Accepted Manuscript

Experimental Studies of 2-Pyridinecarbonitrile as Corrosion Inhibitor for Mild

Steel in Hydrochloric Acid Solution

Reşit Yıldız, Ali Döner, Turgut Doğan, İlyas Dehri

PII: S0010-938X(14)00022-5

DOI: http://dx.doi.org/10.1016/j.corsci.2014.01.008

Reference: CS 5686

To appear in: Corrosion Science

Received Date: 24 September 2013

Accepted Date: 15 January 2014

Please cite this article as: R. Yıldız, A. Döner, T. Doğan, İ. Dehri, Experimental Studies of 2-Pyridinecarbonitrile

as Corrosion Inhibitor for Mild Steel in Hydrochloric Acid Solution, Corrosion Science (2014), doi: http://

dx.doi.org/10.1016/j.corsci.2014.01.008

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1

Experimental Studies of 2-Pyridinecarbonitrile as Corrosion Inhibitor for Mild Steel

in Hydrochloric Acid Solution

Reşit Yıldıza,*, Ali Dönerb, Turgut Doğanc, İlyas Dehria

aÇukurova University, Department of Chemistry, 01330 Adana, Turkey

bŞırnak University, The Faculty of Engineering, Energy Systems Engineering

Department 73000 Şırnak, Turkey

cPetkim Petrochemical Holding Inc.,12 35800 İzmir, Turkey

Abstract

The effect of 2-Pyridinecarbonitrile (2-PCN) was studied on mild steel (MS) corrosion

in 0.1 mol L-1 HCl by electrochemical impedance spectroscopy (EIS), linear polarisation

resistance (LPR) and potentiodynamic polarisation measurements. The surface morphologies

of the MS were investigated in the inhibitor-free and in the presence of 10 mmol L-1 2-PCN

containing corrosive media, at 120 h exposure period by scanning electron microscopy

(SEM). The mechanism of adsorption was determined from the potential of zero charge (Epzc).

2-PCN adsorption on the MS surface obeyed the isotherm of Langmuir and the

thermodynamic parameters Kads; ∆G

ads were also calculated and discussed.

Keywords: A. Mild steel, B. EIS, B. SEM, C. Acid corrosion

*Corresponding Author: e-mail: ryildiz80@gmail.com, tel: + 90 (322) 3386084-2465,

fax:+90 (322) 3386070 (R Yıldız).

2

1. Introduction

One of the main problems in the industrial process is corrosion of metals leading to

increase in manufacturing costs, thereby production costs. Metallic materials are often

exposed to conditions that facilitate corrosive processes. Acid solutions, especially

hydrochloric acid, that are widely used in a range of industries for acid pickling, acid

cleaning, acid descaling and oil refinery equipment cleaning [1,2].

The use of organic inhibitors to prevent the metal corrosion stands as an alternative

method in industrial applications [3-6]. Various inhibitors have been used to prevent

dissolution of metals due to the corrosion [7-10]. The inhibitors are usually capable of

adsorption onto the metal surface through a strong interaction between metal and inhibitor

[11-14]. These interactions depend on the molecule structure of the inhibitor. Molecular

structures showing high interaction properties usually contains electronegative functional

groups and π-electron in triple or conjugated double bonds as well as heteroatoms like

sulphur, nitrogen, phosphorus and oxygen [15-17]. The way of interaction to metal surface at

known concentration of inhibitors could be as chemically, physically or both chemically and

physically [18]. Electronegative functional groups are usually considered the chelation

(coordinate-bonded) center for chemical adsorption [19]. Once the complex is occurred the

tightly chelation will then create a film on the metal surface

Many kinds of nitrogen-containing compounds e.g. pyridine and its derivates have been

studied and reported as effective corrosion inhibitors [20-28]. The nitrogen and π- bonds

containing pyridine ring has high electron density and these structure facilitates the adsorption

action on the metal surface.

3

This work aimed to determine the inhibitory features of 2-Pyridinecarbonitrile (2-PCN)

on the mild steel (MS) surface using some electrochemical techniques. A comparison study

with the literature data was carried out.

2. Experimental

2.1. Preparation of electrodes

The MS samples used herein had the following composition: C (0.17 %), Mn (1.40

%), P (0.045 %), S (0.045 %), N (0.009 %), and Fe (remainder). The MS rod was covered

with polyester excluding its underneath surface with a surface area of 0.50 cm2. The surface

of MS electrodes mechanically abraded prior to use with different emery papers up to 1200

grade. Afterwards, the MS was washed with deionised water and dried with soft paper,

followed by immediate immersion in the test solution.

2.2. Test solution

The molecular structure of 2-pyridinecarbonitrile (2-PCN) is given in Fig. 1. 2-PCN

and other chemicals were purchased from Sigma-Aldrich and used without further

purification. HCl solutions were prepared using by dilution of analytical commercial grade

37% HCl with deionised water. Electrochemical measurements were performed in 0.1 mol L-1

HCI solution in the inhibitor free and in the presence of various 2-PCN concentrations. The

different concentrations of inhibitor were obtained by diluting the correct amounts of this

solution to obtain 10.0, 5.0, 1.0, and 0.5 mmol L-1 solutions.

Figure 1

4

2.3. Electrochemical measurements

The electrochemical methods were carried out by the means of a CHI 660 A.C (serial

number: F1070). The corrosion behavior of the MS specimens were investigated in a 0.1 mol

L-1 HCl solution using EIS, LPR, and cathodic (from open circuit potential, (Eocp) to -1.000 V,

scan rate of 0.001 V s-1) and anodic (from Eocp to 0.000 V scan rate of 0.001 V s-1)

polarisation curves in the inhibitor-free and presence of inhibitor solutions. Corrosion tests

were performed in a three-electrode cell closed to air under stagnant conditions at 25 C,

which was controlled by a thermostat. An Ag/AgCl (3 mol L-1 KCl) electrode was used as a

reference electrode, along with a platinum sheet (with a 2 cm2 surface area) as a counter

electrode. The MS was immersed in the test solution for 1 h to reach the balanced state Eocp.

The EIS measurements were immediately performed under measured Eocp, start-up frequency

range from 100 kHz to 0.009 Hz and amplitude of 7 mV (peak to peak). In the entire

experiments the Nyquist diagrams were fitted using the Zview program. LPR technique was

performed by obtaining the current caused by scanning the potential ±0.010 V on both sides

of its free-corrosion value at a scan rate of 0.001 V s-1. The polarisation resistance Rp was

determined from the slope of the acquired current–potential curves. To understand the

inhibition mechanism, polarisation tests were also performed starting from Eocp, after reaching

a steady state Eocp.

In this study, we aim to determine the time dependent inhibitor efficiency of the MS

electrodes using EIS and LPR with and without a 10 mmol L-1 solution of 2-PCN for 120 h

exposure period. On the other hand, SEM investigation was performed on this specimen.

5

2.4. Measurement of the potential of zero charge (Epzc)

Epzc for the MS was calculated, with help of EIS technique. To illuminate the

mechanism of inhibition of 2-PCN, the potentials were plotted against the values of Rp to

determine the Epzc after MS was immersed in the 0.1 mol L-1 HCl solutions containing 10

mmol L-1 2-PCN.

2.5. Scanning electron microscopy studies

The surface morphologies of the MS samples after subject to 0.1 mol L-1 HCI

solutions in the inhibitor-free and in the presence of 10 mmol L-1 2-PCN solutions for 120 h

were investigated by SEM using a Carl Zeiss Evo 40.

3. Results and Discussion

3.1. Potentiodynamic polarisation measurements

Potentiodynamic polarisation measurements allow determining the corrosion

inhibition mechanism of metals. Hence potentiodynamic polarisation curves are seen in Fig. 2

for both the inhibitor-free and inhibitor-added solutions. As would be expected from Fig. 2,

inhibitor molecules evidently inhibit for both anodic and cathodic reactions which was

indicated by the decrease in current densities with increasing concentration of 2-PCN. As

known, cathodic and anodic regions correspond to hydrogen evolution and iron dissolution

reactions under the experimental conditions, respectively. Considering the addition of 2-PCN,

both the current densities of cathodic and anodic reactions reduces effectively. And this

6

reduction becomes more pronounced with increasing 2-PCN concentration, meaning that this

molecule could be classified as a mixed-type inhibitor. After addition of the inhibitor to a

blank solution, the corrosion potential shifted from -0.595 to the values of -0.601, -0.642, -

0.664 and -0.683 V(Ag/AgCl) which confirmed inhibition of the cathodic and anodic

corrosion processes on the electrode surface. We could not calculate the corrosion parameters

such as corrosion current density, anodic and cathodic Tafel slopes and inhibition efficiency

as there are no linear Tafel regions on both cathodic and anodic current-potential curves. As

seen in Fig. 2, the inhibitor started to desorbed at -0.5 V(Ag/AgCl) which can be qualified as

the desorption potential. The same observations were already made in previous works [29-

31]. When current densities on the both the cathodic and anodic branches in uninhibited and

inhibited solutions are compared, the cathodic current densities decreased from 4.618 to 0.381

mA cm-2 at -0.725 V(Ag/AgCl) and the anodic current densities decreased from 3.180 to

0.470 mA cm-2 at -0.445 V(Ag/AgCl) which are smaller than those in the absence of inhibitor.

The corrosion proceeds more slowly than that of blank solution.

Figure 2

From these results 2-PCN is observed to prevent MS corrosion by blocking the reaction sites

effectively. Similar results for aromatic carbonitrole derivates were reported in the literature,

Khaled et al. [32] reported that inhibition efficiencies of copper corrosion increased by

increasing 2-mercapto-4-(p-methoxyphenyl)-6-oxo-1,6-dihydropyrimidine-5-carbonitrile

(MPD) and 2-carboxymethylthio-4-(p-methoxyphenyl)-6-oxo-1,6- dihy-dropyrimidine-5-

carbonitrile (CPD) concentrations. These values were 94.17 and 88.27 % in the presence of

1x10-3 mol L-1 MDP in 3.5 % NaCl solution according to potentiodynamic techniques [32,

33]. Furthermore, inhibition efficiency of 3-pyridinecarboxaldehyde thiosemicarbazone (3-

7

PCT) on mild steel in 1 mol L-1 HCl was found to be 88 % [34]. Fekry et al [35] studied the

corrosion inhibition effect of 4-(4-methoxyphenyl)-6-thioxo-1,6-dihydro-2,3'-bipyridine-5-

carbonitrile and 4-(4-methoxyphenyl) -6-(thiophen-2-yl)-2-thioxo-1,2-dihydropyridine-3-

carbonitrile on the MS in NaOH in the presence of NaCl. The corrosion inhibition efficiencies

were found to be 86.89 and 67.48 % at 10 mmol L-1 of inhibitors.

3.2. Electrochemical impedance spectroscopy (EIS) measurements

Electrochemical impedance spectroscopy was used to determine the behavior of

metal/solution interface in the absence and in the presence of inhibitor, as depicted in Fig. 3.

Inset shows the behavior of impedance response from the blank solution. One depressed

capacitive loop is observed in the absence of inhibitor from inset in Fig. 3. Charge transfer

controls the corrosion of the MS at the metal/solution interface [36-38]. It is suggested that

the structure of metal/solution interface changes in the existence of inhibitor in the blank

solution but there are no changes in the Nyquist plot, as seen in Fig. 3. As it would be

expected, corrosion rates decreased and diameter of the semicircles increased evidently by an

increase in the concentration of 2-PCN. A high impedance response was obtained up to 510 Ω

cm2, indicating that the protective film heals the corroded area and inhibits the corrosion on

the MS surface. This value increased to 1034 Ω cm2 with increasing inhibitor concentrations

because more inhibitor molecules adsorbed onto the metal surface and resulted in a barrier

effect. This phenomenon was also observed by other workers [39, 40]. Fig. 3b is

corresponding plot to Bode diagrams of uninhibited and inhibited solutions. As seen in Fig.

3b one time constant was observed in Bode plots. The values of phase angles are 72, 69, 63,

62 and 46°. The phase angles increased with increasing inhibitor concentration. The increase

in phase angle confirms the higher protection when increasing the inhibitor concentration. The

8

Nyquist diagrams were fitted using Zview program and impedance parameters listed in Table

1 were obtained from fitting results using the best equivalent electrical circuit which is

commonly used. The fit error values are also given in Table 1, they indicate the good quality

of fittings. Fig. 4 shows the equivalent electrical circuit. In this circuit, Rs corresponds to

solution resistance, Rp corresponds to charge transfer resistance Rct and diffuse layer

resistance Rd at the metal/solution interface in the absence of inhibitor. Rp' includes the

accumulated species Ra, Rp and film resistance Rf, Rp'= Rp (Rct + Rd) + Ra + Rf in the presence

of inhibitor [12, 41, 42]. n corresponds to the phase shift which is related to the in-

homogeneties of metal/solution interface [43, 44]. The metal/solution interface doesn’t

behave like an ideal capacitor [43]. Other than this constant phase element CPE is replaced a

double layer capacitance Cdl [45]. The impedance of the CPE is expressed as [46]:

ZCPE =[Yo(jw)n]-1 (1)

where Yo is a proportionality coefficient, w is the angular frequency and j2= -1 is the

imaginary number. Numerous authors have used CPE in modelling by relating it to different

physical phenomena [47].

Following formula can be used to determine the inhibition efficiency (η) from the

polarisation resistance.

η % ⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

'p

p'p

R

RRx100 (2)

where Rp and Rp' corresponds to polarisation resistances for uninhibited and inhibited

solutions, respectively.

9

Figure 3

Figure 4

Table 1

It is seen on Table 1 that CPE values decrease with decreasing the local dielectric

constant and/or increasing the thickness of metal/solution interface. This may attribute to the

adsorption of inhibitor at the metal/solution interface [49-51]. Formation of inhibitor film is

increasing with increasing inhibitor concentration as more inhibitor molecules adsorbs on the

metal surface. As a consequence lower CPE values were obtained. This features was also seen

in the others studies [49, 52]. According to data presented in Table 2, inhibition efficiency

obtained from sum of Ra, Rf and Rp increased with increasing inhibitor concentration and

highest value, 96 %, is obtained from 10 mmol L-1 2-PCN concentration. We could say that

inhibitor molecules may adsorb and block the available active center of the MS surface. In

other words, interfacial double layer is changed by the adsorption of inhibitor molecules. In

the 2-PCN structure, pyridine ring and –C≡N group have π-electrons and these electrons

occupy the unoccupied orbitals of iron. After this, more adsorption sites form on the iron

surface [36, 53].

Linear polarisation resistance (LPR) was used as an alternative technique for

supporting the EIS results under the same experimental conditions. Electrochemical data

obtained from LPR are listed in Table 1. As indicated in Table 1, inhibition efficiency and Rp'

values are very close to those obtained from EIS data. LPR are all in agreement with EIS

results.

When we compared our study with the literature data, e.g. Bouklah et al. [54] studied

the inhibition effect of pyridine and its derivate. They found Rp and inhibition efficiency as 81

Ω cm2 and 63 %, respectively. Yang et al. [34] also investigated the inhibition performance of

10

pyridine derivate in 1 mol L-1 HCl solution on mild steel. And they showed that the inhibition

efficiency of this inhibitor was around 92 % from the EIS results. As another study,

Hammouti et al. [55] measured the behaviors of inhibition of two organic compounds

pyridine (P1) and pyrazole (P2) in acidic solution. The inhibition efficiencies increased with

the increase of inhibitor concentration and reached 90 % and 80 % for P1 and P2 in 1 mol L-1

HCl solution, respectively. Xiaco-Li et al. [26] carried out the quantum chemical study of the

corrosion inhibition effects of pyridine and its derivates at the aluminum electrode in HCl

media. The results showed that, the compounds adsorbed in their protonated forms on the

metal surface. Emregül and et al. [56] showed for a pyridine derivative that Rp and inhibition

efficiency are 131 Ω cm2 and 91 %, at 2.5x 10-3 mol L-1 concentration of inhibitor in 2 mol L-1

HCl solution. Moreover, Fekry et al. [35] investigated the inhibition efficiencies of

carbonitroles molecules in NaOH in the presence of NaCl on the MS. They found inhibition

efficiencies were around 93.19 and 67.25 %, respectively. Our impedance results will be

improved and supported the literature studies taking into account the physical and chemical

meaning of the inhibition features 2-PCN.

We also performed EIS and LPR techniques to understand the long exposure time

impedance responses. Fig. 5a and b show the impedance responses of uninhibited and

inhibited solutions at different exposure times, respectively. The impedance responses of

blank solution at the MS electrode clearly exhibited one depressed capacitive loop. This

capacitive loop related the charge transfer as we discussed earlier. We calculated the

inhibition efficiency and Rp' values for inhibited solution and Rp values for uninhibited

solution from Fig. 5a and b at different exposure times and these parameters are given in

Table 2. The Rp values of blank solution decreased from 45, 39, 25 and 25 Ω cm2 with

increase of exposure times due to the dissolution of metal. Under the same experimental

conditions, we can see that there are no differences in shapes of diagrams (Fig. 5b) in

11

inhibited solution. All Rp' values of inhibited solution are significantly high in comparison to

the uninhibited solution, characterizing the protective layer against the aggressive acid attack.

But magnitude of Rp' tends to decrease with increasing the exposure times. This could be

explained that small amount of adsorbed inhibitor molecules desorb from the electrode

surface. Other than this, as seen in Table 2 that LPR data support the EIS results.

Figure 5

Table 2

3.3. Adsorption isotherm

The isotherm of adsorption that defines the adsorptive inhibitors behavior is

significant to recognize the corrosion inhibition mechanism. Adsorbed inhibitor on the surface

of the MS can alter the electrical double layer structure. Fundamental teaching coping with

interaction between the molecules of organic inhibitor and the MS surface can be obtained by

adsorption isotherms [57]. The adsorption of inhibitor molecules is accomplished by two main

species of interactions: chemisorption and physisorption [58].

Some attempts have been made to fit the values of surface coverage ratio (θ) to

different isotherms counting Frumkin, Temkin and Langmuir, isotherm. A straight line is

obtained upon plotting Cinh/θ vs. Cinh, as given in Fig. 6. The linear association coefficient

(R2) is equivalent to 1 (R2 = 0.9999) and slope is nearly 1, indicating that the 2-PCN

compound adsorption on the surface of MS obeys the isotherm of Langmuir adsorption. The

strong correlation to the isotherm of Langmuir adsorption may prove the effectiveness of this

application. It is obvious that, the large value shows a physically powerful adsorption of the 2-

12

PCN molecules on the MS surface in 0.1 mol L-1 HCl. The relationship between surface

coverage ratio, θ, and inhibitor adsorption in the corrosive media can be represented as:

where Cinh is concentration of inhibitor, θ was calculated from the EIS data, and Kads is the

equilibrium constant for the adsorption-desorption process. The value of Kads of 2.37 x104 M-1

demonstrates high adsorption of 2-PCN on the surface of MS [59-61]. ∆G

ads, the standard

free energy of adsorption of the organic inhibitor on the surface of MS can be determined

using the following equation;

∆G°ads= –RT (ln55.5 Kads) (4)

where 55.5 is the molar concentration of water in the solution expressed in mol L -1, R the gas

constant and T the absolute temperature K. By Eq. 4, the ∆G

ads value is calculated as -34.92

kJ mol-1. A ∆G

ads value shows that there is a strong interaction between compounds of the

inhibitor and the surface of MS [57]. The value of ∆G

ads less than 40 kJmol-1is usually

commensurate with the existence of physisorption by the structure of an adsorptive film with

electrostatic effects [62, 63].

Figure 6

It is well identified that the values of -20 kJ mol-1 or lower show a physisorption; those

around -40 kJ mol-1 or higher include charge sharing or transfer from the compounds of

organic inhibitor to the surface of the MS to shape a synchronize variety of bond

13

(chemisorption) [64–66]. Conversely, the organic compound of adsorption is not calculated

merely as a purely physisorption or chemisorption [3, 11]. A large spectrum of circumstances,

changing from the electrostatic interaction or chemisorption supremacy, reveals from other

adsorptions tested data [62]. The value of -34.92 kJ mol-1 may give easily the two adsorption

process.

3.4. The potential of zero charge (Epzc) and inhibition mechanism

To clarify the mechanism of inhibition of 2-PCN, concerning the adsorptive effects

with surface of MS, the Epzc was obtained. In Fig. 7, the Eocp value of the MS in inhibitor

containing solution and Epzc values are obtained in the same diagram. The surface charge of

MS is handled by comparing the corrosion potential with the Epzc [31]. The plot of the Rp vs.

the applied potentials is exemplified in Fig. 7. The net surface charge of the MS at the Eocp

can be utilised with respect to the equation:

Er = Eocp-Epzc (5)

where Er is the Antropov’s ‘‘rational’’ potential of corrosion [67].

As it can be shown in Fig. 7, the Eocp value of the MS electrode was obtained as -0.492

V(Ag/AgCl) and the Epzc the maximum point was shown -0.472 V(Ag/AgCl) in presence 10

mmol L-1 2-PCN in acidic test solution [68]. The Eocp of MS in the same circumstances is -

0.492 V(Ag/AgCl), which is more negative than the Epzc and showing a negatively charged

MS surface after 1 h of exposure time (Er = Eocp-Epzc = -0.020 V(Ag/AgCl). In 0.1 mol L-1

HCl solution, the 2-PCN compound may exist in the protonated form in balance with the

14

similar molecular structure. This protonation may be through the nitrogen atoms in acidic

solution in equilibrium with the following equation:

2-PCN + 2H+ ↔ [2-PCNH]2+ (6)

The positively charged molecule of inhibitor can easy reaching the negatively loaded MS

surface because of the electrostatic attraction. Thus, the compounds of inhibitor form a

compressed layer of adsorption and action as an obstacle in opposition to iron corrosion.

Besides to the physisorption, the adsorption of 2-PCN compounds can also occur through

donor–acceptor interactions between free electron pairs of heteroatoms also π-electrons of

several bonds and the unoccupied d orbitals of iron [69]. Furthermore, 2-PCN inhibitor

compounds have electronegative donor atoms the nitrogen which as well attends to aromatic

ring by emancipatory their unshared pair of electrons, to increase electron density in the

inhibitor compounds. [70].

Figure 7

3.5. SEM studies

Surface examination of the MS electrode exposed to a 0.1 M HCl solution in the

uninhibited and inhibited solutions after 120 h was performed by SEM (Fig. 8) analysis. It is

clearly shown in Fig. 8a that, the MS surface is strongly damaged in the inhibitor free due to

the metal dissolution in corrosive media. The metal surface was very rough and depth of pits

is observed on the metal surface. In contrast, the appearance of steel surface is different after

the addition of 10.0 mmol L-1 2-PCN to the corrosive medium after 120 h immersion. It can

15

be seen from Fig. 8b that, the MS surface was improved smooth in the presence of 2-PCN,

less pits and much less damaged were observed in comparison to the MS surface in the

absence of inhibitor.

Figure 8

4. Conclusions

2-Pyridinecarbonitrile: as a new and effective corrosion inhibitor on the corrosion of MS in

0.1 mol L-1 HCI solution was studied using electrochemical techniques and SEM images.

From the results obtained herein, the following points were concluded:

1. 2-PCN has a good inhibition effect for the corrosion of MS in 0.1 mol L-1 HCl solution

and the inhibition efficiency increases with increased inhibitor concentration.

2. 2-PCN acted as mixed-type corrosion inhibitor by decreasing both the anodic metal

dissolution and cathodic hydrogen reduction reactions.

3. 2-PCN on the metal surface obeyed Langmuir adsorption isotherm. The value of the

adsorption equilibrium constant shows that inhibitor is strongly adsorbed on the metal

surface. The ∆G

ads value indicates that the adsorption is more physical than chemical.

4. SEM micrographs showed that the inhibitor compounds form a good protective film

on the metal surface.

5. The inhibition efficiency value determined from EIS results is 97.3 % at 120 h

immersion time in the presence 10.0 mmol L-1 2-PCN in 0.1 mol L-1 HCl.

Acknowledgements

This study was supported by Çukurova University Research Found.

16

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25

Figure captions

Fig.1. 2-Pyridinecarbonitrile (2-PCN).

Fig. 2. Potentiodynamic polarization curves of MS electrode obtained in 0.1 mol L-1 HCI

solution () and containing 0.5 (), 1.0 (♦), 5.0 () and 10.0 mM () 2-PCN at 25 °C.

Fig.3. Nyquist (a) and Bode (b) plots of MS electrode obtained in 0.1 M HCI solution ()

(inset) and containing 0.5 (), 1.0 (♦), 5.0 () and 10.0 mmol L-1 () 2-PCN (solid lines

show fitted results).

Fig.4. Electrical equivalent circuit diagrams used to modeling metal/solution interface. Rs:

solution resistance, Rp: polarization resistance, CPEdl: double layer capacitance and film

capacitance for uninhibited and inhibited solutions, respectively. Rp corresponds to charge

transfer resistance Rct and diffuse layer resistance Rd at the metal/solution interface in the

absence of inhibitor Rp = Rct + Rd. Rp' includes the accumulated species Ra, Rp and film

resistance Rf, Rp'= Rp (Rct + Rd) + Ra + Rf.

Fig.5. Nyquist plots of MS electrodes in 0.1 M HCI solution in the absence (a) 1 h (), 24 h

(), 72 h (♦) and 120 h () and presence of 10.0 mmol L-1 2-PCN after 1 h (), 24 h (), 72

h (♦) and 120 h () exposure time (solid lines show fitted results).

Fig.6. Langmuir adsorption plot of MS in 0.1 mol L-1 HCl solution containing different

concentrations of 2-PCN.

Fig.7. The plot of Rp versus electrode potential for MS containing 10.0 mmol L-1 2-PCN in

0.1 mol L-1 HCl.

Fig.8. SEM images of MS samples: after immersion for 120 h (a) in 0.1 mol L-1 HCI solution

without inhibitor and after immersion 120 h (b) in 0.1 M HCI solution in the presence of 10.0

mmol L-1 2-PCN.

26

Table 1. Electrochemical parameters for MS electrode corresponding to the EIS and LPR data in 0.1 mol L-1 HCl solution in the absence and

presence of various concentrations of 2-PCN.

*Cdl values determined from CPE parameters, Yo and n, as described in Ref. [48].

Cinh (mmol L-1) EIS LPR

Rs

(Ω cm2) Rp

(Ω cm2) CPEdl

Yo ( x106 sn Ω-1 cm-2) n η(%) Cdl*

( x106 s Ω-1 cm-2)

Fit error (chi-squared)

Rp

(Ω cm2)

η(%)

Blank 5.8 45 460 0.90 293 8.1x10-3 53

0.5 8.5 510 191 0.80 91.1 97.7 1.3x10-2 555 90.4

1.0 8.2 568 163 0.84 92.1 101.4 4.1x10-3 625 91.5

5.0 7.6 829 112 0.85 94.6 73.9 6.7x10-3 833 93.6

10 7.7 1034 84 0.89 95.6 61.9 6.7x10-3 1000 94.7

27

Table 2. Polarization resistance values and inhibition efficiencies for MS electrode obtained

in 0.1 mol L-1 HCl in the absence and presence of 10.0 mmol L-1 2-PCN solutions after

different immersion times.

t(h) Blank 2-PCN EIS LPR EIS LPR Rp(Ω cm2) Rp(Ω cm2) Rp(Ω cm2) η(%) Rp(Ω cm2) η(%) 1 45 53 1034 95.6 1000 94.7 24 39 37 896 95.6 833 95.5 72 25 28 814 96.9 756 96.2 120 20 26 734 97.3 714 96.3

28

Fig.1. 2-Pyridinecarbonitrile (2-PCN).

29

Fig. 2. Potentiodynamic polarization curves of MS electrode obtained in 0.1 mol L-1 HCI

solution () and containing 0.5 (), 1.0 (♦), 5.0 () and 10.0 mmol L-1 () 2-PCN at 25 °C.

E / V(Ag/AgCl)

log

I / A

cm

-2

30

Fig.3. Nyquist (a) and Bode (b) plots of MS electrode obtained in 0.1 M HCI solution ()

(inset) and containing 0.5 (), 1.0 (♦), 5.0 () and 10.0 mmol L-1 () 2-PCN (solid lines

show fitted results).

b

10-2 10-1 100 101 102 103 104 105100

101

102

103

104 -75

-55

-35

-15

Frequency / Hz

IZI/

Ωcm

2 theta / θ°

a

0 250 500 750 1000 1250

-1250

-1000

-750

-500

-250

0

Z''

/ Ω c

m2

Z' / Ω cm2

Z' / Ω cm2

Z''

/ Ω c

m2

0 10 20 30 40 50 60

-60

-50

-40

-30

-20

-10

0

0.25 Hz

0.97 Hz

2.1 Hz5.4 Hz

17.4 Hz

81.3 Hz

0.1 Hz

11.9 Hz

3.09 Hz25.7 Hz

31

Fig.4. Electrical equivalent circuit diagrams used to modeling metal/solution interface. Rs:

solution resistance, Rp: polarization resistance, CPEdl: double layer capacitance and film

capacitance for uninhibited and inhibited solutions, respectively. Rp corresponds to charge

transfer resistance Rct and diffuse layer resistance Rd at the metal/solution interface in the

absence of inhibitor Rp = Rct + Rd. Rp' includes the accumulated species Ra, Rp and film

resistance Rf, Rp'= Rp (Rct + Rd) + Ra + Rf.

Rs

CPE

Rp

Rs

CPE

Rp Rp / Rp'

CPEdl

Rs

32

Fig.5. Nyquist plots of MS electrodes in 0.1 mol L-1 HCI solution in the absence (a) 1 h (),

24 h (), 72 h (♦) and 120 h () and presence of 10.0 mmol L-1 2-PCN after 1 h (), 24 h

(), 72 h (♦) and 120 h () exposure time (solid lines show fitted results).

Z' / Ω cm2

Z''

/Ωcm

2

a

0 10 20 30 40 50 60

-60

-50

-40

-30

-20

-10

0

0.1 Hz

3.09 Hz

11.9 Hz

25.7 Hz

81.3 Hz

b

Z''

/ Ω c

m2

Z' / Ω cm20 250 500 750 1000 1250

-1250

-1000

-750

-500

-250

0

0.25 Hz

0.97 Hz

2.1 Hz5.4 Hz

17.4 Hz

33

Fig.6. Langmuir adsorption plot of MS in 0.1 mol L-1 HCl solution containing different

concentrations of 2-PCN.

C (mmol L-1)

C / θ (

mm

ol L

-1)

34

Fig.7. The plot of Rp versus electrode potential for MS containing 10.0 mmol L-1 2-PCN in

0.1 mol L-1 HCl.

E / V(Ag/AgCl)

Rp

/ Ω c

m2

EPZC

EOCP

35

Fig. 8. SEM images of MS samples: after immersion for 120 h (a) in 0.1 mol L-1 HCI solution

without inhibitor and after immersion 120 h (b) in 0.1 M HCI solution in the presence of 10.0

mmol L-1 2-PCN.

b

a

36

Highligts

• The corrosion effect of inhibitor was studied in 0.1 mol L-1 HCl on mild steel.

• The inhibitor efficiency increases with increasing the concentration of inhibitor.

• SEM micrographs showed that the inhibitor has a good protective film on the metal surface.