research article open access corrosion inhibition properties … · tween the inhibitors and the...

Ebadi et al. Chemistry Central Journal 2012, 6:163http://journal.chemistrycentral.com/content/6/1/163

RESEARCH ARTICLE Open Access

Corrosion inhibition properties ofpyrazolylindolenine compounds on coppersurface in acidic mediaMehdi Ebadi1,2*, Wan Jeffrey Basirun1,3, Hamid Khaledi1 and Hapipah Mohd Ali1

Abstract

Background: The corrosion inhibition performance of pyrazolylindolenine compounds, namely 4-(3,3-dimethyl-3H-indol-2-yl)-pyrazole-1-carbothioamide (InPzTAm), 4-(3,3-dimethyl-3H-indol-2-yl)-1H-pyrazole-1-carbothiohydrazide(InPzTH) and 3,3-dimethyl-2-(1-phenyl-1H-pyrazol-4-yl)-3H-indole (InPzPh),) on copper in 1M HCl solution isinvestigated by electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and linear scanvoltammetry (LSV) techniques.

Results: The results show that the corrosion rate of copper is diminished by the compounds with the inhibitionstrength in the order of: InPzTAm> InPzTH > InPzPh. The corrosion inhibition efficiencies for the three inhibitors are94.0, 91.4 and 79.3, for InPzTAm, InPzTH and InPzPh respectively with the same inhibitor concentration (2 mM).

Conclusion: From the EIS, OCP and LSV results it was concluded that pyrazolylindolenine compounds with S-atom (withan amine group) have illustrated better corrosion inhibition performance compared to hydrazine and phenyl group.

Keywords: Corrosion behaviour, Copper, Electro-corrosion, FESEM, Acid inhibition

BackgroundCorrosion reactions are thermodynamically favourable onless noble metals and alloys when exposed to a corrosiveenvironment such as chloridric acid. The inhibition ofthese reactions can be controlled by many types of organicand inorganic compounds [1-4], but organic compoundsare the more common type of corrosion inhibitors. Mostorganic compounds which are efficient corrosion inhibi-tors contain functional groups which incorporate phos-phorus, oxygen, nitrogen, sulfur atoms and multiplebonds [5,6]. The action of these inhibitors are closelyrelated to factors such as: the types of functional groups,the number and type of adsorption sites, the charge distri-bution in the molecules and the type of interaction be-tween the inhibitors and the metal surface [7]. A largenumber of organic compounds have been investigated ascorrosion inhibitors for different types of metals [8-10].

* Correspondence: [email protected] of Chemistry, Faculty of Science, University of Malaya, 50603,Kuala Lumpur, Malaysia2Department of Chemistry, Faculty of Science, Islamic Azad University-Gorgan Branch, Gorgan, IranFull list of author information is available at the end of the article

© 2012 Ebadi et al.; licensee Chemistry CentraCommons Attribution License (http://creativereproduction in any medium, provided the or

With increased awareness towards environmental pollu-tion and control, the search for less toxic and environmentfriendly corrosion inhibitors are becoming increasinglyimportant.We have recently reported the synthesis and the struc-

tures of new pyrazolylindolenine compounds which are4-(3,3-dimethyl-3H-indol-2-yl)-pyrazole-1-carbothioamide(InPzTAm), 4-(3,3-dimethyl-3H-indol-2-yl)-1H-pyrazole-1-carbothiohydrazi (InPzTH) and 3, 3-dimethyl-2-(1-phenyl-1H-pyrazol-4-yl)-3H-indole (InPzPh) [11]. The corrosioninhibition properties of these compounds on copper sur-face in hydrochloric acid media are investigated using elec-trochemical impedance spectroscopy (EIS), open circuitpotential (OCP), and field emission scanning electron mi-croscopy (FESEM) techniques. The chemical structures ofthe compounds are shown in Figure 1.

ExperimentalPreparation of the pyrazolylindolenine compoundsThe three pyrazolylindolenine compounds were synthe-sised through the reaction of 2-(diformylmethylidene)-3,3-dimethylindole with thiosemicarbazide (for InPzTAm),with thiocarbohydrazide (for InPzTH) and with phenylhy-

l Ltd. This is an Open Access article distributed under the terms of the Creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andiginal work is properly cited.

mailto:[email protected]

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Figure 1 Molecular structure of the pyrazolylindolenine compounds.

Ebadi et al. Chemistry Central Journal 2012, 6:163 of 10http://journal.chemistrycentral.com/content/6/1/163

drazide (for InPzPh). The details of the synthetic andcharacterization methods are described elsewhere [11].

Corrosion characterization techniquesAll experiments were done using Cu plates of 99.9% purity(Goodfellow Cambridge) in 1 M HCl aqueous media con-taining various amount of pyrazolylindolenine com-pounds: InPzTAm (0–2.5 mM l-1), InPzTH (0–2.5 mM l-1)and InPzPh (0–4 mM l-1). The Cu plates (0.3× 1× 1 cm)were polished with emery paper (2000 grit), rinsed in dis-tilled water and ultrasound in acetone to remove oilystains. A three-compartment cell was used in the corro-sion measurements with a saturated calomel electrode(SCE) as the reference electrode and Cu plate and plat-inum wire with the same surface area as the working andcounter electrodes respectively. Frequency response ana-lysis (FRA) software was used in the EIS experimental andsimulation process, while general purpose electrochemicalsoftware (GPES) was used in the linear scan voltammetry(LSV, Tafel) and open circuit potential (OCP) techniques.The software was installed in a computer interfaced withan Autolab (302 N) potentiostat/galvanostat instrument.The scan rate for LSV was 10 mV s-1, while the potentialrage was between 0.3 V to −0.4 V). The EIS measurementswere carried out at the OCP value with a frequency do-main of 100 kHz-10 mHz with an amplitude of 5 mV.Prior to all analysis (EIS and LSV) the Cu plates wereimmersed in the corrosive solution (1 M HCl) containingdifferent amount of inhibitors for 1 hr to obtain the OCPvalue. A JEOL JSM-840A field emission scanning electronmicroscopy (FESEM) instrument was used to capture theimages of the copper surface after immersion in the corro-sive solution for 1 week.

Results and discussionsElectrochemical impedance spectroscopyThe Nyquist and Bode plots are used to study the corro-sion inhibition performance of the pyrazolylindoleninecompounds. The Nyquist plots for the copper plates incorrosive media (1 M HCl) in the absence and presenceof different amounts of the pyrazolylindolenine com-pounds are shown in Figure 2 (a, b and c). In the Nyquistplot, the diameter of the semicircles represents the chargetransfer resistance (Rct) between the outer Helmholtzplane (OHP) and the electrode surface [12], and can bedetermined from the difference between the lower and

higher frequency intercepts on the real axis Zr [13]. Withthe correct choice of the equivalent circuits, the closest fitto the experimental data can be done by simulation wherethe chi-squared value is minimised to 10-4 (Figure 2a-c).The solution resistance (Rs) is the resistance between theworking electrode (WE) and the counter (CE) and refer-ence electrode (RE). The ability of the Helmholtz plane toaccumulate charges can be defined as the double layercapacitance (Cdl). The constant phase element (CPE) in-stead of Cdl, is introduced in the simulation of the experi-mental data to obtain a more accurate fit. The impedanceof the CPE (ZCPE) is a function of angular frequency(ω=2πf ) and the magnitude of “Q” and “n”. Furthermore,Rf is the resistance of the inhibition film on the coppersurface. Instead of the capacitance of inhibition film (C), aconstant phase element (CPE, denoted as Qf in the cir-cuit) is introduced and it forms the first parallel combin-ation. The impedance of CPE can be written as [14,15]:

ZCPE ¼ Q�1 jωð Þ�n ð1ÞThe conversion of Q to Cdl is given by the following

equation [16]:

Cdl ¼ Q ωmaxð Þn�1 ð2Þwhere ωmax is the angular frequency when Z imaginary(Zi) is maximum and n is the phase shift and is thedegree of surface in-homogeneity. The value of “n” isalso related to the slope of the log ℜ Z ℜ vs. log f in theBode plot, when n=1, then Q = Cdl.Figure 2 (a, b and c) shows that the semicircle diam-

eter (therefore the charge transfer resistance, Rct)increases with the increase of the inhibitor concentra-tion. The inhibition efficiency of the compounds are cal-culated and tabulated in Table 1.

η% ¼ Rct:ðinhib:Þ � Rct: blankð ÞRct: inhibð Þ

� 100 ð3Þ

where Rct.(inhib.) and Rct.(blank) are the charge transfer re-sistance in the presence and absence of the inhibitors re-spectively [17]. From Table 1 unlike the Rct, the change ofthe Cdl is not consistent with the increase of the inhibitorconcentration, but is a function of the double layer thick-ness (δ). Therefore, the Cdl is affected by the changes inthe double layer thickness (δ) [17], where Cdl = (εε0/δ),where ε is the dielectric constant of the protective layer

Figure 2 The Nyquist plots (vs. SCE) for copper plates immersed in various amounts of pyrazolylindolenine compounds in 1 M HCl.The symbols represent experimental data while the continuous line represent simulation.


Table 1 EIS simulation results of copper surface in 1 M HCl with the presence of different inhibitor concentration

Elements→ Inhib.↓ Inhi. Conc. (mM l-1) Rs (Ω cm2) Rct (kΩ cm2) n Q mS.sn. cm-2 Cdl (μF cm-2) W mS.s1/2. cm-2

Blank 0.0 19.21 0.298 0.7142 7.35×10-3 149.3 4.05×10-4

InPzPh 1.0 357 2.044 0.7448 1.44×10-4 11.90 5.88×10-4

2.0 27.2 2.562 0.8910 4.32×10-5 14.63 5.23×10-3

3.0 59.2 2.923 0.9132 4.41×10-5 18.31 5.26×10-3

4.0 28 4.32 0.9104 1.44×10-4 57.25 5.06×10-3

InPzTH 2.5 31.17 35.92 0.6573 1.55×10-4 2.27

2.0 36.2 21.14 0.6762 2.77×10-4 5.65

1.5 18.92 11.56 0.7060 4.05×10-4 15.49

1.0 27.23 1.8 0.852 4.83×10-4 17.31

InPzTAm 2.0 30.40 174.5 0.8263 2.62×10-6 0.24

1.5 24.23 143.5 0.8782 1.18×10-6 0.23

1.0 17.39 68.7 0.8060 3.13×10-6 0.39

0.5 17.48 56.5 0.8089 3.03×10-6 0.27


and ε0 the permeability of free space respectively. Khaled[18] has reported that the charge resistance of coppersurface was increased from 0.58 (bare copper) to 11.77kΩ/cm2 when N-(5,6-diphenyl-4,5-dihydro-[1,2,4] tria-zin-3-yl)-guanidine (NTG) (0.01 M) was added to H2SO4

(0.5 M) media. It was found that the synthesized com-pounds (specifically InPzTAm) has shown good corrosioninhibition efficiency on copper surface compared to pre-vious results by other researchers [19-22].

Potentiodynamic measurements and thermodynamiccalculationsPotentio-thermodynamic polarization is done on copperplates in 1.0 M HCl at room temperature containing vari-ous amounts of the inhibitor compounds (InPzTAm: 0.5-2.5 mM, InPzTH:1–2.5 mM and InPzPh: 1–4 mM), andis shown in Figure 3 (a, c and e). The electrochemicalparameters such as corrosion rate (CR), corrosion poten-tial (Ecorr.), corrosion current (Icorr..), Tafel slopes and theinhibition efficiency (E%) are obtained from the potentio-dynamic measurements and given in Table 2. Li et al.[23] and Ferreira et al. [24] have suggested that a cath-odic and an anodic type inhibitor will show Ecorr. shift ofmore than 85 mV with respect to the Ecorr. without theinhibitor, and a mixed type of inhibitor will show a shiftof less than 85 mV.In this work the displacement range of Ecorr. for

InPzTAm (Figure 3a) is 15.9-62.9 mV towards the anodicregion, whereas InPzTH shows a 32.8-7.3 mV displace-ment to the anodic region (Figure 3c). Figure 3e showsthat the Tafel curve has a small shift to the cathodic zone(< 85 mV) with the increase of InPzPh (54.4-5.2 mV).Therefore it can be concluded that the pyrazolylindole-nine inhibitors (InPzTAm, InPzTH and InPzPh) showmixed type behaviour. From Figure 3a, the InPzTAm

concentration of 2.5 mM shows maximum inhibition effi-ciency. The corrosion rate of the copper plate decreasesfrom 4.69 mm y-1 to 1.9×10-1 mm y-1 with the addition of0 to 2.5 mM l-1 for InPzTAm; from 4.69 mm y-1 to2.1×10-1 mm y-1 with the addition of 0 to 2.5 mM l-1 forInPzTH; and from 4.69 mm y-1 to 7.2×10-1 mm y-1 withthe addition of 0 to 4.0 mM l-1 for InPzPh. These resultsare consistent with the EIS results. The inhibition effi-ciency (η %) is calculated using the data from the corro-sion potential (Ecorr.), cathodic and anodic Tafel slopes(βc and βa, respectively) and corrosion current density(Icorr.), and are tabulated in Table 2. From Table 2, largerdifferences in the cathodic slopes are shown by InPzTAmand InPzTH. This can be attributed to the thickening ofthe electrical double layer due to the adsorbed inhibitormolecules. According to Bockris and Srinivason [25], thisbehaviour is associated with the changes in the mechan-ism of the cathodic reaction, e.g. hydrogen evolution re-action (HER). On the other hand, there are no significantchanges for the anodic and cathodic Tafel slopes (βa, βc,respectively) for InPzPh. It can be suggested that thereare also no significant changes in the inhibition mechan-ism of cathodic and anodic reactions with the increase ofthe InPzPh concentration.The change of the corrosion rate with temperature

(298 to 318 K) is also examined on the copper plates.Figure 3 (b and d) shows that the activity of the inhibi-tors decreases with the increase of temperature from 298to 318 K. This is due to the decrease of adsorptionstrength of the inhibitor on the metal surface with therise in temperature. The adsorption of the inhibitor isdependent on many factors such as temperature, adsorp-tion of solvent, type of ions, thickness of the electricaldouble layer and charge density. The surface coverage (θ)is an important factor which can be used to describe the

Figure 3 Tafel polarization curves (vs. SCE) for corrosion of Cu plates in 1 M HCl in the presence of different concentration ofpyrazolylindolenine compounds (left side curves: a, c and e), Tafel plots in the right side (b and d) shows corrosion behaviour in thepresence of 2.5 mM inhibitors at different temperatures.

Table 2 Electrochemical impedance parameters for copper plates in 1 M HCl solution in the absence and presence ofdifferent concentrations of the pyrazolylindolenine compounds

Conc. Inhib.(mM)

Ecorr.(mV)

Icorr.(A cm-2)

βc(mV/dec)

βa(mV/dec)

Rp(Ω cm-2)

CR(mm/y)

η% Kads.(mol-1)

ΔG(kJ mol-1)

θ

Blank 0.0 0.6 19×10-4 184 144 368 4.694 - - - -

InPzTAm 0.5 0.300 3.2×10-4 187 114 290 3.7 21.1 2.74×10-4 −6225 0.212

1.0 0.013 3.7×10-5 132 125 190 0.43 90.8 9.9×10-3 −15633 0.909

1.5 0.011 2.8×10-5 107 74 120 0.34 92.7 12.7×10-3 −32501 0.928

2.0 0.008 2.4×10-5 64 72 81 0.28 94.0 15.7×10-3 −33552 0.940

2.5 0.01 1.6×10-5 87 44 99 0.19 95.9 23.4×10-3 −44394 0.960

InPzTH 1.0 0.015 4.3×10-5 218 215 470 0.49 89.5 8.58×10-3 −15279 0.896

1.5 0.02 5.8×10-6 95 138 970 0.67 85.7 6.01×10-3 −21595 0.857

2.0 0.024 2.3×10-6 101 129 1420 0.40 91.4 10.73×10-3 −31666 0.915

2.5 0.034 1.7×10-6 98 121 2900 0.21 95.3 20.33×10-3 −43542 0.953

InPzPh 1.0 0.015 7.0×10-5 193 89 100 3.12 33.9 0.51×10-3 −8235 0.340

2.0 0.010 5.8×10-5 127 89 84 0.97 79.3 3.84×10-3 −26573 0.794

3.0 0.002 3.3×10-5 115 64 97 0.78 83.3 5.02×10-3 −41852 0.834

4.0 0.006 1.9×10-5 142 63 210 0.72 84.6 5.52×10-3 −56744 0.847



type of adsorption mechanism of the inhibitors. Thecommon isotherms which are used to calculate the sur-face coverage (θ) are [26]:Frumkin isotherm: (θ/1-θ)exp(−2Fθ) = Kads. CFreundlich isotherm: θ = Kads. CTemkin isotherm: exp(f.θ) = Kads. CLangmuir isotherm: (θ/1-θ) = Kads. C

where Kads. is the equilibrium constant for the adsorp-tion process, C is the inhibitor concentration, and f isthe energic in-homogeneity. The Langmuir adsorptionisotherm gives the best straight line for the inhibitors

Figure 4 Adsorption isotherm plots in various amounts ofpyrazolylindolenine compounds in 1 M HCl media a); log θ/θ-1vs. log C, b) logCR/T vs. (1000/T), c); logCR vs. (1000/T).

(Figure 4a: log[θ/1-θ] vs. log C). The calculation of θvalue is given by [12]:

θ ¼ CRo � CR

CRo � CRmð4Þ

where CRo, CR and CRm are the corrosion rate of copperplates for the uninhibited, inhibited and the highest concen-tration of the inhibitor in 1.0 M HCl solution respectively.The straight lines in Figure 4a (log[θ/1-θ] vs. log C) suggestthat the inhibitors at the metal/electrolyte interface followsthe Langmuir adsorption isotherm. The thermodynamicparameters such as entropy of activation (ΔS), and enthalpyof activation (ΔH) and activation energy (Ea) are calculatedusing the Arrhenius equation (Eq. 6) [13]:

log CRð Þ ¼ �Ea

2:303RTþ λ ð5Þ

CR ¼ RTNh

exp ΔSR= Þ exp �ΔH

RT

� ��ð6Þ

where T is the temperature, R is the gas constant, N is theAvogadro number and h is the Plank constant. The dia-gram log(CR/T) vs. (1000/T) is plotted and ΔH and ΔS arecalculated from the slope and intercept, from the values of(−ΔH/2.303R) and [log(R/Nh) + (ΔS/2.303R)] respectively(Figure 4b). It can be observed that the ΔS and ΔH areenhanced due to the increase of inhibitors in the corrosiveelectrolyte. The activation energy (ΔE) can be determinedfrom the slope of log CR vs. (1000/T) in Figure 4c.The thermodynamic parameters such as activation en-

ergy (Ea) and the Arrhenius pre-exponential factor (λ) areobtained from the slope and intersection of the straightlines (Figure 4c). The linear regression coefficients (R2) areclose to 1 for all three inhibitors. The values of Ea and λfor the inhibitor compounds are tabulated in Table 3,where the highest and lowest values are shown by theInPzTAm and InPzPh respectively. The higher value of Eashows a lower corrosion rate this is also confirmed by theelectrochemical tests in this work.

Table 3 Computed molecular parameters for thepyrazolylindolenine compounds

InPzTAm Samples InPzTH InPzPh

ΔSads. (kJ mol-1) −16.25 −38.24 −137.76

ΔHads. (kJ mol-1) −105.97 −94.62 −24.80

λ (mg cm-2 h-1) −22.10 ±1.60 −19.24 ±2.19 −2.62 ±0.91

Ea (kJ mol-1) 7.34 ±0.49 6.68 ±0.68 1.67 ±0.28

EHOMO (eV) −0.2297 −0.2268 −0.2046

ELUMO (eV) 0.19846 0.19864 0.18805

ΔE= ELUMO - EHOMO 0.03124 0.02816 0.01655


Gaussian software 9.0 is used to study the quantumchemical behaviour of the inhibitor. The quantumchemical properties of the inhibitors are studied by cal-culating the energy of the highest occupied molecularorbital (EHOMO) and the energy of lowest unoccupiedmolecular orbital (ELUMO). Prior to the calculation usingthe Gaussian 9.0, optimizations of the inhibitor mole-cules are done by the same software.The calculated EHOMO and ELUMO in Table 3 show

that the ELUMO and ELUMO values are almost the samefor InPzTAm and InPzTH. This is not unexpected sincetheir molecular structures are almost similar. The ELUMO

values show that InPzTAm and InPzTH have good elec-tron donor properties compared to InPzPh. This is dueto the presence of sulfur atoms in both InPzTAm andInPzTH as shown in Figure 1, where the electron chargedensity is further away from the nucleus and can bedonated easily for adsorption and bonding on the coppersurface. However, the EHOMO value for InPzPh showsgood electron acceptor behaviour compared to InPzTAmand InPzTH. Frontier molecular orbital (FMO) densitydistributions for InPzTAm, InPzTH and InPzPh areshown in Figure 5. From the FMO results, the mechan-ism of inhibition on the copper surface can be described.It is a general assumption that the organic inhibitors areprotonated in the acidic media. The adsorption of theinhibitors on the activated sites on the metal surfaceoccurs by one or more of the following ways: (I) electro-static interaction between the charged atoms of the inhi-bitors and the activated metal surface. (II) Unshared

Figure 5 Frontier molecular orbital density distribution for InPzTAm ((e; HOMO, f; LUMO).

electron pair of the hetero-atoms (in hetero-cyclic mole-cules) and metal surface atoms with vacant d-orbitals. (III)Interaction of π-electrons with metal surface atoms. (IV)Interaction between protonated inhibitors with absorbedchloride ions. In our previous work [27], we have sug-gested that the nitrogen protonated caffeine moleculeadsorbed the chloride ion while the electron pair ofN-atom was bonded with the metal surface, thus prevent-ing the chloride ion from attacking the metal surface. Theadsorption mechanisms of the synthesized pyrazolylindo-lenine compounds on the copper surface are shown inFigure 6. The different strengths in the inhibition effi-ciency of these compounds are due to the different func-tional groups which are bonded with Pyrazolylindoleninebase. The InPzTAm and InPzTH compounds have similarchemical structure, but InPzTAm shows better inhibitionefficiency. Scheme 1 a shows that the inter-electron trans-fer from S-atom to the amine protonated group whichdecreases the electron density in the S-atom. Comparedto scheme 1 a, the Scheme 1 b shows that both N-atoms(in Hydrazine group) are protonated hence more electronsfrom the S-atom can be transferred. The S-atom inInPzTAm with higher electron density thus better electrondonor capabilities shows better corrosion inhibition pro-perties compared to InPzTH with less electron density.The InPzPh shows the weakest corrosion inhibition effi-ciency which is due to the absence of the C=S group.The hydrazine group in InPzTH with two N-atoms and

the amine group in InPzTAm with one N-atom have slightdifference in energies. Figure 5a and c show higher

a; HOMO, b; LUMO), InPzTH (c; HOMO, d; LUMO) and InPzPh

Figure 6 A schematic model for the adsorption of a): InPzTAm,b): InPzTH, c): InPzPh on copper surface.

S

NH3X

S

NH2

NH3X

X= pyrazolylindolenineScheme 1 Structure and electron transfer mechanism ofprotonated InPzTAm and InPzTH, (X= pyrazolylindolenine).


electron density in the vicinity of the N-atom in InPzTAmcompared to N-atoms in the hydrazine group in InPzTH.Thus InPzTAm can be absorbed on copper activated sitesvia the donation of the unshared electron pair from the Nand S-atoms to the copper metal. It was found that moreelectron density transfer from inhibitors to metal surfaceresults in increased adsorption of the inhibitor moleculeson the surface, thus better corrosion protection properties.Table 3 shows that the ΔE for compounds is in the orderof InPzTAm > InPzTH > InPzPh. It can be shown that theelectron transfer process in InPzTAm is faster comparedto the other compounds.

OCP measurementsThe changes of the open circuit potential (OCP) with timefor the copper electrode in 1 M HCl solution in theabsence and presence of various amount of inhibitor(InPzTAm, InPzTH and InPzPh) concentration at roomtemperature are shown in Figure 7 (a, b and c). Generally,the OCP variation with time with different amount of in-hibitor shows a similar behaviour. Upon the immersion ofthe electrode in the solution, an accentuated displacementof OCP towards negative values was observed. The quick

changes in the OCP curves could be due to the initial dis-solution process of the oxide film formed on the bare cop-per surface. Soon afterwards (approximately 500 sec), theOCP increased towards positive regions while the metalsurface was passivated due to the adsorption of the inhibi-tor compounds on the activated sites of Cu surface. It canbe observed that the OCP shifts to more noble potentialswith the increase of the inhibitor concentration. With thesame concentration for all three inhibitors, the shift of theOCP towards more noble potentials is in the order ofInPzTAm > InPzTH > InPzPh. The electrolyte has a highconcentration of Cl- and the Cl- has a strong tendency toadsorb on the cathode surface, thus the local corrosioncould be due to changes in the surface polarization [27].With the increase of the inhibitor concentration in thecorrosive electrolyte, the aggressive behaviour of Cl- isquenched due to the increased adsorption of the inhibitorson the copper surface, thus the OCP moves towards noblepotentials. Surface coverage (θ) is calculated and tabulatedin Table 2. It shows that the increase of surface coverage isin the order of: InPzTAm > InPzTH > InPzPh, thus the ad-sorption of the inhibitors are also in that respective order.From Figure 7, the lowest shift towards positive poten-

tial is for InPzPh due to the low surface coverage whilethe largest shift of towards positive potential is forInPzTAm. This phenomenon can be ascribed to the de-crease of the charge transfer and subsequently mass trans-port of the Cl- decreases with the formation of the

Figure 7 OCP measurements (vs. SCE) with time for copperplates with different concentration of the inhibitor compoundsin 1 M HCl media for 1 h.

Figure 8 FESEM images (10000X) of the copper surface after immersiinhibitors, b) 2.5 mM InPzTH and c) 2.5 mM InPzTAm.


inhibitor adsorption on the copper surface. From the mea-surements of surface coverage θ and Kads. the strength ofthe inhibitor adsorption on the copper surface is in theorder of InPzTAm > InPzTH >InPzPh.

FESEM analysisFESEM images of the copper surface are taken to estab-lish a link between the surface morphology and theexperimental results obtained for the inhibition perform-ance of the InPzTAm and InPzTH compounds. Figure 8(a, b and c) are surface images of copper plates whichwere immersed in 1.0 M HCl, 1.0 M HCl + 2.5 mMInPzTH and 1.0 M HCl + 2.5 mM InPzTAm for 10 days,respectively. Figure 8a shows the presence of cavities anddepth roughness on the copper surface after 10 days ofimmersion time in 1.0 M HCl, where the surface isstrongly damaged in the absence of the inhibitors. Onthe other hand, surface images of 8b and 8c show thatcavity formation and surface roughness on the coppersurface has decreased with the presence of InPzTAm andInPzTH in the corrosive media (1.0 M HCl). The Cusurface with the presence of InPzTH (2.5 mM) shows fewpit formation after immersion in 1.0 M HCl for 10 days(Figure 8b). The Cu surface with the presence of theInPzTAm did not show pit and cavity formation after10 days of immersion time in the corrosive solution. TheFESEM images for both inhibitor compounds conforms tothe experimental results obtained in the previous sections.

ConclusionThe EIS, OCP and LSV measurements show that all threepyrazolylindolenine compounds (InPzTAm, InPzTH andInPzPh) give good inhibition performance against coppercorrosion in 1 M HCl solution. OCP measurements showthat the OCP moves towards more noble potentialswith the increase in the inhibitor concentration. Tafelpolarization experiments show that the Rp also increaseswith the increase of inhibitors in acidic media. EIS

on in 1.0 M HCl solution for 10 days with: a) the absence of


simulation of the experimental data shows that InPzTAmand InPzTH have the same equivalent circuit but the War-burg element is present in the equivalent circuit for theInPzPh compound. The corrosion inhibition performanceof these compounds for the same concentration (2 mM) isη = 94.0%, η = 91.4% and η = 79.3% for InPzTAm, InPzTHand InPzPh respectively. From the EIS, OCP and LSVresults, the higher corrosion efficiency for both InPzTAmand InPzTH compared to InPzPh is due to the presenceof sulfur atoms in those compounds. From quantumchemical calculations, the presence of the sulfur atomspromote better electron donor ability for both compoundswhich give higher inhibition efficiency.

AbbreviationsInPzTAm: 4-(3,3-dimethyl-3H-indol-2-yl)-pyrazole-1-carbothioamide; InPzTH:4-(3,3-dimethyl-3H-indol-2-yl)-1H-pyrazole-1-carbothiohydrazide; InPzPh:3,3-dimethyl-2-(1-phenyl-1H-pyrazol-4-yl)-3H-indole; EIS: Electrochemicalimpedance spectroscopy; OCP: LSV: Open circuit potential, linear scanvoltammetry; FESEM: Field emission scanning electron microscopy;SCE: Saturated calomel electrode; FRA: Frequency response analysis;GPES: General purpose electrochemical software; OHP: Outer Helmholtzplane; WE: Working electrode; CE: Counter electrode; CPE: Constant phaseelement; HER: Hydrogen evolution reaction; EHOMO: Occupied molecularorbital; ELUMO: Unoccupied molecular orbital.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsME completed the laboratory work, data treatment and drafted themanuscript. HK and HMA synthesized the inhibitors. WJB coordinated thestudy, data analysis and edited the text. All authors have read and approvedthe final manuscript.

AcknowledgmentsThe authors would like to thank the University of Malaya for financialsupport provided by research grants FP039 2010B and RG181-12SUS.

Author details1Department of Chemistry, Faculty of Science, University of Malaya, 50603,Kuala Lumpur, Malaysia. 2Department of Chemistry, Faculty of Science,Islamic Azad University- Gorgan Branch, Gorgan, Iran. 3Nanotechnology &catalysis research centre, Institute of postgraduate studies, University ofMalaya, 50603, Kuala Lumpur, Malaysia.

Received: 26 August 2012 Accepted: 18 December 2012Published: 31 December 2012

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doi:10.1186/1752-153X-6-163Cite this article as: Ebadi et al.: Corrosion inhibition properties ofpyrazolylindolenine compounds on copper surface in acidic media.Chemistry Central Journal 2012 6:163.

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