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Materials Chemistry and Physics 123 (2010) 666–677

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

nvestigation of adsorption of isoniazid derivatives at mild steel/hydrochloriccid interface: Electrochemical and weight loss methods

shish Kumar Singh, M.A. Quraishi ∗

epartment of Applied Chemistry, Institute of Technology Banaras Hindu University, Varanasi 221 005, India

r t i c l e i n f o

rticle history:eceived 11 January 2010eceived in revised form 19 April 2010ccepted 8 May 2010

eywords:FMlectrochemical techniquesorrosion

a b s t r a c t

The corrosion inhibition properties of isoniazid derivatives, namely N-(morpholino methyl) isatin-3-isonicotinoyl hydrazone (MIIH), N-(piperazino methyl) isatin-3-isonicotinoyl hydrazone (PIIH),N-(2-Thio benzimidazolyl methyl) isatin-3-isonicotinoyl hydrazone (TBIH), N-(piperadino methyl)isatin-3-isonicotinoyl hydrazone (PIIIH) for mild steel corrosion in 1 M HCl medium were analysedby electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and weight loss tech-niques. Electrochemical impedance data demonstrated that the addition of the isoniazid derivatives inthe corrosive solution decreased the charge capacitance and simultaneously increased the function of thecharge/discharge of the interface, facilitating the formation of an adsorbed layer over the steel surface.Adsorption of these inhibitors on the steel surface obeyed the Langmuir adsorption isotherm. Potentio-

dsorptiondynamic polarization studies showed that all the tested inhibitors affected both the anodic and cathodicprocess, thus they can be classified as mixed type of inhibitor. The effect of chemical structure of the fourtested inhibitors was discussed. It was found that the efficiency order followed by molecules tested wasTBIH > PIIH > MIIH > PIIIH. Thus TBIH turned out to be the best inhibitor. This fact strongly suggests that,an efficient corrosion inhibitor molecule should be large one, planar, having unoccupied d-orbital and

ive n

also containing an extens

. Introduction

The corrosion of mild steels has received a considerable amountf attention as a result of its industrial concern. Corrosion is ahermodynamically feasible process, because it is associated withegative change of the Gibbs free energy. The inhibitors influ-nce the kinetics of the electrochemical reactions which constitutehe corrosion process and thereby modify the metal dissolutionn acids. The existing data show that most organic inhibitors acty adsorption on the metal surface. They change the structuref the electrical double layer by adsorption on the metal surface.he adsorption itself depends to a great extent on the moleculartructure. Generally, it is assumed that strong adsorption of thenhibitors is a prerequisite. The adsorption of inhibitors leads tohe formation of a physical barrier that reduces the metal reac-ivity in the electrochemical reactions of corrosion [1]. Because of

he general aggressiveness of acid solutions, the use of inhibitorso control the destructive attack of acid environment was found toave widespread applications [2,3]. Nitrogen, oxygen and/or sul-hur containing heterocyclic compounds with various substituents

∗ Corresponding author. Tel.: +91 9307025126; fax: +91 542 2368428.E-mail address: maquraishi.apc@itbhu.ac.in (M.A. Quraishi).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.05.035

umber of �-electrons.© 2010 Elsevier B.V. All rights reserved.

are considered to be effective corrosion inhibitors [4–9]. The intro-duction of sulphur atom in heterocyclic compounds containingnitrogen has proved very good for inhibition of metal corrosionin acidic solutions [10].

This article reports our attempt to use electrochemicalimpedance spectroscopy (EIS), potentiodynamic polarization,weight loss and atomic force microscopy (AFM) to investigate thenature of adsorption of isoniazid derivatives on the mild steel sur-face.

2. Experimental

2.1. Synthesis of isoniazid derivatives

The condensation product of isatin and isoniazid was subsequently stirred withHCHO and morpholine/piperazine/piperidine/2-mercapto-benzimidazole (organiccompounds containing active hydrogen) in ethanol to produce desired compounds[11,12] according to Scheme 1. The melting points and IR data of all the studiedcompounds are given in Table 1.

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 abradedsuccessively with emery papers from 600 to 1200 grade. The specimen were washedthoroughly with double distilled water, degreased with acetone and finally driedin hot air blower. After drying, the specimen were placed in desiccator and thenused for experiment. The aggressive solution 1 M HCl was prepared by dilution of

A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677 667

hetic

ai

2

swii4wi

TM

Scheme 1. Structure and synt

nalytical grade HCl (37%) with double distilled water. All the concentrations ofnhibitors for electrochemical and weight loss studies were taken in ppm by weight.

.3. Electrochemical impedance spectroscopy

The EIS tests were performed at 303 ± 1 K in a three electrode assembly. Aaturated calomel electrode was used as the reference; a 1 cm2 platinum foil

as used as counter electrode. All potentials are reported vs. SCE. Electrochem-

cal impedance spectroscopy measurements (EIS) were performed using a Gamrynstrument Potentiostat/Galvanostat with a Gamry framework system based on ESA00 in a frequency range of 100,000–0.01 Hz under potentiodynamic conditions,ith amplitude of 10 mV peak-to-peak, using AC signal at Ecorr. Gamry applications

nclude software DC105 for corrosion and EIS300 for EIS measurements, and Echem

able 1.P. and IR data of all the studied Isoniazid derivatives.

Name of the compounds M.P. (◦C)

MIIH 298–299

PIIH 293–294

TBIH 240–242

PIIIH 288–290

route of isoniazid derivatives.

Analyst version 5.50 software packages for data fitting. The experiments were mea-sured after 30 min of immersion in the testing solution (no deaeration, no stirring).The working electrode was prepared from a square sheet of mild steel such that thearea exposed to solution was 1 cm2.

The charge transfer resistance values were obtained from the diameter of thesemi circles of the Nyquist plots. The inhibition efficiency of the inhibitor was cal-culated from the charge transfer resistance values using the following equation:

E(%) = R′ct − R0

ct

Rct× 100 (1)

where R0ct and R′

ct are the charge transfer resistance in absence and in the presenceof inhibitor, respectively.

IR (cm−1)

1140 (C–O–C), 1204 (C–N), 1410, 1530, 1575 (C C of aromatic ring),1655 (C O), 1620 (C N), 3012 (Ar–H), 3105 (C–H str. of morpholinering), 3212 (N–H)1230 (C–N), 1400, 1508, 1561 (C C of aromatic ring), 1637 (C O), 1625(C N), 3008 (Ar–H), 3120 (C–H str. of piperazine ring), 3244 (N–H)772 (C–S), 1235 (C–N), 1474, 1545, 1567 (C C of aromatic ring), 1634(C O), 1628 (C N), 3013 (Ar–H), 3285 (N–H of benzimidazole nucleus)1210 (C–N), 1530, 1567 (C C of aromatic ring), 1643 (C O), 1630(C N), 3020 (Ar–H), 3140 (C–H str. of piperidine ring), 3255 (N–H)

668 A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677

he abs

2

iptpol

r

�

wi

2

mi−tapt

�

wr

2

fmSccra

Fig. 1. Nyquist plots obtained for the mild steel in 1 M HCl in t

.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 inhibitors by changing the electrodeotential automatically from −250 to +250 mV vs. corrosion potential at a scan ratef 1 mV s−1. The linear Tafel segments of anodic and cathodic curves were extrapo-ated to corrosion potential to obtain corrosion current densities (Icorr).

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

p% = I0corr − Ii

corr

I0corr

× 100 (2)

here I0corr and Ii

corr 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

nhibitors. The linear polarization study was carried out from cathodic potential of20 mV vs. OCP to an anodic potential of +20 mV vs. OCP at a scan rate 0.125 mV s−1

o study the polarization resistance (Rp) and the polarization resistance was evalu-ted from the slope of curve in the vicinity of corrosion potential. From the evaluatedolarization resistance value, the inhibition efficiency was calculated using the rela-ionship:

Rp % =Ri

p − R0p

Rip

× 100 (3)

here R0p and R′

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

.6. Atomic force microscopy

The surface morphology of mild steel specimen was investigated by using atomicorce microscope (AFM). Atomic force microscopy was performed using a NT-MDT

ultimode AFM, Russia, controlled by Solver scanning probe microscope controller.emi-contact mode was used with the tip mounted on 100 �m long, single beamantilever with resonant frequency in the range of 240,000–255,000 Hz, and theorresponding spring constant of 11.5 N m−1 with NOVA programme used for imageendering [5]. The mild steel strips of 1.0 cm × 1.0 cm × 0.025 cm sizes were prepareds described in Section 2.2. After immersion in 1 M HCl with and without addition of

ence and presence of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

200 ppm of all the isoniazid derivatives at 308 K for 3 h, the specimen were cleanedwith distilled water, dried and then used for AFM.

2.7. Weight loss measurements

Weight loss measurements were performed on rectangular mild steel sam-ples having size 2.5 cm × 2.0 cm × 0.025 cm by immersing the mild steel couponsinto acid solution (100 mL) without and with different concentrations of isoniazidderivatives. After the elapsed time, the specimen were taken out, washed, driedand weighed accurately. All the tests were conducted in aerated 1 M HCl. All theexperiments were performed in triplicate and average values were reported.

3. Results and discussion

3.1. Electrochemical impedance spectroscopy

The corrosion behaviour of mild steel in 1 M HCl in absenceand presence of isoniazid derivatives were investigated by EISafter immersion for 30 min at 303 ± 1 K. Nyquist and Bode plotsof mild steel in uninhibited and inhibited acid solutions contain-ing various concentrations of isoniazid derivatives are presentedin Figs. 1 and 2. EIS spectra obtained consists of one depressedcapacitive loop (one time constant in Bode-phase plot) at higherfrequency range followed by an inductive loop that is observed inthe low frequency region. The increasing diameter of capacitiveloop obtained in 1 M HCl in presence of isoniazid derivatives indi-cated the inhibition of corrosion of mild steel. The high frequencycapacitive loop may be attributed to the charge transfer reaction.The presence of low frequency inductive loop may be attributed tothe relaxation process obtained by adsorption species like Cl−ads andH+

ads on the electrode surface [13,14]. It may also be attributed tothe re-dissolution of the passivated surface at low frequencies [15].

Corrosion kinetic parameters derived from EIS measurementsand inhibition efficiencies are given in Table 2. Double layer capac-itance (Cdl) and charge transfer resistance (Rct) were obtained fromEIS measurements as described elsewhere [16]. It is apparent fromTable 1 that the impedance of the inhibited system amplified with

A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677 669

Fig. 2. Bode-magnitude plots obtained for the mild steel in 1 M HCl in the absence and presence of different concentrations of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

Table 2Electrochemical impedance parameters of mild steel in 1 M HCl in absence and presence of different concentrations of different inhibitors.

Name of inhibitor Inhibitor concentration (ppm) Rct (� cm2) Y0 (�F cm−2) n Cdl (�F cm−2) E (%)

– 0.0 17.24 162.00 0.845 55.09 –

MIIH 50.0 104.30 123.00 0.803 42.24 83.47100.0 271.00 89.00 0.794 33.86 93.63150.0 469.60 65.00 0.792 26.00 96.32200.0 553.10 36.00 0.814 14.70 96.88

PIIH 50.0 109.10 120.00 0.809 43.11 84.19100.0 324.00 111.00 0.773 41.80 94.67150.0 476.10 82.00 0.801 37.05 96.37200.0 723.40 54.00 0.810 25.23 97.61

TBIH 50.0 153.90 95.00 0.873 51.37 88.79100.0 505.50 52.00 0.754 15.86 96.58150.0 675.20 46.00 0.764 15.73 97.44200.0 1170.00 20.00 0.864 11.07 98.52

iwftifsaeneatw

Z

wib

analyse the impedance spectra. Excellent fit with this model wasobtained for all experimental data. As an example, the Nyquistand Bode plots for free acid solution is presented in Fig. 4a andb, respectively. The measured and simulated data fitted very well.

PIIIH 50.0 72.91100.0 186.40150.0 400.50200.0 486.60

ncreasing the inhibitor concentration and the Cdl values decreasedith increasing inhibitor concentration. This decrease in Cdl results

rom a decrease in local dielectric constant and/or an increase in thehickness of the double layer, suggested that inhibitor moleculesnhibit the iron corrosion by adsorption at the metal/acid inter-ace [17]. The depression in Nyquist semicircles is a feature forolid electrodes and often referred to as frequency dispersion andttributed to the roughness and other inhomogenities of the solidlectrode [18]. In this behaviour of solid electrodes, the paralleletwork: charge transfer resistance-double layer capacitance isstablished where an inhibitor is present. For the description offrequency independent phase shift between an applied ac poten-

ial and its current response, a constant phase element (CPE) is usedhich is defined in impedance representation as in Eq. (4)

CPE = Y0−1(iω)−n (4)

here Y0 is the CPE constant, ω is the angular frequency (in rad s−1),2 = −1 is the imaginary number and n is a CPE exponent which cane used as a gauge of the heterogeneity or roughness of the surface

110.00 0.867 52.46 76.3592.00 0.802 33.71 90.7583.00 0.789 33.39 95.6971.00 0.800 30.61 96.75

[19]. Depending on the value of n, CPE can represent resistance(n = 0, Y0 = R), capacitance (n = 1, Y0 = C), inductance (n = −1, Y0 = L)or Warburg impedance (n = 0.5, Y0 = W).

Fig. 3 showed the electrical equivalent circuit employed to

Fig. 3. The electrochemical equivalent circuit used to fit the impedance measure-ments that include a solution resistance (Rs), a constant phase element (CPE) and apolarization resistance or charge transfer (Rct).

670 A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677

Fig. 4. (a) Nyquist plot of iron corrosion in 1 M HCl solution and (b) Bode-phase plots of iron corrosion in 1 M HCl solution.

Fig. 5. Phase impedance plots for mild steel in 1 M HCl in the absence and presence of different concentrations of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

Fig. 6. Typical polarization curves for corrosion of mild steel in 1 M HCl in the absence and presence of different concentrations of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677 671

Table 3Potentiodynamic polarization parameters for the corrosion of mild steel in 1 M HCl in absence and presence of different concentrations of different inhibitor.

Name of inhibitor Conc. of inhibitor Tafel data Linear polarization data E (%)

−Ecorr (mV vs. SCE) Icorr (�A cm−2) ˇa (mV/dec) ˇc (mV/dec) E% Rp (� cm2)

0.00 469 730.0 73 127 – 18.69

MIIH 50.00 477 241.0 66 186 66.98 70.27 71.97100.00 475 117.0 61 208 83.97 199.10 90.61150.00 469 62.3 70 195 91.46 336.70 94.44200.00 481 39.1 67 186 94.64 525.40 96.44

PIIH 50.00 478 235 70 191 67.80 77.92 76.01100.00 471 86.7 70 200 88.12 307.80 93.92150.00 457 33.7 63 154 95.38 469.10 96.01200.0 457 28.6 64 154 96.08 569.70 96.71

TBIH 50.00 490 117.0 62 143 83.97 115.20 83.77100.00 481 38.5 67 185 94.72 476.00 96.07150.00 463 21.1 73 195 97.10 736.90 97.46200.00 472 16.3 56 185 97.76 953.70 98.04

Itpab[

oip

C

atttatrnc

3

ktacttnh

prp

pm

F

PIIIH 50.00 466 226100.00 487 157150.00 469 47200.00 468 42

t is observed that the fitted data follow almost the same pattern ashe original results along the whole diagrams. The high frequencyart of the impedance and phase angle describes the behaviour ofn inhomogeneous surface layer, while the low frequency contri-ution shows the kinetic response for the charge transfer reaction20].

The electrochemical parameters, including Rs, Rct, Y0 and n,btained from fitting the recorded EIS data using the electrochem-cal circuit of Fig. 3 are listed in Table 2. Cdl values derived from CPEarameters according to Eq. (5) are listed in Table 2.

dl = (Y0Rct1−n)

1/n(5)

Phase angle at high frequencies provided a general idea ofnticorrosion performance. The more negative the phase anglehe more capacitive the electrochemical behaviour [21]. Chargeransfer resistance increment could raise current tendency to passhrough the capacitor in the circuit. Also, depression of phasengle at relaxation frequency with decreasing the isoniazid deriva-ives concentration (Fig. 5a–d) indicated the decrease of capacitiveesponse with the decrease of inhibitor concentration. Such a phe-omenon could be attributed to higher corrosion activity at lowoncentrations of inhibitors.

.2. Potentiodynamic polarization measurements

Polarization measurements were carried out in order to gainnowledge concerning the kinetics of the cathodic and anodic reac-ions. Fig. 6a–d presented the results of the effect of MIIH, PIIH, TBIHnd PIIIH concentration on the cathodic and anodic polarizationurves of mild steel in 1 M HCl, respectively. It could be observedhat both the cathodic and anodic reactions were suppressed withhe addition of isoniazid derivatives, which suggested that the iso-iazid derivatives reduced anodic dissolution and also retarded theydrogen evolution reaction.

Electrochemical corrosion kinetics parameters, i.e. corrosionotential (Ecorr), cathodic and anodic Tafel slopes (ˇa, ˇc) and cor-osion current density (Icorr) obtained from the extrapolation of theolarization curves, were given in Table 3.

In hydrochloric acid solution the following mechanism is pro-osed for the corrosion of iron [22]. The anodic dissolutionechanism of iron is:

e + Cl− ⇔ (FeCl−)ads (i)

66 191 69.04 84.91 77.9858 156 78.50 85.90 78.2469 177 93.56 431.00 95.6663 164 94.24 451.00 95.85

(FeCl−)ads ⇔ (FeCl)ads + e− (ii)

(FeCl)ads → FeCl+ + e− (iii)

(FeCl+) ⇔ Fe2+ + Cl− (iv)

The cathodic hydrogen evolution mechanism is:

Fe + H+ ⇔ (FeH+)ads (v)

(FeH+)ads + e− → (FeH)ads (vi)

(FeH)ads + H+ + e− → Fe + H2 (vii)

It is assumed that Cl− ions are first adsorbed onto the pos-itively charged metal surface by columbic attraction and theninhibitor molecules can be adsorbed through electrostatic interac-tions between the positively charged molecules and the negativelycharged metal surface [23]. These adsorbed molecules interact with(FeCl−)ads species to form monomolecular layers (by forming acomplex) on the steel surface. These layers protect mild steel sur-face from attack by chloride ions. Thus the oxidation reaction of(FeCl−)ads as shown by step (ii) → (vi) can be prevented. On theother hand, the protonated inhibitor molecules are also adsorbedat cathodic sites in competition with hydrogen ions that going toreduce hydrogen evolution.

The parallel cathodic Tafel curves in Fig. 6 suggested thatthe hydrogen evolution is activation-controlled and the reductionmechanism is not affected by the presence of the inhibitors. Theregion between linear part of cathodic and anodic branch of polar-ization curves becomes wider as the concentration of inhibitorsincreases. Similar results were found in the literature [24]. Thevalues of ˇc changed with increasing inhibitor concentration, indi-cated the influence of the compounds on the kinetics of hydrogenevolution. The shift in the anodic Tafel slope ˇa may be due to thechloride ions/or inhibitor molecules adsorbed onto steel surface[25]. It is important to note that in anodic domain, for potentialhigher than −350 mV vs. SCE, the presence of isoniazid derivativesdid not change the current-vs.-potential characteristics (Fig. 6).This potential can be defined as the desorption potential. The phe-nomenon may be due to the obvious metal dissolution, whichleading to a desorption of the inhibitor molecule from the elec-

trode surface, in this case the desorption rate of the inhibitors ishigher than its adsorption rate, so the corrosion current increasedmore obviously with rising potential [26]. Due to the presence ofsome active sites, such as aromatic rings, hetero-atoms in the stud-ied compounds for making adsorption, they may act as adsorption

672 A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677

F ) poliss (f) in

iccpgttaisristiM(pdbEiioc

TC

ig. 7. Atomic force micrographs of mild steel surface of (a) mild steel in 1 M HCl, (bteel (1 M HCl + 200 ppm PIIH), (e) inhibited mild steel (1 M HCl + 200 ppm TBIH and

nhibitors. Being absorbed on the metal surface, these compoundsontrolled the anodic and cathodic reactions during corrosion pro-ess, and then their corrosion inhibition efficiencies are directlyroportional to the amount of adsorbed inhibitor. The functionalroups and structure of the inhibitor play important roles duringhe adsorption process. On the other hand, an electron transferakes place during adsorption of the neutral organic compoundst metal surface [27]. As it can be seen from Table 3, the stud-ed inhibitors reduced both anodic and cathodic currents with alight shift in corrosion potential (≈12–21 mV). According to Fer-eira and others [28,29], if the displacement in corrosion potentials more than 85 mV with respect to corrosion potential of the blankolution, the inhibitor can be seen as a cathodic or anodic type. Inhe present study, the maximum displacement was 21 mV whichndicated that all the studied inhibitors are mixed-type inhibitors.

oreover, the recorded polarization curves in the presence of TBIHFig. 6c) are characterized by the presence of anodic breakdownotential, Eb. This is the potential at which sudden rise in currentensity takes place. As a result, the surface film is shifted from sta-le to unstable state. As the concentration of inhibitor increased,

b shifted to noble direction. The noble shift of Eb with increas-ng inhibitor concentration reflected the increased adsorption ofnhibitor on the metal surface. The noble shift of Eb and the decreasef the corresponding current densities with increasing the inhibitoroncentration reflected the formation of anodic protective films on

able 4orrosion parameters obtained from weight loss measurements for mild steel in 1 M HCl

Name of inhibitor Inhibitor concentration (ppm)

1 M HCl –

MIIH 50.00100.00150.00200.00

PIIH 50.00100.00150.00200.00

TBIH 50.00100.00150.00200.00

PIIIH 50.00100.00150.00200.00

hed mild steel, (c) inhibited mild steel (1 M HCl + 200 ppm MIIH), (d) inhibited mildhibited mild steel (1 M HCl + 200 ppm PIIIH).

the electrode surface [30]. The results obtained from Tafel polar-ization showed good agreement with the results obtained fromEIS.

3.3. Linear polarization resistance

Polarization resistance values were determined from the slopeof the potential–current lines,

Rp = AdE

di(6)

where A is the surface area of electrode, dE is change in potential anddi is change in current. The inhibition efficiencies and polarizationresistance parameters are presented in Table 3. The results obtainedfrom Tafel polarization and EIS showed good agreement with theresults obtained from linear polarization resistance.

3.4. Atomic force microscopy

Atomic force microscope (AFM) was used to image mild steel

specimen. Analysis of the images allowed quantification of sur-face roughness over area scales 12 �m × 12 �m. Atomic forcemicroscope was used mainly for measuring three-dimensionaltopography. The three-dimensional AFM images are shown inFig. 7a–f. As can be seen from Fig. 7c–f and Fig. 7a, there is much less

in absence and presence of different concentrations of inhibitors.

Weight loss (mg cm−2) E (%) CR (mm y−1)

10.90 – 40.44

4.00 63.30 14.841.70 84.40 6.301.30 88.07 4.821.00 90.82 3.71

3.90 64.22 14.471.20 88.99 4.820.90 90.64 3.330.70 93.55 2.60

2.20 79.81 8.160.80 92.47 2.960.50 95.08 1.850.40 96.32 1.48

4.90 55.04 18.182.90 73.39 10.751.60 85.32 5.931.10 89.89 4.08

A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677 673

entrat

dtmbafotP

3

3

fai%p

TT

Fig. 8. Variation of surface coverage with conc

amage on the surface of mild steel with all the isoniazid deriva-ives. The average roughness of polished mild steel (Fig. 7b) and

ild steel in 1 M HCl without inhibitor (Fig. 7a) was calculated toe 395 and 66 nm respectively. The mild steel surface in the freecid solution is getting cracked due to the acid attack on the sur-ace (Fig. 7b). However, in the presence of 200 ppm concentrationf all the isoniazid derivatives, the average roughness was reducedo 270, 225, 180 and 290 nm respectively in the presence of MIIH,IIH, TBIH and PIIIH (Fig. 7c–f).

.5. Weight loss studies

.5.1. Effect of inhibitor concentrationThe corrosion parameters and inhibition efficiency obtained

rom weight loss studies for mild steel in 1 M HCl solution inbsence and presence of different concentration of all the stud-ed inhibitors is presented in Table 4. The inhibition efficiency (E) and corrosion rate (CR, mm y−1) were calculated as describedreviously [4].

able 5hermodynamic activation parameters for mild steel in 1 M HCl in absence and presence

Name of inhibitor Conc. of inhibitor (ppm) Ea (kJ mol−1)

– 0 42.21

MIIH 50 62.55100 72.70150 65.14200 65.66

PIIH 50 61.35100 76.00150 71.95200 67.28

TBIH 50 66.44100 79.00150 74.45200 70.04

PIIIH 50 60.37100 62.93150 68.35200 66.73

ion of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

The variation of surface coverage (�) with inhibitor concentra-tion is shown in Fig. 8. It is observed that all the studied inhibitorsshowed maximum efficiency at 200 ppm concentration. Better effi-ciency at higher concentration may be due to larger coverage ofmetal with inhibitor molecules. The order of efficiency at a givenconcentration of inhibitor is: TBIH > PIIH > MIIH > PIIIH.

3.5.2. Effect of temperatureIn order to investigate the effect of temperature on the

performance of studied inhibitors and to derive thermody-namic activation parameters and thermodynamic parameters ofadsorption, weight loss studies were performed at four differ-ent temperatures. The inhibition efficiency of studied inhibitorsdecreased with increasing temperature, indicated desorption ofinhibitor molecules with rising temperature.

3.5.3. Thermodynamic activation parametersThe dependence of corrosion rate at temperature can be

expressed by Arrhenius equation and transition state equation

of different concentrations of different inhibitors.

� (mg cm−2) �H* (kJ mol−1) �S* (J mol−1 K−1)

5.31 × 108 39.57 −86.75

5.28 × 1011 59.89 −29.391.20 × 1013 70.05 −3.414.84 × 1011 62.48 −30.114.49 × 1011 63.01 −30.76

3.22 × 1011 58.70 −33.523.30 × 1013 73.33 4.964.63 × 1012 69.29 −11.335.24 × 1011 64.63 −29.47

1.33 × 1012 63.79 −21.716.53 × 1013 76.35 −10.566.84 × 1012 71.80 −8.101.02 × 1012 67.38 −23.95

2.79 × 1011 57.71 −34.714.49 × 1011 60.27 −30.751.98 × 1012 65.70 −18.407.33 × 1011 64.08 −26.67

674 A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677

rent c

[

l

C

w�orfcwca

Fig. 9. Adsorption isotherm plots for log CR vs. 1/T at diffe

31,32]:

og(CR) = −Ea

2.303RT+ log � (7)

R = RT

Nhexp

(�S∗

R

)exp

(−�H∗

RT

)(8)

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-exponential

actors for a wide range of concentration of all the inhibitors 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 hydrochloriccid can be explained using the kinetic model. Fig. 9a–d depicted

Fig. 10. Plots for CR/� vs. (1 − �/�) at different temperat

oncentrations of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

an Arrhenius plots for mild steel immersed in 1 M HCl in pres-ence of different concentration of all the isoniazid derivatives. Theplots obtained are straight lines and the slope of each straight linegives its apparent activation energy. Table 5 summarized Ea valuesfor a wide range of concentration of isoniazid derivatives. Inspec-tion of Table 5 showed that apparent activation energy increasedwith increasing concentration of inhibitors. The increase in Ea couldbe interpreted as the physical adsorption. Szauer and Brand [33]explained that the increase in activation energy can be attributedto an appreciable decrease in the adsorption of the inhibitor on the

mild steel surface with increase in temperature and a correspond-ing increase in corrosion rates occurs due to the fact that greaterarea of metal is exposed to the acid environment.

The results obtained in the study could be explained by themechanism proposed by Riggs and Hurd [34]. The authors proposed

ures for (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677 675

fferen

ascwao

aal4ge

Favcisioedp

Fig. 11. Adsorption isotherm plots for log(CR/T) vs. 1/T at di

mechanism which can be applied to explain the result of presenttudy. The authors suggested that the corrosion rate, −d[Fe]/dt,an be expressed by sum of two rates: −d[Fe]/dt = k1(1 − �) + k2�,here k1 and k2 are the rate constants for the uninhibited reaction

nd completely covered surface, respectively, and � is the fractionf surface covered by adsorbed inhibitor.

To confirm this proposal, the CR/� was plotted (Fig. 10a–d)gainst the 1 − �/� for each temperature tested. The values of k1nd k2 can be calculated from slopes and intercept of straightines respectively. The corrosion rate of uninhibited mild steel,0.4–174.5 mm y−1 determined using weight loss method showedood agreement with the results obtained from k1 and the hypoth-sis is justified.

The relationship between log(CR/T) and 1/T were shown inig. 11a–d. Straight lines are obtained with a slope (−�H*/2.303R)nd an intercept of

[log(R/Nh) + �S∗/2.303R

], from which the

alue of �H∗ and �S∗ were calculated and presented in Table 5. Onomparing the values of entropy of activation (�S∗) listed in Table 5,t is clear that entropy of activation increased in presence of all thetudied inhibitors compared to free acid solution. Such variation

s associated with the phenomenon of ordering and disorderingf inhibitor molecules on the mild steel surface. The increasedntropy of activation in the presence of inhibitors indicated thatisorderness is increased on going from reactant to activated com-lex.

Fig. 12. (a) Langmuir adsorption isotherm and (b) adsorption isotherm p

t concentrations of (a) MIIH, (b) PIIH, (c) TBIH and (d) PIIIH.

3.5.4. Thermodynamic parameters and adsorption isothermThe adsorption on the corroding surfaces never reaches the

real equilibrium and tends to reach an adsorption steady state.When corrosion rate is sufficiently decreased in the presenceof inhibitor, the adsorption steady state has a tendency toattain quasi-equilibrium state. Now, it is reasonable to considerquasi-equilibrium adsorption in thermodynamic way using theappropriate adsorption isotherm. The degree of surface coverage(�) for inhibitors was obtained from average weight loss data. Lang-muir, Temkin, and Frumkin adsorption isotherms were tested inorder to find the best suitable adsorption isotherm for adsorptionof isoniazid derivatives on the surface of mild steel from 1 M HClsolution. Langmuir adsorption isotherm (Eq. (9)) was found bestfit. With regard to the Langmuir adsorption isotherm the surfacecoverage (�) of the inhibitor on the mild steel surface is related tothe concentration (Cinh) of the inhibitor (mol L−1) in the bulk of thesolution according to the following equation:

� = KadsCinh

1 + KadsCinh(9)

where Kads is the equilibrium constant for the adsorp-tion/desorption process. This equation can be rearrangedtoCinh

�= 1

Kads+ Cinh (10)

lot for �G◦ads

vs. T for adsorption of different isoniazid derivatives.

676 A.K. Singh, M.A. Quraishi / Materials Chemistry and Physics 123 (2010) 666–677

Table 6Thermodynamic parameters for the adsorption of different inhibitors in 1 M HCl on the mild steel at different temperatures.

Name of inhibitor Conc. of inhibitor (ppm) Temperature (K) Kads (104 × M−1) �G◦ads

(kJ mol−1) −�H◦ads

(kJ mol−1) �S◦ads

(J K−1 mol−1)

MIIH 200.00 308 1.80 35.38 25.45 32.30318 1.31 35.68 – –328 0.99 36.06 – –338 0.74 36.33 – –

PIIH 200.0 308 2.57 36.29 26.70 32.50318 2.65 37.55 – –328 1.80 37.67 – –338 1.06 37.33 – –

TBIH 200.0 308 5.63 38.29 27.97 33.20318 3.69 38.42 – –328 2.71 38.79 – –338 2.12 39.28 – –

PIIIH 200.0 308 1.62 35.10 27.62 24.60599

(o

�

asabtif

sew

l

�

Aei

ptompptqaIpaasaairdf

318 1.1328 0.9338 0.5

From the intercepts of the straight lines on the Cinh/�-axisFig. 12a), Kads can be calculated which is related to free energyf adsorption, �G◦

ads, as given by Eq. (11).

G◦ads = −RT ln(55.5Kads) (11)

The negative values of �G◦ads ensured the spontaneity of the

dsorption process and stability of the adsorbed layer on the mildteel surface [35]. It is usually accepted that the value of �G◦

adsround −20 kJ mol−1 or lower indicates the electrostatic interactionetween charged metal surface and charged organic molecules inhe bulk of the solution while those around −40 kJ mol−1 or highernvolves charge sharing or charge transfer between the metal sur-ace and organic molecules [36].

Assuming thermodynamic model, corrosion inhibition of mildteel in the presence of isoniazid derivatives can be betterxplained, therefore, heat of adsorption and entropy of adsorptionere calculated.

The thermodynamic parameters �H◦ads and �S◦

ads can be calcu-ated from the following equation:

G◦ads = �H◦

ads − T �S◦ads (12)

plot of �G◦ads vs. T gives straight lines (Fig. 12b) with the slope

qual to −�S◦ads, and the value of �H◦

ads can be calculated fromntercept.

The sign of enthalpy of adsorption is negative and entropy isositive in the presence of studied inhibitors. The entropy of activa-ion increased as compared to free acid solution. The negative signf entropy is according to what would be expected, i.e. exother-ic process is accompanied with decrease in entropy. But in the

resent case, enthalpy is negative (exothermic) and entropy isositive. This could be explained in the following way: the adsorp-ion of inhibitors from the aqueous solution can be regarded asuasi-substitution process between the organic compound in thequeous phase and water molecules at the mild steel surface [37].n this situation, the adsorption of isoniazid derivatives is accom-anied by desorption of water molecules from the surface. Thus,s the adsorption of inhibitor is believed to be exothermic andssociated with decrease in the entropy of the solute, the oppo-ite is true for solvent. Since, the thermodynamic values obtainedre the algebraic sum of adsorption of organic inhibitor molecule

nd desorption of water molecules [38]. Therefore, gain in entropys attributed to increase in solvent entropy. Inspection of Table 6evealed that decrease in enthalpy and increase in entropy are theriving force for the adsorption of inhibitors on the mild steel sur-ace.

35.35 – –36.04 – –35.69 – –

4. Mechanism of inhibition

As followed from EIS, polarization and weight loss measure-ments, the corrosion of mild steel in 1 M HCl is retarded inthe presence of different concentrations of four studied isoniazidderivatives. The results clearly indicated that the inhibition mech-anism involves blockage of the mild steel surface by the inhibitormolecules via adsorption.

All the studied isoniazid derivatives are organic bases but thepresence of an imine group ( NH–) in the molecule determineswell outlined acidic properties as well. It means that the addition ofacid or a base to the aqueous solution of any of these inhibitors willtransform the neutral molecule into a cation or an anion [39,40].In 1 M HCl, which is the supporting electrolyte used in the study,the inhibitor molecules protonated. Thus, the molecular and cationboth forms played role in the adsorption of inhibitors on the mildsteel surface.

All the studied inhibitors are hetrocycles containing 1-N-substituted isatin-3-isonicotinoyl hydrazone ring. Apart ofsubstituent R, all of them contain four N hetero-atoms, two O-atomsand �-electron cloud of aromatic ring.

Iron has incomplete d-sublevel, which determines its role as anacceptor of electrons. This is verified by the data for its catalyticactivity and its affinity towards metal complex formation [41,42].Thus, all conditions required for the formation of a chemisorp-tive bond are fulfilled. The introduction of different substituent (R)affected the strength of chemisorptive bond.

The basic character of inhibitors affects the adsorption of cationon the surface of mild steel (electrostatic attraction). In the presenceof Cl− which are strongly adsorbed on the metal surface, the metalsurface becomes negatively charged hence favoured the adsorptionof cation type inhibitors. Thus, all the studied inhibitors adsorbedthrough electrostatic interactions between the positively chargedmolecules and negatively charged metal surface.

MIIH contains morpholino group (C4H8NO) as substituent R.Morpholino group has one N and one O-atom. It assists the adsorp-tion of inhibitor molecule through non-bonding electrons locatedon N and O-atom. Thus, it strengthened the adsorption of inhibitormolecule more as compared to piperidino group in the PIIIH whichcontains alone N-atom. Therefore, greater efficiency of MIIH ascompared to PIIIH is justified. The PIIH molecule contains two

N-atoms and since the electro-negativity of N-atom is less thanO-atom hence, the non-bonding electrons of N-atoms are moreavailable as compared to O-atom. Hence, PIIH showed betterefficiency than MIIH. The TBIH molecule, which contains mercapto-benzimidazolyl group as substituent R, has two nitrogen atoms,

hemis

�TmSmftcas

5

1

2

A

s

R

[[

[[

[[[

[

[[[[[[

[[[[[

[[[[[[[[

[[

A.K. Singh, M.A. Quraishi / Materials C

-electron cloud of aromatic ring and S-atom of mercapto group.hus, non-bonding electrons of N23 atom and �-electrons of aro-atic ring facilitated the adsorption. Moreover, the presence of

-atom in the molecular structure of TBIH molecule makes the for-ation of d�–d� bond resulting from the overlap of 3d-electrons

rom Fe atom to the 3d vacant orbitals of the S-atom possible andhus adsorption of TBIH on the mild steel surface is enhanced. Inonclusion, we consider that physical and chemical adsorption bothre involved in the adsorption of the studied inhibitors on the mildteel surface.

. Conclusions

. The inhibition efficiencies obtained by polarization, EIS andweight loss measurements show good agreement. All thesecompounds were found to affect both the anodic and cathodicprocesses and act as mixed-type inhibitors.

. The adsorption of all the studied inhibitors obeyed Langmuir’sisotherm. The negative value of �G◦

ads obtained from this studyindicated that the studied inhibitors were strongly adsorbed onthe steel surface and the decreasing value of �G◦

ads with increas-ing temperature suggested that the adsorption of inhibitors isnot favoured at higher temperature. The value of �G◦

ads for allthe studied inhibitors is in the range of 35–40 kJ mol−1 indicatedthat they are all adsorbed by mixed-mode of adsorption on themetal surface.

cknowledgement

One of the author AKS is thankful to University Grant Commis-ion (UGC), New Delhi, for providing Senior Research Fellowship.

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