Formation mechanism and protective property of corrosion product scale on X70 steel under supercritical CO2 environment

Accepted Manuscript Title: Formation mechanism and protective property of corrosion product scale on X70 steel under supercritical CO2 environment Author: Liang Wei Xiaolu Pang Chao Liu Kewei Gao PII: DOI: Reference:

S0010-938X(15)30048-2 http://dx.doi.org/doi:10.1016/j.corsci.2015.08.016 CS 6444

To appear in: Received date: Revised date: Accepted date:

21-3-2015 4-8-2015 5-8-2015

Please cite this article as: Liang Wei, Xiaolu Pang, Chao Liu, Kewei Gao, Formation mechanism and protective property of corrosion product scale on X70 steel under supercritical CO2 environment, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.08.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Formation mechanism and protective property of corrosion product scale on X70 steel under supercritical CO2 environment Liang Wei, Xiaolu Pang, Chao Liu, Kewei Gao* Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China * Corresponding author: E-mail: [email protected] Tel: +86 1062334909; Fax: +86 1062334909 Abstract The comparison of corrosion mechanism of X70 steel, as well as the formation mechanism of scale, at between 9.5 and 1 MPa were studied. The outer FeCO3 scale formed at 1 MPa presented better protectiveness than that at 9.5 MPa, while the protectiveness of inner layer was just contrary. Under supercritical CO2 condition, localized corrosion occurred at the transitional stage - the amorphous layer evolved into inner FeCO3 scale. General corrosion rate decreased in a logarithm tendency with the Young’s modulus of FeCO3 scale. The fracture toughness of scale at 9.5 MPa was higher than at 1 MPa. Key words: A. steel; B. EIS; B. SEM; B. XPS; C. acid corrosion 1. Introduction The emission of carbon dioxide (CO2) is believed to be the main reason for global climate change. To reduce the emission of CO2 and consequently control global climate warming, carbon capture and storage (CCS) [1] has been under development for decades, it main contains three parts: capturing the CO2 from various emission sources (such as fossil-fuel combustion), transporting it to selected storage sites and injecting it into geological reservoirs (the saline aquifers and oil/gas reservoirs [2]). Moreover, CO2 has been transported via pipelines to be used in enhanced oil recovery (EOR) for many years [3-5], and more than 6,000 km of such pipelines are used for CO2 transportation all over the world, mostly in North America [6]. According to the “Blue Map Scenario” [7] for the mitigation of climate change, 3000 twelveinch or 1000 twenty-inch pipelines will be required to transport 10 Gtons of CO2 per year by 2050 [8]. It is evident that the most cost effective materials for manufacturing such long pipelines are high strength carbon steels, such as X65 and X70, which are already widely used in oil and gas transport [5, 9-10]. In the CO2 transportation pipelines that are mainly used for EOR in the USA, which are operating under strict limits on impurities, no significant corrosion problem has been recorded - only two incidents ascribed to corrosion have occurred over more than twenty years [4, 11]. As a consequence, many more studies have focused on development of capture technology to reduce costs, assess storage sites and the feasibility of injection, and monitor the CO2 in geological reservoirs [12-13], only a limited number of studies have focused on the corrosion behavior of pipelines used in the CO2 transportation and injection process. To make it easier to transport and to avoid two-phase flow regimes [14-15], as well as to reduce costs, CO2 is typically compressed to the supercritical state [16]. It is well known that dry SC CO2 is not corrosive [17-18], and a number of investigations have also confirmed that corrosion was insignificant in SC CO2/H2O systems when the water content is far below its solubility limit in SC CO2 [13, 19-24]. However, drying CO2 can drastically increase the cost of CCS and hinder its application. Therefore, water and other impurities are typically present in actual CO2 transportation processes. When free water (liquid water phase) is present, CO2 can immediately dissolve in the free water and form H2CO3, bringing the free water to a lower pH value (~3.2 [25-26]). Once this SC CO2-saturated water comes in contact with the internal wall of the pipeline, corrosion can occur. If other impurities such as SO2 or NO2 exist are present, the pH value of the free water is further decreased [27-28] resulting in more serious corrosion of the pipelines. Overall, the water content is an especially critical factor in the CCS process that is worthy of greater attention. To assess the corrosion of pipelines, it is necessary to identify the possible operational regimes of SC CO2/H2O systems. Based only on considerations of H2O impurity, the systems can be classified into three regimes:

Regime 1: Either the CO2 is dry or a small amount of water is present, but its content is below the water solubility limit in SC CO2, i.e., no free water exists. Only one phase, a CO2-rich phase, is present in this regime. Regime 2: A small amount of water that nevertheless exceeds the solubility limit of water in SC CO2 is present in the SC CO2/H2O system, i.e., free water exists. This regime is still rich in CO2, but two phases are present: an H2O-saturated CO2 phase and a CO2-saturated H2O phase. Regime 3: A large number of water is present. This regime is rich in H2O, and two phases are present: a CO2-saturated H2O phase and an H2O-saturated CO2 phase. Table 1 summarizes the data of corrosion rate of carbon steels in SC CO 2 environments. The general corrosion rates in Regime 1 were typically very low, below 0.08 mm/y [13, 25]. The general corrosion rates in Regime 2 were higher than those in Regime 1, typically above 0.1 mm/y [13, 19-20, 25, 29-32]. It has been observed that the pitting corrosion in Regimes 1 and 2 (CO2-rich phase) were serious, with a maximum pitting corrosion rate of 1 mm/y [13]. However, in Regime 3 (water-rich phase), general corrosion is the predominant corrosion type and the corrosion rates were much higher than in the CO2-rich phase [20, 29-30, 33-41]. Although pitting corrosion occurs on pipeline steels in CO2-rich phase, the pitting corrosion rate is still much lower than the general corrosion rate in water-rich phase. The formation of a CO2 corrosion scale and its protective properties significantly influence the corrosion rate of steels [41-43]. The investigation of Choi [44] indicated that the corrosion rate of carbon steel was rather high (~20mm/y) without FeCO3 layer but could significantly decrease (~0.2 mm/y) due to the formation of the FeCO3 layer after long-term exposure under SC CO2 condition. Wu [51] found that the corrosion rate of carbon steels decreased with the increasing formation of a dense and completeness corrosion product scale. Moreover, researchers have found that the mechanical properties of such corrosion product scale significantly influence its protective characteristic to steels [46-47]. Chen [48] found that the lower the elastic modulus was, the higher the porosity of corrosion product scale was, and the worse the protectiveness of scale was. Therefore, to evaluate the corrosion of steels in the most severe SC CO2 environment (i. e. SC CO2-saturated water phase), it is very necessary to in-depth investigate the formation mechanism and the mechanical properties of the corrosion product scale. In addition, the tubing used to inject CO2 into saline aquifer reservoirs may come in contact with the brine, which contains high concentrations of chloride and SC CO2, and therefore may also suffer serious corrosion attack [49-50]. However, as listed in Table 1, the investigations of steels immersed in SC CO2saturated brine were limited. The effect of saline ions, especially Cl-, on the corrosion rate of steels has been widely investigated [51-55], it is generally believed that the maximum corrosion rate of carbon steel is obtained with Cl- content of about 25 g/L, similar to the content of Cl- in a 3.5wt% NaCl solution or in seawater. Moreover, the existing models have not well predicted the corrosion rate of steels at SC CO2 conditions [39, 56-57]. Therefore, the objective of this work is to systematically investigate the corrosion mechanism of carbon steel in 3.5 wt% NaCl under SC CO2 conditions. Meanwhile, the formation mechanism and mechanical properties of the corrosion product scales formed at both SC CO2 and low CO2 partial pressure conditions were also discussed. 2. Experimental 2.1 Test material and immersion experiments The test materials used in this study was X70 carbon steel, which composition is presented in Table 2. Prior to the immersion experiments, all specimens were machined to dimensions of 10×10×3 mm3, ground successively with 150, 400 and 800 grit SiC papers, cleaned with alcohol and finally weighed with a precision of 0.1 mg. The immersion experiments were performed in a high temperature and high pressure autoclave. The test specimens were completely immersed in the electrolyte solution, which consisted of deionized water and NaCl (analytical reagent grade). Test conditions are given in Table 3. Before the immersion experiments, N2 was bubbled into the electrolyte solution for 8 h to remove the oxygen in the electrolyte. After each immersion experiment, the corrosion product scales on three specimens were removed using 1000 mL of stripping solution, the compositions of which are given in Table 4, and weighed with a precision of 0.1 mg. The corrosion rates were calculated using the weight-loss method, as presented in the following equation:

=

∆

(1)

where RCorr is the general corrosion rate, mm/y; Δm is the weight loss, g; S is the exposed surface area, cm2; ρ is the density of the steel, g/cm3; and t is the immersion time, h. The corrosion rate corresponding to each immersion time was calculated as an average over three specimens. The surface and cross-section morphologies of the corrosion product scales were observed with Scanning Electron Microscopy (SEM), as were the surface morphology of the specimen after scale was removed. The phase identification of the corrosion product scales was characterized via X-ray Diffraction (XRD), Xray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. 2.2 Electrochemical tests of corrosion product scale In-situ electrochemical tests were performed using a PARSTAT 2273 electrochemical system. Each test specimen, with the same surface area (10×10 mm2) as those used in the immersion experiments, was welded to a copper wire and mounted in epoxy resin. Electrochemical impedance spectroscopy (EIS) was carried out in a typical three-electrode system, in which the steel specimen with corrosion product scale served as the working electrode, an Ag/AgCl electrode was used as the reference electrode and a platinum strip was used as the counter electrode. The EIS measurements were performed at OCP, and the test frequency was selected from 100 kHz to 10 mHz with an amplitude perturbation of 10 mV (peak to peak). To compare the protectiveness of the corrosion products scales formed under SC CO2 and low CO2 partial pressure conditions, the EIS tests were all conducted at 1 bar. The ZSimpWin impedance analysis software was used to fit the EIS data. 2.3 Mechanical properties of corrosion product scales The Young’s modulus (E) of the corrosion product scale was measured using a nano-indentor. The fracture toughness (KIC) of the scale was measured by means of Vickers indentation on the polished crosssection of the scale and was calculated by the following equation [46-47]:

K IC 

0.0294 d EL c3 2

(2)

where d is the diagonal length of the indent, mm; E is the Young’s modulus, GPa; L is the applied indenting load, N; and c is the length between the center of the indent and the tip of the radical crack, mm. 3. Results 3.1 Comparison of corrosion rates at pressures between 9.5 and 1MPa The general corrosion rates of X70 steel as a function of immersion time under SC CO2 (9.5 MPa) and low CO2 partial pressure (1 MPa) conditions are shown in Fig. 1. The corrosion rates of X70 steel at 9.5 and 1 MPa were both found to gradually decrease with increasing immersion time, although some differences were observed. The corrosion rate at 1 MPa slowly decreased with increasing immersion time between 2 and 50 h, followed by a rapid decrease from 50 to 384 h. A similar trend of variation was also observed by Zhang et al. [39]. They studied the corrosion rate of X65 steel in deionized water at 1 MPa and 80 °C, and found that the corrosion rate decreased slowly during the first few hours because of the scale-free CO2 corrosion, and then decreased sharply between 7 and 48 h due to the formation of a FeCO3 scale. However, the variation trend of the corrosion rate in SC CO2 environment is more complex, consisting of four distinct stages: 1. A sharp reduction stage from 0.5 to 7 h; 2. An abnormal increase between 7 and 12 h; 3. A second rapid decrease from 12 to 96 h; 4. A smooth decrease after 96 h. During the first stage, the corrosion rate was sharply reduced from 119 to 49.5 mm/y (a 58% decrease). A similar level of reduction in corrosion rate was also reported by Hua et al. [41]. They performed corrosion tests on X65 steel in 300 mL of distilled water at 8 MPa and 50 °C, and found that the corrosion rate was reduced by 54% between 6.5 and 48 h of immersion. An amorphous layer predominantly consisting of network-like Fe3C and FeCO3 was observed on the matrix surface after 24 h of immersion, which was identified as reason for the reduction in corrosion rate. By contrast, no corrosion product was observed on X65 steel by Zhang et al. [39] at 9.5 MPa and 80 °C after 2 h of immersion; as a result, the corrosion rate

decreased only slightly and remained at a high level. Interestingly, during the second stage, an abnormal increase in corrosion rate was observed from 49.5 to 57.4 mm/y, which was not observed in the results of Zhang et al. [39] and Hua et al. [41]. The corrosion rate decreased sharply during the third stage and more slowly during the fourth stage (from 96 to 384 h), similar behavior was also observed by researchers [39-41]. Hua et al. [41] observed a gradual reduction in the corrosion rate corresponding to the formation of a dense FeCO3 layer after 96 h. Zhang et al. [39] observed that the corrosion rate sharply decreased between 7 and 96 h and then remained essentially constant after 96 h of immersion, consistent with the formation process of the FeCO3 scale. With the increasing immersion time, the coverage percentage, thickness and density of the FeCO3 scale increased, thereby also enhancing its protectiveness capabilities. Cui et al. [40] performed weight loss measurements on carbon steels in produced water at 8.27 MPa and 90 °C, and also observed that the corrosion rate sharply decreased over the initial 96 h and then remained constant after 96 h. At 9.5 MPa and 80 °C and with 0.5 h of immersion, the corrosion rate observed in this study was 119 mm/y, much higher than that observed by Zhang et al. for exposed to deionized water carbon steel (28.3 mm/y) [39]. This finding indicates that the addition of NaCl dramatically increased the dissolution rate of the steel during the initial corrosion stage. Cl- could form intermediate corrosive species and thus accelerate the active dissolution of steels during the scale-free CO2 corrosion process [51, 58-59]. The anodic catalytic reaction process can be described as follows [51]:

Fe  Cl   H 2 O  FeClOHad  H   e  

FeClOH ad  FeClOH  e 

(3) (4)

FeClOH H  Fe2  Cl  H 2O

(5) By comparing the corrosion rates observed at 9.5 and 1 MPa, it can be seen for less than 50 h of immersion, the corrosion rate at 9.5 MPa was much higher than that at 1 MPa, whereas after 50 h of immersion, X70 steel suffered more severe corrosion attack at 1 MPa. This finding differs from the results observed by Zhang et al. [39], who observed that carbon steel always suffered more severe corrosion attack under SC CO2 condition than under low CO2 partial pressure condition. By contrast, Lin et al. [60] observed that the corrosion rate of carbon steels under SC CO2 condition was lower than that under low CO2 partial pressure condition, and they claimed that SC CO2 possessed inhibiting function on the CO2 corrosion process. Pfennig et al. [50, 61] also found that after 700 h of immersion, denser and thinner corrosion scales were observed on the steels under SC CO2 condition (10 MPa), led to a lower corrosion rate than that at 0.1 MPa. Overall, the change in the corrosion rate over time in corrosive environments strongly depends on the formation and protectiveness capabilities of corrosion product scale. Therefore, to better predict the corrosion rate of steels under SC CO2 environment and to compare the differences in corrosion behavior between SC CO2 and low CO2 partial pressure conditions, it is important to study the formation mechanism and microstructure of the corrosion product scale forms in SC CO2 environment. 3.2 Immersion experiment at 9.5 MPa and 80 °C 3.2.1 Formation of original amorphous scale Fig. 2 shows the surface and cross-section morphologies of the corrosion scale formed on X70 steel at 9.5 MPa and 80 °C in an immersion time range of 0.5 to 7 h. During the initial 0.5 h, no corrosion products were observed on the steel surface. XRD analysis also revealed only Fe peaks (as presented in Fig. 3). Because of the selective dissolution of the steel, the morphology of the steel surface was irregular, as shown in Fig. 2(a). After 2 h of immersion, an original corrosion product scale with several cracks and pores formed on the steel surface, as shown in Fig. 2(b). The XRD results presented in Fig. 3 indicated that Fe3C was present in this scale. Notably, signals of O, Mn and trace amounts of Cr were also detected in the scale via EDS analysis, but no peaks corresponding to these elements were detected via XRD, suggested that the scale may have possessed an amorphous structure. After 7 h of immersion, as shown in Fig. 2(c), irregular FeCO3 crystal grains (confirmed by XRD, as shown in Fig. 3) initially precipitated on the corrosion scale. The cross-section morphology of the corrosion scale after 7 h of immersion, shown in Fig. 3(d), indicated that this original corrosion scale was a network-like scale with a thickness of 20 µm. A similar phenomenon was also observed by Hua et al. [41], who reported that a network-like Fe3C layer appeared on the steel surface after 6.5 h of immersion in SC CO2-saturated water because of the selective dissolution of the ferrite phase in the steel. To clearly understand the microstructure of this original scale, XPS was employed for precise phase

analysis. As shown in Fig. 4, high resolution XPS spectra of Fe and C elements were detected. The peaks in the Fe 2s spectra at 711.4 eV and 724.3 eV corresponded to FeCO3, and the peak at 709.9 eV corresponded to Fe, which originated from Fe3C, the peak with a binding energy of 717.4 eV was associated with Fe2O3, which may form as a result of the oxidation of Fe2+ after the specimens were took out from the autoclave. The peaks in the C 1s spectra also confirmed the presence of Fe3C and amorphous FeCO3. Considering both the XPS and XRD results, it could be confirmed that an amorphous scale formed between 0.5 and 7 h of immersion. Guo et al. [62], Hua et al. [41] and Sun et al. [63] also demonstrated the presence of an amorphous scale during the initial corrosion stage. Sun et al. [63] claimed that this scale could significantly reduce the susceptibility to localized attack for X65 steel. Because of the formation of this original amorphous scale, the corrosion rate dramatically decreased. However, as presented in Fig. 2(c) and (d), several cracks and holes were observed on the surface of the amorphous scale, and thus, corrosive ions were capable of reaching the scale/matrix interface through these defects to continue corroding the steel. Meanwhile, as observed in Fig. 2 (d), the amorphous scale demonstrated poor adhesion to the steel matrix. 3.2.2 Formation of the FeCO3 corrosion product scale After 12 h of immersion, a large number of FeCO3 crystal grains, scattered across the surface of the amorphous scale, emerged and formed an incomplete outer layer of scale, in which large holes were observed (Fig. 5(a)). Fe and Fe3C, which should be contained in the inner amorphous scale, were detected via XRD at 12 h (as presented in Fig. 6), also suggesting that the outer FeCO3 layer was incomplete. From 24 to 96 h, the quantity and size of the holes in the outer FeCO3 layer gradually decreased, and the outer FeCO3 layer became denser. After 168 h of immersion, a complete and dense FeCO3 layer formed on the steel surface. Meanwhile, XRD results presented in Fig. 6 indicated that after 24 h of immersion, only FeCO3 could be detected. The cross-section morphology of the corrosion product scale formed between 12 and 384 h in the SC CO2-saturated solution at 9.5 MPa and 80 °C are presented in Fig. 7. FeCO3 crystals began to precipitate in the inner amorphous scale after 12 h of immersion (Fig. 7(a)), and large holes between the FeCO 3 grains in the inner layer scale resulted in regions where the matrix being in direct contact with the solution, where the corrosion was more severe than that regions covered by scale. That was the cause of the abnormal increase in the corrosion rate for at 12 h. After 24 h of immersion, a thick and dense inner scale formed on the steel surface, whereas the outer layer scale remained porous, with poor protectiveness property. Notably, localized corrosion, such as in Fig. 7 (c), was observed after 24 and 50 h of immersion. As presented in Fig. 7(d), (e) and (f), dense corrosion product scales formed on the steel surface after 96 h of immersion, but they all had similar thicknesses. An evident boundary line could be observed between the outer FeCO 3 scale and the inner scale, as shown in Fig. 8(a), indicating that the corrosion product scale formed in SC CO2-saturated solution consisted of two parts. The outer scale was demonstrated to consist of FeCO3 crystal. The elemental composition of the inner layer was analyzed via EDS, and Fe, O and C as well as traces of Mn and Cr were detected, consistent with the elements detected in the amorphous scale. Fig. 8 (b) shows the Raman spectra of this inner scale: the peaks near 180, 282 and 1082 cm-1 could be identified as FeCO3, the peak near 741 cm-1 corresponded to MnCO3, and the peak near 931 cm-1 was identified as Fe3C. Therefore, the inner scale consisted of FeCO3 and small amount of MnCO3 and Fe3C, which evolved from the original amorphous scale. The formation process of the corrosion product scale on X70 steel in SC CO2 environment was found to correspond well to the variation trend of corrosion rate measured by weight loss. At less than 0.5 h of immersion, no corrosion product scale formed on the steel surface, the rate of active dissolution of the steel was high, and much higher corrosion rate was obtained. Between 0.5 and 7 h of immersion, a protective amorphous scale formed on the steel surface, which reduced the corrosion rate dramatically (Stage 1). A transition period was observed between 7 and 12 h, during this time, FeCO3 crystal grains began to precipitate in the amorphous scale. At the same time, holes appeared in the amorphous scale, which accelerated the corrosion of the steel and resulted in localized attack (Stage 2). With the formation of the inner FeCO3 layer, the corrosion rate underwent a second rapid decrease up to 96 h (Stage 3). After 168 h, both the outer FeCO3 layer and the inner scale were denes, and thus, the corrosion rate began to decrease more slowly (Stage 4). 3.3 Immersion experiment at 1 MPa and 80 °C

Fig. 9 shows the surface and cross-section morphologies of X70 steel after 0.5, 2, 12 and 24 h of immersion at 1 MPa and 80 °C. As shown in Fig. 9(a) and (b), no corrosion product scale was observed on the steel surface at less than 7 h of immersion; this finding was supported by the XRD results presented in Fig. 10. Between 12 and 24 h of immersion (from Fig. 9 (c) to (f)), a thin and porous corrosion product scale, with no protectiveness, formed on the steel surface. XRD results presented in Fig 10 indicated that this scale contained Fe3C, which evidently accumulated on the steel surface as a result of the selective dissolution of ferrite. Therefore, the active dissolution of the steel was still proceeding rapidly because of the poor protectiveness of this corrosion product scale, and the corrosion rate was correspondingly high and constant. FeCO3 crystals began to precipitate on the steel surface after 24 h of immersion, as shown in Fig. 9 (d), much later than that observed at 9.5 MPa. The surface and cross-section morphologies of the corrosion product scale formed after 50, 96, 168 and 384 h of immersion are shown in Fig. 11. The XRD results presented in Fig. 12 indicated that after 50 h of immersion, a FeCO3 corrosion product scale completely covered the steel surface. The surface morphologies of the corrosion product scales observed in Fig. 11 (a), (c), (e) and (g) appeared denser than those formed on X70 steel at 9.5 MPa; they contained fewer holes and finer crystal grains. However, although the outer layer of the corrosion product scale formed after 50 h of immersion was dense, some of the area spalled, as shown in the magnified image in Fig. 11(a). Meanwhile, the inner layer was porous and demonstrated poor adhesion to the matrix, as shown in Fig. 11(b). If the growth rate of FeCO 3 was less than the corrosion rate of the steel, then the steel surface would never be covered with a dense FeCO3 corrosion product scale; moreover, the corrosion scale would be porous and non-adherent [64]. Therefore, the corrosion of X70 steel after 50 h of immersion at 1 MPa was still rapid, and the corrosion rate was higher than that at 9.5 MPa. However, the outer layer of the corrosion product scale was capable of blocking the transport of Fe2+ ions from the surface of the matrix to the bulk solution, resulting in a higher concentration of Fe2+ between the outer layer of corrosion scale and the matrix surface and thus accelerating the precipitation of FeCO3 along the interface with the matrix (Fig. 11(d)). After 96 h of immersion, a corrosion product scale with a dense outer FeCO3 layer and a thick but porous inner FeCO3 layer formed. From a comparison of the formation processes of corrosion product scale with immersion time at 9.5 MPa and 1 MPa, it is obvious that the formation mechanisms of corrosion product scale differ between SC CO2 and low CO2 partial pressure conditions. 3.4 EIS tests To a large extent, the corrosion rates of steels in CO 2 environments depend on the formation of FeCO 3 corrosion product scales and their protective characteristics. To further explore the different characteristics of FeCO3 corrosion product scales formed under SC CO2 and low CO2 partial pressure conditions, electrochemical impedance spectroscopy (EIS) was employed. Under SC CO2 condition, a fairly complete FeCO3 scale appeared after 24 h of immersion; therefore 24, 50 and 168 h were selected as the test immersion times to represent the initial, intermediate and stable stage of FeCO3 scale formation. In addition, 50 and 168 h were selected as the test immersion times for low CO2 partial pressure condition. Fig. 13 shows the EIS plots of X70 steel after 24, 50 and 168 h of immersion at 9.5 MPa and the EIS plots of X70 steel after 50 and 168 h of immersion at 1 MPa. As shown in Fig. 13(a), a similar characteristic was observed among the Nyquist plots corresponding to different immersion times at 9.5 MPa - a capacitive reactance arc. The amplitude of the loop increased with increasing immersion time, indicating that the charge transfer process, assumed to be the cathodic reaction or hydrogen reduction, slowed [42]. Similar phenomenon was also observed in the Nyquist plots of X70 steel at 1 MPa, the amplitude of the loop increased with increasing immersion time, as shown in Fig. 13(b). The equivalent circuit (EC) characterizing the impedance signature is shown in Fig. 14, where Rs is the solution resistance; Qi (i=1 at 24 h and, i=1, 2 at 50 and 168 h; 1-inner layer scale, 2-outer layer scale) is the constant phase element (CPE) used to represent the capacitance of corrosion product scale i; Ri is the resistance of corrosion product scale i; Qdl is the CPE used to represent the double-charge layer capacitance; and Rt is the charge transfer resistance that represent the dissolution of the steel matrix, which also represents the corrosion rate [63]. The fitted impedance spectrum shown in Fig. 13 agrees quite well with the measured impedance spectrum. According to the fitted results, only one layer of the corrosion product scale demonstrated a protective function for the steel after 24 h of immersion at 9.5 MPa. By observing the morphologies of scale shown in Fig. 5(b) and Fig. 7(b), it could be confirmed that this protective layer was

the inner scale. After 50 h of immersion, the outer and inner scales formed on X70 steel at 9.5 and 1 MPa both provided protection for the matrix. Fig. 15 compares the trends of variation in the Rt, R2 and R1values obtained after different immersion times at 9.5 and 1 MPa. Apparently, the values of Rt, R1 and R2 all increased with increasing immersion time at both 9.5 and 1 MPa. The variation of Rt with the immersion time indicated that the corrosion rate of the steel decreased with the increasing immersion time after the formation of FeCO3. Meanwhile, the value of Rt at 9.5 MPa was much higher than that at 1 MPa for the same immersion time, again indicating that the corrosion rate at 9.5 MPa was lower than that at 1 MPa after 50 h of immersion. Additionally, the variation in the values of R2 and R1 indicated that the protectiveness of both the outer and inner scales improved with the increasing immersion time. After 168 h of immersion, the R2 value obtained at 9.5 MPa was lower than that at 1 MPa, indicating that the outer scale formed at 1 MPa was more protective than that formed at 9.5 MPa, i.e., R2(9.5MPa) ＜ R2(1MPa). This was consistent with the surface morphologies observed in Fig. 5(e) and Fig. 11(e) - the outer layer formed at 1 MPa was denser and consisted of finer FeCO3 crystal grains than that formed at 9.5 MPa. However, due to the cracks in the outer scale after 50 h of immersion at 1 MPa (Fig. 11(a), magnified image), it was less protective than the scale formed after the same amount of time at 9.5 MPa. The value of R1 at 9.5 MPa was much higher than that at 1 MPa, demonstrating the inner scale formed at 9.5 MPa was much more protective than that formed at 1 MPa, i.e., R1(9.5MPa) > R1(1MPa). This was also consistent with the morphology of inner scale shown in Fig. 7 and Fig. 11 – the inner layer formed at 9.5 MPa was much denser than that formed at 1MPa. Notably, the resistance of the inner scale (R1) was much higher than the resistance of the outer scale (R2) at both 9.5 and 1 MPa, indicating that the inner scale was much more protective than the outer scale during the CO2 corrosion process. Gao et al. [42] also observed similar results, which they attributed to the much greater thickness of the inner layer scale. Moreover, as shown in the magnified image presented in Fig. 11(e), although the inner layer contained several larger holes, it was much denser than the outer layer. Therefore, the inner scale was more protective than the outer scale during the CO2 corrosion process because of both its greater thickness and its denser microstructure. Importantly, the variation trends of R2 and R1 shown in Fig. 15 indicated that the formation process of corrosion product scale differed between SC CO2 and low CO2 partial pressure conditions: the formation of the inner layer was dominant in SC CO2 environment, while the formation of outer layer scale was dominant in low CO2 partial pressure environment. This was consistent with the morphologies of corrosion product scales observed in Fig. 7 (9.5 MPa) and Fig. 11 (1 MPa). 4. Discussion 4.1 Corrosion mechanism of carbon steel and formation mechanism of corrosion product scale in supercritical CO2 environment Once dissolved in water, CO2 becomes hydrated to form H2CO3, and dissociates to form HCO , CO and H . These ions play important roles during the CO2 corrosion process. The equilibrium reactions that proceed during the CO2 dissolution process and the corresponding reaction equilibrium constants are listed as follows [65]:

CO 2 aq   H 2 O  H 2 CO 3 

H 2 CO 3  H  HCO HCO 3  H   CO 32 

 3

K1 

K2  K3 

C H 2 CO3

(6)

C CO 2

γ 22 C HCO  C H  3

(7)

C H 2 CO 3 γ  3 C CO 2  C H  3

C HCO 

(8)

3

H 2O  H   OH

K 4  γ 2 4 C H  C OH 

(9)

where K1, K2, K3 and K4 are the equilibrium constants of reactions (6-9), respectively. The equilibrium constant is a function of temperature, pressure and the ionic strength (I), and can be easily determined from the information provided in the open literature [65]. I can be determined as follows:

I

1 m i z i2  2 i

(10)

where mi is the molar concentration of the ith ion in the solution and zi is its charge. The mean ionic activity coefficients ± , ± and ± in the solution are about 0.662 [66]. is the concentration of CO2 dissolved in the solution, mol/L; , , , and are the equilibrium concentrations of H2CO3, HCO , CO , H+ and OH in the solution (mol/L), respectively. According to electro-neutrality theory, the total positive charge is equal to the total negative charge in the CO2/H2O/NaCl system, which is expressed as follows: + = + 2 + + (11) + where and are respective the concentrations of Na and Cl in the solution (mol/L), and = . By combining the equilibrium constant formulas with Eq. (11), a mathematical equation in terms of can be obtained:

CH3  

K4  K1K 2CCO2 2

0.662

CH 

2K1K 2 K3CCO2 0.6623

0

(12)

Meanwhile, according to the investigation of Duan and Sun [67], following equation:

lnCCO2  ln( yCO2 ψCO2 P)  where

l 0  μCO 2

RT

can be calculated using the

 2 λCO Na  C Na  2 λCO Cl- CCl-  ζ CO  Na Cl- CNa CCl- (13) 2 2 2

is the mole fraction of CO2 in the vapor phase.

is the fugacity coefficient of CO2. P is the

( )

total pressure of the system, bar. is the standard chemical potential of CO2 in the liquid phase; is the interaction parameter between CO2 and Na+; is the interaction parameter between CO2 and Cl ; ζCO2  Na Cl- is the interaction parameter between CO 2 and Na+, Cl . The interaction parameters and the standard chemical potential of CO2 in the liquid phase are all dependent upon the temperature and total pressure in the system, which are available in the literature [67]. R is the gas constant, 8.3145 J/mol/K. T is the temperature, K. By solving Eq. (6)-(9), (12) and (13), the concentrations of CO2 and H2CO3 at 9.5 and 1 MPa, as well as those of the other ions, can all be obtained; the results are listed in Table 5. As reported by Nesic [56], the corrosion rate increased as the partial pressure of CO2 increased in the case of the scale-free CO2 corrosion process. The commonly accepted explanation is that the concentration of H2CO3 increases with increasing CO2 partial pressure, which accelerates the cathodic reactions and, ultimately the corrosion rate. As listed in Table 5, the concentrations of H2CO3, HCO and H+ at 9.5 MPa are much higher than those at 1 MPa, thereby significantly accelerating the cathodic reactions. This is the main reason why the corrosion rate of X70 steel at 9.5 MPa was much higher than that at 1 MPa during the initial corrosion period, as shown in Fig. 1. Commonly, the anodic reaction in the CO2 corrosion process is the dissolution of Fe, and the cathodic reactions consist of the reduction of H2CO3, HCO and H+. The cathodic reactions and the corresponding equilibrium electrode potentials (described with Nernst equation) are presented as follows [68]:

H 2 CO 3  e   HCO 3  H

φ1  φ10 

RT CH 2 CO 3 ln nF C HCO 

(14)

3

HCO 3  e   CO 23   H

φ2  φ20 

RT C HCO 3 ln nF C CO 2 

(15)

3

2 H  2 e  H2

φ3  φ30 

RT ln C H2  nF

(16)

where, , and are respective the standard electrode potential of reactions (14-16), V; n is the number of the electrons transferred during the electrode reaction; F is the Faraday constant, J/mol/V. Based on the concentrations of H2CO3, HCO and H+, the equilibrium electrode potentials of reactions (14)-(16) at both 9.5 and 1 MPa can be calculated, the results are listed in Table 6. With the increase of CO2 partial pressure, the equilibrium electrode potential of each cathodic reaction undergoes a positive

shift, this is consistent with the observations of Jia et al. [69]. With the increase of the CO2 partial pressure, the solubility of CO2 in water increases, and the concentrations of H2CO3, HCO and H+ correspondingly increase. Reactions (14)-(16) are therefore accelerated, and the corrosion current increases. As a result, more electrons are needed, and the dissolution of Fe is thus accelerated. Although the increase of CO2 partial pressure increases the corrosion rate of steels, the equilibrium electrode potentials of reactions (14)-(16) differ only slightly between 9.5 and 1 MPa, which implies that the corrosion mechanism of steels under SC CO2 conditions is basically consistent with that at low CO2 partial pressure. Hassani et al. [70] employed potentiodynamic polarization measurements to investigate the corrosion behavior of X65 steel in CO2-saturated brine at 3 and 8 MPa. They observed that a higher cathodic reaction rate and a higher corrosion rate under SC CO2 conditions. However, the characteristics of the cathodic polarization curve at 8 MPa were the same as those at 3 MPa, which also indicated that the phase change of CO2 (gas to supercritical) did not affect the corrosion mechanism of the steels. Higher corrosion rate under SC CO2 environment results in more Fe2+ formation and a great quantity of Fe3C accelerating on the steel surface, which may affect the formation process of the corrosion product scale. As shown in Table 5, although the concentrations of CO were similar between 9.5 and 1 MPa, the pH value was much lower at 9.5 MPa, which was capable of enhancing the solubility of FeCO3 and decrease the scaling tendency. Therefore, a much higher supersaturation (S) of FeCO3 is achieved under SC CO2 conditions during the initial corrosion period. In addition, it is assumed that the rates of nucleation and crystal grain growth are both related to the relative supersaturation (SR, SR = S-1). Crystal growth dominates at low SR, and at higher SR, nucleation dominates. Consequently, the higher SR reached under SC CO2 conditions prevents the growth of FeCO3 crystals, and a nanocrystalline or amorphous scale forms [62]. As shown in Fig. 2(b) and Fig. 9(b), an amorphous scale formed on the X70 steel surface during the initial corrosion period at 9.5 MPa, while no scale formed at 1 MPa. The formation of the amorphous scale reduced the corrosion rate of X70 steel, and the concentration of Fe2+ consequently decreased. The amorphous scale blocked the transportation of Fe2+, which resulted in a higher concentration of Fe2+ accumulating in the amorphous scale than in the bulk solution near the steel surface. As a result, conditions were favorable for FeCO3 crystals to precipitate in the amorphous scale. After a certain immersion time, the amorphous scale had completely transformed into the inner FeCO3 layer, which was dense and exhibited better adhesion to the matrix, as shown in Fig. 7(b). Cheng et al. [71] observed that FeCO3 scales possessed anion selectivity, which can block Fe2+ penetration, thereby resulting in a lower SR in the bulk solution. Consequently, the growth of FeCO3 crystals dominated on the surface of the inner FeCO3 scale, and a porous outer FeCO3 scale with a larger grain size formed, as shown in Fig. 5. Fig. 16 shows the variation in scale thickness and FeCO3 crystal size with immersion time at 9.5 MPa. It was observed that the size of the FeCO3 crystals indeed increased with the increasing immersion time. Notably, at less than 96 h of immersion, the thickness of the corrosion product scale was increasing gradually, and once the immersion time exceeded 96 h, the thickness of scale remained essentially constant. This indicated that the density of the corrosion product scale continued to increase after 96 h of immersion, thus achieving better protectiveness. However, under low CO2 partial pressure (1 MPa) condition, the formation process of corrosion product scale was very different from that under SC CO2 condition. As shown in Fig. 1, the corrosion rate of X70 during the initial corrosion period at 1 MPa was much lower than that at 9.5 MPa, which resulted in a lower SR in the bulk solution. Therefore, crystal growth was dominant, and a FeCO3 crystal scale with poor adhesion to the matrix formed on the steel surface, as shown in Fig. 11(b). If the growth rate is less than the corrosion rate, then gaps among FeCO3 crystals could fill part of the scale, as a result, the FeCO3 scale formed was porous and exhibited non adhesion to the matrix [64]. Therefore, although an outer FeCO3 scale formed on the steel surface, the corrosion rate at 1 MPa was still high, and more Fe2+ was produced and accumulated in the solution between the FeCO3 scale and the matrix, which accelerated the formation of the inner FeCO3 scale. Although the corrosion mechanism of carbon steel between 9.5 and 1 MPa is basically the same, the formation mechanism of the corrosion product scale is very different. At 9.5 MPa, a dense, more protective inner FeCO3 scale first formed on the steel surface because of the presence of the amorphous layer, and a porous outer scale then formed above this inner scale. A model of the formation mechanism of the corrosion product scale under SC CO2 conditions is shown in Fig. 17. By contrast, at 1 MPa, a less protective outer FeCO3 scale first formed on the steel surface, followed by the formation of a porous inner FeCO3 scale.

4.2 Influence of corrosion product scale on localized corrosion To investigate the relationship between the corrosion type and the corrosion product scale formed at 9.5 and 1 MPa, the surface morphologies of the steel matrix after different immersion times were observed after removing the corrosion product scale, and a profilometer was employed to investigate the surface topography. As an example, the morphologies of X70 steel after 7 and 168 h of immersion at 9.5 MPa and after 24 and 168 h of immersion at 1 MPa are presented in Fig. 18. At 9.5 MPa, an amorphous layer covered the steel surface after 7 h of immersion, no localized corrosion was observed, and general corrosion was the principal corrosion type (Fig. 18 (a)). This indicated that the amorphous layer was capable of preventing localized corrosion. Similar results were observed by Guo et al. [62], who reported that amorphous scale exhibited a strong ability to help prevent localized corrosion. However, an FeCO3 scale covered the steel surface after 24 h of immersion, and localized corrosion was clearly observed, as shown in Fig. 18(b), implying that the formation of the FeCO3 scale at 9.5 MPa may result in localized corrosion of carbon steels. By contrast, regardless of whether FeCO3 precipitation had occurred, only general corrosion was observed on X70 steel at 1 MPa, as shown in Fig. 18(c) and (d). A summary of the general corrosion rate, the localized corrosion rate and the depth of localized corrosion at 9.5 MPa after 24 h of immersion is presented in Fig. 19. The localized corrosion rate was lower than the general corrosion rate, and both decreased with the increasing immersion time. The largest value of the localized corrosion depth was observed at 24 h; afterwards, this depth decreased with the increasing time. This finding implied that the initiation and development of localized corrosion occurred during the transitional period when the amorphous layer was transforming into the inner FeCO3 scale. Once FeCO3 crystals began to precipitate and grow in the amorphous layer, the internal stress in the amorphous layer increased with an increasing quantity of FeCO3 crystals. Therefore, parts of the amorphous layer cracked and spalled, resulting in the formation of holes, where aggressive ions could easily reach the matrix interface and corrode the matrix. Moreover, at the matrix interface where the amorphous layer disappeared, it was relatively difficult for FeCO 3 crystals to precipitate due to the lower pH value. Therefore, the active dissolution of the matrix was faster where the amorphous layer spalled than that it was where the matrix was covered by the FeCO3 scale, meaning that localized corrosion occurred, as shown in Fig. 17 (b). After a certain immersion time, because of the higher concentration of Fe2+ ions accumulated in the localized corrosion pits, FeCO3 precipitated on these localized corrosion pits and formed regions of the FeCO 3 scale that were thicker than those in the other areas where general corrosion occurred. The thicker FeCO3 scale was more protective, therefore these areas then began to corrode more slowly while the other areas with the thinner FeCO3 scale corroded more rapidly. Ultimately, the scale/matrix interface became flat, and the localized corrosion depth decreased. However, as shown in Fig. 19, the localized corrosion depth decreased only gradually after 48 h of immersion at 9.5 MPa. This indicated that once the FeCO3 scale thickness was greater than a certain value (~ 100 μm) under SC CO2 conditions, the effect of the differing FeCO3 scale thickness on the corrosion of the steel matrix was lower, and the localized corrosion depth then remained essentially unchanged with the increasing immersion time. 4.3 The relationship between the general corrosion rate and the mechanical properties of the corrosion product scale Investigations performed by Zhang et al. [16], Gao et al. [46-47] and Schimtt et al. [72] showed that the mechanical characteristics of corrosion product scales, especially the Young’s modulus (E) and fracture toughness (KIC), strongly affect the CO2 corrosion process of steels. Fig. 20 shows the variation in the Young’s modulus of FeCO3 corrosion product scales with immersion time at both 9.5 and 1 MPa. A comparison between the general corrosion rates presented in Fig. 1 and the Young’s modulus results presented in Fig. 20 suggests that the general corrosion rate exhibited and inverse relationship with the Young’s modulus – it decreased as the Young’s modulus increased. The E value of the FeCO3 scales formed at both 9.5 and 1 MPa increased gradually with the increasing immersion time, and the E of the FeCO3 scale formed at 9.5 MPa was higher than that of the scale formed at 1 MPa. As calculated by Ramachandran et al. [73], the Young’s modulus increases with a decrease in the porosity ɛ, which is related to the density of the corrosion product scale. The higher Young’s modulus of the FeCO3 scale formed at 9.5 MPa indicated that this scale was denser than that formed at 1 MPa after the same immersion time, implying that the corrosion product scale formed at 9.5 MPa was more protective. Two semi-empirical formulas have been introduced to describe the relationship between the Young’s modulus and the porosity ɛ, as shown in Eq. (17) and (18) [73]:

E  E0 1  ε 

3

(17)

E  E0 exp  bε 

(18) where E0 is the Young’s modulus of pure FeCO3 crystal, and b is a tunable parameter. According to Ramachandran et al. [73], Eq. (18) demonstrates better reproducibility, and therefore, this is the equation used in this study. The volumetric porosity ɛ, as the principal scale parameter, significantly affects the permeation of species into the corrosion product scale. The relation between the geometry of the pores in the corrosion product scale and the permeability of the scale in terms of porosity is expressed in Eq. (19) [74]. If the corrosion product scale is porous and loose (with a high Vvoid, i.e., ɛ is high), it cannot effectively prevent the permeation of aggressive ions, and a high corrosion rate is expected. If the scale is dense (with a low Vvoid, i.e., ɛ is low), it can effectively block the permeation of aggressive ions, and thus, low corrosion rate is expected [46].

ε

V FeCO 3 V void V total  V FeCO 3  1  V total V total V total

(19)

where Vvoid, and Vtotal are the volume of voids in the corrosion product scale, the volume of FeCO3 crystal in the scale and the entire volume of the corrosion product scale, respectively. Gao et al. [42] investigated the relationship between the porosity ɛ of the corrosion product scale and the corrosion rate, and found that the corrosion rate increased linearly with the increase of porosity ɛ, as expressed in Eq. 20: RCorr  hε  a (20) where h and a are the slope and intercept of the straight line with a unit of mm/y, respectively. Therefore, by combining Eq. (18) with Eq. (20), it can be confirmed that the corrosion rate exhibited a logarithmic relationship with the Young’s modulus. The relationship between the general corrosion rate and the Young’s modulus, as well as the fitted curves, is shown in Fig. 21; the corresponding formulas are presented as follows (Eq. (21) for 9.5 MPa; Eq. (22) for 1 MPa): RCorr  87.25  21.79lnE  94.5 (21)

RCorr  32.85  8.53lnE 101

(22) According to Eq. (21) and (22), when the Young’s modulus of the corrosion product scale is below 101 GPa at 1 MPa or below 94 GPa at 9.5 MPa, the presence of the corrosion product scale will enhance the corrosion rate, indicating that the corrosion product scale has no protectiveness to the steel surface. When the Young’s modulus of the corrosion product scale is greater than 150 GPa both at either 1 or 9.5 MPa, the corrosion rate calculated from Eq. (21) and (22) is close to zero, indicating that the corrosion product scale has an excellent protectiveness to the steel matrix. When the Young’s modulus is between 101 GPa and 150 GPa, the corrosion rate strongly decreases with the increase of Young’s modulus. Notably, because of the higher slope at 9.5 MPa (21.79) compared with that at 1 MPa (8.53), the corrosion rate decreases more rapidly at 9.5 MPa than at 1 MPa, implying that a more protective FeCO3 scale forms under SC CO2 conditions. According to the simulation performed by Ramachandran [73], a Young’s modulus of 101 GPa corresponds to a porosity of 40% and a Young’s modulus of 150 GPa is corresponding to a porosity of 20%. This implies that when the porosity of the FeCO3 scale is greater than 40%, the FeCO3 scale has no protectiveness, and when the porosity of the FeCO3 is lower than 20%, the FeCO3 scale is highly protective and essentially no corrosion occurs on the steel surface. Generally, as summarized in Table 7 [16, 46-48, 75], the Young’s modulus of an FeCO3 corrosion product scale is main between 100 and 150 GPa. The fracture toughness of the corrosion product scales formed after 168 h of immersion at 9.5 and 1 MPa is shown in Fig. 22. The fracture toughness at 9.5 MPa was higher than that at 1 MPa. It is known that during CO2 corrosion process, a scale with a lower fracture toughness will more easily crack and spall, degrading the protectiveness of the scale. Zhang et al. [16] also found that the corrosion rate decreased with the increase of fracture toughness. Therefore, the higher fracture toughness of the scale formed at 9.5 MPa also implied that the corrosion rate at 9.5 MPa was lower than that at 1 MPa after 168 h of immersion. 5. Conclusions The corrosion behavior of X70 steel in CO2-saturated NaCl solutions at 9.5 and 1 MPa were investigated,

and both the corrosion mechanism of the carbon steel and the formation mechanism of the corrosion product scale were considered. From this study, the following conclusions can be drawn: 1. The general corrosion rate of X70 steel under SC CO2 condition was higher than that under low CO2 partial pressure condition during the initial corrosion period before FeCO3 precipitation. Once the FeCO3 corrosion product scale completely covered the steel surface, the corrosion rate under SC CO2 condition was lower than that under low CO2 partial pressure condition. 2. Although the corrosion mechanism of X70 steel is the same under both SC CO2 and low CO2 partial pressure conditions, the formation mechanism of the corrosion product scale was quite different. Under SC CO2 conditions, an amorphous layer preferentially formed on the steel surface, which gradually evolved into the dense inner FeCO3 layer of the corrosion product scale, and finally the relatively porous outer FeCO3 scale formed above this inner layer. By contrast, under low CO2 partial pressure condition, a dense outer FeCO3 scale was first to form, followed by the formation of a porous but thick inner layer. The outer corrosion product scale formed at 1 MPa offered better protectiveness than that formed at 9.5 MPa, while the inner corrosion product scale formed at 9.5 MPa exhibited better protective than that at 1 MPa. The inner layer of the corrosion scale played the primary protective role, especially under SC CO2 conditions. 3. In the SC CO2 environment, the formation of the amorphous layer caused general corrosion to be the predominant corrosion type the prior to FeCO 3 precipitation. Localized corrosion occurred during the transitional stage as the amorphous layer evolved into the inner FeCO3 scale, and the rate of this localized corrosion decreased with the growth of FeCO3 corrosion product scale. By contrast, the predominant corrosion type of X70 steel during CO2 corrosion process under low CO2 partial pressure conditions was always general corrosion. 4. The Young’s modulus of the scale that formed under SC CO2 conditions was higher than that of the scale formed under low CO2 partial pressure conditions, and it exhibits logarithmic relationship with the general corrosion rate. As the increase of immersion time, the number of defects in the corrosion product scale reduced, leading to a decrease in scale porosity. A lower scale porosity implied a higher Young’s modulus, which led to the decrease in the corrosion rate. Moreover, the fewer the defects in scale was, the more complete and denser the scale was, and the lower the fracture toughness was, indicating that the scale exhibited better protectiveness. Therefore, the FeCO3 scale formed under SC CO2 conditions presented better protective characteristics than that formed at low CO2 partial pressure, and the corrosion rate under SC CO2 conditions was lower than that under low CO2 partial pressure conditions. Acknowledgments This study is supported by the Beijing Natural Science Foundation Major Project (No. 2131004) and the National Natural Science Foundation of China (No. 51271024). Reference [1] E. Rubin, L. Meyer, H. Coninck, Carbon dioxide capture and storage: technical summary, IPCC Special Report, 2005. [2] B. O. Metz, H. Davidson, C. De Coninck, M. Loos, L. A. Meyer, IPCC Special Report on Carbon Dioxide Capture and Storage, prepared by Working Group III of the Intergovernmental Panel on Climate Change (Cambridge, U. K. New York, NY, USA: Cambridge University Press, 2005). [3] L. D. Carter, Capture and storage of CO2 with other air pollutant, no. CCC/162, 2010, IEA Clean Coal Centre, London. [4] J. Gale, J. Davison, Transmission of CO2 - safety and economic considerations. In: 6th Int. Conf. Greenh. Gas Control Technol., 29, 2004: p. 1319-1328. [5] I. S. Cole, P. Corrigan, S. Sim, N. Birbilis, Corrosion of pipelines used for CO2 transport in CCS: is it a real problem?, Int. J. Greenh. Gas Control 5, 2011: p. 749-756. [6] J. J. Dooley, R. T. Dahowski, C. L. Davidson, Energy Procedia 1, 1, 2009: p. 1595-1602. [7] IEA, Energy Technology Perspectives 2010, Scenarios & Strategies to 2050. [8] A. Dugstad, M. Halseid, Internal corrosion in dense phase CO2 transport pipelines - state of the art and the need for further R&D, Corrosion/2012, NACE International, Houston/TX, 2012, paper no. 1452. [9] Y. Xiang, Z. Wang, C. Xu, C. Zhou, Z. Li, W. Ni, J. Supercrit. Fluids 58, 2011: p. 286-294. [10] F. Farelas, Y. S. Choi, S. Nesic, Corrosion behavior of API 5L X65 carbon steel under supercritical and liquid carbon dioxide phases in the presence of water and sulfur dioxide, Corrosion 69, 3 (2013): p. 243-250.

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9.5 MPa 1 MPa

General corrosion rate, mm/y

100

10

1

0

50

100

150

200

250

Immersion time, h

300

350

400

Fig. 1 General corrosion rates of X70 steel with immersion time under SC CO2 (9.5 MPa) and low CO2 partial pressure (1 MPa) conditions at 80 °C.

(b)

(a)

100 μm (c)

100 μm (d)

Fig. 2 Surface and cross-section morphologies of corrosion product scales formed on X70 steel at 9.5 MPa Hole immersion times: (a) Surface morphology after 0.5 h of immersion; (b) Surface and 80 °C with different 40 μm(d) Cross-section morphology after 2 h of immersion;100 (c) μm Surface morphology after 7 h of immersion; morphology after 7 h of immersion.

Fe

Fe3C

FeCO3

Intensity, CPS

7h

2h

0.5 h 20

40

60

80

100

2 , deg

Fig. 3 XRD spectra of corrosion product scale formed on X70 steel at 9.5 MPa and 80 °C after 0.5, 2 and 7 h of immersion.

(a)

C 1s spectra Absorbed C

Intensity

7000

6000 FeCO3 Fe3C

5000

4000

Intensity

8000

4.0x10

4

3.5x10

4

3.0x10

4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

(b)

Fe 2p spectra

FeCO3 Fe2O3 Fe

Fe 2p3/2

Fe 2p1/2

Fig. 4 XPS spectra of the original scale formed on X70 steel at 9.5 MPa and 80 °C after 7 h of immersion. 3000 275

280

285

Binding energy, eV

290

295

700

710

720

730

Binding energy, eV

740

(b)

(a)

Hole 200 μm

400 μm

400 μm (d)

(c)

200 μm

400 μm

400 μm (e)

(f)

Fig. 5 Surface morphology of corrosion product scales formed on X70 steel at 9.5 MPa and 80 °C after (a) 12, (b) 24, (c) 50, (d) 96, (e) 168 and (f) 384 h of immersion.

400 μm

400 μm

Intensity, CPS

FeCO3

Fe

Fe3C

384 h 96 h 50 h 24 h 12 h

20

40

60

80

100

2 , deg

Fig. 6 XRD spectra of corrosion product scale formed on X70 steel at 9.5 MPa and 80 °C after 12, 24, 50, 96 and 384 h of immersion.

(b)

(a)

Hole

100 μm

100 μm (d)

(c)

Hole

Localized corrosion 100 μm

100 μm (f)

(e)

Fig. 7 Cross-section morphologies of corrosion product scales formed on X70 steel at 9.5 MPa and 80 °C after (a) 12, (b) 24, (c) 50, (d) 96, (e) 168 and (f) 384 h of immersion.

100 μm

100 μm

(b)

(a)

Inner layer

Outer layer

Inner layer

Boundary line

Intensity, a.u.

200

150

100

50

MnCO3

Fe C

3 20 μm FeCO3 Fig. 8 Boundary line (a) between the outer FeCO3 scale and the inner FeCO scale3 formed on X70 steel at 9.5 500 Raman 1000spectroscopy 1500 2000 2500inner 3000 MPa and 80 °C after 168 h of immersion and the corresponding (b) of the scale. -1

Wavenumbers shift, cm

(b)

(a)

40 μm

40 μm (d)

(c)

40 μm

40 μm (e)

(f)

Fig. 9 Surface and cross-section morphologies of the corrosion product scales formed on X70 steel at 1 MPa and 80 °C with different immersion times: (a) Surface morphology after 0.5 h of immersion; (b) Surface morphology after 2 h of immersion; (c) Surface morphology after 12 h of immersion; (d) Surface morphology after 24 h of immersion; (e) Cross-section morphology after 12 h, (f) Cross-section 20 μm morphology after 24 h of immersion. 20 μm

Intensity, CPS

Fe

Fe3C

24 h 12 h 7h 2h 0.5 h

Fig. 10 XRD spectra of corrosion product scales formed on X70 at 1 MPa and 80 °C steel after 0.5, 2, 7, 12 and 24 h of immersion. 20 40 60 80 100 2, deg

(a)

(b)

200 μm

100 μm

400 μm (d)

(c)

400 μm (e)

Inner layer

100 μm (f)

200 μm

400 μm

100 μm (h)

(g)

Boundary Fig. 11 Surface and cross-section morphologies of corrosion product scales formedline on X70 steel at 1 MPa and 80 °C with different immersion times: (a) and (b) Surface and cross-section morphologies after 50 h of Outer layer immersion; (c) and (d) Surface and cross-section morphologies after 96 h of immersion; (e) and (f) Surface and cross-section morphologies after 168 h of immersion; (g) and (h) Surface and cross-section Inner layer morphology after 384 h of immersion. 400 μm

100 μm

FeCO3

Intensity, CPS

168 h

96 h

50 h

20

40

60

80

100

2, deg

Fig. 12 XRD spectra of corrosion product scales formed on X70 steel at 1 MPa and 80 °C after 50, 96 and 168 h of immersion.

8000

6000

(b)

24 h 50 h 168 h

(a) 6000

50 h 168 h Calculated

Calculated 2

-Zim,  cm

4000 0.1 Hz 1.72 Hz

2000

2000

1.7 Hz 0.5 Hz 0.01 Hz

0.01 Hz

0 0

2000

4000

6000

0

8000

0

2000

2

(c)

4000

6000

2

Zre,  cm

4

Zre,  cm

50 h

4

60

(d)

50 h

80 70

50

2

20

2

log IZI,  cm

30

Phase, deg

2

40

3

50 40

2

30 20

Phase, deg

60

3 log IZI,  cm

-Zim,  cm

2

4000

Fig. 13 EIS plots of X70 steel at 80 °C under SC CO 2 (9.5 MPa) and low CO2 partial pressure (1 MPa) 10 conditions with different immersion times: (a) Nyquist plots of X70 steel after 24, 50 and 168 h10 of immersion at 9.5 MPa; (b) Nyquist plots of5 X7006 steel after 1 1 50 and 168 h of immersion at 1 MPa; (c) Bode 0 -3 -2 -1 0 1 2 3 4 -3 -2 -1 0 1 2 3 4 5 6 10 of X70 10 10 10 10 10 10 steel 10 after 10 10 h 10 10 10 at plot steel 10 after10 50 h10of immersion at 9.510MPa; (d)10 Bode10plot10of X70 50 of immersion Frequency, Hz Frequency, Hz 1 MPa.

Fig. 14 Equivalent circuit used to fit the measured EIS plots.

10000

R,  cm

2

1000

9.5 MPa - 24 h 9.5 MPa - 50 h 9.5 MPa - 168 h 1 MPa - 50 h 1 MPa - 168 h

100

10

Fig. 15 Variations of the resistance of outer layer scale (R2), inner layer scale (R1) and the charge transfer 1 time. resistance (Rt) with immersion R R R 2

1

t

140

60 Thickness

120 50

40

80 Grain size

60

30

40

Grain size, m

Thickness, m

100

20 20 0 thickness of the scale formed on X70 steel at 9.5 MPa 10 and 80 °C with immersion Fig. 16 Variations of the 0 50 100 150 200 250 300 350 400 time and the corresponding grain size of FeCO3 crystals. Immersion time, h

(a)

(b)

(c) Fig. 17 Formation mechanism of the corrosion product scale on steels under supercritical CO2 condition: (a) Formation of the inner amorphous layer; (b) Formation of the inner FeCO3 corrosion product scale; (c) Formation of the outer FeCO3 corrosion product scale.

(b)

(a)

200 μm

200 μm (c)

(d)

Fig. 18 Surface morphologies of X70 steel with different immersion times after corrosion product scales removed: 200 200 μm (a) Surface morphology of X70 steel at 9.5μm MPa after 7 h of immersion; (b) Surface morphology of X70 steel at 9.5 MPa after 168 h of immersion; (c) Surface morphology of X70 steel at 1 MPa after 24 of immersion; (d) Surface morphology of X70 steel at 1 MPa after 168 h of immersion.

Corrosion rate, mm/y

35

70 General corrosion rate Localized corrosion rate

30

60

25 20

50

15 10

40

5

Depth of localized corrosion, m

40

Fig. 19 General corrosion0 rates, localized corrosion rates and the depth of localized corrosion of X70 steel 30 under SC CO2 condition at09.5 MPa and 80 °C with immersion time. 2 4 6 8 10 12 14 16 t, d

150

9.5 MPa 1 MPa

EYm, GPa

140 130 120 110 100

Fig. 20 Variation of Young’s modulus (E) of the corrosion product scales formed on X70 steel at both 9.5 MPa and 1 MPa with immersion 0 2time. 4 6 8 10 12 14 16 t, d

50

120

(a)

(b)

9.5 MPa

1 MPa

40 Corrosion rate, mm/y

Corrosion rate, mm/y

100 80 60 40 20

30

20

10

Fig. 21 Fitted curves for the general corrosion rate of0X70 steel and the Young’s modulus of corrosion 0 100 110 120 130 140 150 90 100 1109.5 MPa, 120 (b)130 150 product scale: (a) 1 MPa.140 E , GPa EYm, GPa

Ym

KIC, MPam

1/2

1.5

1.0

0.5

0.0 1 MPa

9.5 MPa

Fig. 22 Fracture toughness (KIC) of scales formed on X70 steel with 168 h of immersion at both 9.5 and 1 MPa.

Table 1 Summery of corrosion of carbon steel in SC CO2 conditions PCO2 (MPa)

Researcher

Steels

Sim [25]

Carbon steel

8

Hua [13]

X65

Hua [13]

T (°C)

Time (d)

System

40

7

CO2-rich phase with various water content (244-122000ppmv)

8

35

2

CO2-rich phase with various water content (300-3437ppm)

X65

8

50

2

CO2-rich phase with various water content (700-3400ppm)

Choi [19,29-30]

X65

8

50

1

Xiang [31]

X70

10

50

5

Dugstad [20]

X65

10

10-50

14

Alami [32]

X65

15

80

14

Alami [32]

X65

15

80

12

Mohammed [33] Mohammed [33] Suhor [34] Choi [29-30, 35] Choi [29-30] Kongshaug [36] Seiersten [37] Dugstad [20] Dugstad [20] Cabrini [38]

X65 X65 X65 X65 X65 X65 X65 X65 X65 X65

8 8 8 8 8 8 8.5-9.5 10 10 12.3-

25 80 80 50 50 50 40 13 50 25-60

1 1 14 1 5 5-7 7-14 2-4 10-20 48-400h

CO2-rich saturated with H2O CO2-rich phase with 0.01MPa O2 and 0.2 MPa SO2 and various fraction of water (9-100 RH%) CO2-rich phase saturated with water CO2-rich phase saturated with water and 3v%H2S CO2-rich phase saturated with water and 1000ppmSO2 1wt%NaCl 1wt%NaCl 1wt%NaCl Deionized water Deionized water 10g/L NaCl Water-rich phase Water-rich phase Water-rich phase Water-rich phase

Corrosion rate, mm/y General corrosion rate: Water-undersaturated ~0.08 Water-saturated 0.11-0.17 Pitting corrosion rate: ~0.62 General corrosion rate: Water-undersaturated 0.0040.068 Water-saturated ~ 0.1 Pitting corrosion rate: 0.3-0.9 General corrosion rate: Water-undersaturated 0-0.014 Water-saturated ~ 0.024 Pitting corrosion rate: 2650ppm 0.2; 3400ppm 1.4 0.38 Water-undersaturated: 0.0387 - ~0.8 Water-saturated: ~1.5 0.5-2.7 >0.3 0.2-0.9 ~8 ~25 2.7 19.2 10.6 4.6 1.7 3-5 3-40 5-30

Zhang [39] Cui [40] Hua [41]

14.6 9.5 8.274 8

X65 C steel X65

C 0.07

80 90 50

0.5-168h 1-6 6.5h-96h

Deionized water Water-rich phase Deionized water

7.26-28.3 2-10 4-10.8

Table 2 Chemical compositions of X70 steel (wt.%) Mn Mo Cr Nb V Ti 1.58 0.17 0.035 0.048 0.026 0.015

Si 0.23

CO2 pressure (MPa)

Cu 0.14

Fe bal.

Table 3 Immersion conditions in these experiments Temperature (°C) Electrolyte Immersion time (h)

9.5 (Supercritical CO2)

3.5 wt.%NaCl 2L

80 1 (Low CO2 partial pressure)

HCl (ρ=1.19g/ml) (mL) 1000

0.5, 2, 7, 12, 24, 50, 96, 168, 384

Table 4 Compositions of 1L stripping solution Sb2O3 (g/L) SnCl2 (g/L) 20 50

Table 5 Ionic concentrations (mol/L) of all species in CO2-saturated 3.5 wt.% NaCl solution at 80 °C P (MPa) 9.5 1

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0.7778 0.1097

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-3

2.007×10 2.830×10-4

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-3

1.230×10 4.391×10-4

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-10

5.820×10 5.377×10-10

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-10

3.993×10 1.118×10-9

pH -3

1.229×10 4.391×10-4

2.9 3.4

Table 6 Equilibrium electrode potentials (V vs. SCE) of cathodic reactions (14-16) P (MPa) φ1 φ2 φ3 9.5 -0.3611 -0.1678 -0.4079 1 -0.3891 -0.1968 -0.4705 Table 7 Summary of Young’s modulus of CO2 corrosion product scale in the previous literature Young’s modulus (GPa) Reference 115-150 [16] 85-115 [47] 100-110 [46] 100-120 [48] 125 [75]