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Computational and experimental evaluation of the acid corrosion inhibitionof steel by tacrine

S.E. Nataraja a, T.V. Venkatesha a,⇑, H.C. Tandon b

a Department of Chemistry, Kuvempu University, Shankaraghatta 577451, Karnataka, Indiab Department of Chemistry, Sri Venkateswara College, Dhula Kuan, New Delhi 110021, India

a r t i c l e i n f o

Article history:Received 28 July 2011Accepted 26 March 2012Available online 3 April 2012

Keywords:A. Acid solutionsA. SteelB. Electrochemical calculationB. Weight lossC. Acid corrosion

a b s t r a c t

The electronic parameters of tacrine were evaluated by quantum chemical method to assess its corrosioninhibition potential. Further, it was confirmed by chemical and electrochemical studies on steel in 1.0 MHCl and 0.5 M H2SO4. It was found that tacrine performed better than the quantum prediction which isattributed to the planarity of the molecule. Also, tacrine is found to be more efficient in 1.0 M HCl than in0.5 M H2SO4, which according to potential of zero charge study, is due to acid anions Cl� and SO2-

4 . Tacrinepursued Langmuir adsorption isotherm.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Steel is extensively used in industries in varied conditions.Aqueous acidic solutions, that to in hot condition, are highly corro-sive [1]. Acid solutions are widely used in industries for pickling,acid cleaning, acid descaling of boilers, heat exchangers, coolingtowers, etc. to remove scales, deposits and other corrosion prod-ucts [2]. There is possibility that underlying metal undergo corro-sion. To mitigate this, among the various available methods, useof an inhibitor is the most practical and cost-effective choice.Inhibitors are organic molecules having hetero atoms like P, S, N,S and p electrons through which the inhibitor molecule interactwith the metal surface and get adsorbed [3–5]. The extent ofadsorption depends on physico-chemical properties of the inhibi-tor molecule like functional group, geometry, electronic densityat hetero atom, p electrons and planarity. Further, it is reportedthat smaller inhibitor molecule are efficient because they facili-tates electronic interactions and impede steric effects. But manyof them are toxic even though they exhibit good inhibition action[6]. This has led to consider drugs as inhibitors [7]. Tryptamine[8], ketoconazole [9], sulfa drugs [10], antibacterial drugs [11], rho-danine azosulfa drugs [12] have been reported to be good inhibitor.

The planarity of the molecule, besides other factors, decides theefficiency of an inhibitor [13,14]. Thus drugs with planar structurecan be a promising inhibitor which encouraged us to choose a drugtacrine [15,16]. Tacrine (IUPAC name: 1,2,3,4,-tetrahydroacridin-

9-amine, trade name: Cognex, Romotal), is an approved drug forthe treatment of Alzheimer’s disease. It has two nitrogen heteroatoms, two aromatic rings. The pi electrons of aromatic rings andlone pair of electrons of nitrogen atoms are likely to induce greateradsorption of inhibitor molecule on the surface of steel, in aqueousacid medium [3].

The present study was intended to ascertain the inhibition abilityof tacrine by comparing with our earlier work where in the corrosioninhibition effect of 2- [4-(methylthio) phenyl] acetohydrazide(HYD), 2-{[4-(methylthio) phenyl] acetyl} hydrazinecarbothioa-mide (TAD) and 5-[4-(methylthio) benzyl]-4H-1,2,4-triazole-3-thiol (TRD) on steel in 1.0 M HCl was investigated [17] and to predictits inhibition efficiency on the basis of quantum chemical calcula-tions. Later, verifying it by weight loss, tafel polarization, and elec-trochemical impedance spectroscopy (EIS) measurements in 1.0 MHCl and 0.5 M H2SO4. Difference if any, is subjected to reasoning.The role of acid anions Cl� and SO2-

4 were evaluated by potential ofzero charge (PZC) method. Adsorption and thermodynamic param-eters were determined to establish the mechanism of theadsorption.

2. Experimental

2.1. Materials

Steel plates of compositions (wt.%) C: 0.04, Mn: 0.35, P: 0.022, S:0.036 and rest being Fe, were used for all experiments. Plates ofdimension 4 � 2.5 � 0.05 cm were used for weight loss methodand plates with an exposed area of 1 cm2 (rest is covered with

0010-938X/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2012.03.034

⇑ Corresponding author. Tel.: +91 9448855079; fax: +91 08282256262.E-mail address: [email protected] (T.V. Venkatesha).

Corrosion Science 60 (2012) 214–223

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araldite resin) with 2.5 cm long stem were used for polarizationand EIS methods. All plates were abraded by using emery papers(Grade No: 220, 400, 600, 800 and 1200), washed thoroughly withdistilled water, degreased with acetone and dried at room temper-ature. The corrosive media 1.0 M HCl and 0.5 M H2SO4 solutionswere prepared using Merck AR grade HCl, H2SO4 and Milliporewater (Elix-3).

2.2. Inhibitor compound

Tacrine was obtained from Cayman chemicals, Pro lab market-ing Limited, New Delhi, India and used with no further purification.Tacrine solutions of concentration 0.5, 1.0, 2.5 and 5.0 � 10�4 Mwere prepared in 1.0 M HCl and 0.5 M H2SO4 using Millipore water.The three dimensional structure of tacrine as given by drug bankwebsite, is shown in Fig. 1.

2.3. Methods

2.3.1. Quantum chemical studiesQuantum chemical calculations for tacrine were carried out in

gas phase by using parametric method 3(pm3). The Polak-Rieberrealgorithm which is fast and accurate has been used for computa-tion. The energy parameters in the form of root mean square gra-dient were kept at 0.05 kcal/A mol and convergence limit at 0.05.These calculations were performed by using Hyperchem 7.5 pack-age program (Hypercube Inc., Florida, 2003).

2.3.2. Weight loss measurementsWeight loss measurements were done by hanging steel plate in

a glass beaker containing 100 cm3 of corrosive media (1.0 M HCland 0.5 M H2SO4) without and with different concentrations ofinhibitor for an immersion time of 4 h. Then steel plate was takenout and washed well with tap water followed by distilled water,dried, and weighed accurately using digital balance (Accuracy: ±0.1 mg, model No: AA-2200, Anamed Instruments Pvt. Limited,MIDC, Navi Mumbai 400706, India). Weight loss experiments werecarried out at 30, 40, 50 and 60 �C in a digital temperature con-trolled water bath (±0.5 �C accuracy, Raagaa Industries, Chennai61, India). All experiments were carried out in static and aeratedcondition. Each measurement was repeated thrice for reproducibil-ity and an average value was reported.

2.3.3. Electrochemical measurementsThe electrochemical measurements were carried out in CHI

660C electrochemical work station (CH Instruments, Austin, USA)at 30 �C. The cell consists of three electrodes, the working electrode(steel), counter electrode (platinum), and reference electrode (SCE).Before each electrochemical measurement, the working electrodewas allowed to stand for 30 min in test solution to allow stabiliza-tion of open circuit potential (OCP). Each experiment was repeatedfor three times and an average value was reported. All reportedpotentials were with respect to SCE. For Tafel measurements, the

potential-current curves were recorded at a scan rate of 0.001 Vs�1 in the potential range obtained by adding �0.2 and + 0.2 V toopen circuit potential (OCP) value. The corrosion parameters suchas corrosion potential (Ecorr), corrosion current (Icorr) cathodic Tafelslope (bc), and anodic Tafel slope (ba) were obtained from theinstrument.

Impedance measurements were carried out by using AC signalwith amplitude of 5 mV at OCP in the frequency range from100 kHz to 10 mHz. The impedance data were fitted to an appro-priate equivalent circuit by using Z Simp win 3.21 software. Theimpedance parameters were obtained from Nyquist plots. The po-tential of zero charge was evaluated by the impedance response ofsteel in 1.0 M HCl and 0.5 M H2SO4 without and with 2.5 � 10�4 Minhibitor by applying dc potentials in 5 mV increments at a fre-quency of 100 Hz.

3. Results and discussion

3.1. Quantum studies

The optimized structure, Mulliken charges and frontier orbitalenergy distribution of homo and lumo of tacrine are given inFig. 2. The computed quantum chemical parameters like energyof highest occupied molecular orbital (EHOMO), energy of lowestunoccupied molecular orbital (ELUMO), HOMO–LUMO energy gap(DEH-L), dipole moment (l), total negative charge (TNC) on wholemolecule, TNC/n ratio (n = total number of atoms), and TNC/m ratio(m = number of atoms in the respective moiety) are summarized inTable 1. From Fig. 2, it can be observed that HOMO and LUMO en-ergy orbitals were strongly localized on naphthalene moiety andalmost nil, on cyclohexane moiety indicating that the naphthalenemoiety posses good adsorption centres [17–19] consolidating theopinion of several researchers that p electrons and hetero atomsare responsible for inhibition activity [20]. Further, this is validatedby the presence of higher Mulliken charges and higher TNC/m ratio(number of atoms of the moiety) of naphthalene moiety (�0.050)compared to cyclohexane moiety (�0.031). On the other hand,cyclohexane moiety being saturated, with zero distribution, smal-ler charges, and lower TNC/m ratio endorse that it has feebleadsorption centres.

Further, HOMO and LUMO distribution on naphthalene moietyis almost similar but, with a slight dominance of homo. Tanget al. [21] suggest that when a molecule has similar frontier orbi-tals, its inhibition efficiency can be correlated to the energy levelsof HOMO and LUMO and the difference between them. It has beenwell documented in literature that [22] higher the value of EHOMO,greater is the ease for an inhibitor to donate electrons to unoccu-pied d orbital of metal atom and higher is the inhibition efficiency.Further lower the ELUMO, easier is the acceptance of electrons frommetal atom to form feedback bonds. The gap betweenHOMO–LUMO energy levels of molecules was another importantparameter that needs to be considered. Smaller the value ofDEH-L of an inhibitor, higher is the inhibition efficiency of thatinhibitor [23]. It is also documented that, large values of dipolemoment will enhance corrosion inhibition [24].

Analysis of Mulliken population, to probe the adsorption cen-tres present in an inhibitor molecule has been widely reported[25,26]. Several researchers are of the opinion that, higher themagnitude of negative charge on hetero or any atom and higherthe number of such atoms, the more strongly it can be adsorbedon the metal surface [27,28]. These negative charged atoms couldbe adsorption centres. The nitrogen atom of heterocyclic ring hav-ing highest negative Mulliken charge (�0.151) is a potentialadsorption centre. Vetkin et al. is also of the opinion that tacrine(pka value of about 9.8) gets protonated primarily at the nitrogenatom of heterocyclic ring to form inhibitor cation (C13H15N2

+)Fig. 1. 3D structure of tacrine as depicted in drug bank website.

S.E. Nataraja et al. / Corrosion Science 60 (2012) 214–223 215


[15,16]. Thus formed inhibitor cations, C13H15N2+, synergistically

adsorb onto the Cl� or SO2-4 pre adsorbed surface of steel, in HCl

or in H2SO4 medium, respectively.TNC is the sum of all such negative charges present in the mol-

ecule. Higher the TNC value indicates the presence of potentialadsorption centres. Further, TNC/n ratio could be convenientlyused to compare the magnitude of negative charges between twoor more molecules.

We have used same computational methods for the present andprevious work [17]. Thus, an arbitrary comparison can be made,even though compounds of our earlier work [17] and tacrine havevery less similarity. Tacrine has less number of hetero atoms anddiscouraging quantum parameters, even then, it showed higherinhibition efficiency than HYD, TAD and TRD. The anomaly couldbe related to the planar structure of tacrine which can be explainedas follows: an adsorption centre can be an active adsorption centreonly when it is able to interact with the metal atom and contributetowards the adsorption of the molecule on the metal surface. Thisis possible only when the inhibitor molecule is planar. For planarmolecules, in an orientation parallel to the metal surface, alladsorption centres finds good chance to interact with the metalsurface atoms and contribute. On the contrary, for non planar mol-ecules, in any possible orientation, some of the centres will bedeprived of their chance to interact. Quantum study showed thattacrine is planar and drug bank web site confirms it. Thus it can

be concluded that, in spite of discouraging electronic parameters,tacrine showed exceptionally high inhibition efficiency which canonly be attributed to the planarity of the molecule [13,18].

3.2. Weight loss determination

The values of percentage inhibition efficiency (gw) obtainedfrom weight loss experiment for the corrosion of steel in 1.0 MHCl and 0.5 M H2SO4 in the presence of different concentration oftacrine at different temperature are given in Table 2. The gw wascalculated from the following relationship:

gw ¼Wo �W

Wo � 100 ð1Þ

where W� and W are weight loss in the absence and presence ofinhibitor.

The rate of corrosion q (g cm�2 h�1) was calculated fromequation:

q ¼Wo �WSt

� 100 ð2Þ

where S is surface area of steel plate and t is immersion time.

3.2.1. Effect of concentration of the inhibitorThe variation of gw with the concentration of inhibitor, at 30 �C

is shown in Fig. 3 which revealed that tacrine has got significantinhibition ability, both in 1.0 M HCl and 0.5 M H2SO4 media. Itshowed substantial raise in gw values up to 2.5 � 10�4 M concen-tration of inhibitor and there after a small increase, in both acids.The data in Table 2 indicates that, at any temperature (30, 40, 50and 60 �C), in HCl as well in H2SO4, the gw increased with theincrease in inhibitor concentration. The increase of gw with inhib-itor concentration primarily indicates that the magnitude ofadsorption and surface coverage by inhibitor molecules on the sur-face of steel increases with inhibitor concentration [7].

Fig. 2. Quantum parameters for tacrine. (a) Optimized structure (b) Mulliken charges (c) Homo frontier orbital energy distribution (d) Lumo frontier orbital energydistribution.

Table 1Quantum chemical parameters of tacrine. (n = total number of atoms present in themolecule; m = total number of atoms present in the moiety; m = naphthalene fortacrine, m = hydrazide for HYD, m = thioamide for TAD, and m = triazole for TRD).

Inhibitor EH (eV) EL (eV) EH�L l (Debye) TNC TNC/n TNC/m

Tacrine �8.0 �0.5 7.5 6.5 �1.15 �0.088 �0.050HYD �8.5 �0.4 8.1 7.6 �1.43 �0.130 �0.079TAD �8.0 �1.4 6.6 9.2 �1.96 �0.163 �0.097TRD �7.8 �2.4 5.4 14.6 �1.59 �0.144 �0.109

216 S.E. Nataraja et al. / Corrosion Science 60 (2012) 214–223


3.2.2. Effect of solution temperatureThe plot showing the variation of gw with the solution temper-

ature is shown in Fig. 4, which indicated that gw, for both the acids,decreased with the increase of temperature. This suggests that atelevated temperature, desorption of the previously adsorbed inhib-itor molecules takes place resulting in the decrease of gw. This phe-nomenon can be attributed to physical adsorption of inhibitor

molecules [29]. At any experimented temperature, the gw standsin the order 1.0 M HCl > 0.5 M H2SO4.

3.3. Electrochemical studies

3.3.1. Open circuit potentialThe change in Potential as a function of immersion time

(30 min) for steel in 1.0 M HCl and 0.5 M H2SO4 in the absenceand the presence of 2.5 � 10�4 M concentration of inhibitor at30 �C was measured and is presented in Fig. 5. The potential ofthe uninhibited 1.0 M HCl and 0.5 M H2SO4 solution slightly variedin the beginning and later stood at a constant value of �0.484 and�0.489 V respectively. But the potential of the inhibited solutionshave exhibited a nobler(less negative) which tends to shift towardspositive direction with time. The less negative potential stands inthe order HCl < H2SO4 which indicated the higher protection effi-ciency of tacrine in 1.0 M HCl than in 0.5 M H2SO4 [30].

3.3.2. Polarization studiesThe polarization behavior of steel immersed in 1.0 M HCl and

0.5 M H2SO4 at 30 �C in the absence and presence of different con-centration of tacrine is shown in Fig. 6. Electrochemical parameterslike corrosion potential (Ecorr), corrosion current density (Icorr),cathodic Tafel slope (bc), anodic Tafel slope (ba) and percentageinhibition efficiency (gp) according to the polarization studies aredepicted in Table 3. The gp was calculated from following relation:

gP ¼I�

corr � I�

corr

I�

corr

� 100 ð3Þ

Table 2Corrosion parameters obtained from the weight loss measurements for steel in 1.0 M HCl and 0.5 M H2SO4 in the presence of various concentrations of tacrine at differenttemperature.

Medium Inhibitor concentration (� 10�4 M) gw at different temperature

30 �C 40 �C 50 �C 60 �C

HCl 0.5 75.2 67.0 44.4 35.71.0 94.2 88.5 76.1 62.92.5 96.0 94.3 88.0 82.05.0 96.9 94.9 89.9 85.9

H2SO4 0.5 70.0 59.8 40.6 32.51.0 88.7 79.7 66.1 57.02.5 94.5 90.3 83.0 76.55.0 96.0 93.8 87.1 82.7

Fig. 3. The variation of gw with inhibitor concentration at 30 �C.

Fig. 4. The variation of gw with solution temperature for the corrosion of steel in 1.0 M HCl and 0.5 M H2SO4 in the presence of different concentration of tacrine.



where I�corr and Icorr are corrosion current densities in the absenceand presence of inhibitor. The Fig. 6 and Table 3 mainly indicatethe following: (a) Icorr decreased considerably in the presence ofinhibitor in both the acids which indicate good inhibition potentialof tacrine. Icorr decreased continuously with the increase in concen-tration of inhibitor. The decreased Icorr stands in the order 1.0 MHCl < 0.5 M H2SO4 which emphasize that, tacrine is more efficientin 1.0 M HCl than in 0.5 M H2SO4. (b) Ecorr value was shifted towardsless negative (noble) potential which is a necessary to quote theinhibitive action of inhibitor and to classify an inhibitor into an ano-dic, a cathodic, or mixed type. It has been reported that [20], aninhibitor can be classified as an anodic or a cathodic-type inhibitoron the basis of magnitude of shift in Ecorr value. If displacement inEcorr is greater than 85 mV, towards anode or cathode with refer-ence to blank, then an inhibitor is categorized as either anodic orcathodic type inhibitor, respectively. Otherwise an inhibitor is trea-ted as mixed type. In our study, maximum displacement in Ecorr

value was around 65 mV indicating tacrine is a mixed type inhibi-tor, in both acids. (c) Both bc and ba values of inhibited solution havechanged with respect to inhibitor free solution, for both acids whichreiterate that tacrine is mixed type inhibitor. It can also be seenfrom Fig. 5 that shape of anodic and cathodic polarization curve

of steel in HCl looks almost similar to that in H2SO4. This indicatesthat not much difference in inhibitive mechanism of the inhibitor inHCl and H2SO4 [31]. Obtained gp values are in good agreement withgw values.

3.3.3. EIS studiesElectrochemical impedance spectra for steel in 1.0 M HCl and

0.5 M H2SO4 without and with different concentration of inhibitorat 30 �C are presented as Nyquist plot in Fig. 7. An equivalent cir-cuit (Fig. 8) was proposed to fit and analyze EIS data. A similarmodel was previously used by Ying Yan [32]. EIS parameters calcu-lated in accordance with equivalent circuit are listed in Table 3.Sum of charge transfer resistance (Rct) and adsorption resistance(Rad) is equivalent to polarization resistance (Rp) [33]. The inhibi-tion efficiency (gz) was calculated using following equation:

gZ ¼Rp � Ro

p

Rp� 100 ð4Þ

where Rp and Rop are polarization resistance values in the presence

and absence of inhibitor. The diameter of semicircle increased withthe increase in inhibitor concentration reflects the inhibitive actionof the inhibitor in both the acids. The increase in the diameter of thesemicircle is predominate in 1.0 M HCl than in 0.5 M H2SO4, empha-size the higher protective action of the inhibitor in HCl [34]. Thedata in Table 3 revealed that Rp values increased and capacitancevalues decreased with the increase in inhibitor concentration forboth the acids. The decrease in capacitance, which can result froma decrease in local dielectric constant and/or an increase in thethickness of electrical double layer, suggests that the inhibitor mol-ecules act by adsorption at metal/solution interface [35]. This indi-cated the formation of a surface film on steel. Obtained gz are ingood agreement with gp and gw.

3.3.4. PZC studiesAn inhibitor generally act by adsorbing onto the metal surface

and its adsorption mainly depends on surface charge of metal,charge and chemical structure of inhibitor molecule, nature ofsolution, etc. [36]. A metal surface acquires an electric chargedue to the electric field which emerges at the interface when it isimmersed in the electrolyte. The surface charge of metal (a) canbe defined by the position of Ecorr with respect to PZC [37]. The plotof Cdl against applied potential yields a parabolic curve and minimaof the curve correspond to PZC [1]. If the Ecorr value is positive com-pared to the value of PZC under the same conditions, indicates pos-itively charged steel surface. More the positive value of Ecorr, more

Fig. 5. Variation of the potential with immersion time for the corrosion of steel in1.0 M HCl and 0.5 M H2SO4 in the absence and presence of 2.5 � 10�4 M tacrineinhibitor.

Fig. 6. Tafel plots for steel in 1.0 M HCl and 0.5 M H2SO4containing different concentration of tacrine, at 30 �C.



positive is the metal surface. Positive surface charge favors theadsorption of anion while negative surface favors adsorption ofcation. The PZC values of steel in 1.0 M HCl and 0.5 M H2SO4 inthe absence and presence of 2.5 � 10�4 M concentration of tacrineinhibitor was evaluated and the corresponding corrosion potential(Ecorr) are depicted in Fig. 9. It showed that, surface charge of inhib-itor free steel in HCl solution is more positive than in H2SO4, indi-cates that inhibitor free steel surface in HCl largely favors theadsorption of Cl� than steel in H2SO4 do for SO2

4�. More over Cl�

being small and being less hydrated compared to SO24�, extensively

adsorb on steel surface creating an excess negative charge towardsthe bulk solution which in turn favour the adsorption of inhibitorcation.

The surface charge of the inhibited HCl is again more positivecompared to that of inhibited H2SO4, conclude that the adsorbabil-ity of SO2

4� ions on steel surface is less than that of Cl� ions [38] and

as a consequence of this, inhibitor shows more inhibition efficiencyin HCl than in H2SO4.

3.4. Corrosion kinetics

Arrhenius proposed a relation to evaluate effect of temperatureon rate of corrosion [39]:

lnq ¼ lnA�E�app

RTð5Þ

here q is the corrosion rate of steel, calculated in accordance withequation (2), A is the frequency factor and E⁄app is apparent activa-tion energy, R is gas constant (R = 8.314 J mol�1 K�1) and T is abso-lute temperature. The plot of ln q vs. 1/T should be a straight linewith slope equal to �E⁄app /R and an intercept of the line extrapo-lated is equal to ln A. The change in enthalpy (DH⁄) and entropy(DS⁄) of activation for the formation of the activation complex inthe transition state was evaluated using transition-state equation[39]:

lnqT

� �¼ ln

RhN

� �� þ DSast

R

� �� DHast

RTð6Þ

where h is Plank’s constant and N is Avogadro’s number. Plot of ln q/T vs. 1/T gives a straight line with slope �DH⁄/R and an interceptequal to [(ln(R/hN)) + (DS⁄/R)]. The Arrhenius and Transition stateplots for the corrosion of steel in 1.0 M HCl and 0.5 M H2SO4, inaccordance with eqs. (5) and (6) are shown in Figs. 10 and 11. Ki-netic A (frequency factor), E⁄app, DH⁄ and DS⁄ are determined andare depicted in Table 4, from which following conclusions can bemade. The values of E⁄app for the inhibited solutions are much high-er than the uninhibited solution, indicates strong inhibitive action

Table 3Polarization and EIS parameters for steel in 1.0 M HCl and 0.5 M H2SO4 in the presence of different concentration of tacrine.

Polarization EIS

Corrosive medium Inhibitor conn (�10�4 M) Ecorrvs SCE (mV) I corr lA cm�2 bc mV dec�1 ba mV dec�1 gp Rp O cm2 Cdl (lF cm�2) gz

HCl Blank �0.484 170 126 86 – 104 61 –0.5 �0.460 43 112 76 74.7 414 25 74.81.0 �0.469 11 102 151 93.5 1664 11 93.72.5 �0.467 7.0 112 142 95.8 2950 8.7 96.45.0 �0.460 5.1 112 107 97.0 3346 1.0 96.8

H2SO4 Blank �0.489 150 126 86 – 67 71 –0.5 �0.480 46 117 76 69.3 225 56 70.21.0 �0.478 18 96 64 88.0 593 21 88.72.5 �0.459 7.9 116 136 94.7 1202 16 94.45.0 �0.463 5.6 113 119 96.2 1664 11 95.9

Fig. 7. Nyquist plot for steel in 1.0 M HCl and 0.5 M H2SO4 in the presence of different concentration of tacrine at 30 �C.

Fig. 8. Equivalent circuit used to interpret the results of EIS.



[5]. The higher value of E⁄app in the presence of inhibitor can beattributed to the increase in the thickness of double layer which in-crease the E⁄app for corrosion process [40]. Further, E⁄app value forinhibited solutions increased with inhibitor concentration up to1.0 � 10�4 M for HCl and 2.5 � 10�4 M for H2SO4 and thereafter de-creased. This sort of increase in E⁄app and as well as in frequency fac-tor, up to a certain concentration and then decrease, has been welldocumented in the literature [5,41,42]. The large negative values ofDS⁄ implies formation of activated complex is the rate determiningstep which is characterized by the association of molecules ratherthan dissociation meaning that decrease in disorder takes placeon moving from reactant to the activated complex [43]. Thus order-liness increased as reactants are converted to activated complex.Similar observations have been reported by many authors for thecorrosion of steel in HCl and H2SO4 [5,41,42].

3.5. Adsorption isotherm

The adsorption isotherms give good amount of informationregarding the interaction of the inhibitor molecules with steel.The surface coverage (h) defined as gw/100, was tested by fittingto various adsorption isotherms like Langmuir, Temkin, Freundlichand Flory–Huggins. However, the best fit was obtained with Lang-muir isotherm. According to Langmuir’s isotherm, surface coverage

is related to inhibitor concentration (C) by the following equation[44]:

Ch¼ 1

Kadsþ C ð7Þ

where Kads is the equilibrium constant for adsorption process. Theplot of C/h versus C yields a straight line (Fig. 12) with regressioncoefficient close to 1 suggests that the adsorption of inhibitor mol-ecules follow Langmuir adsorption isotherm [45]. The Kads valuescan be calculated from line intercept on C/h axis and is related tostandard free energy change of adsorption (DG�ads) as follows [46]:

DG�

ads ¼ �2:303RT logð55:5KadsÞ ð8Þ

where R is molar gas constant (8.314 J mol�1 K�1), T is absolutetemperature (�K) and value 55.5 is the concentration of water (inmol dm�3) in the solution. Obtained Kads and DG�ads values arelisted in Table 5. The large negative DG�ads and high Kads values in-sure spontaneity of adsorption, stability of the adsorbed film andhence better inhibition efficiency. In our study, negative DG�ads

and high Kads values stood in the order HCl > H2SO4, meaning thatthe tacrine is more efficient in HCl than in H2SO4. DG�ads is relatedto change in enthalpy and entropy of adsorption, DH�ads and DS�ads,

respectively, by following equations [44]:

Fig. 9. Capacitance versus voltage plot of steel in 1.0 M HCl and 0.5 M H2SO4in the absence and presence of 2.5 � 10�4 M tacrine, at 30 �C.

Fig. 10. Arrhenius plots for the corrosion of steel in 1.0 M HCl and 0.5 M H2SO4 in the absence and presence of different concentrations of tacrine.



D�

ads ¼ DH�

ads � TDS�

ads ð9Þ

The value of DH�ads can be calculated by Van’t Hoff equation[47]

ln K�

ads ¼�DH

�

ads

RTþ const: ð10Þ

A plot of ln Kads versus 1000/T gives a straight line, as shown inFig. 13. DH�ads values calculated from the slope of the straight line(slope = DH�ads/R) are depicted in Table 5.

The standard entropy of inhibitor adsorption, DS�ads, were cal-culated from Eq. (9) and recorded in Table 5. As expected, the val-ues of DS�ads are negative, because exothermic adsorption processis to be associated with decrease of entropy. Orderliness increasesas inhibitor molecules adsorb on metal surface, resulting in a de-crease in entropy [48].

3.6. Inhibition mechanism

The anodic reaction in any corrosion process is the oxidation ofthe metal to its ion [49].

Fig. 11. Transition state plots for the corrosion of steel in 1.0 M HCl and 0.5 M H2SO4 in the absence and presence of different concentrations of tacrine.

Table 4Corrosion kinetic parameters of steel in 1.0 M HCl and 0.5 M H2SO4 in the absence and presence of different concentration of tacrine.

Corrosive medium Inhibitor concentration (�10�4 M) A(g cm�2 h�1) E⁄app (kJ mol�1) DH⁄ (kJ mol�1) DS⁄ (J mol�1 K�1)

HCl 0.0 1.1 � 10 19.2 16.6 �175.30.5 1.9 � 105 47.3 44.6 �124.51.0 9.1 � 108 72.2 69.7 �53.72.5 1.9 � 107 63.6 61.1 �86.45.0 1.1 � 107 62.2 60.1 �91.4

H2SO4 0.0 1.8 � 10 18.7 16.1 �171.90.5 1.5 � 104 38.9 35.5 �117.81.0 7.3 � 106 56.7 54.2 �64.02.5 1.1 � 107 59.6 57.1 �60.65.0 3.2 � 106 57.2 53.9 �58.1

Fig. 12. Langmuir isotherms for the adsorption of tacrine on steel in 1.0 M HCl and 0.5 M H2SO4.



M!Mnþ þ ne� ð10:1Þ

However, different cathodic reactions are possible. More commonreactions are

Hydrogen evolution : 2Hþ þ 2e� ! H2 ð10:2Þ

Oxygen reductionsðacidic solutionsÞ : O2 þ 4Hþ þ 4e�

! 2H2O ð10:3Þ

Above reaction is possible because the medium has dissolved air/oxygen and is open to the atmosphere.

Oxygen reductionsðneutral or basic solutionsÞ :

O2 þ 2H2Oþ 4e� ! 4OH� ð10:4Þ

Less common are;Metal ion reduction : M3þ þ e� !M2þ

ð10:5Þ

Metal deposition : Mþ þ e� ! M ð10:6Þ

For the corrosion of steel in acid media, oxidation reaction isFe ? Fe2+ + 2e– while the reduction reactions are 2H+ + 2e– ? H2

and O2 + 4H+ + 4e– ? 2H2O. Any way the kinetics of the steel disso-lution in acid medium depends on the adsorbed FeOHads which areformed by the following reactions

FeþH2O ¡ FeH2Oads ð10:7Þ

FeH2Oads ¡ FeOH�ads þHþ ð10:8Þ

FeOH�ads ¡ FeOHads þ e� ð10:9Þ

FeOHads ¡ FeOHþ þ e� ð10:10Þ

FeOHþ þHþ ¡ Fe2þ þH2O ð10:11Þ

Equally possible is the reaction}} [50]:

FeH2Oads þ inh ¡ FeðinhÞads þH2O ð10:12Þ

FeðinhÞads ¡ FeðinhÞþads þ e� ð10:13Þ

FeOHads þ FeðinhÞþads ¡ FeðinhÞads þ FeOHþ ð10:14Þ

FeOHþ þHþ ¡ Fe2þ þH2O ð10:15Þ

The kinetics of dissolution of steel depends on two intermedi-ates: one adsorbed hydroxyl (FeOHads) and the other adsorbedinhibitor molecule Fe(inh)ads. Displacement of the adsorbed watermolecules by the inhibitor molecule can affect the dissolutionkinetics. Here, every condition, such as molecular size, shape, local-ized partial charges, steric hindrance, etc., would influence thekinetics. The fair size and planarity of tacrine molecule providethe advantage and this accelerates the adsorption of the inhibitormolecule onto the steel surface which finally leads the goodinhibition.

4. Conclusions

The high profile inhibitive property of tacrine is due to its planarstructure. It is an effective inhibitor both in HCl and H2SO4, butstands slightly better in HCl. Inhibition efficiency increased withconcentration and decreased with temperature. Tacrine is a mixedtype inhibitor. Adsorption and thermodynamic study showed thatthe adsorption of tacrine molecule on steel surface follow Lang-muir adsorption isotherm.

Acknowledgements

The authors are grateful to the authorities of Department ofChemistry, Kuvempu University, Karnataka, India for providinglab facilities. The authors also thank Department of Science andTechnology, New Delhi, [DST: Project Sanction No. 100/IFD/1924/2008-2009 dated 2.07.2008] for providing instrumental facilities.The first author is grateful to University Grants Commission, NewDelhi for awarding FIP/FDP teacher fellowship.

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Table 5The parameters for the adsorption of tacrine on steel in 1.0 M HCl and 0.5 M H2SO4 solutions at different temperature.

Corrosive medium Temperature �C Kads � 104 M�1 DG�ads (kJ mol�1) 4H�ads (kJ mol�1) 4S�ads (J K�1 mol�1)

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H2SO4 30 6.45 �38.0 �28.040 3.61 �37.7 �46.5 �28.150 1.73 �36.9 �29.760 1.19 �37.1 �28.2

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