Effect of mercury on corrosion of η′ Cu---Sn phase in dental amalgams

Objectives. The η′ Cu---Sn phase is the most corrosion-susceptible major component of the microstructure in high-copper dental amalgams, even though metallurgically prepared specimens of the phase are corrosion-resistant. The purpose of this study was to determine if mercury is the cause of the degradation. Methods. Specimens of metallurgically prepared η′ Cu---Sn phase were tested in synthetic saliva using electrochemical methods. Mercury was adsorbed on the surface by exposure to mercury vapor, and the changes in the corrosion potential and anodic polarization characteristics were determined. Galvanic currents between η′ specimens and a high-copper dental amalgam were measured for both Hg-free and Hg-contaminated η′ electrodes. Results. The Hg-free η′ phase showed a wide region of passivity. Exposure to mercury made the η′ phase electrochemically more active, and current peaks appeared on the polarization curves. The Hg-free η′ electrode was a cathode to a high-copper dental amalgam. The Hg-contaminated η′ electrode started also as a cathode, but became an anode after a period of exposure. Significance. It was concluded that mercury destabilizes the protective passive film on the η′ phase, making the phase more susceptible to corrosion in the oral environment. The results of the galvanic current measurements seem to indicate that the galvanic interaction between η′ and the other phases facilitates the mercury-related degradation.