in-vitro cyclic potentiodynamic polarization testing. Breakdown in-vivo will not occur unless the rest potential equals or exceeds the breakdown potential. Therefore, the ‘safety margin' against pitting increases as the gap Eb-Er increases. Long-term exposure in-vivo may change the characteristics of the protective passive surface layer and affect the gap between Eb and Er, and therefore affect the safety margin against pitting. This study was implemented to address this question. ASTM standard F 2129 is commonly used to determine Eb and Er, however, the test only provides data following relatively short immersion periods (up to one hour). In this study, mechanically-polished and electropolished nitinol wires were immersed in phosphate-buffered saline at 37 ºC for periods of up to 30 days prior to performing cyclic polarization tests in accordance with ASTM F 2129. The effect of aeration during the immersion periods was also investigated. Results are presented showing the effect of immersion time on Er, Eb, and Eb-Er.