Tarnish films formed on copper plates exposed for a month in a hot spring area present in the atmosphere) were analyzed by coulometric reduction, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The coulometric reduction curve revealed the existence of and another phase; however, the XRD pattern showed only peaks. XPS spectra of the S 2p region indicated the coexistence of sulfide and sulfate. Two typical areas were revealed by using TEM to observe the tarnish product microstructure. One was an area of low sulfur content. The microelectron diffraction (μ-ED) pattern of this area showed diffraction spots caused by cuprite The other was an area of high sulfur content. The μ-ED pattern obtained in this area had a -induced ring pattern superimposed on a diffused pattern. This diffused pattern originates from an amorphous or amorphous-like phase that we believe contains sulfur. This phase may account for the plateau found at −0.75 V vs. Ag/AgCl in the coulometric reduction curve.
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Watanabe et al. (2002) studied this question.
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