In vitro tarnish and corrosion of a consolidated silver material for direct filling applications

Abstract

Objectives: A consolidated silver (CS) material, composed of acid activated silver particles has been developed for direct filling applications. The hypothesis to be tested was that the tarnish discoloration and immersion corrosion of CS was similar to that of a dispersed phase amalgam (DA). Methods: Disk-sized samples of CS and DA were prepared. CS contained a volume porosity of 25%, comparable to the material prepared for clinical uses. CIE L ∗a ∗b ∗ colorimetry was used to evaluate tarnish discoloration, while solution analysis was used to evaluate corrosion susceptibility. Surfaces following tarnish and corrosion were characterized with SEM–EDS and X-ray diffraction. Fusayama solution, defined as an artificial saliva composed of NaCl, KCl, CaCl 2·2H 2O, NaH 2PO 4·H 2O, Na 2S·9H 2O, and urea at component concentrations of (6.8, 5.4, 5.4, 5.0, 0.021 and 16.7 mmol/l, respectively, was used with controlled variations in sulfide-content. Clinically relevant sulfide contents below, equal to, and above the original Fusayama concentration were used. Intermittent exposures to air and saliva and continuous immersions were performed for tarnish discoloration, and agitated continuous immersions in 0.1 mol/l NaCl+0.1 mol/l lactic acid were performed for corrosion testing. The effects on tarnish discoloration by adding mucin and other salivary proteins to saliva were studied. Results: The total color vector change (Δ E ab ∗) for CS was significantly greater than for DA. Intermittent air and saliva exposures significantly increased Δ E ab ∗ for CS relative to continuous immersions. The rate of tarnish discoloration for CS increased with sulfide concentration. With adding protein to saliva, Δ E ab ∗ for CS significantly decreased. Mucin partly inhibited the discoloration from sulfide. The morphology of the tarnished products depended upon the testing method. For CS continuously immersed in sulfide- and protein-free saliva, a porous AgCl layer intermixed with black corrosion products, taken to be Ag 2O, formed, while intermittent exposures to air and sulfide-containing saliva formed a continuous, dense and adherent layer identified as Ag 2S. The concentrations of released silver from CS and DA were similar. However, DA also dissolved concentrations of copper and tin, changed solution pH, and grossly affected solution color, none of which occurred with CS. Significance: CS is most susceptible to tarnish discoloration in protein-free, sulfide-containing saliva under alternating wet and dry conditions. The release of silver from CS is not significantly different to the release of silver from DA.