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Corrosion Inhibitors

International Journal of Corrosion

International Journal of Corrosion


Copyright © 2011 Hindawi Publishing Corporation. All rights reserved.

This is a focus issue published in “International Journal of Corrosion.” All articles are open access articles distributed under the CreativeCommons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the originalwork is properly cited.

Editorial Board

Raman Singh, AustraliaCarmen Andrade, SpainKsenija Babic, USAJose Maria Bastidas, SpainPier Luigi Bonora, ItalyMarek Danielewski, PolandFlavio Deflorian, ItalyOmar S. Es-Said, USASebastian Feliu, Spain

Wei Gao, New ZealandKarl Ulrich Kainer, GermanyW. Ke, ChinaH. K. Kwon, JapanDongyang Y. Li, CanadaChang-Jian Lin, ChinaEfstathios I. Meletis, USAVesna Miskovic-Stankovic, SerbiaRokuro Nishimura, Japan

Michael I. Ojovan, UKF. J. M. Perez, SpainRamana M. Pidaparti, USAWillem J. Quadakkers, GermanyAravamudhan Raman, USAMichael J. Schutze, GermanyYanjing Su, ChinaJerzy A. Szpunar, CanadaYu Zuo, China

Contents

Study of Temperature Effect on the Corrosion Inhibition of C38 Carbon Steel UsingAmino-tris(Methylenephosphonic) Acid in Hydrochloric Acid Solution, Najoua Labjar, Fouad Bentiss,Mounim Lebrini, Charafeddine Jama, and Souad El hajjajiVolume 2011, Article ID 548528, 8 pages

A Comparative Study of the Inhibitory Effect of the Extracts of Ocimum sanctum, Aegle marmelos,and Solanum trilobatum on the Corrosion of Mild Steel in Hydrochloric Acid Medium, M. Shyamala andP. K. KasthuriVolume 2011, Article ID 129647, 11 pages

Corrosion Inhibition of the Galvanic Couple Copper-Carbon Steel in Reverse Osmosis Water,Irene Carrillo, Benjamın Valdez, Roumen Zlatev, Margarita Stoycheva, Michael Schorr, and Monica CarrilloVolume 2011, Article ID 856415, 7 pages

Inhibition Effect of 1-Butyl-4-Methylpyridinium Tetrafluoroborate on the Corrosion of Copper inPhosphate Solutions, M. Scendo and J. UznanskaVolume 2011, Article ID 761418, 12 pages

The Effect of Ionic Liquids on the Corrosion Inhibition of Copper in Acidic Chloride Solutions,M. Scendo and J. UznanskaVolume 2011, Article ID 718626, 13 pages

Hindawi Publishing CorporationInternational Journal of CorrosionVolume 2011, Article ID 548528, 8 pagesdoi:10.1155/2011/548528

Research Article

Study of Temperature Effect on the Corrosion Inhibition of C38Carbon Steel Using Amino-tris(Methylenephosphonic) Acid inHydrochloric Acid Solution

Najoua Labjar,1, 2 Fouad Bentiss,3 Mounim Lebrini,2 Charafeddine Jama,2

and Souad El hajjaji1

1 Laboratoire de Spectroscopie Infrarouge, Faculte des Sciences, University Med V Agdal, avenue Ibn Battouta, BP 1014,Rabat 10000, Morocco

2 Unite Materiaux et Transformations (UMET), Ingenierie des Systemes Polymeres CNRS UMR 8207, ENSCL, BP 90108,59652 Villeneuve d’Ascq Cedex, France

3 Laboratoire de Chimie de Coordination et d’Analytique, Faculte des Sciences, Universite Chouaib Doukkali, BP 20,El Jadida 24000, Morocco

Correspondence should be addressed to Souad El hajjaji, [email protected]

Received 13 March 2011; Revised 17 July 2011; Accepted 12 August 2011

Academic Editor: Carmen Andrade

Copyright © 2011 Najoua Labjar et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Tafel polarization method was used to assess the corrosion inhibitive and adsorption behaviours of amino-tris(methylenephosphonic) acid (ATMP) for C38 carbon steel in 1 M HCl solution in the temperature range from 30 to60◦C. It was shown that the corrosion inhibition efficiency was found to increase with increase in ATMP concentration butdecreased with temperature, which is suggestive of physical adsorption mechanism. The adsorption of the ATMP onto theC38 steel surface was found to follow Langmuir adsorption isotherm model. The corrosion inhibition mechanism was furthercorroborated by the values of kinetic and thermodynamic parameters obtained from the experimental data.

1. Introduction

Corrosion inhibition of steel in acid solutions by differenttypes of inhibitors has been extensively studied. The use ofenvironmentally acceptable inhibitors is favoured. Phospho-nates are known to be environmentally friendly corrosioninhibitors, which form adsorbed layers on oxide- or hydro-xide-covered metal surfaces [1–4]. Many works can be foundin the literature about the interactions between phospho-nates and iron or steels. In particular, Ochoa and al. [2, 4]studied the interaction between phosphonocarboxylic acidsalts (monophosphonates) and carbon steel. Their environ-mental impact at usual concentrations for corrosion inhibi-tion is negligible [5, 6]. Moreover, in contrast to inorganicphosphorous compounds, they do not cause eutrophication.Their high stability to hydrolysis and resistance to degrada-tion is also beneficial. It was found that few inhibitors withacid-metal systems have specific reactions that are still

effective at high temperatures as (or more) they are at lowtemperatures [7, 8]. A large number of investigations havestudied the temperature effects on acidic corrosion andcorrosion inhibition of iron and steel in HCl and H2SO4

solutions [9–17].In previous work [1], the improving of the corrosion

resistance of C38 carbon steel in 1 M HCl solution usingATMP has been investigated at 30◦C by means of gravimetricand electrochemical (ac impedance and Tafel polarisation)methods. We have found that this compound is efficientinhibitor in 1 M HCl and the corrosion inhibition is mainlycontrolled by a physisorption process. The antibacterial acti-vity investigations have been shown that the ATMP has anantibacterial effect against both Gram-positive and Gram-negative bacteria [1]. A great limitation of the inhibitorapplication is the fall down of their efficiencies at high tem-peratures. The effect of temperature on the inhibited acid-metal reaction is highly complex because many charges

2 International Journal of Corrosion

occur on the metal surface such as rapid etching and de-sorption of the inhibitor, and the inhibitor itself, in somecases, may undergo decomposition and/or rearrangement[18]. However, it provides the ability of calculating manythermodynamic functions for the inhibition and/or the ad-sorption processes which contribute in determining the typeof adsorption of the studied inhibitor. The aim of this workis then to study the effect of temperature on C38 carbonsteel corrosion process in 1 M HCl both in the absence andin the presence of amino-tris(methylenephosphonic) acid-(ATMP) using Tafel polarisation method. The thermody-namic parameters for both activation and adsorption pro-cesses were calculated and discussed.

2. Experimental Details

The material used in this study is a C38 carbon steel witha chemical composition (in wt %) of 0.370% C, 0.230% Si,0.680% Mn, 0.016% S, 0.077% Cr, 0.011% Ti, 0.059% Ni,0.009% Co, 0.160% Cu, and the remainder iron (Fe). TheC38 carbon samples were pretreated prior to the experimentsby grinding with emery paper SiC (120, 600, and 1200),rinsed with distilled water, degreased in acetone in anultrasonic bath immersion for 5 min, washed again withbidistilled water, and then dried at room temperature beforeuse. The tested compound, namely amino-tris(methylene-phosphonic) acid (N[CH2P(O)(OH)2]3), (ATMP), obtainedfrom Sigma-Aldrich (50 wt.% in H2O), was tested withoutfurther purification. The molecular structure of the ATMPis shown in Figure 1. The acid solutions (1 M HCl) wereprepared by dilution of an analytical reagent grade 37% HClwith doubly distilled water.

Polarisation curves were conducted using an electro-chemical measurement system Tacussel-Radiometer modelPGZ 301 potentiostat controlled by a PC and supportedby Voltamaster 4.0 software. Electrochemical measurementswere carried out in a conventional three-electrode cylindri-cal Pyrex glass cell. The temperature is thermostaticallycontrolled. The working electrode (WE) in the form of disc-cut from steel has a geometric area of 1 cm2 and is embedd-ed in polytetrafluoroethylene (PTFE). A saturated calomelelectrode (SCE) and a platinum electrode were used, as refer-ence and auxiliary electrodes, respectively. A fine Luggin cap-illary was placed close to the working electrode to mini-mizeIR drop. All test solutions were deaerated in the cell by usingpure nitrogen for 10 min prior to the experiment. Duringeach experiment, the test solution was mixed with a magneticstirrer, and the gas bubbling was maintained. The mildsteel electrode was maintained at corrosion potential for30 min and thereafter prepolarised at −800 mVSCE for10 min. The potentiodynamic current potential curves wereobtained by changing the electrode potential automaticallyfrom −800 to −200 mVSCE with a scan rate of 0.5 mV s−1.

3. Results and Discussion

3.1. Corrosion Kinetic Study. In order to gain more infor-mation about the type of adsorption and the effectiveness

OH

P

O

N

POH

OH

O

P

O

HO

HO

HO

Figure 1: Molecular structure of the amino-tris(methylenephos-phonic) acid (ATMP).

of the ATMP inhibitor at higher temperature, polarisationexperiment was conducted in the range of 30–60◦C withoutand with selected concentrations of the inhibitor. Repre-sentative Tafel polarisation curves for C38 steel electrodein 1 M HCl without and with 0.1 M of ATMP at differenttemperatures are shown in Figure 2. Similar polarisationcurves were obtained in the case of the other concentrationsof ATMP (not given). The analysis of these figures reveals thatraising the temperature increases both anodic and cathodiccurrent densities, and consequently the corrosion rate of C38steel increases.

Electrochemical kinetic parameters (corrosion potential(Ecorr), corrosion current density (Icorr), and cathodic Tafelslope (bc)), determined from these experiments by extrap-olation method [19–23], are reported in Table 1. The Icorr

was determined by Tafel extrapolation of only the cathodicpolarization curve alone, which usually produces a longerand better defined Tafel region [24]. The inhibition efficien-cies, E(%), are calculated from Icorr values as described else-where [18]. The surface coverage θ was calculated from thefollowing equation [25]:

θ = Icorr − Icorr(inh)

Icorr − Isat, (1)

where Icorr, Icorr(inh), and Isat are the corrosion current densityvalues in the absence, the presence of ATMP, and in anentirely covered surface, respectively, (Isat = Icorr for the mostelevated concentration of inhibitor).

As Isat � Icorr, thus

θ = Icorr − Icorr(inh)

Icorr. (2)

Analyse of the results in Table 1 indicates that in the pre-sence of ATMP molecules, the Icorr of C38 steel decreases atany given temperature as inhibitor concentration increasescompared to the uninhibited solution, due to the increase ofthe surface coverage degree. In contrast, at constant ATMPconcentration, the Icorr increases as temperature rises, butthis increase is more pronounced for the blank solu-tion.Hence we can note that the E(%) depends on the tempera-ture and decreases with the rise of temperature from 30 to

International Journal of Corrosion 3

Table 1: Electrochemical parameters and the corresponding inhibition efficiencies at various temperatures studied of C38 steel in 1 M HClcontaining different concentrations of ATMP.

Temperature (C◦) Conc. (M) Ecorr versus SCE mV Icorr (μA cm−2) bc (mV dec−1) E (%) θ

30

1 M HCl −482 569.8 188 — —

5 × 10−5 −482 477.8 186 16.1 0.16

5 × 10−3 −473 175.4 146 69.2 0.69

5 × 10−2 −471 109.3 128 80.8 0.81

1 × 10−1 −464 76.7 122 86.5 0.87

40

1 M HCl −464 800.7 190 — —

5 × 10−5 −470 700.3 189 12.5 0.13

5 × 10−3 −452 478.7 154 40.2 0.40

5 × 10−2 −471 308.3 132 61.5 0.61

1 × 10−1 −462 235.3 129 70.6 0.71

50

1 M HCl −461 999.1 191 — —

5 × 10−5 −455 875.9 188 12.3 0.12

5 × 10−3 −463 737.2 166 26.2 0.26

5 × 10−2 −464 572.7 145 42.7 0.43

1 × 10−1 −454 407.9 134 59.2 0.59

60

1 M HCl −465 1314.7 194 — —

5 × 10−5 −456 1195.5 191 9.0 0.09

5 × 10−3 −461 1046.9 187 20.4 0.20

5 × 10−2 −453 788.0 167 40.1 0.40

1 × 10−1 −445 651.1 152 50.5 0.50

60◦C. This can be explained by the decrease of the strengthof the adsorption process at elevated temperature and wouldsuggest a physical adsorption mode.

The activation parameters for the corrosion reaction canbe regarded as an Arrhenius-type process, according to thefollowing equation:

Icorr = A exp(− Ea

RT

), (3)

where Ea is the apparent activation corrosion energy, R is theuniversal gas constant, and A is the Arrhenius preexponentialfactor. The apparent activation energies (Ea) in the absenceand in the presence of various concentrations of ATMP arecalculated by linear regression between ln (Icorr) and 1/T(Figure 3), and the results are given in Table 2. All the linearregression coefficients are close to 1, indicating that the steelcorrosion in hydrochloric acid can be elucidated using thekinetic model. As observed from Table 2, the Ea increasedwith increasing concentration of ATMP, but all values of Ea inthe range of the studied concentration were higher than thatof the uninhibited solution. The increase in Ea in the pre-sence of ATMP may be interpreted as physical adsorption.Indeed, a higher energy barrier for the corrosion process inthe inhibited solution is associated with physical adsorptionor weak chemical bonding between the inhibitor species andthe steel surface [14, 26]. Szauer and Brand. explained thatthe increase in activation energy can be attributed to anappreciable decrease in the adsorption of the inhibitor on thecarbon steel surface with the increase in temperature. A cor-responding increase in the corrosion rate occurs because of

Table 2: Corrosion kinetic parameters for C38 steel in 1 M HCl inabsence and presence of different concentrations of ATMP.

Concentration(M)

Ea

(kJ mol−1)ΔHa

(kJ mol−1)ΔSa

(J mol−1 k−1)Ea − ΔHa

(kJ mol−1)

Blank 22.92 20.28 −125.22 2.64

5 × 10−5 24.98 22.34 −119.84 2.64

5 × 10−3 48.90 46.26 −48.05 2.64

5 × 10−2 55.25 52.62 −31.04 2.63

1 × 10−1 58.75 56.11 −22.43 2.64

the greater area of metal that is consequently exposed to theacid environment [27].

The enthalpy of activation (ΔHa) and the entropy of acti-vation (ΔSa) for the intermediate complex in the transitionstate for the corrosion of C38 steel in 1 M HCl in the absenceand in the presence of different concentrations of ATMPwere obtained by applying the alternative formulation of Ar-rhenius equation [28]:

Icorr = RT

Nhexp

(ΔSa

R

)exp

(−ΔHa

RT

), (4)

where h is the Plank’s constant and N is the Avogadro’snumber. Figure 4 shows a plot of ln(Icorr/T) versus 1/T . Astraight lines are obtained with a slope of (−ΔHa/R) andan intercept of (lnR/Nh + ΔSa/R) from which the values ofΔHa and ΔSa were calculated (Table 2). The positive valuesof ΔHa in the absence and the presence of ATMP reflect theendothermic nature of the C38 steel dissolution process. Onecan also notice that Ea and ΔHa values vary in the same way


−5

−4.5

−4

−3.5

−3

−2.5

−2

−1.5

−1

−800 −700 −600 −500 −400 −300 −200

ab

c

d Blank

E versus SCE (mV)

LogI

(Acm

−2)

(a)

−5

−4.5

−4

−3.5

−3

−2.5

−2

−1.5

−1

−800 −700 −600 −500 −400 −300 −200

a

b

cd

a: 30◦Cb: 40◦C

c: 50◦Cd: 60◦C

1 × 10−1 M of ATMP

E versus SCE (mV)

LogI

(Acm

−2)

(b)

Figure 2: Effect of temperature on the cathodic and anodic respon-ses for C38 steel in 1 M HCl and 1 M HCl + 0.1 M of ATMP.

as shown in Table 2, indicating that the corrosion process is aunimolecular reaction [29]. This result permits verifying theknown thermodynamic equation between the Ea and ΔHa

[29]

Ea − ΔHa = RT. (5)

The values of activation entropy (ΔSa) are higher forinhibited solutions than that for the uninhibited solutionand increase gradually with increasing ATMP concentrations(Table 2). The positive increment of ΔSa suggests that an in-crease in randomness occurred on going from reactants tothe activated complex [30]. This observation is in agreementwith the findings of other workers [30, 31].

3.2. Adsorption Isotherm and Thermodynamic Parameters.The values of surface coverage θ corresponding to differentconcentrations of AMTP in the temperature range from30 to 60◦C have been used to explain the best isothermto determine the adsorption process. As it is known thatthe adsorption of an organic adsorbate onto metal-solutioninterface can be presented as a substitutional adsorption pro-cess between the organic molecules in the aqueous solution

Org(sol) and the water molecules on the metallic surfaceH2O(ads),

Org(sol) + n H2O(ads) ←→ Org(ads) + n H2O(sol), (6)

where Org(sol) and Org(ads) are the organic molecules inthe aqueous solution and adsorbed on the metallic surface,respectively, H2O(ads) is the water molecules on the metallicsurface, and n is the size ratio representing the numberof water molecules replaced by one molecule of organicadsorbate. When the equilibrium of the process describedin this equation is reached, it is possible to obtain differentexpressions of the adsorption isotherm plots, and thus thesurface coverage degree (θ) can be plotted as a function of theconcentration of the inhibitor under test [32]. The Langmuiradsorption isotherm was found to give the best description ofthe adsorption behaviour of ATMP. In this case, the surfacecoverage (θ) of the inhibitor on the steel surface is related tothe concentration of inhibitor in the solution according tothe following equation:

θ

1− θ= KadsCinh. (7)

Rearranging this equation gives

Cinh

θ= 1

Kads+ Cinh, (8)

where θ is the surface coverage degree, Cinh is the inhibitorconcentration in the electrolyte, and Kads is the equilibriumconstant of the adsorption process. The Kads values may betaken as a measure of the strength of the adsorption forcesbetween the inhibitor molecules and the metal surface [33].To calculate the adsorption parameters, the straight lineswere drawn using the least squares method. The experi-mental (points) and calculated isotherms (lines) are plottedin Figure 5. The results are presented in Table 3. A verygood fit is observed with a regression coefficient (R2) upto 0.99 and the obtained lines have slopes very close tounity, which suggests that the experimental data are welldescribed by Langmuir isotherm and exhibit single-layeradsorption characteristic [18]. This kind of isotherm involvesthe assumption of no interaction between the adsorbedspecies and the electrode surface. From the intercepts of thestraight lines Cinh/θ-axis, the Kads values were calculated andgiven in Table 3. As can be seen from Table 3, Kads valuesdecrease with increasing temperature from 30 to 60◦C. Suchbehaviour can be interpreted on the basis that the increase intemperature results in desorption of some adsorbed inhibitormolecules from the metal surface [18].

The well-known thermodynamic adsorption parametersare the free energy of adsorption (ΔGo

ads), the standardenthalpy of adsorption (ΔHo

ads), and the entropy of adsorp-tion (ΔSo

ads). These quantities can be calculated dependingon the estimated values of Kads from adsorption isotherms,at different temperatures. The constant of adsorption, Kads,is related to the standard free energy of adsorption, ΔGo

ads,with the following equation [34]:

Kads = 155.5

exp(−ΔGo

ads

RT

), (9)


4

4.5

5

5.5

6

6.5

7

7.5

ab

c

d

e

a: Blank

2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35

1000/T (K−1)

lnI c

orr

(μA

cm−

2)

b: 5× 10−5 Mc: 5× 10−3 M

d: 5× 10−2 Me: 1× 10−1 M

Figure 3: Arrhenius plots for C38 steel corrosion rates ln Icorr ver-sus 1/T in 1 M HCl in absence and in presence of different concen-trations of ATMP.

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35

ab

c

de

a: Blank

1000/T (K−1)

lnI c

orr/T

(μA

cm−

2K−

1)

b: 5× 10−5 Mc: 5× 10−3 M

d: 5× 10−2 Me: 1× 10−1 M

Figure 4: Transition-state plots for C38 steel corrosion rates ln Icorr

versus 1/T in 1 M HCl in absence and in presence of different con-centrations of ATMP.

30◦C40◦C

50◦C

60◦C

0

0.04

0.08

0.12

0.16

0.2

0 0.02 0.04 0.06 0.08 0.1 0.12

Cin

h/θ

(M)

Cinh (M)

Figure 5: Langmuir’s isotherm adsorption model of ATMP on theC38 steel surface in 1 M HCl at different temperatures.

where R is the universal gas constant, T is the thermody-namic temperature, and the value of 55.5 is the concen-tration of water in the solution in mol/L. The calculatedΔGo

ads values, at all studied temperatures, are given in Table 3.The negative values of ΔGo

ads indicate the spontaneity of theadsorption process and the stability of the adsorbed layer onthe C38 steel surface [16]. Generally, the adsorption type isregarded as physisorption if the absolute value of ΔGo

ads is inthe range of 20 kJ mol−1 or lower. The inhibition behaviour isattributed to the electrostatic interaction between the organicmolecules and steel surface. When the absolute value ofΔGo

ads is in the order of 40 kJ mol−1 or higher, the adsorptioncould be seen as chemisorption. In this process, the covalentbond is formed by the charge sharing or transferring fromthe inhibitor molecules to the metal surface [35, 36]. Theobtained ΔGo

ads values in the studied temperature domainare in the range of −23.5 to −26.5 kJ mol−1, indicating,therefore, that the adsorption mechanism of the ATMP ontoC38 steel in 1 M HCl solution is mainly due to physisorption(Table 3). This behaviour is in good agreement with thatobtained at 30◦C using ac impedance technique [1]. On theother hand, the obtained values of ΔGo

ads show a regulardependence on temperature, indicating a good correlationamong thermodynamic parameters. However, a limiteddecrease in the absolute value of ΔGo

ads with the increase intemperature values is observed. This behaviour is explainedby the fact that the adsorption is somewhat unfavourablewith increasing experimental temperature, indicating thatthe physisorption has the major contribution while thechemisorption has a minor contribution in the corrosioninhibition mechanism [37]. The other thermodynamic func-tions (ΔHo

ads and ΔSoads) can be calculated from the following

equation:

ΔGoads = ΔHo

ads − TΔSoads. (10)

Figure 6 shows the plot of ΔGoads versus T which gives straight

lines with slopes of −ΔSoads and intercepts of ΔHo

ads. Theobtained values of ΔHo

ads and ΔSoads are given in Table 3.

The obtained value of ΔHoads is negative, reflecting the exo-

thermic nature of the adsorption process on C38 steelsurface. The value of ΔHo

ads can also provide valuable infor-mation about the type of inhibitor adsorption. While anendothermic adsorption process (ΔHo

ads > 0) is attributedunequivocally to chemisorption [38], an exothermic adsorp-tion process (ΔHo

ads < 0) may involve either physisorption orchemisorption or a mixture of both the processes. Inan exothermic process, chemisorption is distinguishedfrom physisorption by considering the absolute valueof ΔHo

ads. For the chemisorption process, ΔHoads approaches

100 kJ mol−1, while for the physisorption process, it is lessthan 40 kJ mol−1 [37]. In the case of ATMP, the calculatedvalue of ΔHo

ads (−56.56 kJ mol−1) is larger than the commonphysical adsorption enthalpy, but smaller than the common-chemical adsorption enthalpy, confirming that the adsorp-tion mechanism of ATMP on carbon steel surface probablyinvolves two types of interactions, predominant physisorp-tion (ionic), and weak chemisorption (molecular). The valueof ΔSo

ads is negative (Table 3), meaning that the inhibitor


−27

−26

−25

−24

−23

−22300 305 310 315 320 325 330 335

T (K)

ΔG

0 ads

(KJm

oI−1

)

Figure 6: Variation of ΔGoads versus T on C38 steel in 1 M HCl con-

taining ATMP.

4

4.5

5

5.5

6

6.5

7

2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35

1000/T (K−1)

InK

ads

(M−1

)

Figure 7: Vant’t Hoff plot for the C38 steel/ATMP/1 M HCl.

molecules move freely in the bulk solution (are chaotic)before adsorption, while as adsorption progresses, the inhi-bitor molecules adsorbed onto the mild steel surface becomemore orderly, resulting in a decrease in entropy [39].

ΔHoads and ΔSo

ads can be also deduced from the integratedversion of the Van‘t Hoff equation expressed by [40]

lnKads = −ΔHoads

RT+ constant. (11)

Figure 7 shows the plot of ln Kads versus 1/T which givesstraight lines with slopes of (−ΔHo

ads/R) and intercepts of(ΔSo

ads/R + ln 1/55.5). The calculated ΔHoads using the Van‘t

Hoff equation is −55.55 kJ mol−1 for ATMP, confirming thephysisorption process and the exothermic behaviour of theadsorption of the ATMP molecule on the steel surface. Valuesof ΔHo

ads obtained by both methods are in good agreement.Moreover, the deduced ΔSo

ads value of −97.16 J mol−1K−1 forATMP is very close to that obtained in Table 3.

4. Conclusion

We studied the inhibitor action of ATMP on corrosion of C38steel in 1 M HCl depending on effect of temperature. We ob-tained the following conclusion.

Table 3: Thermodynamic parameters for the adsorption of ATMPon the C38 steel in 1 M HCl at different temperatures.

Temperature(K)

R2 Kads

(M−1)ΔGo

ads

(kJ mol−1)ΔHo

ads

(kJ mol−1)ΔSo

ads

(J mol−1 K−1)

30 0.99 666.7 −26.50

−54.87 −95.0040 0.99 222.2 −24.52

50 0.99 138.9 −24.04

60 0.98 86.9 −23.49

(1) Based on the Tafel polarization results, the E (%) ofATMP is found to decrease with increasing tempera-ture, and its addition to 1 M HCl leads to an increaseof apparent activation energy (Ea) of the corrosionprocess.

(2) The corrosion process is inhibited by the adsorptionof ATMP on C38 steel surface. This adsorption fitsa Langmuir isotherm model. Thermodynamic adsor-ption parameters show that ATMP is adsorbed onsteel surface by an exothermic and spontaneous pro-cess.

(3) The calculated values of ΔGoads and ΔHo

ads corrob-orate that the adsorption mechanism of ATMP onsteel surface in 1 M HCl solution is mainly due tophysisorption.

(4) At temperatures higher than 30◦C, this inhibitor isnot efficient to control the corrosion of steel in 1 MHCl at the concentration range studied.

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Research Article

A Comparative Study of the Inhibitory Effect of the Extracts ofOcimum sanctum, Aegle marmelos, and Solanum trilobatum onthe Corrosion of Mild Steel in Hydrochloric Acid Medium

M. Shyamala1 and P. K. Kasthuri2

1 Department of Chemistry, Government College of Technology, Tamil Nadu Coimbatore 641013, India2 Department of Chemistry, L.R.G. Government Arts College for Women, Tamil Nadu Tirupur 638604, India

Correspondence should be addressed to M. Shyamala, [email protected]

Received 1 April 2011; Accepted 24 June 2011

Academic Editor: F. J. M. Perez

Copyright © 2011 M. Shyamala and P. K. Kasthuri. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

A comparative study of the inhibitory effect of plant extracts, Ocimum sanctum, Aegle marmelos, and Solanum trilobatum, on thecorrosion of mild steel in 1N HCl medium was investigated using weightloss method, electrochemical methods, and hydrogenpermeation method. Polarization method indicates plant extracts behave as mixed-type inhibitor. The impedance method revealsthat charge-transfer process mainly controls the corrosion of mild steel. On comparison, maximum inhibition efficiency was foundin Ocimum sanctum with 99.6% inhibition efficiency at 6.0% v/v concentration of the extract. The plant extracts obey Langmuiradsorption isotherm. The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the highperformance of inhibitive effect of the plant extracts. From hydrogen permeation method, all the plant extracts were able to reducethe permeation current. The reason for the reduced permeation currents in presence of the inhibitors may be attributed to theslow discharge step followed by fast electrolytic desorption step. Results obtained in all three methods were very much in goodagreement in the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum.

1. Introduction

Mild steel is a structural material widely used in automobiles,pipes and used in most of the chemical industries. Mild steelsuffers from severe corrosion in aggressive medium of acidsand pickling processes. Hydrochloric acid is widely used forpickling, descaling, and chemical cleaning processes of mildsteel. 90% of pickling problems can be solved by introducingappropriate pickling inhibitor to the medium. Generally,organic compounds containing O, N, and S atoms are nor-mally used as inhibitors to reduce the corrosion of mild steelin hydrochloric acid medium [1, 2]. Environmental concernsworldwide are increasing and are likely to influence thechoice of corrosion inhibitors in the present and in future.Environmental requirements are still being developed, butsome elements have been established. One of the methodsto protect metals against corrosion is addition of species tothe solution in contact with the surface in order to inhibit

the corrosion rate. Unfortunately, many of the inhibitorsused are inorganic salts or organic compounds with toxicproperties or limited solubility. Increasing awareness ofhealth and ecological risks has drawn attention to find moresuitable inhibitors which are nontoxic. Accordingly, greaterresearch efforts have been directed towards formulatingenvironmentally acceptable inhibitors.

Due to the diversity of their structures, many extractsof common plants have been used as corrosion inhibitorsfor materials in pickling and cleaning processes. Plantmaterials contain proteins, polysaccharides, polycarboxylicacids, tannin, alkaloids, and so forth. These compounds arepotential acid corrosion inhibitors for many metals [3]. Thecost of using green inhibitors is very low when compared tothat of organic inhibitors which require a lot of chemicals andalso time for its preparation. Chemical inhibitors are moreexpensive and cause more hazard effects. Nowadays due tostrict environmental legislation, emphasis is being focused


on usage of natural products that are corrosion inhibitor. Therecent and growing trend is using plant extracts as corrosioninhibitor. Recently, many plant extracts have been reported aseffective corrosion inhibitors within India and outside India[4–20]. In this study, leaf extracts of three medicinal plants,namely, Ocimum sanctum (Tulasi), Aegle marmelos (Vilvam),and Solanum trilobatum (Thuthuvalai), have been selectedto study the inhibition effect on the corrosion of mild steelin 1N hydrochloric acid medium using weight loss method,the potentiodynamic polarization method, electrochemicalimpedance method, and hydrogen permeation method.

2. Experimental Procedure

2.1. Preparation of Mild Steel Specimen. Mild steel strips weremechanically cut into strips of size 4.5 cm × 2 cm × 0.2 cmcontaining the composition of 0.14% C, 0.35% Mn, 0.17%Si, 0.025% S, 0.03% P, and the remainder Fe and providedwith a hole of uniform diameter to facilitate suspensionof the strips in the test solution for weight loss method.For electrochemical studies, mild steel strips of the samecomposition but with an exposed area of 1 cm2 were used.Mild steel strips were polished mechanically with emerypapers of 1/0 to 4/0 grades, subsequently degreased withtrichloroethylene or acetone and finally with deionised water,and stored in the desiccator. Accurate weight of the sampleswas taken using electronic balance.

2.2. Preparation of the Plant Extract. The leaves of the plantsOcimum sanctum, Aegle marmelos, and Solanum trilobatumwere taken and cut into small pieces, and they were dried inan air oven at 80◦C for 2 h and ground well into powder.From this, 10 g of the sample was refluxed in 100 mLdistilled water for 1 h. The refluxed solution was then filteredcarefully, the filtrate volume was made up to 100 mL usingdouble distilled water which is the stock solution, and theconcentration of the stock solution is expressed in termsof % (v/v). From the stock solution, 2–10% concentrationof the extract was prepared using 1N hydrochloric acid.Similar kind of preparation has been reported in studiesusing aqueous plant extracts in the recent years [21–30].

2.3. Weight Loss Method. The pretreated specimens’ initialweights were noted and were immersed in the experimentalsolution with the help of glass hooks at 30◦C for a periodof 3h. The experimental solution used was 1N HCl inthe absence and presence of various concentrations of theinhibitors. After three hours, the specimens were taken out,washed thoroughly with distilled water, and dried com-pletely, and their final weights were noted. From the initialand final weights of the specimen, the loss in weight wascalculated and tabulated. From the weight loss, the corrosionrate (mmpy), inhibition efficiency (%), and surface coverage(θ) of plant extracts were calculated using the formula

Corrosion rate(mmpy

) = KW

AtD, (1)

where K = 8.76 × 104 (constant), W is weight loss in g,

A is area in cmm2, t is time in hours, and D is density ingm/cmm3 (7.86):

Inhibition efficiency (%) = (CRB − CRI)CRB

× 100,

Surface coverage (θ) = CRB − CRI

CRB,

(2)

where CRB and CRI are corrosion rates in the absence andpresence of the inhibitors.

2.4. Potentiodynamic Polarization Method. Potentiodynamicpolarization measurements were carried out using elec-trochemical analyzer. The polarization measurements weremade to evaluate the corrosion current, corrosion potential,and Tafel slopes. Experiments were carried out in a conven-tional three-electrode cell assembly with working electrodeas mild steel specimen of 1 sq. cm area which was exposedand the rest being covered with red lacquer, a rectangular Ptfoil as the counter electrode, and the reference electrode asSCE. Instead of salt bridge, a luggin capillary arrangementwas used to keep SCE close to the working electrode toavoid the ohmic contribution. A time interval of 10–15minutes was given for each experiment to attain the steadystate open circuit potential. The polarization was carriedfrom a cathodic potential of −800 mV (vs SCE) to ananodic potential of −200 mV (vs SCE) at a sweep rate of1 mV per second. From the polarization curves, Tafel slopes,corrosion potential, and corrosion current were calculated.The inhibitor efficiency was calculated using the formula

IE (%) = ICorr − I∗Corr

ICorr× 100, (3)

where ICorr and I∗Corr are corrosion current in the absence andpresence of inhibitors.

2.5. Electrochemical Impedance Method. The electrochemicalAC-impedance measurements were also performed usingelectrochemical analyzer. Experiments were carried out ina conventional three-electrode cell assembly as that usedfor potentiodynamic polarization studies. A sine wave withamplitude of 10 mV was superimposed on the steady opencircuit potential. The real part (Z′) and the imaginary part(Z′′) were measured at various frequencies in the range of100 KHz to 10 MHz. A plot of Z′ versus Z′′ was made. Fromthe plot, the charge transfer resistance (Rt) was calculated,and the double layer capacitance was then calculated using

Cdl = 12π fmaxRt

, (4)

where Rt is charge transfer resistance, and Cdl is double layercapacitance. The experiments were carried out in the absenceand presence of different concentrations of inhibitors. Thepercentage of inhibition efficiency was calculated using


Table 1: Corrosion parameters obtained from weight loss measurements for mild steel in 1N HCl containing various concentrations of theplant extracts.

Name of the plant extract Conc. of the extract (% in v/v) Corrosion rate (mmpy) Inhibition efficiency (%) Surface coverage (θ)

Ocimum sanctum

Blank 30.67 — —

2.0 2.39 92.2 0.9221

4.0 1.10 96.4 0.9641

6.0 0.12 99.6 0.9961

8.0 1.08 96.5 0.9648

10.0 1.32 95.7 0.9570

Aegle marmelos

Blank 30.67 — —

2.0 3.81 87.6 0.8758

4.0 3.03 90.1 0.9012

6.0 2.02 93.4 0.9341

8.0 0.76 97.5 0.9752

10.0 0.76 97.5 0.9752

Solanum trilobatum

Blank 30.67 — —

2.0 12.75 58.4 0.5843

4.0 8.42 72.5 0.7255

6.0 6.59 78.5 0.7851

8.0 5.21 83.0 0.8301

10.0 3.00 90.2 0.9022

IE (%) = R∗t − Rt

R∗t× 100, (5)

where R∗t and Rt are the charge transfer resistance in thepresence and absence of inhibitors.

2.6. Hydrogen Permeation Method. When metals are incontact with acids, atomic hydrogen is produced. Beforethey combine to produce hydrogen molecules, a fractionmay diffuse into the metal. Inside the metal, the hydrogenatoms may combine to form molecular hydrogen. Thus,a very high internal pressure is built up. This leads toheavy damage of the metal. This is known as “hydrogenembrittlement”. This phenomenon of hydrogen entry intothe metals can occur in industrial processes like pickling,plating, phosphating, and so forth. An inhibitor can beconsidered as completely effective only if it simultaneouslyinhibits metal dissolution and hydrogen penetration intothe metal [31]. Hydrogen permeation study has been takenup with an idea of screening the inhibitors with regardto their effectiveness on the reduction of hydrogen uptake.Hence, the hydrogen permeation study was carried out usingan adaptation of the modified Devanathan-Stachurski two-compartment cell assembly [32, 33] in 1N HCl medium inthe absence and presence of optimum concentration of theextracts. Similar kind of study is reported in the works ofQuraishi and Rawat [34].

2.7. Surface Examination Studies. Surface examination ofmild steel specimens in the absence and presence of theoptimum concentration of the extracts immersed for 3 h at30◦C was studied using JEOL-Scanning electron microscope(SEM) with the magnification of 1000x specimens.


3.1. Weight Loss Studies. The weight loss studies were donein 1N hydrochloric acid in the absence and presence ofvarious concentrations of the plant extracts ranging from2% to 10% v/v. Using the weight loss data, the corrosionrate, inhibition efficiency, surface coverage, and the optimumconcentration of the extract have been calculated. Thecorrosion parameters obtained in the weight loss method arelisted in Table 1.

From Table 1, it was found that with the addition of theplant extract to 1N hydrochloric acid, the weight loss of mildsteel decreased, the corrosion rate also decreased, while theinhibition efficiency increased. The optimum concentrationfor Ocimum sanctum was found to be 6% v/v with maximuminhibition efficiency of 99.6%, Aegle marmelos at 8% v/vwith maximum inhibition efficiency of 97.5%, and Solanumtrilobatum at 10% v/v with maximum inhibition efficiencyof 90.2% for a period of 3 hours of immersion time. Thisresult indicated that the plant extracts could act as effectivecorrosion inhibitors for mild steel in 1N HCl. The effect ofimmersion time studied for a period of 3 h to 24 h as givenin Table 2 reveals that the plant extracts showed maximumefficiency at 3 h of immersion time which is sufficient forpickling process. The order of inhibition effect among thethree plant extracts on mild steel in 1N HCl is found to beOcimum sanctum > Aegle marmelos > Solanum trilobatum.

3.2. Potentiodynamic Polarization Studies. The potentiody-namic polarization parameters for different concentrationsof the plant extracts are given in Table 3, and the polarizationcurves are given in Figure 1. Potentiodynamic polarizationstudies revealed that the corrosion current density (Icorr)


Table 2: Effect of immersion time on percentage inhibition efficiency of mild steel in 1N HCl at 30◦C in the presence of optimumconcentration of the plant extracts.

Name of the plant extract with optimum conc.Inhibition efficiency (%)

Time (h)

3 6 9 12 15 18 21 24

6% v/v of Ocimum sanctum 99.6 98.5 98.0 97.3 96.5 96.0 95.3 94.8

8% v/v of Aegle marmelos 97.5 96.7 95.6 95.0 94.2 93.0 92.6 90.8

10% v/v of Solanum trilobatum 90.2 89.5 89.4 88.6 88.0 87.5 87.0 86.2

Table 3: Potentiodynamic polarization parameters for mild steel in 1N HCl containing various concentrations of the plant extracts.

Name of the plant extractConc. of extract (% in v/v) Ecorr (V) Icorr (mA/cm2)Tafel slope mV/decade

Inhibition efficiency (%)ba bc

Blank — −0.510 3.57 78 122 —

Ocimum sanctum

2.0 −0.515 0.24 74 126 93.3

4.0 −0.498 0.12 76 124 96.6

6.0 −0.496 0.01 74 124 99.7

8.0 −0.499 0.09 74 122 97.5

10.0 −0.500 0.12 76 124 96.6

Aegle marmelos

2.0 −0.493 0.40 78 126 88.5

4.0 −0.492 0.31 76 124 91.3

6.0 −0.497 0.21 74 122 94.1

8.0 −0.483 0.09 74 122 97.5

10.0 −0.492 0.09 76 124 97.5

Solanum trilobatum

2.0 −0.490 1.45 74 126 59.4

4.0 −0.480 0.97 76 128 72.8

6.0 −0.462 0.75 74 126 79.0

8.0 −0.459 0.56 78 130 84.3

10.0 −0.460 0.33 76 128 90.8

Table 4: Impedance parameters for the corrosion of mild steel in 1N HCl in the absence and presence of various concentrations of the plantextracts at 30◦C.

Name of the plant extract Conc. of extract (% in v/v) Rt (Ω cm2) Cdl (μF/cm2) Inhibition efficiency (%)

Blank — 7.58 285.34

Ocimum sanctum

2.0 110.91 19.34 93.2

4.0 253.86 8.44 97.0

6.0 358.80 6.00 97.9

8.0 274.99 7.95 97.2

10.0 239.25 9.02 96.8

Aegle marmelos

2.0 69.85 31.09 89.1

4.0 88.41 24.52 91.4

6.0 136.49 15.86 94.4

8.0 224.80 9.62 96.6

10.0 208.34 10.25 96.4

Solanum trilobatum

2.0 18.62 116.02 59.3

4.0 27.34 79.00 72.3

6.0 37.12 58.35 79.6

8.0 48.31 44.72 84.3

10.0 87.86 24.52 91.4


I (amps/cm2)

10−7 10−6 10−5 10−4 10−3 10−2 10−1−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

E(V

olts

)

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(a)

I (amps/cm2)

10−7 10−6 10−5 10−4 10−3 10−2 10−1−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

E(V

olts

)

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(b)

I (amps/cm2)

10−7 10−6 10−5 10−4 10−3 10−2 10−1−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

E(V

olts

)

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(c)

Figure 1: Potentiodynamic polarization curves for mild steel in 1N HCl solution in the absence and presence of various concentrations ofthe plant extracts (a) Ocimum sanctum, (b) Aegle marmelos, and (c) Solanum trilobatum.

markedly decreased with the addition of the extract andthe corrosion potential shifts to less negative values uponaddition of the plant extract. Moreover, the values of anodicand cathodic Tafel slopes (ba and bc) are slightly changedindicating that this behavior reflects the plant extracts’ abilityto inhibit the corrosion of mild steel in 1N HCl solution viathe adsorption of its molecules on both anodic and cathodic

sites, and, consequently, the extracts act through mixed modeof inhibition [15, 16]. It was observed that with increasein concentration of the plant extract from 2% to 10%, themaximum inhibition efficiency of 99.7% was observed forOcimum sanctum extract at 6% v/v, for Aegle marmelos with97.5% at 8% v/v, and for Solanum trilobatum with 90.8% at10% v/v of the extract.


Z

(oh

ms)

0 100 200 300

100

200

300

4000

400

Z (ohms)

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(a)

Z (ohms)

Z

(oh

ms)

0 100 200 300

100

200

300

4000

400

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(b)

Z (ohms)

Z

(oh

ms)

0 1000

25

25

50

50

75

75

100

(1) Blank(2) 2 (% v/v)(3) 4 (% v/v)

(4) 6 (% v/v)(5) 8 (% v/v)(6) 10 (% v/v)

(c)

Figure 2: Impedance diagrams for mild steel in 1N HCl solution in the absence and presence of various concentrations of the plant extract(a) Ocimum sanctum, (b) Aegle marmelos, and (c) Solanum trilobatum.

3.3. Electrochemical Impedance Studies. Impedance measure-ments were studied to evaluate the charge transfer resistance(Rt) and double layer capacitance (Cdl), and through theseparameters, the inhibition efficiency was calculated. Figure 2shows the impedance diagrams for mild steel in 1N HCl

with different concentrations of the plant extract, and theimpedance parameters derived from these investigations aregiven in Table 4.

As noticed from Figure 2, the obtained impedancediagrams are almost in a semicircular appearance, indicating


Figure 3: Structure of α-bisabolene.

Figure 4: Structure of β-bisabolene.

Figure 5: Structure of β-caryophyllene.

OH

Meo

CH–CH2–NH–CO–CH=CH–C6H5

Figure 6: Structure of aegelin.

HO

H3C

H3C

CH3

CH3N

H

H

H

H

H

H

O

Figure 7: Structure of solasodine.

that the charge-transfer process mainly controls the cor-rosion of mild steel. Deviations of perfect circular shapeare often referred to the frequency dispersion of interfacialimpedance. This anomalous phenomenon may be attributedto the inhomogeneity of the electrode surface arising fromsurface roughness or interfacial phenomena. In fact, in thepresence of the plant extracts, the values of Rt have enhancedand the values of double-layer capacitance are also broughtdown to the maximum extent. The decrease in Cdl showsthat the adsorption of the inhibitors takes place on the metalsurface in acidic solution.

For Ocimum sanctum extract, the maximum Rt valueof 358.80Ω cm2 and minimum Cdl value of 6.00 μF/cm2

are obtained at an optimum concentration of 6% in v/vwith a maximum inhibition efficiency of 97.9%. For Aeglemarmelos extract, the maximum Rt value of 224.80Ω cm2

and minimum Cdl value of 9.62 μF/cm2 are obtained atan optimum concentration of 8% in v/v with a maximuminhibition efficiency of 96.6%. For Solanum trilobatumextract, the maximumRt value of 87.86Ω cm2 and minimumCdl value of 24.52 μF/cm2 are obtained at an optimumconcentration of 10% in v/v with a maximum inhibitionefficiency of 91.4%. A good agreement is observed betweenthe results of weight loss method and electrochemical meth-ods (potentiodynamic polarization method and impedancemethod).

3.4. Kinetics and Reason for the Corrosion Inhibition.The major phytochemical constituents present in Oci-mum sanctum are β-bisabolene (7.6–15.4%), α-bisabolene(9.4–19.6%), and eugenol (24.2–38.2%) as given in Fig-ures 3, 4, and 5, and the other phytochemical con-stituents present are 1,8-cineole (5.6–11%), E-β-ocimene(4.0–4.7%), β-Caryophyllene (1.4–2.5%), α-humulene (2.0–3.5%), methylchavicol (11.6–14.%), and germacrene-D(2.4–4.5%). The major phytochemical constituent presentin Aegle marmelos is Aegelin (Figure 6), and the majorphytochemical constituent present in Solanum trilobatum isSolasodine as shown in Figure 7 [35–37].

Inspection of the chemical structures of the phytochem-ical constituents reveals that these compounds are easilyhydrolysable and the compounds can adsorb on the metalsurface via the lone pair of electrons present on their oxygenatoms and make a barrier for charge and mass transferleading to decrease the interaction of the metal with thecorrosive environment. As a result, the corrosion rate of themetal was decreased. The formation of film layer essentiallyblocks discharge of H+ and dissolution of metal ions. Acidpickling inhibitors containing organic N, S, and OH groupsbehave similarly to inhibit corrosion [38, 39].

It follows that inhibition efficiency (IE) is directlyproportional to the fraction of the surface covered by theadsorbed molecules (θ). Therefore, (θ) with the extract con-centration specifies the adsorption isotherm that describesthe system. Adsorption isotherm gives the relationshipbetween the coverage of an interface with the adsorbedspecies and the concentration of species in solution. Theuse of adsorption isotherms provides useful insight into thecorrosion inhibition mechanism. The values of the degree


R2 = 0.9996SD = 0.09429

0

2

4

6

8

10

12

0 5 10 15

C/θ

C (% v/v)

Ocimum sanctum

(a)

0

4

8

12

0 5 10 15

SD = 0.07591R2 = 0.9986

C/θ

C (% v/v)

Solanum trilobatum

(b)

0

2

4

6

8

10

12

0 5 10 15

R2 = 0.996

SD = 0.15004

C/θ

C (% v/v)

Aegle marmelos

(c)

Figure 8: Langmuir adsorption isotherm plot for the adsorption of various concentrations of the plant extracts on the surface of mild steelin 1N HCl solution.

15 kV WD 15 mm ×1000

Figure 9: SEM Photograph of mild steel immersed in 1N HClsolution (blank).

of surface coverage (θ) were evaluated at different concen-trations of the inhibitors in 1N HCl solution. Attemptswere made to fit θ values to various adsorption isotherm.An inhibitor is found to obey Langmuir, if a plot of logθ/1−θ versus logC is linear. Similarly, for Temkin plot θversus logC, for BDM plot (logC – log θ/1−θ) versus θ3/2,and for Frumkin plot log θ/(1−θ)C versus θ will be linear.

On examining, the adsorption of different concentrations ofOcimum sanctum, Aegle marmelos, and Solanum trilobatumextracts on the surface of mild steel in 1N hydrochloricacid was found to obey Langmuir adsorption isotherm.The Langmuir adsorption isotherm plot for the adsorptionof various concentrations of the plant extracts is given inFigure 8.

3.5. Surface Examination Studies. Surface examination ofthe mild steel specimens was made using JEOL-Scanningelectron microscope (SEM) with the magnification of 1000x.The mild steel specimens after immersion in 1N HCl solutionfor three hours at 30◦C in the absence and presence ofoptimum concentration of the plant extracts were taken out,dried, and kept in a dessicator. The SEM images of mild steelimmersed in 1N HCl in the absence and presence of theoptimum concentration of the plant extracts are shown inFigures 9, 10, 11, and 12. The protective film formed on thesurface of the mild steel was confirmed by SEM studies. Fromthe SEM images, it was found that more grains were found in


15 kV WD 15 mm ×1000

Figure 10: SEM Photograph of mild steel immersed in 1N HClsolution containing an optimum conc. (6% v/v) of Ocimumsanctum.

15 kV WD 15 mm ×1000

Figure 11: SEM Photograph of mild steel immersed in 1N HClsolution containing an optimum conc. (8% v/v) of Aegle marmelos.

SEM image of mild steel immersed in 1N HCl solution in theabsence of the inhibitor, whereas no grains were found in theSEM image of mild steel immersed in 1N HCl solution inthe presence of the plant extracts, which shows the presenceof a protective film over the surface of the mild steel inthe presence of the inhibitors, and the protective film isuniform in the order: Ocimum sanctum > Aegle marmelos >Solanum trilobatum. The SEM morphology of the adsorbedprotective film on the mild steel surface has confirmed thehigh performance of inhibitive effect of the plant extracts.

3.6. Hydrogen Permeation Studies. The behaviour of theinhibitors with regard to hydrogen permeation can beunderstood by measuring the permeation current withand without inhibitors. Those inhibitors which reduce thepermeation current are good at inhibiting the entry ofhydrogen into the metal concerned [31]. There are basicallytwo reaction schemes. Common to both schemes, the firststep is the diffusion of few hydrogen atoms that get ontothe electrode surface. Hydrated protons are reduced to formneutral hydrogen atoms upon those areas of the surface,which are unoccupied. One can say protons are dischargedon to free sites on the electrode to form adsorbed hydrogenatoms

M(e) + H3O+ −→ MHads + H2O, (6)

where M is the cathodic metal surface. The second step is thedesorption step. The two basic reaction paths are

(i) discharge D, followed by chemical desorption, CD,

MHads + MHads −→ 2M + H2 ↑ (7)

15 kV WD 15 mm ×1000

Figure 12: SEM Photograph of mild steel immersed in 1N HClsolution containing an optimum conc. (10% v/v) of Solanumtrilobatum.

1

2

4

3

0 2 4 6 8 10 12 140

5

10

15

20

25

Perm

eati

oncu

rren

t(μ

A)

Time (min)

(1) Blank(2) Solanum trilobatum (10% v/v)(3) Aegle marmelos (8% v/v)(4) Ocimum sanctum (6% v/v)

Figure 13: Hydrogen permeation current versus time plots for mildsteel in 1N HCl solution in the absence and presence of an optimumconcentration of the inhibitors.

(ii) discharge D, followed by electrolytic desorption, ED,

MHads + H3O+ + M(e) −→ 2M + H2O + H2 ↑ . (8)

For transition metals, it has been reported that theelectrolytic desorption is the rate determining step. A part ofthe atomic hydrogen liberated during these processes entersthe metal, when the remainder is evolved as hydrogen gas[40]. Permeation current versus time curves for mild steel in1N HCl in the absence and presence of inhibitors are shownin Figure 13, and their corresponding permeation are givenin Table 5.

From the hydrogen permeation studies on mild steelin 1N HCl in the absence and presence of inhibitors, itwas observed that all the prepared extracts were able toreduce the permeation current compared to the control.The decrease in the permeation current follows the orderOcimum sanctum > Aegle marmelos > Solanum trilobatum.The reason for the reduced permeation currents in presenceof the inhibitors can be attributed to the slow discharge stepfollowed by fast electrolytic desorption step

M(e) + H3O+ slow−−→ MHads + H2O,

MH + H3O+ + M(e)fast−−→ 2M + H2O + H2.

(9)


Table 5: Values of hydrogen permeation current for the corrosion of mild steel in 1N HCl alone and in the presence of inhibitors.

Inhibitor Conc. of the extract (% in v/v) Permeation current (μA) Reduction in permeation current (%)

Blank — 23.0 —

Ocimum sanctum 6.0 2.2 90.43

Aegle marmelos 8.0 6.0 73.91

Solanum trilobatum 10.0 17.3 24.78

The reduction of hydrogen uptake could be attributed toadsorption of the phytochemical constituents present in theplant extracts on the mild steel surface, which preventedpermeation of hydrogen into metal.

4. Conclusion

(i) The leaf extracts of Ocimum sanctum, Aegle marme-los, and Solanum trilobatum act as good and effi-cient inhibitors for corrosion of mild steel in 1Nhydrochloric acid.

(ii) Potentiodynamic polarization studies revealed thatthe extracts act through mixed mode of inhibition.

(iii) The Nyquist diagrams obtained in impedancemethod revealed that charge-transfer process mainlycontrols the corrosion of mild steel.

(iv) The mechanism involved in this study is the phy-tochemical constituents in the plant extracts thathave adsorbed on the mild steel surface forming aprotective thin film layer preventing the dischargeof H+ ions and dissolution of metal ions and hasprevented the small corrosion on the surface of themetal.

(v) The plant extracts obey Langmuir adsorption iso-therm.

(vi) The SEM morphology of the adsorbed protective filmon the mild steel surface has confirmed the highperformance of inhibitive effect of the plant extracts.

(vii) From hydrogen permeation method, it was observedthat all the plant extracts were able to reduce thepermeation current compared to the control.

(viii) The reduction of hydrogen uptake in hydrogen per-meation method could be attributed to adsorption ofthe phytochemical constituents present in the plantextracts on the mild steel surface, which preventedpermeation of hydrogen into metal.

(ix) Results obtained in weight loss method were verymuch in good agreement with the electrochemi-cal methods (potentiodynamic polarization methodand impedance method) and hydrogen permeationmethod in the order Ocimum sanctum > Aeglemarmelos > Solanum trilobatum.

(x) Among the three plant extracts studied, the max-imum inhibition efficiency was found in Ocimumsanctum which showed 99.6% inhibition efficiency at6.0% v/v concentration of the extract.

References

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[6] M. Lebrini, F. Robert, and C. Roos, “Inhibition effect of alka-loids extract from Annona squamosa plant on the corrosion ofC38 steel in normal hydrochloric acid medium,” InternationalJournal of Electrochemical Science, vol. 5, no. 11, pp. 1698–1712, 2010.

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[10] A. O. James and E. O. Ekpe, “Inhibition of corrosion of mildsteel in 2 M hydrochloric acid by Aloe Vera,” InternationalJournal of Pure and Applied Chemistry, vol. 35, no. 10, 2002.

[11] M. Lebrini, F. Robert, and C. Roos, “Alkaloids extract fromPalicourea guianensis plant as corrosion inhibitor for C38 steelin 1 M hydrochloric acid medium,” International Journal ofElectrochemical Science, vol. 6, no. 3, pp. 847–859, 2011.

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[13] E. E. Ebenso, N. O. Eddy, and A. O. Odiongenyi, “Corrosioninhibitive properties and adsorption behaviour of ethanolextract of Piper guinensis as a green corrosion inhibitor formild steel in H2SO4,” African Journal of Pure and AppliedChemistry, vol. 2, no. 11, pp. 107–115, 2008.


[14] I. M. Mejeha, A. A. Uroh, K. B. Okeoma, and G. A. Alozie,“The inhibitive effect of Solanum melongena L. leaf extract onthe corrosion of aluminium in tetraoxosulphate (VI) acid,”African Journal of Pure and Applied Chemistry, vol. 4, no. 8,pp. 158–165, 2010.

[15] A. Y. El-Etre, “Inhibition of aluminum corrosion usingOpuntia extract,” Corrosion Science, vol. 45, no. 11, pp. 2485–2495, 2003.

[16] A. Y. El-Etre, M. Abdallah, and Z. E. El-Tantawy, “Corrosioninhibition of some metals using lawsonia extract,” CorrosionScience, vol. 47, no. 2, pp. 385–395, 2005.

[17] E. E. Oguzie, “Studies on the inhibitive effect of Occimumviridis extract on the acid corrosion of mild steel,” MaterialsChemistry and Physics, vol. 99, no. 2-3, pp. 441–446, 2006.

[18] G. Gunasekaran and L. R. Chauhan, “Eco friendly inhibitorfor corrosion inhibition of mild steel in phosphoric acidmedium,” Electrochimica Acta, vol. 49, no. 25, pp. 4387–4395,2004.

[19] K. O. Orubite and N. C. Oforka, “Inhibition of the corrosionof mild steel in hydrochloric acid solutions by the extracts ofleaves of Nypa fruticans Wurmb,” Materials Letters, vol. 58, no.11, pp. 1768–1772, 2004.

[20] Y. Li, P. Zhao, Q. Liang, and B. Hou, “Berberine as a naturalsource inhibitor for mild steel in 1 M H 2SO4,” Applied SurfaceScience, vol. 252, no. 5, pp. 1245–1253, 2005.

[21] M. A. Quraishi and D. K. Yadav, “Corrosion and its control’by some green inhibitors,” in Proceedings of the 14th NationalCongress on Corrosion Control, 2008.

[22] A. Y. El-Etre, “Inhibition of acid corrosion of carbon steelusing aqueous extract of olive leaves,” Journal of Colloid andInterface Science, vol. 314, no. 2, pp. 578–583, 2007.

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[25] B. Anand and V. Balasubramanian, “Corrosion behaviour ofmild steel in acidic medium in presence of aqueous extract ofAllamanda blanchetii,” E-Journal of Chemistry, vol. 8, no. 1, pp.226–230, 2011.

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Research Article

Corrosion Inhibition of the Galvanic Couple Copper-CarbonSteel in Reverse Osmosis Water

Irene Carrillo, Benjamın Valdez, Roumen Zlatev, Margarita Stoycheva, Michael Schorr,and Monica Carrillo

Instituto de Ingenierıa, Universidad Autonoma de Baja California, Boulevard, Benito Juarez y Calle a la Normal S/N,2180 Mexicali, BCN, Mexico

Correspondence should be addressed to Benjamın Valdez, [email protected]

Received 9 March 2011; Revised 24 May 2011; Accepted 24 May 2011

Academic Editor: Jerzy A. Szpunar

Copyright © 2011 Irene Carrillo et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The purpose of this paper is to evaluate the electrochemical behaviour of corrosion inhibition of the copper-carbon steel galvaniccouple (Cu-CS), exposed to reverse osmosis water (RO) used for rinsing of heat exchangers for heavy duty machinery, duringmanufacture. Molybdate and nitrite salts were utilized to evaluate the inhibition behaviour under galvanic couple conditions.Cu-CS couple was used as working electrodes to measure open circuit potential (OCP), potentiodynamic polarization (PP), andelectrochemical impedance spectroscopy (EIS). The surface conditions were characterized by scanning electron microscopy (SEM)and electron dispersive X-ray spectroscopy (EDS). The most effective concentration ratio between molybdate and nitrite corrosioninhibitors was determined. The morphological study indicated molybdate deposition on the anodic sites of the galvanic couple.The design of molybdate-based corrosion inhibitor developed in the present work should be applied to control galvanic corrosionof the Cu-CS couple during cleaning in the manufacture of heat exchangers.

1. Introduction

The study of inhibition mechanism, electrochemical, andkinetic behaviour of inorganic inhibitors such as molybdatesand other salts applied to protect Cu-CS galvanic couplein aqueous media, contributes to the prevention of corro-sion, particularly in industrial equipment which inevitablyrequires joining pieces of different metals for its construction[1–6]. Heat exchangers are often constructed by dissimilarmetals such as copper fins that cool the fluid by convection,internal copper tubes and carbon steel shells (Figure 1). Theconditions of the cleaning process during the manufacture ofheavy duty heat exchangers promote the dissolution of theanodic metal in a galvanic couple when it is rinsed with ROwater, especially with an unfavourable cathode-anode arearatio of 2.4 to 1.0.

The use molybdate-based corrosion inhibitors repre-sents an environmentally friendly alternative, since sodiummolybdate is considered a nontoxic inhibitor used forcorrosion protection of cooling systems handling softened

water. Sodium molybdate has a good performance as anodicinhibitor, that is incorporated into the metal surface forminga protective film. In combination with other chemical agentsit may promote or inhibit the corrosion process. These resultsare directly dependent on the concentration, temperature,pH, and the oxidizing agent. These studies have a relevantimportance to find an efficient and economical solution tocontrol the galvanic corrosion in industrial processes.

2. Methodology

2.1. Materials. Specimens of carbon steel UNS G10200 andcopper UNS C10300 were used to prepare the galvaniccouples. All the experiments were done with an area ratio 2.4to 1.0 cathode-anode in order to simulate the real dimensionsof heat exchangers for heavy machinery.

2.2. Specimen and Solutions Preparation. The working elec-trode was constructed joining metal coupons with a Cu areaof 4.0 cm2 and 1.7 cm2 for CS, connected to an insulated


Figure 1: Heat exchanger for heavy duty machinery of copper UNSC10300 and carbon steel G10200.

Cu wire. The surfaces were sanded to 400 grit and rinsed withdistilled water and acetone before coupling. The corrosiveenvironment was RO water at pH 5.5 and the appliedtemperature was 77◦C to simulate the real rinse processconditions. The corrosion inhibitor solutions were preparedby adding the solid salts to the RO water: molybdate andnitrite.

2.3. Electrochemical Measurements. The OCP variation alongtime was recorded to analyze the effect of the sodiummolybdate Na2Mo2O4 in different concentrations, addingsodium nitrite NaNO2 as oxidizing agent in RO water.The copper and steel coupons were separated by 7 cm andconnected by an insulated Cu alligator clip which in turnconnects to a multimeter Digital Protek Model B-45 toobtain a response in mV.

Electrochemical polarization plots were produced using athree electrode cell: a Cu-CS working electrode, an Ag/AgClreference electrode and a high density graphite electrode ascounter electrode. The potential-current plots were obtainedas a function of sodium molybdate and sodium nitriteconcentrations for each working electrode applying a scanrate of 5 mV/s in a potential range from −0.5 to 0.5 V versus.a Ag/AgCl electrode.

In order to evaluate the mass transfer process and the filmformed under molybdate effect, electrochemical impedanceanalysis were carried out after achieve the steady statepotential, in the frequency range from 0.01 Hz to 105 Hz with10 points/decade. The Nyquist and Bode plots were obtainedunder potentiostatic conditions.

2.4. Morphology Analysis. Surface analyses of specimenstested in RO water inhibited with the most efficient formu-lations were performed without any previous treatment byscanning electron microscopy (SEM) and electron dispersiveX-ray spectroscopy (EDS). The model SEM used was JeolJSM6360.


3.1. Open Circuit Potential Measurements. The OCP valueswith a potential shift to positive values in the presence of

OC

P(V

vers

us

Ag/

AgC

l)

Time (hr)

0

−0.2

−0.4

−0.6

−0.80 6 7 8 9 29

(1)

(2)

(3)

Figure 2: Variation of the open circuit potential with time: (1)Na2MoO4 : NaNO2 180 : 75 ppm; (2) Na2MoO4 180 ppm; (3) ROwater.

corrosion inhibitors obtained under different conditions forthe galvanic couples tested are shown in Figure 2.

At the moment of immersion in 180 ppm MoO42− a

potential of −140 mV was recorded. This potential movesfast toward negative values around −400 mV as a result ofthe breakdown of the molybdate layer formed on the steelsurface. Simplistically, when iron corrodes, ions, in con-junction with other anions adsorb to form a nonprotectivecomplex with Fe2+ ions [3, 7]. The result is a soluble andshallow protective film due to the poor oxidation ability ofNa2MoO4. Because of dissolved oxygen or other oxidizers inthe water, some of the Fe2+ ions are oxidized to the ferric(Fe3+) state, and the ferrous molybdate is transformed toferric molybdate, which is both insoluble and protective inneutral and alkaline waters [3].

An OCP value of −200 mV was obtained when NaNO2

was added, due to its oxidant properties that improve thecorrosion protection of CS.

3.2. Potentiodynamic Polarization. The effect of the ratiobetween the two inhibitors: Na2MoO4; NaNO2, was studiedby its electrochemical behaviour recording potentiodynamicpolarization (PP) plots. The plots in Figure 3 were obtainedin RO water at 77◦C in the presence of the sodiummolybdate; they clearly show a significant change in thedisplacement of the potential to electropositive values,implying a potential shift driven by kinetic changes in thecathodic process.

As the sodium molybdate concentration increases with-out sodium nitrite (Figure 3), the current density in theanodic curve tends to decrease and the performance inhi-bition process improves. Nevertheless, the inhibition onlyby sodium molybdate is not enough to avoid the corrosiondamage of the CS in the short term due to the galvanic effect.

The addition of sodium nitrite improves the efficiencyof the corrosion inhibition process. The presence of anoxide layer is essential for the corrosion inhibition action of

International Journal of Corrosion 3E

lect

rode

pote

nti

al(V

vers

us

Ag/

AgC

l)

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

Blank150, Na2MoO4

200, Na2MoO4

250, Na2MoO4

0.01 0.1 1 10 100×103

Current density (μA/cm2)

120 : 50, Na2MoO4 : NaNO2

180 : 75, Na2MoO4 : NaNO2

200 : 133, Na2MoO4 : NaNO2

280 : 120, Na2MoO4 : NaNO2

Figure 3: Current-Potential plots varying the concentration ratioNa2MoO4 : NaNO2 in ppm.

Table 1: Current potential data varying the concentration ratioNa2MoO4 : NaNO2.

Concentration (ppm) Current (μA/cm2) E (mV)

Blank 9.85 −534

150 MoO4−2 4.21 −208

200 MoO4−2 4.25 −211

250 MoO4−2 3.77 −200

120 : 150 MoO4−2: NO2

− 2.52 −454

180 : 75 MoO4−2 : NO2

− 5.54 −470

200 : 133 MoO4−2 : NO2

− 2.33 −167

280 : 120 MoO4−2 : NO2

− 3.81 −475

molybdate [3, 7–9] and in order to accelerate and stabilizethe oxidized surface an oxidizing compound such as sodiumnitrite is necessary. Finally, it was noted that in all the cases,the potential shift to positive values and the current densitydecreases, due to the electrostatic attraction force betweenthe MoO4

2− anions and the metal electrons and its oxidepromoting the formation of a polyoxomolybdate protectivelayer [3, 7, 10]. Since nitrite and dissolved oxygen (DO) orother anions seem to promote competitive adsorption overthe anodic surface, the molybdate and nitrite ratio is veryimportant to find the most efficient and economical solution.

The current and potential values of Table 1 were calcu-lated from potentiodynamic tests by the Tafel polarization.The results of the corrosion rates are showed in Figure 4. Theresults showed that the best inhibitor concentration ratio was

0

2

4

6

8

10

12

14

Blank

Cor

rosi

onra

te(m

m/y

)

Concentration (ppm)

M : N180 : 75

M : N280 : 120

M : N150 : 50

M : N200 : 133

Figure 4: Corrosion rates of Cu-CS as a function of MoO4 : NO2

(M : N) ratio concentration.

Ele

ctro

depo

ten

tial

(Vve

rsu

sA

g/A

gCl)

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

Blank

0.01 0.1 1 10 100×103

Current density (μA/cm2)

50 : 33, MoO2 : NO2

100 : 66, MoO2 : NO2

150 : 100, MoO2 : NO2

200 : 133, MoO2 : NO2

250 : 166, MoO2 : NO2

300 : 200, MoO2 : NO2

Figure 5: Current potential polarization plots for the Cu-CSgalvanic couple as a function of Na2MoO4 : NaNO2 concentration,in a 1.5 : 1 ratio.

200 : 133 ppm MoO4 : NO2. The electrochemical behaviourof the proportion 1.5 to 1 for MoO4 and NO2, respectively,was studied varying their concentration.

The effect of both compounds under this ratio concen-tration is shown in Figure 5, which reveals a continuousennobling of the potential that drives an acceleration of theanodic reaction. Table 2 shows the potential and currentvalues from Tafel polarization. On the other hand, the ratio250–166 ppm leads to a potential of−0.146 volts with a lower


Table 2: Current potential data as a function of Na2MoO4 : NaNO2

concentration in a 1.5 : 1 ratio.

Concentration (ppm) Current (μA/cm2) E (mV)

Blank 9.85 −534

50 : 33 Na2MoO4−2 : NO2

− 5.67 −233

100 : 66 Na2MoO4−2 : NO2

− 2.85 −180

150 : 100 Na2MoO4−2 : NO2

− 2.86 −181

200 : 100 Na2MoO4−2 : NO2

− 2.68 −177

250 : 166 Na2MoO4−2 : NO2

− 0.6 −154

300 : 200 Na2MoO4−2 : NO2

− 3.04 −185

0

2

4

6

8

10

12

14

Cor

rosi

onra

te(m

m/y

)

Concentration (ppm)

M150

M250

M200

Blank

M:N

150

:100

M:N

100

:66

M:N

300

:200

M:N

200

:133

M:N

250

:166

Figure 6: Corrosion rate of Cu-CS galvanic couple in function ofthe molybdate-nitrite ratio in RO water solutions.

current density that indicates the presence of CS corrosionprotection.

When carbon steel corrodes, the MoO4−2 ions and other

anions are fixed to the surface by adsorption to form amolybdate-Fe2+ complex, which sometimes is nonprotectivebecause the ferrous compounds are soluble. Due to dissolvedoxygen and another oxidants such as sodium nitrite in theRO water, the Fe2+ ions are oxidized to the ferric state (Fe3+)and ferrous molybdate complex is transformed to ferricmolybdate complex forming an insoluble and protectivelayer [3] in neutral, alkaline, and RO water.

Figure 6 shows a corrosion rate decrease up to a 94%of inhibition efficiency for the galvanic couple regardingthe blank. The excess concentration of Na2MoO4 affects theinhibition process, given by the adsorption competitivenessof other anions.

The potentiodynamic polarization is an accelerated tech-nique for corrosion measurements, therefore was possibleto decrease the concentration inhibitors in real manufacturelines that involves cleaning process where the expositiontime is short. The formulation 180–120 Na2MoO4 : NaNO2

was applied in a real industrial process and the corrosioninhibitors were added at the rinsing where the galvaniccorrosion occurs during the heat exchanges cleaning process.

In Figure 7 it is possible to observe an application of thesecorrosion inhibitors combination. The surface appearance

(a) (b)

Figure 7: Heat exchangers manufacturing process. (a) withoutcorrosion inhibitor in cleaning process. (b) Molybdate and nitritewere applied 180 : 120 ppm to inhibit galvanic corrosion.

of the heat exchanger after the cleaning process addingcorrosion inhibitors is really good.

3.3. Electrochemical Impedance Spectroscopy. Electrochemi-cal impedance spectroscopy assays for the systems: blank,180 : 120 and 250 : 166 Na2MoO4 : NaNO2 were performed toconfirm the inhibition process and study the electrochemicalcorrosion inhibition of molybdate.

An initial process of charge transfer was recorded inthe Nyquist diagram (Figure 8) for the blank represented bythe small semicircle of low impedance at high frequenciesfollowed by a greater semicircle at medium frequencies, thatdominate the process where the carbon steel loss electronsto reach an equilibrium potential with the copper. Finallydiffusion at very low frequencies contributes to the rapidformation of a porous layer which was observed at the end ofthe test. It is very notable that mass transfer by diffusion con-trols the process for the systems containing Na2MoO4 andNaNO2 since medium frequencies, the projection of centresfrom the experimental data points is also characterized by therelatively large angle of tilt which sometimes reaches valuesclose to 45◦ and is feature of Warburg impedance.

At high frequencies a small charge transfer semicircle wasrecorded in these systems, attributed at the oxidizing speciesthat promote the oxidation followed for a diffusion processdue the adsorbed specie where a combination is adjustedbetween the double layer capacitance value represented bya constant phase element (CPE) and the transfer chargeresistance value (Rct) when the molybdate concentrationincreases over the surface and the result is a stable layerformation of great charge transfer resistance. An equivalentcircuit was proposed in order to analyze the Bode andNyquist plot showed in Figure 9. The fit line of Figure 8shows the accuracy of the proposed circuit. Table 3 shows theimpedance data of this arrangement.

The adjustment of these data shows a capacitivebehaviour, dominated by the CPE element and an increasingin the impedance at high frequencies due to the corrosioninhibition process. Every interface has one capacitance anda charger transfer resistance related at the ions migration,the oxidation process, and the oxide film, therefore theequivalent circuit is constituted by two parts: the firstelements represent the solution resistance and the carbon


0 1000 2000 3000 4000 5000

0

−500

−1000

−1500

−2000

−2500

−3000

Blank

Fit180 : 120 MoO4 : NO2

250 : 166 MoO4 : NO 2

Zreal (Ω∗cm2)

−Zim

ag(Ω∗c

m2)

(a)

−40

−30

−20

−10

0

100

1000

10000

0.001 0.01 0.1 1 10 100 1000 10000×102Frequency (Hz)

Blank

Fit180 : 120 MoO4 : NO2

250 : 166 MoO4 : NO 2

|Z|(Ω∗

cm2)

Zph

z(g

rad)

(b)

Figure 8: Nyquist and Bode diagram for Cu-CS galvanic couple inRO water with molybdate sodium and nitrite sodium.

R1

R2

s

Ws1

CPE1

CPE2

wR3

Figure 9: Electrical equivalent circuit proposed to simulate theimpedance behaviour of Cu-CS couple in RO water.

KC

nt

C

OFe

Fe

1.2

1

0.7

0.5

0.2

01 2 3 4 5 6 7 8 9 10

(a)

O

Fe

Fe

Mo

934

747

560

373

186

01 2 3 4 5 6 7 8 9 10

CNa

(b)

Figure 10: SEM and EDS for CS exposed to RO water withoutcorrosion inhibitors.

steel surface portion in contact with the electrolyte and itsrespective polarization resistance increase under the effectof molybdate concentration increment and the oxidationof the Fe2+ to Fe3+ promoted by the nitrite and DO. Thesecond part represents the double layer that contains the Rctelement (R3) which increases significantly because the film isprotecting CS and Cu remains stable; the subsequent studiesof surface analysis by EDS show the molybdenum presencein the surface, nevertheless is not a uniform layer.

4. Morphological Study

The anodic surface was analyzed to evaluate the molybdateincorporated by adsorption to form a polyoxomolybdateferric complex [3, 7, 9] that protect the carbon steel evenunder galvanic conditions.

The microphotography in Figure 10(a) shows an oxideporous thick layer in the carbon steel surface that was incontact with Cu. The porous of the iron oxide black layerwas also observed visually The amount in weight recordedwas of 87% wt iron and 8.9% wt oxygen. An area zoom ofFigure 10(a) was done on a surface defect (b). High oxygen


Table 3: Parameters obtained from the fitting process with the equivalent circuit in Figure 9.

[Na2Mo4](Ppm) [NaNO2](Ppm) R1 (Ω·cm2) R2(Ω·cm2) Y2, s N R3 (Ω·cm2) Y2, S n WS

— — 351.7 38 1.89e − 5 0.86 676.2 1.46e − 4 0.72 1.679

180 120 207.2 908.9 7.45e − 6 0.81 8341 1.16e − 4 0.47 0.800

KC

nt

C

O

Fe

Fe

Mo

1.3

1

0.8

0.5

0.3

01 2 3 4 5 6 7 8 9

NaC

OOO

FeMoNa

(a)

KC

nt

1.3

1

0.8

0.5

0.3

01 2 3 4 5 6 7 8 9

CO

Fe

FeMoNaC

O FeMoNa

(b)

Figure 11: SEM and EDS of carbon steel under galvanic conditionsin inhibited RO water by 180 : 120 Na2MoO4 and NaNO2.

contents (14% wt) and a decreasing of Fe composition(80% wt) was recorded by EDS test as consequence ofthe galvanic corrosion and the precipitation by solublecompounds formation.

The film deposited on CS surface was characterized byEDS and is shown in Figure 11. The EDS test Figure 11(a)recorded small amounts of molybdenum (0.26–0.45%weight) by a reduction reaction of MoO4

2− anion to joinit with the ferric oxide layer [1, 3, 6–8]. The homogeneoussurface improves with the molybdate concentration incre-ment. There was a 30% decreasing of oxygen with respectto the blank and the Fe content is above 92%. The area zoomFigure 11(b) 180 : 120 ppm Na2MoO4 : NaNO2 shows a smallaccumulation of molybdate in a surface defect which seemsas “restorative effect”. This behaviour was reported from alloy

1.1

0.9

0.7

0.4

0.2

0

CFe

Fe

Mo

KC

nt

1 2 3 4 5 6 7 8 9 10

Na

(a)

1.1

0.9

0.7

0.4

0.2

0

CFe

Fe

Mo

KC

nt

1 2 3 4 5 6 7 8 9 10

O

Na

(b)

Figure 12: SEM and EDS of carbon steel under galvanic conditionsin inhibited RO water by 250 : 166 Na2MoO4 and NaNO2.

with molybdenum can repair the defect of the iron passive inborate buffer solution and inhibit pit growth [11].

It is possible to observe in low quantity the samebehaviour in a zoom area of part Figure 12(b) 250 : 166 ppmNa2MoO4 : NaNO2 when the surface has defects, no homo-geneous zones or susceptible zones by copper contact, theadsorption of the MoO4

2− anions increases, taking accountthere is an electrostatic attraction between the Fe cations andthe MoO4

2−, [12, 13] this is deposited in the anodic zones(0.45–0.90%), the most susceptible area portions in contactwith the copper and the solution.

The electrochemical tests showed the effect by increasingthe concentration of sodium molybdate (limited to 250 ppmin combination with the sodium nitrite) [6, 8, 14, 15] todecrease the corrosion rate and the results by EDS confirm


a deposition of molybdenum compounds joined to the oxidelayer and oxide reduction in all the cases. The combination250 : 166 MoO4 : NO2 was totally confirmed as the optimalformulation for copper and carbon galvanic couple.

5. Conclusions

Good performance of the corrosion inhibitor requires thepolyoxomolybdate complex layer formation which is morestable, highly protective and a great charge transfer withferric compounds, necessary to ensure the oxidation of Fe2+

to Fe3+ promoted by the nitrite. The optimal inhibitor con-centration was 250 ppm of sodium molybdate and 166 ppmof sodium nitrite obtaining 94% inhibition efficiency. Theeconomical application for the heat exchanger cleaning was180 : 120 molybdate : nitrite and it takes 24 minutes. Molyb-date excess concentration affects the inhibition related to thenitrite and DO molecules and possible other anions whichpromote the adsorption competitiveness influenced by theconcentration ratio between molybdate and nitrite hence theimportance of the efficient combination. The impedance testshows the diffusion control for the formation of resistantmolybdate film. Molybdate can be physically adsorbed onthe metal surface or on the hydroxide layer available on thesurface and act in anodic zones by electrostatic attraction.

Acknowledgments

The authors thank Honeywell Thermal Systems of Mexicali,for support of the study development and to implementthe corrosion inhibitor in their cleaning process for heatexchangers during the manufacture. Thanks are also dueto CONACYT for the Scholarship 208739 support toI. Carrillo.

References

[1] C. M. Mustafa and S. M. S. I. Dulal, “Molybdate and nitriteas corrosion Iinhibitors for copper-coupled steel in simulatedcooling water,” Corrosion, vol. 52, no. 1, pp. 16–22, 1996.

[2] E. J. Talbot and D. R. Talbot, Corrosion Science and Technology,CRC Press, New York, NY, USA, 2007.

[3] J. R. Davis, “Corrosion fundamentals, testing and protection,”ASM International and The materials Information Society,Ohio, EE.UU, 2000, http://www.asm.intl.org/.

[4] R. Francis, “Galvanic corrosion of high alloy stainless steels insea water,” British Corrosion Journal, vol. 29, no. 1, pp. 53–57,1994.

[5] P. R. Roberge, Corrosion Engineering: Principles and Practice,McGraw–Hill, New York, NY, USA, 2008.

[6] I. Carrillo, Inhibition of the corrosion in galvanic couples copperand carbon steel of Heat Exchangers for heavy machineryindustry, M.S. thesis, Instituto de Ingenieria de la UniversidadAutonoma de Baja California for Engineering Master Degree,2009.

[7] M. R. Ali, C. M. Mustafa, and M. Habib, “Effect of molybdate,nitrite, zinc ions on the corrosion inhibition of Mild steel inaqueous chloride media containing cupric ions,” Journal ofScientific Research, vol. 1, pp. 82–91, 2009.

[8] M. Saremi, C. Dehghanian, and M. M. Sabet, “The effectof molybdate concentration and hydrodynamic effect on

mild steel corrosion inhibition in simulated cooling water,”Corrosion Science, vol. 48, no. 6, pp. 1404–1412, 2006.

[9] V. S. Sastri, Corrosion Inhibitors Principles and Applications,John Wiley & Sons, Ontario, Canada, 1998.

[10] K. C. Emregul and A. A. Aksut, “The effect of sodium mol-ybdate on the pitting corrosion of aluminum,” CorrosionScience, vol. 45, no. 11, pp. 2415–2433, 2003.

[11] E. Fujioka, H. Nishihara, and K. Aramaki, “The inhibition ofpit nucleation and growth on the passive surface of iron in aborate buffer solution containing Cl- by oxidizing inhibitors,”Corrosion Science, vol. 38, no. 11, pp. 1915–1933, 1996.

[12] M. Shams and L. Wang, “Mechanism of corrosion inhibitionby sodium molybdate,” in Proceedings of the Material TestingLaboratory, Government of Abu Dhabi Water, vol. 5, pp. 181–202, Electricity Department, Umm Al Nar Station, Abu Dhabi,UAE, 1996.

[13] M. Meziane, F. Kermiche, and C. Fiaud, “Effect of molybdateions as corrosion inhibitors of iron in neutral aqueoussolutions,” British Corrosion Journal, vol. 33, no. 4, pp. 302–308, 1998.

[14] D. G. Kolman and S. R. Taylor, “Sodium molybdate as acorrosion inhibitor of mild steel in natural waters part 2:molybdate concentration effects,” Corrosion, vol. 49, no. 8, pp.635–643, 1993.

[15] D. G. Kolman and S. R. Taylor, “Sodium molybdate as acorrosion inhibitor of mild steel in natural waters. Part 1: flowrate effects,” Corrosion, vol. 49, no. 8, pp. 622–634, 1993.


Research Article

Inhibition Effect of 1-Butyl-4-MethylpyridiniumTetrafluoroborate on the Corrosion of Copper in PhosphateSolutions

M. Scendo and J. Uznanska

Institute of Chemistry, UJK Kielce, Swietokrzyska Street 15G, 25406 Kielce, Poland

Correspondence should be addressed to M. Scendo, [email protected]

Received 19 November 2010; Accepted 1 February 2011

Academic Editor: Flavio Deflorian

Copyright © 2011 M. Scendo and J. Uznanska. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The influence of the concentration of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) as ionic liquid (IL) on thecorrosion of copper in 0.5 M PO3−

4 solutions of pH 2 and 4 was studied. The research involved electrochemical polarizationmethod, and scanning electron microscopy (SEM) technique. The results obtained showed that the inhibition efficiency ofcorrosion of copper increases with an increase in the concentration of 4MBPBF4 but decreases with increasing temperature.The thermodynamic functions of corrosion analysis and adsorptive behavior of 4MBPBF4 were carried out. During the test, theadsorption of the inhibitor on the copper surface in the phosphate solutions was found to obey the Langmuir adsorption isothermand had a physical mechanism.

1. Introduction

Copper is used as a construction metal in the central heatinginstallations, car industry, energetics, oil refineries, sugarfactories, marine environment, to name only a few of itsvarious applications. This extensive use of copper is due to itsmechanical and electric properties as well as the behaviourof its passivation layer. Acidic solutions are widely used invarious industries for the cleaning of copper. The behaviourof copper in acidic media is extensively investigated, andseveral ideas have been presented for the dissolution process[1, 2]. To avoid the base metal attack and to ensure theremoval of corrosion products/scales alone, inhibitors areextensively used. The most well-known acid inhibitors areorganic compounds containing nitrogen, phosphor, sulfur,and oxygen atoms. The surfactant inhibitors have manyadvantages such as high inhibition efficiency (IE), low price,low toxicity, and easy production [3–5]. The interactionsbetween the inhibitor molecules and the metal surfacesshould by all means be explained and understood in detail.In examining of these interactions, theoretical approaches

applied can be very useful [6–10]. Many N-heterocyclic com-pounds have been used for the corrosion inhibition of metals,such as imidazoline [11], triazole [12–14], tetrazole [15],pyrrole [16], pyridine [17], pyrazole and bipyrazole [18,19], pyrimidine [20], pyridazine [21], and some derivatives.Some heterocyclic compounds containing a mercapto grouphave been developed as copper corrosion inhibitors. Thesecompounds include: 2-mercaptobenzothiazole [22], 2,4-dimercaptopyrimidine [23], 2-amino-5-mercaptothiadzole,2-mercaptothiazoline [24], potassium ethyl xanthate [25–28] and indole and derivatives [29]. Among the numer-ous organic compounds tested and industrially applied ascorrosion inhibitors, nontoxic ones are far more strategicnow than in the recent past. These compounds include suchamino acids [30–32] and derivatives as cysteine [33].

In the past two decades, the research in the field of greencorrosion inhibitors has been addressed towards the goal ofusing cheap effective molecules at low or zero environmentalimpact. These compounds include purine and adenine,which have been tested for copper corrosion in chloride[34, 35], sulfate [36], and nitrate solutions [37].


Ionic liquids (ILs) are molten salts with melting pointsat/or below ambient temperature, which are composedof organic cations and various anions. Configuration ofILs consists of an amphiphilic group with a long chain,hydrophobic tail, and a hydrophilic polar head. Usually, ILshave nitrogen, sulphur, and phosphorus as the central atomsof cations, such as imidazolium, pyrrolidinium, quaternayammonium, pyridinium, piperidinium, sulfonium and qua-ternary phosphonium. Currently, funtionalized IL is a verynoticeable topic in the field of IL research. Introducingdifferent functional groups into cations provides a great dealof ILs with new structures that can markedly change thephysicochemical properties of ILs, and it also affords morechoices for applications of ILs in electrochemical devices.

Imidazolium compounds are reported to show corrosionresistant behavior on mild steel [38], copper [39, 40], andaluminium [41]. It was found that the action of suchinhibitors depends on the specific interaction between thefunctional groups and the metal surface, due to the presenceof the –C = N– group and electronegative nitrogen in themolecule. Ionic liquids and different types of surfactansbase inhibitors are well known to have a high activity inacid medium [42, 43] and therefore are used in an oilfield to minimize carbon-dioxide-induced corrosion [44,45]. Among many kinds of functionalized ionic liquidsether-functionalized ILs have been investigated intensively,and ether groups have been successfully introduced in toimidazolium cations [46–52].

However, no substantial information is available onpyridinium ionic liquids being used as corrosion inhibitorsof copper.

The present work describes a study of the corrosionof copper in 0.5 M PO3−

4 solutions of pH 2 and 4without and with different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4), based oncopper stationary disc electrode voltammetry measurementsand scanning electron microscope. Moreover, the thermody-namic functions were appointed for the adsorption processand to gain more information about the mode of adsorptionof the inhibitor on the surface of copper.

2. Experimental Methods

2.1. Solutions. 1-Butyl-4-methylpyridinium tetrafluorobo-rate (4MBPBF4) (>99.8%) was purchased from Fluka. Themolecular structures of compound are shown in Figure 1. Itis worth to notice that 4MBPBF4 is not flat molecule. The4MBPBF4 is stable in air, water, and in majority organicsolvents. However, this compound is well enough solvable inwater. All the solutions were prepared using analytical gradereagent and triple distilled water (resistivity 13 MΩ cm). The4MBPBF4 was dissolved at concentrations in the range of1.0–50.0 mM in 0.5 M PO3−

4 solutions of pH 2 and 4. Duringthe measurements, the solution was not stirred or deaerated.

2.2. Electrodes and Apparatus. The working electrode wasa home-made stationary disk electrode (SDE) of Specpurecopper (Johnson Matthey Chemicals Ltd.) with r = 0.240 cm

N

B

F F

FF

Figure 1: Molecular structure ionic liquid: 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4).

and A = 0.181 cm2. Prior to each experiment the workingelectrode was mechanically polished to mirror gloss by using1000 and 2000-grade emery papers. Then the electrodewas washed several times interchangeably with bidistilledwater and ethanol. Finally, SDE was dried using a streamof air. Such pretreatment of the disk was repeated after eachvoltammetric measurement. Other details were published in[53–56]. All the surface-area-dependant values are normal-ized with respect to the geometric surface area of the workingelectrode.

Electrode potentials were measured and reported againstthe external saturated calomel electrode with NaCl solution(SCE(NaCl)) coupled with a fine Luggin capillary. Tominimize the ohmic contribution, the capillary was keptclose to the working electrode. A platinum (purity 99.99%)wire was used as an auxiliary electrode. Auxiliary electrodewas individually isolated from the test solution by a glass frit.

All voltammetric experiments were performed usinga Model EA9C electrochemical analyzer, controlled viaPentium computer using the software Eagraph V. 4.0.

2.3. Scanning Electron Microscope. A scanning electronmicroscope (SEM) PHILIPS XL 30 was used to study themorphology of the copper surface in the absence andpresence of the inhibitor. Samples were attached on top ofan aluminum stopper by means of 3 M carbon conductiveadhesive tape (SPI).

2.4. Potentiodynamic Polarization Measurements. Electro-chemical experiments were carried out in a classical three-electrode glass cell. The cell was open to air. The degreasedSDE was quickly inserted into the solution and immediatelycathodically polarized at −1100 mV (SCE(NaCl)) for 3 minto reduce any oxide on the copper surface. The polarizationcurves were obtained using the linear potential sweep (LSV)


−6

−4

−2

0

2

4

j(m

Acm

−2)

−1100 −900 −700 −500 −300 −100 100

E (mV) versus SCE(NaCl)

(d)(c)

(b)

(a)

Figure 2: Some chosen polarization curves of the copper electrodein 0.5 M PO3−

4 solutions containing different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate: (a) 0, (b) 1.0, (c) 10.0,and (d) 50.0 mM, pH 4, dE/dt 1 mV s−1.

technique. The scan started from the cathodic (−1100 mV)to the anodic direction with the scan rate of 1 mV s−1.Electrochemical experiments were repeated many times, andthe average values of the current were used.

All experiments were carried out using an air thermostatwith the forced air circulation.

3. Experimental Results and Discussion

3.1. Polarization Behaviour of Copper. The effect of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) on thecorrosion reactions of copper was determined by polar-ization measurements at 20◦C. Figure 2 shows example ofpolarization curves for the copper electrode in 0.5 M PO3−

4solutions of pH 4 without and with different concentrationsof 4MBPBF4. Similar curves were recorded for solution of pH2. It is clear that the presence of different concentrations ofthe inhibitor decreases the current densities and reduces bothof the cathodic and anodic current densities in comparisonto those recorded in the additive-free solution. However, incase of more acid solutions (pH 2) were observed smallerchanges in the cathodic and anodic current densities. Thedecrease in current densities could be attributed to thedecrease in the phosphate ions attack on the copper surfacedue to the adsorption of the inhibitor molecules at thecopper/solution interface.

3.1.1. Corrosion Parameters. The corrosion kinetic param-eters were calculated on the basis of cathodic and anodicpotential versus current characteristics in the Tafel poten-tial region (Figure 3). The corrosion parameters such ascorrosion potential (Ecorr), corrosion current density ( jcorr),and cathodic (bc) and anodic (ba) Tafel slope are listed inTable 1. It is worth noticing that addition of the 4MBPBF4

0.00001

0.0001

0.001

0.01

0.1

1

7

j(m

Acm

−2)

−1100 −900 −700 −500 −300 −100 100


(d)

(c)

(b) (a)

Figure 3: Some chosen Tafel plots of the copper electrode in 0.5 MPO3−

4 solutions containing different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate: (a) 0, (b) 1.0, (c) 10.0 and (d)50.0 mM, pH 2, dE/dt 1 mV s−1.

causes more negative shift in corrosion potential valuesindependently from pH solutions. Hence small changes inpotentials can be a result of the competition of the cathodicand the anodic inhibiting reactions.

The corrosion current density (Table 1) decreased whenthe concentrations of 1-Butyl-4-methylpyridinium tetraflu-oroborate were increased for both solutions of pH 2 and4. This indicates the inhibiting effect of 4MBPBF4 oncorrosion of copper. The decrease in cathodic (bc) and anodic(ba) or the increase in (ba) only in case of solutions ofpH 4 Tafel slopes (Table 1) indicated that the 1-Butyl-4-methylpyridinium tetrafluoroborate molecules are adsorbedon both the anodic and cathodic sites resulting in aninhibition of both anodic dissolution of copper and cathodicreduction reactions. Moreover, these inhibitors cause smallchange in the cathodic and anodic Tafel slopes, indicatingthat 4MBPBF4 is first adsorbed onto copper surface andtherefore impedes the reaction by merely blocking thereaction sites of copper surface without affecting the cathodicand anodic reaction mechanism [57].

3.1.2. Polarization Resistance. The polarization resistance(Rp) values are related to the corrosion current density ( jcorr),which can be calculated from the equation:

Rp =[

babc2.303(ba + bc)

]×[

1jcorr

]. (1)

The Rp values listed in Table 1 are used to estimate thecorrosion inhibition effect of the inhibitor. The addition of1-Butyl-4-methylpyridinium tetrafluoroborate to the phos-phate solutions produced higher Rp values than the blanksolution indicating the formation of a protective layer on theelectrode surface. Hence, the polarization resistance valuesincrease with an increase in the concentration of 4MBPBF4


Table 1: Corrosion parameters and polarization resistance of copper electrode in 0.5 M PO3−4 solutions in the absence or presence of different

concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) of pH 2 and 4 at 20◦C.

Inhibitor pHConcentrationinhibitor (mM)

−Ecorr (mV) jcorr (μA cm−2) − bc (mV dec−1) ba (mV dec−1) RP (Ω cm2)

Blank 0.0 60 29.0 300 75 898

4MBPBF4

1.0 64 16.0 280 70 1519

2 5.0 79 13.2 277 67 1846

10.0 94 10.5 275 65 2174

25.0 99 6.5 228 63 5158

50.0 104 2.4 180 60 8141

Blank 0.0 60 20.0 240 160 2084

4MBPBF4

1.0 200 10.0 175 190 3955

4 5.0 240 8.0 148 291 5987

10.0 278 5.0 120 400 8016

25.0 273 3.4 115 410 13478

50.0 268 1.2 110 420 27824

for both solutions of pH 2 and 4. It seems that protectivelayer created on surface of copper is the most tight in of lessacid solution about the largest concentration of 1-Butyl-4-methylpyridinium tetrafluoroborate.

3.1.3. Inhibition Efficiency. Inhibition efficiency (IE) can alsobe calculated from polarization tests by using the followingequation [58, 59]:

IE(%) =(jo − jcorr

jo

)× 100, (2)

where jo and jcorr are the corrosion current densities in theabsence and presence of inhibitor, respectively.

The inhibition efficiency depends on both the natureand the concentration of the investigated compounds. Thecalculated inhibition efficiencies are presented in Figure 4.In the presence of 1-Butyl-4-methylpyridinium tetraflu-oroborate solution of pH 2 and 4, the inhibition effi-ciency increases with an increase in the concentration ofinhibitor. This confirms the inhibiting character of 1-Butyl-4-methylpyridinium tetrafluoroborate. However, IE is higherin case of solution of pH 4 than 2. It is obvious that in thepresence of 1-Butyl-4-methylpyridinium tetrafluoroboratesolution of pH 2 the film on copper does not cover tightlythe surface and hence does not protect it prior to corrosionof Cu in an adequate degree.

3.1.4. Corrosion Rate. The corrosion current density ( jcorr)was converted into the corrosion rate (kr) by using theexpression [60]:

kr

(mmyear

)= 3.268× 10−3

(jcorr ×MCu

nρ

), (3)

where MCu is the molecular weight of copper, n is the numberof electrons transferred in the corrosion reaction, and ρ is thedensity of Cu (g cm−3).

0

10

20

30

40

50

60

70

80

90

100IE

(%)

1 5 10 25 50

c (mM)

pH 2pH 4

Figure 4: Inhibition efficiency of corrosion of copper in0.5 M PO3−

4 solution with different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate of pH 2 and 4.

The values of the copper corrosion rate in the absenceand the presence of inhibitor for solution of both pH Valuesare presented in Table 2. The corrosion rate of copper issignificantly reduced as a result of the reduction in thecorrosion current densities. The protective layer on surfaceof metal causes that the corrosion rate to be more diminishesin case of less acid solution of phosphates.

3.2. Scanning Electron Microscopy Studies. The surface mor-phology of copper samples immersed in 0.5 M PO3−

4 (pH 2and 4) for 24 hours in the absence and in the presence of50.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate


Table 2: Corrosion rate of copper in 0.5 M PO3−4 solutions in

the absence or presence of different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) of pH 2 and 4.

Concentration of 1-Butyl-4-methylpyridiniumtetrafluoroborate(mM)

kr (mm/year)

pH 2 pH 4

0 0.67 0.46

1.0 0.37 0.23

5.0 0.31 0.18

10.0 0.24 0.12

25.0 0.15 0.08

50.0 0.06 0.03

was studied by scanning electron microscopy (SEM). Thesolutions were not degassed.

Figure 5 show the surface morphology of copper speci-mens (a) before and (b) after being immersed in corrosivesolution (pH 2). The photograph (b) revels that the surfacewas strongly damaged in absence of the inhibitor. Figures5(c) and 5(d) show SEM images of the surface copperspecimens after immersion (for the same time interval) incorrosive solution containing additionally 50.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate of pH 2 and4, respectively. In the presence of the inhibitor the filmprecipitates on the surface of copper. The SEM photographsshow that protective layer does not cover tightly the surface,and, hence does not protect the Cu surface to an adequatedegree especially in case of solution of pH 2. Phosphateions, oxygen and water penetrate the protective film throughpores, flaws or other weak spots what results in the furthercorrosion of copper. In order to check the results of actionby aggressive solution, the protective layer was removedfrom surface of copper. The layer was well adhered to thesurface of the metal, and the removal of it was really difficult.Therefore ultrasonic water bath was used. The sample wasshaken in diluted acetic acid and rinsed in propanol. Figure 5presents samples after the removal of the inhibiting film forpH 4 (Figure 5(e)) and 2 (Figure 5(f)). However. However,received results indicated that more tight protective layerwas forming in solution of pH 4 (Figure 5(e)). Moreover, inphosphate solution the 4MBPBF4 acts better as the inhibitorin less acidic environment.

3.3. Mechanism of Corrosion Inhibition. Regarding the mech-anism the oxygen reduction reaction on copper in acidicsolutions a lot of work has been carried out [61–67]. Thecathodic global reaction in an aerated aqueous phosphatesolution could be described as follows:

O2 + 4H+ + 4e− ←→ 2H2O. (4a)

However, the first cathodic wave is attributed to reaction:

O2 + 2H+ + 2e− ←→ H2O2. (4b)

In the more negative potential at the electrode, surface occursthe next reaction:

H2O2 + 2H+ + 2e− ←→ 2H2O. (4c)

Furthermore, reaction (4a) is strongly influenced by poten-tial [66].

The dissolution process of copper (anodic corrosionreaction) at low overpotentials runs according to the follow-ing steps [68–70]:

Cu− e− ←→ Cu+ads, (5a)

Cu+ads − e− ←→ Cu2+, (5b)

where the Cu+ads is an adsorbed monovalent species of copper

at the electrode surface.The inhibition effect of 1-Butyl-4-methylpyridinium

tetrafluoroborate on the copper surface could be explainedas follows

The inhibitor of 4MBPBF4 can be protonated in acidicsolutions:

4MBPBF4 + H+ ←→ [4MBPBHF4]+. (5c)

Then the inhibitor molecules adsorb through electrostaticinteractions between the negatively charged copper surfaceand positively charged [4MBPBHF4]+. However, the elec-trode carried the negative charge, therefore [4MBPBHF4]+

ions should be first adsorbed directly on copper to probablyform a protective layer at active sites:

Cu + [4MBPBHF4]+ ←→ [Cu—4MBPBF4]ads + H+, (5d)

and blocks the further oxidation reaction of Cu+ads to Cu2+

(reaction (5b)). Moreover, the inhibitor molecules lead tothe blocking of the transfer of oxygen from the bulk solutionto the copper/solution interface that is going to reduce thecathodic reaction of oxygen (reaction (4a)). This indicatesthat the presence in phosphate solution of 4MBPBF4 affectsboth the cathodic and anodic reactions, therefore the,compound acts as a mixed-type inhibitor. The proposedmechanism of corrosion inhibition of copper by 4MBPBF4

in phosphate solutions (reactions (4a)-(5a)) requires theconfirmation through making additional research.

However, exhausting information regarding mechanismof corrosion inhibition can be obtained on the basis ofthermodynamic measurements.

3.4. Effect of Temperature. The effect of temperature on thecorrosion of copper in 0.5 M PO3−

4 solution in the absenceand presence of 10.0 mM of 1-Butyl-4-methylpyridiniumtetrafluoroborate of pH 2 and 4 at temperature rangingfrom 303 to 343 K was investigated by potentiodynamicpolarization measurements. The corrosion parameters andthe inhibition efficiency are presented in Table 3. The cor-rosion potential and cathodic and anodic Tafel slope changesimilarly in case of low temperature of solutions (Table 1).Therefore, the growth of temperature of solutions doesnot influence the change of inhibition mechanism. Worth


(a) (b)

(c) (d)

10μm750x30 kV

(e)

10μm750x30 kV

(f)

Figure 5: SEM micrographs of the surface of copper: (a) before, (b) after being immersed in 0.5 M PO3−4 (pH 2) for 24 hours, (c), (d)

corrosive solution contained additionally 50.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate of pH 4 and 2, respectively, afterremoval of the inhibiting film for pH: (e) 4 and (f) 2 (magnification 750x).

noticing is that the corrosion current density increases andinhibition efficiency decreases with increasing temperature,which indicates desorption of the inhibitor molecules fromthe surface of copper with rising temperature of solutions.

3.4.1. Thermodynamic Activation Parameters. Thermody-namic activation parameters are important to study theinhibitive mechanism. The mechanism of the inhibitoraction can be deduced by comparing the apparent activationenergies, Ea, in the presence and absence of the corrosioninhibitor. Activation parameters such as Ea, the enthalpy ofactivation, ΔHa, and the entropy of activation, ΔSa, werecalculated from an Arrhenius-type plot [71, 72]:

jcorr = A exp(−EaRT

), (6)

where A is the Arrhenius constant, Ea is the apparentactivation energy, R is the universal gas constant, and Tis the absolute temperature. An alternative formula of theArrhenius equation is the transition state equation [73]:

jcorr =(RT

Nh

)exp

(ΔSaR

)exp

(−ΔHa

RT

), (7)

where N is the Avogadro’s constant, h is the Planck’sconstant, ΔSa is the change of entropy for activation, and

ΔHa is the change of enthalpy for activation. Plots ofln( jcorr) versuss 1/T, and ln ( jcorr/T) versus 1/T give straightlines with slopes of –Ea/R and 1/TΔHa/R, respectively. Theintercepts, which can then be calculated, will be [ln(R/Nh) +(ΔSa/R)] for the Arrhenius and transition-state equations,respectively. Figures 6 and 7 represent the data plots in theabsence and presence of 4MBPBF4 of pH 2 and 4. Thecalculated thermodynamic activation parameters are listedin Table 4. The values of Ea and ΔHa in the presence of10.0 mM 4MBPBF4 are higher than those in black solutions,indicating that more energy barrier for the reaction in thepresence of 4MBPBF4 is attained, especially in case of pH 4.This shows that the energy barrier of the corrosion reactionincreased in the presence of the inhibitor without changingthe mechanism of dissolution of copper [74]. The entropyof activation, ΔSa, in the absence and presence of 4MBPBF4

is large and negative (especially with pH 4), implying thatthe rate-determining step for the activated complex is theassociation rather than the dissociation step, which meansthat a decrease in disordering takes place by going fromreactants to the activated complex [75].

3.5. Adsorption Isotherm. It has been assumed that inhibitormolecules establish their inhibition action via the adsorptionof the inhibitor onto the metal surface. The adsorptionprocesses of inhibitors are influenced by the chemical


Table 3: Corrosion parameters and inhibition efficiency of copper electrode in 0.5 M PO3−4 solutions in the absence or presence of 10.0 mM

of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) of pH 2 and 4 at different temperatures.

Inhibitor pH Temperature (K) −Ecorr (mV) jcorr (μA cm−2) −bc (mV dec−1) ba (mV dec−1) IE (%)

Blank

303 66 41.0 297 76 —

313 79 53.5 299 76 —

323 85 65.0 301 79 —

333 94 77.3 306 82 —

343 104 89.1 304 83 —

2 303 101 15.1 276 66 63.1

4MBPBF4

313 109 22.4 279 69 58.1

323 117 30.6 270 72 52.9

333 124 39.9 279 74 48.4

343 132 49.7 282 74 44.2

Blank

303 69 29.0 241 163 —

313 78 36.3 240 163 —

323 87 47.2 243 166 —

333 97 57.5 246 169 —

343 115 67.0 248 170 —

4 303 286 7.5 121 392 74.1

4MBPBF4

313 295 11.0 123 401 69.6

323 302 16.9 126 406 64.1

333 311 23.6 128 408 59.0

343 319 30.7 130 411 54.2

−12

−11.5

−11

−10.5

−10

−9.5

−9

lnj c

orr

(Acm

−2)

2.9 3 3.1 3.2 3.3

1000/T (K−1)

(d)

(b)

(c)

(a)

Figure 6: Arrhenius plots for copper in 0.5 M PO3−4 solutions

containing: (a), (c) 0 and (b), (d) 10.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate. The pH of solutions was thefollowing: (a), (b) 2 and (c), (d) 4.

structure of organic compounds, the nature and surfacechange of metal, the distribution of charge in molecule andthe type of aggressive media [76]. The adsorption isothermcan provide the basic information on the interaction betweenthe inhibitor and the metal surface, which depends onthe degree of surface coverage, Θ [77]. The values of

−17.5

−17

−16.5

−16

−15.5

−15

ln(j

corr/T

)(A

cm−2

K−1

)

2.9 2.95 3 3.05 3.1 3.15 3.2 3.25 3.3

1000/T (K−1)

(d)

(b)

(c)

(a)

Figure 7: Transition state plots for copper in 0.5 M PO3−4 solu-

tions containing: (a), (c) 0 and (b), (d) 10.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate. The pH of solutions was thefollowing: (a), (b) 2 and (c), (d) 4.

surface coverage for different concentrations of inhibitor in0.5 M PO3−

4 solutions of pH 2 and 4 were evaluated frompolarization curves according equation

Θ = 1 − jcorr

jo. (8)


Table 4: Thermodynamic activation parameters for copper in0.5 M PO3−

4 solutions in the absence or presence of 10.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) for pH 2and 4.

Inhibitor pH Ea (kJ mol−1) ΔHa (kJ mol−1)ΔSa

(J mol−1 K−1)

Blank2

16.64 13.98 −282.65

4MBPBF4 25.66 22.98 −261.19

Blank4

18.47 15.81 −297.69

4MBPBF4 31.02 28.33 −249.59

Table 5: Surface coverage of copper electrode in 0.5 M PO3−4 solu-

tions for different concentrations of 1-Butyl-4-methylpyridiniumtetrafluoroborate (4MBPBF4) for pH 2 and 4.

Concentration

pH 1.0 5.0 10.0 25.0 50.0

(mM), 1-Butyl-4-methylpyridinium tetrafluoroborate

2 0.45 0.55 0.64 0.78 0.92

4 0.50 0.63 0.75 0.82 0.94

Table 6: Slope (b), linear correlation coefficient (R2), equilibriumconstant (K), and standard free energy of adsorption (ΔG0

ads)in 0.5 M PO3−

4 of 1-Butyl-4-methylpyridinium tetrafluoroborate(4MBPBF4) solutions of pH 2 and 4.

pH b R2 K(M−1)ΔG0

ads

(kJ mol−1)

2 1.00 0.9998 2.2× 102 −23.3

4 1.01 0.9998 3.8× 102 −24.7

The values of the degree of surface coverage are listedin Table 5. It can be seen that the values of Θ increasedwith an increase in the concentration of 4MBPBF4. It isalso worth to notice that the degree of surface coverage ishigher in case of solutions of pH 4 (Table 5). Using thesevalues of Θ, different adsorption isotherms can be used todeal with the experimental data. The Langmuir adsorptionisotherm [78, 79] was applied to investigate the adsorption of4MBPBF4 on copper surface given by the following equation:

c

Θ= 1

K+ c, (9)

where K is the adsorption equilibrium constant and c is theconcentration of inhibitor.

Figure 8 represents the adsorption plots of 1-Butyl-4-methylpyridinium tetrafluoroborate on copper. It should beexplained that other adsorption ishotherms (Frumkin andTemkin) were checked. The linear correlation coefficient wasused to choose the isotherm that best fits the experimentaldata. It should be noted that the data fits the straight line witha slope nearly equal unity with linear correlation coefficienthigher than 0.999 (Table 6) indicating that these inhibitorsadsorb according to the Langmuir adsorption isotherm.

0

10

20

30

40

50

60

c/Θ

(mM

)

0 10 20 30 40 50 60

c (mM)

(b)

(a)

Figure 8: Adsorption isotherm of 1-Butyl-4-methylpyridiniumtetrafluoroborate on the copper surface in 0.5 M PO3−

4 solutions ofpH: (a) 2 and (b) 4.

The nature of corrosion inhibition has been deduced interms of the adsorption characteristics of the inhibitor.The metal surface in aqueous solution is always coveredwith adsorbed water dipoles. The adsorption of inhibitormolecules from aqueous solution is a quasi-substitutionprocess between the organic compounds in the aqueousphase and water molecules at the electrode surface [80]. TheLangmuir isotherm is based on the assumption that each siteof metal surface holds one adsorbed species:

4MBPBF4(sol) + H2O(ads) ←→ 4MBPBF4(ads) + H2O(sol).(10)

In this situation, the adsorption of of one molecule of4MBPBF4 is accompanied by desorption one molecule ofH2O from the surface of copper. This kind of isotherminvolves the assumption of no interaction between theadsorbed species on the metal surface.

A graph c/Θ against c leads to values of K , as theequilibrium constant of the adsorption process (Figure 8).The free energies of adsorption, ΔG0

ads were calculated fromthe adsorption equilibrium constant using the equation [81]:

ΔG0ads = −RT ln(55.5×K), (11)

where value 55.5 is the molar concentration of water in thesolution.

The adsorption equilibrium constant and the standardfree energy of adsorption of 4BMPBF4 for solutions of pH 2and 4 on copper are presented in Table 6. The values of K arerelatively low, meaning that interactions between 1-Butyl-4-methylpyridinium tetrafluoroborate and the metal surfaceare weaker. The negative values of ΔG0

ads mean that theadsorption of 4MBPBF4 on copper surface is a spontaneousprocess, and indicates the strong interaction between theinhibitor molecules and the copper surface [82].

Generally, values of ΔG0ads around –40 kJ mol−1 or lower

are consistent with the electrostatic interaction between the


Table 7: Thermodynamic adsorption parameters for copper in0.5 M PO3−

4 solutions in the presence of 10.0 mM of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF4) for pH 2 and 4 atdifferent temperatures.

pHTemperature

(K)K×10−2

(M−1)ΔG0

ads

(kJ mol−1)ΔH0

ads

(kJ mol−1)ΔS0

ads

(J mol−1 K−1)

2

303 1.71 −22.69

−16.69

20.14

313 1.39 −22.17 18.40

323 1.12 −21.65 16.65

333 0.93 −21.20 15.15

343 0.79 −20.78 13.74

4

303 2.86 −23.97

−19.27

15.76

313 2.29 −23.41 13.91

323 1.79 −22.80 11.84

333 1.44 −22.26 10.04

343 1.18 −21.78 8.42

charged molecules and the charged metal surface (physisorp-tion) [83–85]. For investigated inhibitor the values of ΔG0

adsequal −23.6 and −24.7 kJ mol−1 for solutions of pH 2 and 4,respectively (Table 6). The results indicate the 4MBPBF4 tobe physically adsorbed on the copper surface. The adsorptionof the inhibitor at the metal surface is the first step in theaction mechanism of inhibitors in aggressive acid media.The adsorption of 1-Butyl-4-methylpyridinium tetrafluo-roborate on the copper surface makes a barrier for mass andcharge transfers. This situation leads to the protection of thecopper surface against the attack of aggressive solution.

3.6. Thermodynamic Adsorption Parameters. Thermody-namically, the free energy of adsorption, ΔG0

ads, is relatedto the standard enthalpy, ΔH0

ads and entropy, ΔS0ads of the

adsorption process as follows [86, 87]:

ΔG0ads = ΔH0

ads − TΔS0ads. (12)

Moreover, the standard enthalpy of adsorption could becalculated according to the Van’t Hoff equation

ln K = −ΔH0ads

RT+ const. (13)

The adsorption equilibrium constant is related to the degreeof surface coverage by:

K = Θ

c(1−Θ). (14)

It should be noted that the K decreases with increasing tem-perature, (Table 7). This confirms earlier made admissionthat the molecules of 4MBPBF4 are physically adsorbed onsurface of copper. However, desorption process of inhibitorenhances with raising of the temperature of the solution. Thefree energies of adsorption of 1-Butyl-4-methylpyridiniumtetrafluoroborate were calculated at different temperatures(11) and are given in Table 7. The values of ΔG0

ads are around−20 kJ mol−1 indicating that the adsorption mechanism

4

4.5

5

5.5

6

lnK

(M−1

)

2.9 3 3.1 3.2 3.3

1000/T (K−1)

(a)

(b)

Figure 9: The Van’t Hoff plots for the copper in 0.5 M PO3−4 solu-

tions containing 10.0 mM of 1-Butyl-4-methylpyridinium tetraflu-oroborate. The pH of solutions was as the following: (a) 2 and (b) 4.

of 4MBPBF4 in 0.5 M PO3−4 solution of pH 2 or 4 is

physisorption at the studied temperatures.A plot ln K versus 1000/T gives of straight lines, as shown

in Figure 9. The slope of the straight line is −ΔH0ads/R. The

values of the standard enthalpy are given also in Table 7.The ΔH0

ads values are negative, for that reason adsorptionof 1-Butyl-4-methylpyridinium tetrafluoroborate moleculesonto the Cu surface is an exothermic process. Moreover, thevalues of ΔH0

ads are less than −40 kJ mol−1 [88], therefore,once again implying that in investigated solutions, physicaladsorption is taking place.

The standard entropy of inhibitor adsorption, ΔS0ads

can be calculated from (12). The values of ΔS0ads, are

recorded in Table 7. The positive values of ΔS0ads mean

that the increase in disordering takes place by going fromreactants to the Cu/solution interface, which is the drivingforce for the adsorption of 1-Butyl-4-methylpyridiniumtetrafluoroborate onto the copper surface [89].

4. Conclusion

The following results can be drawn from this study.

(1) The investigated 1-Butyl-4-methylpyridinium tetra-fluoroborate (4MBPBF4) exhibits inhibiting prop-erties for the corrosion of copper in 0.5 M PO3−

4solutions of pH 2 and 4.

(2) The inhibition efficiency (IE(%)) increased with theincrease in inhibitor concentration but decreases withincreasing temperature. IE at all concentrations of4MBPBF4 followed the order of pH: 4 > 2.

(3) The of 1-Butyl-4-methylpyridinium tetrafluorobo-rate acts as a mixed-type inhibitor, independentlyfrom pH solutions.


(4) The corrosion inhibition action of 1-Butyl-4-methylpyridinium tetrafluoroborate is mainly due toadsorption of 4MBPBF4 on the surface of copper.

(5) The adsorption of the investigated compound obeysthe Langmuir adsorption isotherm.

(6) The thermodynamic functions of corrosion indicatethat of 1-Butyl-4-methylpyridinium tetrafluorobo-rate adsorbs on the copper surface by a physisorp-tion-based mechanism involving a spontaneous andexothermic process.

Acknowledgment

The authors would like to thank Professor M. Hepel fromthe Department of Chemistry SUNY at Potsdam, USA forthe helpful discussions and a critical reading of the paper.

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Research Article

The Effect of Ionic Liquids on the Corrosion Inhibition ofCopper in Acidic Chloride Solutions

M. Scendo and J. Uznanska

Institute of Chemistry, UJK Kielce, ul. Swietokrzyska 15G, 25406 Kielce, Poland

Correspondence should be addressed to M. Scendo, [email protected]

Received 19 August 2010; Accepted 21 December 2010

Academic Editor: Willem J. Quadakkers

Copyright © 2011 M. Scendo and J. Uznanska. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The influence of the concentration of the 1-Butyl-3-methylimidazolium chloride (BMIMCl) and 1-Butyl-3-methylimidazoliumbromide (BMIMBr) as ionic liquids (ILs) on the corrosion inhibition of copper in 1.0 M Cl− solutions of pH 1.0 was studied.The investigation involved electrochemical polarization methods as well as electrochemical quartz crystal microbalance (EQCM)technique and scanning electron microscopy (SEM). The inhibition efficiency increases with an increase in the concentration ofBMIMCl and BMIMBr. Adherent layers of inhibitors were postulated to account for the protective effect. Both of the compoundsact as a mixed-type inhibitor. The values of standard free energy of adsorption suggest the chemical adsorption BMIMCl andBMIMBr on the copper surface.

1. Introduction

Copper and its alloys are used extensively in many kinds ofchemical equipment. Several corrosion inhibitors for copperand its alloys have been known and applied for corrosionprotection. The inhibition properties of triazole, imidazole,and thiazole derivatives in the corrosion of copper have beenstudied [1–4]. Some heterocyclic compounds containing amercapto group have been developed as copper corrosioninhibitors. These compounds include 2-mercaptobenzot-hiazole [5], 2,4-dimercaptopyrimidine [6], 2-amino-5-mer-captothiadzole, 2-mercaptothiazoline [7], potassium ethylxanthate [8–11], and indole and derivatives [12]. Amongthe numerous organic compounds tested and industriallyapplied as corrosion inhibitors, nontoxic are far more strate-gic now than in the recent past. These compounds includesuch amino acids [13–15] and derivatives as cysteine [16].

In the past two decades, the research in the field of greencorrosion inhibitors has been addressed toward the goal ofusing cheap effective molecules at low or zero environmentalimpact. These compounds include purine and adenine,which have been tested for copper corrosion in chloride[17, 18], sulfate [19], and nitrate solutions [20].

Ionic liquids (ILs) are molten salts with melting points ator below ambient room temperature, which are composedof organic cations and various anions. Ionic liquids possessa large number of physicochemical properties [21–24],mainly, good electrical conductivity, solvent transport, and arelatively wide electrochemical window [24]. Configurationof ILs consists of an amphiphilic group with a long chain,hydrophobic tail, and a hydrophilic polar head. Usually, ILshave nitrogen, sulphur, and phosphorus as the central atomsof cations, such as imidazolium, pyrrolidinium, quaternaryammonium, pyridinium, piperidinium, sulfonium andquaternary phosphonium. Currently, functionalized IL is avery noticable topic in the field of IL research. Introducingdifferent functional groups into cations, which provides agreat deal of ILs with new structures, can markedly changethe physicochemical properties of ILs, and it also affordsmore choices for applications of ILs in electrochemicaldevices.

Imidazolium compounds are reported to show corrosion-resistant behavior on mild steel [25], copper [26, 27], andaluminium [28]. It was found that the action of suchinhibitors depends on the specific interaction between thefunctional groups and the metal surface, due to the presence


1

3

2H

H

H

H

H

HH

HH

H

H

HHH

+

C

C

C

C

C

C

C

CN

N

H

Cl−

(a)

H

H

H

H

H

H

HH

H

H

H

H

HH2

3

1

H

+

C

C

C

C

CC

C

N

N

Br−

(b)

Figure 1: Molecular structures ionic liquids: (a) 1-Butyl-3-methylimidazolium chloride (BMIMCl), (b) 1-Butyl-3-methylimidazoliumbromide (BMIMBr).

4003002001000−100−200−300


0.01

0.1

1

10

100

1000

5000

j(μ

Acm

−2)

(a)

(b)

(c)

(d)

Figure 2: Some chosen polarization curves of copper in 1.0 MCl− solutions containing different concentrations of 1-Butyl-3-methylimidazolium bromide: (a) 0, (b) 1.0, (c) 10.0, and (d)50.0 mM, pH 1.0. The scan rate of 1 mV s−1. Arrows indicatepotential for chronoamperometric measurements.

of the –C=N– group and electronegative nitrogen in themolecule. Ionic liquids and different types of surfactantsbase inhibitors are well known to have a high activityin acid medium [29, 30] and therefore are used in oilfield to minimize carbon dioxide-induced corrosion [31,32]. Among many kinds of functionalized ionic liquidsether-functionalized ILs have been investigated intensively,and ether groups have been successfully introduced intoimidazolium cations [33–39]. Shi et al. [40] have synthesized

5010510.1

c (mM)

0

10

20

30

40

50

60

70

80

90

100

η(%

)

BMIMCIBMIMBr

Figure 3: Inhibition efficiency of copper in 1.0 M Cl− solutions withdifferent concentrations of 1-Butyl-3-methylimidazolium chlorideand 1-Butyl-3-methylimidazolium bromide.

a series of new imidazolium ionic liquids with highly purenaphthenic acids and investigated the relationship betweenthe alkyl connecting with N3 of imidazolium ring andcorrosion inhibition performance in acidic solution. Theinhibition efficiency (η) was found to increase with increas-ing the carbon chain length of the alkyl connecting with N3

of imidazolium ring. However, no substantial informationis available on imidazolium ionic liquids using as corrosioninhibitors in acidic chloride solutions.


1801501209060300

t (s)

−500

−400

−300

−200

−100

0−100

400

900

1400

1900

2400

j(μ

Acm

−2)

(a)

(b)

(e)

(c)

(d)

E = 350 mV

E = 100 mV

E = −250 mV

Figure 4: Chronoamperometric curves for copper in 1.0 M Cl−

solutions: (a) and (d) without, (b), (c) and (e) with 50.0 mM 1-Butyl-3-methylimidazolium chloride. Potential electrode: (a), (b)+350, (c), (d) −250, and (e) +100 mV.

4002000−200−400−600−800−1000


0

0.2

0.4

0.6

0.8

1

1.2

1.4

Δm

(μg

cm−2

)

(a)

(b)

1

3

2

Figure 5: Mass change of copper electrode with potential in1.0 M Cl− solutions: (a) without and (b) with 50.0 mM 1-Butyl-3-methylimidazolium bromide. The scan rate of 1 mV s−1. Arrowsindicate of direction of potential scanning.

The present work describes a study of the corrosion ofcopper in 1.0 M Cl− solutions of pH 1.0 without and withdifferent concentrations of 1-Butyl-3-methylimidazoliumchloride (BMIMCl) or 1-Butyl-3-methylimidazolium bro-mide (BMIMBr), based on copper stationary disk electrode(SDE) voltammetry measurements as well as quartz crystalmicrobalance (EQCM) and scanning electron microscopy(SEM).

2. Experimental

2.1. Solutions. 1-Butyl-3-methylimidazolium chloride,C8H15ClN2 (BMIMCl) and 1-Butyl-3-methylimidazoliumbromide, C8H15BrN2 (BMIMBr) (>99.8%) were purchasedfrom Fluka. The molecular structures of compounds areshown in Figure 1. It is worth to notice that both BMIMCland BMIMBr are not flat molecules. BMIMCl and BMIMBrare stable in air, water, and in the majority of organicsolvents. Both compounds are enough well solvable in water.The solutions were prepared using analytical grade reagentsand bidistilled water (resistivity 12 MΩ cm). BMIMCl andBMIMBr were dissolved at concentrations in the range of0.1–50.0 mM. All studied solutions contained 1.0 M Cl− ofpH 1.0. The solutions were prepared through the mixing upof suitable quantities of NaCl and HCl for all experimentswere used a naturally aerated solutions.

2.2. Electrochemical Measurements. Electrochemical experi-ments were carried out in a classical three-electrode glasscell. The cell was open to air. The working electrode (W) washome made stationary disk electrode of the Specpure copper(Johnson Matthey Chemicals Ltd.) with r = 0.240 cm, A =0.181 cm2. Prior to each experiment, theW was mechanicallyabraded to mirror gloss using in this aim 1000 and 2000grade emery papers. Then electrode was washed several timeson change bidistilled water and ethanol. Finally, SDE wasdried using a stream of air. Such pretreatment of the disk wasrepeated after each voltammetric measurement. Other detailswere published in [12, 41–44]. All the surface area-dependentvalues are normalized with respect to the geometric surfacearea of the working electrode.

Electrode potentials were measured and reported againstthe external saturated calomel electrode with NaCl solution(SCE(NaCl)) coupled to a fine Luggin capillary. To minimizethe ohmic contribution, the capillary was kept close tothe working electrode. A platinum (purity 99.99%) wirewas used as an auxiliary electrode. Auxiliary electrode wasindividually isolated from the test solution by glass frit.

All voltammetric experiments were performed using aModel EA9C electrochemical analyzer, controlled via Pen-tium computer using the software Eagraph v. 4.0. Thepolarization curves were obtained using the linear potentialsweep (LSV) technique. Before each run, the clean copperSDE was quickly inserted into the solution and immediatelycathodically polarized at –800 mV (SCE(NaCl)) for 3 minto reduce any oxide on the copper surface. The scan startedfrom the cathodic to the anodic direction with the scan rateof 1 mV s−1.

The chronoamperometric curves were recorded at thedifferent potentials electrode in solutions without and withinhibitors. The potentials applied for the copper electrodewere chosen on basis of polarization curves.

During the measurements, the solution was not stirred.Electrochemical experiments were repeated many times,until reproducible curves were received.

2.3. Electrochemical Quartz Crystal Microbalance. Electro-chemical quartz crystal microbalance (EQCM) experiments


(a) (b)

(c) (d)

30 kV 750x 10μm

(e)

Figure 6: SEM micrographs of the surface of copper: (a) before, (b) after being immersed for 24 hours in 1.0 M Cl− pH 1.0, (c) corrosivesolution contained additionally 50.0 mM of 1-Butyl-3-methylimidazolium chloride and after the removal of the inhibiting film: (d) 1-Butyl-3-methylimidazolium chloride, (e) 1-Butyl-3-methylimidazolium bromide (magnification 750x).

were performed with an apparatus constructed in theInstitute of Physical Chemistry, Warsaw. The quartz crystalhad a geometric area of 0.432 cm2 and was operated atthe fundamental frequency of 5 MHz (refers to air). Thesensitivity of the EQCM can be a few nanograms per squarecentimeter, which makes it an ideal equipment for corrosionstudies. Other details of the EQCM system used in thisstudy were similar to those previously described [11, 12,44].

Copper was galvanostatically deposited onto one surfaceof the crystal (resonator). The freshly deposited copperelectrode was thoroughly washed with bidistilled water.Then the aggressive solution was immediately added to thecell.

2.4. Scanning Electron Microscope. A scanning electronmicroscope PHILIPS (XL 30) was used to study the morphol-ogy of the copper surface in the absence and presence of theinhibitor. Samples were attached on the top of an aluminumstopper by means of 3 M carbon conductive adhesive tape(SPI).

All experiments were carried out at 25.0± 0.2◦C using anair thermostat with the forced air circulation.


3.1. Polarization Behaviour of Copper. The effect of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimida-zolium bromide on the corrosion reactions of copper wasdetermined by polarization measurements. Figure 2 showsan example of polarization curves for the copper electrodein 1.0 M Cl− solutions of pH 1.0 without and with differentconcentrations of BMIMBr. Similar curves were recorded forBMIMCl.

Regarding the mechanism of the oxygen reduction reac-tion on copper in acidic solutions a lot of work has beendone [45–48]. The cathodic polarization curve (a) may beattributed to the diffusion-controlled reduction of oxygen.It is also worth emphasizing that the curve (a) includes twofaintly visible waves, which do not appear for deoxidizedsolutions [17, 18]. The cathodic global reaction in an aeratedacidic aqueous solution could be described as follow [49–51]:

O2 + 4H+ + 4e− ←→ 2H2O. (1a)

However, the first cathodic wave is attributed to the reaction

O2 + 2H+ + 2e− ←→ H2O2. (1b)


6050403020100

c (mM)

0

10

20

30

40

50

60

(c/Θ

)(m

M)

(a)

(b)

Figure 7: Langmuir’s adsorption plots for copper in 1.0 MCl− solutions containing different concentrations of inhibitors:(a) 1-Butyl-3-methylimidazolium chloride and (b) 1-Butyl-3-methylimidazolium bromide.

In the more negative potential of the electrode surface thenext reaction occurs:

H2O2 + 2H+ + 2e− ←→ 2H2O. (1c)

Furthermore, the reaction (1a) is strongly influenced bypotential [50]. The cathodic parts, curves (b)–(d), show thatin the presence of 1-Butyl-3-methylimidazolium bromide(similarly as in the case of 1-Butyl-3-methylimidazoliumchloride) the cathodic currents decrease as the BMIMCl orBMIMBr concentrations increase. However, the curves (b)–(d) split up into two waves, which correspond to reactions(1b) and (1c). As it is discussed below, this shoulder mayconcern to the adsorption of the BMIMCl or BMIMBron copper surface. After crossing potential at about the−300 mV, (SCE(NaCl)) was observed of the growth ofdensity of current as a result of the formation of gas hydrogenon the electrode surface.

The dissolution process of copper (anodic corrosionreaction) at low overpotentials runs according to the follow-ing steps [15, 52]:

Cu− e− ←→ Cu+ads (2a)

Cu+ads − e− ←→ Cu2+, (2b)

where the Cu+ads is an adsorbed monovalent species of

copper on the electrode surface.In corrosive medium in presence of complexing ions such

as Cl− the dissolution process of Cu proceeds via a two-stepreaction mechanism [17, 18]. During the first step, copperis ionized under the influence of Cl− ion, yielding CuCladsorbed at the electrode

Cu + Cl− − e− ←→ CuClads. (3)

This adsorbed compound dissolves by combining withanother Cl− ion according to reactions

CuClads + Cl− − e− ←→ CuCl2,sol (4a)

or

CuClads + Cl− ←→ CuCl−2,sol. (4b)

Products as a result of reactions (4a) and (4b) move to bulk ofsolution. However, in 1.0 M, the Cl− concentration range ofCuCl−2 is the dominant cuprous species [10–12, 53], while athigher concentrations the reaction is proportional to [Cl−]

x,

where x > 2 [54].Figure 2 shows that the cathodic and anodic currents

decrease with the increase of the concentration of BMIMBr(curves (b)–(d)). Probably the protonated 1-Butyl-3-methylimidazolium chloride [BMIMClH+] and 1-Butyl-3-methylimidazolium bromide [BMIMBrH+] molecules areelectrostatically adsorbed on the cathodic sites of Cu. Cationsof ILs are large; in addition, the alkyl chain covers a wide partof the copper surface [55]. However, the hydrophobic chainmay be oriented horizontally or vertically to the electrodeplane. The adsorption or desorption of Cl− and Br− ionsoccurs on the anodic sites. The adsorbed species such asCuClads interact with the cations of ILs to form molecularlayers as a complex on the copper surface [56]. This indicatesthat the addition of BMIMCl and BMIMBr affects both thecathodic and anodic reactions; therefore, the compounds actas mixed-type inhibitors.

3.1.1. Corrosion Parameters. Electrochemical corrosionkinetic parameters were calculated on the basis of cathodicand anodic potential versus current characteristics in theTafel potential region (Figure 2). The corrosion parameterssuch as corrosion potential (Ecorr), corrosion current density( jcorr), cathodic (bc), and anodic (ba) Tafel slope are listedin Table 1. It is worth noticing that no definite trend wasobserved in the shift of the Ecorr values in the presence ofvarious concentrations of 1-Butyl-3-methylimidazoliumchloride and 1-Butyl-3-methylimidazolium bromide. Thisconfirms earlier advanced admission that both inhibitorsbelong to mixed-type inhibitors. The current density(Table 1) decreased when the concentrations of BMIMCland BMIMBr were increased. This indicates the inhibitingeffect of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide. An increase incathodic (bc) and a decrease in anodic (ba) Tafel slopes(Table 1) indicated a mixed cathodic and anodic effectof the inhibition on the copper corrosion mechanism.Moreover, these inhibitors cause small change in thecathodic and anodic Tafel slopes, indicating that BMIMCland BMIMBr are first adsorbed onto copper surface andtherefore impeded by merely blocking the reaction sites ofcopper surface without affecting the cathodic and anodicreaction mechanism [57]. Generally speaking, the inhibitormolecule blocks the active corrosion sites on the coppersurface.


Charge

Z

X

Y

N3

N1

−0.316

0.416 −0.217

−0.2090.407

2

4

5

(a)

HOMO

(b)

LUMO

(c)

Figure 8: Molecular structure of 1-Butyl-3-methylimidazolium ion (the charges density distribution on nitrogen: N1, N3 and carbon: C2,C4, C5 atoms), and molecular orbital density distribution of BMIM+; highest occupied molecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO).

3.1.2. Inhibition Efficiency. The inhibition efficiency (η) canalso be calculated from polarization tests by using thefollowing equation [58, 59]:

η (%) =(jo − jcorr

jo

)100, (5)

where jo and jcorr are the corrosion current densities in theabsence and presence of inhibitor, respectively.

The inhibition efficiency depends on both the nature andthe concentration of the investigated compounds. The cal-culated inhibition efficiencies are presented in Figure 3.In the presence of 1-Butyl-3-methylimidazolium chlorideand 1-Butyl-3-methylimidazolium bromide, the inhibitionefficiency increases with an increase in for the concentrationof inhibitors. However, for concentration 50.0 mM bothinhibitors η are the highest for 1-Butyl-3-methylimidazolium

chloride. It is obvious that for higher concentration of 1-Butyl-3-methylimidazolium bromide we will get a film whichconsiderably better protects the surface of copper.

3.1.3. Corrosion Rate. The corrosion current density ( jcorr)was converted into the corrosion rate (kr) by using theexpression [60]

kr

(mmyear

)= 3.268× 10−3

(jcorrMCu

nρ

), (6)

where the corrosion current density ( jcorr) should be inμA cm−2, MCu is the molecular weight of copper, n is thenumber of electrons transferred in the corrosion reaction,and ρ is the density of Cu (g cm−3).

The values of the copper corrosion rate in the absenceand presence of inhibitors were calculated from (6) and


Hydrophobictail

Hydrophilicpolar head

N3 N3 N3

N1 N1 N1

Cu

+ + +

Figure 9: Schematic illustration of the mechanism of corrosionprotection of copper by BMIMX (where X is the Cl− or Br− ions).

Table 1: Corrosion parameters of copper in 1.0 M Cl− solutionsin the absence or presence of different concentrations of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidzaoliumbromide, pH 1.0.

InhibitorConcentration(mM)

Ecorr

(mV)jcorr

(μA cm−2)−bc ba

(mV dec−1)

without 0 149 18 50 130

BMIMCl

0.1 162 14 55 126

1.0 125 8 65 120

5.0 183 5 70 114

10.0 207 2 75 110

50.0 165 0.8 85 100

BMIMBr

0.1 197 15 70 115

1.0 247 9 100 90

5.0 248 6 110 80

10.0 224 3 120 70

50.0 249 1.4 140 60

Table 2: Corrosion rate of copper in 1.0 M Cl− solutionsin absence or presence of different concentrations of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazoliumbromide, pH 1.0.

Concentration of BMIMCland BMIMBr (mM)

kr (mm/year)

BMIMCl BMIMBr

0 0.417 0.417

0.1 0.325 0.348

1.0 0.185 0.185

5.0 0.116 0.139

10.0 0.046 0.093

50.0 0.018 0.069

are presented in Table 2. The corrosion rate of copper issignificantly reduced as a result of the reduction in the jcorr.The kr of Cu in 1.0 M Cl− solution without inhibitors isfound to be 0.417 mm/year, which is ≈23 and 6 times lowerin solutions with 50.0 mM 1-Butyl-3-methylimidazolium

Table 3: Values of surface coverage (Θ) of copper in1.0 M Cl− solu-tions for different concentrations of 1-Butyl-3-methylimidazoliumchloride and 1-Butyl-3-methylimidazolium bromide, pH 1.0.

Inhibitor0.1 1.0 5.0 10.0 50.0

(mM)

BMIMCl 0.22 0.56 0.72 0.89 0.96

BMIMBr 0.17 0.56 0.67 0.78 0.83

Table 4: Slope (b), linear correlation coefficient (R2), equilib-rium constant (K), and standard free energies of adsorption(ΔG0

ads) of copper in 1.0 M Cl− solutions for different concen-trations of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide.

Inhibitor b R2 K (M−1) ΔG0ads (kJ mol−1)

BMIMCl 1.0 0.9997 1.0× 103 −27.2

BMIMBr 1.1 0.9999 1.1× 103 −27.4

chloride or 1-Butyl-3-methylimidazolium bromide, respec-tively. Thus, these results reveal the capability especially ofBMIMCl to act as a corrosion protective layer on copper inchloride solutions.

3.2. Chronoamperometric Measurements. The effects of anaddition of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide to the Cl− electrolyteon the behavior of copper were studied by chronoamper-ometry. The chronoamperometric curves in Figure 4 wererecorded at the electrode potential of −250, +100, and+350 mV (SCE(NaCl)) in solutions without (curves (a), (d))and with 50.0 mM of BMIMCl (curves (b), (c), and (e)),pH 1.0. Similar curves were recorded for BMIMBr. Thepotentials applied to the copper electrode were chosen onbasis of polarization curves (Figure 2), and they regarded thecathodic (1b), (1c), and anodic (2a) reactions.

It can be seen that in solutions without inhibitors, curves(a) and (b), a thick film CuClads was formed on the electrodesurface according to the reaction (3). The large currentsuggests that the layer was not compact. However, the filmbreakdown or corrosion products inside it are due to thecontinuous dissolution of copper

CuCl−2,sol

Cu2+ + 2Cl− + e−

Cu+ads + Cl− + CuCl2,sol (7)

In the presence of BMIMCl or BMIMBr, the smallercurrents for reactions (1b), (1c), and (2a) were recorded,especially for BMIMCl (curves (b), (c), and (e)). This mustbe due to the adsorption of BMIMCl or BMIMBr moleculeson the copper surface limiting of the aggressive attack onthe solutions on the metal. In this case, the protectivelayer on the copper surface was the most compact for bothinhibitors.


Table 5: Quantum chemical parameters for BMIM+ calculated by PM3 semiempirical method.

Net atomic charges EHOMO (eV) ELUMO (eV) ΔE (eV)

N1 C2 N3 C4 C5

0.407 −0.316 0.416 −0.217 −0.209 −0.8355 0.6844 1.5199

3.3. Electrochemical Quartz Crystal Microbalance Measure-ments. The electrochemical quartz crystal microbalance(EQCM) has been widely used to observe the response ofmass loading on the surface. The EQCM technique is basedon the piezoelectric effect. The resonant frequency of thequartz crystal lattice vibrations in a thin quartz crystal waferis measured as a function of the mass attributed to the crystalinterfaces [61, 62]. Sauerbrey,s equation is commonly usedto describe the linear relationship between the frequencyshift (Δ f ) and the mass change (Δm). Using the EQCM,the change in surface mass is calculated from the change inresonant frequency according to the equation:

Δm = − Δ f

K/Sp, (8)

where K is the fundamental constant predicted to be56.6 Hz cm2 μg for quartz crystal with the resonant frequencyof ca. 5 MHz, and Sp is the piezoelectrically active area of thequartz crystal covered by the Cu film [63]. It is significantthat a decrease in frequency corresponds to an increase insurface mass. The mass change is reasonably assumed asthe result of metal corrosion as well as the adsorption ofcomponents of studied electrolytes.

The influence of 1-Butyl-3-methylimidazolium chlorideand 1-Butyl-3-methylimidazolium bromide on corrosion ofcopper was additionally tested by EQCM measurements. Theinvestigations were carried out in 1.0 M Cl− without and withthe addition of BMIMCl or BMIMBr of 50.0 mM (pH 1.0)in the presence of air. The results are presented in Figure 5.The curves represent the change of mass (Δm), calculatedfrom the frequency increase/decrease (8). Similar curveswere recorded for BMIMCl. The curve (a) representing thesolution without inhibitors shows an increase in the massof copper electrode from −600 to +120 mV (SCE(NaCl))(Section 2), which may by attributed to the oxide formationfollowing the reactions

2Cu+ +12

O2 + 2e− ←→ Cu2Oads

Cu+2 +

12

O2 + 2e− ←→ CuOads.

(9)

In the range of the potential from +120 to +400 mV,(SCE(NaCl)) (Section 3) was observed to increase, Δm/Sp ofca. 0.22 μg cm−2. However, the copper electrode is partiallycovered by an oxide film, which dissolves slowly. Simulta-neously, the Cl− ions interact with the electrode; therefore,for solutions without inhibitors, Δm increased immediatelybecause of the precipitation of CuClads layer which couldbe formed on the copper surface according to the reaction(3). This effect was observed in numerous studies. In severalinvestigations [64–69], only low concentration of chloride, in

the range from 0.1 to 10.0 mM, was used. In other words [70–72], much greater chloride concentrations were employed,then the protective layer dissolves and passes into thesolutions as the following complexes: CuCl2 and CuCl3

2−. Itis obvious that those conditions were more favorable to theprecipitation of CuClads layer, which could act as a protectivebarrier safeguarding copper from its further oxidation. Theaction of Cu2Oads, CuOads, and CuClads can be physicallyinterpreted as a blockage of the copper surface which afterchanging direction potential on the opposite completelydissolves.

In the next experiment, 1-Butyl-3-methylimidazoliumchloride or 1-Butyl-3-methylimidazolium bromide wereadded to 1.0 M chloride solutions, see Figure 5 curve (b)which concerns BMIMBr. Similar curve was obtained forBMIMCl. In anodic cycle on the curve (b) appear twocharacteristics sections of change of mass in the functionof potential of electrode. After crossing potential –600 mV(SCE(NaCl)), the film on the copper surface was precipitated(Sections 1 and 2). Therefore, it was assumed that BMIMClor BMIMBr molecules are adsorbed directly on the surfaceof copper to form monomolecular layers as a complex on themetal surface. It is clear that the inhibitory action of BMIMCland BMIMBr reduces the surface area available for oxygen.After crossing potential +120 mV (SCE(NaCl)), layer surfaceBMIMX (where X is the Cl− or Br−) becomes more tightas a result of the precipitation of the CuClads and CuBrads

layers on the copper surface (Sections 2 and 3). Moreover,after the change of direction of the potential (+400 mV(SCE(NaCl)), CuClads and CuBrads were dissolved. However,Δm was slightly changed (curve (b)). Probably, precipitatedlayer did not appear on the copper surface. BMIMCl andBMIMBr form adherent layers on the copper surface. Thisleads to the protection of the Cu surface from the attack ofaggressive solutions and oxygen.

It seems to be obvious that 1-Butyl-3-methylimida-zolium chloride or 1-Butyl-3-methylimidazolium bromidemolecules are chemisorbed on the surface of copper. Prob-ably the adsorption of BMIMCl and BMIMBr molecules canoccur through the formation of copper-nitrogen coordinatebond or the π-electron interaction between the aromatic ringand the copper substrate.

3.4. Scanning Electron Microscopy Studies. The surface mor-phology of copper samples immersed in 1.0 M Cl− (pH1.0) for 24 hours in the absence and presence of 50.0 mMof 1-Butyl-3-methylimidazolium chloride or 1-Butyl-3-methylimidazolium bromide was studied by scanning elec-tron microscopy (SEM). The solutions were not degassed.

Figure 6 shows the surface morphology of copper spec-imens (a) before and (b) after being immersed in corrosivesolution. Figure 6(b) reveals that the surface was strongly


damaged in the absence of inhibitors. The presence ofchloride ions and dissolved oxygen contributes to theoxidation of the metal. The copper surface is covered byaggregates of small cubic crystals CuClads and probably Cu2Oor CuO. Figure 6(c) shows SEM image of the surface ofcopper specimens after immersion for the same time intervalin corrosive solution additionally containing 50.0 mM ofBMIMCl. However, in these conditions, the film precipitateson the surface of copper (Figure 6(c)). A similar result wasobtained for BMIMBr. The SEM photographs show thatit does not cover tightly the surface and hence does notprotect the Cu surface to an adequate degree. Chloride ions,oxygen, and water penetrate the protective film throughpores, flaws, or other weak spots what results in furthercorrosion of copper. The results of the action of aggressiveenvironment were visible after the removal of protective layerfrom the surface of copper. It well adhered to surface of themetal, and removal of protective film was difficult enough.In this aim, ultrasonic water baths were used. The samplewas shaken in diluted sulphuric acid and rinsed in propanol.Figure 6 presents samples after the removal of the inhibit-ing film which contained (d) 1-Butyl-3-methylimidazoliumchloride and (e) 1-Butyl-3-methyl-imidazolium bromide.These results indicated that a more tight protective layerwas received for solution with 1-Butyl-3-methylimidazoliumchloride (Figure 6(d)). Moreover, the BMIMCl in chloridesolution was a better inhibitor than the BMIMBr.

3.5. Adsorption Isotherm. It has been assumed that inhibitormolecules establish their inhibition action via the adsorptionof the inhibitor onto the metal surface. The adsorption canbe described by two main types of interaction: physicaladsorption and chemisorption. The adsorption processes ofinhibitors are influenced by the chemical structure of organiccompounds, the nature and surface change of metal, thedistribution of charge in molecule, and the type of aggressivemedia.

Assuming the corrosion inhibition was caused by theadsorption of inhibitor on metal surface, the values of surfacecoverage (Θ) for different concentrations of inhibitors werecalculated on the basis of polarization curves according to theequation:

Θ = 1− jinh

jo. (10)

The values of the surface coverage are listed in Table 3. Itcan be seen that the Θ increased with an increase in theconcentration of BMIMCl and BMIMBr. It is also worthnoticing that at 50.0 mM solution the inhibitors of thesurface coverage are higher for the BMIMCl (Table 3).

Basic information on the interaction between theinhibitors and the metal surface can be provided by theadsorption isotherm. All isotherms can be expressed by

f (Θ, x) exp(−2aΘ) = Kc, (11)

where f (Θ, x) is the configurational factor which dependsupon the physical model and the assumptions underlyingthe derivation of the isotherm [17], x is the size ratio, a is

the molecular interaction parameter, K is the equilibriumconstant of the adsorption process, and c is the inhibitorconcentration.

Figure 7 shows the dependence of the fraction of thesurface covered c/Θ as a function of the concentrationc of 1-Butyl-3-methylimidazolium chloride or 1-Butyl-3-methylimidazolium bromide. It should be explained thatother adsorption isthoterms (Frumkin and Temkin) werealso checked. The linear correlation coefficient was used tochoose the isotherm that fits the best of the experimentaldata. It should be noticed that the data fit the straightline with a slope nearly equal unity with linear correlationcoefficient higher than 0.999 (Table 4) indicates that theseinhibitors adsorb according to the Langmuir adsorptionisotherm [17, 73, 74] given by the following equation:

c

Θ= 1

K+ c, (12)

where K is the adsorption equilibrium constant, related tothe free energy of adsorption, ΔG0

ads

K = 1csolv

exp

(−ΔG0ads

RT

), (13)

where csolv represents molar concentration of the solvent,which in the case of water is 55.5 mol dm−3. The freeenergy of adsorption was calculated from the adsorptionequilibrium constant using the equation [75]

ΔG0ads = −RT ln(55.5K). (14)

The slopes of linear correlation coefficients, equilibriumconstants, and the free energies adsorption of BMIMCl andBMIMBr for copper are presented in Table 4.

The nature of corrosion inhibition has been deducedin terms of the adsorption characteristics of the inhibitor[76]. The metal surface in aqueous solution is always coveredwith adsorbed water dipoles. Therefore, the adsorption ofinhibitor molecules from aqueous solution is a quasisub-stitution process, the thermodynamics of which depend onthe number of water molecules replaced by the inhibitormolecule. The Langmuir isotherm is based on the assump-tion that each site of metal surface holds one adsorbedspecies. This kind of isotherm involves the assumption ofno interaction between the adsorbed species on the metalsurface. The negative values of ΔG0

ads (Table 4) prove thestrong interaction of BMIMCl or BMIMBr molecules on tothe copper surface [58, 77].

Generally, values ΔG0ads around –20 kJ mol−1 or lower are

consistent with the electrostatic interaction between thecharged molecules and the charged metal surface (physisorp-tion). Those more negative than –40 kJ mol−1 involve chargesharing or transfer from the inhibitor molecules to the metalto form a coordinate type of bond (chemisorption) [78–80].For investigated ionic liquids inhibitor, it can be concludedthat calculated ΔG0

ads values equal –27.2 and –27.4 kJ mol−1

for BMIMCl and BMIMBr, respectively (Table 4). However,the free energies of adsorption are situated in range from−20 to −40 kJ mol−1; therefore, it can be accepted with


a large probability, that the adsorption mechanisms ofBMIMCl and BMIMBr in 1.0 M Cl− solution involve first ofall chemical adsorption on the copper surface with negligibleparticipation physisorption.

The adsorption of the inhibitor on the metal surfaceis the first step in the action mechanism of inhibitors inaggressive acid media.The adsorption of 1-Butyl-3-methyl-imidazolium chloride or 1-Butyl-3-methylimidazolium bro-mide on the copper surface makes a barrier for mass andcharge transfers. This situation leads to the protection of thecopper surface from the attack of aggressive solutions. Theorientation of the BMIMX molecules in the adsorbed stateon the copper surface could be resolved by the application ofquantum chemical calculations.

3.6. Quantum Chemical Calculations. Quantum chemicalcalculations have been widely used to study the reactionmechanisms and to interpret the experimental results as wellas to solve chemical ambiguities. The structure and electronicparameters can be obtained by means of theoretical calcu-lations using the computational methodologies of quantumchemistry [81]. The geometry of the inhibitor in its groundstate as well as the nature of their molecular orbitals, highestoccupied molecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO), are involved in the properties ofthe activity of inhibitors. The HOMO is the orbital thatcould act as an electron donor, since it is the outermostorbital containing electrons. The LUMO is the orbital thatcould act as the electron acceptor, since it is the innermostorbital which has place to accept electrons. Highest occupiedmolecular orbital energy (EHOMO) and lowest unoccupiedmolecular orbital energy (ELUMO) are very popular quantumchemical parameters. These orbitals, also called the frontierorbitals, determine the way the molecule interacts with otherspecies. It should be noted that the EHOMO is often associatedwith the electron donating ability of the molecule, whereasELUMO indicates its ability to accept electrons. The energyof the HOMO is directly related to the ionization potential,and the energy of the LUMO is directly related to theelectron affinity. The calculated difference (ΔE = ELUMO −EHOMO) demonstrates inherent electron donating ability andmeasures the interaction of the inhibitor molecules withmetal surface. A large energy gap (ΔE) implies high stabilityfor the molecule in chemical reactions [82].

In current practice semiempirical methods serve asefficient computational tools which can yield fast quanti-tative estimates for a number of properties. The PM3 issemiempirical method based on the neglected differentialdiatomic overlap integral approximation. However, the PM3method uses the same formalism and equations as the AM1method [83].

The quantum chemical parameters for MBIM+ were ob-tained by PM3 method and listed in Table 5. More-over, Figure 8 shows the molecular structure of 1-Butyl-3-methylimidazolium ion, the charge density distribution (δ)for nitrogen and carbon atoms, and molecular orbital densitydistribution, highest occupied molecular orbital (HOMO)and lowest unoccupied molecular orbital (LUMO). The

binding capability of a molecule with metal depends also onthe electronic charge on the chelating atom. From the dataon the electron density of BMIM+, it can be concluded that

(i) the largest positive charge (δ+) is localized at the N1

and N3 atoms;

(ii) the negative charge (δ−) localized at the C4, C3, andC5 atoms. However, the δ−C5

charge is smaller than δ−C4

and δ−C2;

(iii) the BMIM+ interacts perhaps with the surface Cuelectrode loaded negatively with nitrogen atoms andpositively with carbon atoms. It is obvious thatthe interaction with the surface of the electrode isstronger through nitrogen atoms.

The higher the EHOMO (Table 5) of the inhibitor, thegreater the tendency of offering electrons to unoccupied dorbital of the Cu and the higher the corrosion inhibitionefficiency for copper in Cl− acid solutions (Figure 3). Inaddition, the higher the ELUMO, the more difficult theacceptance of electron from the metal surface. However,the LUMO-HOMO energy gap admits low values, what iseffective with this is that the inhibition efficiency of inhibitorswas improved. Figure 8 shows that the highest occupiedmolecular orbital (HOMO) is mainly constituted by N1andN3 atoms, and the lowest unoccupied molecular orbital(LUMO) is constituted by C2, C4, and C5 atoms, whichindicate that N1 and N3 can provide electrons, and C2, C4,and C5 can accept electrons.

Four types of adsorption may take place involvingBMIMX molecules on the copper-solution interface:

(i) electrostatic attraction between charged moleculesand the charged metal,

(ii) interaction of unshared electron pairs in the moleculewith the metal,

(iii) interaction of π-electrons with the metal,

(iv) a combination of the above.

Frontier orbital electron densities on atoms provide auseful means for the detailed characterization of donor-acceptor interactions. Figure 9 shows schematic illustrationof mechanism of corrosion protection of copper by BMIMClor BMIMBr. The mechanism involves the formation ofprotective monolayer inhibitors which protect copper beforecorrosion.

4. Conclusion

The following results can be drawn from this study:

(1) the investigated ionic liquids (ILs): 1-Butyl-3-meth-ylimidazolium chloride (BMIMCl) and 1-Butyl-3-methylimidazolium bromide (BMIMBr) exhibitinhibiting properties for the corrosion of copper in1.0 M Cl− solutions of pH 1.0,

(2) the inhibition efficiency increased with the increasein ILs concentration, followed the order: BMIMCl >BMIMBr;


(3) the adsorption of the investigated compoundswas confirmed to follow the Langmuir adsorptionisotherm;

(4) the values of standard free energies of adsorptionsuggest mainly the chemical adsorption of BMIMCland BMIMBr on the copper surface;

(5) the quantum chemistry calculation results show thatthe imidazoline ring and heteroatoms are the activesites of the both inhibitors;

(6) the inhibition effect is due to the formation ofan insoluble stable film through the process ofadsorption of the inhibitor molecules on the coppersurface.

Acknowledgments

One of the authors would like to thank Professor M. Hepelfrom the Department of Chemistry SUNY at Potsdam, USA,A. Skrobisz, and his wife Heni for helpful discussions and acritical reading of the paper.

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