Corrosion Behavior of Carbon Steel in the MonoethanolamineâH2O

Ind. Eng. Chem. Res. 2009, 48, 10169–10179

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Corrosion Behavior of Carbon Steel in the Monoethanolamine-H2O-CO2-O2-SO2 System: Products, Reaction Pathways, and Kinetics Nattawan Kladkaew,† Raphael Idem,*,‡ Paitoon Tontiwachwuthikul,‡ and Chintana Saiwan† Petroleum and Petrochemical College, Chulalongkorn UniVersity, Bangkok 10330, Thailand, and International Test Centre for CO2 Capture, UniVersity of Regina, Regina, SK, Canada S4S 0A2

This work investigates the effect of operating parameters on corrosion products, reaction pathways, and kinetics for the corrosion of carbon steel in the monoethanolamine-H2O-CO2-O2-SO2 system. Corrosion experiments were conducted using a 273A potentiostat unit under conditions in which monoethanolamine (MEA), O2, and SO2 concentrations and CO2 loading were in the range of 1-7 kmol/m3, 0-100%, 0-204 ppm, and 0-0.5 mol CO2/mol MEA, respectively, at corrosion temperatures of 303-353 K to mimic the absorption-regeneration sections. Analysis, performed for this system for the first time, shows that corrosion products generated from the effect of SO2 include FeSO4 and Fe2O3 · H2O. Also, a higher concentration of SO2 in simulated flue gas stream induces a higher corrosion rate because of the increase in the hydrogen ion concentration generated by reactions of SO2 and H2O as well as SO2, O2, and H2O. A power-law model developed to correlate corrosion rate with the parameters in the MEA-H2O-CO2-O2-SO2 system shows that corrosion rate of carbon steel increases with an increase in O2 and SO2 concentrations in simulated flue gas stream, as well as MEA concentration, CO2 loading, and operating temperature. It was observed that CO2 loading had the highest impact on the corrosion rate, while SO2 and O2 show only slight effects on the corrosion rate. 1. Introduction Coal-fired power plants represent a major source of CO2 emission as well as varying amounts of sulfur dioxide (SO2). Chemical absorption with aqueous amine solutions such as monoethanolamine (MEA) has been found to be an effective CO2 capture technique especially for low-pressure flue gas streams. However, the disadvantage is that aqueous amine-based CO2 capture systems can be prone to corrosion problems. The corrosion rate of this system increases with increasing amine (e.g., MEA) concentration, CO2 loading, process temperature, oxygen concentration,1-4 and solution velocity.3,4 Heat-stable salts also increase the corrosiveness of carbon steel in MEA solution-CO2 environment to various degrees depending on the type and concentration of the salt.5,6 Sulfur dioxide (SO2), also an acid gas, in flue gas can give rise to plant equipment damage. In a previous study,7 the effects of various operating parameters on corrosion of carbon steel in simulated CO2 capture process in the presence of SO2 were investigated. It was found that corrosion rate of carbon steel in a MEA-H2O-CO2-O2-SO2 system varies significantly with the operating parameters. O2, SO2, and MEA concentrations, CO2 loading, and operating temperature can induce corrosion. However, this work did not identify the products that are formed and the possible pathway for SO2 related corrosion or its interaction with corrosion caused by other variables such as CO2 loading, MEA concentration, O2 concentration, and process temperature. In the literature, Rooney and DuPart8 reported that some corrosion products in CO2 absorption with aqueous amine solutions such as iron carbonate and iron hydroxide, which are often insoluble and precipitate, are found in various places in the plant. There are also other soluble products in the corrosion system as well, such as Fe(HCO3)2. However, no direct * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (306)585-4855. † Chulalongkorn University. ‡ University of Regina.

experimental analytical results have been reported to quantify these products. In order to understand how these corrosion products are produced, some corrosion mechanisms or pathways have been postulated. Generally, corrosion can be explained in terms of electrochemical reactions involving the transfer of electrons which results in material deterioration. More specifically, when a metal is immersed in a given solution, electrochemical reactions occur at the surface of the metal, causing the metal to corrode. The corrosion process involves two or more reactions at the electrodes: the oxidation of the metal (anodic partial reaction) and the reduction of an oxidizing agent (cathodic partial reaction). The oxidation reaction leads to the production of electrons whereas the consumption of electrons signifies a reduction reaction.9,10 For example, the corrosion of zinc in an acid environment proceeds according to the following reactions: Overall reaction Zn + 2H+ T Zn2+ + H2

(1)

Oxidation reaction Zn T Zn2+ + 2e-

(2)

2H+ + 2e- T H2

(3)

Reduction reaction

According to Veawab and Aroonwilas,11 several corrosion mechanisms in an aqueous amine-CO2 system have been postulated in the amine treating plant. In this work, five different types of iron dissolution reactions were suggested. The evolved CO2 reacts directly with carbon steel to form iron carbonate (FeCO3) as shown in reaction 4.12 Fe + CO2 + H2O T FeCO3 + H2

(4)

The second one is corrosion involving the reduction of the hydrogen ion13

10.1021/ie900746g CCC: $40.75  2009 American Chemical Society Published on Web 11/02/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009

Fe + 2H+ T Fe2+ + H2

(5)

Bicarbonate ion (HCO3-) in the solution functions as an oxidizing agent in the reduction reaction14 Fe + 2HCO3- T FeCO3 + CO32- + H2

(6)

Corrosion due to the reduction of undissociated carbonic acid (H2CO3)15 Fe + H2CO3 T FeCO3 + H2

(7)

In addition to the above reactions, Kossein et al.16 consider that there is the corrosion mechanism involving the amine itself. Fe + 2RR′NH2+ T Fe2+ + H2 + 2RR′NH

(8)

where RR′NH2+ and RR′NH denote protonated amine ion and amine, respectively. However, this has not been used in any other work. In the present work, the products that are formed and the possible pathway for corrosion caused by SO2 and its interaction with corrosion caused by other variables such as CO2 loading, MEA concentration, O2 concentration, and process temperature are identified and quantified. This work also attempts to postulate possible pathways to account for these products. Analytical techniques were used to obtain information on the organic anions in the tested solution as well as the corrosion products on the metal surface and those that dissolved in the solution. Inductively coupled plasma-mass spectrometry (ICP/MS) was used to determine the amount of dissolved iron in the solution, while capillary electrophoresis (CE) was used to determine the organic anions present in the tested solution. Corrosion products on the metal surface were evaluated by using both scanning electron microscopy and energy dispersive spectrometry (SEM/EDS). In addition, an empirical kinetic evaluation to determine the effects of the process variables on corrosion rate in the presence of SO2 was formulated. The results of these analyses and evaluation are presented and discussed in this paper. 2. Experiments 2.1. Equipment and Chemicals. The electrochemical experiments were carried out using aqueous monoethanolamine (Fisher Scientific, ON, Canada) as the absorption solvent. It was diluted with deionized water to the desired concentration, which was accurately determined by titration with 1.0 N hydrochloric acid (HCl) solution using methyl orange as the titration indicator. The desired concentration of aqueous MEA solution was then preloaded with carbon dioxide to obtain the desired CO2 loading (mol CO2/mol MEA) by purging a stream of CO2 gas (Praxair, research grade, ON, Canada) into the solution. The CO2 loading procedure followed the AOAC method.17 The desired CO2 loading was determined by titrating with a 1.0 N HCl solution using a Chittick apparatus. Carbon steel C1020 (Metal Samples Company, Munford, AL) was used to study the corrosion in the MEA-H2O-CO2O2-SO2 system. The chemical composition of C1020 in percentage was as follows: C, 0.19; Cr, 0.01; Cu, 0.01; Mn, 0.56; Mo, 0.01; N, 0.0036; Ni, 0.01; P, 0.009; S, 0.007; and Fe, balance. The tested specimens are cylindrical in shape with 3/8 in. diameter, 1/2 in. length with a 3-48 threaded hole at one end. The specimens were prepared in accordance with ASTM G1-90.18 The specimens were wet ground with 240 grit silicon carbide paper, wet polished with 600-grit silicon carbide

paper, rinsed with deionized water, dried with air, and kept in a desiccator before use. The surface areas of the specimens were determined by measuring all dimensions with a vernier caliper. Electrochemical techniques are used to study corrosion and corrosion behavior of carbon steel C1020 in the MEA-H2OCO2-O2-SO2 system. The experiment setup for this technique is shown in Figure 1. It consists of an ASTM corrosion cell, potentiostat, water bath with temperature controller, condenser, gas supply set, and data acquisition system. An ASTM corrosion cell model K47 (London Scientific, Ltd., London, ON, Canada), is a 1 L flat bottom flask with ground glass joints. It is composed of one working electrode mounted with specimens, two highdensity carbon graphite rods used as counter electrodes, one reference electrode which is a mercurous sulfate electrode (MSE), one bridge tube, and a glass inlet and outlet for transferring gas to and from the corrosion cell. A potentiostat model 273A (London Scientific, Ltd., London, ON, Canada) was used to control the potential and to read the current accurately. PowerCORR version 2.47 (London Scientific, Ltd., London, ON, Canada) was used to acquire and analyze the experimental data. A water bath with a temperature controller was used to control the operating temperature. A condenser was connected to the corrosion cell to maintain the temperature of the solution in order to keep the correct concentration in the cell, while avoiding evaporation during the experiment. The gas supply set was gaseous mixtures of SO2-O2-N2 that were similar to actual flue gas stream conditions. ASTM G5-9419 was used in evaluating the accuracy of a given electrochemical test apparatus. It was performed by running the experiment with potentiodynamic anodic polarization technique on a 430 stainless steel in a 1 N sulfuric acid (H2SO4) solution at 30 °C. The reliability of experiment is ascertained when the obtained polarization plot appears within the reference band. All the electrochemical experiments were carried out in accordance with ASTM G5-94. 2.2. Typical Experimetal Run. 2.2.1. MEA-H2O-CO2O2 System: The Effect of Oxygen Concentration. The corrosion cell containing about 1 L of 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA was immersed in a water bath with a temperature controller. The temperature of the solution was kept constant at 353 K. Oxygen concentration in a stream of simulated flue gas was varied as 0, 6, 21, and 100% and introduced into the corrosion cell at a flow rate of 150 mL/min for 1.5 h. The carbon rods counter electrodes were placed in the test cell. Then, the salt bridge was filled with the test solution and placed in the corrosion cell. The prepared surface was mounted on the electrode holder rod. Consequently, the specimen was degreased with methanol and rinsed in distilled water just prior to immersion in the test cell. The salt-bridge probe tip was adjusted close to the specimen electrode. All the lines between the corrosion cell and the model 273A potentiostat had been connected before the corrosion potential (ECORR) versus the MSE reference electrode of the test system were measured for at least 1 h to ensure that the corrosion potential value remained constant. Finally, the electrochemical experiment was started. During the running of each experiment, the applied potential and the measured current were continuously recorded. 2.2.2. MEA-H2O-CO2-O2-SO2 System: The Effect of SO2 Concentration. The procedures were the same as described in the effect of oxygen concentration, but a stream of simulated flue gas with varying SO2 concentrations 0, 5, 10, and 204 ppm with 6% oxygen was introduced into the corrosion cell at a flow rate of 150 mL/min for 1.5 h.


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Figure 1. Experimental setup for electrochemical corrosion experiment.

2.2.3. MEA-H2O-CO2-O2-SO2 System: The Effect of CO2 Loading. Just as described for the MEA-H2O-CO2O2-SO2 system, about 1 L of 5 kmol/m3 MEA, preloaded CO2 loadings of 0, 0.2, 0.4, and 0.5 mol CO2 loading/mol MEA were studied with a stream of simulated flue gas of 204 ppm SO2 and 6% O2. The procedures followed here were the same as those described above for the effect of SO2 concentration. 2.2.4. MEA-H2O-CO2-O2-SO2 System: The Effect of MEA Concentration. Just as in the case of the MEA-H2OCO2-O2-SO2 system, about 1 L of MEA solution with varying concentrations of 1, 3, 5, and 7 kmol/m3, CO2 loading of 0.4 mol CO2/mol MEA were studied. The procedures were the same as described in the effect of CO2 loading. 2.2.5. MEA-H2O-CO2-O2-SO2 System: The Effect of Operating Temperature. Just as in the case of the MEA-H2OCO2-O2-SO2 system, the experimental conditions were set at 1 L of 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA. The temperature was adjusted at 303, 328, and 353 K. The procedures were the same as described in the effect of CO2 loading. 2.3. Corrosion Measurement Technique: Tafel Plot.9,20 In our previous work,7 we used two techniques (Tafel plot and potentiodynamic) to evaluate corrosion rates. Both techniques provided very similar trends of information. The corrosion rates obtained from Tafel plot and potentiodynamic techniques were almost identical. In this work, we decided to use the Tafel plot technique. A Tafel plot was generated by beginning the scan from -250 to +250 mV vs corrosion potential (ECORR). The resulting data is plotted as the applied potential vs the logarithm of the measured current. The corrosion current (iCORR) was obtained from the intersection at ECORR and, then, was used to calculate the corrosion rate using eq 9. CR(mpy) )

0.13iCORR(EW) Ad

(9)

where CR is the corrosion rate in mils per year (mpy), iCORR is the corrosion current in microampere (µA), EW is the equivalent weight of the corroding species in grams (g), A is the surface area of the specimen in squared centimeters (cm2), and d is the density of the specimen in grams per cubic centimeter (g/cm3). 2.4. Analysis of Corrosion Products. 2.4.1. Inductively Coupled Plasma-Mass Spectrometry (ICP/MS) Technique. The Varian ICP/MS (Varian, Inc., Mississauga, ON, Canada) was used to determine the amount of dissolved iron (Fe) in the tested solution. The acquired data were used to confirm the fact

Figure 2. Effect of O2 concentrations in simulated flue gas stream on polarization curves obtained with the Tafel plot technique for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA at 353 K.

that higher amounts of dissolved iron represent higher corrosion rates for each studied system. Furthermore, corrosion products are generally composed of dissolved iron. 2.4.2. Capillary Electrophoresis (CE) Technique. A CE instrument equipped with a diode array detector (DAD, HP 3D CE, Hewlett-Packard Canada, Ltd., Montreal, QC, Canada) was employed for the detection of inorganic anions in the tested solution. There are six different anions considered in this work, namely bicarbonate, carbonate, sulfite, bisulfite, sulfate and thiosulfate, which were chosen based on the possible formation of these anions in the studied system. 2.4.3. Scanning Electron Microscopy-Energy Dispersive Spectrometry (SEM-EDS) Technique. SEM (JSM-5600, JEOL USA, Inc., Peabody, MA) was used to examine the surface area of the tested specimen. Then, an EDS (EDAX Genesis 7000, EDAX, Inc., Mahwah, NJ) detector was used to characterize the elemental distribution on the surface of the images taken by SEM. This analytical technique can be used to determine the composition of the corrosion product on the metal surface. 3. Results and Discussion 3.1. Corrosion Rate. 3.1.1. Oxygen Concentration. 3.1.1.1. Experimental Results. Figure 2 shows the polarization curves of the effect of oxygen concentration in the simulated flue gas stream on the corrosion rate of carbon steel in the tested system from the experiments. The small changes of corrosion

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Figure 3. Effect of O2 concentrations in simulated flue gas stream on corrosion rate for system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA at 353 K.

Figure 5. Effect of O2 concentrations in simulated flue gas stream on the carbonate/bicarbonate concentrations in tested solution for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA at 353 K.

Figure 4. Effect of O2 concentration in simulated flue gas stream on the amout of dissolved iron in solution for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA at 353 K.

current density with oxygen concentration in Figure 2 were determined using the PowerCORR program which requires a large number of data points. This was made possible because each graph consists of a large number of raw data. The results obtained from the PowerCORR program are plotted in Figure 3, which demonstrates that the corrosion rate increases with increasing oxygen concentration. 3.1.1.2. Corrosion Products Analysis. The ICP/MS technique examines the amount of dissolved iron in the tested solution. The amount of iron obtained in the tested solution as a function of oxygen concentration in the simulated flue gas stream is shown in Figure 4. It is seen from the figure that the amount of dissolved iron present in the tested solution increases as the oxygen concentration in simulated flue gas stream increases. This is confirmation that a high oxygen concentration is detrimental to the absorption process in terms of a higher corrosion rate. Only carbonate and bicarbonate inorganic anions were detected together by the CE technique in the tested solution. The amount of carbonate/bicarbonate obtained in the tested solution as a function of oxygen concentration in the flue gas is shown in Figure 5. The figure shows that there is no significant change in the amount of carbonate/bicarbonate anions in the tested solution for different oxygen concentrations. This is because each of the tested solutions contained the same amount of dissolved carbon dioxide. It is well-known that the carbonate or bicarbonate are typically derived from dissolved carbon dioxide and not from dissolved oxygen, thus explaining why the amount of carbonate/bicarbonate ions are independent of the oxygen concentration in the flue gas. Generally, the tested specimen surface is changed from shiny to tarnished metal because the specimen surface is covered by some corrosion products. Figures 6 and 7 respectively show representative SEM images of the tested specimen surface in microscopic scale before and after the experiment.

Figure 6. SEM image of the tested specimen surface before the experiment.

Figure 7. SEM image of the tested specimen surface after the experiment.

The representative EDS spectrum (elemental mapping) shown in Figure 8 reveals the elemental compositions of these microscope images. Iron (Fe), carbon (C), oxygen (O), and sulfur (S) are indicated to be the composition of the corrosion products. Therefore, only these elements are considered. The amounts of each element on the metal surface are shown in Figure 9. The first bar represents the amounts of the elements on the specimen surface before the experiments. These amounts


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Figure 10. Effect of SO2 concentrations in simulated flue gas stream on polarization curves obtained with the Tafel plot technique for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 6% O2 at 353 K.

The general anodic reaction is the dissolution of iron or the oxidation of iron to the ferrous (Fe2+) ion given in reaction 11.

Figure 8. EDS spectrum of the tested specimen.

Fe T Fe2+ + 2e-

(11)

Then, the reduction-oxidation reaction among iron, dissolved oxygen, and water occurs as in reaction 12 2Fe + O2 + 2H2O T 2Fe(OH)2

(12)

The produced ferrous hydroxide component (Fe(OH)2) is unstable in systems containing oxygen and thus is oxidized to the ferric salt (Fe(OH)3) or rust21 as Figure 9. Effect of O2 concentration in simulated flue gas stream on the amount of elements on the tested specimen for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA at 353 K.

are compared with those after the experiments for the second, third, and fourth bars for 0, 6, and 100% oxygen, respectively. The amount of Fe on the surface of the tested specimen decreases as the oxygen concentration in simulated flue gas stream increases. On the other hand, the amount of O increases as the oxygen concentration increases, whereas there is no change in the amount of S. This is probably due to the small amount of sulfur in the system relative to those of Fe and O. The amount of C increases slightly from its original value. Therefore, the products that are formed based on O and C on the tested surface are more measurable than those formed from S. These products include Fe(OH)2, Fe(OH)3, and FeCO3. We have attempted to use the XRD technique to determine and verify the corrosion product formed on the specimen surface but could not get any meaningful result from this technique. It is possible that the corrosion products are either not in a crystalline form or are present in very minute amounts. As such, the use of the XRD technique appears not to be appropriate for this system. 3.1.1.3. Mechanisms. The effect of varying the oxygen concentration on corrosion of carbon steel in an MEA-H2OCO2-O2 system is due to the difference in the amount of dissolved oxygen. The higher the oxygen concentrations, the higher the dissolved oxygen present in the tested solution resulting in the dissolution of iron by the reaction of dissolved oxygen as follows: Reduction of dissolved oxygen21 O2 + 2H2O + 4e- T 4OH-

(10)

2Fe(OH)2 + 1/2O2 + H2O T 2Fe(OH)3

(13)

The corrosiveness of the system is due to the amount of dissolved oxygen in the tested solution leading to the oxidation of iron. 3.1.2. Sulfur Dioxide Concentration. 3.1.2.1. Experimental Results. Figure 10 shows the obtained Tafel plot of the effect of sulfur dioxide concentration in the simulated flue gas stream on corrosion rate. The slightly higher current densities are shown in this figure based on the Tafel plot technique. As explained in the oxygen concentration case, the small changes of corrosion current density with SO2 concentration were determined using the PowerCORR program which requires a large number of data points. This was made possible because each graph consists of a large number of raw data. The results obtained from the PowerCORR program are plotted in Figure 11. This figure shows the effect of sulfur dioxide concentration in the flue gas stream on corrosion rate and shows that the corrosion rate increases slightly with the SO2 concentration. 3.1.2.2. Corrosion Products Analysis. Increasing the SO2 concentration in the system results in a higher amount of dissolved iron (as determined by ICP-MS) being present in the solution. This is shown in Figure 12. This shows clearly that SO2 in the flue gas induces corrosion in the amine-based CO2 absorption system. The amount of carbonate/bicarbonate anions in the tested solution against the SO2 concentration is plotted in Figure 13. It is seen that there is no significant difference in the amount of carbonate/bicarbonate in the systems by varying the SO2 concentration. On the other hand, only for the high-concentration (204 ppm) SO2 system, about 20 ppm of sulfate/bisulfite/sulfite anions were obtained using the CE technique.

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Figure 11. Effect of SO2 concentrations in simulated flue gas stream on the corrosion rate of system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 6% O2 at 353 K.

Figure 12. Effect of SO2 concentration in simulated flue gas stream on the amout of dissolved iron in solution for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 6% O2 at 353 K.

Figure 14. Effect of SO2 concentrations in simulated flue gas stream on the amount of elements on the tested specimen for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 6% O2 at 353 K.

3.1.2.3. Mechanisms. The results presented show that a higher sulfur dioxide concentration in the simulated flue gas stream induces more corrosiveness due to the increase in the solubility of SO2, and generally, the formation of hydrogen ion (H+) as shown in reactions 14-16.22-24 SO2 + H2O T H+ + HSO3-

(14)

HSO3- T H+ + SO32-

(15)

SO2 + 1/2O2 + H2O T 2H+ + SO42-

(16)

Generally H+ or the hydronium ion (H3O+) are the reducible ions, the basic reduction reaction of H+ is shown in reaction 3, and the oxidation-reduction reaction of iron and the hydrogen ion is the same as in reaction 5, while the reduction of H3O+ is shown in reaction 17. 2H3O+ + 2e- T 2H2O + H2

(17)

Then, the oxidation-reduction of iron and hydronium ion occurs as in reaction 18. Fe + 2H3O+ T Fe2+ + 2H2O + H2

(18)

SO2 may react with water and oxygen and, then, cause the corrosion of the carbon steel directly as shown in reactions 19-21.25 Fe + SO2 + O2 f FeSO4

(19)

4FeSO4 + O2 + 6H2O f 2Fe2O3 · H2O + 4H2SO4 (20) Figure 13. Effect of SO2 concentration in simulated flue gas stream on the carbonate/bicarbonate concentrations in solution for a system prepared with 5 kmol/m3 MEA and a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 6% O2 at 353 K.

The EDS spectrum which shows the amounts of elements on the metal surface from the effect of SO2 concentration is given in Figure 14. The first bar represents the amounts of the elements on the specimen surface before the experiments. These amounts are compared with those after the experiments for the second, third, and fourth bars for 5, 10, and 204 ppm SO2, respectively. The amount of Fe on the surface of the tested specimen decreases with increasing SO2 concentration while O and C slightly increase from the original amounts. S and O increase with increasing SO2 concentration because of the formation of products that are composed of O, S, and C on the tested surface. These products include Fe(OH)2, Fe(OH)3, 2Fe2O3 · H2O, FeCO3, and FeSO4.

4H2SO4 + 4Fe + 2O2 T 4FeSO4 + 4H2O

(21)

3.1.3. CO2 Loading. 3.1.3.1. Experimental Results. Figure 15 illustrates the effect of CO2 loading on the polarization curves obtained from the Tafel plot technique. It shows that higher CO2 loadings result in both higher anodic and cathodic current densities, implying higher corrosion rates. As stated earlier, the effect of CO2 loading on the corrosion rate is illustrated in Figure 16. The corrosion rate increases dramatically with an increase in CO2 loading. 3.1.3.2. Corrosion Products Analysis. There is an increase in the amount of dissolved iron with increasing CO2 loading as shown in Figure 17. The plot of CO2 loading against the amount of carbonate/bicarbonate anions is shown in Figure 18. This plot indicates that these anions increase with increasing CO2 loading. This is attributed to the increase in the amount of dissolved CO2 in the system. The amounts of carbonate/


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Figure 15. Effect of CO2 loadings on polarization curves obtained with the Tafel plot technique for a system prepared with 5 kmol/m3 MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K. Figure 18. Effect of CO2 loadings on the carbonate/bicarbonate concentrations in solution for a system prepared with 5 kmol/m3 MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

Figure 16. Effect of CO2 loadings on the corrosion rate of system prepared with 5 kmol/m3 MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

Figure 19. Effect of CO2 loadings on the amount of elements on the tested specimen for a system prepared with 5 kmol/m3 MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

3.1.3.3. Mechanism. The effect of CO2 loading on corrosion of carbon steel in MEA-H2O-CO2-O2-SO2 system is due to an increase in the concentration of carbonic acid (H2CO3) and bicarbonate (HCO3-) that can induce the corrosion of iron. The formation of carbonic acid and bicarbonate are explained on the basis of reactions 22-24.26

Figure 17. Effect of CO2 loadings on the amout of dissolved iron in solution for a system prepared with 5 kmol/m3 MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

bicarbonate anions are dominant in this system causing a significant difference in corrosion rates of these systems. The amounts of elements on the tested specimen surface as a function of CO2 loading were obtained from the EDS technique. The results are shown in Figure 19. The first bar represents the amounts of the elements on the specimen surface before the experiments. These amounts are compared with those after the experiments for the second, third and fourth bars for 0.2, 0.4, and 0.5 mol CO2/mol MEA, respectively. The amount of Fe on the surface of the tested specimen decreases as the CO2 loading increased, while O and S do not change much. C increases with increasing CO2 loading because of increasing amounts of dissolved CO2 in the solution. Corrosion products are composed of O, S, and C on the tested surface. These products include Fe(OH)2, Fe(OH)3, 2Fe2O3 · H2O, FeCO3, and FeSO4.

H2O + CO2 f H2CO3

(22)

2H2CO3 + 2e- T H2 + 2HCO3-

(23)

2HCO3- + 2e- T H2 + 2CO32-

(24)

The reduction-oxidation of iron with carbonic acid and bicarbonate ion are given in reactions 25 and 26. Fe + 2H2CO3 T H2 + Fe(HCO3)2

(25)

Fe + 2HCO3- T H2 + FeCO3 + CO32-

(26)

It is also found that the amounts of bicarbonate and hydronium ions generally increase, as shown by reactions 27-29.27 Formation of carbamate 2RR′NH + CO2 T RR′NCOO- + RR′NH2+

(27)

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Hydrolysis of carbamate RR′NCOO- + H2O T RR′NH + HCO3-

(28)

Dissociation of protonated amine RR′NH2+ + H2O T H3O+ + RR′NH

(29)

The reductions of these reducible ions are also shown in reactions 3, 17, 23, and 24; also, the redox reactions are shown in reactions 5, 18, 25, and 26. A higher concentration of these ions implies a higher reduction rate, and consequently, a higher corrosiveness of the system. 3.1.4. MEA Concentration. 3.1.4.1. Experimental Results. Based on Tafel plot technique, it is seen in Figure 20 that a higher MEA concentration resulted in higher anodic and cathodic current densities. The implication is that MEA concentration has a significant effect on the corrosion rate as shown in Figure 21 because the corrosion rate of carbon steel in the tested system increases sharply as MEA concentration increases. 3.1.4.2. Corrosion Products Analysis. The corrosion rate increases as MEA concentration increases. This could be confirmed from Figure 22. Higher MEA concentrations result in larger amounts of dissolved iron being present in the solution. There is also an increase in the amount of carbonate/bicarbonate anions with higher MEA concentrations. This is illustrated in Figure 23. This could be explained on the basis of an increase in the amount of absorbed CO2 with increasing MEA concentration. The increase in carbonate/bicarbonate anion concentration is responsible for the increased corrosiveness of these systems. The amounts of different elements on the tested specimen surface were obtained from the EDS technique. The results are

Figure 22. Effect of MEA concentrations on the amout of dissolved iron in solution for a system prepared with an MEA solution and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

Figure 23. Effect of MEA concentrations on the carbonate/bicarbonate concentrations in solution for a system prepared with a MEA solution, a CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppmSO2 and 6% O2 at 353 K.

Figure 20. Effect of MEA concentrations on polarization curves obtained with the Tafel plot technique for a system prepared with an MEA solution and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K. Figure 24. Effect of MEA concentrations on the amount of elements on the tested specimen for a system prepared with an MEA solution and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

Figure 21. Effect of MEA concentrations on the corrosion rate for a system prepared with an MEA solution and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2 at 353 K.

shown in the bar chart in Figure 24. The first bar represents the amounts of the elements on the specimen surface before the experiments. These amounts are compared with those after the experiments for the second, third, and fourth bars for 3, 5, and 7 kmol/m3 MEA, respectively. The amount of Fe on the surface of the tested specimen decreases as the MEA concentration increases. In contrast, C and S increase with increasing MEA concentration. On the other hand, there was no change in the amount of O with MEA concentration. The products


Figure 25. Effect of operating temperatures on polarization curves obtained with the Tafel plot technique for a system prepared with 5 kmol/m3 MEA and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2.

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Figure 27. Effect of operating temperatures on the amount of dissolved iron in solution for a system prepared with 5 kmol/m3 MEA and CO2 loading of 0.4 mol CO2/mol MEA with a simulated flue gas stream of 204 ppm SO2 and 6% O2.

Figure 26. Effect of operating temperatures on the corrosion rate for a system prepared with 5 kmol/m3 MEA and CO2 loading of 0.4 mol CO2/ mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2).

responsible for the elemental distribution described include Fe(OH)2, Fe(OH)3, 2Fe2O3 · H2O, FeCO3, and FeSO4. 3.1.4.3. Mechanisms. According to reactions 22-29, the higher the MEA concentration, the higher the total amount of CO2 absorbed into the amine solution, resulting in higher amounts of reducible ions, HCO3- and H3O+. These ions enhance the reduction rate, which induces a more rapid corrosion process. These oxidation-reduction processes are described in eqs 5, 17, 24, and 25. 3.1.5. Operating Temperature. 3.1.5.1. Experimental Results. The effect of temperature on polarization curves is shown in Figure 25, which reveals that systems kept at higher temperature have higher anodic and cathodic current densities, resulting in an increase in the corrosion rate. It is illustrated in Figure 26 that a higher temperature increases the corrosion rate of carbon steel in the studied system. 3.1.5.2. Corrosion Products Analysis. The significance of the operating temperature comes on the basis of the amount of dissolved iron in solution. This is shown in Figure 27. As the temperature increases, a larger amount of dissolved iron is present in the solution. Also, a not so significant amount of carbonate/bicarbonate anions are observed with increasing operating system temperature as shown in Figure 28. The amounts of different elements on the tested specimen surface are obtained by the EDS technique. The results are shown in Figure 29. The first bar represents the amounts of the elements on the specimen surface before the experiments. These amounts are compared with those after the experiments for the second, third, and fourth bars for 303, 328, and 353 K, respectively. The amount of Fe on the surface of the tested specimen decreases as the temperature increases because of higher rate

Figure 28. Effect of operating temperatures on the carbonate/bicarbonate concentrations in solution for a system prepared with 5 kmol/m3 MEA and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2.

Figure 29. Effect of operating temperatures on the amount of elements on the tested specimen for a system prepared with 5 kmol/m3 MEA and CO2 loading of 0.4 mol CO2/mol MEA with simulated flue gas stream of 204 ppm SO2 and 6% O2.

of corrosion. O, C, and S increase with increasing operating temperature which results from the products of corrosion that cover the specimen surface, including Fe(OH)2, Fe(OH)3, 2Fe2O3 · H2O, FeCO3, and FeSO4. The reason for the increase in corrosiveness with operating temperature can be explained on the basis of reaction kinetics that temperature generally accelerates the rate of any reaction.28 All reactions mentioned in the previous section can go faster. The dissolved oxygen,

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H+ or H3O+, H2CO3, and HCO3- enhance the reduction rate, and then, more metal is dissolved into the solution, thus leading to a higher corrosion rate. 3.2. Corrosion Rate Correlation. An empirical power lawtype model (eq 30) for the liquid phase has been used to correlate corrosion rate with all the parameters in the MEA-H2OCO2-O2-SO2 system. CR ) Ae(-Ha)/T[SO2]a2[O2]a3[CO2]a4[MEA]a5

(30)

where CR is the corrosion rate (mpy), A is the pre-exponential constant, Ha represents the temperature sensitivity of the electrochemical reaction (K-1), T is the operating temperature (K), [SO2], [O2], [CO2], and [MEA] denote the concentrations of SO2, O2, CO2, and MEA (kmol/m3) in the solution phase, respectively, and a2, a3, a4, and a5 are the partial reaction orders with respect to SO2, O2, CO2, and MEA, respectively. It should be noted that in the gas phase Ha is equivalent to Ea/R. The SO2 concentration in the liquid phase was calculated from SO2 concentration in the gas phase based on its solubility in water at various temperatures. The O2 concentrations in the gas phase were used to calculate the dissolved oxygen in the liquid phase at a given pressure and temperature as shown in eq 31.29 [O2] ) -2.545 + 0.807 × 10-2T - 84.14p + 2.096 × 10-4pT2 + 2.322 × 104p/T + 1.027p2 3.911 × 102p2 /T

Acknowledgment The authors acknowledge the financial support from the Thailand Research Fund (TRF) through the Royal Golden Jubilee Ph.D. Program (Grant PHD/0204/2547) and from the Natural Science and Engineering Research Council of Canada (NSERC). Literature Cited

(31)

where [O2] is concentration of oxygen in millimoles per liter, T is temperature in kelvin, and p is pressure in megapascals. By using a nonlinear regression program (NLREG version 6.3), it was possible to determine all the constants in eq 30. The final corrosion rate correlation was thus obtained as eq 32. × { [ -5955 T ]}

CR ) 1.77 × 109 exp

{[SO2]0.0011[O2]0.0006[CO2]0.9[MEA]0.0001}

for the first time shows that corrosion products generated from the effect of SO2 include FeSO4 and 2Fe2O3 · H2O. • The corrosion products obtained experimentally based on CE and SEM/EDS techniques for the MEA-H2O-CO2O2-SO2 system include Fe(OH)2, Fe(OH)3, 2Fe2O3 · H2O, FeCO3, and FeSO4. • A power-law model developed to correlate corrosion rate with the parameters in the MEA-H2O-CO2-O2-SO2 system shows that corrosion rate of carbon steel increases with an increase in O2 and SO2 concentrations in simulated flue gas stream, as well as MEA concentration, CO2 loading, and operating temperature. The CO2 loading had the strongest impact on the corrosion rate, while SO2 and O2 show only slight effects on the corrosion rate.

(32)

The regression results show good regression statistics, with a coefficient of correlation (R2) of 0.97. Also, a comparison of the experimental results with the corrosion rate model in terms of accuracy of the percentage average absolute deviation (% AAD) was 15.48%. Thus, this coefficient shows that the corrosion rate model adequately fits the corrosion rate data for the MEA, O2, SO2 concentrations, CO2 loading, and temperature ranges studied in this work, namely MEA concentration range of 1-7 kmol/m3, O2 concentration range of 0-100%, SO2 concentration range of 0-204 ppm, CO2 loading range of 0-0.5 mol CO2/mol MEA, and a temperature range of 303-353 K. The regression results show that CO2 loading has the strongest effect on the corrosion of carbon steel in MEA-H2O-CO2O2-SO2 system. On the other hand, the MEA concentration has a small effect on corrosion since MEA itself is not corrosive to carbon steel.30 Also, since small amounts of dissolved SO2 and O2 are present in the aqueous solution, the consequence is slight contributions of these two parameters to corrosion. 4. Conclusions • We have shown for the first time that a higher concentration of SO2 in a simulated flue gas stream will induce a higher corrosion rate because of the increase in hydrogen ion concentration generated by SO2 and H2O as well as SO2, O2, and H2O. Also, iron may also react directly with SO2 and O2 to cause corrosion of carbon steel. Furthermore, analysis done

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ReceiVed for reView May 7, 2009 ReVised manuscript receiVed October 16, 2009 Accepted October 17, 2009 IE900746G

Corrosion Behavior of Carbon Steel in the MonoethanolamineâH2O

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