Localized corrosion behaviour in simulated human body fluids of commercial Ni–Ti orthodontic wires

The corrosion performances in simulated human body fluids of commercial equiatomic Ni–Ti orthodontic wires having various shape and size and produced by different manufacturers were evaluated; for comparison purposes wires made of stainless steel and of cobalt-based alloy were also examined. Potentiodynamic tests in artificial saliva at 40°C indicated a sufficient pitting resistance for the Ni–Ti wires, similar to that of cobalt-based alloy wire; the stainless steel wire, instead, exhibited low pitting potential. Potentiodynamic tests at 40°C in isotonic saline solution (0.9% NaCl) showed, for Ni–Ti and stainless steel wires, pitting potential values in the range ≈200–400 mV and ≈350 mV versus SCE, respectively: consequently, according to literature data (Hoar TP, Mears DC. Proc Roy Soc A 1996; 294: 486–510), these materials should be considered potentially susceptible to pitting; only the cobalt-based alloy should be immune from pitting. The localized corrosion potentials determined in the same environment by the ASTM F746 test (≈0–200 mV and 130 mV versus SCE for Ni–Ti and stainless steel, respectively) pointed out that for these materials an even higher risk of localized corrosion. Slight differences in localized corrosion behaviour among the various Ni–Ti wires were detected.