Electrochimica Acta 50 (2005) 3446–3452

Effect of some pyrimidinic Shciff bases on the corrosionof mild steel in hydrochloric acid solution

H. Ashassi-Sorkhabia,∗, B. Shaabanib, D. Seifzadeha

a Electrochemistry Laboratory, Physical Chemistry Department, Faculty of Chemistry, Tabriz University, Tabriz, Iranb Inorganic Chemistry Department, Faculty of Chemistry, Tabriz University, Tabriz, Iran

Received 30 September 2004; received in revised form 19 December 2004; accepted 19 December 2004Available online 6 February 2005

Abstract

The efficiency of benzylidene-pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-yl-amine (B) and (4-chloro-benzylidene)-pyrimidine-2-yl-amine, as corrosion inhibitors for mild steel in 1 M HCl have been determined by weight loss measurements and electro-chemical polarization method. The results showed that these inhibitors revealed a good corrosion inhibition even at very low concentrations.Polarization curves indicate that all compounds are mixed type inhibitors. The effect of various parameters such as temperature and inhibitorc absence ofi t an efficienti n inhibitioe©

K

1

paaitstSfScaC

a

theived

idinn re-

enin

r no.sheder-acidHCl

0d

oncentration on the efficiency of the inhibitors has been studied. Activation energies of corrosion reaction in the presence andnhibitors have been calculated. The adsorption of used compounds on the steel surface obeys Langmuir’s isotherm. It appears thanhibition is characterized by a relatively greater decrease in free energy of adsorption. Significant correlations are obtained betweenfficiency and quantum chemical parameters using quantitative structure–activity relationship (QSAR) method.2004 Elsevier Ltd. All rights reserved.

eywords:Corrosion inhibitors; Langmuir’s isotherm; Quantitative structure–activity relationship

. Introduction

Acid solutions are widely used in industry. The most im-ortant areas of application are acid pickling, acid cleaning,cid rescaling and oil well cleaning[1]. Corrosion inhibitorsre needed to reduce the corrosion rates of metallic materials

n these media. Most of the efficient inhibitors used in indus-ry are organic compounds, which mainly contain oxygen,ulphur, nitrogen atoms, and multiple bonds in the moleculehrough which they are adsorbed on metal surface[2]. Somechiff bases have been reported earlier as corrosion inhibitors

or steel[3–6], copper[7], aluminum[8,9], and zinc[10].ome research work[11,12] reveal that the inhibition effi-iency of Schiff bases is much greater that of correspondingmines and aldehydes. This may be due to the presence of a

N group in the molecule.

∗ Corresponding author. Tel.: +98 4113355998; fax: +98 4113340191.E-mail addresses:habibashassi@yahoo.com,

shassi@tabrizu.ac.ir (H. Ashassi-Sorkhabi).

The aim of the present investigation is to examineinhibitive properties of three Schiff base compounds derfrom 2-amino pyrimidin and corresponding aldehydes.

2. Experimental details

2.1. Materials

Schiff base compounds as inhibitors, shown inFig. 1, weresynthesized from equimolar amounts of 2-amino-pyrimand the corresponding aldehyde through a condensatioaction in ethanol media[13,14].

The chemical composition (wt.%) of the steel specim(determined by SPECTROLAB quantometer) is givenTable 1. The specimens were polished with emery pape400–1200 grade. They were cleaned with acetone, wawith double-distilled water, and finally dried at room tempature before being immersed in the acid solution. Thesolutions (1 M) were made from analytical grade 37%

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.

oi:10.1016/j.electacta.2004.12.019

H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452 3447

Fig. 1. Structures of studied Schiff bases.

and double-distilled water. The concentration range of in-hibitor employed was 2× 10−4 to 1× 10−2 M in 1 M HCl.

2.2. Weight loss measurements

Weight loss measurements were carried out as it wasshown in literature[15]. The volume of solution was 100 ml.The steel specimens had a rectangular form (length: 2 cm,width: 1 cm, thickness: 0.08 cm). A maximum immersiontime was 24 h in the case of inhibited solutions. Solutionswere not stirred. At the end of the tests, the specimens werecarefully washed in acetone, dried and then weighted. Thetemperature of solution was fixed thermostatically a desiredvalue.

2.3. Electrochemical studies

Electrochemical experiments were carried out using anAutolab, Potentiostat-Galvanostat. A three-electrodes elec-

Table 1Chemical composition of used mild steel specimen (wt.%)

C 0.002Si 1.380Mn 0.203P 0.033SCNMVCNTACWPMSSABF

trochemical cell was used. The working electrode was pre-pared from a mild steel sheet, mounted in polyester such thatthe area exposed to solution was 1 cm2. A saturated calomelelectrode (SCE) and a platinum electrode were used as the ref-erence and the counter electrode, respectively. All potentialsare reported versus SCE. In the case of polarization measure-ments, the potential sweep rate was 2 mV s−1. The immersiontime before measurements was 1 h. The ac impedance mea-surements were carried out in the frequency range of 10 KHzto 10 mHz, at the rest potential, by applying 5 mV sine waveac voltage. The double layer capacitance (Cdl) and the chargetransfer resistance (Rt) were calculated from Nyquist plots asdescribed elsewhere[15]. All experiments were performedunder atmospheric conditions.

2.4. Quantum chemical study

Quantum chemical parameters for inhibitors, obtained us-ing the AM1 semi empirical quantum chemical approach,were correlated with their experimental inhibition efficien-cies. The following quantum chemical indices were consid-ered: the energy of the highest occupied molecular orbital(EHOMO), the energy of the lowest unoccupied molecular or-bital (ELUMO), and the dipole moment (µ).

3

3

ec-t asesa tionso tud-i ctionm addi-tE oten-t sionc es,a

0.009r 0.055i 0.022o 0.019

0.002u 0.061b 0.001i 0.003l 0.365o 0.002

0.010b 0.001g 0.001b 0.001n 0.001s 0.014

0.001e 97.810

. Results and discussion

.1. Linear polarization

Figs. 2–4present polarization curves for mild steel elrode in 1 M HCl, in the presence and absence of Schiff bt various concentrations. Both cathodic and anodic reacn mild steel electrode were inhibited in the presence of s

ed compounds. The Schiff bases affected cathodic reaore than anodic reaction. This result suggests that the

ion of inhibitors retards the hydrogen evolution reaction[1].lectrochemical corrosion parameters, i.e., corrosion p

ial (Ecorr), cathodic and anodic Tafel slopes and corrourrent (Icorr), obtained by extrapolation of the Tafel linre given inTable 2.

3448 H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452

Fig. 2. Polarization curves for mild steel in 1 M HCl in the presence of Schiffbase A at different concentrations.

Fig. 3. Polarization curves for mild steel in 1 M HCl in the presence of Schiffbase B at different concentrations.

The inhibition efficiency,ηp, was calculated from the fol-lowing equation[16]

ηp = [( Io − I)/Io] × 100 (1)

whereIo andI are the corrosion current densities without andwith an inhibitor, respectively.

Fig. 4. Polarization curves for mild steel in 1 M HCl in the presence of Schiffbase C at different concentrations.

The obtained values ofηp are given inTable 3. Theseresults show that all used Schiff base compounds act aseffective inhibitors. Corrosion inhibition increases whenthe inhibitor concentration increases. Maximum inhibi-tion efficiency (99.3%) was obtained at a concentration of1× 10−2 M in the case of C compound.

3.2. Electrochemical impedance spectroscopy (EIS)

Figs. 5–7show a typical set of Nyquist plots for mild steelin 1 M HCl in the absence and presence of Schiff bases atvarious concentrations. It is apparent from these plots thatthe impedance response of mild steel in uninhibited HCl has

Table 3The percentage inhibition efficiency obtained from polarization studies

Concentration Inhibitor efficiency

A B C

0.0002 84.8 63.2 75.40.001 97.0 96.4 97.50.005 97.6 98.1 98.60.01 98.7 98.9 99.3

Table 2Electrochemical corrosion parameters for mild steel in 1 M HCl in the absence and presence of Schiff bases at various concentrations

Inhibitor Concentration (M) Ecorr (mV) ba (mV/de

Blank – −429 98

A 0.0002 −420 550.001 −419 650.005 −475 2050.01 −488 89

B 0.0002 −441 780.001 −443 500.005 −479 920.01 −489 104

C 0.0002 −532 620.001 −479 770.005 −445 1280.01 −437 270 196 2121 2.3× 10−5

c) bc (mV/dec) RP (� cm2) Icorr (A cm−2)

142 7.49 3.4× 10−3

109 31 5.1× 10−4

79 79 1.0× 10−4

158 158 8.2× 10−5

98 98 4.4× 10−5

154 18 1.2× 10−3

76 107 1.2× 10−4

97 328 6.2× 10−5

95 596 3.6× 10−5

133 22 8.3× 10−4

81 204 8.4× 10−5

120 560 4.8× 10−5

H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452 3449

Fig. 5. Nyquist plots for mild steel in 1 M HCl in the presence of Schiff baseA at different concentrations.

significantly changed after the addition of Schiff bases in tothe corrosive solutions. The results described below can beinterpreted in terms of the equivalent circuit of the electricaldouble layer shown inFig. 8 which has been used previ-ously to model the iron/acid interface[17]. The capacitance(Cdl) and the charge transfer resistance (Rt) were calculatedfrom Nyquist plots[15]. In the case of the electrochemicalimpedance spectroscopy, the inhibition efficiency is calcu-lated using the charge transfer resistance as follow[15]:

ηz(%) = [(Rt(inh) − Rt)/Rt(inh)] × 100 (2)

whereRt andRt(inh) are the charge transfer resistance valueswithout and with inhibitor, respectively.

The electrochemical parameters derived from the Nyquistplots and the inhibition efficienciesηz are given inTable 4.As can be seen fromFigs. 5–7, the Nyquist plots are not

F hiffb

Fig. 7. Nyquist plots for the mild steel in 1 M HCl in the presence of Schiffbase C at different concentrations.

Fig. 8. Equivalent circuit of the studied system.

perfect semicircles as expected from the theory of EIS forthe assumed equivalent circuit, and this difference can be ex-plained as follows. The Nyquist plots obtained in the realsystem represent a general behavior where the double layeron the interface of metal/solution does not behave as a realcapacitor. On the metal side, electrons control the charge dis-tribution whereas on the solution side it is controlled by ions.Since ions are much larger than the electrons, the equiva-

Table 4Electrochemical impedance parameters for mild steel in 1 M HCl in thepresence and absence of Schiff bases at different concentrations, at 25◦C

Inhibitor Cinh (M) Rs (� cm2) Rt (� cm2) Cdl (�F/cm2) ηz (%)

Blank – 1.3 6 349 –

A 0.0002 1.4 18 157.9 54.80.001 1.2 60 72.6 79.20.005 1.4 159 38.8 88.90.01 1.1 166 34.9 90.0

B 0.0002 1.3 46 70.0 79.90.001 1.2 88 68.9 80.30.005 1.2 164 53.9 84.60.01 1.8 248 25.3 92.8

C 0.0002 1.3 34 97.8 71.80.001 1.3 98 48.4 86.10.005 1.3 204 31.6 90.90.01 2.3 397 14.6 95.8

ig. 6. Nyquist plots for the mild steel in 1 M HCl in the presence of Sc

ase B at different concentrations.

3450 H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452

lent ions to the charge on the metal will occupy quite a largevolume on the solution side of the double layer[18]. It canbe obtained fromTable 4that, the capacitance of electricaldouble layer (Cdl) decreases in the presence of inhibitors. De-crease in theCdl, which can result from a decrease in localdielectric constant and/or an increase in the thickness of theelectrical double layer, suggests that the inhibitor moleculesact by adsorption at the metal/solution interface[12,19].

The inhibition efficiency follows the order:

C > B > A

As known, some atoms, such as O and N atoms of Schiffbases, which have unoccupied orbitals, so exhibit a tendencyto obtain electrons. The electrons in the d orbitals can easilybe offered because their applied force is small. If an inhibitordoes not only offer electrons to un occupied d orbitals ofmetals, but it can also accept the electrons in d orbitals ofmetallic steel by using their antibond orbital to form stablechelate, then it may be considered an as excellent inhibitor[20]. The presence the electron donating groups on the Schiffbases structure (such as Cl and CH3) increases the electrondensity on the nitrogen of the CN group, resulting highinhibition efficiency. Among the compounds investigated inthe present study, C has been found to give the best perfor-mance as corrosion inhibitor. This can be explained on thebasis of the presence of chloride group on the benzene ring.

3

ob-t s ofi

t thec nsu hibi-t the

TC d ab-s t lossm

I ,

B

A

B

C

Fig. 9. Langmuir’s isotherm for adsorption of Schiff bases on the steel sur-face.

increase in concentration. This fact could be expressed thatthese compounds acts as adsorption inhibitors. Plotting suit-able adsorption isotherm could prove it. In fact, logθ/(1− θ)against logC of Schiff bases gives straight lines with unitslope (Fig. 9) indicates that adsorption of Schiff bases onthe steel surface follows the Langmuir’s adsorption isotherm.From these results it could be concluded that there is no inter-action between the inhibitor molecules adsorbed at the metalsurface[21].

The values of corrosion rate and inhibition efficiency ofSchiff bases at 0.1 M concentration at different temperaturesare given inTable 6. It can be seen that the inhibition effi-ciency for all of the compounds slightly decreases with in-creasing temperature from 25 to 43◦C. It may be explainedby desorption of adsorbed inhibitor from the steel surface.

The values of activation energy (Ea) were calculated usingArrhenius equation[22,23]. The free energy of inhibitor ad-sorption (�Gads) at 25◦C was calculated from the followingequation[21]:

�Gads= −RT ln(55.5K) (3)

Table 6Corrosion parameters of mild steel in 1 M HCl in the presence and absenceof 0.01 M of Schiff bases at different temperature, obtained from weight lossmeasurements

I

B

A

B

C

.3. Weight loss measurements

The values of inhibition efficiency and corrosion rateained from weight loss method at different concentrationnhibitors at 25◦C are summarized inTable 5.

It has been found that all of these compounds inhibiorrosion of mild steel in HCl solution at all concentratiosed in this study. It has also been observed that the in

ion efficiency for all of these compounds increases with

able 5orrosion parameters for mild steel in 1 M HCl in the presence anence of Schiff bases at different concentrations, obtained from weigheasurements at 25◦C

nhibitor Concentration(M)

Surface coverage(θ)

Inhibition efficiencyηw (%)

lank – – –

0.0002 0.582 58.20.001 0.902 90.20.005 0.961 96.10.01 0.990 99.0

0.0002 0.879 87.90.001 0.967 96.70.005 0.985 98.50.01 0.989 98.9

0.0002 0.679 67.90.001 0.962 96.20.005 0.987 98.70.01 0.994 99.4

nhibitor Temperature(◦C)

Corrosionrate (mg/cm2 h)

Surfacecoverage (θ)

Inhibitionefficiency,ηw (%)

lank 25 0.549 – –34 0.982 – –43 1.220 – –

25 0.005 0.990 99.034 0.021 0.978 97.843 0.049 0.960 96.0

25 0.006 0.989 98.934 0.014 0.986 98.643 0.045 0.963 96.3

25 0.003 0.995 99.534 0.016 0.984 98.443 0.029 0.976 97.6

H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452 3451

Table 7Activation energy (Ea) of corrosion and free Gibbs energy (�Gads) of Schiffbases adsorption obtained from weight loss measurements

Inhibitor Temperature (◦C) Ea (kJ/mol) �Gads(kJ/mol)

Blank – 34.92 –

A 25 79.56 −31.5334 −33.0343 −34.00

B 25 88.97 −31.9334 −32.5643 −33.52

C 25 99.36 −32.9934 −33.9843 −34.97

whereK is given by:

K = θ/C(1 − θ) (4)

whereθ is the degree of coverage on the metal surface,C theconcentration of inhibitor in mol l−1 andK the equilibriumconstant. The values of the free energy and the activationenergy are given inTable 7.

It was found that,Ea values for inhibited systems arehigher thanEa for uninhibited system. This results that phys-ical adsorption occurred in the first stage, which explains thenature of organic molecules–metal interactions. On the otherhand, physical adsorption is related to lower values of the ac-tivation energy (30–50 kJ mol−1). So these criteria cannot betaken as decisive to competitive adsorption with water whoseremoval from the surface requires also some activation en-ergy[24]. The low and negative value of�Gadsindicated thespontaneous adsorption of inhibitor on the surface of mildsteel[25,26].

3.4. Quantum chemical study

Since the electronic properties can change by an ad-dition of a functional group to the molecule, the in-hibitor efficiency can be treated as a controlled property.The synthesis of better corrosion inhibitors then can bea ctedg ela-t pa-r tors,w piri-c n-s lecu-l ciffb ordedb nds tiga-t ces( ital,E itala 1

Table 8Quantum chemical parameters for used Schiff bases obtained from AM1method

Inhibitor EHOMO (eV) ELUMO (eV) �E (eV) µ (D)

A −9.3487 −0.9333 −10.2820 0.580B −9.1665 −0.9132 −10.0797 1.835C −9.3598 −1.0908 −10.4506 2.797

semi empirical quantum chemical approach are shown inTable 8.

The average values of inhibition efficiency (ηm), obtainedusing three experimental methods (EIS, linear polarizationand weight loss) were used for QSAR modeling (Table 9).Linear and non-linear QSAR models have been used in thiscase.

Not only the� electron of the Schiff bases enter unoccu-pied orbitals of iron, but the�* orbital can also accept theelectrons of d orbitals of metallic iron to form feed back bonds[12]. In order to prove the forming of feed back bonds, a linearregression analysis was performed on the average inhibitionefficiency versus the HOMO energy (EH) and LUMO energy(EL) of the Schiff bases A, B, C, and the following equationwas obtained.

η (%) = 3.362EHOMO − 7.094ELUMO + 122.761 (5)

Inhibition efficiency increases with increasing values ofEHOMO. The results seem to indicate that charge transferfrom the inhibitor takes place during the adsorption on themetal surface. Increasing values of theEHOMO may facili-tate adsorption (and therefore inhibition) by influencing thetransport process through the adsorbed layer.EHOMO is oftenassociated with the electron donating ability of a molecule.Therefore, the energy of the lowest unoccupied molecular or-b epte att

et s thea on

TI

I

A

B

C

chieved by controlling electronic properties of a seleroup of Schiff base molecules. In addition, the corr

ions between the inhibition efficiency and molecularameters can be used for pre-selection of new inhibihich are, at the moment, taken essentially from emal knowledge’s[19]. Quantum structure–activity relatiohips (QSAR) has been used to study the effect of moar structure on the inhibition efficiency of the used Shase compounds. Satisfactory correlation has been recetween the inhibition efficiency of some inhibitors aome quantum chemical parameters by other invesors [12,27–29]. The calculated quantum chemical indiEHOMO = energy of the highest occupied molecular orbLUMO = energy of the lowest unoccupied molecular orbnd dipole momentµ) of Schiff bases obtained using AM

ital (ELUMO) indicates the ability of the molecules to acclectrons. The lower value ofELUMO, the more probable, th

he molecule would accept electrons[12,18].The coefficients ofEH (positive) andEL (negative) prov

hat the main aspect of the forming of feed back bonds ibility of the inhibitors to offer electrons, so the inhibiti

able 9nhibition efficiency obtained from different methods and their average

nhibitor Cinh Hw (%) ηp (%) ηz (%) ηm (%)

0.0002 58.2 84.8 54.8 65.90.001 90.2 97.0 79.2 88.80.005 96.1 97.6 88.9 94.20.01 99.0 98.7 90.0 95.9

0.0002 87.9 63.2 79.9 77.10.001 96.7 96.4 80.3 91.10.005 98.5 98.1 84.6 93.70.01 98.9 98.9 92.8 96.9

0.0002 67.9 75.4 71.8 77.80.001 96.2 97.5 86.1 93.30.005 98.7 98.6 90.9 96.10.01 99.4 99.3 95.8 98.2

3452 H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452

Fig. 10. Correlation between the experimental inhibition efficiency (ob-tained from LP, EIS and weight loss methods) and the calculated inhibitionefficiency (obtained from QSAR).

efficiency values of the three Schiff bases follow the order:

C > B > A

The non-linear equation was used to correlate all quantumchemical parameters (EH, EL, µ) and inhibitor concentra-tion (Cinh) with the experimental inhibition efficiencies. Thenon-linear model proposed by Lukovits et al.[30] for the in-teraction of corrosion inhibitors with metal surface in acidicsolutions has been used in this part of the study. The fol-lowing proposed relation between inhibition efficiency, andquantum chemical index was used:

η (%) = (Axj + B)Ci/(1 + (Axj + B)Ci) (6)

whereA andB are the regression coefficients to be deter-mined by regression analysis;xj a quantum chemical indexcharacteristic of moleculej; Ci denotes the concentration inan experimenti. In this case,xj is constructed, as a compositeindex of quantum chemical parameters;EHOMO, ELUMO andµ. Following equation is obtained for the three compounds.

η (%) = (AEHOMO + BELUMO + Cµ − D)Cinh

1 + (AEHOMO + BELUMO + Cµ − D)Cinh(7)

where A, B, C, D are −526.325, 977.031, 107.937 and2150.147, respectively. The plot of the experimental and thecalculated inhibition efficiency of compound A, B, and C isp nc ffi-c

4

1 orro-

2 reac-

3. Corrosion inhibition of used compounds increases whentheir concentration increases.

4. Substitution of Cl and CH3 group on the benzene ringincreases the inhibition efficiency.

5. Corrosion inhibition decreases when the temperature in-creases.

6. Adsorption of Schiff bases on the steel surface is sponta-neous and obeys the Langmuir’s isotherm.

7. Results obtained from all applied experimental methodsare in good agreement.

8. Inhibition efficiency of Schiff bases increases whenthe HOMO energy increases and the LUMO energydecreases.

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. Conclusion

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