银器文物防变色缓蚀作用机理的研究

为探讨银器文物防变色缓蚀作用的机理,用AES和XPS方法研究了缓蚀剂在银表面成膜的厚度和各组分的化学状态,测定了PMTA、MBO、MBI的普通拉曼光谱(NRS),利用表面增强拉曼散射光谱(SERS)研究几种唑系缓蚀剂防止银器文物变色的表面成膜过程,复合缓蚀剂组分与银的相互作用,并讨论了其防变色作用的机理。实验结果表明,PMTA、MBI和MBO与银表面存在着较强的相互作用,由于杂环结构的差异,不同缓蚀剂在银表面上呈现不同的吸附取向,形成了更为致密的防变色保护膜。在成膜溶液中,pH值较小时有利于缓蚀剂吸附,处理时间应不少于120min。成膜溶液和含缓蚀剂的硫化钠溶液中,银电极阴极极化时(外加阴极电位≤600mV的测试范围内),复合缓蚀剂的SERS响应均随电位负移而增强,表明其可有效防止银在腐蚀介质中的变色行为。

Study on the mechanism of anti - tranishing for silver antiques

Silver antiquities can be tarnished easily by the actions of oxygen, sulfur compounds, humid and ultra - violet light in atmosphere. Silver tarnishing could result in pattern unclear on the surface of ancient artifacts. The composite corrosion inhibitors(PMM), composed of several azoles compounds such as 1 - phe-ny 1- 5 - mercaptotetrazole ( PMTA ) , 2 - mercaptobenzoxazole ( MBO ) and 2 - mercaptobenzimidazole (MBI), were used to film pretreatment that could improve the antitamishing abilities of silver antiquities obviously.The anti - tarnishing films were studied by using Auger Electron Spectroscopy (AES) and X - ray Photoemission Spectroscopy(XPS) . Test results showed that the synergistic effect of silver anti - tarnishing was produced after treated with PMM, the anti - tarnishing film formed by PMM was more compact and stable than that of mono- component inhibitor, so the composite ramishing inhibitors (PMM) were much more effective. This kind of anti - tarnishing film was formed by the interaction between silver surface atoms and corrosion inhibitors, the thickness of the composite film is about 20nm, in which the main constitution was PMTA - Ag surface complex,MBO and MBI could promote the interaction between silver and PMTA.The normal Raman spectra (NRS) of PMTA,MBO and MBI were measured. Surface - enhanced Raman scattering (SERS) was also used to study the surface filming processes of these azoles corrosion inhibitors for anti - tarnishing of silver, and the interactions between components of composite corrosion inhibitor and silver surface. The mechanism of anti - tarnishing of silver was also discussed. The experiments results show that there are strong interactions between corrosion inhibitors and silver surface. As constituents of the composite inhibitor with different cycle configuration took on different adsorption directions on the silver surface, the much more compact anti - tarnishing film was formed. In the pretreatment solution , weak acid pH value is beneficial to the adsorption of corrosion inhibitor and pretreatment time should not less than 120 min. Under the condition of cathodic polarization with additional potential less than 600mV in the test, in the pretreatment solution and the solution of sodium sulfide, the SERS response of composite corrosion inhibitors became stronger as the potential became lower. It is proved that the composite corrosion inhibitors can prevent silver tarnish effectively from corrosion medium.