Ni-Ti orthodontic archwires at different temperatures

Orthodontic mechanotherapy encompasses the use of several archwires during speci c stages of treatment. Metallic orthodontic appliances consist of bands, arch wires, ligature wires, hooks, tubes, brackets, and springs are daily used in orthodontic practices for the correction of malocclusions in patients ( Oh et al. , 2004 ; Verstrynge et al. , 2006 ; Sabane et al. , 2009 ). One of the most important issues in the use of metallic biomaterials is their corrosion behavio u r. The corrosion of orthodontic appliances in the oral environment may release corrosion products or effects on the physical properties and the clinical performance of orthodontic appliances ( House et al. , 2008 ; Virtanen et al. , 2008 ). Orthodontic appliances and retainers are commonly constructed from stainless steel (SS), cobalt–chromium alloy, nickel–titanium (Ni-Ti) ,and titanium alloys, used singly or in combination. Up until the 1930s, the only orthodontic wires available were made of gold alloy. Austenitic SS , with its greater strength, higher modulus of elasticity, excellent formability, resistance to corrosion , and moderate costs , was introduced as an orthodontic wire in 1929 ( Wilkinson 1962 ; Vaheda et al. , 2007 ). The SS alloys used as orthodontic wires are of the ‘18-8’ austenitic type and are classi ed as AISI type 304. This alloy owes its corrosion resistance property to chromium, a highly reactive base metal. The corrosion resistance of alloy depends on the passive  lm, which spontaneously forms and reforms in air and under most  uid conditions. In vitro evaluation of the electrochemical behavio u r of stainless