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.