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

By Segal, Nadav; Hell, Jess & Berzins, David W.
Published in American Journal of Orthodontics and Dentofacial Orthopedics NULL 2009

Abstract

Introduction The 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. Methods The 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). Results Significant 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. Conclusions Stress 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.

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