Desalination 263 (2010) 45–57

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Desalination

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Synthesis characterization and corrosion inhibition efficiency of N-C2{(2E)-2-[4-(dimethylamino) benzylidene] hydrazinyl} 2-oxo ethyl benzamide onmild steel

Rinki Goel a,⁎, Weqar A. Siddiqi a, Bahar Ahmed b, Mohd. Shahid Khan c, V.M. Chaubey a

a Department of Applied Science & Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia (Central University), New Delhi 110025, Indiab Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Jamia Hamdard, New Delhi 110025, Indiac Department of Physics, Jamia Millia Islamia (Central University), New Delhi 110025, India

⁎ Corresponding author. Tel.: +91 9868252549; fax:E-mail address: rinkigoel_jmi@yahoo.com (R. Goel).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.06.033

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 November 2009Received in revised form 16 June 2010Accepted 17 June 2010Available online 31 July 2010

Keywords:Weight lossPolarizationEISEDXSEMTGA

In the present paper, we report the synthesis of N-C2{(2E)-2-[4-(dimethylamino) benzylidene] hydrazinyl}2-oxo ethyl benzamide (DMB) as a potential inhibitor of mild steel. The study of inhibition effect of DMB onthe mild steel in 1.0 N H2SO4 solution has been carried out using weight loss, galvanostatic polarization andelectrochemical impedance spectroscopy (EIS) techniques. The polarization curves showed that DMBcompound behaves as both cathodic and anodic inhibitor, i.e. a mixed type inhibitor. EDX and SEMobservations of the mild steel surface confirmed existence of a protective adsorbed film of the inhibitor onthe mild steel surface. The temperature dependent studies from 298–318 K showed that the DMB adsorptionfollows the Langmuir isotherm model. The value of activation energy (Ea) for mild steel corrosion andthermodynamic parameters such as adsorption heat (ΔHads), K and ΔG0

ads have been calculated anddiscussed. It has been found that the protection efficiency increases with increasing inhibitor concentrationin the range of 250–1000 ppm, but slightly decreases with increasing temperature. The graphs obtained fromTGA indicate relatively good thermal stability of the inhibitor film on the metal surface. The results show thatDMB is good corrosion inhibitor for mild steel in acidic medium.

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1. Introduction

Addition of inhibitor remains the necessary procedure to securethe metal against acid attack in chemical cleaning and pickling toremove mild scales (oxide scale) from the metallic surface [1]. Asignificant m to protect the metals from corrosion is addition ofspecies to the solution in contact with the surface in order to inhibitthe corrosion reaction and reduce the corrosion rate [2]. Acidsolutions are widely used in industry, the most important fields ofapplication being acid pickling and industrial acid cleaning. Because ofthe general aggressiveness of acid solutions, inhibitors are commonlyused to reduce the corrosive attack on metallic materials. The mostimportant aspect of inhibition normally considered by corrosionscientists is the relation between molecular structure and corrosioninhibition efficiency. Many authors have done studies on such effects[3–5]. Lorenz andMansfeld [6] classified themodes of inhibition effectof interface inhibitors into three categories; that are caused by

i. The geometric blocking effect of adsorbed inhibitive species onthe metal surface.

ii. The effect of blocking the active sites on the metal surface byabsorbed inhibitive species.

iii. The electro catalytic effects of the inhibitor of its reactionproducts.

Heteroatom, for example, N, O and S usually becomes the activecentre. When metal is immersed in the inhibition solution, N, O and Sdonate lone pair electrons to the unoccupied orbits on themetal atom.Stable and strong bonds are formed between inhibitor and the metalsurface during the self-assembling process [7,8]. It has been observedthat the absorption of organic inhibitor depends on the physico-chemical properties of the functional groups and electro density at thedonor atom [9].

The choice of the inhibitors is based mainly on two considerations:

i. They can be synthesised easily from relatively cheap material.ii. The presence of an electron cloud on the aromatic ring, the

electronegative nitrogen, oxygen atoms and the presence of a C Ngroup steel surface prompting effective inhibition [10].

The aim of this work is to investigate the role played by newlysynthesised N-C2 {(2E)-2-[4-(dimethylamino) benzylidene] hydrazi-nyl} 2-oxo ethyl benzamide (DMB) on the corrosion behaviour of mildsteel in 1.0 N H2SO4. In view of this, N-C2 {(2E)-2-[4-(dimethylamino)

46 R. Goel et al. / Desalination 263 (2010) 45–57

benzylidene] hydrazinyl} 2-oxo ethyl benzamide (DMB) was synthe-sised and characterized using IR and NMR analysis technique. Theinhibitive action of this compound on the corrosion behaviour of mildsteel in 1.0 N H2SO4 solution at three levels of concentration and atthree different temperatures was analysed. Corrosion inhibition wasinvestigated using Electro Chemical technique (Galvanostatic Polar-ization, Electrochemical Impedance Spectroscopy) and Weight Lossmeasurements.

2. Experimental

2.1. Inhibitors

N-C2 {(2E)-2-[4-(dimethylamino) benzylidene] hydrazinyl} 2-oxoethyl benzamide (DMB) were synthesised in the laboratory usingwell-established methods and characterized by spectral analysismethods. The IR spectra (in KBr pellets) were recorded on a NicoletProtege 460 spectrometer. 1H NMR spectra were recorded on a300 MHz DPX300 spectrometer.

2.2. Synthesis

In the laboratory N-C2 {(2E)-2-[4-(dimethylamino) benzylidene]hydrazinyl} 2-oxo ethyl benzamide (DMB, 1) was synthesised fromester of hippuric acid (1) using the procedure described below:

A mixture of 0.01(mol) ester of hippuric acid (4), 0.01 (mol) of 2,4Dimethyl aminobenzyl hydrazine, 0.2 ml of glacial acetic acid and 25 mlof absolute ethanol was refluxed for 24 h. The crystalline product thusobtained was filtered, washed, dried and recrystalised inmethanol. Theyield of crystals is 84.5% and their melting point is 203–205 °C. Therequired intermediates were prepared using reported procedures [11].Their purity was monitored by TLC and its structure was established byspectral measurements. The synthesis sequence is given in scheme 1.

2.3. Corrosive medium

The aggressive solutions used were made of AR grade H2SO4 andappropriate concentrations of acid were prepared using doubledistilled water.

2.4. Specimens

Mild steel specimens had composition (% bymass) of C (0.017%), Si(0.017%), Mn (0.85%), P (0.0047%) and Fe (Bal). Rectangular speci-mens of the size 1 cm3 were used for the gravimetric measurements.The working electrodes with an exposed area of 0.5 cm2 in theelectrochemical measurement were covered with epoxy araldite,polished with emery paper (100, 360, 600, 800, and 1000 grit) rinsedwith double distilled water, degreased in acetone and dried for uselater. AR grade chemicals were used for the preparation of solutions.

2.5. Gravimetric measurements

The weight loss of the rectangular steel specimens of size 1 cm3 in1.0 N H2SO4 with and without the addition of different concentrationsof DMB compound were determined after 6 h period of immersion, atthe temperature of 298 K, 308 K and 318 K in air atmosphere withoutbubbling. The inhibition efficiency (%) of the inhibitor was calculatedby Eq. (1).

%IE =W0−W

W0× 100 ð1Þ

Where, W0 and W are weight losses without and with inhibitor.

The corrosion rate of mild steel was calculated by Eq. (2) [12].

μ =3:45 × 106W

ADTð2Þ

Where, W = Weight loss (g), D = Density of mild steel specimen(g cm−3), A = Area of the coupon (cm2), and T = Exposure time (h).

2.6. Electrochemical measurements

Electrochemical experimentswereperformed in a conventional three-electrode glass cell assembly with a mild steel rod as the workingelectrode (We) and platinum foil as the auxiliary electrode (Ce) andsaturatedcalomel as a referenceelectrode. Theelectrolytic solutionwasanacidic solution maintained at 298 K in air atmosphere without bubbling.

2.6.1. Polarization measurementThe polarization curves are recorded with a Keithley 220 program-

mable current source.The potentialswere scannedprimarily in the cathodic direction from

corrosion potential and then subsequently in the anodic direction. Theelectrode was held in the test solution for few seconds, prior tomeasurements, to ensure reliable corrosion potential. The percentageinhibition efficiency (% IE) was calculated using Eq. (3).

%IE =Icorr−I-corr

Icorr× 100 ð3Þ

Where, Iºcorr and Icorr are the corrosion current densities in theabsence and presence of inhibitor.

2.6.2. Electrochemical impedance studies (EIS)Impedance spectroscopy measurements were carried out in the

frequency range from 42 Hz to 5 MHz using LCR HI-Tester (H10K13532-50). The EIS data are presented in the formof complex planes. Thevalues of Polarization resistance (Rp), Solution resistance Rs and Doublelayer capacitance (Cdl) were calculated from complex planes by usingZview software. Various impedance parameters like polarizationresistance (Rp) and double layer capacitance (Cdl) were determined.Nyquist plots of mild steel in inhibited and uninhibited acid solutions,containing different concentrations of DMB compound, were obtained.The percentage inhibition efficiency (% IE) was calculated using Eq. (4).

%IE =Rp0−Rp

Rp0 × 100 ð4Þ

Where, Rp and Rp′ are the polarization resistance values withoutand with inhibitor, respectively.

2.7. Thermal stability of surface film

A series of electrodes were immersed in 1.0 N H2SO4 solutioncontaining 1000 ppm DMB compound for a period of 6 h. After thistime period, the specimens were taken out, washed with distilled waterand dried. The organic thin film was mechanically removed from theelectrode surface and its thermal stabilitywas tested. The thermal analysisexperiments were performed under nitrogen atmosphere using TGPerkin–Elmer thermal analysis system at a heating rate of 293 Kmin−1

over a temperature ranging from 323 K to 873 K.

2.8. Morphological investigation

The composition and surface morphology of the steel samples inthe absence and presence of DMB compound was investigated afteranodic polarization using Energy Dispersive X-ray (EDX) technique(INCAX-Sight oxford instruments) and Scanning Electron Microscopy

47R. Goel et al. / Desalination 263 (2010) 45–57

(SEM) technique (JSM 840 Jeol). The SEM images were taken at2000 μm magnification of the metal surfaces.

2.9. FTIR and UV studies

The corrosion products formed on steel surface during polarizationwas removed by scrapping and was used for recording FourierTransform Infrared (FTIR) and UV spectra. This study reveals thepossibility of the adsorption on the inhibitor on themetal surface. FTIRspectra (in KBr pellets) and UV were recorded on a Nicolet Protege460 spectrophotometer and Shimadzu UV 1601.

2.10. Theoretical calculations

All the theoretical calculations for DMB were made using theHyperChem software version 7.52 acquired from Hypercube Inc., USA.The Ground state nuclear geometry of the molecular system was firstoptimized using the Molecular Mechanics (MM+) method [13]. Themolecular geometry of the DMB so obtained was then optimized fullyagain employing two semi-empirical AM1 [14] and PM3 [15] methods.The equilibriumgeometry of DMBwas located by the search proceduresof the software without imposing any constraints and with the stan-dard parameters as implemented in the software. The computation ofelectronic properties of the DMB using the optimized geometry wasthen performed using the default convergence criteria. The two semi-empirical AM1 and PM3 methods were used for theoretical computa-tions as these methods are known to provide accurate geometries andelectronic properties comparable to those obtained using ab initiomethods but with far less computational efforts [16–22].

3. Results and discussions

3.1. Characterization of DMB, 1

Ester of hippuric acid (4) on treatment with 2,4 Dimetylaminobenzyl hydrazine in ethanol containing trace of glacial acetic acid ascatalyst, yielded N-C2{(2E)-2-[4-(dimethylamino) benzylidene]hydrazinyl} 2-oxo ethyl benzamide (DMB, 1).

The characterization data of (DMB, 1) are given below:

IR KBr cm−1 νmax, 3325(NH), 3048(C CH, Ar), 2850(CH2), 2925(CH2),1674(C O), 1605, 1548(C C), 1367(C–N, Ar), 1226, 1058 (C–N, Al), 684(NC Cb).

1H NMR (300MHz, D2O): δ 2.97 (6H, s, 2×CH3 at N), 3.95 (2H, d,J=6.0 H2), 4.39 (2H, d, J=6.0 H2, –CH2–), 6.75 (2H, d, J=9.0 H2,H-2,3), 7.51(5H, m, Ar-H), 7.91(2H, d, J=9.0 H, H-5,6), 8.66(1H, s,NNH), 8.68(1H, s, NNH), and 11.22(1H, s, NH).

3.2. Gravimetric measurements

The values of percentage inhibition (%IE) and corrosion rate (μcorr)obtained from weight loss method at different concentrations ofinhibitor at different temperatures are summarised in Table 1. It hasbeen found that the DMB compound inhibits the corrosion of mildsteel in 1.0 N H2SO4 solution at all concentrations used in this studyi.e. 250–1000 ppm.

It has also been observed that the inhibition efficiency of the com-pound increases with the increase in concentration as shown in Fig. 1a.Thevariationof IEwithdifferent solution temperatures is shown in Fig. 1b.It can be seen that IE for different concentrations causes a significantdecrease with an increase in temperature from 298 K to 318 K. Thisbehaviour couldbe attributeddue to strong interaction of compoundwiththe metal surface that resulted in the adsorption of inhibitor molecules[23–25]. The lone pair of electron on the nitrogen will coordinate withthe metal atoms of active sites. DMB compound containing electrondonating groups such as N(CH3)2, C N and C O, increases the electrondensity on the benzene ring. Methyl groups in this compound increasemoderately the localization of lone pairs on nitrogen atoms [26,27].

3.3. Electrochemical measurements

3.3.1. Polarization measurementsAnodic and cathodic polarizations were studied in 1.0 N H2SO4 with

and without addition of different concentration of DMB compoundand obtained results are plotted in Fig. 2a.

Table 1Corrosion parameters obtained from weight loss measurements for various concentra-tions of inhibitor at different temperatures in 1.0 N H2SO4.

Temp. (K) Conc. (ppm) Weight loss (g) μcorr (cm h−) IE (%)

298 1.0 N H2SO4 0.0786 2.2620 –

250 0.021 0.5735 69.40500 0.0125 0.3451 84.091000 0.007 0.1935 90.09

308 1.0 N H2SO4 0.158 4.4034 –

250 0.0567 1.5939 64.11500 0.0375 1.0558 76.251000 0.0189 0.5339 88.03

318 1.0 N H2SO4 0.5487 15.3299 –

250 0.209 5.7322 61.90500 0.1597 4.4297 70.891000 0.1083 3.0765 80.26

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The corrosion current (Icorr) and exchange current (I0) densitieswere determined from the intersection of liner parts of cathodiccurves successively with:

i. The stationary corrosion potential (Ecorr).ii. The reversible potential of hydrogen on Pt in the same solution.

Fig. 1. Variation of inhibition efficiency with (a) different temperatures and (b) differentconcentrations of DMB.

Fig. 2. Electrochemical plots (a) Tafel polarization (b) Nyquist plot of electrochemicalimpedance spectroscopy of DMB 1.0 N H2SO4 on mild steel surface.

The effect of various inhibitor concentrations on the corrosionkinetic parameters such as anodic and cathodic tafel slopes (βa, βc),Ecorr, Icorr, surface coverage (θ) and IE obtained from polarizationmeasurement method at different temperatures were recorded andsummarised in Table 2.

An inspection of the results given in Table 2 suggests that theincrease in the concentration of the additive compound has resultedthe following:

i. Increase of both anodic and cathodic tafel slopes indicate amixed anodic and cathodic effect on the corrosion mechanism[28] i.e. mixed inhibitor.

ii. The corrosion potential (Ecorr) has shifted to more positivevalues while the corrosion and exchange current densitiesdecrease with increasing the inhibitor concentration indicatesthe inhibiting effect of DMB compound.

iii. The Inhibition Efficiency (IE), calculated from weight loss andpolarization measurements, was found to increase with increas-ing inhibitor concentration.

It is interesting to note that the values of IE measured by po-larization measurement are higher than those obtained by weightloss measurement. These results may be due to the fact that the

Table 2Corrosion parameters obtained from polarization measurements for various concentrations of inhibitor at different temperatures in 1.0 N H2SO4.

Temp. (K) Conc. (ppm) Ecorr (mV) Log Icorr (μA cm−2) θ βa (mV dec−1) βc (mV dec−1) IE% (%)

298 1.0 N H2SO4 512 3.45 – 145 98 –

250 510 2.95 0.6813 96 45 68.13500 507 2.69 0.8232 65 31 82.321000 504 2.41 0.9084 43 18 90.84

308 1.0 N H2SO4 522 3.35 – 158 115 –

250 514 2.9 0.6403 108 80 64.03500 505 2.75 0.7457 70 65 74.571000 504 2.38 0.8905 54 29 89.05

318 1.0 N H2SO4 515 3.28 – 128 98 –

250 512 2.87 0.6036 71 69 60.36500 511 2.75 0.7018 53 48 70.181000 509 2.56 0.8068 37 29 80.68

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electrochemical measurements were carried out on freshly preparedsolutions.

The inhibition effect of this compound can be attributed totheir parallel adsorption at the metal solution interface. The paralleladsorption is owing to the presence of one or more active centre foradsorption. The chemisorption takes place by the formation of achemical bond between the metal and the adsorbed molecule.Chemisorption involves charge sharing or charge transfer from inhibitormolecule to the metal surface forming coordinate type bond [29].

3.3.2. Electrochemical impedance measurementsThe corrosion behaviour was studied for different concentrations

of inhibitor at room temperature. The Nyquist plots are analyzed inthe term of the equivalent circuit composed with classic parallelcapacitor and resistor and the values of polarization resistance, Rp anddouble layer capacitor, Cdl are calculated [30,31]. The deduced EISparameters are collected in Table 3. Rp values were calculated fromthe difference in impedance at lower and higher frequencies [32]. Cdlvalues were obtained using the Eq. (5) by determining the frequencyat which the imaginary component of the impedance was themaximum.

Cdl =1

2πfð−Z′maxÞRp ð5Þ

Where, Cdl is double layer capacitance, Rp is polarization resistance,f is the frequency at which the imaginary component of the impedanceis maximum (−Z′max).

As seen in Table 3, the inhibition efficiency has increasedmarkedlywith increase in inhibitor concentration, which may have resulteddue to the contact adsorption film. The Cdl value (=ε/4πd) tends todecrease due to the displacement of theH2Omolecules by the inhibitormolecules at the interface of the electrical double layer, whichsuggested that the two-inhibitor molecules function by adsorption atthe metal solution interface [33], and the charge transfer betweenthe solution and metal surface has been inhibited sharply.

It is also clear from Table 3 that Rp has increased with increasinginhibitor concentration, which confirms the increase in corrosion

Table 3Polarization resistance (Rp), capacitance (Cp) and inhibition efficiencies (IE) obtainedusing electrochemical impedance method in 1.0 N H2SO4 at 298 K.

Conc. (ppm) Rp (Ω cm2) Cdl (μF cm2) IE (%)

1.0 N H2SO4 53 600 –

250 172 339 69.2500 353 165 85.01000 607 96 91.3

resistance of the adsorption film formed by the inhibitor on the metalsurface, therefore, the corrosion rate has decreased rapidly. A typicalset of Nyquist plots, which is shown in Fig 2b, confirms that theimpedance spectra were similar to a single circle but not a perfect loop[27]. From the values of Rp and Cdl, it can be deduced that DMBcompound shows good inhibition effect with increase in the workingconcentration.

3.4. Thermal stability of surface film

The thermal stability of the surface inhibitor film is very importantfor their application. Acid pickling of steel is usually carried out atelevated temperatures upto 60 °C in HCl and up to 90 °C in H2SO4

solution [34]. Therefore, the organic inhibitors are expected to bechemically stable to provide a high protective efficiency under theconditions mentioned above. The Thermogravimetric Analysis (TGA)of inhibitor film removed mechanically from the MS surface wastested and obtained TGA curve is shown in Fig. 3a. The TGA curve ofpure organic compound was also performed and given in Fig. 3b. Acomparison of TGA curve for pure DMB compound and that ofinhibitor film removed from the metal surface reveals the differencein thermal events. As seen from Fig. 3b, the pure compound showsthree steps weight loss. The initial weight loss upto 201 °C could beattributed to the loss of moisture and dopans [35]. Between 200 °Cand 406 °C, the maximum mass loss occurs due to the thermaldegradation of DMB compound. A last weight loss step is displayedbetween 406 °C and 600 °C. Whereas the TGA curve of mechanicallyremoved inhibitor film shows small step weight loss, which starts at460 °C, may be corresponding to the metal inhibitor complexdegradation. The remaining weight corresponds to the iron metal.From these results, it can be concluded that the surface inhibitor filmpossesses relatively good thermal stability.

3.5. Thermodynamic parameters

In order to understand the mechanism of corrosion inhibition, theadsorption behaviour of the organic adsorbate on the steel surfacemust be known. The degree of surface coverage (θ) for differentconcentrations of inhibitor has been evaluated from polarizationmeasurements. The data were tested graphically by fitting to variousisotherms. A straight line, obtained on plotting Log (θ/1–θ) againstLog C (Fig. 4), suggests that the adsorption of the compound on mildsteel surface follows Langmuir adsorption isotherm model [36].According to this isotherm, θ is related to the inhibitor concentrationby Eq. (6):

θð1−θÞ = KC ð6Þ

Where, K is the equilibrium constant of the adsorption process.

Fig. 3. TGA curve of (a) DMB removed from MS surface and (b) pure DMB compound.

50 R. Goel et al. / Desalination 263 (2010) 45–57

Fig. 4. Langmuir adsorption isotherm at different temperatures in 1.0 N H2SO4. Fig. 5. Arrehenius plots Log (θ/1−θ) versus 1/T in 1.0 N H2SO4 at various concentrationsof inhibitor.

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Free energy (ΔG0ads) of investigated compound has been calculated

from Eq. (7):

K =1

55:5exp −ΔG0

ads

RT

!ð7Þ

Where, R is the regressive coefficient and T is the absolutetemperature. The standard free energy of adsorption (ΔG0

ads) has beencalculated at different concentrations of inhibitor and given in Table 4.The negative value of ΔG0

ads ensures the spontaneity of the adsorptionprocess and stability of the adsorbed layer on the steel surface. Itis well known that the values of –ΔG0

ads of the order of 20 kJ mol−1 orlower indicate a physisorption; those of order of 40 kJ mol−1 orhigher involve charge sharing or a transfer from the inhibitormolecules to the metal surface to form a coordinate type of bond(chemisorption) [29,37]. The calculated values of ΔG0

ads of slightlymore negative than −20 kg mol−1 indicate, therefore, that theadsorption mechanism of the investigated compound on steel in1.0 N H2SO4 solution is typical of chemisorption.

3.5.1. Adsorption isothermThe inhibition efficiency of DMB compound is determined by their

adsorbability on the surface of the corroding metal. The values ofdegree of surface coverage (θ) of mild steel by adsorption of differentconcentrations of DMB compound were calculated at constantpotential using Eq. (8).

θ = 1− IaddIfree

ð8Þ

Where, Ifree and Iadd are the corrosion current densities in absenceand presence of the additive compound respectively. The values of θhave been inserted into Table 2.

It was found that the degree of coverage ‘θ’ increases withincreasing the concentration of additive compound and decreases as

Table 4Thermodynamic parameters at various concentrations of inhibitor in 1 N H2SO4.

Conc. (ppm) Ea (kJ mol-1) ΔH (kJ mol-1) K (10-3 mol-1) ΔG0ads (kJ mol-1)

1.0 N H2SO4 −15.43 – – –

250 9.93 −13.81 8.5 −22.79500 16.26 −27.86 8.3 −32.591000 22.63 −34.98 9.9 −34.69

the temperature was raised from 298 K–318 K. For a certain rangeof inhibitor concentrations and temperatures, where monolayeradsorption occurs on the steel surface, the Langmuir adsorptionisotherm [35] may be expressed by Eq. (9).

θ1−θ

= AC exp −ΔHRT

� �ð9Þ

Where, T is temperature, A is independent constant, C is inhibitorconcentration, R is gas constant, ΔH is heat of adsorption and θ issurface coverage by the inhibitormolecule. Eq. (9) can be converted tologarithmic scale as given in Eq. (10).

Logθ

1−θ= Log A + Log C− ΔHads

2:303RTð10Þ

The Plots of log θ/(1–θ) versus (1/T) at different additiveconcentrations are shown in Fig. 5. The slope of the linear parts ofcurves is equal to −ΔH/2.303R from which the average heat of

Fig. 6. Variation of corrosion current with temperature in 1.0 N H2SO4.

Fig. 7. EDX spectra of MS specimens (a) after 2 h immersion in 1.0 N H2SO4 solution (a) after 2 h immersion in 1.0 N H2SO4 containing 1000 ppm DMB (c) after 4 h immersion in 1.0 N H2SO4 containing 1000 ppm DMB and (d) after 6 himmersion in 1.0 N H2SO4 containing 1000 ppm DMB.

52R.G

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al./Desalination

263(2010)

45–57

Fig. 8. SEM image of MS surface after 6 h immersion in 1.0 N H2SO4 (a) withoutadditive. (b) with 500 ppm DMB and (c) with 1000 ppm DMB.

53R. Goel et al. / Desalination 263 (2010) 45–57

adsorption, ΔH were calculated at different concentrations ofinhibitor and are given in Table 4. The negative valves of ΔH reflectthe exothermic behaviour of DMB compound on the metal surface.

3.5.2. Activation energyThe activation energy of the corrosion process can be calculated

using Eq. (11):

K = A exp − EaRT

� �ð11Þ

Where, Ea is the activation energy, A is the frequency factor, T is theabsolute temperature, R is the gas constant and K is the rate of metaldissolution reaction which is directly related to corrosion currentdensity Icorr [28]. The values of Ea can be calculated from the slopes ofstraight line obtained from Fig. 6. The values of Ea obtained in 1.0 NH2SO4 and at different concentrations of the DMB compound are listedin Table 4.

This result agrees with the order of IE. The activation energy ishigher in the presence of additives than in its absence. During thecorrosion reaction mechanism, the charge transfer is blocked withadsorption of DMB molecules to the metal surface, causing theincrease in the activation energy [38,39]. The higher values of Ea aregood evidence for the chemisorptions mechanism of DMB compoundon the steel surface.

3.6. Surface analysis

3.6.1. EDX examinations of the MS surfaceEDX survey spectra were used to determine which elements were

present on the electrode surface before and after exposure to theinhibitor solution. EDX spectrums were recorded for MS samplesexposed for 2 to 6 h in 1.0 N H2SO4 solution in the absence andpresence of 1000 ppm concentration of DMB compound. In absence ofDMB compound, the EDX spectrum (Fig. 7a) shows the characteristicpeaks of some of the elements constituting the MS sample. In thepresence of DMB compound (Fig. 7b–d), the EDX spectra did not showadditional lines. However, the carbon and oxygen signals aresignificantly enhanced upon adding DMB compound to H2SO4

solution. This enhancement in carbon and oxygen signals is due tothe carbon and oxygen atoms of the adsorbed DMB species. These datashow that a carbonaceous material containing oxygen atoms hascovered the MS surface. This layer is undoubtedly due to the inhibitor.This high contribution is not present on the MS surface exposed touninhibited H2SO4 solution (see Fig. 7a). In addition, the intensity ofthe carbon and oxygen signals increase with immersion time. Sincemore DMB anions are adsorbed on the metal surface, confirming theresults obtained from chemical and electrochemical measurements.

The spectrums of Fig. 7b–d indicate that the Fe peaks areconsiderably suppressed relative to the samples prepared in 1.0 NH2SO4 solution, and this suppression increases with increasing DMBimmersion time. The suppression of the Fe lines occurs because of theoverlying inhibition film.

3.6.2. SEM observations of the MS surfaceMicrostructural studies of mild steel in 1.0 N H2SO4 in the absence

and presence of a certain concentration of compound at 298 K wereperformed and illustrated in Figs. 8(a–c) The corrosion attack wasmore pronounced in the absence and presence of low inhibitorconcentration (500 ppm) of the studied inhibitors at 298 K temper-ature as shown in Fig. 8a and b respectively, while the film formed onthe metal surface becomes more protective with an increase in theinhibitor concentration (1000 ppm) at 298 K (Fig. 8c). This isattributed to the involvement of compound in the interaction with

the active sites of metal surface. This results in an enhanced surfacecoverage of the metal so that there is a contact betweenmetal and theaggressive medium.

3.7. FTIR and UV studies

FTIR spectrum analysis of inhibitor film removed mechanicallyfrom the MS surface was tested. A comparison of FTIR spectra for pure

Fig. 9. Comparison of FTIR spectrum of pure DMB compound and inhibitor film removed mechanically from the MS surface.

54 R. Goel et al. / Desalination 263 (2010) 45–57

DMB compound and inhibitor film removed mechanically from theMS surface was also performed and given in Fig. 9. It is seen from theFTIR spectra of inhibitor film mechanically removed from the MS

Fig. 10. UV-spectra of the solutions containing 1 N H2SO4 1000 ppm before mild steelimmersion (1) and after 24 h immersion (2).

Fig. 11. Scheme of numbering of atoms as u

surface that the intensity of the peaks stretching frequency isdecreased which implies that the shift peaks in this compound iscoordinated to Fe+2 resulting in the formation of a Fe+2 inhibitorcomplex on the metal surface. UV–visible adsorption spectra obtainedfrom the corrosive solution in the presence of 1000 ppm of dye beforeand after 24 h of steel immersion are shown in Fig. 10. The electronicadsorption spectra of DMB before the steel immersion (curve 1)display two bands in UV region. The shorter wavelength band withλmax at 205 nm and second λmax at 240 nm are ascribed to π–π*transition of the compound. After 24 h of steel immersion (curve 2), itis clearly seen the band maximum of π–π* transition within theheterocyclic moiety underwent a blue shift, suggesting the interactionbetween DMB and Fe+2 ions in the solution. In themean time, there isan increase in the absorbance of this band.

3.8. Theoretical studies

To obtain more information about the inhibition effects of thecompound, the molecular structure of the DMB was optimizedwithout imposing any constraints and the scheme of numbering ofatoms used in the calculations is shown in Fig. 11. Full optimization ofthe full optimization of all bond lengths, bond angles, and dihedralangles of the DMB was carried out at the restricted Hartree-Fock(RHF) level of theory using two semiemirical methods viz., AM1 and

sed in calculations for DMB compound.

Fig. 12. Optimized electronic structure of DMB. The colour coding of the atoms is: Carbon — Cyan, Oxygen — red, Nitrogen — Blue and Hydrogen — white.

55R. Goel et al. / Desalination 263 (2010) 45–57

the PM3method. The optimized structure so obtained is belongs to C1point group and is shown in Fig. 12. The optimized values of the bondlengths, bond angles, and dihedral angles of the DMB are presented inTable 5.

The highest occupied molecular orbital (HOMO) and lowestoccupied molecular orbital (LUMO) energies, LUMO–HOMO energygap the dipole moment and the volume of the DMB as calculated bythe two semi-empirical methods are presented in Table 6. EHOMO isoften associated with the electron donating ability of the molecule

Table 5Computed geometrical parameters of DMB compound.

Computed bond length (A0) Computed bond length (0)

BL AM1 PM3 BA AM1

1–2 1.3935 1.3913 3–2–1 120.15722–3 1.3948 1.3899 4–3–2 119.76003–4 1.3939 1.3914 5–4–3 120.32564–5 1.3947 1.3887 6–5–4 120.18895–6 1.4000 1.3989 7–6–5 123.71556–7 1.4941 1.4944 8–7–6 118.04917–8 1.3773 1.4030 9–8–7 123.14888–9 1.4369 1.4765 10–9–8 110.61749–10 1.5258 1.5140 11–10–9 121.032410–11 1.3975 1.4161 12–11–10 124.656511–12 1.3286 1.3782 13–12–11 124.672312–13 1.3014 1.2985 14–13–12 135.944213–14 1.4597 1.4621 15–14–13 116.769714–15 1.4095 1.4052 16–14–13 126.148114–16 1.3989 1.3972 17–16–14 121.817216–17 1.3885 1.3858 18–17–16 121.183617–18 1.4166 1.4061 19–15–14 122.131315–19 1.3844 1.3827 20–18–17 121.112618–20 1.3887 1.4125 21–20–18 119.937220–21 1.4327 1.4723 22–20–18 120.364720–22 1.4314 1.4716 36–7–8 120.29127–36 1.2497 1.2272 37–11–10 113.754411.37 1.0040 1.0028 25–22–20 110.272022.25 1.216 1.0992 26–22–20 110.492422.26 1.1246 1.0999 27–22–20 110.504422–27 1.1246 1.0997 28–21–20 110.425421–28 1.1213 1.0990 29–21–20 110.499421–29 1.1249 1.0999 30–21–20 110.482921–30 1.1249 1.1001 31–1–2 120.61681–31 1.1031 1–0975 32–2–1 119.84202–32 1.0999 1.0950 33–3–2 120.24983–33 1.1001 1.0949 34–4–3 120.12284–34 1.1002 1.0951 35–5–4 117.57835–35 1.1003 1.1002 23–7–6 121.66007–23 1.2497 1.2272 24–10–9 123.440510–24 1.2480 1.2269 38–16–14 123.080016–38 1.0973 1.0960 39–17–16 118.107817–39 1.1004 1.0969 40–15–14 119.288315–40 1.1025 1.0976 41–19–15 118.864119–42 1.1000 1.0966 42–9–8 110.08249–42 1.1291 1.1114 43–9–8 110.08269–43 1.2191 1.1114 44–13–12 110.253213–44 1.1159 1.1030

and a high value of the EHOMO usually indicates a tendency of themolecule to donate electrons to appropriate acceptor molecules withlow energy, empty molecular orbitals. On the other hand, ELUMO, theenergy of the lowest unoccupied molecular orbital, indicates theability of molecule to accept electrons. The lower the value of ELUMO,the more probable is that the molecule accepts electrons [26,40]. Alow energy band gap value of less than 8.0 eV as estimated by the twoquantum mechanical methods suggests stronger chemisorptions andgood inhibition efficiency, which is in agreement with the observation

Computed dihedral angles (0)

PM3 DA AM1 PM3

120.2687 4–3–2–1 000.0000 000.0000119.8247 5–4–3–2 000.0000 000.0000120.1263 6–5–4–3 000.0000 000.0000120.352 7–6–5–4 179.9982 180.0015121.764 8–7–6–5 000.0351 000.0289117.3897 9–8–7–6 180.0100 180.0224122.9586 10–9–8–7 179.990 179.9857110.8631 11–10–9–8 179.9913 180.0034121.5224 12–11–10–9 000.0000 −00.03337121.4896 13–12–11–10 180.0030 180.0317126.2214 14–13–12–11 000.0000 000.0000133.6889 15–14–13–12 180.0089 179.9949116.0819 16–14–13–12 000.0000 000.0000126.2828 17–16–14–15 000.0000 −00.0148121.4200 18–17–16–14 000.0000 000.0000120.7299 19–15–14–16 000.0000 000.0000121.6993 20–18–17–16 180.0071 180.2191120.7072 21–20–18–17 −00.0453 −00.6706121.1941 22–20–18–17 180.0655 180.5333121.3891 36–7–8–6 180.000 179.9752118.1754 37–11–10–9 180.0109 180.0501118.5269 25–22–20–18 179.9771 179.8666109.6000 26–22–20–18 299.9409 299.3909111.2087 27–22–20–18 060.0050 060.1646111.4158 28–21–20–18 180.014 180.1173109.6824 29–21–20–18 300.0537 299.8504111.4210 30–21–20–18 059.9872 060.5582111.2084 31–1–2–3 179.9982 180.0006119.9965 32–2–1–6 180.0006 179.9999119.7657 33–3–2–1 179.9960 179.9994120.1937 34–4–3–2 179.9998 180.0002120.0642 35–5–4–3 179.9958 180.0009119.1284 23–8–7–6 000.0000 000.0000124.4357 24–10–9–8 000.0000 000.0000125.3620 38–16–14–15 180.0244 179.9725122.3108 39–17–16–14 180.0038 179.9929118.1867 40–15–14–16 180.0053 180.0227119.4986 41–19–15–14 179.9997 180.0234118.6516 42–9–8–23 120.8513 122.0307109.4637 43–9–8–23 239.1283 238.0302109.4703 44–13–12–11 179.9996 180.0072110.5755

Table 6Computed quantum mechanical indices of DMB compound.

Method ofcalculation

EHOMO

(eV)ELUMO

(eV)ELUMO–EHOMO

(eV)μ(DEBYE)

Volume(A3)

AM1 −8.23 −0.28 7.95 5.641 996.98PM3 −8.19 −0.50 7.69 5.785 997.42

56 R. Goel et al. / Desalination 263 (2010) 45–57

by Bentis et al. [31]. The value of dipole moment for DMB is estimatedto be of the order of 5.7 D, which indicates high compound polarityleading to greater inhibition. The calculated electron density distribu-tions for the HOMO and the LUMO for DMB are displayed in Fig. 13.The occurrence of sufficiently high values of HOMO densities aroundthe oxygen and nitrogen, having negative charge of −0.37 and −0.3,respectively, further indicates that these atoms behaves as centres forchemisorptions.

4. Conclusion

In this work, chemical (weight loss) and Electrochemical (Galva-nostatic and Impedance measurement) methods were used to studythe ability of DMB compound to inhibit the corrosion of MS in aerated1.0 N H2SO4 solution. The principal conclusions are:

a. The inhibition efficiency of designed molecules increases byincreasing the inhibitor concentration, but it decreases withincrease in testing temperature. The order of inhibition efficiencywas correlated with the modification of the molecular structure ofinhibitor, while the decrease in inhibitor efficiency with temper-ature was ascribed to the thermodynamic parameters due to thechanges in the nature of molecular interactions.

Fig. 13. Electron density distributions for the H

b. The galvanostatic polarization curve indicates that DMB compoundinhibits both anodic metal dissolution and cathodic hydrogenevolution reaction. This compound acts as mixed type inhibitor.

c. AC impedance plots of mild steel indicate that polarizationresistance increases with increase in inhibitor concentration.

d. The adsorption of DMB molecules on the metal surface from 1.0 NH2SO4 solution obeys Langmuir adsorption isotherm. The negativesign of theΔH indicates that the adsorption process is spontaneousand exothermic.

e. The increase in activation energy after the addition of DMBcompound to the1.0 NH2SO4 solutions indicates that the adsorptionis more physical than chemical.

f. EDX analysis and SEM images suggest that the corrosion of themild steel is mainly through pitting and the addition of inhibitor inthe aggressive solution results in the formation of the protectivefilm on mild steel surface.

g. The results of thermal analysis have shown relatively good thermalstability of surface inhibitor film.

h. Low ELUMO and low energy gap estimated for DMB using thetheoretical calculations supports the high experimental efficiencyreported in the present work. Furthermore, the high inhibitionefficiency is also attributed to high molecular polarity for DMBas indicated by larger dipole moment estimated by theoreticalmethods.

Acknowledgements

The authors are thankful to Prof. Alimuddin and Mr. KhalidMujasam Batoo, Department of Applied Physics, A.M.U. Aligarh, Indiafor providing necessary laboratory facilities to perform Impedancemeasurements.

OMO and the LUMO for DMB compound.

57R. Goel et al. / Desalination 263 (2010) 45–57

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