Tetrahydroacridines as corrosion inhibitor for X80 steel corrosion in simulated acidic oilfield water

Journal of Molecular Liquids 293 (2019) 111478

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Tetrahydroacridines as corrosion inhibitor for X80 steel corrosion in simulated acidic oilfield water Weiwei Zhang a, Hui-Jing Li a,⁎, Meirong Wang b, Li-Juan Wang b,⁎, Qianwen Pan a, Xubiao Ji a, Yaqian Qin a, Yan-Chao Wu a,c,⁎⁎ a b c

School of Marine Science and Technology, Harbin Institute of Technology, Weihai 264209, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, PR China Weihai Institute of Marine Biomedical Industrial Technology, Wendeng District, Weihai 264400, PR China

a r t i c l e

i n f o

Article history: Received 26 April 2019 Received in revised form 2 July 2019 Accepted 30 July 2019 Available online 30 July 2019 Keywords: X80 steel Corrosion inhibition Electrochemical SEM AFM XPS

a b s t r a c t X80 steel is commonly used in pipeline, tubing, casing and other facilities in oilfield construction. Its corrosion behaviors in 15% HCl without or with tetrahydroacridines was investigated by weight-loss tests, electrochemical techniques along with scanning electron microscope (SEM)/energy dispersive spectroscopy (EDX), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The experimental results indicated that tetrahydroacridines have excellent inhibitive performance for X80 steel in 15% HCl, and the ηmax of MPTA and EPTA reached up to 97.87% and 95.96% at 400 ppm, respectively. The properties of tetrahydroacridines are mixed-type based on polarization studies, and the adsorption mode follows Langmuir isotherm and physical adsorption is dominant. On the basis of surface analysis and characterization, the formation and characteristic of an adsorption-related protective inhibitor film on the X80 steel surface have been verified. © 2019 Elsevier B.V. All rights reserved.

1. Introduction In the oil and gas industry, different grades of steel are used in building materials such as pipelines, tubing and sleeves. Steel grades such as API-5L X70, X80, X60, API-5CT J55, N80-P110, and K55 have been widely used in line pipes, welded casings and tubing pipes, and their basic difference is the relative amount of elements in the alloy [1–3]. In practical applications, the key factor in selecting steel grades is the ability to withstand very harsh conditions, with corrosive erosion being one of the priorities. An important grade of steel in oil and gas production is X80 steel because of its superior mechanical quality and high cost-effective [1]. With the increase of oil and gas field exploitation, most newly exploited oilfields are deep wells and ultra-deep wells, and the corrosion factors in the wells are more complex and variable, which greatly affects the efficiency of oil production. Therefore, stimulation of wells, secondary and enhanced production operations become very essential. However, in oil production, the surface of equipment often appear corrosion and scaling problems due to changes in pressure, temperature and composition, etc. [3,4]. Especially during the

⁎ Corresponding authors. ⁎⁎ Correspondence to: Y.-C. Wu, School of Marine Science and Technology, Harbin Institute of Technology, Weihai 264209, PR China. E-mail addresses: [email protected] (H.-J. Li), [email protected] (L.-J. Wang), [email protected] (Y.-C. Wu).

https://doi.org/10.1016/j.molliq.2019.111478 0167-7322/© 2019 Elsevier B.V. All rights reserved.

stimulation treatments, acids (usually 15–28% HCl) are typically introduced into pipelines at elevated temperatures to dissolve stratigraphic rocks, expand flow paths and remove scale [5–7]. In doing so, pipes and other metallic structures are highly exposed to corrosive acids, resulting in damage to component integrity, pipeline rupture and leakage, which greatly affects the efficiency of oil production and produces other negative social effects. In addition, corrosion or failure may require replacement or maintenance of the equipment, which could cause the factory to close, shutdown time and undesirable damage to the company's interests. A practical, economical and effective way to solve these problems is to use corrosion inhibitors. The inhibitors are believed to act by adsorbing their active functionalities on metal surface and form a layer of physical or chemical properties that prevents corrosion of the steel. Generally, excellent corrosion inhibition performances are manifested by inhibitors bearing larger electronegativity atoms (i.e., N, O, S) as well as conjugated-bond structures or aromatic rings, such as imidazoline, quinolones, thioureas, pyridines, thiourea, mercaptans and quaternary salts were frequently utilized as corrosion inhibitors during stimulation treatments [8–10]. Various corrosion inhibitors have been exploited to narrow the gap between corrosion inhibition tests and practical steel protection. Even so, some of them are toxic, very expensive or too harsh synthetics that cannot be overlooked, making the exploitation of an environmental-friendly and efficient inhibitor for steel protection a high priority. Tetrahydroacridines as an Nheterocyclic organic compound is being scrutinized significantly due

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to its low toxicity, easy synthesis, excellent coordination performance and unique antimicrobial activity. It is known that acridines are potential corrosion inhibitors for steel in acidic media [11–13]. We believe that tetrahydroacridine compounds may also be an effective corrosion inhibitor platform for iron and steel protection as it contains the availability of N atom and pi-bonds that can form coordination bonds with iron atoms to form a dense adsorption film on steel surface. Meanwhile, tetrahydroacridines are cost-effective and environmentally friendly [14], so they have great application prospects in the field of corrosion and protection. Despite extensive works performed on heterocyclic molecules as corrosion inhibitors for steel during stimulation treatments, no research has been conducted on the inhibitive effect of tetrahydroacridines as corrosion inhibitors in acidic oilfield water. In this study, 2-methyl-9-phenyl-1,2,3,4-tetrahydroacridine (MPTA) and ethyl 9-phenyl-1,2,3,4-tetrahydroacridine-2-carboxylate (EPTA) were designed as corrosion inhibitors for X80 steel in 15% HCl, which simulate real field acidizing conditions. The corrosion inhibition properties of MPTA and EPTA were studied by utilizing weight-loss tests and electrochemical techniques. The morphological characteristics and elemental compositions of adsorption film on steel surface were investigated by scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDX), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). Moreover, the inhibition mechanism of inhibitors with X80 steel was evaluated by adsorption, thermodynamic. Our finding offers a new light for designing eco-friendly tetrahydroacridine-type inhibitors for X80 steel under oil well acidizing conditions. 2. Experiments

residue was purified by silica gel column chromatography (PE/AcOEt = 1:5), and the molecular structures of compounds were further corroborated by 1H NMR, 13C NMR, and IR, shown below: MPTA: 1H NMR (400 MHz, CDCl3) δ: 8.02 (d, 1H, J = 8.4 Hz), 7.61–7.57 (m, 1H), 7.54–7.44 (m, 3H), 7.32–7.30 (m, 2H), 7.25–7.21 (m, 2H), 3.33–3.14 (m, 2H), 2.69–2.63 (m, 1H), 2.23 (dd, J = 10.8, 17.0 Hz, 1H), 2.08–2.01 (m, 1H), 1.92–1.84 (m, 1H), 1.63–1.53 (m, 1H), 1.00 (d, J = 6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ: 158.7, 146.3, 146.2, 137.0, 129.1, 128.9, 128.5, 128.4, 128.2, 127.8, 127.6, 126.5, 125.7, 125.2, 36.4, 33.7, 31.0, 29.2, 21.7; IR (film): νmax = 2922, 2864, 2361, 1572, 1484, 1454, 1355, 1260, 1153, 1073, 927, 800, 758, 703 cm−1. EPTA: 1H NMR (400 MHz, CDCl3) δ: 8.01 (d, 1H, J = 8.4 Hz), 7.63–7.58 (m, 1H), 7.54–7.44 (m, 3H), 7.32 (d, 2H, J = 3.9 Hz), 7.26–7.21 (m, 2H), 4.16–4.10 (m, 2H), 3.36–3.17 (m, 2H), 2.92–2.83 (m, 2H), 2.49–2.44 (m, 1H), 2.15–1.97 (m, 2H), 1.22 (t, 3H, J = 7.1 Hz)；13C NMR (100 MHz, CDCl3): δ/ppm = 174.9, 157.5, 147.0, 146.4, 136.6, 129.2, 128.9, 128.7, 128.4, 128.0, 126.7, 126.2, 125.9, 125.6, 60.6, 39.8, 32.9, 30.0, 28.5, 25.8, 14.2; IR (film): νmax = 2939, 2359, 2254, 1723, 1572, 1373, 1260, 1075, 1029, 906, 827 cm−1. 2.2. Weight loss experiments The measurement was carried out in a thermostatic water bath containing 100 mL of uninhibited solution and the solution containing MPTA and EPTA at 30 °C for 24 h. Before weighing, the surface samples were carefully washed with ultra-pure water and dried with nitrogen. The procedure was carried out on triplicate samples, and the average corrosion rate was calculated. The corrosion rate (v) was obtained from Eq. (1):

2.1. Specimens and solutions The X80 steel sample composition used for the experiments was C (0.064), Si (0.025), Mn (1.56), P (0.013), S (0.004), Cu (0.01), Cr (0.021), Nb (0.056), V (0.005), Ti (0.025), B (0.0006) and balance Fe. The dimensions of X80 steel materials used for weight loss were cut into 2 cm × 2 cm × 0.02 cm (sectional area = 4 cm2), 0.5 cm × 0.5 cm × 2 cm for electrochemical experiments and 1 cm × 1 cm × 0.02 cm for surface analysis. For electrochemical experiments, the working surface (0.25 cm2) of the electrode was polished with sandpaper of different grades (400–1200) before using them for experiments. The analytical grade 37% HCl was diluted with double distilled water to prepare a 15% HCl solution to simulate the real field acidizing concentration. The concentration of MPTA and EPTA inhibitors in corrosive solution ranged from 100 ppm (100 mg/L) to 400 ppm. MPTA and EPTA were synthesized from 2-aminobenzophenone and cyclohexanones in our laboratory, of which citric acid was the only catalyst, and the reaction process shown in Fig. 1. The synthetic procedure has the advantages of using harmless non-metallic catalyst, easy availability of raw materials, high regioselectivity and so on. The mixture of 2-aminobenzophenone (197.2 mg, 1.0 mmol), cyclohexanones (1.5 mmol) and citric acid (50 mmol%) in toluene (1.5 mL) was stirred at 120 °C under the atmosphere of air for 24 h. After the reaction, the

O

v¼

W 1 −W 2 st

ð1Þ

where W1 and W2 (mg) are the weight loss of specimen before and after immersion, s (cm2) is the area of steel samples, and t (h) is the soaking time. The inhibition efficiency (ηw) was obtained from Eq. (2): ηw ¼

vo −v  100% vo

ð2Þ

where vo and v are the corrosion rate without and with inhibitors, respectively. Besides, the effect of temperature like 50 °C, 70 °C and 90 °C was also studied by weight loss method. 2.3. Electrochemical measurements The CHI 760E workstation of the conventional three−electrode system is used for electrochemical measurement, where X80 steel is a working electrode, saturated calomel electrode is a reference, and the platinum is an auxiliary electrode. The polarization curves were scanned from −750 to −250 mV (vs. SCE) at a scan rate of 1 mV s−1, and linear polarization resistance experiments were done from −20 to +20 mV vs. Ecorr at the scan rate of 0.125 mV s−1. The electrochemistry

O citric acid R

toluene, 120 oC

NH2 R

MPTA: R = CH3; EPTA: R = CO2Et Fig. 1. Synthetic route of the studied tetrahydroacridines.

N

W. Zhang et al. / Journal of Molecular Liquids 293 (2019) 111478

3

impedance tests were conducted under EOCP in the frequency range of 100 kHz−100 mHz with an AC voltage amplitude of 5 mV. Before electrochemical measurements, keep the working electrode in contact with the solution under test for 30 min to reach a stable state. 2.4. Surface analysis The surface morphological of X80 steel in non-inhibiting and inhibiting acid solution at 30 °C for 48 h were studied by SEM (SUPR55, Zeiss, Germany, 2 k magnification), and AFM (MFP-3D-BIO, Asylum Research, America) using tapping mode in the Air, RTESPA probe (k = 40 N/m and fo = 302 kHz). The associated EDX was used to provide qualitative information on the composition of surface elements. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo VG Scientific ESCALAB 250 spectrometer with a monochromatic Mg Ka X-ray source. The XPS spectra were fitted by XPS PEAK 4.1 software with a reference value of C 1s peak at 284.6 eV [15]. 3. Results and discussion 3.1. Weight loss measurements Weight loss is a commonly used method to evaluate the corrosion inhibition performance of inhibitor, and it has high reliability. The corrosion behavior of X80 steel in 15% HCl with different concentrations of MPTA and EPTA was studied by weight loss technique. The corrosion rate (v) and inhibition efficiency (ηw) are shown in Fig. 2. As can be seen from Fig. 2, the v value is obviously reduced and ηw is increased after adding inhibitors. The maximum ηw values of MPTA and EPTA are 96.14% and 94.43%, respectively, and the order of v value is v (MPTA) b v (EPTA). The inhibition behavior of MPTA and EPTA can be explained by the adsorption of their molecules on the steel surface. Moreover, it is noteworthy that the values of ηw followed the order: ηw (MPTA) N ηw (EPTA), showing that the substituent has a certain influence on the corrosion inhibition effect of the inhibitor. Recent studies have confirmed that the contribution of a substituent to the inhibition potential is directly related to the Hammett substituent constant (σ), and an inhibitor with negative σ value is related to the stronger adsorption tendency compared with the positive or smaller negative σ value [16]. The σp value for –CH3 (−0.17) is more negative than that of – CO2C2H5 (+0.45), indicating that –CH3 substituent has a higher electron donating ability than the –CO2C2H5 substituent that favors the bonding of inhibitors with metals, exhibiting higher inhibition efficiency. 3.2. Effect of temperature Surface temperature is usually different from downhole temperature, so understanding the effect of temperature on inhibitor is critical, and temperature studies can be used to understand the kinetics and thermodynamic processes of metal corrosion. Table 1 shows the corrosion rate (v) of X80 steel with or without MPTA and EPTA in 15% HCl measured by weight loss at different temperatures (30–90 °C). As illustrated in Table 1, the v value improved and the ηw reduced after increasing temperature, which is due to the adsorption equilibrium tends to desorption direction with increasing temperature, resulting in a decrease in metal surface coverage. This means MPTA and EPTA are temperature dependent corrosion inhibitors that can be used for corrosion protection of medium and high temperature media. The activation energy (Ea) of corrosion process is calculated from Arrhenius Eq. (3): ln vcorr ¼ ln A−

Ea RT

ð3Þ

where vcorr is the rate of corrosion, R is gas constant, T is absolute temperature, and A is pre-exponential factor. The Ea values in the absence

Fig. 2. Variation of (a) corrosion rate and (b) inhibition efficiency with different concentrations of additives by weight loss. Error bars (┳) represent the standard deviations (%).

and presence of MPTA and EPTA (only studied 400 ppm) obtained from the slope of the plots of lnvcorr vs. 1/T (Fig. 3) are shown in Table 2. As depicted in Table 2, the values of Ea with additives are higher than the uninhibited solution, supporting the physical adsorption mechanism, which is probably associated with the electrostatic adsorption film formed on the X80 steel surface [17,18]. Furthermore, the activation enthalpy (ΔH⁎) and entropy (ΔS⁎) of the corrosion process may be assessed by the influence of temperature, and the values were calculated from Eq. (4):     vcorr R ΔS −ΔH exp ¼ Þ exp Nh T R RT

ð4Þ

where h is Planck's constant and N is Avogadro's number. The plots of ln (vcorr/T) vs. 1/T for blank, MPTA and EPTA are illustrated in Fig. 4. The values of ΔH⁎ and ΔS⁎ were obtained from the slope (ΔH⁎/R) and intercept [ln (R/Nh) + (ΔS⁎/R)] of the linear plots, and the results are presented in Table 2. The positive values of ΔH⁎ indicate the endothermic nature of the steel dissolution process and was retarded by the inhibitor-containing solution. Compared with free acid solution, the increase of ΔH⁎ value in the presence of additives indicates the existence of physical adsorption. The higher ΔS⁎ value with additives can be

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Table 1 Weight loss results of X80 steel in 15% HCl with MPTA and EPTA at different temperatures; the values are the mean of three replicates and (±) corresponds to the standard deviations. Inhibitor/C (ppm)

Blank MPTA

EPTA

30 °C

0.00 100 200 300 400 100 200 300 400

50 °C

70 °C

90 °C

v (mg

ηw

v (mg

ηw

v (mg

ηw

v (mg

ηw

cm−2 h−1)

(%)

cm−2 h−1)

(%)

cm−2 h−1)

(%)

cm−2 h−1)

(%)

57.32 ± 2.7 12.41 ± 1.5 6.67 ± 1.1 3.29 ± 0.4 2.21 ± 0.4 14.73 ± 1.8 9.35 ± 1.3 4.26 ± 0.7 3.19 ± 0.3

– 78.35 88.36 94.26 96.14 74.32 83.69 92.57 94.43

77.73 ± 4.6 19.23 ± 2.1 10.78 ± 1.4 6.19 ± 1.3 4.95 ± 0.8 21.62 ± 2.6 14.75 ± 1.7 8.26 ± 1.2 6.71 ± 1.3

– 75.26 86.13 92.04 93.63 72.18 81.02 89.37 91.37

105.47 ± 4.3 30.08 ± 2.9 18.53 ± 2.2 13.49 ± 1.9 11.44 ± 1.3 33.42 ± 2.7 24.44 ± 2.3 16.99 ± 1.8 14.15 ± 1.2

– 71.48 82.43 87.21 89.15 68.31 76.83 83.89 86.58

156.15 ± 6.1 53.31 ± 3.2 37.10 ± 2.8 28.37 ± 1.9 27.65 ± 2.1 58.81 ± 3.4 44.88 ± 2.3 34.70 ± 2.5 32.77 ± 1.8

– 65.86 76.24 81.83 82.29 62.34 71.26 77.78 79.01

regarded as a quasi-substitution process between the inhibitor molecule and the water molecule at the solution-metal interface [19,20].

associated with the standard adsorption free energy (ΔG0ads), as per following relationship:

3.3. Adsorption isotherms

  ΔG0ads ¼ −RT ln 1  106 K ads

Adsorption isotherms are the most advisable way to learn the adsorption mechanism of inhibitors. Fig. 5 shows the plots of C/θ versus C for MPTA and EPTA at 30–90 °C, and the adsorption parameters are shown in Table 3. In this study, it was found that the Langmuir isotherm showed the optimal fitting results because the linear regression coefficient was almost 1 (Table 3), showing a single layer adsorption characteristics. The Langmuir isotherm is shown in Eq. (5): C 1 ¼ þC θ K ads

ð5Þ

In the formula, C represents the inhibitor concentration, θ is the surface coverage, and Kads is the adsorption equilibrium constant

ð6Þ

where 1 × 106 is the amount of water molecules expressed in ppm, T is the thermodynamic temperature and R is the universal gas constant. As seen in Table 3, the ΔG0ads values are ranged between −25.41 kJ·mol−1 and − 30.56 kJ·mol−1, which indicates the adsorption of two tetrahydroacridines on the X80 steel surface is a mixed from physical and chemical adsorption [21–25]. In addition, the value of Kads follows: MPTA N EPTA, and MPTA shows larger negative ΔG0ads value than EPTA, further confirming that MPTA exhibits stronger adsorption stability than EPTA. From the listed values of Kads and ΔG0ads in Table 3, ΔH0ads (standard enthalpy) and ΔS0ads (entropy) of adsorption can be calculated using Eq. (7):   ΔH 0 ΔS0 ln K ads ¼ − ads þ ads − ln 1  106 RT RT

ð7Þ

The values of ΔH0ads and ΔS0ads can be determined from the slope and intercept of the plot of lnKads vs. 1/T (Fig. 6) and were listed in Table 3. As seen in Table 3, the negative and small values of ΔH0ads indicate that the adsorption of MPTA and EPTA is an exothermic process, leading to

Fig. 3. Arrhenius plots of lnvcorr vs. 1/T for X80 steel in 15% HCl in the absence and presence of inhibitors. Table 2 Activation parameters for X80 steel dissolution in 15% HCl without and with inhibitors. Inhibitors Blank MPTA EPTA

Ea (kJ mol−1)

ΔH* (kJ mol−1)

ΔS* (J mol−1 K−1)

15.06 38.55 35.11

12.31 35.80 32.36

−171.50 −121.28 −129.45

Fig. 4. Arrhenius plots of ln(vcorr/T) vs. 1/T for X80 steel in 15% HCl in the absence and presence of inhibitors.

W. Zhang et al. / Journal of Molecular Liquids 293 (2019) 111478

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Fig. 6. Plot of ln Kads vs. 1/T for X80 steel in 15% HCl solution containing different concentrations of MPTA and EPTA.

3.4. Open circuit potential (EOCP) curves The variation of open circuit potential (EOCP) of X80 steel electrode with time in 15% HCl solution in absence and presence of inhibitor was shown in Fig. 7. It could be observed from OCP curves that the EOCP shifted toward negative potentials with addition of different concentration of inhibitor, which can be explained by the adsorption of inhibitor on the X80 steel surface [11]. By increasing the concentration of the inhibitor, the steady state potentials (EOCP) moves in a more negative direction, indicating that the cathodic reactions is more affected than the anodic reaction. It takes about 20 min to reach a stable state, so we chose a 30-minute immersion time for electrochemical measurements. 3.5. Potentiodynamic polarization and linear polarization resistance (LPR) The polarization curve of X80 steel in 15% HCl solution in the absence or presence of MPTA and EPTA are presented in Fig. 8. Table 4 displays the corrosion potential (Ecorr), corrosion current density (icorr) and Tafel slopes (βa and βc) obtained by Tafel extrapolation method. The ηi was calculated according to Eq. (8): Fig. 5. Langmuir adsorption plots for X80 steel in 15% HCl solution containing different concentrations of (a) MPTA and (b) EPTA at 30–90 °C.

physical adsorption [21,22]. The positive values of ΔS0ads arise from the substitution process, which can be attributed to the increase in the solvent entropy and more positive water desorption entropy [22]. Table 3 Thermodynamic parameters of inhibitors on the X80 steel surface in 15% HCl solution. Compounds

T (°C)

Kads (ppm/mol)

ΔG0ads (kJ mol-1)

ΔH0ads (kJ mol-1)

ΔS0ads (J mol-1)

MPTA

30 50 70 90 30 50 70 90

0.0305 0.0283 0.0271 0.0250 0.0237 0.0232 0.0220 0.0210

-26.05 -27.53 -29.11 -30.56 -25.41 -26.99 -28.51 -30.04

-29.17

76.24

-18.84

77.62

EPTA

ηi ð%Þ ¼

i0corr −icorr i0corr

 100

ð8Þ

where, i0corr and icorr are the current density values of unprotected and protected steel in acidic solution, respectively. The icorr values in Table 4 reduced as the inhibitor concentration increased indicating the inhibitive effects of MPTA and EPTA. Simultaneously, the potential obtained for all studied inhibitors shifted to relatively more negative and less than 58 mV with respect to the blank, showing that MPTA and EPTA could be mixed type inhibitors but predominantly restrained the cathode process. Similar results were obtained for the corrosion inhibition of other nitrogen heterocyclic compounds as a steel corrosion inhibitor in acidic solution [26–28]. Besides, the values of βa and βc were the absence of significant change after adding inhibitors also revealed that the anodic dissolution of steel and hydrogenation reaction are slowed down by the surface blocking effect of inhibitors [29,30]. The shape of the curves is similar, which also demonstrates that the addition of MPTA and EPTA do not lead to changes in the electrochemical corrosion reaction mechanism of electrodes. Moreover, the calculated ηi

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Fig. 7. EOCP – t curves for X80 steel in 15% HCl solutions with different concentrations of inhibitors: (a) MPTA and (b) EPTA.

increased with the increase of MPTA and EPTA concentration, with the maximum ηi of 97.87% and 95.96%, respectively, similar to weight loss results. The LPR experiments were carried out by recording the electrode potential ±20 mV around corrosion potential (Stern plots). The polarization resistance (Rp) obtained by LPR method is used to calculate the corrosion current (icorr) by using the Stern–Geary kinetics equation [30]: icorr ¼

βa βc 1  2:303ðβa þ βc Þ Rp

ð9Þ

Inhibition efficiency (ηLPR) is calculated using the following equation: ηLPR ð%Þ ¼

Rp −R0p Rp

 100

ð10Þ

where R0P and Rp are polarization resistances of uninhibited and inhibited solutions, respectively. The Rp and ηLPR values (Table 4) increased considerably with the increase of MPTA and EPTA

Fig. 8. Potentiodynamic polarization curves for X80 steel in 15% HCl (a) MPTA and (b) EPTA.

concentrations. Besides, the ηLPR values calculated by LPR and potentiodynamic polarization curves are comparable. The coincidence between these two methods can suggest that no significant surface changes occur during the polarization measurements [30].

Table 4 Polarization curve parameters for the corrosion of X80 steel in 15% HCl at 30 °C. Inhibitors C Ecorr (ppm) (mV vs. SCE) – MPTA

EPTA

Blank 100 300 400 100 300 400

−449 −472 −496 −507 −458 −491 −506

icorr (μA cm−2)

-βc (mV dec−1)

βa (mV dec−1)

993.8 219.5 52.4 21.2 241.2 76.9 40.1

140.3 135.9 136.1 144.8 137.7 133.6 138.1

95.1 92.6 91.9 107.1 100.3 101.9 100.4

ηi (%)

Rp (Ω cm2)

ηLPR (%)

– 54.5 – 79.62 256.1 78.72 94.73 1068.3 94.90 97.87 1891.4 97.12 75.73 229.7 76.27 92.26 750.7 92.74 95.96 1384.9 96.06

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3.6. Electrochemical impedance spectroscopy (EIS) EIS were carried out to investigate the corrosion behavior of protected and unprotected X80 steel. The Nyquist plots of X80 steel for MPTA and EPTA are displayed in Fig. 9. As shown in Fig. 9, all Nyquist plots show similar capacitive loops over the entire frequency range, and the capacitive loop diameter with inhibitors is greater than that of noninhibitors, and it increases further with the increase of concentration. Besides, the imperfect semicircular nature with the addition of inhibitors was caused by frequency dispersion and inhomogeneity of electrode surface [24,29,31]. Corresponding bode plots (Fig. 10), the frequency range of maximum phase increases with rising inhibitors' concentration, indicating MPTA and EPTA are effectively adsorbed inhibitor molecules on steel surface. The values of absolute impedance were increased significantly at low frequencies with the increase of inhibitor concentration, and have the same trend as the change of capacitive diameter, which indicates higher protection of X80 steel in inhibited 15% HCl solution. The impedance data are determined according to the equivalent circuit shown in Fig. 11. The chi-squared (χ2) was used to evaluate the

Fig. 10. Bode plots for X80 steel in 15% HCl solution (a) MPTA and (b) EPTA.

precision of the fitted data, the low χ2 values (Table 5) obtained for all the results indicate that the fitted data have good agreement with the experimental data. The fitted dates are listed in Table 5 includes Rs (electrolyte resistance), Rct (charge transfer resistance), Rf (film resistance), Qf and Qdl represent constant phase elements (CPE), representing film capacitance (Cf) and double layer capacitance (Cdl), respectively. The impedance of CPE was evaluated from Eq. (11):  −1 Z CPE ¼ Y 0 ðjwÞn

ð11Þ

where Y0 is proportionality coefficient, j is imaginary number, ω is angular frequency, and n is phase shift. Also, the ηz was calculated by polarization resistance Rp in Eq. (12):

ηz ð%Þ ¼

Fig. 9. Nyquist plots for X80 steel in 15% HCl solution (a) MPTA and (b) EPTA.

  Rp −R0p Rp

 100

ð12Þ

where R0p and Rp are the sum of Rf and Rct in the absence or presence of inhibitor, respectively. As presented in Table 5, the values of Rf and Rct increased significantly with increasing inhibitors' concentration leading to enhanced inhibitor efficiency. This behavior was assigned to the formation of protective layer at metal/solution interface and strongly

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that there was no significant change in the phase shift n value, indicating charge transfer controls the dissolution mechanism of steel. Lastly, it can be concluded that MPTA possesses the higher resistance. Hence the ηz of X80 steel in test solution with MPTA is higher than that of the EPTA, which is consistent with the results of weightlessness and polarization curves. A comparison with other acidizing oilfield inhibitors by EIS measurement for steel was shown in Table 6. MPTA and EPTA, possessing a unique aromatic ring system, are among the excellent corrosion inhibitors.

3.7. SEM and EDX analysis

Fig. 11. Equivalent circuit used to fit the EIS experiment data: (a) without inhibitor and (b) the presence of inhibitor.

hinders local corrosion caused by chloride-induced. While, Cf and Cdl values showed a decreasing trend with the presence of MPTA and EPTA inhibitor, which is due to a decrease in local dielectric constant and/or an increase in the thickness of electric double layer capacitor [11,17,32]. The reduction of capacitance and increase of resistance indicate that the absorbed organic molecule displaces the water molecule on metal surface and reduce the acid solubility of X80 steel, thus hindering further charge and mass transfer. Besides, another finding in Table 5

Representative surface micrographs of X80 steel after immersion in corrosion solution for 48 h without or with 400 ppm MPTA and EPTA were examined using SEM (Fig. 12) and EDX (Fig. 13) so as to determine the formation of protective film on the steel surface. As shown in Fig. 12a, the X80 steel surface was severely corroded and badly damaged in 15% HCl solution. In contrast, the corrosion degree of steel surface was significantly reduced and clearly a smooth surface with a little shallow pitting corrosion in the solution containing MPTA and EPTA (Fig. 12b and c), showing that the corrosion of X80 steel was well safeguarded by the investigated inhibitor. The surface of the MPTA-protected steel was flatter than the EPTA protection and was close to the polished surface of the sample. EDX energy spectrum analysis provides a clear understanding of the elemental composition of steel surface. The EDX spectrum of X80 steel without and with 400 ppm MPTA was shown in Fig. 13. In blank acidic solution, the EDX spectrum illustrated in Fig. 13 (a) show the characteristic peaks of iron and the small peaks that characterize chloride and oxygen, which represent corrosive products. After adding MPTA inhibitor (Fig. 13b), the peaks of Fe were largely suppressed compared to that of blank acid, and the characteristic peak of N appeared around 0.4 keV, which is derived from the N elements in the MPTA inhibitor. This result clearly indicates that MPTA molecule was adsorbed on steel surface.

Table 5 EIS parameters for the corrosion of X80 steel in 15% HCl at 30 °C. Inhibitors

C (ppm)

Rf (Ω cm2)

Rct (Ω cm2)

Cf (n1) (μF cm−2)

– MPTA

Blank 100 300 400 100 300 400

– 14.5 32.1 39.9 9.9 29.0 35.6

51.4 260.7 1100.0 1813.0 225.4 784.3 1385.0

– 59.3 (1) 20.2 (1) 13.4 (1) 77.5 (1) 33.4 (1) 14.5 (1)

EPTA

Cd (n2) (μF cm−2) 526.5 (0.92) 138.6 (0.58) 45.0 (0.61) 37.1 (0.65) 182.7 (0.59) 58.3 (0.64) 40.8 (0.61)

Rp (Ω cm2)

χ2 (10−3)

ηz (%)

51.4 275.2 1132.1 1852.9 235.3 813.3 1420.6

2.19 1.58 2.16 3.90 2.95 4.06 1.37

– 81.32 95.46 97.23 78.16 93.68 96.38

Table 6 Comparison of the inhibition efficiency of MPTA and EPTA with the literature data as acidizing oilfield inhibitor for steel. Inhibitors

Medium of testing

2-Methyl-9-phenyl-1,2,3,4-tetrahydroacridine Ethyl 9-phenyl-1,2,3,4-tetrahydroacridine-2-carboxylate Griffonia simplicifolia Sophorolipids 2,3-Pyrazine dicarboxylic acid Pyrazine carboxamide 2-Methoxy-3-(1-methylpropyl) pyrazine Lemongrass extract Gemini surfactants GS2 Gemini surfactants GS6 Gemini surfactants GS10 5-Hydroxytryptophan

15% HCl 15% HCl 1 M HCl Oilfield water saturated with CO2 15% HCl 15% HCl 15% HCl Oilfield water Oilfield water Oilfield water Oilfield water 15% HCl

Concentration

η (%)

Ref.

400 ppm 400 ppm 500 ppm 70 mg/L 0.2 wt% 0.2 wt% 0.2 wt% 400 ppm 400 ppm 400 ppm 400 ppm 0.1 mM

97.23 96.38 91.77 89.05 41.7 54.9 48.9 60.12 67.47 59.11 67.24 75.2

This paper This paper [2] [3] [6] [6] [6] [7] [27] [27] [27] [31]

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processes at metal/solution interfacial [33]. Analysis of the photographs allowed quantification of surface roughness over an area of 5 μm × 5 μm. The two-dimensional (2D) and three-dimensional (3D) AFM images of uninhibited and inhibited X80 steel surfaces with 400 ppm MPTA and EPTA in 15% HCl solution for 24 h are shown in Fig. 14. It is clearly seen from Fig. 14a that the X80 steel sample shows a relatively uneven and rough structure due to erosion by the hydrochloric acid medium. In presence of MPTA (Fig. 14b) and EPTA (Fig. 14c), the sample surface becomes relatively flat, while MPTA provide a more uniform surface. The maximum height scale of X80 steel in blank solution was 712 nm, while the corresponding values in the presence of MPTA and EPTA were 106 nm and 134 nm respectively. These images and parameters suggest that the inhibitive ability of the studied compounds follows the order of MPTA N EPTA, which is in good agreement with the electrochemical measurements and SEM studies. The AFM results further confirm that the formation of adsorbed protective film of MPTA and EPTA on the X80 steel surface. 3.9. XPS analysis In order to confirm the nature of the organic film adsorbed on the X80 steel surface and the adsorption mode of the inhibitor, XPS test was used to study in the presence of the best inhibitor (MPTA). As shown in Fig. 15a, the XPS spectra with three elements (C, N and Fe) were detected, which provides evidence for the adsorption of the inhibitor on the X80 steel surface. Fig. 15(b–d) illustrates the high-resolution XPS spectra of specific elements (C1s, N1s and Fe2p3/2) protected layers. According to Fig. 15b, the C1s spectrum was fitted to three peaks. The small peak at 284.6 eV was ascribed to C\\C, C_C and C\\H aromatic bonds, and another peak located at 286.3 eV was related to C\\N and/ or C_N bonds in tetrahydroacridine ring. The peak at 288.7 eV, which has higher binding energy was attributed to C=N+ structures may be caused by the proton of the = N– structure in the tetrahydroacridine ring [34–36]. The N 1s peaks (Fig. 15c) at 399.2 eV, 400.7 eV and 401.6 eV were observed, the first peak was assigned to C\\N and the unprotonated N atoms (=N– structure) [17,20], the peak at 400.7 eV, representing N\\Fe groups which due to the coordination of nitrogen in the MPTA ring with iron atom. Indeed, the N\\Fe bond complex can make the peak shift to a higher binding energy compared to the uncoordinated = N– structure [17]. The last peak at 401.6 eV was ascribed to the protonated nitrogen, which gives rise to positive polarization nitrogen atoms, and therefore the binding energy is increased [36]. The Fe 2p3/2 (Fig. 15d) showed three peaks around 710.6 eV, 713.4 eV and 715.2 eV. The first peak at 710.6 eV was attributed to ferric compounds, such as Fe2O3 (i.e., Fe3+ oxide) and FeOOH (i.e., oxyhydroxide). The second peak appeared at 713.4 eV was due to the presence of a small concentration of FeCl3 on X80 steel surface in the hydrochloric acid solution, while that appeared at 715.2 eV may be assigned to the satellite of Fe(III) [20]. The results obtained by XPS analysis support the adsorption of inhibitor on the X80 steel surface. Especially, the presence of the nitrogen species on the X80 steel surface indicates that MPTA can be adsorbed on the X80 steel surface by physisorption simultaneously accompanied by chemisorption in 15% HCl, which corroborates the thermodynamic study. In addition, the Fe 2p3/2 analysis showed that the formation of a stable and insoluble layer (Fe2O3, FeOOH) makes the protected layer become more complete and possess excellent anti-corrosion ability. Fig. 12. SEM images of X80 steel surfaces: (a) 15% HCl, (b) MPTA, (C) EPTA.

3.8. AFM observation AFM is a powerful tool for observing nano- to microscale surface topography and has become a superior choice for discussing the corrosion

4. Conclusions The inhibitive ability and mechanism of MPTA and EPTA for X80 steel corrosion in 15% HCl was studied by systematic methods and characterizations. The main conclusions are as follows: (1) MPTA and EPTA exhibited good inhibition performance for X80 steel corrosion in simulated acidic oilfield water, and the

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Fig. 13. EDX spectrum of X80 steel after immersion in 1 M HCl solution: (a) 15% HCl, (b) 15% HCl with 400 ppm MPTA.

Fig. 14. AFM images of X80 steel surfaces: (a) 15% HCl, (b) MPTA, (C) EPTA.

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Fig. 15. XPS spectra for X80 steel treated with 400 ppm MPTA in 15% HCl: (a) survey scan spectra and narrow scan spectra of (b) C, (c) N, and (d) Fe.

inhibition efficiency is related to the concentration of inhibitors and temperature. (2) Electrochemical results showed that MPTA and EPTA performed as mixed-type inhibitors, which predominantly restrained cathode process. (3) The adsorption of investigated inhibitors on the X80 steel surface is predominantly physisorption and obeys Langmuir adsorption isotherm. (4) Surface analysis (SEM, EDX, AFM and XPS) confirms the adsorption of MPTA and EPTA molecules on X80 steel surface has played a good inhibition performance.

Acknowledgments This work was supported by the Natural Science Foundation of Shandong (ZR2019MB009), the Fundamental Research Funds for the Central Universities (HIT.NSRIF.201701), the National Natural Science Foundation of China (21672046, 21372054), and the Fund from the Huancui District of Weihai City. References [1] E. Ituen, O. Akaranta, A. James, S. Sun, Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid, Sustain. Mater. Technol. 11 (2017) 12–18. [2] H. Tian, W. Li, B. Hou, D. Wang, Insights into corrosion inhibition behavior of multiactive compounds for X65 pipeline steel in acidic oilfield formation water, Corros. Sci. 117 (2017) 43–58. [3] J. Zhang, J. Wang, F. Zhu, M. Du, Investigation of inhibition properties of sophorolipids for X65 steel corrosion in simulated oilfield produced water saturated with carbon dioxide, Ind. Eng. Chem. Res. 54 (2015) 5197–5203.

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