Electrochemical and Surface Characterization of Nickel-Containing Orthodontic Archwires Under In Vitro and In Vivo Conditions

Objectives: To evaluate the corrosion behavior and surface characteristics of four nickel-containing orthodontic archwires (stainless steel (SS), superelastic nickel–titanium (NiTi), copper–nickel–titanium (CuNiTi), and multiforce NiTi) under unused, in vitro artificial saliva-immersed, and clinically used (in vivo) conditions. Methods: Rectangular SS, NiTi, CuNiTi, and multiforce NiTi archwires were analyzed in the following three conditions: as received, after one week of immersion in artificial saliva (pH 6.4), and after clinical use for 6–8 weeks. Corrosion behavior was assessed using cyclic voltammetry (CV), open-circuit voltammetry (OCV), and electrochemical impedance spectroscopy (EIS). Surface morphology was examined by scanning electron microscopy (SEM). Results: Corrosion behavior was dependent on archwire type and exposure condition. SS archwires exhibited reduced impedance response after clinical use, indicating passive-film degradation. In the clinically used NiTi specimen, pronounced electrochemical instability was observed, characterized by a deep OCV transient, slow repassivation, and SEM evidence compatible with localized pitting corrosion. In the CuNiTi specimens, minimal differences were observed between the unused and clinically used conditions, which may be consistent with stable passive-film integrity in these specimens. In the multiforce specimen, clinical use was associated with a higher impedance response than the unused and saliva-immersed conditions of that same specimen. In the saliva-immersed specimens, possible passive-film formation and a higher impedance response were observed relative to the other conditions of the same specimens, but these did not reproduce the electrochemical and morphological changes observed after clinical use. Conclusions: In vitro artificial saliva immersion does not reliably replicate in vivo aging of nickel-containing orthodontic archwires. Corrosion behavior evolves during clinical service in an archwire-specific manner, with NiTi archwires showing susceptibility to clinically induced surface degradation.