银纪念币在大气环境中表面发生变色,失去原有的金属光泽.采用OM,SEM,EMPA,XPS,XRD等现代物理测试技术对变色的银纪念币进行分析.结果表明,银纪念币的变色部位呈均匀的浅褐色,并随机分布着深褐色的斑点.变色银纪念币表面沿着划痕密集分布着蚀孔.表面分析显示,除Ag外,还有S和O元素.XPS,XRD分析发现,变色银纪念币表面存在Ag2S,Ag2SO3和Ag2O.实验室加速变色试验表明,在含S气体环境中,Ag被氧化生成Ag2S,导致银纪念币表面变色.从而验证银纪念币的变色是由电化学腐蚀引起的,S和O元素参与了腐蚀历程.
In natural environment, tarnish was observed on the surface of a commemoration silver coin of China. In order to identify the nature of the tarnishing film, optical microscope(OM), scanning electron microscope(SEM), electron microprobe analyses(EMPA), X-ray photoelectron spectroscopy(XPS), and X-ray diffraction(XRD) were applied to examine the tarnishing areas. It was found that the tarnishing position is uniformly moderate brown and several dark brown spots are randomly distributed. Groups of corrosion holes are distributed over the forging stress zones formed in the struck process. As result of surface analyses using EMPA, sulfur and oxygen were detected besides silver with uniformity of sulfur. Furthermore,distributions of silver and oxygen were in correspondence with the morphology of dark brown spots. XPS and XRD showed that components of spots are Ag2S, Ag2O and Ag2SO3, with a predominance of the first one. It could be estimated that the tarnish of silver is a series of electrochemical process that requires the presence of sulfur and oxygen.
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