Electrochemical, DFT and MD simulation of newly synthesized triazolotriazepine derivatives as corrosion inhibitors for carbon steel in 1 M HCl

Accepted Manuscript Electrochemical, DFT and MD simulation of newly synthesized triazolotriazepine derivatives as corrosion inhibitors for carbon steel in 1 M HCl

Youness El Bakri, Lei Guo, El Hassane Anouar, El Mokhtar Essassi PII: DOI: Reference:

S0167-7322(18)34642-7 https://doi.org/10.1016/j.molliq.2018.11.048 MOLLIQ 9958

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

8 September 2018 11 November 2018 12 November 2018

Please cite this article as: Youness El Bakri, Lei Guo, El Hassane Anouar, El Mokhtar Essassi , Electrochemical, DFT and MD simulation of newly synthesized triazolotriazepine derivatives as corrosion inhibitors for carbon steel in 1 M HCl. Molliq (2018), https://doi.org/10.1016/j.molliq.2018.11.048

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ACCEPTED MANUSCRIPT Electrochemical, DFT and MD simulation of newly synthesized triazolotriazepine Derivatives as corrosion inhibitors for carbon steel in 1 M HCl Youness El Bakria,b*, Lei Guoc, El Hassane Anouard,*, El Mokhtar Essassia a

Laboratoire de Chimie Organique Hétérocyclique, Centre de Recherche des Sciences des Médicaments, Pôle de Compétences Pharmacochimie, URAC 21, Faculté des Sciences, Mohammed V University, Rabat, Avenue Ibn Battouta, BP 1014, Rabat, Morocco

Department of Chemistry, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow, 117198 Russian Federation. c

School of Material and Chemical Engineering, Tongren University, Tongren 554300, China

Department of Chemistry, College of Science and Humanities, Prince Sattam bin Abdulaziz University, P.O.

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Box 83, Al Kharj 11942, Saudi Arabia

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*Corresponding authors: Youness El Bakri Tel: +212677288857 / +79773560777;E-mail: [email protected] El Hassane Anouar Tel: +566537237912; E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract The inhibitive action of three newly synthesized triazolotriazepine derivatives, namely, 9-ethyl-6methyl-7H-1,2,4-triazolo[4,3-b][1,2,4-triazepin-8(9H)-one

(TTY),

1,2,4-triazolo[4,3-b][1,2,4]triazepin-8-one

7,9-ditetradecyl-6-methyl-7H-1,2,4-

(TTY4),

and

7,9-didecyl-6-methyl-7H-

triazolo[4,3-b][1,2,4]triazepin-8-one (TTY5) for carbon steel in 1M HCl is investigated using potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and surface analysis techniques. The results show that the corrosion of carbon steel in HCl solution is

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efficiently inhibited by these new organic inhibitors. The adsorption of tested inhibitors on carbon steel surface is found to be spontaneous and obeyed the Langmuir adsorption isotherm. The anti-

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corrosion mechanism of these new inhibitors on iron was further revealed by quantum chemical

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calculations and molecular dynamics simulations. In agreement with the experimental data, theoretical results showed that the order of inhibition efficiency is TTY5 > TTY4 > TTY.

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Keywords: Triazolotriazepine derivatives; Carbon steel; Corrosion inhibition; Electrochemical;

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Quantum chemical calculation; Molecular dynamics simulations

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ACCEPTED MANUSCRIPT 1. Introduction Corrosion of materials is one of the main problems in the industry that is associated with significant economic losses. Corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of the product, reduction in efficiency, an increase in maintenance needs, and expensive overdesign [1-3]. Acidic solutions are extensively used in a variety of industrial processes such as oil well acidification, acid pickling and acidic cleaning, which generally lead to serious metallic corrosion. The performance of the corrosion inhibitors based on organic

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compounds containing nitrogen, phosphorus, sulfur, and oxygen atoms shows promising results. However, the stability of the inhibitor film formed over the metal surface depends on some

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physicochemical properties of the molecule, related to its functional groups, aromaticity, the

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possible steric effects, electronic density of donors, type of the corrosive medium and nature of the interactions between π-orbital of inhibitors with the d-orbital of iron[4-7]. The inhibitors with

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the polar groups may act as reactive centers, and the play an important role in the adsorption process on the metal surface. Generally, the tendency to form a stronger coordination bond, and consequently resulting in high inhibition efficiency, increases in the following order O < N < S <

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P [8, 9]. Nitrogen-containing heterocyclic organic compounds have been found to be effective corrosion inhibitors. Triazolotriazepine is a heterocyclic aromatic organic compound. This

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bicyclic compound consists of the fusion of triazole and triazepine. Triazolotriazepine derivatives have been used as potent inhibitors of bone resorption [10]. They also exhibit anti-fungal activity

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[11].

Quantum chemical calculations have been widely used to study the reactivity and rationalization reaction mechanisms. They have also proved to be a very important tool for

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studying the corrosion inhibition mechanism [12-14]. In recent times, density functional theory (DFT) has become an attractive theoretical method because it gives exact basic vital parameters

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for even huge complex molecules at low cost. Furthermore, by using sophisticated computational tools, we can understand the reactivity behavior of hard and soft acid-base(HSAB) theory that provides a systematic way for the analysis of the inhibitor/surface interaction. Thus, DFT has become themain source of connecting some traditional empirical concepts with quantum mechanics, it has been a very powerful technique to probe the active sites and to analyze experimental data. In addition, as validated by Refs.[15-17], molecular dynamics (MD) simulation has been another effective tool to study the interaction of inhibitors with the metal surface or similar problems. The present study aimed at the evaluation of the corrosion inhibition performances of three triazolotriazepine derivatives by applying electrochemical and surface analysis techniques. 3

ACCEPTED MANUSCRIPT Furthermore, DFT calculations have been carried out in an attempt to understand the impact of molecular and electronic properties of the synthesized inhibitor over iron surface and to determine the inhibitor/surface interaction active sites, also molecular dynamics simulation method was used to elucidate the adsorption behavior of inhibitors at the iron surface.

2. Material and methods

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2.1. Synthesis of 6-methyl-7H-1,2,4-triazolo[4,3-b][1,2,4-triazepin-8(9H)-oneand its derivatives 2.1.1. 6-methyl-7H-1,2,4-triazolo[4,3-b][1,2,4-triazepin-8(9H)-one

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In a flask containing 10 cm3 of ethyl acetylacetate, 2 g of sodium acetate (0.024 mol) 3.6 g (0.02 mol) of 3,4-dimino[1,2,4]triazole hydrobromide was heated under reflux during 3 hours,

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after cooling, the precipitate formed is filtered off, the precipitate was filtered off under vacuum and the mixture was washed with ethanol to give the product with a yield of 65% (Scheme 1). H NMR (DMSO-d6, 300 MHz) δ 2.28 (s, 3H, CH3); 3.55 (s, 2H, CH2); 8.71 (s, 1H,

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CHtriazolic).13C NMR (DMSO-d6, 75 MHz) δ 24.94; 42.8437; 141.42; 144.08; 164.78; 168.99.

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Scheme 1.Synthesis of 6-methyl-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin-8(9H)-one.

2.1.2. 9-ethyl-6-methyl-7H-1,2,4-triazolo[4,3-b][1,2,4-triazepin-8(9H)-one (TTY)

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To a solution of 1.00 g (0.06 mol) of 6-methyl-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin8(9H)-one in 30 mL of sodium methoxide (prepared from 30 mL of methanol and 0.15 g of

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sodium) was added 1.00 g (0.007 mol) of ethyl iodide and the mixture was heated for 5 h (Scheme 2). The solution was then concentrated to dryness under reduced pressure and the residue was extracted with chloroform. The precipitate obtained was chromatographed on a silica column (eluent: chloroform/ethanol 95:5 v/v). The purified product was crystallized from ethanol to give the product with a yield of 70%. 1

H NMR (DMSO-d6, 300 MHz) δ 1.38(t,3H,CH3), 1.94(s,3H,CH3), 3.43(s,2H,CH2),

4.52(q,2H,CH2), 9.33(s,1H,C-Htriazolic). 13C NMR (DMSO-d6, 75 MHz) δ 13.92, 21.22, 40.5, 46.6, 144.23, 157.2, 159.43, 161.77.

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ACCEPTED MANUSCRIPT 2.1.3. 7,9-didecyl-6-methyl-7H-1,2,4-triazolo[4,3-b][1,2,4]triazepin-8(9H)-one (TTY4) To a solution of 6-methyl-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin-8(9H)-one (0.5 g, 3.03 mmol) in N,N-dimethylformamide (10 mL), was added potassium carbonate (0.42 g, 1.82 mmol), 1-decyl bromide (0.63 ml, 3.03 mmol) and a catalytic amount of tetra n-butyammonium bromide. The reaction mixture was stirred for 12h (Scheme 2). The solution was then concentrated to dryness under reduced pressure and the residue was extracted with dichloromethane. The precipitate formed by cooling was filtered and crystallized from ethanol to give product with a 1

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yield of 66%.

H NMR (DMSO-d6, 300 MHz) δ 0.83(t, 3H, CH3), 1.20(2H, CH2), 1.31(m, 1H, C-H),

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1.45(q, 2H, CH2), 2.14(t,1H, C-H), 3.20(t,2H, CH2), 8.76 (s, 1H, C-Htriazolic). 13C NMR (DMSO-

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d6, 75 MHz) δ 14.13, 22.57, 25.97, 26.13, 29.18, 31.76, 47.32, 141.82, 146.93, 164.04, 168.92.

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2.1.4. 7,9-ditetradecyl-6-methyl-7H-1,2,4-triazolo[4,3-b][1,2,4]triazepin-8(9H)-one (TTY5) To a solution of 6-methyl-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin-8(9H)-one (0.5 g, 3.03 mmol) in N,N-dimethylformamide (10 ml), was added potassium carbonate (0.42 g, 3.03 mmol),

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1-tetradecyl bromide (0.9 ml, 1.82 mmol) and a catalytic amount of tetra n-butyammonium bromide. The reaction mixture was stirred for 12h (Scheme 2). The solution was then

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concentrated to dryness under reduced pressure and the residue was extracted with dichloromethane. The precipitate formed by cooling was filtered and crystallized from ethanol to 1

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give the product with a yield of 64%.

H NMR (DMSO-d6, 300 MHz) δ 0.83(t, 3H, CH3), 1.20(2H, CH2), 1.31(m, 1H, C-H),

1.45(q, 2H, CH2), 2.14(t,1H, C-H), 3.20(t,2H, CH2), 8.76 (s, 1H, C-Htriazolic). 13C NMR (DMSO-

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d6, 75 MHz) δ 14.13, 22.57, 25.97, 26.13, 29.18, 31.76, 47.32, 141.82, 146.93, 164.04, 168.92.

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Scheme 2.Synthetic routes of the target TTY, TTY4 and TTY5inhibitors.

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2.2. Preparation of electrodes and test solution

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Corrosive solutions were prepared by dilution of analytical reagent grade 37% HCl with doubly distilled water. The composition (wt%) of tested steel strips is 0.36 % C, 0.66 % Mn, 0.27 % Si, 0.02 % S, 0.015 % P, 0.21 % Cr, 0.02 % Mo, 0.22 % Cu, 0.06 % Al, and the rest Fe. The

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working area of 1 cm2 was subsequently ground with 100 and 1500 grit grinding papers, cleaned by distilled water and ethanol at hot air. The effect of temperature on the inhibition efficiencies

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for the inhibitor were tested between 303~333 K.

2.3. Electrochemical measurements The effects of all inhibitors studied on the carbon steel corrosion were investigated by electrochemical techniques including EIS and PDP test in the concentration range of 10−6 to 10−3M at 303 K. The electrochemical experiment consisted of a three electrode electrolytic cell consisting of platinum foil as counter electrode, saturated calomel as reference electrode and carbon steel as working electrode with an exposed area of 1 cm2. The carbon steel specimen was immersed in a test solution for 30 min until a steady-state potential was achieved. EIS measurements were performed with a frequency range of 100 kHz to 10 mHz and amplitude of 10 mV with 10 points per decade. The polarization curves were recorded by polarization with a 6

ACCEPTED MANUSCRIPT scanning rate of 0.1667mVs−1. The data obtained by PDP and EIS methods were analyzed and fitted using graphing and analyzing impedance software, version Ec-Lab.

2.4. Scanning electron microscopy studies The carbon steel specimens (0.5 cm × 0.5 cm × 0.25 cm dimension) were subjected to 1 M HCl solution with and without the inhibitors. After 24 hours, carbon steel was treated by deionized water, alcohol wash and dried. The surface morphological analysis was performed

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using the field emission scanning electron microscope (FE-SEM) with JSM-7800F model under

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high vacuum (JEOL Ltd, Japan).

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2.5. Computational details

The optimization and frequency calculations of the ground states for the synthesized 6-

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methyl-7H-1,2,4-triazolo[4,3-b][1,2,4]triazepin-8-one derivatives were carried out using the DFT hybrid functional B3LYP combined with a triple-ζ basis set 6-311++G (d,p) as implemented in Gaussian09 software [18]. The minima of the optimized structures were confirmed by the

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absence of imaginary frequencies. To correlate the corrosion inhibition efficiencies of the synthesized compounds on carbon steel, a set of electronic properties such as HOMO and LUMO

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energies, ionization potential (I), electronic affinity (A), energy gap between the HOMO and LUMO (ΔE), electronegativity (), chemical hardness (), softness (S=1/), and dipole moment

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() were calculated. These electronic parameters were calculated using Koopman’s theorem [19], in which I=−EHOMO and A=−ELUMO.  and  are calculated by the following formula:

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

IA (1) 2

IA 2 The fraction of the transferred electrons (∆N) was calculated by using Pearson theory [20]:

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

N =

 Fe  inh 2 Fe  inh 

(2)

(3)

where Fe andFe are the absolute electronegativity and global hardness of Fe atom, and χinh and

inh are the absolute electronegativity and global hardness of the inhibitor molecules. The theoretical values of χFe (7 eV) and Fe (0 eV) are used to calculate ΔN [21]. To determine the active sites for the titled inhibitors binding to the carbon steel surface, Mulliken atomic charges were calculated. The solvent effects were taken into account implicitly by using the polarizable continuum model (PCM). In PCM, the substrate is embedded into a cavity surrounded by solvent

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ACCEPTED MANUSCRIPT described by its dielectric constant ε (for water ε = 78.3) [22]. The frontier molecular orbitals were visualized with Molden software. Molecular dynamic simulations were performed using the Forcite module of the Materials Studio 8.0 software developed by BIOVIA Inc. For the iron surface, the Fe(110) surface was selected for the study because the (110) surface was densely packed surface and was therefore the most stable [23]. The molecular dynamic simulation was performed in a simulation box (24.8 Å × 24.8 Å × 45.1 Å) with periodic boundary conditions. To construct a more reliable system, water

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molecules were added to the solution layer in the simulated system. Therefore, the adsorption system included 600 H2O molecules, andone inhibitor molecule. The entire system was run at

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303 K, as controlled by the Andersen thermostat, NVT ensemble, with a time step of 1.0 fs and a

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simulation time of 1 ns, using the COMPASS force field [24]. The dynamic process was performed until the entire system reached equilibrium, at which both the temperature and the

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energy of the system were balanced. Each simulation was repeated to ensure the reliability of the results.

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3. Results and discussion 3.1. Electrochemical studies

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As we know, it is important to provide steady state conditions before performing the

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potentiodynamic polarization and impedance measurements. The open circuit potential (EOCP) of the carbon steel electrode with immersion time (t) in 1 M HCl solution at 303 K is depicted in Fig. 1. It is generally considered that the open circuit potential is stable when the change of open circuit potential is less than 3 mV. It is obvious that 30 minutes is enough to achieve this

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equilibrium state [25]. And then, the potentiodynamic polarizationand impedance measurements

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are performed after the attainment of the steady state OCP.

Fig. 1. The change of open circuit potential of the carbon steel electrode with immersiontime in 1 M HCl solution at 303 K. 8

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3.1.1. Electrochemical impedance spectroscopy (EIS) The corrosion behavior of carbon steel in 1 M HCl (blank solution) and in the presence of the TTY, TTY4, and TTY5 inhibitors at different concentrations are shown in Nyquist plots (Fig. 2). The Nyquist plots present capacitive loop that is slightly depressed semicircle due to the roughness and inhomogeneities of carbon steel surface attributed to a phenomenon called “dispersing effect” [26]. It is obvious that the impedance response of carbon steel is significantly

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changed after the addition of the synthesized inhibitors. The diameter of the semicircle increases with increasing the concentrations of the three tested inhibitors resulted from the effective surface

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coverage of the inhibitors on carbon steel surface.

Fig. 2.Nyquist plot for mild steel in 1 M HCl in the presence and absence of different concentrations of synthesized inhibitors at 303 K.

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Fig. 3.Equivalent circuit model used to fit the impedance spectra.

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As shown in Fig. 3, a simple electrical equivalent circuit was used to fit the experimental data, which consists of solution resistance Rs, charge transfer resistance (Rct), as well as the

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constant phase element (CPE). The obtained impedance parameters are summarized in Table 1. The inhibition efficiency (%) was calculated from Rct values using the following relation: Rct  Rct0  100 Rct0

(4)

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 % =

where, Rct0 and Rct are the charge transfer resistance in the absence and presence of inhibitor,

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respectively.In addition, the double layer capacitances, Cdl, for a circuit including a CPE were calculated by using the following formula [27]: Cdl  QRct1n 

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1/ n

(5)

wherein, n is a CPE exponent which can be used as a gauge for the heterogeneity or roughness of

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the CPE constant.

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the surface, if n = 0, the CPE is identical to a pure resistor, and for n = 1, a pure capacitor. Q is

Table 1Impedance parameters for mild steel electrode in 1M HCl in the presence of different concentrations of synthesized inhibitors at 303 K. Q Inhibitor Rct(Ω cm2) n Cdl(μF cm−2) η (%) C(M) (×10−4Ω−1Sncm−2)

TTY

TTY4

Blank

20.24

0.86

2.420

112.04

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1×10−3 1×10−4 1×10−5 1×10−6

665 460 213 150

0.86 0.88 0.85 0.83

0.280 0.307 0.673 0.901

14 17 31 37

96.9 95.6 90.4 86.5

1×10−3 1×10−4 1×10−5 1×10−6

710 542 336 67

0.89 0.86 0.88 0.85

0.189 0.300 0.408 1.654

11 15 22 71

97.1 96.2 93.9 69.7

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TTY5

958 475 431 350

0.88 0.84 0.81 0.83

0.169 0.351 0.501 0.591

9.6 16 20 26

97.8 95.7 95.3 94.2

From the impedance data in Table 1 we remarked that the value of Rct increases with increasing the inhibitor concentration. As a result, inhibition efficiencies increase with increasing the concentration of inhibitors and reach 96.9% for TTY, 97.1% for TTY4, and 97.8% for TTY5,

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respectively. Then the order of optimum inhibition ability is that TTY < TTY4 < TTY5, which suggests that these green inhibitors can display more effective protection for iron corrosion with

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increasing the length of alkyl chain. Thisanti-corrosion behavior was according to a minimum value of surface heterogeneity which attributed to the adsorption of our compounds. These results

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leading to suggest that a layer protective film was formed on the electrode surface.

3.1.2. Potentiodynamic polarization test

The tafel plots of carbon steel in 1 M HCl at various concentrations of TTY, TTY4, and

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TTY5 are shown in Fig. 4. Several electrochemical parameters such as corrosion current density (icorr), corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βc and βa) derived from the

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extrapolation of the polarization curves are obtained. These parameters and inhibition efficiency

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(%), defined as Eq. (6), are summarized in Table 2.  % =

0 i I corr  I corr  100 (6) 0 I corr

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0 i where I corr and I corr are the corrosion current densities for uninhibited and inhibited solutions,

1000 100

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1 0.1 0.01

0.001

Blank -3 1 x 10 M -4 1 x 10 M -5 1 x 10 M -6 1 x 10 M

-700

-600

1 0.1 0.01

0.001 1E-4 -500 -400 E (mV/SCE)

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10 2

10

2

I (mA/cm )

100

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1000

I (mA/cm )

respectively.

-300

Blank -3 1 x 10 M -4 1 x 10 M -5 1 x 10 M -6 1 x 10 M

1E-5 -600

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-500 -400 E (mV/SCE)

-300

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2

I (mA/cm )

10 1 0.1 Blank -3 1 x 10 M -4 1 x 10 M -5 1 x 10 M -6 1 x 10 M

0.01 0.001 -700

-600

-400 -500 E (mV/SCE)

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-300

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Fig. 4. Tafel plot for mild steel in 1 M HCl in the presence and absence of different concentrations of synthesized inhibitors at 303 K.

From Fig. 4, it can be seen that the addition of TTY, TTY4, and TTY5 into the acidic media

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affects both polarization curves associated to the anodic and cathodic current density, which decreases with increasing concentration of inhibitors. This result indicates that dissolution of iron on the carbon steel surface is retarded hence reducing the corrosion rate. As can be seen from

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Table 2, the increased concentration of inhibitors induced a decrease of the corrosion current density which indicates that the adsorptive film of the inhibitors on carbon steel surfaces becomes more stable [28]. For three studied inhibitor, the corrosion potentials shift towards positive

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potential compared with the blank, but the displacements of the Ecorr are less than 85 mV.

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Theseresults reveal that these triazolotriazepine derivatives act as mixed-type inhibitors [29]. It is worth mentioning that the values of  for these inhibitors follow the order: TTY
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these compounds can be seen as effective inhibitors for iron corrosion in hydrochloric acid solution, and longer length of alkyl chain can improve their inhibitive performance, which is in

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perfect accordance with the findings obtained from EIS measurements. Table 2Polarization parameters for mild steel electrode in 1 M HCl in the presence of different concentrationsof synthesized inhibitorsat 303 K. Inhibitor C(M) −Ecorr(mV/SCE) icorr(µA cm−2) βa(mV dec−1) −βc(mV dec−1) η (%) Blank

492

507

100

122

/

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1×10−3 1×10−4 1×10−5 1×10−6

435 430 424 420

28 36 84 102

89 93 80 90

160 163 159 161

94.4 92.8 83.4 79.8

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1×10−3 1×10−4 1×10−5 1×10−6

465 460 459 463

25 31 56 134

71 69 74 73

154 156 165 161

95.0 93.8 88.9 73.5

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TTY5

1×10−3 1×10−4 1×10−5 1×10−6

481 480 478 482

15 37 43 56

15 37 43 56

151 156 148 153

97.0 92.7 91.5 88.9

3.2. Adsorption isotherm To describe the nature of the interaction between the synthesized inhibitors and carbon steel surface in the corrosion process, adsorption isotherm regression analyses were applied. Several

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adsorption isotherms were tested for the studied compounds and it was found that they obey the Langmuir adsorption isotherm. The surface coverage (θ, defined as ) at various concentrations

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of the synthesized inhibitors were calculated. The plots of C/θvs. C yielded straight lines as

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shown in Fig. 5, indicating monolayer formation of TTY, TTY4, and TTY5 on carbon steel surface. The adsorption equilibrium constants Kads of the synthesized inhibitors were calculated

1  C (7) K ads

TTY TTY4 TTY5

0.0010

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0.0008

C/





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0.0012

0.0006

C

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by using the following formula [30]:

0.0004 0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

C (M)

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Fig. 5. Langmuir plots for synthesized inhibitors on carbon steel in 1 M HCl.

From the intercept of Fig. 5, the values of Kads were calculated. Large values of Kads obtained for three studied inhibitors imply efficient adsorption and good corrosion inhibition efficiency. The higher value of Kads for TTY5indicates a higher adsorption capability on carbon steel surface compared to TTY and TTY4. The free adsorption energies (ΔGads) for the inhibitors were calculated based on the following equation [31]: Gads   RT ln  55.5K ads 

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(8)

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chemical phenomenon. In addition to the chemical adsorption, inhibitor molecules can also be adsorbed on the metal surface via physical interactions [34].As given in Table 3, the ΔGads value

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is closer to −40 kJ mol−1,which indicates the contribution of physical adsorption. Therefore, it is

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concluded that, the adsorption of triazolotriazepine molecules on the carbon steel surface is a mixed type of chemical and physical adsorptions with predominantly the first one. Meanwhile,

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the negative ΔGads values for synthesized inhibitors indicate the spontaneity in the adsorption process on carbon steel surface.

3.3. Temperature effect

Kads(L mol−1) 555425 760867 1363484

ΔGads (kJ mol−1) −43 −44 −45

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Linear regression coefficient (R2) 0.997 0.998 0.999

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Inhibitor TTY TTY4 TYY5

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Table 3 Adsorption thermodynamics properties of the synthesized inhibitors.

The effect of temperature on the inhibition efficiency of carbon steel in acidic medium in the absence and presence of three triazolotriazepine derivatives at optimum concentrations were

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investigated. As can be seen from Fig. 6 and Table 4, at any given temperature, the corrosion current density in the absence of inhibitors is higher than that in its presence. The inhibition

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efficiency decreases with increase in the solution temperature from 303 to 333 K. Therefore, increasing the temperature increases the corrosion current density from 28 to 570µA cm−2 for TTY, from 25 to 607µA cm−2for TTY4, and from 15 to 370µA cm−2 for TTY5, respectively. This result can be explaining by desorption of inhibitor molecules from carbon steel surface [35]. Increase in temperature is accompanied by an increase in cathodic as well as anodic current density.

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TTY

100 10

2

10

I (mA/cm )

1

303 K 313 K 323 K 333 K

0.1

1 0.1

303 K 313 K 323 K 333 K

0.01

0.01

0.001 -550

-500

-450

-400

-300

-350

-700

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-600

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2

I (mA/cm )

1

303 K 313 K 323 K 333 K

1E-4

0.1

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0.1

0.001 1E-4

-600 -550 -500 -450 -400 -350 -300

E (mV/SCE)

303 K 313 K 323 K 333 K

0.01

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2

I (mA/cm )

10

0.001

-200

TTY5

10

0.01

-300

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100

-500 -400 E (mV/SCE)

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E (mV/SCE)

-650 -600 -550 -500 -450 -400 -350 -300 -250 E (mV/SCE)

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Fig. 6. Effect of temperature on the behavior of carbon steel/1M HCl interface in (a) uninhibited solution, at 10−3M of (b) TTY, (c) TTY4, and (d) TTY5.

Table 4Influence of temperature on the corrosion rate and inhibition efficiency of carbon steel in 1 M HCl at 10−3M of TTY, TTY4, and TTY5.

TTY

TTY4

TTY5

icorr(µA cm−2)

η (%)

303 313 323 333

−452 −454 −443 −450

507 860 1840 2800

/ / / /

303 313 323 333

−435 −440 −441 −439

28 95 330 570

94.4 88.9 82.1 79.6

303 313 323 333

−466 −483 −484 −482

25 90 300 607

95.0 89.5 83.6 78.3

303 313 323 333

−481 −492 −493 −491

15 57 201 370

97.0 93.3 89.1 86.7

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Blank

Ecorr (mV/SCE)

T (K)

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Inhibitor

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ACCEPTED MANUSCRIPT Arrhenius plots for the corrosion rate of carbon steel are given in Fig.7. Values of apparent activation energy of corrosion (Ea) for carbon steel in 1 M HCl with the absence and the presence of various concentrations of compounds tested were determined from the slope of ln(icorr) vs. 1/T plots of inhibitors and shown in Table 5.To calculate the activation parameters for the corrosion process based on Arrhenius Eq. (9) and transition state Eq. (10) were used [36]:

 E  ln I corr  ln A    a   RT 

RT  S   H a  exp  a  exp    (10) Nh  R   RT 

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I corr 

(9)

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where A is the pre-exponential factor, ∆Ha is the enthalpy, ∆Sa is the entropy of activation, T is

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the absolute temperature in Kelvin, h is the Plank constant, N is the Avogadro number, and R the

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molar gas constant.

8

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6 5 4 3

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3.00 3.05 3.10 3.15 3.20 3.25 3.30

1000/T (K-1)

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1

Blank TTY TTY4 TTY5

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ln (icorr) (A/cm2)

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Fig. 7. Arrhenius plots of lnicorrvs. 1/T for carbon steel in 1M HCl in the absence and the presence of 10−3 M of TTY, TTY4, and TTY5.

Analysis of the results in Table 5, we can conclude that the values of Ea in the presence of the inhibitors are higher than those in the uninhibited acid. The studied inhibitors retard corrosion at ordinary temperatures, but inhibition is diminished at an elevated temperature. The decrease in the steel corrosion rate is mostly determined by the activation energy [37]. Additionally, the positive values of ∆Ha showed that the corrosion process is endothermic. There is also an agreement between the values of ΔHa and Ea as they change in the same manner which is qualified by following equation ΔHa = Ea–RT [38]. The higher value of ΔSa in the presence of inhibitors suggests the formation of the activated complex, and the rate determining step is dissociation rather than association. Hence, the increase in entropy is attributed to increase in solvent entropy,which drives the adsorption of inhibitors on the metal surface [39]. 16

ACCEPTED MANUSCRIPT Table 5Activation parameter of the synthesized inhibitors Inhibitor Blank TTY TTY4 TTY5

Linear regression coefficient (R2) 0.995 0.991 0.989 0.992

ΔHa (kJ mol−1) 46.77 83.88 87.79 88.87

Ea (kJ mol−1) 49.38 86.54 90.45 91.53

ΔSa (J mol−1 K−1) −38.96 60.52 72.33 71.91

Ea−ΔHa (kJ mol−1) 2.6 2.6 2.6 2.6

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3.4. SEM analysis The formation of protective film of the inhibitors on carbon steel surface was confirmed by SEM analysis. Fig.8 shows the micrographs of the carton steel specimens exposed to 1 M HCl

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solution for 24 h without and with 10−3M TTY, TTY4 and TTY5, respectively. As shown in Fig.

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8a, the iron specimens in the blank solution is strongly damaged with dense holes. Compared to the blank, the TTY has retarded the corrosion rate of iron, but there are still some corrosion

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products on the substrate. The specimen in Fig. 8c was smoother than Fig.8b, yet with some notches. Moreover, the addition of TTY5 in Fig. 8d was smoother than TTY4 and TTY.

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Therefore, it can be concluded that the corrosion of carbon steel is inhibited in presence of

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inhibitors and the inhibited sequence follows TTY5>TTY4 > TTY.

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Fig.8. SEM images of the carbon steel surface dipped in 1 M HCl solution without and with inhibitors for 24 h: (a) blank, (b) TTY, (c) TTY4, and (d) TTY5.

3.5. DFT calculations

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In order to further explore the inhibition mechanism of synthesized compounds, quantum chemical calculations were carried out. Simple linear regression curves between the experimental corrosion inhibition efficiency (exp) and the electronic parameters of the synthesized compounds are displayed in Fig. S1. The results showed that the best correlations were obtained with the ionization potential (I) and number of the transferred electron (ΔN) with correlation coefficients (R2) of 99.54 and 99.54%, respectively. Moderate correlations were obtained with an energy gap, softness, and dipole moment with R2 of 83.16, 83.24, and 75.2%, respectively. Weak correlations were obtained with electronic affinity and electrophilicity with R2 of 17.06and 4.9 %, respectively. The lower inhibition efficiency of TTY compared to TTY4 and TTY5 is well explained by the lower ionization energy of TTY compared TTY4 and TTY5 (Table 6). The 17

ACCEPTED MANUSCRIPT positive value of transferred electron (ΔN>0) indicates that the transfer of electrons from the inhibitor molecule to the metallic surface. However, ΔN value does not give information on the exact number of electrons transferred between the inhibitor and the metal surface. Herein, the SLR reveals that the efficiency of the synthesized derivatives decreases with ΔN (Table 6 and Fig. S1). Table 6Quantum chemical parameters calculated at the B3LYP/6-311++G (d, p) level for studied inhibitors. χ

5.59 5.64 5.60

4.19 4.20 4.21



S

ΔN

µ (Debye)

0.502 0.496 0.498

6.38 6.14 6.36

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6.99 7.01 7.02

ΔE(eV)

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TTY TTY4 TTY5

A (eV)

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I (eV)

Fig. 9. Frontier molecular orbitals of the synthesized inhibitors (isovalue of 0.04 a.u. for TTY, and 0.03 a.u. for TTY4 and TTY5).

HOMO and LUMO orbitals are useful for prediction of adsorption centers of the interaction between the inhibitor and metal surface. The HOMO and LUMO molecular orbitals of the synthesized inhibitors are shown in Fig. 9. As can be seen from Fig. 9, the HOMO orbitals of the synthesized inhibitors are mainly delocalized over the triazolering and the N(CO)(C2H5 )moietiesof triazepine ring. However, the LUMO orbitals are mainly delocalized over the triazepine ring. The efficient inhibitors are those donating electrons (Nucleophilic attack) to d orbital of the metal, and accepting electrons from the metal surface (Electrophilic attack)[40]. 18

ACCEPTED MANUSCRIPT The electronic density delocalization of HOMO and LUMO orbitals clearly showed that the current synthesized may have donor/acceptor electron of atom sites in triazoleandtriazepine moieties to/from the d-orbitals of the iron metal. To determine the main active sites for the tilted compounds, Mulliken atomic charges were calculated at the B3LYP level of theory (Figs. S2−S4). As can be seen in Figs. S2-4, some of the atomiccenterspossess negative charges, while others positive ones. For instance, for the synthesized the oxygen atom of the carbonyl group play the role of the nucleophile or electron

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donator (with atomic charges in the range [−0.37,−0. 31]). The number of transferred electrons from the inhibitor to the metal (∆N) was calculated. As mentioned above, the correlation curve

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between the inhibition efficiency and ∆N is highly correlated. Hence, if we consider TTY, TTY4

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and TTY5 inhibitors, the higher inhibition efficiency of (i) TTY5 compared to TTY4 and (ii) TTY4 compare to TTY are in good agreement with the increased ∆N values (0.502, 0.496 and

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0.498 for TTY, TTY4 and TTY5, respectively). Theseresults are in good agreement with previous studies, which reported that the inhibition efficiency increased with the electron-

3.6. Molecular dynamics simulation

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donating ability to the metal surface (∆N).

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To get more knowledge on the interaction between triazolotriazepine derivatives and the iron substrate, molecular dynamics simulations in a solvent-containing system were performed.

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The most stable low energy adsorption configurations of the inhibitors on the Fe(110)/H2O system using For cite simulations are depicted in Fig. 10. Careful inspection of Fig. 10 shows that for all the cases, the polar triazole and triazepine rings are attached to the metal surface

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horizontally, while the apolar tails stretch toward the water slab. Generally, the adsorption energy (Eads) is the energy released (or needed) when the relaxed

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adsorbate component is adsorbed on the substrate. The values of Eads between studied inhibitors and Fe(110) surface were evaluated when systems reach equilibrium. Edas was defined as follows[41]:

Eads  Etotal  ( Esurface  Einhibitor  Ewater )

(11)

wherein, Etotal denotes the total energy of ironcrystal together with the adsorbed inhibitor molecule and water molecules, Einhibitor and Esurface represent the total energy of isolated inhibitor molecule and clean Fe-slab, respectively, and Ewater is the energy of water molecules. The binding energy of the inhibitor molecule is expressed as Ebinding = −Eads. The calculated adsorption energies for TTY, TTY4, TTY5 on Fe(110) surface are −135.7, −137.9, and −140.5 kcal mol−1, respectively. Thus, the value of Ebinding for different studied 19

ACCEPTED MANUSCRIPT inhibitors follows the order:TTY5 > TTY4 > TTY, which is in accordance with the order of their

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inhibition efficiencies obtained experimentally.

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3.7. Mechanism of adsorption and inhibition

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Fig. 10. The most stable low energy configurations for the adsorption ofTTY, TTY4, and TTY5 on Fe(110) surface.

Finally, it is necessary toelucidate the corrosion inhibition mechanism of researched

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triazolotriazepinederivatives. Due to the activation effect of Cl−, the pitting corrosion of the carbon steel under HCl proceeded according to the following steps[25]: Fe  Cl    FeCl ads  e 

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 FeCl ads   FeCl+   e FeCl +  Fe2+  Cl 

(11) (12) (13)

For specimens in the inhibited HCl solutions, a [Fe(0)Inh] layer is formed at the flat area via

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thefollowing reaction:

Inh + Fe(0)   Fe(0)Inh

(14)

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As shown in Fig. 11, the inhibitors (Inh for short) quickly react with Fe(0) and form a strong protective [Fe(0)Inh] layer in the non-corroded area. This layer is very thin and is presumably a single monlayer. While in the low-lying area, Cl− anions are first adsorbed on to the positively charged iron surface. Since triazolotriazepine derivatives are organic bases, then TTY, TTY4, andTTY5 molecules could be partially protonated (defined as InhH+) in HCl solution. Consequently, InhH+can react with Fe(II) and form a thick and protective [FeCl−InhH+] complex. Ultimately, the corrosive ions were blocked by the protective film and thus carbon steel was effectively protected.

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Fig. 11. Proposedadsorption model of inhibitors on carbon steel surface inHCl medium.

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4. Conclusions

Three new triazolotriazepine derivatives have been synthesized and investigated for their inhibition performances on carbon steel corrosion in 1 M HCl solution. The results revealed that

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they are relatively good inhibitors. Potentiodynamic polarization data indicated that synthesized derivatives are of mixed type inhibitor. EIS data reveals the inhibition of corrosion on getting

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adsorbed on the carbon steel surface. Langmuir model is best fitted into the obtained results. Quantum chemical parameters suggested the inhibition of carbonsteel corrosion in 1 M HCl

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solution by the studied compounds is essentially due to the effective transfer of electrons from the inhibitors to Fe atom, which facilitates donor-acceptor interactions between the inhibitors and the metal. Molecular dynamics simulation results reveal thatthe interaction energy values of the three

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inhibitor compounds with the Fe(110) surface obey the order TTY5>TTY4 >TTY, in accordance witht he experimental inhibition efficiency. We hope that the present study will provide a

inhibitors.

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distinctive perspective on developing new type of triazolotriazepine derivatives as corrosion

Acknowledgments This research was supported by the National Natural Science Foundation of China (21706195), the Science and Technology Program of Guizhou Province (QKHJC2016-1149), the Guizhou Provincial Department of Education Foundation (QJHKYZ2016-105), and the student's platform for innovation and entrepreneurship training program (2018521250). The publication was prepared with the support of the “RUDN University Program 5-100”.

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Res., 54 (2015) 12242-12253.

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Graphical abstract

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Highlights Synthesis and spectroscopic characterization of new triazolotriazepine derivatives. Corrosion inhibition on carbon steel in 1M HCl is achieved using EIS and PDP methods. The corrosion inhibition efficiency increases with inhibitor concentration.

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MD simulation results are in accordance with the observed inhibition efficiencies.

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