In vitro corrosion resistance of orthodontic super stainless steel wire-The effect of stress relieving heat treatment

Generally, orthodontic stainless steel wires are made from 18-8 stainless steels, but because of their low resistance to localized corrosion, corrosion can still occur easily in the oral cavity. Therefore for the present study, rectangular wire was produced experimentally with super stainless steel (SR-50A), which then underwent stress relieving heat-treatment in air, argon, or vacuum, at 500°C, for 6 min, followed finally by either quenching or furnace cooling. The corrosion resistance and ion release properties of the wires were investigated. SR-50A exhibited a low current density of about 1 μA/cm2 and a stable passive region in artificial saliva. Meanwhile, the comparative wires displayed a high current density because of the pitting that occurred above the breakdown potential. SR-50A heat-treated in either vacuum or argon demonstrated high corrosion resistance and a low ion release rate irrespective of the cooling method, but the SR-50A heat-treated in air exhibited low corrosion resistance. The comparative wires, heat-treated in air or quenched, showed very low corrosion resistance and a high ion release rate due to their porous and nonprotective film. SR-50A was less sensitive to the heat-treatment conditions than the comparative wires, and further was able to inhibit the release of metallic ions in the oral cavity. Therefore, these properties of SR-50A are considered to satisfy all the requirements for the safe application of orthodontic wire. © 2002 The Electrochemical Society. All rights reserved.ope