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Available online at www.sciencedirect.com

Electrochimica Acta 53 (2008) 3484–3492

Molecular simulation, quantum chemical calculations andelectrochemical studies for inhibition of mild steel by triazoles

K.F. Khaled ∗Electrochemistry Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt

Received 19 October 2007; received in revised form 5 December 2007; accepted 6 December 2007Available online 15 December 2007

bstract

The inhibition performance of three triazole derivatives on mild steel in 1 M HCl were tested by weight loss, potentiodynamic polarizationnd electrochemical impedance spectroscopy (EIS). The adsorption behavior of these molecules at the Fe surface was studied by the molecularynamics simulation method and the quantum chemical calculations. Results showed that these compounds inhibit the corrosion of mild steeln 1 M HCl solution significantly. Molecular simulation studies were applied to optimize the adsorption structures of triazole derivatives. The

ron/inhibitor/solvent interfaces were simulated and the charges on the inhibitor molecules as well as their structural parameters were calculated inresence of solvent effects. Aminotriazole was the best inhibitor among the three triazole derivatives (triazole, aminotriazole and benzotriazole).he adsorption of the inhibitors on the mild steel surface in the acid solution was found to obey Langmuir’s adsorption isotherm.2007 Elsevier Ltd. All rights reserved.

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eywords: Molecular simulation; Quantum chemical calculation; Mild steel; C

. Introduction

Iron and its alloys are widely used in many applications,hich have resulted in research into the corrosion resistance inarious aggressive environments. In efforts to mitigate electro-hemical corrosion, the primary strategy is to isolate the metalrom corrosive agents. Among the different methods available,he use of corrosion inhibitors is usually the most appropriateay to achieve this objective [1]. Corrosion inhibition occursia adsorption of the organic molecule on the corroding metalurface following some known adsorption isotherms with theolar groups acting as the reactive centers in the molecules.he resulting adsorption film acts as a barrier that isolates

he metal from the corroding environment and efficiency ofnhibition depends on the mechanical, structural, and chemicalharacteristics of the adsorption layers formed under particular

onditions. Although various experimental and theoretical tech-iques [2] have been developed to study the structural propertiesf inhibitor molecules, little is known about the interactions that

∗ Present address: Chemistry Department, Faculty of Science, Taif University,88 Hayia, Saudi Arabia. Tel.: +966550670425; fax: +9667255529.

E-mail address: khaledrice2003@yahoo.com.

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.12.030

ion inhibition; Triazoles; EIS

ccurred between the adsorbed molecules and metal surfaces. Aractical route to study these complex processes are computerimulations of suitable models. In recent years, the quantumhemistry computing method has become an effective way totudy the correlation of the molecular structure and its inhibi-ion properties and much achievement was reached [3–8]. Theuantum chemistry computing method is often used to studyhe simple systems. For more complex systems, such as systemsnvolving a relatively large number of molecules, the quantumhemistry computing method is not suitable anymore. Khaledt al. [9] concluded that the quantum mechanical approach mayell be able to foretell molecule structures that are better for cor-

osion inhibition purposes if it is taken into account that (i) theffect depends only on the inhibitor molecule properties and (ii)verything else in its vicinity is uninvolved either with respecto competition for the surface or with respect to itself. Also, its clear that there is no general way for predicting compoundsefulness to be good corrosion inhibitor or find some univer-al type of correlation. A number of excluded parameters thathould be involved as effect of solvent molecules, surface nature,

dsorption sites of the metal atoms or oxide sites or vacancies,ompetitive adsorption with other chemical species in the fluidhase and solubility give at least simplified inspection. In this cir-umstance, a molecular simulation method is the best choice in

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K.F. Khaled / Electrochim

n attempt to take into account the effect some of these excludedarameters.

Molecular simulation has been used to investigate the forma-ion of adsorption layers on metals and provide some valuablenformation about the microstructures of the surfaces [10,11].t maybe has a potential application towards the design andevelopment of organic corrosion inhibitors in corrosion field.olecular modeling has not been used widely for inhibition

tudies. Edwards et al. [12] investigated the adsorption of the oileld pipeline inhibitor oleic imidazoline using both molecularrbital and molecular mechanics methods whilst Fitzwater [13]mployed molecular mechanics and molecular dynamics to sim-late the adsorption of both poly(acrylic acid) and poly(asparticcid) on various CaCO3 surfaces. Ramachandran et al. [14] stud-ed the oleic imidazolines and reported on the use of molecularrbital calculations and subsequent molecular dynamics simu-ations to investigate the adsorption of 1,2-dimethylimidazolinen an Fe(OH)3(H2O)2 surface. These methods were also used toimulate the adsorption of imidazolines to the Fe site of Fe2O315], whilst in another article [16] molecular mechanics wasmployed to model the interactions between imidazolines ande3O4 surface. Further work on the adsorption of imidazolinesas been done by Wang et al. [17]. These workers designed threemidazoline derivatives and their MO predictions as to inhibitionfficiency were in good agreement with experimental corrosiontudies. Triazole derivatives have been studied extensively in theiterature as corrosion inhibitors for iron and copper in acidic

edia [18–22]. Relation between inhibition performance anduantum chemical calculations have been reported by severaluthors [23–26].

In this study, molecular simulation studies were performedo simulate the adsorption of some triazole derivatives on ironurface and advance the understanding of interactions betweenhese molecules and iron surface. In essence molecular mechan-cs has been coupled with molecular dynamics to simulatedsorption of triazoles with a clean (1 1 0) iron surface. In eachase interaction of a single, triazole derivatives molecule onlyas been considered. The aim of this work is to investigatehe inhibition mechanism of these molecules on mild steel inn aerated solution of 1 M HCl, using chemical (weight loss)lectrochemical techniques such as electrochemical impedancepectroscopy (EIS) and potentiodynamic polarization as well asolecular modeling of the corrosion system which allow deter-ining the possible anchoring site suitable for the inhibitor to

ond with steel surface.

. Experimental

.1. Structure of triazole derivatives

The structure of the studied compounds are listed below:

2

v

ta 53 (2008) 3484–3492 3485

All compounds investigated were obtained from Aldrichhemical Co. They were put in the 1 M HCl (Fisher Scientific)ithout pretreatment at concentrations of 10−4, 10−3, 5 × 10−3

nd 10−2 M.

.2. Modeling calculations

The molecular simulation product called MS Modeling 4.0as used to perform MS Modeling using the amorphous cellodule which provides a comprehensive set of tools to per-

orm atomistic simulations on complex systems containingense amorphous polymers, liquids and other non-crystallineaterials. Using these tools, one can construct systems con-

aining bulk solvent and phases, perform structural analysisnd property calculation to obtain estimates of experimen-ally measurable quantities and parameters, including estimatesf properties that are sometimes difficult or even impossi-le to obtain in the laboratory. Also, the study was carriedut using Dewar’s linear combinations of atomic orbitals–self-onsistent field–molecular orbital (LCAO–SCF–MO) [27,28].emi-empirical PM3 method [29,30] with commercially avail-ble quantum chemical software HyperChem, Release 7.5 [31].full optimization of all geometrical variables without any sym-etry constraint was performed at the restricted Hartree–Fock

RHF) level [32,33]. It develops the molecular orbitals on aalence basis set and also calculates electronic properties, opti-ized geometries and total energy of the triazoles molecules.olecular structures were optimized to a gradient <0.01 in

he solvent phase. As an optimization procedure, the built-inolak–Ribiere algorithm was used [34].

.3. Weight loss measurements

The experiments were carried out using mild steel (99.14%)pecimens. The steel coupons of 3.0 cm × 1.0 cm × 0.20 cmith an exposed total area of 7.6 cm2 were used for weight losseasurement studies. A mild steel rod of the same compositionas mounted in Teflon with an exposed area of 0.28 cm2 used

or polarization and electrochemical impedance EIS measure-ents the coupons were polished, dried and weighted and then

uspended in a 100-cm3 aerated solution of 1 M HCl with andithout the different concentrations of triazoles for exposureeriod (8 h). After the designated exposure to the test solution,he specimens were rinsed with distilled water, washed withcetone to remove a film possibly formed due to the inhibitor,ried between two tissue papers, and weighted again. Weightoss measurements were made in triplicate and the loss of weightas calculated by taking an average of these values.Prior to all measurements, the steel samples are abraded with

series of emery paper up to 0000 grit size. The specimens areashed thoroughly with bidistilled water, degreased and driedith acetone.

.4. Electrochemical measurements

Electrochemical experiments were carried out using a con-entional electrolytic cell with three-electrode arrangement:

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more efficient than triazole and bensotriazole, respectively. Thecharges on nitrogen atoms and other quantum chemical param-eters for the three triazoles are presented in Fig. 2 and Table 1,respectively. Fig. 2 shows the molecular orbital plots (Fig. 2c) as

Table 1The calculated quantum chemical properties for triazole derivatives

Compound

Aminotriazole Triazole Benzotriazole

EHOMO (eV) −10.395 −9.309 −8.433ELUMO (eV) 0.438 0.1057 −0.571

486 K.F. Khaled / Electrochim

aturated calomel reference electrode (SCE), platinum meshs a counter electrode, and the working electrode (WE) hadhe form of rod. The counter electrode was separated from theorking electrode compartment by fritted glass. The reference

lectrode was connected to a Luggin capillary to minimize IRrop. Solutions were prepared from bidistilled water of resis-ivity 13 M� cm, Prior to each experiment, the specimen wasolished with a series of emery papers of different grit sizes upo 0000 grit size, polished with Al2O3 (0.5 mm particle size),ashed several times with bidistilled water then with acetone

nd dried using a stream of air. The electrode potential wasllowed to stabilize 30 min before starting the measurements.he aggressive environment used was 1 M HCl solution withifferent concentrations of triazole derivatives. All experimentsere conducted at 300 K. The exposed electrode area to the

orrosive solution is 0.28 cm2.Potentiodynamic polarization curves were obtained by

hanging the electrode potential automatically from (−1100 to400 mV vs. SCE) at open circuit potential with scan rate ofmV s−1.

EIS measurements were carried out in a frequency range of00 kHz to 50 mHz with amplitude of 5 mV peak-to-peak usingc signals at open circuit potential.

Measurements were performed with a Gamry Instrumentotentiostat/Galvanostat/ZRA. This includes a Gamry Frame-ork System based on the ESA400, Gamry applications that

nclude DC105 for dc corrosion measurements, EIS300 for elec-rochemical impedance spectroscopy measurements to calculatehe corrosion current and the Tafel constants along with a com-uter for collecting the data. Echem Analyst 4.0 Software wassed for plotting, graphing and fitting data.

. Results and discussions

.1. Molecular modeling

Here, molecular simulation studies were performed to sim-late the adsorption structure of the triazole derivatives andress on the understanding of interactions between these tria-oles and iron surface. Molecular structure of triazoles showshat it is likely for these molecules to adsorb on iron surfacey sharing the electrons of nitrogen atoms with iron to formoordinated bonds with nitrogen and �-electron interactions ofhe aromatic rings [35]. Both interactions can make it possi-le for the triazoles to form coordination bond with iron. Thedsorption progress of triazoles on iron surface is investigatedy performing molecular mechanics (MM) using MS Modelingoftware. As the three kinds of Fe surfaces (1 1 0, 1 0 0, 1 1 1),e (1 1 1) and Fe (1 0 0) surfaces have relatively open struc-

ures while Fe (1 1 0) is a density packed surface and has theost stabilization, so we choose Fe (1 1 0) surface to simulate

he adsorption process [36]. The periodic boundary conditionsPBC) are applied to the simulation cell. The size of simulation

ox is 23.0 A × 23.0 A × 19.79 A. The force field used in theurrent MM is COMPASS (condensed phase optimized molec-lar potentials for atomistic simulation studies) force field. Allolecules are energy optimized, iron surface and solvent layers

EMDM

ig. 1. The amorphous cell containing the iron substrate, the solvent moleculend the triazole derivatives.

as constructed using the amorphous cell module, the wholeystem was energy optimized and the possibility of triazolesdsorption on the iron surface were simulated as in Fig. 1.

Fig. 1 shows that the close contact between aminotriazole is

(total energy) (kcal mol−1) −29341.47 −21824.7 −17724.14aximum charge on N atoms −0.795 −0.51 −0.204ipole moment (D) 1.2 2.9 3.8olecular weight 84.8 69.07 119.13

K.F. Khaled / Electrochimica Acta 53 (2008) 3484–3492 3487

lar or

waawsfiatcsjc

bmiuLt

tμ

im[aa

iaaoztitnp

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Fig. 2. Structure of triazole derivatives, molecu

ell as the charge density distribution (Fig. 2d) on the three tri-zoles. It is worth noting that the charge density distribution onminotriazole are more intense than triazole and benzotriazolehich enhance the possibility of aminotriazole to adsorb more

trongly on iron surface than triazole and benzotriazole. It is con-rmed that the more negative the atomic charges (Fig. 2b) of thedsorbed centre, the more easily the atom donates its electronso the unoccupied orbital of metal [37]. So these negative atomicharges indicated that nitrogen atoms are the active adsorptionites. The electronegativity of the nitrogen atoms and the con-ugated �-electrons of the triazoles make it possible to formoordination bond with iron surface as can be seen in Fig. 1.

The structure and electronic parameters can be obtainedy means of theoretical calculations using the computationalethodologies of quantum chemistry. The geometry of the

nhibitor in its ground state, as well as the nature of their molec-lar orbitals, HOMO (highest occupied molecular orbital) andUMO (lowest unoccupied molecular orbital) are involved in

he properties of activity of inhibitors.Table 1 shows the calculated quantum chemical properties for

riazole compounds, EHOMO (eV), ELUMO (eV), dipole moment,(D) total energy. In fact the adsorption power (efficiency of the

at

bital plots and the charge density distribution.

nhibitor) increases with lower dipole moments, with decreasingolecular size and with increasing nitrogen charge, respectively

38,39]. EHOMO is often associated with the electron donatingbility of a molecule, whereas ELUMO indicates its ability toccept electrons.

As EHOMO is often associated with the electron donating abil-ty of the molecule, high values of EHOMO are likely to indicate

tendency of the molecule to donate electrons to appropri-te acceptor molecules with low energy and empty molecularrbital. From Table 1, it is clear that EHOMO in case of aminotria-ole is higher than both triazole and benzotriazole which enhancehe assumption that aminotriazole will adsorb more strongly onron surface than both triazole and benzotriazole. Fig. 2 showshe molecular orbital distribution of triazole compounds. Theegative sign of the EHOMO indicates that the adsorption ishysisorption [39].

.2. Weight loss measurements

The corrosion parameters such as inhibition efficiency, EI (%)nd corrosion rate (mg cm−2 h−1) at different concentration ofriazoles in 1 M HCl at 300 K are presented in Table 2. As can

3488 K.F. Khaled / Electrochimica Acta 53 (2008) 3484–3492

Table 2Corrosion rate and efficiency data obtained from weight loss measurements formild steel in 1 M HCl solutions in absence and presence of different concentra-tions from triazole derivatives

Concentration(M)

CR (mg cm−2 h−1) Coverage(�)

Ew (%)

Benzotriazole

0 2.15 0 0.000010−4 1.21 0.44 44.000010−3 0.98 0.54 54.00005 × 10−3 0.45 0.79 79.000010−2 0.27 0.87 87.0000

Triazole

10−4 0.75 0.65 65.000010−3 0.47 0.78 78.00005 × 10−3 0.23 0.89 89.000010−2 0.17 0.92 92.0000

A

10−4 0.60 0.72 72.0000−3

bavc

E

θ

wpiwaTo.taecibaa

3

issebrcz

Fig. 3. Potentiodynamic polarization curves of the mild steel electrode in 1 MHCl without and with various concentrations from benzotriazole.

Fig. 4. Potentiodynamic polarization curves of the mild steel electrode in 1 MHCl without and with various concentrations from triazole.

minotriazole10 0.41 0.81 81.00005 × 10−3 0.13 0.94 94.000010−2 0.06 0.97 97.0000

e seen in Table 2, triazoles inhibit the corrosion of mild steel atll concentrations in 1 M HCl. From the determined weight lossalues, the inhibition efficiencies, Ew(%) and coverage θ, werealculated using the following equations [40]:

w(%) =(

1 − w

w0

)× 100 (1)

=(

1 − w

w0

)(2)

here w0 and w are the weight loss in absence and in theresence of triazoles, respectively. Inspection of these datan Table 2, reveal that the inhibition efficiency increasesith increasing the concentration of triazoles, and aminotri-

zole is better inhibitor than both triazole and benzotriazole.he corrosion inhibition can be attributed to the adsorptionf triazoles molecules at mild steel acid solution interfaceAdsorption of triazole derivatives can be explained on the basishat adsorption of these compound are mainly via the nitrogentoms in the triazole ring, in addition to the availability of �-lectrons (by resonance structures) in the aromatic system. Inase of aminotriazole, the presence of an amino group enhancests adsorption more than triazole itself, while the presence ofenzene ring in benzotriazole lower the efficiency of benzotri-zole than triazole compound due to the electron withdrawingbility of benzene ring.

.3. Polarization measurements

Figs. 3–5 are showing typical polarization curves for thenhibition characteristics of triazole derivatives. These curveshow anodic and cathodic polarization plots recorded on mildteel electrode in 1 M HCl at various concentrations in the pres-nce and absence of triazole derivatives. As would be expected

oth anodic and cathodic reactions of mild steel electrode cor-osion were inhibited with the increase of triazole derivativesoncentration. This result suggests that the addition of tria-ole derivatives reduces anodic dissolution and also retards the

Fig. 5. Potentiodynamic polarization curves of the mild steel electrode in 1 MHCl without and with various concentrations from aminotriazole.

K.F. Khaled / Electrochimica Acta 53 (2008) 3484–3492 3489

Table 3Electrochemical kinetic parameters obtained form potentiodynamic polarization curves shown in Figs. 3–5 for the mild steel electrode in 1 M HCl in differentconcentrations of triazole derivatives

Concentration (M) icorr (�A cm−2) −Ecorr (mV) −βc (mV dec−1) βa (mV dec−1) Ep (%)

Benzotriazole

0.00 489 450 117 114 0.0010−4 381 459 119 100 22.110−3 320 458 128 100 34.65 × 10−3 130 465 130 102 73.410−2 80 469 137 108 83.6

Triazole

10−4 331 459 138 119 32.310−3 280 468 139 120 42.75 × 10−3 101 463 143 124 79.310−2 65 475 142 121 86.7

Aminotriazole

10−4 309 460 149 101 36.810−3 220 475 149 110 55.15 × 10−3 85 476 151 114 82.6

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ydrogen evolution reaction. Table 3 shows the electrochemi-al corrosion kinetic parameters, i.e., corrosion potential (Ecorr),athodic and anodic Tafel slopes (βc, βa) and corrosion currentensity icorr obtained by extrapolation of the Tafel lines. Thealculated inhibition efficiency, Ep (%) are also reported fromhe following equation:

p(%) =(

1 − icorr

i0corr

)× 100 (3)

here i0corr and icorr correspond to uninhibited and inhibited cur-ent densities, respectively. The best inhibition efficiency wasbout 90.2% at concentration 10−2 M. It can be seen that byncreasing inhibitor concentration, the corrosion rate decreasednd inhibition efficiency Ep (%), increased. No definite trendas observed in the shift of Ecorr values, in the presence ofarious concentrations of triazole derivatives, suggesting thathese compounds behave as mixed-type inhibitors. Moreover,hese inhibitors cause no change in the anodic and cathodic Tafellopes, indicating that the inhibitors are first adsorbed onto steelurface and therefore impedes by merely blocking the reactionites of iron surface without affecting the anodic and cathodiceaction mechanism [40].

.4. Electrochemical impedance measurements

Results obtained from EIS can be interpreted in terms ofhe equivalent circuit of the electrical double layer shown inig. 6, which was used previously to model the iron/acid inter-ace [41]. The effects of triazole derivatives concentrations on

Fig. 6. Suggested equivalent circuit model for the studied system.Fd

75 160 115 90.2

he impedance behavior of steel in 1 M HCl solutions are givenn Figs. 7–9.

These curves show a typical set of Nyquist plots for mild steeln 1 M HCl in the absence and presence of various concentra-ions of triazole derivatives. It is clear from these plots that thempedance response of mild steel has significantly changed afterhe addition of triazole derivatives in the corrosive media. Thisndicates that the impedance of an inhibited substrate increasesith increasing concentration of inhibitor in 1 MHCl. It is worthoting that the change in concentration of triazole derivativesid not alter the profile of the impedance behavior, suggest-ng similar mechanism for the corrosion inhibition of mild steely triazole derivatives, Figs. 7–9. The impedance parameterserived from Figs. 7–9 are given in Table 4. The charge trans-er resistance Rct, double layer capacitance Cdl and inhibitionfficiency EI (%) are calculated from the following equations:

ig. 7. Nyquist plots for mild steel in 1 M HCl in absence and presence ofifferent concentrations of benzotriazole.

3490 K.F. Khaled / Electrochimica Acta 53 (2008) 3484–3492

Table 4Electrochemical kinetic parameters obtained by EIS technique for mild steel in 1 M HCl in absence and presence of various concentrations of triazole derivatives

Concentration (M) Rs (� cm) Rct (� cm) Cdl (�F cm−2) EI (%)

Benzotriazole

0.00 2.3 112.5 93.1210−4 2.5 173.2 73.55 35.0410−3 2.3 214.4 74.27 47.525 × 10−3 2.9 475.6 52.89 76.3410−2 3.0 717.5 49.76 84.32

Triazole

10−4 2.1 272.5 46.74 58.7110−3 2.5 400.0 39.81 71.875 × 10−3 2.1 637.0 39.49 82.3310−2 1.9 1084.0 32.93 89.62

Aminotriazole

10−4 1.7 306.0 41.63 63.2310−3 1.95 × 10−3 2.110−2 2.6

Fd

f

wH

Fd

zc

gmsstoltdideb

v

ig. 8. Nyquist plots for mild steel in 1 M HCl in absence and presence ofifferent concentrations of triazole.

(−Z′′img) = 1

2πCdlRct(5)

here R0ct and Rct are the charge transfer resistances in 1 M

Cl solution without and with different concentrations of tria-

ig. 9. Nyquist plots for mild steel in 1 M HCl in absence and presence ofifferent concentrations of aminotriazole.

mit[

C

wpope

aa

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tti

432.0 36.86 73.95848.0 29.66 86.73

1446.0 24.69 92.21

ole derivatives, respectively, −Z′′img is the maximum imaginary

omponent of the impedance.The Nyquist plots obtained in the real system represent a

eneral behavior where the double layer on the interface ofetal/solution does not behave as a real capacitor. On the metal

ide electrons control the charge distribution whereas on theolution side it is controlled by ions. As ions are much larger thanhe electrons, the equivalent ions to the charge on the metal willccupy quite a large volume on the solution side of the doubleayer [42]. From Table 4, it was clear that charge transfer resis-ance Rct values were increased and the capacitance values Cdlecreased with increasing inhibitors’ concentration. Decreasen the capacitance, which can result from a decrease in localielectric constant and/or an increase in the thickness of thelectrical double layer, suggests that the inhibitor molecules acty adsorption at the metal/solution interface [43].

The addition of triazole derivatives provides lower Cdlalues, probably as a consequence of replacement of waterolecules by triazoles at the electrode surface. Also the

nhibitor molecules may reduce the capacitance by increasinghe double layer thickness according to the Helmholtz model44]:

dl = εε0A

δ(6)

here ε is the dielectric constant of the medium, ε0 is the vacuumermittivity, A is the electrode surface area and δ is the thicknessf the protective layer. The value of Cdl is always smaller in theresence of the inhibitor than in its absence, as a result of theffective adsorption of the triazole derivatives.

The results obtained from EIS measurements are in goodgreement with that obtained from potentiodynamic polarizationnd weight loss measurements.

.5. Adsorption isotherm

It has been assumed that organic inhibitor molecules establishheir inhibition action via the adsorption of the inhibitor ontohe metal surface. The adsorption processes of inhibitors arenfluenced by the chemical structures of organic compounds, the

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K.F. Khaled / Electrochim

ature and surface charge of metal, the distribution of charge inolecule and the type of aggressive media.In general, two modes of adsorption can be considered. The

roceeding of physical adsorption requires the presence of elec-rically charged metal surface and charged species in the bulkf the solution. Chemisorption process involves charge shar-ng or charge transfer from the inhibitor molecules to the metalurface. The presence, with a transition metal, having vacant,ow-energy electron orbital, of an inhibitor molecule having rel-tively loosely bound electrons or heteroatoms with lone-pairlectrons facilitates this adsorption [45,46].

Assuming the corrosion inhibition was caused by the adsorp-ion of triazole derivatives, and the values of surface coverageθ) for different concentrations of inhibitors in 1 M HCl werevaluated from weight loss measurements from Eq. (2).

Adsorption isotherms are very important in determining theechanism of organic electrochemical reactions. The most fre-

uently used adsorption isotherms are Langmuir, Temkin andrumkin. So several adsorption isotherms were tested for theescription of adsorption behaviour of studied compounds andt is found that adsorption of triazoles on mild steel surface inCl solution obeys the Langmuir adsorption isotherm given by

he following equations [47]:

Cinh

θ= 1

b+ Cinh (7)

= 1

55.5exp

(−Gads

RT

)(8)

inh is the inhibitor concentration, θ is the fraction of the surfaceovered, b is the adsorption coefficient and Gads is the standardree energy of adsorption.

Fig. 10 shows the dependence of the fraction of the surfaceovered C/θ as a function of the concentration (C) of triazoleerivatives.

It should be explained that other adsorption isothermsFrumkin and Temkin) were checked and Langmuir adsorptionsotherm is the best approximate between them. This is why thessumption is true for Langmuir adsorption isotherm.

ig. 10. Langmuir’s adsorption plots for mild steel in 1 M HCl containingarious concentrations from triazole derivatives.

•

•

•

•

•

R

ta 53 (2008) 3484–3492 3491

The obtained plots of the inhibitors is linear with corre-ation coefficient higher than 0.99. The intercept permits thealculation of the equilibrium constant b which are 9980.1,615.38 and 2898.5 M−1 for aminotriazole, triazole and benzo-riazole, respectively. The values of b which indicate the bindingower of the inhibitor to the steel surface can lead to calculatehe adsorption energy. Values of Gads = −14.323, −14.283,

12.984 kJ mol−1.The negative value of Gads means that the adsorption of

riazoles on mild steel surface is a spontaneous process, andurthermore the negative values of Gads also show the strongnteraction of the inhibitor molecule onto the mild steel surface48,49].

Generally, values of Gads around −20 kJ mol−1 or lower areonsistent with the electrostatic interaction between the chargedolecules and the charged metal (physisorption). Those more

egative than −40 kJ mol−1 involve charge sharing or transferrom the inhibitor molecules to the metal surface to form a coor-inate type of bond (chemisorption) [50,51]. For investigatedminotriazole inhibitor, one can see that the calculated Gadsalues, equals −14.323 kJ mol−1, indicating, that the adsorp-ion mechanism of the aminotriazole on mild steel in 1 M HClolution was typical of physisorption.

. Conclusions

The following results can be drawn from this study:

This study is unique in that the simulation included the pres-ence of solvent molecules; traditionally the solvent has beenignored to save computational time. The work is a good exam-ple of how computational chemistry can not only be used asa screening tool to test several different molecules, but moreimportantly to develop an understanding on the behavior ofdifferent systems as a function of their molecular character-istics. This reduces the number of experiments required andallows one to do more intelligent experiments.Molecular modeling techniques incorporating molecularmechanics and molecular dynamics can be used to simu-late the adsorption from 1 M HCl solution of a single targetmolecule from triazole derivatives on iron (1 1 0) surface.The quantum mechanical approach may well be able to fore-tell molecule structures that are better for corrosion inhibition.Double layer capacitances decreases with respect to the blanksolution when these inhibitors are added. This fact may beexplained on the basis of adsorption of these inhibitors on thesteel surface.In determining the corrosion rates, electrochemical studiesand weight loss measurements give similar results.Triazole derivatives can be used as corrosion inhibitors formild steel in 1 M HCl.

eferences

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[2] A. Halperin, M. Tirell, T.P. Lodge, Adv. Polym. Sci. 100 (1992) 31.

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[3] S.L. Li, Y.G. Wang, S.H. Chen, R. Yu, S.B. Lei, H.Y. Ma, D.X. Liu, Corros.Sci. 41 (1999) 1769.

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