Corrosion of orthodontic wires according to heat treatment conditions

Heat treatments are applied to orthodontic wires in order to relieve the stresses arising during orthodontic work. Four types of wires were heated in air, argon or vacuum environments, and cooled in the furnace or in a water bath. The susceptibility to corrosion and mechanical properties of the heat-treated wires were investigated. Heat treatment marginally increased the yield strength and elastic modulus of all wires. After heat treatment in air, both water- and furnace-cooled wires had similar low corrosion resistances. Corrosion resistances of wires heated under vacuum differed significantly according to cooling methods, such that furnace-cooled wires had higher corrosion resistances than water-cooled wires. Water-cooled wires consistently showed low corrosion resistance whether heated in vacuum, argon, or air environments. After heat treatment under vacuum, furnace-cooled wires had low current densities and high pitting potentials similar to control wires, but other heat-treated wires generally exhibited high current densities and low pitting potentials. Wires heat-treated under vacuum or argon and then cooled in the furnace were the least susceptible to surface oxidation and corrosion, and showed a marginal improvement in mechanical properties. Therefore, we conclude that wires treated in these way can reduce metal ion release and wire fracture, which can minimize adverse effects arising from orthodontic practices.