Tarnishing and corrosion of silver-based casting alloys in synthetic salivas of different compositions

Abstract

The aim of this work is to study the electrochemical response of Ag and Ag-based alloys in synthetic salivas of different compositions. The effect of several anions present in saliva on the dissolution of Ag and some of the alloy components, such as Pd and Cu, was evaluated. Potentiostatic and potentiodynamic techniques were used and were complemented with atomic adsorption and XPS analysis. Chloride, fluoride and thiocyanate were particularly analysed because of their aggressiveness. Corrosion, pigmentation and/or tarnishing processes were enhanced when they were present in the synthetic saliva. A dependence of the selectivity of Pd dissolution from Ag–15Pd binary alloy with the potential was noticed. Ag microsegregations and precipitates formed on casting alloys showed higher activity than the matrix and rendered the alloy more susceptibility to deterioration. The need to use a normalised saliva to make comparisons of electrochemical results is highlighted.