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AD

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Materials Chemistry and Physics 129 (2011) 68– 76

Contents lists available at ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

nhibitive effect of ceftazidime on corrosion of mild steel in hydrochloric acidolution

shish Kumar Singh, Sudhish Kumar Shukla, Manjeet Singh, M.A. Quraishi ∗

epartment of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India

r t i c l e i n f o

rticle history:eceived 22 October 2010eceived in revised form 12 March 2011ccepted 23 March 2011

eywords:FM

a b s t r a c t

The corrosion inhibition properties of ceftadizime (CZD) for mild steel corrosion in HCl solution wereanalysed by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and gravimet-ric methods. These results were supplemented by atomic force microscopy (AFM) and scanning electronmicroscopy (SEM). Mixed mode of adsorption (physisorption and chemisorption) is proposed for the inhi-bition and the process followed the Langmuir adsorption isotherm and kinetic/thermodynamic model ofEl-Awady et al. The mechanism of adsorption inhibition and type of adsorption isotherm were proposed

◦

EMdsorptionorrosionISnhibitoreftazidime

from the trend of inhibition efficiency with temperature, Ea and �Gads. Potentiodynamic polarizationstudy clearly revealed that CZD acted as mixed type inhibitor. The experimental data showed a frequencydistribution and therefore a modelling element with frequency dispersion behaviour and a constant phaseelement (CPE) have been used.

© 2011 Elsevier B.V. All rights reserved.

ild steel

. Introduction

The corrosion of steel is the most common form of corrosion,specially in acid solution. Hydrochloric acid is also an importantineral acid with many uses in the same systems and leads the

esearchers to the study of the effect of the corrosion inhibitors1–4]. Organic materials act as corrosion inhibitors according toheir functional group, which are adsorbed on the metal surface.

ost of the efficient inhibitors used in industry are organic com-ounds which mainly contain oxygen, sulphur, nitrogen atoms andultiple bonds in the molecule through which they are adsorbed

n metal surface [5,6]. Though the existing data show that numer-us N-heterocyclic organic compounds have good anticorrosivectivity, some of them are highly toxic to both human beings andnvironment. The safety and environmental issues of corrosionnhibitors arisen in industries has always been a global concern. Dueo increasing environmental awareness and adverse effect of somehemicals, research activities in recent times are geared towardseveloping cheap, non-toxic and environmentally safe corrosion

nhibitors [7–11].The present article is devoted to study ceftadizime (CZD) as

orrosion inhibitor for mild steel in hydrochloric acid solution

∗ Corresponding author. Tel.: +91 9307025126; fax: +91 542 2368428.E-mail addresses: maquraishi@rediffmail.com, maquraishi.apc@itbhu.ac.in

M.A. Quraishi).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.03.054

using electrochemical impedance spectroscopy (EIS), potentiody-namic polarization, weight loss, atomic force microscopy (AFM)and scanning electron microscopy (SEM) techniques. The surveyof literature reveals that it is a non-toxic compound (LD50 dose is710 mg kg−1 in rats) [12,13].

Ceftadizime is a commercial name of (6R,7R,Z)-7-(2-(2-aminothiazol-4-yl)-2-(2-carboxypropan-2-yloxyimino)acetamido)-8-oxo-3-(pyridinium-1-ylmethyl)-5-thia-1-aza-bicyclo [4.2.0] oct-2-ene-2-carboxylate. It is a third-generationcephalosporin antibiotic and does not contain any toxic metallicelements. It is sold under the trade name ceptaz. It can be taken asintramuscular or intravenous dose. The structure of ceftadizime isshown in Fig. 1.

2. Experimental

2.1. Inhibitor

Stock solution of CZD was made in 10:1 ratio water:ethanol mixture by volumeto ensure solubility. This stock solution was used for all experimental purposes.

2.2. Corrosion measurements

Prior to all measurements, the mild steel specimens, having composition (wt%)C = 0.17, Mn = 0.46, Si = 0.26, S = 0.017, P = 0.019 and balance Fe, were abraded suc-

cessively with emery papers from 600 to 1200 mesh in.−1 grade. The specimen werewashed thoroughly with double distilled water, degreased with acetone and finallydried in hot air blower. After drying, the specimen were placed in desiccator andthen used for experiment. The aggressive solution of 1 M HCl was prepared bydilution of analytical grade HCl (37%) with double distilled water and all exper-

A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76 69

ftadiz

idsa

2

uuiPaaiAr

r

�

wi

2

iptpre

r

�

wi

2

mi−0w

Fig. 1. Structure of ce

ments were carried out in unstirred solutions. The rectangular specimens withimension 2.5 cm × 2.0 cm × 0.025 cm were used in weight loss experiments and ofize 1.0 cm × 1.0 cm (exposed) with a 7.5 cm long stem (isolated with commerciallyvailable lacquer) were used for electrochemical measurements.

.3. Electrochemical impedance spectroscopy

The EIS tests were performed at 308 ± 1 K in a three electrode assembly. A sat-rated calomel electrode was used as the reference; a 1 cm2 platinum foil wassed as counter electrode. All potentials are reported versus SCE. Electrochemical

mpedance spectroscopy measurements were performed using a Gamry instrumentotentiostat/Galvanostat with a Gamry framework system based on ESA 400 in

frequency range of 10−2 Hz to 105 Hz under potentiodynamic conditions, withmplitude of 10 mV peak-to-peak, using AC signal at Ecorr. Gamry applicationsnclude software DC105 for corrosion and EIS300 for EIS measurements, and Echemnalyst version 5.50 software packages for data fitting. The experiments were car-ied out after 30 min of immersion in the testing solution (no deaeration, no stirring).

The inhibition efficiency of the inhibitor was calculated from the charge transferesistance values using the following equation:

Rct % = Rict − R0

ct

Rict

× 100 (1)

here, R0ct and Ri

ct are the charge transfer resistance in absence and in presence ofnhibitor, respectively.

.4. Potentiodynamic polarization

The electrochemical behaviour of mild steel sample in inhibited and non-nhibited solution was studied by recording anodic and cathodic potentiodynamicolarization curves. Measurements were performed in the 1 M HCl solution con-aining different concentrations of the tested inhibitor by changing the electrodeotential automatically from −250 to +250 mV versus corrosion potential at a scanate of 1 mV s−1. The linear Tafel segments of anodic and cathodic curves werextrapolated to corrosion potential to obtain corrosion current densities (icorr).

The inhibition efficiency was evaluated from the measured icorr values using theelationship:

p% = i0corr − iicorr

i0corr

× 100 (2)

here, i0corr and iicorr are the corrosion current density in absence and presence ofnhibitor, respectively.

.5. Linear polarization measurement

The corrosion behaviour was studied with polarization resistance measure-

ents (Rp) in 1 M HCl solution with and without different concentrations of studied

nhibitor. The linear polarization study was carried out from cathodic potential of20 mV versus OCP to an anodic potential of +20 mV versus OCP at a scan rate.125 mV s−1 to study the polarization resistance (Rp) and the polarization resistanceas evaluated from the slope of curve in the vicinity of corrosion potential. From

ime (CZD) molecule.

the evaluated polarization resistance value, the inhibition efficiency was calculatedusing the relationship:

�Rp % =Ri

p − R0p

Rip

× 100 (3)

where, R0p and Ri

p are the polarization resistance in absence and presence of inhibitor,respectively.

2.6. Weight loss measurements

Weight loss measurements were performed on rectangular mild steel sampleshaving size 2.5 cm × 2.0 cm × 0.025 cm by immersing the mild steel coupons intoacid solution (100 mL) in absence and presence of different concentrations of CZD.After the elapsed time, the specimen were taken out, washed, dried and weighedaccurately. All the tests were conducted in aerated 1 M HCl. All the experimentswere performed in triplicate and average values were reported. From the evaluatedweight loss, surface coverage (�) was calculated using:

� = w0 − wi

w0(4)

where, w0 is weight loss in free acid solution and wi is weight loss in acid solutionin presence of inhibitor, respectively.

2.7. Atomic force microscopy

The surface morphology of mild steel specimen was investigated by using atomicforce microscope (AFM). Atomic force microscopy was performed using a NT-MDTmultimode AFM, Russia, controlled by Solver scanning probe microscope controller.Semi-contact mode was used with the tip mounted on 100 �m long, single beamcantilever with resonant frequency in the range of 2.4 × 105–2.5 × 105 Hz, and thecorresponding spring constant of 11.5 N m−1 with NOVA programme used for imagerendering [14]. The mild steel strips of 1.0 cm × 1.0 cm × 0.025 cm sizes were pre-pared as described in Section 2.2. After immersion in 1 M HCl with and withoutaddition of 1.83 × 10−4 M CZD at 308 K for 3 h, the specimen were cleaned withdistilled water, dried and then used for AFM.

2.8. Scanning electron microscopy

The morphology of the corrosion products formed on the surface of mild steelin 1 M HCl solution in absence and presence of 1.83 × 10−4 M of CZD was also testedby SEM examinations using a Traktor TN-2000 energy dispersive spectrometer anda Jeol-Jem-1200 EX II electron microscope in the vacuum mode by instrument oper-ated at 10 kV.

3. Results and discussion

3.1. Electrochemical impedance spectroscopy

Impedance method provides information about the kinetics ofthe electrode processes and simultaneously about the surface prop-erties of the investigated systems. The shape of impedance gives

70 A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76

F teel ine

mtpdftf(cta

attta(dai

s

ig. 2. (a) Nyquist plots, (b) Bode-magnitude plots, (c) phase angle plots for the mild squivalent circuit used to fit the impedance measurements.

echanistic information. The method is widely used for investiga-ion of the corrosion inhibition processes [15]. Nyquist and Bodelots of mild steel in 1 M HCl solution in absence and presence ofifferent concentrations of CZD are presented in Fig. 2a and b. Itollows from Fig. 2a that a high frequency (HF) depressed charge-ransfer semicircle was observed (one time constant in Bode plot)ollowed by a well defined inductive loop in the low frequencyLF) regions. The HF semicircle is attributed to the time constant ofharge transfer and double-layer capacitance [16,17]. The LF induc-ive loop may be attributed to the relaxation process obtained bydsorption species as Clads

− and Hads+ on the electrode surface [18].

Phase angle at high frequencies provided a general idea ofnticorrosion performance. The more negative the phase anglehe more capacitive the electrochemical behaviour [19]. Chargeransfer resistance increment could raise current tendency to passhrough the capacitor in the circuit. Also, depression of phase anglet relaxation frequency with decreasing the CZD concentrationFig. 2c) indicated the decrease of capacitive response with theecrease of inhibitor concentration. Such a phenomenon could be

ttributed to higher corrosion activity at low concentrations ofnhibitor.

To get more accurate fit of these experimental data, the mea-ured impedance data were analysed by fitting in to equivalent

-10 0 0 100 20 0 30 0 40 0 50 0-10 0

0

100

200

300

400

500

-3 -2 -1 0 1 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0Experi men talSimula ted

Z i/Ω c

m2

Zr/Ω cm2

log Z r/Ω

cm

2

log

a

Fig. 3. (a) Nyquist plot, (b) Bode-magnitude plot and (c) phase an

absence and presence of different concentrations of CZD and (d) the electrochemical

circuit given in Fig. 2d. Excellent fit with this model was obtained forall experimental data. As an example, the Nyquist and Bode-phaseplots in presence of 1.83 × 10−4 M CZD are presented in Fig. 3a–crespectively. The equivalent circuit consists of the double-layercapacitance (Cdl) in parallel to the charge transfer resistance (Rct),which is in series to the parallel of inductive elements (L) and RL.The presence of L in the impedance spectra in the presence of theinhibitor indicates that mild steel is still dissolved by the directcharge transfer at the CZD-adsorbed mild steel surface [20].

One constant phase element (CPE) is substituted for the capaci-tive element to give a more accurate fit, as the obtained capacitiveloop is a depressed semi-circle. The depression in Nyquist semi-circles is a feature for solid electrodes and often referred to asfrequency dispersion and attributed to the roughness and otherinhomogenities of the solid electrode [21]. The CPE is a specialelement whose admittance value is a function of the angular fre-quency (ω), and whose phase is independent of the frequency. Theadmittance and impedance of CPE is given by;

n

YCPE = Y0(iω) (5)

where, Y0 is the magnitude of CPE, i is an imaginary number(i2 = − 1)˛ is the phase angle of CPE and n = ˛/(�/2) in which ̨ isthe phase angle of CPE.

2 3 4 5 6 -3 -2 -1 0 1 2 3 4 5 6

0

-20

-40

-60

-80

f /Hz

Phas

e an

gle

/o

log f /Hz

b c

gle plot of iron in 1 M HCl in presence of 1.83 × 10−4 M CZD.

A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76 71

Table 1Impedance parameters for mild steel in 1 M HCl in absence and presence of different concentrations of CDZ.

Inhibitor Conc. of inhibitor Rs Q n L Rct RL �Rct %(M × 10−4) (� cm2) (10−6 × �−1 sn cm−2) (H) (� cm2) (� cm2)

– 1.3 164 0.811 13 16 1 –CDZ 0.36 0.6 87 0.857 53 65 9 75

0.73 0.6 49 0.842 308 231 33 931.46 1.0 41 0.860 165 261 49 941.83 0.9 32 0.864 300 374 59 96

Table 2Potentiodynamic polarization parameters for mild steel without and with different concentrations of CDZ in 1 M HCl.

Inhibitor Conc. Tafel data Linear polarization data

(M × 10−4) −Ecorr icorr ba bc �P % Rp �Rp %(mV vs. SCE) (�A cm−2) (mV dec−1) (mV dec−1) (� cm2)

– 469 730 73 127 – 19 –CDZ 0.36 506 208 63 282 72 85 78

0.73 480 91 74 147 88 147 8777

aiuvi

C

wiq

3

kto1awr

Fa

1.46 503 38

1.83 493 25

The point of intersection between the inductive loop and the realxis represents (Rs + Rct). The electrochemical parameters, Includ-ng Rs, Rct, RL, L, Y0 and n, obtained from fitting the recorded EISsing the electrochemical circuit of Fig. 2d are listed in Table 1. Cdlalues derived from CPE parameters according to Eq. (8) are listedn Table 1.

dl = Y0(ωmax)n−1 (6)

here, ωmax is angular frequency (ωmax = 2�fmax) at which themaginary part of impedance (−Zi) is maximal and fmax is AC fre-uency at maximum.

.2. Potentiodynamic polarization measurements

Polarization measurements were carried out in order to gainnowledge concerning the kinetics of the cathodic and anodic reac-ions. Fig. 4 presented the results of the effect of CZD concentrationn the cathodic and anodic polarization curves of mild steel in

M HCl, respectively. It could be observed that both the cathodicnd anodic reactions were suppressed with the addition of CZD,hich suggested that the CZD reduced anodic dissolution and also

etarded the hydrogen evolution reaction.

-7

-6

-5

-4

-3

-2

-1

0

1

2

-200-400-600-800

1 M HCl0.36 × 10-4 M0.73 × 10-4 M1.46 × 10-4 M1.83 × 10-4 M

E/mV versus SC E

logi

/mA

cm

-2

ig. 4. Typical polarization curves for corrosion of mild steel in 1 M HCl in thebsence and presence of different concentrations of CZD.

2 168 95 324 943 132 97 438 96

Electrochemical corrosion kinetics parameters, i.e., corrosionpotential (Ecorr), cathodic and anodic Tafel slopes (ba, bc) and cor-rosion current density (icorr) obtained from the extrapolation of thepolarization curves, were given in Table 2.

It followed from Table 2 that the values of bc changed withincreasing inhibitor concentration, indicated the influence of thecompounds on the kinetics of hydrogen evolution. The shift in theanodic Tafel slope bc may be due to the chloride ions/or inhibitormolecules adsorbed onto steel surface. It is important to note thatin anodic domain, for potential higher than −300 mV versus SCE,the presence of CZD did not change the current-versus-potentialcharacteristics (Fig. 4). This potential can be defined as the desorp-tion potential. The phenomenon may be due to the obvious metaldissolution, which leading to a desorption of the inhibitor moleculefrom the electrode surface, in this case the desorption rate of theinhibitor is higher than its adsorption rate, so the corrosion cur-rent increased more obviously with rising potential. Due to thepresence of some active sites, hetero-atoms in the studied com-pound for making adsorption, it may act as adsorption inhibitor.Being adsorbed on the metal surface, CZD controlled the anodicand cathodic reactions during corrosion process, and then its cor-rosion inhibition efficiency is directly proportional to the amountof adsorbed inhibitor. The functional groups and structure of theinhibitor play important roles during the adsorption process. On theother hand, an electron transfer may take place during adsorptionof the neutral organic compounds at metal surface. Similar resultswere found by Ozcan et al. [22]. As it can be seen from Table 2, thestudied inhibitor reduced both anodic and cathodic currents witha slight shift in corrosion potential (≈11–37 mV). According to Fer-reira and others [23,24], if the displacement in corrosion potentialis more than 85 mV with respect to corrosion potential of the blanksolution, the inhibitor can be seen as a cathodic or anodic type. Inthe present study, the maximum displacement was 37 mV whichindicated that the studied inhibitor is a mixed type inhibitor. Theresults obtained from Tafel polarization showed good agreementwith the results obtained from EIS.

3.3. Linear polarization resistance

Polarization resistance values were determined from the slope

of the potential-current lines,

Rp = AdE

di(7)

72 A.K. Singh et al. / Materials Chemistry

Table 3Corrosion parameters for mild steel in 1 M HCl without and with different concen-trations of CZD at 308 K.

Inhibitor Conc. Surface coverage Weight loss CR

(M × 10−4) (�) (mg cm−2) (mm y−1)

– – – 10.9 40.4CZD 0.36 0.75 27.2 10.1

0.73 0.88 13.1 13.71.10 0.90 10.6 3.90

waiow

3

psoi1simCc1mt12

3

Tswtwop

3

3

cCctti

3

maci

1.46 0.95 5.70 2.111.83 0.96 4.20 1.55

here, A is the surface area of electrode, dE is change in potentialnd di is change in current. The inhibition efficiencies and polar-zation resistance parameters are presented in Table 2. The resultsbtained from Tafel polarization and EIS showed good agreementith the results obtained from linear polarization resistance.

.4. Atomic force microscopy

AFM is a powerful technique to investigate the surface mor-hology at nano to micro-scale and has become a new choice totudy the influence of inhibitor on the generation and the progressf the corrosion at the metal/solution interface. Analysis of themages allowed quantification of surface roughness over area scales2 �m × 12 �m. Atomic force microscope was used mainly for mea-uring three-dimensional topography. The three-dimensional AFMmages are shown in Fig. 5a–c. As can be seen from Fig. 5, there is

uch less damage on the surface of mild steel with 1.83 × 10−4 MZD (Fig. 5a). The average roughness of bare mild steel surface wasalculated as 165 nm and the average roughness of mild steel in

M HCl without inhibitor (Fig. 5b) was calculated as 660 nm. Theild steel in the free acid solution is getting cracked due to due

o acid attack on the surface (Fig. 5b). However, in presence of.83 × 10−4 M of inhibitor the average roughness was reduced to30 nm (Fig. 5a).

.5. Scanning electron microscopy

The SEM images of mild steel surfaces are given in Fig. 5d–f.he inspection of Fig. 5d, which is the micrograph of the mild steelurfaces exposed to 1 M HCl revealed that the specimen surfaceas strongly damaged in the absence of inhibitor. Fig. 5e showed

he SEM image of the mild steel surface exposed to the 1 M HClith the addition of 1.83 × 10−4 M CZD. There are no pits and cracks

bserved in the micrograph except polishing lines. Fig. 5f showedolished mild steel surface.

.6. Weight loss measurement

.6.1. Effect of inhibitor concentrationTable 3 presented weight loss, corrosion rate and inhibition effi-

iency in the absence and presence of different concentrations ofZD. It followed from Table 3 that the weight loss decreased (i.e.,orrosion rate is suppressed), and therefore the corrosion inhibi-ion strengthened, with increase in inhibitor concentration. Thisrend may result from the fact that adsorption and surface coveragencreased with the increase in CZD concentration.

.7. Adsorption isotherm

Adsorption depends mainly on the charge and nature of the

etal surface, electronic characteristics of the metal surface,

dsorption of solvent and other ionic species, temperature oforrosion reaction and on the electrochemical potential at solution-nterface. Adsorption of inhibitor involves the formation of two

and Physics 129 (2011) 68– 76

types of interaction responsible for bonding of inhibitor to ametal surface. The first one (physical adsorption) is weak undi-rected interaction and is due to electrostatic attraction betweeninhibiting organic ions or dipoles and the electrically chargedsurface of metal. The potential of zero charge plays an impor-tant role in the electrostatic adsorption process. The charge onmetal surface can be expressed in terms of potential difference(˚) between the corrosion potential (Ecorr) and the potential ofzero charge (Epzc) of the metal (� = Ecorr − Epzc). If ̊ is negative,adsorption of cations is favoured. On the contrary, the adsorp-tion of anions is favourable if ̊ is positive. The second typeof interaction (adsorption) occurs when directed forces governthe interaction between the adsorbate and adsorbent. Chemicaladsorption involves charge sharing or charge transfer from adsor-bates to the metal surface atoms in order to form a coordinatetype of bond. Chemical adsorption has a free energy of adsorptionand activation energy higher than physical adsorption and, hence,usually it is irreversible [25]. Adsorption isotherms are usuallyused to describe the adsorption process. The most frequently usedisotherms include: Langmuir, Frumkin, Temkin, Flory–Huggins,Dhar–Flory–Huggins, Bockris–Swinkels and the recently formu-lated thermodynamic/kinetic model of El-Awady et al. [26,27]. Theestablishment of adsorption isotherms that describe the adsorp-tion of a corrosion inhibitor can provide important clues to thenature of the metal–inhibitor interaction. Adsorption of the organicmolecules occurs as the interaction energy between molecule andmetal surface is higher than that between the H2O molecule andthe metal surface [28].

In order to obtain the adsorption isotherm, the degree of sur-face coverage (�) for various concentrations of the inhibitor hasbeen calculated according to Eq. (4). Langmuir isotherm was testedfor its fit to the experimental data. Langmuir isotherm is given byfollowing equation:

Cinh

�= 1

Kads+ Cinh (8)

where, Kads is the equilibrium constant of theadsorption–desorption process, � is the degree of surface coverageand Cinh is molar concentration of inhibitor in the bulk solution.Though the plot of Cinh versus Cinh/� was linear (Fig. 6a) (correla-tion > 0.9), the deviation of the slopes from unity (Table 4) (for idealLangmuir adsorption isotherm) can be attributed to the molecularinteraction among the adsorbed inhibitor species, a factor whichwas not taken in to consideration during the derivation of theLangmuir equation. Langmuir isotherm assumes that:

(i) The metal surface contains a fixed number of adsorption sitesand each site holds one adsorbate;

(ii) �G◦ads is the same for all sites and it is independent of �;

(iii) The adsorbates do not interact with one another, i.e., there isno effect of lateral interaction of the adsorbates on �G◦

ads [29].

Though the linearity of the Langmuir plot may be interpreted tosuggest that the experimental data for CZD obeyed the Langmuiradsorption isotherm, the deviation of the slope from unity showedthat the isotherm cannot be strictly applied.

Hence, the experimental data were fitted into the El-Awady’skinetic/thermodynamic model. The characteristic of the isothermis given by:

log

(�

1 − �

)= log K + y log Cinh (9)

where, Cinh is the molar concentration of inhibitor, � is the degreeof surface coverage, Kads is the equilibrium constant of adsorptionprocess and Kads = K1/y. In this model, the number of active sites y is

A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76 73

Fig. 5. (a) Atomic force micrograph of inhibited mild steel surface (1 M HCl + 1.83 × 10−4 M CZD), (b) atomic force micrograph of mild steel surface exposed to free acidsolution, (c) atomic force micrograph of polished mild steel surface, (d) scanning electron micrograph of mild steel surface exposed to free acid solution and (e) scanningelectron micrograph of inhibited mild steel (1 M HCl + 1.83 × 10−4 M CZD) and (f) scanning electron micrograph of polished mild steel surface.

74 A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76

0.000200.000160.000120.000080.00004

0.00006

0.00012

0.00018

0.00024

0.00030

-3.6-3.8-4.0-4.2-4.4-4.60.0

0.4

0.8

1.2

1.6308 Ka b318 K328 K338 K

C inh/θ

/M

log θ/

1-θ

rm an

iwotsietah

3

e

l

C

w�orfcwcepetTtesaiii

TC

Cinh/M

Fig. 6. (a) Langmuir adsorption isothe

ncluded. Values of 1/y less than one imply multilayer adsorption,hile 1/y greater than one suggests that a given inhibitor molecule

ccupies more than one active site. Curve fitting of the data to thehermodynamic–kinetic model is shown in Fig. 6b. This plot gavetraight lines which clearly showed that the data fitted well to thesotherm. The values of 1/y and Kads calculated from the El-Awadyt al. model curve is given in Table 4. It is also seen from Table 4hat Kads decreased with increase in temperature indicating thatdsorption of CZD on the mild steel surface was unfavourable atigher temperatures.

.8. Kinetic and thermodynamic considerations

The dependence of corrosion rate at temperature can bexpressed by Arrhenius equation and transition state equation:

og(CR) = −Ea

2.303RT+ log (10)

R = RT

Nhexp

(�S∗

R

)exp

(−�H∗

RT

)(11)

here, Ea apparent activation energy, the pre-exponential factor,H* the apparent enthalpy of activation, �S* the apparent entropy

f activation, h Planck’s constant and N the Avogadro number,espectively. The apparent activation energy and pre-exponentialactors for a wide range of concentration of the inhibitor can bealculated by linear regression between log CR and 1/T, the resultsere shown in Table 5. All the linear regression coefficients are

lose to 1, indicating that corrosion of mild steel in 1 M HCl can bexplained using the kinetic model. Fig. 7a depicted an Arrheniuslots for mild steel immersed in 1 M HCl in presence of differ-nt concentration of CZD. The plots obtained are straight lines andhe slope of each straight line gives its apparent activation energy.able 5 presented Ea and values for a wide range of concentra-ion of CZD. Inspection of Table 5 showed that apparent activationnergy increased on addition of CZD in comparison to the blankolution. The increase in Ea could be interpreted as the physical

dsorption. Szauer and Brandt [30] explained that the increasen activation energy can be attributed to an appreciable decreasen the adsorption of the inhibitor on the mild steel surface withncrease in temperature and a corresponding increase in corrosion

able 4alculated parameters for Langmuir and El-Awady adsorption isotherm.

Temperature (K) Langmuir’s isotherm

Kads (mol−1) −�G◦ads

(kJ mol−1) Slope

308 10.0 × 104 −40 0.96

318 2.50 × 104 −37 0.90

328 1.25 × 104 −37 0.77

338 0.50 × 104 −35 0.49

logCinh/M

d (b) El-Awady adsorption isotherm.

rates occurs due to the fact that greater area of metal is exposed tothe acid environment.

According to Eq. (11), corrosion rate (CR) is being affected byboth Ea and . In general, the influence of Ea on the mild steel cor-rosion is higher than that of . However, if the variation in wasdrastically higher than that of Ea, the value of might be the dom-inant factor to determine the mild steel corrosion. In present case,Ea and increased with concentration (the higher Ea and lower led to lower corrosion rate). As it can be seen from Fig. 7a, the corro-sion rate of steel decreased with increasing concentration; hence,it is clear that increment of Ea is the decisive factor affecting thecorrosion rate of mild steel in 1 M HCl.

The relationship between log(CR/T) and 1/T were shown inFig. 7b. Straight lines are obtained with a slope (−�H*/2.303R) andan intercept of [log(R/Nh) + (�S*/2.303R)], from which the value of�H* and �S* were calculated and presented in Table 5. The positivesign of enthalpy reflect the endothermic nature of steel dissolutionprocess meaning that dissolution of steel is difficult. On compar-ing the values of entropy of activation (�S*) listed in Table 5, it isclear that entropy of activation increased in presence of the studiedinhibitor compared to free acid solution. Such variation is associ-ated with the phenomenon of ordering and disordering of inhibitormolecules on the mild steel surface. The increased entropy of acti-vation in the presence of inhibitor indicated that disorderness isincreased on going from reactant to activated complex.

An estimate of heat of adsorption was obtained for the trend ofsurface coverage with temperature as follows:

log

(�

1 − �

)= log A + log Cinh −

(�H◦

ads2.303RT

)(12)

A plot of log(�/1 − �) versus 1/T is given in Fig. 7c. The slope(−�H◦

ads/2.303R) of the graph gives the value of heat of adsorp-tion. The values of heat of adsorption are presented in Table 5.According to Bhajiwala and Vashi [31], negative �H◦

ads values implythat inhibitor adsorption and hence efficiency decreases at hightemperature while positive values suggest increased efficiency at

high temperature. From Table 5, it is evident that in all cases thevalues of �H◦

ads are negative and in the range of −24 kJ mol−1 to−45 kJ mol−1. Since these values are near to 40 kJ mol−1 hence,suggesting the mixed mode of adsorption (involvement of both

El-Awady isotherm

R2 Kads (mol−1) �G◦ads

(kJ mol−1) 1/y R2

0.99 6.17 × 104 −38 0.77 0.970.99 5.25 × 104 −39 0.86 0.970.97 3.58 × 104 −39 0.92 0.960.75 3.01 × 104 −40 1.11 0.99

A.K. Singh et al. / Materials Chemistry and Physics 129 (2011) 68– 76 75

Table 5Thermodynamic parameters for mild steel in absence and presence of different concentrations of different inhibitors.

Inhibitor Conc. (M × 10−4) Ea (kJ mol−1) (mg cm−2) H* (kJ mol−1) S* (J mol−1 K−1) �H◦ads

(kJ mol−1)

– 42 5.31 × 108 40 −87 –CZD 0.36 58 6.98 × 1010 56 −46 −24

0.73 66 8.10 × 1011 64 −26 −301.10 68 10.96 × 1011 65 −23 −301.46 75 8.79 × 1012 72 −6 −361.83 83 15.88 × 1013 80 18 −45

3.33.23.13.02.90.8

1.2

1.6

2.0

2.4

2.8

3.2 a b c1 M HCl0.36 × 10-4M0.73 × 10-4M1.10 × 10-4M1.46 × 10-4M1.83 × 10-4M

log C

R/m

mye

ar-1

-1 1000 / -1 -13.33.23.13.02.9

-1.6

-1.2

-0.8

-0.4

0.0

log C

R/T

/mm

year

-1K

-1

3.33.23.13.02.9

0.4

0.8

1.2

1.6308 K318 K328 K338 K

log θ/

1-θ

F ns of C

p

mee

K

w5Ttwtlpimt

4

titiFoaimctahfpch

1000 /T/K

ig. 7. Adsorption isotherm plots in absence and presence of different concentratio

hysisorption and chemisorption) [32,33].The equilibrium constant for the adsorption process from Lang-

uir and Kinetic/thermodynamic isotherm model of El-Awadyt al. is related to the standard free energy of adsorption by thexpression:

ads = 155.5

exp

[−�G◦ads

RT

](13)

here, R is the molar gas constant, T is the absolute temperature and5.5 is the concentration of water in solution expressed in molar.he standard free energy of adsorption, �G◦

ads, which can charac-erize the interaction of adsorption molecules and metal surface,as calculated by Eq. (13). The negative values of �G◦

ads ensurehe spontaneity of adsorption process and stability of the adsorbedayer on the mild steel surface. The adsorption is enhanced by theresence of N atoms with lone pairs of electrons, S atoms in the

nhibitor molecules that make it adsorbed electrostatically on theetal surface forming insoluble stable films on the metal surface

hus decreasing metal dissolution.

. Mechanism of inhibition

Generally, Corrosion inhibition mechanism in acid medium ishe adsorption of inhibitor onto the metal surface. As far as thenhibition process is concerned, it is generally assumed that adsorp-ion of the inhibitor at the metal/solution interface is the first stepn the action mechanism of the inhibitors in aggressive acid media.our types of adsorption may take place during inhibition involvingrganic molecules at the metal/solution inter-face: (1) electrostaticttraction between charged molecules and the charged metal, (2)nteraction of unshared electron pairs in the molecule with the

etal, (3) interaction of �-electrons with the metal, and (4) aombination of the above [34]. Concerning inhibitors, the inhibi-ion efficiency depends on several factors; such as the number ofdsorption sites and their charge density, molecular size, heat ofydrogenation, mode of interaction with the metal surface, and the

ormation metallic complexes [35]. Physical adsorption requiresresence of both electrically charged surface of the metal andharged species in the bulk of the solution; the presence of a metalaving vacant low-energy electron orbital and of an inhibitor with

T/K 1000 /T/K

ZD for (a) log CR versus 1/T, (b) log (CR/T) versus 1/T and (c) log (�/1 − �) versus 1/T.

molecules having relatively loosely bound electrons or heteroatomwith lone pair electrons. However, the compound reported can beprotonated in an acid medium. Thus they become cations, existingin equilibrium with the corresponding molecular form:

CZD + xH+ ↔ [CZDx]x+ (14)

The protonated CZD, however, could be attached to the mild steelsurface by means of electrostatic interaction between Cl− and pro-tonated CZD since the mild steel surface has positive charge in theHCl medium [36]. This could further be explained based on theassumption that in the presence of Cl−, the negatively charged Cl−

would attach to positively charged surface and thereby protonatedCZD being adsorbed to the metal surface.

5. Conclusions

The following main conclusions are drawn from the presentstudy:

1. CZD was found to be a good inhibitor for mild steel corrosion inacid medium.

2. The increasing values of n indicated the decreasing roughness ofmild steel surface with CZD concentration.

3. The inhibition efficiency of CZD decreased with temperature,which leads to an increase in activation energy of the corrosionprocess.

4. The adsorption of CZD followed El-Awady’s kinetic–thermodynamic model.

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