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Analysis of Carboneceous Deposits from Thermal Stressing of a JP-8 Fuel on Superalloy Foils in a Flow Reactor Orhan Altin† and Semih Eser*,†,‡ Laboratory for Hydrocarbon Process Chemistry, The Energy Institute, 209 Academic Projects Building, and Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
A flow reactor study was carried out to investigate the carbon deposition on nickel-, cobalt-, molybdenum-, and iron-based alloy foils during thermal stressing of a JP-8 fuel at 500 °C wall temperature and 34 atm (500 psig), for 5 h at a fuel flow rate of 4 mL/min. Temperatureprogrammed oxidation (TPO) analysis and SEM examination showed that the amount and the nature of the carbonaceous deposits on the foils depend strongly on the chemical composition of the foil surface. The amount of carbon deposited on metal foils decreased in the order Inconel 600 > Havar > Fecralloy > Waspaloy > Hastelloy-C > MoRe > Inconel 718. The presence of minor components, such as Ti, Al, and Nb, in the alloys appears to be responsible for reducing carbon deposition from jet fuel thermal stressing. This effect can be attributed to the formation of a passive layer on alloy surfaces that limits the access of deposit precursors to base metals, Ni, Fe, and Co, that catalyze deposit formation. 1. Introduction The formation of carbonaceous deposits on metallic surfaces from thermal decomposition of jet fuels is a major concern in the development of high-speed aircraft in which the fuel is also used as the principal heat sink.1 The jet fuel instability has been studied for almost five decades.2-5 Solid deposits from fuel degradation can attach to the surface of the flow lines or plug filters and create problems with the fuel system operation. The carbonaceous deposit formation appears to depend on a combination of different conditions, such as the reactivity of the starting fuel, the temperature, the pressure, the concentration, and the nature of the substrate surface. Temperature is one of the most important parameters that affect the rate and reaction mechanisms of fuel degradation. Autoxidation of fuels takes place at temperatures less than 260 °C, decomposition of oxygenated products at intermediate temperatures between 290 and 350 °C, and pyrolysis at temperature greater than 350 °C.6 At high temperatures, surface and/or fluid-phase reactions produce carbonaceous deposits with different morphologies and properties.7-10 The composition of jet fuels is very complex. The JP-8 fuel used in this study consists mainly of long-chain paraffins, with lower concentrations of alkylcyclohexanes, alkylbenzenes, and alkylnaphthalenes.11 Many studies have been carried out with light hydrocarbons12-16 to understand the mechanisms of carbon formation on metals. It is well-known that, at elevated temperatures, the transition metals, such as Ni and Fe catalyze, carbon deposition through the formation of filamentous17,18 and graphitic carbon.19 One distinguish* Author to whom correspondence should be addressed. Telephone: 814-863-1392. Fax: 814-865-3573. E-mail: sxe2@ psu.edu. † Laboratory for Hydrocarbon Process Chemistry. ‡ Department of Energy and Geo-Environmental Engineering.
ing property of carbon deposits is their oxidation reactivity, which depends on the chemical composition and the structure of the deposits. Differences in oxidation reactivity of the deposits are manifested in the evolution of CO2 peaks at different temperatures during temperature-programmed oxidation (TPO) experiments.20,21 The TPO analysis is, therefore, useful for characterization of the deposits to help understand the role of the metal surface in carbon deposit formation. Holm and Evans22-24 studied different alloyed steels exposed to CO2/CO-based gases at 650-850 °C to observe the role of grain size, oxidation, and silicon content of alloys on carbon deposit formation. Various metal surfaces were examined by Linne et al.,25 including 304 stainless steel and superalloys, to test JP-7 fuel thermal stability. Superalloys, with their good mechanical strength, thermal conductivity, and corrosion resistance at high temperatures, constitute promising materials for building aircraft fuel systems.26 The objective of this study is to investigate the surface effects of the superalloys Havar, Hastelloy-C, Waspaloy, Inconel 600, Inconel 718, MoRe, and Fecralloy on solid deposition from high-temperature thermal stressing of a JP-8 fuel in a flow reactor. Scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO) analyses were used to characterize the solid deposits produced on the different alloy foils. 2. Experimental Section 2.1. Thermal Stressing Experiments. The thermal stressing of JP-8 in the presence of different superalloy foils were performed in 1/4-in. (o.d.) glass-lined flowthrough reactor. The foils, 15 × 0.3 × 0.015 cm were placed at the bottom of a 20-cm-long reactor. The reactor containing the foil samples was heated to 500 °C and maintained at that temperature for 2 h in flowing nitrogen at 34 atm before the fuel was introduced. The fuel was preheated to 250 °C in a valve oven before entering the reactor. Throughout the experiments, the reactor wall temperature, fuel pressure, and liquid fuel
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Table 1. Composition of Alloy Foils (Goodfellow Ltd.) contents wt % In 718 In 600 Havar Waspaloy Hastelloy-C Fecralloy MoRe
Ni
Fe
Cr
Mo
Co
Al
Ti
Nb
Cu
Mn
Si
C
S
W
Y
Re
52.5 74.4 13.0 59.0 57.0 -
18.5 8.0 18.9 2.0 5.5 72.6 -
19.0 15.5 20.0 19.5 16.0 22.0 -
3.05 2.0 4.2 17.0 52.5
42.5 13.5 -
0.5 1.2 4.8 -
0.9 3.0 -
5.13 -
0.15 0.5 -
0.18 1.0 1.6 0.7 -
0.18 0.5
0.04 0.15 0.2 0.07 0.015 0.03 -
0.0008 0.0015 -
2.8 4.2 -
0.3 -
47.5
flow rate were kept at 500 °C, 34 atm, and 4 mL/min, respectively. The stainless steel preheating section is 2 mm in i.d. (1/8-in. o.d.) and 2 m in length. The fuel residence time in this preheating zone is 1.57 min at a liquid fuel flow rate of 4 mL/min. The fuel residence time in the reactor (4-mm i.d., 1/4-in. o.d., and 20-cm length) is 0.42 min at the same fuel flow rate. At the end of the reaction period (5 h), the foils were cooled under a nitrogen flow in the reactor. The details about the flow reactor system can be found in the companion paper.10 2.2. Materials. All of the superalloy foils were obtained from Goodfellow Metals Ltd (Cambridge, U.K.). The composition of the foils is given in Table 1. A brief description of each alloy follows. Inconel 600 is a nickel-based austenitic solid solution alloy that has good mechanical strength, thermal conductivity, and corrosion resistance at high temperatures. Inconel 718 provides high corrosion resistance and strength with excellent welding properties. Ti, Nb, and Al elements are added for precipitation strengthening because of their small but finite solubility in the alloy structure. Hastelloy-C is highly corrosion resistant at high temperatures. The high molybdenum content makes this alloy particularly resistant to pitting and corrosion. Waspaloy is a nickel-based alloy that contains very small amount of Fe. Addition of aluminum and titanium increases its strength. Havar is a cobalt-based alloy with good high-temperature mechanical properties. These alloys are not as strong as nickel-based alloys for short-time tests, but they are competitive in strength and corrosion resistance with nickel-based alloys at high temperatures and for long periods of operation. Fecralloy is a heat-resistant alloy with excellent resistance to oxidation at elevated temperatures. Protection from corrosion is afforded by an R-alumina layer that is superior to chromia. MoRe is a highly ductile material. It is not stable at temperatures higher than 400 °C in highly oxidative atmospheres.26,27 No significant difference was observed in the surface roughness of the alloy foils. All of the foils were used as received, after being washed with hexane and dried before each experiment. 2.3. Carbon Deposit Analysis. The deposited foils were analyzed using a LECO RC-412 multiphase carbon analyzer to determine the total carbon content and TPO profiles of the deposits on the foils.10 The surface morphology of the deposits was examined with an ISIDS 130 dual-stage scanning electron microscope (SEM). To verify the reproducibility of the experimental results, duplicate experiments of jet fuel stressing were carried out in the presence of the Inconel 600, Inconel 718, Fecralloy, and Hastelloy-C foils. The results of duplicate experiments showed that the TPO profiles were reproducible with respect to individual peak posi-
0.3 -
-
Figure 1. Total carbon deposit amounts from JP-8 decomposition on different superalloy surfaces at 500 °C and 34 atm (500 psig) with 4 mL/min JP-8 flow for 5 h.
tions and relative peak intensities. The total amount of deposition measured on different foil surfaces were reproducible to within (10 wt % of the deposit mass. 3. Results and Discussion A wide range of variation was observed in the amount of carbon deposits obtained on the different superalloy foils used in this study. Figure 1 shows the amount of carbon deposits produced on different foils, reported in terms of micrograms of carbon deposit per square centimeter of foil. The two Inconel alloys, Inconel 600 and Inconel 718 gave the highest and lowest carbon depositions, respectively, among the eight alloy foils studied. This large difference in deposition tendency under the same stressing conditions can be attributed to the differences in the composition of the two alloys, considering that the surface roughness of the two alloy foils was not very different. Inconel 718 has a much lower Ni content and a higher Fe content compared to those of Inconel 600. In addition, Inconel 718 contains Nb, Mo, Ti, and Al as minor components, which are not present in Inconel 600. These elements were added for precipitation strengthening because of their small but finite solubility in the alloy structure. The low surface reactivity of this alloy toward carbon deposition can be ascribed to the relatively low Ni content and/or the presence of the minor components in the alloy structure. The data obtained for the other foils, presented later, suggest that the composition of the minor components in the alloys, not the nickel content per se, strongly affects the extent of deposition. Figure 2 shows the TPO profiles for the deposits on the two Inconel alloy foils. The low temperature peaks around 150 and 350 °C can be attributed to adsorbed liquids on the deposits and hydrogen-rich amorphous deposits, respectively. The intense sharp peaks observed in the TPO profile for Inconel 600, especially in the hightemperature region, are very similar to those seen from the analysis of the stressed nickel tubes.10 The presence of high-temperature (>500 °C) peaks in the TPO profiles is ascribed to the catalytic activity of the foil surface in carbon deposition during thermal stressing. In contrast to Inconel 600, the stressed Inconel 718 foil gave a broad
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Figure 2. TPO profiles of the deposits collected on Inconel 718 and 600 alloys from thermal stressing of JP-8 AT 500 °C and 34 atm (500 psig) with 1 mL/min flow for 5 h.
Figure 3. SEM micrographs of deposits formed on (a) Inconel 600 and (b) Inconel 718 surfaces at 500 °C and 34 atm (500 psig) with 4 mL/min JP-8 flow for 5 h.
low-intensity band mainly in the low-temperature region. This profile can be attributed to incipient carbon deposition by heterogeneous reactions. The SEM micrographs in Figure 3 show the differences in appearance of the deposits on the two foil surfaces, indicating more extensive deposition on Inconel 600 with many protrusions, in contrast to small quantities of incipient
deposits seen on Inconel 718 in isolated regions. Carbon filaments, some coated with amorphous carbon, are seen on Inconel 600, together with platelike crystallites. The surfaces of some filaments appear to be coated with a fine grain material, probably through subsequent deposition on the filaments. The presence of these two different deposit morphologies can explain the lowtemperature and high-temperature peaks seen in the TPO profile of this deposit (Figure 2). Thin filaments, which most likely contain metal particles at their tips, and amorphous carbon would oxidize at relatively low temperatures, giving the broad low-temperature peaks. Compared to amorphous carbon, carbon filaments would have a lower oxidation reactivity. However, the metal particles found at the tip of the filaments could be exposed and could catalyze the oxidation reactions, to produce CO2 evolution peaks at relatively low temperatures. The platelike crystallites with a more ordered carbon structure would oxidize at higher temperatures than the amorphous (i.e., disordered) deposits or thin filamentous carbons. The platelike crystallites might, therefore, be responsible for the sharp high-temperature peak seen in the TPO profile. Compared to the stressed Inconel 600 alloy surface, the stressed Inconel 718 surface contains much less deposit, seen as isolated regions of incipient filaments which are 0.3 µm in length and 0.1 µm in diameter. Similar deposit morphologies were also observed on Inconel 718 by Bleekkan and Holmen28 during the pyrolysis of heavy oils and anthracene around 500 °C. As mentioned before, the lower catalytic activity of the Inconel 718 foil surface can be related to the presence of Ti, Al, Nb, and Ta as minor components in the alloy composition. Baker and Chludzinski17 reported, for example, that TiO2, Al2O3, and MoO3 coating of a nickel-iron surface suppressed the carbon deposition from acetylene decomposition at 850 °C. The bulk Ni content of the alloy foils alone does not explain the distribution in the amount of carbon deposits measured on the different foils. For example, Waspaloy with a much higher Ni content compared to Havar, collected a significantly lower amount of deposit than the Havar foil. Both Havar and Waspalloy alloys contain cobalt in relatively high concentrations, 42.5 and 14%, respectively, and also show variations in the composition of their minor components, as shown in Table 1. Hastelloy-C has a similar chemical composition to that of Waspalloy, except that the Co in Waspalloy is replaced with Mo in Hastelloy-C. The amount of deposition on these foils is comparable. It is known that Fe and Co are also catalytically active toward carbon deposition during the thermal stressing of hydrocarbons.12,29-33 Chambers et al.34 studied the cobaltcopper binary system and observed that increasing the Co content decreases the carbon deposition during ethylene thermal stressing. These results suggest that the carbon deposition activity of superalloys can depend strongly on their chemical composition, including the major and minor components. Figure 4 shows the TPO profiles for the stressed foils Havar, Waspaloy, and Hastelloy-C. The deposits on all three superalloys gave two common broad bands at around 300 and 550 °C. In addition, the deposit formed on Havar gave an intermediate band with a well-defined peak at 425 °C. The Havar foil stands out among the three superalloys because of the
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Figure 4. TPO profiles of the deposits collected on Havar, Hastelloy, and Waspaloy alloys from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 4 mL/min flow for 5 h.
significantly higher deposition and the presence of much higher intensity of high-temperature peaks around 550 °C in the TPO profiles. One can conclude from Figure 4 that the surface catalytic activity for carbon deposition decreased in the order Havar > Hastelloy > Waspaloy on the basis of the decreasing relative intensity of hightemperature peaks. Figure 5 shows the SEM micrographs of the three foils stressed under the same conditions. As can be seen in Figure 5a, the deposits on Havar contained platelike structures and filaments. The Hastelloy surface was covered by both filamentous- and amorphous-type deposits (Figure 5b). The micrograph in Figure 5c shows more extensive coating of the filamentous carbons with apparently amorphous material. It can be seen from the micrographs in Figure 5 that the proportion of structurally more ordered solids in the deposits decreased in the order Havar > Hastelloy > Waspaloy. This is the same order observed for the decreasing relative intensity of the high-temperature peaks in TPO profiles. The sharp high-temperature peak around 750 °C in the Inconel 600 profile does not appear in the profiles of these three alloys. The relatively low catalytic activity of the Waspaloy foil can be related to the minor element composition, including Ti and Al, which are also present in Inconel 718. The stressed Fecralloy foil (aluminized iron and chromium alloy) showed three distinct carbon phases in the TPO profile, as shown in Figure 6. The most intense peak evolved at approximately 450 °C. The carbon deposits associated with this phase might have resulted from the iron catalysis of the deposition reactions. The SEM examination showed large crystallites on the stressed Fecralloy surface, similar in structure to the “chunks of deposits” reported by Graff and Albright.35 The deposits formed on Fecralloy contain faceted structures with sharp edges, as shown in Figure 7. It is known that both iron and iron oxide surfaces catalyze carbon deposition. The active surface of iron can be passivated by the addition of other elements, such as Al, Cr, or Ti. Fecralloy is one of the typical examples of passivated iron containing alloys. It appears, however, that, under the stressing conditions used in this study Fecralloy, displays high surface activity that can be attributed to the catalytic effect of iron in deposition reactions.
Figure 5. SEM micrographs of deposits formed on (a) Havar, (b) Hastelloy, and (c) Waspaloy surfaces at 500 °C and 34 atm (500 psig) with 4 mL/min JP-8 flow for 5 h.
Molybdenum-rhenium alloy gave the second lowest amount of carbon deposits among the superalloy foils, but it does not seem to be stable in oxygen atmosphere above 500 °C. The alloy started to melt and changed form into a low-density glassy particulate upon heating to 500 °C in the carbon analyzer furnace. At 500 °C and 34 atm, JP-8 fuel produced a small amount of deposit on the molybdenum and rhenium binary metal surface, as shown in Figure 7. There are two phases observed around 350 and 700 °C, as shown in the TPO profile in Figure 6. Mo-Re alloy is inactive in an oxygen-free atmosphere under the thermal stressing conditions used. As shown in Figure 7, the formation of carbon deposits was observed only on isolated regions of the
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Figure 6. TPO profiles of the deposits collected on FeCrAl and MoRe alloys from thermal stressing of JP-8 at 500 °C and 34 atm (500 psig) with 4 mL/min flow for 5 h.
Figure 8. SEM images of remaining surfaces after burning of deposits on (a) Inconel 600 and (b) Inconel 718. The deposits were burnt away under 750 mL/min O2 flow by heating from 100 to 900 °C with a 30 °C/min heating rate.
Figure 7. SEM micrographs of deposits formed on (a) FeCrAl, (b) and MoRe surfaces at 500 °C and 34 atm (500 psig) with 4 mL/min JP-8 flow for 5 h.
surface where the roughness was very high. The deposits observed appear to have rodlike forms. To investigate the surface damage resulting from deposit formation on Inconel 600 and Inconel 718, the deposited foils were examined by SEM after the deposits were burned off in TPO analyses. As shown in Figure 8, the foil surfaces became porous and rough possibly
because of carbide formation and/or lifting of metal particles during deposit formation. During TPO, metalcontaining carbon deposits were oxidized and the associated metals remained on the surface, being seen as white rounded structures in Figure 8a. The damage on the Inconel 600 surface was ubiquitous and more extensive (Figure 8a) than that seen in isolated regions on Inconel 718 (Figure 8b). Note that the magnification of the micrograph of the Inconel 718 surface (Figure 8b) is twice that of Inconel 600 (Figure 8a), indicating the formation of larger holes on the Inconel 600 surface. The low extent of deposition and surface damage on Inconel 718 can be attributed to the presence of Ti, Al, Nb, and Ta as minor components in the alloy composition. As suggested by Altin and Eser,36 these minor elements might have physically covered the active metal elements or acted to reduce the carbon solubility at the reaction temperature of 500 °C. The increasing Cr content and the presence of some minor elements, such as Ti, Al, Nb, and Ta, appear to improve an alloy’s resistance to carbon deposit formation.37,38 For example, it has been found that addition of Ti, Al, Nb, and Ta to various binary alloys reduces carbide formation40-43 because of the lower solubility and diffusivity of C in Ti, Al, Nb, and Ta oxides. Oxides of Cr, Ti, and Al are thermodynamically more stable than those of Ni and Fe. These stable oxides form during the annealing step of alloy manufacturing.
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4. Conclusions Thermal stressing of superalloy foils with a JP-8 fuel in a flow reactor at 500 °C and 34 atm for 5 h produced carbon deposits on the alloy surfaces. Temperatureprogrammed oxidation (TPO) analysis and SEM examination showed that the amount and nature of the carbonaceous deposits on the foils depend strongly on the chemical composition of the foil surface. The amount of carbon deposited on the alloy foils decreased in the order Inconel 600 > Havar > Fecralloy > Waspaloy > Hastelloy-C > MoRe > Inconel 718. The SEM examination of the stressed foils showed crystalline, or highly faceted, deposit structures and/or filamentous carbon on most of the stressed alloys, indicating catalytic activity of the metal components in deposit formation. The TPO analysis of the deposited foils pointed out the presence of multiple “carbon phases” that have different oxidation reactivities. The differences in oxidation reactivity of the deposit components have been ascribed to the different levels of structural order (from amorphous to crystalline) present, as was observed by SEM. The presence of minor components, such as Ti, Al, and Nb, in the alloys appears to play a significant role in suppressing carbon deposition from jet fuel thermal stressing. This effect can be attributed to the formation of a passive layer on the alloy surfaces that inhibits the access of deposit precursors (reactive species from thermal decomposition reactions) to the active metal elements (i.e., Ni, Fe, and Co) that catalyze carbon deposition. Acknowledgment This work was funded by the Air Force Research Laboratory/Aero Propulsion and Power Directorate, Wright Patterson AFB. We thank Prof. H. H. Schobert of PSU for his support and Dr. T. Edwards of AFWL/ APPD for helpful comments and discussions. Literature Cited (1) Hazlett, R. N. Physicochemical Aspects of Aviation Fuel Thermal Stability. In Aviation Fuel: Thermal Stability Requirements; Kirklin, P. W., David, P., Eds.; ASTM STP 1138; ASTM: Philadelphia, PA, 1992. (2) Bol’shakov, G. F. The Physico-Chemical Principles of the Formation of Deposits in Jet Fuel; Technical Report; WrightPaterson Air Force Base, Dayton, OH, 1972. (3) Schimdt, J. E. Air Frame Considerations in Fuel Thermal Stability for Commercial Supersonic Flight. In Aviation Fuel: Thermal Stability Requirements; Kirklin, P. W., David, P., Eds.; ASTM STP 1138; ASTM: Philadelphia, PA, 1992. (4) Heneghan, S. P.; Martel, C. R.; Williams, T. F.; Ballal, D. R. Studies of Jet Thermal Stability in a Flowing System. Trans. ASME J. Eng. Gas Turbines Power 1993, 115, 480. (5) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanisms. Ind. Eng. Chem. Res. 1993, 32, 3117. (6) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Pyrolytic Degradation Studies of a Coal-Derived and a Petroleum-Derived Aviation Jet Fuel. Energy Fuels. 1993, 7 (2), 234. (7) Li, J.; Eser, S. Carbonaceous Deposit Formation on Metal Surfaces from Thermally Stressed Dodecane. Carbon ‘95, Extended Abstracts; 22nd Biennial Conference on Carbon; American Carbon Society: San Diego, CA, 1995; p 314. (8) Edwards, T.; Atria, J. V. Thermal Stability of HighTemperature Fuels; ASME Report 97-GT-143; ASME: New York, 1997. (9) Li, J.; Eser, S. Surface Effects of Copper on Deposit Formation from Jet-Fuel Range Hydrocarbons. Carbon ‘97, Extended Abstracts; 23rd Biennial Conference on Carbon; American Carbon Society, Pennsylvania State University: State College, PA, 1997; Vol. 2, p 290.
(10) Altin, O.; Eser, S., Analysis of Carboneceous Deposits from Thermal Stressing of a JP-8 Fuel on Different Tube Surfaces in a Flow Reactor. Ind. Eng. Chem. Res. 2001, 40, 596. (11) Yu, J.; Eser, S. Thermal Decomposition of C10-C14 Normal Alkanes in Near-Critical and Supercritical Regions: Product Distributions and Reaction Mechanisms. Ind. Eng. Chem. Res. 1996, 36 (3), 574. (12) Trimm, L. D. The Formation and Removal of Coke from Nickel Catalyst. Catal. Rev.-Sci. Eng. 1977, 16, 155. (13) Trimm, D. L. Fundamantal Aspects of the Formation and Gasification of Coke. In PyrolysissTheory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (14) Baker, R. T. K.; Harris, P. S. The Formation of Filamentous Carbon. In The Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker Inc.: New York, 1978. (15) Albright, L. F.; Tsai, C.-H. T. Importance of Surface Reactions in Pyrolysis Units. In PyrolysissTheory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (16) Froment, G. F. Coke Formation in the Thermal Cracking of Hydrocarbons. Rev. Chem. Eng. 1990, 16 (4), 293. (17) Baker, R. T. K.; Chludzinski, J. J., Jr. Filamentous Carbon Growth on Nickel-Iron Surfaces: The Effect of Various Oxide Additives. J. Catal. 1980, 64, 464. (18) Figueiredo, J. L., Carbon Deposition Leading to Filament Growth on Metals. Mater. Corros. 1998, 49, 373. (19) Rostrup-Nielsen, J.; Trimm, D. L. Mechanisms of Carbon Formation on Nickel-Containing Catalysts. J. Catal. 1977, 155, 48. (20) McCarty, J. G.; Wise, H. Hydrogenation of Surface Carbon on Alumina-Supported Nickel. J. Catal. 1979, 57, 406. (21) Marecot, P.; Akhachane, A.; Micheaud, C.; Barbier, J. Deactivation by Coking of Supported Palladium Catalysts. Effect of Time and Temperature. Appl. Catal. A: Gen. 1998, 169, 189. (22) Holm, R. A.; Evans, H. E. The Resistance of 20 Cr/25 Ni Steels to Carbon Deposition. I. The Role of Surface Grain Size. Werkst. Korros. 1987, 38, 115. (23) Holm, R. A.; Evans, H. E. The Resistance of 20 Cr/25 Ni Steels to Carbon Deposition. II. Internal Oxidation and Carburisation. Werkst. Korros. 1987, 38, 166. (24) Holm, R. A.; Evans, H. E. The Resistance of 20 Cr/25 Ni Steels to Carbon Deposition. III. Cold Work and Selective Preoxidation. Werkst. Korros. 1987, 38, 219. (25) Linne, D. L.; Meyer, M. L.; Edwards, T.; Eitman, D. A. Evaluation of Heat Transfer and Thermal Stability of Supercritical JP-7 Fuel; NASA Technical Memorandum 107485, AIAA-97-3041; 33rd Joint Propulsion Conference and Exhibit, Seattle, WA, July 6-9, 1997. (26) Sims, C. T.; Stoloff, N. S.; Hagel, W. C. Superalloys; John Wiley & Sons: New York, 1987. (27) Boyer, H.; Gall, L. T. Metals Handbook; American Society for Materials: Metals Park, OH, 1985. (28) Bleekkan, A. E.; Holmen, A. Coke Formation on Metal Foils from Heavy Oils. Carbon 1987, 25 (6), 827. (29) Bartholomew, C. H. Carbon Deposition in Steam Reforming. Catal. Rev.-Sci. Eng. 1982, 24, 67. (30) Cnossen, G. R.; Lee, S.; Sacco, A., Jr. Carbon Deposition Over Transition Metal Alloys-I. Generation of 〈NiCo〉, 〈FeNi〉, and 〈CoFe〉 Phase Diagrams. Carbon 1994, 32 (6), 1143. (31) Geurts, F. W. A. H.; Cnossen, G. R.; Sacco, A., Jr. Carbon Deposition Over Transition Metal Alloys-I I. Kinetics of Deposition Over 〈FeNi〉 and 〈FeCo〉 Alloy Foils. Carbon 1994, 32 (6), 1151. (32) Jablonski, A. G.; Geurts, W. F. A. H.; Sacco, A., Jr.; Biederman, R. R. Carbon Deposition over Fe, Ni, and Co Foils from CO-H2-CH4-CO2-H2O, CH4-H2, and CO-H2-H2O Gas Mixtures: I. Morphology. Carbon 1992, 30 (1), 87. (33) Jablonski, A. G.; Geurts, W. F. A. H.; Sacco, A., Jr. Carbon Deposition over Fe, Ni, and Co foils from CO-H2-CH4-CO2-H2O, CO-CO2, CH4-H2, and CO-H2-H2O Gas Mixtures: II. Kinetics. Carbon 1992, 30 (1), 99. (34) Chambers, A.; Rodriquez, M. N.; Baker, R. T. K. Modification of the Catalytic Behavior of Cobalt by the Addition of Copper. J. Phys. Chem. 1995, 99, 10581. (35) Graff, J. M.; Albright, I. L. Coke Deposition from Acetylene, Butadiene, and Benzene Decompositions at 500-900 °C on Solid Surfaces. Carbon 1982, 20 (4), 319.
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Received for review April 28, 2000 Revised manuscript received October 25, 2000 Accepted November 6, 2000 IE0004489
