Tafel pourbaix

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Materials Chemistry and Physics 112 (2008) 290300

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Materials Chemistry and Physics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s

Application of electrochemical frequency modulation for monitoringcorrosion and corrosion inhibition of iron by some indolederivatives in molar hydrochloric acid

K.F. KhaledElectrochemistry Research Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt

a r t i c l e i n f o

Article history:

Received 27 April 2008Received in revised form 14 May 2008Accepted 23 May 2008

Keywords:

EFMCorrosion inhibitionEISMolecular simulationQuantum chemical calculation

a b s t r a c t

The corrosion inhibition effect of four indole derivatives, namely indole (IND), benzotriazole (BTA), ben-zothiazole (BSA) and benzoimidazole (BIA), have been used as possible corrosion inhibitors for pure ironin 1 M HCl. In this study, electrochemical frequency modulation, EFM was used as an effective methodfor corrosion rate determination in corrosion inhibition studies. By using EFM measurements, corrosioncurrent density was determined without prior knowledge of Tafel slopes. Corrosion rates obtained usingEFM, were compared to that obtained from other chemical and electrochemical techniques. The resultsobtained from EFM, EIS, Tafel and weight loss measurements were in good agreement. Tafel polarizationmeasurements show that indole derivatives are cathodic-type inhibitors. Molecular simulation studieswere applied to optimize the adsorption structures of indole derivatives. The inhibitor/iron/solvent inter-faces were simulated and the adsorption energies of these inhibitors were calculated. Quantumchemicalcalculations have been performed and several quantum chemical indices were calculated and correlatedwith the corresponding inhibition efficiencies.

2008 Elsevier B.V. All rights reserved.

1. Introduction

It is well known that iron and iron-based alloys are used mostwidely in industry. Consequently, great attention has been paid tostudies on the corrosion of iron and its alloys. Acid solutions areextensively used in industry, the most important of which are acidpickling, industrialacid cleaning, acid-descaling and oil wellacidiz-ing. The commonly used acids are hydrochloric acid, sulfuric acid,nitricacid,etc.Sinceacidsareaggressive,inhibitorsareusuallyusedto minimize the corrosive attack on metallic materials. Inhibitorsare widely used in the corrosion protection of metals in severalenvironments[1].

Electrochemical frequency modulation, EFM is used as a new

technique for corrosion rate measurements[24].In EFM, two acvoltage waveforms are summed and applied to an iron electrode.The frequencies of the two sinusoidal waveforms must share nocommon factors. Normally, 2 and 5 Hz are suitable frequencies forEFM. While simple in concept, EFM yields an impressive amountof information on the corrosion process including corrosion rate,Tafel constantsand causalityfactors. The corrosionrateis calculatedfrom the corrosion current density that is measured by EFM[3].It

E-mail address:[email protected].

is worth emphasizing that the Tafel constants are not needed tomeasure the corrosion current density. For this reason, EFM enjoysa significant practical advantage compared to polarization tech-niques. EFM provides an independent measure of the anodic andcathodic Tafel constants. The causality factors are used to validatethe data. They are similar to an internal check of the consistency ofthe measurement process. There are two causality factors 2 and 3;if the calculated causality factors are higher than 2 and 3, respec-tively, the quality of the measured data is not valid. Additionally,EFM may be less susceptible to errors in applied potential from iReffects than polarization measurements.

Bogaertset al. [3,4] proposedEFMasanoveltechniqueforonlinecorrosion monitoring. In this technique, current responses due to

a potential perturbation by one or more sine waves are measuredat more frequencies than the frequency of the applied signal, forexample at zero, harmonic and intermodulation frequencies. Thissimple principle offers various possibilities for corrosion rate mea-surements. With this novel EFM technique, the corrosion rate canbe determined from the corrosion system responses at the inter-modulation frequencies. The EFM approach requires only a smallpolarizing signal, and measurements can be completed in a shortperiod of time. EFM can be used as a rapid and non-destructivetechnique for corrosion rate measurements without prior knowl-edge of Tafel constants. EFM can be used successfully for corrosionrate measurements undervarious corrosionconditions, suchas iron

0254-0584/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.matchemphys.2008.05.056
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K.F. Khaled / Materials Chemistry and Physics 112 (2008) 290300 291

Fig. 1. Chemical structures for indole derivatives.

in acidic environment without and with inhibitors and mild steelin a neutral environment. However, Mansfeld and co-workers[5]evaluate the EFM technique for several corrosion systems includ-ing active and passive systems and found that EFM measurementscan be applied successfully only for a limited number of corrosionsystems with fairly high corrosion rates and the technique shouldbe usedonly with great caution for corrosion monitoring. Recently,Hanetal. [6], setupasystemforEFMmeasurementfortheearlycor-rosion rate of mild steel in see water system. It was shown that thecorrosion rates determined with EFM technique were higher than

the real values, especially at longer exposed times when diffusion-controlled effect became obvious. But they were still at the sameorder of magnitude as those obtained with polarization curve anal-ysis or weight loss method.

In continuation of our previous work on evaluation for EFM asa new non-linear distortion technique as well as the developmentof new corrosion inhibitors for acidic solutions[2,710].Corrosioninhibition data will be calculated using EFMtechnique andit will becompared with that obtained from traditional chemical and elec-trochemical techniques like weight loss, Tafel polarization and EISmeasurements.

In this work, EFM measurements were used to study the corro-sion and corrosion inhibition of iron in 1M HCl using some indolederivatives namely, indole (IND), benzotriazole (BTA), benzothia-zole (BSA) and benzoimidazole (BIA), in order to evaluate the EFMas a new approach for online corrosion rate monitoring in iron/HClsystems, which might be helpful for online application of elec-trochemical techniques. Also, molecular simulation and quantumchemical calculations were used to study the adsorption of thesecompounds on iron surface.

2. Experimental

Experiments werecarried out using pureiron (Puratronic 99.9999%) fromJohn-son Mattey Ltd., as the electrode material. Iron rods were mounted in Teflon withsurface area of0.28cm2 andare usedfor electrochemical measurements.Iron sheetswith dimensions5.0 cm1.0 cm0.1 cm with totalsurfacearea equals 11.2cm2 areused for weight loss measurements. The surface was abraded using emery papersof (180, 120, 0, 4/0) grit size, polished with Al2O3 (0.5m particle size), cleanedin 18 M cm water in an ultrasonic bath, and subsequently rinsed in acetone and

bidestiled water.A conventional electrolytic cell, as described elsewhere [11],was used for all

experiments with a platinum counter electrode and a saturated calomel electrode(SCE) as a reference electrode. All reported potential values are on the SCE scale.Electrochemical experiments were carried out under static conditions at 25 1 Cinaeratedsolutions,withafineLuggincapillarytipcloselyplacedtominimizeohmicresistance.

The structuresof the heterocycliccompounds studied arepresented in Fig. 1. Allcompounds investigated were obtained from Aldrich chemical co. They were put in1 M HCl (Fisher Scientific)without pretreatment at concentrationof 104 M,103 M,5103 M and 102 M. Theelectrode wasimmersed in these solutionsfor onehourbefore starting measurements.

EFMmeasurementscarriedout usingtwo frequencies of2 and5 Hz.The basefre-quencywas 1 Hz.EIS measurementswerecarriedout ina frequencyrangeof 100kHzto 50mHz with amplitude of 5mV peak-to-peak using ac signals at respectivecorrosion potentials. Polarization curves were obtained by changing the electrodepotentialautomaticallyfrom(250mVSCE to+250mVSCE)attherespectivecorrosion

potentials with a scan rate of 1 mV s1

.

Electrochemical measurements were performed with a Gamry InstrumentPotentiostat/Galvanostat/ZRA.Theseinclude Gamryframeworksystembasedon theESA400, Gamry applications that include EFM140 to perform EFM measurements,EIS300 for electrochemical impedance spectroscopy measurements and DC105 fordc corrosion measurements along with a computer for collecting the data.

For weight loss measurements, the iron coupons were left hanged in the testsolutionfor6 h at 251 C before recording theloss of their weights. Thecorrosionrate was calculated, in milligram per square centimeter per hour (mgcm2 h1),on the basis of the apparent surface area. The inhibition efficiencies calculationswere based onthe weightloss measurementsat theendof theexposure period. Theresults of theweight loss experimentsare themean of three runs, each with a freshiron coupon and fresh acid solution.

Forapplyingmolecularmodelingtechniques to thesecompounds, theiron crys-talwas cleavedalong with(0 0 1)plane, thusrepresentingtheiron surface.The liquidphase consisted of 400 water molecule and a single dissolved indole derivative. Onthetop of this aqueous layer, an additional layer of 200 water molecule servesas anupperlimitfortheaqueouslayer actinglikea wall with thesamephysical andchem-ical properties. To obtain this configuration we use Accelyrs molecular dynamicssoftware with boundary conditions described elsewhere[12].

Molecular orbital calculations (MO) are based on the semi-empirical self-consistent methods (SCF). A full optimization of all geometrical variables (bondlengths, bond angles and dihedral angles) without any symmetry constraint wasperformed at the restricted Hartree-Fock level (RHF). We used PM3, AM1, MONDOand MINDO/3 semi-empirical SCF-MO methods in the Hyperchem 8.03 program,implemented on an Intel Pentium IV, 3.6 GHz computer.

3. Results and discussion

3.1. Electrochemical frequency modulation (EFM)

Electrochemical frequency modulation is a nondestructive cor-rosion measurement technique that can directly give values of thecorrosion current without prior knowledge of Tafel constants. LikeEIS, it is a small signal ac technique. Unlike EIS, however, two sinewaves (at different frequencies) are applied to the cell simulta-neously. Because current is a non-linear function of potential, thesystem respondsin a non-linear wayto the potential excitation. Thecurrent response contains not only the input frequencies, but alsocontains frequency components which are the sum, difference, andmultiplesofthetwoinputfrequencies.Thetwofrequenciesmaynotbe chosen at random. They must both be small, integer multiplesof a base frequency that determines the length of the experiment.

Fig. 2shows representative examples for the waveform when thetwo input frequencies are 2 and 5 Hz.

The higher frequency must be at least two times the lower one.The higher frequency must also be sufficiently slow that the charg-ing of the double layer does not contribute to the current response.Often, 10 Hz is a reasonable limit. Intermodulation spectra obtainedfrom EFM measurements are presented in Fig. 3.Each spectrumis a current response as a function of frequency. The two largepeaks,withamplitudesofabout200microampere,aretheresponseto the 2 and 5Hz excitation frequencies. Those peaks between1 microampere and 20 microampere are the harmonics, sums, anddifferences of the two-excitation frequencies. These peaks are usedby the EFM140 software package to calculate the corrosion cur-rentandtheTafelconstants.Itisimportanttonotethatbetweenthe

peaks thecurrentresponse isverysmall.There isnearly noresponse


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292 K.F. Khaled / Materials Chemistry and Physics 112 (2008) 290300

Fig. 2. Wave form spectrum for the input frequencies 2 and 5 Hz for iron in 1 M HCl in absence and presence of various concentrations of indole derivatives.

(


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Fig. 3. Intermodulation spectrum for iron in 1 M HCl in absence and presence of various concentrations of indole derivatives.

Table 1shows the corrosion kinetic parameters such as inhibi-tion efficiency (EEFM%), corrosion current density (A cm2), Tafelconstants (a, c) and causality factors (CF-2, CF-3) at differentconcentration of indole derivatives in mol L1 HCl at 251 C.

It is obvious fromTable 1that, the corrosion current densitiesdecrease with increase in concentrations of these compounds. Theinhibition efficiencies increase with increase in indole derivativesconcentrations. The causality factors inTable 1are very close totheoretical values which according to the EFM theory [3] shouldguarantee the validity of Tafel slopes and corrosion current den-

sities. Inhibition efficiency (EEFM%) depicted inTable 1calculatedfrom the following equation.

EEFM% =

1 icorri0corr

100 (6)

wherei0corrand icorrare corrosion current densities in the absenceand presence of the studied compounds, respectively.

Thegreatstrength of theEFM is thecausality factors which serve

as an internal check on the validity of the EFM measurement [4].


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Table 1

Electrochemical kinetic parameters obtained by EFM technique for iron in absence and presence of various concentrations of indole derivatives in 1 M HCl at 25 1 C

Inhibitor Concentration (M) icorr(A cm2) a(mVdec1) c(mVdec1) EEFM% C.F-2 C.F-3

Benzoimidazole(BIA)

0 177.8 89.81 278.4 1.97 1.02104 131.6 112.8 196.9 25.9 2.03 2.59103 92.5 106 189.1 47.9 1.70 2.955103 67.6 99.37 196.9 61.9 1.70 1.92102 55.12 98.12 195.9 68.9 1.92 2.1

Benzothiazole(BSA)

104 120.9 108.4 197.1 32.0 2.00 2.27103 80.01 118.9 196.4 55.0 1.75 2.0215103 58.7 99.05 189.1 66.9 1.93 1.19102 39.12 89.411 196.52 77.9 2.01 2.25

Benzotriazole(BTA)

104 106.7 109.6 196.8 39.9 1.99 2.15103 62.23 99.12 194.0 65.0 1.92 2.925103 37.34 108.1 189.9 78.9 1.80 2.32102 24.9 97.97 187.01 85.9 1.30 1.93

Indole(IND)

104 97.8 99.7 175.7 44.9 1.49 1.51103 44.45 98.5 181.5 75.0 1.45 1.925103 30.23 108.0 187.2 82.9 2.02 3.02102 8.9 96.82 196.02 94.9 2.06 1.98

Fig. 4. Nyquist plots for iron in 1M HCl in absence and presence of various concen-trations of BIA.

With the causality factors the experimental EFM data can be veri-fied. Thecausality factors in Table 1 indicate thatthe measured dataare of good quality. The standard values for CF-2 and CF-3 are 2.0and 3.0, respectively. To evaluate the EFM technique as an effective

Fig. 5. Nyquist plots for iron in 1M HCl in absence and presence of various concen-

trations of BSA.

Fig. 6. Nyquist plots for iron in 1M HCl in absence and presence of various concen-trations of BTA.

corrosion monitoring technique, several traditional corrosion tech-niques implied to study the corrosion inhibition of iron by indolederivativesin 1 M HCl solutions. EIS, Tafel extrapolation andweightloss measurementsare usedto calculatethe inhibition efficiencyaswell as other corrosion kinetic parameters.

Fig. 7. Nyquist plots for iron in 1 M HCl in absence and presence of various concen-

trations of IND.


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Fig. 8. Equivalent circuit used to model impedance data in 1 M HCl solutions, OHP,outer Helmholtz plane; Cdl, double layer capacitance; Rs, solution resistance; Rct,charge-transfer resistance.

3.2. Electrochemical impedance spectroscopy

Nyquist plots recorded forironelectrodein 1 M HClsolutionandcontaining various concentrations of indole derivatives are showninFigs. 47.As can be seen from Figs. 47,the Nyquist plots do

not yield perfect semicircles as expected from the theory of EIS.The deviation from ideal semicircle was generally attributed tothe frequency dispersion[13]as well as to the inhomogenities ofthe surface and mass transport resistant[7].In the evaluation ofNyquist plots, the difference in real impedance at lower and higherfrequencies is generally considered as charge-transfer resistance.The resistances between the metal and outer Hemlholtz plane(OHP) must be equal to the Rct.

Fig. 9. Anodic and cathodic Tafel polarization curves for iron in the absence andpresence of various concentrations of compound BIA in 1 M HCl.

Figs. 47clearly demonstrate that the shapes of the impedance

plots for inhibited electrodes are not substantially different fromthose of uninhibited electrodes. The presence of indole derivativesincreases the impedance (the semicircle diameter) but does notchange other aspects of the corrosion behavior. The impedancespectra of inhibited solutions consist of one depressed semicirclewith a considerable deviation from an ideal semicircle. Inhibitormolecules adsorb on the iron surface and modify the interface. Theadsorption of inhibitormoleculeson the metalsurfacedecreasesitselectrical capacity because they displace the water molecules andother ions originally adsorbed on the metal surface [14]. This mod-ification results in an increase of charge-transfer resistance. TheRct values increased with indole derivatives concentrations maysuggest the formation of a protective layer on the iron electrodesurface. This layer makes a barrier for mass and charge-transfer.

The double layer can be represented by the electrical equivalentcircuit diagrams to model metal/solution interface. The corre-sponding electrical equivalent circuit model for iron in uninhibitedsolution is given in Fig. 8 [7]. According to Fig. 8, the charge-transfer resistance, which corresponds to the diameter of Nyquistplot, determines the corrosion rate and represents the resistancebetween the metal/OHP (outer Helmholtz plane). Fig. 8describesthe potential distributions on the metal/solution interface and pro-

Table 2

Circuit element Rs , Rct, n and CPE values obtained using equivalent circuit in Fig. 8for iron in 1 M HCl and different concentrations of indole derivatives in molL1 HCl at251 C

Inhibitor Concentration (M) Rs( cm2) Rct( cm2) n CPE/Cdl(F cm2) Eimp(%)

Benzoimidazole(BIA)

0 2.1 595.1 0.83 93.52

104 3.2 708.3 0.89 22.5 16.0103 3.4 901.5 0.91 17.7 34.05103 3.2 1101.8 0.84 14.4 46.0102 3.3 1322.2 0.85 12.1 55.0

Benzothiazole(BSA)

104 2.1 743.7 0.83 21.4 20.0103 2.1 1081.8 0.83 14.7 45.05103 2.5 1239.6 0.81 12.8 52.0102 5.1 1700.0 0.79 9.37 65.0

Benzotriazole(BTA)

104 1.9 793.3 0.84 20.1 25.0103 2.8 1239.6 0.83 12.8 52.05103 3.1 1652.7 0.82 9.63 64.0102 3.5 2203.7 0.78 7.22 73.0

Indole(IND)

104 2.1 875.0 0.91 18.2 32.0103 1.9 1608.12 0.90 9.9 63.05103 2.5 2051.72 0.92 7.7 71.0102 4.5 4576.92 0.90 3.5 87.0


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Fig. 10. Anodic and cathodic Tafel polarization curves for iron in the absence andpresence of various concentrations of compound BSA in 1M HCl.

Fig. 11. Anodic and cathodic Tafel polarization curves for iron in the absence andpresence of various concentrations of compound BTA in 1 M HCl.

posedelectricalequivalent circuit diagram for the corrosion systemblank solution.

The values of impedance parameters determined from Nyquistplots, such as Rctand Eimp% are listed in Table 2. It is apparent from

Fig. 12. Anodic and cathodic Tafel polarization curves for iron in the absence andpresence of various concentrations of compound IND in 1 M HCl.

Table 2that the impedance of the inhibited system amplified withincreasing the inhibitors concentration and CPE values decrease

with increasing the inhibitors concentrations. This decrease inCPE results from a decrease in local dielectric constant and/or anincrease in the thickness of the double layer, suggesting that indolederivatives inhibit the iron corrosion by adsorption at iron/acidinterface[15].It is well known that the capacitance is inverselyproportional to the thickness of the double layer [16]. A low capac-itance may result if water molecules at the electrode interface arelargely replaced by organic inhibitor molecules through adsorp-tion [16]. The larger inhibitormoleculesalso reduce the capacitancethrough the increase in the double layer thickness. The thicknessof this protective layer increases with increase in inhibitor con-centration. This process results in a noticeable decrease in CPE/Cdl.This trend is in accordance with Helmholtz model, given by thefollowing equation:

Cdl=0A

d (7)

whered is the thickness of the protective layer, is the dielectricconstant of the medium, 0 is the vacuum permittivity and A isthe effective surface area of the electrode. The value of CPE/Cdl isalways smaller in the presence of the inhibitor than in its absence,as a result of the effective adsorption of the inhibitor. It is apparent

Table 3

Electrochemical kinetic parameters obtained by Tafel polarization technique for iron in absence and presence of various concentrations of indole derivatives in mol L1 HClat 251 C

Inhibi tor Concentration (M) jcorr(A cm2) Ecorr(mV) a(mVdec1) c(mVdec1) ETafel(%)

Benzoimidazole(BIA)

0 165.4 507.3 105.2 232

104 130.60 506.1 95.35 200 21.17103 94.3 517.1 100.1 229.7 43.005103 67.8 531.2 114.3 227.3 59.02102 61.2 533.4 114.5 235.0 63.01

Benzothiazole(BSA)

104 119.1 512.4 99.05 203.8 28.01103 79.4 520.1 83.05 221.2 52.015103 61.2 525.3 102.7 199.8 63.01102 41.4 533.7 104.3 186.7 74.97

Benzotriazole(BTA)

104 109.2 502.2 92.31 196.0 34.00103 64.5 514.5 103.5 206.4 61.015103 44.6 521.3 105.2 219.2 73.04102 31.4 526.7 105.8 214.2 81.02

Indole(IND)

104 100.8 504.0 108.7 203.3 39.07103 52.9 506.5 102.1 200.8 68.025103 34.7 505.3 109.8 217.2 79.02

102

13.2 510.3 109.9 222.5 92.02


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Table 4

Corrosion rate in (mgcm2 h1), inhibition efficiency data obtained from weight loss measurements iron in absence and presence of various concentrations of indolederivatives in mol L1 HCl at 251 C

Inhibitor Concentration (M) Weight loss (mg) Corrosion rate (mg cm2 h1) Ew(%)

Benzoimidazole(BIA)

0 57 0.848 104 46.74 0.695 18.03103 37.05 0.551 35.035

103 29.64 0.441 48.01

102 25.08 0.373 56.02

Benzothiazole(BSA)

104 43.89 0.653 23.03103 29.64 0.441 48.015103 25.65 0.382 55.02102 18.24 0.271 68.0

Benzotriazole(BTA)

104 40.47 0.602 29.03103 26.22 0.390 54.015103 18.81 0.279 67.01102 14.25 0.212 75.00

Indole(IND)

104 37.05 0.551 35.03103 19.95 0.296 65.015103 15.39 0.229 73.01102 6.84 0.102 88.00

that causal relationship exists between adsorption and inhibition.The inhibitionefficiencywas calculated usingcharge-transfer resis-tance as follows:

Eimp% =

1 R0ct

Rct

100 (8)

whereR0ct and Rct arethe charge-transfer resistances foruninhibitedand inhibited solutions, respectively.

3.3. Tafel polarization measurements

Polarization curves for iron in 1M HCl in the absence and pres-ence ofBIA, BSA, BTA and IND ofvarious concentrations at 251 C

are shown inFigs. 912.As can be seen the cathodic curves weremore polarized than anodic curves where the cathodic reactionis remarkably affected by the inhibitors, whereas the anodic oneis slightly shifted toward lower currents. Electrochemical kineticparameters obtained by Tafel polarization technique for iron inabsence and presence of various concentrations of indole deriva-tives in 1M HCl at 251 C are listed inTable 3.As can be seen inTable 3,higher cvalues revealed that cathodic reduction rate wasretarded[17].This indicated that the indole derivatives influencedcathodic reaction more than anodic reaction and hence the addi-tion of these compounds controls the rate of hydrogen evolutionreaction on iron surface. BIA, BSA, BTA and IND are considered ascathodic inhibitors due to the negative shift in thecorrosion poten-tial andnoticeable decrease of thecathodiccurrent byadding themto 1 M HCl solution. The inhibition efficiency was evaluated fromTafel polarization measurements and listed in Table 3using thefollowing equation:

ETafel% =

1 jcorrj0corr

100 (9)

where j0corr and jcorr are the corrosion current densities for unin-hibited and inhibited solutions, respectively. It is clear from Table 3that, corrosion current densityjcorrdecreased by addition of indolederivatives in 1 M HCl. The obtained inhibition efficiencies given inTable 3show that indole derivatives act as effective inhibitors.

It is obvious fromTable 3that the slopes of the anodic (a) andcathodic (c) Tafellines remainalmost unchangedupon addition ofindolederivatives.Thustheadsorbedinhibitorsactbysimpleblock-

ing of active sites for both anodic and cathodic processes. In other

words, the inhibitors decrease the surface area for corrosion with-out affecting the corrosion mechanism of iron in 1 M HCl solution,and only causes inactivation of a part of the surface with respecttothe corrosive medium[18].

Fig. 13. Comparison of inhibition efficiencies obtained with EFM, EIS, Tafel extrap-

olation and weight loss measurements for indole derivatives.


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Fig. 14. The amorphous cell containing the iron substrate, the solvent molecule and indole derivatives.

3.4. Weight loss measurements

Theweight loss of iron coupons in 1 M HCl without and withtheaddition of indole derivatives was used to calculate the inhibitionefficiencyEw% by using the following equation:

Ew% =w0 ww0

100 (10)

wherew0and w are the weight loss of iron coupons in HCl solutionwithout and with the addition of indole derivatives, respectively.

Table 4 shows the values of inhibition efficiencies and corrosionrates obtained from weight loss method at different concentrationsof BIA, BSA, BTA and IND in 1M HCl at 251 C. The amount ofweight loss is found to decrease with increasing additive concen-trationsfor these compounds. It is obvious from Table 4 that, indole

derivatives inhibit the corrosion of iron in 1 M HCl solutions at allconcentrations used in this study. The weight loss measurementsrevealedthe excellent stabilityof theinhibitors in theacid medium.To assess the stability, the experiments were conducted by takingacid solution containing inhibitor which was kept for10 days underair agitation and no change in inhibition efficiency values wereobserved.

Fig. 13 represents comparisons of the inhibition efficienciesobtained for IND, BTA, BSA and BIA at concentrations (102 M,

103

M) using EFM, EIS, Tafelpolarization andweight loss measure-ments. The calculated inhibition efficiency obtained from weightloss, Tafel polarization and EIS measurements are in good agree-ment with that obtained from EFM measurements. As can be seenin Fig. 13, the corrosion ratesdetermined with EFM technique werehigh, but they were still at the same order of magnitude as thoseobtained with other conventional chemical and electrochemicaltechniques. These results are comparable with that mentioned inthe literature[6].

3.5. Molecular modeling and quantum chemical calculations

Molecular simulation studies were performed to simulate theadsorption of thefour indolederivatives on theiron surface in pres-

ence of solvent effects. Molecular structures of indole derivativesshow that it is likely for these molecules to adsorb on iron sur-face by sharing the electrons of nitrogen atoms and/or -electroninteractions of the aromatic rings[7].Both interactions can makeit possible for indole derivatives to form coordinated bond withiron. The adsorption progress of indole derivatives on iron surfaceis investigated by performing molecular mechanics (MM) using

Table 5

Theoretical quantum chemical calculations using semi-emprical SCF-MO method (PM3) in Hyperchem 8.03 program as well as adsorption energy calculations

Inhibitor Molecular area (A2) EHOMO(eV) ELUMO(eV) (D) Volume (A3) Adsorption energy (kJmol1)

Indole (IND) 220.4 8.355 0.1737 2.005 418.1 28.6Benzotriazole (BTA) 215.8 8.889 0.5714 3.846 389.9 26.4Benzothiazole (BSA) 216.9 9.237 0.7321 2.251 409.7 26.1benzoimidazole (BIA) 218.3

9.309

0.07001 3.38 405.2

24.6


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Fig. 15. Structure of indole derivatives, molecular orbital plots and the charge density distribution.

MS Modeling Software. As the three kinds of Fe surfaces (110,100, 111), Fe (111) and Fe (100) surfaces have relatively openstructures while Fe (1 1 0) is a density packed surface and has themost stabilization, so we choose Fe (1 1 0) surface to simulate theadsorption process[19].The periodic boundary conditions (PBC)are applied to the simulation cell. The size of simulation box is21.3 21.3 21.3. The force field used in the current MM isCOMPASS (condensed phase optimized molecular potentials foratomistic simulation studies) force field. All molecules are energyoptimized, iron surface and solvent layers was constructed using

the amorphous cell module, the whole system was energy opti-mized and the possibility of indoles adsorption on the iron surfacewere simulated as inFig. 14.

Fig. 14shows the different steps of indole derivatives adsorp-tion on iron surface with its different configurations. Through thesimulationprocess, the configurations of the fourindole derivativeschanged greatly. The four indole derivatives move towards the ironsurface which enhances the adsorption mechanism of their actionas corrosion inhibitors. These molecules adsorbed on the iron sur-face through the heteroatoms (nitrogen and/or sulphur). It is foundthat through the simulation course, the benzene ring fluctuate upanddowntheironsurfacewhiletherestofthemoleculeisattachedto the iron surface. The mean adsorption energy obtained from themolecular simulation were calculated and tabulated inTable 5.Ascan beseenfrom Table 5, the adsorption energy for indole molecule

was the highest and equals 28.6kJmol1. High values of adsorp-tion energy indicates that the indole molecule will give the highestinhibition efficiency which is consistent with the inhibition effi-ciencyobtainedfrom the chemical and electrochemicaltechniques.Theclosecontactsbetweenindole (IND) andironsurface are showninFig. 14,indicate that the adsorption of indole occurred throughthenitrogenatom andenhanced by theparallel benzene ring to theiron surface.

The adsorption of these inhibitor molecules on the iron surfacecan be explained on the basis of the donor acceptor interaction

between electrons of donor atoms N, S and aromatic rings of theinhibitors and the vacant d orbitals of iron surface atoms[2022].All investigated compounds are bicyclic containing a benzene ringfused to a five-membered heterocyclic ring without substituent(Fig. 1).So the influence of the chemical structure is limited to themoleculararea in theadsorbed state, because it determines theareaof the metal, shielded by the inhibitor.The approximate area, occu-pied by these molecules at planar adsorption has been calculatedby the approximate method[23,24]and presented inTable 5.Theapproximatemethod is fast, andgenerally accurate within10% foragiven set of atomic radii. As inTable 5the approximate area valuesarevery close to each other and hence this parameterdoes notplaya decisive role when comparing the inhibition properties of thesecompounds, the same results were given in calculating the molec-ular volume. This different behavior should be therefore prescribed


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