Characterization of Solid Deposits Formed from Jet Fuel Degradation under Pyrolytic Conditions: Metal Sulfides Ramya Venkataraman † and Semih Eser* The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 Reaction of the organic sulfur compounds in Jet A with Fe- and Ni-based alloy substrates under pyrolytic conditions formed significant amounts of metal sulfides. Pyrrhotite (Fe (1-x) S) and heazlewoodite (Ni 3 S 2 ) were formed on SS316 and Inconel 600 surfaces, respectively, in the short duration experiments. After extended periods of thermal stressing, an additional crystal phase, pentlandite (Fe,Ni) 9 S 8 , was also observed on both surfaces. The lack of FeS 2 (pyrite) formation over extended periods of stressing indicates that the amount of sulfur reacting with the substrates decreased with the increasing thermal stressing time. A focused ion beam (FIB)/SEM analysis showed that the metal sulfide formation can extend up to 2 μm depth from the surface in 2 h of thermal stressing. The formation of metal sulfides on alloy surfaces degrades the alloy surfaces and affects solid carbon deposition from jet fuel. Introduction Studies in the past have clearly shown that the organosulfur compounds present in hydrocarbon fuels react with metal substrates to form metal sulfides. 1-3 Surface reconstruction upon addition of sulfur compounds to Fe-Ni surfaces has also been reported in several earlier studies. 4 These surface changes play a significant role in solid deposit formation from hydrocarbon degradation, including a dramatic increase in the amount of carbon formed in some cases, 5 and clearly suppressed deposit formation in others. 5-8 Corrosion of metal surfaces due to sulfide formation is expected to be as problematic as solid carbon deposition from fuel degradation especially under the high temperature-high pressure conditions in future advanced aircraft. Two important metals used in the construction of jet engine components are Fe, Ni, and their alloys. This study characterizes the nature of metal sulfides formed on an iron- based alloy, SS316, and a nickel-based alloy, Inconel 600, from the thermal stressing of a jet fuel sample under pyrolytic conditions for varying reaction times. This was done in an attempt to understand the nature of metal-sulfur interactions under conditions of flight operation in advanced aircraft. Experimental Section An iron-based alloy, SS316, and Ni-based alloy, Inconel 600, were used in this study. Individual coupons measuring 13 cm × 0.3 cm × 0.025 cm were placed in a 1/4 in. (OD), 20 cm long, glass-lined, stainless steel tube reactor inserted in a vertical block heater. The fuel used in the study was commercial aviation Jet A with a sulfur content of 0.1 wt %. The sulfur content of this fuel is higher than usually encountered, but it lies within the sulfur level specifications for commercial aviation fuel (<0.3 wt %). The elemental composition of the alloys, the hydrocarbon composition of the fuel, and the organic sulfur compounds present in it have been discussed in the accompanying Article. The glass-lined stainless steel reactor (described in the accompanying Article) containing a single SS316 coupon was heated to 470 °C under flowing argon at 34 atm (500 psig) before the fuel was introduced. The fuel was preheated to 250 °C before entering the reactor. The start time for the experiment was noted after the fuel bulk temperature reached the wall temperature of 470 °C. The fuel temperature and pressure were kept constant for the duration of the experiment. The thermal stressing on the SS316 coupons was carried out for periods of 15 min (0.25 h), 30 min (0.5 h), 2 h, 5 h, and 10 h. The thermal stressing on the Inconel 600 surface was carried out for periods of 15 min and 2 h. To make sure that 15 min experiments captured the initiation of solid deposits from Jet A decomposi- tion, 5 and 10 min reactions were also conducted, and the surface was examined thereafter. No chemical or morphological changes were observed on the surface for these time periods. The fuel flow rate into the reactor for the thermal stressing experiments was 4 mL/min. A detailed description of the flow reactor setup including its schematic diagram is described elsewhere. 9 Morphology of Deposits. Field emission scanning electron microscopy (FESEM) was used to observe the morphology of the deposits under high magnification (∼100 000×). The FESEM used in this study was a JEOL 6700F located at the Materials Research Institute at Penn State. A Philips EM - 420ST transmission electron microscope (TEM) was used to study the nanostructure of the solid deposits. This was operated at 120 kV. The solid deposits were either scraped off the coupons or dispersed in alcohol and deposited onto a 200 mesh copper grid with a lacey carbon film. An EDAX Detector with 138 eV resolution, Si(Li) detector with Super Ultra Thin Window for detecting down to Be was used to obtain elemental analysis of the particles observed under the TEM. FIB/SEM. An FEI Company Quanta focused ion beam (FIB) instrument containing a highly focused (10 nm) ion gun combined with a conventional scanning electron microscope (SEM) was used to extract cross-sectional micron sized slices from the solid deposited foils. An omniprobe nanomanipulator was welded to the FIB sample by depositing Pt. After the specimen was transferred to the sample holder, it was weld cut using a Ga + beam. Chemical Characterization. X-ray diffraction was used to identify changes in the substrate composition and phases of the solids formed on the surface with increasing thermal stressing time. The X-ray beam was used at both normal and grazing angle incidence to obtain depth-wise information from the samples. Data were collected using a Sintag model X2 (θ-θ goniometer), Cu KR (1.540562 Å) radiation, with a Si(Li) Peltier detector at the Materials Research Institute in Penn State. * To whom correspondence should be addressed. Tel.: (814) 863- 1392. E-mail: seser@psu.edu. † Current address: Sinfin-A, Rolls-Royce plc, Derby DE24 8BJ, U.K. Ind. Eng. Chem. Res. 2008, 47, 9351–9360 9351 10.1021/ie801007r CCC: $40.75  2008 American Chemical Society Published on Web 11/01/2008