Research ArticleInhibition of API 5L X52 Pipeline Steel Corrosion in AcidicMedium by Gemini Surfactants: Electrochemical Evaluation andComputational Study

IbrahimHamed,1 Magda Mohamed Osman,2 Omnia Hassan Abdelraheem ,3

Maher Ibrahim Nessim,2 andMaryam Galal El mahgary4

1Chemical Engineering Department, Faculty of Engineering, Minia University, Egypt2Analysis and Evaluation Department, Egyptian Petroleum Research Institute, Egypt3Basic Engineering Sciences Department, Faculty of Engineering, Beni-Suef University, Egypt4Chemical Engineering Department, Faculty of Engineering, BUE University, Egypt

Correspondence should be addressed to Omnia Hassan Abdelraheem; omnia hassan55@eng.bsu.edu.eg

Received 17 November 2018; Accepted 3 January 2019; Published 21 March 2019

Academic Editor: Francisco Javier Perez Trujillo

Copyright © 2019 IbrahimHamed et al.This is an open access article distributed under the Creative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 carbonsteel API 5L X52 grade in 1MHCl.The relation between the experimental inhibition efficiency and theoretical chemical parametersobtained by computational calculation in order to predict the behavior of the organic compounds as corrosion inhibitorswas instigated. The chemical structures were elucidated using 1HNMR spectra. Inhibition performance was investigated bypotentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and weight loss tests. The polarization curves showthat applied surfactants act as mixed type inhibitors. Nyquist plots showed the semicircle capacitive loop with different surfactantsand 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 inhibitoryeffect 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 theoreticalcalculated quantum parameters were subjected to correlation analysis.

1. Introduction

In the petroleum production industry, failures of pipelinesas a result of corrosion have led to heavy loss [1]. Themetallurgy of pipelines, tanks, ships, etc. in oil productionsystems is often based on carbon steel [2]. The API 5L gradesteel is one of the most common pipeline materials in theoil industry. However, the carbon steel is heavily exposedto corrosion attack because these operations usually induceserious corrosion of equipment, tubes, and pipelines made ofsteels [3]. In oil industry and petrochemical processes acidsolutions are widely applied in acid pickling, so the use of

corrosion inhibitors is one of the most applied methods forcorrosion prevention [4].

Gemini surfactants are a category of surfactants con-sisting of two conventional surfactant head groups, bondedtogether by a spacer [5]. Cationic surfactants form compactadsorption layers that hydrophobize the surfaces of metals[6]. Due to their more efficient surface properties, recentlymany researchers paid attention to Gemini surfactants ascorrosion inhibitors in acidic medium [7–10].

Generally, inhibitor molecules may physically or chem-ically adsorb on a corroding metal surface, forming anadsorption protective layer. The power of the inhibition

HindawiInternational Journal of CorrosionVolume 2019, Article ID 4857181, 12 pageshttps://doi.org/10.1155/2019/4857181

2 International Journal of Corrosion

i) R Br NMe

MeC

O

H

EtOH

Reflux 6 h N

Me

MeC

O

HR

BrN-Alkyl bromide 4-(Dimethylamino)benzaldehyde 4-formyl-N,Ndimethyl-N-alkylbenzeneaminium bromide

ii)H2N NH2

Benzene-1,4-diamine

NMe

MeC

O

HR

Br

EtO

HRe

flux

6 h

N CH

NMe

Me Br

N CH

NMe

Me

Br

R R

4,4-[1,4-phenylenebis(azanylidene)bis(methanyllyidene)bis(N,N-dimethyl-N-alkylbenzeneaminium] bromide

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.

depends on themolecular structure of the inhibitor. Presenceof the lone electron pairs in the heteroatoms is an importantfeature that controls the adsorption on the metal surface [11].

Computational chemistry has proven to be a very power-ful tool of evaluating the efficiency of corrosion inhibitor andof investigation of corrosion inhibition mechanism. More-over it is a theoretical prediction tool which provides a pre-diction of the possibilities of newly synthesized compoundsto act as corrosion inhibitors and permits the preselectionof compounds with the necessary structural characteristics,chemical intuition, and experience into a mathematicallyquantified and computerized form [12–15].

Certain quantum chemical indices which are calculatedby computational chemistry programs can be associatedwith metal/inhibitor reactions. These are the HOMO energy,LUMO energy, and the gap energyΔE (ΔE = ELUMO - EHOMO)and dipole moment (𝜇). HOMO energy (highest occupiedmolecular orbital) is often associated with the capacity ofa molecule to donate electrons, and high EHOMO valuesindicate that the molecule has the ability to donate electronsto suitable acceptor molecules with low-energy molecularorbits [16]. ELUMO indicates the ability of the molecules toaccept electrons. In the same way low values of the energygap ΔE = EHOMO - ELUMO will indicate good inhibitionefficiencies, because the energy needed to remove an electronfrom the last occupied orbital will be low [17]. The dipolemoment (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 inhibitionefficiency of three synthesized cationic Gemini surfactants,namely, A312, A314, and A316, in which the two cationichead groups are linked by rigid spacer containing Schiff base,on the corrosion behavior of API X52 steel pipeline in 1 MHCl. The second goal is to identify the relation between theexperimental inhibition efficiency and theoretical chemical

parameters obtained by computational calculation in orderto predict of the behavior of the organic compounds ascorrosion inhibitors.

2. Experimental

2.1. Synthesis and Characterization. The three cationicGemini surfactants were synthesized in high purity throughtwo consecutive steps according to [19]. The first step is thesynthesis of the cationic surfactants 4-formyl-N,N-dimethyl-N-alkylbenzeneaminium bromide, by heating three alkylhalides with different chain lengths, (dodecyl-, tetradecyl-,and hexadecyl-bromide, respectively) with equimolaramount of 4-(dimethylamino) benzaldehyde, in absoluteethanol. The components mixture was allowed to refluxfor 12 h and was left to cool to room temperature. Schiffbase compounds were synthesized in the second step bya condensation reaction between the products out of thefirst step and benzene-1,4-diamine in ethanol of 2:1 molarratio. Finally the product was recrystallized from ethanol.The chemical structures of the three synthesized cationicGemini surfactants (named as surfactants A312, A314, andA316) are shown in Figure 1. Confirmation of the synthesizedstructures was elucidated by using two different tools:elemental analysis (using elemental analyzer Perkin Elmer240C) and 1H-NMR spectroscopy (using Jeol-EX-270 MHz1H-NMR spectroscopy).

2.2. Materials. Pipeline steel coupons of type API X52 havethe 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, whichwas prepared with analytical reagent (37%) in distilled water.The synthesized inhibitors concentration ranged from 5 to200 ppm.

International Journal of Corrosion 3

CH

N N CH

N N

H3C

H3C CH3

CH3

H2C

H2C

H2C

H2C (CH2)n(CH2)n CH3H3C

abc de

fghi

Figure 2: 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 C

48H76Br2N4

868 66.35 66.55 8.82 8.61 18.41 18.44 6.45 6.40A314 C

52H84Br2N4

924 67.52 67.18 9.15 9.42 17.28 17.11 6.06 6.29A316 C

56H92Br2N4

980 68.55 68.51 9.45 9.58 16.29 16.33 5.71 5.58

2.3. Weight Loss Measurements. The weight loss of carbonsteel coupons was determined after immersion period of 72h.The coupons having 2 cm2 cross-sectional area were polishedwith grit emery papers (grade 300-1000), dried, and thenweighted. The freshly prepared specimens were immersedin 100 ml of 1 M HCl solution with and without differentconcentrations of the three cationic Gemini surfactants A312,A314, and A316.

2.4. Electrochemical Polarization Measurements. Electro-chemical polarization experiments were performed at roomtemperature using a conventional three-electrode cell witha platinum counter electrode, standard calomel electrode(SCE) as the reference electrode, and cylindrical workingelectrode with 1 cm diameter. Polarization measurementswere conducted potentiodynamically using a corrosion mea-surement system SP 150 Potentiostat/galvanostat with soft-ware EC-lab version 9.9. The impedance experiments werecarried out in the frequency range of 100 kHz–30 Hz, at theOCP by applying small amplitude ac signal of 10 mV. Tafelpolarization measurements were carried out at a scan rate of10 mV/s in the potential range from –850 to -200 mV.

2.5. Computational Theoretical Calculations. Quantumchemical calculations were performed with completegeometry optimizations using Chem Bio Draw Ultra 12software, using ab initio (HF/3-21G and MP2/3-21G) andsemiemperical (MNDO, AM1, and PM3) methods.

3. Results and Discussion

3.1. Characterization. Results of elemental analysis ofthe synthesized three cationic Gemini surfactants A312,A314, and A316 with chemical formulas (C

48H76Br2N4),

(C52H84Br2N4), and (C

56H92Br2N4), respectively, are shown

in Table 1, where the observed data are compatible with thecalculated ones. 1H-NMR spectra analysis is represented inTable 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.

3.2. Weight Loss. The corrosion rate CR of API X52 carbonsteel in acidic solutions in the presence and absence ofinhibitors were calculated according to the following equa-tions [20]:

CR = ΔWS.t (1)

where t is the specimen immersion time (hour), S is thesurface area of the test specimen (cm2), andΔW is the weightloss (mg), CR in mg/cm2.hr.

EW % = [Wo-WWo] × 100 (2)

where 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 increas-ing of surfactant concentration.The changes in the inhibitionefficiency with concentration are given in Table 3. Thecorrosion rate CR decreases in the order: A316> A314> A312.For a constant concentration, the corrosion rate reveals thatthe increase of alkyl chain length of hydrophobic part from12 to 16 carbon atoms increases the corrosion rate of theinhibitor.

3.3. Electrochemical Polarization. Corrosion parametersobtained from polarization curves (Figure 3) includingcorrosion current density (𝑖corr, in mA.cm−2), corrosionpotential (𝐸corr, in mV), inhibitor efficiency (E%), and anodicand cathodic Tafel slopes (ba and bc, respectively, in mV) areshown in Table 3. For the inhibited systems the inhibitionefficiency is calculated from the following equation [21]:

EP% = [ i∘corr − icorr

i∘corr] × 100 (3)

4 International Journal of Corrosion

Table 2: 1H-NMR spectroscopy of A312, A314 and A316.

Cpd.

H+TypeChemical 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 0.89A314 8.28 7.82 7.27 6.84 3.52 3.04 2.87 1.32 0.92A316 8.43 7.85 7.31 6.81 3.51 3.08 2.90 1.31 0.91

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

−0.9−0.8−0.7−0.6−0.5−0.4−0.3−0.2−0.1

0

E vs

. SCE

(mV

)

10 2 3−2 −1−3log (i, mA cm-2)

(a)

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

10 2 3−2 −1−3log (i, mA cm-2)

−0.9

−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0

E vs

. SCE

(mV

)

(b)

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

10 2 3−2 −1−3log (i, mA cm-2)

−0.9

−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0

E vs

. SCE

(mV

)

(c)

Figure 3: Potentiodynamic polarization of API X52 steel in 1MHCl with different concentrations of the synthesized corrosion inhibitors, (a)A312, (b) A314, and (c) A316.

where (icorr and i∘corr) are the corrosion current densities incase of the presence and absence of the inhibitor, respectively.

From obtained data it can be observed that with theaddition of the three inhibitors (A312, A314, and A316) tothe acidic solution, the corrosion current densities decrease,while inhibition efficiencies increase depending on the

concentration. There is no clear trend in the shifting of Ecorr(vs. SCE); on the other hand, both anodic and cathodic curvesshift to lower current densities.

These results suggest that synthesized surfactants retardthe anodic dissolution reaction as well as delay the cathodicreaction of hydrogen evolution. Therefore, these compounds

International Journal of Corrosion 5

Table3:Th

eweightlossa

ndelectro

chem

icalpo

lariz

ationmeasurementsof

API

X52carbon

steelin

1MHCl

with

additio

nof

synthesiz

edinhibitorsA312,A314,andA316at25∘C.

Surfa

ctant

Con

centratio

n(ppm

)Electro

chem

icalPo

lariz

ation

WeightL

oss

Icorr,

mAcm−2

Ecorr,

mV

ßa,

mVdec−1

ßc,

mVdec−1

Ep%

Corr.Ra

te(m

g.cm−2.hr−1)

Ew%

Blank

0.00

801.7

5-510.9

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

261.4

1-494.0

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

0.04

4790.86

100.0

48.18

5-514.8

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

359.0

8-500.4

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

0.110

977.36

50.0

91.31

1-502.1

149.3

167.2

88.64

0.0693

85.86

100.0

74.32

2-507.5

143.8

157.3

90.73

0.04

0291.80

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

361.9

0-527.8

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

171.9

7-519.3

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

6 International Journal of Corrosion

Table 4: Electrochemical impedance spectroscopy (EIS) for the corrosion of API X52 steel in 1 M HCl at various concentrations of thesynthesized 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 65.8925.0 2.119 126 17.55 81.6650.0 2.433 87.0 31.46 89.21100.0 2.325 49.9 54.72 93.79200.0 2.175 48.1 65.44 94.81

A314

5.00 2.18 158 9.241 63.2625.0 1.987 170.6 16.5 79.4250.0 2.237 159.6 21.87 84.48100.0 2.073 131.8 29.72 88.58200.0 2.207 79.7 55.55 93.89

A316

5.00 2.034 269 7.031 51.7125.0 2.021 180 11.73 71.0650.0 1.885 119.5 15.79 78.50100.0 1.977 218 20.02 83.04200.0 2.11 146 32.66 89.60

act as mixed type inhibitors [4, 22]. A slight decrease in bothTafel slopes (ba and bc) indicates that the inhibitors decreasethe surface area exposed for corrosion without altering thecorrosion mechanism [23].

In all cases, the order of corrosion inhibition efficiencyof the surfactants is A312,> A314, > A316. The higher thehydrophobic alkyl chain length is, the lower the inhibitorefficiency is at a constant concentration.This is a consequenceof the adsorption process. This result is in accordance withprevious work [24].

3.4. Electrochemical Impedance Spectroscopy. Electrochemi-cal impedance spectroscopy (EIS) was applied to investigatethe electrode/electrolyte interface and processes that occuron the API X52 pipeline steel surface in the presence andabsence of the acid solution. The spectra were recorded afterthe 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 versusthe real component of impedance (Zr) for each excitationfrequency. The electrode impedance, Zr, is related to thefrequency of the AC signal in Hz (�), resistance of (Rp), thedouble layer capacity of double layer (Cdl), and resistanceof solution (RS) and is represented by the mathematicalformulation [25]:

Zr = Rs + Rct[1 + (2.𝜋.�.Rct.Cdl) 𝛼] (4)

The percentage inhibition efficiencies (Ei%) are determinedby the following relation [26]:

E% = [(R∘ct − Rct)R∘ct

] × 100 (5)

where Rct and R∘ct are the charge transfer resistanceswith and without the inhibitors, respectively. Impedanceparameters: double layer capacitance (Cdl), charge transferresistance (Rct), and the inhibition efficiency Ei% are givenin Table 4. The EIS spectra of the API X52 steel electrode arerecorded in the absence and presence of inhibitors at differentconcentrations in Figure 4.

All the Nyquist plots obtained (Figure 4) were with onesingle capacitive loop semicircle in nature. The semicirclediameter (Rct) increases with the increase in inhibitor con-centration. Data in Table 4 show that the Rs values are verysmall compared to the Rct values. Also the calculated Cdlvalues decrease by increasing the inhibitor concentrations.The high Rct values are generally associated with slowercorroding system and hence an increase in the calculatedinhibition efficiencies Ei% [27].

The obtained semicircle shaped plots show that corrosioninhibition of Gemini surfactants is controlling only by chargetransfer process, as well as electrode surface homogeneitysuch that the adsorption of inhibitors is formed by compactlayers [28].

The inhibition efficiencies, calculated from EIS results,show the same trend as those obtained from polarizationmeasurements. The change in concentration of the inhibitordid not alter the profile of the impedance behavior, referringto similar mechanism for the corrosion inhibition reaction[29]. This confirms the behavior observed in potentiody-namic polarization measurement, such that the inhibitors donot change themechanismofmetal dissolution but only affectthe rate.

Differences in inhibition efficiency obtained from twomethods may be attributed to the different surface statusof the electrode in two measurements. EIS was performed

International Journal of Corrosion 7

0 50 100 150 200 250

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

Zr (Ω cＧ2)−10

0

10

20

30

40

50

60

70-Z

i (Ω

cm2)

(a)

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

0 10 20 30 40 50 60 70

Zr (Ω cＧ2)−5

0

5

10

15

20

25

-Zi (Ω

cm2)

(b)

Blank5 ppm25 ppm

50 ppm100 ppm200 ppm

0 5 10 15 20 25 30 35 40Zr (Ω cＧ2)

−2

0

2

4

6

8

10

12

14

-Zi (Ω

cm2)

(c)

Figure 4: Nyquist plots for API X52 steel in 1 MHCl with different concentrations of the synthesized corrosion inhibitors, (a) A312, (b) A314,and (c) A316.

Table 5: Adsorption parameters of synthesized inhibitors on the steel surface.

Surfactant Linear correlationcoefficient, r2 Slope Kads x 10−3

(l. mol−1)ΔG

kJ. mol−1

A312 0.9997 1.034 251.130 -40.57A314 0.9996 1.049 163.941 -39.69A316 0.9993 1.083 128.336 -39.09

at the rest potential, while in polarization measurementsthe electrode potential was polarized to high overpotential;nonuniform current distributions, resulting from cell geom-etry, solution conductivity, counter and reference electrodeplacement, etc., will lead to the difference between theelectrode area actually undergoing polarization and the totalarea [30].

EIS results can be elucidated in terms of the equiva-lent circuit of the electrical diagram (Figure 5) to model

the iron/acid interface [31]. The equivalent circuit com-posed of a constant phase element Cdl, in parallel witha resistor, Rct, which corresponds to a single capacitiveloop. The resistor Rs is in series to the Cdl and Rct.Rs is the uncompensated resistance between the work-ing steel electrode and SCE reference electrode or solu-tion resistance, Rct is the polarization resistance at theelectrode/solution interface, and Cdl is the double layercapacitance at the interface. The double layer capacity is

8 International Journal of Corrosion

Cdl

Rs

RCT

Figure 5: Equivalent circuit model to fit the metal/acid interfacecontaining different concentrations of the synthesized corrosioninhibitors.

A312A314A316

50 100 150 200 2500C (ppm)

0

50

100

150

200

250

C/

(ppm

)

Figure 6: Langmuir isotherm plots for API X52 steel in 1M HClat presence of different concentrations of synthesized corrosioninhibitors A312, A314, and A316.

in parallel with the impedance due to the charge transferreaction.

3.5. Adsorption Isotherm. Adsorption isotherm for the threeGemini surfactants can provide important informationwhich highlight the interaction between the organic com-pounds and the steel surface.

In this study, various isotherms models were applied inattempts to fit the degree of surface coverage (𝜃) using lossin weight data. The best fit was obtained with the Langmuirisotherm (Figure 6) by the following relation [32]:

C𝜃 =1

Kads+ C (6)

where C is the concentration in mol/l, � is the surfacecoverage, and Kads is the adsorption equilibrium constant.Table 5 shows the parameters and the regression factorscalculated from Langmuir adsorption isotherm and also thecalculated values of ΔGads. All linear correlation coefficients(r2) exceeded 0.999 indicating that the corrosion inhibition ofAPI X52 steel by the synthesized cationic Gemini surfactantswas attributed to adsorption of these compounds on the

metal surface. However, the slopes of the C/𝜃 versus Cplots show a little deviation from unity. The deviation maybe due to interaction between the adsorbed species on thesteel surface by mutual attraction or repulsion. High valuesof the adsorption constant Kads can be reasoned for betteradsorption and high inhibition efficiency [33].

The standard adsorption free energy (ΔG∘ads) was calcu-lated by the following equation [34]:

-ΔG∘ads = RT ln (55.5Kads) (7)

whereR is the gas constant (8.314 J /Kmol. 1) andT is the abso-lute 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 withincreasing the number of ethylene groups in the hydrophobicalkyl chain, which means the decrease in adsorption capacityof inhibitors with lengthening the alkyl chain from 12 to 16.

Generally, it is known that the values of ΔGads up to -20kJ.mol−1 are regarded as physisorption which is the adsorp-tion type associatedwith electrostatic interaction between thecharged molecules and the charged metal. Values around -40kJ.mol−1 or more negative are considered as chemisorptions,which is a result of the charge sharing or a transfer of electronpair or 𝜋 electrons from the inhibitor molecules to the steelsurface 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 explainedas a result of either the 𝜋-electrons of the aromatic structureor the electronegative N atoms and cathodic sites on themetallic surface.

The calculated adsorption parameters and hence theinhibiting properties of the investigated surfactants appearto decrease by increasing size of the alkyl group withhigher electronic charge density on the nitrogen atom. As aconsequence the inhibitive action of the positive head groupshould decrease as a result of a less tightly held layer ofpositive ions adjacent to the adsorbed bromide ions. Thisobserved opposite behavior may be due to the effect of VanderWaal’s forces on attraction action between the alkyl chainsof adjacently adsorbed positive head group ions [37].

3.6. Computational Study. In order to support the experi-mental findings, the calculated quantum chemical parame-ters, namely, the energy of highest occupiedmolecular 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 6

FromTable 6, it was found that the𝐸HOMO and the𝐸LUMOchanged rulelessly, while it was observed that the highestvalue of calculated ΔE was obtained by A316 and the lowestenergy gap value was that of A312. It is well known thatthe lower the values of energy gap, the better the corrosioninhibition, because the ionization potential will be low [38–41].This behavior pointed to that the shorter the chain lengthof the hydrophobic part, the lower the energy gap ΔE andthe higher the inhibition efficiency of the Gemini surfactant,

International Journal of Corrosion 9

y = -134.4x + 542.63

HF(3-21G)89

90

91

92

93

94

95

96

Expt

E %

3.33 3.34 3.35 3.36 3.373.32ΔE (eV)

２2= 0.7554

(a)

y = -216.42x + 818.59

MNDO

89

90

91

92

93

94

95

96

Expt

E %

3.35 3.36 3.373.34ΔE (eV)

２2= 0.9492

(b)

y = -240.85x + 900.48

PM389

90

91

92

93

94

95

96

Expt

E %

3.35 3.36 3.373.34ΔE (eV)

２2= 0.8651

(c)

y = -266.32x + 986.12

AM1

89

90

91

92

93

94

95

96

Expt

E %

3.35 3.355 3.36 3.3653.345ΔE (eV)

２2= 0.8012

(d)

y = -216.42x + 818.59

MP( 3-21G)88

90

92

94

96

Expt

E %

3.35 3.36 3.373.34ΔE (eV)

２2= 0.9492

(e)

Figure 7: Correlations between experimental inhibition efficiency (Ew%) of synthesized corrosion inhibitors and calculated quantumparameter (energy gap; ΔE).

because the energy needed to remove an electron from thelast occupied orbital will be low, which facilitates adsorption(and therefore inhibition), and that agrees well with theexperimental findings.

The dipolemoment (𝜇) is an important electronic param-eter which results from the nonuniform distribution of thecharges on the different atoms of the molecule, so theinhibitive ability of a molecule is related to it [42]. Thereis no agreement in the literature on the use of dipolemoment (𝜇) as a predictor for the trend of the inhibitionreaction. Some authors reported that the high values of thedipole 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 dipolemoments of the investigated compounds reveals that the low-est has better inhibition efficiency; moreover, no significantrelationship between these values has been identified in someother cases [46].The found results show increase in inhibitionefficiencies with decreasing value of the dipole moment.

An attempt to search correlations between the experi-mentally obtained inhibition efficiency resulted from weightloss (Ew%) at concentration 200 ppm and theoreticallycalculated energy gap (ΔE) values is illustrated in Table 7.The low values of gap energy (ΔE) favor the adsorptionof the three Gemini surfactants on API X52 steel surface

10 International Journal of Corrosion

Table 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

𝐸HOMO(eV) -4.988 -5.276 -5.278 -5.205 -5.276𝐸LUMO(eV) -1.657 -1.933 -1.832 -1.862 -1.932𝐸LUMO-𝐸HOMO(eV) 3.331 3.343 3.346 3.343 3.344𝜇(Debye) 15.29 15.41 19.76 20.20 19.59

A314

𝐸HOMO(eV) -5.278 -5.278 -5.280 -5.279 -5.279𝐸LUMO (eV) -1.919 -1.920 -1.920 -1.921 -1.920𝐸LUMO-𝐸HOMO(eV) 3.359 3.358 3.360 3.358 3.359𝜇(Debye) 23.24 23.19 23.73 24.17 23.81

A316

𝐸HOMO(eV) -5.279 -5.283 -5.280 -5.278 -5.283𝐸LUMO (eV) -1.918 -1.917 -1.918 -1.912 -1.920𝐸LUMO-𝐸HOMO(eV) 3.361 3.366 3.362 3.366 3.363𝜇(Debye) 27.55 27.65 27.72 28.62 27.98

Table 7: The relation between the energy gap ΔE and the experimental inhibition efficiency (Ew %) of the synthesized corrosion inhibitors.

Inh. Ew% ΔEHF(3-21G) MP2(3-21G) AM1 MNDO PM3

A312 94.87 3.331 3.343 3.346 3.343 3.344A314 92.52 3.359 3.358 3.360 3.358 3.359A316 89.69 3.361 3.366 3.362 3.366 3.363

and enhance the protecting power as shown in Table 7. Theplotting (Figure 7) shows that the best correlation is obtainedusing semiempirical methods MNDO and AM1. It can beconcluded that quantum chemical calculations can be usedto predict the effectiveness of the used inhibitors.

4. Conclusions

(1) The investigated three novel synthesized cationicGemini surfactants act as corrosion inhibitors forAPI 5L X52 steel in 1 M HCl. The increasing ofsurfactant concentration (in the studied range) haspositive effect on the inhibition efficiency, while theincreasing of length of the alkyl chain attached to thesurfactants molecules has a negative effect accordingto order: A312 > A314> A316.

(2) Electrochemical evaluation measurements showedthat the synthesized surfactants act as mixed typeinhibitors. Polarization and impedance measure-ments results are in good agreement with thoseobtained from weight loss measurements.

(3) The adsorption isotherm obeys Langmuir’s model.

(4) Quantum chemical computational study revealedthat the theoretical calculated parameters can besuccessfully used as a predictive tool of the investi-gated experimental behavior of the novel synthesizedinhibitors.The experimental inhibition efficiency wascorrelated with energy gap (ΔE).

Data Availability

The data used to support the findings of this study areincluded within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors gratefully acknowledge the financial supportfrom Beni-Suef University, Minia University, and EgyptianPetroleum Research Institute.

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