Abstract
Anodic polarization of nitinol in acetic acid under galvanostatic conditions produces oxide films composed mainly of TiO2. An exponential current-field relation is valid during ionic conduction through the growing oxide, in which the field coefficient is related to the jump distance. Transport processes in anodic films have been discussed in terms of a cooperative mechanism developed for amorphous oxide films on valve metals, in which both metal and oxygen ions were involved in ionic conduction. For more crystalline oxide structure of passive films on nitinol, formed during a prolonged potentiostatic conditions, the charge transfer takes place only through the oxygen vacancies as mobile species via a high-field-assisted mechanism. Based on the results of the Mott–Schottky analysis, these films behave as n-type semiconductors indicating that oxygen vacancies formed during the film formation and growth act as electron donors. The barrier/protecting and electronic/semiconducting properties of the passive films as well as their chemical composition were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.
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