Tarnishing Mechanism And Anti-tarnishing Techniques Of Copper And Brass Coins

In natural environment,copper and brass coins transform from their natural colors to a progressively darker brown and finally to green,which is a serious problem in mint,museum and archaeology.Because of the particularity of coins'use value,Residual sweat on surface is one of the main causes of tarnishing of coins.So the researches on tarnishing mechanism and anti-tarnishing techniques of copper and brass are of great significance.In order to investigate the tarnishing mechanism of copper and brass in simulated sweat, different kinds of analysis techniques are used,such as Scanning Electron Microscope(SEM), Optical Microscopy(OM),X-Ray Diffraction(XRD),Fourier Transform Infrared Spectrometer(FT-IR),and so on.It is found that the existence of Cl^- is the main cause of corrosion for copper and brass.For the copper in sweat,uniform corrosion occurs;but for brass in sweat,pitting and selective corrosion occur.The kinetic equation of pitting development of brass is i₀=0.3735(t+185.93)^-1/2,this process is controlled by the dissolution of salt deposit.A new anti-tarnishing method is developed to protect brass coin.The anti-tarnishing solution is composed of PMTA 0.5g/L.The optimize temperature and treating duration are 55℃and 10min,respectively.Accelerated tarnishing test and electrochemical tests results show that the brass treated with PMTA represents better anti-tarnishing property than that treated with traditional anti-tarinshing reagent KL-1.According to the results of DFT calculation,it can be inferred that PMTA(thione form) can adsorb on brass surface thtough N3 and S6 atom of molecule,and PMTA(thiol form) can adsorb on brass surface thtough N3 atom.The 3-alkyl-4-amino-5-mercapto-1,2,4-triazole(AAMT) synthesized is assembled onto copper surface,and the effects of ultrasonic irradiation,modification with alkanthiol and length of alkane to the anti-tarnishing property are studied.The 3-undecyl -4-amino-5-mercapto-1,2,4-triazole(UAMT) self assembled monolayers-forming mechanism is mainly researched.The process of UAMT SAMs formation can be divided into 2 steps:the initially rapid adsorption and the following rearrangement.According to the results of X-Ray Photoelectron Spectroscopy(XPS) and DFT calculation,it can be inferred that the electron of p orbital of S andÏ€(C=N) orbital of UAMT can be donated to band with d orbital of copper. The heterocyclic ring in molecule lie on copper surface,and the alkane exists in the form of lying on copper surface or inclining with an angle of 33°between the transition dipole moment and the surface surface.The method of preparing alkanethiol SAMs on copper surface in aqueous solution is proposed,and the SAMs-forming mechanism is researched.The process of dodecanethiol (DT) SAMs formation also contains 2 steps:the initially rapid adsorption and the following rearrangement.The initially rapid adsorption of DT obeys to the first-order Langmuir model, and the kinetic equation isθ(t)=l-e^-10.3ct.The relationship between constants of the adsorption (k_a) and temperature(T) is ln(k_a)=-5324.2/T+19.58,and the apparentactivation energy is 44.27 kJ/mol,which means the process of self-assembly is controlled by chemical reaction. Furthermore,the properties of SAMs are studied,it is found that the DT SAMs formed in aqueous mecellar solution represent comparative properties with that formed in ethanol solution,and can effectively prevent the tarnishing of copper.