Silver-tin alloys and amalgams: electrochemical considerations.

The corrosion potential and anodic polarization profiles of a representative number of silver-tin alloys and their corresponding amalgams in a physiological solution were determined and compared to their microstructures. For the alloys with tin-content greater than 27%(wt) and for all amalgams, the corrosion process is related to the attack of free tin for the alloys and to the gamma-2 tin for the amalgams. The gamma-2 concentration in the amalgams increases with an increase in tin-content. For alloys with tin-content less than 27%, the corrosion process is even more restricted than for the process observed with pure silver. From a developed theory based upon the potential-time and polarization results, association of the O2 reduction process on a SnO cathodic film to an intermediate specie of H2O2 is made. The rate of H2O2 decomposition on a SnO surface in a four electron process is thought to control the O2 reduction overvoltage. The O2 reduction overvoltage decreases with increases in the silver-content of the amalgam, particularily seen with the 8 and 12% tin compositions. Due to the polarization induced corrosion process, a phase with high silver and high mercury concentrations was observed over the unreacted particles.