Galvanic corrosion between N80 carbon steel and 13Cr stainless steel under supercritical CO2 conditions

Accepted Manuscript Title: Galvanic corrosion between N80 carbon steel and 13Cr stainless steel under supercritical CO2 conditions Authors: Y.Y. Li, Z.Z. Wang, X.P. Guo, G.A. Zhang PII: DOI: Reference:

S0010-938X(17)31378-1 https://doi.org/10.1016/j.corsci.2018.11.025 CS 7781

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

28 July 2017 29 July 2018 19 November 2018

Please cite this article as: Li YY, Wang ZZ, Guo XP, Zhang GA, Galvanic corrosion between N80 carbon steel and 13Cr stainless steel under supercritical CO2 conditions, Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.11.025 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.

Galvanic corrosion between N80 carbon steel and 13Cr stainless steel under supercritical CO2 conditions

Z. Z. Wang1

X. P. Guo G. A. Zhang*

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Y. Y. Li1

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

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Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and

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Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China

These authors contributed equally to this work.

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

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Highlights:

Galvanic corrosion behaviour in supercritical CO2 (SC-CO2)-H2O system was

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studied.

Corrosion of N80 carbon steel was enhanced by coupled with 13Cr stainless steel.

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Galvanic effect under dynamic condition is bigger than that under static condition.

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Galvanic current decreases with time due to the formation of protective FeCO3

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

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film.

Abstract: The galvanic effect between N80 carbon steel and 13Cr stainless steel in formation water under supercritical CO2 conditions was studied by electrochemical measurements and surface characterization. It is demonstrated that the galvanic effect

obviously promotes the corrosion of N80 carbon steel and lowers the protection of corrosion products under static and dynamic conditions. The fluid flow under dynamic conditions not only accelerates the corrosion of N80 carbon steel, but also strengthens the galvanic effect with higher galvanic current density. The galvanic current density

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decreases with increasing immersion time due to the decrease of potential difference

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and the increase of resistance.

Keywords: A. Carbon steel; A. Stainless steel; B. EIS; B. SEM; C. Acid corrosion

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1. Introduction

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Fossil fuels will continue to be a dominant source of energy in foreseeable future.

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However, combustion of carbon-based fuels will produce greenhouse gases

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(particularly CO2) that adversely affect the global climate. This problem has aroused

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public concern [1, 2]. Concerning this issue, Carbon Capture and Storage (CCS) technology, which has the potential to prevent CO2 emissions into the atmosphere, is

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believed to be an effective measure to control the CO2 level in the Earth’s environment

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[3-6]. CCS usually involves capturing CO2 from large industrial point sources, following by compressing and transporting the fluid to geological reservoirs for sequestration [7-10]. The captured CO2 can also be injected into oil fields to enhance

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the oil recovery, i.e., CO2 enhanced oil recovery (CO2-EOR). With the widespread application of CO2-EOR technique [11, 12], especially the exploitation of deep oil wells under the environment of high temperature and high pressure, the supercritical CO2 (SC-CO2) corrosion problem of tubular goods, such as

carbon steel and stainless steel, has become increasingly significant [13-16]. Therefore, it is necessary to elucidate the SC-CO2 corrosion mechanism of steels for developing an appropriate countermeasure. The study on the corrosion of steels in SC-CO2 environment has been a hot topic over the past few years. Due to the high corrosion rate

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of carbon steel in SC-CO2/H2O environment [16-24], low Cr alloy steels and Corrosion Resistant Alloys (CRAs) have been used for oil and gas exploitation [25-30]. Wei et al.

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[31] reported that compared with P110 carbon steel and 3Cr low alloy steel, 316L stainless steel presented excellent corrosion resistance in a supercritical CO2/H2S/H2O

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system. Hassani et al. [26] also confirmed that 13Cr stainless steel showed better

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corrosion resistance than 1018 carbon steel and 5Cr low alloy steel in SC-CO2

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environment. When carbon steel and stainless steel are used for downhole pipes and

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equipment simultaneously, and they are in electrical contact, there will be galvanic

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effect between these two dissimilar metals due to their potential difference [32-35]. The corrosion of carbon steel will be promoted by the galvanic effect.

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The galvanic corrosion behaviour between carbon steel and stainless steel induced

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by their potential difference was confirmed by Varela et al. [36]. Some researchers have investigated the galvanic corrosion behaviour between carbon steel and stainless steel under low pressure (less than 1 MPa). Dong et al. [37, 38] studied the galvanic corrosion

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behaviour between 1020 carbon steel and 304L stainless steel at atmospheric pressure and determined the synergism of galvanic corrosion and erosion-accelerated corrosion of carbon steel in a flowing sand-containing chloride solution. Scott and his co-workers [39] revealed the effect of environmental variables on the galvanic corrosion behaviour

of carbon steel and stainless steel in the environments with 100 psi of CO2 and predicted the distribution of galvanic corrosion by finite element analysis. Yao et al. [40] found that an increase of temperature enhanced the galvanic effect between P110 carbon steel and super martensitic 13Cr steel at atmospheric pressure. However, the galvanic

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corrosion behaviour between carbon steel and stainless steel under SC-CO2 environment is still not clear. Especially the evolution of corrosion products of carbon

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steel during the corrosion process in SC-CO2 environment will determine the potential difference between carbon steel and stainless steel, and then determine their galvanic

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effect.

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The objective of this work is to investigate the galvanic effect between the N80

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carbon steel and 13Cr stainless steel under static and dynamic SC-CO2 conditions by in

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situ electrochemical measurements and surface characterization. Meanwhile, the

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galvanic corrosion behaviour was determined to provide some information for suitable

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selection of materials in the process of CO2-EOR.

2. Experimental

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2.1. Materials and solution The materials used in this experiment were N80 carbon steel and 13Cr stainless

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steel, and their nominal chemical compositions were listed in Table 1. The specimens were machined into 5 mm × 4 mm × 2.5 mm with an exposed area of 0.2 cm 2 for electrochemical measurements and 8 mm × 10 mm × 2.5 mm with an exposed area of 0.8 cm2 for weight loss test. A copper wire was soldered to the back of specimen to ensure the electric connection for electrochemical measurements and weight loss test

under galvanic effect. The specimens were embedded into epoxy resin except the exposed working surface, and then abraded with 800 grit silicon carbide paper, cleaned with acetone and deionized water. The test solution, simulating the formation water in an oil field, was prepared from

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analytical grade reagents and deionized water. Its chemical composition was listed in Table 2. The solution was deoxygenated by purging CO2 (99.99%) for 12 h before the

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test. In the experiments, 1.4 L test solution was added into the autoclave. Therefore, the ratios of the surface area of specimen to the volume of solution for weight loss and

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2.2. Electrochemical measurements

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electrochemical measurements were 0.0057 and 0.0014, respectively.

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Electrochemical measurements were performed under high pressure dynamic and

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static conditions in a 3 L autoclave. The experimental setup, which is composed of two concentric cylinders, i.e., an outer Teflon cylinder holder and an inner Teflon cylinder

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rotator (as shown in Fig. 1), was described elsewhere [41]. The specimens were

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mounted into the inner surface of the outer Teflon cylinder holder. During the tests, the outer cylinder was static, while the inner cylinder rotated, which drove the flow of the solution between these two concentric cylinders. Under dynamic conditions, the

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rotation rate of inner cylinder was set as 530 rpm, which corresponds to a linear velocity at the surface of inner cylinder of 2 m/s. According to the calculated fluid dynamics (CFD) simulation [41], the corresponding flow velocity near the specimen surface (inner surface of the outer Teflon cylinder holder) was about 0.5-0.9 m/s and the shear

stress on the specimen surface was about 3.4 Pa. An electrochemical workstation was used for electrochemical measurements with a three-electrode system that was mounted in the inner surface of the outer Teflon cylinder holder, as shown in Fig. 1. N80 carbon steel electrode and/or 13Cr stainless

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steel electrode were as working electrodes, a platinum plate as counter electrode and a Ag/AgCl electrode (0.1 M KCl solution) as reference electrode. Potentiodynamic

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polarisation curves were conducted from cathodic to anodic polarisation at a scanning rate of 0.5 mV/s with a data interval of 0.5 mV. Electrochemical impedance

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spectroscopy (EIS) was measured at open circuit potential (OCP) with a sinusoidal

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potential excitation of 5 mV amplitude in the frequency range from 10,000 Hz to 10

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mHz. The impedance data were fitted with ZsimpWin software using an equivalent

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circuit. The galvanic current density and coupled potential between N80 carbon steel

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and 13Cr stainless steel were measured by using a zero resistance ammeter (ZRA).

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2.3. Weight loss measurements

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After corrosion tests, the corrosion products on the N80 carbon steel specimens were removed and then the weight loss corrosion rate was calculated according to following equation:

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Vcorr 

m S t

(1)

where Vcorr is the weight loss corrosion rate (g/(m2 h); Δm is the weight loss of specimen after corrosion (g); S is the exposed surface (m2); and t is the exposed time (h). The weights of the specimens before and after corrosion test were weighed using an

electronic balance with a precision of 0.1 mg. Three parallel specimens were used for each condition. The weight loss tests lasted 1 h, 12 h, 18 h and 34 h, respectively. The weight loss measurement of 13Cr stainless steel was not performed because its weight

2.4. Characterization of corrosion products after corrosion

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loss was too small to be weighed.

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After immersion corrosion test, the specimens for surface analysis were removed from autoclave, and rinsed with deionized water. The corrosion morphologies of the

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specimens were observed by Scanning Electron Microscope (SEM) and the

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composition of corrosion products was analyzed by X-ray diffraction (XRD),

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respectively.

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3. Results

3.1. Weight loss corrosion rate measurements

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Fig. 2 shows the weight loss corrosion rate of N80 carbon steel after coupled or

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uncoupled with 13Cr stainless steel in supercritical CO2-containing formation water under static and dynamic conditions for different times. It is seen that the corrosion rate of N80 carbon steel is fairly high in supercritical CO2-containing formation water. The

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corrosion rate under dynamic conditions decreases with the prolongation of immersion time no matter whether the N80 carbon steel is coupled with 13Cr stainless steel or not. This may be attributed to the formation of a protective corrosion product film on the surface of N80 carbon steel during the corrosion process. Furthermore, the corrosion

rate of N80 carbon steel coupled with 13Cr stainless steel is higher than that without coupling, which indicates that there is a galvanic effect to accelerate the corrosion of N80 carbon steel. The difference between the corrosion rates of N80 carbon steel coupled and uncoupled with 13Cr stainless steel also decreases with increasing

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immersion time, i.e., the galvanic effect weakens with increasing immersion time. Under static conditions, the corrosion rates of N80 carbon steel coupled or uncoupled

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with 13Cr stainless steel are lower than those under dynamic conditions. Furthermore, the difference between the corrosion rates of N80 carbon steel coupled and uncoupled

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with 13Cr stainless steel is also less than that under dynamic conditions. Therefore, the

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galvanic effect between N80 carbon steel and 13Cr stainless steel is stronger under

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dynamic conditions, i.e., the fluid flow strengthens the galvanic effect between N80

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carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water.

3.2. Open circuit potential measurements

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Fig. 3 shows the time dependence of the open circuit potentials (OCPs) of N80

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carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water under static and dynamic conditions when they are not coupled. It can be seen that under static conditions, the OCP of N80 carbon steel shifts to positive direction before 8 h,

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and then reaches a relatively stable value until 20 h. After 20 h, a prominently positive shift of OCP is observed, which is associated with the formation of protective corrosion products on the surface of N80 carbon steel [42]. The positive shift of OCP slows down after 24 h. The OCP of 13Cr stainless steel shifts from -0.565 V vs. Ag/AgCl (0.1 M

KCl) to about -0.380 V vs. Ag/AgCl (0.1 M KCl) sharply in the initial period (before 4 h) and then reaches a relatively stable value. Under dynamic conditions, the change of OCP is similar to that under static conditions. The OCP of N80 carbon steel is more positive than that under static conditions, which may be attributed to the acceleration

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of cathodic process under flow conditions. Furthermore, the prominently positive shift of the OCP of N80 carbon steel (at about 17 h) is earlier than that under static conditions

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(at about 20 h). This situation may be resulted from the fast dissolution of Fe in the

initial period, i.e., higher Fe2+ concentration in the solution, which results in the easy

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formation of FeCO3 film on the steel surface under dynamic conditions. For 13Cr

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stainless steel, the OCP shifts positively in the initial period and then reaches a relatively

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stable value. The OCP under dynamic conditions is more negative than that under static

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conditions, which may be attributed to the formation of less stable passive film under

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dynamic conditions. Furthermore, under both static and dynamic conditions, the OCP of N80 carbon steel is more negative than that of 13Cr stainless steel. Therefore, N80

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carbon steel will act as anode and its corrosion will be promoted after coupled with

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13Cr stainless steel. Fig. 3(c) shows the time dependence of the potential difference (calculated from Fig. 3 (a, b)) between the N80 carbon steel and 13Cr stainless steel. It is seen that the potential difference under static conditions is larger than that under

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dynamic conditions when N80 carbon steel and 13Cr stainless steel are not coupled. The potential difference under dynamic conditions decreases with increasing immersion time. Fig. 4 shows the time dependence of the OCPs of N80 carbon steel and 13Cr

stainless steel in supercritical CO2-containing formation water under static and dynamic conditions when they are coupled. The OCP measurements were conducted with the time interval of half an hour. For the OCP measurements, the couple was disconnected and the stable OCP was recorded. After OCP measurements, the couple was connected

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again. The time for each disconnection was about 1 min, and the total time for the disconnection during the whole OCP measurements was about 1 h. It is seen that the

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changes of the OCPs of N80 carbon steel and 13Cr stainless steel are similar to the case

that these two electrodes were not coupled. However, the time at which the OCP of N80

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carbon steel shifts to positive direction significantly (at about 18 h and 16 h under static

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and dynamic conditions, respectively) is a little earlier than that of N80 carbon steel

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without coupled with 13Cr stainless steel. Under dynamic conditions, compared with

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the OCPs of N80 carbon steel and 13Cr stainless steel when they are not coupled, the

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OCP of 13Cr stainless steel is more positive, while the OCP of N80 carbon steel is more negative when they are coupled together. Therefore, there is a larger potential difference

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between the N80 carbon steel and 13Cr stainless steel, i.e., larger galvanic corrosion

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driving force, when they are coupled together. Fig. 4(c) shows the time dependence of the potential difference (calculated from Fig. 4 (a, b)) between the N80 carbon steel and 13Cr stainless steel. It is seen that the potential difference decreases with the

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prolongation of immersion time under both static and dynamic conditions. Therefore, the driving force for galvanic corrosion decreases with increasing immersion time. Furthermore, the potential difference under dynamic conditions is higher than that under static conditions in the initial period (before 14 h), i.e., a larger galvanic corrosion

driving force under dynamic conditions in the initial period. However, the potential difference under dynamic conditions is less than that under static conditions at 14-24 h. This is ascribed to the more prominently positive shift of the OCP of N80 carbon steel

3.3. Coupled potential and galvanic current density measurements

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under dynamic conditions at this time (Fig. 4(b)).

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Fig. 5 shows the time dependence of the coupled potential and galvanic current

density between N80 carbon steel (WE1) and 13Cr stainless steel (WE2) in supercritical

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CO2-containing formation water under static and dynamic conditions. It is seen that the

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change of coupled potential is similar to the changes of the OCPs of N80 carbon steel

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under both static and dynamic conditions. The coupled potential under dynamic

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conditions is more positive than that under static conditions. Furthermore, the

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prominently positive shift of the coupled potential under dynamic conditions is prior to that under static conditions. The galvanic current densities are relatively high in the

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initial period under both static and dynamic conditions. The maximum galvanic current

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densities are about 0.84 mA/cm2 and 1.25 mA/cm2 under static and dynamic conditions, respectively. Under static conditions, the galvanic current density decreases rapidly in the initial period (before 10 h), and then reaches a relatively stable value. After about

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18 h, the galvanic current density decreases rapidly again. The rapid decrease in galvanic current density corresponds to the prominently positive shift of the OCP of N80 carbon steel (Fig. 4(a)), which results in a rapid decrease in potential difference between N80 carbon steel and 13Cr stainless steel. In the late period (after 24 h), the

galvanic current density decreases to small value. The change of galvanic current density under dynamic conditions is similar to that under static conditions, only with an earlier change time. The galvanic current density under dynamic conditions is larger than that under static conditions, which indicates a stronger galvanic effect under

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dynamic conditions. Comparing the galvanic current density with the weight loss corrosion rate, it is

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found that the galvanic current density is not exactly equal to the weight loss corrosion

rate. The measured weight loss corrosion rate is higher than the galvanic current density.

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This could be attributed to the fact that the coupled potential is close to the OCP of N80

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carbon steel (Fig. 4 and Fig. 5). In this case, the cathodic current density of N80 carbon

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steel cannot be neglected at the coupled potential. The galvanic current density should

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be equal to the difference between the anodic and cathodic current densities. Therefore,

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the anodic current density (the dissolution rate) of N80 carbon steel should be higher than the galvanic current density. However, the variation trends of the weight loss

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corrosion rate and the galvanic current density are similar, i.e., both the weight loss

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corrosion rate and galvanic current density decrease with time, as shown in Fig. 2 and Fig. 5.

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3.4. Polarisation curves measurements Fig. 6 shows the polarisation curves of N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water under static and dynamic conditions after these two electrodes were coupled or uncoupled for 34 h. For N80 carbon steel

under various conditions, the cathodic process is under activation control while a pseudo-passive behaviour is observed in the anodic process. This pseudo-passive behaviour may be associated with the formation of a protective corrosion product film on the electrode surface, as described in the literature [43, 44]. For 13Cr stainless steel,

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passivation behaviour is observed under both static and dynamic conditions. The corresponding electrochemical parameters in polarisation curves, such as corrosion

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potential (Ecorr), corrosion current density (icorr), Tafel slopes (ba, bc) were determined by the Tafel extrapolation method. The obtained values of these electrochemical

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parameters, including pitting potential (Epit) and passive current density (ip), were listed

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in Table 3. The corrosion current density of N80 carbon steel coupled with 13Cr

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stainless steel is higher than that without coupling under both static and dynamic

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conditions, which indicates that the galvanic effect between N80 carbon steel and 13Cr

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stainless steel promotes the corrosion of N80 carbon steel. The corrosion rate of 13Cr stainless steel also increases after coupled with N80 carbon steel. This situation may be

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film.

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attributed to the fact that the cathodic polarisation decreases the protection of passive

Fig. 7 shows the polarisation curves of N80 carbon steel and 13Cr stainless steel

in the supercritical CO2-containing formation water under dynamic conditions (2 m/s)

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after coupled for different times. The corresponding electrochemical parameters were listed in Table 4. It is seen that for N80 carbon steel, both the anodic and cathodic processes are under activation control after coupled with 13Cr stainless steel for 1 h, 12 h, 18 h. With the prolongation of immersion time, there is a prominent decrease in

anodic current density, which results in a significant decrease in the corrosion current density and positive shift of corrosion potential. The decrease of corrosion current density and the positive shift of corrosion potential may be attributed to the gradual formation of a protective corrosion product film on the steel surface. For 13Cr stainless

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steel, passivation behaviour is present at various immersion times. An active/passive transition is observed in the potential range of around -0.4 V~ -0.3 V in the anodic

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polarisation curves measured at 1 h, 12 h, 18 h, but not at 34 h. The great galvanic effect

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in the initial period deteriorates the passivity of 13Cr stainless steel.

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3.5. EIS measurements

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Fig. 8 shows the EIS of N80 carbon steel and 13Cr stainless steel in supercritical

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CO2-containing formation water under static and dynamic conditions after coupled or

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uncoupled for 34 h. It is seen that a depressed capacitive loop is observed on the Nyquist plots of N80 carbon steel coupled or uncoupled with 13Cr stainless steel under both

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static and dynamic conditions. This depressed capacitive loop is composed of two

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overlapping capacitive loops with close time constants. The capacitive loop at high frequency may be attributed to double layer capacitance and charge transfer resistance while the capacitive loop at low frequency may related to the corrosion products film

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formed on the steel surface [41]. The capacitive loop of N80 carbon steel coupled with 13Cr stainless steel is smaller than that without coupling, i.e., the galvanic effect promotes the corrosion of N80 carbon steel after coupled with 13Cr stainless steel. The decrease in the capacitive loop is more significant under dynamic conditions, i.e., a

more significant increase in the corrosion rate of N80 carbon steel under dynamic conditions due to the galvanic effect. This is in accordance with the weight loss measurements. For 13Cr stainless steel, the Nyquist plots are also characterized by a depressed capacitive loop consisting of two capacitive loops. The capacitive loop at

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high frequency may be related to the double layer capacitance and charge transfer resistance while the capacitive loop at low frequency may be attributed to the passive

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film formed on the 13Cr stainless steel surface. Compared with the impedance of 13Cr stainless steel without coupling, the impedance of 13Cr stainless steel coupled with N80

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carbon steel decreases, i.e., the corrosion rate of 13Cr stainless steel increases after

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coupled with N80 carbon steel. This may be attributed to the fact that the cathodic

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polarisation is adverse to the formation of protective passive film on 13Cr stainless steel.

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Fig. 9 shows the EIS of N80 carbon steel and 13Cr stainless steel in supercritical

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CO2-containing formation water under dynamic conditions (2 m/s) after coupled for different times. For N80 carbon steel, the Nyquist plot after coupled for 1 h is

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characterized by three time constants, i.e., two capacitive loops at high and intermediate

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frequencies and one inductive loop at low frequency. These two capacitive loops at high and intermediate frequencies may be associated with the double layer capacitance and corrosion products, respectively, while the inductive loop may related to the adsorbed

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intermediate products during the dissolution of steel [42]. With the increase of immersion time (12 h, 18 h), the inductive loop disappears and the Nyquist plot becomes double capacitive loops. After coupled for 34 h, the two capacitive loops merge into a depressed capacitive loop due to their close time constants. The impedance

increases with the prolongation of immersion time. Especially after coupled for 34 h, a significant increase in impedance is observed, which should be attributed to the formation of protective corrosion products on the steel surface in the late period. For 13Cr stainless steel, a depressed capacitive loop, which is composed of two capacitive

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loops, is present in the Nyquist plots. The impedance also increases with the increase of immersion time, i.e., a more protective passive film is formed on 13Cr stainless steel

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surface in a longer time.

To determine the electrochemical parameters, equivalent circuits shown in Fig. 10

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were used to fit the EIS data, where the equivalent circuit in Fig. 10(a) was used for the

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EIS of N80 carbon steel after coupled with 13Cr stainless steel for 1 h under dynamic

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condition and the equivalent circuit in Fig. 10(b) was used for the EIS of N80 carbon

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steel under other conditions, while the equivalent circuit in Fig. 10(c) was used for the

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EIS of 13Cr stainless steel. In the equivalent circuits, Rs is solution resistance; Qdl is constant phase element (CPE) representing the double layer capacitance; Rct is charge

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transfer resistance; Qf is constant phase element representing the capacitance of

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corrosion products film or passive film; Rf is the resistance of corrosion products film or passive film; L is inductance; RL is the resistance of inductance. The values of corresponding fitted parameters are listed in Table 5 and Table 6. It is seen that both the

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impedances (Rct + Rf) of N80 carbon steel and 13Cr stainless steel when they are coupled are less than those without coupling. Furthermore, the impedances of N80 carbon steel and 13Cr stainless steel increase with the prolongation of immersion time.

3.6. Surface morphologies of the specimens after corrosion Fig. 11 shows the SEM surface morphologies of N80 carbon steel and 13Cr stainless steel after corrosion in supercritical CO2-containing formation water under static and dynamic conditions for 34 h when they were coupled or uncoupled. It is seen

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that typical FeCO3 crystals are observed on the N80 carbon steel surface. The corrosion products on the N80 carbon steel coupled with 13Cr stainless steel are less compact

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than those without coupling. Especially under dynamic conditions (Fig. 11(d)), the corrosion products of N80 carbon steel coupled with 13Cr stainless steel are not

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compact and then have less protection for the corrosion of steel. For 13Cr stainless steel,

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no obvious corrosion products are observed on the steel surface without coupling (Fig.

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11(e, g)). The scratches produced by the abrasion process before corrosion test are still

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present. After coupled with N80 carbon steel, slight corrosion and a thin layer of

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corrosion products are observed on the steel surface, especially under dynamic conditions. Therefore, the galvanic effect is adverse to the formation of protective

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corrosion products on N80 carbon steel and passive film on 13Cr stainless steel.

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Fig. 12 shows the SEM surface morphologies of N80 carbon steel after coupled or uncoupled with 13Cr stainless steel in supercritical CO2-containing formation water under dynamic conditions (2 m/s) for different times. It is seen that a corrosion products

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film with cracks but no FeCO3 crystals are observed before 12 h no matter whether the N80 carbon steel is coupled with 13Cr stainless steel or not. Obviously, the corrosion products film has less protection for the corrosion of steel. After 18 h, FeCO3 crystals are present in the corrosion products. However, the formed FeCO3 crystals are not

compact under dynamic conditions.

3.7. XRD analysis After corrosion test, XRD was carried out to determine the composition of the

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corrosion products on N80 carbon steel surface after coupled with 13Cr stainless steel in supercritical CO2-containing formation water under dynamic conditions (2 m/s) for

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different times, and the diffraction patterns were shown in Fig. 13. It is seen that after coupled for 1 h, Fe and Fe3C are detected on the steel surface. It is obvious that Fe is

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derived from the steel substrate and Fe3C is the remainder after the preferential

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dissolution of ferrite. With the increase of immersion time to 12 h, corrosion products

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film becomes thicker and the Fe peaks disappear. Only Fe3C is detected on the steel

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surface. After coupled for 18 h, aside from the Fe3C, FeCO3 is also detected in the

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corrosion products. This corresponds to the observation of FeCO3 crystals in SEM image. With further corrosion for 34 h, the corrosion products are mainly FeCO3 with

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a small amount of Fe3C.

4. Discussion

4.1. Evolution of the galvanic corrosion behaviour of N80 carbon steel and 13Cr

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stainless steel in SC-CO2-containing formation water under dynamic conditions For the corrosion of steels under CO2 environment, the cathodic reactions mainly include the following reactions [9, 14, 45]: 2H+ + 2e- → H2

(2)

2H2CO3 + 2e- → H2 + 2HCO3-

(3)

2HCO3- + 2e- → H2 + 2CO32-

(4)

While the anodic reaction is the dissolution of steel: Fe → Fe2+ + 2e-

(5)

Fe2+ + CO32- → FeCO3

(6)

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product of [Fe] × [CO32-] exceeds the solubility product of FeCO3:

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During the corrosion process, the precipitation of FeCO3 will occur when the

For the galvanic corrosion of N80 carbon steel coupled with 13Cr stainless steel,

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it can be simplified as an equivalent electric circuit, as shown in Fig. 14, and the

E Ra  Rc  Rs

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(7)

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Ig 

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galvanic current density can be calculated by the following equations:

E  Ec  Ea

(8)

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where Ig is galvanic current; ΔE is the potential difference between 13Cr stainless steel

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and N80 carbon steel; Ec is the potential of 13Cr stainless steel; Ea is the potential of N80 carbon steel; Ra and Rc are the reaction resistance (the polarization resistance,

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which is equal to Rct + Rf) of N80 carbon steel and 13Cr stainless steel, respectively; Rs is the solution resistance. This equation was also applied to evaluate the galvanic

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current and determine the galvanic corrosion mechanism in the literature [46-48]. It is clear that Ig is determined by the potential difference (ΔE) and the total resistance in the whole circuit (Ra + Rc + Rs). As mentioned in section 3.3, with the prolongation of immersion time, the changes of coupled potential and galvanic current density under dynamic conditions present

different stages. A schematic diagram of the galvanic corrosion process between N80 carbon steel and 13Cr stainless steel under SC-CO2 environment is shown in Fig. 15(ae). The corresponding different stages (a-e) in the change of galvanic current density under dynamic conditions are shown in Fig. 5 (b). In the initial stage (before 1 h), the

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formation of passive film leads to a rapidly positive shift of the OCP of 13Cr stainless steel (Fig. 4). Then, the potential difference (ΔE) between N80 carbon steel and 13Cr

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stainless steel increases dramatically, i.e., an enhancement of driving force for galvanic corrosion. Therefore, the galvanic current density increases rapidly and the weight loss

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of N80 carbon steel coupled with 13Cr stainless steel is much higher than that without

N

coupling before 1 h. The EIS and polarisation curve of N80 carbon steel coupled with

A

13Cr stainless steel at 1 h reveal the relatively high corrosion rate. After 1 h, the

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galvanic current density decreases rapidly, which is associated with the positive shift of

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the OCP of N80 carbon steel while the OCP of 13Cr stainless steel is relatively stable. Then the potential difference (ΔE) between N80 carbon steel and 13Cr stainless steel,

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i.e., the driving force for galvanic corrosion, decreases rapidly. The positive shift of the

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OCP of N80 carbon steel may be attributed to the accelerated cathodic process due to the accumulated cathodic phase Fe3C on steel surface after the preferential dissolution of ferrite [49]. XRD analysis confirms the presence of Fe3C on steel surface. After about

A

5 h, the positive shift of the OCP of N80 carbon steel slows down because most the steel surface has been covered by Fe3C. Then a gentle decrease of galvanic current density is observed at this stage (about 5-16 h). After corrosion for 16 h, the galvanic current density decreases rapidly again,

which is attributed to the rapidly positive shift of the OCP of N80 carbon steel due to the formation of protective FeCO3 film on the steel surface. XRD analysis and SEM observation demonstrate the presence of FeCO3 in the corrosion products on the steel surface. The positive shift of the OCP of N80 carbon steel results in a rapid decrease in

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the potential difference (ΔE) between N80 carbon steel and 13Cr stainless steel. Furthermore, the formation of protective FeCO3 film on the steel surface results in a

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significant increase of reaction resistance (Ra), as shown in the EIS measurement.

Therefore, according to Eq (7), the sharp decrease of potential difference (ΔE) and the

U

increase of reaction resistance (Ra) should be responsible for the significant decrease in

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galvanic current density. After corrosion for about 20 h, the OCP of N80 carbon steel

A

positively shifts to a relatively stable value, while the reaction resistance (Ra) of N80

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carbon steel prominently increases due to the thick FeCO3 film on the steel surface.

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Therefore, the galvanic current density deceases to a relatively small stable value.

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4.2. Comparison of galvanic corrosion behaviour of N80 carbon steel under static and

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dynamic conditions

In static and dynamic supercritical CO2-containing formation water, the cathodic

and anodic reactions will occur simultaneously, resulting in the corrosion problem of

A

N80 carbon steel. Under dynamic conditions, the fluid flow accelerates the transportation of species to the steel surface (such as H+, H2CO3, and HCO3-) and the diffusion of Fe2+ to the bulk solution, which leads to the increase of corrosion rate under dynamic conditions. Therefore, the weight loss corrosion rate under dynamic conditions

is much higher than that under static conditions. Compared with static conditions, the high dissolution rate of steel under dynamic conditions in the initial period results in high Fe2+ concentration in the solution, which facilitates the formation of FeCO3 film on the steel surface. Therefore, the time at which the OCP of N80 carbon steel

is earlier under dynamic conditions than under static conditions.

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prominently shifts to positive direction due to the formation of protective FeCO3 film

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When the N80 carbon steel are coupled with 13Cr stainless steel under static and

dynamic conditions, the corrosion of N80 carbon steel is promoted by the galvanic

U

effect between the N80 carbon steel and 13Cr stainless steel. The galvanic effect under

N

dynamic conditions is more significant than that under static conditions due to the larger

A

potential difference and smaller impedance under dynamic conditions, which results in

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larger galvanic current density, as shown in Fig. 5. The weight loss measurements also

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confirm a larger galvanic effect under dynamic conditions, and a corrosion product film with less protection is formed on the steel surface, as shown in SEM images (Fig. 11).

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Therefore, the dynamic condition not only accelerates the corrosion rate of N80 carbon

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steel, but also strengthens the galvanic effect between N80 carbon steel and 13Cr stainless steel.

With the increase of immersion time, the variation trend of coupled potential and

A

galvanic current density under static conditions is similar to the case under dynamic conditions. The time at which prominently positive shift of coupled potential and then sharp decrease of galvanic current density appear is later than that under dynamic conditions. This situation could be attributed to the earlier formation FeCO3 under

dynamic conditions. Therefore, the difference of the coupled potential and galvanic current density under static and dynamic conditions is associated with the weight loss and electrochemical measurements, and SEM observation under static and dynamic

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conditions.

5. Conclusions

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The galvanic effect between N80 carbon steel and 13Cr stainless steel was studied

in supercritical CO2-containing formation water under static and dynamic conditions.

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The corrosion rate increases and a corrosion product film with less protection is formed

N

on the N80 carbon steel after coupled with 13Cr stainless steel. The corrosion of N80

A

carbon steel is promoted by the galvanic effect.

M

Under dynamic conditions, the fluid flow no only accelerates the corrosion of N80

ED

carbon steel and 13Cr stainless steel, but also strengthens their galvanic effect with higher galvanic current density. With the prolongation of immersion time, the galvanic

PT

current density decreases due to the decrease of potential difference between N80

CC E

carbon steel and 13Cr stainless steel and the increase of resistance because of the formation of protective corrosion products on N80 carbon steel surface. The galvanic current density under static conditions is less than that under dynamic

A

conditions. With the increase of immersion time, the variation trend of coupled potential and galvanic current density under static conditions is similar to that under dynamic conditions. The time at which prominently positive shift of coupled potential and then sharp decrease of galvanic current density appear is later than that under dynamic

conditions. This situation could be attributed to the earlier formation FeCO3 under dynamic conditions.

Acknowledgements

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The authors thank the support of National Natural Science Foundation of China (Nos. 51371086, 51571097). The authors also thank the support of analytical and

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M

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testing center of Huazhong University of Science and Technology.

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[47] J. Amri, E. Gulbrandsen, R.P. Nogueira, Pit growth and stifling on carbon steel in CO2-containing media in the presence of HAc, Electrochim. Acta 54 (2009) 7338-7344. [48] Y.Z. Li, N. Xu, X.P. Guo, G.A. Zhang, Inhibition effect of imidazoline inhibitor on

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the crevice corrosion of N80 carbon steel in the CO2-saturated NaCl solution containing acetic acid, Corros. Sci. 126 (2017) 127-141. [49] J.L. Mora-Mendoza, S. Turgoose, Fe3C influence on the corrosion rate of mild steel in aqueous CO2 systems under turbulent flow conditions, Corros. Sci. 44 (2002)

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1223-1246.

Figure captions Fig. 1. Schematic diagram of the experimental setup for in situ electrochemical measurements under dynamic supercritical CO2-water environments

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Fig. 2. Weight loss corrosion rate of N80 carbon steel after coupled or uncoupled with 13Cr stainless steel in supercritical CO2-containing formation water under static and

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dynamic conditions for different times

Fig. 3. Time dependence of the open circuit potentials of N80 carbon steel and 13Cr

N

U

stainless steel (without coupled) in supercritical CO2-containing formation water under

M

A

static and dynamic conditions: (a) static, (b) 2 m/s, (c) potential difference

Fig. 4. Time dependence of the open circuit potentials of N80 carbon steel and 13Cr

ED

stainless steel (when they are coupled) in supercritical CO2-containing formation water

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under static and dynamic conditions: (a) static, (b) 2 m/s, (c) potential difference

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Fig. 5. Time dependence of the coupled potential (a) and galvanic current density (b) between N80 carbon steel (WE1) and 13Cr stainless steel (WE2) in supercritical CO2

A

formation water under static and dynamic conditions

Fig. 6. Polarisation curves of N80 carbon steel (a) and 13Cr stainless steel (b) in supercritical CO2-containing formation water under static and dynamic conditions after these two electrodes were coupled or uncoupled for 34 h

Fig. 7. Polarisation curves of N80 carbon steel (a) and 13Cr stainless steel (b) in supercritical CO2-containing formation water under dynamic conditions (2 m/s) after

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these two electrodes were coupled for different times

Fig. 8. EIS of N80 carbon steel (a, b) and 13Cr stainless steel (c, d) in supercritical CO2-

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containing formation water under static and dynamic conditions after these two

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electrodes were coupled or uncoupled for 34 h

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Fig. 9. EIS of N80 carbon steel (a, b) and 13Cr stainless steel (c, d) in supercritical CO2-

M

ED

were coupled for different times

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containing formation water under dynamic conditions (2 m/s) after these two electrodes

Fig. 10. Equivalent circuits for EIS fitting of N80 carbon steel and 13Cr stainless steel

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in supercritical CO2-containing formation water under various conditions: (a) for the

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EIS of N80 carbon steel after coupled with 13Cr stainless steel for 1 h, (b) for the EIS of N80 carbon steel under other conditions, (c) for the EIS of 13Cr stainless steel

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Fig. 11. SEM surface morphologies of N80 carbon steel (a-d) and 13Cr stainless steel (e-h) after corrosion in supercritical CO2-containing formation water under static and dynamic conditions for 34 h when they were coupled or uncoupled: (a, e) static, uncoupled, (b, f) static, coupled, (c, g) 2 m/s, uncoupled, (d, h) 2 m/s, coupled

Fig. 12. SEM surface morphologies of N80 carbon steel after uncoupled (a-d) or coupled (e-h) with 13Cr stainless steel in supercritical CO2-containing formation water

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at 2 m/s for different times: (a, e) 1 h, (b, f) 12 h, (c, g) 18 h, (d, h) 34 h

Fig. 13. XRD of N80 carbon steel after coupled with 13Cr stainless steel in supercritical

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CO2-containing formation water at 2 m/s for different times

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Fig. 14. The equivalent circuit for the galvanic corrosion between N80 carbon steel and

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13Cr stainless steel

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Fig. 15. Schematic diagram of the galvanic corrosion process between N80 carbon steel

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and 13Cr stainless steel in supercritical CO2-containing formation water under dynamic conditions at different stages: (a) rapid increase of galvanic current density in the initial

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period (before 1 h), (b) rapid decrease of galvanic current density (1-5 h), (c) gentle

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decrease of galvanic current density (5-16 h), (d) rapid decrease of galvanic current density due to the formation of protective FeCO3 film (16-20 h), (e) relatively stable

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galvanic current density in the late period (after 20 h) (Note: The number of upward arrows ( ) represents the extent of increase in parameters; the number of downward arrow ( ) represents the extent of decrease in parameters.)

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

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ED

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A

N

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measurements under dynamic supercritical CO2-water environments

2 m/s, uncoupled 2 m/s, coupled static, uncoupled static, coupled

150

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100

50

0

0

4

8

12

16 20 Time (h)

24

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2

Weight loss corrosion rate (g/(m h))

200

28

32

Fig. 2. Weight loss corrosion rate of N80 carbon steel after coupled or uncoupled with

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13Cr stainless steel in supercritical CO2-containing formation water under static and

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PT

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M

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dynamic conditions for different times

-0.25

Potential (V vs. AgCl (0.1M KCl))

(a)

static

-0.30 -0.35 -0.40 -0.45

N80 carbon steel 13Cr stainless steel

-0.50 -0.55 -0.60

-0.70 -0.75

0

4

8

12

16 20 Time (h)

24

28

32

N80 carbon steel 13Cr stainless steel

-0.35 -0.40 -0.45

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-0.50

N

-0.55 -0.60 -0.65

0

4

8

12

16

20

24

28

32

Time (h)

400

static 2 m/s

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350 300 250

(c)

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Potential difference (mV)

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-0.70 -0.75

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(b)

2 m/s

-0.30

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Potential (V vs. AgCl (0.1M KCl))

-0.25

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-0.65

200 150

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100

50 0

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-50

0

4

8

12

16

20

24

28

32

Time (h)

Fig. 3. Time dependence of the open circuit potentials of N80 carbon steel and 13Cr stainless steel (without coupled) in supercritical CO2-containing formation water under static and dynamic conditions: (a) static, (b) 2 m/s, (c) potential difference

-0.40

(a)

-0.45 -0.50

N80 carbon steel 13Cr stainless steel

-0.55 -0.60 -0.65 -0.70 -0.75

0

4

8

12

16

20

24

28

32

Time (h) -0.40

(b)

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-0.50

N80 carbon steel 13Cr stainless steel

-0.55

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-0.60

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-0.65 -0.70

0

4

8

12

16 20 Time (h)

24

28

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-0.75

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Potential (V vs. AgCl (0.1M KCl))

2 m/s -0.45

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Potential (V vs. AgCl (0.1M KCl))

static

300

32

(c)

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static 2 m/s

200

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Potential difference (mV)

250

150

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100

50

0

4

8

12

16 20 Time (h)

24

28

32

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Fig. 4. Time dependence of the open circuit potentials of N80 carbon steel and 13Cr stainless steel (when they are coupled) in supercritical CO2-containing formation water under static and dynamic conditions: (a) static, (b) 2 m/s, (c) potential difference

(a) -0.50

2 m/s

-0.55 -0.60

static -0.65 -0.70 -0.75 0

4

8

12

16 20 Time (h)

24

28

32

a

c

b

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1.2

2 m/s

1.0

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0.8 0.6

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static

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0.4 0.2 0.0 0

4

8

12

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-0.2

(b)

e

d

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Galvanic current density (mA/cm2)

1.4

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Coupled potential (V vs. AgCl (0.1M KCl))

-0.45

16 20 Time (h)

24

28

32

Fig. 5. Time dependence of the coupled potential (a) and galvanic current density (b)

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between N80 carbon steel (WE1) and 13Cr stainless steel (WE2) in supercritical CO2

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formation water under static and dynamic conditions

-0.2

(a)

-0.3 -0.4 -0.5

-0.7 -0.8 -7 10

-6

-5

10

-4

-3

10 10 10 2 Current density (A/cm )

-0.2

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-0.1

static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled

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-0.3

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-0.4 -0.5 -0.6 -7

10

-6

-5

10 10 2 Current density (A/cm )

-4

10

-3

10

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-0.7 -8 10

-1

10

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0.0

10

(b)

13Cr stainless steel

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

0.1

-2

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-0.6

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

N80 carbon steel static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled

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Fig. 6. Polarisation curves of N80 carbon steel (a) and 13Cr stainless steel (b) in supercritical CO2-containing formation water under static and dynamic conditions after

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these two electrodes were coupled or uncoupled for 34 h

-0.3 -0.4

(a)

N80 cabon steel

1h 12 h 18 h 34 h

-0.5 -0.6

-0.8 -0.9 -5 10

-4

10

-3

-2

0.1

-0.2

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-0.1

N

-0.3

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-0.4

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-0.5 -0.6 -0.7 -8 10

-7

10

-6

-5

10 10 2 Current density (A/cm )

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

1h 12 h 18 h 34 h

10

(b)

13Cr stainless steel 0.0

-1

10 10 2 Current density (A/cm )

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-0.7

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

-0.2

-4

10

-3

10

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Fig. 7. Polarisation curves of N80 carbon steel (a) and 13Cr stainless steel (b) in

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supercritical CO2-containing formation water under dynamic conditions (2 m/s) after

A

these two electrodes were coupled for different times

1000

static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled Fitting line

100

0.051 Hz

60

40 10

100

20

IP T

0.051 Hz

1 0

0

100

200 300 2 Z' cm )

400

500

0.01

0.1

1

10 100 Frequency (Hz)

25000

SC R

(c)

13 Cr stainless steel

13 Cr stainless steel

10000

N

10000 0.033 Hz

5000

10

A

0.02 Hz 0.02 Hz

0

5000

10000 15000 2 Z' cm )

20000

M

0

100

25000

1 0.01

0.1

10

20

100

1000

Frequency (Hz)

ED

Fig. 8. EIS of N80 carbon steel (a, b) and 13Cr stainless steel (c, d) in supercritical CO2-

PT

containing formation water under static and dynamic conditions after these two

CC E

electrodes were coupled or uncoupled for 34 h

80

(d)

40

static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled Fitting line

1

0 10000

60

U

0.033 Hz

1000 2

static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled Fitting line

|Z| (cm )

15000

A

Z'' cm2)

20000

1000

Phase angle (degree)

200

(b)

N80 carbon steel

2

300

80

(a)

N80 carbon steel

0 10000

Phase angle (degree)

static, uncoupled static, coupled 2 m/s, uncoupled 2 m/s, coupled Fitting line

|Z| (cm )

Z'' cm2)

400

0 2

4

6

50

1h 12 h 18 h 34 h Fitting line

100

2 Z' cm )

40

0.051 Hz

0

50

100

150

0.01

200

0.1

1

2

10000

(c)

SC R

2

|Z| (cm )

10

A

1000

2000 3000 2 Z' cm )

4000

M

0.065 Hz 0.065 Hz

1000

5000

1 0.01

0.1

1h 12 h 18 h 34 h Fitting line

1

20

10

100

1000

Frequency (Hz)

ED

PT

containing formation water under dynamic conditions (2 m/s) after these two electrodes were coupled for different times

80

40

Fig. 9. EIS of N80 carbon steel (a, b) and 13Cr stainless steel (c, d) in supercritical CO2-

CC E

10000

60

U

0.082 Hz

100

N

0.082 Hz

0

1000

13Cr stainless steel (d)

1000

2000

A

2

Z'' cm )

3000

100

0

Frequency (Hz)

13Cr stainless steel 1h 12 h 18 h 34 h Fitting line

4000

10

IP T

20

Z' cm )

0

60

10

1 0

(b)

0 10000

Phase angle (degree)

1.36 Hz

100

N80 carbon steel

1h 12 h 18 h 34 h Fitting line

1.36 Hz

2

(a)

Phase angle (degree)

N80 carbon steel

2

Z'' cm2)

Z'' cm2)

80

4

|Z| (cm )

150

(a)

Rs

IP T

Q dl

(b)

Qf R ct

SC R

Rf

N

U

(c)

A

Fig. 10. Equivalent circuits for EIS fitting of N80 carbon steel and 13Cr stainless steel

M

in supercritical CO2-containing formation water under various conditions: (a) for the

ED

EIS of N80 carbon steel after coupled with 13Cr stainless steel for 1 h, (b) for the EIS

A

CC E

PT

of N80 carbon steel under other conditions, (c) for the EIS of 13Cr stainless steel

(a)

(b)

`

(c)

30 µm

(e)

(d)

30 µm

(f)

30 µm

30 µm

(h)

30 µm

SC R

IP T

(g)

30 µm

30 µm

30 µm

U

Fig. 11. SEM surface morphologies of N80 carbon steel (a-d) and 13Cr stainless steel

N

(e-h) after corrosion in supercritical CO2-containing formation water under static and

A

dynamic conditions for 34 h when they were coupled or uncoupled: (a, e) static,

A

CC E

PT

ED

M

uncoupled, (b, f) static, coupled, (c, g) 2 m/s, uncoupled, (d, h) 2 m/s, coupled

(c)

30 µm

(e)

(d)

30 µm

30 µm

(g)

(f)

(h)

30 µm

30 µm

SC R

30 µm

30 µm

IP T

(b)

(a)

Fig. 12. SEM surface morphologies of N80 carbon steel after uncoupled (a-d) or

U

coupled (e-h) with 13Cr stainless steel in supercritical CO2-containing formation water

A

CC E

PT

ED

M

A

N

at 2 m/s for different times: (a, e) 1 h, (b, f) 12 h, (c, g) 18 h, (d, h) 34 h

30 µm

0

2

8000

4

6

8

10



 FeCO3

6000





4000

                   



2000

 

 

  

 





34 h



18 h







12 h







1h

00-006-0696 Fe

SC R

00-035-0772 Fe3C

IP T

Intensity

 Fe Fe3C

00-029-0696 FeCO3 20

30

40

50

60

70

80

2(degree)

90

100

N

U

Fig. 13. XRD of N80 carbon steel after coupled with 13Cr stainless steel in supercritical

A

CC E

PT

ED

M

A

CO2-containing formation water at 2 m/s for different times

CPE c

CPE a

Rc

IP T

Rs

Ra

ΔE

Fig. 14. The equivalent circuit for the galvanic corrosion between N80 carbon steel and

A

CC E

PT

ED

M

A

N

U

SC R

13Cr stainless steel

Flow

(a) Solution

Fe2+

Fe2+

HCO3- H+ 2-

CO3

2e+ Ra

2e+

Rs

Rc

Ig

H+

Passive Film

Ec

High potential (Cathode) 13Cr stainless Steel

ΔE

N80 carbon Steel

Flow Fe2+

Fe2+

2-

CO3

2e+

Fe2+

Rs

CO32Rc

Ea

Ra

H+ HCO3-

H2CO3

H+

Flaky Fe3C Ig

Low potential (Anode)

13Cr stainless Steel

Flow Fe2+ Fe2+ CO32H+ 2e+

Fe2+

2e+ Corrosion Products

Ra Ea

Low potential (Anode)

H2CO3

HCO3-

Passive Film

H+

Rs

Rc

Flaky Fe3C Ig

N80 carbon Steel

Ec

N

Solution

U

(c)

Ec

High potential (Cathode)

ΔE

N80 carbon Steel

Passive Film

SC R

Solution

IP T

(b)

High potential (Cathode) 13Cr stainless Steel

A

ΔE

Flow

Corrosion Products

Fe2+ CO32- + Fe2+ 2CO3

ED

Solution

Fe2+

M

(d)

Ra

Ea

FeCO3

Low potential (Anode)

PT

N80 carbon Steel

CC E

Solution

Fe2+

Corrosion Products

A

HCO3-

Flaky Fe3C

Ea

Low potential (Anode)

(e)

H2CO3

CO32-

Fe2+

Rs

Rc

CO32-

H+

Passive Film

Ec

Flaky Fe3C High potential (Cathode)

Ig

13Cr stainless Steel

ΔE

Fe2+

H2CO3 Rs

Ea FeCO3 Flaky Fe3C

Low potential (Anode)

N80 carbon Steel

HCO3-

Flow

CO32- + Fe2+ Fe2+ CO32-

Ra

H2CO3

Ig ΔE

CO32-

HCO3H+ Rc

Passive Film

Ec

High potential (Cathode) 13Cr stainless Steel

Fig.15. Schematic diagram of the galvanic corrosion process between N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water under dynamic conditions at different stages: (a) rapid increase of galvanic current density in the initial

period (before 1 h), (b) rapid decrease of galvanic current density (1-5 h), (c) gentle decrease of galvanic current density (5-16 h), (d) rapid decrease of galvanic current density due to the formation of protective FeCO3 film (16-20 h), (e) relatively stable galvanic current density in the late period (after 20 h) (Note: The number of upward

A

CC E

PT

ED

M

A

N

U

SC R

arrow ( ) represents the extent of decrease in parameters.)

IP T

arrows ( ) represents the extent of increase in parameters; the number of downward

Table 1 The nominal chemical composition (wt%) of N80 carbon steel and 13Cr stainless steel C

Si

Mn

P

S

Cr

Cu

Mo

Ni

Fe

N80 carbon steel

0.34

0.20

1.45

0.02

0.015

0.15

0.008

0.18

0.03

bal.

13Cr stainless steel

0.08

1.00

1.00

0.035

0.030

13.05

/

/

0.50

bal.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Elements

Table 2 Composition of the formation water used for test NaCl

CaCl2

Na2SO4

MgCl2.6H2O

NaHCO3

Concentration (g/L)

41.20

0.76

0.71

3.52

0.51

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Chemical

Table 3 Fitted electrochemical parameters of the polarization curves of N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water under static and dynamic conditions after coupled or uncoupled for 34 h Conditions

N80 carbon steel

static, uncoupled

-0.561

1.86×10-4

-107

static, coupled

-0.588

2.19×10-4

-103

2 m/s, uncoupled

-0.495

1.74×10-5

123

-91.0

2 m/s, coupled

-0.560

1.63×10-4

206

-108

static, uncoupled

-0.553

5.43×10-6

static, coupled

-0.525

1.89×10-5

2 m/s, uncoupled

-0.565

7.48×10-6

2 m/s, coupled

-0.533

1.64×10-5

bc (mV/dec)

N A M ED PT CC E A

Epit (V vs. Ag/AgCl (0.1 M KCl))

ip (A/cm2)

-59.7

0.049

3.94×10-6

-100

-0.029

5.08×10-5

-58.6

-0.079

1.19×10-5

-133

-0.121

1.06×10-5

SC R

ba (mV/dec)

U

13Cr stainless steel

icorr (A/cm2)

IP T

Electrodes

Ecorr (V vs. Ag/AgCl (0.1 M KCl))

Table 4 Fitted electrochemical parameters of the polarization curves of N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water at 2 m/s after coupled for different times Electrodes

Times

Ecorr (V vs. Ag/AgCl (0.1 M KCl))

N80 carbon steel

1h

-0.689

9.29×10-3

246

-259

12 h

-0.654

7.89×10-3

228

-228

18 h

-0.611

3.27×10-3

338

-159

34 h

-0.560

1.63×10-4

206

-108

1h

-0.501

1.11×10-5

-93

-0.044

1.91×10-5

12 h

-0.477

1.29×10-5

-140

-0.003

3.03×10-5

18 h

-0.484

1.43×10-6

-87

-0.095

7.68×10-6

34 h

-0.533

1.64×10-5

-132

-0.121

1.06×10-5

N A M ED PT CC E A

54

Epit (V vs. Ag/AgCl (0.1 M KCl))

ip (A/cm2)

IP T

bc (mV/dec)

SC R

ba (mV/dec)

U

13Cr stainless steel

icorr (A/cm2)

Table 5 Fitted electrochemical parameters of the EIS of N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water under static and dynamic conditions after coupled or uncoupled for 34 h Rs

Qdl

Qf

N80 carbon steel

static, uncoupled

1.38

1.13×10-2

0.89

298.1

4.59×10-2

0.76

254.6

static, coupled

1.58

9.78×10-3

0.89

287.8

9.54×10-2

0.86

120.6

2 m/s, uncoupled

1.61

1.02×10-2

0.89

423.8

2.25×10-2

0.79

290.3

2 m/s, coupled

1.49

1.17×10-2

0.89

151.7

8.97×10-2

0.98

25.1

static, uncoupled

0.24

1.16×10-2

0.75

31.02

3.65×10-4

0.85

95060

static, coupled

1.55

4.07×10-4

0.85

2 m/s, uncoupled

1.79

5.42×10-4

0.76

2 m/s, coupled

1.71

4.22×10-3

0.73

N A M ED PT CC E A

55

(Ω-1

cm-2 s-n2)

(Ω cm2)

IP T

(Ω

cm2)

SC R

cm-2 s-n1)

545.8

1.53×10-3

0.65

4551

4451

1.19×10-3

0.89

19530

40.28

7.23×10-5

0.93

6733

U

13Cr stainless steel

(Ω

(Ω-1

n2

Rf

Conditions

cm2)

n1

Rct

Electrodes

Table 6 Fitted electrochemical parameters of the EIS of N80 carbon steel and 13Cr stainless steel in supercritical CO2-containing formation water at 2 m/s after coupled for different times Rs

Qdl

Qf

RL

N80 carbon steel

1h

1.43

9.42×10-3

0.81

2.37

0.378

0.98

0.43

12 h

1.43

5.19×10-3

0.84

1.78

9.37

0.93

1.14

18 h

1.49

4.77×10-2

0.79

3.70

8.53

0.99

1.13

34 h

1.49

1.17×10-2

0.89

151.7

8.97×10-2

0.98

25.1

1h

1.53

3.79×10-4

0.82

749.8

2.01×10-3

0.75

3453

12 h

1.56

6.47×10-4

0.81

769.4

1.46×10-3

0.72

2329

18 h

1.42

6.25×10-4

0.89

1262

1.02×10-3

0.71

3271

34 h

1.71

4.22×10-3

0.73

40.28

7.23×10-5

0.93

6733

(Ω-1

cm-2 s-n2)

N A M ED PT CC E A

56

(Ω

cm2)

(

L cm2)

1.95

(H/cm2) 5.84

IP T

(Ω

cm2)

SC R

cm-2 s-n1)

U

13Cr stainless steel

(Ω

(Ω-1

n2

Rf

Time

cm2)

n1

Rct

Electrodes