Erratum to the Influence of Gaseous Pollutants on Silver Artifacts Tarnishing, Vol. 6 (2017), 135-148

Yussri Salem

Conservation Department, Faculty of Archaeology, South Valley University, Qena, Egypt.

DOI: 10.4236/ojap.2022.113004                

Abstract

The original online version of this article Salem, Y. (2017) The Influence of Gaseous Pollutants on Silver Artifacts Tarnishing. Open Journal of Air Pollution, 6, 135-148. doi: 10.4236/ojap.2017.64011 unfortunately contains grammar mistakes. The author wishes to correct the errors.

The present work investigated the effect of common gaseous pollutants on silver artifacts. The study was carried out on coupons made of a silver alloy (91 silver and 9% copper) with chemical composition similar to ancient Egyptian silver artifacts. These coupons were exposed to gaseous pollutants such as sulfur dioxide, nitrogen dioxide, carbon dioxide, hydrogen sulfide and chlorine, each gas separately. The exposure period was four weeks inside a climate chamber with 10 PPM concentration of each gas. After each test, examinations by SEM and PM were used to evaluate the effect of each gas and observe the formed tarnish layers. The results revealed that all gases reacted with the surface except carbon dioxide. The formed tarnish layers varied in coverage and density rate, and the heaviest layer was of H₂S coupons. The tested coupons were analyzed by XRD and the results revealed Ag₂S, AgCl, Ag₂SO₄, Ag(NO₃)₃(NO)₃, AgO and Ag₂O as corrosion products.

Keywords

Silver, Artifacts, Atmospheric Corrosion, Gaseous Pollutants

The original online version of this article Salem, Y. (2017) The Influence of Gaseous Pollutants on Silver Artifacts Tarnishing. Open Journal of Air Pollution, 6, 135-148. doi: 10.4236/ojap.2017.64011 unfortunately contains grammar mistakes. The author wishes to correct the errors.

1. Introduction

Archaeological silver and its alloys have relatively high resistance against atmospheric environment corrosion compared to copper and iron objects. Nevertheless, silver is susceptible to tarnish and corrosion in the atmospheric environment especially in the presence of sulphur containing pollutants and humidity. Many corrosion products such as Ag₂S, Ag₂SO₄, and AgCl have been identified on silver objects whether in museum environment (displayed in showcases and stored in cabinets) or excavated from the burial environment. Silver objects corrosion in the atmospheric environment attributes to the reaction with the gaseous pollutants such as chloride anions, sulfates, carbonates, and sulfides that lead to the metal dissolution [1].

Among the family of ancient metals, silver has received the fewest number of studies related to the laboratory and field exposure of the gaseous pollutants. Previous laboratory exposure tests have often focused on silver tarnish with sulfur-containing pollutants [2] - [7]. These studies were in agreement with their results which confirmed the formation of silver tarnish as a black layer of Ag₂S as a main corrosion product and Ag₂SO₄ in a negligible quantity. Few laboratory studies have included the effect of other gaseous pollutants such as Cl, NO₂, and CO₂, whereas the results were different and varied. The differences were as follow:

· Results of previous studies indicated that nitrogen dioxide is not considered a corrosion factor for silver and does not react with it [8] [9] [10] [11] [12]. Conversely, a previous study identified silver nitrate Ag₂NO₃ as corrosion products on exposed silver to 1.2 ppm NO₂ for 40 hours [2].

· Ag₂SO₄ as corrosion product was detected on silver coupons exposed to outdoor environment [13] , although some studies indicated that silver sulfate (Ag₂SO₄) forms only in artificially high levels of sulfur dioxide [14] [15].

· Silver is sensitive to chloride and chlorine, and silver chloride is formed as a result of the reaction [10] [11] [12] [15] [16] [17]. On the other hand, a study revealed that silver chloride product was not identified on silver coupons after the exposure to an ASTM B117 salt spray chamber [18]. other previous studies mentioned that silver does not react directly with chlorine gas and the presence of silver chloride as a corrosion product is due to burial in a chloride-enriched environment [19] [20].

· Laboratory studies of the effect of CO₂ gas on silver artifacts are very few. although CO₂ is found as natural constituent and artificial pollutant in the atmospheric environment surrounding of silver artifacts. Ag₂CO₃ is distinguished as corrosion product of silver as result of the reaction with CO₂. the formation mechanism of Ag₂CO₃ depends on the presence of strong alkaline solutions [21]. Ag₂CO₃ product has not been detected as corrosion product on the silver artifacts, whereas it was identified in a recent study on the silver coupons exposed to various outdoor environments and this product was detected only in one site distinguished as frozen desert environment [1].

The current study, the effect of H₂S, SO₂, NO₂, Cl, and CO₂ on silver was presented as a laboratory study and the results were compared with previous studies. NO₂, Cl, and CO₂ gases did not undergo sufficient laboratory studies to evaluate their effect on the silver artifacts although they were common gaseous pollutants in outdoor and indoor atmospheric environments. Hydrogen sulfide and sulfur dioxide were tested as they are the main gases in tarnishing process of silver and lead to a silver sulfide product which is distinguished as a predominant corrosion product of silver artifacts.

2. Experimental Procedures

2.1. Coupons Preparation

Silver coupons should be similar to archaeological silver of ancient Egypt civilization. Various chemical compositions were found in Ancient Egyptian Silver. The elemental analysis of a number of Egyptian silver artefacts showed that the concentration of silver was between 83% to 90% of 10 objects and between 90% to 95% of 19 other objects [22] [23]. Also analyses of Babylonian coins from a silver copper alloy were about 87% - 90% purity [21]. Therefore, the chemical composition of the experimental coupons was Ag 89.4% and Cu 10.5%. This concentration was not artificially available and was obtained by alloying a mixture of silver (pieces from the pure silver) and pure copper pieces. This mixture was melted in a crucible and then poured into a rectangular mold. The thick rectangular rod was obtained after solidification. Therefore, successive processes of annealing and drawing on a metal rolling machine were performed to obtain thin thickness about 0.8 mm (Figure 1) [24]. The dimensions were 3 cm × 5 cm × 0.08 cm and all coupons were holed for hanging in the test chamber, Figure 2. XRF analysis was used to determine the chemical composition. Five coupons were used for each test of gas.

Figure 1. The preparation of silver coupons: (a) pure silver pieces, (b) the shape of the resulting rod after casting and (c) cutting the coupons after drawing and hammering processes.

Figure 2. Silver alloy coupons before the tests.

Thin thickness of the coupons about 0.8 m will be suitable and similar to silver artifacts thickness. many silver artefacts were manufactured from sheets whether thick or thin such as slippers, crowns, thin sheets for royal garments, hollow statues, horse saddles items, jewellery items, funeral items, cosmetic items, and Household items of daily life including spoons, jugs, cups, vessels, pots, covered wooden object, and bowls [21].

2.2. Design and Preparation of Climate Chamber

The Climate chamber was designed according to ASTM [25]. It is made of glass with dimensions of 1 cubic meter.

Humidity inside the chamber was controlled gradually by a cup of saturated salt solution. Approximately 85% RH was obtained with a saturated solution of potassium chloride [26] and the chamber was only opened to remove the coupons. A cartridge heater was placed inside the room to heat the air if necessary. The air inside the chamber was distributed by a fan, which was hanged from the ceiling of the chamber. The temperature and relative humidity inside the chamber are continuously measured by a data logger device [27] [28]. Few studies were presented on the silver deterioration tests inside climate chamber, these studies used two types of the deterioration factors: high relative humidity [10] [29] or gaseous pollutants in the presence of high relative humidity [2] [25] [29]. Most studies of silver deterioration were carried out with corrosive solutions as deterioration factors, such as BaS 5 g/l solution for 24 hours [18] , and Na₂S that was used as tarnish solution [30] [31]. Acetic acid solutions were used as simulation of emissions vapors in wooden cabinets. CuCl₂ 50 g/l for 20 min and NaCl [29] were used for AgCl patina [19].

2.3. The Test Gases

The studied gases were as follows: sulfur dioxide SO₂, nitrogen dioxide NO₂, carbon dioxide CO₂, chlorine gas Cl and hydrogen sulfide H₂S. The gas was mixed with the air inside chamber. Those gas types are the most effective in the deterioration of silver artefacts.

2.4. Test Procedures

Five coupons were exposed to humidified air (85%) containing a concentration of 10 ppm for each gas. The procedures for each test were as shown in Table 1. The test period and proportion of relative humidity, temperature, and gas concentration were chosen after a survey of such previous studies concerning ancient metals family (copper, bronze, silver, steel and lead) as shown in Table 2. Most tests shown in Table 2 were performed at room temperature (22˚C - 5˚C)

Table 1. Conditions and procedures of the tests.

Table 2. Summarizes the laboratory exposure conditions for gaseous pollutants with ancient metals in previous studies.

and relative humidity between 80 - 90, whereas the gases concentration and the exposure period were various.

Cylinders with 99% concentration were used to obtain CO₂, SO₂ and NO₂ gases. The gas has flowed from cylinder into the exposure chamber after the calculation of the flow and time of the required concentration. H₂S and Cl⁻ were prepared in lab, H₂S was prepared by the reaction of hydrochloric acid with ferrous sulfide (Equation (1)) [51] and Cl⁻ prepared by the reaction of concentrated hydrochloric acid with manganese dioxide (Equation (2)) [52]. For the required concentration of H₂S and Cl, it was obtained according to the molar volume of this concentration inside the test chamber. According to the law of Avogadro and Lussac, The molar volume occupied by the required concentration (10 ppm) inside the chamber can be calculated, Where molecular weight (one mole) of any gas under standard conditions occupies 22,400 ml (molar volume), Equation (3)) [53] , Based on the molecular weight of H₂S Equation (4) which occupies 22,400 ml, the volume molar of H₂S can be calculated Equation (5).

$FeS+2HCl\to FeC{l}_2+H_{2}S$ (1)

$4HCl+Mn{O}_2\to MnC{l}_2+2H_{2}O+C{l}_2$ (2)

1 mole of a gas at STP = 22.4 liters of a gas (3)

2 X 1 + 32 = 34 g = 22400 ml (4)

10 ppm (19 mg/m³) = X (5)

Gas syringe was used to obtain the required volume and inject its into inside the chamber Figure 3 and a small fan was used to distribute the gas inside the chamber.

3. Results and Discussion

3.1. Examination of the Samples after the Test

All coupons reacted with the gases from the first week, except the exposed coupons to carbon dioxide. The reaction behavior inside the chamber and the growth rate of the tarnish layer were similar among Cl, H₂S, and NO₂. The reaction began as a very thin tarnish layer on the surface. The surface appearance of the coupons converted to dark gray then black film, finally. The tarnish layers of H₂S and Cl were heavy, whereas were slight with SO₂, NO₂ as shown in Figure 4.

The coupons were examined after each test by visual examination, polarizing microscope, and scanning electron microscope to observe the formed tarnish

Figure 3. The syringe and the method for taking the required volume of the gas resulting from the reaction.

Figure 4. The coupons after the tests: (a) H₂S coupons, (b) Cl coupons, (c) SO₂ coupons, (d) NO₂ coupons and (e) CO₂ coupons.

layer and evaluate the reaction between the coupons surface and the test gases. The investigation results revealed that the thickness, density and coverage of the formed tarnish layer on the surface were differed among the coupons as shown in Figure 5. H₂S coupons showed heavy tarnish layer and complete coverage of the surface. Moreover, H₂S caused pitting on the coupons surface. Cl coupons were completely covered by a uniform dense layer of silver tarnish. CO₂ coupons revealed very weak effect of the gas. A slight tarnish layer with green spots was

Figure 5. Investigation by SEM and polarizing microscope ×500 show the most important characteristics of tarnish layer on the surface such as H₂S coupons are covered by dense tarnish layer (a1)-(a5); Cl coupons reveal a uniform and thick tarnish layer (b1)-(b5); CO₂ coupons reveal very weak effect of the gas (c1)-(c5); SO₂ coupons reveal the crystal structure of product of tarnish (d1)-(d5); Green spots on the surface of NO₂ coupons (e1)-(e5).

observed on the surface of NO₂ and SO₂ coupons and the reaction was very slow [35].

3.2. The Analysis of Corrosion Products by X-Ray Diffraction and Raman Spectroscopy

The coupons were exposed to X-ray diffraction to analyze the formed tarnish layer.

XRD results showed many of the corrosion products as shown in Figure 6 and Table 3. The Raman spectrum of the tarnish formed of H₂S gas showed four intensive bands in the range of 80 - 274 cm⁻¹. The bands related to silver lattice vibrations were shown at 93 and 147 cm⁻¹, the others can be assigned to Ag-S-Ag symmetric stretching mode in particular at 93, 188 and 243 cm⁻¹ with a shoulder at 273 cm⁻¹ [54]. The Raman spectrum confirmed XRD result because the bands revealed acanthite product Figure 7(a).

The Raman spectrum of Cl gas coupon showed a sharp and high intensive band at 236 cm⁻¹ and two weak bands at 145 and 349 cm⁻¹ Figure 7(b). These beaks are in agreement with the main beaks of AgCl bands, as reported in ref.

Table 3. Corrosion products on the coupons after the test.

Figure 6. XRD patterns of tarnish layers on the coupons: (a) H₂S, (b) Cl, (c) CO₂, (d) SO₂ and (e) NO₂.

[54]. The tarnish layer of other gas coupons was so slight that it was not identified by Raman.

Figure 7. Raman spectrum of tarnishing of H₂S (a)and Cl (b).

Silver sulfide (Ag₂S acanthite) is predominant product of silver artifacts and it often forms as a result of the reaction between gas H₂S and the silver surface, as shown in Equations (6) and (7).

$4Ag+O_2+2H_{2}S\to 2A{g}_{2}S+2H_{2}O$ [55] [56] (6)

$2Ag+H_{2}S\to A{g}_{2}S+H_2$ [5] (7)

Silver chloride (AgCl Cerargyrite, chlorargyrite): the prevailing theory interpreting the formation mechanism of chloroargyrite AgCl is the transformation of Ag₂O to AgCl as shown in Equation (8) [13] [14]

$A{g}_{2}O+2C{l}^{-}+H_{2}O\to 2AgCl+2O{H}^{-}$ [13] (8)

Silver sulfite (Ag₂SO₄): This product was identified in a previous study as a corrosion product of silver [1]. The Equations (9)-(12) were suggested for the formation mechanism of Ag₂SO₄ on the coupons surface.

$2A{g}^{+}+2O{H}^{-}\to A{g}_{2}O+H_{2}O$ (9)

$O{H}^{-}+S{O}_2\to HS{O}_3^{-}$ (10)

${\text{Ag}}_{\text{2}}\text{O}+{\text{HSO}}_3^{-}\to {\text{Ag}}_{\text{2}}{\text{SO}}_{\text{3}}+{\text{OH}}^{-}$ (11)

$A{g}_{2}S{O}_3\to A{g}_{2}S{O}_4$ (12)

Silver oxide Ag₂O: Silver artifacts react with oxygen whether by the electrochemical reactions in the presence of humidity (Equations (13)-(15)) or in the absence of humidity (Equation (16)). Therefore, Ag₂O and AgO are often formed on silver artifacts surfaces.

$Ag\to A{g}^{+}+e^{-}$ (13)

$\frac{1}{2}{O}_2+H_{2}O+e^{-}\to 2O{H}^{-}$ (14)

$2A{g}^{+}+2O{H}^{-}\to A{g}_{2}O+H_{2}O$ (15)

$Ag+\frac{1}{2}{O}_2\to A{g}_{2}O$ (16)

Silver ammine nitrate (Ag(NO₃)₃(NO)₃): a previous laboratory study of silver coupons exposed to NO₂ gas identified Ag₂NO₃ product as a corrosion product of silver from nitrates anions [2] , Therefore, Ag₂NO₃ was expected to be a corrosion product of the tarnish layer of NO₂ gas, whereas analysis showed a Silver ammine nitrate product (Ag(NO₃)₃(NO)₃).

Copper Nitrate Hydroxide (Rouaite, Cu₂(NO₃)(OH)₃): Silver and copper are the coupons alloy elements. The formation of rouaite corrosion product is contributed to selective corrosion of copper by the reaction of with NO₂ gas.

4. Conclusions

In atmospheric environment, the silver artifacts are susceptible to the reaction with several air pollutants and a black tarnish layer is formed on a surface. The tarnish layer is heavily formed in the presence of sulfur containing pollutants and other gaseous such as Cl and NO₂.

Except for the CO₂ coupons, all tested gas coupons showed a tarnish layer which was formed on the surface as a blackish tarnish layer. The coupons of H₂S and Cl showed a sever tarnish layer, whereas the coupons of SO₂ and NO₂ showed slight tarnish layer. The XRD analysis revealed several corrosion products and Raman confirmed the identification Ag₂S and AgCl.

Acknowledgements

I would like to thank Dr. Mai Rifai Con. Dep. Arch. Fac. Cai. Uni, for all assistance and support, Thanks also need to go to Dr. Adel Abdelkader Chem. Dep. Sci. Fac. Sou. Val. Uni. and prof. A. A. Shakour, Air Pollution Department, National Research Center, Egypt, for their helpful to adjust the gases concentration inside the chamber test.

Founding

This research was founded by South Valley University (Egypt).

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References

[1]	Sanders, C.E., Verreault, D., Frankel, G.S. and Allen, H.C. (2015) The Role of Sulfur in the Atmospheric Corrosion of Silver. Journal of the Electrochemical Society, 162, 630-637. https://doi.org/10.1149/2.0051512jes
[2]	Kim, H. (2003) Corrosion Process of Silver in Environments Containing 0.1 ppm H2S and 1.2 ppm NO2. Materials and Corrosion, 54, 243-250. https://doi.org/10.1002/maco.200390053
[3]	Kleber, C., Wiesinger, R., Schnller, J., Hilfrich, U., Hutter, H. and Schreiner, M. (2008) Initial Oxidation of Silver Surfaces by S2- and S4-Species. Corrosion Science, 50, 1112-1121. https://doi.org/10.1016/j.corsci.2007.12.001
[4]	Pope, D., Gibbens, H.R. and Moss, R.L. (1986) The Tarnishing of Silver at Naturally Occurring H2S and SO2 Levels. Corrosion Science, 8, 883-887. https://doi.org/10.1016/S0010-938X(68)80141-6
[5]	Graedel, T.E., Franey, J.P., Gualtieri, G.J., Kammlott, G.W. and Malm, D.L. (1985) On the Mechanism of Silver and Copper Sulfidation by Atmospheric H2S and OCS. Corrosion Science, 25, 1163-1180. https://doi.org/10.1016/0010-938X(85)90060-5
[6]	Derdall, G. and Hyne, J.B. (1979) The Production of H2S by Hydrolysis of Entrained COS in Hydrocarbon Liquids. Canadian Journal of Chemical Engineering, 57, 112-114. https://doi.org/10.1002/cjce.5450570119
[7]	Lin, H. and Frankel, G.S. (2013) Accelerated Atmospheric Corrosion Testing of Ag. Corros, 69, 1060-1072. https://doi.org/10.5006/0926
[8]	Guinement, J. and Fiaud, C. (1986) Laboratory Study of the Reaction of Silver and Copper with Some Atmospheric Pollutants. Proceedings of the 13th ICEC Conference, Pattaya, 28-29 December 2017, 383-390.
[9]	Abbott, W.H. (1987) The Development and Performance Characteristics of Mixed Flowing Gas Test Environments. IEEE Transactions on Components, Hybrids, and Manufacturing Technology, 11, 22-35.
[10]	Graedel, T.E. (1992) Corrosion Mechanisms for Silver Exposed to the Atmosphere. Journal of the Electrochemical Society, 139, 1963-1970. https://doi.org/10.1149/1.2221162
[11]	Abbott, W.H. (1974) Effects of Industrial Air Pollutants on Electrical Contact Materials. IEEE Transactions on Parts, Hybrids, and Packaging, 10, 24-27. https://doi.org/10.1109/TPHP.1974.1134830
[12]	Myers, M. (2009) Overview of the Use of Silver in Connector Applications. Technical Paper, Interconnection & Process Technology Tyco Electronics, Harrisburg, PA, 503-516.
[13]	Wan, Y., Wang, X., Wang, X., Li, Y., Sun, H. and Zhang, K. (2015) Determination and Generation of the Corrosion Compounds on Silver Exposed to the Atmospheres. International Journal of Electrochemical Science, 10, 2336-2354.
[14]	Abbott, W.H. (1968) The Influence of Environment on Tarnishing Reactions. Proceedings of the 4th ICEC Conference, Paris, 1968, 35-39.
[15]	Rice, D., Peterson, P., Rigby, E., Phipps, P., Cappell, R. and Tremoureaux, R. (1981) Atmospheric Corrosion of Copper and Silver. Journal of the Electrochemical Society, 128, 275-284. https://doi.org/10.1149/1.2127403
[16]	Liang, D., Allen, H.C., Frankel, G.S., Chen, Z.Y., Kelly, R.G., Wu, Y. and Wyslouzil, B.E. (2010) Effects of Sodium Chloride Particles, Ozone, UV, and Relative Humidity on Atmospheric Corrosion of Silver. Journal of the Electrochemical Society, 157, 146-156. https://doi.org/10.1149/1.3310812
[17]	Ingo, G.M., Angelini, E., Riccucci, C., de Caro, T., Mezzi, A., Faraldi, F., Caschera, D., Giuliani, C. and Di Carlo, G. (2015) Indoor Environmental Corrosion of Ag-Based Alloys in the Egyptian Museum (Cairo, Egypt). Applied Surface Science, 326, 222-235. https://doi.org/10.1016/j.apsusc.2014.11.135
[18]	Wan, Y., Macha, E.N. and Kelly, R.G. (2012) Modification of ASTM B117 Salt Spray Corrosion Test and Its Correlation to Field Measurements of Silver Corrosion. Corrosion, 68. https://doi.org/10.5006/1.3693699
[19]	Novakovic, J., Georgiza, E. and Vassiliou, P. (2013) Nano-Alumina Modified Acrylic Coatings for Silver Protection. School of Chemical Engineering, NTUA, Athens, 23-25.
[20]	Al-Saad, Z. and Bani Hani, M. (2007) Corrosion Behavior and Preservation of Islamic Silver Alloy Coins.
[21]	Vassilio, P. and Gouda, V. (2013) Ancient Silver Artefacts: Corrosion Processes and Preservation Strategies. Corrosion and Conservation of Cultural Heritage Metallic Artefacts. In: European Federation of Corrosion (EFC) Series, Woodhead Publishing Limited and CRC Press, Oxford, 213-235. https://doi.org/10.1533/9781782421573.3.213
[22]	Gale, N.H. and Stos-Gale, Z.A. (1981) Ancient Egyptian Silver. The Journal of Egyptian Archaeology, 67, 103-115. https://doi.org/10.2307/3856605
[23]	Lucas, A. (2011) Ancient Egyptian Materials and Industries. 4th Edition, Dover Publications, London.
[24]	Abu-Baker, A.N., MacLeod, I.D., Sloggett, R. and Taylor, R. (2013) A Comparative Study of Salicylaldoxime, Cysteine and Benzotriazole as Inhibitors for the Active Chloride-Based Corrosion of Copper and Bronze Artifacts. European Scientific Journal, 9, 1857-7881.
[25]	ASTM D5116 (1997) Standard Guide for Small-Scale Environmental Chamber Determinations of Organic Emissions from Indoor Materials/Products.
[26]	Young, J.F. (1967) Humidity Control in the Laboratory Using Salt Solutions—A Review. Journal of Applied Chemistry, 17, 241-245. https://doi.org/10.1002/jctb.5010170901
[27]	Kim, M.N., Yu, H.S. and Lee, S.E. (2003) A Small Chamber Test and Oddy Test on Medium Density Fiberboard Grade (E0, E1). Indoor Air Quality in Museums and Historic Properties University of East Anglia, Norwich.
[28]	Kim, S., Kim, J.A., An, J.Y., Kim, H.J., Kim, S.D. and Park, J.C. (2007) TVOC and Formaldehyde Emission Behaviors from Flooring Materials Bonded with Environmental-Friendly MF/PVAc Hybrid Resins. Indoor Air, 17, 404-415. https://doi.org/10.1111/j.1600-0668.2007.00488.x
[29]	Wiesinger, R., Martina, I., Kleber, C. and Schreiner, M. (2013) Influence of Relative Humidity and Ozone on Atmospheric Silver Corrosion. Corrosion Science, 77, 69-76. https://doi.org/10.1016/j.corsci.2013.07.028
[30]	Bernard, M.C., Dauvergne, E., Evesque, M., Keddam, M. and Takenouti, H. (2005) Reduction of Silver Tarnishing and Protection against Subsequent Corrosion. Corrosion Science, 47, 663-679. https://doi.org/10.1016/j.corsci.2013.07.028
[31]	Lin, H., Frankel, G.S. and Abbott, W.H. (2013) Analysis of Ag Corrosion Products. Journal of the Electrochemical Society, 160, 345-355. https://doi.org/10.1149/2.055308jes
[32]	Franey, J.P., Kammlott, G.W. and Graedel, T.E. (1985) The Corrosion of Silver by Atmospheric Sulfurous Gases. Corrosion Science, 25, 133-143. https://doi.org/10.1016/0010-938X(85)90104-0
[33]	Sasaki, T., Kanagawa, R., Ohtsuka, T. and Miura, K. (2003) Corrosion Products of tin in Humid Air Containing Sulfur Dioxide and Nitrogen Dioxide at Room Temperature. Corrosion Science, 45, 847-854. https://doi.org/10.1016/S0010-938X(02)00151-8
[34]	Tran, T.T.M., Fiaud, C. and Sutter, E.M.M. (2005) Oxide and Sulphide Layers on Copper Exposed to H2S Containing Moist Air. Corrosion Science, 47, 1724-1737. https://doi.org/10.1016/j.corsci.2004.08.019
[35]	Seo, M., Ishikawa, Y., Kodaira, M., Sugimoto, A., Nakayama, S., Watanabe, M., Furuya, S., Minamitani, R., Miyata, Y., Nishikata, A. and Notoya, T. (2005) Cathodic Reduction of the Duplex Oxide Films Formed on Copper in Air with High Relative Humidity at 60°C. Corrosion Science, 47, 2079-2090. https://doi.org/10.1016/j.corsci.2004.09.016
[36]	Niklasson, A., Johansson, L.G. and Svensson, J.E. (2007) Atmospheric Corrosion of Lead: The Influence of Formic Acid and Acetic Acid Vapors. Journal of the Electrochemical Society, 154, 618-625. https://doi.org/10.1149/1.2775173
[37]	Lenglet, M., Lopitaux, J., Leygraf, L., Odnevall, I., Carballeira, M., Noualhaguet, J.C., Guinement, J., Gautier, J. and Boissel, J. (1995) Analysis of Corrosion Products Formed on Copper in Cl2/H2S/NO2 Exposure. Journal of the Electrochemical Society, 142, 3690-3696. https://doi.org/10.1149/1.2775173
[38]	Astrup, T., Wadsak, M., Leygraf, C. and Schreinerb, M. (2000) In Situ Studies of the Initial Atmospheric Corrosion of Copper Influence of Humidity, Sulfur Dioxide, Ozone and Nitrogen Dioxide. Journal of the Electrochemical Society, 147, 2543-2551. https://doi.org/10.1149/1.1393566
[39]	Samie, F., Tidblad, J., Kucera, V. and Leygraf, C. (2005) Atmospheric Corrosion Effects of HNO3-Method Development and Results on Laboratory Exposed Copper. Atmospheric Environment, 39, 7362-7373. https://doi.org/10.1149/1.1393566
[40]	Rickett, B.I. and Payer, J.H. (1995) Composition of Copper Tarnish Products Formed in Moist Air with Trace Levels of Pollutant Gas: Hydrogen Sulfide and Sulfur Dioxide/Hydrogen Sulfide. Journal of the Electrochemical Society, 142, 3723-3728. https://doi.org/10.1149/1.2048404
[41]	Rickett, B.I. and Payer, J.H. (1995) Composition of Copper Tarnish Products Formed in Moist Air with Trace Levels of Pollutant Gas: Sulfur Dioxide and Sulfur Dioxide/Nitrogen Dioxide. Journal of the Electrochemical Society, 142, 3713-3722. https://doi.org/10.1149/1.2048403
[42]	Tétreault, J., Cano, E., Bommel, M., Scott, D., Dennis, M., Barthés, L., Minel, L. and Robbio, L. (2003) Corrosion of Copper and Lead by Formaldehyde, Formic and Acetic Acid Vapours. Studies in Conservation, 48, 237-250. https://doi.org/10.1179/sic.2003.48.4.237
[43]	Lopez-Delgado, A., Cano, E., Bastidas, J. and López, F. (2001) A Comparative Study on Copper Corrosion Originated by Formic and Acetic Acid Vapours. Journal of Materials Science, 36, 5203-5211. https://doi.org/10.1023/A:1012497912875
[44]	Samie, F., Tidblad, J., Kucera, V. and Leygraf, C. (2007) Atmospheric Corrosion Effects of HNO3-Comparison of Laboratory-Exposed Copper, Zinc and Carbon Steel. Atmospheric Environment, 41, 4888-4896. https://doi.org/10.1016/j.atmosenv.2007.02.007
[45]	Castano, J.G., de la Fuente, D. and Morcillo, M. (2007) A Laboratory Study of the Effect of NO2 on the Atmospheric Corrosion of Zinc. Atmospheric Environment, 41, 8681-8696. https://doi.org/10.1016/j.atmosenv.2007.07.022
[46]	Oesch, S. and Faller, M. (1997) Environmental Effects on Materials: The Effect of the Air Pollutants SO2, NO2, NO and O3 on the Corrosion of Copper, Zinc and Aluminium. A Short Literature Survey and Results of Laboratory Exposures. Corrosion Science, 39, 1505-1530. https://doi.org/10.1016/S0010-938X(97)00047-4
[47]	Strandberg, H. and Johansson, L.G. (1997) Role of O3 in the Atmospheric Corrosion of Copper in the Presence of SO2. Journal of the Electrochemical Society, 144, 2334-2342. https://doi.org/10.1149/1.1837814
[48]	Eriksson, P. and Johansson, L.G. (1986) The Role of NO2 in the Atmospheric Corrosion of Different Metals. Proceeding of 10th Scandinavian Corrosion Congress, Stockholm, 43.
[49]	Campin, M.J. (2003) Microstructural Investigation of Copper Corrosion: Influence of Humidity. Ph.D. Dissertation, New Mexico State University, Las Cruces.
[50]	Mariaca, L., de la Fuente, D., Feliu, S., Simancas, J., Gonzalez, J.A. and Morcillo, M. (2008) Interaction of Copper and NO2: Effect of Joint Presence of SO2, Relative Humidity and Temperature. Journal of Physics and Chemistry of Solids, 69, 895-904. https://doi.org/10.1016/j.jpcs.2007.10.003
[51]	Wikipedia (2017) Hydrogen Sulfide. https://en.wikipedia.org/wiki/Hydrogen_sulfide
[52]	Wikipedia (2017) Chlorine. https://en.wikipedia.org/wiki/Chlorine#cite_ref-Greenwood789_7-1
[53]	Vonderbrink, S.A. (2006) Laboratory Experiments for Advanced Placement Chemistry. 2nd Edition, Flinn Scientific, Inc., Batavia, 87.
[54]	Martina, I., Wiesinger, R., Simbürger, D.J. and Schreiner, M. (2012) Micro-Raman Characterization of Silver Corrosion Products: Instrumental Set Up and Reference Darabase. Preservation Science, 9, 1-8.
[55]	Sharma, S.P. (1978) Atmospheric Corrosion of Silver, Copper, and Nickel-Environmental Test. Journal of the Electrochemical Society, 125, 2005-2011. https://doi.org/10.1016/j.jpcs.2007.10.003
[56]	Volpe, L. and Peterson, P.J. (1989) The Atmospheric Sulfidation of Silver in a Tubular Corrosion Reactor. Corrosion Science, 29, 1179-1196. https://doi.org/10.1016/0010-938X(89)90065-6