Study On Protection Technology Of Gilt Relics And Silver Relics

More than20pieces of gold, silver and gilt relics were unearthed fromthe Changgan Temple underground palace of Nanjing in2008as one especialpiece reflected the highest level of silver processing technique of the SongDynasty Emperor and had high heritage value. It was covered by gilt silverplates inlaid with diamonds, which were made by hammering and modeling.The colour of Asoka Pagoda was black or gray because of Asoka Pagodasubstrate corrosion. Silver samples based on composition of the Pagodasubstrate were simulated by traditional smelting and processing technique byourselves. The corrosion potential, the corrosion current of silver samples andthe composition, the micro-shape, the size of corrosion products in differentconcentrations of NaCl and Na2S corrosion medium were examined andcompared with each other by means of electrochemical technique, XRD, XRF,SEM, EDS and XPS. Inhibitors and protective coatings were studied bygravimetric method, potentiodynamic method and electrochemical impedancespectroscopy（EIS）. Silver samples and gilt samples based on composition ofthe Pagoda were simulated by traditional smelting and processing techniqueby ourselves. There are some conclusions as follows:（1） The increase concentrations of the NaCl solutions from lower0.1（wt）%to higher3.5（wt）%led to the increase of the corrosion current density icand the corrosion potential φcwas shifted towards less noble values. So theincreasement of corrosion thermodynamical inclination and the kinetic ratewere both not great. Silver samples in corrosion passive region maintained thepassivation with the corrosion rate about71.2-308.8mm/a. Corrosion productswere AgCl and pitting corrosion presented after+0.12VSCEconstant potentialpolarized time intervals in0.1%and3.5%NaCl solution. The amount ofactive corrosion sites and the growth rate of AgCl crystals on the silver samplesurface increased with the increase of the NaCl concentrations from0.1%to3.5%, and the size of AgCl crystals was reduced from1-2um to100nm. Thecolor of corrosion product layers turned from grayish white to black. Theprism-like pentahedron structure of AgCl crystals was clearly observed in0.1%NaCl solution, and the size of AgCl particles became bigger with thepolarization time increase.（2） The increase concentrations of the Na2S solutions from lower20mg/Lto higher5%（5.0×104mg/L） led to the increase of the corrosion currentdensity icand the corrosion potential φcwas shifted towards less noble values. So the increasement of corrosion thermodynamical inclination and the kineticrate were both great. Silver samples were still in the active condition whenpolarization potential was shifted from φc（-0.31V） to2.5V in20mg/LNa2Ssolution. Current density of corrosion active region increased rapidly, and thensilver samples were switched into corrosion passive region and maintained thepassivation with higher corrosion current density in5%Na₂S solution.Corrosion products of silver were mainly Ag₂S after+0.3VSCE（20mg/LNa2S）or-0.8VSCE（5%Na₂S） constant potential polarized time intervals, respectively.The amount of S^2-as corrosion active center and the growth rate of Ag2Scrystals on the silver sample surface increased with the increase of the Na2Sconcentrations from20mg/L to5%. However, the size of Ag2S crystals wasreduced from20um to1.86-5.10nm.（3） A lauryl imidazoline was synthesized. Corrosion inhibition of LM wasstudied in Na₂S corrosion medium for silver samples by pre-filming,gravimetric method, potentiodynamic method and electrochemical impedancespectroscopy. In addition, the corrosion inhibition was compared withbenzotriazole（BTA） and1-phenyl-5-mercaptotetrazole（PMTA）. There was aconclusion that the sequence of corrosion inhibition on silver wasPMTA>LM>BTA, which was proved at the same time by the results ofgravimetric method, potentiodynamic method and EIS. The inhibition rate ofthe built inhibitor of LM and BTA exhibited synergistic effect.（4） Water-baesd acrylic latex（TX） modified nano-TiO2on the Agprotective properties was studied.The result showed than the protectiveproperties of Ag was improve by in addition in TX. The built protectivecoating of0.1%nano-TiO2with2%TX was the most prominent one for silverin5mg/L Na2S solution.（5） Silver samples were prefilmed by the built inhibitor of LM and BTAfirst and then by TX+0.1%TiO2. In this way the proctective effect on silverwas better than LM, BTA, or TX alone.（6） Gilt layer would accelerate the corrosion of silver substrate for giltsamples. For the same inhibitor or protective coating, the protective effect wasbest when it was coated on both of silver side and gilt side.