Corrosion of dental alloys is usually evaluated by the uniform corrosion test. However, in dental clinics, localized corrosion is widely observed at the cervical and proximal area of metal restorations or at the interface between restoration and cavity wall. This study evaluated the crevice and galvanic corrosion of dental alloys using models simulating, the oral enviroment to understand localized corrosion behavior of metal restorations.
Two dental commercial alloys, Au-Pd-Ag-Cu and Ag-Sn-Zn were used as the specimen electrode. For the uniform corrosion test, the usual model was used, for the crevice corrosion test, a model with a 50 μm space between the metal surface and glass plate was designed and for the galvanic corrosion test, a model coupling both alloys by a copper wire was designed. The models were immersed in 0.9% NaCl solution adjusted at pH2. The natural potential (Evs. Ag/AgCl) and polarization resistance (Rp) were measured every 2 days for 60 days by the coulostatic method. Galvanic current between P and M was measured. The potential sweep method was also applied to the models to determine the rate of reaction of the crevice and uniform corrosion. After 60 days, the specimens were elementally analyzed by ESCA (Electron Spectroscopy for Chemical Analysis). The following results were obtained:
In crevice corrosion, the E and Rp values of Au-Pd-Ag-Cu were low compared with those in uniform corrosion and the rate of reaction decreased. The E value of Ag-Sn-Zn markedly decreased compared with that in uniform corrosion. However, in the crevice corrosion both the Rp and the rate of reaction showed values similar to those in the uniform corrosion.
In the galvanic corrosion, the E and Rp values of Au-Pd-Ag-Cu decreased compared with those uniform corrosion. On ESCA, a marked amount of Sn was found on the surface of the alloy which contained no Sn.
The E and Rp of Ag-Sn-Zn were similar to the uniform corrosion. On ESCA, a Sn rich surface layer was detected. Even on the 60th day, galvanic current was still detected indicating that the galvanic corrosion may continue for long period.
The results suggest that the corrosion rate of precious alloys depends on enviromental conditions.