Influence of stress and phase on corrosion of a superelastic nickel-titanium orthodontic wire.
BACKGROUND
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 degrees 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 degrees C. Nondeflected segments were also tested. Sentalloy was additionally examined for bending and corrosion at 5 degrees C, where it exists as martensite and is devoid of stress-induced phase transformation. OCP at 2 hours and corrosion current density (i(corr)) were analyzed by using ANOVA and Tukey tests (alpha = .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 degrees C, but not for Sentalloy at 37 degrees C. Significant differences (P < 0.05) in i(corr) with deflection were also observed. All 3 wire groups had their lowest mean i(corr) values when not deflected. The i(corr) for superelastic Sentalloy (37 degrees 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 i(corr) values for TMA and Sentalloy at 5 degrees 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.
Segal
Nadav
N
Department of Developmental Sciences, School of Dentistry, Marquette University, Milwaukee, WI 53201-1881, USA.
Hell
Jess
J
Berzins
David W
DW
D002152:Calorimetry, Differential Scanning; D003343:Corrosion; D003722:Dental Alloys / Q000737:chemistry*; D004548:Elasticity; D055664:Electrochemical Techniques; D006801:Humans; D008422:Materials Testing; D055595:Mechanical Phenomena; D009532:Nickel / Q000737:chemistry*; D016910:Orthodontic Brackets; D009969:Orthodontic Wires; D018583:Pliability; D012464:Saliva, Artificial / Q000737:chemistry; D013314:Stress, Mechanical; D013696:Temperature; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*
D003722:Dental Alloys; D012464:Saliva, Artificial; C023988:beta titanium; C040654:titanium nickelide; C093824:Martensite; C093826:Austenite; D009532:Nickel; D014025:Titanium