Experimental and quantum chemical studies on corrosion inhibition performance of synthesized Schiff base for mild steel in acidic media

1

Experimental and quantum chemical studies on corrosion inhibition performance of synthesized Schiff base for mild steel in acidic media

S. Issaadi ^a*, T. Douadi^a and S. Chafaa^a

a: Laboratory of Electrochemistry of molecular materials and Complexes (LEMMC),

Department of Engineering Process, Faculty of Technology, University of Setif-1, DZ-19000, Setif-Algeria.

*e-mail: [email protected]

Abstract

The effect of synthesized Schiff base 4,4’-bis(2- carboxaldehyde thiophene) diphenyl diimino ethane (L) on the corrosion of mild steel in acidic media 1 M HCl has been investigated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. These studies have shown that (L) is a good corrosion inhibitor for mild steel in 1M HCl. The adsorption of inhibitor on mild steel surface was found to follow Langmuir isotherm model and the adsorption isotherm parameters (Kads, ΔG⁰ads) were determined. Quantum chemical calculations were further applied to reveal the adsorption structure and explain the experimental results.

Keywords: Mild steel, Corrosion inhibition, Adsorption isotherm, Quantum chemical studies

I.INTRODUCTION

Acid solutions are extensively used in a variety of industrial processes such as oil well acidification, acid pickling and acidic cleaning [1], which generally lead to a serious metallic corrosion. Organic compounds, corrosion inhibitors, are widely used in industry to prevent corrosion in acidic environments [2-6].

Some Schiff bases have been recently reported as effective corrosion inhibitors for steel in acidic media [7-10]. Due to the presence of the -CH=N- group, electronegative nitrogen (N), sulphur (S) and/or oxygen (O) atoms in the molecule, Schiff bases should be good corrosion inhibitors [11-13]. The action of such inhibitors depends on the specific interaction between the functional groups and the metal surface. So it is very important to clarify the interactions between inhibitor molecules and metal surfaces in order to develop new and efficient corrosion inhibitors.

Quantum chemical calculations have been

2 widely used to study reaction mechanisms and to interpret the experimental results as well as to resolve chemical ambiguities. Recently, more corrosion publications contained substantial quantum chemical calculations have appeared [14-17]. Such calculations were usually used to correlate the relationship between the inhibitor molecular properties and their corrosion inhibition efficiencies.

The objective of this investigation is to determine the corrosion inhibition efficiency of 4, 4’-bis (2-carboxaldehyde thiophene) biphenyl diimino ethane (L) as a novel inhibitor for the corrosion of mild steel in 1 M HCl using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The experimental results were associated with a theoretical method and discussed.

II. EXPERIMENTAL

a. Preparation of of 4,4’-bis(2-carboxaldehyde thiophene) biphenyl diimino ethane (L)

4, 4’-bis (2-carboxaldehyde thiophene) biphenyl diimino ethane ligand (L) was synthesized by the condensation of thiophene 2-carboxaldehyde derivative with an equimolar amount of the diamine in ethanol according to the published procedures as follow [18, 19].

(0.2 g ,1 mmol) of 4,4’- diamine biphenyl ethane was dissolved in 20 mL in ethanol and added dropwise to a 10 mL ethanolic solution of (0.24 g, 2 mmol) thiophene 2- carboxaldehyde. The mixture was heated to 40

°C for 3h. Ten millilitres of solvent was distilled out from the reaction mixture. During this time a yellow colored microcrystalline product separated. The solid was isolated by filtration, washed with minimum volume of

ethanol. The structure of the product was confirmed by ¹H-RMN (250 MHz in DMSO- d6, Grenoble- France) and FT-IR spectroscopy (KBr). The molecular structure of the obtained compound L is shown in Fig.1. Yield = 90%;

M.P = 184 ⁰C; IR bands: ν (-CH=N-) = 1618 cm^-1, ν (C-S) = 1507 cm^-1 and ν (CH2) =1425 cm^-1. The ¹H-NMR spectrum of L exhibits a singlet at 2.95 ppm, corresponding to the four aliphatic protons. The aromatic protons appeared as multiplets at 7.3-7.5 (6H, m, Ar-H at thiophene ring). A singlet at 8.58 ppm is due to the imino proton.

Fig. 1. The chemical structure of the synthesized inhibitor (L)

b. Electrochemical measurements

The 1 M HCl electrolyte solution was prepared from analytical grade 37% HCl (Merck) and double distilled water. The inhibitor concentration range used was 1 .10^-4 to 5.10^-3 M in 1 M HCl. All tests have been performed in desecrated solutions and at 25 ± 1 °C.

Electrochemical measurements were conducted in a conventional three-electrode thermostated cell. A platinum disk was used as counter electrode and standard calomel electrode (SCE) as the reference electrode. A cylindrical Mild steel electrode with the composition (in wt. %) C: 0.076, P: 0.012, Si:

0.026, Mn: 0.192, Cr: 0.050, Cu: 0.135, Al:

0.023, Ni: 0.050 and Fe balance was used as a working electrode. The electrode was mechanically abraded with a series of emery papers (800 and 1200 grad). Then it was rinsed

CH₂-CH₂

N N S

S

3 with acetone and double distilled water before their immersion in the solution.

The potentiodynamic curves were recorded using a PGZ 301 voltalab 40 system. The working electrode was first immersed into the test solution for 30 min to establish a steady state open circuit potential. After measuring the open circuit, potential dynamic polarization curves were obtained with a scan rate of 0.5 mV/s. Corrosion rates (corrosion current densities) were obtained from the polarization curves by linear extrapolation of the anodic and cathodic branches of the Tafel plots at points 75 mV higher and lower than the corrosion potential (Ecorr). Electrochemical impedance (EIS) measurements were performed at potential open circuit in the frequency range (100 kHz to 10 MHz) with a signal amplitude perturbation of 10 mV.

Measurements were carried out using a computer controlled Volta Lab PGZ 301 system with and Voltamaster 4 software.

c. Theoretical study

Theoretical calculations were performed on ligand L to calculate the energy of the highest occupied molecular orbital (E_HOMO), the lowest unoccupied molecular orbital (E_LUMO), the number of transferred electrons (∆N) and the dipole moment (μ). The calculations were carried out using the Gaussian 03 [20]

program. The DFT/B3LYP using 6-31G (d, p) basis set was used for the geometry optimization and electronic structure determination.

III. RESULTS AND DISCUSSION

a. Tafel polarization measurements

Anodic and cathodic polarization curves of mild steel in 1M HCl in absence and presence

of various concentrations of synthesized inhibitor L at 25 °C are shown in Fig. 2. At low over potential, polarization curves shows that the Tafel relationship for both anodic and cathodic reactions are activation controlled [21]. In general, increasing the concentration of L leads to a decrease in the cathodic current density and an increase in the anodic current density. However the inhibited systems curves (fig. 2) were shifted towards cathodic potentials, emphasizing that the studied compound acts predominately as a cathodic inhibitor [21]. The electrochemical parameters such as corrosion potential (E_corr), corrosion current density (icorr), anodic (ba) and cathodic (b_c) Tafel slopes are estimated by using Tafel ruler (Table 1). The percentage inhibition efficiency IE (%) can be given by the following equation (1):

100 (%)

₀

0

 

 i i i

corr corr

IE

corr (1)

Where

i

^{corr and}

i

^ocorr are respectively uninhibited and inhibited current densities.

Fig. 2. Tafel polarisation curves for mild steel obtained at 25 ⁰C in 1 M HCl containing different C

-600 -525 -450 -375 -300

-3 -2 -1 0 1

5 432 1

1: Blank 2: 1.10^-4 M 3: 5.10^-4 M 4: 1.10^-3 M 5: 5.10^-3 M logi(mA/cm2)

E(mVvs.SCE)

4

TABLE I. Polarisation Parameters and Ccorresponding Inhibition Efficiency for the Mild Steel in 1 M HCl with and without addition of various Concentrations of L.

b. Electrochemical impedance measurements (EIS)

The corrosion behavior of mild steel in acidic solution in the presence of L was investigated by the EIS at 25 °C. Nyquist plots for various concentrations of L are given in Fig. 3. The electrochemical parameters derived from the Nyquist plots are given in Table 2. The percentage inhibition efficiency IE (%) was calculated from the values of R_p using the following equation:

100 (%)

0

 

 R

R R

p p

IE

p (2)

Where

R

^pand

R

^0p are the polarization resistances in the presence and absence of L, respectively.

Capacitance values (C_dl) are obtained at the maximum frequency (f_max) and at maximum imaginary impedance of the component (-Z

max) using the following equation:

R C f

p dI

1 2

1

max







(3)

0 100 200 300 400

0 40 80 120 160

5

4

3 2 1

1. Blank 2. 1.10^-4 M 3. 5.10^-4 M 4. 1.10^-3 M 5. 5.10^-3 M

-Zi(ohm.cm-2)

Zr(ohm.cm²)

Fig. 3. Nyquist impedance diagrams for mild steel obtained at 25 ⁰C in 1 M HCl solution containing different concentrations of

Table 2 shows that the polarization resistance R_p values were increased and the capacitance values C_dl decreased with increasing inhibitor (L) concentration. The increase in R_p value can be attributed to the formation of protective film on the metal/solution interface. The decrease in the C_dl values may be caused by a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, indicating that the inhibitor acts by adsorption at the metal surface [22].

The highest inhibition is observed at a concentration of 5.10^-3 M of L. The inhibition efficiencies, calculated from impedance results, show the same trend as those obtained from polarization measurements. [23, 24].

c. Adsorption isotherm

Useful information about the inhibition mechanism of mild steel corrosion process can be gained from adsorption isotherm, by determining the standard Gibbs free energy (ΔG⁰_ads) value that characterizes the type of interaction existing between inhibitor molecules and metal surface. The ΔG⁰_ads was calculated with equation (4) [25].











 

 

RT

K

^ads

G

^ads

0

5exp . 55

1 (4) Compound C -Ecorr icorr η θ

(M) (mV/SCE) (µAcm^-2) %

Blank 1 441 786 - - 1.10^-4 452 421 42.4 0.42 L 5.10^-4 454 221 71.8 0.72 1.10^-3 467 173 77.9 0.78 5.10^-3 498 55 93.0 0.93

5 Where R is the universal gas constant 8.314 J mol^-1 K^-1, T the thermodynamic temperature in K, K_ads is the equilibrium constant for the adsorption process and 55.5 represents the molar concentration of water in the solution. In the case of Schiff base adsorption, the most suitable isotherm was found to be the Langmuir model, written in the linear form equation as (5) [26]

C C

K

^ads

^

 1



(5) TABLE II . Electrochimical Impedance Parameters for Mild Steel in 1 M HCl without and with addition of various Concentrations of L.

From the linear dependence C_inh/θ = f (c_inh) plotted using the experimental parameters computed from EIS data and Tafel slopes the value of the adsorption constant was calculated. The regression coefficient R² for all methods is around 1, indicating that the adsorption process of L on metal surface obeys the Langmuir isotherm. The values of ΔG⁰_ads determined by the two methods are - 31.22 kJ mol^-1 by Tafel slopes and -31.25 kJ mol^-1 by EIS measurements, revealing that the adsorption of inhibitor molecules on metal surface is due to electrostatic interactions between the molecule L and the electrode surface. Generally, values of the free energy less negative than - 40 kJ mol^-1 are associated with processes that occur by physical

adsorption of the inhibitor on metal surface [27].

d. Quantum chemical calculation

Quantum chemical calculations were used to find a relationship between the molecular structure of the synthesized inhibitor and its inhibition effect. The structure parameters and adsorptive performance of the synthesized inhibitor are used to elucidate the inhibition mechanism in the present work. The corresponding molecule geometries were optimized and the energies were calculated at B3LYP/6-31G (d, p) level. The optimized geometry is shown in Fig. 4, the frontier molecule orbital density distributions of the molecule are shown in Fig. 5, and the quantum chemical parameters are listed in Table 3.

E_HOMO is often associated with the capacity of a molecule to donate electron. High value of E_HOMO probably indicates a tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy and empty molecular orbital. E_LUMO indicates the ability of the molecule to accept electrons. The lower the value of E_LUMO, the more probable is that the molecule would accept electrons [28, 29].

So, the smaller gap (∆E) between E_HOMO and E_LUMO is the more probable to donate and accept electrons. The values of ∆E (3.88 eV) suggesting that the strongest ability of the synthesized inhibitor to form coordinate bonds with d-orbitals of metal through donating and accepting electrons. Additionally, the large values of the dipole moment μ (2.91 D) indicate the enhancement of corrosion inhibition. It has been reported that the more negative the atomic charges of the adsorbed centre, the more easily the atom donates its electron to the unoccupied orbital of the metal.

Compound C Rp Cdl η (M) (Ω cm² ) (µF cm^-2) %

Blank 1 54.4 292.2 - 1.10^-4 105.5 67.5 48.4

L 5.10^-4 125.2 80.2 56.5 1.10^-3 205.5 55.1 73.5

5.10^-3 308.2 72.2 82.3

6 The two nitrogens as well as some carbons atoms have negative charge centers which make them electron rich to form a coordinate bond with the mild steel surface. This shows that the two atoms N (-0.489 e) of azomethine groups are the probable reactive sites for the adsorption of mild steel. The number of transferred electrons (∆N) was also calculated

depending on the quantum chemical method.

^{N 2}

^  _

^Fe_Fe^

_ ^

^inh_inh



(6)

TABLE III. Quantum Parameters of L cCalculated Using DFT at the B3LYP/6-31G (d, p) Basis set.

Fig. 4. Optimized geometry of L using DFT at the B3LYP/6-31G (d,p) basis set level.

Fig. 5. Frontier molecule orbital density distributions of L using DFT at the B3LYP/6-31G (d,p) basis set level. Top: LUMO, bottom: HOMO

Where χFe and χinh denote the absolute electronegativity of iron and the inhibitor molecule, respectively; ηFe and ηinh are the absolute hardness of iron and the inhibitor molecule, respectively. These quantities are related to electron affinity (A) and ionization potential (I)

 

2 A I 

 

2 A I 

 

I and A are related in turn to E_HOMO and E_LUMO I = -E_HOMO and A = -E_LUMO

Values of χ and η were calculated by using the values of I and A obtained from quantum chemical calculation. Using a theoretical χ value of 7 eV/ mol and η value of 0 eV /mol for iron atom, ∆N, the fraction of electrons transferred from inhibitor to the iron molecule, was calculated. The synthesized inhibitor shows the highest inhibition efficiency because it has the highest HOMO energy (-5.61 eV) and this reflects the greatest ability (the lowest

∆E) of offering electrons. It can be seen from Table 3 that the ability of the synthesized inhibitor to donate electrons to the metal surface.

Quantum parameter Calculated value

E_HOMO (eV) -5.61 ELUMO (eV) -1.72 EHOMO -ELUMO (eV) 3.88 dipole moment μ (D) 2.91

Transferred electrons ∆N (e) 0.85

7 IV. CONCLUSIONS

-From the overall experimental results and discussion, the following conclusions can be deduced:

-The synthesized Schiff base L is a good inhibitor for the mild steel corrosion in 1 M HCl solution showing more than 93 % inhibition efficiency at 5.10^-3 M.

-Results obtained from dc polarization and impedance techniques are in a reasonably good agreement and show increased inhibitor efficiency with increasing inhibitor concentration.

-The adsorption of inhibitor at mild steel/acid solution interface obeyed the Langmuir adsorption isotherm model.

-Quantum chemical study reveals that the benzene ring and N atoms can be suitable sites for adsorption onto surface and the smaller gap between E_HOMO and E_LUMO favors the adsorption of the synthesized Schiff base on mild steel surface and enhancement of corrosion inhibition.

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