ABSTRACT INTRODUCTION

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1 C Mehanisti Modeling of Carbon Steel Corrosion in a MDEABased CO 2 Capture Proess YoonSeok Choi Institute for Corrosion and Multiphase Tehnology, Department of Chemial and Biomoleular Engineering, Ohio University 342 West State Street Athens, OH, USA Deli Duan Institute of Metal Researh, Chinese Aademy of Sienes 72 Wenhua Road Shenyang, China Srdjan Nešić Institute for Corrosion and Multiphase Tehnology, Department of Chemial and Biomoleular Engineering, Ohio University 342 West State Street Athens, OH, USA Shengli Jiang Institute of Metal Researh, Chinese Aademy of Sienes 72 Wenhua Road Shenyang, China ABSTRACT A preditive model was developed for orrosion of arbon steel in CO 2 loaded aqueous methyldiethanolamine (MDEA) systems, based on modeling of thermodynami equilibria and eletrohemial reations. The onentrations of aqueous arboni and amine speies (CO 2, HCO 3, CO 3 2, MDEA and MDEAH ) as well as ph values in the MDEA solution were alulated. The water hemistry model showed a good agreement with experimental data for ph and CO 2 loading, with an improved orrelation upon use of ativity oeffiients. The eletrohemial orrosion model was developed by modeling polarization urves based on the given speies onentrations. The required eletrohemial parameters (e.g., exhange urrent densities, Tafel slopes and reation orders) for different reations were determined from experiments onduted in glass ells. Iron (Fe) oxidative dissolution, biarbonate (HCO 3 ) redution and protonated alkanolamine (MDEAH ) redution reations were implemented to build a omprehensive model for orrosion of arbon steel in an MDEACO 2 H 2 O environments. The model is appliable to uniform orrosion when no protetive films are present. A solid foundation is provided for orrosion model development for other aminebased CO 2 apture proesses. Key words: MDEA, orrosion model, CO 2 apture, arbon steel, arbon apture and storage INTRODUCTION Aminebased CO 2 apture proess has gained more interest reently as the immediate tehnologial solution that an be used for apturing CO 2 from flue gas streams emitted from oalfired power plant. 1,2

2 Although aminebased CO 2 apture proess has been proven in urrent industrial proesses suh as natural gas prodution, syngas srubbing, et., the amine proess is assoiated with several tehnial hallenges. 3 One of the major problems is orrosion of proess omponents whih results in unexpeted downtime, prodution loss and even fatalities. Corrosiveness of an amine solution after CO 2 absorption depends on the type and onentration of amine, CO 2 loading, temperature, solution turbulene, et. 4 From a orrosion standpoint, methyldiethanolamine (MDEA, CH 3 N(C 2 H 4 OH) 2 ) is the most forgiving alkanolamine beause it is a tertiary amine and it does not form arbamate (R 3 NCOO ) with CO Although there are extensive researh data available on orrosion and orrosion inhibition in amineco 2 systems, 812 minimal information has been reported in the literature whih ould aid in establishing a orrosion model for arbon steel in suh systems. Veawab and Aroonwilas 13 reported a mehanisti orrosion model to identify the oxidizing agents responsible for orrosion reations in the monoethanolamine (MEA, (CH 2 ) 2 OHNH 2 ) system. Results indiated that biarbonate ion and water are the primary oxidizing agents and hydrogen ion played an insignifiant role in the redution reation. The objetive of the present study was to develop a preditive model for orrosion of arbon steel under operating onditions in the absorber with MDEA related to the CO 2 apture proess in fossil fuelfired power plants. Thermodynami framework SPECIATION MODEL FOR AN MDEA/CO 2 /H 2 O SYSTEM When CO 2 is dissolved and reated with the MDEA, one an identify eight main speies in the solution (MDEA, H 2 O, CO 2 (aq), MDEAH, HCO 3, CO 3 2, H 3 O, and OH ). Carboni aid (H 2 CO 3 ) is not inluded here as it has a muh lower onentration ompared to other arboni speies (HCO 3 and CO 3 2 ), and the ativity oeffiient data for this speies were not available in the open literature. Furthermore, in the followup eletrohemial work it was onfirmed that this H 2 CO 3 does not ontribute muh to the overall orrosion proess and ould be omitted from the analysis. The following hemial reations were onsidered: K CO 2 (g) 1 CO 2 (1) K CO 2 2H 2 O 2 HCO 3 H 3 O (2) HCO K 3 H 2 O 3 CO 2 3 H 3 O (3) MDEAH K H 2 O 4 MDEA H 3 O (4) K 2H 2 O 5 OH H 3 O (5) The reations shown above an be desribed by equilibria reations whih an be solved by using the known values of the equilibrium onstants (K), to obtain the onentrations of speies ( i ). The equilibrium onstants are a funtion of the temperature and are available in the open literature: 14,15 = (6) CO2 K1 pco2 K K K HCO3 = (7) CO2 HCO 3 H3O 2 H2O 2 CO3 H3O = (8) MDEAH H O 2 MDEA H3O = (9) H2O

3 K 5 = (10) OH H3O 2 H 2O where pco 2 is the partial pressure of CO 2. Sine the solution annot have a net harge, an eletroneutrality equation is: = 2 2 (11) MDEAH H3O HCO3 CO3 OH In addition, a mass balane an be written for MDEA and arboni speies in the solution: MDEA MDEAH = onstant (12) = onstant (13) 2 HCO3 3 CO2 CO The onstants in the two mass balane equations above depend on the given onentration of MDEA and CO 2 loading in the aqueous solution, respetively. The onentrations of all speies an be alulated by solving above eight equations (6) (13). Calulation of ativity oeffiients In order to aount for the nonideality of the solution, in the present study, the DeshmukhMather model is used to evaluate ativity oeffiient for the speies in the MDEA/CO 2 /H 2 O solution: 16 2 i AZ I lnγi = 2 βi.ji (14) 1 B I where γ i is the ativity oeffiient for speies i in the solution used to orret the onentration of speies i. The first term on the rhs is based on DebyeHukel theory, whih aounts for the ontribution due to the eletrostati fores among all ions in solution. Z i is the eletrial harge of ion i; B equals to 1.2; I is the ioni strength of the solution and A is taken as a funtion of temperature as proposed by Lewis et al. 17 The seond term on the rhs expresses the ontribution from shortrange interation fores among speies in the solution. β ij are the interation parameters between the different speies i and j in the solution. Verifiation: Comparison with experiments In order to verify the speiation model, CO 2 loading and ph measurements were onduted at different CO 2 partial pressures from 5 bar to 1.0 bar. The work was arried out in a 2L glass ell with 50 wt.% MDEA at 50 o C. The CO 2 loading was measured by the methanoi KOH titration method. The ph eletrode and meter were alibrated at the testing temperature (50 o C) with ph 7 and 10 buffer solutions. CORROSION MODEL FOR CARBON STEEL IN AN MDEA/CO 2 /H 2 O SYSTEM The orrosion model was based on desribing the eletrohemial proess taking plae at the steel surfae exposed to an MDEA/CO 2 /H 2 O environment, as shematially illustrated in Figure 1.

4 CO 2 (g) CO 2 (aq) CO 2 H 2 O H 2 CO 3 H 2 CO 3 H 3 O HCO 3 HCO 3 H 3 O CO 3 2 MDEAH H 2 O MDEA H 3 O 2H 2 O OH H 3 O Chemial reations Eletrohemial reations Aqueous MDEA solution Fe 2 HCO3 H2 CO3 2 H2 MDEA H2 OH H2 H2 H2CO3 HCO3 MDEAH H2O H e Steel Figure 1: Shemati of orrosion proess in MDEACO 2 H 2 O environments Eletrohemial reations at the steel surfae As shown in Figure 1, the eletrohemial reations ourring simultaneously at the steel surfae are dissolution of iron and redution of the various oxidizing agents : Anodi (oxidation) reation o Fe Fe 2 2e (15) Cathodi (redution) reations: o 2H 3 O 2e 2H 2 O H 2 (16) o 2H 2 O 2e 2OH H 2 (17) o 2H 2 CO 3 2e 2HCO 3 H 2 (18) o 2HCO 3 2e 2CO 2 3 H 2 (19) o 2MDEAH 2e 2MDEA H 2 (20) Sine MDEA/CO 2 /H 2 O solution under the absorber ondition is alkaline (lose to ph 9), it an be shown that the ontributions of H 3 O redution and H 2 CO 3 redution reations are quite small due to the very low onentrations in solution, when ompared to other speies. In addition, H 2 O redution kinetis is very slow and thus it was not onsidered in the present orrosion model. Thus, only HCO 3 and MDEAH redution reations (19 and 20) were onsidered as the key athodi reations in this system. The rates of the eletrohemial reations at the steel surfae depend on the eletrial potential of the surfae, the surfae onentrations of speies involved in the reations and temperature. Sine eletrohemial reations involve exhange of eletrons, the reation rate an be onveniently expressed as a rate at whih the eletrons are onsumed or released (i.e., in terms of an eletrial urrent density, i). Fundamental rate equations of eletrohemistry relate i to the potential at the steel surfae (E), via an exponential relationship: 18 o E Erev ± b i = i 10 (21) whih an be written down for eah of the eletrohemial reations involved in the orrosion proess. The positive sign applies for the anodi reation while the negative sign applies for the athodi reations. i o is the exhange urrent density, E rev is the reversible potential and b is the Tafel slope. In most ases, i o and E rev are nonlinear funtions of the surfae onentration of speies involved in a partiular reation, while all three parameters are funtions of temperature.

5 Implementation of the model The model requires as input: ph, HCO 3 onentration, and MDEAH onentration. One the input parameters are determined, the model alulates individual and total athodi and anodi urrents (rates). At the balane of the total athodi and anodi urrents (rates) one an find orrosion potential (E orr ) by solving: ife = i i (22) HCO 3 MDEAH Corrosion urrent density (i orr ) is alulated from the anodi reation urrent (i Fe ) and the known E orr. Finally the orrosion rate is then reovered by using Faraday s law. If the unit A/m 2 is used for the orrosion urrent density, then onveniently the orrosion rate for arbon steel expressed in mm/y takes almost the same numerial value, preisely: CR= i orr. Verifiation: Comparison with experiments The speimens made of arbon steel (ASTM (1) A36) with a hemial omposition of 0.23% C, 0.79% Mn, 2% P, 3% S, 0.29% Cu, 0.20% Si, and balane Fe. The speimens were ground with 600 grit silion arbide (SiC) paper, leaned with isopropyl alohol (C 3 H 8 O) in an ultrasoni bath, and dried prior to exposure. An aqueous solution of MDEA with a onentration of 50% by weight was prepared from a 99% pure MDEA reagent and deionized (DI) water. The test solution was purged with 12% CO 2 gas (pco 2 =0.12 bar: CO 2 loading= 0.13 mol CO 2 /mol amine). Corrosion tests were arried out in a 2L glass ell at 50 C under atmospheri pressure. Further details of the experimental setup an be found elsewhere. 19 Speiation model for an MDEA/CO 2 /H 2 O system RESULTS Figure 2 shows the ativity (whih is defined as the ativity oeffiient (γ i ) times onentration ( i )) for the speies in a 50 wt.% MDEA system at 50 o C at different CO 2 partial pressures. As shown in Figure 2 2, ativities of MDEA and CO 3 dereased with CO 2 partial pressure whereas they inreased for MDEAH, and HCO 3. The alulated ph of 50 wt.% MDEA solution at 50 o C under different CO 2 partial pressures is ompared with the measurements in Figure 3. The omparison of the alulated CO 2 loading as a funtion of partial pressure of CO 2 with our own and open literature 20 data is shown in the same figure. There it an be seen that the speiation model performs reasonably well. ( 1 ) Amerian Soiety for Testing Materials (ASTM) International, 100 Barr Harbor Dr., West Conshohoken, PA

6 Ativity (mol/l) MDEA MDEAH HCO 3 CO CO 2 Partial Pressure (bar) Figure 2: Ativity of speies in 50% MDEA system at 50 o C as a funtion of partial pressure of CO Experiment Model ph CO 2 loading (mol CO 2 /mol amine) Model Experiment Experiment (literature) CO 2 Partial Pressure (bar) CO 2 partial pressure (bar) (a) (b) Figure 3: Comparison between experimental data and alulations; (a) ph and (b) CO 2 loading at different CO 2 partial pressures (50% MDEA system at 50 o C). Corrosion of arbon steel in an MDEA/CO 2 /H 2 O system The eletrohemial parameters for the reations whih were onsidered in the present study are summarized in Table 1 and Table 2. They were found in the open literature, 18,21 and determined from the eletrohemial data obtained in the present study.

7 Table 1 Eletrohemial Parameters for the Exhange Current Density Inluded in the Model i o (A/m 2 i ) o,ref (A/m 2 H,ref HCO3,ref MDEAH ) (molar) (molar) (molar) Fe oxidation (15) HCO 3 redution (19) MDEAH redution (20) i o,fe = i o,fe,ref HCO3 HCO3,ref α H H,ref HCO3 = i o,hco3 o,hco3,ref HCO3,ref i δ β MDEAH = i o, MDEAH o,mdeah,ref MDEAH,ref i κ Table 2 Eletrohemial Parameters for the Reversible Potential and Tafel Slope Inluded in the Model Fe oxidation (15) HCO 3 redution (19) MDEAH redution (20) E rev (V) b (V) 2.3RT E rev = E 2 log o,fe /Fe Fe 2F 2.3RT 2.3RT E rev = ph b = F 0.5F 2.3RT E rev = ph F 2.3RT b = 0.5F Determination of reation order Iron dissolution: In order to determine reation orders with respet to HCO 3 and H for iron dissolution reation, polarization tests were onduted under different test onditions shown in Table 3. Figure 4 shows the anodi polarization urves of arbon steel at different HCO 3 onentrations. It an be learly seen that anodi urrent density inreased and orrosion potential dereased with inreasing HCO 3 onentration. Figure 5 is a plot of the log of the urrent density at onstant potential (0.74 V) versus the log of the onentration of HCO 3 at ph 9.1. The slope is 1.86, indiating that the reation order (α) is lose to 2. Figure 6 shows the measured anodi polarization urves and the alulated Tafel lines at different HCO 3 onentrations. The Tafel lines were produed with the reation order of 2 and the Tafel slope of 0.12 V/de. A reasonable agreement is seen, onsidering that there is very little linearity in the measured urves, whih is likely due to passivation of the steel surfae aused by polarization. Figure 7 shows the anodi polarization urves of arbon steel at different ph values. The anodi urrent density inreased and the orrosion potential dereased with inreasing ph. Figure 8 is a plot of the log of the urrent density at onstant potential (0.7 V) versus the log of the ph. The slope of the plot showed a slope of approximately 0.5, indiating that the reation order (β). Figure 9 shows the measured anodi polarization urves and the alulated Tafel lines at different ph. The Tafel lines were produed with the reation order of 0.5 and the Tafel slope of 0.12 V/de. Again the agreement an be onsidered being reasonable onsidering the nonlinearity of the experimental urves due to passivation. Based on the results presented above, reation orders of HCO 3 and H for iron dissolution reation were determined as:,ref

8 i o,fe = i o,fe,ref HCO HCO H (23),ref H,ref Table 3 Test Conditions for Determining Iron Dissolution Reation Orders Solutions HCO 3 onentration (mol/l) ph Temperature ( o C) α NaHCO 3 /Na 2 CO 3 /H 2 O 0.5, 1.0, β NaHCO 3 /Na 2 CO 3 /H 2 O 1.0 7, 8, mol/l HCO mol/l HCO mol/l HCO 3 logi Slope = Figure 4: Anodi polarization urves for arbon steel at different HCO 3 onentrations log Figure 5: Determination of iron dissolution reation order wrt HCO 3 onentration. ph 9.1, 50 o C mol/l HCO mol/l HCO mol/l HCO 3 Reation order= 2 Slope= 0.12V/de Figure 6: Measured anodi polarization urve and alulated Tafel lines for iron dissolution at different HCO 3 onentrations.

9 ph 7 ph 8 ph 9 logi Slope = log Figure 7: Anodi polarization urves of arbon steel at different ph. Figure 8: Determination of the iron dissolution reation order wrt H onentration ph 7 ph 8 ph mol/l HCO 3, 50 o C Reation order= 0.5 Slope= 0.12 V/de Figure 9: Measured anodi polarization urves and alulated iron Tafel lines for dissolution at different ph. Biarbonate redution: In order to determine the reation order for the HCO 3 redution reation (19), athodi polarization tests were onduted using a range of test onditions as shown in Table 4. Figure 10 shows the athodi polarization urves of arbon steel at different HCO 3 onentrations at ph 9.1. The athodi urrent density slightly inreased and the orrosion potential dereased with inreasing HCO 3 onentration. Figure 11 is a plot of the log of the urrent density at onstant potential (0.95 V) versus the log of the onentration of HCO 3. The slope of the line in the plot is 0.39, indiating that the reation order (δ) is lose to 0.5. Figure 12 shows the measured athodi polarization urves and the alulated Tafel lines at different HCO 3 onentrations. The Tafel lines were produed with the reation order of 0.5 and the Tafel slope of V/de. A reasonable agreement between experimental polarization urves and alulated Tafel lines is seen, although one an argue that this effet is within the margins of experimental error. Based on these result, the reation order for HCO 3 redution reation was determined as:

10 0.5 HCO3 i = i (24) o,hco3 o,hco3,ref HCO3,ref Table 4 Test Conditions for Determining the HCO 3 Redution Reation Order Solutions HCO 3 onentration (mol/l) ph Temperature ( o C) δ NaHCO 3 /Na 2 CO 3 /H 2 O 0.5, 1.0, M HCO 3 1.0M HCO 3 1.5M HCO 3 logi Slope = log Figure 10: Cathodi polarization urves for arbon steel at different HCO 3 onentrations. Figure 11: Determination of the reation order for HCO 3 redution M HCO 3 1.0M HCO 3 1.5M HCO 3 Reation order= 0.5 Slope= V/de Figure 12: Measured athodi polarization urve and alulated Tafel lines for HCO 3 redution at different HCO 3 onentrations.

11 Protonated MDEA redution: In order to determine the reation order for the MDEAH redution reation (20), athodi polarization tests were onduted at different test onditions shown in Table 5. Figure 13 shows the athodi polarization urves for arbon steel at different MDEAH onentrations at ph 9.1. The athodi urrent density inreased with inreasing MDEAH onentration. Figure 14 is a plot of the log of the urrent density at onstant potential (0.8 V) versus the log of the onentration of MDEA. The slope of the line in the plot is approximately 1.3, indiating that the reation order (κ) is lose to 1. Figure 15 shows the omparison of the measured athodi polarization urves and the alulated Tafel lines at different MDEAH onentrations. The Tafel lines were produed with the reation order of 1 and the Tafel slope of V/de. A good agreement between experimental polarization urve and alulated Tafel line at different MDEAH onentrations is seen. Based on these result, a reation order for MDEAH redution reation was determined as: 1 MDEAH i = i (25) o, MDEAH o,mdeah,ref MDEAH,ref Table 5 Test Conditions for Determining the Reation Order for MDEAH Redution Solutions MDEAH onentration (mol/l) ph Temperature ( o C) κ MDEA/HCl/H 2 O 0.33, 0.63, M MDEAH 0.66M MDEAH 1.0M MDEAH logi Slope = Figure 13: Cathodi polarization urves for arbon steel at different MDEAH onentrations log Figure 14: Determination of the reation order for MDEAH redution.

12 M MDEAH 0.66M MDEAH 1.0M MDEAH Reation order= 1 Slope= V/de Figure 15: Measured athodi polarization urves and alulated Tafel lines for MDEAH redution at different MDEAH onentrations Validation of the overall orrosion model Performane of the overall orrosion model was validated by omparing the preditions with results from experiments. Figure 16 ompares orrosion rates between experiment and predition at different HCO 3 onentrations and ph. The predited orrosion rates showed good agreement with experimental data with an error not larger than 2030%, whih an be onsidered to be within the experiential error range for the urrent LPR measurements Model LPR Model LPR Corrosion rate (mm/y) Corrosion rate (mm/y) HCO 3 onentration (mol/l) (a) ph (b) Figure 16: Comparison of orrosion rates between experiments and preditions at different onditions: (a) at different HCO 3 onentrations (b) at different ph Figure 17 shows the omparison of experimental and alulated polarization urves 19 and orrosion rates for a 50 wt.% MDEA / 12% CO 2 ondition. Although predited polarization urves indiate a higher orrosion potential than seen in the experiments, given the omplexity of the system one an aept this result, partiularly in the light of the reasonable agreement of the orrosion urrent / rate, as indiated in the same figure. Many similar omparisons were made for other onditions overed in this study with similar results. This indiates that the urrent orrosion model is appliable to uniform

13 orrosion of arbon steel in the absorber onditions, as long as there is no major deviation from the onditions studied here: MDEA onentration (50 wt.%) and temperature (50 o C). Further work is ongoing in order to extend the validity of the model to over a broader range of onditions suh as those seen in the regenerator Experimental urve Anodi Tafel Cathodi Tafel Model urve (a) Corrosion rate (mm/y) Experiment (b) Model Figure 17: Comparison between experimental data and preditions in a 50 wt.% MDEA/12% CO 2 system at 50 o C: (a) polarization urves, (b) orrosion rates. CONCLUSIONS A preditive model was developed for orrosion of arbon steel in CO 2 loaded aqueous methyldiethanolamine (MDEA) systems based on modeling of solution speiation and eletrohemial reations. The following onlusions are drawn: Ativities of MDEA and CO 3 2 dereased with CO 2 partial pressure whereas they inreased with CO 2 partial pressure for MDEAH, and HCO 3.

14 The speiation model showed a good agreement with experimental data for ph and CO 2 loading. The required eletrohemial parameters (e.g., exhange urrent densities, Tafel slopes and reation orders) for the Fe dissolution, HCO 3 redution and MDEAH redution reations were determined by experiments, and used to suessfully build a orrosion model for orrosion of arbon steel in MDEA/CO 2 /H 2 O environments. The orrosion model showed a good agreement with experimental data for various environmental onditions inluding pure CO 2 and MDEA/CO 2 solutions. ACKNOWLEDGEMENTS The authors would like to aknowledge the finanial support from Alstom Power In. to the Institute for Corrosion and Multiphase Tehnology at Ohio University on this projet. REFERENCES 1. E. Fleury, J. Kittel, B. Vuillemin, R. Oltra, F. Ropital, Corrosion in Amine Units for Aid Gas Treatment: A Laboratory Study, EUROCORR 2009, SS 12O7888 (Dehema e.v. 2009). 2. W. Tanthapanihakoon, A. Veawab, B. MGarvey, Eletrohemial Investigation on the Effet of Heatstable Salts on Corrosion in CO 2 Capture Plants Using Aqueous Solution of MEA, Ind. Eng. Chem. Res. 45 (2006): p W. Liu, D. King, J. Liu, B. Johmson, Y. Wang, Z. Yang, Critial Material and Proess Issues for CO 2 Separation from CoalPowered Plants, JOM 61 (2009): p M.R. Khorrami, K. Raeissi, H. Shahban, M.A. Torkan, A. Saathi, Corrosion Behavior of Carbon Steel in Carbon DioxideLoaded Ativated Methyl Diethanol Amine Solution, Corrosion 64 (2008): p M.S. DuPart, T.R. Baon, D.J. Edwards, Understanding Corrosion in Alkanolamine Gas Treating Plants: Part 2, Hydroarb. Proess. (1993): p Y. Tomoe, M. Shimizu, H. Kaneta, Ative Dissolution and Natural Passivation of Carbon Steel in Carbon DioxideLoaded Alkanolamine Solutions, CORROSION/96, paper no. 395 (Houston, TX: NACE International, 1996). 7. X.P. Guo, Y. Tomoe, The Effet of Corrosion Produt Layers on the Anodi and Cathodi Reations of Carbon Steel in CO 2 Saturated MDEA Solutions at 100 o C, Corros. Si. 41 (1999): p A. Veawab, P. Tontiwahwuthikul, A. Chakma, Corrosion Behavior of Carbon Steel in the CO 2 Absorption Proess Using Aqueous Amine Solutions, Ind. Eng. Chem. Res. 38 (1999): p M. Nainar, A. Veawab, Corrosion in CO 2 Capture Proess Using Blended Monoethanolamine and Piperazine, Ind. Eng. Chem. Res. 48 (2009): p A. Veawab, P. Tontiwahwuthikul, A. Chakma, Investigation of LowToxi Organi Corrosion Inhibitors for CO 2 Separation Proess Using Aqueous MEA Solvent, Ind. Eng. Chem. Res. 40 (2001): p W. Tanthapanihakoon, A. Veawab, Polarization Behavior and Performane of Inorgani Corrosion Inhibitors in Monoethanolamine Solution Containing Carbon Dioxide and HeatStable Salts, Corrosion 61 (2005): p. 371.

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