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 degrees 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 degrees C in isotonic saline solution (0.9% NaCl) showed, for Ni-Ti and stainless steel wires, pitting potential values in the range approximately 200-400 mV and approximately 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 (approximately 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.