Surface characterization of passive film on NiCr-based dental casting alloys

Surface characterization of passive film formed on different NiCr-based dental casting alloys in an acidic artificial saliva was investigated by using electrochemical impedance spectroscopy (EIS) measurement and potential decay test. The polarization resistance (Rp), which is inversely proportional to corrosion rate, of passive film was obtained from the EIS data. The stable passive current density, passivating rate, and passivity decay rate of passive film were evaluated by potential decay test. Surface chemical analyses of passive film were performed by using X-ray photoelectron spectrometer and Auger electron spectrometer. Results showed that the Rp, stable passive current density, passivating rate, and passivity decay rate, respectively, were significantly different among the various NiCr-based alloys (p<0.001). The presence of higher Cr (maximum 31–37 at%; mainly as Cr2O3) and Mo (maximum 10–11 at%; mainly as MoO3) contents in the passive film led to a higher Rp, lower stable passive current density, faster passivating rate, and lower passivity decay rate. The presence of Be (>20 at%; mainly as BeO) in the passive film was detrimental to both the corrosion resistance and the stability of passive film. The presence of small amount of Ti (<6 at%; mainly as TiO2) in the passive film did not show any influence on the characterization of passive film.