Effects of O2 and SO2 on Water Chemistry Characteristics and

Subscriber access provided by READING UNIV

Article

Effects of O2 and SO2 on water chemistry characteristics and corrosion behavior of X70 pipeline steel in supercritical CO2 transport system Jianbo Sun, Chong Sun, and Yong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04870 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Effects of O2 and SO2 on water chemistry characteristics and corrosion behavior of X70 pipeline steel in supercritical CO2 transport system Jianbo Sun †, §, *, Chong Sun †, ‡, §, Yong Wang † †

School of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, PR China ‡

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada

*

Corresponding author: Jianbo Sun Tel.: +86 532 86983503-8625. Fax: +86 532 86983300. E-mail: [email protected].

§

J.B. Sun and C. Sun contributed equally.

Abstract: The water chemistry characteristics of supercritical CO2 streams and the corrosion behavior of X70 steel in supercritical CO2 system containing O2 and/or SO2 impurities were investigated by thermodynamic simulation and corrosion test. The results show that O2 concentration in supercritical CO2 streams contributes to the negligible influence on the water chemistry characteristics, corresponding to a slight change in the corrosion rate, whereas the rising SO2 concentration noticeably deteriorates the water chemistry characteristics, in accordance with a remarkable increase in the corrosion rate. The coexistence of O2 and SO2 synergistically accelerates the corrosion of X70 steel due to the fact that the formation of H2SO4 makes the condensed water highly acidified. The corrosion products were further characterized by surface analysis techniques. It is found that O2 and/or SO2 remarkably affect the corrosion film characteristics by changing the chemistry characteristics of condensed water. The corrosion model corresponding to this phenomenon was proposed. Keywords: supercritical CO2; impurity; water chemistry; synergistic effect; X70 steel 1. Introduction CO2 emission originated from the combustion of fossil fuels is recognized as a leading source of greenhouse gases, which adversely brings the global climate change and environmental issue.1 Therefore, reasonably limiting CO2 emission is highly desirable to 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tackle the global climate change. To this end, carbon capture and storage (CCS) technology holds the promise to alleviate CO2 emission without limiting the further utilization of fossil fuels, where CO2 transport, as the intermediate link that transports captured CO2 from large point sources to the storage locations, is a crucial segment of the CCS process.2, 3 However, carbon steel pipelines, utilized for supercritical CO2 (SC CO2) transmission, potentially experience the internal corrosion once forming free water phase within the pipelines, because the water saturated and acidified by SC CO2 is more aggressive to pipeline than that by the gaseous or liquid CO2.4, 5 Nevertheless, the pipeline corrosion caused by liquid or SC CO2 is not prominent, even under the harsh condition of water-saturated SC CO2.6-11 It is noteworthy that the corrosion situation of pipeline is hardly optimistic when a range of impurity gases (e.g., O2, SO2, H2S, and NO2), mineral acids (e.g., HCl, HNO3, and H2SO4) or salts (Na2SO4, NaCl, and NaNO3) are present in SC CO2, and these impurities have been reported to generally exacerbate the corrosion of pipeline, even under the condition of water-unsaturated SC CO2.12-21 Therefore, the corrosion issues of pipeline in SC CO2 streams containing impurities have been a hot research topic in the past several years. It is well known that the increase of SC CO2 solubility in aqueous phase enhances the H2CO3 concentration and reduces the pH value of aqueous phase, corresponding to an increase in the corrosion rate.22,

23

Similar to SC CO2, SO2 contributes to the enhanced

corrosion effect on the steel by reacting with H2O to form H2SO3 that further lowers the pH value of aqueous phase,24, 25 especially this corrosion effect is intensified with the increase of SO2 concentration in SC CO2 streams.16, 17, 25, 26 Likewise, as a commonly existing impurity in captured CO2 streams, O2 also affects the pipeline corrosion. Related research reports that low concentration of O2 has no obvious effect on the corrosion rate of carbon steel in water-unsaturated SC CO2 system,27 but mildly increases the corrosion rate in water-saturated SC CO2 system.28, 29 Additionally, Choi et al.24 reports the O2-enhanced corrosion effect in water-saturated SC CO2-O2 system, because the corrosion rate of X65 steel increases from 0.38 mm/y to 1.05 mm/y with the rising O2 concentration from 0 to 4%. Similar results are also reported by Tang et al.30. The reason credited for this effect of O2 is that O2 can provide an additional cathodic reaction for corrosion process and inhibit the formation of protective FeCO3 film by forming less protective Fe2O3 products. However, Hua et al.31 shows the 2


Page 2 of 36

Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O2-inhibited corrosion effect in water-saturated SC CO2-O2 system; the results reveal that increasing O2 concentration from 0 to 1000 ppm leads to the decrease of the general corrosion rate of X65 steel from 0.1 mm/y to 0.01 mm/y, because the formation of thin amorphous Fe2O3 layer provides improved protection for the steel. It is, thus, concluded that controversies of O2 effect are left behind on the corrosion of pipeline. Apparently, the formation of aqueous phase within the pipeline is the prerequisite for the occurrence of pipeline corrosion, while the changes in chemistry characteristics of aqueous phase due to the dissolution of SC CO2 or impurity gases plays a critical role in inducing the severe corrosion of the pipeline. However, due to the trace existence of water in SC CO2 streams,32, 33 difficult in directly measuring the chemical species in the aqueous phase on the steel surface makes it challenge to establish the direct correlation of the changed corrosion rate and the variation of water chemistry characteristics caused by O2 or SO2. Thus, most previous studies for evaluating the influences of O2 and SO2 on the corrosion rate and corrosion mechanism mainly focus on weight loss test combining with surface analysis of corrosion products. To have an in-depth understanding of the corrosion mechanism of CO2 transport pipeline it is highly desirable to determine the chemistry distribution of impurities and understand the corrosivity of the aqueous phase in SC CO2 streams. As reported by Ayello et al.27 and Cole et al.34, attention should be paid on the OLI Analyzer software since it supports the numerical computation to simulate the electrolyte thermodynamics and allows the coexistence of a second liquid phase and a gas phase. This, therefore, makes the water chemistry study of CO2 streams to be possibly executed regarding to CCS. In this study, our goal is to understand the effects of O2 and SO2 on the chemistry characteristics of aqueous phase and the corrosion behavior of X70 steel in water-saturated SC CO2-impurity system. To this end, the water chemistry characteristics were analyzed by thermodynamic software, the corrosion rate was measured using weight loss test, and the corrosion products were characterized by Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD). Accordingly, the correlation between the water chemistry and the steel corrosion as well as the corrosion mechanism of X70 steel was discussed.

3



2. Experimental methods 2.1. Modeling water chemistry The OLI Analyzer software was adopted to model the water chemistry (e.g., chemical species, ionic concentrations, and pH) of CO2 streams with various impurities. The parameters and conditions used in thermodynamic simulation are listed in Table 1. 1000 g CO2 and 10 g H2O were used as the basic components of the CO2 streams. The influences of temperature and pressure on the water chemistry characteristics of CO2-H2O system were firstly investigated, and then the impurities with different concentrations were further introduced into the streams to determine their influences on the water chemistry characteristics of SC CO2 streams. 2.2. Material and pretreatment X70 pipeline steel, with a composition (mass %) of 0.05 C, 0.186 Si, 1.90 Mn, 0.305 Cr, 0.093 Mo, 0.018 Ni, 0.014 Cu, 0.045 Al, 0.018 P, 0.001 S, 0.042 V and Fe balance, was used in this study. The test specimen was machined into a dimension of 40 mm × 15 mm × 3 mm, and its surface was ground sequentially up to 1000 grit silicon carbide (SiC) paper, and subsequently washed with deionized water and acetone. Lastly, the specimens were placed in a vacuum desiccator for 24 h to remove water. Prior to the corrosion tests, the specimen was weighed using an analytical balance with an accuracy of 0.1 mg. Four parallel specimens were used for each test. 2.3. Corrosion test and surface characterizations Corrosion tests were performed in a 3 L hastelloy autoclave under the static condition to investigate the corrosion behavior of X70 steel. Figure 1 depicts the schematic diagram of the autoclave. As seen, the specimens were placed on the specimen holder and exposed to water-saturated SC CO2 streams. The water used in the study was deionized water which was preferentially purged by N2 for 12 h to remove the dissolved oxygen. Table 2 lists the test conditions. After the corrosion tests, three of the four corroded specimens were descaled in a pickling solution (100 mL hydrochloric acid, 3.5 g hexamethylenetetramine and 900 mL deionized water) at room temperature, and then weighed using the analytical balance. Finally, 4


Page 4 of 36

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the corrosion rate was calculated according to the weight loss of the specimen. Furthermore, the rest corroded specimen was employed to investigate the surface morphology and elemental compositions of the corrosion products using a SEM equipped with EDS. The phase compositions of the corrosion products were identified by means of XRD. After that, the film-covered specimen was cut along its cross-section. It was then sealed using epoxy and ground sequentially up to 2000 grit SiC paper. The cross-sectional morphologies and the elemental distributions in cross-section of the corrosion products were correspondingly investigated using SEM and EDS. 3. Results 3.1. Water chemistry characteristics of CO2-H2O system Figure 2 shows the pH value and ionic strength of aqueous phase in CO2-H2O system. The ionic strength, which can indirectly reflect the total ionic concentrations,35 increases with the rising pressure and decreases with the rising temperature, whereas the change of pH is opposite to that of ionic strength. Figure 3 shows the solubility of CO2 in H2O calculated by a well-accepted thermodynamic model for mutual solubilities of CO2 and H2O as depicted by Choi and Nešić.22 As seen, the variations of CO2 solubility with the pressure or temperature agree well with those of ionic strength. The decrease of temperature and the increase of pressure enhance the solubility of CO2 in H2O. Since the presence of H2O in CO2 streams can cause the formation of condensed aqueous phase, CO2 can dissolve in H2O and then partially dissociate into HCO3-, CO32- and H+. Therefore, the increase of CO2 solubility promotes the dissociation of H2CO3, and consequently increases the ionic concentrations (ionic strength) and decreases the pH value of aqueous phase. Furthermore, the change of CO2 solubility with pressure shows two distinct trends in two different phases: gaseous CO2 below the critical pressure (7.38 MPa) and SC CO2 above this pressure, as exhibited in Figure 3. Likewise, the critical pressure also points to significant changes in ionic strength and pH value (Figure 2). This is because when the pressure is over 7.38 MPa at the test temperatures, CO2 is transformed from gaseous state into supercritical state which possesses special physical and chemical properties of both a gas and a liquid CO2. As a result, the solubility of CO2 in H2O at supercritical pressure is much larger than that at 5



low pressure, corresponding to the considerable deterioration of the water chemistry characteristics. For example, the pH value of aqueous phase at 1 MPa and 50 ºC is 3.5 while that at 10 MPa and 50 ºC is 3.1 (decreased by about 12 %). As reported by Zhang et al.,36 although the corrosion under supercritical conditions is identical with the typical CO2 corrosion pattern at low pressure and the fundamental corrosion mechanism is not changed with the pressure, the corrosion rate of X65 steel after corrosion in distilled water for 168 h at 1 MPa CO2 and 80 ºC is 1.64 mm/y, whereas that at 9.5 MPa CO2 and 80 ºC increases to 7.26 mm/y. The sharp change in corrosion rate mainly derives from the variation of the water chemistry characteristics caused by the transformation of CO2 phase state. It is evidently that the CO2 streams transported under supercritical pressure conditions is more aggressive to carbon steel pipeline than that transported at low pressures. 3.2. Effects of O2 and SO2 on water chemistry characteristics 3.2.1. Distributions of various chemical species in aqueous and SC CO2 phases Figure 4 shows the distributions of 10 g H2O, 1000 g CO2 and 0-2000 ppm O2 or SO2 in aqueous phase and SC CO2 phase at 10 MPa and 50 ºC, respectively. As seen in Figure 4a, the amounts of H2O and CO2 distributed in aqueous phase increases with the rising O2 concentration in SC CO2-H2O-O2 system, while those in SC CO2 phase decreased accordingly. However, with the increasing SO2 concentration in SC CO2-H2O-SO2 system (Figure 4b), the variations of H2O and CO2 distributed in both aqueous and SC CO2 phases reverses those in SC CO2-H2O-O2 system. Apparently, the addition of O2 leads to the decline of H2O content in SC CO2 phase, whereas the presence of SO2 results in the increase of H2O content in SC CO2 phase. As exhibited in Figure 4c, the amounts of both O2 and SO2 distributed in different phases increase with their concentrations in SC CO2 streams. When 2000 ppm O2 or 2000 ppm SO2 is added into SC CO2 streams, about 0.08 ppm O2 or 13.15 ppm SO2 is in aqueous phase while most of O2 or SO2 distributes in SC CO2 phase. Nevertheless, the amount of SO2 in aqueous phase is much higher than that of O2 under same conditions. As well known, the formation of aqueous phase is the prerequisite for the occurrence of steel corrosion, and the impurities distributed in aqueous phase play critical roles in the corrosion process. Therefore, based on the amount of aqueous phase formed in 6


Page 6 of 36

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SC CO2-H2O-impurity system, the effects of O2 and SO2 distributed in aqueous phase on the water chemistry characteristics were further investigated. 3.2.2. Water chemistry characteristics In SC CO2-H2O system, the main chemical species in aqueous phase include CO2(aq), H+, HCO3-, CO32- and OH-. When O2 is added into the system, besides the above species, O2(aq) will be present in aqueous phase. Whereas the presence of SO2 makes the chemical species more complicated. SO2 reacts with H2O to form H2SO3 which further dissociates into HSO3-, SO32- and H+.25, 37 Figures 5-8 show the variations of the above-mentioned chemical species in aqueous phase with O2 or SO2 concentrations at 10 MPa and 50 ºC. As seen in Figure 5, the equilibrium concentration of O2 in aqueous phase increases with the rising O2 concentration in SC CO2 streams. When 2000 ppm O2 is introduced into SC CO2 streams, the equilibrium concentration of O2(aq) is only about 11 ppm. Nevertheless, the presence of O2(aq) inhibits the dissolution of SC CO2 in aqueous phase to some extent and reduces CO2(aq) equilibrium concentration in aqueous phase. Likewise, the concentrations of HCO3- and H+ in aqueous phase decrease with the increase of O2 concentration, as shown in Figures 6 and 7, which consequently leads to the slight increase of pH value. As exhibited in Figure 6, the pH value of aqueous phase in SC CO2-H2O system with 2000 ppm O2 is only 0.02% higher than that in the system without O2. It can be determined that the change of O2 concentration in the test ranges has negligible influence on the water chemistry characteristics. When SO2 is present in SC CO2-H2O system, similar to the system containing O2, the increase of SO2(aq) equilibrium concentration and the decrease of CO2 solubility in aqueous phase with the rising SO2 concentration in the system are observed in Figure 5. Particularly, SO2 has a higher solubility in aqueous phase than O2. After adding 2000 ppm SO2 in SC CO2-H2O system, the equilibrium concentration of SO2(aq) is about 1089 ppm which is 99 times higher than that of O2(aq). The formation of H2SO3 significantly enhances the H+ concentration in aqueous phase. As exhibited in Figure 6, H+ concentration rises with the increase of SO2 concentration in SC CO2 streams, which consequently reduces the pH value of aqueous phase. When 10 ppm SO2 exists in SC CO2-H2O system, the pH value is reduced to 2.95 and decreased by about 4.9% than that in the system without SO2. While SO2 7



concentration increases to 2000 ppm, the pH achieves a value of 1.94 which is 37.3 % lower than that in the system without SO2. Apparently, low concentrations of SO2 as well as the increase of its concentration in SC CO2-H2O system notably deteriorate the water chemistry characteristics and enhance the aggressiveness of the aqueous phase. A further inspection on the differences of HCO3-, HSO3- and SO32- concentrations in aqueous phase before and after the addition of SO2 in Figures 7 and 8, it is found that HSO3and SO32- concentrations derived from the dissociation of H2SO3 rise with the increase of SO2 concentration in SC CO2 streams, meanwhile the HCO3- concentration originated from the dissociation of H2CO3 significantly decreases. The addition of 2000 ppm SO2 reduces the HCO3- concentration by one order of magnitude. Unlike the O2-containing system, the small change of CO2 solubility causes the slight variation of HCO3- concentration. The reason credited for the considerable decline of HCO3- mainly derives from the increase of H+ due to the dissociation of H2SO3, replying that the dissociation processes of H2CO3 are remarkably inhibited in SO2-containing system. The results of water chemistry analysis make it clear that although most of O2 and SO2 in SC CO2 streams distribute in SC CO2 phase, small amounts of O2(aq) and SO2(aq) in aqueous phase can change the chemical characteristics of aqueous phase to varying degrees, which presumably have different influences on the corrosion of SC CO2 transport pipeline. Therefore, the effects of O2 and SO2 on the corrosion behavior of X70 steel were further investigated to establish the correlation between the water chemistry and the steel corrosion. 3.3. Effects of O2 and SO2 on corrosion rate and film characteristics of X70 steel 3.3.1. Corrosion rate The corrosion rate of X70 steel exposed to water-saturated SC CO2 system with different impurities for 120 h at 10 MPa and 50 ºC, as shown in Figure 9. The corrosion rate is 0.014 mm/y in SC CO2-H2O system and the addition of O2 and/or SO2 in the system enhances the corrosion rate of X70 steel. Under same concentration conditions, 1000 ppm O2 increases the corrosion rate from 0.014 mm/y to 0.027 mm/y, whereas 1000 ppm SO2 makes the corrosion rate increase to 0.423 mm/y. When 1000 ppm O2 and 1000 ppm SO2 are simultaneously present in the system, the corrosion rate of X70 steel achieves a value of 0.842 mm/y which 8


Page 8 of 36

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is even higher than the sum of that in the individual O2- or SO2-containing system, indicative of the presence of a synergistic effect between SO2 and O2. A further investigation on the corrosion rate of X70 steel in the system with addition of different O2 concentrations indicates that considerably increasing O2 concentration from 1000 ppm to 10000 ppm only causes a mild increase in the corrosion rate from 0.027 mm/y to 0.029 mm/y (Figure 9), suggesting that the rising O2 concentration makes no significant difference on the corrosion rate, in accordance with its influence on the water chemistry characteristics (Figures 5-7). However, the corrosion rate of X70 steel exhibits a high value of 0.269 mm/y only with the presence of 200 ppm SO2 in SC CO2-H2O system. Moreover, the corrosion rate confirms to a linear relationship with the increase of SO2 concentration (Figure 9), which varies inversely to the pH value of aqueous phase calculated by thermodynamic software in Figure 6. This confirms that the impurities in SC CO2 streams can affect the corrosion through changing the chemistry characteristics of aqueous phase on the steel surface. 3.3.2. Corrosion film characteristics Figure 10 shows the macroscopic morphologies and SEM surface morphologies of the steels after corrosion for 120 h in water-saturated SC CO2 system with 1000 ppm O2 and/or 1000 ppm SO2 at 10 MPa and 50 ºC. A significant change in the color for the surface corrosion products between O2-containing and O2-free systems is found: black corrosion products in SC CO2 system (Figure 10a) and SC CO2-SO2 system (Figure 10c) while rust-colored corrosion products in SC CO2-O2 system (Figure 10b) and SC CO2-O2-SO2 system (Figure 10d). It means that O2 appears to play an important role in changing the corrosion film characteristics. As exhibited in Figure 10a, some black regions with small crystalline grains stacked, similar to “water stains”, are found on the local surface of X70 steel in SC CO2-H2O system, suggesting that the condensed water randomly accumulates on some regions of the steel. Moreover, the cross-sectional morphology (Figure 11a) shows that the corrosion film on the black region is quite thin, only about 2-3 µm thickness. XRD analysis reveals that the crystalline film consists of FeCO3 (Figure 12a). After being corroded in the system with 1000 9



ppm O2, the phenomenon of “water stains” also appears on the local steel surface (Figure 10b). SEM surface and cross-sectional morphologies show that the deposition of the corrosion products is not uniform on the steel surface, which can be divided into two typical regions (Figure 10b and Figure 11b): a thicker film in region 1 and a thinner film in region 2, which point to the inhomogeneous precipitation of condensed water on the steel surface. As seen in Figure 11b, the steel substrate in region 1 is subject to more severe corrosion than that in region 2, indicating that more condensed water accumulates on region 1. Based on the Fe/O atomic ratio by EDS and the phase compositions by XRD (Figure 12b), it can be determined that the crystalline products in region 1 are FeCO3 (16 at% Fe, 49.98 at% O and 34.02 at% C), whereas the amorphous worm-like products in region 2 mainly consist of Fe2O3 (28.10 at% Fe, 44.97 at% O and 26. 93 at% C). However, the phenomenon of local “water stains” disappears in SC CO2-H2O-SO2 system (Figure 10c), the corrosion film with cracks forms on overall steel surface, indicating that an relatively homogeneous water film may be formed on the steel surface. The cross-sectional morphology (Figure 11c) and EDS line scanning analysis (Figure 11e) reveal that the corrosion film mainly contains Fe, O, and S elements and presents a single-layer structure. The main components of corrosion products are FeSO3·xH2O or FeSO3 and a small amount of FeCO3, as shown in Figure 12c. When 1000 ppm O2 is further introduced into the SC CO2-H2O-SO2 system, the corrosion film has a double-layer structure (Figures 11d and f): the outer layer, showing the worm-like and opening-like morphology (Figure 10d), mainly contains Fe and O elements, whereas the inner layer contains Fe, S and O elements. XRD analysis in Fig. 12d implies that the phase compositions of the corrosion products are FeSO4·4H2O, FeOOH and small amounts of FeSO3·xH2O and FeCO3. It is, thus concluded that the corrosion products in the inner layer are mainly FeSO4, FeSO3 and FeCO3 while the rust-colored products in the outer layer are primarily FeOOH. 4. Discussion 4.1. SC CO2 corrosion of X70 steel in impurity-free system In this study, the water chemistry analysis shows that the condensed aqueous phase under supercritical conditions demonstrates strong causticity because it can be saturated and 10


Page 10 of 36

Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


acidified by SC CO2. Nevertheless, X70 steel exposed to water-saturated SC CO2 at 10 MPa and 50 ºC for 120 h is corroded slightly (0.014 mm/y). Related research on the corrosion of pipeline steel in SC CO2-saturated water under similar pressure and temperature conditions is indicative of considerably high corrosion rate,11,

36, 38

such as Zhang et al.36 reports a

corrosion rate of about 5 mm/y for X65 steel exposed to SC CO2-saturated water at 9.5 MPa and 50 ºC for 168 h. On a basis of the morphology observation in Figure 10a, the corrosion difference between water-saturated SC CO2 and SC CO2-saturated water is apparently related to the amount of the available water provided for the corrosion reaction of the steel as well as the contact area of the steel surface and the water. As reported previously, the steel surface exposed to CO2 streams acts as condensation nucleus, and the water in CO2 streams can condense on the steel surface in the form of small fog droplets or thin film.20,

29, 39

In

water-saturated SC CO2 system, only a small amount of condensed water accumulates on the local surface of steel, which causes the corrosion. Concurrently, FeCO3 can deposit on the regions covered with the condensed water. With the proceeding of the corrosion and the depletion of the electrolyte, the condensed water is filled with the compact FeCO3 crystals, preventing the steel from further corrosion and leaving a morphology feature similar to “water stains” on the steel surface (Figure 10a). Thus, X70 steel shows a quite low corrosion rate in water-saturated SC CO2 system. 4.2. The roles of O2 and SO2 in corrosion process of X70 steel When impurity gases exist in CO2 streams, the interaction of impurity with H2O can further promote the precipitation of an impurity-containing aqueous phase from the CO2 streams.17,

20, 29, 39, 40

The differences in the corrosion rate as well as the deposition of

corrosion products on the steel surface (e.g., uniformity and thickness) in Figures 9-11 suggest that more aqueous phase accumulates on the steel surface in SC CO2-H2O-SO2 system which contributes to the formation of a relatively homogeneous aqueous film, whereas the condensation of aqueous phase is inhomogeneous on the steel surface in SC CO2-H2O-O2 system due to the lower amount of condensed aqueous phase. The reason credited for this phenomenon is presumably attributed to the differences in the influences of O2 and SO2 on the water distribution and the aqueous phase precipitation. As seen in Figures 11



4a and b, the amount of H2O in SC CO2 phase in SO2-containing system is higher than that in O2-containing system under same conditions, This, thus, provides more H2O for the condensation of aqueous phase on the steel surface which is exposed to SC CO2 phase (Figure 1). Moreover, SO2 more readily interacts with H2O in SC CO2 phase than O2 due to its higher solubility in H2O (Figure 5), which favors the precipitation of H2SO3-containing aqueous phase from CO2 streams. In order to further interpret the effects of the above phenomenon on the corrosion and film characteristics of X70 steel, a corrosion model is proposed according to the results of SEM, EDS and XRD analyses, as depicted in Figure 13. In water-saturated SC CO2-O2 system, O2 has negligible influence on the pH value of condensed water (Figure 6), but as a strong depolarizing agent, O2 can promote the corrosion of the steel by providing an additional cathodic reaction [Eq. (1)] for the corrosion process in acid environment.31 However, the amount of O2 distributed in aqueous phase is much lower than that of O2 in SC CO2 streams according to the distributions of O2 in different phases (Figure 4c), for example, only 0.04 ppm of added 1000 ppm O2 distributes in aqueous phase, whereas the amount of H2O actually condensed on steel surface should be lower than that of aqueous phase formed in SC CO2 streams under test conditions. Therefore, the amount of O2 actually involved in corrosion reaction is very limited, namely, although O2 can enhance the corrosion rate of X70 steel, the effect of O2 concentration on the corrosion rate is not prominent due to the limitation of low concentration in H2O. As exhibited in Figure 13a, since the precipitation of condensed water on the steel surface is inhomogeneous, slight corrosion occurs in the regions covered with a small amount of condensed water (e.g., Region 2). Meanwhile, the formed Fe2+ is easily oxidized to Fe3+ due to the presence of O2, which results in the decline of Fe2+ concentration in condensed water. Given that the solubility product of Fe(OH)3 (4.0×10-38) is far less than that of FeCO3 (3.13×10-11),41 the precipitation of Fe(OH)3 is more favorable than that of FeCO3. As a result, the products of Fe2O3 associated with oxygen corrosion are formed on region 2 via the reactions of Eqs. (2) and (3),41, 42 while the formation of FeCO3 is inhibited. However, the anodic dissolution of steel is exacerbated in the regions covered with a large amount of condensed water (e.g., Region 1), which leads to the formation of more Fe2+. Despite the fact that some Fe2+ can be oxidized by O2, the product of the concentrations of residual Fe2+ and CO32- is sufficient to exceed the 12


Page 12 of 36

Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solubility product of FeCO3 because of the quite low concentration of O2 in condensed water. Therefore, FeCO3 can be formed on region 1. O2 + 4H+ + 4e- → 2H2O

(1)

4Fe2+ + O2 + 10H2O → 4Fe(OH)3 + 8H+

(2)

2Fe(OH)3 → Fe2O3 + 3H2O

(3)

In water-saturated SC CO2-SO2 system, the presence of SO2 induces the formation of homogeneous H2SO3-containing aqueous film on the steel surface, as shown in Figure 13b. Unlike the effect of O2 on the corrosion rate, the presence of SO2 in water-saturated SC CO2 system remarkably increases the corrosion rate through deteriorating the chemistry characteristics of condensed water. H2SO3 derived from the dissolution of SO2 in H2O not only significantly lowers the pH value of condensed water, but also greatly inhibits the dissociation of H2CO3. Furthermore, XRD analysis indicates the corrosion products formed in SO2-containing system are mainly FeSO3 with a small amount of FeCO3. Apparently, the result of water chemistry analysis agrees well with that of XRD analysis, confirming that the corrosion process of the steel is mainly controlled by SO2, whereas the corrosion effect of CO2 is greatly reduced. Accordingly, a single-layer FeSO3 film with a small amount of FeCO3 can be formed on the steel surface, as exhibited in Figure 13b. 4.3. Synergistic effect between O2 and SO2 When O2 and SO2 simultaneously exist in water-saturated SC CO2 system, an outer FeOOH film and an inner FeSO4 film with small amounts of FeSO3 and FeCO3 are formed on the steel, as shown in Figure 13c. The presence of FeSO4 in the corrosion products confirms that H2SO4 is able to form in the system containing O2 and SO2. Two possible mechanisms are expected to result in the formation of H2SO4: SO2 dissolves in condensed water to form H2SO3 which further reacts with the dissolved O2 to produce H2SO4 [Eq. (4)];16, 26, 43 Or SO2 first reacts with O2 in SC CO2 phase to form SO3 [Eq. (5)], and subsequently SO3 dissolves in condensed water to produce H2SO4 [Eq. (6)].20, 40 However, considering that O2 and SO2 have quite low concentrations in aqueous phase and most of O2 and SO2 distribute in SC CO2 phase (Figures 4c), the second mechanism [Eqs. (5) and (6)] probably dominates the

13



Page 14 of 36

formation of H2SO4. 2H2SO3(aq) + O2(aq) → 2H2SO4

(4)

2SO2 + O2 → 2SO3

(5)

SO3(aq) + H2O → H2SO4

(6)

Figure 14 shows the variations of H+ concentration and pH value of aqueous phase with H2SO4 concentration in SC CO2-H2SO4 system at 10 MPa and 50 ºC. It can be seen that the H2SO4 remarkably increases H+ concentration and reduces the pH value. Compared with the pH value of 3.08 in 1 ppm SO2-containing system (Figure 6), the presence of 1 ppm H2SO4 lowers the pH to a value of 2.58. It means that the effect of H2SO4 derived from the interaction of O2 and SO2 on the water chemistry characteristics is higher than that of individual O2 or SO2. This is consistent with its effect on the corrosion rate. In water-saturated SC CO2-O2-SO2 system, X70 steel demonstrates highest corrosion rate that is even higher than the sum of that in the individual O2- or SO2-containing system (Figure 9). It can be concluded that the essence of the synergistic effect between O2 and SO2 is that the interaction of O2 and SO2 promotes the formation of H2SO4 and makes the condensed water highly acidified, thereby aggravating the corrosion. Therefore, the corrosion process of steel is mainly controlled by H2SO4, and the corrosion effects of SC CO2 and SO2 are weakened, forming the corrosion film mainly consisting of FeSO4 with small amounts of FeSO3 and FeCO3 on the steel, as shown in Figure 13c. Since O2(aq) is present in condensed water, FeSO4 at the corrosion film/condensed water interface can be further oxidized to produce FeOOH via the reaction of Eq. (7).24, 28 Therefore, the rust-colored products of FeOOH are developed on the surface of the inner FeSO4 layer to form the outer layer (Figure 13c). 4FeSO4 + 6H2O + O2(aq) → 4FeOOH + 4H2SO4(aq)

(7)

The above statements indicate that although O2 has negligible influence on the water chemistry characteristics and its effect on the corrosion rate of X70 steel is far less than SO2, O2 will play a critical role in the corrosion process once O2 and SO2 simultaneously exist in SC CO2-H2O system. This is because that O2 can contribute to a synergistic effect on the corrosion by converting SO2 to SO3 with subsequent formation of strong corrosive H2SO4.

14


Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5. Conclusions In water-saturated SC CO2-O2 system, the change of O2 concentration has negligible influence on the water chemistry characteristics. Increasing O2 concentration from 1000 ppm to 10000 ppm only changes the corrosion rate of X70 steel from 0.027 mm/y to 0.029 mm/y. However, O2 can mildly increase the corrosion rate by inducing the oxygen corrosion, and control the corrosion process of X70 steel together with CO2 to form Fe2O3 and FeCO3 products on the steel. In water-saturated SC CO2-SO2 system, the rising SO2 concentration significantly worsens the water chemistry characteristics. The corrosion rate of X70 steel changes from 0.269 mm/y to 0.423 mm/y as the SO2 concentration increased from 200 ppm to 1000 ppm. SO2 not only remarkably lowers the pH value of condensed water, but also greatly inhibits the dissociation of H2CO3. The corrosion process is mainly dominated by SO2 while CO2 corrosion effect is weakened, forming FeSO3 film with small amounts of FeCO3 on the steel. In water-saturated SC CO2-O2-SO2 system, a synergistic effect between O2 and SO2 mainly controls the corrosion of X70 steel and contributes to a highest corrosion rate of 0.842 mm/y due to the fact that the formation of H2SO4 makes the condensed water highly acidified. A double-layer film consisting of the inner FeSO4 layer with small amounts of FeSO3 and FeCO3 and the outer FeOOH layer is developed on the steel. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51471188) and Natural Science Foundation of Shandong Province (No. ZR2014EMM002). The authors would like to express their appreciation to Dr. Zhichao Qiu (Research Institute of Petroleum Exploration and Development, Beijing, China) for his technical support in the use of OLI Analyzer software. References (1) Bhave, A; Taylor, R. H. S.; Fennell, P.; Livingston, W. R.; Shah, N.; Dowell, N. M.; Dennis, J.; Kraft, M.; Pourkashanian, M.; Insa, M.; Jones, J.; Burdett, N.; Bauen, A.; Beal, C.; Smallbone, A.; Akroyd, J. Screening and techno-economic assessment of biomass-based power generation with CCS technologies to meet 2050 CO2 targets. Appl. 15



Energy 2017, 190, 481. (2) Sim, S.; Cole, I. S.; Choi, Y.-S.; Birbilis, N. A review of the protection strategies against internal corrosion for the sate transport of supercritical CO2 via steel pipelines for CCS purposes. Int. J. Greenh. Gas Control 2014, 29, 185. (3) Hua, Y.; Jonnalagadda, R.; Zhang, L.; Neville, A.; Barker, R. Assessment of general and localized corrosion behavior of X65 steel and 13Cr steels in water-saturated supercritical CO2 environments with SO2/O2. Int. J. Greenh. Gas Control 2017, 64, 126. (4) Wei, L.; Pang, X. L.; Zhou, M.; Gao, K. W. Effect of exposure angle on the corrosion behavior of X70 steel under supercritical CO2 and gaseous CO2 environments. Corros. Sci. 2017, 121, 57-71. (5) Cole, I. S.; Corrigan, P.; Sim, S.; Birbilis, N. Corrosion of pipelines used for CO2 transport in CCS: is it a real problem. Int. J. Greenh. Gas Control 2011, 5, 749. (6) Dugstad, A.; Halseid, M. Internal corrosion in dense phase CO2 transport pipelines-state of the art and the need for further R & D. NACE International: Houston, TX, 2012 (In: Corrosion 2012, Paper No. 1452). (7) McGrail, B. P.; Schaef, H. T.; Glezakou, V.-A.; Dang, L. X.; Owen, A. T. Water reactivity in the liquid and supercritical CO2 phase: has half the story been neglected. Energy Procedia 2009, 1, 3415. (8) Dugstad, A.; Morland, B.; Clausen, S. Corrosion of transport pipelines for CO2-effect of water ingress. Energy Procedia 2011, 4, 3063. (9) Hua, Y.; Barker, R.; Neville, A. Effect of temperature on the critical water content for general and localized corrosion of X65 carbon steel in the transport of supercritical CO2. Int. J. Greenh. Gas Control 2014, 31, 48. (10) Sun, C.; Wang, Y.; Sun, J. B.; Lin, X. Q.; Li, X. D.; Liu, H. F.; Cheng, X. K. Effect of impurity on the corrosion behavior of X65 steel in water-saturated supercritical CO2 system. J. Supercrit. Fluids 2016, 116, 70. (11) Hua, Y.; Barker, R.; Neville, A. Comparison of corrosion behaviour for X-65 carbon steel in supercritical CO2-saturated water and water-saturated/unsaturated supercritical CO2. J. Supercrit. Fluids 2015, 97, 224. (12) Zeng, Y. M.; Pang, X.; Shi, C.; Arafin, M.; Zavadil, R.; Collier, J. Influence of impurities on corrosion performance of pipeline steels in supercritical carbon dioxide. NACE International: Houston, TX, 2015 (In: Corrosion 2015, Paper No. 5755). (13) Xu, M. H.; Zhang, Q.; Wang, Z.; Liu, J. M.; Li, Z. Effect of high-concentration O2 on corrosion behavior of X70 steel in water-containing supercritical CO2 with SO2. Corrosion 2017, 73, 290. (14) Wei, L.; Pang, X. L.; Gao, K. W. Effect of small amount of H2S on the corrosion behavior of carbon steel in the dynamic supercritical CO2 environments. Corros. Sci. 2016, 103, 132. 16


Page 16 of 36

Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Choi, Y. S.; Hassani, S.; Vu, T. N.; Nesic, S.; Abas, A. Z. B. Effect of H2S on corrosion behavior of pipeline steels in supercritical and liquid CO2 environments. NACE International: Houston, TX, 2015 (In: Corrosion 2015, Paper No. 5927). (16) Xiang, Y.; Wang, Z.; Xu, C.; Zhou, C. C.; Li, Z.; Ni, W. D. Impact of SO2 concentration on the corrosion rate of X70 steel and iron in water-saturated supercritical CO2 mixed with SO2. J. Supercrit. Fluids 2011, 58, 286. (17) Dugstad, A.; Halseid, M.; Morland, B. Effect of SO2 and NO2 on corrosion and solid formation in dense phase CO2 pipelines. Energy Procedia 2013, 37, 2877. (18) Hua, Y.; Barker, R.; Neville, A. The influence of SO2 on the tolerable water content to avoid pipeline corrosion during the transportation of supercritical CO2. Int. J. Greenh. Gas Control 2015, 37, 412. (19) Sun, C.; Sun, J.B.; Wang, Y.; Sui, P. F.; Lin, X. Q.; Liu, H. F.; Cheng, X. K.; Zhou, M. N. Effect of impurity interaction on the corrosion film characteristics and corrosion morphology evolution of X65 steel in water-saturated supercritical CO2 system. Int. J. Greenh. Gas Control 2017, 65, 117. (20) Ruhl, A. S.; Kranzmann, A. Corrosion in supercritical CO2 by diffusion of flue gas acids and water. J. Supercrit. Fluids 2012, 68, 81. (21) Sim, S.; Cole, I. S.; Bocher, F.; Corrigan, P.; Gamage, R. P.; Ukwattage, N.; Birbilis, N. Investigating the effect of salt and acid impurities in supercritical CO2 as relevant to the corrosion of carbon capture and storage pipelines. Int. J. Greenh. Gas Control 2013, 17, 534. (22) Choi, Y.-S.; Nešić, S. Determining the corrosive potential of CO2 transport pipeline in high pCO2-water environments. Int. J. Greenh. Gas Control 2011, 5, 788. (23) Sim, S.; Bocher, F.; Cole, I. S.; Chen, X.-B.; Birbilis, N. Investigating the effect of water content in supercritical CO2 as relevant to the corrosion of carbon capture and storage pipelines. Corrosion 2014, 70, 185. (24) Choi, Y.-S.; Nesic, S.; Young, D. Effect of impurities on the corrosion behavior of CO2 transmission pipeline steel in supercritical CO2-water environments. Environ. Sci. Technol. 2010, 44, 9233. (25) Farelas, F.; Choi, Y. S.; Nešić, S. Corrosion behavior of API 5L X65 carbon steel under supercritical and liquid carbon dioxide phases in the presence of water and sulfur dioxide. Corrosion 2013, 69, 243. (26) Hua, Y.; Barker, R.; Neville, A. Understanding the influence of SO2 and O2 on the corrosion of carbon steel in water-saturated supercritical CO2. Corrosion 2015, 71, 667. (27) Ayello, F.; Evans, K.; Thodla, R.; Sridhar, N. Effect of impurity on corrosion of steel in supercritical CO2. NACE International: Houston, TX, 2010 (In: Corrosion 2010, Paper No. 10193). (28) Sun, C.; Sun, J. B.; Wang, Y.; Lin, X. Q.; Li, X. D.; Cheng, X. K.; Liu, H. F. Synergistic 17



effect of O2, H2S and SO2 impurities on the corrosion behavior of X65 steel in water-saturated supercritical CO2 system. Corros. Sci. 2016, 107, 193. (29) Sun, J. B.; Sun, C.; Zhang, G. A.; Li, X. D.; Zhao, W. M.; Jiang, T.; Liu, H. F.; Cheng, X. K.; Wang, Y. Effect of O2 and H2S impurities on the corrosion behavior of X65 steel in water-saturated supercritical CO2 system. Corros. Sci. 2016, 107, 31-40. (30) Hua, Y.; Barker, R.; Neville, A. The effect of O2 content on the corrosion behaviour of X65 and 5Cr in water-containing supercritical CO2 environments. Appl. Surf. Sci. 2015, 356, 499. (31) Tang, Y.; Guo, X. P.; Zhang, G. A. Corrosion behavior of X65 carbon steel in supercritical-CO2 containing H2O and O2 in carbon capture and storage (CCS) technology. Corros. Sci. 2017, 118, 118. (32) de Visser, E.; Hendriks, C.; Barrio, M.; Mølnvik, M. J.; de Koeijer, G.; Liljemark, S.; Gallo, Y. L. Dynamis CO2 quality recommendations. Int. J. Greenh. Gas Control 2008, 2, 478. (33) Oosterkamp, A.; Ramsen, J. State-of-the-art overview of CO2 pipeline transport with relevance to offshore pipelines. In: POLYTEC, 2008, pp. 20. (34) Cole, I. S.; Paterson, D. A.; Corrigan, P.; Sim, S.; Birbilis, N. State of the aqueous phase in liquid and supercritical CO2 as relevant to CCS pipelines. Int. J. Greenh. Gas Control 2012, 7, 82. (35) Sun, W.; Nešić, S.; Woollam, R. C. The effect of temperature and ionic strength on iron carbonate (FeCO3) solubility limit. Corros. Sci. 2009, 51, 1273. (36) Zhang, Y. C.; Pang, X. L.; Qu, S. P.; Li, X.; Gao, K. W. Discussion of the CO2 corrosion mechanism between low partial pressure and supercritical condition. Corros. Sci. 2012, 59, 186. (37) Xiang, Y.; Li, C.; Long, Z. W.; Guan, C. Y.; Wang, W.; Hesitao, W. Electrochemical behavior of valve steel in a CO2/sulfurous acid solution. Electrochim. Acta 2017, 258, 909. (38) Wei, L.; Pang, X. L.; Liu, C.; Gao, K. W. Formation mechanism and protective property of corrosion product scale on X70 steel under supercritical CO2 environment. Corros. Sci. 2015, 100, 404. (39) Huijbregts, W. M. M.; Leferink, R. G. I. Latest advances in the understanding of acid dewpoint corrosion: corrosion and stress corrosion cracking in combustion gas condensates. Anti-Corros. Method. M. 2004, 51, 173. (40) Ruhl, A. S.; Kranzmann, A. Investigation of corrosive effects of sulphur dioxide, oxygen and water vapour on pipeline steels. Int. J. Greenh. Gas Control 2013, 13, 9. (41) Zhang, J.; Liu, W.; Lin, X. Q.; Dong, S.; Lu, S. L.; Yang, C.; Wang, T. T.; Lu, M. X. Corrosion behavior and mechanism of N80 steel under high temperature and high 18


Page 18 of 36

Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pressure CO2-O2 coexisting condition. NACE International: Houston, TX, 2013 (In: Corrosion 2013, Paper No. 2479). (42) Sun, H.; Wu, X. Q.; Han, E. H.; Wei, Y. Z. Effects of pH and dissolved oxygen on electrochemical behavior and oxide films of 304SS in borated and lithiated high temperature water. Corros. Sci. 2012, 59, 334. (43) Xiang, Y.; Xu, M. H.; Choi, Y.-S. State-of-the-art overview of pipeline steel corrosion in impure dense CO2 for CCS transportation: mechanisms and models. Corros. Eng. Sci. Technol. 2017, 52, 485.

19



Figure 1. Schematic diagram of the autoclave for the corrosion test.

20


Page 20 of 36

3.6

Gaseous CO2

7.38 MPa

Supercritical CO2

3.5 3.4 pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31.1 C o 40 C o 50 C

3.3

o

31.1 C o 40 C o 50 C

pH: 3.2 3.1 3.0

4

8

12

-5

1.6x10

-5

1.4x10

-5

1.2x10

-5

1.0x10

-5

8.0x10

-6

6.0x10

-6

4.0x10

-6

o

Ionic strength:

0

1.8x10

16

20

Ionic strength (mol/mol)

Page 21 of 36

Pressure (MPa)

Figure 2. The effects of pressure and temperature on the pH and ionic strength of aqueous phase in SC CO2-H2O system.

21



3.0 Solubility of CO2 in H2O × 100 (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

o

31.1 C o 40 C o 50 C

2.5

Page 22 of 36

7.38 MPa

2.0 1.5 1.0 0.5 Gaseous CO2 0.0

0

4

Supercritical CO2 8 12 Pressure (MPa)

16

20

Figure 3. The effects of pressure and temperature on the solubility of CO2 in H2O calculated by the thermodynamic model for mutual solubilities of CO2 and H2O as reported by Choi and Nešić.22

22


Page 23 of 36

1000

6.90

6.85 999 6.80 H2O in aqueous phase H2O in SC CO2 phase CO2 in aqueous phase CO2 in SC CO2 phase

3.20

0.360

3.15

3.10

0.355

0

500

1000

CO2 concentration in different phases (g)

H2O content in different phases (g)

(a)

0.350 2000

1500

O2 concentration (ppm) 1000

6.85

6.80

999 H2O in aqueous phase H2O in SC CO2 phase CO2 in aqueous phase CO2 in SC CO2 phase

0.360 3.20 0.355

3.15

0

500

1000

1500

CO2 concentration in different phases (g)

H2O content in different phases (g)

(b)

0.350 2000

SO2 concentration (ppm)

Impurity concentration in different phases (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

4

10

3

10

2

10

1

10

0

(c)

10

-1

10

-2

10

-3

10

-4

10

-5

O2 in aqueous phase O2 in SC CO2 phase SO2 in aqueous phase SO2 in SC CO2 phase

0

500

1000

1500

2000

Impurity concentration (ppm)

Figure 4. Distributions of 10 g H2O, 1000 g CO2 and 0-2000 ppm O2 or SO2 in aqueous phase and SC CO2 phase in SC CO2-H2O-impurity system at 10 MPa and 50 ºC: (a) H2O and CO2 distributions in SC CO2-H2O-O2 system; (b) H2O and CO2 distributions in SC CO2-H2O-SO2 system and (c) O2 and SO2 distributions in SC CO2-H2O-O2 system and SC CO2-H2O-SO2 system, respectively.

23


Page 24 of 36

49440

1000

49410

100

10

49380

1

49350 O2 (a)

0.1

49320

CO2 (a) SO2 (b) CO2 (b)

0.01 0

500

49290 1000

1500

CO2 solubility in aqueous phase (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Impurity solubility in aqueous phase (ppm)


2000


Figure 5. Variations of the solubility of impurity and SC CO2 in aqueous phase with impurity concentrations in SC CO2-H2O-impurity system at 10 MPa and 50 ºC: (a) SC CO2-H2O-O2 system and (b) SC CO2-H2O-SO2 system.

24


Page 25 of 36

3.2

14

3.0

12 O2

+

SO2

10 8

2.8

0.7854

3.1016

0.7852

3.1015

2.6

3.1014

0.7850

200

3.1013

6

0.7848

3.1011

0.7844

4 +

2 0

600

3.1010 1

H:

2.4

3.1012

0.7846

10

100

1000

1000

O2

2.2 2.0

SO2 1

pH

pH: H concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

100

1000

1.8


Figure 6. H+ concentration and pH value of aqueous phase in SC CO2-H2O-impurity system at 10 MPa and 50 ºC.

25



50

47.54

40

47.52 47.50

30

47.48 1

10

100

1000

20

-

HCO3 concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

O2 SO2

0

1

10

100

1000


Figure 7. HCO3-concentration of aqueous phase in SC CO2-H2O-impurity system at 10 MPa and 50 ºC.

26


Page 26 of 36

Page 27 of 36

6

1200 -

HSO3 2-

SO3

5

4

600

3

400

2

200

1

2-

-

-3

800

SO3 concentration ×10 (ppm)

1000 HSO3 concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1

10

100

1000

0

SO2 concentration (ppm)

Figure 8. HSO3- and SO32- concentrations of aqueous phase in SC CO2-H2O-SO2 system at 10 MPa and 50 ºC.

27



1.0 SC CO2-H2O

0.842

SC CO2-H2O-O2 SC CO2-H2O-SO2

0.8

SC CO2-H2O-O2-SO2 Corrosion rate (mm/y)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

0.6 0.423 0.345

0.4 0.269 0.2

0.0

0.014

0.027 0.029

0

1000 10000

200

600 1000

1000+1000


Figure 9. Corrosion rate of X70 steel exposed to water-saturated SC CO2 system containing O2 and/or SO2 for 120 h at 10 MPa and 50 ºC.

28


Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Macroscopic morphologies and SEM surface morphologies of the corroded X70 steels exposed to water-saturated SC CO2 system containing O2 and/or SO2 for 120 h at 10 MPa and 50 ºC: (a) CO2; (b) CO2 + 1000 ppm O2; (c) CO2 + 1000 ppm SO2 and (d) CO2 + 1000 ppm O2 + 1000 ppm SO2.

29



(e)

Fe O S C

(f)

Corrosion products

0

10

20 30 Line scanning distance (um)

40

50

Fe O S C Outer layer

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

0

10

Inner layer

20 30 40 50 60 Line scanning distance (um)

70

80

Figure 11. (a-d) SEM backscattered electron images and (e and f) EDS line scanning analysis of the cross-sections of the corroded X70 steels exposed to water-saturated SC CO2 system containing O2 and/or SO2 for 120 h at 10 MPa and 50 ºC: ((a) CO2; (b) CO2 + 1000 ppm O2; (c) CO2 + 1000 ppm SO2; (d) CO2 + 1000 ppm O2 + 1000 ppm SO2; (e) elemental distributions denoted by the white arrow line in (c) and (f) elemental distributions denoted by the white arrow line in (d). 30


Page 31 of 36

Θ

Fe • FeCO3 ο Fe2O3 ♥ FeOOH

∆ FeSO3⋅xH2O ∇ FeSO3 ♦FeSO4⋅4H2O

(a)

Θ

Θ

•

•

•

(b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Θ Θ

•ο ο •

∆

∆

∆ ∆

∇ ∇∆

∆ ∆•

•

(d)

10

∆

•

•

♦

•

∆

(c)

♦♦ • • ♦ ∆ ♦ ♦ ♦ ♦♥♦ ∆ ♥ ♥

20

30

40 50 2θ (degree)

♥

60

♥

70

80

Figure 12. XRD spectra of corrosion products on X70 steels exposed to water-saturated SC CO2 system containing O2 and/or SO2 for 120 h at 10 MPa and 50 ºC: (a) CO2; (b) CO2 + 1000 ppm O2; (c) CO2 + 1000 ppm SO2 and (d) CO2 + 1000 ppm O2 + 1000 ppm SO2.

31



Figure 13. Schematic model for the corrosion and film characteristics in water-saturated SC CO2-impurity system: (a) CO2-H2O-O2 system; (b) CO2-H2O-SO2 system and (c) CO2-H2O-O2-SO2 system

32


Page 32 of 36

Page 33 of 36

3.0

2500 +

H

pH 2.5

2000 2.0

1.0 1000

pH

1.5

1500

0.5

+

H concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 500 -0.5 0

1

10

100

1000

-1.0

H2SO4 concentration (ppm)

Figure 14. H+ concentration and pH value of aqueous phase in SC CO2-H2O-H2SO4 system at 10 MPa and 50 ºC.

33



Page 34 of 36

Table 1. Conditions for modeling the water chemistry of CO2 streams. Condition

Temperature Pressure (ºC) (MPa)

O2 (ppm)

SO2 (ppm)

H2SO4 (ppm)

CO2 (g)

H2O (g)

1 2 3 4

31.1, 40, 50 50 50 50

0-2000 -

0-2000 -

0-2000

1000

10

1-20 10 10 10

34


Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Test conditions for the corrosion tests. Test

O2 (ppm)

SO2 (ppm)

CO2 (MPa)

H2O (g)

Temperature (ºC)

Test time (h)

1 2 3 4

1000, 10000 1000

200, 600, 1000 1000

10

10 (Sat.)

50

120

35



Table of Contents Graphic:

36


Page 36 of 36

Effects of O2 and SO2 on Water Chemistry Characteristics and

Recommend Documents