Abstract
Corrosion of nitinol (NiTi) is a major factor in the failure of implantable materials. Recently, as the importance of corrosion of metals has increased, testing according to international guidelines is essential. The purpose of this study was to evaluate the corrosion resistance of NiTi wire through heat treatment and passivation process. In this study, NiTi wire used two commercially available products and a self-manufactured stent. Experimental consideration was carried out according to ASTM standards. Heat treatment was carried out in an air or a salt furnace, and the corrosion was measured after additional process, such as passivation and scratch tests. As a result, the metal potential was rapidly decreased in the air furnace group. On the other hand, the potential of wires was dramatically increased in the salt furnace group compared to the air furnace group. The dislocation decreased below the acceptance criteria (>600 mV) within 60 s of heat treatment time in the air furnace. Moreover, the potential was dramatically improved, even after only 20 min of passivation treatment (1076 mV, 442% compared to the non-passivated group), and it continued to rise until 180 min. This phenomenon was similarly observed in the group of self-manufactured stents. The potential slightly decreased by the scratch process (93.1%) was significantly reduced by the air furnace process (315 mV, 24.4% of the nontreated group). In the passivated group of the air furnace sample with reduced potential, the potential was restored to the level before the air furnace (scratch stage) (1032 mV). In conclusion, the heat treatment is preferably carried out in a salt furnace rather than an air furnace, and the passivation process can be an advantageous tool to improve corrosion resistance by suppressing the oxidation process.
Highlights
Nitinol (NiTi) is a metallic alloy of nickel (Ni) and titanium (Ti) in which the two elements are present in approximately equal atomic ratios
The corrosion test was performed according to the cyclic potentiodynamic test method of ASTM 2129
Air furnace was performed at 500 ◦C with 0.007 and 0.008 of Furukawa and NDC NiTi each
Summary
Nitinol (NiTi) is a metallic alloy of nickel (Ni) and titanium (Ti) in which the two elements are present in approximately equal atomic ratios. NiTi alloys are more susceptible to corrosion due to ion release than stainless steel, cobalt–chromium or β-titanium. It was reported that Ni release occurred after treatment with NiTi orthodontic archwires, bands, and brackets and was associated with an increase in Ni ion concentration in patients’ saliva, which persisted for 10 weeks, and gradually decreased [8]. This phenomenon may cause various adverse reactions in the human body, such as foreign body reaction, allergy, and inflammation [9,10]
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