Effective inhibition on the corrosion of X65 carbon steel in the oilfield produced water by two Schiff bases

Journal of Molecular Liquids 285 (2019) 223–236

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Effective inhibition on the corrosion of X65 carbon steel in the oilfield produced water by two Schiff bases Q.H. Zhang a, B.S. Hou a, N. Xu a, W. Xiong b,⁎, H.F. Liu a,c, G.A. Zhang a,⁎ a Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b College of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430200, PR China c State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 15 January 2019 Received in revised form 29 March 2019 Accepted 15 April 2019 Available online 18 April 2019 Keywords: Carbon steel Schiff bases Corrosion inhibitors Electrochemical measurements Surface analysis Theoretical calculations

a b s t r a c t The failure of carbon steel pipelines due to corrosion is a serious problem in the oil and gas exploitation. In this study, two new Schiff bases, 5-((3-phenylallylidene)amino)-1,3,4-thiadiazole-2-thiol (PATT) and 5((thiophen-2-ylmethylene)amino)-1,3,4-thiadiazole-2-thiol (TATT), were synthesized as inhibitors to inhibit the corrosion of X65 carbon steel in the CO2-saturated oilfield produced water. Electrochemical measurements and surface analysis show that both PATT and TATT exhibit significant inhibition effect by predominant inhibition of anodic process. The considerable negative standard Gibbs free energy indicates that the adsorptions of PATT and TATT are primarily chemisorption. Molecular dynamics simulations show that both PATT and TATT adsorb on Fe surface in flat orientation. PATT exhibits a higher inhibition effect than TATT, which has been demonstrated by experimental measurements and theoretical calculations. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Carbon steel is extensively applied in various industrial processes owing to its obvious advantages, such as good mechanical performance and cost-effectiveness [1,2]. However, when the carbon steel is exposed to acidic or salty solutions, serious corrosion attack will occur because generally no protective film can be formed, which may cause enormous economic loss [3]. For example, during the oil and gas exploitation, the failure of carbon steel pipelines due to corrosion has posed great threat to the safety in oil and gas production. Therefore, developing more effective corrosion inhibitors is urgent to solve the corrosion problem of carbon steel [4]. As is well-known, plenty of compounds have been explored their potentials as corrosion inhibitors, such as inorganic compounds [5–8], plant extracts [9–11], amino acid derivatives [12–14], and Schiff bases [15–17]. From the environment and health point of view, traditional inorganic corrosion inhibitors, such as chromates, are highly toxic, while the uses of phosphates, metaphosphates and other phosphoruscontaining compounds would lead to environmental problems, i.e., eutrophication of water and red tides. Hence, plenty of organic compounds have been utilized to prevent the corrosion of different metals including aluminum [18,19], mild steel [20–22], and copper [23,24]. ⁎ Corresponding authors. E-mail addresses: [email protected] (W. Xiong), [email protected] (G.A. Zhang).

https://doi.org/10.1016/j.molliq.2019.04.072 0167-7322/© 2019 Elsevier B.V. All rights reserved.

The inhibition effects of these organic compounds are mainly dependent upon the active sites in their molecular structures such as oxygen, nitrogen and sulfur, which can adsorb on the metal surface to form a protective adsorbed film [25]. Schiff bases are one of the most common organic inhibitors not only because of the relatively simple and low-cost synthesis, but also because of the low toxicities and environment-friendly nature [15,26]. It is also worth noting that Schiff bases generally exhibit higher inhibition efficiencies than amines and aldehydes, which may be ascribed to the imine functional group (\\C_N\\) in Schiff bases [27]. In addition, the Schiff bases containing heterocyclic thiadiazole are environmentfriendly compounds, which have been widely used as chemosensors [28], intermediate of drugs [29], and antibacterial agents [30]. In the past decades, thiadiazole and its derivatives have been widely used as corrosion inhibitors owing to their low toxicities and high inhibition efficiencies [31]. Therefore, the Schiff bases containing heterocyclic thiadiazole have great prospect as inhibitors for industrial applications. In this work, we synthesized two Schiff bases, namely, 5-((3phenylallylidene)amino)-1,3,4-thiadiazole-2-thiol (PATT) and 5((thiophen-2-ylmethylene) amino)-1,3,4-thiadiazole-2-thiol (TATT). Their inhibition effects on the corrosion of X65 carbon steel in the CO2-saturated oilfield produced water were studied by experimental measurements and theoretical calculations. The inhibition efficiencies of PATT and TATT were determined using electrochemical polarization and impedance measurements. Quantum chemical calculations and molecular dynamics (MD) simulations were conducted to reveal the

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Scheme 1. Synthesis of the two Schiff bases (PATT, TATT).

inhibitive mechanisms of these two Schiff bases on the corrosion of X65 steel. 2. Experimental 2.1. Synthesis of Schiff bases inhibitors The synthetic route of the PATT and TATT inhibitors was shown in Scheme 1 and their structures were presented in Table 1. For the synthesis of PATT, 5-amino-1,3,4-thiadiazole-2-thiol (ATT) (1.66 mmol, 221 mg), MgSO4 (0.83 mmol,100 mg) and diethyl ether (4.0 mL) were added to a 25 mL round bottom flask, and then cinnamaldehyde (1.66 mmol, 210 μL) was added dropwise. After the mixture was stirred for 12 h, dichloromethane (30 mL) was added and the mixture was filtered. Then, the organic solvent was removed by rotary evaporation, and the residue was purified by column chromatography to obtain the target product (PATT) (333 mg, 81% yield). 1H NMR analysis was conducted on a Bruker AV-400 spectrometer with tetramethylsilane as the internal reference. The 1H NMR data of PATT were shown in Fig. 1 (a). 1H NMR (400 MHz, DMSO‑d6): δ (ppm) = 7.204–7.266 (m, 1H), 7.467 (d, 3H), 7.649–7.743 (m, 3H), 8.444 (d, 1H), 14.494 (s, 1H). For the synthesis of TATT, ATT (6.0 mmol, 799.2 mg) and thiophene2-carbaldehyde (6.0 mmol, 0.561 mL) was dissolved in ethanol (50 mL), then refluxed at 90 °C with stirring for 6 h. After removing the solvent by rotary evaporation, petroleum ether (50 mL) was added. Then, the mixture was filtrated, and the filter cake was further purified by column chromatography to obtain the TATT product (1036 mg, 76% yield). The 1H NMR data of TATT were shown in Fig. 1(b). 1H NMR (400 MHz, DMSO‑d6): δ (ppm) = 7.600–7.604 (m, 1H), 7.934 (t, 1H), 8.076 (d, 1H), 8.874 (s, 1H), 14.479 (s, 1H). 2.2. Material and solution for corrosion tests The X65 carbon steel samples used in this work were machined into the dimensions of 1.0 cm × 0.8 cm × 0.3 cm. The composition (wt%) of X65 steel was: C 0.04%, Si 0.2%, Mn 1.5%, P 0.011%, S 0.003%, Mo 0.02% and Fe balance. As the working electrode, the steel samples were embedded in epoxy resin with the exposed area of 0.8 cm2. Prior to all electrochemical measurements, the samples were abraded with 800 grit SiC paper, and then cleaned with deionized water. The test solution, which contains 62.36 g L−1 NaCl, 3.4 g L−1 KCl, 4.46 g L−1 MgCl2·6H2O, 0.57 g L−1 CaCl2, 0.64 g L−1 Na2SO4, and 0.52 g L−1 NaHCO3, was used to simulate the oilfield produced water in an oilfield. The concentration of each species was determined by inductive coupled plasma atomic emission spectrometry (ICP-AES) and

Fig. 1. 1H NMR spectra of the two Schiff bases: (a) PATT (b) TATT.

ion chromatography (IC) analyses. The measured pH value of the simulated CO2-saturated oilfield produced water was 5.4. 2.3. Electrochemical measurements Electrochemical measurements were conducted by using the conventional three-electrode cell. A saturated calomel electrode (SCE) and a platinum electrode were utilized as reference electrode (RE) and counter electrode (CE), respectively. The steel sample was employed as the working electrode (WE). Before the electrochemical measurements, the solution was deoxygenated by purging CO2 (99.99%) for 1 h. Then, the WE was immersed in the test solution and CO2 purging was continued to ensure the whole test under CO2saturated condition. After the WE was exposed to the solution for 1 h, electrochemical impedance spectroscopy (EIS) measurements were

Table 1 Structures and molecular weights of the two Schiff bases inhibitors. Code

Name

Structure

Molecular weight (g mol−1)

PATT

5-((-3-Phenylallylidene)amino)-1,3,4-thiadiazole-2-thiol

247.4

TATT

5-((Thiophen-2-ylmethylene)amino)-1,3,4-thiadiazole-2-thiol

227.2

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Fig. 2. Variations of the OCPs of mild steel during 1 h of immersion in CO2-saturated oilfield produced water with different concentrations of PATT or TATT at 60 °C: (a) PATT and (b) TATT.

implemented at open circuit potential (OCP) by applying a sine wave potential signal with the frequency from 100,000 to 0.1 Hz and amplitude of 5 mV. The measured EIS data were fitted with an equivalent circuit to obtain the polarization resistances (RP). Then the inhibition efficiencies (ηR%) of inhibitors were determined with the polarization resistances [32]: ηR % ¼

RP −R0P  100 RP

ð1Þ

where R0p and RP are the polarization resistances of X65 steel in the absence or presence of inhibitors, respectively. Potentiodynamic polarization curves were measured from −200 to 200 mV vs. OCP with a sweep rate of 0.5 mV s−1. The inhibition efficiencies (ηP%) of inhibitors were calculated by following equation [33]:

ηP % ¼

i0corr −icorr i0corr

 100

ð2Þ

Fig. 3. EIS of mild steel in the CO2-saturated oilfield produced water with different concentrations of PATT or TATT at 60 °C: (a) Nyquist plot, PATT, (b) Bode plot, PATT, (c) Nyquist plot, TATT, (d) Bode plot, TATT.

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E i

Fig. 4. Equivalent circuits for EIS fitting: (a) without or with low concentration of inhibitors, (b) with high concentration of inhibitors.

where i0corr and icorr are the corrosion current densities of X65 steel without or with inhibitors, respectively. Meanwhile, the potential of zero charge (Epzc) of electrode was also obtained by EIS measurements under different polarization potentials. After EIS measurements, the Rp values were determined and the Epzc was obtained by plotting the values of Rp with respect to the applied potentials [34]. All the tests were conducted at 60 °C except the tests to determine the effect of temperature. 2.4. Surface characterization After being immersed in the test solution with and without 0.01 mM PATT or TATT at 60 °C for 72 h, the specimen was analyzed using a contact angle meter (JC2000D) to determine the contact angle by dropping 5 μL deionized water. Three positions were selected to repeat three times for each sample. Quanta 200 Environmental scanning electron microscope (FEI Corporation, Dutch) and SPM9700 atomic force microscope (Shimadzu Corporation, Japan) were employed to capture the surface morphology and roughness of the sample. X-ray photoelectron spectroscopy (XPS) was conducted to determine the elemental composition and valence state of the sample surface, using a monochromatic X-ray Al Kα source with a pass energy of 25 eV. 2.5. Quantum chemical calculations Quantum chemical calculations were conducted to determine the optimized structures of PATT and TATT based on density functional

Fig. 5. Polarization curves of mild steel in the CO2-saturated oilfield produced water with different concentrations of PATT or TATT at 60 °C: (a) PATT (Inset shows the linear portion selected for fitting Tafel equation in polarization curve), (b) TATT.

theory (DFT) with B3LYP in conjunction with 6-311++ G (d, p) basis set using Gaussian 09 [35]. To ensure the optimized structures with the minimum potential energy, the geometry optimizations were performed without symmetry and vibrational constraints [36]. Water was set as the solvent by considering the solvent effects in the calculations.

Table 2 Fitted parameters of the EIS of mild steel in the CO2-saturated oilfield produced water in the absence or presence of PATT or TATT at 60 °C. Inhibitors

Blank PATT

TATT

Concentration (mM)

Rs (Ω cm2)

0 0.001 0.0025 0.005 0.01 0.025 0.05 0.001 0.0025 0.005 0.01 0.025 0.05

1.89 2.44 2.58 2.25 2.41 2.25 2.37 2.32 2.39 2.22 2.33 2.27 2.42

CPEf Y0 (μΩ−1 sn cm−2)

n

Rf (Ω cm2)

56.3 52.7 49.3 45.1

0.95 0.93 0.93 0.95

3458 6766 7761 5329

77.4 66.0 52.8 64.3

0.94 0.93 0.93 0.93

4568 6463 7609 8177

CPEdl Y0 (μΩ−1 sn cm−2) 601.8 448.1 271.6 84.9 156 56.5 99.8 451.4 288.2 220 361 139 101

n 0.76 0.90 0.92 0.67 0.95 0.99 0.83 0.89 0.91 0.66 0.98 0.98 0.99

Rct (Ω cm2) 88.7 219 964.4 3689 1913 3839 2867 175.7 430.5 2183 1940 1591 576

ηR (%)

59.50 90.80 98.76 98.98 99.24 98.92 49.52 79.40 98.69 98.94 99.04 98.99

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Table 3 Fitted parameters of the polarization curves of mild steel in the CO2-saturated oilfield produced water in the absence or presence of PATT or TATT at 60 °C. Inhibitors

Concentration (mM)

Ecorr (V vs. SCE)

0 0.001 0.0025 0.005 0.01 0.025 0.05 0.001 0.0025 0.005 0.01 0.025 0.05

−0.732 −0.708 −0.711 −0.691 −0.681 −0.670 −0.685 −0.720 −0.715 −0.698 −0.680 −0.692 −0.679

Blank PATT

TATT

2.6. Molecular dynamics simulations Molecular dynamics (MD) simulations were conducted to investigate the adsorption progress of the two Schiff bases on Fe surface using Material Studio software. In the simulation process, the most densely packed plane, Fe (1 1 0), was selected because this plane was generally considered as the most stable plane [37,38]. The MD simulations were performed with periodic boundary conditions and without any arbitrary boundary effects [39]. Besides, since the corrosion inhibitors were dissolved in aqueous solution, the consideration of water molecules was essential [40]. In this model, 300 H2O molecules were built on Fe (1 1 0) surface. Finally, the equilibrium structures for the adsorptions of PATT and TATT on Fe surface were obtained by MD simulations at 333.15 K by adopting a canonical ensemble (NVT) with a time step of 1.0 fs, and a total simulation time of 500 ps. 3. Results and discussion 3.1. Open circuit potential and electrochemical impedance measurements Open circuit potential (OCP) vs. time curves for X65 steel in the CO2saturated oilfield produced water in the absence and presence of inhibitors with different concentrations are shown in Fig. 2. It is seen that after immersion for 1 h, the OCPs of the specimens reach a relatively steady state. Furthermore, the OCPs of the specimens in the presence of PATT and TATT shift positively compared with that in blank solution,

icorr (A cm−2) 1.80 × 10−4 6.46 × 10−5 9.99 × 10−6 2.09 × 10−6 1.59 × 10−6 1.24 × 10−6 1.39 × 10−6 8.68 × 10−5 1.78 × 10−5 2.22 × 10−6 1.77 × 10−6 1.35 × 10−6 1.38 × 10−6

ba (mV dec−1)

bc (mV dec−1)

149 162 174 169 179 182 173 171 188 233 263 185 234

−435 −225 −179 −131 −126 −163 −110 −282 −178 −130 −141 −131 −143

ηP (%)

64.11 94.46 98.84 99.12 99.31 99.23 51.78 90.11 98.77 99.01 99.25 99.23

which may be due to the adsorption of inhibitor molecules on the active sites of the specimens surface [41]. To determine the inhibition effects of the two Schiff bases, EIS measurements were conducted at the end of the OCP measurements. The Nyquist and Bode plots of X65 steel in the solutions without or with different concentrations of inhibitors are shown in Fig. 3. From the Nyquist plots, depressed capacitive loops are observed under all different conditions, which may be ascribed to the surface roughness and heterogeneity [42]. In this situation, the double layer capacitance (Cdl) is substituted with constant phase element (CPE) [43]. Furthermore, the capacitive loop enlarges with the increasing inhibitor concentration, indicating the increase in the inhibitive efficiency. To obtain the parameters, EIS were fitted with the equivalent circuits in Fig. 4. From the Bode plots, the phase angles without or with low concentration of inhibitors (0.001 mM, 0.0025 mM) exhibit a narrow phase angle peak, indicating only one time constant, and then an equivalent circuit in Fig. 4(a) was used, where Rs is the solution resistance, Rct and CPEdl are the charge transfer resistance and double layer capacitance, respectively. The polarization resistance Rp is equal to Rct (RP = Rct). In the relatively high concentration of PATT or TATT, the wide phase angle peaks are observed, which may consist of two overlapped phase angle peaks [44]. That is to say, these two overlapped phase angle peaks are too close to be separated and detected [45,46]. Therefore, an equivalent circuit with two time constants in Fig. 4 (b) was used. The time constant at high frequency is related to the adsorbed inhibitor film while the time constant at low frequency is associated with the electric double layer. In this equivalent circuit, Rf and CPEf are the resistance and capacitance of the adsorbed inhibitor film, respectively. In this case, Rp is equal to the sum of Rf and Rct [26,47–49]. Table 2 presents the fitted parameters for EIS. It could be seen that the value of Rp in the solution without inhibitors is much smaller than those in the solutions with inhibitors, which indicates the considerable inhibition effects of the two Schiff bases. The maximum inhibition efficiency reaches 99.24% for PATT and 99.04% for TATT even with a low concentration of 0.025 mM. Furthermore, compared with TATT, PATT presents a higher inhibition efficiency. 3.2. Polarization curves measurements

Fig. 6. Corrosion current density of mild steel and the inhibition efficiencies of PATT and TATT in the CO2-saturated oilfield produced water at 60 °C.

The polarization curves of X65 steel without or with PATT and TATT are presented in Fig. 5. It is seen that the presence of inhibitors reduces both the anodic and cathodic current densities, and the decrease in anodic current density is more prominent than that in cathodic current density, which causes the positive shift of corrosion potential [50]. From Fig. 5, in the blank solution, there is slight effect of diffusion on the cathodic reaction due to the fast reaction rate under the polarization condition. However, in the inhibited solution, no effect of diffusion on the cathodic reaction is observed due to the reduced reaction rate after the adsorption of inhibitor molecules. The diffusion of reducing

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Table 4 Comparison of the inhibition efficiencies (η%) of PATT and TATT with other Schiff bases. Inhibitor

Medium

Material

Concentration

η%a

Ref.

(E)-4-(4-nitrobenzylideneamino)benzoate (E)-4-(4-nitrobenzylideneamino)benzoate N,N′-(pyridine-2,6-diyl)bis(1-(4-methoxyphenyl)methanimine) N,N′-(pyridine-2,6-diyl)bis(1-(4-methoxyphenyl)methanimine) 1-[(2-Hydroxyethyl)amino]-2-(salicylideneamino)ethane PATT TATT

10 mM NaCl 10 mM NaCl CO2-saturated 3.5%NaCl CO2-saturated 3.5%NaCl CO2-saturated 3.0% NaCl CO2-saturated oilfield produced water CO2-saturated oilfield produced water

Fe Carbon steel N80 steel J55 steel Carbon steel X65 steel X65 steel

10 mM 10 mM 400 mg l−1 (1.16 mM) 400 mg l−1 (1.16 mM) 500 ppm (0.24 mM) 0.025 mM 0.025 mM

81.0 91.0 90.0 91.0 86.2 99.31 99.25

[39] [39] [52] [52] [53] This work This work

a

The maximum inhibition efficiency (η%) was determined by polarization curve measurements.

species from bulk solution is fast enough to support the reaction. The fitted parameters, including corrosion current densities (icorr, which is determined by cathodic branch), corrosion potential (Ecorr), anodic and cathodic Tafel slope (ba, bc), and inhibition efficiency (η%), are listed in Table 3. It is seen that the corrosion potentials in the presence of PATT and TATT shift to the positive direction, which suggests that PATT and TATT primarily inhibit the anodic dissolution of X65 steel. Furthermore, the anodic Tafel slopes (ba) in the presence of inhibitors are higher than that in the blank solution, which suggests the inhibition of anodic dissolution of steel by adsorption of inhibitors. However, there is an irregular change in the Tafel slopes with increasing inhibitor concentration, which indicate that the inhibition effects of PATT and TATT may be attributed to a mixed effect including the adsorption on active sites and involvement of some other anions in the solution [49,51]. Fig. 6 shows the corrosion current densities of X65 steel in the solutions without or with different concentrations of PATT or TATT, and the corresponding inhibition efficiencies. As shown in Fig. 6, both PATT and TATT exhibit high inhibition efficiencies even at low concentration. With the increasing inhibitor concentration (less than 0.025 mM), the icorr decreases and the η% increases. Furthermore, the inhibition efficiency of PATT is higher than that of TATT. Table 4 lists the inhibition efficiencies of PATT and TATT, as well as others Schiff bases in NaCl solution. Although there are differences in the materials and media, it still could see that the inhibition efficiencies of PATT and TATT are much higher than those Schiff bases reported in the literature [39,52,53].

3.3. Adsorption isotherm To gain the inhibition mechanism of PATT and TATT for corrosion of X65 steel, the adsorption isotherm is evaluated. By the analysis of the electrochemical measurements data, both the adsorptions of PATT and TATT follow the Langmuir isotherm: C inh 1 ¼ þ C inh K ads θ

ð3Þ

where Cinh is the concentration of inhibitors, Kads is the adsorption equilibrium constant of inhibitors, θ is the coverage of inhibitors (θ = ηR). Fig. 7 presents the plots of Cinh/θ vs. Cinh for PATT and TATT, and the corresponding linear fitting parameters are summarized in Table 5. The good linear correlation of Cinh/θ vs. Cinh confirms that the adsorptions of PATT and TATT follow the Langmuir isotherm. The values of Kads for PATT and TATT are 3.928 × 106 and 2.078 × 106, respectively. The high values of Kads suggest the strong interactions of PATT and TATT with carbon steel surface [54,55]. The standard free energy of adsorption (ΔG0ads), as another important thermodynamic parameters to describe the adsorption process, can be calculated with Kads: ΔG0ads ¼ −RT ln ð55:5K ads Þ

c

c

ð4Þ

c

c

Fig. 7. Langmuir adsorption isotherms of PATT and TATT in the CO2-saturated oilfield produced water at 60 °C: (a) PATT, (b) TATT.

Table 5 Thermodynamic adsorption parameters for the adsorptions of PATT and TATT on mild steel surface in the CO2-saturated oilfield produced water at 60 °C. Inhibitors

Slope

Intercept (mM)

PATT TATT

1.003 0.997

2.546 × 10−4 4.813 × 10−4

Linear correlation coefficient 0.9998 0.9995

Kads (L mol−1) 3.928 × 106 2.078 × 106

ΔG0ads (kJ mol−1) −53.18 −51.42

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3.5. Potential of zero charge (Epzc) measurements

i

It is well-known that the adsorption process of inhibitors on metal surface is dependent upon many factors, such as the structures and dipole moments of inhibitor molecules, the charge state of metals [60]. Potential of zero charge (Epzc), which corresponds to the maximum polarization resistance or minimum capacitance [25,61,62], can reflect the adsorption mechanism of inhibitors based on the charge state of metals surface. To determine the Epzc of X65 steel without or with PATT and TATT, EIS measurements were performed in the solutions without or

E

T

R

Fig. 8. Arrhenius plots for mild steel in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT.

E

where T is the absolute temperature and R is the gas constant. The calculated ΔG0ads for PATT and TATT are −53.18 kJ mol−1, −51.42 kJ mol−1, respectively, which suggests the chemisorption of PATT and TATT on the X65 steel surface [49,56]. 3.4. Activation energy of corrosion process To investigate the effect of temperature on the inhibition performances of PATT and TATT for the corrosion of X65 steel, potentiodynamic polarization measurements were conducted at different temperatures without or with 0.01 mM PATT or TATT. According to the corrosion current densities at different temperatures, the activation energy (Ea) of the corrosion process can be obtained by the Arrhenius equation:

E

R

  Ea icorr ¼ A exp − RT

E

ð5Þ

E E

R

where A is the frequency factor. Fig. 8 presents the plots of lnicorr vs. 1/T and the corresponding linear fitting straight lines. The values of Ea, which are obtained from the slopes of the plots of lnicorr vs. 1/T, are listed in Table 6. From Table 6, the activation energies in the solutions with 0.01 mM PATT and TATT are lower than that in the blank solution, which indicates the strong chemisorption between the inhibitors and X65 steel [57]. A similar situation was observed on the adsorptions of thiadiazole derivatives [58]. Therefore, both the activation energy and corrosion rate decrease after adding inhibitors. The corrosion rate decreases with decreasing the activation energy in the presence of inhibitors may be owing to the decrease in the frequency factor. The effective collision between the aggressive ions and metal surface may decrease due to the strong chemisorption of inhibitor molecules on the metal surface, which results in a decreased frequency factor [59]. Moreover, the activation energy of PATT is lower than that of TATT, which confirms the better inhibition effect of PATT.

Table 6 Activation energy for the corrosion of mild steel in the CO2-saturated oilfield produced water in the absence or presence of 0.01 mM PATT or TATT. Inhibitors

Ea (kJ mol−1)

Linear correlation coefficient

Blank PATT TATT

36.47 13.68 23.86

0.89 0.98 0.94

Fig. 9. Plots of Rp vs. applied electrode potential of mild steel in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT at 60 °C: (a) blank, (b) PATT, (c) TATT.

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3.6. Contact angle measurements Generally, the inhibition effects of inhibitors are related to the hydrophobicity of the adsorbed inhibitor films, and contact angle test can recognize the hydrophobic nature of the adsorbed inhibitor films [64]. Fig. 10 shows the contact angle of X65 steel after exposed to the solutions without or with 0.01 mM PATT and TATT for 72 h. In the blank solution, the contact angle is small (26°), which may be attributed to the formation of hydrophilic corrosion products and the increase in the roughness of the steel surface after corrosion [65]. In the solution with inhibitors, the contact angles (69° for PATT and 57° for TATT) increase significantly, which may be due to the hydrophobic inhibitor films on the steel surface by the adsorption of PATT and TATT. Additionally, the higher contact angle in the presence of PATT means the higher hydrophobicity of PATT inhibitor film, which provides better inhibition effect on the corrosion of X65 steel. 3.7. Surface morphology after corrosion

Fig. 10. Contact angles images for mild steel after immersed in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT at 60 °C for 72 h: (a) blank, (b) PATT, (c) TATT.

The SEM micrographs of the X65 steel surface after exposed to the solutions without or with 0.01 mM inhibitors for 72 h are shown in Fig. 11. It is seen that relatively serious corrosion is observed in the blank solution with plenty of corrosion products covered on the sample surface (Fig. 11(a)). However, in the presence of PATT or TATT, the corrosion of the sample is slight and the scratches produced by the abrasive pretreatment process before corrosion tests are still visible (Fig. 11(b, c)). This confirms that PATT and TATT have significant inhibition effects on the corrosion of X65 steel. 3.8. Atomic force microscope (AFM) analysis

with 0.01 mM PATT or TATT at various polarization potentials. Fig. 9 shows the plots of Rp vs. the applied polarization potential. It is seen that Eocp is more positive than Epzc in both blank and inhibited solutions, which indicates that the electrode surface is positively charged and the adsorption of anions is favored [63].

In order to characterize the microstructure of the sample surfaces after corrosion test, the corroded sample surfaces were observed by atomic force microscope. Fig. 12 shows the 3D AFM micrographs of the sample surfaces after exposed in the solutions without or with 0.01 mM inhibitors at 60 °C for 72 h. The AFM micrograph shows the

Fig. 11. SEM morphologies of mild steel after immersed in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT at 60 °C for 72 h: (a) blank, (b) PATT, (c) TATT.

Fig. 12. AFM images of mild steel after immersed in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT at 60 °C for 72 h: (a) blank, (b) PATT, (c) TATT.

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Fig. 13. XPS high resolution spectra of mild steel after immersed in the CO2-saturated oilfield produced water without or with 0.01 mM PATT or TATT at 60 °C for 72 h: (a) Fe 2p3/2, (b) O 1s, (c) C 1s, (d) N 1s, (e) S 2p.

uneven surface of the sample with the average surface roughness of 84.69 nm after corrosion in the blank solution. However, the sample surfaces are significantly smoother and relatively flatter in the presence of inhibitors. The average surface roughness are only 28.46 nm and 40.81 nm in the presence of PATT and TATT, respectively. This may be due to the formation of the inhibitor films as a protective barrier against corrosion, thereby reducing the surface roughness of the sample. 3.9. X-ray photoelectron spectroscopy (XPS) analysis Since the adsorption of inhibitors on the carbon steel surface is crucial to the inhibition effect, XPS was employed to analyze the composition and valence state of the adsorbed protective film on the X65 steel surface. Fig. 13 shows the high resolution peaks of Fe 2p3/2, O 1s, C 1s, N 1s, S 2p. The Fe 2p3/2 spectrum in the absence of inhibitors is deconvoluted into two peaks. The peak at 709.9 eV is attributed to Fe (II), such as FeO, FeCO3. The peak at 711.5 eV is indicative of the presence of Fe3+ such as FeOOH [1,66], which may be ascribed to the

oxidation of Fe(II) in the process of sample preparation for XPS analysis after corrosion test. However, in the presence of inhibitors, besides these two peaks, a peak at 707 eV is present, which is assigned to the metallic iron. This indicates that PATT and TATT can inhibit the corrosion of X65 steel by adsorption. The O 1s spectra of the samples without or with PATT and TATT consist of two peaks. The peak at 529.7 eV is ascribed to O2−, and the peak at 531.3 eV is assigned to OH−, which can be related to the presence of FeOOH [67,68]. Fig. 13(c–e) show the C 1s, N 1s, S 2p spectra of the samples in the presence of 0.01 mM PATT and TATT. The C 1s spectrum consists of three peaks. The peak at 284.5 eV could be assigned to C_C and C\\H in the aromatic ring, and the peak at 286.3 eV represents C_N and C\\S of inhibitors [69]. The peak at 288.2 eV corresponds to C_N+ of the protonated imine functional group [25]. The N 1s spectrum is deconvoluted into three peaks. The peaks at 398.6 eV, 399.2 eV are assigned to the N\\N and C_N [70], while the peak at 400 eV may be related to protonated N atom in the PATT and TATT [71]. For the S 2p spectrum, the peaks at 162 eV and 164.1 eV could be assigned to the S

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Table 7 Binding energies and element contents of Fe 2p3/2, O 1s, C 1s, N 1s, S 2p in the absence or presence of 0.01 mM PATT or TATT. Inhibitors

Elements

BE (eV)

Assignment

At.%

Blank

Fe 2p3/2

709.9 711.5 529.7 531.2 – 706.8 709.6 711.5 529.5 531.7 284.6 286.3 288.3 398.7 399.2 400 162.0 164.1 706.9 709.5 711.2 529.4 531.0 284.6 286 288 398.5 399.4 400.3 161.5 163.3

FeO,FeCO3 FeOOH O2− OH− – Fe0 FeO,FeCO3 FeOOH O2− OH− C_C/C\ \H C_N/C\ \S + C_N N\ \N C_N C_N+ \ \SH C\ \S\ \C Fe0 FeO,FeCO3 FeOOH O2− OH− C_C/C\ \H C_N/C\ \S + C_N N\ \N C_N C_N+ \ \SH C\ \S\ \C

16.85

O 1s

PATT

C 1s Fe 2p3/2

O 1s C 1s

N 1s

S 2p TATT

Fe 2p3/2

O 1s C 1s

N 1s

S 2p

50.14 33.17 4.04

27.87 62.21

2.93

2.95 13.69

39.67 42.66

2.09

1.89

3.10. Quantum chemical calculations It is acknowledged that quantum chemical calculations are an effective method to explore the interaction mechanism between inhibitors and metal surface [39]. Based on the frontier molecular orbital theory, the highest occupied molecular orbital (HOMO) corresponds to the electron-donating ability, while the lowest unoccupied molecular orbital (LUMO) corresponds to electron-accepting ability. The higher energy of HOMO, the lower energy of LUMO, and the smaller the energy gap (ΔE = ELUMO − EHOMO) suggest the higher inhibition effects of inhibitors [72]. Figs. 14 and 15 show the optimized structures of the two Schiff bases and their HOMO and LUMO distributions. It is seen that both the optimized structures of PATT and TATT are planar, except the H atom in the \\SH group. For both PATT and TATT, the HOMO is distributed on the whole molecule except the S atom of thiadiazole ring, and the LUMO is also distributed on the whole molecule. Table 8 summarizes the obtained parameters by quantum chemical calculations. It can be seen that PATT exhibits higher HOMO and lower LUMO compared with TATT, i.e., a smaller energy gap ΔE of PATT, which means the higher inhibition effect of PATT. Additionally, the elecLUMO HOMO tronegativity (χ ¼ − EHOMO þE ), global hardness (γ ¼ ELUMO −E ) and 2 2

softness (σ ¼ γ1) are also the important parameters for the inhibition effects of inhibitors. The higher χ, the lower γ, the larger σ, then the better inhibition performance. In addition, based on the electronegativities and global hardnesses of inhibitors (χinh and γinh) and Fe (χFe and γFe), the fraction of electrons transferred (ΔN) could be obtained: ΔN ¼

in sulfhydryl group and heterocycle ring, respectively [70]. In addition, it should be pointed out that the contents of the N, S atoms absorbed on the sample surface in the solution with PATT are higher than those in the solution with TATT (see Table 7), which suggests the favorable adsorption of PATT.

χ Fe −χ inh 2ðγFe þ γ inh Þ

ð6Þ

The theoretical values of χFe and γFe are 4.82 eV and 0 eV, respectively. A higher value of ΔN corresponds to better inhibition performance of inhibitors when ΔN b 3.6 [73]. Therefore, from Table 8, all the parameters of quantum chemical calculations favor a better inhibition effect of PATT compared with TATT.

Fig. 14. Optimized structures of the two Schiff bases: (a) PATT; (b) TATT.

Fig. 15. The frontier molecule orbital density distributions of PATT and TATT: (a) HOMO-PATT; (b) LUMO-PATT; (c) HOMO-TATT; (d) LUMO-TATT.

Q.H. Zhang et al. / Journal of Molecular Liquids 285 (2019) 223–236 Table 8 Quantum chemical parameters for PATT and TATT calculated by DFT/B3LYP method with 6-311++ G (d, p) basis set. Inhibitors

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

χ (eV)

γ (eV)

σ (eV−1)

PATT TATT

−6.367 −6.456

−2.939 −2.842

3.428 3.614

4.653 4.649

1.714 1.807

0.583 0.553

ΔN

233

Table 9 Adsorption energies (Eads (kJ mol−1)) of PATT and TATT on Fe (1 1 0) surface. Inhibitors

Adsorption energy Eads (kJ mol−1)

PATT TATT

−561.94 −479.95

0.0487 0.0473

which is closer than TATT. It further confirms that PATT has better inhibition effect than TATT. 3.11. Molecular dynamics (MD) simulations 3.12. Adsorptive mechanism of Schiff bases To further investigate the interaction between these two Schiff bases and the Fe surface, MD simulations were conducted. Fig. 16 presents the adsorption modes of PATT and TATT on Fe (1 1 0) surface. The adsorption of PATT is in a flat way, which provides the maximum surface coverage. The adsorption mode of TATT is similar to PATT. This is probably because they both have heteroatoms and π electrons [74]. The value of the adsorption energy (Eads) of the adsorptions of PATT and TATT on Fe surface can be determined: Eads ¼ Etotal −ðEinh þ Esurf Þ

ð7Þ

where Etotal represents the total energy of Fe (1 1 0) surface and Schiff base molecules after adsorption. Einh and Esurf represent the energies of Schiff base molecules and Fe (1 1 0) surface before adsorption. Table 9 lists the Eads of the two Schiff bases. Generally, the higher absolute adsorption energy, the more stable system and the stronger adsorption abilities of inhibitors [72]. It can be seen that the absolute adsorption energy of PATT is higher than that of TATT, which suggests a better inhibition effect of PATT. In addition, the radial distribution function (RDF, g(r)), which is used to calculate the distance between the atoms of inhibitors and Fe surface after adsorption of inhibitor molecules, is shown in Fig. 17. It is seen that the distance between PATT and Fe surface at the first peak is only 2.5 Å,

The inhibition of inhibitors to metal corrosion is primarily dependent upon the adsorbed protective inhibitor film on the metal surface. According to the measurement of Epzc, the X65 steel surface is positively charged, which promotes the adsorption of anions (e.g. Cl−) through electrostatic interaction (Fig. 18). The inhibitor molecules of PATT and TATT contain N and S atoms, which are prone to protonation in the solution. The protonated inhibitor molecules become positively charged and then adsorb on the anions-adsorbed steel surface by physisorption. Finally, the molecules of PATT and TATT form a chemical bond (chemisorption) with X65 steel. The unprotonated inhibitor molecules could also adsorb on the steel surface because the N, S atoms and the conjugated system can provide electrons to the unfilled orbital of Fe to form chemical bond. Furthermore, compared with TATT, PATT has a bigger conjugated system. Therefore, PATT could adsorb readily by the interactions of thiadiazole ring,\\SH, the imine functional group (\\C_N\\), \\C_C\\ and benzene ring, which leads to a better inhibition effect of PATT [75]. For the adsorption of inhibitors, inhibitor molecules could adsorb on the steel surface by forming coordination bonds at the locations of HOMO and feekback bonds at the positions of LUMO. Quantum chemical calculations show that the HOMO and LUMO of PATT and TATT are almost distributed on the whole molecule (Fig. 15), which may be the reason of the high inhibition performances of PATT and TATT. It should

Fig. 16. Configurations for the adsorptions of PATT and TATT on Fe (1 1 0) surface.

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Fig. 17. Radial distribution function (g(r)) of PATT and TATT adsorbed on the Fe (1 1 0) surface.

be noted that compared with TATT, PATT has more active sites for adsorption. Especially, the \\C_C\\ functional group in PATT is linked to the benzene ring, which forms a bigger conjugated system and increases the electron cloud. Then, it is more likely for PATT to adsorb on the steel surface by electron donation. Therefore, PATT exhibits a higher inhibition efficiency than TATT. 4. Conclusions Two Schiff bases, PATT and TATT, were synthesized as inhibitors to inhibit the corrosion of X65 steel in the CO2-saturated oilfield produced water. Both PATT and TATT exhibit high inhibition performance by primarily inhibiting the anodic process of the X65 steel corrosion. PATT exhibits better inhibition effect than TATT. The adsorption processes of PATT and TATT on X65 steel surface follow the Langmuir adsorption isotherm. The large negative values of ΔG0ads indicate that the adsorptions of PATT and TATT are chemisorption. All the parameters obtained from quantum chemical calculations favor a better inhibition effect of PATT compared with TATT. Molecular dynamics simulations indicate that the adsorptions of PATT and TATT on Fe surface are in a flat way. The adsorption energy (Eads) of PATT is more negative than that of TATT, which suggests the higher inhibition performance of PATT. Acknowledgements The authors are grateful to the support ofNational Natural Science Foundation of China (Nos. 51571097, 51371086). The authors also thank the support of analytical and testing center ofHuazhong University of Science and Technology

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