Effect of Stress Believing Heat Treatment Conditions on Corrosion Resistance of Orthodontic Stainless Steel Wires

Heat treatment is applied to orthodontic wires in order to relieve the stress that results from cold working and the orthodontists manipulations. In this study, 4 types of wires were heat-treated in an air, argon or vacuum environment, and were either cooled in a furnace or a water bath. Corrosion properties of the heat-treated wires were investigated by potentiodynamic method. The difference of corrosion resistance between the wires heat-treated in an ambient according to cooling methods was not significant. After heat-treated in an ambient, both water-cooled and furnace-cooled wires had a similar and low corrosion resistance. The difference of corrosion resistance between the wires heat-treated in a vacuum environment with cooling methods was significant. After heat-treated in a vacuum, the furnace-cooled wires had high corrosion resistance, but the water-cooled wires low. After heat-treated in a vacuum, argon, or air environments, the water-cooled wires had low corrosion resistance. It was considered because the irregular and unstable film was formed on the surface of the wires when cooled in a water bath. The control wires showed a low current density and high pitting potential, but most of the heat-treated wires did high current density and low pitting potential except for the wire heat-treated in vacuum and then cooled in a furnace. Conclusively, orthodontic wires are considered to have to be heat treated in vacuum condition or inert gas environment and then to be cooled in a furnace in order to inhibit surface oxidation, minimize the decrease of corrosion resistance and improve the mechanical properties.ope