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Corrosion monitoring of mild steel in sulphuric acid solutions in presenceof some thiazole derivatives – Molecular dynamics, chemical and electrochemicalstudies

K.F. Khaled a,*, Mohammed A. Amin b

a Electrochemistry Research Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egyptb Chemistry Department, Faculty of Science, Ain Shams University, Al-Khalefah Al-Maamoon St., 11566 Abbassia, Cairo, Egypt

a r t i c l e i n f o

Article history:Received 24 March 2009Accepted 15 May 2009Available online 23 May 2009

Keywords:A. Mild steelB. Molecular dynamicsB. Tafel extrapolationB. EISB. EFMC. Acid corrosion

a b s t r a c t

The physical behavior of three selected thiazole derivatives, namely 2-Amino-4-(p-tolyl)thiazole (APT), 2-Methoxy-1,3-thiazole (MTT) and Thiazole-4-carboxaldehyde (TCA) at iron (1 1 0) surface dissolved inaqueous solution were studied via molecular dynamics (MD) simulations. From the calculated bindingenergies, APT showed preferred adsorption on the steel surface among the three tested thiazole deriva-tives. The inhibition performance of the three thiazoles on the corrosion of mild steel in 0.5 M H2SO4 solu-tions was investigated at 25 �C. Measurements were conducted under various experimental conditionsusing weight loss, Tafel polarization and electrochemical impedance spectroscopy. Electrochemical fre-quency modulation (EFM) technique was also employed here to make accurate determination of the cor-rosion rates and test validation of the Tafel extrapolation method for measuring corrosion rates.Polarization curves showed that the three thiazole derivatives were of mixed-type inhibitors for mildsteel corrosion in 0.5 M H2SO4 solution. EFM results were in agreement with other traditional chemicaland electrochemical techniques used in corrosion rate measurements. Chemical and electrochemicalmeasurements are consistent with computational study that APT is the most effective inhibitor amongthe tested thiazoles.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Despite the experimental work in corrosion inhibition studies, anunderstanding of the atomic level interactions at the inhibitor/metalinterface is still needed. Although various experimental and theoret-ical techniques [1–4] have been developed to elucidate the struc-tural properties of molecules at interfaces little is known about theinteractions that occur between adsorbed molecules and solidsurfaces [5]. A practical route in order to study these very complexprocesses is computer simulations [6–12] of suitable models. Inter-actions at metal surfaces and interfaces are critical to understand thecorrosion inhibition phenomena. Properties of interest to corrosionscientists include the structure of the interface or interphase, howit differs from the bulk and the interaction of inhibitor moleculeswith metal surface. Due to the relatively large number of molecules(atoms) involved in adsorption on metal surface, a moleculardynamics method using a proper force field is the best choice.Contrary, quantum chemistry computing methods, such as ab initioand semi-empirical methods, are computational expensive and areusually only applied to systems containing no more than 100 atomsor small molecules. Recently, molecular dynamics (MD) simulations

are performed to study the interaction of several corrosion inhibitorswith metal surfaces [6,11,12–14]. From these studies, moleculardynamics simulations provide insights into the design of inhibitorsystems with superior properties.

In general, most of the effective and efficient inhibitors in usageare organic compounds having p bonds in their structures. Theefficiency of an organic compound as a successful inhibitor ismainly dependent on its ability to get adsorbed on the metalsurface. 4-acetyl pyridine [15], aliphatic amines [16] such asdimethylamine, ethylamine, diethylamine, butylamine, butyldi-ethylamine and other derivatives of octylamine inhibit the corro-sion of steel in acid solution by donating the unshared pair ofelectrons from the N atom and form a surface complex. Alkylenepyridinium compounds inhibit mild steel corrosion in 0.5 MH2SO4 due to inter-molecular synergism by the introduction ofsubstituents in the pyridine ring [17]. The inhibitive effect of ani-line and alkylamine [18], p-substituted anilines [19] and N-substi-tuted anilines [20] are attributed to the interaction of p-electroncloud of aromatic ring on iron and steel surface through vacant‘d’orbital of iron leading to the formation of co-ordination bondbetween Fe–N. The corrosion inhibition property of low molecularweight straight chain amines for steel corrosion has also beenstudied [21]. Ortho-substituted anilines [22] and polyanilines[23–24] were also studied.

0010-938X/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2009.05.023

* Corresponding author. Tel.: +20 27650485; fax: +20 222581237.E-mail address: [email protected] (K.F. Khaled).

Corrosion Science 51 (2009) 1964–1975

Contents lists available at ScienceDirect

Corrosion Science

journal homepage: www.elsevier .com/locate /corsc i


In this paper, the inhibition effect of three selected thiazolederivatives, namely, 2-Amino-4-(p-tolyl)thiazole (APT), 2-Meth-oxy-1,3-thiazole (MTT) and Thiazole-4-carboxaldehyde (TCA) onthe corrosion of mild steel in 0.5 M H2SO4 was discussed. Theinhibitive properties of these three thiazoles were investigatedexperimentally, using measurements based on weight loss, Tafelpolarization, impedance and electrochemical frequency modula-tion techniques. Corrosion rates (in lm y�1) determined from theEFM technique were compared with those recorded from the Tafelextrapolation method. It was also the purpose of the present workto study the adsorption of these compounds on iron (1 1 0) surface,using molecular dynamics simulations. The objective of thistheoretical study is to explain the mechanism of adsorption, andtherefore offer theoretical information regarding designing andsynthesizing novel corrosion inhibitors.

2. Theoretical models and methods

Accurate simulation of atomic and molecular systems generallyinvolves the application of quantum mechanical theory. However,quantum mechanical techniques are computationally expensiveand are usually only applied to small systems containing between10 and 100 atoms, or small molecules. It is not practical to modellarge systems such as corrosion systems containing many thou-sands of molecules in this way. Even if such a simulation were pos-sible, in many cases much of the information generated would bediscarded. This is because in simulating large systems, the goal isoften to extract bulk (statistical) properties, such as diffusion coef-ficients or Young’s modulus, which depend on the location of theatomic nuclei or, more often, an average over a set of atomic nucleiconfigurations. Under these circumstances the details of electronicmotion are lost in the averaging processes [25].

The Discover molecular dynamics module in Materials Studio4.3 software from Accelrys Inc. [26] allows selecting a thermody-namic ensemble and the associated parameters, defining simula-tion time, temperature and pressuring and initiating a dynamicscalculation. The first step to calculate the interaction energy be-tween the inhibitor molecule and the metal surface is to buildthe iron (1 1 0) surface by importing the iron crystal and thencleave its surface through the cleavage plane (1 1 0). To get anaccurate result, the thickness of the surface must be more thanthe non-bond cut-off distances in the force field. After buildingthe iron (1 1 0) surface, the surface must be relaxed by minimizingits energy using molecular mechanics. The next step is to increasethe surface area of the iron (1 1 0) and changing its periodicity byconstructing a super cell and building a vacuum slab with zerothickness. Thiazole derivatives are built using the sketching toolsin Materials Visualizer and their geometries are optimized.

The solvent effect on molecular structure of solute can be stud-ied by a model, which is known as polarized continuum model(PCM) [27]. In this model, solvent is treated as a continuum dielec-tric media and the solute considered as a trapped molecule in acavity surrounded by solvent. Furthermore, the solvent dielectricconstant is decreased (from about 80 to around 6 for aqueous solu-tions) when going from bulk of solution towards electrical doublelayer EDL [28]. Thus a realistic model of corrosion inhibitor musttake these facts into considerations. After minimizing the surfaceand the thiazole derivative as well as the aqueous layer, the corro-sion system will be built by layer builder to place the thiazolederivatives on iron (1 1 0) surface. A molecular dynamics (MD)simulation is run to equilibrate the system. The MD simulationwas performed under 298 K, NVT ensemble, with a time step of1.0 fs and simulation time of 30 ps to allow the system to fullyequilibrate and finally calculating the interaction energy betweenthe thiazole derivative and the iron (1 1 0) surface.

The interaction energy EFe�inhibitor of the Fe surface with the thi-azole derivatives was calculated according to the followingEquation:

EFe�inhibitor ¼ Ecomplex � ðEFe þ EinhibitorÞ ð1Þ

with Ecomplex is the total energy of the Fe crystal together with theadsorbed inhibitor molecule, EFe and Einhibitor are the total energyof the Fe crystal and free inhibitor molecular, respectively. Thebinding energy of the inhibitor molecule is the negative value ofthe interaction energy, Ebiniding ¼ �EFe�inhibition [6] then; the systemwas simulated in water solution with a density of 1.0 g cm�3.

For the whole simulation procedure, COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic SimulationStudies) forcefield [29] which used to optimize the structures ofall components of the system of interest (Fe/solvent/thiazolesderivatives) and represents a technology break-through in force-field method. COMPASS is the first ab initio forcefield that enablesaccurate and simultaneous prediction of chemical properties(structural, conformational, vibrational, etc.) and condensed-phaseproperties (equation of state, cohesive energies, etc.) for a broadrange of chemical systems. It is also the first high quality forcefieldto consolidate parameters of organic and inorganic materials.

3. Experimental procedures

Experiments were carried out using mild steel rods (C = 0.12%;Mn = 0.85%; S = 0.055%; P = 0.05%; Si = 0.09% and the remainderiron), as the electrode material. Steel rods were mounted in Teflonwith surface area of 0.28 cm2. The surface was abraded using em-ery papers of (180, 120, 0, 4/0) grit size, polished with Al2O3

(0.5 lm particle size), cleaned in 18 MX water in an ultrasonicbath, and subsequently rinsed with acetone and bidestilled water,the same procedure was also, used in case of gravimetricexperiments.

Gravimetric experiments were carried out in a double glass cellequipped with a thermostated cooling condenser. The solution vol-ume was 100 ml. The steel specimens used have a rectangular form(length = 2 cm, width = 1 cm, thickness = 0.06 cm). The duration oftests was 6 hours at 25 �C. Duplicate experiments were performedin each case and the mean value of the weight loss is reported.Weight loss allowed calculation of the mean corrosion rate in(mg cm�2 h�1).

A conventional electrolytic cell, as described elsewhere [30],was used for all of the experiments with a platinum counterelectrode and a saturated calomel electrode (SCE) as a referenceelectrode. All reported potential values are on the SCE scale.Electrochemical experiments were carried out under stagnantconditions at 25 �C in aerated solutions, with a fine Luggin capillaryclosely placed to minimize ohmic resistance.

The structures of the studied thiazole derivatives are presentedbelow:

S

N

O

Thiazole-4-carboxaldehyde

S

N O

2-Methoxy-1,3-thiazole

S

N

NH2

2-Amino-4-(p-tolyl)thiazole

(APT) (MTT) (TCA)

All compounds investigated were obtained from Sigma–AldrichChemical Co. They were put in the 0.5 M H2SO4 (Fisher Scientific)without pretreatment at concentration of 10�4 M, 10�3 M,5.0 � 10�3 M and 10�2 M. The electrode was immersed in thesesolutions for one hour before starting measurements.

K.F. Khaled, M.A. Amin / Corrosion Science 51 (2009) 1964–1975 1965


Fig. 1. Energy fluctuation curves performed for the three thiazole derivatives on Fe (1 1 0) surface obtained by molecular dynamic simulations.

1966 K.F. Khaled, M.A. Amin / Corrosion Science 51 (2009) 1964–1975


Potentiodynamic cathodic and anodic polarization curves wereobtained by changing the electrode potential automatically from(�250 up to +250 mVSCE) versus open circuit potential with a scanrate of 1.0 mV s�1. EIS measurements were carried out in a fre-quency range of 100 kHz–30 mHz with an amplitude of 5.0 mVpeak-to-peak using ac signals at open circuit potential. EFM mea-surements carried out using two frequencies of 2.0 and 5.0 Hz.The base frequency was 1.0 Hz.

Electrochemical measurements were performed using GamryInstrument Potentiostat/Galvanostat/ZRA. This includes a GamryFramework system based on the ESA400, Gamry applications thatinclude DC105 for dc corrosion measurements, EIS300 for electro-chemical impedance spectroscopy measurements along with acomputer for collecting data. Echem Analyst 4.0 Software was usedfor plotting, graphing and fitting data.

4. Results and discussion

4.1. Computational study

The molecular dynamics simulations are performed to study theadsorption behavior of some thiazole derivatives (APT, MTT andTCA) on Fe (1 1 0) surface. The studied system reaches equilibriumonly if both of the temperature and energy reach balance. The geom-etry optimization of the studied system is carried out using aniterative process, in which the atomic coordinates are adjusted untilthe total energy of a structure is minimized, i.e., it corresponds to alocal minimum in the potential energy surface. Fig. 1 shows theenergy fluctuation curves. It can be seen from Fig. 1 that the systemtends to equilibrium, by the end of the simulation process. The val-ues of the interaction energy and the binding energy of the three thi-azole derivatives on iron (1 1 0) surface are listed in Table 1.

As can be seen from Table 1, the binding energy, in all cases, hasa positive value. As the value of the binding energy increases, themore easily the inhibitor adsorbs on the metal surface, the higherthe inhibition energy [6]. It follows from the data presented in Ta-ble 1 that, during the simulation process, APT possesses the maxi-mum binding energy to the iron (1 1 0) surface among the threetested inhibitors. High values of binding energy indicate that APTmolecule will give the highest inhibition energy from the theoret-ical point of view. From the structure of iron surface, it is evidentthat the presence of unoccupied d-orbital exhibits a tendency toobtain electrons from the adsorbed molecules. Thiazole derivativesstudied in this work have a number of lone pair of electrons on the

S, N and O atoms as well as the p-electron clouds on the aromaticrings. This will make it possible to provide electrons to the unoccu-pied orbitals of iron surface to form a stable co-ordination bonds.Therefore the studied thiazole derivatives are likely to chemisorbon the iron surface, protecting it from corrosion. In addition, basedon the equilibrium configuration of the three thiazole derivativesadsorbed on Fe (1 1 0) surface, presented in Fig. 2, we can draw aconclusion that the studied thiazole derivatives can be absorbedon the Fe surface through the thiazole ring and heteroatoms. In thisway, the exposed part of Fe surface can be reduced by the coveringof the thiazole ring and heteroatoms, consequently preventing thesurface from the acid.

4.2. Weight loss measurements

Table 2 presents the values of the corrosion rates (in g cm�2 h�1)and the inhibition efficiency, nw (%), derived from weight loss mea-surements, for mild steel corrosion in 0.5 M H2SO4 solutions as afunction of the concentration of the three tested thiazole derivav-tives. Data in Table 2 indicate that, in all cases, corrosion ratedepends on the nature and concentration of the thiazoles underinvestigation. The inhibition efficiency, nw (%), was evaluated usingEq. (2):

nwð%Þ ¼ ½ðw0 �wÞ=w0� � 100 ð2Þ

where w0 and w are the weight losses of mild steel in uninhibitedand inhibited H2SO4 solutions, respectively. It is obvious that the

Table 1Interaction and binding energy of the thiazole derivatives on iron (1 1 0) surface.

Inhibitor EFe�inhibitor/kJ mol�1 Ebinding (kJ mol�1)

APT �398.1 398.1MTT �327.6 327.6TCA �261.7 261.7

Table 2Corrosion rate (in g cm�2 h�1) and inhibition efficiency, nw (%), values obtained fromweight loss measurements for mild steel in 0.5 M H2SO4 solutions without and withvarious concentrations of thiazole derivatives at 25 �C.

Inhibitor Concentration(M)

(Corrosion rate)(mg cm�2 h�1)

nw (%)

Blank 5.21 –

TCA 10�5 2.89 44.510�4 2.69 48.25 � 10�4 2.46 52.610�3 1.84 64.615 � 10�3 1.11 78.6310�2 4.01 92.3

MTT 10�5 2.54 51.210�4 2.28 56.35 � 10�4 2.09 59.810�3 1.59 69.45 � 10�3 0.80 84.610�2 0.34 93.4

APT 10�5 1.96 62.310�4 0.93 82.15 � 10�4 0.54 89.710�3 0.34 93.55 � 10�3 0.29 94.410�2 0.01 98.1

Fig. 2. Equilibrium configuration performed for the three thiazole derivatives on Fe (1 1 0) surface obtained by molecular dynamic simulations.



inhibition efficiency values for the three tested thiazole derivativesincrease with increase in their concentrations, and this increase inthe inhibition efficiency, at a given inhibitor concentration, en-hances, as will be discussed later, following the order:APT > MTT > TCA. Thus, weight loss measurements, in accordancewith theoretical study, demonstrate that all studied thiazoles exhi-bit inhibition properties, and the most effective one is the APTderivative. Indeed, we see that the skeleton of the three tested thi-azole derivatives is approximately similar (except for the presenceof a substituted phenyl group in the APT structure). The high inhi-bition performance recorded for APT, therefore can be explainedon the basis of the larger steric body of APT caused by the phenylring. This larger steric body of APT in comparison to MTT and TCAmay result in a higher surface coverage area during the chemicaladsorption process. The phenyl ring in APT molecule may also sup-port electron donation to the vacant d-orbital of the Fe atoms, andconsequently chemical adsorption of this molecule is favored.

4.3. Tafel Polarization measurements

Tafel polarization curves recorded for mild steel electrode in0.5 M H2SO4 solutions without and with various concentrationsof APT, MTT or TCA are shown in Fig. 3. It is clear from Fig. 3 thatboth cathodic and anodic reactions of mild steel electrode corro-sion were inhibited with the increase of thiazole derivatives con-centrations in 0.5 M H2SO4. It is obvious that the polarizationplots display a cathodic region of Tafel behavior, see later.

On the other hand, the anodic polarization curves do not displayan extensive Tafel region, may be due to the deposition of thecorrosion products or impurities on the steel (e.g., Fe3C) to forma non-passive surface film [31]. The deposition of corrosion prod-ucts and the existence of such impurities do not result in a well-de-fined experimental anodic Tafel region. The cathodic curves are,therefore preferred, as will be seen, over the anodic ones for deter-mining corrosion currents using the Tafel extrapolation method.

It also follows from the polarization plots presented in thisfigure that the shapes of the polarization plots for inhibited elec-trodes are not substantially different from those of uninhibitedelectrodes. The presence of these compounds decreases the corro-sion rate but does not change other aspects of the behavior. Thismeans that these inhibitors do not alter the electrochemical reac-tions responsible for corrosion. In addition, as will be seen, theabsence of significant changes in the cathodic Tafel slope in thepresence of inhibitor indicates that the hydrogen evolution reac-tion is activation controlled. This reaction is, therefore sloweddown by the surface blocking effect of the three inhibitors. Thisindicates that the inhibitive action of these three thiazole deriva-tives may be related to their adsorption and formation of a barrierfilm on the electrode surface.

Based on E. McCafferty, who presented a valuable study con-cerning validity of the Tafel extrapolation method for measuringcorrosion rates [32], the use of both the anodic and cathodic Tafelregions is undoubtedly preferred over the use of only one Tafelregion for corrosion rates determination by this method. However,McCafferty pointed out that the corrosion rate can also be deter-mined by Tafel extrapolation of either the cathodic or anodic polar-ization curve alone. If only one polarization curve alone is used, itis generally the cathodic curve which usually produces a longerand better defined Tafel region. Anodic polarization may some-times produce concentration effects, due to passivation and disso-lution, as noted above, as well as roughening of the surface whichcan lead to deviations from Tafel behavior.

Due to the absence of linearity in our anodic branches, accurateevaluation of the anodic Tafel slope by Tafel extrapolation of theanodic branch is impossible [33–37]. There is, therefore an uncer-tainty and source error in the numerical values of the anodic Tafel

slope (ba), and may be in values of jcorr too, calculated by the soft-ware; the reason why we did not introduce here jcorr as well as ba

values recorded by the software. On the same basis, we cannotdetermine jcorr from impedance plots, since these require accurateTafel slopes. For these reasons we used, as will be seen later, theelectrochemical frequency modulation, EFM, technique (featuring

-700

-600

-500

-400

-300

-5 -4 -3 -2 -1

1

7

7

log (j / A cm-2)

E /

mV

(S

CE

)

1

ATP

-700

-600

-500

-400

-300

-5 -4 -3 -2 -1

1

7

7

log (j / A cm-2)

E /

mV

(S

CE

)1

MTT

-700

-600

-500

-400

-300

-5 -4 -3 -2 -1

1

7

7

log (j / A cm-2)

E /

mV

(S

CE

)

1

TCA

Fig. 3. Anodic and cathodic polarization curves recorded for mild steel electrode in0.5 M H2SO4 solutions without and with various concentrations of the threethiazole derivatives at 25 �C. (1) Blank; (2) 10�5 M; (3) 10�4 M; (4) 5 � 10�4 M; (5)10�3 M; (6) 5 � 10�3 M; (7) 10�2 M.



of obtaining the Tafel slopes directly, small polarization and shortmeasuring time, offers a novel way for electrochemical corrosionmonitoring) to evaluate accurate values of the corrosion ratesand compare them with those obtained from the Tafel extrapola-tion method. The aim here is to test validation of corrosion ratesmeasured by Tafel extrapolation method; see details in Section 4.5.

We believe that the existence of the anodic Tafel region,together with the cathodic Tafel region, in the polarization plotsis essential for accurate evaluation of the corrosion rates. For thisreason we adopted here McCafferty method for evaluation ofcorrosion rates using Tafel extrapolation method [32]. This methodwas successfully applied in our previous study [38]. Based on thismethod, the cathodic Tafel region is extrapolated back to zeroovervoltage to give the net rate of the cathodic reaction at the cor-rosion potential; this is also the net rate of the anodic reaction atthe corrosion potential. Consequently, it is possible to calculatethe anodic Tafel line from the experimental data, as shown inFig. 4, as a representative example. The Tafel line of the cathodicpolarization curve is first extended, since it exhibits a linear Tafelregion, to electrode potentials below the corrosion potential, andthen the anodic current density ja is calculated from:

ðjnetÞa ¼ ½ðjFeÞa þ ðjHÞa� � ½ðjjFejÞc þ ðjjHjÞc� ð3Þ

or, simply:

Jaðnet experimentalÞ ¼ ja � jjcj ð4Þ

where the subscripts a and c refer to the anodic and cathodic direc-tion, respectively. Thus, (jFe)a and (|jH|)c refer, respectively, to therates of the anodic dissolution of Fe and the cathodic hydrogenevolution reactions; see more details in Ref. [32]. |jc| is the cathodiccurrent density. Thus, the anodic current density ja is the sum of theexperimentally observed anodic current density and the extrapo-lated cathodic current density. The result from Fig. 4 is thatjcorr = 4.4 � 10�4 A cm�2. This jcorr value is converted into the corro-sion rate in lm y�1, see later, using Eq. (5) [39].

Corrosion rate ðlm y�1Þ ¼ 3280� jcorr � ðM=ndÞ ð5Þ

where M is the atomic weight of Fe (55.85 g), n the number of elec-trons transferred in the corrosion reaction (n = 2) and d the densityof Fe (7.88 g cm�3).

It well known that Tafel slopes have units of Volts per current-density-decade, where a decade is one order of magnitude currentdensity, such as from 0.10 to 1.0 A cm�2. Fig. 4 also illustrates howto estimate the anodic and cathodic Tafel slopes for inhibitor-freesulphuric acid solutions. In this example, the Tafel slopes were esti-

mated to be ba = 34 mV dec�1 and bc = �170 mV dec�1. The rest val-ues for jcorr, ba and bc (together with the corrosion potential, Ecorr)were also recorded for various concentrations of the three inhibi-tors, Table 3, using the respective polarization curves based onthe same calculations presented in Fig. 4. Inhibition efficiency val-ues, nTafel (%), calculated from Eq. (6), are also presented in Table 3.

nTafelð%Þ ¼ ½ðj0corr � jcorrÞ=jcorr� � 100 ð6Þ

As it can be seen from these polarization results, the jcorr values, inall cases, decrease in the presence of the three thiazoles, and thisdecrease enhances with increasing inhibitor concentration. It isobvious that, under the same conditions, the corrosion currentdecreases in the order: TCA > MTT > APT. This sequence againreflects the increased ability of APT to inhibit steel corrosion in sul-phuric acid solutions as compared to MTT and TCA. No definite trendwas observed in the shift of Ecorr values, in the presence of variousconcentrations of these inhibitors in 0.5 M H2SO4 solutions. Basedon these results, thaizole derivatives are considered as mixed-typeinhibitors. Meaning that the addition of thaizole derivatives to0.5 M H2SO4 solutions reduces the anodic dissolution of mild steeland also retards the cathodic hydrogen evolution reaction. Furtherinspection of Table 3 reveals that the inhibition efficiency for thethree thiazoles enhances with increase in their concentrations. Thehighest efficiency was also recorded for APT. These findings confirmtheoretical study and weight loss measurements that APT is the mosteffective inhibitor among the three thiazoles used.

It is well known that the spontaneous dissolution of steel in acidmedium give Fe2+ ions, this reaction accompanied by hydrogenevolution reaction which is a cathodic reaction. The corrosion ofiron in uninhibited acidic solutions is controlled by the hydrogenevolution reaction [40–42]. In presence of an adsorbed inhibitormolecules, hydrogen ionization as well as the proceeding of elec-trochemical reactions can be neglected on the surface sites coveredby the inhibitor molecules. Besides, it is accepted that the presenceof Fe2+ (due to metal dissolution) on the uncovered surface has al-most no effect [41]. The studied thiazole derivatives affect the rateof the corrosion process mainly through the variation of the degreeof surface coverage, i.e., by blocking effect they exert, as previouslymentioned. This is in turn results in a decrease of the concentrationof the hydrogen ions in the surface layer and a correspondingincrease in the hydrogen evolution overpotential.

-700

-600

-500

-400

-300

-5 -4 -3 -2 -1

jcorr = 4.40 x 10-4

A cm-2

βa = 34 mV dec-1

jcorr

One decade

Calculated

anodic

Anodic Tafel

line

Experimental

anodic

Experimental

cathodic

log (j / A cm-2)

E /

mV

(S

CE

)

βc = -170 mV dec-1

Extrapolated

cathodic

Fig. 4. Determining corrosion current and Tafel slopes from Tafel plots using Tafelextrapolation method for the cathodic and anodic polarization curves recorded formild steel electrode in 0.5 M H2SO4 solutions at a scan rate of 1.0 mV s�1 and 25 �C.

Table 3Electrochemical parameters, together with the inhibition efficiencies, nTafel (%),calculated from polarization measurements on the steel electrode in 0.5 M H2SO4

solutions without and with various concentrations of the three thiazole derivatives at25 �C.

Inhibitor [Inhib.]/M

�Ecorr/mVSCE

�bc/mV dec�1

ba/mV dec�1

jcorr /l A cm�2

nTafel (%)

Blank 513 170 34 440 –

TCA 10�5 497 172 36 283 35.6910�4 494 168 35 272 38.095 � 10�4 503 169 37 252 42.8210�3 502 171 38 183 58.485 � 10�3 498 168 36 151 65.5810�2 505 167 37 79 82.05

MTT 10�5 524 168 33 250 43.1010�4 522 167 34 222 49.515 � 10�4 534 166 35 210 52.2710�3 537 167 34 166 62.315 � 10�3 551 169 36 91 79.3010�2 526 167 35 52 88.29

APT 10�5 525 165 34 201 54.3110�4 515 166 33 103 76.495 � 10�4 521 168 34 74 83.0910�3 520 165 35 54 87.795 � 10�3 512 168 34 43 90.1210�2 514 166 34 21 95.19



Fig. 5. Complex-plane impedance plots recorded for mild steel electrode in 0.5 M H2SO4 solutions without and with various concentrations of the three thiazole derivatives at25 �C.



4.4. Impedance measurements

It is evident that the more efficient the inhibitor, the lower thecorresponding corrosion rate and the higher the charge-transferresistance, Rct. EIS technique was used to evaluate the efficiencyof the three tested thiazoles against the sulphuric acid corrosionof steel. In most cases either the values of the corrosion rates orinhibition efficiencies obtained from Rct values were comparedwith those obtained using other techniques.

The results of the EIS measurements were presented in Fig. 5 asNyquist plots. Impedance measurements were conducted in 0.5 MH2SO4 solutions without and with different concentrations of thethree thiazoles at the respective corrosion potentials at 25 �C. Com-plex-plane impedance plots presented in Fig. 5 showed a capacitiveloop in the high frequency (HF) range and an inductive loop in thelow frequency (LF) range. The capacitive loop was related tocharge-transfer in corrosion process [43]. The depressed form ofthe higher frequency loop reflects the surface inhomogeneity ofstructural or interfacial origin, such as those found in adsorptionprocesses [44]. The presence of the LF inductive RL-L loop may beattributed to the relaxation process obtained by adsorption specieslike Hads

+ on the electrode surface. It may also attribute to the re-dissolution of the passivated surface at low frequencies [45].

The two equivalent circuits, depicted in Fig. 6, were used to fitthe EIS data. In these two circuits, the solution resistance, Rs, thecharge-transfer resistance, Rct, the constant phase element, CPE,and the inductive arrangement, RL and L, were fitted and their

values were listed in Table 4. The Rct values increase with theincrease in concentration of thiazole derivatives, indicating the for-mation of an insulated adsorption layer. Indeed, the inhibition effi-ciencies of these inhibitors, as will be seen, were evaluated by Rct

and Cdl values of the impedance. The more densely packed themonolayer of the inhibitor, the larger the diameter of the semicir-cle, which results in higher Rct and lower Cdl values. Data presentedin Table 4 showed that, in all cases, the values of Rct increase withincrease in inhibitor concentration, while those of Cdl (derived, aswill be discussed from CPE) tend to decrease.

The Rct values were used to calculate the inhibition efficiency, nz

(%), as a function of the concentration of the three tested thiazoles,using Eq. (7):

nzð%Þ ¼ fðRct � R0ctÞ=RctÞg � 100 ð7Þ

where R0ct and Rct are the charge-transfer resistance values in the

absence and presence of the inhibitor, respectively. It is apparentthat, in all cases, the inhibition efficiency increases with inhibitorconcentration. A maximum inhibition efficiency of about 97% wasrecorded for APT at a concentration of 0.01 M. These results againconfirm previous findings that APT was better inhibitor than MTTand TCA under these conditions.

CPE is used as a substitute for capacitor to fit more accuratelythe impedance behavior of the electric double layer. The imped-ance of the CPE is expressed as:

ZCPE ¼1

Y0ðjxÞnð8Þ

where Y0 is the magnitude of the CPE, �1 6 n 6 1. The higher fre-quency range loops have depressed semicircular appearance,0.5 6 n 6 1, which is often referred to as frequency dispersion as aresult of the non-homogeneity or the roughness of the solid surface[44,46]. The HF capacitive loop, Rct-CPE, can be attributed to thecharge-transfer reaction.

The capacitance values can be calculated from CPE parametervalues Y and n using the expression [47].

Cdl ¼Yxn�1

sinðnðp=2ÞÞ ð9Þ

According to the expression of the layer capacitance presented inthe Helmholtz model [48].

Cdl ¼e0ed

A ð10Þ

where d is the thickness of the film, A is the surface area of the elec-trode, e0 is the permittivity of the air and e is the local dielectricconstant. The decrease in Cdl is due to the gradual replacement ofwater molecules by the adsorption of the thiazole derivatives atmild steel/0.5 M H2SO4 solution interface, leading to a protectivefilm on the steel surface, then decreasing the extent of dissolutionreaction [49]. Also, the increase of n values, Table 4, with concentra-tion of the three thiazoles could be explained by decreasing in

Fig. 6. Equivalent circuits used to fit experimental EIS data recorded for mild steel electrode in 0.5 M H2SO4 solutions in the absence and presence of various concentrations ofthe three thiazole derivatives at 25 �C. (a) Without inductive loop (b) with inductive loop.

Table 4Electrochemical parameters, together with the inhibition efficiencies, nz (%), calcu-lated from EIS measurements on mild steel electrode in 0.5 M H2SO4 solutionswithout and with various concentrations of the three thiazole derivatives at 25 �C.

Inhibitor [Inhib.]/M

Rs/X cm2

CPE/Y0

(lF cm�2)n Rct/

X cm2RL/X cm2

L/H cm2

nz (%)

Blank 1.2 85.6 0.81 11.82 – – –

TCA 10�5 0.173 83.17 0.83 20.81 – – 43.2010�4 1.58 79.26 0.82 21.53 – – 45.105 � 10�4 1.91 68.64 0.88 24.2 0.5 0.0235 51.2010�3 2.51 55.07 0.91 30.70 0.77 0.0701 61.505 � 10�3 1.30 48.07 0.86 47.85 0.81 0.237 75.3010�2 6.5 44.35 0.93 108.0 2.1 0.391 89.10

MTT 10�5 0.147 79.44 0.79 23.31 – – 49.3010�4 0.224 76.01 0.78 25.97 – – 54.505 � 10�4 0.157 69.12 0.76 29.26 – – 59.6010�3 0.235 50.27 0.81 36.25 0.6 0.611 67.405 � 10�3 0.478 45.78 0.83 70.35 0.52 2.6 83.2010�2 1.5 42.52 0.85 149.6 1.2 3.2 92.10

APT 10�5 0.266 70.21 0.85 30.3 0.11 0.611 61.0010�4 0.330 57.66 0.81 57.1 0.10 4.707 79.305 � 10�4 6.23 39.95 0.79 105.5 4.2 1.894 88.8010�3 1.3 37.65 0.78 134.3 0.8 1.58 91.205 � 10�3 6.31 32.65 0.76 173.8 2.1 2.44 93.2010�2 15.35 28.45 0.77 369.4 1.3 4.1 96.80



surface heterogeneity, due to the adsorption of the inhibitor on themost active adsorption sites [50].

4.5. Electrochemical frequency modulation, EFM measurements

The corrosion rate and Tafel parameters can be obtained with onemeasurement by analyzing the harmonic frequencies. Because cur-rent is a non-linear function of potential, the system responds in anon-linear way to the potential excitation. The current response con-tains the input frequencies, frequency components which are thesum, difference, and multiples of the two input frequencies.

In corrosion research, it is known that the corrosion process isnon-linear in nature, a potential distortion by one or more sine

waves will generate responses at more frequencies than thefrequencies of applied signal. Virtually no attention has been givento the intermodulation or electrochemical frequency modulation.However, EFM showed that this non-linear response containsenough information about the corroding system so that the corro-sion current can be calculated directly. The great strength of theEFM is the causality factors which serve as an internal check onthe validity of the EFM measurement [51] .With the causality fac-tors the experimental EFM data can be verified.

The results of EFM experiments are a spectrum of currentresponse as a function of frequency. The spectrum is called theintermodulation spectrum and examples for corrosion of mild steelin absence and presence of thiazole derivatives in 0.5 M H2SO4 are

Fig. 7. Representative examples for intermodulation spectra recorded for mild steel electrode in 0.5 M H2SO4 solutions in the absence and presence of various concentrationsof the three thiazole derivatives at 25 �C.



shown in Fig. 7. The spectra contain current responses assigned forharmonical and intermodulation current peaks. The larger peakswere used to calculate the corrosion current density (jcorr), theTafel slopes (bc and ba) and the causality factors (CF-2 and CF-3).These electrochemical parameters were simultaneously deter-mined by Gamry EFM140 software, and listed in Table 5.

As can be seen from Table 5, the corrosion current densitiesdecrease by increasing the concentrations of the studied inhibitors.The inhibition efficiencies, nEFM (%), calculated from Eq. (11),increase by increasing the studied inhibitor concentrations.

nEFMð%Þ ¼ ½ðj0corr � jcorrÞ=jcorr� � 100 ð11Þ

where j0corr and jcorr are corrosion current densities in the absence

and presence of inhibitors, respectively. The causality factors inTable 5 indicate that the measured data are of good quality. Thestandard values for CF-2 and CF-3 are 2.0 and 3.0, respectively[51]. Here again, the values of jcorr recorded by the EFM technique

were converted into corrosion rates (in lm y�1) following Eq. (5).Table 6 presents values of corrosion rates monitored for mild steelin 0.5 M H2SO4 solutions without and with various concentrationsof the three tested thiazoles, using both EFM and Tafel extrapolationmethods at 25 �C. Data collected in Table 6 showed that the corro-sion rates obtained with the EFM technique are almost in agree-ment with those recorded using the Tafel extrapolation method.These results may confirm validation of corrosion rates measuredby the Tafel extrapolation method.

Fig. 8 showed the inhibition efficiencies recorded for APT, MTTand TCA at a concentration of 5 � 10�3 M using the four differenttechniques, namely weight loss, Tafel extrapolation, EIS and EFM.The calculated inhibition efficiencies obtained are in good agree-ment with that obtained from EFM. The corrosion rates calculatedby EFM were higher, but still of the same order of magnitude asthose obtained using other traditional techniques. These resultsare in consistence with our previous study [13,38,52–56]. Basedon these results, the electrochemical frequency modulation(EFM) technique appears capable of monitoring the corrosion inhi-bition of mild steel in 0.5 M H2SO4 solutions with different concen-trations of thiazole derivatives at 25 �C.

5. Conclusion

In the present work, molecular dynamics simulations methodusing a COMPASS forcefield is used to calculate the interactionenergy, and hence the binding energy of some thiazole derivativeson mild steel Chemical (weight loss) and electrochemical (Tafelpolarization, electrochemical impedance spectroscopy and electro-chemical frequency modulation) methods were used to study thecapability of theses thiazole derivatives to inhibit the corrosionof mild steel in aerated stagnant 0.5 M H2SO4 solutions at 25 �C.The following are the main conclusions of this study:

The molecular dynamics simulation results show that the threethiazole derivatives can adsorb on the iron (1 1 0) surface throughthe thiazole ring and the heteroatoms as well as p-electrons in thearomatic structure. Addition of thiazole derivatives reduced signif-icantly the corrosion of mild steel in 0.5 M H2SO4. The highest inhi-bition efficiency achieved with APT. Polarization curvesdemonstrated that the thiazole derivatives were of mixed-typeinhibitors for mild steel corrosion in these solutions. EIS plots indi-cated that the addition of inhibitors increases the charge-transferresistance of the corrosion process, and this increase in the

Table 5Electrochemical parameters, together with the inhibition efficiencies, nEFM (%), calculated from EFM measurements on mild steel electrode in 0.5 M H2SO4 solutions without andwith various concentrations of the three thiazole derivatives at 25 �C.

Inhibitor [Inhib.]/M jcorr/l A cm�2 �bc/mV dec�1 ba/mV dec�1 CF-2 CF-3 nEFM (%)

Blank 459 178 45 1.92 2.87 –

TCA 10�5 257 177 44 1.98 3.01 43.9710�4 262 179 43 1.99 2.99 43.005 � 10�4 234 178 42 2.01 2.89 48.9810�3 188 177 45 2.03 2.87 58.985 � 10�3 124 179 46 1.96 2.95 72.9910�2 60 178 42 1.87 3.04 87.00

MTT 10�5 230 175 41 1.94 3.0 49.9910�4 216 174 42 1.92 2.79 52.985 � 10�4 202 176 41 1.99 2.99 55.9710�3 147 175 40 1.99 2.89 68.005 � 10�3 73 177 44 2.01 2.78 83.9910�2 32 176 45 2.03 2.86 92.97

APT 10�5 175 174 47 1.89 2.96 61.9810�4 96 173 41 1.79 2.88 78.985 � 10�4 60 172 42 1.96 2.88 87.0010�3 37 174 39 1.87 2.86 91.995 � 10�3 32 176 43 1.94 2.98 92.9810�2 18 175 44 1.95 2.79 95.99

Table 6Corrosion rates (in lm y�1) monitored, based on EFM and Tafel extrapolationmethods, for mild steel electrode in 0.5 M H2SO4 solutions without and with variousconcentrations of the three thiazole derivatives at 25 �C.

Inhibitor [Inhib.]/M Corrosion rate/lm y�1

EFM method Tafel extrapolationmethod

Blank 5324 5104

TCA 10�5 2981 328310�4 3039 31555 � 10�4 2714 292310�3 2181 21235 � 10�3 1438 175210�2 696 916

MTT 10�5 2668 290010�4 2505 25755 � 10�4 2343 243610�3 1705 19265 � 10�3 847 105610�2 371 603

APT 10�5 2030 233410�4 1114 11955 � 10�4 696 85810�3 429 6265 � 10�3 371 49910�2 209 244



charge-transfer resistance, and hence the inhibition performance,enhances with inhibitor concentration and depends on the typeof the adsorbed molecule. EFM results were in good agreementwith other traditional chemical and electrochemical techniquesused. Corrosion rate monitored using the EFM technique was con-sistence with those obtained from the Tafel extrapolation method.

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