Impact of Fluoride on the Surface Degradation and Corrosion Behavior of Orthodontic Wires

This study evaluated the corrosion behavior and ion release of assembled orthodontic appliances incorporating different archwire alloys in artificial saliva with and without fluoride, focusing on localized degradation and the susceptibility of welded joints. Two additional appliances with titanium brackets and stainless steel archwires were evaluated exclusively in fluoridated saliva as an exploratory, descriptive assessment. Thirty-two fixed orthodontic appliances (30 stainless steel and 2 titanium brackets) were tested under controlled electrochemical conditions. Half of the stainless steel samples were exposed to fluoridated saliva (0.2% NaF), while the other half served as controls. Titanium brackets were tested exclusively in fluoridated saliva. Electrochemical characterization was performed using polarization measurements and chronoamperometry at 37 °C to assess the electrochemical behavior of the assembled appliances. Surface degradation was analyzed via optical and scanning electron microscopy (SEM), and ion release was quantified using atomic absorption spectroscopy. In fluoride-containing artificial saliva, the highest measured nickel concentration was associated with appliances incorporating thermoactivated NiTi archwires (147 ng/g), whereas the highest chromium concentration was associated with appliances incorporating β-Ti archwires (122 ng/g). These concentrations represent release from the complete assemblies rather than from the archwires alone. Stainless steel brackets showed localized corrosion in spot-welded areas, with steady-state potential shifting from +60 mV (ECS) in non-fluoridated saliva to approximately 0 mV (ECS) in fluoridated conditions. The two appliances with titanium brackets exhibited different current–time responses, which were interpreted descriptively and did not establish the comparative corrosion resistance of the bracket material. Fluoride accelerates the localized degradation of nickel–titanium archwires and spot-welded stainless steel components, though non-fluoridated saliva demonstrated greater overall electrochemical aggressiveness due to chloride activity. The exploratory findings for appliances with titanium brackets require confirmation using larger samples and matched control groups.