Inhibitive effect of some thiadiazole derivatives on C-steel corrosion in neutral sodium chloride solution

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Materials Chemistry and Physics 125 (2011) 26–36

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Inhibitive effect of some thiadiazole derivatives on C-steel corrosion in neutralsodium chloride solution

F. El-Taib Heakala,∗, A.S. Foudab, M.S. Radwanc

a Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egyptb Chemistry Department, Faculty of Science, El-Mansoura University, El-Mansoura, Egyptc Petrogulf Misr Company, Maadi, Cairo, Egypt

a r t i c l e i n f o

Article history:Received 20 February 2010Received in revised form 8 August 2010Accepted 18 August 2010

Keywords:C-steelThiadiazoleCorrosion inhibitionThermodynamic propertiesEISNeutral NaCl

a b s t r a c t

Electrochemical techniques were used to investigate the effect of concentration of three new thiadia-zole derivatives (I–III) on the corrosion behavior of C-steel in 0.5 M NaCl solution through the analysisof electrochemical measurements including open circuit potential (OCP), Tafel polarization and electro-chemical impedance spectroscopy (EIS). Polarization curves showed that the compounds studied act asanodic type inhibitors, where the inhibition efficiency increases with increase in inhibitor concentrationand decreases with rise in temperature. An adherent layer of inhibitor molecules on the surface is pro-posed to account for their inhibitive action in which the organic molecules adsorb on the active anodicsites following Langmuir isotherm. The thermodynamic parameters of adsorption and corrosion pro-cesses were determined and discussed. The results also indicated that pitting potential at higher anodicpolarization of C-steel in 0.5 M NaCl solution becomes more positive the higher the concentration of theadditive, suggesting that these inhibitors acts as retarding catalyst for pitting corrosion. EIS data confirmwell the electrochemical dc results and the results are all in good agreement with the calculated quan-tum chemical HOMO and LUMO energies of the tested molecules, as well as with surface examinationvia scanning electron microscope.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Corrosion is a naturally occurring phenomenon which deterio-rates a metallic material or its properties because of a reaction withits environment. Corrosion can cause dangerous and expensivedamage to everything from pipelines, bridges and public buildingsto vehicles, water and wastewater systems, and even home appli-ances. It is one of the most serious problems in the oil and gasindustry. The use of organic inhibitors is one of the most widelypractical methods for protection of metals and alloys against cor-rosion, since adding inhibitors does not cause disruption of theindustrial process. There are many studies deal with the corrosioninhibition effects of organic compounds in acidic solutions, espe-cially in the treatment of steel and ferrous alloys by hydrochloricor sulphuric acid [1–4] however; researches concerning steel cor-rosion in neutral chloride medium are meager [5–8]. This is despitethe fact that extracted crude oil often contains water contaminatedwith significant concentration of Cl− ions, which leads to extensivegeneral and localized pitting corrosion of C-steel [9].

It has been established that sulphur and/or nitrogen containingheterocyclic compounds with various substituents are considered

∗ Corresponding author. Tel.: +20 102449048; fax: +20 235728099.E-mail address: [email protected] (F. El-Taib Heakal).

to be effective corrosion inhibitors in different solutions over awide pH range. The inhibition efficiency of an organic compound ismainly depending on its ability to get adsorbed on metal surface.The protective nature of the adsorbed compact barrier film is influ-enced by the nature of the metal surface and electronic structureof inhibiting molecules. The adsorption of the nitrogenous com-pounds is ascribed to the effects of the functional groups connectedwith aromatic rings parallel to the metal surface [10].

The present work aims to investigate the inhibitive effect ofsome new thiadiazole derivatives for C-steel corrosion in NaClsolutions using different electrochemical measuring techniquesincluding open circuit potential (OCP), potentiodynamic polariza-tion and electrochemical impedance spectroscopy (EIS). The effectsof inhibitor concentration and temperature on the efficiencies ofthe tested compounds and on the corrosion inhibition behaviorwere examined. It was also the purpose of this work to addressthe relationship between the inhibition efficiency and quantumchemical parameters, namely, the energies of the highest occupiedmolecular orbital (EHOMO) and the lowest unoccupied molecularorbital (ELUMO), in order to support our findings.

2. Experimental methods

2.1. Cell and materials

A three-electrode glass cell with a capacity of 100 ml was used in all exper-iments. A large platinum sheet of size 20 mm × 10 mm × 2 mm and a saturated

0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2010.08.067


F. El-Taib Heakal et al. / Materials Chemistry and Physics 125 (2011) 26–36 27

Fig. 1. Chemical structure and molecular weights of the investigated thiadiazole derivatives.

calomel electrode (SCE) served as counter and reference electrodes, respectively.The tip of the luggin capillary included in the design was made very close to thesurface of the working electrode to minimize IR drop. The working electrode wasmachined to have a fixed exposed surface area of 0.12 cm2 from C-steel samplewith composition in wt% of: 0.200 C, 0.350 Mn, 0.024 P, 0.003 Si and the balanceFe. The test electrode was polished to 600 and then to 1000 grade emery paper toensure the same surface roughness, degreased with acetone, washed in distilledwater and dried in air. Before polarization and impedance experiments, OCP of theworking electrode was measured vs. time till reach a quasi-stationary value.

The corrosive medium (0.5 M NaCl) was prepared from a stock 4.0 M NaCl solu-tion by dilution with bi-distilled water. The compounds examined in this studywere synthesized according to a previously described experimental procedure [11],and purified with acetone. After preparation and recrystallization the compoundswere characterized by melting point, elemental analysis, NMR and IR spectroscopiesbefore use; their molecular structure formulae and molecular weights are given inFig. 1. An appropriate weighted amount from each compound was dissolved in ace-tone to prepare 0.001 M stock solution. From each inhibitor very low concentrationrange (1–11 �M) was employed, and if necessary a calculated volume from ace-tone was added to each test solution to keep the same acetone level at all inhibitorconcentrations.

2.2. Potentiodynamic polarization

Cathodic and anodic polarization curves were recorded in the potential regionOCP ± 500 mV(SCE) as a function of concentration or temperature at a sweep rateof 1 mV s−1 using Gamry potentiostat/galvanostat/ZRA (version 3.20, USA). Polar-ization curves in the vicinity of OCP were analyzed to determine the corrosionparameters using Gamry framework/analysis DC105 corrosion software. The corro-sion inhibition efficiency (%IE) was evaluated from the measured icorr values usingthe relationship (1):

%IE =(

1 − i′corr

icorr

)× 100 (1)

where icorr and i′corr are the corrosion current densities in the absence and presenceof various concentrations form the inhibitor, respectively [4].

2.3. Electrochemical impedance spectroscopy (EIS)

EIS experiments were conducted at 298 ± 1 K at the OCP over a frequency domainfrom 100 kHz down to 1.0 Hz, using ac signal amplitude perturbation of 10 mV peakto peak. All impedance data were fitted to an appropriate equivalent circuit usingpotentiostat/galvanostat/ZRA analyser (Gamry PCI300/4) provided with a personalcomputer with EIS300 software and Echem Analyst 5.21 for fitting the data andcalculations. Nyquist and Bode plots were obtained from the results of these exper-iments. The charge transfer resistance (Rct) was determined from those plots as thedifference in the electrode impedance at low and high frequencies, as described pre-viously [12]. The double layer capacitance (Cdl) was calculated from the frequency(fmax) where the impedance imaginary component (−Z′′) was maximum on theNyquist plot, using the formula Cdl = 1/2�fmaxRct, or by using the equality Cdl = 1/|Z|on the Bode plot when f = (1/2�) s−1, i.e. when ω = 1 or log ω = 0 (ω = 2�f rad s−1).From EIS data, the %IE was calculated using the expression (2):

%IE =(

1 − 1/R′ct

1/Rct

)× 100 (2)

where Rct and R′ct denote the charge transfer resistance values without and with the

addition of inhibitor, respectively [12].

2.4. Scanning electron microscopy (SEM)

Carbon steel samples was treated as mentioned in Section 2.1, immersed in0.5 M NaCl solution in absence and presence of 11 �M of inhibitors (I–III) for 2 hat Eoc then SEM micrographs were collected using a JEOL JXA-840A electron probemicroanalyzer.


28 F. El-Taib Heakal et al. / Materials Chemistry and Physics 125 (2011) 26–36

Fig. 2. Potentiodynamic cathodic and anodic polarization scans for C-steel in NaClsolutions with various concentrations at 298 K.

3. Results and discussions

3.1. Polarization measurements

3.1.1. Effect of NaCl concentrationThis part was carried out in an attempt to determine the appro-

priate concentration of NaCl solution to be used for studying thecorrosion and corrosion inhibition of C-steel. Fig. 2 shows cathodicand anodic polarization scans recorded for C-steel in NaCl solutionswith various concentrations over the range 0.01–1.0 M. The impor-tant corrosion parameters derived from these curves are presentedin Table 1. As can be seen from the figure, at all NaCl concentrationsC-steel electrode gives the same curve pattern, where the currentchanges linearly around the corrosion potential (Ecorr) exhibitingcathodic and anodic Tafel type behavior. The cathodic and anodicbranches of the polarization curves are shifted towards highercurrents with increasing NaCl concentration up to 0.5 M, whileEcorr moves significantly in the negative direction of the potentialcommensurate with the increase in the corrosion rate (Table 1).However, at still higher NaCl concentrations (>0.5 M) the change inthe above behavior becomes very little. Therefore, 0.5 M NaCl solu-tion (∼3% by wt) would be a suitable test medium to investigatethe inhibitive influence of the studied thiadiazole compounds on

Fig. 3. Variation of the open circuit potential (Eoc) with time for C-steel in 0.5 MNaCl solutions free or containing different concentrations of inhibitor (III) at 298 K.

the corrosion of C-steel under aerated stagnant near neutral con-ditions. It should be mentioned that the anodic polarization curvespresented here do not display the expected log i–E linear behav-ior over the complete applied potential range. At any tested NaClconcentration there is a current plateau in the anodic branch thatends at a clear inflection beyond which the anodic current increasesrapidly. As will be seen, the steep rise of current after the inflectionpotential may be due to passivation breakdown and pitting. Otherresearchers also noted [13] similar inflection in the anodic domainof cold rolled steel (CRS) in HCl solution.

3.1.2. Open circuit potentialsPrior to each polarization experiment the open circuit poten-

tials (Eoc) of C-steel in 0.5 M NaCl solution in the absence andpresence of different concentrations of compounds I, II or III wasmonitored for 30 min to access the free corrosion potential or thequasi-stationary Eoc value. The OCP transients obtained for com-pound (III) are shown in Fig. 3 as a typical example. Practically inall cases, C-steel potential drifts with time in the active directionbefore it tends to stabilize within 8 min. The negative shift of poten-tials is related to continuous dissolution of C-steel sample in sodiumchloride solution due to the non-protective nature of its native sur-face film. The stabilized Eoc value is always more negative than

Table 1Electrochemical parameters associated with polarization measurements recorded for C-steel in NaCl solutions with various concentrations of at 298 K.

[NaCl] (M) −Eoc vs. SCE (mV) −Ecorr vs. SCE (mV) icorr (�A cm−2) −ˇc (mV dec−1) ˇa (mV dec−1) Rp (k� cm2) CR (mm year−1)

0.01 560 809 0.4 142 193 96.20 0.0040.05 563 740 0.8 232 202 59.10 0.0090.10 657 703 1.1 282 213 50.20 0.0120.50 711 1008 77.2 101 550 0.48 0.8971.00 713 1009 80.5 101 555 0.46 0.934



the immersion value (Eoc at t = 0), suggesting that before achiev-ing stabilized conditions the pre-immersion air-formed oxide filmhas to dissolve [14]. Generally, the stabilized Eoc becomes signifi-cantly more positive the higher the concentration of the inhibitor.Under these conditions, the studied thiadiazole compounds (I–III)act as anodic type inhibitors characterized by progressive enno-blement of the free corrosion potential of C-steel with increasingconcentration of each additive. These results may be interpreted onthe basis of forming protective layer from the adsorbed thiadiazolemolecules on the active anodic sites of the steel surface.

3.1.3. Effect of inhibitor concentrationTafel polarization curves (E–log i) for C-steel in 0.5 M NaCl

solution containing various concentrations from thiadiazole com-pounds (I–III) at 298 K over the potential range 500 mV(SCE) ± Eoc

were recorded. Fig. 4 shows as a typical example for the resultsof compound (III), and similar behavior was noted for the others(I) and (II). The important corrosion parameters and %IE calculatedaccording to Eq. (2) as a function of concentrations of the testedthiadiazoles (I–III) are given in Table 2. As can be seen, icorr for C-steel in 0.5 M NaCl blank solution equals 77 �A cm−2, and decreaseswith increase in the concentration of each added compound. Thisindicates that thiadiazole derivatives act as inhibitors for C-steelcorrosion, and the degree of inhibition depends on their concen-trations, as well as on the type of the substituents attached to thethiadiazole ring (Fig. 1). In all cases, the increase in the inhibitorconcentration from 1 �M to 11 �M was accompanied by a decreasein icorr and an increase in %IE from 48 to 91% for the most efficientcompound (III). These results lead to the conclusion that the inves-tigated organic compounds are fairly efficient inhibitors towardsC-steel dissolution in NaCl medium even when added at �M con-centration range. As demonstrated in Table 2, the decrease in eithericorr or the corrosion rate (CR) with increasing concentration ofcompounds (I–III) is mainly due to an increase in the polarizationresistance (Rp). This is further evident from the positive shift inEcorr indicating an increase in the effectiveness of the cathodic pro-cess, which in turn decreases the rate of the anodic one [15]. Ourmeasured free corrosion potential was found to be −1008 mV(SCE)for C-steel in 0.5 M NaCl solution. Ibrahim et al. [16] found thatEcorr is varying from −930 to −995 mV(SCE) for some Egyptianaustenitic stainless steels in 0.5 M NaCl. On the other hand, Sahin etal. [17] found it to be −675 and −650 mV(SCE) for low carbon steel

Fig. 4. Potentiodynamic cathodic and anodic polarization scans for C-steel in 0.5 MNaCl solutions free or containing different concentrations of inhibitor (III) at 298 K.

in 2.5 and 3.5% NaCl, respectively and Ai et al. [18] found it to be−705 mV(SCE) for C-steel in 1.0% NaCl at pH 4. From these results,one can conclude that the free corrosion potential depends on thetype of steel samples used, as well as on the composition and pH ofthe test solution. Addition of thiadiazole derivatives shift Ecorr val-ues from −1008 to −910 mV(SCE), i.e. by about 100 mV(SCE) in thenoble direction, confirming the intrinsic anodic action of the usedinhibitors. It is to be noted that Ecorr values determined from poten-tiodynamic polarization are somewhat different from the stabilized

Table 2Electrochemical parameters and percent inhibition efficiencies (%IE) associated with polarization measurements recorded for C-steel in 0.5 M NaCl solutions free or containingdifferent concentrations of inhibitors I, II or III at 298 K.

Inhibitor Conc. (�M) −Eoc vs. SCE (mV) −Ecorr vs. SCE (mV) icorr (�A cm−2) −ˇc (mV dec−1) ˇa (mV dec−1) Rp (k� cm2) CR (mm year−1) %IE

Blank 0 712 1008 77 101 550 0.48 0.90 –I 1 702 993 52 97 408 0.66 0.60 33

3 700 900 41 100 428 0.91 0.47 475 696 997 35 85 310 0.82 0.41 557 685 985 30 89 355 1.04 0.34 619 671 973 24 85 321 1.21 0.28 69

11 658 941 13 85 268 1.80 0.15 83

II 1 699 961 46 117 557 1.00 0.53 403 691 949 38 111 505 1.03 0.45 515 686 927 36 118 513 1.22 0.42 537 681 923 25 113 432 1.28 0.29 689 661 939 15 98 332 2.13 0.18 81

11 639 910 9 78 209 9.15 0.11 88

III 1 700 997 41 101 494 0.90 0.47 473 689 992 39 104 548 1.05 0.46 495 684 988 32 100 451 1.16 0.37 587 677 957 17 99 346 1.93 0.20 789 660 960 14 101 354 3.37 0.16 82

11 639 998 7 72 163 8.70 0.09 91



Eoc values, which is likely attributed due to partial removal of thepassive film as polarization scanning was started at more negativepotential relative to Eoc value [19].

Table 2 shows also that while the cathodic Tafel slope (ˇc)remains almost constant the anodic Tafel slope (ˇa) is higherthan ˇc and slightly decreases on increasing the concentrationof the tested compounds. This indicates that addition of thiadia-zole derivatives dose not modify the mechanism of the corrosionreaction of C-steel in neutral chloride medium. The fact that ˇa

value is higher than ˇc can be attributed to a surface kinetic pro-cess rather than a diffusion-controlled one [20], indicating that theinhibition mode of the tested thiadiazole derivatives is by simpleblockage of the surface via adsorption. Increasing inhibitor concen-tration increases the degree of surface coverage by the adsorbedinhibiting species, which hinders the diffusion of ions to or fromthe electrode surface [21]. This in turn leads to a decrease in theanodic dissolution current density of C-steel, suggesting that thecorrosion inhibition by the tested molecules in NaCl solution is pre-dominantly under anodic control. Moreover, for one and the samethiadiazole derivative the polarization resistance (Rp) was found tobe higher in neutral NaCl than its previously reported [22] value inH2SO4 solution, indicating that the alloy corrodes faster in acidicmedium and that the stability of the protective adsorbed barrierfilm on the steel surface in neutral solution is superior to that inacidic one.

3.1.4. Adsorption isothermAssuming that the studied inhibitors affect the rate of the corro-

sion process mainly through the variation of the degree of surfacecoverage (�). Consequently, the inhibition efficiency is a functionof the electrode surface covered by the inhibitor molecules, i.e.� = 10−2 × %IE. Attempts were made to fit the relationship between� and the bulk concentration of the inhibitor used at a certain giventemperature in order to give an insight into the adsorption process.Several adsorption isotherms are commonly tried to characterizethe inhibitor performance and the best fit was obtained using Lang-muir isotherm, which assumes that the solid surface contains afixed number of adsorption sites and each site holds one adsorbedspecies [22], in good agreement with the following Eq. (4):

C

�= 1

Kads+ C (4)

where Kads is the equilibrium constant of the adsorption process.Variation of (C/�) with inhibitor concentration (C) of compounds(I–III) presented in Fig. 5 suggests that there are no attraction orrepulsion forces between the adsorbed molecules, since a linearrelationship is obtained with a slope equal to unity and interceptequal to (1/Kads), Kads being related to the standard free energy ofadsorption (�G

◦ads) by the relation (5):

Kads = 155.5

exp−�G◦

adsRT

(5)

where the constant 55.5 is the molar concentration of water in solu-tion and R is the universal gas constant. By applying Eq. (5), differentvalues of �G

◦ads for inhibitors (I–III) as a function of temperature in

the domain 298–323 K were obtained and listed in Table 3. Fig. 6shows that the dependence of �G

◦ads on the absolute temperature

(T) gives linear plot with a slope equal to the entropy of adsorption(�S

◦ads) and intercepts equal to the enthalpy of adsorption (�H

◦ads),

according to the following Eq. (6):

�G◦ads = �H

◦ads − T�S

◦ads (6)

The various thermodynamic functions for the adsorption pro-cess (Kads, �G

◦ads, �H

◦ads and �S

◦ads) are given in Table 3. It is

generally obvious that �G◦ads has increasingly negative value, as

the %IE increases, which indicates that the tested compounds are

Fig. 5. Langmuir adsorption isotherm for C-steel in 0.5 M NaCl solution in the pres-ence of different concentrations of inhibitors (I–III) at 298 K.

adsorbed spontaneously on C-steel surface forming a relativelystable adsorbed layer. The value of �G

◦ads is approximately equal

−42 kJ mol−1, indicating that the adsorption mechanism of the thia-diazole derivatives on C-steel surface involves physical adsorption.The Kads follows the same trend in the sense that larger valueof Kads means more efficient adsorption and hence better inhibi-tion efficiency. The negative sign of �H

◦ads reveals that adsorption

of inhibitors (I–III) on C-steel surface from 0.5 M NaCl solution isan exothermic process, which implies that %IE for each inhibitordecreases with the rise in temperature as shown in Table 3. Suchbehavior can be explained on the basis that temperature rise causesdesorption of some adsorbed inhibitor molecules off the metal sur-face and consequently leads to formation of surface organic layerhaving lower protection.

3.1.5. Activation parametersThermodynamic activation parameters have an important role

in understanding the inhibitive mechanism of organic additives.Results of %IE obtained in the presence of 11 �M of inhibitors (I–III)at different temperatures (Table 3) reveal again that increasingtemperature increases the corrosion rate and reduces %IE for a giveninhibitor concentration. Such trend implies that these compoundsare adsorbed physically on C-steel surface and the extent of theiradsorption decreases with raising the ambient temperature. Theapparent activation energies (Ea) for the corrosion process of C-steel in 0.5 M NaCl solution in the absence and presence of 11 �M ofinhibitors (I–III) were determined from the slopes of the linear plotsof log k versus 1/T (not shown) according to the familiar ArrheniusEq. (7):

log k = constant − Ea

2.303RT(7)



Table 3Thermodynamic adsorption parameters of thiadiazole inhibitors (I–III) on C-steel surface in 0.5 M NaCl solution as a function of temperature.

Inhibitor T (K) %IE Kads (×10−4 mol−1) −�G◦ads

(kJ mol−1) −�H◦ads

(kJ mol−1) −�S◦ads

(J mol−1 K−1)

I 298 83 32.4 41.4 54.0 42.2303 79 22.7 41.2308 73 16.3 41.0313 68 11.4 40.7318 64 8.3 40.6323 58 6.0 40.3

II 298 88 39.7 41.9 54.4 41.9303 84 27.8 41.7308 79 19.3 41.5313 73 13.7 41.2318 68 10.0 41.1323 61 7.3 40.9

III 298 91 44.4 42.2 54.5 41.0303 87 29.8 41.9308 83 21.2 41.7313 79 14.9 41.5318 74 11.0 41.3323 65 8.1 41.1

As can be seen in Table 4, Ea value increases in the same orderof increasing inhibition efficiency of the inhibitor and parallel alsothe order of decreasing the corrosion rate. This gives further supportthat inhibitor (III) exhibits the best performance and the inhibitiveeffect of the tested compounds decreases in the sequence III > II > I.The increase in Ea with addition of 11 �M of inhibitors (I–III) isrelated to a concurrent increase in the energy barrier of the acti-vated complex for the corrosion reaction and its value indicate thatthe whole process is under surface reaction control, since the acti-

Fig. 6. Effect of temperature on the free energy of adsorption (�G◦ads

) for inhibitors(I–III) on C-steel in 0.5 M NaCl solution.

vation energy of the corrosion process is over 20 kJ mol−1 [23]. Theobtained activation energy for C-steel in inhibitor-free 0.5 M NaClsolution is equal 13.5 kJ mol−1 in good agreement with the valueof 15.5 kJ mol−1 for 316 L stainless steel in 0.5 M NaCl solution [24]and the other values of 14.0, 16.3, 18.8 and 21.7 kJ mol−1 calculatedin 2.5 M NaCl solution for some Egyptian austenitic stainless steels[16]. Generally, one can say that composition of the electrolyte andtype of the corroding metallic material affect greatly the activationenergy for the corrosion process.

Enthalpy and entropy of activation (�H*, �S*) were calculatedbased on the transition state theory, being an alternative formulaof the Arrhenius equation and takes the form [10]:

k = RT

Nhexp

(�S∗

R

)exp

(−�H∗

RT

)(8)

where h is Planck’s constant and N is Avogadro’s number. Thusplot of log (k/T) versus 1/T for C-steel in 0.5 M NaCl solutionin the absence and presence of 11 �M of inhibitors (I–III) gavestraight lines as shown in Fig. 7. Values of �H* and �S* werecalculated, respectively, from the slopes (−�H*/2.303R) and theintercepts (log RT/Nh + (�S*/2.303R)) of the linear plots and listedin Table 4. The value of �H* reflects the strong adsorbability of thetested compounds on C-steel surface and indicates the endothermicnature of the process. In the absence and presence of thiadiazoleinhibitors (I–III) �S* values are large and negative, indicating thatthe activated complex in the rate-determining step represents anassociation rather than dissociation. This means that a decreasein disorder of the system takes place on going from reactants tothe activated molecules which are in higher ordered state thanmolecules at the initial state. The order of the inhibition efficiencyof the investigated thiadiazole derivatives as gathered from theincrease in Ea and �H* and the decrease in �S* values remainsthe same (III > II > I).

Table 4Activation parameters of the corrosion process of C-steel in 0.5 M NaCl solution inthe absence and presence of 11 �M of inhibitors (I–III).

Inhibitor Ea (kJ mol−1) �H* (kJ mol−1) −�S* (J mol−1 K−1)

Blank 13.5 11.0 256.6I 40.7 38.4 178.8II 49.0 46.9 153.1III 51.3 48.9 148.4



Fig. 7. log (k/T)–(1/T) plots for the corrosion process of C-steel in 0.5 M NaCl solutionin the absence and presence of 11 �M of inhibitors I, II or III.

3.2. Pitting corrosion behavior

Fig. 8a shows a typical positive going linear potential scansthat swept from −1.2 V up to 0.0 V(SCE) and recorded after 30 minimmersion of C-steel electrode in NaCl solutions with various con-centrations (0.01–1.0 M) at a scan rate of 1 mV s−1. The curves in thisfigure illustrate the influence of NaCl concentration on the anodicbehavior of C-steel. As can be seen, the cathodic current corre-sponding to oxygen reduction decreases gradually with the appliedpotential and reaches zero at the corrosion potential (Ecorr). It isalso obvious that the cathodic current density is higher the higherthe ambient concentration, indicating that oxygen reduction reac-tion is more enhanced in concentrated NaCl solution (≥0.5 M). Onthe other hand, after Ecorr the anodic current remains at a lowervalue around 100 �A cm−2, which corresponds to the formation ofa certain passive film on the electrode surface. Beyond this passiveregion the figure reveals a sudden rise in the anodic current den-sity without any sign for oxygen evolution. This occurs at a certainpotential value (Epit) depending on the solution concentration dueto passivity breakdown and pitting attack. Visual examination ofthe electrode surface after each polarization experiment revealedobservable pits. In 0.01 M NaCl solution no pitting effect can beobserved for C-steel up to 0.0 V(SCE), however, on increasing NaClconcentration pitting corrosion becomes evident and Epit moves inthe negative direction of potential indicating more enhancement ofthe process. It was believed [25] that the pitting corrosion of met-als in halide-containing solution involves four steps: (i) formationof the passive layer, (ii) breakdown of passive layer, (iii) growth ofunstable pits which can be repassivated and (iv) growth of largerand stable pits. At Epit, breakdown of the permanent passive layerand initiation of pitting attack by the aggressive Cl− ions couldbe ascribed to competitive adsorption between Cl− ions and oxy-genated species (OH− and H2O dipoles) at the active sites on oxide

Fig. 8. Potentiodynamic anodic polarization scans for C-steel at 298 K (a) in NaClsolution of various concentrations, and (b) in 0.5 M NaCl solutions free or containingdifferent concentrations of inhibitor (III) at 298 K.



Fig. 9. Pitting potential (Epit) as a function of inhibitors (I–III) concentrations forC-steel anodized in 0.5 M NaCl solution at 298 K.

covered layer [26]. The adsorbed Cl− ions can penetrate through thepassive layer especially at its point defects and flaws with the assis-tance of high electric field across the passive film to reach the basemetal surface and accelerate the local anodic dissolution [9,27].

The effect of adding different concentrations of inhibitors (I–III)on the anodic dissolution behavior of C-steel in 0.5 M NaCl solu-tion was also recorded as shown in Fig. 8b for the most efficientcompound (III) as a typical example. The variation of Epit as a func-tion of inhibitor concentration (Fig. 9) follows the following linearrelationship:

Epit = a + b log Cinh (9)

where a and b are two empirical constants dependable on the testedsystem. Similar relation was previously reported in the study ofpitting corrosion of stainless steel in media containing aggressiveanions as a function of pH [28]. The increase in inhibitor concen-tration moves Epit progressively towards more positive potential(Table 5), indicating suppression of passive film breakdown. The

Table 5Pitting potentials of C-steel anodized in 0.5 M NaCl solution containing various con-centrations of inhibitors (I–III) at 298 K.

Conc. (�M) −Epit vs. SCE (mV)

I II III

1 377 373 3723 364 360 3575 356 353 3467 349 344 3419 342 336 331

11 335 331 324

Epit for the blank = −391 vs. SCE(mV).

Fig. 10. EIS spectra for the corrosion of C-steel in 0.5 M NaCl solutions free or con-taining different concentrations of inhibitor (III) at 298 K, (a) the Nyquist plots, and(b) the Bode plots. The insert in (a) represents the equivalent circuit model with aconstant phase element (CPE) used to fit the experimental EIS data.

mitigation of pitting corrosion by addition of thiadiazole deriva-tives (I–III) can be attributed to the adsorption of these compoundson the film surface acting as barrier layers to diffusion of Cl− anionsfrom attacking the passive film.

3.3. Electrochemical impedance spectroscopy

Impedance diagrams (Nyquist and Bode) at frequencies rangingfrom 100 kHz to 1 Hz for C-steel in inhibitor-free and inhibitor-containing 0.5 M NaCl solution were traced at the stabilizedEoc value. The equivalent circuit model which describes themetal/electrolyte interface of the present corroding system isshown as insert in Fig. 10a, where Rs, Rct and CPE refer to solutionresistance, charge transfer resistance and constant phase elementrepresenting the double layer capacitance (Cdl) of the interface,respectively.

A typical example of EIS data obtained for compound (III) is pre-sented as Nyquist plot in Fig. 10a. The complex impedance diagramis characterized by a single full semicircular appearance indicatingthat corrosion of C-steel is controlled by a charge transfer process[29]. The obtained diameters of the capacitive loops increase inpresence of thiadiazole derivatives, and are indicative of the extentof inhibition of the corrosion process. The same EIS data obtainedfor compound (III) as a function of its concentration are also pre-sented as Bode plots in Fig. 10b. The spectra (log |Z| vs. log f) areall characterized by two resistive arrests at higher and lower fre-quency limits for which the phase shift (�) tends to zero, while



Table 6Electrochemical impedance parameters of C-steel in 0.5 M NaCl solutions free orcontaining different concentrations from inhibitor I, II or III at 298 K.

Inhibitor Conc. (�M) Rct (� cm2) Cdl (�F cm−2) �max (◦) %IE

Blank 0 75 41.25 57.5 –I 1 123 37.33 59.3 39

3 152 36.58 61.1 517 217 35.22 62.8 66

11 345 33.98 67.0 78

II 1 137 35.02 59.6 453 161 34.85 60.8 537 253 33.93 63.7 70

11 436 31.92 67.0 83

III 1 148 34.80 59.0 493 164 34.21 60.7 547 362 32.85 63.7 79

11 551 30.83 67.8 86

at the intermediate frequencies their behavior becomes capacitivewith a maximum in the �–log f plot. The contribution of impedanceat the low frequency range shows the kinetic response of chargetransfer reaction [12].

The electrochemical impedance parameters obtained from theexperimental EIS data and %IE calculated using Eq. (2) are all listedin Table 6. It can be seen from this table that, the more denselypacked the monolayer of the inhibitor on the electrode surface thelarger the diameter of the semicircle, which results in higher Rct

and lower Cdl values. It is well known that �max should be 90◦ for acorroding system represented by a simple RC parallel combinationwhen Rs = 0. However, depressed semicircles are usually obtainedfor a practical electrode/solution interface, which has been knownto be associated with a rough electrode surface [30]. Corrosion ofC-steel in NaCl solution increases the roughness of the electrodesurface and therefore reduces the value of �max, which amountingto 57.5–67.8◦ (Table 6). Additionally, a less depressed semicirclewith higher �max indicates a better quality of the inhibitor mono-layer. For the three tested inhibitors, the data in Table 6 revealedclearly that both Rct and �max increase while Cdl decreases withincreasing inhibitor concentration and consequently leads to anincrease in the inhibition efficiency. The decrease in Cdl value maybe due to the gradual replacement of water molecules in the doublelayer by the adsorbed inhibitor molecules which form an adher-ent film on the metal surface and leads to a decrease in the localdielectric constant of the metal/solution interface.

4. Inhibition mechanism

It has been established that steel corrodes when Fe atomsdecompose into positive ions and electrons [14]; Fe = Fe2+ + 2e−. Innaturally aerated neutral chloride solution, the released electronsare absorbed in oxygen reduction; H2O + ½O2 + 2e− = 2OH−.

The present results indicate that thiadiazole derivatives inhibitC-steel corrosion via physisorption mechanism. The extent of anadsorption process is influenced by the charge of the metal andthe chemical structure of the inhibitor [13]. Knowing the potentialof zero charge (EPZC), the charge on the metal surface at the freecorrosion potential (Ecorr) can be determined [31] on the correla-tive scale: ϕ = Eoc − EPZC. Amin et al. [32] determined EPZC for lowC-steel in 1.0 M HCl as equal to −725, −742 and −765 mV(SCE) atpH 6, 7 and 8, respectively. However, values of Eoc obtained in thiswork is −712 mV(SCE) in the blank solution (0.5 M NaCl). There-fore, the value of ϕ is positive, indicating that the metal surfaceacquires slight positive charge that promoting direct adsorption ofthiadiazole derivatives in the form of neutral molecules on the basisof donor–acceptor interactions. This involves the displacement ofwater molecules from the metal surface and sharing the electrons

between the nitrogen and sulphur atoms in the molecules, whichare the centers of concentrated negative charge, as well as their pla-nar p�-orbital on the metal surface having vacant d�-bital [10] toinhibit the corrosion and retarding the pitting of the passive metalat higher anodization. In neutral chloride medium, the probabilityof the formation of adsorbed ion pairs due to electrostatic attractionbetween the protonated thiadiazole molecules and the specificallyadsorbed Cl− ions on the metal surface cannot also be excluded.This favors more adsorption and thus leads to higher inhibitiontowards C-steel even at a very low concentration range (1–11 �M).A representation for the proposed adsorption model of the investi-gated thiadiazole molecules is shown in Fig. 11 and clearly indicatesthe active adsorption centers which are S and N atoms carryinglone pair of electrons. Excellent corrosion inhibitors are usuallythose organic compounds who not only offer electrons to unoc-cupied orbital of the metal, but also accept free electrons from themetal.

The results show conclusively that among the investigated com-pounds inhibitor (III) exhibits the best performance because of thepresence of highly electron releasing para two methyl groups (withHammett constant = −0.17) [33]. The –CH3 groups increase theelectron density on the active centers, which leads to greater sur-face coverage, thereby giving higher inhibition efficiency of morethan 90%IE. Compound (II) has the same structure as compound(III) with the replacement of the two methyl groups by H atomand Br atom [with Br = 0.23 and H = 0.0] coming next to com-pound (III) in the order of %IE. This is due to the operation of amesomeric effect (+M) involving electron pairs on Br atom whichact in opposite direction to the inductive effect (−I), and hence theoverall effect of Br atom is to facilitate release of electrons fromthe molecule but to lesser extent than compound (III). Compound(I) comes after compound (II) in %IE due to its smaller molecularweight compared with compound (II).

The above results are further supported from quantum the-oretical calculations which were performed [34] with Gauissian98 software package for PC, by using of the drawing programHyperchem and optimization to be more stable with PM3. Basedon the energy of the highest occupied molecular orbital (EHOMO)and that of the lowest unoccupied molecular orbital (ELUMO), thelower −EHOMO of the molecule means higher electron donatingability to appropriate acceptor molecules having empty molecularorbital with lower energy. For thiadiazole derivatives I, II and III, theobtained values are: −EHOMO = 3.3514, 3.1672 and 3.0952 eV andELUMO = 8.0093, 8.0164 and 8.0253 eV, respectively. The decrease inEHOMO (i.e. with decreasing ionization potential) and the increasein ELUMO or the electron affinity (i.e. the energy released when anelectron is added to a molecule), are both in orders commensuratewith the increase in %IE obtained experimentally, which confirmsthat thiadiazole compound III is the most efficient inhibitor.

The surface morphologies of C-steel specimen in 0.5 M NaClsolution free or containing 11 �M thiadiazole derivatives (I–III)after 2 h immersion were examined using SEM as displayed inFig. 12a–d, respectively. In the absence of inhibitors (Fig. 12a), avery rough surface was observed due to rapid corrosion attackof carbon steel by chloride anions. There are a large number ofpits surrounded by iron oxide layer which almost fully covers thecarbon steel surface, revealing that pit formation under these con-ditions occurs continuously during the exposure period while ironoxide builds up over the surface. It is important to stress out thatwhen thiadiazole derivatives (I–III) are present in the solution, themorphology of the carbon steel surface are quite different fromthe previous one and the rough surface (amount of the formediron oxide and the number of pits) is visibly reduced in the orderI > II > III, indicating the formation of a protective film with inhibit-ing power that increases in the reverse order. These conspicuousvariations in the microstructures of the films reflect the same trend



Fig. 11. Representation for the proposed adsorption model of the thiadiazole derivatives on C-steel surface.

Fig. 12. SEM morphologies for C-steel surface after 2 h immersion in 0.5 M NaCl solutions in the absence (a) and presence of 11 �M of inhibitors I (b), II (c) and III (d).

of the inhibitive properties of the added compounds as obtainedfrom the electrochemical measurements.

5. Conclusion

Open circuit potential, polarization and impedance measure-ments showed that the investigated thiadiazole derivatives (I–III)can passivate C-steel against general and pitting corrosion by

decreasing the corrosion current density and corrosion rate, aswell as shifting the corrosion and pitting potentials in the nobledirection. The three heterocyclic thiadiazoles exhibit inhibitiveproperties for C-steel corrosion in 0.5 M NaCl solution with effi-ciency directly proportional to the concentration of each additive.At all tested concentrations and temperatures, the best inhibitiveperformance follows the ranking order: III > II > I. The three com-pounds behave like anodic type inhibitors through adsorption onthe active sites of the electrode surface, thus retarding the anodic



dissolution reaction of C-steel. The adsorption of these moleculeswas found to follow Langmuir isotherm. The determined ther-modynamic adsorption and activation functions point out thatthese compounds are spontaneously adsorbed on C-steel surfacevia physisorption mechanism. The results obtained from dc and acmeasurements are in good agreement and they accord also withthe theoretical calculated values of EHOMO and ELUMO of the testedmolecules.

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Inhibitive effect of some thiadiazole derivatives on C-steel corrosion in neutral sodium chloride solution