Corrosion resistance of titanium-containing dental orthodontic wires in fluoride-containing artificial saliva

This study was to investigate the corrosion resistance of different Ti-containing dental orthodontic wires (including Ni-Ti, Ni-Ti-Cu, Ti-Mo-Zr-Sn, and Ti-Nb alloys) in acidic fluoride-containing artificial saliva using cyclic potentiodynamic polarization curve measurements. Different NaF concentrations (0%, 0.2%, and 0.5%), simulating the fluoride contents in commercial toothpastes, were added to the artificial saliva. Surface characterization was analyzed using X-ray photoelectron spectrometry. Cyclic potentiodynamic polarization curves showed that the presence of fluoride ions, especially 0.5% NaF, was detrimental to the protective ability of the TiO2-based film on the Ti-containing wires. This might lead to a decrease in the corrosion resistance of the tested alloys, i.e. an increase in the corrosion rate and anodic current density and a decrease in the passive film breakdown potential. Among the tested Ti-containing wires, the Ni-Ti and Ni-Ti-Cu wires containing mainly TiO2 on surface film were more susceptible to fluoride-enhanced corrosion, while the Ti-Mo-Zr-Sn and Ti-Nb wires containing MoO3/ZrO2/SnO and Nb2O5, respectively, along with TiO2 on surface film were pitting corrosion resistant and showed a lower susceptibility to fluoride-enhanced corrosion. The difference in corrosion resistance of the tested commercial Ti-containing dental orthodontic wires was significantly dependent on the passive film characteristics on wires' surface.