1438
Ind. Eng. Chem. Res. 1990, 29, 1438-1442
(23) Sanderson, R. T. Chemical Bonds and Bond Energy, 2nd ed.; Academic Press: New York, 1976; p 75. (24) Barthomeuf, D. J. Phys. Chem. 1984,88, 42. (25) Giordano, N.; Pino, L.; Cavallaro, S.; Vitarelli, P.; Rao, B. S. Zeolites 1987, 7, 131.
(26) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions; Van Nostrand: New York, 1970; p 214. Receiued for review October 4, 1989 Accepted February 21, 1990
Corrosion of Aluminized Low Carbon Steel Exhaust System in Vehicles Equipped with Three-way Catalytic Converters and Development of a Protective Polymeric Coating Mohinder S. Chattha,* James Perry, Robert L. GOSS, Charles R. Peters, and Haren S. Gandhi Scientific Research Staff, Ford Motor Company, 20000 Rofunda Drive, Dearborn, Michigan 48121
In automobiles equipped with three-way catalytic converters (TWCCs) with no secondary air, the cause of corrosion of the exhaust system has been identified t o be ammonium sulfate formation. Ammonia is formed over three-way catalysts (TWCs) under reducing conditions and reacts with sulfur trioxide, formed during lean conditions and stored on high surface area A1,0,, to form ammonium sulfate. The condensate solution of ammonium sulfate thus formed in the exhaust system reacts with the aluminized surface of the aluminized low-carbon steel, stripping it away and corroding the low carbon steel components. A coating composition has been developed from a new phenolic-type epoxy resin and diaminodiphenyl sulfone. This coating has a softening temperature higher than 200 OC and is thermally stable in air up to 375 "C. The coating adheres well to aluminized low-carbon steel and provides excellent corrosion protection during aqueous ammonium sulfate exposure in bench tests and during aging in a stimulated exhaust for 15000 simulated miles. Field testing of this polymer-coated exhaust system is planned on prototype vehicles. Aluminized low carbon steel mufflers on several prototype vehicles showed excessive corrosion of the interior tubes and baffles after 4 000-10000 miles of driving. All these vehicles were equipped with three-way catalystic converters (TWCCs) without secondary air. The mechanism of corrosion has been investigated, and a corrosion preventive coating is being developed. Since phenolic-type epoxies have recently been reported to provide acid-resistant coatings (White and Leidheiser, 1985), a new phenolic-based low-viscosity epoxy resin was chosen for this application. Aminophenyl sulfone was employed as a cross-linking agent so that a higher glass transition temperature of the cured coating could be attained than that of the network formed with aliphatic amines or acids. The elucidation of the muffler corrosion mechanism, development of the composition of the protective coating, and its evaluation are discussed in this paper.
Experimental Section Materials. The epoxy employed in this development is a new phenolic-type multifunctional resin (XU252, Ciba-Geigy). It is a clear liquid with epoxy value of 0.51-0.54 equiv/ 100 g. 4,4'-Diaminodiphenyl sulfone (DDS) was bought from Aldrich Chemical Company and used as received. Reagent grade acetone, butyl acetate, dimethylacetamide, dimethylformamide, isopropyl alcohol, and xylene were used as solvents. The cure temperature of the composition was estimated with a Du Pont 910 differential scanning calorimeter (DSC) a t a heating rate of 5 "C/min under argon. The thermal decomposition of the cured composition was studied in air with a Du Pont 990A thermal gravimetric analyzer (TGA), and the softening point was determined with a Du Pont Model 943 thermal mechanical analyzer (TMA) by employing the penetration mode at a heating OS8S-~SS5/90/2629-1438$02.50/0
rate of 10 "C/min. The dynamic mechanical thermal behavior (tensile storage modulus, E', and loss tangent, tan 6 ) was examined by using a dynamic mechanical thermal analyzer (DMTA) in the dual cantilever mode. The sample was subjected to oscillating deformation at 0.1, 1.0, and 10 Hz while the temperature was increased from 50 to 250 "C a t a rate of 1 "C/min. Sample Preparation for DSC, TGA, TMA, and DMTA. Thirty grams of the epoxy resin (XU252, CibaGeigy) was mixed with 9.6 g of 4,4'-diaminodiphenyl sulfone in a beaker. The mixture was whipped with a spatula to make a paste, and a small quantity was used for the DSC experiment (Figure 1). The remaining mixture was heated a t 100 OC with continuous stirring for 15 min to obtain a homogeneous melt. The material was poured into three aluminum pans and placed in an oven at 100 "C for half an hour. The samples were heated at 180 "C for 1 h and subsequently a t 200 " C for 2 h and were allowed to cool to room temperature overnight. The cured samples were used in TGA, TMA, and DMTA studies (Figures 2-5). The Coating Formulation and Application. The amino sulfone (4.8 g) was dissolved in a mixture of 10 mL of acetone and 10 mL of dimethylacetamide. Fifteen grams of the epoxy resin XU252 was added to the above solution, and the resulting mixture was stirred at room temperature to obtain a homogeneous composition. This mixture was further diluted with 5 mL of 2-propanol. Five panels of aluminized low carbon steel (1 cm2 each) were washed with acetone and then with xylene. The panels were coated on one side by drawing the coating composition on them. The panels were dried in air a t room temperature for 5 min and heated in an oven at 100 "C for half an hour and at 180 "C for 1 h. The final bake was at 200 "C for 1 h. 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1439 1
20
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Figure 4. DMTA analysis of the cured composition at the heating rate of 1 "C/min.
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Figure 5. DMTA analysis (log E' vs temperature) of the cured composition at the heating rate of 1 OC/min.
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Figure 3. TGA analysis of the cured composition in air a t the heating rate of 10 "C/min.
Evaluation of Coated a n d Uncoated Aluminized Low Carbon Steel Ammonium Sulfate Aerosol. Air was passed at 38 cm3/min through a sintered disk spurger immersed in a solution of 100 g of ammonium sulfate in 400 mL of distilled water placed in a round-bottom flask. The flask was heated a t 90 "C. The aluminized steel test panels were placed in a Pyrex glass tube of 1.5-cm diameter and 33-cm length; the tube was attached to the flask. The tube was heated with a tape to 90 "C, the air flow was continued, and the exhaust was vented through a laboratory hood. The volume and the concentration of the ammonium sulfate solution were maintained nearly constant throughout the test. After 200 h the uncoated panel was removed, washed with distilled water, and examined by scanning electron microscopy (SEM). The ammonium
sulfate aerosol treatment of the coated panel was continued for 3 months. The panel was then cooled to room temperature and washed with distilled water. In order to examine the surface under the coating, removal of the coating was attempted with adhesive tape but the coating did not peel off. The panel was cooled in liquid nitrogen, and the adhesive tape test was repeated. The coating maintained an excellent bond to the aluminized steel surface. The coating was then removed with a blade, and the surface was analyzed with SEM. Pulsator Evaluation of t h e Coated a n d Uncoated Aluminized Steel. Two panels coated on one side were placed downstream of the TWCC in the exhaust of a pulsator tube (Williamson et al., 1979), where 212 "C temperature was maintained. The pulsator cycle used during the simulated mileage accumulation included cycling at temperatures 340,515,and 730 OC for 17%, 77%, and 6% of the time, respectively, with pulsator modulation of f l air/fuel a t 0.5-Hz frequency. In this test procedure, the simulated mileages are 30 mph or 5000 miles/week at a nominal space velocity of 40000 h-l. The fuel used in the simulated test consisted of isooctane doped with 0.2 mg of P/L, 1.5 mg of Pb/L, and 0.03 w t % S. The fuel phosphorus was derived from cresyl diphenyl phosphate, sulfur was derived from diethyl sulfide, and the source of lead was tetraethyllead. The dopants are added to assess the effect of misfueling. The doped isboctane fuel was injected with a nebulizer directly into the hot portion of the pulsator furnace for combustion. The panels were aged in the pulsator with simulated exhaust for 3 weeks (15000 simulated miles). One panel was washed with distilled water to remove the deposits.
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1440 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
1590-1
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The panel was then cooled in liquid nitrogen and the coating was removed with a razor blade. The newly exposed surface and the uncoated surface of the panel were examined by SEM. Exposure of Aluminized Steel to Ammonia. Pieces of aluminized low carbon steel were exposed to an exhaust downstream of a TWCC operated under strongly reducing conditions (A = 0.97), where >30% of the NO in the exhaust is converted to NH3. (A 0.97 X value means a gets mixture that is 3% fuel rich. Typically a stoichiometric air/fuel ratio for gasoline is 14.6; i.e., it requires 14.6 lb of air for complete combustion of 1 lb of gasoline.) Exposure to this NH,-rich exhaust for 160 h at 130 "C followed by
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Figure 11. SEM analysis of the aluminized surface under the coating after 3 months of aging in aqueous ammonium sulfate saturated air at 90 "C.
SEM analysis did not detect any loss of aluminum coating. Similar exposure to aqueous ammonium hydroxide for 120 h at 25 OC did not result in any dissolution of aluminum, as observed by SEM. All the SEM results are shown in Figures 6-13. Analysis of the Corrosion Products. The corrosion products found in the interior of prototype car mufflers were analyzed with X-ray diffraction (XRD) and X-ray fluorescence (XRF). Homogenized samples yielded dif-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1441 4000
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UNCOATED SIDE OF THE PANEL AFTER 15,000 SIMULATED MILES IN PULSATOR EXPERIMENT
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Figure 13. SEM analysis of the uncoated side of the aluminized steel panel after lSOO0 simulated miles in the pulsator test.
fraction spectra typical of the oxides of iron: Fe203(both hematite and maghemite) and a much smaller amount of Fe30,. No crystalline aluminum-containing compounds were identifiable although the element was found in all the qualitative XRF spectra along with Fe (Major) plus S, Ca, and Pb, in low to trace amounts. Optical examination of the muffler debris revealed small accretions, orange to white in color, with a geode-like morphology. The rust exhibited domed regions and some glazed surface, Figure 14. The fragile domes were often hollow. One of the small (=1-mm-diameter) geodes was examined by XRD using a Gandolfi powder diffraction camera. The resulting photographically recorded pattern was typical of the alunite mineral group: A+R+'((S04)2(OH)6where A+ = Na+, K+, Pb+, Rb+, NH4+,Ag+, or H30+ and B+' = A13+ or Fe3+. The specific alunite found in the muffler has the formula (H30)A13(S04)2(0H)6.The mineral is insoluble in H20, is slightly soluble in HC1 and HN03, and dissolves slowly in H2S04. An IR spectrum of the powdered alunite particles verified the presence of the anions and revealed a trace of NH4+ ions.
Results and Discussion Exposure of aluminized low carbon steel to ammoniarich exhaust in pulsator experiments and to ammonium hydroxide solution in a bench test did not result in any detectable loss of the aluminum coatings as observed by SEM analyses, which are comparable to those of the original sample (Figures 6-8). However, when pieces of
aluminized low carbon steel were exposed to aqueous ammonium sulfate or to air saturated with ammonium sulfate and water, severe loss of aluminum occurred as evidenced by SEM (Figures 9 and 10). The mechanism of this corrosion process appears to be that ammonia formed over the TWCC under reducing conditions (rich air/fuel ratio spikes) reacts with SO3 formed during lean conditions and stored on the alumina surface to form a highly corrosive ammonium sulfate solution. The low pH (2.5) of warm condensate attests to the presence of an acidic species, which is the probable cause of corrosion. The acidic solution removed the aluminized surface and subsequently results in the corrosion of the low carbon steel components. XRD analysis of the corrosion products was identified as the mineral alunite, (H30)A13(S04)2(OH)6.This mineral may be formed by the chemical reaction of steam and sulfuric acid resulting from the decomposition of ammonium sulfate. Thus, in the emission system equipped with a TWCC without secondary air and oxidation catalyst, the corrosive ammonium sulfate formed during cyclic air/fuel ratio over the TWCC could mineralize the aluminized steel. This corrosion problem was particularly aggravated in the case of prototype vehicles due to an efficient engine, producing a relatively cool exhaust, and due to the muffler being placed far from the engine and the catalyst. This caused significant amounts of condensate to accumulate in the cold parts of the exhaust system during short-trip, low-speed driving. The vehicles equipped with a TWCC and without secondary air, which had higher exhaust temperatures, did not experience exhaust system corrosion; this is most likely due to a lower volume of the condensate. Upon application of the coating and baking, the amino groups of the diphenyl sulfone react with the epoxy groups to produce a cross-linked polymer network (Steiner, 1968). Since the sulfone group is situated a t the para position to the amino groups and is strongly electron withdrawing, the amino groups have a reduced reactivity toward the epoxy groups. This is shown by the differential scanning calorigram (Figure 1) of the reaction mixture: there is no reaction up to 123 "C. At 125 "C the curing starts, the peak temperature of the exotherm is 208 OC, and the heat of the reaction is 158J/g. This DSC analysis indicates that no reaction occurs a t room temperature and therefore the coating would have a long shelf life. The thermal mechanical analysis shows (Figure 2) that this cross-linked polymer softens only a t 213 "C. Thermal gravimetric analysis shows that there is no weight loss up to 375 "C in air and the polymer degradation takes place only around
1442 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
400 "C (Figure 3). Large carbon residue up to 500 "C (Figure 3) suggests that even partially decomposed polymer may continue to provide corrosion protection. The thermal mechanical analysis results are further supported by dynamic mechanical thermal analysis of the polymer (Figure 4), at three different frequencies, 0.1, 1, and 10 Hz, which shows that the polymer softens between 212 and 227 "C. The dynamic mechanical thermal analysis further shows (Figure 5) that, even though the material undergoes softening around 200 "C, upon further heating it transforms into a rubbery phase with a fairly high modulus (log E' = 7.5) and continues to maintain this modulus to at least 250 "C. Thus, even beyond the softening point, the polymer would continue to provide an excellent barrier between the substrate and the corrosive species. In order to assess the extent of corrosion of the aluminized low carbon steel, SEM analysis of the as-received steel (Figure 6) was compared with the analyses of the treated surfaces. Figure 6 shows that the electron microscope sees essentially aluminum on the as-received sample and that the steel surface is well protected with aluminum. When this aluminized low carbon steel was exposed to aqueous ammonium hydroxide solution and to ammoniarich exhaust under reducing conditions in the pulsator experiment, no loss of aluminum had occurred (figures 7 and 8). However, when the aluminized steel was exposed to hot air saturated with ammonium sulfate for 200 h and then examined by SEM, most of the aluminum coating had been removed and the microscopic observation noted largely iron (Figure 9). Similarly exposure to ammonium sulfate solution causes significant loss of aluminum (Figure 10). When the aluminized steel was coated with the epoxy composition and then exposed to aqueous ammonium sulfate aerosol for 3 months, the coating did not show any blistering, peeling, or loss of adhesion. Removal of the epoxy coating and SEM analysis of the substrate showed that the aluminum layer was intact (Figure 11) and that there was no sign of corrosion. The same coating composition, when applied to aluminized steel and aged in the pulsator for 15000 simulated miles, showed only corrosion at the edges where there was no epoxy coating. Removal of the coating and subsequent SEM analysis of the surface showed that there was no corrosion and the aluminum
layer was unaffected (Figure 12). SEM analysis of the uncoated side of the same panel aged in the pulsator experiment showed that all the aluminum had disappeared (Figure 13) and the surface had rusted excessively. The aged coating exhibited excellent adhesion to the aluminized steel and could not be peeled off even with a tape. The adhesion may be due to the polarity of the sulfone groups and the hydroxy groups formed during reaction of the amino groups with epoxy functionality (Johnson, et. al., 1985). The softening point, which is above 200 "C, may be due to high the cross-link density, the aromatic nature of the reactants, and the polarity of the sulfone groups. Concluding Remarks This paper elucidates a mechanism for the removal of aluminum coating in aluminized low carbon steel exhaust parts of vehicles that were equipped with TWCCs. Several laboratory procedures are outlined in detail to provide simple but reproducible evaluation techniques. Finally, a polymeric protective coating is developed which offers, in a laboratory evaluation, a lower cost alternative to the use of a stainless stell exhaust system. It is estimated that the material and application cost of the polymeric coating can be around 20-2570 of the cost penalty incurred by the use of an all stainless steel exhaust system. Registry No. (DDS)(XU-252)(copolymer), 123415-84-9; NH3, 7664-41-7;Al, 7429-90-5; (NH4)$04, 7783-20-2; steel, 12597-69-2. Literature Cited Johnson, M. D.; Chattha, M. S.; Robertson, R. E. Organophosphorus Enamine: A New Class of Epoxy Hardeners and Adhesion Promoters. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 565. Steiner, T. E. Epoxy Resins Technology; Bruins, P. F., Ed.; Interscience: New York, 1968; p 193. White, M. L.; Leidheiser, H. The Evaluation of Coating Resins for Corrosion Protection of Steel Exposed to Dilute Sulfuric Acid. Polym. Mat. Sci. Eng. 1985, 53, 480. Williamson, W. B.; Stepien, H. K.; Watkins, W. L. H.; Gandhi, H. S. Poisoning of Platinum-Rhodium Automotive Three-way Catalysts by Lead and Phosphorus. Enuiron. Sci. Technol. 1979,13, 1109. Received for review September 25, 1989 Accepted February 26, 1990
