Micro-Raman innovative methodology to identify Ag-Cu mixed sulphides as tarnishing corrosion products

Mixed Cu-Ag alloys with different compositions have been produced and subjected to an accelerated sulphidation process which causes the development of a mixed sulphide-rich corroded film on their surface. It was called tarnishing, that is, the formation of a blue-brownish patina when Cu-Ag alloys are exposed in a sulfur-containing atmosphere. The structures of the pristine alloys have been determined by the combined analytical techniques as scanning electron microscopy energy dispersive X-ray microanalysis and X-ray diffraction. The experimental conclusions confirmed the occurrence of micro phase separation with the formation of different dendritic domains of about 10 µm in width. The sulphidized samples were firstly investigated by optical microscopy and X-ray diffraction in order to verify the homogeneity of the patina and to identify the different AgCuS phases appearing on the alloy surfaces. It was observed that, despite the inherent micro-heterogeneity of the alloys, the sulphide layer was throughout uniform in composition at the micro-scale. The complex scenario of the relative stability of all the various mixed sulphides involved was then explored by micro-Raman spectroscopy (?-RS), pointing out that the Cu-for-Ag substitution in the crystal lattice of the mixed Ag-Cu sulphides caused a monotonous blue shift of the vibrational wavenumbers in Raman spectra. This study has unveiled microscopic details of the tarnishing process, furnishing an innovative, cheap and non-destructive methodology based on ?-Raman spectroscopy for the evaluation of the silver-copper artefacts via the compositions of their corroded products.