Energy & Fuels 1991,5, 263-268
263
Adsorption of a Metal Deactivator Additive onto Metal Surfaces John A. Schreifels,?Robert E. Morris,* Noel H. Turner, Robert L. Mowery, and Steven M. Hues Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000, Department of Chemistry, George Mason University, Fairfax, Virginia 22030, and Geo-Centers, Inc., Fort Washington, Maryland 20744 Received October 15, 1990. Revised Manuscript Received January 1 1 , 1991
The adsorption of a metal deactivator (MDA) onto metal surfaces in a dodecane model fuel was studied. X-ray photoelectron spectroscopic (XPS) analysis of polished metal surfaces indicated that nitrogen levels were at the detection limits on 6061 aluminum, 304 stainless steel, and copper. Thus, only small amounts of the compound could have adsorbed. Evidence for the presence of carboxylic acids were obtained during Fourier transform infrared analysis of these surfaces, but no MDA could be found until the dodecane was treated with silica gel to remove the acidic constituents prior to ita use. Secondary ion mass spectrometric analysis of these surfaces showed that MDA was present, but its exact coverage could not be determined. The surfaces of tubes that had been heated in the jet fuel thermal oxidation tester (JFTOT) were also found to have a very weak nitrogen signal and a thick carbonaceous layer as low as 260 "C.
Introduction In modern aircraft fuel systems, fuel is used as a heattransfer medium to cool avionics and hydraulic systems. Under these conditions, the fuel can undergo autoxidation. Autoxidation or jet fuel can result in the formation of insoluble gum and sediment which can impair operation of the jet engine. In addition, hydroperoxides, which are formed during autoxidation, are known to attack certain elastomeric fuel systems components. Thus, the thermal oxidation stability of the fuel becomes an important consideration. Trace quantities of certain transition metals will catalyze fuel oxidation. Dissolved copper is the most reactive.'f Significant reductions in the thermal stability of aviation fuels have been demonstrated by Kendall and Mills in a single-tube heat exchanger rig after addition of 10 pg L of dissolved coppel.3 and in the JFTOT by Morris et al. with 1.3pg of Cu/L. Copper contamination can and does occur, particularly in shipboard fuel handling systems from contact with copper lines, brass fittings, admiralty metal, and other copper-bearing alloys. Metal deactivator additives (MDA) were developed to counteract the catalytic activity of dissolved metals. These additives can act' as polydentate ligands for divalent copper. Chelate complexes derived from similar hydroxyaromatic Schiff bases are thermally stable! The additive currently approved for use by the military specification MIL-T-5624 is N,"-disalicylidene-l,2-propanediamine at up to 5.8 mg/L in JP-4 and JP-5. The ASTM specification6 for aviation turbine fuels allows up to 5.7 mg/L. Laboratory scale tests, such as the Jet Fuel Thermal Oxidation Tester (JFTOT), have been relied upon to evaluate the thermal oxidation stability of aviation fuels. In the ASTM method' for assessing thermal oxidation stability of aviation fuels, the fuel is passed once over the outside of a resistively heated aluminum tube (the heater tube) at 3 mL/min. In previous work,l'*we have observed that the addition of the metal deactivator resulted in significant reductions in deposits on 304 stainless steel
JFTOT heater tubes from 280 to 310 "C. In tests conducted by Kendall and Earlsg with an injector feed-arm simulator, metal deactivator reduced deposit formation in two fuels, onto clean steel surfaces, by 2- and 14-fold, respectively. However, after an induction period, rapid deposition ensued at a rate similar to that observed without MDA. From this observation they concluded1° that MDA passivated the clean steel surface toward thermal deposits but was ineffective once the surface became coated with an organic deposit. The influence exerted by MDA during JFTOT testing of metal-free hydrotreated fuels was also attributed by Clark" to passivation of the clean metal surface of the JFTOT heater tube. The short test duration and laminar flow (low Reynolds number) conditions of the JFTOT test, com-
l
'George Mason University and Geo-Centers, Inc.
(1) Pederson, C. J. Ind. Eng. Chem. 1949,41,924. (2) Smith, J. D. J. Aero. Eng. 1967,33(4), 19. (3) Kendall D. R.: Mills. J. S. SAE Technical PaDer 851871: The Society of Automotive Engineers: Warrendale, PA, i985; SP-633. (4) Morris, R. E.;Tumer, N. H. Fuel Sci. Technol. Int. 1990,8(4), 327. (5) Marvel, C. S.;Aspey, S. A,; Dudley, E. A. J. Am. Chem. SOC.1956, 78, 4905. (6) ASTM 'Specification for Aviation Turbine Fuels". In Annual Book of ASTM Standards; ASTM Philadelphia, 1987; part 23, ASTM D1655-na. (7) ASTM 'Standard Method for Thermal Oxidation Stability of Aviation Turbine Fuels (JETOT Procedure)". In Annuul Book of ASTM Standards; ASTM Philadelphia, 1988; Vol. 05.02, ASTM D 3241-85. (8) Morris, R. E.; Hazlett, R. N.; McIlvaine, C. L. I11 Ind. Eng. Chem. 1988,27(8), 1524. (9) Kendall, D. R.; Earls, J. W. "The Assessment of Aviation Fuel Thermal Stabilitv". Preaented at the 25th IATA Aviation Fuel Subcommittee, Septembk 17-18, Geneva, 1985. (10) Kendall, D. R.; Houlbrcmk, G.; Clark, R. H.; Bullock,S. P.; Lewis, C. "The Thermal Degradation of Aviation Fuels in Jet Engine Injector Feed-Arms,Part I Results from a Full-Scale Rig". h n t a d at the 1987 Tokyo International Cas Turbine Congrees, October 26-31, Tokyo, Japan, 1987. (11) Clark, R. H. "The Role of Metal Deactivator in Improving the Thermal Stability of Aviation Kerosines". In Proceedings of 3rd International Conferenceon Stability and Handling of Liquid Fuels; Hiley, R. W., Penfold, R. E., Pedley, J. F., Eds.; The Institute of Petroleum: London, 1989; p 283. (12) Clark, R. H.; Delargy, K. M.; Heins, R. J. Prepr.-Pap. Am. Chem. SOC.,Diu. Fuel Chem. 1990,35(4), 1223.
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Q887-0624/91/25Q5-Q263$Q2.5Q/Q0 1991 American Chemical Society
264 Energy & Fuels, Vol. 5, No. 2, 1991
pared to the rig test, would be entirely within the induction period resulting from MDA passivation of the heater tube surface. This would mean that, when MDA was present in t h e fuel, the results of thermal stability testing would be no more than an artifact of the influence exerted by the additive on the test apparatus and would not represent the thermal stability of t h e fuel. Surface studies conducted by Clark et a1.12of aluminum JFTOT tubes after JFTOT testing of fuel containing MDA showed that t h e amounts of MDA on the surface increased with initial MDA concentration in the fuel and with temperature. They concluded that, during JFTOT testing, when MDA is present at the specification limit of 5.7 mg/L, relatively few surface sites would be occupied. Concerns about strong responses of the JFTOT to MDA have raised questions about the applicability of the method for assessing thermal oxidation stability in the presence of metal deactivators. Thus, we have directed our efforts at ascertaining the various mechanisms by which MDA can act, particularly in accelerated stability testing. One objective of this study was to determine to what extent interactions with the metal surfaces of the test apparatus govern t h e effectiveness of metal deactivators t o reduce deposits. This paper describes a n examination of metal surfaces exposed to solutions of MDA in a dodecane model fuel t o determine under what conditions would MDA be chemically bound t o t h e metal. Experimental Section The experimental conditions were chosen to model as carefully as possible an environment which would provide conditions which were optimal for adsorption of MDA onto the metal surfaces. Thus, surface contamination was minimized through careful cleaning with the assumption that, if no chemisorption of MDA was observed under these conditions, then it would be even less likely in JFTOT testing of jet fuels. In order to minimize the risk of surface contamination from trace impurities in a fuel, a model fuel consisting of n-dodecane (Phillips 66, pure grade, 99 mol % minimum) was used. Flat substrates were used in the initial phase of the study to provide reproducibly clean surfaces and for the Fourier transform infrared (FTIR)and secondary ion mass spectrometry (SIMS) analyses which were limited to flat specimens in the instruments used. JFTOT heater tubes used to stress the model fuel in the JFTOT apparatus were also examined by X-ray photoelectron spectroscopy (XPS). Analyses were conducted on substrates exposed to the model fuel and the model fuel containing MDA. N,N'-Disalicylidene-1,2propylenediamine (MDA) (Pfalz and Bauer) was dissolved into the dodecane to obtain a final concentration of 5.8 mg/L. An MDA-copper complex was prepared from MDA and copper(I1) ethylacetoacetate. Instrumental Methods. X-ray photoelectron spectroscopy was used to determine the presence of MDA on metal surfaces. Analyses were performed using a Surface Science Instruments SSX-100-03 X-ray photoelectronspectrometer. Initial wide-scan spectra were taken with an X-ray beam size of 0.6 mm and a pass energy of 100 V to identify the elements in the surface region. Narrow region scans of the identified elements were made with a beam size of 0.3 mm and a pass energy of 50 V. Under these latter conditions the Au 4f,/, peak fwhm was about 0.9 eV. Semiquantitativeestimates of surface compition were calculated from narrow scans with the nonlinear least-squares curve fitting program supplied with the instrument. The base pressure was at least 2 X lo4 Torr for all samples analyzed. Fourier transform infrared spectra were acquired on a Digilab FTS-90 Fourier transform infrared spectrometer (Digilab Division, Bio-Rad Inc., Cambridge, MA) with a wide-band mercury-cadmium telluride detector. Reflection spectra on the metal substrates were acquired at 85' from the normal in p polarization (wire grid polarizer, Perkin Elmer Corp., Norwalk, CT) using a modifed Versatile Reflection Attachment (Harrick Scientific Inc., Ossining, NY). This optical configuration allows one to acquire high-quality IR spectra of films on metals with monolayer sen-
Schreifels et al. sitivity. Most spectra were recorded at either 8 or 2 cm-' resolution and 512 or 1024 scans were coadded to obtain adequate signal to noise. Triangular apodization was used and all computations were done in double precision (32 bits). Reference spectra of freshly cleaned metal substrates were used as required to obtain absorbance spectra. Secondary ion mass spectrometry was conducted in a static mode (SSIMS) with the time-of-flight system that has been previously de~cribed.'~In brief, the instrument utilizes a pulsed alkali-metal ion gun, containing a thermoionic emitter source, which produces approximately 24-15 pulses of 14.0-keVcesium ions having a beam divergence of less than 0.5'. These primary ions are than directed onto a sample which is floated at the secondary ion accelerating voltage of h0.6 kV, resulting in a primary ion beam energy of 20.0 keV when negative secondary ions are detected or a beam energy of 8.0 keV when positive secondary ions are analyzed. The data collection system is based upon commercially available CAMAC modules that are controlled by a dedicated Micro VAX-I1 computer (Digital Equipment Corp., Maynard, MA). The CAMAC modules are used for timing and pulse detection, all the data storage and numerical manipulations being performed by the computer. A LeCroy Model 4208 timeto-digital converter (6) allows up to eight secondary ions to be detected per pulse with the flight time being measured to *1 ns. The secondary ion mass spectra can be collected at a rate of approximately 5 kHz. The analysis chamber was pumped using a 450 L/s turbomolecular pump, and samples were introduced via a cryosorption/ion pumped sample loadlock. Base pressure of the instrument was approximately 8 X Torr. Surface Preparation. Flat substrates measuring 2.5 X 1.2 cm were prepared from OFHC copper, 6061 aluminum, and 304 stainless steel for the XPS analyses. The copper and stainless steel surfaces were ground to 4000 grit by using silicon carbide paper with triply distilled water as the lubricant. Samples for FTIR analysis were polished with 0.05-pm alumina. The aluminum substrates were first ground to a 4000 grit finish with silicon carbide paper, using dodecane as the lubricant, polished with 1-pm diamond paste and then sonicated in dodecane followed by toluene (10 min each). Immediately before use they were immersed in a 10% sulfuric acid solution for 3 min, then washed thoroughly with triply distilled water, and then allowed to air dry. Samples were considered sufficiently clean when hydrophilic. The absence of sulfuric acid residues was verified by XPS. The substrates for SSIMS examination were 1 X 1 cm and prepared as described above. After polishing, the surfaces were rinsed with triply distilled water and allowed to air dry in acidcleaned glassware. Samples were immersed in either pure dodecane or dodecane containing MDA at ambient temperature for 150 min. Afterwards the samples were withdrawn from this solution and rinsed with EPA grade toluene (Baker),covered with aluminum foil, and analyzed within an hour. The following procedure was used to obtain a metal surface that was suitable for XPS and FTIR analysis and represented the exposure to a solution of MDA at elevated temperatures. A block of aluminum was heated with a cartridge heater in an argon atmosphere. The block was constructed such that a 2.5 X 1.2 cm metal substrate could be attached with minimal disturbance of the surface. The temperature of the substrate was monitored with a copper-constantan thermocouple inserted through the heating block to a point just below the sample. After heating the block to the desired temperature, the metal sample was mounted onto the block. When the substrate had thermally equilibrated with the heating block, the block was immersed into a dodecane solution for 15 s. The sample was removed from the heating block and again immersed in the dodecane solution until the solution and sample had cooled to ambient temperature. The solution was decanted off and the substrate rinsed with EPA grade toluene. Prepared substrates were stored in an acid cleaned vial until they could be examined. Stainless steel and aluminum JFTOT heater tubes were also examined by XPS. Dodecane and solutions of MDA at 5.8 mg/L in dodecane were stressed at 260 OC and at 310 "C in the JFTOT (13) Hues,S. M.; Colton, R. L.; Mowery, R. L.; McGrath, K. J.; Wyatt, J. R.A p p l . Surf. Sci. 1988, 35,507.
Energy & Fuels, Vol. 5,NO.2, 1991 265
Adsorption of a Metal Deactivator Additive onto Metal
Table I. Composition of Copper Surfaces after Exposure to MDA at Ambient Temmrature atom percent condition C 0 Cu N freshly polished 38.8 26.6 34.6 nda 22.8 12.2 1.8 dodecane, 2.5 h 63.2 MDA/dodecane, 2.5 h 54.1 27.9 17.5 0.6
N 1%
I
CUZP
1
o
1s
a
1
C 16 I Cu
1
1
-
MDA COMPLEX
Dodecane * MDA C" 3s C" 3p
"nd = not detected.
1000
Do d e c P ne k
0 BINDING ENERGY leVl
Figure 1. XPS broad scans of the copper-MDA complex and copper exposed to MDA. for 1 h. The tubes were rinsed with hexane immediately after a JETOT experiment. Since the JFI'OT heater tube temperature is not isothermal, the tubes were sectioned so that XPS analyses were performed on segments representing known temperature
ranges.
Results In principle, it should be possible to detect all the elements (except hydrogen) that comprise the MDA molecule through XPS analysis. Nitrogen, carbon, oxygen, and any metals that may be chelated should be observed, with peak intensities proportional to the relative atomic amounts. Any film greater than about 100 A in thickness would be indistinguishable by XPS from a sample of the pure compound. When there is a thin film on a metal substrate, the base metal signal can still be observed easily. The relative intensities of the other peaks should remain the same if there are no impurities on the surface. However, nearly any surface that has been exposed to air for even a short time will have adsorbed oxygen and carbon. The oxygen band comes from oxide formation and the absorption of some oxygen-containingcompounds such as CO and COP The carbon species likewise can come from CO, C02, or other carbon-containing species. Carbon originating from the environment or possibly from the vacuum system has been referred to as "adventitious carbon" by many workers for several years and is used as a binding energy reference for any charging that might occur on nonconducting surfaces. However, the use of adventitious carbon as a binding energy standard can lead to complications, as indicated by Swift.14 Additionally, the samples that were analyzed in these experiments have been exposed to dodecane and, as a result, would be expected to have a residual carbon signal. As a consequence, the signal intensity ratios for carbon and oxygen cannot be relied upon as a quantitative measure of MDA content. Thus it was necessary to rely on the intensity of the nitrogen signal alone as the most reliable indication of the presence of MDA on the surface, since it is not present in the pure dodecane. This approach would be limited in a jet fuel, since there are other nitrogen-bearing compounds in fuels, and it is known that heteroatoms tend to concentrate in the deposits. The surface binding energies from XPS high-resolution scans of the copper-MDA complex were similar to those reported15for a copper-Schiff base analogue of MDA. It is clear from the XPS broad scan (Figure 1,top spectrum) that all the expected elements are present. The nitrogen peak was relatively intense compared with the carbon and oxygen peaks. The relative intensities of the elements (14) Swift, P.Surf. Interface Anal. 1982,447, (15) Dillard, J. G.; Taylor, L. T. J.Electron Spectrwc. Relat. Phenom. 1974, 3, 455.
detected should be similar to the intensity ratios of the chelate if it were present as a very thick film. Analysis of signal intensities from high-resolution scans of these peaks allowed an estimation of the relative atomic amounts. The amount of copper calculated was 2 times higher than expected, while the atomic ratios of the other elements were as anticipated. Although the exact reason for this discrepancy is not clear, the relative signal intensities should provide a reasonable indication of the amount of MDA present on an uncontaminated surface. Flat Metal Specimens. Copper. Flat substrates consisting of OFHC copper were prepared and exposed to dodecane and dodecane solutions of MDA at room temperature as described above and then examined by XPS and FTIR. The substrates were introduced into the vacuum system within 2 h after exposure to the dodecane solutions by using a quick insertion system. The XPS survey scan from the freshly cleaned copper surface indicated that the sample preparation procedure produced an acceptably clean surface. Exposure to the MDA solution (Figure 1,middle spectrum) had increased the amount of carbon on the surface but nitrogen was not observed in the broad scan. This indicated that at must there could only be a very thin film of the MDA on this surface. Otherwise, the nitrogen intensity would be similar to that found in the copper-MDA complex. The results of this are shown in Table I for the freshly polished surface and after exposure to MDA. The carbon and oxygen levels observed on the freshly polished copper, although still quite high, are small with respect to those found from any other cleaning procedures that were tried. As expected, the calculated carbon contents on surfaces that were exposed to the organic solutions were higher than on the freshly cleaned surfaces. In addition, the quantities of carbon were about the same for the surfaces of both the sample exposed to pure dodecane and to MDA. Nitrogen levels were at or below the detection limit of approximately one atomic percent. The lack of any nitrogen signal suggests that the formation of an MDA coating on the copper surface was not occurring, since XPS can resolve monolayer thicknesses. FTIR analysis of the copper surfaces exposed to dodecane showed that an organic acid had chemisorbed onto the surface. This is evident from absorptions in the C-H stretching region (below 3000 cm-') and between 1400 and 1750 cm-'. The former absorptions can be assigned to CH, and CH2 groups and the latter to vibrations of the C=O of the free acid and to motions of a chemisorbed species (COO-). In addition, the ratio of the intensities of the bands due to CH2 and CH3 (i.e., CH2/CH3) is about 2. This implies that the adsorbed film is a carboxylic acid and that this acid has a straight-chained tail which is partially aligned with respect to the copper surface.16 The presence of a few parts per million of dodecanoic acid was confirmed by HPLC analysis of the base-extractable component of the dodecane. Thus, the acid adsorbed on the metal surface was most likely a dodecanoic acid, derived from (16) Allara, D.L.;Nuzzo,
R. G. Langmuir
1985, 1 , 52.
Schreifels et al.
266 Energy & Fuels, Vol. 5, No. 2, 1991 Table 11. Composition of 304 Stainless Steel Surfaces after Exuosure to MDA at Ambient Temwrature atom percent condition C 0 Fe N 57.8 14.0 1.4 freshly polished 21.3 37.0 7.7 0.7 dodecane, 2.5 h 45.7 48.8 10.9 0.6 MDA/dodecane, 2.5 h 35.1
Table 111. Compositions of Stainless Steel Surface Films after Exposure to Silica Gel Treated Dodecane Solutions atom percent condition Fe 0 C N Cr Ambient Temperature freshly cleaned 14.0 57.8 21.3 1.4 4.2 dodecane, 2 h 11.0 55.8 28.3 0.4 3.7 MDA/dodecane 8.1 50.2 34.3 1.1 3.2 freshly cleaned dodecane, 20 s
260 O C 14.7 67.8 12.9 61.8 13.4 61.5
16.8 23.7 23.3
nda 1.1 1.4
0.7 0.4 0.3
freshly cleaned dodecane
310 "C 16.1 68.1 15.9 66.6 13.7 59.0
15.0 17.1 24.3
0.5 nd 1.8
0.2 0.4 1.2
MDA/dodecane
DODECANE X
I
(SILICA-GEL TREATED1
MDA/dodecane "nd = not detected.
(MDAIC~I)'
GO0
3400
2i00
2200 CM
1600
1000
400
'
Figure 2. FTIR spectrum of stainless steel surfaces exposed to MDA in dodecane at ambient temperature.
autoxidation of the dodecane. Varying the MDA content between 2.9 and 14.4 mg/L had no discernible effect on the magnitudes of the C-H and C=O stretches that characterized the surface adsorbed acids. Stainless Steel. Flat 304 stainless steel substrates were prepared and exposed to dodecane- and dodecahe-containing MDA as described above at ambient temperature and at elevated temperatures. The exposed samples were examined by the XPS, FTIR and SSIMS techniques. The results from the XPS examination of 304 stainless steel exposed to MDA at ambient temperature are shown in Table 11. There were higher levels of oxygen on the freshly cleaned stainless steel surface than on the copper surface, probably from metal oxides. Nitrogen content was at the detection limit of 1% . FTIR surface analysis of stainless steel exposed to dodecane again revealed the presence of carboxylic acids on the surface. When the experiment was repeated using dodecane which had been treated with activated silica gel, there was no evidence of surface adsorbed acids in the FTIR spectra (Figure 2). The characteristic in-plane C=N stretching and the out of plane benzylic C-H bending frequencies seen in the infrared spectrum of the bulk MDA were not detected on the metal surface. MDA did not seem to exert any influence on the surface acid concentration. However, bands which could be associated with MDA were detected by FTIR when the carboxylic acids had been removed from the dodecane prior to exposure to the metal surface. The nitrogen concentrations by X P S were slightly higher in the presence of MDA and thus are consistent with the FTIR findings (Table 111). This indicates that the metal surfaces have a greater affinity for carboxylic acids and possibly other highly polar species which would interfere with the chemisorption of MDA. SSIMS analysis was performed on a stainless steel surface which had been exposed to a dodecane solution of MDA for 120 min at ambient temperature (Figure 3). The positive secondary ion spectra of this sample showed peaks at a massfcharge ratio of 284 and 334. These peaks would correspond to the molecular ions (MDA + 2H)+and (MDA[Cr])+,respectively. The mass spectrum of a blank
250
270
290
310
330
350
MASS (AMU)
Figure 3. Positive secondary ion mass spectrum of 304 stainleas steel after exposure to MDA in dodecane.
sample made with neat dodecane did not show this peak. Since the dodecanef MDA-treated surface was rinsed with toluene, it may be assumed that the MDA was not a residual contaminant and has undergone some type of interaction with the metal surface. When the stainless steel was heated in an inert atmosphere, the amount of chromium on the surface decreased with temperature. The iron and oxygen signals increased in intensity, but the carbon signal became weaker at higher temperatures. This indicates that residual carbon was probably being removed either by thermal desorption or by chemically reacting with some species on the surface and then undergoing thermal desorption or by diffusion of carbon into the metal. Thus, at elevated temperatures the surface was cleaner and was predominantly an iron oxide. Unoxidized iron was not observed by XPS on the heated samples, so that the oxide layer was thicker than on the unheated samples. Since the heated samples had lower background levels, lower carbon and nitrogen concentrations were found when the surface was exposed to dodecane. Exposure to MDA resulted in observable increases in the carbon and nitrogen signals (Table 111). While the nitrogen signal is still very close to the detection limit, it was consistently higher on those samples that had been exposed to MDA. The nitrogen concentrations increased with temperature, with the most found on samples heated to 310 OC. This may indicate that there was more MDA or its decomposition products on the surface at the higher temperatures. Aluminum. Specimens were prepared from 6061 aluminum and treated with dodecane and a dodecane solution of MDA at ambient temperature as described above and
Adsorption of a Metal Deactivator Additive onto Metal
Energy & Fuels, Vol. 5, No. 2, 1991 267
Table IV. Composition of Aluminum Surfaces exposed to Dodecane Solutions of MDA for 150 min at Ambient Temperature atom percent condition C 0 A1 N dodecane 24.6 53.7 20.8 0.9 32.0 48.6 18.3 1.1 MDAfdodecane Table V. Surface Composition of Aluminum Surface Films after Exposure to Silica Gel Treated Dodecane Solutions atom percent treatment Al 0 C N Ambient Temperature 28.1 60.9 10.7 0.3 freshly cleaned dodecane 26.9 50.5 22.3 nd" 1.2 MDA/dodecane 24.7 54.0 19.3 freshly cleaned dodecane MDA/dodecane
260 "C 27.6 54.6 26.6 51.6 26.3 54.1
15.7
nd 0.6 0.6
freshly cleaned dodecane MDA/dodecane
310 O C 27.2 59.3 27.2 58.6 26.3 56.4
7.2 10.5 12.0
nd nd nd
13.6 18.1
"nd = not detected.
at elevated temperatures. The prepared samples were examined by XPS, FTIR and SSIMS. The results from area analysis of XPS peaks are summarized in Table IV. The nitrogen content was very low, but somewhat higher on surfaces exposed to MDA, particularly in those instances when the dodecane was treated with silica gel to remove acids (Table V, ambient temperature). The results of FTIR examinations of the aluminum surfaces were similar to those from the stainless steel. Untreated dodecane deposited a layer of carboxylic acid while the silica gel treated dodecane did not. MDA was not found on the aluminum surfaces by FTIR. SSIMS analysis revealed the presence of a peak corresponding to m l e of 307, which could be an MDA-aluminum ion (MDA[A1]-2H)+), observed only on aluminum exposed to MDA in silica gel treated dodecane and not on the dodecane blanks. Treatment of the dodecane with silica gel reduced the quantities of hydrocarbons on the surface of the aluminum, which would be consistent with the FTIR findings. Unlike the stainless steel, the carbon content of the aluminum surface did not decrease as much after heating in an inert atmosphere and increased when exposed to dodecane (Table V). There was no significant effect of MDA on surface carbon content. The nitrogen level was higher on the sample that had been exposed to MDA at ambient temperature and decreased with increasing temperature. The binding energies were found to be consistent for all the samples analyzed and were as precise as could be expected, i.e., *0.2 eV. Two peaks were observed in the aluminum 2p region that corresponded to the oxidized and reduced states. The oxidized state represented nearly 90% of the total aluminum 2p peak area region. Since the reduced state was still apparent, the oxide layer probably was less than 5 nm thick. JFTOT Heater Tubes. Stainless Steel. While 6061 aluminum is the material most commonly used in the construction of JFTOT heater tubes, 304 stainless steel is often used in research efforts involving the JFTOT apparatus and many fuel system components are constructed from stainless steel. Areas from stainless steel JFTOT heater tubes were examined by XPS after only 1 h of
Dodecane
\ EINDING ENERQY (evl
1000
0
Figure 4. XPS broad scan of stainless steel JFTOT heater tube surfaces after 1 h stress at 260 O C . Table VI. Composition of Surface Films Formed on Stainless Steel Heater Tubes after 60 min of JFTOT Testing atom percent temperature, O C C 0 Fe Cr N Dodecane, 260 O C 174-208 56.7 38.4 2.8 0.5 nda 208-238 41.4 49.2 6.5 nd nd 238-257 51.8 39.8 5.0 nd nd 257-260 78.3 20.4 1.3 nd nd 5.8 mg of MDA/L in Dodecane, 260 OC 174-208 56.0 37.6 2.2 0.6 1.0 208-238 42.8 46.0 4.0 nd 1.2 238-257 48.2 42.3 3.5 0.2 1.6 257-260 82.0 17.6 0.4 nd nd 5.8 mg of MDA/L in Dodecane, 310 O C 212-253 39.6 45.2 11.4 0.3 1.2 253-287 56.2 37.5 4.4 0.5 1.5 287-306 81.9 13.8 nd nd nd 305-310 93.3 6.2 nd nd nd "nd = not detected. Table VII. Comparison of Films Formed on Aluminum Heater Tubes after 60 min of JFTOT Testing atom percent temperature, " C C 0 A1 N Dodecane, 260 O C 141-193 30.5 47.2 20.4 nd' nd 193-232 28.9 47.2 20.6 232-256 31.9 45.9 nd 18.4 nd 256-260 28.1 47.8 19.4 141-193 193-232 232-256 256-260
5.8 mg of MDA/L 30.8 33.0 33.7 44.1
in Dodecane, 260 O C 45.1 19.1 43.2 18.4 43.4 18.5 35.5 17.3
0.8 1.0 1.3 nd
"nd = not detected.
stressing samples of dodecane with and without MDA. Nitrogen on heater tubes heated at 260 "C was near the detection limit. The XPS broad scans show that the concentrations of chromium and manganese on the tube surfaces were very low (Figure 4) and that the iron content was significantly reduced (Table VI). Therefore, a multilayer deposit was forming whether the MDA was present or not. At 310 "C in the presence of MDA, similar results were obtained except that the carbon levels were higher. The iron peak, characteristic of oxidized iron (2p3/,, 710.5 eV), showed that, at elevated temperatures, the surface was comprised of iron oxide. Aluminum. XPS examinations of 6061 aluminum JFTOT heater tubes revealed that carbon levels were lower than on the stainless steel tube surfaces (Table VII). Accordingly, there was no evidence of a multilayer structure of thermally degraded dodecane over the full length of the tube. This substantiates earlier reports that the
268 Energy & Fuels, Vol. 5, No. 2, 1991
liquid-phase chemistry is identical for both tube materials17 but heavier deposits tend to form on stainless steel heater t ~ b e s . ~ JNitrogen * was at or below the detection limit in both cases. However, slightly more nitrogen was evident on tubes heated in the MDA solution. The oxide coating thickness was no greater on the aluminum JFTOT tubes than on the flat specimens, i.e., about 5 nm. Discussion Several attempts at obtaining XPS spectra of pure MDA were unsuccessful. It appeared that the compound was subliming in the vacuum chamber. Cooling the sample within the instrument may have solved this problem, but that technique was not available. We were able to detect MDA on stainless steel surfaces in the high-vacuum chamber with SSIMS and at atmospheric pressure with infrared spectroscopy. Chemisorbed MDA would not be expected to readily sublime; therefore, the SSIMS was probably detecting chemisorbed MDA which remained on the surface. The FTIFt would not necessarily discriminate between physisorbed and chemisorbed MDA, although physisorbed MDA should have been removed when the substrates were rinsed with solvent in which the MDA was very soluble. Therefore, the possibility of evaporative loss of physisorbed MDA limits the accuracy of any estimate of surface coverage when such an estimate is based on measurements obtained under high vacuum, i.e., SSIMS or XPS. However, while we cannot accurately estimate the percentage of the surface which is coverqd with a film of MDA, the findings from this study do indicate that the coverage was less than a monolayer. This rules out the possibility that the MDA acts to suppress thermal deposition onto the hot metal surfaces simply by forming a contiguous barrier between the fuel components and the metal. The metal deactivator has been shown to effectively chelate soluble copper but we found no evidence of attachment of MDA to a clean copper surface. Increases of copper dissolution attributed to the additive must be occurring through formation of a soluble complex. It is known that MDA will suppress deposition onto unused JFTOT tubes. Within 1 h of JFTOT testing, multilayer coatings of carbonaceous thermal oxidation products were formed on the stainless steel JFTOT heater tubes from a relatively unreactive "fuel", Le., dodecane. Although the nature of the surface can exert an effect on deposition, MDA seems to inhibit this process on a wide variety of substrates, both metallic and organic. The findings of this study indicate that when the liquid phase contains only a few ppm of carboxylic acids, a significant portion of the metal surface is coated with chemisorbed acids. While MDA did not affect the acid coverage over a concentration range expected in fuels (i.e,, 2.7-14.4 mg/.L), there was more MDA on the surface when acids were not present to begin with, by silica gel treatment. If surface-adsorbed acids play a role in the formation of (17) Hazlett, R. N., Hall,J. M.; Matson, M. Znd. Eng. Chem. R o d . Rea. Dev. 1977, 16, 171. (18) Faith, L. E.; Ackerman, C. H.; Henderson, H. T.; Richie, A. W.; Ryland, L. B. 'Hydrocarbon Fuele for Advanced Syatems". Shell Development Co.; Air Force Report AFAPL-TR-70-71, Part 111, 1972.
Schreifels et al.
deposits, there are ample opportunities for organic acids from fuels to reach the metal surface. Acids can also be thermally generated during autoxidation. The nature of the metal does seem to have some impact on deposition rate. An aluminum surface accumulates significantly less deposit than does a stainless steel surface under identical conditions of thermal stress. In addition, a magnesium-rich surface, such as that produced on aluminum after thermal stress, has been shown14 to be even less reactive toward fuel deposition. In that context, it would seem that surface acidity may play a role in the interactions between fuel and metal that lead to formation of adherent deposits. Since attachment of MDA to the metal surface does not seem to be a prerequisite for reducing deposition, the additive may function by interacting with acidic sites on the metal or deposit surface. In addition, interactions with active sites or surface defects cannot be excluded from consideration either. An understanding of the nature of this interaction will be necessary in order to define how and why this additive so strongly suppresses deposition in the JFTOT. Conclusions Surface analyses of copper, aluminum, and stainless steel exposed to MDA in static solutions and in JFTOT testing point toward a different mechanism than what would be expected if the surface were deactivated toward deposition by a contiguous layer of MDA. While these findings suggest that the surface contains some chemisorbed MDA, it is sparsely distributed; i.e., monolayer coverage was not attained. There was no evidence that MDA was coating a pure copper surface. It is noteworthy that this result agrees with the findings of Clark et a1.,12 obtained from SIMS examinations of aluminum surfaces exposed to comparable levels of MDA in fuel. In the JFTOT, there was more deposition onto the 304 stainless steel tubes than on the 6061 aluminum. If metal surface passivation was responsible for the strong response of the JFTOT to MDA, then one would expect to observe at least a monolayer of MDA on the metal surface over the entire test duration of 150 min. However, in this study, while MDA suppressed visually detectable amounts of thermal oxidation deposits, surface analysis revealed that multilayer deposits were produced on 304 stainless steel JFTOT tubes after only 1 h of JFTOT testing at the specification test temperature, 260 "C. Therefore, the time required for monolayer deposition of thermal oxidation products onto stainless steel heater tubes during JFTOT testing was well within the limits of the test. While this study has shown that passivation by chemisorbed MDA does not occur, the mechanisms through which MDA a d s is still not completely understood. Future efforts in this area will be directed toward determining how MDA can reduce deposition in the absence of dissolved metals. Acknowledgment. We thank Mr. Bruce Black of Geo-Centers, Inc. for the determination of carboxylic acids in the dodecane. This work was funded by the Office of Naval Research and the Naval Air Propulsion Center. Registry NO.MDA,94-91-7; CU,7440-50-8; AI,7429-90-5; 304 steel, 11109-50-5.
