Influence of stress and phase on corrosion of a superelastic nickel-titanium orthodontic wire

IntroductionThe purpose of this investigation was to study the effect of stress and phase transformation on the corrosion properties of a superelastic nickel-titanium orthodontic wire.MethodsThe phase transformation profiles of superelastic nickel-titanium (Sentalloy, GAC International, Bohemia, NY) and beta-titanium (TMA, Ormco, Orange, Calif) archwires were analyzed by using differential scanning calorimetry. The force/deflection behavior of the wires at 37°C was measured in a 3-point bending test per modified American Dental Association specification no. 32. Electrochemical testing consisted of monitoring the open circuit potential (OCP) for 2 hours followed by polarization resistance and cyclic polarization tests on archwire segments engaged in a 5-bracket simulation apparatus with bend deflections of 0.75, 1.5, or 3 mm in artificial saliva at 37°C. Nondeflected segments were also tested. Sentalloy was additionally examined for bending and corrosion at 5°C, where it exists as martensite and is devoid of stress-induced phase transformation. OCP at 2 hours and corrosion current density (icorr) were analyzed by using ANOVA and Tukey tests (α = .05) (n = 10 per deflection).ResultsSignificant differences (P < 0.05) in OCP with deflection were found for the TMA and the Sentalloy wires at 5°C, but not for Sentalloy at 37°C. Significant differences (P < 0.05) in icorr with deflection were also observed. All 3 wire groups had their lowest mean icorr values when not deflected. The icorr for superelastic Sentalloy (37°C) peaked at 0.75 mm deflection before the wire's stress-induced phase transformation point and then decreased with further deflection and transformation. The icorr values for TMA and Sentalloy at 5°C, both of which do not undergo phase transformation with deformation, continuously increased from 0 to 1.5 mm deflection before decreasing at the 3.0-mm deflection.ConclusionsStress increased the corrosion rate in nickel-titanium and beta-titanium orthodontic wires. Alterations in stress/strain associated with phase transformation in superelastic nickel-titanium might alter the corrosion rate in ways different from wires not undergoing phase transformation.