Corrosion of Various High-Copper Amalgams

Part 2. Effect of Corrosion on the Metallurgical Structure

Abstract

The corrosion behavior of several types of high-copper amalgam specimens attacked by the crevice corrosion during one year was microstructurally investigated by using SEM, WDX and EDX, comparing with those of the conventional amalgams.
1) In the conventional amalgams, the corrosion was observed through the whole layer of the specimens. It was proved that the γ₂ phase was selectively corroded, but γ₁ or γ phase was highly resistant.
2) In the high-copper dispersant amalgam, the demarcated corrosion front showed the irregular line. The reaction zone (η′phase+γ₁ phase) around the Ag-Cu eutectic particles seemes to be primarily attacked by corrosion. This selective corrosion of the reaction zone might retard the proceeding of the inward corrosion.
3) In the high-copper single composition amalgams, the corroded layer was markedly demarcated with the corrosion front line. Both η′phase and the original alloy particle appear to be readily corroded. Until the η′phases and original alloy particles will be completely destroyed by this localized corrosion, the corrosion front would not proceed toward the deeper portion.
4) Accumulation of tin-rich corrosion products and disappearance of copper component were observed at the corroded layers.