14MI31

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

Electrochemical and Quantum Chemical Studies on Corrosion Inhibition of Imidazole Derivative on N80 Steel in 15% HCl Solution

M. Yadav*1 T. K. Sarkar1, R. R. Sinha1 1Department of Applied Chemistry, Indian School of Mines, Dhanbad-826004, India

P. N. Yadav2

2Department of Physics, Post Graduate College Ghazipur- 233002, India

Email: yadav_drmahendra@yahoo.co.in

ABSTRACT

The inhibition effect of synthesized N-((2-(4-methoxyphenyl)H-imidazo[1,2-a]pyridin-3-yl)methylene)benzenamine (MPIPB) on corrosion of N80 steel in 15% HCl solution was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The inhibition efficiency increases as the concentration of the inhibitor was increased. The polarization measurements indicate that the studied inhibitor act as mixed inhibitor. The Density function theory (DFT) was employed for theoretical calculations and the obtained results were found to be consistent with the experimental findings. Keywords: N80 steel; EIS; Polarization ; Acid inhibition; DFT

1. INTRODUCTION

Acid solutions are frequently used in industrial processes such as pickling, cleaning, descaling, and oil well acidizing thus the chances of N80 steel corrosion is particularly high in these industrial processes 1, 2. To control the corrosion of N80 steel in these processes corrosion inhibitors are used. Mainly nitrogen, sulphur, oxygen containing compounds and their derivatives are reported in the literature as corrosion inhibitors 3–6. It is observed that the presence of hetero-atoms (N, S, O), aromatic rings or long alkyl chains generally improves corrosion inhibition efficiency of organic compounds 7. Besides the number of hetero-atoms, the projected surface area and molecular weight of these organic molecules play an important role in determining their inhibition efficiency 8. Keeping in view the importance of organic compounds as corrosion inhibitors, we synthesized N-((2-(4-methoxyphenyl)H-imidazo[1,2-a]pyridin-3-yl)methylene)benzenamine (MPIPB) and studied its corrosion inhibition effect on N80 steel in 15% HCl solution.

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

2. EXPERIMENTAL PROCEDURE 2.1 SYNTHESIS OF N-((2-(4-METHOXYPHENYL) H-IMIDAZO [ 1,2-a PYRIDIN-3 YL) METHYLENE) BENZENAMINE The compound MPIPB was synthesized by the reported methode 9 as shown is Scheme 1.

Scheme 1. Synthetic route of MPIPB.

2.2 N80 STEEL SAMPLE AND TEST SOLUTION PREPARATION

The composition (wt.%) of N80 steel used for studies was : C, 0.31; Mn, 0.92; Si, 0.19; S, 0.008; P, 0.010; Cr, 0.20 and Fe balance.. Prior to each experiment, the specimens were abraded successively with different grades (320–2000) of emery papers, washed with double distilled water, degreased with acetone dried and stored in a desiccator. Analytical reagent (AR) grade HCl was diluted with triple distilled water to obtain 15% HCl solution. The concentration range of inhibitor employed was varied from10 to 80 ppm (mg L-1) and the volume of the electrolyte used was 150 mL . 2.3. ELECTROCHEMICAL MEASUREMENTS

The electrochemical experiments were performed using a conventional three-electrode cell assembly. N80 steel samples for electrochemical experiments were machined into rectangular specimens and fixed in polytetrafluoroethylene (PTFE) rods by epoxy resin in such a way that only 1.0 cm2 surface area was left uncovered. The cell consisted of a platinum counter electrode and a calomel electrode as the reference electrode. The polarization and impedance studies were performed using CHI Electrochemical Analyzer (Model 660D). The polarization studies were carried out over a potential of +250 to -250 mV/SCE with respect to the open circuit potential at a scan rate of 0.5 mV s-1. The linear Tafel segments of the cathodic and anodic Tafel lines were extrapolated to the point of intersection to obtain the corrosion current density (icorr). The percentage inhibition efficiency (η %) was evaluated from the measured icorr values using the relationship:

0

0(%) 100corr corr

corr

i i

i

(1 )

where, i0corr and icorr are the values of corrosion current density in absence and presence of inhibitors, respectively. The electrochemical impedance spectroscopic (EIS) measurements were carried out using AC signals of 5 mV amplitude for the frequency spectrum from 100 kHz to 0.01 Hz. The charge transfer resistance (Rct) values were obtained by fitting the experimental data in equivalent circuit. The

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

percentage inhibition efficiency of the inhibitor has been found out from the charge transfer resistance values using the following equation:

( )

( )

(%) 100ct inh ct

ct inh

R R

R

(2)

where Rct(inh) &Rct are charge transfer resistance in presence and absence of inhibitor respectively.

The values of electrochemical double layer capacitance (Cdl) were calculated by using the following equation : Cdl = (Y0 Rct

1-n)1/n (3) where Y0 is CPE constant and n is CPE exponent . The value of n represents the deviation from the ideal behavior and it lies between 0 and 1. 2.4 QUNATUM CHEMICAL STUDY

Complete geometrical optimizations of the investigated molecules are performed using density functional theory (DFT) with the Beck’s three parameter exchange functional along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set is implemented in Gaussian 03 program package10.Theoretical parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE) and dipole moment (µ) were calculated.

3. RUSULTS AND DISCUSSION 3.1 ELECTROCHEMICAL STUDIES

The polarization curves of N80 steel in 15% HCl solution without and with addition of

different concentrations of MPIPB are shown in Fig.1. The value of the anodic Tafel slope (βa), cathodic Tafel slope (βc) corrosion current density (icorr) and corrosion potential (Ecorr) were calculate by Tafel extrapolation of the anodic and cathodic curves to the point of intersection . The corrosion parameters viz. Ecorr, icorr, βa and βc obtained from these curves are summarized in Table 1. The curves in Fig.1 illustrate that the nature of the curve remains almost same even after the addition of the inhibitor and but the anodic as well as cathodic curves shifted towards lower current density on increasing the concentration of the inhibitor, indicating that the inhibitor molecules retard the corrosion process. 11. The minor shift in the value of corrosion potential (Ecorr.) in presence of the MPIPB support the mixed nature of the inhibitor 12. The corrosion inhibition performance at various concentrations of MPIPB on N80 steel in 15% HCl solution was studied by EIS at 298 K after 6 h immersion time. The Nyquist plots of MPIPB is shown in Figure 2. The existence of a single depressed semicircle with its centre below x-axis in Nyquist plots (Fig. 1) indicates the presence of a single time constant. The depressed nature of the semicircle is due to the presence of roughness and other inhomogeneties of the solid electrode formed during the process of corrosion 13. The electrochemical parameters derived from Nyquist plot are listed in Table 1.

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

Table 1 Corrosion parameters for N80 steel in 15% HCl solution in the absence and presence of different concentrations of inhibitors at 303 K.

Figure 1. Potentiodynamic polarization curves for N80 steel in 15% HCl solution in the presence and absence of MPIPB at 303 K.

Conc.

(ppm)

Tafel extrapolation data EIS data

Ecorr

(V vs SCE)

βa

(mV dec

-1)

βc

(mV dec

-1)

icorr

(µAcm-2

)

η % Rs

(Ω cm

2)

Rct

(Ω cm

2 )

Y0

(µFcm-2

)

n Cdl

(µF

cm2 )

η %

Blank

MPIPB

-504 145 136 6128 - 0.48 5.09 1471 0.87 707 -

10 -527 163 128 1124 81.6 0.54 25 1057 0.86 596 80.1

20 -483 164 112 845 86.2 0.76 44 846 0.88 556 88.6

40 -510 162 113 287 95.3 0.79 100 543 0.89 380 95.0

60 -499 115 111 252 95.8 1.07 162 227 0.92 172 96.9

80 -502 159 107 158 97.4 1.08 234 82 0.94 65 97.8

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

Figure 2. Nyquist plot for N80 steel in 15% HCl solution containing various concentrations of MPIPB at 303 K.

Figure 3. Equivalent circuit applied for fitting of the impedance spectra.

The increase in Rct values in presence of MPIPB is attributed to the formation of an insulating protective film at the metal/solution interface. The decrease in Cdl values in presence of MPIPB can be attributed to a decrease in local dielectric constant and/or to an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules are adsorbed at the metal/solution interface 14.

3.2. THEORETICAL STUDIES In order to study the effect of molecular structure on the inhibition efficiency, quantum chemical calculations were performed by using DFT and all the calculations were carried out with the help of complete geometry optimization. Optimized structure, EHOMO and ELUMO of MPIPB are shown in Figure 4 (a, b, c). The quantum chemical parameters such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE) and the dipole moment (µ) obtained are summarized in Table 2. The frontier molecular orbital energies (EHOMO and ELUMO) are significant parameters for the prediction of the reactivity of a chemical species. The EHOMO is often associated with the electron donating ability of a molecule. The inhibition efficiency increases with the increasing EHOMO, values. High EHOMO values indicate that the molecule has a tendency to donate electrons to appropriate acceptor molecules with low energy empty molecular orbital. The lower value of ELUMO, suggesting that the molecule easily accepts electrons from the donor molecules 15. It was reported previously by some researchers that smaller values of ΔE and higher values of EHOMO and dipole moment (μ)

NIGIS * CORCON 2014 * Nov 12 – 15, 2014 * Mumbai

are responsible for enhancement of inhibition efficiency 16. The value of EHOMO, ELUMO, ΔE and µ for MPIPB were found as -4.1212 eV, -1.3824 eV, 2.7408 eV and 3.458 D, respectively.

Figure 4. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for

MPIPB. [Atom legend: white = H; Grey =C; blue = N; red = O].

Figure 4, reveals that the HOMO location in MPIPB is mostly distributed in vicinity of the imidazopyridine ring and C=N group indicating that these are the coordinating site of the inhibitor for the interaction between inhibitor molecules and N80 steel surface.

4. CONCLUSIONS 1. The synthesized imidazole derivative showed good inhibition efficiencies for the corrosion of

N80 steel in 15% HCl solution and the inhibition efficiency increases on increasing the concentration of inhibitor.

2. The variation in the values of βa and βc (Tafel slopes) and the minor deviation of Ecorr of inhibitor with respect to Ecorr of the blank, indicated that studied inhibitot act as mixed inhibitor.

3. EIS measurements showed that charge transfer resistance (Rct) increases and double layer capacitance (Cdl) decreases in presence of inhibitor, suggested the adsorption of the inhibitor molecules on the surface of mild steel.

4. The results obtained from SEM and Langmuir adsorption isotherm suggested that the mechanism of corrosion inhibition is occurring mainly through adsorption process.

5. Quantum chemical results correlate with the experimental results.

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