Corrosion behavior and microhardness of three amalgams

Abstract – The marginal microhardness of three different types of amalgam was tested after 2 months' immersion in an aqueous solution of NaCl (85 mM) and phosphates (Na₂HPO₄ 100 mM and NaH₂PO₄ 100 mM). Amalgams immersed in distilled water were used as controls. The microhardness tests were conducted at a distance of 50 μm from the margins and at the bulk of each specimen. The solutions were analyzed for Sn, Cu, Zn, Ag, and Hg by means of atomic absorption spectrophotometry (AAS). A statistically significant reduction in the marginal microhardness after immersion in the test solution was found for the conventional and the high-Cu single composition amalgam but not for the high-Cu blended amalgam. SEM-examination of cross-sections of the amalgams revealed small areas of subsurface grain boundary corrosion, no deeper than 10 μm for all the amalgams. The SEM-examination of the specimens and AAS analysis of the solutions indicated that the reduction in marginal microhardness was attributed mainly to corrosion of the Cu-rich phases for the high-Cu single composition amalgam and to corrosion of the γ₂ phase for the conventional amalgam. The phosphates reduced the corrosion of the amalgams in the presence of NaCl. It is concluded that the marginal strength of dental amalgams in a corrosive environment is largely dependent upon their corrosion resistance.