Corrosion Inhibition of N80 Steel Using Novel Diquaternary

Article pubs.acs.org/IECR

Corrosion Inhibition of N80 Steel Using Novel Diquaternary Ammonium Salts in 15% Hydrochloric Acid Xiaoyun Zhang, Yunxiang Zheng, Xiangpeng Wang, Yufeng Yan, and Wei Wu* College of Science, China University of Petroleum (East China), Qingdao 266580, China S Supporting Information *

ABSTRACT: Three novel diquaternary ammonium salts with alkanediyl spacers of varying chain length were synthesized, and their corrosion inhibition effects on N80 steel in 15 wt % HCl solution were studied by weight loss measurement, electrochemical polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX). The results indicated that the inhibition efficiency increased with the inhibitor concentration and the length of hydrophobic spacer of the inhibitor. At 90 °C, the inhibition efficiency of diquaternary ammonium salt N,N′-octane-1,8-diyl-bisquinolinium dibromide reached about 91% at the inhibitor concentration of 0.01 mol/L. Potentiodynamic polarization curves indicated that all synthesized compounds acted as mixed-type inhibitors. The inhibition mechanism involved the formation of an inhibitor protective layer on the N80 steel surface by a Langmuir-type adsorption process. The presence of Br and N in chemical composition detected by EDX confirmed the adsorption of inhibitors on the N80 steel surface. industry every year.21 Thus, the addition of corrosion inhibitors during the acidizing process is required to reduce the aggressive attack of acid on tubing and casing materials (N80 steel, which is the widely used pipe steel22). Also, inhibitors with good thermal stability and maintained efficiency are required under the circumstance of high temperature application. Diquaternary ammonium salt, which has high surface activity, low toxicity, and good water solubility,23−25 is a promising acidizing corrosion inhibitor. It features two cationic quaternary nitrogen atoms and a hydrophobic chain between the two cationic centers. The diquaternary ammonium salt could possibly be a stronger barrier to prevent the contact of the metal surface with the acidic environment, compared to its monoquaternary ammonium salt analogues.26 In the present work, we have synthesized three novel diquaternary ammonium salts with different lengths of CH2 groups and investigated their inhibiting properties in 15 wt % HCl for N80 steel on the basis of weight loss, Tafel polarization measurements, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The effects of temperature, inhibitor dose, and the hydrophobic spacer between the two heterocyclic rings are discussed.

1. INTRODUCTION A great number of scientific studies have been devoted to the subject of corrosion inhibitors for mild steel in acidic media.1−5 It has been well recognized that the organic inhibitors can be adsorbed on the metal surface, and hence form a protective layer that impedes the access of corrosive ions to the surface of metal substrate. Among many different types of inhibitors, organic compounds containing electron-rich atoms, such as oxygen, nitrogen, sulfur, phosphor, and multiple bonds or aromatic rings, have been studied in detail during the process of design and selection of proper corrosion inhibitors to maximize the inhibition efficiency. The organic structure moieties, such as imidazole,6 benzimidazole,7 triazole,8 benzotriazole,9 pyridine,10 quinoline,11 Schiff base,12 cationic ammonium salt,13 etc., can serve as adsorption sites in potential inhibitors due to their capability to share free electrons for complexing with a metal substrate.14 It has also been reported that hydrophobic inhibitor molecules can effectively prevent the metal surface from acid corrosion by decreasing the contact surface area available for H+ ions.15 However, most of the reported inhibitors often function well only at low acid concentration and ambient temperature. Inhibitors which could withstand concentrated acid (>15 wt % HCl) and higher temperature are in great demand in the oil and gas industry as essential additives in acidizing fluids and pipeline cleaning solutions. Acidizing is a common production technique that involves pumping a large amount of hydrofluoric or hydrochloric acid down to wells, to restore the natural permeability of reservoir rock. Compared to other mineral acids, hydrochloric acid is the most often used acid, as it is economical and practical.16−19 The acidizing (stimulation) fluids which are usually dosed at high concentration can cause severe metal corrosion and lead to stress cracking by hydrogen and chloride ions. Damages caused by the strong acid generate high cost for inspection,20 repair, and replacement of acid-damaged equipment in the petroleum © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Diquaternary Ammonium Salts. The diquaternary ammonium salts were synthesized via quaterization reaction of quinoline with dibromoalkanes. Quinoline (0.048 mol) was mixed with 10 mL of distilled ethanol in a 250 mL three-necked flask. The solution was Received: Revised: Accepted: Published: 14199

June 17, 2014 August 7, 2014 August 29, 2014 August 29, 2014 dx.doi.org/10.1021/ie502405a | Ind. Eng. Chem. Res. 2014, 53, 14199−14207

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1.3 × 10−2 M. The volume of the acid solution used was 270 mL for each test. 2.4. Weight Loss Measurements. N80 steel specimens were immersed in 15 wt % HCl solutions in the absence or presence of different concentrations of diquaternary ammonium salt for 4 h at atmospheric pressure. Then the removed specimens were washed thoroughly with water and then acetone, dried, and weighed precisely. Triplicates were performed in each test, and the mean value of weight loss was calculated. The corrosion rates were calculated using the following equation:

stirred using a magnetic stirrer and heated to reflux. Then dibromohydrocarbon 1,4-dibromobutane/1,6-dibromohexane/ 1,8-dibromooctane (0.02 mol) dissolved in 5 mL of distilled ethanol was added through a dropping funnel to the above solution. The reaction continued for 3 days, followed by cooling to room temperature. The pink solid product was filtered and recrystallized three times from distilled ethanol. Pure diquaternary ammonium salt was obtained as colorless crystal. The chemical structures of the three synthesized diquaternary ammonium salts were confirmed by element analysis (VarioElementar Analyzer), Fourier transform infrared spectroscopy (FTIR, Genesis Fourier transformer FTIR), and nuclear magnetic resonance spectroscopy (NMR, Bruker NMR-600 MHz). N,N′-Butane-1,4-diyl-bisquinolinium dibromide (Q4Q): Yield 91.6%. mp 183−185 °C. 1H NMR (600 MHz, D2O): δ 9.10 (dd, J = 6,1.2 Hz, 2H), 8.95 (d, J = 8.4 Hz, 2H), 8.23 (m, 4H), 8.10 (m, 2H), 7.92(t, J = 7.8 Hz, 2H), 7.86 (dd, J = 8.4, 6 Hz, 2H), 5.01 (m, 4H), 2.17 (m, 4H). 13C NMR (150 MHz, D2O): δ 148.57, 147.98, 137.44, 136.11, 130.93, 130.13, 129.99, 121.59, 117.98, 57.17, 25.58. IR (KBr) ν: 3411, 3073, 2960, 1579, 1523, 1375, 777 cm−1. Anal. Calcd for C22H22N2Br2: C, 55.72; H, 4.68; N, 5.91. Found: C, 55.59; H, 4.57; N, 5.82. N,N′-Hexane-1,6-diyl-bisquinolinium dibromide (Q6Q): Yield 92.8%. mp 190−193 °C. 1H NMR (600 MHz, D2O): δ 9.12 (d, J = 5.4 Hz, 2H), 9.01 (d, J = 8.4 Hz, 2H), 8.30 (d, J = 9.0 Hz, 2H), 8.26 (d, J = 8.4 Hz, 2H), 8.13 (t, J = 8.4 Hz, 2H), 7.92 (m, 4H), 4.94 (t, J = 7.2 Hz, 4H), 2.01 (m, 4H), 1.39 (m, 4H). 13C NMR (150 MHz, D2O): δ 148.36, 147.60, 137.69, 135.90, 130.69, 129.99, 121.59, 118.16, 57.98, 28.92, 25.31. IR (KBr) ν: 3439, 3030, 2938, 1593, 1500, 1375, 777 cm−1. Anal. Calcd for C24H26N2Br2: C, 57.39; H, 5.22; N, 5.58. Found: C, 57.25; H, 5.31; N, 5.92. N,N′-Octane-1,8-diyl-bisquinolinium dibromide (Q8Q): Yield 89.7%. mp 195−196 °C. 1H NMR (600 MHz, D2O): δ 9.13 (d, J = 6 Hz, 2H), 9.01 (d, J = 6 Hz, 2H), 8.32 (d, J = 9.0 Hz, 2H), 8.25 (d, J = 7.8 Hz, 2H), 8.14 (t, J = 7.2 Hz, 2H), 7.93 (m, 4H), 4.93 (t, J = 6 Hz, 4H), 1.98 (m, 4H), 1.31 (m, 4H), 1.24 (m, 4H). 13C NMR (150 MHz, D2O): δ 148.33, 147.53, 137.66, 135.86, 130.67, 129.99, 129.98, 121.60, 118.20, 58.14, 29.13, 27.93, 25.50. IR (KBr) ν: 3432, 3016, 2917, 1593, 1523, 1375, 777 cm−1. Anal. Calcd for C26H30N2Br2: C, 58.88; H, 5.70; N, 5.28. Found: C, 58.95; H, 5.79; N, 5.38. 2.2. Preparation of Specimens. The specimens for weight loss experiments and working electrodes for electrochemical measurements were prepared from N80 steel strips with a composition (in wt %) of C, 0.31; Mn, 0.92; Si, 0.19; P, 0.01; S, 0.008; Cr, 0.20; and Fe, balance. The size of specimens for the weight loss experiment was 50 × 10 × 3 mm3. The working electrodes soldered with copper wire at one side were sealed with epoxy resin, and only a 1 × 1 cm2 flat surface was left open to the corrosive environment. Prior to the experiments, the open surface was abraded with a series of emery papers ranging from 400 to 1200 grift, then washed with double-distilled water, and degreased with acetone. The specimens were finally dried and stored in a vacuum desiccator before use. 2.3. Preparation of Solutions. All the tests were performed in 15 wt % HCl solutions with or without diquaternary ammonium salt as a corrosion inhibitor. The test solutions were made by diluting analytical grade 37 wt % HCl with distilled water. The concentration range of the diquaternary ammonium salts was varied from 2.0 × 10−3 M to

Vcorr =

W1 − W2 S×t −2

(1) −1

where Vcorr (g·m ·h ) is corrosion rate and W1 (g) and W2 (g) are the weights of a specimen before and after immersion in corrosion media, respectively. S (m2) is the area of a specimen, and t (h) is immersion time. 2.5. Electrochemical Measurements. The electrochemical experiments were performed on a CHI604C electrochemical analyzer (Shanghai Chen Hua instrument limited company, China). A conventional three-electrode cell consisting of N80 steel as a working electrode (WE), platinum as a counter electrode (CE), and a saturated calomel electrode (SCE) as a reference electrode (RE) were used. A stabilization time of 30 min was allowed prior to each measurement. After this period, a steady-state open circuit potential (OCP) was obtained, corresponding to the corrosion potential (Ecorr) of the working electrode. Potentiodynamic polarization curves were obtained by scanning the electrode potential automatically from −150 mV to +150 mV vs OCP with a rate of 0.5 mV/s. Corrosion current density (Icorr) was calculated by extrapolating the linear Tafel segment of cathodic and anodic curves to corrosion potential. The impedance measurements were carried out at the corrosion potential (Ecorr) using a 5 mV sine wave as excitation signals with a frequency ranging from 100 kHz to 5 mHz. All potentials were measured versus the SCE. Experimental data were analyzed using ZSimpWin software. 2.6. Surface Analysis. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis were carried out to study the changes of the surface morphology of the N80 steel with or without an inhibitor. After 4 h of immersion in 15 wt % HCl at 90 °C, the specimens were cleaned with distilled water and dried using a cold air blaster. The surface was examined by using a HITACHI S-4800 Fieldemission Scanning Electron Microscope. Chemical compositions of the sample surfaces were recorded by an EDX detector.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Diquaternary Ammonium Salts. The syntheses of diquaternary ammonium salts were carried out in ethanol (Figure 1). The molar ratio of quinoline to dibromide was 2.4:1, so as to minimize the yield of mono quaternary product. One favorable characteristic of the reaction is that the diquaternary product can be precipitated from the

Figure 1. Synthesis of diquaternary ammonium salts. 14200

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reaction mixture because of its low solubility in ethanol. Therefore, the isolation and purification is easy and highly pure product can be yielded. All three diquaternary ammonium salts showed clear melting points higher than 180 °C, indicating that they are thermally stable compounds. A typical FTIR spectrum of Q6Q (Figure 2)

Figure 4. 13C NMR spectrum of N,N′-hexane-1,6-diyl-bisquinolinium dibromide (Q6Q).

spectra of Q4Q and Q8Q (Supporting Information Figures S1−S4) where two very close signals around 129.99 ppm can be seen, matching the total number of nine aromatic carbons. These spectroscopic data together with the elemental analysis results prove that the syntheses were successful. 3.2. Weight Loss Measurements. The efficiency of an organic inhibitor for impeding metallic corrosion depends on several factors, such as the chemical structure, the concentration of inhibitor, nature of the metal, temperature, and property of the corrosive medium. In this study, N80 steel was used. The percentage inhibition efficiency (IE) and surface coverage (θ) were calculated using the following equation:

Figure 2. FTIR spectrum of N,N′-hexane-1,6-diyl-bisquinolinium dibromide (Q6Q).

shows the following peaks at 3000−3100 cm−1 (C−H stretching in quinoline ring), 2850−2960 cm−1 (aliphatic C− H stretching), 1579 cm−1 (CN stretching), 1523 cm−1 (C C stretching), 1375 cm−1 (N−CH2 bending), 1100−1250 cm−1 (C−C skeletal), and 777 cm−1 (CH2 rocking). The 1H NMR spectrum of Q6Q (Figure 3) in D2O shows three groups of

θ = IE% =

V0 − Vcorr V0 −2

−1

(2) −2

−1

where V0 (g·m ·h ) and Vcorr (g·m ·h ) were the corrosion rates without and with addition of the corrosion inhibitor, respectively. The corrosion rate Vcorr, the inhibition efficiency (IE), and surface coverage (θ) obtained from weight loss measurements of N80 steel in 15 wt % HCl under various concentrations of inhibitor and temperatures are presented in Table 1. It is demonstrated that the synthesized diquaternary ammonium salts were effective in inhibiting the corrosion of N80 steel. At a relatively high temperature of 90 °C, the inhibition efficiency increased with the inhibitor concentration. The highest inhibition efficiency of about 91% was achieved at a concentration of 1.0 × 10−2 M using Q8Q. Considering the acid concentration and temperature, this inhibition efficiency value is rather favorable. Besides, the effect of the hydrophobic chain in diquaternary ammonium salts on corrosion behavior can be concluded. As shown in Table 1, when the inhibitor concentration was kept constant, the corrosion rate decreased gradually with the increase of the length of the hydrophobic chain. The inhibition efficiencies of the inhibitors follow the sequence Q4Q < Q6Q < Q8Q, indicating that the inhibitory effect is directly related to the hydrophobicity of inhibitor molecules.27 The CH2 spacer between the two quinoline rings significantly contributes to the inhibition efficiency of the diquaternary ammonium salts. 3.3. Effect of Temperature on the Inhibition Efficiency. Table 1 also shows the temperature effect on the inhibition efficiency of the three diquaternary ammonium salts for N80 steel in 15 wt % HCl solution. At the same

Figure 3. 1H NMR spectrum of N,N′-hexane-1,6-diyl-bisquinolinium dibromide (Q6Q).

signals at 4.94, 2.01, and 1.39, corresponding to the CH2 at α-,β-,γ- positions to the N atom, respectively. Signals at 9.12 (1H), 9.01 (1H), 8.30 (1H), 8.26 (1H), 8.13 (1H), and 7.92 (2H) are assigned to the seven protons on a quinoline ring. From the 13C NMR of Q6Q (Figure 4), three aliphatic carbons at 57.98, 28.92, and 25.31 ppm and eight aromatic carbon signals in the range of 148.36−118.16 belong to the carbon atoms on the quinoline ring. The peak at 129.99 ppm with exceptional high intensity can be ascribed to the overlapping of two carbons. This is supported by observation of the 13C NMR 14201



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adsorption of inhibitor molecules on the metal surface leads to the decrease in entropy, the accompanying desorption of the solvent molecules leads to an increase in entropy. This phenomenon has been found in several inhibition systems.11,30 Here, the increase in entropy is mainly a contribution from the solvent molecules. 3.4. Potentiodynamic Polarization. The inhibition efficiencies from the electrochemical polarization method were evaluated using the following equation:

Table 1. Weight Loss Results in 15 wt % HCl in the Absence and Presence of Inhibitor substance 15% HCl

Q4Q

Q6Q

Q8Q

c (M) 0 0 0 2.0 2.0 2.0 5.0 1.0 1.3 2.0 2.0 2.0 5.0 1.0 1.3 2.0 2.0 2.0 5.0 1.0 1.3

× × × × × × × × × × × × × × × × × ×

10−3 10−3 10−3 10−3 10−2 10−2 10−3 10−3 10−3 10−3 10−2 10−2 10−3 10−3 10−3 10−3 10−2 10−2

T (°C)

V (g·m−2·h−1)

θ

IE (%)

25 60 90 25 60 90 90 90 90 25 60 90 90 90 90 25 60 90 90 90 90

12.39 270.53 1235.21 1.78 49.91 371.30 298.81 236.03 233.29 1.60 41.17 312.51 220.42 183.40 171.16 1.32 39.04 239.32 152.55 122.32 111.42

0.86 0.81 0.70 0.76 0.81 0.81 0.88 0.83 0.75 0.82 0.85 0.86 0.89 0.86 0.81 0.88 0.90 0.91

85.7 81.0 69.9 75.9 80.9 81.1 87.9 83.0 74.7 82.2 85.2 86.1 89.2 85.6 80.6 87.7 90.1 91.0

IE(%) =

′ Icorr − Icorr × 100 Icorr

(3)

where Icorr is corrosion current in the absence of inhibitor and Icorr ′ is the corrosion current in the presence of an inhibitor. The anodic and cathodic polarization curves of N80 steel in 15 wt % HCl solution with or without the diquaternary ammonium salts are presented in Figure 5. The corrosion

concentration, 2.0 × 10−3 M, as the temperature increased from 25 °C, to 60 °C, then to 90 °C, the inhibition efficiency decreased with rising temperature. This could be a result from the desorption of inhibitor molecules at an elevated temperature.28,29 To date, the opposite tendency, namely, the inhibition efficiency increases with increasing temperature, was only found in very diluted acid solution and slightly elevated temperature. In order to understand the interaction between the diquaternary ammonium salt and N80 steel surface, the apparent activation energy Ea of the corrosion process was calculated based on weight loss results. The Ea value was determined by the slope of the ln V vs 1/T plot (see Supporting Information for the equations and Figure S5) and are listed in Table 2. The Ea in the presence of diquaternary ammonium salt is larger than that in the blank acid solution, indicating enhanced activation energy for the corrosion process. Table 2. Thermodynamic Activation Parameters from Weight Loss Measurements inhibitors

Ea (kJ mol−1)

ΔHa (kJ mol−1)

ΔSa (J K−1 mol−1)

blank Q4Q Q6Q Q8Q

64.28 74.21 73.22 72.48

61.56 71.49 70.50 69.09

−16.53 −7.73 −4.06 0.24

From the effect of temperature on the inhibition efficiency, the enthalpy ΔHa and the entropy ΔSa of the activation for the corrosion process were also obtained (Supporting Information Figure S6). The positive value of ΔHa for both with and without the addition of the inhibitors suggests the endothermic nature of the N80 steel dissolution process in acid solutions. The addition of the studied inhibitor in 15 wt % HCl solution leads to an increase in the value of ΔSa, as in the following order: Q4Q < Q6Q < Q8Q. The ΔSa value reflects the ordering and disordering of the inhibition process. Although the

Figure 5. Electrochemical polarization curves for N80 steel in 15 wt % HCl with different concentrations of (A) Q4Q, (B) Q6Q, and (C) Q8Q at 25 °C. 14202



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Table 3. Electrochemical Parameters and Inhibition Efficiency in 15 wt % HCl at 25 °C substance 15% HCl Q4Q

Q6Q

Q8Q

c (M) 0 2.0 5.0 1.0 1.3 2.0 5.0 1.0 1.3 2.0 5.0 1.0 1.3

× × × × × × × × × × × ×

10−3 10−3 10−2 10−2 10−3 10−3 10−2 10−2 10−3 10−3 10−2 10−2

−Ecorr (mV vs SCE)

ba (mV)

−bc (mV)

Icorr (μA·cm−2)

IE (%)

400 403 404 400 410 407 408 406 414 408 401 404 411

86.1 84.5 87.9 90.7 96.7 101.2 105.2 107.8 117.5 104.2 108.2 112.8 115.5

93.6 116.4 119.4 120.2 125.3 105.3 106.6 111.5 112.3 103.5 120.1 110.8 116.7

830 118 101 94 80 108 86 73 59 89 68 40 37

0 85.8 87.8 88.7 90.4 87.0 89.6 91.2 92.9 89.3 91.8 95.2 95.5

potential (Ecorr), the anodic Tafel slopes (ba), the cathodic Tafel slopes (bc), the values of corrosion current densities (Icorr), and the inhibition efficiency (IE) are calculated from the curves and are collected in Table 3. From Figure 5A−C, it appears that the nature of the polarization curves remains almost the same regardless of the inhibitors. But, with the addition of inhibitors, both cathodic and anodic polarization curves notably shifted to a lower current density. This behavior indicated that the addition of diquaternary ammonium salt impacted both cathodic and anodic reactions of the corrosion process. As shown in Table 3, the presence of diquaternary ammonium salts does not remarkably shift the corrosion potential (Ecorr), suggesting a mixed-type inhibitor in 15 wt % HCl. The anodic Tafel slopes (ba) and the cathodic Tafel slopes (bc) both increase with the inhibitor concentration, probably due to the adsorption of the cationic nitrogen on quinoline rings at the active sites of the metal surface which slow down the corrosion reactions. It can also be found that the corrosion current (Icorr) decreases and the inhibition efficiency increases with the increase of the hydrophobic chain length in diquaternary ammonium salt. This phenomenon may be due to the hydrophobic methylene segments effectively preventing the metal from acid contact, thus reducing the surface area available for H+ ions.31 3.5. EIS Measurements. The Nyquist plots for N80 steel in 15 wt % HCl solution with or without Q4Q, Q6Q, and Q8Q at different concentrations are presented in Figure 6. It shows that the impedance response of N80 steel was changed significantly after the addition of diquaternary ammonium salts. But the profile of the impedance did not alter, indicating the synthesized inhibitors have a similar anticorrosion mechanism. For all inhibitors, there is only one depressed capacitive semicircle at high frequency with their centers below the real axis. The capacitive semicircle at high frequency is due to the double layer capacitance and the charge transfer resistance. Their corresponding Bode plots show only one time constant (Supporting Information Figure S7). The impedance of N80 steel increases with inhibitor concentration. This implies that the high concentration inhibitor can enhance surface coverage by inhibitor molecules, hence increasing the corrosion inhibition ability. From the diameter of the semi circles of the Nyquist plots, the values of charge transfer resistance were obtained. The inhibition efficiencies from EIS using charge transfer resistance were calculated as follows:

Figure 6. Nyquist plot for N80 steel in 15 wt % HCl with and without inhibitor at 25 °C. (A) Q4Q, (B) Q6Q, and (C) Q8Q (a, Blank; b, 2.0 × 10−3 M; c, 5.0 × 10−3 M; d, 1.0 × 10−2 M; e, 1.3 × 10−2 M).

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R ct(Inh) − R ct R ct(Inh)

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Temkin, and Freundlich models were screened to fit the experimental data. The correlation coefficient (R2) of the fitting was used to choose a suitable model. The degree of surface coverage (θ) was obtained from weight loss measurements. It was found that the adsorption process of the diquaternary ammonium salt on the N80 steel surface was best fitted by the Langmuir adsorption isotherm:

× 100 (4)

where Rct is the charge transfer resistance in the absence of inhibitor and Rct(Inh) is the charge transfer resistance in the presence of inhibitor. A simple equivalent circuit model which could well fit the experimental results is shown in Figure 7, consisting of the

θ = K adsc 1−θ It can also be expressed with the following equation: c 1 = +c θ K ads

Figure 7. Equivalent circuit used to fit the Nyquist plot.

ln K ads = ln

15% HCl Q4Q

Q6Q

Q8Q

0 2.0 5.0 1.0 1.3 2.0 5.0 1.0 1.3 2.0 5.0 1.0 1.3

× × × × × × × × × × × ×

10−3 10−3 10−2 10−2 10−3 10−3 10−2 10−2 10−3 10−3 10−2 10−2

Rct (Ω cm2)

Cdl (μF cm−2)

IE (%)

34.23 235.9 278.7 291.5 378.7 265.1 315.9 390.2 487.3 320.1 400.9 697.7 750.5

397.9 215.9 196.9 174.8 158.3 212.6 183.1 148.7 127.5 132.9 118.0 90.6 84.1

0 85.5 87.7 88.3 91.0 87.1 89.2 91.2 93.0 89.3 91.5 95.1 95.4

0 ΔGads 1 − 55.5 RT

(7)

where 55.5 is the value of the molar concentration of water in the solution expressed in molarity units in mol/L.36 Figure 8 shows the relationship between c/θ and c of three diquaternary ammonium salts in contact with N80 steel at 90

Table 4. EIS Parameters for N80 Steel in 15 wt % HCl with and without Inhibitor at 25 °C c (M)

(6)

where Kads is the adsorption constant, c is the inhibitor concentration, and θ is the degree of surface coverage. 0 The standard free energy of adsorption ΔGads of the adsorption process is related to the adsorption constant Kads. The equation is as follows:

solution resistance between the working electrode and the reference electrode Rs, the charge transfer resistance Rct, and the double layer capacitance Cdl. The value of Rct is a measure of electron transfer across the surface and is inversely proportional to corrosion rate.32 EIS parameters were calculated from the experiments to study the effect of diquaternary ammonium salts. As shown in Table 4, the values of Rct increased, while the double layer

substance

(5)

Figure 8. Adsorption isotherms on the surface of N80 steel at 90 °C. (a) Q4Q, (b) Q6Q, and (c) Q8Q.

capacitance Cdl dropped gradually with increasing the concentration of inhibitors. The increase of Rct can lead to a decrease of the corrosion rate of N80 steel. The decrease of Cdl could be attributed to the reduction of the local dielectric constant and the increase of the thickness of the electrical double layer.33 These results mean that the inhibitor molecules could form a protective film at the metal surface by the adsorption of inhibitor molecules under an acid environment through the interactions, such as the cationic N+ ion and the π electrons of the quinoline ring with the metal surface.34,35 The fitted result for the impedance spectrum measured in a 15 wt % HCl solution with a 5.0 × 10−3 M inhibitor is shown in Figure S8. Q6Q was chosen as a representative example. It can be seen that the fitted and measured results match quite well in both Nyquist and Bode plots. 3.6. Adsorption Isotherms. The adsorption pattern was studied to further understand the inhibition process. The adsorption of diquaternary ammonium salt molecules on the N80 steel surface is a crucial step in the anticorrosion process. Various adsorption isotherms, such as Frumkin, Langmuir,

°C. Table 5 gives the thermodynamics parameters derived from the Langmuir adsorption isotherm. Table 5. Standard Free Energy ΔG0ads of the Adsorption Process substance

R2

ΔG0ads

Q4Q Q6Q Q8Q

0.9998 1.0000 1.0000

−35.58 kJ mol−1 −36.10 kJ mol−1 −36.81 kJ mol−1

From Figure 8, a good linear correlation between c/θ and c was observed for all diquaternary ammonium salts (with an R2 of 0.9998, 1.000, and 1.0000 for Q4Q, Q6Q, and Q8Q, respectively), which suggests that the adsorption process obeys the Langmuir adsorption isotherm. On the basis of the slopes of 1.19, 1.13, and 1.07 for each inhibitor, it is reasonable to deduce that each diquaternary ammonium molecule occupied 14204



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Figure 9. SEM images of N80 steel surfaces after 4 h of immersion in 15 wt % HCl without an inhibitor (a) and with the inhibitor of Q4Q (b), Q6Q (c), and Q8Q (d).

and spread out on the metal surface. Thus, the collaboration of the cationic centers and the hydrophobic spacer could effectively prevent the metal from acid corrosion and enhance inhibition efficiency. 3.8. Microstructure Characterization. The SEM micrographs of the N80 steel surface treated in 15 wt % HCl with and without inhibitors are shown in Figure 9. Figure 9a is the N80 steel surface after being exposed 4 h at 90 °C without an inhibitor, which shows a very rough surface with deep and large pores and cracks due to rapid corrosion. Figure 9b, c, and d show the micrographs of the metal surface in the presence of different diquaternary ammonium salt inhibitors in the same corrosion environment. Compared with the sample from the acid solution without the addition of an inhibitor, the corroded surfaces become relatively smoother with increased spacer chain length, and only a small amount of corrosion particles can be observed. This change proves the adsorption of inhibitors on the metal surface, and a passive film formed spread over the metal surface. To confirm the presence of the protective layer on the metal surface, the chemical composition of the metal surface was measured by energy dispersive X-ray spectroscopy (EDX). Figure 10 shows the EDX spectra of the N80 steel surface immersed for 4 h in 15 wt % HCl solutions with and without inhibitors. The percentage atomic contents of the chemicals on the N80 steel surface is listed in Supporting Information Table S1. The EDX spectra of the sample treated in acid solution with inhibitors (Figure 10b−d) show characteristic peaks of nitrogen and bromine. The presence of Br and N in chemical composition confirms the adsorption of inhibitors on the N80 surface. This is also an indication that the inhibitor molecules formed a stable protective layer on the N80 steel surface.

more than one adsorption site on the steel surface. A modified Langmuir adsorption isotherm could be applied to this phenomenon with the following equation:37 c n = + nc θ K ads

(8)

The standard free energy of adsorptions ΔG0ads are −35.58 kJ mol−1, −36.10 kJ mol−1, and −36.81 kJ mol−1 for Q4Q, Q6Q, and Q8Q, respectively. These large negative ΔG0ads values, indicate that the adsorption process of the diquaternary ammonium salts on the N80 steel surface is favorable. The absolute ΔG0ads values of the three diquaternary ammonium salts are all in the range of 20−40 kJ mol−1, according to the literature;38,39 this could mean that the interaction of diquaternary ammonium salts with the surface of N80 steel in 15 wt % HCl involved both a physical sorption and a chemical sorption process. 3.7. Mechanism of Inhibition. Generally, the mechanism of inhibition is closely related to the interaction between the inhibitor and the metal surface. For the organic corrosion inhibitors, the polar units with atoms of nitrogen, sulfur, and/or oxygen are regarded as the reaction center to the adsorption process. Furthermore, the steric effect and electric charge of the inhibitor molecules can also affect the degree of adsorption and hence the effectiveness of the inhibitor. The diquaternary ammonium salt studied contains two nitrogen atoms in its molecular structure. It should have more adsorption sites and electric charge than its monoquaternary analogues, thus providing stronger adsorption on the metal surface. On the basis of the experimental observations, the adsorption types of the diquaternary ammonium salts at the metal-acidic solution interface may involve (1) electrostatic attraction, (2) the cationic N+ ion with metal surface, and (3) π-electrons of the quinoline ring with the metal surface. In addition, the hydrophobicity of inhibitor molecules is an important factor for a corrosion inhibitor. If two quinoline moieties were adsorbed on the N80 steel surface, the hydrophobic CH2 spacer between them could be stretched

4. CONCLUSIONS Three novel diquaternary ammonium salts were synthesized through a quaterization reaction of quinoline and dibromoalkanes. Their inhibition efficiencies for the N80 steel in 15 wt % 14205



Article

51374230), and Key Technologies Research and Development Program of China (Grant 2011ZX05051-003-003).

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Figure 10. EDX spectra of N80 steel surface before (a) and after 4 h of immersion in 15 wt % HCl solution with the inhibitor Q4Q (b), Q6Q (c), and Q8Q (d).

HCl solution were investigated by weight loss and electrochemical measurements. The inhibition efficiencies of the diquaternary ammonium salts increase with increasing the length of a hydrophobic spacer chain. At the same concentration, the inhibitor efficiencies follow the sequence: N,N′-butane-1,4-diyl-bisquinolinium dibromide < N,N′-hexane-1,6-diyl-dibromide < N,N′-octane-1,8-diyl-bisquinolinium dibromide. At 90 °C, the inhibition efficiency of diquaternary ammonium salt N,N′-octane-1,8-diyl-bisquinolinium dibromide reached about 91% at the inhibitor concentration of 0.01 mol/ L. The electrochemical results reveal that diquaternary ammonium salts are mixed-type inhibitors and the adsorption of diquaternary ammonium salts on the N80 steel surface obeys the Langmuir adsorption isotherm. The reported diquaternary ammonium salts are potentially useful as acidizing inhibitors working in a relatively high concentration acid media and under high temperature conditions.

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ASSOCIATED CONTENT

* Supporting Information S

Supporting Information: (i) The 1H NMR and 13C NMR spectra of Q4Q and Q8Q. (ii) Bod plots for all three diquaternary ammonium salts. (iii) A comparison of Nyquist plots and Bode plots by experiment and by the equivalent circuit. (iv) The plots and the equations used for the calculations of the apparent activation energy Ea, the enthalpy ΔHa, and the entropy ΔSa. (v) Percentage atomic contents from EDX analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-532-86981571. Fax: +86-532-86981791. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Project No.21172264, No. 14206



Article

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Corrosion Inhibition of N80 Steel Using Novel Diquaternary

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