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Corrosion and polarization behavior of carbon steel in MEA-based CO2 capture process
international journal of greenhouse gas control 2 (2008) 553–562
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Corrosion and polarization behavior of carbon steel in MEA-based CO2 capture process Immanuel Raj Soosaiprakasam, Amornvadee Veawab * Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2
article info
abstract
Article history:
This work reveals levels of corrosion rate and polarization behavior of carbon steel
Received 4 September 2007
immersed in aqueous solutions of monoethanolamine (MEA) used in the absorption-based
Received in revised form
carbon dioxide (CO2) capture process for greenhouse gas reduction from industrial flue gas
26 January 2008
streams. Such information was obtained from electrochemical-based corrosion experi-
Accepted 3 February 2008
ments under a wide range of the CO2 capture process conditions. The corrosion of carbon
Published on line 8 April 2008
steel was evaluated in respect to process parameters including partial pressure of oxygen
Keywords:
and metal surface condition. Results show that the aqueous MEA solution containing CO2
CO2 capture
provides a favorable condition for the corrosion of carbon steel to proceed. Corrosion rate is
Alkanolamine
increased by all tested process parameters. These parametric effects were explained by the
Corrosion
electrochemical kinetic data obtained from polarization curves and by the thermodynamic
Carbon steel
data obtained from Pourbaix diagram.
(O2), CO2 loading in solution, solution velocity, solution temperature, MEA concentration
# 2008 Elsevier Ltd. All rights reserved.
Electrochemical test Polarization
1.
Introduction
The increasingly alarming levels of carbon dioxide (CO2) in the atmosphere have triggered the international community to take necessary steps with a series of immediate actions to protect the environment. One of the actions is to promote the use of non-fossil fuels or renewable energy sources, such as hydro, nuclear, biomass and solar energy. However, with existing technologies, none of these can, at the present time, become a single energy source satisfying all our energy requirements, as the fossil fuels have achieved. The renewable energy sources still suffer from disadvantages related to cost and safety. Many groups of researchers have directed their attention to a concept of post-combustion CO2 capture and sequestration. This concept involves recovering CO2 from industrial flue gas before releasing it into the atmosphere, without disturbing the fossil fuel combustion processes, and utilizing the recovered
CO2 in enhanced oil recovery operations or storing it in depleted oil/gas reservoirs and deep oceans. There are various ways to recover or capture CO2 from industrial flue gas. The most promising, based on its practicality, is gas absorption process using aqueous solutions of alkanolamines, often referred to as amine treating process. The amine treating process is not a new technology. In fact, it has been in practice in gas processing industries for 60–70 years for the removal of acidic impurities such as CO2 and hydrogen sulfide (H2S) from natural gas streams. However, using this amine treating process for the CO2 capture from industrial flue gas for the purpose of environmental cleanup, is a new adventure for practitioners due to the differences in operating conditions and compositions of natural gas and industrial flue gas. All amine treating plants have experiences on corrosion problems (Kohl and Nielsen, 1997). Corrosion occurs in the forms of general, galvanic, crevice, pitting, intergranular, selective leaching, erosion and stress corrosion cracking. The
* Corresponding author. Tel.: +1 306 585 5665; fax: +1 306 585 4855. E-mail address: [email protected] (A. Veawab). 1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.02.009
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international journal of greenhouse gas control 2 (2008) 553–562
plant areas susceptible to corrosion are the bottom of absorbers, regenerators, reboiler bundles, pumps and valves where the acid gas loading and temperatures are high (DuPart et al., 1993). According to the survey conducted for refinery plants (Rampin, 2000), the process equipment that is out of service due to severe corrosion problems is reboiler, rich amine exchanger, regenerator, condenser, absorber and amine cooler. Although a number of factors contribute to severe corrosion, the major causes reported are (1) poor plant design and operation, such as high flow velocity in pipelines, high operating temperature in reboiler and insufficient steam for solvent regeneration and (2) the presence of process contaminants. Corrosion problems essentially lead to substantial expenditure in addition to the process costs. According to the CC Technologies & NACE International (Koch, 2001), in 1998, the plant expenditure due to corrosion in the United States was estimated at US$276 billions while that for petroleum refining alone was US$3.7 billion. Of this total, the maintenance-related expenses were estimated at $1.8 billion, the vessel turnaround expenses were at $1.4 billion, and the fouling-related costs were approximately $0.5 billion annually. This reflects a significant impact of corrosion problems in plant operations. In addition to the extra expenditure, corrosion also has an adverse impact on the safety of plant personnel. Often raised as a well-known event in the amine treating plant history, an incident caused by severe corrosion occurred on 23 July 1984 (Mogul, 1999). A refinery at Romeoville, IL, owned and operated by the Union Oil Co. of California, experienced a disastrous explosion and fire. An amine absorber pressure vessel ruptured and released large quantities of flammable gases and vapors. Seventeen lives were lost, seventeen individuals were hospitalized, and more than US$100 million in damages was a result. All of these were caused by hydrogen-induced cracking and non-stress relieved repair welds. Even though no incidents as severe have been reported since, this incident is a serious indicator of the danger posed by the corrosion in the amine treating units. This work explores corrosion of carbon steel in the CO2 absorption process using an aqueous solution of monoethanolamine (MEA). Its objectives are to generate baseline corrosion data of the uninhibited MEA–CO2 system which can be used for further study of corrosion control, and to gain an understanding of corrosion mechanism and behavior in respect to process parameters simulating the actual plant operating conditions. Corrosion was experimentally evaluated as a function of six process parameters using electrochemical techniques under a wide range of test conditions summarized in Table 1. The tested parameters were partial pressure of oxygen (O2), CO2 loading in solution, solution velocity, solution temperature, MEA concentration and metal surface condition (precorrosion).
2.
Experiments
2.1.
Preparation of specimen and solution
The specimen was made of carbon steel 1018 with a chemical composition of 0.175% carbon, 0.75% manganese and balance
Table 1 – Tested parameters and conditions for uninhibited MEA–H2O–CO2 systems Parameter Partial pressures of O2 (kPa) CO2 loading in solution (mol/mol) Solution velocity (rpm) Solution temperature (8C) MEA concentration (kmol/m3) Duration of precorrosion (days)
Condition 0.00, 5.07 and 10.13 0.20 and 0.55 0, 1000 and 2000 40 and 80 5.0, 7.0 and 9.0 1, 7, 14 and 28
iron. Carbon steel was chosen as it is widely used in the construction of equipment and piping in amine treating plants (DuPart et al., 1993). The electrochemical specimens were cylindrical in shape with a height, outside diameter and center hole diameter of 0.80, 1.20 and 0.60 cm, respectively. Prior to the experiments, the specimens were prepared by wet grinding with 600 grit silicon carbide papers and deionized water, degreased with high purity methanol and then dried with hot air in accordance with the ASTM Standard G1-90 (1999a). An aqueous solution of monoethanolamine with a concentration of 5.00 0.10 kmol/m3 (approximately 30% by weight) was prepared from a 99% MEA reagent and deionized water. The test solution was loaded with 0.20 0.01 mol of CO2/mol of MEA. If the experimental conditions demanded the CO2 saturation loading, the solution was then loaded to saturation levels with CO2.
2.2.
Electrochemical experiment
Fig. 1 illustrates the experimental setup for the electrochemical corrosion tests used in this work. It consisted of a 100 ml double-walled corrosion cell (Model 636-ring disk electrode (RDE) assembly, Princeton Applied Research, USA), a water bath with a temperature controller, a gas supply set connected to flow meters, a condenser, a rotator with a speed controller, a potentiostat and a data-acquisition system. The corrosion cell was a three-electrode assembly with a cylindrical working electrode made of carbon steel-1018 with a surface area of 3.0 cm2, an silver/silver chloride (Ag/AgCl) reference electrode and a platinum (Pt) counter electrode. The reference electrode was introduced into the cell using a bridge tube containing test solution. The heated water was circulated through the outer jacket of the corrosion cell to keep the test solution at the desired temperature. The temperature of the heated water was adjusted to a desired temperature using the temperature controller. A water-cooled condenser was used to prevent any change in solution concentration due to water evaporation. The effect of solution velocity was studied by keeping the rotating cylinder electrode (RCE) assembly in the dynamic mode of operation at 1000 and 2000 rpm. A computercontrolled potentiostat (Basic Electrochemical System (BES), Princeton Applied Research, USA) was used for the electrochemical measurements. The results were recorded and analyzed using Powercorr corrosion software (Princeton Applied Research, USA). Prior to the electrochemical experiments, the experimental setup and procedure were validated by performing potentiodynamic anodic polarization and conforming to the ASTM Practice G5-94 (1999b).
international journal of greenhouse gas control 2 (2008) 553–562
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Fig. 1 – Schematic diagram of experimental setup for electrochemical corrosion tests.
2.3.
Experimental procedure
100 cm3 of aqueous MEA solution was prepared with a specific test composition and CO2 loading. A corrosion cell was purged with nitrogen (N2) gas to de-aerate the test solution and maintain the CO2 loading of 0.20 mol/mol, and was purged with CO2 in case of saturated CO2 loading. A condenser was fitted to the corrosion cell to avoid any vaporization loss. The temperature of the cell was gradually raised to a set value (40 or 80 8C) by circulating hot water through the outer jacket. A specimen was prepared according to ASTM Standard G1-90 (1999a) and then degreased with high purity methanol and dried with hot air. The pH and temperature of the solution in the corrosion cell was measured after at least 60 min of N2 purging. Once the temperature was within 0.10 8C of the set temperature, three solution samples were taken for titration in order to measure the MEA concentration and the CO2 loading with hydrochloric acid (HCl) using methyl orange as the pH-end point indicator. The corrosion cell was then assembled with a working electrode, an auxiliary electrode and a reference electrode. The corrosion cell and the potentiostat were connected electrically. Type and settings of corrosion measuring techniques were specified on the corrosion measuring software. The open circuit potential was recorded until it reached steady state at the corrosion potential with 1 mV between successive readings. After attainment of the steady state corrosion potential, the electrochemical experiment was started. A potentiodynamic cyclic polarizations scan was initiated with a scan rate of 0.166 mV/s. When the experiment was complete,
the data were saved. The pH, temperature, MEA concentration and CO2 loading were measured again at the end of the experiment. The Tafel extrapolation method was used to estimate corrosion current density (icorr), which was subsequently converted to corrosion rate. In this method, only 250 mV from the corrosion potential (Ecorr) of the obtained potentiodynamic polarization curve was considered. The extrapolation of linear portions of the anodic and cathodic curves to Ecorr provided an estimate of the corrosion current density (icorr). This icorr was used to calculate the corrosion rate by the following equation:
CR ¼
3:27 103 icorr E:W: AD
(1)
where CR is the corrosion rate in mmpy, E.W. is the equivalent weight of specimen in g/equivalent, A is the area of working electrode in cm2 and D is the density of the specimen in g/cm3.
3.
Results and discussion
3.1.
Typical corrosion behavior in the absence of O2
Fig. 2 illustrates a typical cyclic polarization curve for carbon steel immersed in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2, loading at 80 8C. The curve demonstrates transitions of the metal state from active to passive and transpassive as the system potential is increased.
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international journal of greenhouse gas control 2 (2008) 553–562
Formation of ferrous hydroxide : Fe2þ þ 2OH $ FeðOHÞ2
(11)
Formation of ferrous carbonate : Fe2þ þ CO3 2 $ FeCO3
Fig. 2 – Cyclic polarization curve of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/ mol CO2 loading at 80 8C in the absence of O2 ((") forward scan; (#) reverse scan).
Under the test condition, the carbon steel specimen is clearly in the active state where corrosion takes place on the free-film surface at the corrosion potential (Ecorr) of 0.853 V Ag/AgCl. The cyclic polarization curve in Fig. 2 shows negative hysteresis, i.e. the reverse polarization scan is on the left of the forward scan. This suggests no pitting tendency on the tested specimen in this environment. The corrosion mechanism at the interface between the carbon steel surface and the CO2-loaded MEA aqueous solution was examined using the obtained results and previous work published in the literature. Let us consider chemical reactions taking place in the bulk solution due to CO2 absorption (Reactions (2)–(6)), possible electrochemical reactions due to corrosion (Reactions (7)–(10)), and possible chemical reactions due to formation of corrosion products (Reactions (11) and (12)) (Veawab and Aroonwilas, 2002). Dissociation of water : Hydrolysis of CO2 :
2H2 O $ H3 Oþ þ OH
(2)
2H2 O þ CO2 $ H3 Oþ þ HCO3
(3)
Dissociation of bicarbonate ion : H2 O þ HCO3 $ H3 Oþ þ CO3 2
(4)
Dissociation of protonated-amine : RNH3 þ þ H2 O $ RNH2 þ H3 Oþ
(5)
Carbamate reversion :
RNHCOO þ H2 O $ RNH2 þ HCO3 (6)
Iron dissolution :
Fe $ Fe2þ þ 2e
(7)
(12)
where H2O = water, H3O+ = hydronium ion, OH = hydroxyl ion, HCO3 = bicarbonate ion, CO32 = carbonate ion, alkanolamine, RNH2+COO = zwitterions, RNH2 = primary RNHCOO = carbamate ion, Fe = iron, Fe2+ = ferrous ion, H2 = hydrogen, Fe(OH)2 = ferrous hydroxide and FeCO3 = ferrous carbonate. Essentially, corrosion occurs due to electrochemical reactions comprising anodic and cathodic reactions. The anodic reaction is iron dissolution (Reaction (7)) while the cathodic reactions are reductions of oxidizers available in the solution. In the absence of O2, some possible oxidizers in this system are H3O+, undissociated H2O and HCO3. According to Veawab and Aroonwilas (2002), undissociated H2O and HCO3 are major oxidizers whereas H3O+ plays a minor role. As such, possible primary corrosion reactions are Reactions (7), (9) and (10). A Pourbaix diagram was used to predict possible corrosion products. From Fig. 3 at pH 9.0 and corrosion potential (Ecorr) = 0.853 V Ag/AgCl, FeCO3 is thermodynamically stable and predominant in the aqueous solution as the corrosion product. FeCO3 was also observed during the experiment as loose black slime. Note that Fe(OH)2 is not thermodynamically stable under this Ecorr. Therefore, the formation of corrosion products is primarily Reaction (12), not Reaction (11). If the system potential is raised to the passive region by any means for the purpose of corrosion control, two possible passive films would be formed on the metal surface as postulated from the Pourbaix diagram in Fig. 3. Above 0.700 V Ag/AgCl, passive film, namely magnetite (Fe3O4) is likely to form on the surface of the corroding metal. It can be gradually converted to hematite (g-Fe2O3) when the potential is increased beyond 0.575 V Ag/AgCl. The hematite is stable in the potential range from 0.575 to +0.525 V Ag/AgCl before the breakdown potential. The film formation reactions can be written below (Ahmad, 2006). 3Fe þ 4H2 O ! Fe3 O4 þ 4H2
(13)
2Fe3 O4 þ H2 O ! 3Fe2 O3 þ H2
(14)
This suggests that the carbon steel can be protected and corrosion can be controlled if the system potential is raised and maintained within the passive region.
3.2.
Typical corrosion behavior in the presence of O2
Reduction of hydronium ion : 2H3 Oþ þ 2e $ 2H2 O þ H2 ðgÞ
(8)
Reduction of bicarbonate ion : 2HCO3 þ 2e $ 2CO3 2 þ H2 ðgÞ
(9)
Reduction of undissociated water : 2H2 O þ 2e $ 2OH þ H2 ðgÞ
(10)
O2 has an influence on the corrosion behavior of carbon steel. As shown in Fig. 4, the cyclic polarization curve of carbon steel in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the presence of 10% O2 in gas (10.13 kPa O2) is similar to the curve produced in the absence of O2 (Fig. 2). The carbon steel specimen is in the active state where corrosion takes place and produces FeCO3 as postulated from the Pourbaix diagram in Fig. 5 at pH 9.14 and
international journal of greenhouse gas control 2 (2008) 553–562
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Fig. 3 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the absence of O2 (created using corrosion analyzer software, OLI Systems Inc.).
Ecorr = 0.833 V Ag/AgCl. No pitting tendency is exhibited as the cyclic polarization curve demonstrates negative hysteresis, i.e. the reverse polarization curve is on the left of the forward curve. To study the effect of O2 on corrosion, polarization curves of the systems containing various partial pressures of O2 were obtained and compared. As illustrated in Fig. 6, in the active region, there are no significant shifts of the polarization curves from the system under 0.00 to 5.07 and 10.13 kPa O2. Raising the O2 partial pressure causes a slight increase in corrosion current density (icorr), indicating a slight increase in corrosion rate (Table 2). The increase in corrosion rate is due to an increase in the oxidizer concentration in the solution. That is, as O2 is dissolved into the solution, it is reduced to receive electrons as shown in the following reaction: Reduction of dissolved O2 :
O2 þ 2H2 O þ 4e ! 4OH
(15)
This results in an additional reduction reaction which in turn induces a greater rate of iron dissolution and an acceleration of the corrosion process. The additional reduction, due to dissolved O2, is evidenced by a significant change in the cathodic Tafel slope (bc) from 107 mV/decade (at 0 kPa O2) to 134 mV/decade (at 10.13 kPa O2). The presence of cathodic loops in Fig. 6 also suggests multiple cathodic reactions. Anodic Tafel slopes remain unchanged indicating no alteration of the anodic reaction mechanism. It should be noted that the increase in corrosion rate found here is not significant. This is because the dissolved O2 is in such a small quantity, it is not sufficient enough to provide a much stronger oxidizing potential to significantly increase the system’s corrosion rate. In addition to the corrosion behavior in the active region described above, Fig. 6 also provides information on corrosion behavior under a passivation condition between 0.673 and +0.448 V Ag/AgCl where passive film is developed on the metal surface. It is apparent the passivation current densities (reflecting corrosion rate) of the systems containing dissolved O2 are generally lower than that of systems without O2. This may be because the passive film of hematite (g-Fe2O3) established on the metal surface is strengthened in the presence of O2 due to the inclusion of O2 or species produced by the O2 reduction reaction. This behavior implies the dissolved O2 will be required in the system to achieve maximum efficiency if corrosion control is applied by raising the system potential to passivation.
3.3.
Fig. 4 – Cyclic polarization curve of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/ mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C. (") Forward scan; (#) reverse scan.
Effect of solution velocity
Solution velocity has a measurable effect on corrosion rate and corrosion behavior of carbon steel. As shown in Table 2, an increase in solution velocity (agitation speed) causes carbon steel to corrode faster. This can be explained by considering the polarization behavior of carbon steel under the tested
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international journal of greenhouse gas control 2 (2008) 553–562
Fig. 5 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C (created using corrosion analyzer software from OLI Systems Inc.).
condition in the active region. Fig. 7 shows a greater agitation speed leading to higher cathodic current densities, reflecting a greater rate of reduction of oxidizing agents, or an enhancement of the mass transport to and away from the corroding metal. Significant changes in anodic and cathodic Tafel slopes were observed, suggesting a different iron dissolution mechanism and cathodic reduction reactions under dynamic conditions. Fig. 7 also projects the corrosion behavior of the system during passivation. If any corrosion control technique is to be applied to raise the corrosion potential to the passivation state, the corrosion rate would increase with the solution velocity. This can be observed from the increase in passivation current densities (iPass in Table 2). The increase in the corrosion rate is due to the deterioration of film in a more turbulent environment which reduces the resistance of the passive film to the diffusion of oxidizers to the metal surface.
3.4.
Solution temperature was found to have a significant effect on corrosion in a MEA–H2O–CO2 system. Experiments were carried out at 40 and 80 8C, representing low and elevated temperatures in the CO2 absorption process, respectively. The results in Table 2 show the carbon steel specimen corrodes at a lower rate at 40 8C rather than at 80 8C. This is attributed to the lower rates of iron dissolution and oxidizer reduction which are evidenced by the lower anodic and cathodic current densities in Fig. 8. The differences in anodic Tafel slopes between low and high temperatures suggest different mechanisms for iron dissolution. It is worthwhile to note the pH of the solution (Table 2) is higher at 40 8C than at 80 8C. This higher alkalinity may partly contribute to the lower iron corrosion rate at the lower temperature. Fig. 8 also provides information on passivation of the metal surface if any corrosion control is applied. The passive current densities at 40 8C are lower than those at 80 8C, indicating greater protection of the passive film at 40 8C. However, higher fluctuations in current densities at 40 8C suggest that the passive film may not be as stable as that at 80 8C. It undergoes continuous dissolution and precipitation. This is supported by the presence of ionic species (Fe(OH)4) in the passive region (Fig. 9). It can be postulated that when the potential is increased in the anodic direction to the passive region, hematite (g-Fe2O3) is formed via the transformation of magnetite (Fe3O4).
3.5.
Fig. 6 – Effect of O2 partial pressure on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C.
Effect of solution temperature
Effect of CO2 loading in solution
CO2 loading in solution is another important parameter affecting corrosion behavior and the corrosion rate in the CO2 absorption process. Corrosion experiments were carried out by varying CO2 loading from 0.20 to 0.55 mol/mol, to cover the typical operational range. The results in Table 2 show
Table 2 – Summary of pH, conductivity, electrochemical parameters and corrosion rate for uninhibited MEA–H2O–CO2 systems Experimental condition
pH
40 8C, a = 0.20
10.19 0.00
80 8C, a = 0.55
7.95 0.07
1000 rpm, a = 0.20, 80 8C
9.23 0.13
2000 rpm, a = 0.20, 80 8C
9.25 0.11
10.13 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.14 0.02 40 8C, a = 0.20
10.50 0.57
80 8C, a = 0.55
8.17 0.11
10.13 kPa O2, a = 0.55, 80 8C 7 kmol/m3 MEA, a = 0.55 9 kmol/m3 MEA, a = 0.55
8.52 0.04 8.32 0.00
5.07 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.13 0.03
ba (mV/ decade)
bc (mV/ decade)
Ecorr (mV Ag/AgCl)
icorr (mA)
Epp (mV Ag/AgCl)
icrit (mA)
ipass (mA)
Eb (mV Ag/AgCl)
23.18 0.33 20.29 0.02 43.68 2.17 21.81 1.85 22.48 2.69
72.92 0.78
106.93 1.11
662.00 1.50 686.00
9.44E + 00 3.25E 01 1.42
8.45E + 01 4.5E + 00 15.00
0.31 0.03
101.71
8.02E + 01 7.63E + 00 2.82E + 01
522.00 4.50
92.88 0.00
853.00 3.50 846.00
580.00
0.11
88.00 4.80
113.11 1.045
2.55E + 02 1.5E + 01 4.72E + 02 4E + 00 7.25E + 02 2.5E + 01
1.21 0.15
127.81 8.65
2.52E + 01 1.82E 00 1.4E + 01 1.79E + 00 1.42E + 01 5.25E 01
604.00 19.00
110.25 1.77
573.00 36.00 634.00 10.00 583.00 6.00
0.67 0.09
122.35 1.71
1.75E + 02 2.41E + 01 3.15E + 02 3.90E + 01 3.66E + 02 1.05E + 01
635.00 26.00
97.67 8.96
775.00 4.00 826.86 8.59 815.13 5.83
708.00 31.00
1.43 0.07
1.99E + 02 3.41E + 00 2.12E + 01
673.00 4.00 –
8.94E + 00 7.25E 01 –
2.60E + 01 0.00E + 00 1.40E + 01
448.00 4.00
0.43 0.03
175.00
833.00 7.00 356.00
76.00
113.00
788.00
1.41E + 02
507.00
2.52E + 01
105.90
120.90
795.00
1.87E + 02
507.00
117.00
150.42
795.00
2.74E + 02
76.85 3.15
136.00 5.00
839.00 1.00
63.49
135.8
99.34 84.63
22.99 0.46 19.69 1.00 46.24 1.38
32.62 3.16 21.89 0.52
23.36 0.58
10.13 kPa O2, 5.0 kmol/m3 MEA, 80 8C, a = 0.20 7 days 9.15 0.06 23.03 0.03 14 days 9.14 0.07 23.31 0.31 28 days 9.10 0.12 24.04 0.36
76.00 10.00 –
134.50 8.50
Corrosion rate (mmpy)
408.00
0.08
1.41E + 01
705.00
0.55
2.60E + 04
7.80E + 01
699.00
0.72
501.00
2.04E + 04
7.10E + 01
699.00
1.06
1.59E + 02 4.5E + 01
653.00 1.00
1.59E + 02 4.5E + 00
2.56E + 01 0.26E + 00
481.00 23.00
0.37 0.01
780.00
2.07E + 02
648.00
8.05E + 03
1.96E + 02
528.00
0.80
116.64
789.00
4.74E + 02
653.00
7.59E + 03
1.04E + 03
523.00
1.83
152.16
790.00
8.31E + 02
642.00
1.31E + 04
2.83E + 03
546.00
1.94
international journal of greenhouse gas control 2 (2008) 553–562
0.00 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.06 0.02
s (mS/ cm2)
s: conductivity; ba: anodic Tafel slope; bc: cathodic Tafel slope; Ecorr: corrosion potential; icorr: corrosion current density; EPP: primary passivation potential; icrit: critical current density; ipass: passivation current density; Eb: breakdown potential; a: CO2 loading (mol/mol).
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Fig. 7 – Effect of solution velocity on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the absence of O2.
increasing CO2 loading in the solution increases the corrosion rate of carbon steel. The corrosion rate in 0.55 mol/mol CO2 loading system doubles the rate of 0.20 mol/mol, in the absence of dissolved O2. This is due to the increase in concentrations of oxidizers (HCO3 and H+) in the MEA–H2O– CO2 environment which causes rates of oxidizer reduction to increase. This can be seen when viewing the increase in cathodic current densities in Fig. 10. The increase in H+ concentration was observed during the experiment due to the reduction of pH from 9.06 at 0.20 mol/ mol to 7.95 at 0.55 mol/mol, while the increase in the corrosion rate was suggested by the increase in solution conductivity from 23 mS/cm at 0.20 mol/mol to 45 mS/cm at 0.55 mol/mol (Table 2). Despite the changes in oxidizer reduction rate, the anodic and cathodic Tafel slopes do not change significantly with CO2 loading (Table 2).
Fig. 8 – Effect of solution temperature on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading in the absence of O2.
The corrosion potential is also shifted to the positive direction when CO2 loading is raised from 0.20 to 0.55 mol/mol. This indicates the solution’s oxidizing power (i.e. increased corrosion tendency) has increased due to an increase in oxidizing agent concentration. Fig. 10 also shows the corrosion rate of the system, containing a greater CO2 loading, would still be greater than one containing a lower CO2 loading even when corrosion inhibition is applied to raise the system potential to the passivation region. This is shown by the greater passivation current densities in the system containing 0.55 mol/mol CO2 loading.
3.6.
Effect of MEA concentration
MEA concentration has an effect on the corrosion of carbon steel. As the MEA concentration is increased from 5.0 to 7.0
Fig. 9 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading and 0 kPa O2 partial pressure at 40 8C (created using corrosion analyzer software from OLI Systems Inc.).
international journal of greenhouse gas control 2 (2008) 553–562
Fig. 10 – Effect of CO2 loading on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA at 80 8C, in the absence of O2.
and 9.0 kmol/m3, the corrosion rate rises from 0.55 to 0.72 and 1.06 mmpy, respectively, in an environment under 0.55 mol/ mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C. Such a corrosion effect is illustrated in Fig. 11. The anodic polarization curves shift slightly to the right where the current densities are greater. Changes in cathodic and anodic Tafel slopes suggest changes in the corrosion mechanism (Table 2). The increase in corrosion rate is a result of the increase in HCO3 available in the solution which induces a greater rate of iron dissolution. Fig. 11 also shows all curves with O2 show multiple cathodic loops, suggesting multiple cathodic reactions.
3.7.
Effect of precorrosion
All previous results and discussion relate to the corrosion behavior of carbon steel specimens in which their surface was clean and had no corrosion products before the experiments. Such a clean surface simulates new process equipment and piping at the beginning of plant operation but not the process components under long-term service. Therefore, it is neces-
561
Fig. 12 – Effect of precorrosion on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA at 80 8C, in the presence of 10.13 kPa of O2.
sary for this work to include the effect of precorrosion, to reveal corrosion behavior of carbon steel that is not clean and has been in plant operation for certain periods of time. A series of electrochemical corrosion tests were carried out using an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading under 10.13 kPa O2 at 80 8C. Carbon steel specimens were precorroded under the identical environment for 7, 14 and 28 days. Results in Table 2 show that when a specimen is precorroded for a longer time, its corrosion rate becomes greater. This is evidenced by the increases in both anodic and cathodic current densities as shown in Fig. 12. Variation in the anodic and cathodic Tafel slopes (Table 2) suggests changes in anodic and cathodic reaction mechanisms. The passivation current densities also increase with precorrosion duration. All curves exhibit at least two cathodic loops before passive region, suggesting multiple cathodic reactions.
4.
Conclusion
Under typical CO2 absorption process conditions, carbon steel is in the active state where corrosion process is thermodynamically in favor. All parameters tested in this work affect corrosion rate and polarization behavior of carbon steel as summarized below.
Fig. 11 – Effect of MEA concentration on polarization behavior of carbon steel 1018 in an aqueous solution of MEA containing 0.55 mol/mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C.
Increasing O2 partial pressure accelerates corrosion due to the increasing oxidizer concentration in the solution. Dissolved O2 is required for the corrosion control that raises the system potential to passivation where passive film of hematite (g-Fe2O3) is established on the metal surface. A greater solution velocity causes a higher corrosion rate due to the enhancement of the transport rates of corroding agents between metal surface and bulk solution. Raising solution temperature enhances corrosion rate. This is the result of the increases in rates of iron dissolution and oxidizer reduction during corrosion process. An increase in CO2 loading in solution causes corrosion rate to increase. This is due to the increase in concentrations of
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corroding agents (HCO3 and H+), which causes rates of oxidizer reduction to increase. An increase in amine concentration makes solution more corrosive due to the increase in HCO3 available in the solution, which induces a greater rate of iron dissolution. The precorroded carbon steel corrodes faster than the nonprecorroded steel due to the faster rates of both iron dissolution and oxidizer reduction reactions.
Acknowledgments Authors gratefully acknowledge the Natural Science and Engineering Research Council of Canada (NSERC) and the Natural Resources Canada (NRCan) for financial support.
references
Ahmad, Z., 2006. Principles of Corrosion Engineering and Corrosion Control, 1st edition. Elsevier.
ASTM Standard G1-90, 1999a. Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens. ASTM, Philadelphia, PA (Re-approved 1999). ASTM Standard G5-94, 1999b. Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. ASTM, Philadelphia, PA (Reapproved 1999). DuPart, M.S., Bacon, T.R., Edwards, D.J., 1993. Understanding corrosion in alkanolamine gas treating plants. Part 2. Case histories show actual plant problems and their solutions. Hydrocarbon Process. (April), 75–80. Koch, G., 2001. Corrosion Cost Preventive Strategies in the Unites States. CC Technologies & NACE international (Sponsored by Office of Infrastructure and Development Federal Highway Administration). Kohl, A.L., Nielsen, R., 1997. Gas Purification, 5th edition. Gulf Publishing Company, TX. Mogul, M.G., 1999. Reduce corrosion in amine gas absorption columns. Hydrocarbon Process. 47–50 53–54, 56. Rampin, P., 2000. Amine units: results of a survey on structural reliability. In: Proceedings of International Conference Corrosion in Refinery, Petrochemical and Power Generation Plants, Venezia, May 2000, pp. 18–19. Veawab, A., Aroonwilas, A., 2002. Identification of oxidizing agents in aqueous amine–CO2 systems using a mechanistic corrosion model. Corros. Sci. 44, 967–987.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ijggc
Corrosion and polarization behavior of carbon steel in MEA-based CO2 capture process Immanuel Raj Soosaiprakasam, Amornvadee Veawab * Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2
article info
abstract
Article history:
This work reveals levels of corrosion rate and polarization behavior of carbon steel
Received 4 September 2007
immersed in aqueous solutions of monoethanolamine (MEA) used in the absorption-based
Received in revised form
carbon dioxide (CO2) capture process for greenhouse gas reduction from industrial flue gas
26 January 2008
streams. Such information was obtained from electrochemical-based corrosion experi-
Accepted 3 February 2008
ments under a wide range of the CO2 capture process conditions. The corrosion of carbon
Published on line 8 April 2008
steel was evaluated in respect to process parameters including partial pressure of oxygen
Keywords:
and metal surface condition. Results show that the aqueous MEA solution containing CO2
CO2 capture
provides a favorable condition for the corrosion of carbon steel to proceed. Corrosion rate is
Alkanolamine
increased by all tested process parameters. These parametric effects were explained by the
Corrosion
electrochemical kinetic data obtained from polarization curves and by the thermodynamic
Carbon steel
data obtained from Pourbaix diagram.
(O2), CO2 loading in solution, solution velocity, solution temperature, MEA concentration
# 2008 Elsevier Ltd. All rights reserved.
Electrochemical test Polarization
1.
Introduction
The increasingly alarming levels of carbon dioxide (CO2) in the atmosphere have triggered the international community to take necessary steps with a series of immediate actions to protect the environment. One of the actions is to promote the use of non-fossil fuels or renewable energy sources, such as hydro, nuclear, biomass and solar energy. However, with existing technologies, none of these can, at the present time, become a single energy source satisfying all our energy requirements, as the fossil fuels have achieved. The renewable energy sources still suffer from disadvantages related to cost and safety. Many groups of researchers have directed their attention to a concept of post-combustion CO2 capture and sequestration. This concept involves recovering CO2 from industrial flue gas before releasing it into the atmosphere, without disturbing the fossil fuel combustion processes, and utilizing the recovered
CO2 in enhanced oil recovery operations or storing it in depleted oil/gas reservoirs and deep oceans. There are various ways to recover or capture CO2 from industrial flue gas. The most promising, based on its practicality, is gas absorption process using aqueous solutions of alkanolamines, often referred to as amine treating process. The amine treating process is not a new technology. In fact, it has been in practice in gas processing industries for 60–70 years for the removal of acidic impurities such as CO2 and hydrogen sulfide (H2S) from natural gas streams. However, using this amine treating process for the CO2 capture from industrial flue gas for the purpose of environmental cleanup, is a new adventure for practitioners due to the differences in operating conditions and compositions of natural gas and industrial flue gas. All amine treating plants have experiences on corrosion problems (Kohl and Nielsen, 1997). Corrosion occurs in the forms of general, galvanic, crevice, pitting, intergranular, selective leaching, erosion and stress corrosion cracking. The
* Corresponding author. Tel.: +1 306 585 5665; fax: +1 306 585 4855. E-mail address: [email protected] (A. Veawab). 1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.02.009
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plant areas susceptible to corrosion are the bottom of absorbers, regenerators, reboiler bundles, pumps and valves where the acid gas loading and temperatures are high (DuPart et al., 1993). According to the survey conducted for refinery plants (Rampin, 2000), the process equipment that is out of service due to severe corrosion problems is reboiler, rich amine exchanger, regenerator, condenser, absorber and amine cooler. Although a number of factors contribute to severe corrosion, the major causes reported are (1) poor plant design and operation, such as high flow velocity in pipelines, high operating temperature in reboiler and insufficient steam for solvent regeneration and (2) the presence of process contaminants. Corrosion problems essentially lead to substantial expenditure in addition to the process costs. According to the CC Technologies & NACE International (Koch, 2001), in 1998, the plant expenditure due to corrosion in the United States was estimated at US$276 billions while that for petroleum refining alone was US$3.7 billion. Of this total, the maintenance-related expenses were estimated at $1.8 billion, the vessel turnaround expenses were at $1.4 billion, and the fouling-related costs were approximately $0.5 billion annually. This reflects a significant impact of corrosion problems in plant operations. In addition to the extra expenditure, corrosion also has an adverse impact on the safety of plant personnel. Often raised as a well-known event in the amine treating plant history, an incident caused by severe corrosion occurred on 23 July 1984 (Mogul, 1999). A refinery at Romeoville, IL, owned and operated by the Union Oil Co. of California, experienced a disastrous explosion and fire. An amine absorber pressure vessel ruptured and released large quantities of flammable gases and vapors. Seventeen lives were lost, seventeen individuals were hospitalized, and more than US$100 million in damages was a result. All of these were caused by hydrogen-induced cracking and non-stress relieved repair welds. Even though no incidents as severe have been reported since, this incident is a serious indicator of the danger posed by the corrosion in the amine treating units. This work explores corrosion of carbon steel in the CO2 absorption process using an aqueous solution of monoethanolamine (MEA). Its objectives are to generate baseline corrosion data of the uninhibited MEA–CO2 system which can be used for further study of corrosion control, and to gain an understanding of corrosion mechanism and behavior in respect to process parameters simulating the actual plant operating conditions. Corrosion was experimentally evaluated as a function of six process parameters using electrochemical techniques under a wide range of test conditions summarized in Table 1. The tested parameters were partial pressure of oxygen (O2), CO2 loading in solution, solution velocity, solution temperature, MEA concentration and metal surface condition (precorrosion).
2.
Experiments
2.1.
Preparation of specimen and solution
The specimen was made of carbon steel 1018 with a chemical composition of 0.175% carbon, 0.75% manganese and balance
Table 1 – Tested parameters and conditions for uninhibited MEA–H2O–CO2 systems Parameter Partial pressures of O2 (kPa) CO2 loading in solution (mol/mol) Solution velocity (rpm) Solution temperature (8C) MEA concentration (kmol/m3) Duration of precorrosion (days)
Condition 0.00, 5.07 and 10.13 0.20 and 0.55 0, 1000 and 2000 40 and 80 5.0, 7.0 and 9.0 1, 7, 14 and 28
iron. Carbon steel was chosen as it is widely used in the construction of equipment and piping in amine treating plants (DuPart et al., 1993). The electrochemical specimens were cylindrical in shape with a height, outside diameter and center hole diameter of 0.80, 1.20 and 0.60 cm, respectively. Prior to the experiments, the specimens were prepared by wet grinding with 600 grit silicon carbide papers and deionized water, degreased with high purity methanol and then dried with hot air in accordance with the ASTM Standard G1-90 (1999a). An aqueous solution of monoethanolamine with a concentration of 5.00 0.10 kmol/m3 (approximately 30% by weight) was prepared from a 99% MEA reagent and deionized water. The test solution was loaded with 0.20 0.01 mol of CO2/mol of MEA. If the experimental conditions demanded the CO2 saturation loading, the solution was then loaded to saturation levels with CO2.
2.2.
Electrochemical experiment
Fig. 1 illustrates the experimental setup for the electrochemical corrosion tests used in this work. It consisted of a 100 ml double-walled corrosion cell (Model 636-ring disk electrode (RDE) assembly, Princeton Applied Research, USA), a water bath with a temperature controller, a gas supply set connected to flow meters, a condenser, a rotator with a speed controller, a potentiostat and a data-acquisition system. The corrosion cell was a three-electrode assembly with a cylindrical working electrode made of carbon steel-1018 with a surface area of 3.0 cm2, an silver/silver chloride (Ag/AgCl) reference electrode and a platinum (Pt) counter electrode. The reference electrode was introduced into the cell using a bridge tube containing test solution. The heated water was circulated through the outer jacket of the corrosion cell to keep the test solution at the desired temperature. The temperature of the heated water was adjusted to a desired temperature using the temperature controller. A water-cooled condenser was used to prevent any change in solution concentration due to water evaporation. The effect of solution velocity was studied by keeping the rotating cylinder electrode (RCE) assembly in the dynamic mode of operation at 1000 and 2000 rpm. A computercontrolled potentiostat (Basic Electrochemical System (BES), Princeton Applied Research, USA) was used for the electrochemical measurements. The results were recorded and analyzed using Powercorr corrosion software (Princeton Applied Research, USA). Prior to the electrochemical experiments, the experimental setup and procedure were validated by performing potentiodynamic anodic polarization and conforming to the ASTM Practice G5-94 (1999b).
international journal of greenhouse gas control 2 (2008) 553–562
555
Fig. 1 – Schematic diagram of experimental setup for electrochemical corrosion tests.
2.3.
Experimental procedure
100 cm3 of aqueous MEA solution was prepared with a specific test composition and CO2 loading. A corrosion cell was purged with nitrogen (N2) gas to de-aerate the test solution and maintain the CO2 loading of 0.20 mol/mol, and was purged with CO2 in case of saturated CO2 loading. A condenser was fitted to the corrosion cell to avoid any vaporization loss. The temperature of the cell was gradually raised to a set value (40 or 80 8C) by circulating hot water through the outer jacket. A specimen was prepared according to ASTM Standard G1-90 (1999a) and then degreased with high purity methanol and dried with hot air. The pH and temperature of the solution in the corrosion cell was measured after at least 60 min of N2 purging. Once the temperature was within 0.10 8C of the set temperature, three solution samples were taken for titration in order to measure the MEA concentration and the CO2 loading with hydrochloric acid (HCl) using methyl orange as the pH-end point indicator. The corrosion cell was then assembled with a working electrode, an auxiliary electrode and a reference electrode. The corrosion cell and the potentiostat were connected electrically. Type and settings of corrosion measuring techniques were specified on the corrosion measuring software. The open circuit potential was recorded until it reached steady state at the corrosion potential with 1 mV between successive readings. After attainment of the steady state corrosion potential, the electrochemical experiment was started. A potentiodynamic cyclic polarizations scan was initiated with a scan rate of 0.166 mV/s. When the experiment was complete,
the data were saved. The pH, temperature, MEA concentration and CO2 loading were measured again at the end of the experiment. The Tafel extrapolation method was used to estimate corrosion current density (icorr), which was subsequently converted to corrosion rate. In this method, only 250 mV from the corrosion potential (Ecorr) of the obtained potentiodynamic polarization curve was considered. The extrapolation of linear portions of the anodic and cathodic curves to Ecorr provided an estimate of the corrosion current density (icorr). This icorr was used to calculate the corrosion rate by the following equation:
CR ¼
3:27 103 icorr E:W: AD
(1)
where CR is the corrosion rate in mmpy, E.W. is the equivalent weight of specimen in g/equivalent, A is the area of working electrode in cm2 and D is the density of the specimen in g/cm3.
3.
Results and discussion
3.1.
Typical corrosion behavior in the absence of O2
Fig. 2 illustrates a typical cyclic polarization curve for carbon steel immersed in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2, loading at 80 8C. The curve demonstrates transitions of the metal state from active to passive and transpassive as the system potential is increased.
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international journal of greenhouse gas control 2 (2008) 553–562
Formation of ferrous hydroxide : Fe2þ þ 2OH $ FeðOHÞ2
(11)
Formation of ferrous carbonate : Fe2þ þ CO3 2 $ FeCO3
Fig. 2 – Cyclic polarization curve of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/ mol CO2 loading at 80 8C in the absence of O2 ((") forward scan; (#) reverse scan).
Under the test condition, the carbon steel specimen is clearly in the active state where corrosion takes place on the free-film surface at the corrosion potential (Ecorr) of 0.853 V Ag/AgCl. The cyclic polarization curve in Fig. 2 shows negative hysteresis, i.e. the reverse polarization scan is on the left of the forward scan. This suggests no pitting tendency on the tested specimen in this environment. The corrosion mechanism at the interface between the carbon steel surface and the CO2-loaded MEA aqueous solution was examined using the obtained results and previous work published in the literature. Let us consider chemical reactions taking place in the bulk solution due to CO2 absorption (Reactions (2)–(6)), possible electrochemical reactions due to corrosion (Reactions (7)–(10)), and possible chemical reactions due to formation of corrosion products (Reactions (11) and (12)) (Veawab and Aroonwilas, 2002). Dissociation of water : Hydrolysis of CO2 :
2H2 O $ H3 Oþ þ OH
(2)
2H2 O þ CO2 $ H3 Oþ þ HCO3
(3)
Dissociation of bicarbonate ion : H2 O þ HCO3 $ H3 Oþ þ CO3 2
(4)
Dissociation of protonated-amine : RNH3 þ þ H2 O $ RNH2 þ H3 Oþ
(5)
Carbamate reversion :
RNHCOO þ H2 O $ RNH2 þ HCO3 (6)
Iron dissolution :
Fe $ Fe2þ þ 2e
(7)
(12)
where H2O = water, H3O+ = hydronium ion, OH = hydroxyl ion, HCO3 = bicarbonate ion, CO32 = carbonate ion, alkanolamine, RNH2+COO = zwitterions, RNH2 = primary RNHCOO = carbamate ion, Fe = iron, Fe2+ = ferrous ion, H2 = hydrogen, Fe(OH)2 = ferrous hydroxide and FeCO3 = ferrous carbonate. Essentially, corrosion occurs due to electrochemical reactions comprising anodic and cathodic reactions. The anodic reaction is iron dissolution (Reaction (7)) while the cathodic reactions are reductions of oxidizers available in the solution. In the absence of O2, some possible oxidizers in this system are H3O+, undissociated H2O and HCO3. According to Veawab and Aroonwilas (2002), undissociated H2O and HCO3 are major oxidizers whereas H3O+ plays a minor role. As such, possible primary corrosion reactions are Reactions (7), (9) and (10). A Pourbaix diagram was used to predict possible corrosion products. From Fig. 3 at pH 9.0 and corrosion potential (Ecorr) = 0.853 V Ag/AgCl, FeCO3 is thermodynamically stable and predominant in the aqueous solution as the corrosion product. FeCO3 was also observed during the experiment as loose black slime. Note that Fe(OH)2 is not thermodynamically stable under this Ecorr. Therefore, the formation of corrosion products is primarily Reaction (12), not Reaction (11). If the system potential is raised to the passive region by any means for the purpose of corrosion control, two possible passive films would be formed on the metal surface as postulated from the Pourbaix diagram in Fig. 3. Above 0.700 V Ag/AgCl, passive film, namely magnetite (Fe3O4) is likely to form on the surface of the corroding metal. It can be gradually converted to hematite (g-Fe2O3) when the potential is increased beyond 0.575 V Ag/AgCl. The hematite is stable in the potential range from 0.575 to +0.525 V Ag/AgCl before the breakdown potential. The film formation reactions can be written below (Ahmad, 2006). 3Fe þ 4H2 O ! Fe3 O4 þ 4H2
(13)
2Fe3 O4 þ H2 O ! 3Fe2 O3 þ H2
(14)
This suggests that the carbon steel can be protected and corrosion can be controlled if the system potential is raised and maintained within the passive region.
3.2.
Typical corrosion behavior in the presence of O2
Reduction of hydronium ion : 2H3 Oþ þ 2e $ 2H2 O þ H2 ðgÞ
(8)
Reduction of bicarbonate ion : 2HCO3 þ 2e $ 2CO3 2 þ H2 ðgÞ
(9)
Reduction of undissociated water : 2H2 O þ 2e $ 2OH þ H2 ðgÞ
(10)
O2 has an influence on the corrosion behavior of carbon steel. As shown in Fig. 4, the cyclic polarization curve of carbon steel in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the presence of 10% O2 in gas (10.13 kPa O2) is similar to the curve produced in the absence of O2 (Fig. 2). The carbon steel specimen is in the active state where corrosion takes place and produces FeCO3 as postulated from the Pourbaix diagram in Fig. 5 at pH 9.14 and
international journal of greenhouse gas control 2 (2008) 553–562
557
Fig. 3 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the absence of O2 (created using corrosion analyzer software, OLI Systems Inc.).
Ecorr = 0.833 V Ag/AgCl. No pitting tendency is exhibited as the cyclic polarization curve demonstrates negative hysteresis, i.e. the reverse polarization curve is on the left of the forward curve. To study the effect of O2 on corrosion, polarization curves of the systems containing various partial pressures of O2 were obtained and compared. As illustrated in Fig. 6, in the active region, there are no significant shifts of the polarization curves from the system under 0.00 to 5.07 and 10.13 kPa O2. Raising the O2 partial pressure causes a slight increase in corrosion current density (icorr), indicating a slight increase in corrosion rate (Table 2). The increase in corrosion rate is due to an increase in the oxidizer concentration in the solution. That is, as O2 is dissolved into the solution, it is reduced to receive electrons as shown in the following reaction: Reduction of dissolved O2 :
O2 þ 2H2 O þ 4e ! 4OH
(15)
This results in an additional reduction reaction which in turn induces a greater rate of iron dissolution and an acceleration of the corrosion process. The additional reduction, due to dissolved O2, is evidenced by a significant change in the cathodic Tafel slope (bc) from 107 mV/decade (at 0 kPa O2) to 134 mV/decade (at 10.13 kPa O2). The presence of cathodic loops in Fig. 6 also suggests multiple cathodic reactions. Anodic Tafel slopes remain unchanged indicating no alteration of the anodic reaction mechanism. It should be noted that the increase in corrosion rate found here is not significant. This is because the dissolved O2 is in such a small quantity, it is not sufficient enough to provide a much stronger oxidizing potential to significantly increase the system’s corrosion rate. In addition to the corrosion behavior in the active region described above, Fig. 6 also provides information on corrosion behavior under a passivation condition between 0.673 and +0.448 V Ag/AgCl where passive film is developed on the metal surface. It is apparent the passivation current densities (reflecting corrosion rate) of the systems containing dissolved O2 are generally lower than that of systems without O2. This may be because the passive film of hematite (g-Fe2O3) established on the metal surface is strengthened in the presence of O2 due to the inclusion of O2 or species produced by the O2 reduction reaction. This behavior implies the dissolved O2 will be required in the system to achieve maximum efficiency if corrosion control is applied by raising the system potential to passivation.
3.3.
Fig. 4 – Cyclic polarization curve of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/ mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C. (") Forward scan; (#) reverse scan.
Effect of solution velocity
Solution velocity has a measurable effect on corrosion rate and corrosion behavior of carbon steel. As shown in Table 2, an increase in solution velocity (agitation speed) causes carbon steel to corrode faster. This can be explained by considering the polarization behavior of carbon steel under the tested
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international journal of greenhouse gas control 2 (2008) 553–562
Fig. 5 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C (created using corrosion analyzer software from OLI Systems Inc.).
condition in the active region. Fig. 7 shows a greater agitation speed leading to higher cathodic current densities, reflecting a greater rate of reduction of oxidizing agents, or an enhancement of the mass transport to and away from the corroding metal. Significant changes in anodic and cathodic Tafel slopes were observed, suggesting a different iron dissolution mechanism and cathodic reduction reactions under dynamic conditions. Fig. 7 also projects the corrosion behavior of the system during passivation. If any corrosion control technique is to be applied to raise the corrosion potential to the passivation state, the corrosion rate would increase with the solution velocity. This can be observed from the increase in passivation current densities (iPass in Table 2). The increase in the corrosion rate is due to the deterioration of film in a more turbulent environment which reduces the resistance of the passive film to the diffusion of oxidizers to the metal surface.
3.4.
Solution temperature was found to have a significant effect on corrosion in a MEA–H2O–CO2 system. Experiments were carried out at 40 and 80 8C, representing low and elevated temperatures in the CO2 absorption process, respectively. The results in Table 2 show the carbon steel specimen corrodes at a lower rate at 40 8C rather than at 80 8C. This is attributed to the lower rates of iron dissolution and oxidizer reduction which are evidenced by the lower anodic and cathodic current densities in Fig. 8. The differences in anodic Tafel slopes between low and high temperatures suggest different mechanisms for iron dissolution. It is worthwhile to note the pH of the solution (Table 2) is higher at 40 8C than at 80 8C. This higher alkalinity may partly contribute to the lower iron corrosion rate at the lower temperature. Fig. 8 also provides information on passivation of the metal surface if any corrosion control is applied. The passive current densities at 40 8C are lower than those at 80 8C, indicating greater protection of the passive film at 40 8C. However, higher fluctuations in current densities at 40 8C suggest that the passive film may not be as stable as that at 80 8C. It undergoes continuous dissolution and precipitation. This is supported by the presence of ionic species (Fe(OH)4) in the passive region (Fig. 9). It can be postulated that when the potential is increased in the anodic direction to the passive region, hematite (g-Fe2O3) is formed via the transformation of magnetite (Fe3O4).
3.5.
Fig. 6 – Effect of O2 partial pressure on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C.
Effect of solution temperature
Effect of CO2 loading in solution
CO2 loading in solution is another important parameter affecting corrosion behavior and the corrosion rate in the CO2 absorption process. Corrosion experiments were carried out by varying CO2 loading from 0.20 to 0.55 mol/mol, to cover the typical operational range. The results in Table 2 show
Table 2 – Summary of pH, conductivity, electrochemical parameters and corrosion rate for uninhibited MEA–H2O–CO2 systems Experimental condition
pH
40 8C, a = 0.20
10.19 0.00
80 8C, a = 0.55
7.95 0.07
1000 rpm, a = 0.20, 80 8C
9.23 0.13
2000 rpm, a = 0.20, 80 8C
9.25 0.11
10.13 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.14 0.02 40 8C, a = 0.20
10.50 0.57
80 8C, a = 0.55
8.17 0.11
10.13 kPa O2, a = 0.55, 80 8C 7 kmol/m3 MEA, a = 0.55 9 kmol/m3 MEA, a = 0.55
8.52 0.04 8.32 0.00
5.07 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.13 0.03
ba (mV/ decade)
bc (mV/ decade)
Ecorr (mV Ag/AgCl)
icorr (mA)
Epp (mV Ag/AgCl)
icrit (mA)
ipass (mA)
Eb (mV Ag/AgCl)
23.18 0.33 20.29 0.02 43.68 2.17 21.81 1.85 22.48 2.69
72.92 0.78
106.93 1.11
662.00 1.50 686.00
9.44E + 00 3.25E 01 1.42
8.45E + 01 4.5E + 00 15.00
0.31 0.03
101.71
8.02E + 01 7.63E + 00 2.82E + 01
522.00 4.50
92.88 0.00
853.00 3.50 846.00
580.00
0.11
88.00 4.80
113.11 1.045
2.55E + 02 1.5E + 01 4.72E + 02 4E + 00 7.25E + 02 2.5E + 01
1.21 0.15
127.81 8.65
2.52E + 01 1.82E 00 1.4E + 01 1.79E + 00 1.42E + 01 5.25E 01
604.00 19.00
110.25 1.77
573.00 36.00 634.00 10.00 583.00 6.00
0.67 0.09
122.35 1.71
1.75E + 02 2.41E + 01 3.15E + 02 3.90E + 01 3.66E + 02 1.05E + 01
635.00 26.00
97.67 8.96
775.00 4.00 826.86 8.59 815.13 5.83
708.00 31.00
1.43 0.07
1.99E + 02 3.41E + 00 2.12E + 01
673.00 4.00 –
8.94E + 00 7.25E 01 –
2.60E + 01 0.00E + 00 1.40E + 01
448.00 4.00
0.43 0.03
175.00
833.00 7.00 356.00
76.00
113.00
788.00
1.41E + 02
507.00
2.52E + 01
105.90
120.90
795.00
1.87E + 02
507.00
117.00
150.42
795.00
2.74E + 02
76.85 3.15
136.00 5.00
839.00 1.00
63.49
135.8
99.34 84.63
22.99 0.46 19.69 1.00 46.24 1.38
32.62 3.16 21.89 0.52
23.36 0.58
10.13 kPa O2, 5.0 kmol/m3 MEA, 80 8C, a = 0.20 7 days 9.15 0.06 23.03 0.03 14 days 9.14 0.07 23.31 0.31 28 days 9.10 0.12 24.04 0.36
76.00 10.00 –
134.50 8.50
Corrosion rate (mmpy)
408.00
0.08
1.41E + 01
705.00
0.55
2.60E + 04
7.80E + 01
699.00
0.72
501.00
2.04E + 04
7.10E + 01
699.00
1.06
1.59E + 02 4.5E + 01
653.00 1.00
1.59E + 02 4.5E + 00
2.56E + 01 0.26E + 00
481.00 23.00
0.37 0.01
780.00
2.07E + 02
648.00
8.05E + 03
1.96E + 02
528.00
0.80
116.64
789.00
4.74E + 02
653.00
7.59E + 03
1.04E + 03
523.00
1.83
152.16
790.00
8.31E + 02
642.00
1.31E + 04
2.83E + 03
546.00
1.94
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0.00 kPa O2, 5.0 kmol/m3 MEA 80 8C, a = 0.20 9.06 0.02
s (mS/ cm2)
s: conductivity; ba: anodic Tafel slope; bc: cathodic Tafel slope; Ecorr: corrosion potential; icorr: corrosion current density; EPP: primary passivation potential; icrit: critical current density; ipass: passivation current density; Eb: breakdown potential; a: CO2 loading (mol/mol).
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Fig. 7 – Effect of solution velocity on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading at 80 8C in the absence of O2.
increasing CO2 loading in the solution increases the corrosion rate of carbon steel. The corrosion rate in 0.55 mol/mol CO2 loading system doubles the rate of 0.20 mol/mol, in the absence of dissolved O2. This is due to the increase in concentrations of oxidizers (HCO3 and H+) in the MEA–H2O– CO2 environment which causes rates of oxidizer reduction to increase. This can be seen when viewing the increase in cathodic current densities in Fig. 10. The increase in H+ concentration was observed during the experiment due to the reduction of pH from 9.06 at 0.20 mol/ mol to 7.95 at 0.55 mol/mol, while the increase in the corrosion rate was suggested by the increase in solution conductivity from 23 mS/cm at 0.20 mol/mol to 45 mS/cm at 0.55 mol/mol (Table 2). Despite the changes in oxidizer reduction rate, the anodic and cathodic Tafel slopes do not change significantly with CO2 loading (Table 2).
Fig. 8 – Effect of solution temperature on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading in the absence of O2.
The corrosion potential is also shifted to the positive direction when CO2 loading is raised from 0.20 to 0.55 mol/mol. This indicates the solution’s oxidizing power (i.e. increased corrosion tendency) has increased due to an increase in oxidizing agent concentration. Fig. 10 also shows the corrosion rate of the system, containing a greater CO2 loading, would still be greater than one containing a lower CO2 loading even when corrosion inhibition is applied to raise the system potential to the passivation region. This is shown by the greater passivation current densities in the system containing 0.55 mol/mol CO2 loading.
3.6.
Effect of MEA concentration
MEA concentration has an effect on the corrosion of carbon steel. As the MEA concentration is increased from 5.0 to 7.0
Fig. 9 – Pourbaix diagram of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading and 0 kPa O2 partial pressure at 40 8C (created using corrosion analyzer software from OLI Systems Inc.).
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Fig. 10 – Effect of CO2 loading on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA at 80 8C, in the absence of O2.
and 9.0 kmol/m3, the corrosion rate rises from 0.55 to 0.72 and 1.06 mmpy, respectively, in an environment under 0.55 mol/ mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C. Such a corrosion effect is illustrated in Fig. 11. The anodic polarization curves shift slightly to the right where the current densities are greater. Changes in cathodic and anodic Tafel slopes suggest changes in the corrosion mechanism (Table 2). The increase in corrosion rate is a result of the increase in HCO3 available in the solution which induces a greater rate of iron dissolution. Fig. 11 also shows all curves with O2 show multiple cathodic loops, suggesting multiple cathodic reactions.
3.7.
Effect of precorrosion
All previous results and discussion relate to the corrosion behavior of carbon steel specimens in which their surface was clean and had no corrosion products before the experiments. Such a clean surface simulates new process equipment and piping at the beginning of plant operation but not the process components under long-term service. Therefore, it is neces-
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Fig. 12 – Effect of precorrosion on polarization behavior of carbon steel 1018 in an aqueous solution of 5.0 kmol/m3 MEA at 80 8C, in the presence of 10.13 kPa of O2.
sary for this work to include the effect of precorrosion, to reveal corrosion behavior of carbon steel that is not clean and has been in plant operation for certain periods of time. A series of electrochemical corrosion tests were carried out using an aqueous solution of 5.0 kmol/m3 MEA containing 0.20 mol/mol CO2 loading under 10.13 kPa O2 at 80 8C. Carbon steel specimens were precorroded under the identical environment for 7, 14 and 28 days. Results in Table 2 show that when a specimen is precorroded for a longer time, its corrosion rate becomes greater. This is evidenced by the increases in both anodic and cathodic current densities as shown in Fig. 12. Variation in the anodic and cathodic Tafel slopes (Table 2) suggests changes in anodic and cathodic reaction mechanisms. The passivation current densities also increase with precorrosion duration. All curves exhibit at least two cathodic loops before passive region, suggesting multiple cathodic reactions.
4.
Conclusion
Under typical CO2 absorption process conditions, carbon steel is in the active state where corrosion process is thermodynamically in favor. All parameters tested in this work affect corrosion rate and polarization behavior of carbon steel as summarized below.
Fig. 11 – Effect of MEA concentration on polarization behavior of carbon steel 1018 in an aqueous solution of MEA containing 0.55 mol/mol CO2 loading and 10.13 kPa O2 partial pressure at 80 8C.
Increasing O2 partial pressure accelerates corrosion due to the increasing oxidizer concentration in the solution. Dissolved O2 is required for the corrosion control that raises the system potential to passivation where passive film of hematite (g-Fe2O3) is established on the metal surface. A greater solution velocity causes a higher corrosion rate due to the enhancement of the transport rates of corroding agents between metal surface and bulk solution. Raising solution temperature enhances corrosion rate. This is the result of the increases in rates of iron dissolution and oxidizer reduction during corrosion process. An increase in CO2 loading in solution causes corrosion rate to increase. This is due to the increase in concentrations of
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corroding agents (HCO3 and H+), which causes rates of oxidizer reduction to increase. An increase in amine concentration makes solution more corrosive due to the increase in HCO3 available in the solution, which induces a greater rate of iron dissolution. The precorroded carbon steel corrodes faster than the nonprecorroded steel due to the faster rates of both iron dissolution and oxidizer reduction reactions.
Acknowledgments Authors gratefully acknowledge the Natural Science and Engineering Research Council of Canada (NSERC) and the Natural Resources Canada (NRCan) for financial support.
references
Ahmad, Z., 2006. Principles of Corrosion Engineering and Corrosion Control, 1st edition. Elsevier.
ASTM Standard G1-90, 1999a. Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens. ASTM, Philadelphia, PA (Re-approved 1999). ASTM Standard G5-94, 1999b. Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. ASTM, Philadelphia, PA (Reapproved 1999). DuPart, M.S., Bacon, T.R., Edwards, D.J., 1993. Understanding corrosion in alkanolamine gas treating plants. Part 2. Case histories show actual plant problems and their solutions. Hydrocarbon Process. (April), 75–80. Koch, G., 2001. Corrosion Cost Preventive Strategies in the Unites States. CC Technologies & NACE international (Sponsored by Office of Infrastructure and Development Federal Highway Administration). Kohl, A.L., Nielsen, R., 1997. Gas Purification, 5th edition. Gulf Publishing Company, TX. Mogul, M.G., 1999. Reduce corrosion in amine gas absorption columns. Hydrocarbon Process. 47–50 53–54, 56. Rampin, P., 2000. Amine units: results of a survey on structural reliability. In: Proceedings of International Conference Corrosion in Refinery, Petrochemical and Power Generation Plants, Venezia, May 2000, pp. 18–19. Veawab, A., Aroonwilas, A., 2002. Identification of oxidizing agents in aqueous amine–CO2 systems using a mechanistic corrosion model. Corros. Sci. 44, 967–987.