Microstructure and surface composition effects on the transpassivation of NiTi wires for implant purposes

The microstructure and surface composition effects on the transpassivation behavior of untreated and heat treated at 500 degrees C NiTi wires used for implant purposes were investigated by electrochemical techniques ( open circuit potential and potentiodynamic polarization curves), X- ray diffraction ( XRD) and scanning electron microscopy ( SEM). The potential at which the passive film breaks down ( E (b)) ( transpassivation) was found to be strictly dependent on both the chemical composition of oxide covering layer and the microstructure of the biomaterial. It could be concluded that the disordered microstructure of untreated NiTi is highly resistant to transpassivation ( E b similar to 1.00 V/ SCE), but the presence of a native oxygen rich oxide film onto it makes the dissolution easier ( E (b) similar to 0.00 V/ SCE). Upon thermally treating, the microstructure becomes ordered and less resistant to dissolution ( E (b) similar to 0.12 V/ SCE), but in this case the presence of an oxygen rich native film influenced E b in a lower extent ( E (b) similar to 0.03 V/ SCE).