icro

eimtrol61, S

Keywords:Mild steel

entbonmol suinv

adsorption energy followed the order: VI >AI. Theoretical conclusions were subsequently validatedexperimentally using potentiodynamic polarization, linear polarization resistance, electrochemicalimpedance spectroscopy, and surface analytical techniques (SEM and AFM).

ively iide ap

ce, in oing, ac

el [2022]. Suchantum chemicalmulations. Theset modern tool into make great

addition, the useition efcacies ofals and alloys andy is scarce in the

Journal of Industrial and Engineering Chemistry 21 (2015) 13281339

Contents lists available at ScienceDirect

Journal of Industrial and

journal homepage: www.etreatment. In this case, the exposure of these metals to aqueousacidic solution causes corrosion. The use of inhibitors is one of themost practical approaches to protect steel from corrosion in theseacidic environments [15]. Organic inhibitors containing polargroups (including N, S and O), heterocyclic compounds with polarfunctional groups, and conjugated double bonds can effectivelyinhibit the corrosion of steel, due to their chelating action and theformation of an insoluble physical diffusion barrier on thesubstrate surface [610]. The existing data reveal that mostorganic inhibitors act by adsorption on the metal surface. Thisadsorption is inuenced by the nature and surface charge of metal,the type of aggressive electrolyte and the chemical structure of

zoles as corrosion inhibitors are relatively few anvital to the detailed understanding of the minhibition on metal surface at the molecular levstudies can be undertaken by the use of qucalculations supported by molecular dynamics simethodologies are widely reported as importancorrosion inhibition studies and will continueimpact in corrosion science research [2327]. Inof theoretical methodologies in predicting inhiborganic molecules as corrosion inhibitors for metthereafter validate the prediction experimentallliterature [2830].corrosive environments. For instanacidic solution is used for descalIntroduction

Mild steel is the most extenscorrosion studies because of its wDFTVinylimidazoleAllylimidazoleMolecular dynamics simulationsinhibitors [11]. Unfortunately, moently toxic and potential health haeffect of aromatic heterocyclic com

* Corresponding author. Tel.: +966 53341E-mail address: obot@kfupm.edu.sa (I.B

http://dx.doi.org/10.1016/j.jiec.2014.05.0491226-086X/ 2014 The Korean Society ofst organizards, supounds

3506; fax:. Obot).

Industrial 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rightsreserved.

nvestigated metal forplication in differentil industries, aqueousid pickling, and acid

Imidazoles have attracted attention recently because of theirgood environmental prole and their excellent corrosion inhibi-tion performance [1319]. Most of the inhibition performances ofimidazoles were mainly studied by experimental methods, such asweight lossmeasurement, polarization curves and electrochemicalimpedance spectroscopy. However, theoretical studies on imida-

d is increasinglyechanism of itsTheoretical prediction and electrochemvinylimidazole and allylimidazole as corin 1M HCl

I.B. Obot a,*, S.A. Umoren a, Z.M. Gasem a, Rami SulaCentre of Research Excellence in Corrosion, Research Institute, King Fahd University of PebDepartment of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 312

A R T I C L E I N F O

Article history:Received 15 April 2014Received in revised form 27 May 2014Accepted 31 May 2014Available online 11 June 2014

A B S T R A C T

Corrosion inhibition potallylimidazole (AI) for carchemical calculations andmore reactive towards steeFe2O3 (010) surface wasc inhibitors are inher-ch as the carcinogenicon humans [12].

+966 3 860 3996.

and Engineering Chemistry. Publial evaluation ofsion inhibitors for mild steel

an a, Bassam El Ali b

eum and Minerals, Dhahran 31261, Saudi Arabiaaudi Arabia

ials of two imidazole derivatives namely, vinylimidazole (VI) andsteel in 1M HCl at 25 C were predicted theoretically using quantumlecular dynamics (MD) simulations. DFT calculations indicated that VI isrface than AI. Equilibruim adsorption behaviour of VI and AI molecules onestigated using molecular dynamics (MD) simulations. The equilibrium

Engineering Chemistry

l sev ier .com/ locate / j iecIn thepresent communication, quantumchemical calculations inconjuctionwithmolecular dynamics simulations were employed topredict the inhibition potentials of vinylimidazole (VI) andallylimidazole (AI) on carbon steel and to understand the inter-actions between the inhibitor molecules and steel surface. Theinhibitionperformanceof the two imidazole inhibitors formild steelin 1M HCl were subsequently validated experimentally by usingelectrochemical measurements and surface analytical techniques.

shed by Elsevier B.V. All rights reserved.

Experimentals

Materials and sample preparation

The mild steel specimens with the following chemicalcomposition (weight percentage) were used in the experiments:C, 0.073; Mn, 1.36; P, 0.004; Ti, 0.004; S, 0.003; balance Fe. Testcoupons were cut into 3 cm3 cm0.25 cm dimensions. Thesecoupons were abraded with silicon carbide abrasive paper (fromgrade no. 320 to 800), rinsed with distilled water, placed in anultrasonic acetone bath for about 5min to remove possible residueof polishing, rinsed with acetone, dried in warm air, and thenstored in moisture-free desiccators prior to use. The solutions of1MHCl were prepared by dilution of 37% analytical grade HCl withdouble distilled water. The inhibitors tested was vinylimidazoleand allylimidazole obtained from SigmaAldrich as high grade

I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1329reagent and was used without further purication. The employedconcentration range of the inhibitors was 0.0010.01M. Allexperiments were conducted at 25 C. The molecular structuresare dipicted in Fig. 1.

Quantum chemical calculations and molecular dynamics (MD)simulations

Quantum chemical calculations and molecular dynamicssimulations were conducted with Materials Studio 6.0 [31]. Allelectron calculations of VI and AI molecules were accomplished bydensity functional theory (DFT) hybrid (B3LYP) method with adouble zeta plus polarization (DNP) basis set and the choice ofconvergence accuracy was set to ne. Frequency analysis wasperformed to ensure the calculated structure was at the minimumpoint onpotential energy surface (without imaginary frequency). Acontinuum solvation method (COSMO) was employed to study theeffect of solvent (water). DMol3 module code was employed in theDFT calculations. The following quantum chemical parameterswere calculated from the obtained optimized molecular structure:the energy of the highest occupied molecular orbital (EHOMO), theenergy of the lowest unoccupied molecular orbital (ELUMO), theenergy band gap (DE = ELUMO EHOMO), the dipole moment (m),hardness (h), softness (S) and electrophilicity (v). These param-eters were employed to rank the reactive abilities of VI and AI.Mulliken population analysis was used for the presentation ofFukui functions.

Adsorption locator module present in Materials Studio 6.0 wasemployed in the molecular dynamics simulations using simulatedannealing procedure. The adsorption of VI and AI on Fe2O3(010)surface was simulated to nd the low-energy adsorption sites onthe iron oxide surface and to investigate the preferentialadsorption of the studied compounds. The Fe2O3 surface layer

[(Fig._1)TD$FIG]

Fig.1. 2Dmolecular structures of (a) vinyl imidazole (VI) and (b) allylimidazole (AI).was selected for the simulation based on the fact that the ironmetal already oxidized before introducing the acid solution [32]. VIand AI were optimized (i.e., energy minimizing) using the Forcitemodule and the Condensed-Phase Optimized Molecular Potentialsfor Atomistic Simulation Studies (COMPASS) ab initio force eldand the String Matching Algorithms Research Tool (smart) in asimulation box (88.7654.1939.44) with periodic bound-ary conditions to model a representative part of the interfacedevoid of any arbitrary boundary effects. The simulated annealingprocedure uses the Monte Carlo method to determine theadsorption density and binding energy of the adsorbate on thesubstrate. The Fe2O3 (010) surface was rst built and relaxed byminimizing its energy using molecular mechanics; then thesurface area of Fe2O3 (010) was increased, and its periodicitywas changed by constructing a super cell (99). A vacuum slabwith a thickness of 40 was then built on the Fe2O3 (010) surface.The six layers in the structure were chosen so that the depth of thesurface was greater than the nonbonded cutoff used in thecalculations. Once the Fe2O3 (010) surface had been built, VI and AIwere adsorbed on the metal surface using the adsorption locatormodule. Extensive details of the simulation process have beenelaborated elsewhere [3335].

Electrochemical measurements

All electrochemical experiments were performed in a one-compartment cell with three electrodes connected to Gamryinstrument potentiostat/galvanostat/ZRA (Reference 3000) with aGamry framework system based on ESA410. Gamry applicationsinclude softwareDC105 for corrosion, EIS300 for EISmeasurements,and theEchemAnalyst6.0 softwarepackage fordatatting.Themildsteel was the working electrode with an exposed surface area of0.7855 cm2 in the corrosiveenvironment; platinumwirewasusedasthe counter electrode and saturated calomel electrode (SCE) as thereference electrode. All potentials were measured versus the SCEreference electrode. Tafel curves were obtained by changing theelectrode potential automatically from250 to +250mV versus theopen-circuit potential (Ecorr) at a scan rate of 1mVs1. Linearpolarization resistance (LPR) experiments were done from 20 to+20mVversusEcorr at a scan rate of 0.125mVs1. EISmeasurementswere carried out under potentiostatic conditions in a frequencyrange from 100kHz to 100mHz, with an amplitude of 10mV peak-to-peak, using an alternating-current (ac) signal at Ecorr. Allexperiments were measured after immersion for 30min in 1MHCl with and without the addition of the inhibitors.

Surface morphology using scanning electron microscopy (SEM)

Morphological studies of the mild steel electrode surface wereundertaken by SEM examinations of electrode surfaces exposed todifferent test solutions using a JSM-5800 LV scanning electronmicroscope. Mild steel specimens of dimensions 330.25 cm3were abraded successively with silicon carbide paper of differentgrades (no. 320800) and thereafter using cloth with 1mmdiamond paste to a near m irror nished surface. The precleanedcoupons were immersed for 18h in the blank solutions in 1M HClwithout and with highest concentration of VI and AI (0.01M) at25 C, then washed with distilled water, dried in warm air, andsubmitted for SEM surface examination.

Atomic force microscopy (AFM)

Surface topological studies of the mild steel surface wasinvestigated by atomic forcemicroscopy. For AFM analysis themildsteel specimens of size 330.25 cm3 were immersed in the testsolution in the absence and presence of 0.01M inhibitors (VI and

this parameter and inhibition efciency.It has been reported that molecules with atomswith the highest

negative charges are often considered to have the highest tendencytodonate electrons to themetal surface [13]. Therefore, the inhibitoris likely to interact with the metal surface through such atoms.Table 2 shows theMullikenpopulation analysis performed for for VIand AI. The results show that (VI: N1, N2, C7) and (AI: N1, N2, C3, C6,C8)are atomswithnegative charge centers that couldofferelectronsto the iron atoms on themetal surface to forma coordinate bond.Onthe other hand, (VI: C3, C4, C5, C6) and (AI: C4, C5, C7) are atoms ofpositive centers that can accept electrons easily from3dorbital of Featoms to form feedback bonds. It can be seen from Table 2 thatalthough VI has less negative centers than AI, its total negativechargesareabout thesame(VI:1.13)and(AI:1.16).VIontheotherhand, has the highest total positive charges (0.378) as opposed to(0.357) for AI. These high positive centers can be use by VI to acceptelectrons from3dorbital of Fe, thus strengthening thebondbetweenVI and Fe than between AI and Fe.

The Fukui functionwhichmeasures reactivity in a local sense, isby far the most important local reactivity index [46]. Using ascheme of nite difference approximations, this procedurecondenses the values around each atomic site into a single valuethat characterizes the atom in the molecule. With this approxi-mation, the condensed Fukui function becomes:

fk qkN 1 qkNfor nucleophilic attack (1)

fk qkN qkN 1for electrophilic attack (2)where qk is the gross charge of atom k in the molecule, Ncorresponds to the number of electrons in the molecule, with N+1

1330 I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339AI) for 3h at ambient temperature. Then the specimens werecleaned with distilled water, dried, and used for AFM. The AFManalyses were carried out using a 5420 atomic force microscope(AFM) (N9498S) (Agilent Technologies, UK).

Results and discussion

Quantum chemical study

Quantum chemical calculation has beenwidely used to evaluatethe inhibition performance of corrosion inhibitors, which canquantitatively study the relationship between inhibition efciencyand molecular reactivity [3639]. With this method, the capabilityof inhibitor molecules to donate or accept electrons can bepredicted with analysis of global reactivity parameters, such as theenergy gap between HOMO and LUMO, chemical hardness, dipolemoment, and electrophilicity index, etc.

The reactive abilities of VI and AI are closely related to theirfrontier molecular orbtals including the highest occupied molecu-lar orbitals (HOMO) and the lowest unoccupied molecular orbital(LUMO). EHOMO indicates the tendency of an organic molecule todonate electrons. The higher the value of EHOMO, the greater theability of a molecule to donate electrons while ELUMO indicates thepropensity of a molecule to accept electrons. The lower ELUMO is,the greater is the ability of that molecule to accept electrons. Thus,the binding ability of organics to the metal surface increases withan increase in energy of the HOMO and a decrease in the value ofenergy of the LUMO. The energy gap, DE, is an importantparameter which indicates the reactivity tendency of organicstowards the metal surface [40]. As DE decreases, the reactivity ofthe molecule increases, leading to an increase in adsorption onto ametal surface. A molecule with low energy gap is more polarizableand is generally associated with high chemical reactivity and lowkinetic stability. Thus, DE, has been used in literature tocharacterize the binding ability of organics to the metal surface[41]. The reactivity of corrosion inhibitors may also be discussed interms of chemical hardness and softness parameters. Thesequantities are often associated with the Lewis theory of acidand bases and Pearson's hard and soft acids and bases theory [42];a hard molecule has a largeDE and therefore is less reactive; a softmolecule has a smallDE and is thereforemore reactive. Adsorptionoccurs most probably at the region of the molecule where softness(S) has the highest value [43]. In the study of corrosion inhibitorsand their ability to bind to the metal surface, the inhibitor isconsidered as a soft base and the metal surface as a soft acid. Theelectrophilicity index (v), on the other hand denotes the electron-accepting capability of a molecule [44]. High values of v ensuresgreater binding ability of corrosion inhibitors to metal surfaces.

The optimization progress curves for VI and AI are shown inFig. 2., while the frontier molecular orbital density distributionsare presented in Fig. 3. As can be clearly seen in Fig. 3, the HOMOorbitals of VI and AI are entirely on the imidazole ring. In the case ofLUMO orbitals, the distribution is mainly on the imidazole ring ofVI whereas for AI it is on the allyl group. This is an indication of theavailability of more adsorption centers for VI than AI. Theelectronic parameters related to the reactivity of the VI and AIas corrosion inhibitors are reported in Table 1. It is evident fromTable 1 that VI and AI has similar EHOMO. VI on the other hand hasthe lowest ELUMO andDE values, the highest S andv values, makingit to have more reactive and binding potentials towards steelsurface than AI as calculated theoretically. The dipole moments ofVI (4.76 D) and AI (5.69) aremore than that of water (1.85 D) whichshows the ability of the inhibitors to displacewatermolecules fromthe steel surface thereby inhibiting the corrosion of steel againstaqueous acidic medium. Although the dipole moment of AI isgreater than that of VI from this study, some degree of confusionexists when dealingwith dipolemoment data in the interpretationof inhibition efciency data, since the scientic literature providesboth positive [2] as well as negative [45] relationships between

[(Fig._2)TD$FIG]

Fig. 2. Optimization progress curves of (a) vinyl imidazole and (b) allylimidazolecalculated using DFT.

[(Fig._3)TD$FIG]

I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1331corresponding to a singly-charged anion, with an electron added tothe LUMO of the neutral molecule; and N1 corresponding tosingly-charged cationwith an electron removed from the HOMO ofthe neutral molecule.

An analysis of the Fukui indices for nucleophilic and electrophilicsites are presented inTables 3 and4, respectively. InVI, atomsC4, C6,C7 and in AI, atoms C6, C8 are the most susceptible sites for

Fig. 3. HOMO and LUMO

Table 1Calculated quantum chemical properties for the most stable conformations of VIand AI calculated using B3LYP/DNP level of theory in water.

Properties VI AI

Total energy (Ha) 321.60 363.33EHOMO (eV) 6.61 6.58ELUMO (eV) 0.87 0.16DE (eV) 5.74 6.42m (D) 4.76 5.69h 2.86 3.21S 0.34 0.31v 9.74 7.08

Table 2Mulliken charges of VI and AI.

Atoms Charges

VI AI

N1 0.383 0.383N2 0.492 0.498C3 0.003 0.041C4 0.226 0.035C5 0.038 0.269C6 0.111 0.058C7 0.258 0.053C8 0.183TNC (Total negative charge) 1.133 1.163TPC (Total positive charge) 0.378 0.357nucleophilicattacks.Ontheotherhand,atomsC3,C4,C5,C7 inVIandatomsC4, C5, C7 inAI are themost probable centers for electrophilicattacks. Nevertheless, in VI, the atom C7 has the highest value of f

whereas inAI, theatomC8has thehighestvalueof f. These sitesarethe most reactive sites for neucleophilic attacks. As for f+,electrophilic attacks, C5 is the highest value for both VI and AI.

This site is the most reactive for electrophilic attacks.

orbitals of VI and AI.

Table 3Nucleophilic Fukui function (f+) calculated for VI and AI at B3LYP/DNP.

Atoms f+

VI AI

N1 0.025 0.023N2 0.075 0.033C3 0.093 0.017C4 0.160 0.022C5 0.037 0.055C6 0.107 0.177C7 0.211 0.018C8 0.277

Table 4Electrophilic Fukui function (f+) calculated for VI and AI at B3LYP/DNP.

Atoms f+

VI AI

N1 0.021 0.001N2 0.060 0.080C3 0.130 0.011C4 0.132 0.212C5 0.158 0.177C6 0.047 0.009C7 0.100 0.167C8 0.020

Molecular dynamics simulation

Many corrosion inhibition studies nowadays contain the use ofmolecular dynamics simulation as an important tool in under-standing the interaction between adsorbate-metal surface [4750]. Thus Monte Carlo and molecular dynamics simulations wereperformed on a system containing the inhibitors (VI and AI)adsorbed on a Fe2O3 (010) surface as shown in Fig. 4. Also Fig. 5shows the energy prole diagram for the adsorption progress of VIand AI on Fe2O3 surface. The energy prole consist of the totalenergy, average total energy, Van der Waals energy, electrostaticenergy and intermolecular energy for the systems. The MonteCarlo docking was done on each of the 100 conformations, andeach of the docked structures were energetically relaxed. Theoutputs and descriptors calculated by the Monte Carlo simulation,including the total adsorption, rigid adsorption and deformationenergies, are presented in Table 5. The total energy is dened as thesum of the energies of the adsorbate components. The adsorptionenergy is dened as the sum of the rigid adsorption energy and thedeformation energy for the adsorbate components. The rigidadsorption energy reports the energy, in kJmol1, released (orrequired) when the unrelaxed adsorbate components (VI and AI)(i.e., before the geometry optimization step) are adsorbed on theFe2O3 surface. The deformation energy is the energy releasedwhenthe adsorbed adsorbate components are relaxed on the Fe2O3surface. Table 5 also shows (dEads/dNi), which is the energy ofFe2O3-adsorbate congurations where one of the adsorbatecomponents has been removed [51].

It has been reported that the more negative the adsorptionenergies, the stronger the adsorbate-metal interaction [34]. It is

[(Fig._4)TD$FIG]

1332 I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339Fig. 4. Equilibrium adsorption conguration of (a) VI and (b) AI on Fe2O3(010)surface.clear from Table 5, that VI gave the maximum negative adsorptionenergy. Therefore, VI is expected to exhibit greater inhibitionabilities as compared to AI.

In order to validate results obtained from the theoreticalstudies, experimental evaluations using contemporary electro-chemical techniques such as linear polarization resistance,potentiodynamic polarization, electrochemical impedance spec-troscopy (EIS) aswell as surface analytical techniques like scanningelectronic microscopy (SEM) and atomic force microscopy (AFM)were undertaken to study the inhibition performance of VI and AItowards mild steel corrosion in 1M HCl.

OCP versus time

Fig. 6(a) and (b) shows the variation of OCP of the workingelectrodewith time (1800 s) in 1MHCl without andwith optimumconcentration of VI and AI, respectively at 25 C. From the gures,the OCP of 1M HCl was found to be around0.4675V. This type ofbehavior reects the breakdown of the pre-immersion air formedoxide lm on the electrode surface and attack of electrolyte on baremetal. The result is the attainment of a steady state potential whichcorresponds to be corrosion of the bare metal [52]. This processmay be represented as follows:

Fe2O3 6H 2e ! 2Fe2 3H2O (3)With the adittion of 0.01MVI and 0.01MAI, the OCP shifted to a

noble direction indicating that VI and AI controls mainly anodicmetal dissolution reaction.

Potentiodynamic polarization measurements

The anodic and cathodic polarization curves for the corrosion ofcarbon steel in 1M HCl solution in the presence and absence ofvarying concentrations of inhibitors (VI and AI) at 25 C are shownin Fig. 7. The corrosion current densities and corrosion potentialswere calculated by extrapolation of linear parts of cathodic andanodic curves to the point of intersection. The electrochemicalparameters such as corrosion potential (Ecorr), corrosion currentdensity (icorr), anodic Tafel slope (ba), and cathodic Tafel slope (bc),and h (%) determined from polarization curves are summarized inTable 6. All potentials were measured against SCE. h%, wascalculated using the equation [53]:

h% iocorr icorr

iocorr 100 (4)

where iocorr and icorr are the values of corrosion current density inthe absence and presence of inhibitors, respectively.

Polarization measurements are suitable for monitoring theprogress and mechanisms of the anodic and cathodic partialreactions as well as identifying the effect of an additive on eitherpartial reaction [54]. Thedata inTable6 clearly showthat the currentdensity decreases in the presence of the VI and AI investigated ascorrosioninhibitors.This isanindicationthatthe inhibitorsadsorbedon themetal surface. It is also clear that therewasmore reduction incorrosion current density in the presence of VI in all theconcentrations investigated than in the presence of AI. In acidicmedia, the anodic reaction of corrosion is the passage of metal ionsfrom themetal surface into solution, and the cathodic reaction is thedischargeofhydrogen ions,whichproduceshydrogengasor reducesoxygen. The inhibitors may affect either the anodic reaction or thecathodic reaction or both [55]. The anodic Tafel slope (ba) andcathodic Tafel slope (ba) of VI and AI were observed to changedepending on the inhibitor concentration. Thus, the addition ofinhibitors in blank solution affected anodic (dissolution of carbonsteel) as well as cathodic (evolution of hydrogen) reactions. The

[(Fig._5)TD$FIG]

I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1333variation in values of anodic and cathodic Tafel slopes in thepresence of inhibitors suggest that both the studied inhibitors aremixed type inhibitors. Nodenite trendwas observed in the shift ofEcorr values in the presence of different concentrations of the VI andAI, suggesting that the compounds behave asmixed-type inhibitors[56]. However, the minor shift of Ecorr values toward positivedirection on increasing the concentration of inhibitors suggests thepredominant anodic control over the reaction. The order ofinhibitiors efciency obtained is VI >AI.

Fig. 5. Energy prole diagram for the adsorption pro

Table 5Outputs and descriptors calculated by the Monte Carlo simulation for adsorption ofVI and AI on Fe2O3 (010) surface.

Properties VI AI

Total energy (kJmol1) 154.62 112.04Adsorption energy (kJmol1) 137.31 132.80Rigid adsorption energy (kJmol1) 139.49 132.80Deformation energy (kJmol1) 2.17 9.07dEads/dNi 137.31 132.80Linear polarization resistance (LPR) measurements

Stern and Geary on the basis of a detailed analysis of thepolarization curves of the anodic and cathodic reactions involvedin the corrosion of a metal, and on the assumption that bothreactions were charge-transfer controlled (transport overpotentialnegligible) and that the IR drop involved in determining thepotential was negligible, derived the expression [57]:

Rp DEDi

babc2:3icorr babc

(5)

where Rp is the polarization resistance determined at potentialsclose to Ecorr, andba,bc are the Tafel constants; note that in the caseof bc the negative sign is disregarded. This equation shows that thecorrosion rate is inversely proportional to Rp (or directlyproportional to the reciprocal slope of the DE versus Di curve)at potentials close to Ecorr (typically within 10mV), and that icorrcan be evaluated provided the Tafel constants are known.Electrochemical corrosion kinetic parameters obtained frompolarization resistance (Rp) in 1M HCl in the presence and absenceof VI and AI are presented in Table 7. The inhibition efciency canbe obtained as follows:

gress of (a) VI and (b) AI on Fe2O3(010) surface.

[(Fig._7)TD$FIG]

1334 I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339

[(Fig._6)TD$FIG]h% 1 Rop

Rp

! 100 (6)

where Ropand Rp are the polarization resistances in the absence andpresence of inhibitors (VI and AI), respectively. The resultsobtained show that the polarization resistance increases with anincrease in the concentration of VI at all concentrations butwith AI,polarization resistance increases up to 0.0075M and thereafterdecreased when AI concentration was increased to 0.01M in thecorrosive medium. VI has the highest inhibition efciency amongthe two imidazole derivatives investigated using LPR technique.

Electrochemical impedance spectroscopy (EIS) measurements

Mechanistic information about the kinetics of elctrochemicalreactions at the surface can be obtained using EIS. This method ismost widely used to study the corrosion inhibition process. Figs. 8and 9 show the impedance spectra represented in plots a, b and cas Nyquist, Bode and phase angle plots, respectively, in the absenceand presence of different concentrations of VI (Fig. 8) and AI (Fig. 9)for carbon steel corrosion in 1MHCl. The charge transfer resistance(Rct) and double-layer capacitance (Cdl) were obtained from theimpedance spectroscopy. The inhibition efciency is calculatedfrom the Rct values using the following formula [58]:

Fig. 6. Variation of the open circuit potential (OCP) as a function of time, recordedfor a carbon steel in 1M HCl at 25 C, in the absence and presence of highestconcentration of (a) vinyl imidazole and (b) allyl imidazole.h% Roct RctRoct

100 (7)

where Roct and Rct are the charge-transfer resistances in the absenceand presence of the inhibitors, respectively. The calculatedelectrochemical impedance parameters are given in Table 7. Datafrom Table 7 show that Rct values increased while Cdl valuesdecreased with increase in concentrations for VI and up till0.0075M for AI. This may be due to the increase in the surfacecoverage on the mild steel by the inhibitors, which led to the

Fig. 7. Potentiodynamic polarization curves for carbon steel in 1M HCl in theabsence and presence of different concentrations of (a) vinyl imidazole and (b) allylimidazole.

Table 6Potentiodynamic polarization parameters for carbon steel in 1MHCl solution in theabsence and presence of vinyl imidazole and allyl imidazole.

Inhibitor/concentration (M) Ecorr(mV/SCE)

icorr(mAcm2)

bc(mVdec1)

h%

1M HCl 470.9 72.8 101.1

Vinyl imidazole0.001 466.6 39.4 98.4 52.70.0075 447.9 15.7 88.5 78.40.01 443.8 12.9 96.7 82.3

Allyl imidazole0.001 465.9 45.6 93.3 37.40.00750.01

459.3468.2

33.338.3

89.1101.6

54.347.3

1M

n

0.

0.0.0.

0.0.0.

I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1335increase in inhibition efciency. The the double-layer capacitance(Cdl) is related to the thickness of the protective layer (d). This is inaccordance with the Helmholtz model and is given by thefollowing equation [59]:

Cdl eeoAd

(8)

where e is the dielectric constant of the medium, e0 is thepermittivity of the free space (8.8541012 Fm1), and A is theeffective surface area of the electrode. The decrease in Cdl leads toan increase in thickness of the double layer, which conrms thatthe VI and AI molecules inhibit the corrosion by adsorption at themild steel/solution interface. The changes in Cdl values are due tothe replacement of water molecules by VI and AI.

The Nyquist plots (Figs. 8 and 9(a)) show depressed semi-circles, indicating a non-ideal capacitive behavior of the electro-chemical solid/liquid interface [60]. Such phenomenon is knownasthe dispersing effect, usually attributed to the surface roughness,the chemical in-homogeneities, the adsorption of inhibitormolecules and the degree of polycrystallinity [61]. In addition,the diameters of the capacitive loops increase with increasinginhibitor concentration, which can be related to the increase ofsurface coverage of inhibitive molecules onmild steel surface [53].Thus, double layer behavior can be approximated by a constant-phase element (CPE) rather than a pure capacitor. The CPE isusually substituted for the capacitor to t the Nyquist depressedsemicircles more accurately. The admittance, YCPE, and impedance,ZCPE of a CPE are expressed as follows [6264]:

YCPE Yo jv n (9)and

ZCPE 1Yo

j 2pfmax n

1 (10)

Table 7Electrochemical impedance and linear polarization parameters for carbon steel in

Inhibitor/concentration (M) Rs(V cm2)

Yo(V sn cm2)106

1M HCl 2.97 15.1

Vinyl imidazole0.001 2.82 122.20.0075 2.87 99.70.01 2.87 79.8

Allyl imidazole0.001 2.92 126.80.0075 3.02 128.10.01 2.98 101.9where Y0 is the amplitute comparable to a capacitance, j is thesquare root of 1, fmax is the AC frequency at maximum and n, thephase shift (1n1), when n = 0, the CPE represents pureresistor, if n = +1, the CPE represents pure capacitor, and if n =1,the CPE represents inductor.

The bode plots for both VI and AI is shown in Figs. 8 and 9(b),respectively. The increase of absolute impedance at low frequen-cies in Bode plots conrms the higher protection of steel withincrease in the concentration of the investigated inhibitors. This isdue to the adsorption of VI and AI on steel surface. In the same vein,phase angle plots for VI and AI are depicted in Figs. 8 and 9(c),respectively. It is evident from the plots that only one phase peakclose to 90 at the middle frequency point can be observed. This isan indication that there is only a one time constant for VI and AI[62]. Such observationmay be related to the electrical double layerformation at steel/solution interface [65,66].The equivalent circuit used for the study is Randles circuit,shown in Fig. 10. In this circuit, Rct is the charge transfer resistance,Cdl is the double-layer capacitance, and Rs is the solutionresistance. The result of the three independent electrochemicaltechniques employed are in good agreement with each other. VIhas a higher inhibition efciency than AI for mild steel corrosion in1M HCl.

Adsorption isotherm studies

Results so far obtained indicate that the primary mode ofinteraction of VI and AI on steel surface is by adsorption. Theadsorption of organic inhibitor molecules from the aqueoussolution can be considered as a quasi-substitutionprocess betweenthe organic compounds in the aqueous phase [Org(sol)] and watermolecules associated with the metallic surface [H2O(ads)] asrepresented by the following equilibrium [67]:

Orgsol xH2Oads$Orgads xH2Osol (11)where x is the number of water molecules replaced by one organicmolecule. In this situation, the adsorption of VI and AI wasaccompanied by desorption of watermolecules from themild steelsurface as conrmed by theoretical studies. The degree of surfacecoverage (u) was evaluated from the potentiodyanamic polariza-tion measurements as follows:

u h%100

(12)

It is necessary to determine empirically which adsorptionisotherm ts best to the surface coverage data in order to use thecorrosion rate measurements to calculate the thermodynamicparameters pertaining to inhibitor adsorption. Attempts weremade to t surface coverage values into different adsorption

HCl solution in the absence and presence of vinylimidazole and allylimidazole.

Rct(V cm2)

CdlmF cm2

h% Rp(V cm2)

h%

85 335.6 3.45 314.5

89 461.3 1.93 27.3 464.7 32.389 1036.0 0.37 67.6 1001.0 68.689 1306.0 0.24 74.3 1415.0 77.8

85 435.8 2.05 22.9 410.1 23.383 555.3 1.31 65.5 497.8 36.885 444.4 2.94 24.5 444.9 29.3isotherm models. The models considered were [2]:

Temkinisothermexp f u KadsC (13)

Langmuirisothermu=1 u KadsC (14)

Frumkinisothermu=1 u exp2f u KadsC (15)

andFreundlichisothermu KadsC (16)where Kads is the equilibrium constant of the adsorption process, Cthe inhibitor concentration and f the factor of energetic inhomo-geneity. The correlation coefcient (R2) was used to choose theisotherm that best t experimental data. Best results from the plotswere obtained for Langmuir adsorption isotherm. The linearrelationships of C/u versus C, depicted in Fig. 11, suggest that the

1336 I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339

[(Fig._8)TD$FIG]adsorption of VI and AI on the steel surface obeyed the Langmuiradsorption isotherm. This isotherm can be represented as:

Cu 1

Kads C (17)

The strong correlation (R20.96) of the Langmuir adsorptionfor VI and AI was observed. This isotherm postulates that there isno interaction between the adsorbed molecules and the energy ofadsorption is independent on the surface coverage (u). It assumesthat the solid surface contains a xed number of adsorption sitesand each holds one adsorbed species [68]. The slopes of thestraight lines obtained from the plots of Langmuir isotherm for VI

Fig. 8. Impedance plots for carbon steel in 1M HCl in the absence and presence ofdifferent concentrations of vinyl imidazole exemplied as (a) Nyquist (b) Bode and(c) phase angle plots.[(Fig._9)TD$FIG]and AI are more than unity (Table 8). So, it could be concluded thateach VI and AI unit occupies more than one adsorption site on thesteel surface. Amodied Langmuir adsorption isotherm [69], couldbe applied to this phenomenon, which is given by the correctedequation:

Cu n

Kads nC (18)

The divergence from pure monolayer adsorption can beattributed to interactions between adsorbate species on the metalsurface as well as changes in the adsorption heat with increasingsurface coverage [70], factors which were not taken into

Fig. 9. Impedance plots for carbon steel in 1M HCl in the absence and presence ofdifferent concentrations of allyl imidazole exemplied as (a) Nyquist (b) Bode and(c) phase angle plots.

consideration in derivation of the isotherm.The free energy ofadsorption DG

ads of the inhibitors on aluminium surface was

determined using the following equation:

DGads RTln Kads 55:5 (19)

where DGads is the standard free energy of adsorption, Kads is the

equilibrium constant of adsorption and the value of 55.5 is theconcentration of water in solution expressed in mol l1. Thecalculated DG

ads and Kads results are also listed in Table 8.

The values of equilibrium adsorption constant obtained fromthe Langmuir plots were 1.29103M1 and 3.18103M1, for VIand AI, respectively. On the other hand, the values of DG

ads were

calculated to be 27.57 kJ/mol and 29.93 kJ/mol, for VI and AI,

[(Fig._10)TD$FIG]

Fig. 10. Equivalent circuit diagram used to t impedance data in the absence andpresence of VI and AI in 1M HCl.

[(Fig._11)TD$FIG]

Fig. 11. Langmuir adsorption isotherm for vinyl imidazole and allyl imidazole oncarbon steel in 1M HCl 25 C.

Table 8Langmuir adsorption parameters for carbon steel in 1M HCl at 25 C.

Inhibitor DGads(kJmol

1) Kads(M1) Slope R2

Vinyl imidazole 27.57 1.29103 1.14 0.996Allyl imidazole 29.93 3.18103 1.99 0.964

[(Fig._12)TD$FIG]

I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1337Fig. 12. SEM images of (a) polished steel (b) steel immersed in 1M HCl (c) steel in 1M HCl +VI (d) steel in 1M HCl +AI.

respectively. DGadsvalues obtained in this study reveal that in the

presence of 1M HCl, comprehensive adsorption involving bothphysisorption and chemisorption adsorption of VI and AI on steel isinvolved. The adsorption of VI and AI is not physisorption norchemisorption. This is because it is generally believed in theliterature that DG

adsvalues around 20kJmol1 or lower are

consistent with the electrostatic interaction between chargedorganic molecules and the charged metal surface (physisorption);those around 40 kJmol1 or higher involve charge sharing ortransfer from the organic molecules to the metal surface to form aco-ordinate type of bond (chemisorption).

Scanning electronic microscopy (SEM)

Fig. 12(a) depicts the morphologies of polished mild steel. SEMmicrographs obtained from mild steel surface after 18h ofimmersion in 1M HCl for the untreated mild steel and treated mildsteel in 0.01M VI and 0.01M AI are shown in Fig. 12(b)(d),respectively. It canbeclearlyobservedthat themildsteel surfacewasstronglydamagedwithareasofuniformcorrosionwherethemetal isattackedmore or less evenly over the entire surface. Examinationonthe surfacemorphologyof treatedmild steel in 0.01 VI and 0.01MAI

reveals that the metal surface was in a better conditions by havingsmooth surfaces compared to the untreatedmild steel. However, VIoffered better protection to the steel surface in 1M HCl than AI asobserved in Fig. 12(c) and (d). This may be due to the stronginteraction or adsorptionof VI onmild steel surface forminga strongbarrier against corrosion.

Atomic force microscopy

The AFM is one of the foremost tools for imaging,measuring, andmanipulating matter at the nano- to micro-scale. It has become anewmethod to investigate the nature of the protective layer formedon themild steel surface [71]. TheAFMimagesofpolishedmild steel,mild steel in 1M HCl without and with 0.01M VI and 0.01M AI areshown in Fig. 13(a)(d), respectively. It is evident from the guresthat the mild steel surface immersed in 1M HCl appears severelydamaged than the steel surface immersed into 1M HCl containingthe optimum concentration (0.01M) of VI and AI. Moreover, thevalues of the average roughness ofmild steel immersed in blankHClsolutionwascalculated tobe1600nm.With theadditionofVIandAI,the average roughness was reduced to 156.3 and 296.8nm,respectively. This is an indication of protection abilities of theinhibitors but with AI forming stronger lm on mild steel surface.

[(Fig._13)TD$FIG]

sed

1338 I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339Fig. 13. 3D AFM images of (a) polished steel (b) steel immer in 1M HCl (c) steel in 1M HCl +VI (d) steel in 1M HCl +AI.

Conclusions

We have employed DFT and MD simulations to predict andranked the inhibition efciencies of vinylimidazole and allylimi-dazole as possible green corrosion inhibitor for mild steel in 1MHCl. The ranking of inhibition efciency theoretically follows theorder: VI >AI. Electrochemical evaluations using LPR, PDP, EISvalidate the theoretical ranking. VI is a more potent corrosioninhibitor than AI. Surface morphological studies using SEM andAFM conrm that the steel surface containing VI in 1M HCl wasmore inhibited than the one in the presence of AI in the samesolution. This study has shown that theoretical calculations can beused as a reliable approach to screen organic corrosion inhibitorsprior to experimental validation.

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I.B. Obot et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 13281339 1339Acknowledgments

The authors gratefully acknowledged the Center of ResearchExcellence in Corrosion (CORE-C), King Fahd University ofPetroleum and Minerals (KFUPM), Saudi Arabia for funding.

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Theoretical prediction and electrochemical evaluation of vinylimidazole and allylimidazole as corrosion inhibitors for mil ...IntroductionExperimentalsMaterials and sample preparationQuantum chemical calculations and molecular dynamics (MD) simulationsElectrochemical measurementsSurface morphology using scanning electron microscopy (SEM)Atomic force microscopy (AFM)

Results and discussionQuantum chemical studyMolecular dynamics simulationOCP versus timePotentiodynamic polarization measurementsLinear polarization resistance (LPR) measurementsElectrochemical impedance spectroscopy (EIS) measurementsAdsorption isotherm studiesScanning electronic microscopy (SEM)Atomic force microscopy

ConclusionsAcknowledgmentsReferences