# Inhibition of API 5L X52 Pipeline Steel Corrosion in Acidic Medium by Gemini Surfactants: Electrochemical Evaluation and Computational Study

Authors: Ibrahim Hamed; Magda Mohamed Osman; Omnia Hassan Abdelraheem; Maher Ibrahim Nessim; Maryam Galal El mahgary
Journal: International Journal of Corrosion
 (2019)

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## Abstract

The efficiency of three new synthesized Gemini surfactants (namely, A312, A314, and A316) of the type 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-alkylaminium] bromide is evaluated as corrosion inhibitors for carbon steel API 5L X52 grade in 1M HCl. The relation between the experimental inhibition efficiency and theoretical chemical parameters obtained by computational calculation in order to predict the behavior of the organic compounds as corrosion inhibitors was instigated. The chemical structures were elucidated using1HNMR spectra. Inhibition performance was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and weight loss tests. The polarization curves show that applied surfactants act as mixed type inhibitors. Nyquist plots showed the semicircle capacitive loop with different surfactants and concentrations. The inhibition efficiency orders are A312 > A314 > A316 with the highest efficiency of 94.87% for A312. Adsorption of inhibitors on API X52 steel surface was found to obey Langmuir isotherm. Theoretical evaluation of the inhibitory effect was performed by computational quantum chemical calculations. The molecule structural parameters (EHOMO), (ELUMO), energy gap (ΔE), and the dipole moment (μ) were determined. The results of experimental inhibition efficiency and theoretical calculated quantum parameters were subjected to correlation analysis.


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## Body

## 1. Introduction

In the petroleum production industry, failures of pipelines as a result of corrosion have led to heavy loss [1]. The metallurgy of pipelines, tanks, ships, etc. in oil production systems is often based on carbon steel [2]. The API 5L grade steel is one of the most common pipeline materials in the oil industry. However, the carbon steel is heavily exposed to corrosion attack because these operations usually induce serious corrosion of equipment, tubes, and pipelines made of steels [3]. In oil industry and petrochemical processes acid solutions are widely applied in acid pickling, so the use of corrosion inhibitors is one of the most applied methods for corrosion prevention [4].Gemini surfactants are a category of surfactants consisting of two conventional surfactant head groups, bonded together by a spacer [5]. Cationic surfactants form compact adsorption layers that hydrophobize the surfaces of metals [6]. Due to their more efficient surface properties, recently many researchers paid attention to Gemini surfactants as corrosion inhibitors in acidic medium [7–10].Generally, inhibitor molecules may physically or chemically adsorb on a corroding metal surface, forming an adsorption protective layer. The power of the inhibition depends on the molecular structure of the inhibitor. Presence of the lone electron pairs in the heteroatoms is an important feature that controls the adsorption on the metal surface [11].Computational chemistry has proven to be a very powerful tool of evaluating the efficiency of corrosion inhibitor and of investigation of corrosion inhibition mechanism. Moreover it is a theoretical prediction tool which provides a prediction of the possibilities of newly synthesized compounds to act as corrosion inhibitors and permits the preselection of compounds with the necessary structural characteristics, chemical intuition, and experience into a mathematically quantified and computerized form [12–15].Certain quantum chemical indices which are calculated by computational chemistry programs can be associated with metal/inhibitor reactions. These are the HOMO energy, LUMO energy, and the gap energy ΔE (ΔE =ELUMO - EHOMO) and dipole moment (μ). HOMO energy (highest occupied molecular orbital) is often associated with the capacity of a molecule to donate electrons, and high EHOMO values indicate that the molecule has the ability to donate electrons to suitable acceptor molecules with low-energy molecular orbits [16]. ELUMO indicates the ability of the molecules to accept electrons. In the same way low values of the energy gap ΔE = EHOMO - ELUMO will indicate good inhibition efficiencies, because the energy needed to remove an electron from the last occupied orbital will be low [17]. The dipole moment (μ) is a measure of the polarity of a covalent bond, which is related to the distribution of electrons in a molecule [18].The first goal of this study is to examine the inhibition efficiency of three synthesized cationic Gemini surfactants, namely, A312, A314, and A316, in which the two cationic head groups are linked by rigid spacer containing Schiff base, on the corrosion behavior of API X52 steel pipeline in 1 M HCl. The second goal is to identify the relation between the experimental inhibition efficiency and theoretical chemical parameters obtained by computational calculation in order to predict of the behavior of the organic compounds as corrosion inhibitors.


## 2. Experimental

### 2.1. Synthesis and Characterization

The three cationic Gemini surfactants were synthesized in high purity through two consecutive steps according to [19]. The first step is the synthesis of the cationic surfactants 4-formyl-N,N-dimethyl-N-alkylbenzeneaminium bromide, by heating three alkyl halides with different chain lengths, (dodecyl-, tetradecyl-, and hexadecyl-bromide, respectively) with equimolar amount of 4-(dimethylamino) benzaldehyde, in absolute ethanol. The components mixture was allowed to reflux for 12 h and was left to cool to room temperature. Schiff base compounds were synthesized in the second step by a condensation reaction between the products out of the first step and benzene-1,4-diamine in ethanol of 2:1 molar ratio. Finally the product was recrystallized from ethanol. The chemical structures of the three synthesized cationic Gemini surfactants (named as surfactants A312, A314, and A316) are shown in Figure 1. Confirmation of the synthesized structures was elucidated by using two different tools: elemental analysis (using elemental analyzer Perkin Elmer 240C) and 1H-NMR spectroscopy (using Jeol-EX-270 MHz 1H-NMR spectroscopy).Figure 1
Schematic synthesis of A312, A314 and A316. n = 10{A312} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-dodecylaminium] bromide. n = 12 {A314} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-tetradecylaminium] bromide. n = 14 {A316} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-hexadecylaminium] bromide.


### 2.2. Materials

Pipeline steel coupons of type API X52 have the following chemical composition (wt%) C: 0.07%, Si: 0.24%, Mn: 1.24%, P: 0.013%, Cr: 0.02%, Ni: 0.02%, Al: 0.03% and the remainder Fe. The coupons were polished, degreased, and dried. The acidic medium is 1M HCl, which was prepared with analytical reagent (37%) in distilled water. The synthesized inhibitors concentration ranged from 5 to 200 ppm.


### 2.3. Weight Loss Measurements

The weight loss of carbon steel coupons was determined after immersion period of 72h. The coupons having 2 cm2 cross-sectional area were polished with grit emery papers (grade 300-1000), dried, and then weighted. The freshly prepared specimens were immersed in 100 ml of 1 M HCl solution with and without different concentrations of the three cationic Gemini surfactants A312, A314, and A316.


### 2.4. Electrochemical Polarization Measurements

Electrochemical polarization experiments were performed at room temperature using a conventional three-electrode cell with a platinum counter electrode, standard calomel electrode (SCE) as the reference electrode, and cylindrical working electrode with 1 cm diameter. Polarization measurements were conducted potentiodynamically using a corrosion measurement system SP 150 Potentiostat/galvanostat with software EC-lab version 9.9. The impedance experiments were carried out in the frequency range of 100 kHz–30 Hz, at the OCP by applying small amplitude ac signal of 10 mV. Tafel polarization measurements were carried out at a scan rate of 10 mV/s in the potential range from –850 to -200 mV.


### 2.5. Computational Theoretical Calculations

Quantum chemical calculations were performed with complete geometry optimizations using Chem Bio Draw Ultra 12 software, using ab initio (HF/3-21G and MP2/3-21G) and semiemperical (MNDO, AM1, and PM3) methods.


## 2.1. Synthesis and Characterization

The three cationic Gemini surfactants were synthesized in high purity through two consecutive steps according to [19]. The first step is the synthesis of the cationic surfactants 4-formyl-N,N-dimethyl-N-alkylbenzeneaminium bromide, by heating three alkyl halides with different chain lengths, (dodecyl-, tetradecyl-, and hexadecyl-bromide, respectively) with equimolar amount of 4-(dimethylamino) benzaldehyde, in absolute ethanol. The components mixture was allowed to reflux for 12 h and was left to cool to room temperature. Schiff base compounds were synthesized in the second step by a condensation reaction between the products out of the first step and benzene-1,4-diamine in ethanol of 2:1 molar ratio. Finally the product was recrystallized from ethanol. The chemical structures of the three synthesized cationic Gemini surfactants (named as surfactants A312, A314, and A316) are shown in Figure 1. Confirmation of the synthesized structures was elucidated by using two different tools: elemental analysis (using elemental analyzer Perkin Elmer 240C) and 1H-NMR spectroscopy (using Jeol-EX-270 MHz 1H-NMR spectroscopy).Figure 1
Schematic synthesis of A312, A314 and A316. n = 10{A312} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-dodecylaminium] bromide. n = 12 {A314} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-tetradecylaminium] bromide. n = 14 {A316} 4,4-[1,4phenylenebis(azanylylidene)bis(N,N-dimethyl-N-hexadecylaminium] bromide.


## 2.2. Materials

Pipeline steel coupons of type API X52 have the following chemical composition (wt%) C: 0.07%, Si: 0.24%, Mn: 1.24%, P: 0.013%, Cr: 0.02%, Ni: 0.02%, Al: 0.03% and the remainder Fe. The coupons were polished, degreased, and dried. The acidic medium is 1M HCl, which was prepared with analytical reagent (37%) in distilled water. The synthesized inhibitors concentration ranged from 5 to 200 ppm.


## 2.3. Weight Loss Measurements

The weight loss of carbon steel coupons was determined after immersion period of 72h. The coupons having 2 cm2 cross-sectional area were polished with grit emery papers (grade 300-1000), dried, and then weighted. The freshly prepared specimens were immersed in 100 ml of 1 M HCl solution with and without different concentrations of the three cationic Gemini surfactants A312, A314, and A316.


## 2.4. Electrochemical Polarization Measurements

Electrochemical polarization experiments were performed at room temperature using a conventional three-electrode cell with a platinum counter electrode, standard calomel electrode (SCE) as the reference electrode, and cylindrical working electrode with 1 cm diameter. Polarization measurements were conducted potentiodynamically using a corrosion measurement system SP 150 Potentiostat/galvanostat with software EC-lab version 9.9. The impedance experiments were carried out in the frequency range of 100 kHz–30 Hz, at the OCP by applying small amplitude ac signal of 10 mV. Tafel polarization measurements were carried out at a scan rate of 10 mV/s in the potential range from –850 to -200 mV.


## 2.5. Computational Theoretical Calculations

Quantum chemical calculations were performed with complete geometry optimizations using Chem Bio Draw Ultra 12 software, using ab initio (HF/3-21G and MP2/3-21G) and semiemperical (MNDO, AM1, and PM3) methods.


## 3. Results and Discussion

### 3.1. Characterization

Results of elemental analysis of the synthesized three cationic Gemini surfactants A312, A314, and A316 with chemical formulas (C48H76Br2N4), (C52H84Br2N4), and (C56H92Br2N4), respectively, are shown in Table 1, where the observed data are compatible with the calculated ones. 1H-NMR spectra analysis is represented in Table 2. Data illustrates that the structures of the prepared compounds are in good agreement with the proposed ones. Figure 2 represents the 1H-NMR spectra of A312.Table 1
Elemental analysis of A312, A314, and A316.
Compound MolecularFormula Mw(g/mole) C% H% N% Br% Calc. Obs. Calc Obs. Calc. Obs. Calc. Obs. A312 C48H76Br2N4 868 66.35 66.55 8.82 8.61 18.41 18.44 6.45 6.40  A314 C52H84Br2N4 924 67.52 67.18 9.15 9.42 17.28 17.11 6.06 6.29  A316 C56H92Br2N4 980 68.55 68.51 9.45 9.58 16.29 16.33 5.71 5.58Table 2
1H-NMR spectroscopy of A312, A314 and A316.
Cpd. H+Type Chemical Shift (δ ppm) aSinglet bDoublet cDoublet dSinglet eSinglet fTriplet gMultiplet hMultiplet iTriplet A312 8.36 7.79 7.26 6.74 3.48 3.07 2.91 1.29 0.89  A314 8.28 7.82 7.27 6.84 3.52 3.04 2.87 1.32 0.92  A316 8.43 7.85 7.31 6.81 3.51 3.08 2.90 1.31 0.91Figure 2
1H-NMR spectra of A312.


### 3.2. Weight Loss

The corrosion rate CR of API X52 carbon steel in acidic solutions in the presence and absence of inhibitors were calculated according to the following equations [20]:(1)CR=ΔWS.twhere t is the specimen immersion time (hour), S is the surface area of the test specimen (cm2), and ΔW is the weight loss (mg), CR in mg/cm2.hr.(2)EW%=Wo-WWo×100where Wo and W are the weight losses per unit area (mg/cm2), in the absence and presence of the inhibitors, respectively.In all runs, the corrosion rate decreases with the increasing of surfactant concentration. The changes in the inhibition efficiency with concentration are given in Table3. The corrosion rate CR decreases in the order: A316> A314> A312. For a constant concentration, the corrosion rate reveals that the increase of alkyl chain length of hydrophobic part from 12 to 16 carbon atoms increases the corrosion rate of the inhibitor.Table 3
The weight loss and electrochemical polarization measurements of API X52 carbon steel in 1 M HCl with addition of synthesized inhibitors A312, A314, and A316 at 25°C.
Surfactant Concentration(ppm) Electrochemical Polarization Weight Loss Icorr,mA cm−2 Ecorr,mV ßa,mV dec−1 ßc,mV dec−1 Ep % Corr. Rate(mg.cm−2.hr−1) Ew% Blank 0.00 801.75 -510.9 226.0 259.8 0.00 0.4903 0.00  A312 1.00 551.12 -528.0 220.4 228.2 31.26 0.3462 29.37 5.00 402.31 -504.2 172.6 193.5 49.82 0.2628 46.38 10.0 261.41 -494.0 175.1 208.3 67.39 0.1441 70.60 20.0 142.15 -518.0 168.1 195.1 82.27 0.0772 84.25 25.0 119.86 -523.0 150.1 170.1 85.05 0.0633 87.07 50.0 55.080 -512.1 142.2 167.4 93.13 0.0447 90.86 100.0 48.185 -514.8 130.5 160.5 93.99 0.0289 94.10 200.0 46.260 -514.4 128.2 157.0 94.23 0.0251 94.87  A314 1.00 610.85 -533.0 216.9 226.5 23.81 0.3728 23.95 5.00 428.57 -505.3 180.7 199.5 46.54 0.2888 41.07 10.0 359.08 -500.4 172.9 207.0 55.22 0.2054 58.13 20.0 215.26 -501.2 167.8 188.6 73.15 0.1310 73.29 25.0 180.47 -493.7 149.3 174.0 77.49 0.1109 77.36 50.0 91.311 -502.1 149.3 167.2 88.64 0.0693 85.86 100.0 74.322 -507.5 143.8 157.3 90.73 0.0402 91.80 200.0 96.832 -502.9 127.0 155.6 91.29 0.0366 92.52  A316 1.00 624.80 -526.8 225.8 236.4 22.07 0.3842 21.63 5.00 536.02 -522.3 221.9 234.0 33.14 0.3193 34.86 10.0 361.90 -527.8 215.7 226.2 54.86 0.2265 53.78 20.0 295.76 -514.9 180.0 198.6 67.60 0.1611 67.13 25.0 244.85 -512.3 181.7 204.9 69.46 0.1425 70.92 50.0 171.97 -519.3 170.1 188.1 78.55 0.1019 79.19 100.0 119.94 -490.2 150.2 173.9 85.04 0.0732 85.06 200.0 82.820 -523.8 165.4 174.2 89.67 0.0505 89.69


### 3.3. Electrochemical Polarization

Corrosion parameters obtained from polarization curves (Figure3) including corrosion current density (icorr, in mA.cm−2), corrosion potential (Ecorr, in mV), inhibitor efficiency (E%), and anodic and cathodic Tafel slopes (ba andbc, respectively, in mV) are shown in Table 3. For the inhibited systems the inhibition efficiency is calculated from the following equation [21]:(3)EP%=i°corr-icorri°corr×100where (icorr and i°corr) are the corrosion current densities in case of the presence and absence of the inhibitor, respectively.Figure 3
Potentiodynamic polarization of API X52 steel in 1M HCl with different concentrations of the synthesized corrosion inhibitors, (a) A312, (b) A314, and (c) A316.
(a) (b) (c)From obtained data it can be observed that with the addition of the three inhibitors (A312, A314, and A316) to the acidic solution, the corrosion current densities decrease, while inhibition efficiencies increase depending on the concentration. There is no clear trend in the shifting ofEcorr (vs. SCE); on the other hand, both anodic and cathodic curves shift to lower current densities.These results suggest that synthesized surfactants retard the anodic dissolution reaction as well as delay the cathodic reaction of hydrogen evolution. Therefore, these compounds act as mixed type inhibitors [4, 22]. A slight decrease in both Tafel slopes(ba andbc) indicates that the inhibitors decrease the surface area exposed for corrosion without altering the corrosion mechanism [23].In all cases, the order of corrosion inhibition efficiency of the surfactants is A312,> A314,> A316. The higher the hydrophobic alkyl chain length is, the lower the inhibitor efficiency is at a constant concentration. This is a consequence of the adsorption process. This result is in accordance with previous work [24].


### 3.4. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was applied to investigate the electrode/electrolyte interface and processes that occur on the API X52 pipeline steel surface in the presence and absence of the acid solution. The spectra were recorded after the stabilization of electrode at open circuit potential (OCP) for 30 min. Nyquist plots were obtained. In Nyquist plots, the imaginary component of impedance (Zi) is plotted versus the real component of impedance (Zr) for each excitation frequency. The electrode impedance, Zr, is related to the frequency of the AC signal in Hz (ƒ), resistance of (Rp), the double layer capacity of double layer (Cdl), and resistance of solution (RS) and is represented by the mathematical formulation [25]:(4)Zr=Rs+Rct1+2.π.ƒ.Rct.CdlαThe percentage inhibition efficiencies (Ei%)are determined by the following relation [26]:(5)E%=R°ct-RctR°ct×100where Rct and R°ct are the charge transfer resistances with and without the inhibitors, respectively. Impedance parameters: double layer capacitance (Cdl), charge transfer resistance (Rct), and the inhibition efficiency Ei% are given in Table 4. The EIS spectra of the API X52 steel electrode are recorded in the absence and presence of inhibitors at different concentrations in Figure 4.Table 4
Electrochemical impedance spectroscopy (EIS) for the corrosion of API X52 steel in 1 M HCl at various concentrations of the synthesized inhibitors at 25°C.
Surfactant Conc.(ppm) Rs(Ω cm2) Cdl 10∧-6(Ω–1.cm–2 sn) Rct(Ω. cm2) Ei % Blank 0.00 2.207 786 3.395 0.00  A312 5.00 2.191 412 7.859 65.89 25.0 2.119 126 17.55 81.66 50.0 2.433 87.0 31.46 89.21 100.0 2.325 49.9 54.72 93.79 200.0 2.175 48.1 65.44 94.81  A314 5.00 2.18 158 9.241 63.26 25.0 1.987 170.6 16.5 79.42 50.0 2.237 159.6 21.87 84.48 100.0 2.073 131.8 29.72 88.58 200.0 2.207 79.7 55.55 93.89  A316 5.00 2.034 269 7.031 51.71 25.0 2.021 180 11.73 71.06 50.0 1.885 119.5 15.79 78.50 100.0 1.977 218 20.02 83.04 200.0 2.11 146 32.66 89.60Figure 4
Nyquist plots for API X52 steel in 1 M HCl with different concentrations of the synthesized corrosion inhibitors, (a) A312, (b) A314, and (c) A316.
(a) (b) (c)All the Nyquist plots obtained (Figure4) were with one single capacitive loop semicircle in nature. The semicircle diameter (Rct) increases with the increase in inhibitor concentration. Data in Table 4 show that the Rs values are very small compared to the Rct values. Also the calculated Cdl values decrease by increasing the inhibitor concentrations. The high Rct values are generally associated with slower corroding system and hence an increase in the calculated inhibition efficiencies Ei% [27].The obtained semicircle shaped plots show that corrosion inhibition of Gemini surfactants is controlling only by charge transfer process, as well as electrode surface homogeneity such that the adsorption of inhibitors is formed by compact layers [28].The inhibition efficiencies, calculated from EIS results, show the same trend as those obtained from polarization measurements. The change in concentration of the inhibitor did not alter the profile of the impedance behavior, referring to similar mechanism for the corrosion inhibition reaction [29]. This confirms the behavior observed in potentiodynamic polarization measurement, such that the inhibitors do not change the mechanism of metal dissolution but only affect the rate.Differences in inhibition efficiency obtained from two methods may be attributed to the different surface status of the electrode in two measurements. EIS was performed at the rest potential, while in polarization measurements the electrode potential was polarized to high overpotential; nonuniform current distributions, resulting from cell geometry, solution conductivity, counter and reference electrode placement, etc., will lead to the difference between the electrode area actually undergoing polarization and the total area [30].EIS results can be elucidated in terms of the equivalent circuit of the electrical diagram (Figure5) to model the iron/acid interface [31]. The equivalent circuit composed of a constant phase element Cdl, in parallel with a resistor, Rct, which corresponds to a single capacitive loop. The resistor Rs is in series to theCdl and Rct. Rs is the uncompensated resistance between the working steel electrode and SCE reference electrode or solution resistance, Rct is the polarization resistance at the electrode/solution interface, andCdl is the double layer capacitance at the interface. The double layer capacity is in parallel with the impedance due to the charge transfer reaction.Figure 5
Equivalent circuit model to fit the metal/acid interface containing different concentrations of the synthesized corrosion inhibitors.


### 3.5. Adsorption Isotherm

Adsorption isotherm for the three Gemini surfactants can provide important information which highlight the interaction between the organic compounds and the steel surface.In this study, various isotherms models were applied in attempts to fit the degree of surface coverage (θ) using loss in weight data. The best fit was obtained with the Langmuir isotherm (Figure 6) by the following relation [32]:(6)Cθ=1Kads+Cwhere C is the concentration in mol/l, ϴ is the surface coverage, and Kads is the adsorption equilibrium constant. Table 5 shows the parameters and the regression factors calculated from Langmuir adsorption isotherm and also the calculated values of ΔGads. All linear correlation coefficients (r2) exceeded 0.999 indicating that the corrosion inhibition of API X52 steel by the synthesized cationic Gemini surfactants was attributed to adsorption of these compounds on the metal surface. However, the slopes of the C/θ versus C plots show a little deviation from unity. The deviation may be due to interaction between the adsorbed species on the steel surface by mutual attraction or repulsion. High values of the adsorption constant Kads can be reasoned for better adsorption and high inhibition efficiency [33].Table 5
Adsorption parameters of synthesized inhibitors on the steel surface.
Surfactant Linear correlation coefficient, r2 Slope K a d s x 10-3  (l. mol−1) ΔGkJ. mol-1 A312 0.9997 1.034 251.130 -40.57  A314 0.9996 1.049 163.941 -39.69  A316 0.9993 1.083 128.336 -39.09Figure 6
Langmuir isotherm plots for API X52 steel in 1M HCl at presence of different concentrations of synthesized corrosion inhibitors A312, A314, and A316.The standard adsorption free energy (ΔG°ads) was calculated by the following equation [34]:(7)-ΔG°ads=RTln⁡55.5Kadswhere R is the gas constant (8.314 J /Kmol. 1) and T is the absolute temperature. The calculated ΔG°ads value was kJ.mol-1. As shown in Table 5, the adsorption free energy (ΔG°ads) increases and the adsorption coefficient (Kads) decreases with increasing the number of ethylene groups in the hydrophobic alkyl chain, which means the decrease in adsorption capacity of inhibitors with lengthening the alkyl chain from 12 to 16.Generally, it is known that the values ofΔGads up to -20 kJ.mol-1 are regarded as physisorption which is the adsorption type associated with electrostatic interaction between the charged molecules and the charged metal. Values around -40 kJ.mol-1 or more negative are considered as chemisorptions, which is a result of the charge sharing or a transfer of electron pair or π electrons from the inhibitor molecules to the steel surface to form a chemical bond [35, 36].According to the obtained adsorption free energy values, the adsorption on the API X52 steel surface can be explained as a result of either theπ-electrons of the aromatic structure or the electronegative N atoms and cathodic sites on the metallic surface.The calculated adsorption parameters and hence the inhibiting properties of the investigated surfactants appear to decrease by increasing size of the alkyl group with higher electronic charge density on the nitrogen atom. As a consequence the inhibitive action of the positive head group should decrease as a result of a less tightly held layer of positive ions adjacent to the adsorbed bromide ions. This observed opposite behavior may be due to the effect of Van der Waal’s forces on attraction action between the alkyl chains of adjacently adsorbed positive head group ions [37].


### 3.6. Computational Study

In order to support the experimental findings, the calculated quantum chemical parameters, namely, the energy of highest occupied molecular orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO), dipole moment (μ), and energy gap (ΔE=ELUMO - EHOMO) values were obtained by using ab initio (HF/3-21G) and semiemperical (MNDO, AM1, MP3) methods. Calculated data are given in Table 6Table 6
Quantum chemical parameters of synthesized corrosion inhibitors A312, A314 and A316.
Surfactant Quantum parameters HF(3-21G) MP2(3-21G) AM1 MNDO PM3 A312 E H O M O(eV) -4.988 -5.276 -5.278 -5.205 -5.276 E L U M O(eV) -1.657 -1.933 -1.832 -1.862 -1.932 E L U M O-EHOMO(eV) 3.331 3.343 3.346 3.343 3.344 µ(Debye) 15.29 15.41 19.76 20.20 19.59  A314 E H O M O(eV) -5.278 -5.278 -5.280 -5.279 -5.279 E L U M O (eV) -1.919 -1.920 -1.920 -1.921 -1.920 E L U M O-EHOMO(eV) 3.359 3.358 3.360 3.358 3.359 µ(Debye) 23.24 23.19 23.73 24.17 23.81  A316 E H O M O(eV) -5.279 -5.283 -5.280 -5.278 -5.283 E L U M O (eV) -1.918 -1.917 -1.918 -1.912 -1.920 E L U M O-EHOMO(eV) 3.361 3.366 3.362 3.366 3.363 µ(Debye) 27.55 27.65 27.72 28.62 27.98From Table6, it was found that the EHOMO and the ELUMO changed rulelessly, while it was observed that the highest value of calculated ΔE was obtained by A316 and the lowest energy gap value was that of A312. It is well known that the lower the values of energy gap, the better the corrosion inhibition, because the ionization potential will be low [38–41]. This behavior pointed to that the shorter the chain length of the hydrophobic part, the lower the energy gap ΔE and the higher the inhibition efficiency of the Gemini surfactant, because the energy needed to remove an electron from the last occupied orbital will be low, which facilitates adsorption (and therefore inhibition), and that agrees well with the experimental findings.The dipole moment (μ) is an important electronic parameter which results from the nonuniform distribution of the charges on the different atoms of the molecule, so the inhibitive ability of a molecule is related to it [42]. There is no agreement in the literature on the use of dipole moment (μ) as a predictor for the trend of the inhibition reaction. Some authors reported that the high values of the dipole moment pointed to high inhibition efficiency [41–43].Others as Abdallah et al. [44] and Olasunkanmi et al. [45] found that the comparison between the calculated dipole moments of the investigated compounds reveals that the lowest has better inhibition efficiency; moreover, no significant relationship between these values has been identified in some other cases [46]. The found results show increase in inhibition efficiencies with decreasing value of the dipole moment.An attempt to search correlations between the experimentally obtained inhibition efficiency resulted from weight loss (Ew %) at concentration 200 ppm and theoretically calculated energy gap (ΔE) values is illustrated in Table 7. The low values of gap energy (ΔE) favor the adsorption of the three Gemini surfactants on API X52 steel surface and enhance the protecting power as shown in Table 7. The plotting (Figure 7) shows that the best correlation is obtained using semiempirical methods MNDO and AM1. It can be concluded that quantum chemical calculations can be used to predict the effectiveness of the used inhibitors.Table 7
The relation between the energy gap ΔE and the experimental inhibition efficiency (Ew %) of the synthesized corrosion inhibitors.
Inh. Ew% ΔE HF(3-21G) MP2(3-21G) AM1 MNDO PM3 A312 94.87 3.331 3.343 3.346 3.343 3.344  A314 92.52 3.359 3.358 3.360 3.358 3.359  A316 89.69 3.361 3.366 3.362 3.366 3.363Figure 7
Correlations between experimental inhibition efficiency (Ew%) of synthesized corrosion inhibitors and calculated quantum parameter (energy gap; ΔE).
(a) (b) (c) (d) (e)


## 3.1. Characterization

Results of elemental analysis of the synthesized three cationic Gemini surfactants A312, A314, and A316 with chemical formulas (C48H76Br2N4), (C52H84Br2N4), and (C56H92Br2N4), respectively, are shown in Table 1, where the observed data are compatible with the calculated ones. 1H-NMR spectra analysis is represented in Table 2. Data illustrates that the structures of the prepared compounds are in good agreement with the proposed ones. Figure 2 represents the 1H-NMR spectra of A312.Table 1
Elemental analysis of A312, A314, and A316.
Compound MolecularFormula Mw(g/mole) C% H% N% Br% Calc. Obs. Calc Obs. Calc. Obs. Calc. Obs. A312 C48H76Br2N4 868 66.35 66.55 8.82 8.61 18.41 18.44 6.45 6.40  A314 C52H84Br2N4 924 67.52 67.18 9.15 9.42 17.28 17.11 6.06 6.29  A316 C56H92Br2N4 980 68.55 68.51 9.45 9.58 16.29 16.33 5.71 5.58Table 2
1H-NMR spectroscopy of A312, A314 and A316.
Cpd. H+Type Chemical Shift (δ ppm) aSinglet bDoublet cDoublet dSinglet eSinglet fTriplet gMultiplet hMultiplet iTriplet A312 8.36 7.79 7.26 6.74 3.48 3.07 2.91 1.29 0.89  A314 8.28 7.82 7.27 6.84 3.52 3.04 2.87 1.32 0.92  A316 8.43 7.85 7.31 6.81 3.51 3.08 2.90 1.31 0.91Figure 2
1H-NMR spectra of A312.


## 3.2. Weight Loss

The corrosion rate CR of API X52 carbon steel in acidic solutions in the presence and absence of inhibitors were calculated according to the following equations [20]:(1)CR=ΔWS.twhere t is the specimen immersion time (hour), S is the surface area of the test specimen (cm2), and ΔW is the weight loss (mg), CR in mg/cm2.hr.(2)EW%=Wo-WWo×100where Wo and W are the weight losses per unit area (mg/cm2), in the absence and presence of the inhibitors, respectively.In all runs, the corrosion rate decreases with the increasing of surfactant concentration. The changes in the inhibition efficiency with concentration are given in Table3. The corrosion rate CR decreases in the order: A316> A314> A312. For a constant concentration, the corrosion rate reveals that the increase of alkyl chain length of hydrophobic part from 12 to 16 carbon atoms increases the corrosion rate of the inhibitor.Table 3
The weight loss and electrochemical polarization measurements of API X52 carbon steel in 1 M HCl with addition of synthesized inhibitors A312, A314, and A316 at 25°C.
Surfactant Concentration(ppm) Electrochemical Polarization Weight Loss Icorr,mA cm−2 Ecorr,mV ßa,mV dec−1 ßc,mV dec−1 Ep % Corr. Rate(mg.cm−2.hr−1) Ew% Blank 0.00 801.75 -510.9 226.0 259.8 0.00 0.4903 0.00  A312 1.00 551.12 -528.0 220.4 228.2 31.26 0.3462 29.37 5.00 402.31 -504.2 172.6 193.5 49.82 0.2628 46.38 10.0 261.41 -494.0 175.1 208.3 67.39 0.1441 70.60 20.0 142.15 -518.0 168.1 195.1 82.27 0.0772 84.25 25.0 119.86 -523.0 150.1 170.1 85.05 0.0633 87.07 50.0 55.080 -512.1 142.2 167.4 93.13 0.0447 90.86 100.0 48.185 -514.8 130.5 160.5 93.99 0.0289 94.10 200.0 46.260 -514.4 128.2 157.0 94.23 0.0251 94.87  A314 1.00 610.85 -533.0 216.9 226.5 23.81 0.3728 23.95 5.00 428.57 -505.3 180.7 199.5 46.54 0.2888 41.07 10.0 359.08 -500.4 172.9 207.0 55.22 0.2054 58.13 20.0 215.26 -501.2 167.8 188.6 73.15 0.1310 73.29 25.0 180.47 -493.7 149.3 174.0 77.49 0.1109 77.36 50.0 91.311 -502.1 149.3 167.2 88.64 0.0693 85.86 100.0 74.322 -507.5 143.8 157.3 90.73 0.0402 91.80 200.0 96.832 -502.9 127.0 155.6 91.29 0.0366 92.52  A316 1.00 624.80 -526.8 225.8 236.4 22.07 0.3842 21.63 5.00 536.02 -522.3 221.9 234.0 33.14 0.3193 34.86 10.0 361.90 -527.8 215.7 226.2 54.86 0.2265 53.78 20.0 295.76 -514.9 180.0 198.6 67.60 0.1611 67.13 25.0 244.85 -512.3 181.7 204.9 69.46 0.1425 70.92 50.0 171.97 -519.3 170.1 188.1 78.55 0.1019 79.19 100.0 119.94 -490.2 150.2 173.9 85.04 0.0732 85.06 200.0 82.820 -523.8 165.4 174.2 89.67 0.0505 89.69


## 3.3. Electrochemical Polarization

Corrosion parameters obtained from polarization curves (Figure3) including corrosion current density (icorr, in mA.cm−2), corrosion potential (Ecorr, in mV), inhibitor efficiency (E%), and anodic and cathodic Tafel slopes (ba andbc, respectively, in mV) are shown in Table 3. For the inhibited systems the inhibition efficiency is calculated from the following equation [21]:(3)EP%=i°corr-icorri°corr×100where (icorr and i°corr) are the corrosion current densities in case of the presence and absence of the inhibitor, respectively.Figure 3
Potentiodynamic polarization of API X52 steel in 1M HCl with different concentrations of the synthesized corrosion inhibitors, (a) A312, (b) A314, and (c) A316.
(a) (b) (c)From obtained data it can be observed that with the addition of the three inhibitors (A312, A314, and A316) to the acidic solution, the corrosion current densities decrease, while inhibition efficiencies increase depending on the concentration. There is no clear trend in the shifting ofEcorr (vs. SCE); on the other hand, both anodic and cathodic curves shift to lower current densities.These results suggest that synthesized surfactants retard the anodic dissolution reaction as well as delay the cathodic reaction of hydrogen evolution. Therefore, these compounds act as mixed type inhibitors [4, 22]. A slight decrease in both Tafel slopes(ba andbc) indicates that the inhibitors decrease the surface area exposed for corrosion without altering the corrosion mechanism [23].In all cases, the order of corrosion inhibition efficiency of the surfactants is A312,> A314,> A316. The higher the hydrophobic alkyl chain length is, the lower the inhibitor efficiency is at a constant concentration. This is a consequence of the adsorption process. This result is in accordance with previous work [24].


## 3.4. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was applied to investigate the electrode/electrolyte interface and processes that occur on the API X52 pipeline steel surface in the presence and absence of the acid solution. The spectra were recorded after the stabilization of electrode at open circuit potential (OCP) for 30 min. Nyquist plots were obtained. In Nyquist plots, the imaginary component of impedance (Zi) is plotted versus the real component of impedance (Zr) for each excitation frequency. The electrode impedance, Zr, is related to the frequency of the AC signal in Hz (ƒ), resistance of (Rp), the double layer capacity of double layer (Cdl), and resistance of solution (RS) and is represented by the mathematical formulation [25]:(4)Zr=Rs+Rct1+2.π.ƒ.Rct.CdlαThe percentage inhibition efficiencies (Ei%)are determined by the following relation [26]:(5)E%=R°ct-RctR°ct×100where Rct and R°ct are the charge transfer resistances with and without the inhibitors, respectively. Impedance parameters: double layer capacitance (Cdl), charge transfer resistance (Rct), and the inhibition efficiency Ei% are given in Table 4. The EIS spectra of the API X52 steel electrode are recorded in the absence and presence of inhibitors at different concentrations in Figure 4.Table 4
Electrochemical impedance spectroscopy (EIS) for the corrosion of API X52 steel in 1 M HCl at various concentrations of the synthesized inhibitors at 25°C.
Surfactant Conc.(ppm) Rs(Ω cm2) Cdl 10∧-6(Ω–1.cm–2 sn) Rct(Ω. cm2) Ei % Blank 0.00 2.207 786 3.395 0.00  A312 5.00 2.191 412 7.859 65.89 25.0 2.119 126 17.55 81.66 50.0 2.433 87.0 31.46 89.21 100.0 2.325 49.9 54.72 93.79 200.0 2.175 48.1 65.44 94.81  A314 5.00 2.18 158 9.241 63.26 25.0 1.987 170.6 16.5 79.42 50.0 2.237 159.6 21.87 84.48 100.0 2.073 131.8 29.72 88.58 200.0 2.207 79.7 55.55 93.89  A316 5.00 2.034 269 7.031 51.71 25.0 2.021 180 11.73 71.06 50.0 1.885 119.5 15.79 78.50 100.0 1.977 218 20.02 83.04 200.0 2.11 146 32.66 89.60Figure 4
Nyquist plots for API X52 steel in 1 M HCl with different concentrations of the synthesized corrosion inhibitors, (a) A312, (b) A314, and (c) A316.
(a) (b) (c)All the Nyquist plots obtained (Figure4) were with one single capacitive loop semicircle in nature. The semicircle diameter (Rct) increases with the increase in inhibitor concentration. Data in Table 4 show that the Rs values are very small compared to the Rct values. Also the calculated Cdl values decrease by increasing the inhibitor concentrations. The high Rct values are generally associated with slower corroding system and hence an increase in the calculated inhibition efficiencies Ei% [27].The obtained semicircle shaped plots show that corrosion inhibition of Gemini surfactants is controlling only by charge transfer process, as well as electrode surface homogeneity such that the adsorption of inhibitors is formed by compact layers [28].The inhibition efficiencies, calculated from EIS results, show the same trend as those obtained from polarization measurements. The change in concentration of the inhibitor did not alter the profile of the impedance behavior, referring to similar mechanism for the corrosion inhibition reaction [29]. This confirms the behavior observed in potentiodynamic polarization measurement, such that the inhibitors do not change the mechanism of metal dissolution but only affect the rate.Differences in inhibition efficiency obtained from two methods may be attributed to the different surface status of the electrode in two measurements. EIS was performed at the rest potential, while in polarization measurements the electrode potential was polarized to high overpotential; nonuniform current distributions, resulting from cell geometry, solution conductivity, counter and reference electrode placement, etc., will lead to the difference between the electrode area actually undergoing polarization and the total area [30].EIS results can be elucidated in terms of the equivalent circuit of the electrical diagram (Figure5) to model the iron/acid interface [31]. The equivalent circuit composed of a constant phase element Cdl, in parallel with a resistor, Rct, which corresponds to a single capacitive loop. The resistor Rs is in series to theCdl and Rct. Rs is the uncompensated resistance between the working steel electrode and SCE reference electrode or solution resistance, Rct is the polarization resistance at the electrode/solution interface, andCdl is the double layer capacitance at the interface. The double layer capacity is in parallel with the impedance due to the charge transfer reaction.Figure 5
Equivalent circuit model to fit the metal/acid interface containing different concentrations of the synthesized corrosion inhibitors.


## 3.5. Adsorption Isotherm

Adsorption isotherm for the three Gemini surfactants can provide important information which highlight the interaction between the organic compounds and the steel surface.In this study, various isotherms models were applied in attempts to fit the degree of surface coverage (θ) using loss in weight data. The best fit was obtained with the Langmuir isotherm (Figure 6) by the following relation [32]:(6)Cθ=1Kads+Cwhere C is the concentration in mol/l, ϴ is the surface coverage, and Kads is the adsorption equilibrium constant. Table 5 shows the parameters and the regression factors calculated from Langmuir adsorption isotherm and also the calculated values of ΔGads. All linear correlation coefficients (r2) exceeded 0.999 indicating that the corrosion inhibition of API X52 steel by the synthesized cationic Gemini surfactants was attributed to adsorption of these compounds on the metal surface. However, the slopes of the C/θ versus C plots show a little deviation from unity. The deviation may be due to interaction between the adsorbed species on the steel surface by mutual attraction or repulsion. High values of the adsorption constant Kads can be reasoned for better adsorption and high inhibition efficiency [33].Table 5
Adsorption parameters of synthesized inhibitors on the steel surface.
Surfactant Linear correlation coefficient, r2 Slope K a d s x 10-3  (l. mol−1) ΔGkJ. mol-1 A312 0.9997 1.034 251.130 -40.57  A314 0.9996 1.049 163.941 -39.69  A316 0.9993 1.083 128.336 -39.09Figure 6
Langmuir isotherm plots for API X52 steel in 1M HCl at presence of different concentrations of synthesized corrosion inhibitors A312, A314, and A316.The standard adsorption free energy (ΔG°ads) was calculated by the following equation [34]:(7)-ΔG°ads=RTln⁡55.5Kadswhere R is the gas constant (8.314 J /Kmol. 1) and T is the absolute temperature. The calculated ΔG°ads value was kJ.mol-1. As shown in Table 5, the adsorption free energy (ΔG°ads) increases and the adsorption coefficient (Kads) decreases with increasing the number of ethylene groups in the hydrophobic alkyl chain, which means the decrease in adsorption capacity of inhibitors with lengthening the alkyl chain from 12 to 16.Generally, it is known that the values ofΔGads up to -20 kJ.mol-1 are regarded as physisorption which is the adsorption type associated with electrostatic interaction between the charged molecules and the charged metal. Values around -40 kJ.mol-1 or more negative are considered as chemisorptions, which is a result of the charge sharing or a transfer of electron pair or π electrons from the inhibitor molecules to the steel surface to form a chemical bond [35, 36].According to the obtained adsorption free energy values, the adsorption on the API X52 steel surface can be explained as a result of either theπ-electrons of the aromatic structure or the electronegative N atoms and cathodic sites on the metallic surface.The calculated adsorption parameters and hence the inhibiting properties of the investigated surfactants appear to decrease by increasing size of the alkyl group with higher electronic charge density on the nitrogen atom. As a consequence the inhibitive action of the positive head group should decrease as a result of a less tightly held layer of positive ions adjacent to the adsorbed bromide ions. This observed opposite behavior may be due to the effect of Van der Waal’s forces on attraction action between the alkyl chains of adjacently adsorbed positive head group ions [37].


## 3.6. Computational Study

In order to support the experimental findings, the calculated quantum chemical parameters, namely, the energy of highest occupied molecular orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO), dipole moment (μ), and energy gap (ΔE=ELUMO - EHOMO) values were obtained by using ab initio (HF/3-21G) and semiemperical (MNDO, AM1, MP3) methods. Calculated data are given in Table 6Table 6
Quantum chemical parameters of synthesized corrosion inhibitors A312, A314 and A316.
Surfactant Quantum parameters HF(3-21G) MP2(3-21G) AM1 MNDO PM3 A312 E H O M O(eV) -4.988 -5.276 -5.278 -5.205 -5.276 E L U M O(eV) -1.657 -1.933 -1.832 -1.862 -1.932 E L U M O-EHOMO(eV) 3.331 3.343 3.346 3.343 3.344 µ(Debye) 15.29 15.41 19.76 20.20 19.59  A314 E H O M O(eV) -5.278 -5.278 -5.280 -5.279 -5.279 E L U M O (eV) -1.919 -1.920 -1.920 -1.921 -1.920 E L U M O-EHOMO(eV) 3.359 3.358 3.360 3.358 3.359 µ(Debye) 23.24 23.19 23.73 24.17 23.81  A316 E H O M O(eV) -5.279 -5.283 -5.280 -5.278 -5.283 E L U M O (eV) -1.918 -1.917 -1.918 -1.912 -1.920 E L U M O-EHOMO(eV) 3.361 3.366 3.362 3.366 3.363 µ(Debye) 27.55 27.65 27.72 28.62 27.98From Table6, it was found that the EHOMO and the ELUMO changed rulelessly, while it was observed that the highest value of calculated ΔE was obtained by A316 and the lowest energy gap value was that of A312. It is well known that the lower the values of energy gap, the better the corrosion inhibition, because the ionization potential will be low [38–41]. This behavior pointed to that the shorter the chain length of the hydrophobic part, the lower the energy gap ΔE and the higher the inhibition efficiency of the Gemini surfactant, because the energy needed to remove an electron from the last occupied orbital will be low, which facilitates adsorption (and therefore inhibition), and that agrees well with the experimental findings.The dipole moment (μ) is an important electronic parameter which results from the nonuniform distribution of the charges on the different atoms of the molecule, so the inhibitive ability of a molecule is related to it [42]. There is no agreement in the literature on the use of dipole moment (μ) as a predictor for the trend of the inhibition reaction. Some authors reported that the high values of the dipole moment pointed to high inhibition efficiency [41–43].Others as Abdallah et al. [44] and Olasunkanmi et al. [45] found that the comparison between the calculated dipole moments of the investigated compounds reveals that the lowest has better inhibition efficiency; moreover, no significant relationship between these values has been identified in some other cases [46]. The found results show increase in inhibition efficiencies with decreasing value of the dipole moment.An attempt to search correlations between the experimentally obtained inhibition efficiency resulted from weight loss (Ew %) at concentration 200 ppm and theoretically calculated energy gap (ΔE) values is illustrated in Table 7. The low values of gap energy (ΔE) favor the adsorption of the three Gemini surfactants on API X52 steel surface and enhance the protecting power as shown in Table 7. The plotting (Figure 7) shows that the best correlation is obtained using semiempirical methods MNDO and AM1. It can be concluded that quantum chemical calculations can be used to predict the effectiveness of the used inhibitors.Table 7
The relation between the energy gap ΔE and the experimental inhibition efficiency (Ew %) of the synthesized corrosion inhibitors.
Inh. Ew% ΔE HF(3-21G) MP2(3-21G) AM1 MNDO PM3 A312 94.87 3.331 3.343 3.346 3.343 3.344  A314 92.52 3.359 3.358 3.360 3.358 3.359  A316 89.69 3.361 3.366 3.362 3.366 3.363Figure 7
Correlations between experimental inhibition efficiency (Ew%) of synthesized corrosion inhibitors and calculated quantum parameter (energy gap; ΔE).
(a) (b) (c) (d) (e)


## 4. Conclusions

(1)
The investigated three novel synthesized cationic Gemini surfactants act as corrosion inhibitors for API 5L X52 steel in 1 M HCl. The increasing of surfactant concentration (in the studied range) has positive effect on the inhibition efficiency, while the increasing of length of the alkyl chain attached to the surfactants molecules has a negative effect according to order: A312 > A314> A316. (2)
Electrochemical evaluation measurements showed that the synthesized surfactants act as mixed type inhibitors. Polarization and impedance measurements results are in good agreement with those obtained from weight loss measurements. (3)
The adsorption isotherm obeys Langmuir’s model. (4)
Quantum chemical computational study revealed that the theoretical calculated parameters can be successfully used as a predictive tool of the investigated experimental behavior of the novel synthesized inhibitors. The experimental inhibition efficiency was correlated with energy gap (ΔE).



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