Corrosion resistance properties of 304 stainless steel and nickel-titanium orthodontic wires in artificial saliva solution

The most commonly used alloys in orthodontic wires are stainless steel and titanium due to their corrosion-resistant properties. While these alloys generally exhibit good corrosion behavior in mildly aggressive environments, unexpected breakages and allergic reactions can sometimes occur. This is mainly due to the degradation of the passive film of orthodontic wires, which is caused by the complex and variable conditions within the oral cavity. The composition of human saliva and the duration of exposure can differ significantly depending on the patient. This study evaluated the corrosion resistance properties of two commercially available orthodontic wires: 304 steel and a nickel-titanium alloy. Their electrochemical behavior was investigated using cyclic potentiodynamic polarization curves recorded in artificial saliva at 37 °C and a pH of 5.5. The results showed that 304 stainless steel orthodontic wires exhibit variable corrosion resistance properties, ranging from excellent to poor. Furthermore, when the passive film on 304 stainless steel breaks down, the repassivation potential is lower than the corrosion potential. This indicates that damage can easily propagate, leading to significant failure and the release of metal ions. This poses risk of allergies. On the other hand, the nickel-titanium orthodontic wires never showed localized corrosion behavior, demonstrating superior corrosion resistance properties compared to 304.