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Tarnishing Mechanism and Anti-tarnishing Techniques of Gold and Silver Coins

Author: YangChangJiang
Tutor: LiangChengHao
School: Dalian University of Technology
Course: Chemical Engineering
Keywords: Gold Coin Silver Coin Tarnish Passivation Self-assembled Monolayers
CLC: TG172
Type: PhD thesis
Year: 2008
Downloads: 241
Quote: 3
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Abstract


In natural environment, the phenomenon of tarnish is observed on the surface of some gold and silver coins. It is a serious problem in mint, museum, numismatics and archaeology for alteration of the original esthetical aspect. Surface treatments are important anti-tarnishing techniques for historicl gold and silver coins which are invaluable in art and numismatic fields.In order to know the characteristic of the tarnished film, different analysis techniques are used to investigate the surface of the tarnished coins, which included a panda gold coin of China, a commemoration silver coin of 45th anniversary of aviation industry foundation of China and a commemoration silver coin of resistting US aggression aidding Korea and safeguarding their homeland of DPRK. These analysis techniques are optical Microscopy (OM), Scanning Electron Microscope (SEM), Electron Microprobe Analysis (EMPA), X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD). It is found that the components of red stains on the gold coin are Ag2S and Ag2SO4. The former is much predominant than the latter. The elements of C, O, Mg and Si introduced in the polishing procedure accelerate the tarnishing process.As for the tarnish silver coin, the surface behaves uniformly moderate brown except several dark brown spots randomly distributed. The components of the tarnish parts are mainly Ag2S, which is companied by little of Ag2O and Ag2SO3. The results are confirmed by the accelerated tarnish test of silver coins.The cathode reduction method could not only recover the original lustrous and esthetics from the tarnished gold and silver coins, but also preserve their appearance. So, it is an important method to recover tarnished coins, especially for the invaluable historic coins.When silver metal is exposed to environment containing sulfur contamination, tarnishing will happen and Ag2S forms on the surface. With the thickness of Ag2S increasing, the color of silver gradually changes from silvery to pale yellow, brown, seal brown, purple blue and stone yellow. In laboratory, the exposure tests have been performed to determine the effect of the surface state, relative humidity, temperature and illumination. The results indicate that mechanical polishing and illumination accelerate the sulfuration of silver. The tarnish rate slightly increases with the relative humidity increasing, and accelerates as a function of temperature. A tarnish dynamic model is proposed based on these results. The relationship between the constants of the tarnish reaction and temperature is lnk=-10968.4/T+28.76, and the apparent activation energy is 91.2kJ/mol. The tarnish reaction of silver is chemical reaction control, which is in accordance with the result deduced from the dynamic model.A new chemical passivation technique was developed to protect silver from tarnishing. The chemical passivation solution is composed of 20g/L hydrogen peroxide, 3.75g/L sodium silicate, 1.25g/L sodium carbonate, 3.75g/L lactic acid and 0.75g/L 8- hydroxyquinoline. The optimize temperature is 30℃. The anti-tarnishing time of the treated silver reaches to 3 min and is 8 times longer than the blank one in draping test, and 4 times longer than the blank one in accelerated test. The treatment retards the depolarization of oxygen in cathodic reaction and sulfuration of silver in anodic reaction.The formation of SAMS in aqueous solution having octadecanethiol provides a practical method to protect silver from tarnishing without organic solvent. The monolayer films provided resistance to atmospheric without affecting the appearance of the silver. The psychometric lightness L value, the chromaticity coordinates, and the reflectance of the SAMs covered silver coin is similar to that of the blank one. The thiols transfer to the silver surface by the surfactants. The thickness and the average angle between the transition dipole moment and the surface normal are influenced by the surfactant, which are 0.9-1.36nm in and 51-62°, respectively. Electrochemical tests and accelerated tarnishing tests indicated that the SAMs provide significant tarnish resistance. The kinetic process of forming SAMs in micellar aqueous solution can be divided into 2 steps. The first one is adsorption of thiol which fits to the second-order Langmuir model, the other one is rearrangement of adsorbed thiol which fits to the first-order Langmuir model. The relationship between constants of the tarnish reaction and temperature is lnk=-5826/T+26.5, and the apparentactivation energy is 48.4kJ/mol.With the concentration of thiol and surfactant increasing, the anti-tarnishing quality of SAMs is improved. Compared SAMs from CTAB solution with SAMs from organic solvent, SAMs of phytic acid and silicate tungstenic acid, it is found that the SAMs from CTAB is similar to that from organic solvent and has more excellent inhibition than SAMs of phytic acid and silicate tungstenic acid.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Various types of metal corrosion
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