Insights into corrosion inhibition behavior of a triazole derivative For mild steel in hydrochloric acid solution

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ScienceDirect Materials Today: Proceedings 13 (2019) 1008–1022

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ICMES 2018

Insights into corrosion inhibition behavior of a triazole derivative For mild steel in hydrochloric acid solution I. Merimi1*, R. Benkaddour2, H. Lgaz3, N. Rezki4, M. Messali4, F. Jeffali5, H.Oudda1, and B. Hammouti2 1 Laboratory of Separation Processes, Université Ibn Tofail, Faculté des Sciences, Kenitra, Morocco Laboratory of Applied Analytical Chemistry, Materials and Environment (L2ACME), Faculty of Science, B.P. 717, 60000 Oujda, Morocco 3 Department of Applied Bioscience, College of Life & Environment Science, Konkuk University, 120Neungdong-ro, Gwangjin-gu, South Korea 4 Department of Chemistry, Faculty of Sciences, Taibah University, Al-Madinah Al-Munawarah 30002, Saoudi Arabia. 5 Laboratoire de Dynamique et d’Optique des Matriaux (LDOM), Faculty of Sciences, Mohammed first University, Oujda,Morocco 2

Abstract In order to search for new molecules and use them as corrosion inhibitors of mild steel in 1M HCl, (Z) -5-methyl-4 - ((3nitrobenzylidene) amino) -2,4-dihydro- 3H-1,2,4-triazole-3-thione (C10H9N5O2S) (2m), is a novel inhibitor. Its inhibitory action was studied using gravimetric measurement, electrochemical impedance spectroscopy, potentiodynamic polarization, scanning electron microscopy (SEM) and theoretical calculations. The methods cited above, reveal that (2m) is an excellent inhibitor. Its maximum inhibition efficiency reaches up to 89.74 % at 10-6M. The inhibition of corrosion is due to the adsorption of the inhibitor on the mild steel. Potentiodynamic polarization shows that (2m) acts as a mixed type inhibitor with a certain cathodic predominance. Scanning electron microscopy shows an improved morphology of the surface of mild steel in the presence of the inhibitor studied. The inhibition mechanism was explored by the potential of zero charge (Epzc) measurement at the solution/metal interface. The quantum chemical parameters were calculated using the DFT method to correlate the electron properties with the adsorption and inhibition behavior of (Z) -5-methyl-4 - ((3-nitrobenzylidene) amino) -2, 4-dihydro- 3H-1,2,4triazole-3-thione. © 2019 Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018. Keywords: Acid medium; Corrosion of mild steel; Electrochemical studies; Surface Analysis; Epzc; Theoretical study; Triazole;Weight loss.

* Corresponding author. E-mail address: [email protected] 2214-7853 © 2019 Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.

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1. Introduction Mild steel is considered an important raw material in various applications. Its wide use in several industries, because of its low cost, excellent mechanical properties and availability for the manufacture of various materials. In recent years, corrosion of iron and steel has attracted the attention of several researchers, because corrosion causes significant material damage. Acid solutions are commonly used, for example, pickling, industrial acid cleaning, acid scaling, etc. On the other hand the excessive use of acid, corrodes iron and its alloys which results in a terrible waste of money. The best solution for preventing electrochemical corrosion is the metallic isolation of corrosive agents. Currently, there are many methods available, but the use of inhibitors is one of the most practical methods for corrosion protection. Several studies on inhibitors have been carried out, in particular nitrogen-containing inhibitors. The effect of these inhibitors on the corrosion behavior of iron and steel in acid solutions has been well documented [1]. It can be mentioned that our laboratory is also interested in inhibitors based on natural products [2]. The data show that most organic inhibitors act by adsorption on the metal surface. Due to the presence of electronegative functional groups and electron п in triple or conjugated double bonds, these compounds are generally good corrosion inhibitors. It is added that heteroatoms such as sulfur, phosphorus, nitrogen and oxygen as well as aromatic rings in their structure are the main adsorption centers. There are four types of adsorption by organic molecules at the metal / solution interface. One begins with the electrostatic attraction between the charged molecules and the charged metal, then the interaction of the uncharged electron pairs in the molecule with the metal, then the interaction of the п electrons with the metal, finally a combination between the first and the third type of adsorption [3]. The aim of this study is to investigate the inhibition effect (Z) -5-methyl-4 - ((3-nitrobenzylidene) amino) -2,4-dihydro- 3H-1,2,4-triazole-3-thione (C10H9N5O2S) (2m), on inhibition properties on the corrosion of steel in 1M HCl. Weight loss measurements and electrochemical methods were carried out to study the mechanism of corrosion inhibition. The thermodynamic adsorption parameters were calculated and discussed using weight loss measurement. Scanning Electron Microscope (SEM) images of the inhibited mild steel were taken to better understand the surface morphology upon inhibition. Efficiency is related to quantum parameters. 2. Experimental Details 2.1. Inhibitor 2.1.1. Synthesis procedure and inhibitor structure A mixture of 4-amino-5-methyl-2,4-dihydro-1,2,4-triazole-3-thione (10 mmol) in ethanol (50 ml) and 3nitrobenzaldehyde (10 mmol) with few drops of hydrochloric acid (conc. HCl) was refluxed for 6 h. After cooling, the product was filtered and recrystallized from ethanol to obtain the desired Schiff Base in 86% yield. IR ( , cm−1): 1324 (C=S), 1723 (C=N), 3277 (NH), 1H-NMR (400 MHz, DMSO-d6):δ 2.41 (s, 3H, CH3), 7.52-7.59 (m, 2H, Ar-H), 7.87 (d, 1H, J = 8 Hz, Ar-H), 8.16 (s, 1H, Ar-H), 10.17 (s, 1H, HC=N), 13.89 (s, 1H, NH). N

NH

S N N N O2

(Z)-5-methyl-4-((3-nitrobe nzylidene )a mino)-2,4-dihydro-3 H -1,2,4-tria zole -3-thione Che mic al Formula : C 10H 9N5O 2S M olec ular Weight: 263,28

2.2. Materials and solutions In order to obtain reliable and reproducible results, the samples are of mild steel type, before each test by polishing finer grain size abrasive paper "TINGIS" (grade 100-400-800-600-1200), followed by a Rinsing with double-distilled water, and finally the sample is dried under a stream of air. In this study we use mild steel as material; its mass composition is specified in Table 1.

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I.Merimi et al / Materials Today: Proceedings 13 (2019) 1008–1022 Table 1.Mass content of impurities present in mild steel. Elements Fe C Si % by mass

99.21

0.21

0.38

P

Mn

Al

S

0.09

0.05

0.01

0.05

Analytical grade chlorhydric acid 37% (d = 1.19) and doubled distilled water were used for preparing test solutions for all the experiments. All experiments were repeatedly performed to ensure reproducibility. 10-6, 10-5, 104 and 10-3mol/L are the concentrations used of the triazole compound to evaluate the inhibition of corrosion. 2.3. Measurements 2.3.1. Gravimetric study Unfortunately, this method does not allow the approach of the mechanisms involved in corrosion. Its principle described in our early work [1]. The corrosion rate (W) is given by the relation (1):

W= Δm S.t

(1)

Where W, Δm, T and S represent the corrosion rate (mg cm-2 h-1), loss of mass expressed (mg), time of immersion (h), and area of the sample (cm2) respectively. Then, the inhibitory efficacy of an organic compound is determined as following:

E(%)=

w-w inh  100 W

(2)

Where W and Winh are corrosion rates in the absence and presence of the inhibitor respectively.In the present study, the mild steel samples in cuboidal form (1.5 cm × 1.5 cm × 0.05 cm) were immersed vertically for 6 hours in the corrosive solution at 308 °K. 2.3.2. Electrochemical procedure Electrochemical experiments were recorded by using a Radiometer analytical type (Voltalab-PGZ 100), driven by a computer equipped with software Voltamaster 4. The electrochemical cell is assembled by a mild steel working electrode (WE), platinum as counter electrode (Pt) and a saturated calomel electrode used as a reference electrode. The working electrode (WE) was a flat specimen with an exposure area of 1.0 cm2 and it was mechanically polished using different grades of emery paper (120-1200). The specimens must be degreased with acetone and then washed with bi-distilled water. The inhibitor studied was added to the aggressive HCl medium (1M) which was first purged with nitrogen gas for 30 min to eliminate dissolved oxygen in the medium. The electrochemical tests were carried out at room temperature under static conditions. All the experiments were performed after dipping the working electrode into aggressive medium containing the inhibitor at the open-circuit potential, Ecorr, with respect to a SCE reference electrode. For both electrochemical polarization curves and impedance spectroscopy, we are employed the same arrangement. In order to apply the electrochemical Tafel extrapolation, polarization curves were recorded by the potential-dynamical method at a rate of 1.0 mV/s from -800 mV to -200 mV versus the open circuit potential, the scan rate (1.0 mV/seg) allows the quasi-stationary state measurements. Electrochemical impedance spectroscopy (EIS) was carried from 100 kHz to 10 mHz, with a 15 mV peak-to peak. 2.3.3. Scanning Electron Microscopy (SEM) After 24 hours immersion of the samples of the mild steel surface, in a 1M HCl solution with and without inhibitor, the morphology of these samples was examined by scanning electron microscopy (SEM). 2.3.4. Quantum chemical calculations The inhibition potentials of (2m) have been elucidated using quantum chemical calculations based on density functional theory (DFT). All calculations were performed using the GAUSSIAN 09 program package [4] with the aid of the Gauss View visualization program [5].The ground state geometry of EMOTP was fully optimized using

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the hybrid B3LYP functional methods [6,7] in combination with the 6-31G (d,p) basis set. The quantum chemical calculation parameters such as; EHOMO, ELUMO, ΔE and ΔN are placed in Table 6. The electron affinity (EA) and ionization potential (IP) are derived from EHOMO and ELUMO by the Eqs. (3) and (4) [8]:

IP   E HOMO EA   E LUMO

(3) (4)

Mulliken electronegativity (χ) and Absolute hardness (η) can be approximated as [9,10]:

IA 2 IA η 2

χ

(5)

(6)

To calculate the number of transferred electrons (ΔN), the Pearson method was applied using Eq (7) [11] as under:

N 

χ Fe  χ inh 2  ηFe  ηinh 

(7)

Where χFe and χinh show the absolute electronegativity of iron and inhibitor molecule ηFe and ηinh denote the absolute hardness of iron and the inhibitor molecule respectively. A theoretical value of χFe = 7 eV and ηFe = 0 (since for bulk metallic atoms I = A) had been considered in order to calculate the N values [12]. Recently, it has been reported that the value of χFe= 7 eV is theoretically not acceptable as electron-electron interactions had not been considered, only free electron gas Fermi energy of iron is considered [13]. Hence, the researchers are recently using work function () of the metal surface instead of χFe, as it is more appropriate measure for its electronegativity [13]. Thus, Eq.8 is rewritten as follows:

N 

φ  χ inh 2  ηFe  ηinh 

(8)

2. 3.5. Molecular Dynamics (MD) simulations Currently, Molecular Dynamics (MD) simulations have been emerged as a very popular exploratory technique to studies of interaction between inhibitor and the concerned metal surfaces. In this present investigation, calculations were carried out using Materials studio package [14] using the COMPASS force field and the Smart algorithm with NVT ensemble has been employed to study the interactions between the Fe (110) surface and the inhibitor molecules [15] with periodic boundary conditions to model a representative part of the interface devoid of any arbitrary boundary effects. The first step was to import the iron crystal, its surface cleaved along (1 1 0) plane with a thickness of 10.435 Å, their lattice parameters are a = b = 24.82 Å c = 10.13Å. The Fe (110) surface was relaxed by minimizing its energy using smart minimizer method. The second step was to create a super cell (10×10) to increase the Fe (1 1 0) surface and change its periodicity. A vacuum slab with zero thickness was built. The third step, a super cell with a size of a = b = 25.72 Å c = 26.38 Å, contains 491 H2O, 9Cl-, 9 H3O+ and one inhibitor molecule was created. Finally, the layer builder was used to create the entire model with a size of a = b = 25.72 Å c = 37.83 Å. The simulation was carried out in a simulation box (25.72×25.72×37.83 Å3). Herein, Fe (110) surface has been preferred as it has a packed surface and high stability compared to other Fe surfaces [16]. The Temperature was fixed at 303 K with a time step of 1fs and simulation time of 2000 ps. An Andersen thermostat was used to maintain the system temperature at 303K. The system was quenched every 5000 steps with the Fe (110) surface atoms constrained. Optimized structure of the inhibitor molecule was used for the simulation. More detail of MD simulation is referenced from our published articles [17, 18]. 3. Results and discussion 3.1. Gravimetric measurements, adsorption isotherm and thermodynamic parameters Table 2 shows that the efficiencyof (Z) -5-methyl-4 – ((3-nitrobenzylidene) amino) -2,4-dihydro- 3H-1,2,4triazole-3-thione increases from 73.02% to 89.74% with the increase in inhibitor concentration from 10-6 to 10-3 M.

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In addition to this, corrosion rate values ofmild steel decreases from 0.0863 to 0.0328 mg/cm2.h on the addition of 10-6 to 10-3M of (2m). The increase in efficiency may be due to the blocking effect of the surface by both adsorption and a film formation mechanism, which decreases the effective area of corrosion attack [19]. The results confirm that (2m) is an efficient corrosion inhibitor. Table 2.Weight loss values of various concentrations of compound (2m) in 1.0 M HCl solution at 308 K. Medium Concentration (M) W (mg/cm2 h) HCl Triazolic compound 2m

IE (%)

1 10-3

0.32 0.0328

— 89.74

10-4

0.055

82.8

10-5

0.0771

75.9

-6

0.0863

73.02

10

Furthermore, it is found that the adsorption process of (2m) over the metal surface follows Langmuir isotherm model [20, 21].with yielding a straight line for the plot of Cinh/θ vs. Cinh (Figure 1).

Cinh 1 = +Cinh θ K

(9)

The slope of the plot was nearly 1.0, indicating that the (2m) molecule interacts significantly with metallic surface to form an inhibitor film over the iron surface that corresponds to single layer [22]. 0,0014 0,0012

2m

0,0010

C/

0,0008 0,0006 0,0004

y= 1.11106x + 3.80748E-6 R= 0.9999

0,0002 0,0000 0,0000

0,0002

0,0004

0,0006

0,0008

0,0010

C (M)

Fig.1. Langmuir adsorption isotherm for mild steel immersed in 1.0 M HCl solution in presence of various concentrations of compound 2m.

Adsorption free energy of Gibbs, ∆G°ads, describes the stability of the adsorption bond between the compound and the metal. This parameter was determined using Kads in the following equation:

G °ads = -RTLn(55.5K ads )

(10)

R, T, 55.5 are gas constant, absolute temperature of experiment the molar concentration of water in solution in respectively. Generally, if the ∆G°ads value is resulted around -20.0 Kj/mol, the ligand-metal interaction is classified as physisorption, so there is an electrostatic interaction of the inhibitor molecule with the metal surface; otherwise, if the ∆G°ads is around -40.0 Kj/mol or above, there is presence of chemisorptions between ligand and metal, where a covalent bond is formed between the donor atom of the inhibitor and iron [23]. From table 3, the ∆G°ads value resulted for the adsorption of inhibitor (2m) was -42.21 Kj/mol and its adsorption constant was 2.6 E5 M-1, indicating that a strong bond formed between the inhibitor studied and metal through chemisorption.

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Table 3. Thermodynamic parameters for the adsorption of inhibitor (2m) in HCl on steel at different concentrations.  (KJ/mol) Inhibitor R2 Slopes Kads (L/mol)  Gads Compound 2m

0.9999

1.11106

2.6 E5

-42.21

3.2. Polarisation measurements The potentiodynamic polarization curves of the mild steel in the solution of 1M HCl in the absence and the presence of various concentrations of triazole compound are shown schematically in figure 2. It should be noted that the anodic and cathodic current densities obtained in presence of inhibitor are less than those obtained in it absence. This indicates the adsorption of this triazole derivative on the metal surface. This adsorption is carried out on the cathodic and anodic sites, which leads to the inhibition of mild steel. So, we can conclude that triazole compound behave as mixed type of corrosion inhibitors. The slopes of the Tafel lines are almost equal in the presence of inhibitor. The latter indicates that the hydrogen evolution reaction is controlled by activation. Polarisation curves allows to know the electrochemical parameters. Corrosion potential (Ecorr), corrosion current density (Icorr), cathodic Tafel slope (βc), anodic Tafel slope (βa) and corresponding inhibition efficiency (EI%) values at different inhibitor concentrations are given in Table 4. The corrosion current densities were determined by the extrapolation of cathodic and anodic Tafel slopes to the respective corrosion potentials. To calculate the inhibition efficiency of the inhibitor studied, we use the following equation (11):

E I (%)=

ICorr  ICorr (i) ICorr

 100

(11)

Icorr and Icorr(i) are the corrosion current densities for steel in the uninhibited and inhibited solutions, respectively. 2

2m

2

Log mA/cm )

1

0

-1

HCl -6 10 -5 10 -4 10 -3 10

-2

-3

-4 -8 0 0

-7 0 0

-6 0 0

-5 0 0

-4 0 0

-3 0 0

1M M M M M

-2 0 0

E (m V )

Fig.2. Polarisation curves of mild steel in 1 M HCl for various concentrations of compound 2m at 308K Table 4. Polarization data of mild steel in 1.0 M HCl without and with addition of inhibitor traizolic compound 2m at 308 K Concentrations (M) HCl 1M Triazolic compound 2m

----

10-3 10-4 10-5 10-6

-Ecorr (mV/SCE) 490.1 450.2 454.4 478.8 465.5

Icorr (mA/cm2) 0.655

-βc (mV/dec) 155.6

Βa (mV/dec) 77.6

EI (%) ----

0.0506 0.0845 0.112 0.655

199.5 243 152.9 210

100.2 85.8 79.9 96.5

92.27 87.1 80.21 75.32

According to the EI values given in Table 4, reveal that, firstly, inhibition efficiency increases with an increase in the concentration of inhibitor. Secondly, we observed the increase in EI % at higher inhibitor concentration, it means that more inhibitor molecules is adsorbed on the metal surface, thus providing wider surface coverage and this compound is acting as adsorption inhibitor. The values of Icorr decrease with the increase of triazolic concentrations. We add that the corrosion current densities were more significantly reduced and became only 50.6 µA/cm2 at 10−3 M. The best efficiency 92.27 % is obtained.

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3.3. Electrochemical impedance spectroscopy We investigated the corrosion inhibition behavior of mild steel in 1M HCl solution with different concentrations of the studied inhibitor and its blank solution after immersion for 30 min by EIS at 308 K. When analyzing Nyquist diagrams (Fig.3), we show that these curves consist of a half-circle of capacitive loop depressed with a capacitive time constant in Bode-phase plots for any study of compounds, illustrated in Fig.4a and b. Charge transfer resistance Rt, fmax and double layer capacitance Cdl values were obtained from Nyquist plots. These parameters associated with the diagrams impedance are given in Table 5. To calculate the inhibition efficiency of the inhibitor studied, we use the following equation (12):

(R Ct  R Ct )  100 R Ct

E Rt (%)=

(12)

Where, Rct and R°ct are the charge transfer resistance in absence and in presence of inhibitor, respectively. 130

HCl -6 10 -5 10 -4 10 -3 10

120 110

2m

100

80

2

-ZIM (cm )

90

1M M M M M

70 60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

160

180

200

2

Z R (   cm )

Fig.3. Nyquist plots in absence and presence of different concentrations of 2m in HCl 1M

(a)

2.2

20

HCl 1M -6 10 M -5 10 M -4 10 M -3 10 M

2.0 1.8

(b)

10 0

Phase (Degrée)

1.6

Log |Z|

1.4 1.2 1.0 0.8 0.6

-10 -20 -30 -40 HCl 1M 10-6 M 10-5 M 10-4 M 10-3 M

-50

0.4

-60

0.2 0.0

0.1

1

10

100

Log fréquence

1000

10000

100000

-70 0.01

0.1

1

10

100

1000

10000

100000

Log Fréquence

Fig.4. (a) and (b): The Bode and phase angle plots for carbon steel in 1M HCl line the absence and presence of different concentrations of inhibitors 308 K, along with an immersion time of 30 min

As it can be seen from figure 3, the Nyquist plots contain depressed semi-circles with the centre under the real axis. This phenomenon is known as dispersing effect. Sizes increase with the inhibitor concentration, indicating a charge transfer process mainly controlling the corrosion of steel. Moreover, the presence of the inhibitor did not alter the mechanism of the dissolution of the mild steel [24].The equivalent electrical circuits used provide a good fit over the impedance experimental data and are shown in Fig. 5 (c)-(d). Table 5 shows clearly that, Rt values increased with the increasing concentrations of the inhibitor. The values of Cdl decreased with an increase in the inhibitor concentration and inhibition efficiencies. This is due to an increase in the surface coverage by the inhibitor, which led to an increase in the inhibition efficiency. Decrease in CDi Values, can result from many reasons.

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Beginning with a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggested that the triazolic molecules function by adsorption at the metal/solution interface. Or, it was caused by the gradual displacement of water molecules by the adsorption of the organic molecules on the metal surface, decreasing the extent of the metal dissolution [25].

(c)

(d)

(e)

Fig.5. (c) Fitted Nyquist plot and Bode plot for PZ-1; (d) Fitted phase angle plot; (e) Fitted Bode plot

Table 5.EIS parameters for the corrosion of mild steel in 1.0 M HCl containing triazolic compound 2m at 308 K. Concentrations (M) Rt (Ω.cm2) Fmax (Hz) Cd l(µf/cm2) ER ( %) -HCl 1M 16.27 100 97.82 -

Triazolic compound 2m

10-3 10-4 10-5 10-6

179.3 106.1 69.14 66.61

31.64 40 50 50

28.05 37.5 46.04 47.78

90.92 84.66 76.47 75.57

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The equivalent circuit model used to fit the experimental results is given in figure 6 as described elsewhere [1].

Fig.6. Equivalent electrical circuit model.

Finally, the inhibition efficiencies calculated from electrochemical impedance spectroscopy, polarization and gravimetric measurements are in very good agreement. 3.4. Effect of temperature Temperature is a factor susceptive that may affect the behavior of a material in a corrosive environment and may also affect the interaction between the inhibitory metals. Therefore, it is essential to study the effect of this factor on the rates of protection, determine the mechanism of inhibition, and calculate the activation energies of the corrosion process. The changes in the values of the corrosion rate (W) and the percent inhibition efficiency (E%) of the triazole compound studied at its optimal concentration (10-3 M) as a function of the solution temperature are indicated in table 6. Table 6. Various corrosion parameters for steel in 1.0 M HCl in absence and presence of optimum concentration of triazolic compound (2m) at different temperatures. Medium

Temp(°K)

W (mg/cm2 h)

Surface Coverage (Θ)

E (%)

HCl 1M

318 328 338 348 318 328 338 348

2.1804 4.1887 7.2009 9.9982 0.4448 1.1665 2.5858 4.3882

— — — — 0.79 0.72 0.64 0.56

— — — — 79.6 72.15 64.09 56.11

Triazolic compound 2m

From the results shown in table 6, it is found that the inhibition efficiency of the triazole compound (2m) decreases with increasing the temperature of the solution. This decrease can be attributed to an increased kinetic energy of the inhibitor which causes the decrease of intermolecular force of interaction between adsorbate (2m) and adsorbent (metal surface) [26].In addition to this, rearrangement and molecular decomposition may also decrease inhibitor inhibition performance [27]. The study of the influence of temperature on the rate of corrosion inhibition of mild steel by our inhibitor (2m) was performed at temperatures 318, 328, 338 and 348K in the absence and in the presence of inhibitor at 10-3M, to determine the activation energies, enthalpies and entropies of activation of the corrosion process and thus provides information on the mechanism of inhibition. The corresponding data are shown in table 7. The following Arrhenius equation was used to determine the activation energy (Ea).

w  A exp  E a RT 

(13) -1

Activation energy was obtained by plotting variation of lnW versus reciprocal temperature T (Figure 5) [28]. Ln (W )  Ln (A ) 

Ea RT

(1 4 )

W, R, T and A are corrosion rate, gas constant, the absolute temperature and the pre-exponential factor respectively. On the other hand, enthalpy (ΔHa) and entropy of activation (ΔSa) were obtained from the Arrhenius equation transition [29]: W 

RT   Sa exp  Nh  R

  H a   exp   RT    

(15)

N, h, Ha and Sa, are the Avogrado’s number, the Plank’s constant, Enthalpy of activation and the entropy of activation respectively. Figure 6 shows the variation of Ln (W / T) versus (1 / T) as a straight line with a slope of (Aha / R) and the intersection with the Ln (W / T) axis give [Ln (R / Nh) + (Δsa / R)] (Fig. 7 and 8). Thus, values of Δsa and Δha can be calculated.

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2,4

HCl 1M

2,2

2n

1017

2,0 1,8

2

Ln W (mg/cm .h)

1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 -0,2 -0,4 2,85

2,90

2,95

3,00

3,05

3,10

3,15

-1

1000/T (K )

Fig.7.Arrhenius plots of Ln W versus T-1 for steel in 1.0 M HCl in the absence and the presence of triazolic compound 2m at optimum concentration -3,4

HCl 1M 2n

-3,6 -3,8 -4,0

2

Ln W/T(mg/cm .h.K)

-4,2 -4,4 -4,6 -4,8 -5,0 -5,2 -5,4 -5,6 -5,8 -6,0 -6,2 0,00285

0,00290

0,00295

0,00300

0,00305

0,00310

0,00315

-1

1/T (K )

Fig.8.Arrhenius plots of Ln (CR/T) vs. 1/T for steel in 1.0 M HCl in the absence and the presence of compound 2m at optimum concentration Table 7.Activation parameters for the steel dissolution in 1.0 M HCl in the absence and the presence of triazolic compound 2m at 10-3M. Ea(Kj/mol)

H°a (Kj/mol)

S°a (J/mol.K)

Ea-Ha(KJ/mol)

HCl 1M

47.19

44.42

-98.79

2.77

Triazolic compound 2m

70.73

67.96

-37.81

2.77

The calculated values of Ea were 47.19Kj mol−1 and 70.73Kj mol−1 in the absence and presence of triazolic compound (2m), respectively. The increased value of Ea in presence of 2m is attributed to increased energy barrier for mild steel corrosion in acid solution in presence of inhibitor [30]. Generally, an organic inhibitor forms surface metal-inhibitor complex which acts as barrier for corrosion process and there by increases the value of activation energy (Ea) [31]. The endothermic enthalpy has a positive sign which reflects the nature of the dissolution of the steel. The activation energy Ea and the enthalpy of ΔHa varies in the same way with the inhibitor concentrations, which satisfies the relationship between Ea and thermodynamics as ΔHa [32]: Ea–ΔHa = RT. The ΔSa value for the compound 2m is lower compared to the white, because the rate-determining step for the activated complex was the association rather than the dissociation step [33]. It is clear that ΔSa value moves to more positive values in the presence of compound 2m, which leads to increased disorder (a driving force), which can overcome the barriers for adsorption of the inhibitor on the metal surface.

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3.5. Scanning electron microscopy (SEM) Analysis of the surface area of the mild steel samples after 24 h of immersion at 308 K in 1M HCl alone (figure 9a), and with addition of 10-3 M triazole compound (figure 9b) was carried out by scanning electron microscopy. The examination of the plates of Scanning Electron Microscopy, enabled us to demonstrate the formation of a protective film on the surface of the steel in the presence of the inhibitor studied. Figure (9b) shows the morphology of the layer of corrosion products formed after 24 hours of immersion in the 1M HCl solution.

(a) (b) Fig.9.Surface morphology of mild steel after immersion in 1M HCl for 24 hours, in absence (a) and presence (b) of inhibitor.

3.6. The potential of zero charge and the inhibition mechanism Inhibition mechanisms of corrosion are interpreted by adsorption phenomena which depends on the surface charge of the metal, the charge or dipole moment of the inhibitor ions/molecules and the other ions that are specifically adsorbed on to the metal surface [34]. The surface charge of the metal is defined by the position of the open circuit potential with respect to the PZC [35]. The double layer capacitance value depends on the applied DC potential is graphically denoted in Figure 10. It can be determined according to Antropov et al. [36] by comparing the potential of zero charge (PZC) and the corrosion potential of the metal in the electrolytic medium. 0.8

2m 0.7

2

Cdc (µF/cm )

0.6

0.5

0.4

EPCN

0.3 -560

-540

-520

-500

-480

-460

E (mV/ECS)

Figure.10. Plot of Cdl vs. applied electrode potential (E (mV/ECS)) in 1 M HCl containing 10-3 M 2m.

As PZC corresponds to a state at which the surface is free from charges, at the stationary (corrosion) potential state, so the metal area will be positively or negatively charged. Hence, it is essential to have reliable data about PZC. When carbon steel is immersed in acid solution containing (2m), three kinds of species can be adsorbed on its surface, as described below:  If the metal surface is positively charged with respect to PZC, at first, the chloride ions will be adsorbed on the metal surface. Then After this adsorption step, the steel surface will become negatively charged. Hence, the positively charged derived triazolic cationic forms will form an electrostatic bond with the Cl- ions which are already adsorbed on steel Moreover, the excess positive charge on the electrode surface, Φ (Φ = EPZC - Ecorr) increases as more inhibitor molecules adsorbed on it [37].  If the metal surface is negatively charged with respect to PZC, the protonated water molecules and derived triazolic cationic forms will be directly adsorbed on the metal surface. So, increasing negative charges on the

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metal surface lead to increasing adsorption of derived triazolic molecules, hence, its concentration in solution should decrease.  When the metal obtains the potential at which the surface charge becomes zero, none of the ions (neither cations nor anions) adsorb on the surface through their ionic center. However, a few indole molecules may be physically adsorbed through their planar π orbitals on the metal surface (with vacant π orbitals). In this study, EPZC = -500 mV, Ecorr = -450 mV, for carbon steel with addition 10-3 M of 2m. It can be said that Φ (Φ = EPZC – Ecorr) potential is positive in this case. From the above result, it follows that anions (Cl- ions) in aqueous hydrochloric acid solution will be first adsorbed on the steel surface. After this first adsorption step, the steel surface will become negatively charged. Hence, the positively charged of triazolic derivative cationic forms will form an electrostatic bond with the Cl- ions already adsorbed on steel surface. 3.7. Theoretical studies 3.7.1. Quantum chemical calculation It is well known that the molecular reactivity is mainly analyzed via frontier orbital theory; therefore, it is necessary to investigate the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) for exploring the adsorption behavior of corrosion inhibitor on the metal surface. Geometric structures and electronic properties of (2m) derivative have been calculated by DFT method using B3LYP level and 6-31G (d,p) basis set. Figure 11 shows optimized molecular structures, HOMO and LUMO of triazole derivative. Optimized geometry

HOMO Orbitals

Electrostatic potential surface

LUMO Orbitals

Fig.11. The optimized geometry, HOMO, LUMO and molecular electrostatic potential (MEP) orbitals of molecules (2m) derivative at the BLYP level

The geometrically optimized structure and HOMOs, LUMOs with molecular electrostatic potential l (MEP) of (2m) triazole molecule are depicted in Figure 11. The quantum chemical parameters such as EHOMO, ELUMO, ΔE = EHOMO-ELUMO and the fraction of electrons transferred (ΔN110) of inhibitor are presented in Table 8. The inhibition effect of inhibitor molecule is usually attributed to the adsorption of (2m) triazole molecules on the metal surface. The analysis of frontier molecular orbital (HOMO and LUMO) is useful to predict the adsorption center of (2m) triazole molecules responsible for the interaction with metal surface [38]. HOMO and LUMO are associated with the electron donor and acceptor ability of (2m) triazole molecules, respectively. Molecules with higher value of EHOMO (less negative) and lower value of ELUMO (more negative) show more donor and acceptor tendency of electrons with appropriate metal d-orbital, respectively [39]. From Figure 11, it can be observed that HOMO are distributed over the triazole ring and. Therefore, the triazole ring is responsible for donating the electron to the available vacant 3d orbital of Fe (110). LUMOs are mainly distributed over the benzylidene ring and nitro group of the inhibitor molecules. The reactive sites of inhibitors were further confirmed by the study of molecular electrostatic potential (MEP). MEP is made the electron density visible and is a useful tool to understanding sites of

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electrophilic and nucleophilic attack [40].In MEP, the red (negative) regions are subjected to nucleophilic attack, while blue (positive) regions are related to the electrophilic reactivity. Figure 11, red and blue region of MEPs are corresponding to the HOMO and LUMO orbital distribution of inhibitor molecules, respectively. The quantum-chemical parameters related to the (2m) derivative molecule are listed in Table 8. EHOMO gives information about the tendency of the molecule to donate electrons to an electron poor species [41].The molecule with the highest EHOMO is considered to have the highest tendency to donate electrons to an electron poor species. ELUMO, on the other hand, shows the tendency of a molecule to accept electrons and the lower the EHOMO value, the greater the tendency of that particular system to accept electrons [42]. The energy gap ΔE (ELUMO− EHOMO) was presumed to be an indicator of the energy of the intermediate excited state which provides reliable information for the ability of a corrosion inhibitor to effectively adsorb on the metal surface. The smaller value of ΔE, the greater the reactivity of an inhibitor molecule [43]. ΔN value, on the other hand, indicates the ability of tested inhibitor to transfer its electrons to metal if ΔN > 0 and vice versa if ΔN<0 [44]. According to this criterion, it is obvious from results in Table 8 that (2m) derivative has higher tendency to donate electrons to a metal surface. Table 8.The computed quantum chemical parameters for tested (2m) compound EHOMO (eV) ELUMO (eV) ∆Egap(eV) µ (debye) ∆N -6.007 -2.696 3.311 6.4721 0.141

3.7.2. Molecular dynamics (MD) simulation The MD simulations were carried out to better understand the interaction between the inhibitor molecule and Fe (110) surface. The simulations were performed in the system containing 491 water molecules, 9Cl-, 9H3O+ and one molecule of the inhibitor molecule. After 2000 ps, the system reaches the equilibrium which explains that both the temperature and energy reach a balance [45]. By inspection of the Figure 12, it could be observed that the 2m adsorbs nearly to Fe (110) surface, were a chemical interactions can possibly occur through reactive sites in the molecule as interpreted in experimental and theoretical study. In this case, the inhibitor molecule protects the steel surface from the aggressive medium by adsorbing on the Fe (110) surface, thus reducing the MS dissolution [46].

Fig.12. Equilibrium configuration of 2m in aqueous systems.

The interactions and binding energies are calculated using Equation (16) and Equation (17) [65]: E in teractio n  E total  ( E su rface+ so lution + E inh ibitor ) (16)

E Binding   E interaction (17) Where Etotal is the total energy of the entire system Esurface+solution referred to the total energy of Fe (110) surface and solution without the inhibitor and Einhibitor represents the total energy of inhibitor. The corresponding interaction energy value is -289.5KJ/mol indicating that 2m can more tightly adsorb on iron surface and achieve better corrosion inhibition effectiveness [47]. 4. Conclusion According to our study carried out of (Z) -5-methyl-4 - ((3-nitrobenzylidene) amino) -2,4-dihydro- 3H-1,2,4triazole-3-thione, several observations can be made. Among these observations, we quote the most important ones.

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

     

1021

From weight loss, electrochemical, surface analysis and quantum chemical studies, it is concluded that the investigated compound acts as a good corrosion inhibitor for mild steel in acid solution. Its inhibition efficiency increases with increasing concentration. Maximum inhibition efficiency of 89.74% was obtained at 10-3 M/L concentration. Adsorption of triazolic compound on mild steel surface obeyed the Langmuir adsorption isotherm. Potentiodynamic polarization study revealed the compound (2m) acted as a mixed type inhibitor with cathodic predominance. EIS results showed that the presence of inhibitor molecule decreased the value of Cdl and increased the value of Rct which indicated its adsorption over the metal/electrolyte interface. Optical microscopy investigation analyses showed the existence of protective film of inhibitor molecule over the metallic surface. EIS showed that the charge transferring controls the corrosion inhibition process in the uninhibited and inhibited solutions. Different inhibition mechanisms were proposed for 2m molecules, based on their PZC value in studied conditions. Quantum chemical study confirmed the experimental results and showed that the investigated inhibitor has a strong tendency of adsorption over the metallic surface.

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