627
Ind. Eng. Chem. Prod. Res. Dev. lB86, 25, 627-630
On an equimolar basis, ZDTPs exhibit lower antioxidant capabilities than MPH. Antioxidant capabilities of equimolar mixtures of ZDTPs and MPH are determined by capabilities of MPH and are not improved by the presence of ZDTPs despite a significant decrease of hydroperoxide concentration due to ZDTP-induced decomposition. This behavior of ZDTPs could be explained by free radical formation from ZDTP-induced decomposition of hydroperoxides and by depletion of ZDTP before MPH capabilities are exhausted. Antioxidant capabilities of MPH can be improved by addition of i-C,ZDTP in an amount greater than the amount of MPH. It was not determined what this limiting amount must be; however, when the amount of i-C3ZDTP was greater by a factor of 2 (on a molar basis), significant improvement of antioxidant capabilities was observed. Similarly, the method can be used to characterize the antioxidant capabilities of base oils, to assess consistency of engine oil formulation, and to determine oxidation properties of engine oils. The effects of base oil-additive package interactions and formulation changes on hightemperature antioxidant capability can be investigated.
Also, the degree of oxidative degradation of used oils during service or laboratory testing can be monitored. Registry No. MPH, 128-37-0;n-C8ZDTP, 7059-16-7;iCSZDTP, 2929-95-5. Literature Cited ASTM Spectral Technical Publication, 315H, Part 2, "Multlcyllnder Test
Se-
quences for Evaluating Automotive Englne Oils"; American Society for Testing and Materials: Philadelphia, PA, 1980. Hsu, S. M.; Ku, C. S.;Becker, D. A. SA€ Tech. Pap. Ser. 1082, No. 821240.
Jensen, R. K.; Korcek, S.;Mahoney, L. R.; Zinbo, M. J . Am. Chem. SOC. 1870, 101 7574-7584. Johnson, M. D.; Korcek, S.; Zinbo, M. SA€ Tech. Pap. Ser. 1083, No. I
83 1 684.
Johnson, M. D.; Korcek. S.; Zinbo, M. ASLE Trans. 1986, 2 9 , 136-140. Korcek, S.; Mahoney, L. R.; Johnson, M. D.; Hoffman, S. SA€ Trans. 1070, 8 7 , 3568-3596. Korcek, S.;Mahoney. L. R.; Johnson, M. D.; Siegl, W. 0. SA€ Tech. Pap. Ser. 1081, No. 810014. Mahoney, L. R.; Korcek, S.;Hoffman, S.; Willermet, P. A. Ind. €ng. Chem. Prod. Res. Dev. 1878, 17, 250. Mahoney. L. R.; Otto, K.; Korcek, S.;Johnson, M. D. Ind. Eng. Chem. Prod. Res. Dev. 1080, 19, 11-15. Murray, D. W.; Clarke, C. T.; MacAlpine, G. A.; Wright, P. G. SA€ Tech. Pap. Ser. 1082 No. 821236. Wlllermet, P. A. ASLE Trans. 1070, 2 2 , 301-306.
Received for review March 28, 1986 Accepted July 31, 1986
Corrosivity of Diethanolamine Solutions and Their Degradation Products Amitabha Chakma and Axel Meisen" Department of Chemical €ngineering, The University of Britlsh Columbia, Vancouver, British Columbia V6T 1 W5, Canada
Weight loss and potentiodynamic tests were performed to determine the corrosivity of AISI-ASE 1020 carbon steel in aqueous solutions containing diethanolamine (DEA) and/or its principal degradation products. The solutions were either free from or saturated with CO,. The amine solutions were found to be corrosive in the presence of CO,, and corrosion rates at 100 O C and atmospheric pressure r a n w from approximately 1.6 to 2.1 mm/year for DEA concentrations of 30-60 wt %, respectively. 3-(Hydroxyethyl)-2-oxazolidone, which is a major degradation compound of DEA, was found to be most corrosive. The corrosivity of DEA solutions in the presence of C02 is explained qualitatlvely in terms of the reduction in solution pH and the enlargement of the corrosion region in the Pourbaix diagram due to metal complexing.
Introduction Corrosion is a major problem for alkanolamine-based gas-sweetening planta since it may Tesult in metal failure (especially in heat-exchanger tubes and absorber/regenerator trays), equipment fouling, and foaming. Diethanolamine (DEA) plants generally experience less corrosion than mqmoethanolamine plants (Kohl and Riesenfeld, 1979), but corrosion is also a concern for DEA planta (Polderman and Steele, 1956; Moore, 1960; Fitzerald and Richardson, 1966; Smith and Younger, 1972; Hall and Barron, 1981). It is now well established (McMin and Farmer, 1969; Maddox, 1977) that the corrosion rate increases with temperature and "acid gas loading", i.e., the concentration of C02 and H2Sin solution. Industrial solutions usually contain, apart from DEA, water, and absorbed acid gases, significant quantities of amine degradation products. The latter are formed by irreversible reactions which DEA undergoes mainly with C02. The 0196-4321/06/ 1225-0627$01.50/0
corrosivity of degradation products is a matter of considerable controversy and practical importance. In 1956 Polderman and Steele reported simple experiments which indicated that the products corrode steel. Moore (1960) subsequently published some industrial data and showed that the corrosion rate increases with the concentration of degradation products. Values as high as 1 mm/year (40 mpy) were found for carbon steel. The corrosivityof degradation products has also been reported by others (Smith and Younger, 1972; Nonhebel, 1972). However, Blanc et al. (1982) recently published data in support of their claim that DEA degradation products are not corrosive under conditions typically encountered in gas-treating plants. Their main argument was that the pH of a DEA solution of 30%-all percentages used in this paper are weight percent-(or 3 mol/L) lies between 11.5 and 10 for temperatures ranging from 20 to 100 "C; the precise pH value depends on the concentration of the 0 1986 American Chemical Society
828
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 2.4
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0.8
1
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4
0.4
L
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IO
a
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-0.8
I
CORROSION/ IMMUNITY
- I .6 -I
-2
0
2
4
8
6
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12
14
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PH
Figure 1. Pourbaix potential-pH diagram for the iron-water system at 25 "C (Pourbaix, 1974).
degradation products. They concluded that, under these conditions, iron and carbon steel are either noncorrosive or passive according to the Pourbaix potential-pH diagram (see Figure 1;Pourbaix, 1974). However, Pourbaix diagrams do not provide absolute proof of the feasibility of corrosion. A better understanding of corrosion is obtained from kinetic studies, such as plotting potentiodynamic polarization curves (Pourbaix, 1974). Furthermore, the Pourbaix diagram that Blanc et al. (1982) used was only representative of the iron-water system at 25 OC. Industrial DEA systems are more complicated because they consist of iron, water, DEA, carbon dioxide, hydrogen sulfide, degradation products, and heat-stable salts. The heat-stable salts as well as the degradation products may form metal complexes which impair the use of Pourbaix diagrams. In addition, the Pourbaix diagrams are temperature dependent. As the temperature increases, the regions of corrosion widen and the region of passivity narrows for the iron-water system. Blanc et al. (1982) also performed corrosion experiments by immersing SI 1018 carbon steel coupons in 30% aqueous DEA solutions at 80 OC and a Ha partial pressure of 2000 kPa (290 psi). After 500 h, the weight-loss measurements implied a corrosion rate of 0.05 mm/year (2 mpy). In another similar test, Blanc et al. (1982) used a mixture of DEA and the degradation compound N,Nbis(hydroxyethy1)piperazine (BHEP) and obtained a corrosion rate of 0.02 mm/year (0.8 mpy). Recent extensive work on DEA degradation by Kennard and Meisen (1985) revealed that, in addition to BHEP, 3-(hydroxyethyl)-2-oxazolidone(HEOD) and N,N,N'tris(hydroxyethy1)ethylenediamine (THEED) are major DEA degradation products. Therefore, the statement by Blanc et al. (1982) that degradation products of amines, DEA and MDEA, have no practical effect upon the corrosion rate of carbon steel cannot be regarded as proven since some major products were excluded from their tests. The basic objective of this paper is therefore to report results of corrosion studies undertaken with DEA solutions and their degradation products. Experimental Procedures 1. Preparation of Partially Degraded DEA Solution. A solution containing initially 30% DEA in water was degraded in a heat-transfer loop under a CO, partial pressure of 4.13 MPa. Details of the degradation exper-
7
0
40
120 160 T I M E (Hours)
80
200
240
Figure 2. Typical pH change of a DEA solution degrading in a heat-transfer loop (Chakma, 1984). (Conditions: initial DEA concentration 30%; inlet temperature 60 OC, outlet temperature 195 "C, heating fluid temperature 250 "C; flow rate 0.011 L/s.)
iment have already been reported (Chakma, 1984). The final degraded solution contained about 8.7% of degradation products; its composition was 2.13 mol/L of DEA, 0.58 mol/L of HEOD, 0.10 mol/L of THEED and 0.02 mol/L of BHEP. As the degradation progressed, the pH of the solution decreased substantially as shown in Figure 2. 2. Potentiodynamic Tests. Polished coupons of AISI-ASE 1020 carbon steel with a surface area of 6.44 cm2 were immersed in corrosion cells containing either undegraded or partially degraded DEA solutions. The corrosion cells were then connected to a corrosion measurement system (Model 350A, Princeton Applied Research, Princeton, NJ). A calomel electrode containing saturated KC1 solution was used as the reference electrode. The electrode was equipped with a heat-shrink Teflon bridge tube having a 4-mm-diameter Vycor disk at the tip (Model K65, Princeton Applied Research) in order to minimize diffusion of chloride ions to the specimen while a low electrical resistance was maintained. Potentiodynamic polarization curves were obtained at 1 mV/s scanning rates, and the free corrosion currents were determined by the built-in microcomputer via Tafel slope extrapolation (Uhlig, 1965). The experiments were conducted at 25 "C. 3. Weight-Loss Tests. Weight-loss tests were carried out in accordance with procedures specified by the National Association of Corrosion Engineers (1969). All but the high-pressure experiments were performed by using a Pyrex flask. The high-pressure corrosion experiments were conducted using a 600-mL stainless steel autoclave containing a Pyrex liner (Model 4563, Parr Instrument Co., IL). The solutions for the atmospheric pressure tests were either free of or saturated with COz at room temperature by bubbling COz through an extra-coarse fritted glass gas dispersion tube (8-mm diameter, 20 nim long) placed inside the Pyrex flask. The solutions for the high-pressure experiments were saturated with C 0 2 by introducing CO, into the autoclave containing the solutions. Results and Discussion 1. Corrosion Rates in Undegraded DEA Solution. The corrosion rate of carbon steel in an undegraded C0,-free DEA solution was determined via Tafel slope
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 629 1
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z a c v)
CURRENT D E N S I T Y ( n A / c m 2 )
CURRENT D E N S I T Y ( n A / c m 2 )
Figure 3. Potentiodynamic polarization curve of a 30% undegraded DEA solution (corrosion potential = -0.897 V SCE, corrosion current = 5.751 X lo3nA/cm2, corrosion rate 2.654 mpy, temperature 25 "C). Table I. Corrosivity of DEA Solutions" at 100 "C (Boiling Condition) Using Weight-Loss Tests corrosion rates 30% 40% 60% 30% 40% 60%
solution DEA DEA DEA DEA + Cop DEA + COB DEA + COS
mm/year
mPY
0.0025 0.003 0.0035 1.60 1.84 2.070
0.098 0.12 0.137 63.10 72.32 81.60
a The DEA solutions were either free from or initially saturated with C o pat room temperature and atmospheric pressure.
Table 11. Effect of C 0 2 Partial Pressure at 100 "C on Corrosion Rate of 30% DEA Solutions" Using Weight-Loss Tests total pressure, MPa
C O partial ~ pressure, MPa
1.48 2.88 4.23
1.38 2.78 4.13
corrosion 'at's mm/year mpy 0.61 0.65 0.8
24.0 25.6 31.5
"The solutions were saturated with COPinside the autoclave at various partial pressures.
extrapolation of the polarization curve obtained at 25 "C at the free current potential. The rate was found to be 0.067 mm/year or 2.65 mpy (see Figure 3). This value is very similar to that reported by Blanc et al. (1982) for one of their tests using the Fe-H2S-DEA system. When the solutions were saturated with COz at room temperature and atmospheric pressure before they were heated to 100 "C, they became far more corrosive. This can be concluded from results summarized in Table I. The corrosion rates increase with the DEA concentration, which is probably due to the fact that the COz concentration in the solution increases as the DEA concentration is raised. The corrosion rates obtained in the presence of C 0 2are very high. The probable reason is that all experiments were carried out at 100 "C and atmospheric pressure so that the solutions were nearly boiling. The presence of vapor bubbles disturbed the layer of protective corrosion products on the steel surfaces and therefore enhanced the corrosion rates. When the tests were conducted at elevated pressures, boiling was prevented and the corrosion rates were significantly reduced (see Table 11). Table I1 also shows that the corrosion rates increase with COz partial pressure.
Figure 4. Potentiodynamic polarization curve of a partially degraded aqueous DEA solution containing 22.5% DEA, 4.65% HEOD, 2.49% THEED, and 0.37% BHEP (corrosionpotential = -0.832 V SCE, corrosion current = 3.49 X lo4 nA/cm2, corrosion rate = 16.10 mpy, temperature = 25 "C). Table 111. Effect of Degradation Compounds on Corrosion Rates at 100 "C (Boiling Condition) Using Weight-Loss Tests" corrosion rates sample mm/year mpy 15% DEA + COP 0.13 5.1 15% BHEP + COS 0.16 6.3 15% HEOD + COz 1.95 76.6 30% DEA + COZ 1.60 63.1 30% DEA + 5% BHEP + Cop 1.57 62.0 30% DEA + 5% HEOD + COZ 1.91 75.0 "The solutions were initially saturated with COz at room temperature and atmospheric pressure.
2. Corrosion Rates in Solutions Containing DEA and/or Its Degradation Products. The partially degraded DEA solution yielded a corrosion rate of 0.4 mm/year (16.1 mpy) at 25 "C according to the potentiodynamic method (see Figure 4). This value is about 6.1 times higher than that obtained for an undegraded solution and proves that degraded DEA solutions containing HEOD, THEED, and BHEP are, in fact, corrosive toward carbon steel. The earlier claims by Blanc et al. (1982) are thereby contradicted. Table I11 summarizes the results of weight-loss tests with DEA and various degradation compounds. The corrosion rate in a solution containing DEA and BHEP is about the same as when BHEP is absent. This confirms the noncorrosive nature of BHEP and agrees with the findings of Blanc et al. (1982). However, the corrosion rates in solutions containing HEOD were significantly higher than those in other solutions. 3. Passivity. Amines are known to be fair corrosion inhibitors. For example, Ballard (1966) claims that DEA used in gas-sweetening plants forms good, but not tenacious, films on metal surfaces. He therefore recommends low fluid velocities to avoid destroying the protective films. An examination of the polarization curves for both the undegraded and the degraded samples (Figures 3 and 4) indicates that, although regions of passivity do exist, the films are not quite stable. Particularly in the case of undegraded DEA solutions, the films seem to be very unstable. Furthermore, the passive films seem to offer very little protection since the corrosion current in the passive region is very close to the critical corrosion current. 4. Pitting. Unlike undegraded DEA solutions, the pitting potential of degraded DEA solutions is clearly
630 I d . Eng. Chem. Rod. Res. Dev.. Vol. 25. No. 4. 1986
The reduction in this equilibrium potential enlarges the corrosion region corresponding to the left corrosion triangle in the pH-potential diagram (see Figure 1). Hence the pH reduction due to CO, absorption and degradation-product formation as well as metal-complex formation enhances the corrosivity of DEA solutions.
Finure 5. Electron mieroeraohic Dhoto of AISI-ASE 1020 carbon
Conclusions The following conclusions concerning the corrosivity of DEA solutions toward AISI-ASE 1020 carbon steel may be drawn:
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. .
detectable (see Figure 4). This indicates that degraded DEA solutions mieht induce Dittine corrosion under certain conditions. Eledron micrograpkc photos of mild steel coupons revealed pitting after exposure to DEA and HEOD solutions. The pitting is more severe in the case of HEOD as shown by Figure 5 (left) and Figure 5 (right), respectively.
sponsible for corrosion. (3) in the nresence of COOand deeradation . ~Corrosion , products can be attributed to the redugion in soiution pH and to the enlargement of the corrosion region due to metal complexing.
Qualitative Explanation The corrosivity of DEA solutions in the presence of C02 and degradation compounds can be explained as follows: The pH of typical industrial DEA solutions usually falls between 10 and 12. As C02 is absorbed hy the solutions, their pHs decrease. In addition, the solution pH falls as DEA degrades because the principal degradation compounds are less alkaline than DEA (Hakka et al., 1968). Hall and Barron (1981) presented industrial data showing a gradual reduction in solution pH upon the formation of heat-stable salts. These fmdings are also in agreement with our experimental results as shown by Figure 2. The initial sharp decrease is due to C02 absorption, and the subsequent gradual decrease results from the formation of degradation compounds. The reduction in pH gradually brings the system close to the left corrosion triangle of the pH-potential diagram. Metal complexing also plays an important role in the corrosion due to the presence of degradation products. Degradation products probably form iron chelates because Comeaux (1962) reported the formation of such chelates with similar compounds. Hall and Barron (1981) also reported the presence of iron chelates in industrial DEA solutions. The main effect of metal complexing is the reduction of the potential of the metal-ionjmetal equilibrium represented by Fez+ -k 2eFe
The financial support of the Canadian Gas Processors Association and the Natural Sciences and Engineering Research Council of Canada are gratefully acknowledged. Registry No. DEA, 111-42-2; HEOD, 3356-885: AIS1 1020,
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Acknowledgment
12725-36-9; CO,, 124-38-9.
Literature Cited Ballad. D. Hyaocarba, R-s. 1988, 45(4). 137. Blanc. C.; olall. M.; Demarak. 0. R o c . Gas Cond. Conf. 1982. 32. Chakma. A. M.A.Sc. TMsk. UnlvsrsHy 01 81Moh miumbla, 1984. Comeaux. R. V. Roc.-Am. Pel. I n d . . Mv. Rem. 1982. 42, 4.31. Fn2eram. K. J.; Richardson. J. A. HyhocarBon R-ss. 1988. 45(7). 125. Hakka. L. E.: Slcgh. K. P.: Bata. 0. L.: Testart. A. 0.: AMejchyshyn. W. M. Gas Proms. Can. 1988, 6 1 . 3 2 . Hall. W. 0.;Barnon. J. G. Roc. Gas cond.Conf. 1981. 37. Kennard. M. L.: Me)sL)n. A. Id. Eng. Chem. Furlem. 1985. 2 4 , 129. Kohl. A. L.; Rlesenleld. F. C. Gas Rriricatiar: Gult Houslon, TX. 1979. Maddcx. R. N. Gas and L @ M Sweetening: Campbell Petroleum: Naman. OK, 1977. McMin. R. E.: Farm%. F. Conhnence on S W Rscovsry RoMsdings: Oklahoma state unksrsny: s1111water. M(. 196s. W e . K. L. carosion(Hwot0n) 1080, 76. 111. N.A.C.E. Standard TM01-69 "Laborat- Cmosbn Testing 01 Metals lw mS Process Industries": National ASSOClatkm 01 Cwrorion Enginems: Houstan. TX. 1969. Nanhebei. 0. Gas pL*IAcaHon IW A* PoMlllon Contml. 2nd ed.; NewnessBunemorths: ~ d m1972. . Polderman. L. D.: Steele. A. S. OU uls J . 1950. 54. 206. Pourbaix. M. AHas Of E/EqUlIlbriB bl A q m s S h i b r k S . 2nd English ed.; National ASSOClatlon 01 Conorion Engineers: Houslm. TX.
,-,..
."7,
SmMh. R. F.; Younw, A. H. WkOCBrba, Rocepo. 1972. 51(7). 98. uhlig. H. H. camsian and Conosion ConfrokWiley: New Yo*. 1965
Received for reuiew November 25,1985 Accepted April 7, 1986
