Passivity of 13Cr Stainless Steel in 1% NaCl Solution under Dynamic

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Passivity of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition Yanyan Li, Zhunzhang Wang, Mifeng Zhao, and Guoan Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01364 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Industrial & Engineering Chemistry Research

Passivity of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition Yanyan Li,†

Zhunzhang Wang, † Mifeng Zhao,‡

and Guoan Zhang∗, †

†

Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of

Education, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China ‡

Oil and Gas Engineering Research Institute of PetroChina Tarim Oilfield Company, Korla,

Xinjiang 841000, P.R. China

Abstract: The passivity of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 (SC-CO2) condition without or with different partial pressures of O2 was studied by electrochemical measurements and surface characterization. Electrochemical measurements indicate that both the flow fluid and the increase in CO2 pressure lead to the decrease in the stability of passive film. Introduction of 0.1 MPa O2 into the dynamic SC-CO2 system facilitates the formation of stable and integrated passive film, which enhances the corrosion resistance of 13Cr stainless steel. However, the addition of 1 MPa O2 reduces the stability of passive film and results in the formation of passive film with high donor density, which decreases the corrosion resistance of 13Cr stainless steel.

Keywords: 13Cr stainless steel; Supercritical CO2; Passivity; XPS; Mott-Schottky analysis

∗

Corresponding author; Tel.: +86-27-87559068; Fax: +86-27-87543632 E-mail address: [email protected] (G.A. Zhang) 1

ACS Paragon Plus Environment

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1. Introduction Petroleum, as a kind of fossil fuel, has the highest consumption rate in today’s world. Therefore, the technology for oilfield production is the topic that human concerns. For the extra-low permeability oilfield, it is hard to implement normal water flooding for oil recovery efficiently. Air injection is a new technology to enhance the oil recovery significantly, i.e., air injection enhanced oil recovery (EOR), which could solve the exploitation difficulty of extra-low permeability oilfield.1, 2 Compared to other gases (N2, CO2) injection technology, air injection is becoming the development trend for EOR due to its unique advantages, i.e., abundant source and low cost. 3, 4 Air injection EOR is a cost-effective technology for oil exploitation. However, as the injection of air into the oilfield, the oxidation of crude oil would produce carbon dioxide and lead to an environment with the coexistence of CO2 and O2, which causes a serious corrosion problem of steel pipes. The corrosion failure of steel pipes in supercritical CO2 (SC-CO2) environment is widely recognized.5-10 Choi et al.11 revealed that under SC-CO2 condition, the corrosion of steel was significantly aggravated due to the addition of O2. The corrosion of steel pipes greatly retards the development of air injection technology. Some studies found that stainless steels have better corrosion resistance than carbon steels in SC-CO2 environment.11-13 Hua et al.13 compared the corrosion resistance of X65, 1Cr, 5Cr, 13Cr steels in SC-CO2 environment containing H2O/SO2/O2, and found that 13Cr stainless steel exhibited better corrosion resistance. Choi et al.11 also confirmed that 13Cr stainless steel presented excellent corrosion resistance in SC-CO2 environment without or with O2.

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13Cr stainless steel has been used in oilfield because of its better corrosion resistance in SC-CO2 environment compared to carbon steels and its lower cost compared to other stainless steels.14-19 In a corrosive CO2 system, 13Cr stainless steel presents superior corrosion resistance due to the formation of protective passive film on the surface, which prevents the contact between the steel and corrosive environment. The passive film on stainless steel generally has a semiconductor property, and the p-type or n-type behavior of the passive film is dependent upon the principal defect in the passive film.20-23 Previously, some studies on the corrosion of 13Cr stainless steel under SC-CO2 condition were mainly focused on weight loss test. For better understanding the corrosion resistance of 13Cr stainless steel, it is crucial to investigate its electrochemical behavior and the semiconductor properties of its passive film. However, there are few works on the electrochemical properties of the passive film formed on 13Cr stainless steel in SC-CO2 environment. Hence, the electrochemical behavior and semiconductor properties of 13Cr stainless steel in SC-CO2 environment are still not clear. For air injection EOR, although the O2 content in producing well during the normal production process is low,2 severe channeling and the earlier O2 breakthrough into producing well due to the incomplete oxidation and the unfavorable high mobility ratios of fluid in the highly heterogeneous reservoir could lead to a high oxygen content in producing well.24-27 Therefore, in order to understand the passivity of 13Cr stainless steel used in the oilfield with air injection EOR, it is necessary to determine

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the role of oxygen in the passivity of 13Cr stainless steel. Some researchers have investigated the effect of oxygen on the electrochemical behavior of stainless steel. Feng et al.28 found that dissolved oxygen increased the donor density of the passive film formed on 316L stainless steel in borate buffer solution, and accelerated the cathodic and anodic processes. However, it is still not clear how the oxygen affects the properties of passive film formed on 13Cr stainless steel in SC-CO2 environment. This work aims to study the passivity of 13Cr stainless steel in SC-CO2 environment by electrochemical measurements and surface characterization. The corrosion resistance and semiconductor properties of the formed passive film were investigated to understand the effects of flow fluid and O2 on the properties of the passive film formed on 13Cr stainless steel in SC-CO2 environment.

2. Experimental 2.1. Material and solution The material used in this experiment was 13Cr stainless steel, which was composed of 0.08 wt% C, 1.00 wt% Si, 1.00 wt% Mn, 0.035 wt% P, 0.030 wt% S, 13.05 wt% Cr, 0.50 wt% Ni and Fe balance. For electrochemical measurements, specimens were machined into 5 mm × 4 mm × 2.5 mm, and then embedded into epoxy resin with an exposed area of 0.2 cm2. Meanwhile, for surface characterization, two specimens with the dimension of 8 mm × 10 mm × 2.5 mm were also used for each condition. Before the corrosion test, all specimens were abraded with 800 grit SiC paper, cleaned with acetone and deionized water. The test solution was 1% NaCl

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solution.

2.2. Setup for electrochemical measurements in SC-CO2 environment Electrochemical measurements were conducted under high pressure dynamic or static conditions in a 3 L autoclave. For conducting in situ electrochemical measurements, two PTFE concentric cylinders, i.e., an outer cylinder with an internal diameter of 76 mm as specimens’ holder and an inner cylinder with a diameter of 72 mm as rotator, were used, as shown in Figure 1. The measurements under dynamic condition were realized by this setup when the inner cylinder rotated and the outer cylinder (specimens’ holder) kept static. The linear velocity at the surface of rotator was 2 m/s with a rotation rate of 530 rpm. According to field investigation, the mole fraction of O2 in the producing well usually varies in the range from 0.64 % to 1.34 %.2 Therefore, an oxygen partial pressure of 0.1 MPa (1.25%) was selected in this work. Additionally, considering that severe channeling and the earlier O2 breakthrough into producing well may occur, which results in a high concentration of oxygen (up to approximate 8%) in producing well,24-27 the steel pipes may encounter the corrosion risk of high O2 concentration, especially in the bottom of producing well. Furthermore, to explore the effect of high O2 partial pressure on the passivity of 13Cr stainless steel during the air injection EOR process, O2 partial pressures of 1 MPa was also selected. Before the corrosion test, for the experiment only with CO2 (without O2), the solution was purged with CO2 (99.99%) for 12 h to remove the dissolved O2, while for the experiment with O2 and

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CO2, the solution was purging O2 (99.99%) for 12 h. Then, specimens for electrochemical measurements and surface analysis were installed in the inner surface of the outer cylinder holder, and the solution was added into the autoclave. O2 was injected to the desired pressure, and then CO2 was injected into the autoclave to obtain the desired pressure (8 MPa) through a booster pump.

2.3. Electrochemical measurements For in situ electrochemical measurements, an electrochemical workstation was used with a three-electrode system. A L-like PTFE tube with a porous ceramics cap at the end (filled with 0.1 M KCl solution), a 13Cr stainless steel specimen and a platinum plate were embedded in the inner wall of the outer cylinder. The 13Cr stainless steel specimen and platinum plate were used as working electrode and counter electrode, respectively. A Ag/AgCl electrode was inserted into the PTFE tube as reference electrode. In this study, all potentials were referred to this Ag/AgCl electrode (0.1 M KCl solution). Potentiodynamic polarization curves were recorded at a scanning rate of 0.5 mV/s. Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential (OCP) with the frequency range from 10,000 Hz to 0.01 Hz and an amplitude of 5 mV. The obtained impedance spectra were analyzed using an equivalent circuit. Mott-Schottky analyses were conducted from -0.6 V to 0.6 V at a frequency of 1000 Hz using an amplitude of 5 mV with a step rate of 10 mV. Mott-Schottky relationship between the potential and the capacitance of space-charge layer can be expressed as:28

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κT 1 2 ( E − ϕ fb − ) = eε r ε 0 N D e C 2SC 1 C

2 SC

=−

2

eε r ε 0 N A

( E − ϕ fb −

κT e

)

for n-type semiconductor

(1)

for p-type semiconductor

(2)

where Csc is the capacitance of space-charge layer, which is calculated according to the frequency (f) and the imaginary part of impedance (Z") ( C sc =

1 );29 e is '' 2 π fZ

electron charge (1.6×10-19 C); ε r is dielectric constant of Fe oxide (15.6);

30

ε0 is the

vacuum permittivity (8.85×10-14 F/cm); E is the applied potential; φfb is flat-band potential; κ is Boltzmann constant (1.38×10-23 J/K) and T is absolute temperature; ND and NA are the donor and acceptor densities, which are calculated from the slope of the linear plot of Csc-2 vs. E. For all electrochemical experiments, the electrodes were polarized potentiostatically at -1.5 V vs. Ag/AgCl (0.1 M KCl solution) for 5 min at the beginning to remove the air-formed oxide film. All the experiments were performed at 60 oC.

2.4. Surface characterization After corrosion test, the specimens for surface characterization were removed from the autoclave, and rinsed with deionized water. The corrosion morphologies were observed by scanning electron microscope (SEM) with an accelerating voltage of 15 kV. The composition of passive film was analyzed by X-ray photoelectron spectroscopy (XPS) using a monochromatic X-ray Al-Ka source with a hemispherical electron analyzer operating at a pass energy of 25 eV. The binding energy values were calibrated with reference to the C1s peak at 284.6 eV. The peak fitting was performed with the commercial XPSpeak software with Shirley background subtraction.

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3. Results 3.1. Open circuit potential measurements Figure 2 shows the time dependence of the open circuit potentials (OCPs) of 13Cr stainless steel under static or dynamic supercritical CO2 (SC-CO2) conditions, or dynamic atmospheric pressure CO2 (AP-CO2, 0.1 MPa) condition. Usually, there is a close relationship between the OCP and the stability of the passive film of stainless steel. The more positive OCP, the better passivity of stainless steel.31 It can be seen that under static SC-CO2 condition, the OCP shifts to positive direction rapidly before 8 h. The OCPs under dynamic SC-CO2 and AP-CO2 conditions also show a sharp positive shift at the beginning (before about 2 h). The positive shift of OCP is associated with the formation of passive film on 13Cr stainless steel surface. After the rapidly positive shift, the OCPs under static SC-CO2 and dynamic AP-CO2 conditions reach a relatively stable value, indicating the stability of passive film. The OCP under dynamic SC-CO2 condition also presents a platform at the period of 4-8 h and then shifts rapidly to negative direction until it reaches a stable value. This situation suggests that flow fluid lowers the stability of passive film formed under dynamic SC-CO2 condition. Therefore, the OCP under static SC-CO2 condition is more positive than that under dynamic SC-CO2 condition. Figure 3 shows the time dependence of the OCPs of 13Cr stainless steel under dynamic SC-CO2 condition with different O2 partial pressures. In the SC-CO2 system with 0.1 MPa O2, the OCP shifts rapidly to positive direction due to the formation of

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passive film and then presents a slowly positive shift until it reaches a stable value about -0.140 V at 15 h. For dynamic SC-CO2 condition with 1 MPa O2, the OCP also shifts rapidly to positive direction at the beginning, and then decreases sluggishly to a stable value (about -0.112 V), which may be related to the gradual degradation of the passive film. The OCP under SC-CO2 condition containing O2 is more positive than that under SC-CO2 condition without O2, which may be attributed to the acceleration of cathodic process in the presence of O2. Moreover, the OCP of 13Cr stainless steel increases with increasing O2 partial pressure.

3.2. EIS measurements EIS Nyquist plots of 13Cr stainless steel under static or dynamic SC-CO2, or dynamic AP-CO2 conditions for different times are shown in Figure 4. It can be seen that all impedance spectra present an extended loop, which is actually composed of two overlapping capacitive loops with close time constants. The capacitive loop in high frequency range is attributed to the double layer capacitance and charge transfer resistance while the capacitive loop in low frequency range is related to the formed passive film. The similar impedance spectra under these conditions indicate the same corrosion mechanism of 13Cr stainless steel in CO2-water system. Figure 5 shows the EIS Nyquist plots of 13Cr stainless steel under dynamic SC-CO2 with different O2 partial pressures for different times. It can be seen that the impedance spectra present the same feature as that under dynamic SC-CO2 condition without O2, i.e., an extended loop consisting of two capacitive loops at high and low frequencies.

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To quantify the electrochemical parameters, an electrical equivalent shown in Figure 6 was used to fit the EIS data, where Rs represents solution resistance; Qdl and Rct are the double layer capacitance and charge transfer resistance; Qf and Rf are the capacitance and resistance of passive film. The values of the fitted parameters are listed in Table 1. As both Rf and Rct represent the resistance of 13Cr stainless steel to corrosion process, the polarization resistance, Rp (Rp = Rf + Rct), is used to evaluate the corrosion resistance of 13Cr stainless steel. Figure 7 shows the variation of Rp vs. time for the corrosion of 13Cr stainless steel in CO2-water environment. The Rp increases sharply in the initial period due to the formation of passive film. After this rapid increase, the Rp reaches a stable value under static SC-CO2 and dynamic AP-CO2 conditions, while the Rp under dynamic SC-CO2 condition declines in the late period, i.e., the corrosion resistance of 13Cr stainless steel under dynamic SC-CO2 condition decreases in the late period. In the presence of O2, the Rp increases significantly before 10 h and then reaches a stable value under dynamic SC-CO2 condition with 0.1 MPa O2, while the Rp under dynamic SC-CO2 condition with 1 MPa O2 also increases at the beginning and then a gradual decrease is observed after 5 h. The time dependence of impedance (Rp) is in accordance with the variation tendency of OCP. Furthermore, the Rp under static SC-CO2 condition is higher than that under dynamic SC-CO2 condition, i.e., the flow fluid accelerates the corrosion of 13Cr stainless steel. Moreira et al.32 also found the similar situation that the impedance of 13Cr stainless steel under flow condition (1 m/s) was much lower than the impedance

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under static condition. Additionally, the Rp under dynamic AP-CO2 condition is also higher than that under dynamic SC-CO2 condition, suggesting a higher corrosion rate of 13Cr stainless steel in SC-CO2 environment. This may be ascribed to the fact that the higher concentration of corrosive species (e.g., H+, HCO3- and H2CO3) accelerates the corrosion of 13Cr stainless steel under SC-CO2 condition. Therefore, both the flow fluid and the increase in CO2 pressure aggravate the corrosion of 13Cr stainless steel. Under dynamic SC-CO2 condition with 0.1 MPa or 1 MPa O2, the Rp in the initial period is higher than that under dynamic SC-CO2 condition without O2, indicating that the presence of O2 promotes the formation of passive film. However, with prolongation of immersion time, the Rp under dynamic SC-CO2 condition with 0.1 MPa O2 remains relatively stable, while the Rp under dynamic SC-CO2 condition with 1 MPa O2 decreases gradually. The Rp under dynamic SC-CO2 condition with 0.1 MPa O2 is always higher than that under dynamic SC-CO2 condition without O2, while the Rp under dynamic SC-CO2 condition with 1 MPa O2 is lower than that under dynamic SC-CO2 condition without O2. This suggests that the introduction of little amount of O2 promotes the stability of the passive film, while the excessive O2 reduces the stability of the passive film of 13Cr stainless steel.

3.3. Polarization curves measurements Figure 8 shows the polarization curves of 13Cr stainless steel under static or dynamic SC-CO2, or dynamic AP-CO2 conditions for 24 h. Passivity of 13Cr stainless steel is observed under all these three conditions and the corresponding

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electrochemical parameters, such as corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (bc) determined by Tafel extrapolation method with the cathodic branch are listed in Table 2. Compared to static SC-CO2 condition, both the anodic and cathodic current densities increase under dynamic SC-CO2 condition, which results in the increase of corrosion current density, i.e., flow fluid reduces the stability of passive film and promotes the corrosion of 13Cr stainless steel. The anodic and cathodic current densities under dynamic AP-CO2 condition are less than those under dynamic SC-CO2 condition. Therefore, a higher corrosion current density is observed under SC-CO2 condition, i.e., the increase in CO2 pressure results in the formation of less stable passive film on 13Cr stainless steel surface. Figure 9 shows the polarization curves of 13Cr stainless steel under dynamic SC-CO2 conditions with different O2 partial pressures. It is seen that 13Cr stainless steel is still in passive state in the presence of O2. Under dynamic SC-CO2 condition with 0.1 MPa O2, the anodic current density decreases significantly, which indicates that the introduction of little amount of O2 facilitates the formation of stable passive film and then reduces the corrosion current density of 13Cr stainless steel. However, under dynamic SC-CO2 condition with 1 MPa O2, the cathodic process is promoted significantly due to the cathodic reduction of O2, which leads to the increase in the corrosion current density (Table 2).

3.4. Semiconductor properties of the passive film Figure 10 shows the Mott-Schottky plots of the passive film formed on 13Cr

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stainless steel under dynamic SC-CO2 condition with different O2 partial pressures. In the potential range around -0.1~0.3 V, a linear relationship between C-2 and the applied potential (E) with a positive slope is present under all these three conditions, which reveals the n-type semiconductor property of the passive film. Moreover, the slope increases with introducing 0.1 MPa O2, but decreases in the presence of 1 MPa O2. The value of donor density (Nd) is listed in Table 3. Compared with dynamic SC-CO2 condition without O2, the donor density decreases with adding 0.1 MPa O2 but increases in the presence of 1 MPa O2. Generally, the carrier density is associated with the point defects and the nonstoichiometry of the space charge region of passive film. A high donor density means high defect density in the passive film.33,

34

Therefore, the introduction of little O2 (0.1 MPa) increases the stability of the passive film, while the high concentration of O2 (1 MPa) causes the higher donor density and then accelerates the degradation of 13Cr stainless steel.

3.5. Surface morphologies after corrosion Figure 11 shows the SEM images of 13Cr stainless steel after exposed in CO2-water environment for 24 h. No significant difference is observed on the surface of 13Cr stainless steel after corrosion under different conditions. The scratches produced by the abrasive pretreatment process are still visible, indicating the insignificant corrosion. This indicates that 13Cr stainless steel has high corrosion resistance under SC-CO2 environment even in the presence of O2, and this high corrosion resistance is due to the passivity of 13Cr stainless steel.

13



3.6. XPS analysis For further characterization of passive film, XPS analysis was used to reveal the composition of the passive film formed on the specimen surface. Figure 12 shows the XPS spectra (Fe 2p3/2, Cr 2p3/2, Ni 2p3/2, and O 1s) of the passive film formed on 13Cr stainless steel before and after corrosion under dynamic SC-CO2 condition without or with 1 MPa O2 for 24 h. For the specimen without corrosion, the Fe 2p3/2 peak is deconvoluted into four peaks, corresponding to Fe (706.2 eV), FeO (708.0 eV), Fe3O4 (709.4 eV) and Fe2O3 (710.8 eV). The Fe 2p3/2 peak under dynamic SC-CO2 condition is also deconvoluted into these four peaks, i.e., Fe (706.2 eV), FeO (708.2 eV), Fe3O4 (709.4 eV) and Fe2O3 (710.9 eV) 35. However, besides Fe (706.2 eV), FeO (708.2 eV) and Fe3O4 (709.9 eV), FeCl3 (711.3 eV) is also identified in the Fe2p3/2 peak under dynamic SC-CO2 condition with 1 MPa O2,36 which may be related to more defects in the passive film in the presence of 1 MPa O2. In Figure 12 (b), the Ni2p3/2 peak under all conditions is deconvoluted into the peaks of Ni (852.2 eV) and NiO (856.0 eV).37 The fitting of Cr 2p3/2 peak confirms that Cr (574.0 eV), Cr2O3 (575.9 eV) and Cr(OH)3 (577.3 eV) are present before corrosion, while no metallic Cr is detected under dynamic SC-CO2 condition, i.e., the formation of relatively continuous, protective chromium-rich passive film under dynamic SC-CO2 condition, which accounts for the weak peak intensity of Fe 2p3/2 and Ni 2p3/2. In the presence of 1 MPa O2, a peak representing CrO3 (578.3 eV) species appears.38-40 Generally, the presence of CrO3 indicates the poor corrosion resistance of passive film.31, 35, 41, 42 Besides,

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metallic Cr is also detected in the Cr 2p3/2 spectrum under dynamic SC-CO2 condition with 1 MPa O2, which may be associated with the destruction of passive film. The O 1s spectra in Figure 12(d) present three peaks, i.e., the peak at about 530.2 eV corresponding to O2- in the oxides, the peak at about 531.5 eV corresponding to OHin the metallic hydroxides, and the peak at about 532.5 eV corresponding to the adsorbed water (H2O). Table 4 lists the composition of the passive film of 13Cr stainless steel under different conditions. It is seen that almost no Fe and Ni are detected on the specimen surface, i.e., an integrated Cr oxide (and/or hydroxide) film is formed under dynamic SC-CO2 condition without O2, which provides good protection for the corrosion of 13Cr stainless steel. However, in the presence of 1 MPa O2, the Fe and Ni contents are obviously higher than those under dynamic SC-CO2 condition without O2. The relatively high contents of Fe and Ni under dynamic SC-CO2 condition with 1 MPa O2 may be derived from steel substrate as the high metallic Fe and Ni peaks in the XPS spectra (Figure 12 (a, b)). Therefore, the passive film formed under dynamic SC-CO2 condition with 1 MPa O2 may be not integrated, and has less protection for 13Cr stainless steel.

4. Discussion When 13Cr stainless steel is exposed to CO2-water environment, the cathodic reactions may include the reduction of H+, HCO3- and H2CO3: 2H+ + 2e- → H2

(3)

2H2CO3 + 2e- → H2 + 2HCO3-

(4)

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2HCO3- + 2e- → H2 + 2CO32-

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(5)

Meanwhile, the anodic dissolution process of 13Cr stainless steel and growth of passive film are as follows: Fe + 2H2O → Fe(OH)2 + 2H+ + 2e-

(6)

Cr + 3H2O → Cr(OH)3 + 3H+ + 3e-

(7)

Ni + 2H2O → Ni(OH)2 + 2H+ + 2e-

(8)

The formed Fe/Cr/Ni hydroxide and oxide are the main composition of the passive film of 13Cr stainless steel under dynamic SC-CO2 condition. The positive shift of OCPs and increase of Rp in the initial period (Figure 2 and Figure 7) indicate the rapid formation process of passive film. Under dynamic condition, the transfer of species (such as arrival and departure from the specimen surface) and the reactions occur at the velocity boundary layer and concentration boundary layer. Generally, the concentration boundary layer, also called diffusion boundary layer, i.e., the mass transfer process of species, is the control factor for the corrosion and the formation of passive film.43 The impact of flow fluid on the specimen surface would reduce the thickness of diffusion boundary layer, which accelerates the mass transfer process.44,

45

Therefore, the diffusion of the species

involved in the formation of passive film is promoted under dynamic condition, which leads to the faster growth of passive film in the initial period. Then, the time when the OCP and Rp reach the peak value is earlier than that under static condition. However, the flow fluid also accelerates the diffusion of corrosive species. Furthermore, under dynamic SC-CO2 condition, the specimen surface also suffers the shear stress and

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turbulent energy resulted from flow fluid, which may reduce the stability and integrity of passive film and then lower the corrosion resistance of 13Cr stainless steel. Therefore, a negative shift of OCP and decrease of Rp are observed under dynamic SC-CO2 condition in the late period. The electrochemical measurements confirm that the corrosion resistance of the passive film under dynamic SC-CO2 condition is less than that under static SC-CO2 condition in the late period. Under static SC-CO2 condition, due to lacking of the destructive effect induced by flow fluid, the OCP and Rp keep relatively stable. Compared to AP-CO2 condition, the increase in CO2 pressure under SC-CO2 condition leads to the increase in CO2 solubility and the decrease in solution pH value, which promotes the anodic dissolution of steel and increases the instability of passive film.46 Furthermore, the increase in CO2 pressure also accelerates the cathodic reaction by increasing the concentrations of cathodic species (H+, H2CO3, and HCO3-).47 This situation is confirmed by the polarization curve (Figure 8) with higher anodic and cathodic current densities under SC-CO2 condition, compared to AP-CO2 condition. Therefore, the passive film formed under AP-CO2 condition has better corrosion resistance. When O2 is introduced to SC-CO2 system, the cathodic process is accelerated by the reduction of O2: O2 + 2H2O + 4e- → 4OH-

(9)

The promotion of cathodic process causes the positive shift of the OCP of 13Cr stainless steel in the presence of O2.48, 49 In the presence of low concentration of O2

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(0.1 MPa), O2 could oxidize Cr/Fe/Ni directly to form the corresponding oxide in the formation of passive film. Furthermore, the reduction of O2 (reaction (9)) results in the increase in the local solution pH value on the steel surface, which facilitates the formation of stable passive film. Therefore, the formed passive film in the presence of 0.1 MPa O2 is relatively continuous and integrated. The EIS and polarization curve measurements confirm the better corrosion resistance of passive film, compared with dynamic SC-CO2 condition without O2. The Mott-Schottky plots also indicate the lower donor density (i.e., less defects) in the passive film formed under SC-CO2 condition with 0.1 MPa O2. However, since O2 is a strong oxidant, in the presence of 1 MPa O2, the superabundant O2 could also lead to the transformation from Cr2O3 to soluble Cr6+ compounds,50-52 reducing the stability of passive film. XPS spectrum of Cr2p3/2 indicates the presence of CrO3 in the presence of 1 MPa O2. XPS analysis also reveals the presence of metallic Cr and Fe, which may be ascribed to the destruction of passive film and then leads to the exposure of Cr and Fe in the substrate. Electrochemical measurements indicate the lower Rp and higher donor density (i.e., more defects) of the passive film in the presence of 1 MPa O2, compared to SC-CO2 without O2. The higher donor density (Nd) may result in easier incorporation of chloride ions in the oxygen vacancies in the passive film,31,

53-55

leading to the

destruction of passive film. Therefore, the presence of 1 MPa O2 reduces the corrosion resistance of 13Cr stainless steel under SC-CO2 condition.

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5. Conclusions The effects of flow fluid and O2 partial pressure on the passivity of 13Cr stainless steel under SC-CO2 condition were investigated. Although flow fluid can accelerate the formation of passive film in the initial period, it also imposes shear stress and turbulent energy on the specimens, resulting in the decrease in the stability of passive film. Therefore, a higher corrosion rate is observed under dynamic SC-CO2 condition. Compared to dynamic AP-CO2 condition, the increase in CO2 pressure under dynamic SC-CO2 condition leads to the decrease of pH value and the higher concentration of corrosive species, and then increases the instability of passive film. Introduction of 0.1 MPa O2 to the dynamic SC-CO2 system facilitates the formation of stable and integrated passive film, which enhances the corrosion resistance of 13Cr stainless steel. However, addition of 1 MPa O2 reduces the stability of passive film and results in the formation of passive film with high donor density. Therefore, compared to dynamic SC-CO2 condition without O2 or with 0.1 MPa O2, the presence of 1 MPa O2 promotes the corrosion of 13Cr stainless steel.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 51571097, 51371086) and Self-innovation Foundation of Huazhong University of Science and Technology (No. 2017KFYXJJ164).

References

19



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43. Zeng, L.; Zhang, G. A.; Guo, X. P., Effect of Hydrodynamics on the Inhibition Effect of Thioureido Imidazoline Inhibitor for the Flow Accelerated Corrosion of X65 Pipeline Steel. CORROSION 2016, 72, (5), 598-614. 44. Sasaki, K.; Burstein, G. T., Erosion–corrosion of stainless steel under impingement by a fluid jet. Corros. Sci. 2007, 49, (1), 92-102. 45. Wang, J.; Giridharan, V.; Shanov, V.; Xu, Z.; Collins, B.; White, L.; Jang, Y.; Sankar, J.; Huang, N.; Yun, Y., Flow-induced corrosion behavior of absorbable magnesium-based stents. Acta Biomater. 2014, 10, (12), 5213-5223. 46. Crolet, J. L.; Bonis, M. R., pH Measurements in Aqueous CO2 Solutions under High Pressure and Temperature. CORROSION 1983, 39, (2), 39-46. 47. Ikeda, A.; Mukai, S.; Ueda, M., Corrosion Behavior of 9 to 25% Cr Steels in Wet CO2 Environments. CORROSION 1985, 41, (4), 185-192. 48. Anselmo, N.; May, J. E.; Mariano, N. A.; Nascente, P. A. P.; Kuri, S. E., Corrosion behavior of supermartensitic stainless steel in aerated and CO2-saturated synthetic seawater. Mater. Sci. Eng., A 2006, 428, (1), 73-79. 49. Marcelin, S.; Pébère, N.; Régnier, S., Electrochemical characterisation of a martensitic stainless steel in a neutral chloride solution. Electrochim. Acta 2013, 87, (Supplement C), 32-40. 50. Li, Y.; Wang, S.; Sun, P.; Xu, D.; Ren, M.; Guo, Y.; Lin, G., Early oxidation mechanism of austenitic stainless steel TP347H in supercritical water. Corros. Sci. 2017, 128, 241-252. 51. Gómez-Briceño, D.; Blázquez, F.; Sáez-Maderuelo, A., Oxidation of austenitic

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and ferritic/martensitic alloys in supercritical water. J. Supercrit. Fluids 2013, 78, (Supplement C), 103-113. 52. Holcomb, G. R., Steam Oxidation and Chromia Evaporation in Ultrasupercritical Steam Boilers and Turbines. J. Electrochem. Soc. 2009, 156, (9), C292-C297. 53. Macdonald, D. D., The Point Defect Model for the Passive State. J. Electrochem. Soc. 1992, 139, (12), 3434-3449. 54. Amri, J.; Souier, T.; Malki, B.; Baroux, B., Effect of the final annealing of cold rolled stainless steels sheets on the electronic properties and pit nucleation resistance of passive films. Corros. Sci. 2008, 50, (2), 431-435. 55. Jinlong, L.; Hongyun, L., Electrochemical investigation of passive film in pre-deformation AISI 304 stainless steels. Appl. Surf. Sci. 2012, 263, (Supplement C), 29-37.

27



Figure 1. Schematic diagram of the experimental setup for in situ electrochemical

measurements under dynamic high pressure CO2-water environment

28


Page 28 of 44

Page 29 of 44

-0.15 Potential (V vs. Ag/AgCl (0.1M KCl))

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


static, 8 MPa CO2 2 m/s, 8 MPa CO2 2 m/s, 0.1 MPa CO2

-0.20 -0.25 -0.30 -0.35 -0.40 -0.45 -0.50 -0.55

0

5

10

15 Time (h)

20

25

Figure 2. Open circuit potential of 13Cr stainless steel in 1% NaCl solution under static or dynamic supercritical CO2, or dynamic atmospheric pressure CO2 conditions

29



0.0 Potential (V vs. Ag/AgCl (0.1M KCl))

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 44

-0.1 -0.2 2m/s, 8 MPa CO2 2m/s, 8 MPa CO2 + 0.1 MPa O2 2m/s, 8 MPa CO2 + 1 MPa O2

-0.3 -0.4 -0.5 -0.6

0

5

10

15

20

25

Time (h)

Figure 3. Open circuit potential of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition with different partial pressures of O2

30


Page 31 of 44

10000

(a)

2

Z" (Ω cm )

8000

6000 1h 3h 5h 8h 14 h 24 h fitted line

4000

2000

0

0

2000

4000

6000

8000

10000

12000

14000

2

Z' (Ω cm ) 6000

(b)

2

Z" (Ω cm )

4000 1h 3h 5h 8h 14 h 24 h fitted line

2000

0

0

2000

4000

6000

8000

2

Z' (Ω cm ) 14000

(c) 12000 10000 2

Z" (Ω cm )

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


1h 3h 5h 8h 14 h 24 h fitted line

8000 6000 4000 2000 0

0

2000

4000

6000

8000

10000 12000 14000 16000 2

Z' (Ω cm )

Figure 4. EIS of 13Cr stainless steel in 1% NaCl solution under static or dynamic supercritical CO2, or dynamic atmospheric pressure CO2 conditions for different times: (a) static, 8 MPa CO2, (b) 2 m/s, 8 MPa CO2, (c) 2 m/s, 0.1 MPa CO2

31



16000

(a)

2

Z" (Ω cm )

12000

1h 3h 5h 8h 14 h 24 h fitted line

8000

4000

0

0

4000

8000

12000

16000

20000

2

Z' (Ω cm ) 10000 1h 3h 5h 8h 14 h 24 h fitted line

8000

2

Z" (Ω cm )

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 32 of 44

6000

(b)

4000

2000

0

0

2000

4000

6000

8000

10000

12000

14000

2

Z' (Ω cm )

Figure 5. EIS of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition with different partial pressures of O2: (a) 2 m/s, 8 MPa CO2 + 0.1 MPa O2, (b) 2 m/s, 8 MPa CO2 + 1 MPa O2

32


Page 33 of 44 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


Q dl

Qf

R ct

Rf

Rs

Figure 6. Equivalent circuit for EIS fitting of 13Cr stainless steel in 1% NaCl solution under different conditions

33



60000 static, 8 MPa CO2 2 m/s, 8 MPa CO2 2 m/s, 0.1 MPa CO2 2 m/s, 8 MPa CO2 + 0.1 MPa O2 2 m/s, 8 MPa CO2 + 1 MPa O2

50000

2

Rp (Ω cm )

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 34 of 44

40000

30000

20000

10000 0

5

10

15

20

25

Time (h)

Figure 7. Time dependence of the polarization resistance (Rp) of 13Cr stainless steel in 1% NaCl solution under different conditions

34


Page 35 of 44

0.2 Potential (V vs. Ag/AgCl (0.1 M KCl))

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 0.0

static, 8 MPa CO2 2 m/s, 8 MPa CO2 2 m/s, 0.1 MPa CO2

-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -8 10

-7

10

-6

-5

10 10 2 Current density (A/cm )

-4

10

-3

10

Figure 8. Polarization curves of 13Cr stainless steel in 1% NaCl solution under static or dynamic supercritical CO2, or dynamic atmospheric pressure CO2 conditions for 24 h

35



0.3 Potential (V vs. Ag/AgCl (0.1 M KCl))

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 36 of 44

0.2 0.1

2 m/s, 8 MPa CO2 2 m/s, 8 MPa CO2 + 0.1 MPa O2 2 m/s, 8 MPa CO2 + 1 MPa O2

0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -8 10

-7

10

-6

-5

10

-4

10

10

-3

10

2

Current density (A/cm )

Figure 9. Polarization curves of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition with different partial pressures of O2 for 24 h

36


Page 37 of 44

9

1.2x10

2 m/s, 8 MPa CO2 2 m/s, 8 MPa CO2 + 0.1 MPa O2 2 m/s, 8 MPa CO2 + 1 MPa O2 8

8.0x10

2

4

2

1/C (cm /F )

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


8

4.0x10

0.0 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Applied Potential (V vs. Ag/AgCl (0.1 M KCl))

Figure 10. Mott-Schottky plots of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 condition with different partial pressures of O2 for 24 h

37



(a)

Page 38 of 44

(b)

10 µm

10 µm

100 µm

100 µm

(c)

(d)

10 µm

10 µm

100 µm

100 µm

(e)

10 µm

100 µm

Figure 11. SEM surface morphologies of 13Cr stainless steel after exposed in 1% NaCl solution under different conditions for 24 h: (a) static, 8 MPa CO2, (b) 2 m/s, 8 MPa CO2, (c) 2 m/s, 0.1 MPa CO2, (d) 2 m/s, 8 MPa CO2 + 0.1 MPa O2, (e) 2 m/s, 8 MPa CO2 + 1 MPa O2

38


Page 39 of 44

FeCl3

(b)

Fe (a)

Fe3O4

FeO

NiO

Ni

2 m/s, 8 MPa CO2 + 1 MPa O2

2 m/s, 8 MPa CO2 + 1 MPa O2 860

712

Fe2O3

710

708

CPS

Fe3O4

FeO

A

706

712

856

Fe

854

852

NiO

2 m/s, 8 MPa CO2 714

858

704

2 m/s, 8 MPa CO2

Fe2O3

710

708

FeO

Fe3O4

706

704

Fe

860

858

856

854

852

NiO

Without Corrosion 714

850

Ni

CPS

714

850

Ni

Without Corrosion

712

710

708

706

704

860

858

856

Binging Energy (eV)

854

852

850

Binding Energy (eV) 2 m/s, 8 MPa CO2 + 1 MPa O2 -

(c)

Cr2O3

Cr(OH)3

(d)

OH

2-

O

6+

Cr

H2O Cr

2 m/s, 8 MPa CO2 + 1 MPa O2 538 582

580

578

576

574

572

536

534

530

528

526

524

528

526

524

526

524

OH

CPS

Cr2O3

532

-

570

Cr(OH)3 CPS

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


H2O

2-

O

2 m/s, 8 MPa CO2 2 m/s, 8 MPa CO2 582

580

578

576

574

572

570

538

Cr2O3

Cr(OH)3

536

534

532

-

530

2-

OH Cr

O

H2O Without Corrosion

Without Corrosion 582

580

578

576

574

572

538

570

536

534

Bingding Energy (eV)

532

530

528

Binding Energy (eV)

Figure 12. XPS of 13Cr stainless steel before and after exposed in 1% NaCl solution under dynamic supercritical CO2 condition without or with 1 MPa O2 for 24 h: (a) Fe 2p3/2, (b) Ni 2p3/2, (c) Cr 2p3/2, (d) O 1s

39



Page 40 of 44

Table 1 Fitted electrochemical parameters of the EIS of 13Cr stainless steel in 1% NaCl solution under different conditions Conditions static, 8 MPa CO2

2 m/s, 8 MPa CO2

2 m/s, 0.1 MPa CO2

2 m/s, 8 MPa CO2 + 0.1 MPa O2


Time

Rs (Ω cm2)

Qf (Ω-1 cm-2 s-nf)

nf

Rf (Ω cm2)

Qdl (Ω-1 cm-2 s-ndl)

ndl

1h

4.99

0.000338

0.77

1950

0.00120

0.70

7338

3h

5.02

0.000321

0.75

5222

0.00109

0.70

10270

5h

5.46

0.000311

0.77

6872

0.000914

0.70

10750

8h

5.45

0.000296

0.79

10460

0.00316

0.70

10740

14 h

5.41

0.001170

0.98

2011

0.000405

0.79

27010

24 h

5.36

0.000259

0.83

13470

0.000625

0.70

10710

1h

4.93

0.000236

0.82

696

0.00178

0.70

6575

3h 5h 8h

6.39 6.35 6.72

0.000208 0.000226 0.000229

0.81 0.80 0.78

1926 2533 3691

0.000929 0.000988 0.000808

0.64 0.70 0.72

25270 20500 21760

14 h 24 h

7.22 6.57

0.000252 0.000348

0.76 0.72

4443 2992

0.000605 0.000852

0.70 0.69

21470 10890

1h

4.85

0.000259

0.82

6361

0.000632

0.69

19250

3h 5h 8h 14 h

6.31 8.45 7.08 8.49

0.000243 0.000230 0.000232 0.000224

0.82 0.80 0.80 0.80

14210 23020 15030 17180

0.000694 0.001085 0.001002 0.000827

0.83 0.99 0.99 0.98

23140 20440 16030 11920

24 h

9.85

0.000231

0.81

24810

0.001101

0.98

12910

1h

3.91

0.000377

0.77

6511

0.000822

0.63

9312

3h

4.13

0.000340

0.78

9965

0.000226

0.82

22065

5h

4.12

0.000249

0.86

10185

0.000312

0.80

25415

8h

4.51

0.000269

0.84

14260

0.000753

0.98

27760

14 h

4.16

0.000258

0.84

9394

0.000106

0.70

35150

24 h

3.81

0.000374

0.79

9440

0.000263

0.97

30480

1h

4.74

0.000274

0.87

0.000859

0.86

3h 5h 8h 14 h

4.87 5.11 5.09 5.18

0.000371 0.000309 0.000419 0.000379

0.96 0.94 0.95 0.86

5049 3222 4956 3295 9195

0.000371 0.000463 0.000364 0.000459

0.82 0.82 0.83 0.86

7487 13840 13800 13490 2761

24 h

5.09

0.000274

0.84

6797

0.000965

0.72

2144

40


Rct (Ω cm2)

Page 41 of 44 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 Fitted electrochemical parameters of the polarization curves of 13Cr stainless steel in 1% NaCl solution under different conditions for 24 h Ecorr (V vs. Ag/AgCl (0.1 M KCl))

icorr (A/cm2)

bc (mV/dec)

static, 8 MPa CO2

-0.250

1.69 × 10-6

-146

2 m/s, 8 MPa CO2

-0.505

3.41 × 10-6

-91

2 m/s, 0.1 MPa CO2

-0.512

1.58 × 10-6

-95

2 m/s, 8 MPa CO2 + 0.1 MPa O2

-0.155

1.47 × 10-6

-114


-0.181

1.55 × 10-5

-268

Conditions

41



Page 42 of 44

Table 3 Donor density (Nd) of the passive film of 13Cr stainless steel in 1% NaCl solution under dynamic SC-CO2 condition with different O2 partial pressures O2 partial pressure

0 MPa

0.1 MPa

1 MPa

Nd (cm-3)

8.23 × 1021

5.12 × 1021

1.33 × 1022

42


Page 43 of 44 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 4 The composition (wt%) of the passive film of 13Cr stainless steel before or after corrosion in 1% NaCl solution under dynamic SC-CO2 condition without or with 1 MPa O2 for 24 h by XPS analysis Conditions

O

Fe

Cr

Ni

Without corrosion

39.03

46.79

12.96

1.22

2 m/s, 8 MPa CO2

67.87

1.34

30.53

0.26


52.60

11.55

33.09

2.77

43



TOC graphic A setup was developed to study the passivity of 13Cr stainless steel in 1% NaCl solution under dynamic supercritical CO2 (SC-CO2) condition without or with different O2 partial pressure by in situ electrochemical measurements and surface analysis.

44


Page 44 of 44

Passivity of 13Cr Stainless Steel in 1% NaCl Solution under Dynamic

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