Electrochemistry of the saline corrosion of conventional dental amalgams

The electrochemical behaviour of the component phases of dental amalgam, viz. Ag3Sn(gamma),Ag2Hg3(gamma1), Sn7-8Hg(gamma2),Cu3Sn, Cu6Sn5 and a composite of gamma1 + gamma2 phases were evaluated in saline solution by standard anodic polarization techniques. Using multi-electrode theory a theoretical polarization diagram was constructed from the composite phases and compared with an anodic polarization profile attained from conventional dental amalgam. A strong agreement was obtained. From analysis of the electrochemical reactions it was found that the passivity of gamma2 is due to the formation of stannous and/or stannic oxide (hydroxide) and that the corrosion failure of dental amalgam occurs through the dissolution of this passive oxide in gamma2 at potentials of minus 250 mV Vs. SCE with the formation of tine oxychloride. The corrosion behaviour of Cu6Sn5 is similar to that of gamma2 phase and its presence impairs the corrosion resistance of dental amalgam. If the copper containing phase is Cu3Sn, it will be passive in Ringer's solution, possibly as a result of Tamman's "multiple rule of eight".