In situ monitoring of the transpassivation and repassivation of the passive film on nitinol biomaterial by scanning electrochemical microscopy

Abstract

In our recent experiments the behavior of a nitinol sample used as an implant was examined using electrochemical impedance spectroscopy (EIS), scanning electrochemical microscopy (SECM) and atomic absorption spectroscopy (AAS). The rate of formation and stability of the passive surface layer were studied in neutral (pH = 7) and in acidic (pH = 3.0), 0.1 M NaCl solutions. Approach curves to the metal surface were recorded. The nature of the feedback proved that the surface is passive in neutral media. When the sample was exposed to acidic NaCl solution, however, it showed a soaking-time-dependent behavior. The feedback curves recorded at certain time intervals after exposure to acidic media allowed the rate of passivation to be estimated. AAS measurements showed that at the beginning of acidic exposure, Ni2+ ions were released. The experiments demonstrate that the acidic medium breaks down the passive protective surface layer and during this process a measurable amount of Ni2+ ions are released. Over a time scale of minutes, however, the nitinol repassivates by forming another passive protective layer.