Corrosion of dental alloys in artificial saliva with Streptococcus mutans

Abstract

A comparative study of the corrosion resistance of CoCr and NiCr alloys in artificial saliva (AS) containing tryptic soy broth (Solution 1) and Streptococcus mutans (S. mutans) species (Solution 2) was performed by electrochemical methods, including open circuit potential measurements, impedance spectroscopy, and potentiodynamic polarization. The adherence of S. mutans to the NiCr and CoCr alloy surfaces immersed in Solution 2 for 24 h was verified by scanning electron microscopy, while the results of electrochemical impedance spectroscopy confirmed the importance of biofilm formation for the corrosion process. The R(QR) equivalent circuit was successfully used to fit the data obtained for the AS mixture without S. mutans, while the R(Q(R(QR))) circuit was found to be more suitable for describing the biofilm properties after treatment with the AS containing S. mutans species. In addition, a negative shift of the open circuit potential with immersion time was observed for all samples regardless of the solution type. Both alloys exhibited higher charge transfer resistance after treatment with Solution 2, and lower corrosion current densities were detected for all samples in the presence of S. mutans. The obtained results suggest that the biofilm formation observed after 24 h of exposure to S. mutans bacteria might enhance the corrosion resistance of the studied samples by creating physical barriers that prevented oxygen interactions with the metal surfaces.

Fig 1. OCP values obtained for CoCr and NiCr alloys treated in Solutions 1 and 2 at 37°C.

Fig 2. (a) Nyquist and (b) Bode plots obtained for CoCr and NiCr alloys treated in Solutions 1 and 2 at 37°C.

Fig 3. Equivalent circuits utilized for fitting the EIS results obtained in the (a) absence and (b) presence of S. mutans species.

R_s: solution resistance in the vicinity of the test sample, R₁: charge-transfer resistance of CoCr and NiCr alloys, CPE₁: double-layer capacitance of CoCr and NiCr alloys, R₂: resistance of the biofilm binding layer, CPE₂: capacitance of the biofilm binding layer.

Fig 4

(a) A Nyquist plot fitted using the R(QR) circuit. (b) A Bode plot fitted using the R(QR) circuit. (c) A Nyquist plot fitted using the R(Q(R(QR)))) circuit. (d) A Bode plot fitted using the R(Q(R(QR)))) circuit.

Fig 5. SEM images of the (a) CoCr and (b) NiCr alloy specimens exposed to the mixture of AS with S. mutans species for 24 h (magnification: 3, 000×).

Fig 6. Typical potentiodynamic polarization curves obtained for CoCr and NiCr alloys treated with Solutions 1 and 2.