The corrosion of nickel–titanium rotary endodontic instruments in sodium hypochlorite X. R. No ´ voa 1 , B. Martin-Biedma 2 , P. Varela-Patin ˜o 2 , A. Collazo 1 , A. Macı´as-Luaces 3 , G. Cantatore 4 , M. C. Pe ´ rez 1 & F. Maga ´ n-Mun ˜ oz 5 1 E.T.S.E.I., University of Vigo, Spain; 2 Faculty of Medicine and Deontology, University of Santiago de Compostela, Spain; 3 Faculty of Chemistry, University of Santiago de Compostela, Spain; 4 School of Dentistry, University of Verona, Italy; and 5 E.S.E.I., University of Vigo, Spain Abstract No ´ voa XR, Martin-Biedma B, Varela-Patin ˜ o P, Collazo A, Macı ´as-Luaces A, Cantatore G, Pe ´ rez MC, Maga ´ n- Mun ˜ oz F. The corrosion of nickel–titanium rotary endodontic instruments in sodium hypochlorite. International Endodontic Journal, 40, 36–44, 2007. Aim To evaluate the corrosion resistance of nickel– titanium (NiTi) endodontic rotary instruments immersed in 5.25% sodium hypochlorite (NaOCl) solution. Methodology The corrosion performance of NiTi instruments (S1 25 mm, ProTaper Dentsplay Maillefer, Ballaigues, Switzerland) was evaluated using commer- cial 5.25% NaOCl solution (pH ¼ 12.3), and the same solution partially neutralized adding H 2 SO 4 to reach pH ¼ 10.1. Electrochemical measurements were car- ried out using a potentiostat equipped with a five- channel zero resistance ammeter (ZRA) for galvanic current measurements. The instruments were sec- tioned into three parts (cutting part, noncutting part and shank) and degreased with acetone and rinsing with demineralized water prior to being immersed in NaOCl solution for testing. Each set of the three parts constituted one ‘virtual’ instrument through the ZRA, giving access to the galvanic currents that circulate between the three parts. Nine instruments were employed to check the reproducibility of the electro- chemical measurements. Results The corrosion potential (E corr ) of the NiTi alloy reached the passive domain in approximately 20 s of immersion in the solution having a pH 10.1. After this initial period the potential remained steady, indicating that stable passivation was achieved. However, at pH 12.3 no stationary state was achieved even after 6000 s of immersion time. Thus, the alloy was not stable in this medium from a corrosion point of view. Conclusions The corrosion resistance of NiTi alloy was enhanced by lowering the pH of NaOCl solution to 10.1, which allows the system to reach the stability domain of the passivating species TiO 2 and NiO 2 . Keywords: corrosion resistance, rotary endodontic instrument, sodium hypochlorite. Received 2 December 2005; accepted 9 June 2006 Introduction The potential risk of nickel–titanium (NiTi) endodontic rotary instruments to fracture during root canal shaping is of concern. NiTi endodontic rotary instru- ments usually fracture through two mechanisms: torsion or through metal fatigue caused by flexure. Fracture due to torsion occurs when the tip of the file or any part of the instrument becomes locked in the root canal, whilst the shank continues to rotate in the handpiece (Martin et al. 2003). When this occurs, the elastic limit of the metal is surpassed; the metal becomes deformed and finally fractures. The other type of fracture is caused by stress, and by the fatigue of the metal itself (cyclical fatigue) (Luebke et al. 2005). The smaller the radius of curvature, the more critical the cyclical fatigue becomes (Lopes et al. 1994, Zelada et al. 2002, Varela-Patin ˜o et al. 2005). Those instruments considered to be most resistant to cyclical stress are Correspondence: Dr Benjamı ´n Martı ´n Biedma, Ru ´ a Entrerrios s/n Facultade de Odontoloxı ´a, University of Santiago de Compostela, Santiago de Compostela, 15705 Spain (fax: 3498 156 2226; e-mail: bjmbptd@usc.es). International Endodontic Journal, 40, 36–44, 2007 ª 2006 International Endodontic Journal doi:10.1111/j.1365-2591.2006.01178.x 36