2-Mercapto-1-methylimidazole as corrosion inhibitor forn

2-Mercapto-1-methylimidazole as corrosion inhibitor for

copper in hydrochloric acid

https://www.360docs.net/doc/c611022957.html,rabi a ,*,O.Benali b ,S.M.Mekelleche a ,Y .Harek a

a

De

′partement de Chimie,Faculte ′des Sciences,Universite ′Abou Bakr Belka?¨d,13000Tlemcen,Algeria b

De ′partement de Biologie,Centre Universitaire de Sa?¨da,20000Sa?¨da,Algeria Received 12December 2005;received in revised form 7February 2006;accepted 7February 2006

Available online 11April 2006

Abstract

The inhibition of corrosion of copper in hydrochloric acid by 2-mercapto-1-methylimidazole was investigated by dc polarization,ac impedance and weight loss techniques.A signi?cant decrease in the corrosion rate of copper was observed in the presence of the investigated compound.The corrosion rate was found to depend on the concentration of the inhibitor.The degree of surface coverage of the adsorbed inhibitor is determined by ac impedance technique,and it was found that the results obey the Langmuir adsorption isotherm.The potentiodynamic polarization data indicated that the inhibitor was of mixed type,but the anodic effect is more pronounced.The slopes of the cathodic and anodic Tafel lines are approximately constant and independent on the inhibitor concentration.This inhibitor lowered the corrosion reaction by blocking the copper surface through physical and chemical adsorption.The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitor.Also,some thermodynamic data for the adsorption and dissolution processes are calculated and discussed.The reactivity of the compound under investigation was analyzed through Fukui indices,which are reactivity descriptors derived from density functional theory (DFT),to explain the great ef?ciency of this compound as corrosion inhibitor comparatively to other imidazole derivatives.#2006Elsevier B.V .All rights reserved.

Keywords:Copper;Corrosion inhibition;Hydrochloric acid;2-Mercapto-1-methylimidazole;Fukui indices

1.Introduction

Copper has an excellent thermal conductivity,good corrosion resistance and mechanical workability and is widely used in heating and cooling systems.Such systems are cleaned by acid-pickling processes,using hydrochloric acid [1].The use of the inhibitors in the pickling operation is of the very recent origin [2].Corrosion inhibitors effectively eliminate the undesirable destructive effect and prevent metal dissolution.Most acid corrosion inhibitors are nitrogen-,sulfur-or oxygen-containing organic compounds.

Several investigators have studied the inhibitory effects of ring-substituted benzotriazole in different aqueous solutions [3–6].Some other heterocyclic compounds such as imidazole derivatives are of interest as corrosion inhibitors for Cu metals and alloys [7–11].

Since the S atom has strong adsorption on copper,many heterocyclic compounds containing a mercapto group have been used as copper corrosion inhibitors for different industrial applications.Previous research has shown the inhibiting properties of 2-mercaptobenozothiazole [12],2,4-dimercapto-pyrimidine [13],2-amino-5-mercaptothiazole and 2-mercap-tothiazoline [14].Despite these numerous studies,until now,it is still questionable what role of the mercapto group is in relation to corrosion inhibition.It has been suggested that corrosion inhibitor is chemisorbed on the Cu surface through S atom [15].Other researchers have proposed that interaction of the S atom with the metal surface results in the formation of an insoluble protective complex [16,12].Zhang et al.[17]stated that introduction of mercapto group to a heterocyclic compound can vary the disturbances and orbital energy con?gurations of electrons,thus enhancing the inhibitory effects on copper corrosion in HCl solutions.Recently,Dafali et al.[18]have studied the effect of 2-mercapto-1-methylimidazole (MMI)on the copper corrosion behavior in aerated 3%NaCl solution using electrochemical polarization,impedance spectra,cyclic voltammetry methods and IR spectra.It was observed that MMI

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*Corresponding author.Tel.:+21343286149;fax:+21343286149.E-mail address:larabi_lahcene@yahoo.fr (https://www.360docs.net/doc/c611022957.html,rabi).

0169-4332/$–see front matter #2006Elsevier B.V .All rights reserved.doi:10.1016/j.apsusc.2006.02.013

acts through the formation of a ?lm on the surface of the copper.In addition,Zhao and Pang [19,20]have studied the corrosion inhibition property of 2-mercapto-1-methylimidazole for copper in 5%HCl using the weight loss method.It was shown that the adsorption of these inhibitors on copper surface was a heat-adsorbing reaction.

In order to obtain a better understanding of the mode of inhibitory action of 2-mercapto-1-methylimidazole on the pure Cu corrosion,in the present work,gravimetric measurements,potentiodynamic polarization experiments,ac impedance spectroscopy were carried out in 2-mercapto-1-methylimida-zole free 1M HCl solutions and those containing 2-mercapto-1-methylimidazole.Theoretical calculations based on the Fukui indices [21],which are descriptors derived from density functional theory (DFT),were performed,in order to give further insight into the experimental results.Imidazole (Im)and p -tolyl-3methylimidazole (TMI)were also included for comparison.The ef?ciency of these compounds for corrosion inhibition of copper in 0.5M HCl at 208C was investigated by Gas

ˇparac et al.[9].2.Experimental 2.1.Materials

The copper working electrode was of 99.94purity.2-Mercapto-1-methylimidazole (Merck)was used as received.Fig.1shows the molecular structures and atom order of MMI (two tautomers),Im and TMI.The acid solutions were made from AR grade HCl (Prolabo).Appropriate concentration of acid was prepared by using double-distilled water.2.2.Electrochemical measurements

Disc electrode was prepared from the investigated copper.The electrode was polished with emery papers (from 400to 1000),rinsed with distilled water,degreased by acetone,washed thoroughly with bi-distilled water and dried at room temperature.The concentration range of inhibitor employed was 2.5?10à4to 10à3M.The studies were carried out potentiodynamically in a thermostated electrolytic cell (CEC/TH-Radiometer)with a capacity of 500ml.Platinum disc was used as a counter-electrode (CE)and a saturated calomel electrode as a reference electrode (SCE).The latter was

connected through a Luggin’s capillary to the cell.The working electrode (WE)was in the form of a disc cut from copper under investigation and was embedded in a Te?on rod with an exposed area of 0.5cm 2.This electrode was immersed in 200ml of a test solution for 1h until a steady state open-circuit potential (E ocp )was obtained.Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)were conducted in an electrochemical measurement system (V olta-Lab40-Radiometer)which comprises a PGZ301potentiostat,a personal computer and V oltaMaster 4and Zview software.The potentiodynamic current–potential curves were recorded by changing the electrode potential automatically from à450to 50mV versus SCE with scanning rate of 0.166mV/s under static conditions.All experiments were carried out in freshly prepared solution at constant temperatures,30,40,50and 60?0.18C using a thermostat.The ac impedance measure-ments were performed at corrosion potentials (E corr )over a frequency range of 10kHz to 1mHz,with a signal amplitude perturbation of 10mV .The data were plotted and analyzed using software V oltaMaster 4,version 4.The impedance parameters were calculated using software Z-View,version 2.80,2002,Scribner Associates Inc.2.3.Gravimetric measurements

The specimens for weight loss measurements were 20mm by 10mm by 1mm.They were polished with emery paper (from 400to 1000grade).Each run was carried out in a glass vessel containing 100ml test solution.A clean weighed mild steel specimen was completely immersed at an inclined position in the vessel.After 2h of immersion in 1M HCl with and without addition of MMI at different concentrations,the specimen was withdrawn,rinsed with distilled water,washed with acetone,dried and weighed using an analytical balance accurate to 0.01mg.Three measurements were performed in each case and the mean value of the weight loss has been reported.The weight loss was used to calculate the corrosion rate in milligrams per square centimeter per hour.2.4.Theoretical computations

The equilibrium geometries of the MMI,TMI and Im molecules were optimized by the AM1semi-empirical method [22]using HyperChem program [23].The atomic electronic

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1372Fig.1.Molecular structures of 2-mercapto-1-methylimidazole,imidazole and 1-(p -tolyl-4-methylimidazole).

populations were calculated by the B3LYP/6-31G*method using Gaussian94W program[24].The chemical reactivity of the different sites of the molecules was evaluated by Fukui indices,which are de?ned by[21]:

for nucleophilic attack

ft

k

??q keNt1Tàq keNT (1) for electrophilic attack

fà

k

??q keNTàq keNà1T (2) where q k(N),q k(Nà1)and q k(N+1)denote electronic pop-ulations of the atom k in neutral,cationic and anionic systems, respectively.These quantities were calculated using the natural population analysis(NPA)[25].

3.Results and discussion

3.1.Polarization curves measurements

Fig.2shows cathodic and anodic polarization curves for Cu

carried out at308C in1M HCl.In the inhibitor-free solution an exponentially increasing Cu dissolution current is observed in the anodic scan at potentials positive ofà200mV followed by a decrease in current above%à42mV and a second increase aboveà14mV.Similar behavior has been found previously by Tromans and Sun[26],Lee and Nobe[27],Modestov et al.[28] and analyzed by Bacarella and Griess[29].The decrease in current betweenà42andà14mV was attributed to the formation of a CuCl?lm arising from the lateral growth and thickening of CuCl nuclei[30].The anodic curves in the absence of inhibitor are characterized by a quasi-Tafelian behavior,i.e.the anodic copper dissolution is not solely activation-controlled,but is under diffusion of soluble CuCl2àspecies from the outer Helmholtz plane into the bulk solution [29].

The cathodic polarization curve displayed a current plateau fromà300toà430mV,which may be attributed to the diffusion controlled reduction of dissolved oxygen.According to Zhang et al.[17],the cathodic corrosion reaction in an aerated acidic chloride solution is:

4HttO2t4e!2H2O(3) Fig.3shows cathodic and anodic polarization curves for Cu carried out at308C in1M HCl without and with various concentrations of MMI.It is observed that the addition of MMI affects both the anodic and cathodic parts of the curves.There-fore,MMI acts as mixed-type inhibitor,although the small shift of the corrosion potential in the positive sense as compared to the uninhibited solutions shows that the effect on the anodic reaction is somewhat more pronounced that on the cathodic reaction.Moreover,we remark from Fig.3that although a difference in the position of the cathodic branch is observed upon addition of MMI,the cathodic curves in presence of the inhibitor almost coincide.Thus,a further enhancement of the corrosion inhibition with increasing concentration is almost entirely due to a change in the anodic branch.This permit us to state that for low concentrations,the adsorption preferentially takes place at cathodic sites on the copper surface,or alter-natively,that from a certain concentration onwards,further adsorption mainly occurs at anodic sites.

Table1represents the electrochemical parameters:corrosion potential(E corr),corrosion current density(j corr),the anodic Tafel slope(b a)and inhibition ef?ciency(P%)for corrosion of

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1373

Fig.2.Polarization curves for copper in1M

HCl.

Fig.3.Polarization curves for copper in1M HCl in the presence of MMI at

different concentrations.

Table1

Electrochemical parameters of copper in1M HCl with and without MMI at

various concentrations and the corresponding inhibition ef?ciency

C inh(M)E corr(mV/SCE)b a(mV/dec)j corr(m A/cm2)P(%)

Blankà235.5?0.566.9?0.220.4?0.4–

2.5?10à4à231.5?1.571.6?0.211.5?0.54

3.6

5?10à4à224.0?2.068.7?0.78.9?0.156.3

7.5?10à4à222.0?2.064.3?1.87.7?0.262.2

10à3à214.5?1.560.1?1.2 6.0?0.270.5

copper in 1M HCl containing different concentrations of MMI.From this table,it is clear that the corrosion current densities (j corr )decrease with increasing inhibitor concentration.The effect was maximal for the concentration 10à3M MMI,which is the optimum concentration of inhibitor required to achieve the ef?ciency (P =70%).The inhibition ef?ciency (P %)was calculated using the equation:P %?

j corr àj corr einh T

j corr

?100

(4)

where j corr and j corr(inh)are the corrosion current density values without and with inhibitor,respectively,determined by extra-polation of the slopes anodic Tafel lines to the corrosion potential.The slopes of the anodic Tafel lines (b a )remain almost unchanged,indicating that [31,32]the presence of MMI in the solution has no effect on the mechanism dissolution of copper,but that the adsorbed molecules mechanically screen the coated part of the electrode and thus protect it from the action corrosion medium.

In order to compare our results with those obtained by Gas

ˇparac et al.[9]we have also performed potentiodynamic experiments at 208C in the potential region ?200mV from the corrosion potential at a sweep rate of 0.166mV/S.Table 2depicts the electrochemical parameters:corrosion potential (E corr ),corrosion current density (j corr ),Tafel slopes (b c and b a )and inhibition ef?ciency (P %)for corrosion of copper in 0.5M HCl containing different concentrations of MMI.We remark that j corr decreases with the increase of inhibitor concentration and the ef?ciency increases with the product content and attains

87%.From Table 2and results given by Gas

ˇparac et al.[9]showing that the value of the inhibition ef?ciency is 33and 54%for imidazole and 1-(p -tolyl)-4-methylimidazole at 0.1M,respectively,it is clear that the percentage inhibition decreases in the order:

MMI e87:5%T>TMI e54%T>Im e33%T

3.2.Electrochemical impedance spectroscopy (EIS)Nyquist plots of copper in the 1M HCl at 308C in the presence and absence of MMI are given in Fig.

4.These curves have been obtained after 1h of immersion in the corresponding solution.All the plots show evidence for tow semicircles.Similar Nyquist plots were obtained by Diard et al.[33]and Gas

ˇparac et al.[9],in their investigations of copper corrosion in 1and 0.5M HCl,respectively.According to Crundwell [34]

and Diard et al.[33]the mechanism of dissolution of copper in HCl solutions is Cu tCl à$CuCl ads te (5)CuCl ads tCl à$CuCl 2à

(6)

We attributed the high frequency loop to the charge transfer resistance and the double capacitance Eq.(5)and the low frequency loop to convective and diffusion mass transport of CuCl 2àfrom the surface Eq.(6)in accordance with most authors’views [9,33].Moreover,the capacitive loop observed in the highest frequency range is attributed to the relaxation of the double layer since its charging–discharging is a very rapid process.The diameter of the high-frequency capacitive loop is regarded as the charge transfer resistance.The smaller the charge transfer resistance,the faster the rate of an electrochemical reaction.According to Ma et al.[35]the impedance spectra shown in Fig.4may be analyzed with the electrical circuit in Fig.5.The corresponding ?tting results are listed in Table 3.R is the electrolyte resistance,R 1the charge transfer resistance,Q 1(CPE 1)the capacitance of the double layer and R 2and Q 2(CPE 2)are associated with the adsorbed CuCl layer,as described in detail by Diard et al.[33].The data were ?tted using a constant phase element;Z CPE ,which is described by the expression [36–38]:Z CPE ?Q à1ej v Tàn

(7)

where Q is a proportional factor while n has a meaning of phase shift.The above equation provides information about the

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1374Table 2

Electrochemical parameters of copper in 0.5M HCl with and without MMI at various concentrations and the corresponding inhibition ef?ciency C inh (M)E corr (mV/SCE)b a (mV/dec)j corr (m A/cm 2)P (%)Blank à182.0?1.049.1?0.115.3?0.5–10à4

à190.0?1.053.4?1.3 4.3?0.171.95?10à4à210.0?2.051.2?0.5 2.0?0.286.910à3

à225.0?1.0

54.5?0.1

1.9?0.2

87.5

Fig.4.Nyquist plots for copper in 1M HCl in the absence and presence of different concentrations of

MMI.

Fig.5.The electrochemical equivalent circuit used to ?t the impedance spectra.

degree of non-idealibility in capacitance behavior.Its value makes it possible to differentiate between the behavior of an ideal capacitor(n=1)and of a CPE(n<1).n can be used as a measure of the surface inhomogeneity[39,40].The introduc-tion of such a CPE is often used to interpret data for rough solid electrodes[41].

It is seen from Table3that the value of the charge transfer resistance R1increases with the inhibitor concentration.This result is related to the corrosion protection effect of the molecules.The value of the proportional factor Q of CPE varies regularly as well.Two dimensions of Q are presented,too (including that of the capacitance,i.e.F/cm2),which provides a comparison,i.e.a tendency toward a decrease is observed with increasing concentration,suggesting that the MMI molecules function by adsorption at the metal solution/interface.The value of n grows as well(0.59–0.71),when compared to that obtained in pure HCl.This can be attributed to a certain decrease of the initial surface inhomogeneity resulting from the inhibitor’s adsorption on the most active centers[40].Note that the value of capacitance,C,can be calculated for a parallel circuit composed of a CPE(Q)and a resistor(R1),according to the following formula[42,43]:

Q?eCR tTn

R t

(8)

The percent inhibition ef?ciency is calculated by charge transfer resistance obtained from Nyquist plots,according to the Eq.(9)[44]:

P%?R0

t

àR t

R

t

?100(9)

where R t and R0t are the charge transfer resistance values without and with inhibitor,respectively.Percentage inhibition values for the MMI on copper are also listed in Table3.We remark that the value of inhibition ef?ciency increases with increase in MMI concentration reaching a maximum value at10à3M.It can be concluded that the inhibition ef?ciency found from polarization curves and electrochemical impedance spectro-scopy measurements are in good agreement.

3.3.Weight loss measurements

Table4collects the values of inhibitor ef?ciency and corrosion rate obtained from weight loss measurements for different concentrations of the inhibitor in1M HCl at 30?0.18C.From the calculated weight loss values,the percentage inhibitor ef?ciency P%was calculated using the equation:

P%?

1à

W

W0

?100(10)

where W and W0are rates of corrosion(mg/cm2/h)with and without inhibitor,respectively.

Table4shows that inhibitor ef?ciency increases with increase in inhibitor concentration.At this purpose,one observes that the optimum concentration of inhibitor required to achieve the ef?ciency is found to be10à3M(P=67.86%). Further study on Table4shows that the values of inhibition ef?ciency obtained from weight loss measurements are in satisfactory agreement with those obtained from electroche-mical measurements.

3.4.Adsorption isotherm

To understand the mechanism of corrosion inhibition,the adsorption behavior of the organic adsorbate on the metal surface must be known.If simple adsorptive behavior is assumed for MMI,a direct relationship between inhibition ef?ciency and surface coverage,u,of the inhibitor,takes place and electrochemical impedance spectroscopy data were used to evaluate the surface coverage values,which are given by Eq.(11)[45]:

u?

R0

t

àR t

t

(11)

where R t and R0t are the charge transfer resistance values without and with inhibitor,respectively.

The surface coverage values(u)were tested graphically to allow?tting of a suitable adsorption isotherm.The plot of C/u versus C(Fig.6)yielded a straight line with nearly unit slope,

https://www.360docs.net/doc/c611022957.html,rabi et al./Applied Surface Science253(2006)1371–13781375 Table3

Values of the elements of equivalent circuit in Fig.4required for?tting the EIS for copper in1M HCl with MMI and the corresponding inhibition ef?ciency

C inh(M)R(V cm2)CPE1?104

(V/cm2/s n)n1?10R1(V cm2)C dl?104

(F/cm2)

CPE2?103

(V/cm2/s n)

n2?10R2(V cm2)P(%)

Blank 3.8?0.345.0?3.2 5.9?0.2150.0?11.034.3?4.130.0?5.08.1?0.1759.0?50.0–

2.5?10à4

3.2?0.117.0?1.0 6.7?0.1255.0?1

4.011.2?0.944.0?1.09.1?0.1897.0?90.041.1 5?10à4 6.4?0.3

5.4?0.3

6.7?0.1295.0?10.0 2.0?0.130.0?0.38.0?0.21255.0?80.049.1

7.5?10à4 4.3?0.2 2.8?0.37.1?0.1510.0?10.0 1.2?0.144.0?0.9

8.7?0.41450.0?78.070.6 10à3 1.6?0.3 6.0?0.3 6.5?0.2525.0?16.0 3.2?0.325.0?2.0

9.1?0.12000.0?110.071.4

Table4

Inhibition ef?ciency for various concentrations of MMI for the corrosion of

copper in1M HCl obtained from weight loss measurements

C inh(M)Corrosion rate?102(mg/cm2h)P(%)

Blank 5.6?0.3–

2.5?10à4

3.3?0.141.07

5?10à4 3.1?0.144.64

7.5?10à4 2.4?0.257.14

10à3 1.8?0.167.86

clearly proving that the adsorption of the MMI from 1M HCl solution on the copper obeys the Langmuir adsorption isotherm where:u ?

KC (12)

with K ?

155:5 exp àD G ads

RT

(13)

where K designates the adsorption equilibrium constant and D G ads is the free energy of adsorption.

The value of equilibrium adsorption constant obtained from the Langmuir plot is about 2.5?103l/mol.The value of D G ads is calculated to be à30.2kJ/mol.The large negative value of D G ads indicated that MMI was strongly adsorbed on the copper surface [46].It is well known that values of àD G ads of the order of 20kJ/mol or lower indicate a physisorption;those of order of 40kJ/mol or higher involve charge sharing or a transfer from the inhibitor molecules to the metal surface to form a co-ordinate type of bond [47,48].On the other hand,Metikos-Hukovic et al.[41]describe the interaction between thiourea and iron (D G ads =à39kJ/mol)as chemisorption.The same conclusion was given by Wang et al.concerning the interaction between mercapto-triazole and mild steel (D G ads =à32kJ/mol)[49].Moreover,Bayoumi and Ghanem consider that the adsorption of naphthalene disulfonic acid on the mild steel was principally by chemisorption (D G ads =à28.47kJ/mol)[50].The D G ads value obtained here shows that in the presence of 1M HCl,chemisorption of MMI may occur.Although we have proposed a chemisorption mechanism for the action of MMI,this may not rule out the physical adsorption mechanism for the inhibition,particularly as imidazole and its derivatives physisorb on the copper surface in 0.5M HCl solution [9].Thus,MMI may be adsorbed in the form of neutral molecules involving replacement of water molecules from the metal surface and sharing of electrons between the ‘‘N’’and ‘‘S’’atoms of the inhibitor molecule and metal surface.Adsorption

of MMI can occur through p electron interactions between the imidazole group structure of the molecule and the metal surface and can also occur by the electrostatic interaction between the positively charged part of the molecule and negatively charged metal surface.Due to adsorption,inhibitor molecules block the reaction sites and reduce the rate of corrosion reaction [51].3.5.Effect of temperature

To investigate the mechanism of inhibition and to determine the activation energy of the corrosion process,polarization curves of copper in 1M HCl were determined at various temperatures (303à333K)in the absence and presence of 7.5?10à4M of MMI.Fig.7presents the Arrhenius plots of the natural logarithm of the current density versus 1/T ,for 1M solution of hydrochloric acid,without and with addition of MMI.Straight lines with coef?cients of correlation (c.c.)high to 0.99are obtained for the supporting electrolyte and MMI.The values of the slopes of these straight lines permit the calculation of the Arrhenius activation energy,E A ,according to:ln j corr

?àE ?A

RT

tA (14)

where R is the universal gas constant and A is the Arrhenius factor.

The calculated values of E ?

A are 21.56kJ/mol (blank)and

22.50kJ/mol (MMI).We remark that the E ?

A value remains practically constant in the presence and the absence of MMI.According to Oguzie et al.[52]and Abd El Rehim et al.[53],unchanged or lowered activation energy in inhibited system compared to the blank is indicative of chemisorption possibly,because some energy is used up in chemical reaction.

The enthalpy of activation (D H a )can be obtained from Eyring-type equation:ln

j corr T ?àD H a

RT

tA 0(15)

https://www.360docs.net/doc/c611022957.html,rabi et al./Applied Surface Science 253(2006)1371–1378

https://www.360docs.net/doc/c611022957.html,ngmuir’s isotherm for adsorption of MMI on the copper surface in 1M

HCl.

Fig.7.ln j vs.1/T for copper dissolution in 1M HCl in the absence and presence of 7.5?10à4M MMI.

The Eyring equation is:

k ?exp àD G ads RT ?RT N a h exp D S a R exp àD H a

RT

(16)

where k is the rate constant,R the universal gas constant,D S a

the entropy of activation and h is the Planck’s constant.A plot of ln (j corr /T )versus 1/T gave straight lines,of slope of àD H a /R ,as shown in Fig.8.The D H a values for the corrosion reaction in the absence and presence of MMI are 19.07and 20.31kJ/mol,respectively.We remark that the addition of inhibitor has very little effect on the D H a ,which remains unaffected.3.6.Theoretical calculations

It was shown from Section 3.1that the percentage inhibition decreases in the order:

MMI e87:5%T>TMI e54%T>Im e33%T

The inhibition ef?ciency depends on many factors [54]including the number of adsorption centers,mode of interaction with metal surface,molecular size and structure.

In order to give further insight into the experimental results,the Fukui indices are used for predicting the preferential sites of electrophilic attack on MMI,TMI and Im (we recall that TMI and Im are included for comparison).It is known that the Fukui indices were widely used as descriptors of site selectivity for the soft–soft reactions [55,56].According to Li and Evans [57],the favorite reactive site is that which possesses a high value of Fukui indices.Hence,relationships between electronic structure and

ef?ciency of MMI,TMI and Im can be deduced from Fukui indices calculations.In this way,we have calculated the electrophilic Fukui indices f àk ,de?ned by Eq.(2),for heteroatoms in the three systems.The results are given in Table 5.It turns out from the tabulated values that the sulfur atom,in MMI tautomers 1and 2,possesses the largest value of Fukui indices (0.301,and 0.475for tautomer 1and 2,respectively).Thus,the sulfur atom in MMI is more reactive for electrophilic attack than nitrogen atoms in the three systems.Indeed,the nitrogen atoms,in both MMI,TMI and Im systems have very small values of electrophilic Fukui indices (less than 0.082).These results seem to indicate that the S atom of MMI,which has a great nucleophilic character,is involved in the chemical reactivity of this molecule with the metal surface while the N atoms of the all compounds seem not to participate in the reactivity [58].Therefore,MMI has the strongest interaction with copper and has the best inhibitory effect on corrosion of copper in an acidic chloride solution.Thus,the effect of the mercapto group on inhibition of copper in an acidic chloride solution by MMI could be mainly attributed to chemical adsorption of the S atom.

On the other hand,it is interesting to note that Gas

ˇparac et al.[9,59]have studied the nature of the adsorption processes of,among other,TMI and Im.These studies indicate that these inhibitors are physisorbed on the copper surface and the physisorption is higher for the higher molecular weight inhibitor.This result,which has been justi?ed by theoretical calculations,was con?rmed by the fact that XPS data show that nitrogen N1s is not present on the copper surface when copper samples were exposed to the inhibitors from solutions that were 0.5M in HCl [59].4.Conclusions

The main conclusions drawn from this study are: MMI inhibits the corrosion of copper in 1M HCl.

The inhibition is due to adsorption of the inhibitor molecules on the copper surface and blocking its active sites.

Adsorption of the inhibitor ?ts a Langmuir isotherm model. Results obtained from dc polarization,ac impedance and weight loss techniques are in reasonably good agreement and show increased inhibitor ef?ciency with increasing inhibitor concentration.

The addition of MMI has very little effect on the activation corrosion energy.

Theoretical calculations show that the effect of the mercapto group on inhibition of copper in an acidic chloride solution by

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1377

Fig.8.ln j /T vs.1/T for copper dissolution in 1M HCl in the absence and presence of 7.5?10à4M MMI.

Table 5

Calculated Fukui indices Heteroatom

Fukui indices

Imidazole methylimidazole

1-(p -Tolyl)-4-methylimidazole 2-Mercapto-1-methylimidazole (tautomer 1)2-Mercapto-1-(tautomer 2)N10.0030.0570.0180.081N30.064

0.041

0.0790.068S

0.302

0.475

MMI could be mainly attributed to chemical adsorption of the S atom.

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