Corrosion of dental alloys in vitro by differential oxygen concentration

The corrosion of a conventional and a high Cu dental amalgam, a Cu-Zn dental alloy, and pure silver (control) were studied in vitro under conditions of differential oxygenation, resembling a partially covered dental filling. The experimental method enabled the simultaneous recording of separate pH changes at anordic and cathodic areas and of the corrosion current generated; the corrosion products formed and the corroded alloys were examined by powder X-ray diffraction, optical and scanning electron microscopy, and EDAX elemental analysis. Marked anodic pH drops and cathodic pH rises were seen which, with the total electric change integrated from the recordings, formed the basis for a qualitative evaluation (electrochemical reaction schemes) and a quantitative evaluation (amounts of metal corroded) of the progress of corrosion. The corrosion products found were SnO2, ZnSn(OH)6, and Cu2O of the amalgams, and Zn5(OH)8Cl2 of the Cu-Zn alloy; twice as much tin corroded from the conventional as from the high Cu amalgam, and zinc corroded selectively from the Cu-Zn alloy. These findings were substantiated by the microscopic and EDAX observations.