2-Mercaptobenzothiazole as a corrosion inhibitor for carbon steel in supercritical CO2-H2O condition

Applied Surface Science 476 (2019) 422–434

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Full Length Article

2-Mercaptobenzothiazole as a corrosion inhibitor for carbon steel in supercritical CO2-H2O condition

T

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Hongyu Cena, Jiaojiao Caoa, Zhenyu Chena, , Xingpeng Guoa,b a

Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China

A R T I C LE I N FO

A B S T R A C T

Keywords: 2-Mercaptobenzothiazole Corrosion inhibition Supercritical CO2 Quantum chemical calculations

The corrosion inhibition performance of 2-mercaptobenzothiazole (MBTH) for carbon steel in a CO2-H2O system was investigated by weight loss, surface analysis, electrochemical measurements, and quantum chemical calculations. Results showed that MBTH could effectively protect carbon steel from CO2 corrosion and inhibition efficiency of MBTH in supercritical CO2 was higher than that in non-supercritical CO2 situation. The differential capacitance curves revealed that excess positive charges would be on the surface of carbon steel in the supercritical-CO2 system while negative charges accumulated on carbon steel surface in non-supercritical CO2 condition. The adsorption model was proposed to explain the inhibition mechanism of MBTH.

1. Introduction With the accelerating industrial production, abundant CO2 will be emitted into the air every year, which intensifies the global greenhouse effect directly and attracts more and more attentions of people [1–3]. Environmental problems caused by CO2 emission, such as extreme weather and natural disasters, brought great threats to human survival and development. But fortunately, the Carbon Capture and Storage (CCS) technology, which has been implemented by capturing CO2 from the industrial sources, compressing the CO2 for transmission and injecting the CO2 into underground geological storage has the potential to prevent CO2 emissions into the atmosphere through the application such as enhanced oil recovery (EOR), being praised as the only process to create significant and immediate impact for CO2 level at present [4,5]. Notably, CO2 is ordinarily compressed into supercritical state for transportation, whose temperature exceeds 31.1 °C and the pressure is higher than 7.38 MPa. The supercritical CO2 is between the gas and liquid and has characteristics of both gas and liquid. The density of supercritical CO2 is close to that of liquid and the diffusion coefficient is nearly 100 times bigger than that of liquid [6,7]. Since supercritical CO2 has no surface tension, small viscosity and great diffusion coefficient, it can penetrate into various micro-pores after being injected to strata and expel oil in pores. Meanwhile, the supercritical CO2 has strong salvation ability. It can extract heavy hydrocarbons from crude

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oil [8,9], and improve flow of strata. Therefore, oil driving by supercritical CO2 can solve CO2 sealing problem and increase the oil extraction efficiency. This technique is environmental-friendly and will not cause pollution to the natural environment, becoming a popular technology of petroleum exploitation in the world [10–13]. Nevertheless, the corrosion of pipeline metal during oil driving by supercritical CO2 should be paid attention to [1,4]. Casing pipes and gathering pipeline in oil and gas fields as well as the transportation pipelines in the CO2 enhanced oil recovery (EOR) technology all adopt to carbon steel with high strength and low cost. Supercritical CO2-H2O has strong electrochemical corrosion to carbon steel [1,14–16]. Similar with CO2 corrosion of carbon steel under non-supercritical CO2 conditions, supercritical CO2 corrosion is determined by material properties and environmental factors to a large extents, such as temperature, pressure, solution composition, flow velocity and alloy structures [3,6,17]. Existing studies have demonstrated that influences of fluid on corrosion velocity of carbon steel would not change suddenly as CO2 enters into the supercritical state and the corrosion mechanism still remained same [17,18]. Corrosion inhibitor is one of the most effective methods to inhibit CO2 corrosion in oil and gas fields. It has significant advantages, such as simple processing and low cost [19–22]. Great progresses have been achieved in development and mechanism study of corrosion inhibitor in the non-supercritical CO2 environment. However, there are rare studies about corrosion inhibitor under supercritical CO2 conditions. Zhang

Corresponding author. E-mail address: [email protected] (Z. Chen).

https://doi.org/10.1016/j.apsusc.2019.01.113 Received 10 August 2018; Received in revised form 10 January 2019; Accepted 12 January 2019 Available online 16 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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conventional CO2 corrosion inhibitor in supercritical CO2 environments medium and acquiring the corrosion inhibitor in supercritical CO2 environments have become an important content to solve the corrosion of supercritical CO2-H2O in EOR and even CCS technology. 2-mercaptobenzothiazole (MBTH) has been widely used as rubber accelerator, bactericide, corrosion inhibitor [25,26]. MBTH and its derivatives can process sulfides very effectively and used for floating collection of heavy metals and gold-silver ores [27,28]. MBTH is also a high-efficiency corrosion inhibitor for the containing of N and S heterocyclic rings in the molecules [26,29–32]. Karpagavalli [33] et al. studied the brass corrosion inhibition performance of MBTH and Tween-80 compound in 0.2 mol/L NaCl medium, finding that the inhibition efficiency can reach 97%. Matjaž Finšgar [29] et al. studied the copper corrosion inhibition performance of MBTH in 3 wt% NaCl medium. Based on the electrochemical testing, they reported that the use of MBTH can reduce the corrosion current density to 10−7 A·cm−2. Existing studies on performance of MBTH as the corrosion inhibitor mainly focus on metallic copper fields for a long time. The mechanism was widely believed that S atoms outside the heterocyclic ring and N atoms in the heterocyclic ring form stable chemisorptions on copper metal surface to isolate the corrosion medium effectively [30,31]. However, there are rare studies on carbon steel corrosion performance of MBTH under CO2 conditions. In this paper, corrosion inhibition differences of MBTH in oil field simulation water system under supercritical CO2 and non-supercritical CO2 were studied by weight loss method, XPS analysis, SEM morphological analysis, electrochemical measurement, and quantum chemistry calculation. The adsorption model of MBTH on carbon steel surface and the corrosion inhibition mechanism of MBTH under supercritical CO2 were elaborated according to experimental results.

SH N Fig. 1. Structures of MBTH. Table 1 Weight loss experiment results of carbon steel under different CO2 partial pressure at 50 °C. Pressure (MPa)

Corrosion rate (mm/a)

Blank

2.0 4.0 7.0 8.5

1.87 2.36 2.68 3.00

MBTH

2.0 4.0 7.0 8.5

0.061 0.100 0.137 0.069

[23,24] et al. evaluated 4 corrosion inhibitors in the supercritical CO2 environments, which are effective in non-supercritical CO2 environment, including two tetrahydroglyoxaline corrosion inhibitors, one amides corrosion inhibitor and one quaternaries corrosion inhibitor. Results demonstrated that the corrosion rate of carbon steel after the selected corrosion inhibitors were added was higher than 1.5 mm/a at 50 °C, indicating that conventional CO2 corrosion inhibitor may fail in supercritical CO2 environment. However, the failure mechanism still remained a mystery [8,23,24]. Exploring the failure mechanism of

Fig. 2. Corrosion morphologies of carbon steel under different pressure CO2 conditions: (a) blank at 7.0 MPa, (b) blank at 8.5 MPa, (c) MBTH at 7.0 MPa, (d) MBTH at 8.5 MPa. 423

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Fig. 3. Wide-scan XPS spectra of carbon steel after immersion in solution containing 100 mg·L−1 MBTH for 72 h under (a) 7.0 MPa, (b) 8.5 MPa pressure CO2.

2. Experimental method

2.3. Surface analysis

2.1. Material

After the end of weight loss experiment, the sample surface was cleaned by deionized water and stored in a vacuum drier for surface analysis. Sample surface morphology was investigated using the SEM instrument (Phillips Quanta 200 SEM system). Corrosion product film on sample surface was analyzed by AXIS-ULTRA DLD-600W XPS instrument and aluminum target material was used.

Carbon steel was used as the experimental material. The main chemical composition (mass fraction, %) of carbon steel was C 0.3407, Si 0.2923, Mn 1.3898, P 0.0152, S 0.0132, Cr 0.45, Ni 0.0282, Mo 0.3 and Fe (rest proportion). All experimental materials were processed into 50 mm × 10 mm × 3 mm pieces for the weight loss experiment, the exposure area was 13.6 cm2. They were made into cylinder electrode for electrochemical test and the exposed area was 0.19 cm2. The working electrodes were grinded to mirror surface by 400 #, 800 # and 1200 # abrasive papers successively before each use, scrubbed by acetone and dried by cold air. Analytical pure MBTH (Aladdin Reagent Company) was used as the corrosion inhibitor. The molecular structure is shown in Fig. 1. The experimental medium was the extruded water from simulated oil field strata. Its composition was 4.84 g/L NaCl, 1.76 g/L NaHCO3, 0.3 g/L KCl, 0.28 g/L CaCl2, 0.24 g/L MgCl·6H2O and 0.2 g/L Na2SO4. Water samples were saturated by ventilation with 99.99% CO2 for 8 h.

2.4. Electrochemical measurements Electrochemical experiment was performed in the Gamary interface 3000 electrochemical working station under the experimental temperature of 50 ± 1 °C. Experimental test device used the high-pressure testing device which was designed by us previously [34]. The same procedure was adopted to control pressure and temperature of autoclave as the weight loss test. The traditional three-electrode system was used and the carbon steel electrode was used as the working electrode. Ag/AgCl electrode and Pt electrode were used as the reference electrode and auxiliary electrode. When the open circuit potential (OCP) reaches the stability, the electrochemical test was carried out and the electrochemical impedance spectra were performed continuously as a function of time to determine a stable corrosion rate. The potentiodynamic polarization curves of cathode and anode were measured separately after 72 h and the scanning potential range was divided into 0 ∼ −250 mV and 0 ∼ +250 mV in relative to the OCP with the scanning rate of 0.5 mV/s. Electrochemical impedance spectra were measured using a sinusoidal potential excitation amplitude of 5 mV within a frequency range of 100 kHz to 10 mHz. The impedance data were fit using ZsimpWin software with an equivalent circuit.

2.2. Weight loss The hanging time of weight loss is 72 h and experimental temperature is 50 ± 1 °C. Before the experiment, samples were cleaned by acetone, rinsed by absolute ethyl alcohol and dried by cold air. Autoclave need to be preheated around 50 °C before the samples and solution were added. Then, the samples were fixed to avoid touches with autoclaves and adding solution subsequently. Purging CO2 removes air continually for 30 min at ambient temperature and pressure from autoclave after it was sealed and heating the device in succession with the heating-up time within 30 min. CO2 was injected into autoclave at designated pressure via a booster pump when the temperature of device was stable at 50 ± 1 °C. The actual pressures could be regulated via a pressure indicator on autoclave with a precision of 0.1 MPa. After the hanging test, samples were taken out to eliminate corrosion products on the surface. Samples were rinsed by deionized water and acetone successively, dried by cold air and then weighted.

Vcorr =

8.76 × 10 4 × Δm ρAt

2.5. Quantum chemical studies The molecular structure was optimized by the standard Gaussian 09 software packet and the calculation used the DFT/B3LYP method. Molecular structure was optimized on the level of 6-311++G(d,p) basis set. The highest occupation momentum orbit HOMO (EHOMO), the lowest occupation moment orbit LUMO (ELUMO) and difference between HOMO and LUMO (ΔE), dipole moment (μ) and charge transfer number (ΔN) are important parameters to calculate molecular performance of MBTH.

(1)

where Vcorr is the corrosion rate (mm/y), Δm is the weight loss mass of samples (g), ρ is the material density (g/cm3), A is the contact area between samples and medium (cm2), and t is the hanging time (h). 424

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Fe2(OH)2CO3

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Fe2O3

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Fig. 4. High-resolution XPS spectra obtained by immersing the carbon steel in solution containing 100 mg·L−1 MBTH for 72 h in 7.0 MPa and 8.5 MPa pressure CO2 of C 1s (a-b), N 1s (c-d), O 1s (e-f), Fe 2p (g-h) and S 2p (i-j), respectively.

3. Results

the corrosion rate of samples declined sharply. Before the CO2 partial pressure reaches the supercritical pressure, the corrosion rate of samples with MBTH is positively correlated with CO2 partial pressure. However, the corrosion rate decreases when the CO2 partial pressure increases from non-supercritical condition (< 7.38 MPa) to the supercritical state. This might be related with changes of the adsorption state of corrosion inhibitor on carbon steel surface under supercritical CO2 conditions.

3.1 . Weight loss measurements Under the experimental temperature of 50 °C, the corrosion rates of carbon steel in blank solution and MBTH-containing solution under different CO2 partial pressure are listed in Table 1. Under blank conditions, the corrosion rate of carbon steel increased gradually with the increase of CO2 partial pressure. After 100 mg·L−1 MBTH was added, 425

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716

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SO4

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Binding Energy (eV)

Binding Energy (eV) Fig. 4. (continued)

3.2. SEM analysis

Table 2 Fitting parameters for C 1s, N 1s, O 1s, Fe 2p, and S 2p XPS spectra in the outermost corrosion product films after 72 h immersion. Valence state

Pressure

Binding energy (eV)

Proposed components

C 1s

7.0 MPa

288.1 286.2 284.6 288.0 286.0 284.6 399.1 399.3 531.7 531.0 529.8 531.4 531.1 529.6 712.2 710.9 709.8 712.4 711.7 710.2 168.4 164.3 168.6 164.1 162.0

C]N, C]S CeN, CeS CeC, CeH C]N, C]S CeN, CeS CeC, CeH CeN, C]N, NeH CeN, C]N, NeH FeOOH Fe2(OH)2CO3 Fe2O3 FeOOH Fe2(OH)2CO3 Fe2O3 FeOOH Fe2O3 FeeS FeOOH Fe2O3 Fe-S SO42− C]S, CeS SO42− C]S, CeS SeFe

8.5 MPa

N 1s O 1s

7.0 MPa 8.5 MPa 7.0 MPa

8.5 MPa

Fe 2p

7.0 MPa

8.5 MPa

S 2p

7.0 MPa 8.5 MPa

After the weight loss tests, samples were collected from the autoclave and dried by cold air. Microscopic morphologies of samples were analyzed by SEM (Fig. 2). Blank samples have similar surface morphologies under 7.0 MPa and 8.5 MPa. Corrosion products adhered onto the carbon steel are loose and the surface product film has been broken, accompanied with falloff of local films. The corrosion product scale showed poor protectiveness owing to the local defects and failures such as pore in the scale, which was easily for aggressive medium to approach to the metal surface [17,35]. After addition of MBTH, no obvious corrosion products were adhered onto carbon steel surface and the carbon steel surface was relatively even. In particular, scratches left on carbon steel surface after grinding can be seen clearly under 8.5 MPa, indicating that carbon steel is protected well and MBTH adsorption on carbon steel surface forms a dense protective film.

3.3. X-ray photoelectron spectroscopy (XPS) analysis Based on above results, MBTH has excellent corrosion inhibition effect in the CO2 corrosion environment, which is because MBTH can form a dense chemical adsorption film on carbon steel surface that separate invasion of corrosion medium effectively. To explore the corrosion inhibition mechanism of MBTH better, characteristic of films on the carbon steel surface were studied by XPS. The carbon steel specimens were hanged in the solution containing 100 mg·L−1 MBTH at 50 °C for 72 h under 7.0 MPa and 8.5 MPa pressure CO2 conditions. Samples were rinsed by deionized water and scrubbed by ethyl alcohol 426

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Fig. 5. Polarization curves for carbon steel measured at different CO2 pressure in solution in the absence (blank, a) and presence (b) of MBTH.

molecules and Fe [27,37,46,47]. It can be seen from Fig. 4 (g) and (h) that the Fe-S peak intensity under 8.5 MPa is higher than that under 7.0 MPa. This is mainly because after CO2 partial pressure reaches the supercritical state, steel surface state changes and the adsorption of MBTH molecules on surface is affected. The adsorption performance is significantly better than that under non-supercritical state. It can be seen from spectra of S 2p that peaks of S-Fe bonds can be fitted at 162.0 eV under 8.5 MPa [26,37,48,49]. The occurrence of SO42− at 168.6 eV might be the SO42− in the experimental solution left on the steel surface by combining positive ions [22,37,48,50]. The peaks at 164.3 eV are attributed to C]S and CeS structures.

Table 3 Electrochemical parameters obtained from polarization curves for carbon steel. PCO2 (MPa)

Ecorr vs. Ag/ AgCl (mV)

Icorr (A·cm−2)

ba·(mV/dec)

bc·(mV/dec)

Blank

2.0 4.0 7.0 8.5

−737 −698 −681 −681

2.50 × 10−4 3.45 × 10−4 3.84 × 10−4 5.13 × 10−4

69 66 51 50

−112 −98 −173 −187

MBTH

2.0 4.0 7.0 8.5

−692 −628 −653 −601

7.80 × 10−6 2.79 × 10−5 3.47 × 10−5 1.22 × 10−5

61 64 71 63

−111 −101 −89 −105

3.4. Potentiodynamic polarization cotton balls, dried by cold air and stored in vacuum bags for XPS test. Fig. 3 shows the broadband scanning spectra. Fig. 3 (a) and Fig. 3 (b) are test results under 7.0 MPa and 8.5 MPa. There are obvious peaks of N and S, indicating the adsorption of MBTH molecules on sample surface. The high-resolution spectra of C 1s, N 1s, O 1s, Fe 2p and S 2p under 7.0 MPa and 8.5 MPa are shown in Fig. 4. Specific parameters and corresponding chemical structure are listed in Table 2. In the high-resolution C 1s spectra, three signal peaks of C element were fitted under two pressures. The binding energy of 284.6 eV is corresponding to CeC and CeH, while the binding energies of 286.2 eV and 288.1 eV are assigned to CeS/CeN and C]N/C]S, respectively [22,36–38]. Since MBTH molecules has resonant structural changes in the solution, C]N and C]S can develop simultaneously [29,31]. In the N 1s spectra, the binding energies of 399.1 eV and 399.3 eV under two pressures develop the peak intensities, which proved the adsorption of MBTH molecules on carbon steel surface [22,39]. The binding energies of CeN, C]N and NeH are 399.5 eV, 399.6 eV and 399.1 eV, respectively. Due to slight difference among them, it is difficult to distinguish them even in highresolution spectra. Therefore, there’s only peak in two N 1s spectra, which is developed at 399.1 eV and 399.3 eV, respectively [22,36–38,40]. The fitting of O 1s spectra demonstrated that O mainly exists in FeOOH, Fe2O3 and Fe2(OH)2CO3 [1,37,41–43]. The occurrence of Fe2O3 might be the consequences of sample oxidation in the air before the XPS test. Fe2(OH)2CO3 peaks are fitted at 531 eV and 531.1 eV, respectively. According to relevant studies [44,45], corrosion products in the CO2 environment under 60 °C are difficult to form FeCO3 and is likely to exist in Fe2(OH)2CO3. The occurrence of Fe2 (OH)2CO3 is formed by the reaction between the Fe2+ which is generated after sample corrosion and corrosion medium in solution. The spectra of Fe 2p fitted peaks of FeeS bond at 709.8 eV and 710.2 eV, indicating the formation of bonds between sulfydryl in MBTH

The polarization curves of carbon steel electrodes in solution with and without MBTH under different CO2 partial pressure at 50 °C are shown in Fig. 5(a) and Fig. 5(b). It can be seen from Fig. 5(a) that in blank solution, polarization curves under different pressures have no significant differences and the anode polarization curves are basically consistent. Since the depolarizer H2CO3 content increases with the increase of CO2 partial pressure, the corrosion potentials move positively. The corresponding current density of cathodic polarization curve increases and the corrosion is aggravated. The mass transfer-controlled cathodic process was obvious in 2 MPa CO2 condition, while the rates of diffusion processes were accelerated in the higher CO2 pressures. The fitting results of polarization curves via weak polarization region are shown in Table 3. Fig. 5(b) depicts the polarization curves in MBTHcontaining solution. The OCP moves towards the positive region and corrosion current density (icorr) decrease rapidly in contrast to blank condition, indicating that MBTH belongs to the mixed-type inhibitor and inhibits the anodic reaction more evidently. Under non-supercritical conditions (< 7.38 MPa), icorr increases with the increase of CO2 partial pressure, indicating that the corrosion rate is positively related with pressure. Once the CO2 partial pressure increases to 8.5 MPa (supercritical condition), the OCP moves positively and the corresponding icorr decreases sharply. Furthermore, the anode region of the polarization curve of 2 MPa and 8.5 MPa has evident adsorption and desorption characters for corrosion inhibitors, indicating the compact effect for MBTH in the special conditions. 3.5. Electrochemical impedance spectroscopy measurements Impedance spectra of carbon steel electrodes in blank solution and MBTH-containing solution under different CO2 partial pressure and various test time at 50 °C are shown in Figs. 6 and 7. The electrochemical impedance spectra were performed continuously as a function 427

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Fig. 6. Nyquist plots for carbon steel measured under different CO2 pressure after various time in blank solution for (a) 2 MPa, (b) 4 MPa, (c) 7 MPa, (d) 8.5 MPa.

spectra stabilizing approximately at 24 h. The adsorptions of MBTH have occurred mainly in first 6 h and maintain stability, while the longterm immersion makes negative effect for inhibition film and results in the slight decrease. Even through under different pressures, all the capacitive loop is relatively flat, which is caused by dense coverage of MBTH on carbon steel electrode. At this moment, iron ions can only penetrate the corrosion inhibitor layer in local regions and form the anode current, thus resulting in extremely uneven distribution of current. Before the supercritical state, CO2 pressure is negatively related with the diameter of the capacitive semicircle. Once the CO2 pressure increases to 8.5 MPa and enters into the supercritical state, the diameter of the capacitive semicircle increases significantly. Two equivalent circuits, shown in Fig. 8, were proposed to fit the EIS data measured in a blank solution (a) and in solutions containing 100 mg·L−1 MBTH (b), where Rs is the solution resistance, Rf is inhibition film resistance, Rct is charge transfer resistance, CPEct is the constant phase element representing the double layer capacitance, RL is the inductance resistance and L is the inductance. CPEf is the constant phase element representing the capacitance of film and n is the deviation parameter representing the phase shift. The fitting results of impedance are shown in Tables 4 and 5. The sum of inhibition film resistance and charge transfer resistance (Rf + Rct) for MBTH, which is inversely proportional to corrosion rate, reduce generally after 72 h immersion with the CO2 pressure increasing before supercritical state. However, when the pressure increases from the non-supercritical state (7.0 MPa) to supercritical state (8.5 MPa), Rf increase sharply, indicating that the MBTH adsorption on electrode surface is denser and

of time to determine a stable corrosion rate and investigate the performance change of corrosion for carbon steel in different time. It can be seen from Fig. 6 that all the impedance spectra in blank environment are characterized by capacitive and inductive loops in the high and low frequency ranges, respectively, under supercritical or non-supercritical conditions. The inductive loop is formed by deposition of corrosion products on electrode surface in blank environments. Even after 72 h test, the existence of inductive loop represents the less compact corrosion product and sustained active dissolution of the steel surface, which means the ions in solution can still permeate the corrosion product film and reach the reactive interface easily [35,51]. With the immersion time from 2 h to 12 h, the diameter of capacitive semicircle decrease gradually owing to the preferential dissolution of ferrite, which increases surface area accessible for cathodic reaction and galvanic corrosion effect with cementite [34,52]. After that, capacitive semicircle increase by degrees from 12 h to 72 h due to the high ferrous ions promoting the formation of scale. In addition, the diameter of the capacitive semicircle in blank conditions decreases gradually after 72 h test with the pressure from 2 MPa to 8.5 MPa, indicating the strengthening corrosion. Before and after the CO2 entering into the supercritical state, no significant changes of impedance have been observed between 7 MPa and 8.5 MPa. After MBTH was added, the diameter of the capacitive semicircle in Fig. 7 increases significantly in contrast to blank condition under different pressures, showing the prominent inhibition effect. With the immersion time increases, the capacitive semicircles increase at the initial stage and then decrease in petty, with the maximum impedance 428

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-400 -200

0

-200 0 0

200

400

600

Z' ohm cm

2

800

1000

1200

0

200

400

600

800

1000

Z' ohm cm

1200

1400

1600

2

Fig. 7. Nyquist plots for carbon steel measured under different CO2 pressure after various time with MBTH for (a) 2 MPa, (b) 4 MPa, (c) 7 MPa, (d) 8.5 MPa.

Fig. 8. Electrochemical equivalent circuit used to fit the impedance spectra measured in solution in the absence (blank, a) and presence (b) of MBTH.

molecules and metals, the structure of MBTH molecules was optimized by Gaussian program and parameters (e.g. energy) of MBTH molecules were calculated by B3LYP/6-311++G(d,p) basis set. The optimized molecular structure of MBTH and distributions of HOMO and LUMO are shown in Fig. 10. Calculated values of EHOMO, ELUMO, ΔE, μ and ΔN are listed in Table 6. According to Mulliken charge distribution data in Fig. 11, two S atoms in MBTH molecules are negatively charged. In these atoms, although S atoms on the ring are more negatively charged than those out of the ring, the steric hindrance of thiazole ring influences the electronic cloud density of S atoms significantly. Hence, S atoms out of the ring own the maximum negative charge [29,30,56]. In MBTH, the electron cloud density of S14 atom is the highest and it is the easiest to fill the provided electrons to unoccupied orbits of iron. Therefore, S atoms of the ring are most likely to be the active adsorption sites of MBTH molecules. Based on effects of S atoms, MBTH molecules are adsorbed onto the iron surface in chemical way. The frontier orbital theory believed that the electron transference among reactants mainly occurs between frontier orbits of reactant

the inhibition of electrode reaction intensifies. 3.6. Differential capacitance curves The differential capacitance curve is used to study the electric layer structure on electrode surface under different CO2 pressures. Potentials at extremely small capacitance is the potentials of zero charge (PZC) [53–55]. Results are shown in Fig. 9. Clearly, PZC vs. OCP of carbon steel electrode in the non-supercritical CO2 condition is about +50 mV, and the PZC vs. OCP in the supercritical CO2 condition is about −30 mV. This reflects that in the non-supercritical CO2 aqueous solution, the carbon steel surface carries excess negative charges. However, carbon steel surface carries excess positive charges in the supercritical CO2 system. The excess charge on metal surface is changed, which influences the adsorption behavior of corrosion inhibitor significantly. 3.7. Quantum chemical calculations To further study the interaction between corrosion inhibitor 429

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Table 4 Electrochemical parameters obtained from EIS measurements for blank under different CO2 pressure at 50 °C. Time (h)

CPEct-T (sn1·Ω−1·cm−2)

n1

Rct (Ω·cm2)

RL (Ω·cm2)

2.0

2 6 12 24 48 72

8.7 × 10−4 7.8 × 10−4 8.3 × 10−4 7.9 × 10−4 7.5 × 10−4 6.9 × 10−4

0.80 0.78 0.80 0.82 0.77 0.77

64.9 59.2 46.7 61.7 81.3 92.6

342.8 233.1 268.5 300.2 524.2 444.7

2 6 12 24 48 72

7.6 × 10−4 7.5 × 10−4 8.0 × 10−4 7.6 × 10−4 6.7 × 10−4 6.9 × 10−4

0.77 0.77 0.78 0.77 0.77 0.77

73.5 67.5 54.9 70.2 86.4 83.9

344.1 301.8 241.3 298.0 239.5 318.8

2 6 12 24 48 72

8.2 × 10−4 8.5 × 10−4 7.3 × 10−4 8.4 × 10−4 1.3 × 10−3 1.1 × 10−3

0.76 0.77 0.80 0.78 0.70 0.72

42.1 34.7 33.1 35.8 44.2 46.7

126.5 101.4 108.2 100.0 119.2 132.2

2 6 12 24 48 72

9.4 × 10−4 1.1 × 10−3 9.3 × 10−4 8.6 × 10−4 8.9 × 10−4 1.1 × 10−3

0.77 0.75 0.77 0.78 0.76 0.73

35.3 33.1 33.9 37.7 41.2 44.2

103.3 110.1 100.3 112.2 120.2 96.7

4.0

7.0

8.5

0.0010

Cdl ( F cm-2 )

PCO2 (MPa)

7.0 MPa 8.5 MPa

0.0011

0.0009 0.0008 0.0007 0.0006 0.0005 -0.09

-0.06

-0.03

0.00

0.03

E (V) VS. E OCP

0.06

0.09

Fig. 9. The differential capacitance curves for carbon steel at 7.0 and 8.5 MPa pressure CO2.

ratio (△N) can evaluate electron flow between two systems with different negative charges under specific circumstances. △N can be calculated according to the Pearson theory [56,62]:

χFe − χinh 2(γFe + γinh)

ΔN =

(2)

where χFe and χinh are the absolute electronegativity of Fe and corrosion inhibitor. γFe and γinh are the absolute hardness of Fe and corrosion inhibitor respectively. These terms are related to electron affinity (A) and ionization potential (I) as presented in Eqs. (3) and (4) [56,65]:

molecules. To analyze adsorption process of corrosion inhibitor on metal surfaces, it is necessary to consider HOMO and LUMO of adsorbed molecules [57–59]. The EHOMO of molecules reflects the ability to lose electrons and the higher the EHOMO is, the easier the molecules to give electrons to electron acceptors with low energies or empty orbits [60,61]. The ELUMO of molecules is related with ability to gain electrons. Lower ELUMO is beneficial for molecules to accept the outside electrons. △E = abs(ELUMO -EHOMO), where △E is the index of molecular stability [62]. The lower △E can cause stronger chemisorption of the inhibitor molecule on metal surface [63,64]. The electron transfer

χ=

I+A 2

γinh =

(3)

I−A 2

(4)

where according to Koopan’s theorem [56], the I and A are related to the frontier orbital energies according to Eqs. (5) and (6) [62,66]: (5)

I = −EHOMO

Table 5 Electrochemical parameters obtained from EIS measurements for MBTH under different CO2 pressure at 50 °C. PCO2 (MPa)

Time (h)

CPEct-T (sn1·Ω−1·cm−2)

n1

Rct (Ω·cm2)

CPEf-T (sn2·Ω−1·cm−2)

n2

Rf (Ω·cm2)

2.0

2 6 12 24 48 72

2.3 × 10−4 5.2 × 10−4 5.8 × 10−4 4.4 × 10−4 3.8 × 10−4 4.3 × 10−4

0.84 0.74 0.70 0.74 0.75 0.71

92.5 98.5 84.8 92.1 164.8 74.7

3.9 × 10−4 1.4 × 10−4 9.1 × 10−5 1.0 × 10−4 1.2 × 10−4 1.1 × 10−4

0.86 0.94 0.95 0.98 0.93 0.96

1435 1523 1743 2071 1898 1865

4.0

2 6 12 24 48 72

3.8 × 10−4 2.6 × 10−4 1.1 × 10−4 2.3 × 10−4 9.0 × 10−5 2.2 × 10−4

0.71 0.68 0.86 0.80 0.91 0.77

200.6 51.6 47.1 24.7 60.8 83.7

3.9 × 10−5 7.4 × 10−5 2.6 × 10−4 3.8 × 10−5 2.8 × 10−4 1.6 × 10−4

0.99 0.81 0.72 0.88 0.77 0.74

1220 1586 1527 1450 1588 1506

7.0

2 6 12 24 48 72

6.0 × 10−4 8.7 × 10−4 8.7 × 10−4 9.4 × 10−4 9.6 × 10−4 9.1 × 10−4

0.35 0.61 0.62 0.64 0.59 0.62

7.6 55.8 52.0 53.1 35.6 37.3

5.0 × 10−4 6.2 × 10−5 1.0 × 10−4 1.3 × 10−4 1.7 × 10−4 2.2 × 10−4

0.73 0.98 0.92 0.95 0.96 0.91

888.5 1002 1091 1123 1188 1112

8.5

2 6 12 24 48 72

4.7 × 10−4 7.4 × 10−4 6.8 × 10−4 9.9 × 10−4 8.0 × 10−4 8.1 × 10−4

0.24 0.15 0.36 0.42 0.45 0.37

61.9 35 58.1 84.3 52.7 57.1

8.0 × 10−4 5.1 × 10−4 4.5 × 10−4 1.2 × 10−4 1.6 × 10−4 3.0 × 10−4

0.69 0.72 0.71 0.89 0.80 0.79

1119 1940 2029 2821 2577 2613

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HOMO

LUMO

Fig. 10. Optimized structures and the frontier molecular orbital density distributions (HOMO and LUMO) obtained by the B3LYP/6-311G++ (d,p) method of MBTH.

Under PCZ, carbon steel surface carry neither excess positive charge nor excess negative charge. Therefore, whether carbon steel surface accumulates positive or negative charges according to the deviation direction and degree of the mixed potentials of carbon steel in corrosion medium from PCZ. Excess charge on electrode surface is important to adsorption of corrosion inhibitor on electrode surface. When charges on the corrosion inhibitor molecules are consistent with accumulative charge property on carbon steel surface, it will surely make the adsorption of corrosion inhibitor molecules on carbon steel surface difficult due to electrostatic repulsion. On the contrary, if carbon steel and corrosion inhibitor have opposite charges, the adsorption of the corrosion inhibitor can be strengthened. It can be seen from Fig. 9 that when the system increases from nonsupercritical state (7.0 MPa) to the supercritical state (8.5 MPa), the PCZ vs. OCP of carbon steel changes from +50 mV to −30 mV, indicating that the accumulative charge on carbon steel surface changes from negative to positive [23]. Therefore, charge changes on carbon steel surface may influence corrosion inhibitor performance significantly. This might be one of causes why many cationic corrosion inhibitors fail in the supercritical CO2 environment.

Table 6 Calculated quantum chemical parameters of MBTH obtained from the B3LYP/6311G++ (d,p) method. Parameters

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

μ(D)

ΔN

MBTH

−6.4739

−1.3883

5.1356

0.8586

0.6034

A = −ELUMO

(6)

χinh and γinh of MBTH molecules can be calculated from data in Table 5. Theoretically, χFe =7 eV and γFe = 0 eV under most circumstances [56,58]. It is calculated that ΔN = 0.6034 > 0 for MBTH molecules, indicating that MBTH molecules transfer electrons to metals. This implies that 3d orbits of Fe atoms which are not filled with electrons can accept electrons of MBTH molecules to form the coordinate bonds. Since the 3d electrons can interact with LUMO orbit of MBTH molecules to form the feedback bonds. These two interactions make MBTH molecules form stable chemical adsorption on metal surface. 4. Discussion 4.1. Variations of excess charge on carbon steel surface Carbon steel surface often forms double electrode layer in solutions.

Fig. 11. Mulliken charge data of MBTH. 431

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Fig. 12. Adsorption simulation of MBTH for (a) non-supercritical (b) supercritical condition.

cannot be adhered onto the carbon steel surface closely due to electrostatic repulsion. MBTH acts on the carbon steel surface through chemical adsorption in two isomers without charges. When the pressure increases to the supercritical state, the carbon steel surface accumulates positive charges (Fig. 12 (b)), besides chemical adsorption, MBT− can be adsorbed onto carbon steel surface quickly and closely under physical effects due to the electrostatic attraction, which can separate invasion of corrosion medium more effectively. Therefore, the corrosion behavior changes sharply before and after the supercritical CO2 state.

4.2. Corrosion and inhibition behaviors before and after supercritical CO2 states In blank environment, the corrosion rate of carbon steel reached 1.87 mm/a at 2.0 MPa and 2.68 mm/a at 7.0 MPa (non-supercritical state). After entering into the supercritical state (8.5 MPa), the corrosion rate just increases slightly to 3.0 mm/a. Similarly, the corresponding corrosion current density under 7.0 MPa is 3.84 × 10−4 A·cm−2 and 5.13 × 10−4 A·cm−2 under 8.5 MPa, indicating that the corrosion rate changes slightly before and after the supercritical CO2 state of carbon steel. Many studies [2,9,15] have reported that the corrosion mechanism of carbon steel remains same before and after the supercritical CO2 states. When MBTH is added into the system, the corrosion rate of carbon steel under 7.0 MPa is 0.137 mm/a, but it drops sharply to 0.069 mm/a once entering to the supercritical CO2 states, indicating that MBTH can protect the carbon steel under supercritical CO2 state better. Electrochemical test data showed that after MBTH is added, the corrosion current density is 3.47 × 10−5 A·cm−2 at 7.0 MPa and 1.22 × 10−5 A·cm−2 at 8.5 MPa, reflecting that current density decreases significantly. It found in EIS fitting data that after MBTH is added, Rf is 1112 Ω·cm2 at 7.0 MPa and 2613 Ω·cm2 at 8.5 MPa after 72 h test, indicating that the adsorption of corrosion inhibitor is compact. SEM images in Fig. 2 show that the carbon steel at 8.5 MPa is smoother than that at 7.0 MPa after MBTH is added. Based on above analysis, the corrosion behavior of carbon steel before and after the supercritical CO2 state remains basically same. However, the corrosion behavior of carbon steel changes greatly after MBTH is added. The corrosion is inhibited after supercritical CO2 state more than that before.

5. Conclusion The corrosion rate of carbon steel under supercritical CO2 state decreases significantly after MBTH is added. The corrosion rate after supercritical CO2 state is smaller than the non-supercritical CO2 environment. There’s a sudden change of corrosion rate before and after supercritical CO2 state. According to differential capacitance curve, the excess negative charges on carbon steel surface under non-supercritical state change to excess positive charges in the supercritical CO2-water system. Based on quantum chemical calculation of MBTH and discussion of corrosion inhibition mechanism, MBT− is easier to cover on the carbon steel surface under the electrostatic attraction when the carbon steel surface carries excess positive charges. Besides chemisorption, there’s attraction of hetero charges, thus resulting in the stronger corrosion inhibition. Acknowledgements The authors thank the National Natural Science Foundation of China (Nos. 51571098) for their financial support and the Analysis Support of the Analytical and Testing Center, Huazhong University of Science and Technology.

4.3. Corrosion inhibition mechanism of MBTH It can be seen from charge distribution data in MBTH molecules from quantum chemical calculation that S atoms out of the ring are the highest. Due to resonance of molecular structure, N atoms on the ring have high activity. These two places are both active adsorption sites. This has been verified by abundant experiments by using MBTH as the copper corrosion inhibitor [29–31]. In XPS test (Fig. 4), studies on Fe can find peaks of Fe-S bonds of 709.8 eV and 710.2 eV at 7.0 MPa and 8.5 MPa. The Fe-S peak intensity under 8.5 MPa is significantly higher than that under 7.0 MPa, indicating that the adsorption of MBTH on Fe surface under supercritical state is significantly better than that under non-supercritical state. In addition, analysis of C and N can disclose the existence of C]N and C] S. Therefore, MBTH molecules use S atoms out of the thiazole ring and N atoms in ring as the active adsorption sites under supercritical CO2 state. MBTH molecules are adsorbed onto the carbon steel surface through chemical pattern. Benzene ring in molecules has hydrophobic interaction to separate corrosion medium (Fig. 12 (a)). Under supercritical CO2 state, the carbon steel surface accumulates negative charges. MBT− [26,32] which is dissociated from MBTH molecules

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