The paper roots in the production of the mint. Silver tarnishing affects silver coin collection. As a noble metal, silver is good in decorate, electro conduction, reflection, anticorrosion and have a silver white appearance. But once silverware exposed in industry atmosphere or gas involving sulphur, it would be tarnishing. Treating with silver surface could be good for silver anti-tarnishing. For pure silver goods, coating protection film is a logical method. Some thiol compounds and other new technology bring a new way for silver anti-tarnishing.
In this way, this paper adopt sodium sulfide as the accelerate medium and study the tarnishing mechanism of silver in sodium sulfide solution and the effect of the roughness on silver tarnishing. Besides, choose an appropriate way to protect silver from tarnishing.
Morphological check of samples and analysis of corrosion products, electrochemical methods of polarization and electrochemical impedance spectrum has been used to study the behavior of silver in sodium sulfide solution, the effect of the roughness and choose a way of anti-tarnishing. The result shows that the coverage of surface grains result in the variation of silver luster. The thiner of the surface grains, the lighter-colored. Besides, the diluter of the solution, the smaller of the coverage grains. The corrosion potential and occurrence are variable. In different sodium sulfide solution, the mechanism is the same.
In sodium sulfide solution, surface roughness could affect silver sulfide grains form, grow, and deposit. The effect is referring to the grain size. If the roughness is close to the grain size, the effect is remarkable. In all kinds of surface, 1500# sand paper polished surface is anti-tarnishing best. Polishing surface is easiest to tarnish. Besides, 150#, 400#, 800# sand paper polished surfaces are in the middle. Different roughness surface production all were silver sulfide. Electro chemical analysis also shows the same results as before. Surface roughness could affect silver tarnish for sure.
The silver specimen has been immersed in an isopropanol solution with 0.01mol/L C18H35SH during 2h at 30℃. Then silver surface forms a protection film. This film is transparence and is an ideal coating material. The silver specimen with protection film tarnished lightly in 0.01mol/L sodium sulfide. The result is similar with chromate passivation. Electrochemical results show that immersing in an isopropanol solution with 5×10-3 or 8×10-3mol/L C18H35SH during 0.5h at 30℃ could obtain a best coating film.
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