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Ind. Eng. Chem. Res. 1996, 35, 2576-2580
MATERIALS AND INTERFACES Silver Corrosion and Sulfur Detection Using a Quartz Crystal Microbalance with Silver Electrode Surfaces Steven Zabarnick,* Paula Zelesnik, and Shawn D. Whitacre Aerospace Mechanics Division, University of Dayton Research Institute, 300 College Park/KL-463, Dayton, Ohio 45469-0140
A quartz crystal microbalance (QCM) with silver electrodes is used for detection of sulfur and measurement of silver corrosion by sulfur in jet fuels. It is found that the device responds very sensitively to elemental sulfur by reaction at the silver surface, resulting in the formation of surface silver sulfide. It is also expected that the device will respond to hydrogen sulfide. The response of the QCM is found to depend on the fuel matrix. The presence of oxygen inhibits the surface reaction, resulting in a lower mass accumulation. It is also found that the sulfur compounds which produce a response in this test are not players in the formation of autoxidative surface deposition. It is shown that the QCM response (surface mass per unit time) is proportional to the concentration of elemental sulfur present in the fuel. Introduction In the early days of gas turbine aviation engines it was thought that these engines could run on almost any type of fuel. Over the intervening years jet fuel specifications have become increasingly more complicated and numerous due to the increased complexity of the turbine engine and its control. Increasing demands for improved performance, overhaul life, and economy will continue this trend, despite the necessary compromise between fuel quality and availability (Bishop and Henry, 1993). Of the chemical property specifications, some of the most important are the limits placed on concentrations of various sulfur compounds. The military specification for JP-8 fuel limits the total sulfur content to 0.3 mass % and mercaptan sulfur to 0.002 mass %. Sulfur content is strictly limited in jet fuel because high sulfur content can adversely affect deposition in combustion chambers and sulfur oxides in combustion gases can lead to corrosion problems. Also, sulfur compounds have been implicated in liquid-phase corrosion (Schreifels et al., 1989; Tripathi et al., 1973). Mercaptan sulfur is limited because of objectionable odor, elastomer degradation, and corrosiveness toward certain metals. For JP-8 fuel, direct corrosion due to sulfur is tested by ASTM Method D130, the copper strip corrosion test. In addition, service experience with corrosion of silver components has led to the British military Silver Corrosion Test, IP 227. Previous work in our laboratory has demonstrated the usefulness of the quartz crystal microbalance (QCM) with an oxidation monitoring technique for studying the deposition and oxidation of jet fuels (Zabarnick, 1994). The QCM, and piezoelectric transducers in general, have proven to have multitudinous applications as detectors of interfacial chemical processes (Ward and Buttry, 1990). We have noted unusual behavior of the QCM in tests conducted with high sulfur fuels using silver crystal electrode surfaces (Zabarnick et al., 1995, * To whom correspondence should be addressed. E-mail:
[email protected].
1996). This behavior was attributed to reaction of sulfur compounds in the fuel with the silver surface. In the current work we investigate the details of this interaction of fuel sulfur with silver surfaces in the QCM. This interaction provides the possibility of using the QCM as a silver corrosion test for jet fuel and/or a very sensitive test for fuel sulfur. This study reports on work which addresses the following questions as they pertain to using the QCM with a silver electrode as a silver corrosion test and/or sulfur detection test. What type of sulfur compounds in fuel are responsible for the QCM response? What is the sensitivity and selectivity of the test to these compounds? What is the effect of oxygen on this interaction? What is the effect of the fuel matrix on the response? Are these sulfur compounds responsible for autoxidative deposition? Experimental Section The QCM/Parr bomb system has been described in detail previously and will only be outlined briefly here (Zabarnick, 1994; Zabarnick and Grinstead, 1994). The Parr bomb is a 100 mL stainless steel reactor. It is heated with a clamp-on band heater and its temperature is controlled by a PID controller through a thermocouple immersed in the fuel. The reactor contains an RF feedthrough, through which the connection for the quartz crystal resonator is attached. The crystals are 2.54 cm in diameter and 0.33 mm thick and have a nominal resonant frequency of 5 MHz. The crystals were acquired from Maxtek Inc. and are available in crystal electrode surfaces of gold, silver, platinum, and aluminum. The QCM measures deposition (i.e., an increase in mass) which occurs on overlapping sections of the two-sided electrodes. Thus, the device responds to deposition which occurs on the metal surface and does not respond to deposition on the exposed quartz. The device is also equipped with a pressure transducer to measure the absolute headspace pressure and a polarographic oxygen sensor to measure the headspace oxygen concentration. Previous studies have demonstrated the value of these detectors for determining the
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2577 Table 1. Properties of Fuels Studied fuel no. and type
total sulfur (ppm)
2827 (jet A) 2747 (jet A-1) 2980 (jet A)
763 37 614
oxidation characteristics of fuels and fuels with additives, although in the present study most runs were conducted in the absence of oxygen. A personal computer is used to acquire data at 1 min intervals during the experimental run. The following data are recorded during a run: temperature, crystal frequency, headspace pressure, headspace oxygen concentration, and crystal damping voltage. The reactor is charged with 60 mL of fuel, which is sparged with the appropriate gas for 1 h before each test. The reactor is then sealed, and the heater is started. All runs in this study were performed at 140 °C; heat-up time to this temperature is 40 ( 5 min. Most runs are conducted for 15 h, after which the heater is turned off and the reactor allowed to cool. Surface mass measurements can only be determined during the constant temperature ((0.5 deg) portion of an experimental run. The crystal frequency is converted to a surface mass measurement using the process described below. The theory that relates the measured frequency changes to surface mass has been presented in detail elsewhere (Martin et al., 1991). The frequency change of a crystal immersed in a liquid fuel can be due to two effects: the first results from changes in the surface mass density, the second is due to changes in the liquid density and viscosity. At constant temperature and relatively small extents of chemical conversion the liquid properties remain constant and the frequency change can be related to surface deposition via the equation
∆f Fs ) -(2.21 × 105 g/(cm2 s)) 2 f0
(1)
where f0 is the unperturbed resonant frequency, ∆f is the change in resonant frequency, and Fs is the surface mass density (mass/area). In general, the reproducibility of the mass deposition measurements on fuels is limited to (20% for the QCM technique. The fuels studied and some of their properties are listed in Table 1. The fuels were acquired from the Fuels and Lubricants Division of Wright Laboratory, Wright-Patterson AFB, Dayton, OH, and are referred to by the Wright Lab assigned ascension number. Results and Discussion When jet fuel is heated in an oxygen-containing environment, autoxidation chemistry occurs, which ultimately leads to formation of deposits on surfaces. Elemental analyses have shown that these deposits consist primarily of carbon and oxygen with smaller amounts of sulfur, nitrogen, and hydrogen (Hazlett, 1991). Removal of oxygen results in a significant reduction in deposit formation (Zabarnick, 1994). In a previous QCM paper, we studied the effect of surface metal on jet fuel deposition (Zabarnick et al., 1995, 1996). We found that gold and aluminum produced similar deposition, while platinum surfaces yielded increased deposition for some fuels. The most startling observation was extremely high deposition observed for silver crystal electrode surfaces. These data are shown in Figure 1. The “normal” deposition of ≈4 µg/cm2 after
Figure 1. Plots of mass accumulation vs time for two jet fuels on silver and gold crystal electrode surfaces.
15 h at 140 °C for fuel 2827 on gold surfaces is shown. When a silver crystal is substituted, the resulting deposition approaches 17 µg/cm2 at 15 h. Visually, the silver surface appeared to be discolored with a blue/ black film in contrast to the usual “yellowing” or brownish color which results from autoxidative deposits. In order to confirm that this blue/black film was not due to an autoxidative deposit the test was run in the absence of oxygen by sparging the fuel with nitrogen. Figure 1 shows that the removal of oxygen results in a substantial increase in the deposition rate to ≈30 µg/ cm2 after only 3 h. Possible reasons for this increase in deposition in the absence of oxygen will be discussed later. Also shown in the figure is deposition for a second fuel, fuel 2747, on a silver electrode in the absence of oxygen. This deposition is similar to that normally observed on gold electrodes, and a blue/black film was not observed. Table 1 shows that one major difference in these two fuels is their sulfur level; fuel 2827 is a relativity high sulfur fuel at 763 ppm, while fuel 2747 has a low sulfur level of 37 ppm. The difference in sulfur level of these fuels and the well-known tarnishing of silver surfaces by atmospheric sulfur compounds led us to suspect sulfur as the cause of this behavior. In order to substantiate this hypothesis, Auger electron spectroscopy (AES) analyses with argon-ion sputtering of the surfaces were performed. Figure 2 shows the AES depth profile of the surface film produced from a 15 h run with fuel 2827 in an oxygen-free environment. The elemental composition at the surface (at 0 Å sputter depth) consists primarily of silver (70%) and sulfur (25%) with smaller amounts of carbon. The well-known tarnishing of silver by atmospheric hydrogen sulfide results in the formation of silver sulfide, Ag2S. It is apparent that the surface consists of silver sulfide almost exclusively; the small amount of carbon at the surface may be due to residual fuel. As one sputters through the film, there is an increasing amount of carbon and oxygen, most likely due to a small autoxidative deposit. It is believed that the formation of silver sulfide tarnish on silver surfaces occurs by the reaction of sulfur with surface silver atoms, and subsequent migration of subsurface silver atoms to the surface
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2578 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996
Figure 2. Auger electron spectroscopy depth profile of silver electrode for fuel 2827: (9) silver; (O) sulfur; (2) carbon; (0) oxygen. Table 2. Sulfur Compounds Tested in n-Dodecane sulfur compd
concn (mg/L)
surface mass (µg/cm2) after 15 h at 140 °C
3-methylthiophene diphenyl sulfide diphenyl disulfide 1-hexanethiol 1-hexanethiol 3,4 dimethylthiophenol elemental sulfur elemental sulfur elemental sulfur
81 81 100 67 337 82 0.11 1.1 10.0
0.9 0.9 0.8 0.4 0.6 0.5 1.1 15.1 16.0
(Ricciardiello and Riotti, 1972). This behavior would account for the fact that although sulfur is observed to a depth of at least 230 Å, silver is still the major atomic species at the surface. In the case of fuel 2747, the AES analysis demonstrated (not shown here) that the surface consists almost entirely of silver (95%). A third fuel, 2980, which has a relatively high sulfur content (see Table 1), was also tested. This fuel, despite having 614 ppm sulfur, produced only 0.8 µg/cm2. These results suggest that the device responds selectively to a particular type or types of sulfur compounds rather than universally to all sulfur-containing species. Petroleum fuels can contain a wide variety of sulfur compounds. Some of these come from the original crude and are not removed during the refining process. Sulfur compounds can also result from sweetening processes to which some fuels are subjected. The corrosivity of a given sulfur compound is a function of its chemical structure; some sulfur compounds have even been shown to act as corrosion inhibitors (Garcia-Anton et al., 1995; Tripathi et al., 1973). In order to determine the types of sulfur compounds to which the silver electrode QCM responds, we spiked n-dodecane with various sulfur compounds. These compounds are listed in Table 2. The compounds tested represent the major sulfur compound types normally found in fuel: mercaptans, sulfides, disulfides, thiophenes, thiophenols, and elemental sulfur. Although the compounds were chosen to represent the classes of sulfur compounds in fuel, they may not be representative of specific individual compounds that have actually been identified in fuel. The table shows that none of the organosulfur compounds produced significant deposition (i.e.,
