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Polymers, polymer-graphite composites and different carbon materials are being considered for many of the fuel cell stack components. Exposure to concentrated phosphoric acid in the fuel cell environment and to high anodic potential results in corrosion. Relative corrosion rates of these materials, failure modes, plausible mechanisms of corrosion and methods for improvement of these materials are investigated.
The effects of trace metal impurities in coal-derived liquids on deposition, high temperature corrosion and fouling were examined. Alloys were burner rig tested from 800 to 1100 C and corrosion was evaluated as a function of potential impurities. Actual and doped fuel test were used to define an empirical life prediction equation. An evaluation of inhibitors to reduce or eliminate accelerated corrosion was made. Barium and strontium were found to limit attack. Intermittent application of the inhibitors or silicon additions were found to be effective techniques for controlling deposition without losing the inhibitor benefits. A computer program was used to predict the dew points and compositions of deposits. These predictions were confirmed in deposition test. The potential for such deposits to plug cooling holes of turbine airfoils was evaluated. Tests indicated that, while a potential problem exists, it strongly depended on minor impurity variations.
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The effects of potential impurities such as Na, K, Mg, Ca, and Cl, in coal-derived liquid fuels on accelerated corrosion of IN-100, U-700, IN-792, and Mar M-509 were investigated using a Mach 0.3 burner rig for times to 200 hours in one hour cycles. These impurities were injected in combination as aqueous solutions into the combustor. Other variables were time, temperature, and fuel-to-air ratio. The experimental matrix was a central composite fractional fractorial design divided into blocks to allow modification of the design as data was gathered. The extent of corrosion was determined by metal consumption. The time exponent was near 1.0 for the least corrosion resistant alloys, U-700 and IN-100; near 0.8 for the moderately resistant IN-792; and close to Mar M-509, the most corrosion resistant alloy. As anticipated, corrosion rapidly increased with increasing temperature as well as Na and K concentrations, while corrosion decreased somewhat as the Ca concentration increased for all alloys. Mg was beneficial for the Ni-base alloys but had little effect on the Co-base alloy. Surprisingly, the effect of increasing Cl was to decrease the corrosion of all alloys. Little interaction among the dopants was noted.
The effect of modifications in hydrocarbon jet fuels specifications on engine performance, component durability and maintenance, and aircraft fuel system performance is discussed. Specific topics covered include: specific fuel consumption; ignition at relight limits; exhaust emissions; combustor liner temperatures; carbon deposition; gum formation in fuel nozzles, erosion and corrosion of turbine blades and vanes; deposits in fuel system heat exchangers; and pumpability and flowability of the fuel. Data that evaluate the ability of current technology aircraft to accept fuel specification changes are presented, and selected technological advances that can reduce the severity of the problems are described and discussed.
The purpose of the DOE/NASA CRT (Critical Research and Technology) Project at the Lewis Research Center is to provide an R&T data base for utility gas turbine systems burning coal-derived fuels. Coal-derived fuels present difficult problems in combustion (because of the high organically-bound nitrogen in the fuel), and materials (because of the trace metal contaminant levels in the fuels, leading to corrosion and deposition in the turbine hot section). The combustion task includes (1) an effort to model the rich-lean combustion process, to predict NOx emissions; (2) a two-stage flame tube experiment to study the effects of combustion operating conditions and fuel properties on NOx conversion; and (3) scaled combustor tests to evaluate the NOx-reduction potential of several staged-combustion concepts. The materials-corrosion resistance effort attacks the problem on three sides.
The sodium sulfate hot corrosion of silicon-based ceramics at 1000 C has been extensively studied. Deposition of the sodium sulfate corrodant from combustion products is discussed in relation to sodium air impurity and sulfur fuel impurity content. Corrosion occurs by the combined processes of oxidation to form protective silica scales and dissolution of these scales to form nonprotective sodium silicates. The chemical corrosion mechanisms are presented in terms of acidic/basic dissolution of oxides in molten salts. The reactions are strongly influenced by the presence of free carbon in the ceramic. Strength reductions have been measured and are attributed to pitting in SiC and grain boundary attack in Si3N4. Initial results of burner corrosion of two ceramic matrix composites are consistent with the models developed for monolithic ceramics.
Samples of a cobalt-base alloy, Mar M-509, were subjected to hot corrosion in a Mach-0.3 burner rig. The corrodent was NaCl added as an aqueous solution to the combustion products of a sulfur-containing Jet-A fuel. The metal temperature was fixed at 900 C. The extent of hot corrosion increased by a factor of three as the fuel-to-air mass ratio was increased from 0.033 to 0.050. Because the depositing salt was always Na2SO4, the increased attack appeared to be related to the gas composition.
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Prior research conducted by United Technologies Research Center and Rockwell International Rocketdyne Division encountered severe copper corrosion while flowing hydrocarbon fuels through copper cooling tubes. These experimental results have very important implications for the development of low-cost, regeneratively cooled, oxygen/hydrocarbon bipropellant booster engines. Aerojet TechSystems undertook a program sponsored by NASA/Lewis Research Center in 1986 with two objectives: (1) to define the corrosive interaction process that occurs between hydrocarbon fuels and copper combustion chamber liner materials, using both static and dynamic test methods, and (2) to identify and demonstrate protective measures against this corrosive process. The results of the first program task demonstrated that the most damaging corrosive process was caused by trace amounts of sulfur-containing impurities in the hydrocarbon fuel which react with the copper chamber liner material to form cuprous sulfide (Cu2S), as reported in Paper No. AlAA 88-3215. Experiments were conducted which demonstrated this corrosive process with Mil-Spec RP-1, propane, and methane fuels in contact with OFHC, NASA-Z, and ZrCu coppers. The preliminary results of the second program task demonstrated that electrodeposited gold and platinum coatings held promise of preventing corrosion of copper chamber liners by methane contaminated with sulfur-containing impurities, as reported in Paper No. AIAA 89-2738. Experiments were conducted which demonstrated greatly reduced corrosion of copper by hydrogen sulfide (H2S) and methyl mercaptan (CH3SH) as measured impurities in methane. This paper presents the concluding, confirmatory results from the second program task. The efficacy of a protective gold coating on NASA-Z copper cooling channel walls was conclusively demonstrated. Tests were conducted in the Aerojet Carbothermal Test Facility which provide realistic simulations of booster engine cooling channel conditions such as temperature, pressure, flow velocity, and heat flux. Dynamic test series were performed with methane containing 5 and 10 ppm (by volume) hydrogen sulfide. Gold-plated and unplated copper alloy specimens were evaluated. The tests demonstrated that gold coatings were effective in preventing corrosion of the copper. Posttest metallographic examinations of the unplated specimens showed severe corrosion as a result of reaction with the sulfur containing contaminant in the fuel. In contrast, posttest metallographic examination of the gold-coated specimens showed no corrosion under similar operating conditions. The findings from metallurgical and chemical analyses of the copper specimens, chemical analysis of the methane, and description of all test conditions are provided. Conclusions are reached as to the efficacy of the plating methods and the effectiveness of the metal coatings in the prevention of corrosion in hydrocarbon-fueled booster engine combustion chamber liners.
Hydrogen-oxygen electrolytic regenerative fuel cell development and tests - material corrosion studies, cycle controller assembly, and parts fabrication
Increasing fuel efficiency has been a goal for chemists for several decades. Particularly, a more efficient fuel can increase the range of liquid-hydrocarbon-fueled ram-jets and cruise missiles. A storable high-energy fuel that spontaneously ignites upon addition of an oxidizer is defined as a hypergolic fuel. Hypergolic storable fuels provide an increase in energy per unit volume of fuel and eliminate the need for an external ignition system. Several classes of functionalized hydrocarbons such as amines, boranes, and phosphines are known to be hypergolic with nitric acid oxidizers, but only hydrazine and its simple derivatives5 have been found to exhibit true hypergolic behavior with H2O2. Hydrogen peroxide is a good candidate for an oxidizer due to its reduced toxicity and improved storage capability. Hydrazine-based fuels are expensive, highly corrosive, and toxic, thus providing the need for investigation of other fuels that may be hypergolic with H2O2. Strained hydrocarbons have been studied as high-density fuels. Some examples including benzvalene and cubane, exhibit an increase in heat of combustion as the density of the fuel increases. Many conventional hydrocarbon fuels, such as JP-5 and JP-10, show a decrease in heat of combustion as density of the fuel increases. Strained hydrocarbons can therefore increase the range of the missile by increasing the combustion efficiency per volume of fuel. The goal of this research is to investigate hypergolic behavior of strained hydrocarbons by adding an amine functional group which has been found to hypergolic with nitric acid oxidizers. N,N-Dimethyl-[3]-triangulane-7-methylamine (l), cyclopropylamines (2), cyclobutylamines (3), propylamines (4), and butylamines (5) were synthesized and investigated. The amino group should react with oxygen, providing the initiation step for ring decomposition. The highly exothermic reactions will accumulate energy and potentially lead to spontaneous ignition of the fuel. Triangulanes possess high strain energy, while the less strained cyclopropane and cyclobutanes are more volatile and may show shorter ignition times.
Tests of the mechanical properties, fuel compatibility, sodium corrosion and irradiation behavior were made for vanadium and vanadium alloy. Improved methods for consolidation and fabrication of bar, rod, sheet, and high-quality, small diameter, thin-wall tubing of vanadium-20 without titanium are reported.
High performance channel sealant is based on NASA patented cyano and diamidoximine-terminated perfluoroalkylene ether prepolymers that are thermally condensed and cross linked. The sealant contains asbestos and, in its preferred embodiments, Lithofrax, to lower its thermal expansion coefficient and a phenolic metal deactivator. Extensive evaluation shows the sealant is extremely resistant to thermal degradation with an onset point of 280 C. The materials have a volatile content of 0.18%, excellent flexibility, and adherence properties, and fuel resistance. No corrosibility to aluminum or titanium was observed.
Activities performed in order to provide an R&T data base for utility gas turbine systems burning coal-derived fuels are described. Experiments were run to determine the corrosivity effects of trace metal contaminants (and potential fuel additives) on gas turbine materials and these results were correlated in a corrosion-life prediction model. Actual fuels were burned in a burner rig hot corrosion test to verify the model. A deposition prediction model was assembled and compared with results of actual coal-derived fuel deposition tests. Thermal barrier coatings were tested to determine their potential for protecting gas turbine hardware from the corrosive contaminants. Several coatings were identified with significantly improved spallation-resistance (and, hence, corrosion resistance).
Steel and tantalum apparatus contains various ternary alloys of cadmium, zinc, and magnesium used in pyrochemical processes for the recovery of uranium-base reactor fuels. These materials exhibit good corrosion resistance at the high temperatures necessary for fuel separation in liquid metal-molten salt solvents.
From current projections of the availability of high-quality petroleum crude oils, it is becoming increasingly apparent that the specifications for hydrocarbon jet fuels may have to be modified. The problems that are most likely to be encountered as a result of these modifications relate to engine performance, component durability and maintenance, and aircraft fuel-system performance. The effect on engine performance will be associated with changes in specific fuel consumption, ignition at relight limits, at exhaust emissions. Durability and maintenance will be affected by increases in combustor liner temperatures, carbon deposition, gum formation in fuel nozzles, and erosion and corrosion of turbine blades and vanes. Aircraft fuel-system performance will be affected by increased deposits in fuel-system heat exchangers and changes in the pumpability and flowability of the fuel. The severity of the potential problems is described in terms of the fuel characteristics most likely to change in the future. Recent data that evaluate the ability of current-technology aircraft to accept fuel specification changes are presented, and selected technological advances that can reduce the severity of the problems are described and discussed.
A Mach 0.3 burner rig test program was conducted to determine how the fuel to air mass ratio affects the durability of ZrO2-Y2O3/Ni-16Cr-6Al-0.31Y thermal barrier coating systems in combustion products containing 5 ppm Na and 2 ppm V. As the fuel to air mass ratio was increased from 0.039 to 0.049, the durability of ZrO2-6Y2O3, ZrO2-8Y2O3 and ZrO2-12Y2O3 coatings decreased. ZrO2-8Y2O3 coatings were approximately 2X and 1.3X more durable than ZrO2-12Y2O3 and ZrO2-6Y2O3 coatings respectively at the fuel to air mass ratio of 0.039. The number of one hour cycles endured by ZrO2-8Y2O3 coatings varied from averages of 53 to 200 for the fuel to air mass ratios of 0.049 and 0.039, respectively. At the fuel to air mass ratio of 0.049, all ZrO2-Y2O3 coated specimens failed in 40 to 60 one hour cycles