Cange in Microstructure of Amalgams by Corrosion

Conventional lathe cut and spherical amalgams were fully covered with heavily protuberant corrosion products when placed in saline solution for a six-month period. The products were mostly composed of two distinct layers : the inner consisting primarily of Sn and Cl, and the outer primarily of Sn or Sn with small amount of Cu. Cl was detected together with Sn from the traces of corroded γ_2 phase in the specimen, indicating that γ_2 phase was selectively attacked in succession inward. Residual γ alloy particles were also attacked to a lesser extent than γ_2 phase. High-copper amalgams composed of lathe cut Ag_3Sn and admixed spherical Ag-Cu alloys did not yield so marked corrosion products as the conventional ones by virtue of the absence of γ_2 phase. Their surfaces were mostly covered with Cu-rich small, angular crystals and newly grown-up curly γ_1 crystals. Cl was detected together with Sn from the traces of corroded Cu_3Sn phase and from the vicinity of the residual γ alloy particles. Cl was also found in the γ particles. On the contrary, another system of high-copper amalgam composed of spherical Ag-Sn-Cu ternary alloy including a small amount of In yielded big protuberant corrosion products on the surface regarded as verdigris. Most of the original alloy particles within a 1000μm depth from the surface were severely destroyed leaving In together with Cu and/or Cl and many microcracks. The presence of In seems to induce the corrosive attack in the amalgam after three months. Cl penetration into the amalgam specimen was generally deeper in Zn free amalgams than in Zn containing amalgams.