Evaluation by electrochemical tests of the passive film stability of equiatomic Ni-Ti alloy also in presence of stress-induced martensite

Abstract

In this study, potentiodynamic polarization scans, potentiostatic scratch tests, and modified American Society for Testing and Materials F746 tests were carried out in simulated body fluids on commercial orthodontic wires made of different classes of materials and on titanium used as a reference. The stability of passivating film, evaluated by electrochemical techniques that abruptly damage it, e.g., potentiostatic scratch test, increased in the following order: Ni-Ti </= stainless steel < Co-Cr alloy < Ti. Because satisfactory biocompatibilty of implants relies on the presence of a stable and efficient self-repairing passive film, this ranking should be considered in view of in vivo applications. Moreover, scratch tests show that it is not possible to enhance the performance of Ni-Ti samples by modifying surface-passive film by dipping in an HF/HNO(3) mixture. Finally, straining of Ni-Ti wires under superelastic conditions and consequent presence of stress-induced martensite does not substantially modify their localized corrosion resistance.