Corrosion of Aluminum Fuel System Components by Reaction with

108

Energy & Fuels 1996, 10, 108-116

Corrosion of Aluminum Fuel System Components by Reaction with EGME Icing Inhibitor D. B. Skoropinski The Boeing Company, P.O. Box 3999, M/S 73-09, Seattle, Washington 98124-2499 Received August 14, 1995. Revised Manuscript Received October 30, 1995X

Aluminum structures in sealed fuel systems into which dried, deaerated JP-10 had been introduced were found to have corroded after several years in storage. An investigation into the cause of corrosion revealed that the aluminum had been chemically attacked by the weakly acidic alcoholic hydrogen on the ethylene glycol monomethyl ether icing inhibitor additive in the fuel. Under dry, anaerobic conditions, the ethylene glycol monomethyl ether was found to attack each of several aluminum alloys that were tested, even in instances where they had been treated with a chromate conversion coating. Hydrogen gas and aluminum alkoxides have been identified as the primary byproducts of the corrosion reaction. Methane, methanol, ethanol, ethers, and ether alcohols were also observed. Water and oxygen in sufficient concentrations inhibited attack, probably by virtue of the protective aluminum oxide that is maintained in their presence.

Introduction Aluminum structures in sealed fuel containment and transfer systems that were filled with fuel were discovered to have corroded after several years in storage. Aluminum alloys A357, 2024, 6061, and 7075 with an Alodine chromate surface treatment were all present in the system, as were bare 6061 and bare 2017-T3 (rivets). The synthetic JP-10 fuel used in the system consists of no less than 98.5% exo-tetrahydrodicyclopentadiene (the saturated hydrocarbon octahydro[3a,4b,7b,7a]-4,7-methano-1H-indene). Ethylene glycol monomethyl ether (EGME or methyl cellosolve) is added to JP-10 as a fuel system icing inhibitor at the rate of 0.10-0.15 vol %. The EGME is expected to do double duty by inhibiting growth of microorganisms. Butylated hydroxytoluene (BHT) at a concentration of 90-115 ppm serves as an antioxidant intended to inhibit degradation of the fuel. Isomers of the main component and other hydrocarbons make up the balance of the fuel. When the system was filled with fuel, the JP-10 was deaerated and was passed through a molecular sieve bed to dry it to a level of less than 10 ppm water. Dry nitrogen had been installed in the ullage space. As a result of the care taken in configuring the system, oxidation or corrosion mechanisms driven by aqueous media were not anticipated, assuming the physical integrity of the system remained intact, and the fuel system was expected to be ready for use even after long periods of inactivity. Instead, when the tanks were checked, the ullage pressure in some of them had increased, and, when the systems were opened for inspection, aluminum surfaces in the ullage space and below the liquid level were found to have pitted, yielding solid corrosion byproducts. Results of an investigation into the cause of corrosion, reported here, show the EGME icing inhibitor to be the aggressive agent by virtue of the weak acidity of its alcoholic hydroxyl functional group. Although aluminum resists corrosion by (reaction with) most organic X

Abstract published in Advance ACS Abstracts, December 1, 1995.

0887-0624/96/2510-0108$12.00/0

compounds under wet or dry conditions, the lower alcohols and phenols are known to corrode aluminum in an anhydrous environment.1-3 The susceptibility of aluminum to attack by alcohols in the absence of water has been attributed to the fact that, without water, the protective oxide film on the aluminum surface cannot be maintained or repaired.1 An anaerobic environment would be expected to exacerbate this situation. Corrosion of aluminum by EGME, ethyl cellosolve, and butyl cellosolve has been documented, with reaction reportedly being suppressed by the presence of water.4 Additionally, EGME has been used as a reagent for the synthesis of Al(OCH2CH2OCH3)3 from powdered aluminum alloy in toluene.5 These considerations plus the results presented below show that attention must be given to internal conditions in order to preclude corrosion of aluminum alloys in fuel systems employing ether alcohols as icing inhibitors or as agents to suppress biological growth. Experimental Section Materials. Sheet aluminum alloys 7075, 2024, and 6061 treated with Alodine 1200 chromate conversion coating, bare alloys 7075 and 6061, and clad aluminum 7075 were chosen for exposure testing. Sheet A357 alloy was not available, so portions of an A357 casting were treated with Alodine 1200 and used instead. Exposure media were prepared from JP10 fuel (Koch Chemical Co.), Fisher Certified EGME with a water content of 0.1% (Fisher Scientific), anhydrous 99.9+% EGME (Aldrich), BHT (Eastman Kodak), and Fisher Optima purified water. Exposure Testing. Those coupons that were to be tested without application of a protective chemical conversion coating (1) Shreir, L. L., Ed. Corrosion, Volume 1: Metal/Environment Reactions; Newnes-Butterworths: London, 1976; p 4:28. (2) Mears, R. B. In The Corrosion Handbook; Uhlig, H., Ed.; Wiley: New York, 1948; p 49. (3) Hollingsworth, E.; Hunsicker, H. Metals Handbook Ninth Edition, Volume 2, Properties and Selection: Nonferrous Alloys and Pure Metals; American Society for Metals: Metals Park, OH, 1979; pp 231235. (4) Yoshimura, C; Ogura, T. Keikinzoku 1982, 32(9), 443-450. (5) Iwao, T.; Osaka, S.; Sakaki, T.; Nishida, T.; Yamamura, K.; Koga, S.; Hirai, R.; Nakagawa, H. International Patent Appl., PCT/W090/ 05128, May 1990 Mitsui Toatsu Chemicals, Inc.; 35 pp.

© 1996 American Chemical Society

Corrosion of Aluminum Fuel System Components

Energy & Fuels, Vol. 10, No. 1, 1996 109

Table 1. Exposure Test Set 1: Matrix and Results for Alodined 7075 Coupons after 3 days at 140 °F Using EGME with 0.1% Water trichloroethane degreased

MEK cleaned

test liquid

air

nitrogen

air

nitrogen

EGME EGME + 5% water EGME + JP-10, 50/50 EGME + BHT, 50/50

1-1, no attack 1-5, no attack 1-9, no attack 1-13, two pits

1-2, corroded 1-6, no attack 1-10, corroded 1-14, corroded

1-3, one pit 1-7, no attack 1-11, no attack 1-15, one pit

1-4, four pits 1-8, no attack 1-12, corroded 1-16, corroded

Table 2. Exposure Test Set 2: Matrix and Results for Bare and Clad 7075 Coupons All in EGME with 0.1% Water material

air atmosphere

nitrogen atmosphere

bare 7075 clad 7075

2-1, no attack 2-3, one pit

2-2, corroded 2-4, corroded

Table 3. Exposure Set 3: Matrix and Results for Various Aluminum Alloys Exposed to Anhydrous EGME Icing Inhibitor in Dry Nitrogen coupon

description

time at 140 °F

3-1 3-2 3-3 3-4

Alodined A357 Alodined A357 Alodined 7075 Alodined 7075

3-5

Alodined 2024

3-6 3-7 3-8 3-9 3-10

Alodined 2024 Alodined 6061 Alodined 6061 Bare 6061 Bare 6062

42 h, corroded 42 h, corroded 18 h, corroded none, corroded prior to placement in tank none, corroded prior to placement in tank 18 h, corroded 18 h, corroded 18 h, corroded 18 h, corroded 18 h, corroded

were cleaned with methyl ethyl ketone (MEK) and were not otherwise cleaned or modified prior to testing. Except as noted below, the coupons to which Alodine 1200 chromate surface treatment was to be applied were first vapor degreased with trichloroethane (as the fuel system components had been cleaned). The Alodine 1200 was applied per product specifications in the same manner as the fuel system components had been processed. Test samples varied in size from approximately 2 by 6 in. to about 1 by 4 in., depending on material availability. All coupons of alloys 7075 and 2024 were fabricated from 20 mil sheet stock. The aluminum 6061 coupons were 75 mil thick, and the A357 casting was 0.35 in. thick. Susceptibility to corrosion under various conditions, rather than corrosion rate, was considered to be of primary importance for this study, so that uniformity of sample dimensions was not critical. The test samples were placed into wide-mouth quart jars for exposure testing. To ensure exclusion of oxygen when desired, the samples were prepared in an argon-filled glovebox into which Grade 5 nitrogen had been plumbed in order to backfill the jars, mimicking conditions in the fuel system. Dry air was also plumbed into the glovebox for preparation of test samples intended to show the effect of oxygen on susceptibility to corrosion in the absence of water. The samples under dry air were prepared last so as to preclude contamination of the other containers with oxygen. Each container was purged for

2 min by flowing a stream of the dry gas of choice into the container before closing its lid. The coupons in the jars were partially immersed in a volume of 50 mL of one of six test fluids, which were EGME, EGME plus deionized water, EGME plus JP-10, EGME plus BHT, JP-10, and JP-10 plus BHT. The test containers were sealed using lids with Teflon liners and were thermostated at 140 °F in an explosion-proof tank that is routinely used for fuel exposure testing. Test duration at elevated temperature varied from 18 to 42 h for samples which exhibited corrosion and to more than 35 days for coupons which did not corrode. The test matrix is summarized in Tables 1-4. Gas Phase Analysis. The ullage composition in a corroded fuel containment and transfer system as well as in the test jars was determined by mass spectrometry using a VG Micromass PC 300D residual gas analyzer (RGA). In the case of the fuel system, a deactivated, empty fused-silica open tubular column was plumbed directly from the RGA into the fuel tank ullage space. The gas in the test jars was accessed by bonding a Teflon-lined silicone septum to the surface of the lid with quick-setting epoxy, drilling through the septum and lid, and inserting a syringe needle that was interfaced directly to the RGA vacuum system through the septum and lid. The RGA was calibrated prior to the analyses using Grade 5 nitrogen. The fuel system ullage gas was analyzed for sulfur gases using Dra¨ger indicator tubes for hydrogen sulfide and sulfur dioxide. A short length of Tygon tubing was used to connect each Dra¨ger tube directly to the fuel system plumbing, and a Dra¨ger sampling pump was used to meter 1 L of gas through each tube. The ullage gas from the fuel system and from selected test jars was also analyzed by gas chromatography-infrared spectrometry (GC-IR) and/or gas chromatography with flame ionization detection (GC-FID) using a BioRad (Digilab Division) FTS-60 Fourier transform infrared (FTIR) spectrophotometer interfaced to a Digilab GC-IR light-pipe attachment and a Hewlett-Packard 5980A gas chromatograph (GC). The GC was fitted with a 25 m HP-5 fused silica capillary column (5% phenyl-methyl siloxane stationary phase, 0.52 µm film thickness, 0.32 mm internal diameter). The FID detector follows the infrared light-pipe in series. The fuel system ullage gas was expanded into an evacuated stainless steel gas sampling tube for transport to the FTIR laboratory, while selected test containers were sampled directly through the drilled septa with a gas-tight syringe. Gas volumes of 0.25 mL were injected into the GC column, which was heated from 50 to 180 °C at a rate of 10 °C/min, and with a 1 min temperature hold at each end of the program. The injector and detector were held at 250 °C, while the infrared light-

Table 4. Exposure Test Set 4: Matrix and Results for Alodined 7075 Coupons Exposed to a Variety of Test Fluids

a

coupon

test fluid

atmosphere

time at 140 °F

result

4-1 and 4-2 4-3 and 4-4 4-5 and 4-6 4-7 and 4-8 4-9 and 4-10 4-11 4-12 4-13 4-14

conditioneda JP-10 BHT + JP-10, 50/50 JP-10 + 4% EGME anhydrous EGME JP-10 + 4% EGME EGME + 0.5% water EGME + 1.0% water EGME + 3.0% water EGME + 5.0% water

nitrogen nitrogen nitrogen air air nitrogen nitrogen nitrogen nitrogen

35 days 24 days 2 days none >1 month >1 month >1 month >1 month >1 month

no corrosion no corrosion corroded corroded no corrosion no corrosion no corrosion no corrosion no corrosion

“Conditioned” here means dried and deaerated in the same manner as fuel that was introduced into the fuel systems in question.

110

Energy & Fuels, Vol. 10, No. 1, 1996

Skoropinski

Figure 1. Test set 1, consisting of Alodined 7075 aluminum alloy coupons, after exposure to the test fluids, including EGME containing 0.1% water. pipe and GC-to-IR interface were heated to 200 °C. The GC was operated in the splitless mode, opening the septum purge valve 1 min after injection. Solid and Liquid Phase Analyses. Solid corrosion product from the test coupons and from the fuel system were analyzed by infrared microspectroscopy using the FTS-60 with a BioRad (Digilab Division) UMA 300A infrared microscope

attachment, and by energy-dispersive X-ray spectrometry (EDX) with a JEOL JXA-8600 electron microprobe. Fluid from three selected containers (3-1, 3-5 and 3-9) was distilled to separate the liquid phase components from solid corrosion/ reaction products. Fuel from the corroded fuel system, freshly conditioned (deaerated and dried) JP-10, and samplings of test fluid distillate were analyzed by GC-IR using the conditions



described above, except that the analyses were done in the split mode using injection volumes of 2.0 µL, and the column was heated to 200 °C at a rate of 10 °C/min rather than to only 180 °C.

Results and Discussion Exposure Testing. Early exposure testing was inconclusive, but pointed to EGME as a possible contributor to the corrosion problem, leading to the tests described here. Because the significance of water and oxygen in the corrosion process was not understood at the beginning of the investigation, the driest EGME readily available, EGME that contained 0.1% water, was used for the initial exposure tests. The first two sample sets totaled 20 coupons that were all exposed to this “wet” EGME. Sixteen of the coupons (set 1) were 7075-T6 alloy which had been treated with Alodine. One subset of eight had been vapor degreased with trichloroethane and a second subset had been cleaned by flushing with MEK prior to Alodining. Set 2 included two non-Alodined (bare) 7075 coupons and two nonAlodined aluminum-clad 7075 coupons. As listed in Table 1, the Alodined coupons were exposed to EGME with 0.1% water, EGME plus an added 5% distilled/ deionized water, and EGME plus JP-10 in a 50:50 ratio by volume or EGME plus BHT antioxidant in approximately equal masses. The non-Alodined set 2 coupons were all partially immersed only in the EGME. Half of each subset were prepared in air, and the other half of the containers were backfilled with Grade 5 nitrogen. The coupons are shown in Figures 1 and 2 as they appeared after three days at 140 °F in the fuel soak tank. None of the coupons exposed to 5% water plus EGME exhibited any corrosion. Minor corrosion occurred on three of the samples that were blanketed with air (coupons 1-3, 1-13, 1-15) and one that was in a nitrogen environment (1-4). The five coupons that corroded significantly were all under nitrogen. Two in EGME plus JP-10 were completely consumed by the reaction, while the two which were partially immersed in the mixture of EGME and BHT had been more than half consumed. The identity of the cleaning solvent used prior to application of Alodine appeared to have no significant effect. The bare and clad set 2 coupons under nitrogen were severely attacked, while only one edge pit was observable on one of those in air. These results are indicative of a protective effect by both oxygen and water. They also seem to indicate an augmented corrosion rate due to the presence of the JP10 itself and/or the BHT, but the data presented below do not support such a conclusion. An effect of JP-10 and BHT in shielding the aluminum surface from water in the EGME is a more likely explanation for these observations. Exposure test sets 3 and 4 were more definitive, since these samples were all prepared using anhydrous EGME. Set 3 consisted of the specimens listed in Table 3, all of which were partially immersed in the anhydrous EGME and blanketed with Grade 5 nitrogen. Results for this set are easy to interpret. As the coupons shown in Figure 3 demonstrate, attack by EGME was not specific to any particular aluminum alloy. Test set 4 specimens, all consisting of Alodined 7075, are listed in Table 4, and selected representative coupons are shown in Figure 4. Like test set 3, these

Figure 2. Test set 2, consisting of bare and clad 7075 aluminum alloy coupons, after exposure to EGME containing 0.1% water.

samples confirm that EGME will attack aluminum under anhydrous conditions and further show that it is the only component of JP-10 that will do so. The 50 mL of JP-10 with no added EGME to which coupons 4-1 and 4-2 were exposed only contained 50-75 µL of EGME as received from Koch Chemical, an insufficient volume to yield observable corrosion in this test, but showing that the JP-10 hydrocarbon components do not attack the aluminum. Nor did high concentrations of BHT dissolved in JP-10 corrode coupons 4-3 or 4-4. But coupons 4-5 through 4-8 were all attacked by EGME, with and without added JP-10. The 2 mL of EGME added to the JP-10 to which coupons 4-5 and 4-6 were exposed was sufficient to substantially darken their surfaces in the inert test atmosphere (see Figure 4). Coupons 4-7 and 4-8 show that the oxygen in air will not adequately protect aluminum from attack by high concentrations of EGME in an anhydrous environment, but coupons 4-9 and 4-10 show that air does appear to inhibit attack by a sufficiently low concentration of EGME, equal to that which darkened coupons 4-5 and 4-6. Furthermore, test set 4 shows that the presence of sufficient water, as low as 0.5% (the lowest concentration evaluated), will protect the aluminum even under anaerobic conditions. Gas Phase Analysis: Fuel System Ullage Space. The ullage gas composition in two unopened fuel systems was determined by standardless quantitative analysis with the RGA. The fuel systems included one which exhibited an ullage gas pressure that was el-

112


Skoropinski

Figure 3. Representative coupons from set 3 after exposure to anhydrous EGME under anaerobic conditions. This test demonstrated that EGME will react with a number of aluminum alloys.

Figure 4. Representative Alodined 7075 aluminum alloy coupons from set 4 after exposure to the test fluids under anaerobic, anhydrous conditions except as specified otherwise here and in Table 4. Table 5. Fuel System Ullage Gas Volume Percent Composition gas composition of species

m/z

high-press. tank

low-press. tank

grade 5 nitrogen

H2 CH4 H2O N2 O2 Ar

2 16 18 28 32 40

5.0 0.5 0.4 94.0 0.02 0.08

0.01 0.07 0.3 97 1.8 0.3

0.003 0.01 0.07 99.8 0.04 0.02

evated by 6 psi, and a second system which had not experienced an increase in pressure. Concentrations of the compounds in the ullage gas were calculated using vendor sensitivity data for selected ions, and the results were normalized to 100 vol %. RGA results are summarized in Table 5. The most surprising results were the presence of approximately 5% hydrogen by volume in the system with elevated pressure, and the methane concentration of about 0.5%. EGME and the JP-10 hydrocarbons were not effectively transported through the unheated 30 m trans-

fer line to the RGA. By GC-IR, EGME and fuel hydrocarbons were detected in the vapor phase, and the presence of methane was confirmed. A spectrum of methane obtained by GC-IR analysis of the ullage gas from the fuel system with elevated internal pressure is presented in Figure 5A, and a spectrum of a reference sample of methane is presented in Figure 5B for comparison. Note in the spectrum of the methane from the fuel system sample that there are unassigned absorptions at 2180 wavenumbers and between 800 and 1100 wavenumbers that are absent in the methane reference spectrum, indicating that at least one other compound coeluted with the methane. Dra¨ger tube analyses for sulfur gases, which should have been capable of detecting 2 ppm of hydrogen sulfide and 1 ppm of sulfur dioxide, were negative. Gas Phase Analysis: Exposure Test Containers. The atmospheres in nine test containers were analyzed with the RGA. Referring to Figures 1-4 and Tables 1-4, the containers so analyzed were, from set 1, only the MEK-cleaned sample number 1-4, exposure to EGME under nitrogen (the containers of the more



Figure 5. Infrared spectra of (A) methane in the ullage space of a corroded fuel system and (B) a reference sample of pure methane. These spectra were obtained by GC-IR analysis. Note the peaks near 2180 and between 800 and 1100 wavenumbers in (A) that are absent in (B), indicating co-elution of an as yet unidentified compound along with the methane from the ullage gas sample. Some gas phase water (3500-4000 and 1300-2000 wavenumbers), probably introduced by sampling, also co-eluted with the methane from the fuel system; and weak carbon dioxide peaks are visible in (A) between 2300 and 2400 wavenumbers.

severely corroded coupons in set 1 had vented, precluding analysis of their atmospheres); from set 2, the gas over the two clad coupons 2-3 and 2-4, which had been exposed to EGME under air and nitrogen, respectively; from set 3, containers 3-2, 3-4, 3-6, 3-8, and 3-10; and from set 4, container 4-6. Hydrogen (mass/charge ) 2) was detected in every container and was the major gas phase component detected by RGA in containers in which the coupons had experienced the most corrosion. Methane was detectable by RGA analysis in all but three instances, which were containers 1-4 and 2-3, both of which contained coupons that had experienced only minimal corrosion, and container 4-6, which had only 4% EGME mixed with JP-10. Gas samplings from the eight containers 2-3, 2-4, 3-1, 3-3, 3-5, 3-7, 3-9 and 4-5 were analyzed by GC-IR, by which the presence of methane was detected in all but 2-3. The GC-IR chromatogram for sample 3-1 (gas evolved by corrosion of Alodined A357 alloy in anhydrous EGME under nitrogen) is presented in Figure 6A. Infrared spectra of methane and EGME eluting at the indicated retention times are presented in Figure 6, B and C, respectively. Methane was the principal component detectable by infrared and in fact was the only peak clearly observable above the noise level in the Gram-Schmidt chromatogram shown in Figure 6A (the Gram-Schmidt chromatogram represents the infrared signal over the entire mid-infrared spectral range vs elution time of the sample6). Note that the same

unassigned peaks are present in the methane spectrum in Figure 6B as were observed in the fuel system sample (Figure 5A). This result adds credence to the conclusion that the same chemical process occurred in the test containers as in the enclosed fuel system. These results are significant in that they demonstrate that chemical reaction of high-purity anhydrous EGME with each aluminum alloy tested produces the same gaseous byproducts as were generated in the enclosed fuel system. The evolution of hydrogen points to acidic attack on the aluminum by the alcoholic hydroxyl group on the EGME. The strongly reducing environment resulting from generation of a high concentration of hydrogen in dry, anaerobic conditions has apparently led to the production of significant quantities of methane. If the methane is a product of the reaction of hydrogen with EGME itself, then ethanol and perhaps methanol and ethylene glycol might be expected as liquid phase byproducts. Liquid Phase Analysis: Fuel Sample. JP-10 from the corroded fuel system and freshly conditioned JP-10 (dried and deaerated fuel as was originally installed in the system) were analyzed by GC-IR and GC-FID. EGME and BHT were detected and EGME was quantified in both liquid samples. The EGME was found to (6) Griffiths, P. R.; de Haseth, J. A. Chemical Analysis, Volume 83, Fourier Transform Infrared Spectrometry; John Wiley & Sons: New York, 1986; pp 604-607.

114


Skoropinski

Figure 6. (A) A GC-IR Gram-Schmidt chromatogram of the headspace gases above coupon 3-1 from set 3 after exposure to anhydrous EGME. (B) The infrared spectrum of the methane peak eluting at 0.75 min in (A). Note the same unassigned peaks near 2180 and between 800 and 1100 wavenumbers as were observed in the fuel tank ullage gas GC-IR data presented in Figure 5A; a low concentration of water vapor is also indicated. (C) The methane peak was the only one obvious in the chromatogram in (A), but EGME was detectable in the GC-IR data at a very low concentration 1.4 min into the run.

be present at a concentration of 0.10% in the sample from the corroded fuel system, but only 0.08% in the freshly conditioned fuel (the fuel in the fuel system did not originate from the same batch of fuel as did the conditioned material). By specification, EGME is required to be present at a concentration of 0.10 to 0.15 vol %. If corrosion results from chemical attack on the aluminum by the alcoholic hydrogen in the EGME, then a valid equation for the

corrosive process will be

6HO(CH2)2OCH3(l) + 2Al(s) f 2Al(O(CH2)2OCH3)3(l) + 3H2(g) Three moles of EGME are used for every mole of aluminum oxidized, and 2 mol of EGME yield 1 mol of hydrogen gas. The fuel system under study had an ullage space to fuel volume ratio of approximately 0.1. Assuming the entire 6 psi increase in pressure in the



Figure 7. A typical infrared spectrum of the solid corrosion product generated by reaction of aluminum alloys with EGME, in this instance from a 7075 aluminum alloy component of a corroded fuel system. Some spectra indicated a higher organic content (2800-3000 wavenumbers), but the organic material was not separable by extraction with solvents.

fuel system was due to evolution of hydrogen (which was approximately true, discounting the relatively small percentage of methane), reaction of a volume of EGME equal to 0.03% of the total fuel volume would have been required to obtain the observed pressure elevation at an ambient temperature of 72 °F. If so, the JP-10 in the system would have initially had an EGME concentration of 0.13%, a value that is within specification. The fact that an EGME concentration of 0.10% was found in the fuel from the corroded system does not invalidate the above evidence pointing to EGME as the corrosive agent. Only minor changes in the composition of the JP-10 from the corroded fuel tank relative to the freshly conditioned fuel were indicated by the direct GC analyses. No lower alcohols were identified, which is not surprising given the very low concentrations that would have been generated. The data indicated that low concentrations of a number of hydrocarbons of higher molecular weight than the main component had been formed, but none of these compounds have yet been identified with certainty. Liquid Phase Analysis: Test Solution Distillate. The liquid phase from three selected containers was distilled and the distillate collected for analysis. The liquid from container 3-5 boiled during distillation at 124 to 125 °C, the boiling temperature of EGME. GC analysis confirmed that most of the liquid phase consisted of unreacted EGME, but a low concentration of methanol was also detected by GC-IR. Container 3-5 had not been heated during exposure testing because the coupon exhibited corrosion before the sample could be transported to the exposure tank (Figure 3). Containers 3-1 and 3-9 had both been heated at 140 °F in the exposure test. Liquid from container 3-1 distilled over the range of 85 to 100 °C, after which the viscous material remaining exhibited signs of decomposition and distillation was stopped. Chromatographic analysis suggests that the liquid distilled azeotropically, since peaks with corresponding spectra indicative of higher molecular weight ethers and ether alcohols, plus a few compounds with carbonyl functionality, were prevalent in the chromatogram for that container. The lower alcohols were not detectable, perhaps suggesting that they had been consumed by other reactions.

The liquid in container 3-9 distilled near 98 °C, probably also as an azeotrope. Methanol and a very low concentration of ethanol were detectable, as were higher molecular weight ethers and ether alcohols in lesser quantities than in the distillate from container 3-1. Identification of the ethers and ether alcohols has not been pursued further, but these results show that a good deal of chemical activity can go on in the strongly reducing, hydrogen-rich environment created by attack of EGME on aluminum in a fuel system. The side reactions responsible for the higher molecular weight compounds were probably relatively unimportant in the fuel system, since it had remained at ambient temperatures throughout its life. Solid Phase Analysis. An infrared spectrum typical of colorless corrosion product from the fuel system is presented in Figure 7. The spectra of solid material sampled from a number of test coupons were similar. The spectrum is indicative of a good deal of hydroxyl functional group, probably as water (3450 and 1610 wavenumbers). Weak indication of organic functionality is also evident (2800-3000 wavenumbers). Rinsing with solvent such as MEK was ineffective in separating an organic phase, suggesting that the organic moiety was chemically bound to the aluminum corrosion product. Interestingly, the liquid phase in some test containers was quite viscous, but solidified almost immediately upon exposure to air, yielding spectra like that in Figure 7. It appears that the aluminum tri(methoxyethoxide) or other alkoxides formed by the corrosion reaction were very hygroscopic, yielding spectra similar to ordinary hydrated aluminum oxides generated by other corrosion processes (such as halide initiated oxidation in a wet environment). Early in the investigation, similar analytical results led to speculation that water had entered the fuel system to cause the corrosion observed here, perhaps due to contamination or by a biological process, suppositions which could not be supported and were eventually shown to be incorrect by the data presented here. The elemental composition of samplings of the solid corrosion products as determined by energy dispersive X-ray spectroscopy varied with composition of the alloys, as would be expected. Low concentrations of chlorine were detectable in a some samplings from the corroded

116


fuel system, and low to trace concentrations of sulfur were detected in many samplings, including, for example, bare 7075 cleaned with MEK and exposed to EGME under nitrogen. The role of the low levels of contaminants is uncertain. Some coupons and some fuel system components exhibited a greater tendency to corrode than did others, suggesting that some areas were more susceptible to attack, possibly due to contaminant ions or local variations in alloy composition. Conclusions Ethylene glycol monomethyl ether alone and as an additive to JP-10 fuel has been definitively shown to chemically attack the five aluminum alloys that were tested when water is absent or present at very low concentrations, whether or not the aluminum has been treated with a chromate conversion coating. Water at higher concentrations probably serves to protect the aluminum by maintenance and repair of the protective oxide coating. Oxygen appears to have some protective effect against attack by EGME, but not enough to ensure against corrosion in a strictly anhydrous environment. The EGME will attack the aluminum regardless of whether other fuel components, including BHT, are present or absent. It is the weakly acidic alcoholic hydrogen in the EGME molecule that attacks the aluminum, yielding hydrogen as the principal gaseous byproduct of the corrosion reaction. Methane, methanol, ethanol, a variety of ethers and ether alcohols, and dissolved and solid aluminum alkoxy compounds were also identified as byproducts of the interaction of EGME with aluminum, and of secondary reactions which occur in the strongly reducing environment created by the process in the near absence of oxygen and water.

Skoropinski

EGME has been used as an anti-icing agent and inhibitor of biological growth not only in JP-10, but also in JP-4 and probably in other fuel formulations. The results presented here show that EGME should not be used as an additive in fuel systems from which water is excluded if aluminum components are present. Preliminary study suggests that diethylene glycol monomethyl ether (di-EGME) is significantly less aggressive toward aluminum. Nevertheless, a thorough evaluation should be conducted if di-EGME or other compounds with alcoholic hydroxyl functional groups are candidate additives for a fuel system such as that described here, or if alcohols are to be used in any system with aluminum structure from which water is to be excluded. The strategy adopted for the systems under study here was to eliminate the EGME additive from the fuel altogether, in conjunction with periodic testing to ensure that the water content of the fuel remains low enough to preclude detrimental ice formation at low temperatures. Acknowledgment. Financial support of the United States Air Force (F34601-88-D-0961) and the technical assistance of Mr. Thomas R. Begley, Mr. Donald R. Cressey, Mr. Michael Johnson, and Mr. Robert S. Roper are gratefully acknowledged. Special thanks to Mr. Jeffrey A. Brewer for the photography included here; and to Dr. Mark M. Thornton for his work in planning, coordinating, and conducting the RGA analyses and interpreting the data, and for his valuable insights into other phases of this investigation. EF950164E

Corrosion of Aluminum Fuel System Components by Reaction with

Recommend Documents