Effect of corrosion on the strength of dental silver amalgam

Abstract

ObjectivesTo characterize the effect of crevice corrosion on the strength of dental silver amalgam as determined by the Hertzian ‘ball on disc’ method, with a view to providing a test method for use in standards compliance testing. Materials & methodsSixteen dental silver amalgam products were tested using the ‘ball on disc’ protocol at 30d after setting at 37°C in air or immersed in artificial saliva at pH 6.2. The mixed materials were packed into a tapered steel disc mold (10mm diameter, 3mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10min and placed immediately into an incubator at 37°C. For corrosion specimens, the disc was laid on a flat polystyrene surface, immersed in artificial saliva, to create a spontaneous crevice corrosion cell. Testing was in Hertzian mode, using a 20mm steel ball, with the specimen resting on a disc of glass-filled polyamide (E=10GPa) at a cross-head speed of 0.2mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced. ResultsRadial cracking into 2–4 pieces, in a clinically relevant (non-explosive) mode was observed in all cases. On average, corrosion caused a decrease in load at failure of ∼10%, although the interaction with alloy (analysis of variance) was significant (P∼0.03) indicating variation between products. Comparison of the 30d dry (uncorroded) results with those at 24h obtained earlier showed that there was highly significant increase on average (P∼5×10−12), but again a significant variation between products (P∼5×10−6), the maximum effect being +22%. SignificanceThe ball-on-disc test provides a facile means of ascertaining the sensitivity of dental silver amalgam to corrosion under clinically relevant conditions, and is viable as a standards compliance test.