Tarnishing of silver in environments with sulphur contamination
The Authors
Chang Jiang Yang, School of Chemical Engineering, Dalian University of Technology, Dalian, China
Cheng Hao Liang, School of Chemical Engineering, Dalian University of Technology, Dalian, China Electromechanics and Materials Engineering, Dalian Maritime University, Dalian, China
Xia Liu, School of Chemical Engineering, Dalian University of Technology, Dalian, China
Abstract
Purpose – This paper seeks to investigate work on silver sulphide formed on the surface of silver metal which was exposed to an environment containing sulphur contamination.
Design/methodology/approach – Laboratory tests were performed to determine the effect of the surface state, relative humidity, temperature and illumination.
Findings – It was indicated that sulphuration of silver was accelerated by mechanical polishing and illumination. The exposure was performed at 0, 54, 75 per cent relative humidity (rH) and at 16, 25, 37, 47 and 57°C. The tarnish rate at 54 per cent rH was more than two times than that at 0 per cent rH or 75 per cent rH, and the rate increased as a function of temperature.
Originality/value – A tarnish model was proposed. The tarnish reaction of silver is chemical reaction controlled, which is in accordance with the result deduced from the model.
Article Type:
Research paper
Keyword(s):
Silver; Sulphides; Tarnish; Corrosion.
Journal:
Anti-Corrosion Methods and Materials
Volume:
54
Number:
1
Year:
2007
pp:
21-26
Copyright ©
Emerald Group Publishing Limited
ISSN:
0003-5599
1 Introduction
Silver is an attractive, lustrous, noble metal whose electrical and thermal conductivity are the highest of any of the elements. Silver and its alloys possess attractive electrical and mechanical properties. As a consequence, of these characteristics, silver has a wide array of applications in electronics, solder, batteries, conductive pastes, dental alloys, silverware, jewellery and decorative objects of various kinds. However, silver often tarnishes when exposed to indoor or outdoor atmospheres containing H2S, SO2, NO2, Cl2, as well as ammonia and a whole ill-defined range of organic species.
Considerable experience and a number of analytical techniques have been explored over the years concerning silver's tarnish and found sulphuration of silver caused tarnishing to occur (Fukuda and Fukushima, 1991; Vassiliou and Dervos, 1999; Mehan and Mansingh, 2000; Hindin and Fernandez, 2003). Tarnish means a darkening of the surface of silver and rendering it aesthetically unacceptable. Furthermore, tarnish can form an insulating layer on the contact surface, thus causing trouble in the function of electrical devices.
In recent years, a number of authors studied mechanisms of the tarnishing of silver. Fang and Cai (1988) and Kim (2003) showed that silver will not tarnish without oxygen. Other environmental factors effecting the tarnishing of silver include H2S (Mehan and Mansingh, 2000; Kim, 2003), sulphur (Reagor and Sinclair, 1981), organic sulphur (Sinclair, 1982), SO2 (Mao and Tian, 1995), relative humidity (rH) (Fang and Cai, 1988) and illumination (Mao and Tian, 1995). However, the understanding of the chemical processes involved in tarnishing remains rudimentary. A better understanding of tarnishing behaviour in atmospheric environments is needed.
In the present work, tarnishing of silver was investigated in accelerated tests. The goal was to study the influence of climatic parameters such as the rH and temperature on the tarnishing behaviour of silver. The influence of illumination was investigated as well as the surface state of silver (as-received and polished). A model was proposed on the tarnishing reaction kinetics.
2 Methodology
2.1 Test materials
The composition of silver materials is given in Table I . The silver plates were machined to obtain smaller coupons (dimensions: 10 × 10 × 1 mm). These plates were embedded into epoxy resin together with a brass piece that ensured an electrical contact to the back side.
As-received and polished surfaces were tested. The polishing was performed with XL12-2 swirl flow type polisher (Longzheng Polishers, China). As received and polished samples were degreased in acetone (CH3COCH3) in an ultrasonic bath, rinsed with deionized water, cleaned with ethanol, dried carefully using nitrogen gas, and stored in dry conditions.
All reagents were analytical grade, and deionized water was used for all preparations. Conductivity was 1.41 μm/cm and pH was 6.8.
2.2 Exposure condition
Samples were placed in desiccators where a fluorescent lamp (8 W) was fixed on the top to simulate sunlight. A saturated salt solution, or “Drierite” contained in a 10 ml beaker, was placed at the bottom of the desiccator in the centre of the crystallization dish for humidity control. The salts used were Mg(NO3)2 and CH3COONa. The rH was 0, 54 and 75 per cent, respectively, (ASTM E104-02). The tarnish was accelerated by thioacetamide. The experiments were carried out at 16, 25, 37, 47 and 57°C. The samples were suspended vertically using a polytetrafluorothylene (PTFE) holder. Three replicates were used for each exposure condition, and the results are shown as mean values from all measurements.
2.3 Methods of analysis
The thickness of the tarnish film was obtained by electrolytic cathodic reduction (Fukuda and Fukushima, 1991). A conventional three-electrode electrochemical cell was used to measure potential-time curve. The working electrode was the specimen. The platinum foil was used as the counter electrode, and the reference electrode was saturated Ag/AgCl electrode. 0.1M potassium chloride (KCl) was used as the electrolyte after deoxygenating for 10 min with purified nitrogen. If a constant cathodic current was applied to the tarnish sample and its potential-time transient followed one reduction plateau near −0.7 V (vs Ag/AgCl) due to silver chloride was observed. Silver oxide is reduced at −0.05 V, while silver sulphide requires the more negative cathodic potential, −1.5 V. From the length of plateau, using Faraday's laws, film thicknesses are expressed in nanometres by the relationship: Equation 1 where T is thickness in nanometres; i is current in mA/cm2; t is time in seconds of the plateau; M is the gram-molecular weight of the species forming the film; N is the number of Faradays for reduction of 1 g mol of that species, and ρ is the film density in g/cm3.
Measurement of the surface state and roughness were performed with a profilometer Zygo View 5000 and Surfcom470A roughness instrument (Tokyo Seimitu, Japan). Both as-received and polished were investigated after degreasing of the samples.
3 Results
3.1 Morphology of tarnish film
Figure 1 shows the morphology of tarnish film after exposure at 75 per cent rH and 25°C for 8 h. There was a pale brown film on the surface of silver. Some stains with royal purple in centre were surrounded by dark brown, olive brown and red brown. The colour of film depended on its thickness. X-ray photoelectron spectrum (XPS) and X-ray diffraction (XRD) indicated the components of tarnish film were mainly silver sulphide (Yang et al., 2006).
3.2 Influence of the surface state
By inspecting the surface of both as-received and polished with a profilometer, maps of the surface topography were obtained (Figure 2). The results show that the as-received surfaces were more rough than polished. The arithmetic mean deviation of surface values determined for as-received and polished were 0.402 and 0.029 μm, respectively.
Table II shows the tarnish grade of both as-received and polished as a function of exposure time at 75 per cent rH and 25°C. A visual inspection of corroded materials conformed with the standard of GB 12335-90.
Results show that more corrosion developed upon the polished surfaces than upon the as-received ones. After a test for 30 h, many dispersed pits stains were visible upon the as-received surfaces, whereas the corrosion appeared more uniform upon the polished ones. Indeed, tarnishing of polished surfaces developed more slowly than was the case on as-received surfaces. The results were interpreted as follows:
A corrosion active surface layer resulting from the polishing process may be present on the as-received samples. Corrosion cells formed on the polished surfaces faster than on the as-received ones. After 30 h, thick tarnish films on the polished surfaces were more compact and uniform than on the as-received ones, and the film had a beneficial effect on the tarnish of silver as a diffusion barrier for attack agents. Therefore, tarnishing rates of the polished samples progressed more slowly than on as-received surfaces.
3.3 Influence of illumination
The influence of illumination on the corrosion rate on as-received specimens is shown in Figure 3 for both illuminated and in darkness at 37 and 75°C rH. The average rates of silver sulphide film formation were linear and 4.396, 5.767 nm/hr, respectively, in darkness and in illuminated conditions. The accelerated tarnish rate under illumination was in good agreement with the findings reported by Mao and Tian (1995).
3.4 Influence of relative humidity
The sulphide tarnish film thickness on as-received silver coupons that were exposed at 0, 54 and 75 per cent rH and at 37°C, respectively, are shown in Figure 4 as a function of exposure time. The data confirmed that in a closed environment, relative humidity has a linear relationship with the average thickness of silver sulphide. The average rates of silver sulphide film formation were 3.985, 10.997 and 4.396 nm/hr at 0, 54 and 75 per cent rH, respectively. As can be seen, the tarnish rate at 54 per cent rH was more than two times than that at 0 or 75 per cent rH.
It is known that rH plays an important role on the thickness of the surface electrolyte and therefore on the rate of corrosion. In the absence of a surface electrolyte film at 0 per cent rH, direct chemical attack of silver metal was a plausible explanation of the slow tarnish rate. An increase of the rH from 0 to 54 per cent resulted in an increase of the tarnish rate. It should be noted that, at 75 per cent rH, a continuous surface film of electrolyte perhaps formed and restricted the diffusion of pollutants. Therefore, the tarnish rate decreased abruptly with the rH from 54 to 75 per cent.
3.5 Influence of temperature
Figure 5 shows the sulphide tarnish film thickness on as-received silver coupons as a function of exposure time at 75 per cent rH and at 16, 25, 37, 47 and 57°C on the as-received surfaces. Clearly, there was a linear relationship between environment temperature and the average thickness of silver sulphide. The average rates of silver sulphide film formation were 0.324, 1.009, 4.396, 24.977 and 9.094 nm/hr at 16, 25, 37, 47 and 57°C, respectively. The results indicate that the tarnish rate increased from 16 to 57°C.
4 Discussion
There are many dependence factors which have effects on silver tarnish and in consequence it is difficult to give a completely dynamic theory of the tarnish mechanism. In order to further understand the dynamic mechanism, a semi-experiential model based on shrinking unreacted-core model was proposed and was shown in Figure 6.
Sulphidation of silver is a typical gas-solid reaction. The steps can be reasonably postulated as follows. First, pollutant gas is transferred through the gaseous film to the solid surface. Then, it passes through the solid product of silver sulphide, and reacts to silver at the interface. Subsequently, the gaseous product is transferred through the solid product and the gaseous film to the environment.
From the gaseous transfer equation, the diffuse rate of the gaseous reactant (A) can be obtained when it was transferred through the gas boundary layer: Equation 2 where n A,i is the diffuse rate of A in mol/s, k G,A is the mass transfer constants in cm/s, S is the diffuse area in cm2 which can be regarded as a constant for almost no change of volume of specimens. C A,b and C A,s are concentrations of the gaseous reactant (A) in bulk and at the surface of solid in mol/cm3.
For the gaseous reactant (A) passed through the solid product of silver sulphide, the diffusion process can be described by the Fick equation: Equation 3 If the diffusion of A was steady, the concentration distribution of A remains stable.
After integrating equation (2): Equation 4 where D e is the Fick constant, C A,i is the concentration of gaseous reactant (A) at the interface of reaction in mol/cm3, r 0 and r are the thickness of silver metal in initial time and final time in cm which were measured from the centre of specimens, respectively.
Indeed, the tarnish process was complex and included many reactions, for example: Equation 5 Equation 6 Equation 7 Therefore, it is feasible to postulate that the chemical reaction between silver and the pollutant is a first-order irreversible reaction with respect to the concentration of A. We can get the following reaction rate equation: Equation 8 where k is the reaction rate constant, n A,r is consumed rate of A in mol/s.
If the tarnish was steady, the quantities of every step must be equal. Then: Equation 9 where r A is reaction rate of A to consume.
Combining equations (1)–(5) and eliminating of C A,i and C A,s , we obtain: Equation 10 The tarnish reaction between pollutant agent (A) and sliver can be described as follows: Equation 11 where a, b, c and d are coefficients of reactant and product, respectively.
Then Equation 12 where r Ag2 S is the formation rate of silver sulphide. P (mol/cm3) is defined as density of the silver sulphide film. There is also: Equation 13 Combining equations (6)-(8), we get: Equation 14 and after integrating equation (9): Equation 15 The silver sulphide film thickness, R, is defined as: R=r 0−r. Substitution of R in equation (10) with r 0−r results into: Equation 16 where the first term gives the diffusion time in silver sulphide film and the second term describes the time of both diffusion in the gaseous and interface reactions. These results indicate that the silver sulphide film thickness has a parabolic relationship with time when the process is internal diffusion control and a linear relationship with time when the process is under external diffusion or chemical reaction control.
The laboratory tests suggest there is a linear relationship between the silver sulphide film thickness and time. Therefore, it can be concluded that the tarnish process is under external diffusion or chemical reaction control. Hence, first term of equation (11) can be ignored, and thus: Equation 17 where: Equation 18 The appearance reaction rate constants at various temperatures are presented in Table III.
Plotting ln K′ as a function of 1/T results in a linear correlation, as shown in Figure 7. From the Arrhenius equation, the dynamic equation is ln K′=−10968.4/T+28.76, and the appearance activation energy of tarnish reaction is 91.2 kJ/mol. It is known that there are relationships between the appearance reaction rate constant and control steps of reactions. When the appearance reaction rate constant is between 42 and 420 kJ/mol, the control step is a chemical reaction. This means the tarnish reaction of silver is under chemical reaction control, which is accordance with the result deduced from the model.
5 Conclusion
In this study, tarnish of silver was investigated in accelerated tests. The appearance and lustre of tarnish films depends on the thickness of silver sulphide film. The thicker the film, the darker it is. The tarnish of silver was influenced strongly by the surface state.
It was determined that the tarnish of silver was accelerated by mechanical polishing and illumination. The exposures were performed at 0, 54, 75 per cent relative humidity (rH) and at 16, 25, 37, 47, and 57°C. The tarnishing rate at 54 per cent rH was more than two times than that at 0 or 75 per cent rH and increased as a function of temperature.
A tarnish model was proposed. The tarnish reaction of silver is under chemical reaction control, which is accordance with the result deduced from the model.
Equation 1
Equation 2
Equation 3
Equation 4
Equation 5
Equation 6
Equation 7
Equation 8
Equation 9
Equation 10
Equation 11
Equation 12
Equation 13
Equation 14
Equation 15
Equation 16
Equation 17
Equation 18
Figure 1Morphology of the tarnish surface of as-received silver (×400)
Figure 2Surface image of (a) as-received and (b) polished silver sample
Figure 3Sulphide tarnish film formation on silver in darkness and under illumination
Figure 4Sulphide tarnish film formation on silver at 37°C
Figure 5Sulphide tarnish film formation on silver at 75 per cent rH
Figure 6 A schematic representation of the sulphidation processes of silver
Figure 7ln K′ of silver sulphidation vs 1/T
Table IThe chemical composition of silver sample
Table IIEvaluation of tarnish of both as received and polished silver
Table IIIThe appearance reaction rates constants at various temperatures
References
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Further Reading
Wu, Z., Cui, C. (1980), "Tarnish and protection of silver", Corrosion and Protection, Vol. 3 pp.20-5.
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