Thermal Nitriding Treatment Increases the Corrosion Resistance of Ni-Ti File

Objectives: This study investigated the effect of thermal nitriding treatment on the corrosion resistance of nickel-titanium (Ni-Ti) rotary file in 5.25% sodium hypochlorite (NaOCl) solution. Methods: The surface of Ni-Ti rotary files was treated using thermal nitriding treatment at 200, 250, and 300^oC, which were designated as group-200, group-250, group-300, respectively. The untreated as-received Ni-Ti file was designated as group-as. Surface morphology and chemical composition of the Ni-Ti files were analyzed using scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. The corrosion resistance of the Ni-Ti files was evaluated using linear polarization method and potentiodynamic polarization curve measurement. One-way analysis of variance was used to analyze the difference in corrosion resistance among the tested groups. Results: The presence of TiN layer on Ni-Ti files after nitriding treatment at 200, 250, and 300^oC was found to increase the corrosion resistance in a 5.25% NaOCl solution as compared to the as-received Ni-Ti file (P < 0.01). Although group-300 had the highest corrosion resistance, its clinical application was not suggested because its super-elastic property would decay. Therefore, group-250, which had the second place in corrosion resistance among the thermal-nitrided files, might be the optimal nitrided Ni-Ti file for clinical application in terms of the corrosion resistance. Conclusion: This study showed that the presence of TiN on Ni-Ti files through thermal nitriding treatment can increase the corrosion resistance of files in 5.25% NaOCl solution.