In order to investigate the gold coin's tarnish mechanism, the tarnish and corrosion mechanism on Chinese gold coins was investigated by optical microscopy (OM), scanning electron microscope energy dispersive spectrometry (SEM-EDS), electron microprobe analysis (EMPA), XPS and X-ray diffraction (XRD). The results indicated that the tarnish on the gold coin was caused by silver on its surface. The main component of the tarnish stain on the gold coin was silver sulfide and silver sulfate. The stain and the contamination originate from the polishing procedure, which accelerated the tarnish process. Two measures can be taken to prevent the gold coin's tarnish. First, the mintage of gold and silver coins should be separated and, second, a chemical cleaning procedure should be introduced.
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