Atmospheric Corrosion of Silver and Its Relation to Accelerated Testing

Atmospheric Corrosion of Silver and Its Relation to Accelerated Testing E.B Neiser and R.G. Kelly University of Virginia [email protected] D. Liang, G. Frankel, H.C. Allen Ohio State University ABSTRACT Understanding, recreating in the lab, and predicting atmospheric corrosion would be of great assistance to anyone whose materials or structures are exposed to the atmosphere. A first step in the development of the accelerated aging tests which would provide this information must be an understanding of the corrosive agents present in a given environment and their effects on the exposed material. In this work silver is exposed both to field and laboratory conditions. Laboratory exposures controlled relative humidity, ambient atmospheric gas (i.e., air or nitrogen), and exposure to UV light. The presence of both air and UV illumination was necessary for corrosion for the limited exposure times here. A complex relationship between the amount of corrosion and relative humidity was found; increasing the relative humidity did not necessarily increase the amount of corrosion. Field exposures showed that silver chloride and silver sulfide are common corrosion products and silver chloride is the main product from coastal exposures. Coastal corrosion rates were found to be higher than inland corrosion rates. Keywords: Atmospheric corrosion, corrosion of silver, field exposures, laboratory exposures

INTRODUCTION Atmospheric corrosion is a complex problem because of the large number of variables present in the natural environment. Because of the complexities of natural corrosion, simpler systems are often necessary to provide basic understanding of corrosion mechanisms. Simpler environments are commonly created for laboratory exposures to isolate the variable or variables of interest and determine the relative importance of the variables under study[1]. In order for the laboratory exposures to provide meaningful and helpful data, corrosion obtained in laboratory exposures must resemble field corrosion otherwise the corrosive processes observed in the laboratory may differ from those active in the field. Comparisons between field exposures and lab exposures are therefore critical. Silver is a useful tool for examining both laboratory- and field-induced corrosion. For several decades silver has been exposed at many sites as part of a worldwide corrosion sensing project[2]. This has provided both a wealth of data about the amounts and types of corrosion in different regions

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and an easy way of obtaining field-exposed samples to compare with laboratory exposures and other field sites. Silver’s corrosion products are also reducible and tenacious, allowing the use of coulometric reduction to identify the corrosion products formed and the amounts of each compound[2-5]. Actually identifying the corrosion products rather than only determining the amount of corrosion may provide important information about the active corrosion mechanism not available through techniques such as mass loss which simply measure the total amount of corrosion. Previous studies have established reduction potentials of several commonly found compounds. [4] Relative humidity[6-8], concentration of ozone present in the environment[9, 10], and the presence of UV [7] have all been identified previously as factors influencing the rates and amounts of corrosion, making them good candidates for study in a controlled laboratory environment. The relative humidity (RH) of an environment establishes the thickness of the surface water layer on a metal surface. Most corrosion processes occur in this water layer, making it an important component in the understanding of corrosion. [7, 8] Ozone is formed from molecular oxygen when light with a wavelength of 242nm photolyzes molecular oxygen (O2) into two molecules of atomic oxygen. These atoms of oxygen are extremely reactive and combine quickly with molecular oxygen to form ozone (O 3). This ozone molecule is relatively unstable and readily breaks down into O2 and atomic oxygen when the molecule absorbs a photon with a wavelength of 340nm or shorter[11]. EXPERIMENTAL PROCEDURE Field Exposures To obtain real-world corrosion products, silver samples exposed at three locations were obtained from Dr. W. H. Abbott (Battelle Columbus). Pure silver samples measuring 1.3 by 7.6 cm were suspended above inert, polymeric cards. Multiple cards, containing different metallic and inert samples were then inserted into a box which allowed the free movement of gases along the longer surface of the sample. These samples were then installed at exposure stations and left for the requisite period of time. After exposure, these samples were removed and shipped for coulometric reduction and analysis. The locations studied here were Daytona Beach, Florida, a coastal location; Coconut Island, Hawaii, a region close to the shore; and West Jefferson, Ohio, located about 25 km outside of Columbus, Ohio. Laboratory Exposures The laboratory exposures were performed in one of two chambers which controlled relative humidity, the ambient atmospheric gas (i.e., air or nitrogen), and exposure to UV light. In both chambers, a large pool of saturated salt solution established the humidity for above-ambient humidities. A RH of around 65% was obtained by a saturated solution of sodium nitrite, and the approximately 85%RH was obtained with a saturated solution of potassium chloride. [12]. The lowest humidity, around 6%, was controlled by replacing the salt solution with a desiccant. For all exposures, the silver samples were arrayed above the solution or desiccant and had no contact with the solutions. After the completion of the exposure, the sample was reduced within 15 minutes of being removed from the exposure chamber. The UV light for the UV exposures was provided by a PenRay lamp. The samples were arranged so that all samples were the same distance from the light and had the same angle to the light. The UV light was controlled in a 10 hour cycle. The chamber was left sealed for two hours before the UV light was turned on to allow the chamber to reach its ambient RH and to allow the equilibrium water layers to form on the silver surfaces. After two hours, the UV light was turned on for 8 hours. At the end of the 8 hour illumination, the light was turned off, the chamber opened, a sample removed, and the chamber was then sealed again for its next two-hour rehumidification period. For the exposures in which air was the ambient gas, the humidity was controlled solely by the pool of saturated solution at the bottom of the chamber and the chamber was only opened to remove

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samples. For the nitrogen-ambient exposures, nitrogen was flowed constantly into the chamber to prevent the accidental leakage of oxygen into the chamber allowing the creation of ozone (O3). To establish a stable chamber humidity with the flowing gas, the incoming nitrogen was bubbled through the salt solution to give it the appropriate relative humidity prior to introduction to the chamber. The chamber for the exposures without UV was a simple desiccating chamber. In order to limit the exposure to all light, this chamber was isolated in a dark cabinet during the exposures. Exposures were still measured in cycles, although without the UV light, the chamber was simply opened every 10 hours for the removal of a sample for reduction. Coulometric Reduction Coulometric reduction was used to analyze corrosion products formed during field and laboratory exposures. Coulometric reduction was performed in a deaerated three-electrode cell in which the silver sample was the working electrode and a platinum-plated mesh was the counter electrode. A constant cathodic current density of 0.1mA/cm2 was applied to the sample by a Parstat 2263 potentiostat. Voltages were measured against a mercury/mercury sulfate reference electrode. The reduction electrolyte was 0.1M Na2SO4 with the pH adjusted to 10 by the drop-wise addition of 1M NaOH. Prior to reduction, nitrogen was bubbled through the electrolyte for at least an hour for deaeration. Nitrogen was also flowed through the reduction cell for 10 minutes prior to the introduction of the reduction electrolyte to remove oxygen from the cell. The electrolyte was then introduced to the cell, and current was applied within thirty seconds. Constant current was applied and the voltage was monitored until the voltage dropped to approximately -1.2VNHE, at which hydrogen was evolved. The compound was identified by the reduction potential, and the total charge passed at one potential indicated the amount of that compound present on the sample.

RESULTS AND DISSCUSSION

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Coulometric Reduction ASTM Standard B825 [5] describes the basis for the reduction method used. In the current work, the reduction electrolyte was changed from 0.1M KCl to pH 10 0.1M Na2SO4. Previous experiments [4] have shown that in chloride-containing solutions, silver oxides transformed very quickly into silver chloride. Silver chlorides also transform into silver oxides in 1M NaOH, pH 14. This is shown in figure 1 which shows three reductions, one of as-formed AgCl and two in which the AgCl has been soaked in 1M NaOH, causing transformation to silver oxide. The transformation from AgCl to silver oxide is incomplete after the 0.25 hour soak. The transformation is complete after the 1.25 hour soak. 1.25 Hour Soak in 1M NaOH 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

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Figure 1: Three reduction curves showing the transformation of silver chloride to silver oxide after soaking in a 1M NaOH solution. Figure 1a shows the reduction of as-produced silver chloride, 1b shows the reduction of silver chloride soaked in NaOH for 0.25 hours, and 1c shows the reduction of silver chloride soaked in 1M NaOH for 1.25 hours. All reductions were performed in 0.1M pH 10 Na2SO4. The soaking times were either 0.25 or 1.25 hours.

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Laboratory Exposures By exposing silver to three different relative humidity levels, 6%, 65%, and 85%, in an ambient air environment with UV irradiation, the effect of relative humidity (RH) on the amount of silver corrosion, as measured by the reduction charge, was investigated. A graph showing the reduction charges of samples exposed to different humidity levels as a function of the number of UV cycles the sample was exposed to is shown in figure 2. This also shows the increase in reduction charge with increasing exposure time.

Figure 2: Graph showing the reduction charges for three exposure relative humidities. Samples were exposed for the indicated number of 10 hour cycles at the indicated RH and then reduced. The corrosion observed here is believed to be caused by the breakdown of ozone producing atomic oxygen which oxidizes the silver. UV light with a wavelength less than 242 nm splits molecular oxygen into atomic oxygen which reacts quickly with surrounding molecular oxygen to form ozone[11]. Some ozone adsorbs onto the silver surface. Light with a wavelength less than 340 nm breaks down the fairly unstable ozone molecule into O2 and O[11]. This atomic oxygen, already close to the silver surface, is believed to attack the silver. It is interesting to note the effect of RH on the amount of corrosion. Increasing the humidity did not necessarily increase the amount of corrosion. Higher relative humidites produce thicker water layers on metal surfaces [1, 7, 8] The lowest humidity studied here would have very little adsorbed water, making the silver surface vulnerable to attack by O by providing large areas for ozone adsorbtion. Increasing the RH to 65% would produce thin or incomplete water layers which might partially protect the surface from atmospheric attack. Thicker water layers, such as those produced in an environment of 85% RH might dissolve ozone and produce other oxidative species upon the photoyzation of the dissolved ozone[13]. Laboratory exposures also investigated the effect of different atmospheric gases and UV lighting conditions on silver corrosion. Samples were exposed in one of three environments with the same RH: Ambient air atmosphere with UV illumination, ambient air without UV illumination, and nitrogen with UV illumination. The reduction charges for samples exposed to each of these three environments for two cycles of UV illumination are shown in Figure 3. The most corrosion was evident on samples which had been exposed to UV in air. This is believed to be due to the presence of ozone in this

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Reduction Charge, mC/cm

environment. The amounts of corrosion evident on the sample exposed in nitrogen and the sample with no UV exposure were similar to the amount of corrosion observed on a freshly prepared sample, suggesting that no corrosion had occurred on either of these samples.

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Figure Three: Reduction charges of samples exposed to two 10-hour cycles in each of the three test environments, air with UV, air without UV, and nitrogen with UV. The reduction charges of the air without UV and the nitrogen with UV were very similar to reduction charges of unexposed, as-prepared samples, suggesting little or no corrosion on these samples.

Field Exposures Reductions of samples exposed in various field locations for different periods of time show that different locations form different corrosive products. For example, samples exposed in coastal regions have more silver chloride than the sample exposed in Ohio. Also, the Ohio sample shows silver sulfide which is not present on either of the two coastal exposures. Coastal locations also experienced more corrosion than observed in Ohio. The corrosion products formed at each location were the same for the two exposure times. Sample reductions for each of these locations and different exposure times are shown in figure 4.

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West Jefferson, Ohio 0.4

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Figure 4: Sample reduction curves for samples exposed in the field for one and three months. a shows reductions of samples exposed in Coconut Island, Hawaii. b shows reductions of West Jefferson, Ohio exposed samples, and c shows curves obtained from samples at Daytona Beach, Florida CONCLUSIONS In order to develop a more reliable method of analyzing corrosion products formed on silver, modifications to the standard coulometric reduction method were necessary. The transformations of silver compounds in solution show that the modified coulometric method described above provides accurate information about the types of corrosion products formed on silver and the amount of these products. The alkaline pH and absence of chloride ions in the reduction electrolyte prevent transformation of corrosion compounds in the time scales relevant to the coulometric reduction process. By examining the amounts of corrosion formed in three different environments, it was observed that the only environment which produced significant corrosion for the short exposure lengths studied here had UV illumination in an air atmosphere. Relative humidity was also found to affect the amount of corrosion, but the amount of corrosion did not increase monotonically with relative humidity. This suggests a complex interaction between the silver surface water layer, UV light, and atmospheric compounds. The field exposures demonstrated that coastal sites had higher corrosion rates than inland sites, but the variation between coastal sites is substantial, indicating that other factors influence corrosion of silver. Common corrosion products formed on silver include silver chloride and silver sulfide.

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ACKNOWLEDGEMENTS The authors wish to acknowledge and thank Daniel J.Dunmire, Director of the DoD Corrosion Policy and Oversight, OUSD(AT&L), Office of the Secretary of Defense for sponsoring this work. The assistance of Prof. Lloyd Hihara, George Hawthorn, and Ryan Sugamoto (Univ. of Hawai'i) in providing field samples is gratefully acknowledged. The provision of samples and technical discussions with Dr. Bill Abbott (Battelle Memorial Institute) are also gratefully acknowledged. Special thanks go to Dr. Zhuoyuan Chen whose initial work provided direction for much of this work, and Jason Lee and fellow members of the Center for Electrochemical Science and Engineering.

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11. 12. 13.

Christofer Leygraf, T.E.G., Atmosperic Corrosion. 1 ed. The Electrochemistry Society. 2000: John Wiley & Sons Inc. 354. Abbott, W.H., Aircraft Corrosion Sensing and Monitoring Program. Krumbein, S.J., B. Newell, V. Pascucci, Monitoring Environmental Tests by Coulometric Reduction of Metallic Control Samples. Journal of Testing and Evaluations, 1989. 17(6): p. 357-367. Chen, Z., R.G. Kelly, Atmospheric Corrosion Driven by Naturally Generated Reactive Halogens. 2007. ASTM B825-02 2008, Standard Test Method for Coulometric Reductionof Surface Films on Metallic Test Samples. Christfer Leygraf, T.E.G., Atmosperic Corrosion. 1 ed. The Electrochemistry Society. 2000: John Wiley & Sons Inc. 354. Graedel, T.E., Corrosion Mechanisms for Silver Exposed to the Atmosphere. J. Electrochemical Society, 1992. 139(7): p. 1963-1970. Phipps P.B.P., D.W.R., The Role of Water in Atmospheric Corrosion. American Chemical Society, 1979. Waterhouse G.I.N., G.A.B., J.B. Metson, Oxidation of a polycrystalline silver foil by reaction with ozone. Applied Surface Science, 2001. 183: p. 191-204. Rice D.W., P.P., E.B. Rigby, P.B.P/ Phipps, R.J. Cappell, and R. Tremoureux, Atmospheric Corrosion of Copper and Silver. Journal fo the Electrochemical Society, Electrochemical Science and Technology, 1981. 128(2): p. 275-284. Chapman, S., The Photochemistry of Atmospheric Oxygen. Reports on Progress in Physics, 1942. Young, J.F., Humidity Control in the Laboratory Using Salt Solutions-A Review. Journal of Applied Chemistry, 1967. 17. Dallenbach, R., J. Painot, and P. Tissot, Synthesis of silver (II) oxide by oxidation of silver or silver oxide by means of ozone. Polyhedron, 1982. 1(2): p. 183-186.

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