M. Dartar Oztan, A. A. Akman, L. Zaimoglu & S. Bilgic
Department of Endodontics, Faculty of Dentistry, AnkaraUniversity.
Department of Physical Chemistry, Faculty of Science, Ankara University, Ankara,Turkey.

Introduction.
Chemomechanical canal preparation during the rootcanal treatment involves cleaning and shaping procedures with endodontic instruments and irrigating solutions. The aim of the root-canal instrumentation is to obtain a continuous tapering funnel shape, flowing with the original canal from the coronal access to the apex (Walcott & Himel 1997). The functions of irrigants are to act as lubricants during the mechanical debridement of pulpal and dentinal tissues, a medium to remove debris, a solvent to dissolve tissue, an agent to promote root-canal sterility and patent dentinal tubules on the root-canal walls (Mueller1983).
Many solutions, such as sodium hypochlorite, hydrogen peroxide, citric acid, ethylene diamine tetraacetic acid and physiological saline, have been used for irrigating root canals. In recent years, chlorhexidine gluconate, because of its antimicrobial effects, was also advocated as an irrigating solution (Delany et al. 1982, Jeansonne & White 1994). Even though the benefits of irrigating solutions are essential for chemomechanical preparation, the chemical and electrochemical aggressiveness of these solutions on the instruments must also be taken into account to prevent the decreased service performance of the instruments with usage. The cutting ability of endodontic instruments is affected by several factors, being a complex interaction of different parameters, such as material, metallurgical properties, cross-sectional design, number of flutes, chip-removal capability and helical angle (Scha' ffer 1999). The chemical effects of the irrigating solutions on endodontic files may also hinder their performance (Mueller 1983). Corrosion adversely affects the metallic surfaces by causing pitting and porosity, and decreases the cutting efficiency of endodontic files (Stokes et al. 1999). Neal et al. (1983) evaluated the effect of sterilization and irrigants on the cutting ability of stainless-steel files and concluded that sodium hypochlorite, hydrogen peroxide, and EDTA-urea peroxide irrigants caused a decrease in the cutting ability of the files.
Stokes et al. (1999) evaluated the corrosive effect of 5.25% NaOCl on stainless-steel and Ni-Ti files using five commercial brands. They reported that no relationship existed between the metal alloy and corrosion, as both the corroding- and non-corroding files were present in the same package. They also concluded that the difference could possibly be due to the variations in the manufacturing process and quality control.
The purpose of the present study was to evaluate and compare the electrochemical corrosion rate of stainless- steel endodontic files when immersed in different irrigating solutions.

Materials and methods.
Twenty five mm ISO size 25 stainless-steel K-files (Mani Inc.,Tochigi-Ken, Japan) were used for the present study. The irrigating solutions studied were:

1. 0.2% Chlorhexidine gluconate (pH 5.72) (prepared in the laboratory);
2. 5.25% Sodium hypochlorite solution (NaOCl, pH 12.10) (Sultan Chemists, Inc., Englewood, NJ, USA);
3. Chlorinated soda with KOH (5.25% NaOCl юKOH, pH 12.09) (Sultan Chemists Inc., Englewood, NJ, USA); and
4. 17% EDTA, pH 5.25 (prepared in the laboratory).

The corrosion rates of the stainless-steel K-files in the irrigating solutions were determined electrochemically by the Tafel extrapolation method. Electrochemical experiments were carried out in a Pyrex cell with three compartments. The cell was water-jacketed, and connected to a constant temperature circulator. The experiments were carried out at a stable 37 8C. Stainless-steel files were used as experiment electrodes. The cutting flutes of the files were immersed in the irrigating solutions, and kept for 20 min in the cell prior to each experiment, so that the rest potential of the electrode could be attained. A saturated calomel electrode (SCE) was used as a reference and a platinum plate as counter electrode. All potentials were referred to SCE. During each experiment, the solutions were mixed with a magnetic stirrer to ensure the contact of solutions with the whole file surface.
Data was obtained using a combined system containing a potentiostat (Wenking LB 75 L, Gottingen, Germany), a voltage scan generator (Wenking VSG 72) and a recorder (Yokogawa 3077, Tokyo, Japan). The potential scan rate was chosen as 2.5 mV.
In order to determine corrosion rates, the linear part of anodic currents, obtained from electrochemical current potential curves (Elog) was extrapolated to corrosion potentials. Seven experiments were done for each solution, and statistical analysis of the data was performed using the Kruskal-Wallis one-way anova. The files from the each group, and also one untreated control file (Fig.1) were photographed under the scanning electron microscope (SEM).

Figure 1. SEM of stainless-steel control file revealed no evidence of corrosion (1000x).

The corrosion rates of the stainless-steel K-files in four different irrigating solutions are presented in Table 1.
Differences between the corrosion rates of files were not found to be statistically different for chlorinated soda with KOH and 17% EDTA (P > 0.05), but statistically significant differences were found amongst other groups.
Results showed that the corrosion rates of stainless steel files were highest in the chlorhexidine gluconate solution. General corrosion, especially pitting corrosion, was observed on the surfaces of files, which were immersed into 0.2% chlorhexidine solution (Fig. 2). Corrosion rates of files were higher in 5.25% NaOCl solution than in chlorinated soda with KOH. It was observed that local anode places increased in 5.25% NaOCl solution (Fig. 3). SEM investigation of the files which were immersed into chlorinated soda with KOH revealed general corrosion (Fig. 4). Stainless-steel files which were immersed in EDTA solution demonstrated the lowest corrosion rate. After the SEM investigation, it was seen that the accumulation grew on the micro anode region, and corrosion occurred in local regions (Fig. 5).

Table 1. Corrosion current density values of stainless steel files in tested solutions.



Figure 2. SEM of stainless-steel file, which was immersed in 0.2% chlorhexidine gluconate solution, showing pitting with the corrosion products (1000x).



Figure 3. SEM of stainless-steel file, which was immersed in 5.25% NaOCl solution, showing severe corrosion (1000x).



Figure 4. SEM of stainless-steel file, which was immersed in chlorinated soda with KOH, demonstrating general corrosion (1000x).



Figure 5. SEM of stainless-steel file, which was immersed in 17% EDTA solution, demonstrating locally corroded area (1000x).

Discussion.
Tafel extrapolation is one of the electrochemical tests to assess the corrosion properties of metals and alloys, and was used in the present study. Mueller (1983) demonstrated that electrochemical techniques based on the polarization profiles and polarization-resistance methods were efficient and reliable to study the mechanisms of corrosion of endodontic instruments, and to judge the corrosion susceptibility of newly developed and improved materials for instrument manufacture. As the findings of the present study indicated, the corrosion rates of stainless-steel files in the tested solutions from the highest to the lowest were: 0.2% chlorhexidine solution > 5.25% NaOCl > chlorinated soda with KOH >17% EDTA. The highest corrosion rate of files in chlorhexidine solution may depend on its acidic pH (5.72), as the acidic environment increases the corrosion rate (Matamala 1987). Secondly, the corrosion rate of stainless-steel files was high in 5.25% NaOCl solution. NaOCl contains active Cl_ions, and it is well-known that Cl_ is an aggressive ion, which generally increases corrosion rates (Katayama et al.2000). Iron is the major element in the stainless steel at _70 wt.% and has poor corrosion-resistance in chloride solutions (Sutow et al. 1999).The corrosion rate of stainless-steel files was lower in chlorinated soda with KOH, which contains 5.25% NaOCl, than 5.25% NaOCl solution. Although it is a chloride solution, the reason for this difference may depend on Kюand OH_ ion contents of chlorinated soda with KOH, because K and OH_ ions have a particularly passivating effect on the corrosion (Skoog et al. 1992). The lowest corrosion rate of stainless-steel files was observed in17% EDTA solution. EDTA forms complexes with metal ions (Fe, Ni, Cr, Co, etc.) at pH values <4. EDTA’s ability to protect and passivate instruments is due to its ability to complex with iron to for man inhibiting barrier to oxidation and corrosion (Reinhard et al. 1992, Skoog et al.1992).

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