Corrosion Science 51 (2009) 744–751

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Corrosion Science

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The inhibition of CO2 corrosion of N80 mild steel in single liquid phase andliquid/particle two-phase flow by aminoethyl imidazoline derivatives

X. Liu a,b, P.C. Okafor a,c, Y.G. Zheng a,*

a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, Liaoning Province, P.R. Chinab Department of Applied Chemistry, Shenyang Institute of Chemical Technology, Shenyang 110142, P.R. Chinac Department of Pure and Applied Chemistry, University of Calabar, P. M. B. 1115, Calabar, Nigeria

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 July 2008Accepted 23 December 2008Available online 14 January 2009

Keywords:A. Mild steelB. ErosionB. PolarizationB. EISB. SEM

0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.12.024

* Corresponding author. Tel.: +86 24 23928381; faxE-mail addresses: ygzheng@imr.ac.cn, icpmkaist@y

The inhibitory behaviour of 2-undecyl-1-aminoethyl imidazoline (AEI-11) and 2-undecyl-1-aminoethyl-1-hydroxyethyl quaternary imidazoline (AQI-11) on CO2 corrosion of N80 mild steel in single liquid phaseand liquid/particle two-phase flow was investigated using weight loss, linear polarization, potentiody-namic polarization, EIS methods and SEM observations. The two compounds inhibited the CO2 corrosionof N80 steel and the extent of inhibition was dependent on the flow conditions and particulate content. Inboth phases, AQI-11 exhibited better inhibition ability than AEI-11 due to the polycentric adsorption siteson its structure. Theoretical calculation on the inhibition abilities of the compounds and their modes ofadsorption are reported.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction that the critical flow velocity varies from one inhibitor to another

With the exploitation of oil and gas well containing carbondioxide, CO2 corrosion has become one of the universal problemsin the oil and gas industries responsible for lost production andexpensive repairs. It occurs at all stages of production from down-hole to surface equipment and processing facilities [1–2]. Theproblems arising from CO2 corrosion have led to the developmentof various method of corrosion control which includes the injectionof corrosion inhibitors that has proven to be a practical and eco-nomical method to control CO2 corrosion [3]. Nitrogen-based or-ganic surfactants, such as imidazoline and its derivatives, havebeen used successfully as inhibitors in combating CO2 corrosion[3–8]. These organic compounds inhibit the corrosion of mild steel(the preferred pipeline material due to cost) by adsorption on themetal-solution interface thereby creating a barrier that preventsthe active ions in the corrosion reactions to get to the surface.

Most researches on CO2 corrosion inhibitors reported in openliteratures are generally investigated under static conditions or atlow flow rate using rotating cylinder electrodes or low velocityflow loops [5,9–10], and inhibitors efficient in these systems maynot perform optimally in industrial scaled systems (of highervelocities and multiphase flow). Corrosion rate and corrosion inhi-bition efficiency of organic compounds depends on flow velocityand pattern, presence of entrained particles, size of particles andother flow-influenced parameters [11–14]. It has also been shown

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and is a function of the structure and subsequently its adsorptionstrength [12]. The optimum concentration of inhibition of variousorganic compounds is also flow dependent [12].

In this paper, two imdazolines, 2-undecyl-1-aminoethyl imi-dazoline (AEI-11) and 2-undecyl-1-aminoethyl-1-hydroxyethylquaternary imidazoline (AQI-11) with different but related chemi-cal structures were chosen to investigate their inhibition perfor-mances in combating the corrosion of N80 mild steel in 3% NaClsolutions de-aerated with CO2 under static single liquid phase, sin-gle liquid phase flow and liquid/particle two-phase flow condi-tions. Furthermore, the inhibition performances of theimidazolines were correlated with quantum-chemical calculationsin anticipation that the correlation will be helpful in the design andsynthesis of new inhibitors with higher inhibition efficiency undersingle phase and multiphase flow conditions.

2. Experimental

2.1. Materials and medium

N80 mild steel cut from its parent pipe was used as the testmaterial for these experiments and has the chemical compositionshown in Table 1. The test coupons were prepared, degreasedand cleaned as previously described [12–13]. The graphite elec-trode was used as the counter electrode and a saturated calomelelectrode (SCE) as the reference electrode. CO2-saturated 3% NaClsolution was used as the test medium. Before each test, the systemwas de-aerated by flushing with CO2 gas for more than an hour and

Table 1Chemical composition (wt%) of the tested N80 mild steel.

Element C Si Mn P S Al Cu Nb Ni

Composition(wt%)

0.24 0.32 1.58 0.022 0.012 <0.01 0.006 <0.008 0.05

X. Liu et al. / Corrosion Science 51 (2009) 744–751 745

kept saturated with CO2 by a continuous flow of the gas at atmo-spheric pressure. The structures of the inhibitors used are shownin Fig. 1. Based on previous experience with imidazoline deriva-tives in CO2-saturated systems [12–13], 100 ppm of the inhibitors(AEI-11 and AQI-11) and 1% commercial quartz particles withmean diameter of 125 lm were used for the investigation. All testswere conducted at room temperature.

2.2. Test apparatus and procedure

The test apparatus (a modified rotating disk electrode (RDE))and procedure for this investigation have been previously de-scribed [12–13]. The main advantage of the modified RDE is itssimplicity. It affords a high linear rotation speed which the tradi-tional RDE are unable to provide, and also enables the simulta-neous determination of the in situ corrosion rate using both theweight loss and electrochemical techniques.

The exposed surface area of the specimen was 2.01 cm2. Elec-trochemical measurements were conducted using an EG & GPrinceton Applied Research Potentiostat/Galvanostat Model 263Aand 5210 lock-in-amplifier with software M398. Prior to any elec-trochemical measurement, the working electrodes were immersedin the CO2-saturated solutions for 1 h.

The polarization resistance (Rp) measurements were conductedby polarizing the working electrode ±5 mV from the free corrosionpotential and scanning at 0.166 mV/s hourly for a total of 5 h. EISmeasurements were performed over the frequency range of100 kHz to 10 MHz with a signal amplitude perturbation of 5 mV atthe corrosion potential and the potentiodynamic polarization sweepsconducted at a sweep rate of 0.166 mV/s after 5 h of immersion. Forthe weight loss coupons, post-treatment were carried out by immers-ing the specimens in 5% hydrochloric acid + 0.5% benzalkonium bro-mide for 5 min, wiping off the surface with absorbent cotton. Afterwhich the specimen was rinsed with distilled water, cleansed withethanol and air dried with blower. For each experimental condition,two to three measurements were performed to estimate the repeat-ability. The repeatability was quite good, and the changes observed inthe results reflect influences of various parameters beyond the exper-imental error. Scanning electron microscopy (SEM) was used in thecorrosion morphology observation.

3. Results and discussion

3.1. Inhibition performance under static condition

The potentiodynamic polarization curves for N80 mild steelcoupons in 3% NaCl solution saturated with CO2 containing AEI-

N N C2H4NH2

C11H23

N NC2H4OH

C11H23

C2H4NH2Cl

AEI-11 AQI-11

Fig. 1. Molecular structures of two tested inhibitors.

11 and AQI-11 under static condition are presented in Fig. 2. It isobserved that the presence of the imidazoline derivatives inhibitsthe anodic as well as the cathodic reactions. The decrease in corro-sion rate associating with a shift of both cathodic and anodicbranches of the polarization curves towards lower current densi-ties indicates that the imidazoline derivatives act as a mixed-typeinhibitor [15] in the system studied, blocking the active anodic andcathodic sites available for corrosion reaction [16]. It could also beobserved that the quaternary imidazoline (AQI-11) inhibited thereactions more than AEI-11.

In the polarization curves shown in Fig. 2, a current ‘‘plateau” isobserved on the anodic polarization curves in the presence of theinhibitors. This behaviour has been reported for iron in halide con-taining inhibitor solutions [17] and is ascribed to the desorption ofstrongly adsorbed inhibitor and halide molecules, as well as thecarbonic species on the surface of metal with potentials. At poten-tial around �0.5 V, in the inhibited solution, there is a slight indi-cation of passivity due probably to the interaction between theinhibiting molecules and the corrosion product.

From the polarization curves, corrosion potential (Ecorr) andcorrosion current density (Icorr) were deduced. Icorr was deter-mined graphically from the cathode part of polarization curve,as shown in Fig. 2. Similar method has been employed by otherresearchers for non-Tafel dependence curves in CO2-saturatedsystems with acceptable deviation of less than 10% from othermethods of corrosion rate determination [18]. The results ob-tained are as shown in Table 2 and confirm that the presenceof the compounds inhibits the corrosion process by reducingthe corrosion current densities to lower values. Icorr values ofAQI-11 were also observed to be lower than that of AEI-11. Fromthe calculated values of Icorr, the inhibition efficiencies (g%) of thecompounds for the CO2 corrosion of N80 mild steel were calcu-lated from the equation:

g% ¼ Iocorr � Icorr

Iocorr

� 100% ð1Þ

where Iocorr and Icorr are the uninhibited and inhibited corrosion cur-

rent densities, respectively. It can be seen from the calculated re-sults (Table 2) that the compounds inhibit the corrosion of N80carbon steel to an appreciable extent, with g% values of AQI-11greater than that of AEI-11.

The impedance spectra for N80 mild steel in CO2-saturated 3%NaCl solution in the absence and presence of the inhibitors are pre-sented in Fig. 3 as Nyquist plots. The diameter of semicircle in thehigh frequency range for AQI-11 was much larger than that for AEI-11 which indicates that AQI-11 reduced the corrosion rates moreeffectively under static condition as also observed from the polar-ization curves. The values of the polarization resistance (Rt) weredetermined from the impedance plots and are as shown in Table3. Rt value of AQI-11 was found to be higher than that of AEI-11and indicates higher inhibition performance. The inhibition effi-ciency (g%) imparted by the imidazolines was calculated usingthe equation:

g% ¼ Rti � Rto

Rti

� �� 100% ð2Þ

where Rti and Rto are the charge transfer resistance in the presenceand absence of the imidazolines, respectively. The results obtainedare as shown in Table 3 and confirm that AQI-11 is a better inhibitorthan AEI-11. The values of g% obtained from the EIS technique is ingood agreement with those obtained from the polarizationmeasurements.

Fig. 4 shows the dependence of linear polarization resistance(Rp) on immersion time for N80 mild steel in CO2-saturated 3%NaCl solutions in the absence and presence of the inhibitors. Rp val-

10-8 10-7 10-6 10-5 10-4 10-3 10-2-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

Icorr

(AEI-11)

Icorr

(AQI-11)

E ,

VS

CE

I , A / cm2

Blank AEI-11 AQI-11

Fig. 2. Polarization curves for N80 mild steel coupons in 3% NaCl CO2-saturated solutions in the presence and absence of inhibitors under static condition.

Table 2Polarization parameters for N80 mild steel in 3% NaCl CO2-saturated solution containing AEI-11 and AQI-11.

Inhibitor Static condition Single phase flow Two-phase flow

Ecorr (V) Icorr (lA cm�2) g% Ecorr (V) Icorr (lA cm�2) g% Ecorr (V) Icorr (lA cm�2) g%

Blank �694 72.4 – �605 549.0 – �539 701.0 –AEI-11 �710 6.6 90.8 �495 349.0 36.4 �555 571.0 19.6AQI-11 �650 0.6 99.2 �501 105.0 80.9 �567 185.0 73.9

0 2000 4000 6000 8000 100000

2000

4000

6000

8000

10000

0 50 100 150 200 250

0

50

100

150

200

250

Z re /

ΩΩ . c

m2

Zre / Ω . cm2

blank

Zre / ΩΩ . cm2

− Z

re /

ΩΩ . c

m2

Blank AEI-11 AQI-11

Fig. 3. Nyquist plots for N80 mild steel in 3% NaCl CO2-saturated solution in thepresence and absence of inhibitors under static condition.

Table 3Charge transfer resistance Rt, inhibition efficiency g% and percentage change DC (%)values for N80 mild steel in 3% NaCl CO2-saturated solution containing AEI-11 andAQI-11.

Inhibitor Static condition Single phase flow Two-phase flow

Rt (X) g% Rt (X) g% DC1 (%) Rt (X) g% DC2 (%)

Blank 210 – 36.8 – – 28.0 – –AEI-11 1660 87.0 54.4 32.4 96.7 51.1 45.2 3.2AQI-11 8472 98.0 166.5 77.9 98.0 110.1 74.6 33.9

1 2 3 4 50

2000

4000

6000

8000

10000

12000

R P / ΩΩ

Time / h

Blank AEI-11 AQI-11

Fig. 4. Variation of linear polarization resistance (Rp) with test time for N80 mildsteel in 3% NaCl CO2-saturated solution in the presence and absence of inhibitorsunder static condition.

746 X. Liu et al. / Corrosion Science 51 (2009) 744–751

ues obtained in the presence of inhibitors were observed to in-crease with time and appreciably compared to the values of theblank solution. At any give period, the Rp followed the trend,AQI-11 > AEI-11 > blank. This observation conforms with thepotentiodynamic and impedance measurements pointing to thefact that the quaternary imidazoline amine (AQI-11) had betterinhibition performance.

X. Liu et al. / Corrosion Science 51 (2009) 744–751 747

3.2. Inhibition performance in single liquid phase and liquid/particletwo-phase flow

The effects of the imidazoline derivatives on the corrosion ofN80 mild steel in a single liquid phase and liquid/particle two-phase flow systems were studied using potentiodynamic polariza-tion measurements at a rotational velocity of 5 m/s. The results ob-tained in the absence and presence of 1% sand particles are asshown in Figs. 5 and 6, respectively and the polarization parame-ters deduced from the curves are shown in Table 2. In both sys-tems, the imidazolines were found to inhibit both the anodic andcathodic reactions of the iron corrosion, and shift the corrosion

10-6 10-5

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E , V

SCE

I ,

Blank AEI-11 AQI-11

Fig. 5. Polarization curves for N80 mild steel in 3% NaCl CO2-satura

1E-7 1E-6 1E-5

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E , V

SCE

I ,

Blank AE-11 AQI-11

Fig. 6. Polarization curves for N80 mild steel in 3% NaCl CO2-saturated sol

current densities to lower values. AQI-11, as also observed in thesingle phase system, was found to suppress the reactions morethan AEI-11. However, the extents of reduction in the reactionsare not as effective as that obtained in the single phase. This is inagreement with earlier conclusion [6,19] that the high shear stressby high velocity could degrade the effectiveness of corrosion inhib-itors, and thus significantly enhance the corrosion rate. This degra-dation of the effectiveness of the corrosion inhibitors by shearstress may have prevented the strong adsorption of the inhibitorthat resulted in the current ‘‘plateau” and the slight passivity ob-served in the static system. Also observed in the potentiodynamicpolarization plots is the remarkable positive shift of Ecorr (vs the

10-4 10-3

Icorr(blank)

Icorr(AQI-11)Icorr(AEI-11)

A / cm2

ted solution in the presence and absence of inhibitors at 5 m/s.

1E-4 1E-3

Icorr(AEI-11)

Icorr(AQI-11)

A / cm2

ution in the presence and absence of inhibitors and 1% sand at 5 m/s.

748 X. Liu et al. / Corrosion Science 51 (2009) 744–751

SCE) when compared with the static system. This shift is probablythe effect of flow as previously reported [13].

On addition of the entrained sand particles into the system(two-phase flow), there was a remarkable acceleration of the cor-rosion rate as evident in the increased corrosion current densities(Table 2) and reduction in the inhibition efficiency. The Ecorr (vsthe SCE) was also shifted in the negative direction when comparedwith the blank. However, in both single and two-phase system, thequaternary imidazoline amine (AQI-11) showed better inhibitionperformance than the imidazoline (AEI-11).

From the impedance spectra for N80 steel in the CO2-saturated3% NaCl solution in the absence and presence of inhibitors at 5 m/swith and without 1% silica sand presented as Nyquist plots in Figs.7 and 8, respectively, it could be observed that the diameter ofsemicircles in the high frequency range for the inhibition testswere much larger than those of the blank tests under both singlephase and two-phase flow conditions. This shows that the corro-sion rate is reduced by the addition of the corrosion inhibitor.The Rt values calculated from the spectra and showed in Table 3also indicated the inhibition abilities of the compounds in the sys-tems. Comparing the two-phase flow values in Table 3 with thoseof the single phase flow system indicates that the inhibition abili-ties of the compounds in the systems follow the trend: static single

0 50 100 150 2000

50

100

150

200

Zre / ΩΩ . cm2

Blank AEI-11 AQI-11

− Z

re /

ΩΩ . c

m2

Fig. 7. Nyquist plots for N80 mild steel in 3% NaCl CO2-saturated solution in thepresence and absence of inhibitors at 5 m/s.

00 20 40 60 80 100

20

40

60

80

100

- Zre /

ΩΩ . c

m2

Zre / ΩΩ . cm2

Blank AEI-11 AQI-11

Fig. 8. Nyquist plots for N80 mild steel in 3% NaCl CO2-saturated solution in thepresence and absence of inhibitors and 1% sand at 5 m/s.

liquid phase > single liquid phase flow > liquid/particle two-phaseflow and the inhibition efficiencies (calculated using Eq. (1)) main-tains the trend: AQI-11 > AEI-11.

The percentage change in the impedance values for the N80mild steel in the systems studied were calculated using the follow-ing equation:

DC1ð%Þ ¼ 1� Rtðsf Þ

RtðsÞ

� �� 100% ð3Þ

DC2ð%Þ ¼ 1� Rtðtf Þ

Rtðsf Þ

� �� 100% ð4Þ

where, DC1 (%) is the percentage change in charge transfer resis-tance from static to single phase flow systems, and DC2 (%) is thepercentage change from single phase flow to two-phase flow sys-tem. Rt(s), Rt(sf) and Rt(tf) are the charge transfer resistances understatic, single phase flow and two-phase flow systems, respectively.The percentage change helps to fully appreciate the effects of theconditions of the systems on the inhibition ability of the molecules.The calculated values of the percentage changes are indicated in Ta-ble 3. It can be observed that the percentage change in impedancebetween static and single phase flow conditions DC1 (%) for thetwo inhibitors were very high (above 96%) and greater than the per-centage change in impedance between single phase and two-phaseflow systems DC2 (%). This indicates that flow impact on the inhibi-tion abilities of the imidazoline derivatives is greater than the par-ticle impact. The values of DC (%) for AQI-11 were found to behigher than those of AEI-11 indicating that the later could resistflow and the impact of the sand particle more than AQI-11.

The variation of linear polarization resistance (Rp) with immer-sion time for N80 mild steel in 3% NaCl solutions saturated by CO2

in the absence and presence of the inhibitors at 5 m/s with andwithout 1% silica sand are as shown in Figs. 9 and 10, respectively.Rp values for the single phase as well as the two-phase flow sys-tems depicted in Figs. 8 and 9, respectively were found to increasewith time to a maximum value and then decrease slightly. Thisbehaviour is not unconnected with the impact of an unprotectivecorrosion product formed on the surface of the metal with time.A closer look at Figs. 9 and 10 shows that the Rp values for thetwo-phase system is lower than those of the single phase system.This also confirms that multiphase flow has more negative impacton inhibitor performance due to the destruction of the protectivefilm formed on the surface of the mild steel as previously reported[8,19].

1 2 3 4 50

50

100

150

200

250

300

R P / Ω

Time / h

Blank AEI-11 AQI-11

Fig. 9. Variation of Rp with time for N80 mild steel in 3% NaCl solution saturatedwith CO2 at 5 m/s in the absence and presence of inhibitors.

0

50

100

150

200 Blank AEI-11 AQI-11

1 2 3 4 5

R P / Ω

Time / h

Fig. 10. Variation of Rp with time for N80 mild steel in 3% NaCl solution containing1% sand saturated with CO2 at 5 m/s in the absence and presence of inhibitors.

X. Liu et al. / Corrosion Science 51 (2009) 744–751 749

From the cumulative weight loss for N80 mild steel in 3% NaClsolutions saturated by CO2 with or without 1% quartz sand at 5 m/spresented in Fig. 11, it can be observed that the inhibitors are effec-tive in reducing cumulative weight loss. Generally, the weight lossvalues for the coupons in inhibited and uninhibited liquid/particletwo-phase flow systems are larger than those in single liquid phaseflow. This is due to the damage on the film formed on the surface ofthe metal by the particles in the multiphase system therebyincreasing the corrosion rate. This is in accordance with previousreports [13,20]. Comparing the cumulative weight loss for thetwo inhibitors, AQI-11 was found to inhibit the weight loss andconsequently, the corrosion rate than AEI-11.

3.3. Surface morphology

The surface morphologies of N80 mild steel in 3% NaCl solutionssaturated with CO2 in the absence and presence of the imidazolinesat 5 m/s is presented in Fig. 12. The features of the samples in theabsence of inhibitor (Fig. 12a) are loose corrosion product layerwith lots of cracks. But, the corrosion product layer became denserwith the presence of inhibitors (Fig. 12b and c), which suggestedthat the defects on the metal surface were gradually blocked. Thescales in the presence of the inhibitors provide some level of pro-tection which is evident in the electrochemical measurementsand reduction in the cumulative weight loss. In the presence of

0

2

4

6

8

10

12

14

16

Blank AE

Cum

mul

ativ

e m

ass

loss

/(mg/

cm2 )

Fig. 11. Cumulative weight loss for N80 mild steel in 3% NaCl CO2-saturated solutio

1% sand (Fig. 13) rougher surfaces with pits were observed show-ing the damage caused by the impact of the sand. Some inhibitorsmolecules impacted by the sand were easily stripped away fromthe metal surface by the sand particles and creates on uneven sur-face which may be favourable for pit initiation. From both Figs. 12and 13, it could be seen that the quaternary imidazoline amine,with larger surface activity, exhibits a much more dense and intactlayer than AEI-11 in both single liquid phase and liquid/particletwo-phase flow.

3.4. Adsorption mechanism

Fig. 1 gives the molecular structures of the two tested inhibi-tors. Both compounds have C-11 saturated hydrophobic group at-tached to the C2 atom and an ethylamino pendent groupattached to N1. In addition to the C-11 and the ethylamino groups,the quaternary imidazone amine (AQI-11) have an ethylcarboxylicgroup attached to the N1 atom giving it a positive charge. Theeffectiveness of the compounds as inhibitors can be ascribed tothe adsorption of the molecules on the metal surface. The inhibi-tors can be adsorbed on the metal surface by the formation of aniron–nitrogen co-ordination bond, by a p-electron interaction be-tween the p-electron in the head group and iron, and by coulombicattraction (physical adsorption). AQI-11 was more effective thanAEI-11 because the compound can be adsorbed on the mild steelsurface by electrostatic interaction between the negative chargeon the metal surface (as a result of the specific adsorption of Cl-)and the positive charge on the quaternary amine cation, and alsoby interaction of the p-system as well as the formation of Fe–Nco-ordination bond, while AEI-11 can only be adsorbed by thep-electron interaction and by the formation of Fe–N co-ordinationbond. The presence of oxygen atom in AQI-11 structure may alsocontribute an adsorption site for the interaction of the moleculeon the mild steel surface making the quaternary imidazoline aminea better inhibitor.

3.5. Quantum-chemical calculations

In literatures the inhibition efficiencies of inhibitors with differ-ent chemical structures have been correlated with the quantum-chemical parameters such as HOMO (highest occupied molecularorbital), LUMO (lowest unoccupied molecular orbital) and the en-ergy gap between the LUMO and HOMO (DE = ELUMO � EHOMO)[6,8,21–24]. High EHOMO (less negative) is associated with thecapacity of a molecule to donate electrons to an appropriated

I-11 AQI-11

Single liquid phase flow

Liquid/particle two-phase flow

n with or without 1% sand at 5 m/s in the absence and presence of inhibitors.

Fig. 12. Erosion–corrosion morphology of N80 mild steel in 3% NaCl solutionsaturated with CO2 in the (a) absence (blank) and presence of (b) AEI-11 and (c)AQI-11 at 5 m/s.

Fig. 13. Erosion–corrosion morphology of N80 mild steel in 3% NaCl solutioncontaining 1% sand saturated with CO2 in the (a) absence (blank) and presence of(b) AEI-11 and (c) AQI-11 at 5 m/s.

Table 4The HOMO and LUMO energies, the energy gap between the LUMO and HOMO (DE) oftwo inhibitors.

Molecule EHOMO (eV) ELUMO (eV) DE (eV)

AEI-11 �8.74 1.70 10.44AQI-11 �3.27 �1.93 1.34

750 X. Liu et al. / Corrosion Science 51 (2009) 744–751

acceptor with empty molecular orbitals [8], that facilitate theadsorption process [25–26] and therefore indicates better perfor-mance of the corrosion inhibitor [21–27].

The molecular modelling package Chemoffice 2004 was used toconstruct and view all molecular structures and the output ofchemical properties for the imidazolines used for this investiga-tion. Molecular geometry was optimized with the AM1 method.Molecular orbital calculations were carried out within the semi-empirical quantum chemical parametric method 3(PM3) modelusing the program package MOPAC available from Chemoffice2004. The HOMO and LUMO energies, as well as the energy gap be-tween the LUMO and HOMO (DE) for the inhibitors were deter-mined and are as shown in Table 4. The calculated results

indicated that EHOMO of the quaternary imidazoline amine (AQI-11) is less negative than that of AEI-11 and thus exhibits betterinhibition efficiency. Also, AQI-11 has a lower energy gap (DE) va-lue than AEI-11. Low values of DE will provide good inhibition effi-ciencies, because the excitation energy to remove an electron from

X. Liu et al. / Corrosion Science 51 (2009) 744–751 751

the last occupied orbital will be low [6,28] and confirms the betterinhibition efficiency of AQI-11.

The location of HOMO in a molecule indicates the preferredsites for electrophilic attack and thus the orientation of the adsor-bate on the metal surface. The HOMO location in the molecules ismostly distributed in the imidazoline rings which is oriented to-wards the surface of the metal. Similar view is held by Jorancicevicet al. [6] and Martınez and Stagljar [29] and the long chain hydro-phobic hydrocarbon tail could be associated with the formation ofa protective film that reduces drastically the corrosion process[30].

4. Conclusions

This work has shown that the CO2 corrosion and corrosion inhibi-tion of N80 mild steel in 3% NaCl solution by 2-undecyl-1-aminoethylimidazoline (AEI-11) and 2-undecyl-1-aminoethyl-1-hydroxyethylquaternary imidazoline (AQI-11) are strongly related to flowcondition and the presence of sand particles. The flow system candegrade the inhibitor performance and increase the corrosion ratesbecause of intense mechanical abrasion. Moreover, multiphase flowmakes the corrosion worse due to the sand impact.

The effectiveness of the compounds as inhibitors was ascribedto the adsorption of the molecules on the metal surface. AQI-11was more effective than AEI-11 due to additional electrostaticinteraction and the presence of oxygen atom which contributesan adsorption site for the interaction of the molecule on the mildsteel surface.

The calculated higher EHOMO and lower energy gap (DE) valuesof AQI-11 supported the experimental results that AQI-11 per-forms better than AEI-11 as a corrosion inhibitor.

Acknowledgements

The authors acknowledge the financial support of the NationalNatural Science Foundation of China (50271078). P. C. Okaforacknowledges the Chinese Academy of Sciences (CAS) and theAcademy of Sciences for the Developing World (TWAS) for theCAS-TWAS Postdoctoral Fellowship to IMR.

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