Assessing the impact of electron-donating-substituted chalcones on inhibition of mild steel corrosion in HCl solution: Experimental results and molecular-level insights

Journal Pre-proof Assessing the Impact of Electron-Donating-Substituted Chalcones on Inhibition of Mild Steel Corrosion in HCl Solution: Experimental Results and Molecular-level Insights A. Chaouiki, H. Lgaz, R. Salghi, M. Chafiq, H. Oudda, Shubhalaxmi, K.S. Bhat, I. Cretescu, I.H. Ali, R. Marzouki, I-M. Chung

PII:

S0927-7757(19)31364-0

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124366

Reference:

COLSUA 124366

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

30 October 2019

Revised Date:

16 December 2019

Accepted Date:

16 December 2019

Please cite this article as: Chaouiki A, Lgaz H, Salghi R, Chafiq M, Oudda H, Shubhalaxmi, Bhat KS, Cretescu I, Ali IH, Marzouki R, Chung I-M, Assessing the Impact of Electron-Donating-Substituted Chalcones on Inhibition of Mild Steel Corrosion in HCl Solution: Experimental Results and Molecular-level Insights, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124366

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Assessing the Impact of Electron-Donating-Substituted Chalcones on Inhibition of Mild Steel Corrosion in HCl Solution: Experimental Results and Molecular-level Insights

A. Chaouiki1,2, H. Lgaz3*, R. Salghi2*, M. Chafiq2, H. Oudda1, Shubhalaxmi4, K. S. Bhat4, I. Cretescu5, I. H. Ali6, R. Marzouki6,7,8, I-M. Chung3* 1Laboratory

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separation processes, Faculty of Science, University Ibn Tofail PO Box 242, Kenitra, Morocco 2Laboratory of Applied Chemistry and Environment, ENSA, University Ibn Zohr, PO Box 1136, Agadir, Morocco 3Department of Crop Science, College of Sanghur Life Science, Konkuk University, Seoul 05029, South Korea 4Department of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, India 5Department of Environmental Engineering and Management, Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical University of Iasi, Romania 6Department of Chemistry, College of Science, King Khalid University, P. O. Box 9004, Postal Code 61413, Abha, Kingdom of Saudi Arabia 7Laboratory of Materials, Crystal Chemistry and Applied Thermodynamics, LR15ES01, Faculty of Sciences of Tunis, University of Tunis El Manar, 2092, Tunisia. 8Chemistry Department, Faculty of Sciences of Sfax, University of Sfax, 3038, Tunisia.

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Graphical abstract:

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*Corresponding authors: H. Lgaz; [email protected], R. Salghi; [email protected], I-M. Chung; [email protected].

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ABSTRACT

Material corrosion is one of the outstanding challenging problems in the industry, and it

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strongly influences refining and petrochemical plants lifetime. Therefore, prevention of the corrosion of different metals and alloys is imperative in the viewpoint of industrial safety

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and productivity. For this purpose, the application of suitable corrosion inhibitors is one of the most applicable solutions. The present paper focuses on the anticorrosive properties of

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three biologically active chalcones, namely (E)-2-(4-(3-(2,5 dimethoxyphenyl)acryloyl) phenoxy)acetic acid (AA-3), 2-(4-(3-(4-methoxyphenyl)propanoyl)phenoxy)acetic acid (AA2) and (E)-2-(4-(3-(p-tolyl)acryloyl)phenoxy)acetic acid (AA-1) for mild steel in hydrochloric acid at temperature range 303-333 K. The corrosion inhibition performances of chalcones were evaluated by electrochemical tests, gravimetrical method, SEM, molecular orbital theory and molecular dynamics (MD) simulations. Results show that at the concentration 2

of 5×10-3 molL-1, chalcone derivatives show high corrosion inhibition activities. All compounds are found to act via adsorption at the metal/solution interface, and their adsorption follows Langmuir isotherm model. Electrochemical tests indicate that the three chalcones act as mixed-type inhibitors. The thermodynamic data of adsorption were determined and discussed. Density Functional Theory (DFT) and MD simulations were used to assess the active sites of adsorption of the three inhibitors and their interaction with the iron surface, respectively. Scanning electron microscope (SEM) was used to

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analyze the surface morphological changes that chalcones induce in the corroded mild steel. The comparison of experimental results with theoretical data indicates that methoxy functional groups have a considerable influence on the anticorrosive properties of tested

Chalcone derivative; Corrosion inhibition; Mild steel; Fukui function; DFT;

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Keywords:

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chalcones.

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Molecular dynamics.

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1. Introduction

Generally, iron and its alloys are commonly used materials in several industries due to

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several advantages, including low cost, excellent mechanical properties, availability, etc.

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However, mild steel corrosion provoked by pickling acids and industrial acid cleaners, especially hydrochloric acid generates massive economic losses and severe ecological pollution [1,2]. Nowadays, considerable efforts have been made to develop powerful and green approaches to reduce the dissolution of metals in these environments. For that purpose, the addition of small concentrations of corrosion inhibitors to acidic solutions is one of the most practiced methods. These inhibitors, especially organic compounds that contain nitrogen, oxygen, sulfur, phosphorus heteroatoms, and conjugated aromatic rings 3

have been extensively used as corrosion inhibiting additives for mild steel given their high ability to adhere to metal surfaces [3–5]. At present, plenty of researches have been conducted to explore and understand the inhibition mechanism of such type of corrosion inhibitors. It has been reported that the effective adsorption of an inhibitor on a metal surface is strongly dependent on the electrostatic interaction, adsorption mode, molecular structure of inhibitor as well as the electrolyte solution, among others [6]. This led to the development of many useful corrosion inhibitors. However, due to environmental and other

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concerns, the use of a large variety of chemical compounds as corrosion inhibitors has been restricted. In an attempt to overcome this disadvantage, alternative compounds, such as plant extracts, essential oils, and natural products are used owing to many excellent

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properties such as biodegradability, eco-friendliness, and multiple adsorption centers [7– 10]. However, green synthetic approaches for the preparation of environmentally friendly

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heterocyclic compounds have demonstrated to be one of the most valuable tools for highly

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effective corrosion inhibitors because of their higher stability and effectiveness compared with the aforementioned products. In this regard, chalcones are ubiquitous substances from the flavonoid family that exhibit diverse biological activities [11,12]. Chalcone

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compounds were explored for a variety of applications in the field of medicinal chemistry, materials science and corrosion inhibitors [13–15]. Interests in these compounds are due

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to ease of synthesis in good yield, simple work up procedure, and scalability. Moreover,

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there are several synthetic options to prepare substituted compounds by selecting appropriate starting materials. Recently, we synthesized functionalized chalcone derivatives to screen them for antimicrobial applications and further carried out single crystal analysis to explore them as potential optically active materials. Further, we explored corrosion inhibition potential of three chalcone derivatives among them especially having an ester group [16]. In continuation of our ongoing research, we designed and successfully

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synthesized three chalcone derivatives possessing aryloxyacid group as the substituent following the synthetic procedure developed and reported by us [16,17]. These compounds expected to offer better solubility and miscibility in aqueous medium when compared with chalcones without such functionality. The current work is aimed at the investigation of the inhibition effect of synthesized chalcone derivatives, AA-3, AA-2, and AA-1 on the corrosion of mild steel surface in 1.0 M HCl. Chemical and electrochemical techniques have been used to address a range of

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questions from the estimation of the corrosion rate of mild steel in different conditions to the study of the inhibition mechanism. The change in surface morphology of the mild steel has been analyzed by scanning electron microscope (SEM) in aiming to get better insights

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on inhibitors' effectiveness. Chemical, electrochemical as well as surface characterization techniques are very convenient in the evaluation and the understanding of an inhibitor’s

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performance. Nevertheless, questions about corrosion inhibition mechanisms and intrinsic

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electronic properties of each inhibitor raise the need for reliable methods to explore such insights, thus ensuring the correctness of the synthesis strategy of corrosion inhibitors. For this purpose, computer-assisted simulation methods have been extensively used to

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evaluate the structure-reactivity relationship of corrosion inhibitors from a theoretical point of view [18,19]. In this context, DFT-based quantum chemical calculations, molecular

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dynamics (MD) simulation, and radial distribution function (RDF) methods have been used

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in the present work to explore the mechanism of corrosion inhibition and the effect of electron-donating groups on the reactivity of developed compounds [20,21]. Given all of the above, our objectives are, therefore, on one side, the synthesis of new green ecofriendly chalcone derivatives substituted with electron-donating groups and aryloxyacid group, and on the other hand, the evaluation of their potentialities as corrosion inhibitors for mild steel in 1.0 M HCl, and mechanisms of their interactions with the iron surface.

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2. Experimental procedures

2.1. Materials, electrolytes and inhibitors

The inhibition abilities of AA-1, AA-2 and AA-3 molecules used in this study were tested on the mild steel (MS) with the following chemical composition of (wt %): 0.370 % C, 0.230 %

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Si, 0.680 % Mn, 0.016% S, 0.077 % Cr, 0.011 % Ti, 0.059 % Ni, 0.009 % Co, 0.160 % Cu, and balance Fe. To carry out the tests (electrochemical and surface analysis), the sample of mild steel was prepared by cutting them in appropriate dimensions. After that, the

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sample was mechanically abraded by the working polishing belt grinding machine. Prior to the conduct of the entire test, the mild steel surface was prepared by employing emery

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papers of varying grades from 400-1600 to obtain a homogeneous surface. Before being used in all the experiments, MS washed with de-ionized water, cleaned and air-dried. The

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analytical grade 37% HCl with ultrapure water was used for the preparation of 1.0 M HCl solution. The concentration range of inhibitors tested was between 10 -3 - 10-6 M, and the

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volume of thermostated cooling condenser used for all studies was 80 mL. The molecular structures of chalcone compounds are represented in Table 1. The synthesis and

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[22].

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characterization of these compounds have already been reported in our earlier publication

Table 1: Chemical structures and abbreviation of chalcone inhibitors. Chalcones derivatives

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Notation

AA-1

(E)-2-(4-(3-(p-tolyl)acryloyl)phenoxy)acetic acid

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AA-2

2-(4-(3-(4-methoxyphenyl)propanoyl)phenoxy)acetic acid

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AA-3

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(E)-2-(4-(3-(2,5 dimethoxyphenyl)acryloyl) phenoxy)acetic acid

2.3. Weight loss experiments

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It is one of the oldest and important methods which still being applied to depict the actual

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corrosion taking place in the real state. Weight loss tests were performed, respecting the ASTM standard [23]. The steel specimens are in the form of a square of dimension 2 cm x 2 cm x 0.08 cm. They were immersed in a pyrex glass cell in a ventilated medium containing the prepared aggressive solution without and with different inhibitors concentrations of AA-1, AA-2, and AA-3 at 303 K for 24h. The MS specimens were prepared and weighted as previously signaled using a precision balance. Following the

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soaking process, the steel samples were removed from the analysis solutions, rinsed thoroughly with deionized water, dried and weighted accurately again. The experiments were repeated three times for each group of samples and the average weight loss was reported to calculate the corrosion rate values in millimeters per year (mm y-1), which is determined using the following equation (1) [24].



K W A t  

(1)

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Where K = 8.76×104 was used as constant. W and t are the mass loss in gram and the time of exposure in hours, respectively. The density of mild steel is 7.86 g cm -3 and A is the exposed area in cm2 [25].

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The protection abilities ( WL % ) and degree of surface coverage (θ) of AA-1, AA-2, and AA3 were calculated from the corrosion rate (v) using the following expressions [26]:    WL   0  100  0 

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(2)

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     0   0 

(3)

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respectively.

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Where  0 and are corrosion rates without and with the studied inhibitors concentrations,

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2.4. Electrochemical measurements

Electrochemical methods such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) were used to investigate the electrochemical behavior of mild steel in a hydrochloric medium. For this purpose, electrochemical measurements were performed in a thermostated cell composed of three electrode assembly which includes working electrode (mild steel of circular shape of dimension 1 cm2 in contact with 8

the electrolyte), auxiliary electrode (platinum plate) alongside a reference electrode (saturated calomel electrode). These tests were performed by Volta Lab (TacusselRadiometer PGZ 100) potentiostat combined with "Voltamaster 4" software. First, the mild steel was immersed for 30 min into the examined solution at the temperature of 303K before all measurement to obtain a stable working surface. After this steady state, (EIS) experiments were carried out by configuring the software package to the amplitude of 5 mV (peak-to-peak) and varying the frequency from 10 mHz to 100 kHz. For PDP

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measurements, the entire potential scan was programmed to take place in the range from -750 to -250 mV of the corrosion potential with a scan rate of 1 mV s-1. Results were reproducible in all the experiments carried out. Note that all the experiments reported here

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were controlled via the use of cell containing 80 mL of the solution at 303 K.

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2.5. Surface study

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Scanning electron microscopy (SEM) is a surface characterization technique, which was used to know the morphology of the metal surface when immersed in a different medium.

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Before SEM analysis, the samples of mild steel were put in 1.0 M HCl without and with the addition of inhibitor for a period of 24 hours at 303 K. Thereafter, the samples were

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removed, rinsed with distilled water, air-dried, and finally analyzed by SEM. The SEM

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analysis was done using JEOL JSM - 6480 LV.

2.6. Molecular modeling methods

2.6.1. DFT details

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The use of DFT methods and MD simulations for corrosion inhibition studies is crucial to predict which adsorption mechanism on the metal surface is preferred for inhibitor molecules. Firstly, the conceptual DFT was performed using the Gaussian 9.0 Program [27].

In this work, with the aid of this computer program, geometry optimization of

chalcone derivatives was carried out in the aqueous phase with 6-31G ++ (d, p) basis set as a higher level in order to evaluate the ground state properties of the studied compounds for giving most stable ground state conformers.

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All DFT details were achieved for the neutral and protonated molecules of AA-1, AA-2, AA3 at a number of different levels like HF and DFT/B3LYP with SDD, 6-31G(d, p) and 6311++ G (d, p) basis sets. DFT calculations were performed in the aqueous phase (water) coupled with polarized continuum model

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using self-consistent reaction field (SCRF)

(PCM) [28]. From optimized neutral and protonated chalcone molecules, we compute

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some electronic parameters such as HOMO, LUMO energies, and the gap energy (ΔE =

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ELUMO - EHOMO). Herein, HOMO and LUMO energies can broadly be defined as energy estimated from highest occupied molecular orbital and lowest unoccupied molecular orbital respectively, and the term ΔE refers to reactivity or stability indices of molecules. IP

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(ionization potential) and EA (electron affinity) have been calculated using Koopmans theorem [29] which explores a liaison, on one hand between HOMO energy and EA, and

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on the other hand between LUMO and IP, i.e. simply EA = −E

HOMO

and IP= –E LUMO.

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Other quantum chemical parameters could be used to describe the reactivity of the studied molecules such as absolute electronegativity (χ), chemical hardness (ɳ), softness (σ), and the fraction of electron transfer (ΔN). All these chemical terms were deduced by applying Eqs. (4-6) [30,31]:



IP  EA 2

(4)

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 

1

(5)

 IP  EA 2

(6)

The tendency of the electron-transfer process of the inhibitors to a metal surface was calculated by the fraction of electron transfer (ΔN) according to the equation (7) [32]:

N 

  inh 2( Fe  inh )

(7)

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A theoretical value of  = 4.82 eV used for the Fe (110) surface, inh is the electronegativity of chalcones, while Fe  0 for bulk iron atom.

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2.6.2. Fukui indices

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To study the local reactivity at various sites with which our molecules might adsorb on the

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metallic surface, we calculated the nucleophilic and electrophilic (f – and f+) attacks. To this end, DFT calculations were achieved in the aqueous phase by the condensed Fukui

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functions. Furthermore, the Fukui indices were calculated based on Hirshfeld population analysis as follows [24]:

f k  qk ( N 1)  qk ( N )

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(8)

f k  qk ( N )  qk ( N 1)

(9)

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In the above equations, qk ( N ) , qk ( N  1) and qk ( N 1) are the AAs charge on atom k of the neutral, the anion at the protonated form, and the cation at the protonated form, respectively. Fukui function calculations of the three inhibitors in their protonated forms in the aqueous phase molecules were performed by Dmol3 module implemented in Material Studio (MS) program [33]. Descriptive data were generated for all calculations using DFT

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at the GGA (generalized gradient approximation) level and DNP basis set (double numeric with polarization) [34].

2.6.3. Molecular dynamics simulations and radial distribution function

In the present work, we are going to focus our attention on the interaction modes of studied chalcones into Fe (110) surface. As far as modeling of corrosion systems,

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molecular dynamics (MD) were performed for the three studied chalcones (AA-1, AA-2, and AA-3) in their neutral and protonated forms using Materials Studio software (Accelrys Inc.). This was done in a simulation box where the studied chalcones derivatives are in

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contact with the iron surface. The first step in the simulation process was to import the Fe metal and it is cleaved along (110) plane with a slab of 6 Å. Before proceeding to start the

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simulations, it is important to enlarge Fe (110) crystal up to (10×10) super cell in order to construct a large surface area to help better show the interaction between the inhibitors

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and metal surface. After that, the molecular structure is refined in a super-cell with a size of 24.82×24.82×25.14 Å3 contained 491H2O, 9H3O+, 9Cl− and 1 inhibitor molecule. The

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simulation procedures with a time step of 1 fs and simulation time of 2000 ps via Discover module thermo-stated at 303K were performed in a simulated volume element

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(24.82×24.82×35.69 Å3). All molecular simulations were performed on systems assembled

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using COMPASS force field (condensed phase optimized molecular potentials for atomistic simulation studies) and NVT ensemble (constant number of the molecule, volume, and temperature) [35]. This approach is well established in the metal - inhibitor interaction analysis, and we applied this methodology in our dynamic simulation model to calculate the interaction and binding energies using Eqs. (10) and (11) [36–38]: Einteraction  Etotal  ( Esurface + H O + H O+ + Cl- +Einhibitor ) 2

(10)

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and

EBinding   Einteraction (11)

Where Etotal gives the total energy of the whole system, Esurface + H O + H O+ + Cl- is assigned as 2

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the total energy of Fe (110) surface with H2O, H3O+, Cl- leaving out the AAs inhibitors, and

Einhibitor denotes the total energy of the inhibitor alone. We must take into consideration that numerous studies became interested in understanding and suggesting solutions to the corrosion phenomenon by interpreting the complex physiochemical process in simulation conditions. Even so, further studies are needed to clarify the inhibitor-iron interaction. Thus, RDF analysis (radial distribution

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function), also known as the pair distribution function, computed from MD simulations results provides a further insight into the mode of action between tested chalcone compounds and Fe (110) surface. In this case, we succeeded in obtaining the calculation

1 N A NB (rij  r ) g AB (r )    B local N A iA jB 4 r 2

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of the RDF using the following mathematical formula (12) [39]:

(12)

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Where  Blocal represents the particle density of B averaged over all shells around particle

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A.

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3. Results and Discussion

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3.1. Gravimetric experiments

It is very interesting to study the weight loss of MS samples without and with the presence of various concentrations of AA-1, AA-2, and AA-3 at 303 K. This simple method presents high reliability. The corrosion indices, i.e. corrosion rate, inhibitory efficiency and the degree of surface coverage identified from this experiment are presented in Table 2.

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Table 2: Weight loss parameters of the MS immersed in the solution without and with a variety of concentration of chalcone derivatives at 303 K.

AA-3

AA-2

θ

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130.4

(%) -

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

8.88 14.95 16.93 25.25

93.19 88.53 87.01 80.63

0.93 0.88 0.87 0.80

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

12.97 19.09 22.53 36.70

90.05 85.36 82.72 71.85

0.90 0.85 0.82 0.71

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

20.47 32.36 40.73 50.80

84.30 75.18 68.76 61.04

0.84 0.75 0.68 0.61

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AA-1

WL

(mm/y)

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HCl

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Concentration (mol/L)

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Inhibitor

According to the results presented in Table 2, we can clearly observe that the corrosion

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rate   of MS decreased significantly with the addition of different concentrations of chalcone derivatives compared to the uninhibited solutions. This decrease is more

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pronounced at a higher concentration (10-3) of inhibitors. In addition, at this concentration, the maximum numerical values of inhibition efficiency of AA-1 (84.30 %,), AA-2(90.05%)

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and AA-3 (93.19%) were

observed. In addition, we note that these inhibition

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performances have shown a noticeable interest in increasing the concentrations of chalcone derivatives following the order of AA-3 > AA-2 > AA-1. This might be explained by the fact that the adsorption of the three tested compounds on the metal surface increases with the increase in the chalcones concentrations. Further, this adsorption phenomenon justifies the development of a film on the MS surface, which protects the metal surface, and therefore inhibition of any form of corrosion predominating at the MS

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and the acid solution interface. On the other side, it is interesting to understand why AA-3 inhibitor performance is better among the three tested molecules. To examine this difference in more details, we examined various techniques in the next sections in this work. However, as the first vision of results obtained by the gravimetric tests in relation with the molecular structure of inhibitors, we can say that the presence of electron donating –OCH3 group increases the adsorption of three investigated molecules. Introduction of two electron-donating groups at the meta-position of the phenyl cycle

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enhanced the activity of the molecule and therefore the inhibition performance.

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3.2. Data using potentiodynamic polarization (PDP) measurements

The behaviors of the three tested chalcones in this study were investigated by potentio-

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dynamic polarization technique to acquire more information, which could produce interesting findings of the kinetics of the anodic as well as cathodic branch reactions of

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mild steel in 1.0 M HCl at different concentrations of chalcones derivatives. Fig. 1 shows the resulting polarization plots for MS corrosion alone and with inhibitor concentrations.

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The related kinetic parameters such as corrosion electrode potential Ecorr (mV), anodic and cathodic slopes  a and  c (mV/dec) was estimated from the Figure by extrapolation of

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the Tafel slopes, and the corrosion current density icorr (µA cm-2) was mentioned in Table

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3. The values of inhibition efficiency, which was estimated using the next equation (13) were also enumerated in Table 3 [24].

 i  EPDP (%)  1  corr 100 °  i corr  

(13)

° Where icorr and icorr are the corresponding current densities of the MS sample without and

with the addition of AAs inhibitors, respectively. The polarization plots presented in Fig. 1 15

reveal that the presence of three inhibitors effectively suppressed the anodic dissolution and retarded the cathodic hydrogen evolution. The influence was more pronounced with the increasing of the concentrations of inhibitor molecules. Obviously, it can be directly seen that the anodic and the cathodic current density in acid solution containing chalcones have lower values than that of the blank solution. On the other hand, the parallel cathodic polarization curves indicated that the hydrogen evolution was activation-controlled and the

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reduction mechanism was not affected by the addition of inhibitors [16].

Table 3: Electrochemical polarization results for MS samples in 1.0 M HCl in the absence and presence of the concentration of tested inhibitors at 303 K. −𝑬𝒄𝒐𝒓𝒓 (mV/SCE)

HCl

1.0

496

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

479.3 484.4 478.7 471.8

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

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AA-1

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

150.19

𝒊𝒄𝒐𝒓𝒓 (μA cm-2)

𝑬𝐏𝐃𝐏 (%)

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95.4

564

-

-

81.9 78.2 78.2 86.0

31.3 50.3 64.9 82.8

94.45 91.08 88.49 85.31

0.94 0.91 0.88 0.85

485.8 490.9 485.1 482.2

150.1 145.4 150.6 162.6

104 88.4 92.3 90.5

56.57 84.6 109.8 147.8

89.96 85.0 80.53 73.79

0.89 0.85 0.80 0.73

483.3 484.1 482.4 477.2

185.4 172.5 159.4 151.5

93 92.9 90.8 82.5

86.5 118.2 174.3 230.5

84.66 79.04 69.09 59.13

0.84 0.79 0.69 0.59

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168.8 166.4 147.3 173.1

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AA-2

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AA-3

−𝜷𝒄 𝜷𝒂 -1 (mV dec ) (mV dec-1)

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Concentration (M)

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Inhibitor

The data in Table 3 show that by increasing chalcone derivatives concentration, the corrosion current density (icorr) significantly decreased to reach the values of 31.3, 56.57, and 86.5 μA cm-2 at 10-3 M for AA-3, AA-2, and AA-1, respectively, which indicates that the anodic and cathodic reactions were impeded in the presence of studied inhibitors. It is 16

important to mark that the shift in the values of Ecorr for three inhibitors are less than 85 mV (maximum displacement at 24.2 mV); which suggests that the studied compounds behave as mixed-type inhibitors. The inhibitive performance of the studied chalcone derivatives for mild steel protection follows the order EPDP (%) (AA-3) > EPDP (%) (AA-2) > EPDP (%) (AA1). This order can be attributed to the inhibitors structure, i.e. functional groups, present substituted groups, number of heteroatoms and π-electrons. The presence of two methoxy groups (-OCH3) enhances the protective property of AA-3; therefore, the adsorption of AA-

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3 at the MS surface is more pronounced and the active sites are blocked when compared to only one electron-donating -OCH3.

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3.3. Data using electrochemical impedance spectroscopy (EIS)

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The EIS experiment was also conducted to investigate the protective effects of tested chalcones against mild steel corrosion under the conditions described previously. The

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kinetics of electrochemical reactions at the electrode and adsorption process can better be understood by EIS. Nyquist diagrams of AA-1, AA-2, and AA-3 are shown in Fig. 2. Fig 3

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shows the Bode plots at the optimum concentration of inhibitors (10 -3 M). It is apparent from Fig. 2 that all impedance spectra represent the semicircular shapes that showed that

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the dissolution of MS in hydrochloric acid is principally regulated by the charge transfer.

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[40]. In addition, from these graphs (Fig. 2), similar curves were obtained with the investigated compounds; meaning that the MS corrosion mechanism not changing when the three compounds are added [41]. Depending on the concentration of inhibitors, the diameter of the capacitive loop is changed, i.e. with increasing inhibitors concentration, the diameter is becoming larger. The adsorption of inhibitor molecules on the metallic surface suggests the formation of a stable protective film on the addition of chalcone molecules [42]. 17

Upon inspection of the Bode graph, it can be seen that the corrosion process is occurring through one time-constant in single step. In addition, as the concentration of three inhibitors increases, phase angle values increase up to 65° as compared to the uninhibited system, thus shows an advancement in the inhibition performance by the adsorption of the AAs compounds on the MS surface [43,44]. The equivalent circuit with one-time constant was fitted and the structural model was demonstrated in Fig. S1 (Supplementary material). The presented equivalent circuit comprises Rs; uncompensated solution resistance, CPE;

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interfacial capacitance and Rp; polarization resistance. Where, Rp and CPE (constant phase element) are in parallel with one another. The CPE is described by the following expression [16]:

1 Q( j)n

(14)

-p

ZCPE 

re

Where Q is the CPE constant, ω is angular frequency, j is the imaginary number and n is the exponential term related to CPE. The electrical double layer values (C dl) is defined by

lP

the relation [38]: Cdl  n Q  R1pn

(15)

equation (16) [16]:

na

The inhibitory efficacies of the studied inhibitors were calculated from Rp using the

ur

 Rip  Rp   EIS (%)    100 i  Rp 

Jo

(16)

i Where R p and R p respectively represent the polarization resistances for MS without and

with tested chalcones in the corrosive medium. The fitted electrochemical parameters such as Rs, Cdl, CPE, n using the equivalent circuit as well as the  EIS % are presented in Table 4.

18

Table 4: EIS data obtained from Nyquist plots for MS metal at different concentration of AA-1, AA-2, and AA-3 at 303 K.

AA-2

AA-1

 EIS

29.35

0.89

1.7610

91.86

-

-

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

727.2 375.4 286.3 239.3

0.86 0.84 0.87 0.88

0.1994 0.3378 0.4174 0.6754

10.00 14.70 21.54 38.48

95.96 92.18 89.74 87.73

0.95 0.92 0.89 0.87

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

381.1 240.5 163.9 124.3

0.80 0.79 0.82 0.81

0.4156 0.6441 0.8619 1.4142

1 × 10−3 1 × 10−4 1 × 10−5 1 × 10−6

200.7 133.9 103.1 88.96

0.85 0.88 0.84 0.83

0.5624 0.6822 1.208 1.5899

1.0

Ɵ

(%)

ro of

AA-3

𝑪𝒅𝒍 (𝝁𝑭/𝒄𝒎𝟐 )

𝑹𝒑 (𝜴 × 𝒄𝒎𝟐 )

14.74 21.27 33.83 54.80

92.29 87.79 82.09 76.38

0.92 0.87 0.82 0.76

25.49 35.96 52.39 66.46

85.37 78.08 71.53 67.00

0.85 0.78 0.71 0.67

-p

Blank

𝒏

𝑸 × 𝟏𝟎−𝟒 (𝑺𝒏 𝜴−𝟏 𝒄𝒎−𝟐 )

Concentration (𝑴)

re

Inhibitor

lP

Table 4 clearly shows an increase in the value of Rp with the increasing of the concentration of the studied chalcones derivatives. There is a significant decrease in the

na

Cdl values with the increasing of the dosage of the three chalcones derivatives. Thus, the tested inhibitors were adsorbed onto the MS surface forming an insulating protective

ur

barrier film onto the MS, thereby retarding the charge transfer and therefore decreasing the dissolution of metal. The inhibition performances of AA-1, AA-2 and AA-3 were

Jo

increased with the increasing of inhibitor concentrations and reached 85.37 for AA-1, 92.29 for AA-2 and 95.96 % for AA-3 at 10-3 M. According to these results, we can infer that the best inhibition efficiency was the one observed in the case of AA-3 inhibitor among the three studied compounds. As we have seen from PDP and gravimetric results, the AA3 inhibitor substituted by two methoxy groups (strong electron-donating group) indicates the importance of conjugated system (π-bond parts), electron density at phenyl ring, which 19

simply adsorbed on MS surface for retarding the corrosion process. In an effort to understand the direct effect of the substituent of chalcone derivatives, we also supported the electrochemical studies with well-detailed theoretical studies (see theoretical section).

3.4. Adsorption isotherm

It is imperative to know the adsorption type of organic compounds on the heterogeneous

ro of

surfaces of MS. The inhibition mechanism in the corrosive medium may be in sighted by various adsorption isotherms. For this purpose, many different adsorption models are available to fit the adsorption data in order to describe the adsorption process, but

-p

Langmuir, Frumkin, Freundlich as well as Flory-Huggins models are usually used. It is necessary to note that physical and chemical adsorptions are the two types of interactions

re

that can be described when we studied the inhibitors contact with the metal surface. From

WL 100

, all adsorption models

lP

the measurements of surface coverage (θ) obtained by

mentioned above were tested (Fig. S2, supplementary material). In this case, the results

na

show that the model of Langmuir (Fig. 4) represents the best adsorption isotherm. The Langmuir isotherm is represented by the following equation: [45]:





1  Cinh Kads

(17)

ur

Cinh

Jo

where Cinh is the chalcones’ concentration and Kads is the equilibrium constant of the adsorption process. ° The free energy of adsorption ( Gads ) for AA-1, AA-2, and AA-3 were determined using the

equation (18) [36]: ° Gads   RT ln( Kads  55.5)

(18)

20

° In the above formula, the terms are defined as follows: ( Gads ) is Gibbs free energy of

adsorption, T is thermodynamic temperature (here, 303 K), R represents the universal gas constant and 55.5 is the numerical value of the molar concentration of H 2O in mol/L. Fig. 4 denotes the plots of Cinh/θ versus Cinh which gave a straight line with a slope approximately equal to 1 and correlation coefficient value near to one (R2> 0.999). This result suggests that the adsorption of the three compounds tested on the MS surface is well followed by Langmuir isotherm. In addition, we also see from this data (Table 5) that

ro of

the Kads values are very high, indicating that these compounds show a higher adsorption tendency.

The adsorption measurements for three chalcone compounds on the MS

-p

Table 5:

Slope

AA-3 AA-2 AA-1

1.05 1.06 1.09

lP

Inhibitors

Kads (L/mol) 645.544 379.740 261.670

re

surface in 1.0 M HCl at 303 K.

∆𝑮ads (kJ/mol) -43.79 -42.46 -41.52

na

On the other hand, to define the mode of inhibitor-metal interactions by the adsorption test, the free energy of adsorption was calculated (Table 5). Different research exists in the

ur

° literature [46–49] regarding the relationship between the value of Gads and the mode of ° In this field, if the value of Gads is around -20 kJ.mol-1 or less

Jo

inhibitors adsorption.

negative, the organic inhibitors are mainly adsorbed by electrostatic attraction ° (physisorption). Different to that, if Gads approximately −40 kJ.mol-1 or more negative, the

adsorption of the inhibitor is essentially acted as chemisorptions, i.e. the electron transfer from organic molecules to the metallic surface. From the results obtained in this part ° (Table 5), the values of Gads are −41.52, −42.46, and −43.79 kJ/mol for AA-1, AA-2, and

21

AA-3, respectively. This finding suggests that the AAs molecules may be adsorbed on the metallic surface by the chemisorption mechanism.

3.5. Mechanism of inhibition harmonized by computational simulations

3.5.1. Global reactivity parameters

ro of

In an effort to deepen the understanding of the adsorption ability and electronic properties of the studied chalcone derivatives, DFT calculations have been conducted at various theoretical levels. The DFT method is one of the most common and versatile techniques

-p

that can give exact information about inhibitor-metal interactions. Fig. 5 shows the optimized molecular structures, HOMO, LUMO orbitals, and MEP (molecular electrostatic

re

potential) of AA-1, AA-2, and AA-3 inhibitors for the neutral form in the aqueous phase. Tables 6&7 list the quantum chemical parameters obtained from neutral and protonated

lP

AAs.

The energy densities of HOMO and LUMO are key electronic parameters for

na

characterizing the chemical reactivity of inhibitor molecules. Usually, a high HOMO energy refers to a stronger donating capability of a molecule [24]. While inhibitor molecules with a

ur

higher ability to accept electrons are those that have a lower LUMO energy [24]. It reflects that inhibitors molecules have more sites for accepting electrons. As shown in Fig. 5, the

Jo

HOMO and LUMO densities of inhibitors are located and distributed around the entire molecular structure such as the phenyl rings and the different substituent groups. It means that chalcones derivatives could be adsorbed effectively onto the metal surface due to the rich π-electron cloud of the aromatic system and the pronounced electron-donating and electron-accepting effects over the entire molecules.

22

Key insight into understanding donor and acceptor capacities of inhibitor molecules would be gained by quantum chemical parameters. According to the results listed in Table 6, we can infer that the inhibition efficiency in terms of HOMO energies decreases in the order: AA-3 > AA-2 > AA-1. This means that the capacity to donate electrons to the steel surface follows the same order i.e. AA-3 > AA-2 > AA-1. Conversely, the order of decrease in LUMO energy values is AA-1 > AA-2 > AA-3. With regard to these results, the order of ELUMO energies is not compatible with experimental data. Present results confirm the key

strengthening the adsorption capacities of molecules.

ro of

role played by the electron-donating effect of the methyl and methoxy groups in

Besides, other important factors such as the chemical hardness, the softness, and the

-p

energy gap have an influence on the chemical reactivity of inhibitor molecules. Chemical hardness (η) is resistance of a chemical entity to electron cloud polarization or the

re

resistance to charge transfer. While the global softness (σ) is defined as the inverse of the

lP

global hardness. A chemical compound that has a lower hardness and a higher softness usually has a high inhibition performance [50]. In the case of all B3LYB levels, we found that the chemical hardness values are of the same order as seen for HOMO energy. It

na

reveals that the chalcone with a methyl group is very difficult to liberate the electrons, followed by that with one methoxy group while the other compound with two methoxy

ur

groups is an excellent candidate to give electrons to the metal surface. Importantly, these

Jo

theoretical predictions match well with experimental results. The energy gap is an important indicator, which is usually associated with chemical stability [50]. A small HOMO-LUMO energy gap is an essential requirement for corrosion inhibitors [24]. The low ΔE (ELUMO-EHOMO) value refers to the high reactivity of an inhibitor molecule [51]. Based on the results illustrated in Table 6, we noticed that the values of the energy band gap for neutral molecules are found to obey the following order: AA1 > AA-2

23

> AA-3, except in HF level. The lowest ΔE value is observed for the case of AA-3, which exhibits a higher inhibition efficiency, followed by AA-2, and AA-1 has the lowest inhibition efficiency. The presence of the methyl group (AA-1) or methoxy group (AA-2) gave a moderate inhibitive performance while two methoxy groups in AA-3 gave high corrosion inhibition. We believe that introducing electron-donating methoxy groups resulted in an increase in the chemical reactivity of AA-3, thus leading to enhancement of the corrosion inhibition activity.

ro of

In aqueous acidic solution, the presence of the lone pair of electrons that exist on the heteroatoms like nitrogen and oxygen present in the molecular skeleton leads to a high tendency for protonation. Thus, in order to determine the heteroatom that has a higher

-p

tendency to be protonated, Mulliken atomic charges of neutral molecules have been calculated, and the results are listed in Table 8. Generally speaking, higher negatively

re

charged heteroatom is more likely to be protonated in acidic medium [52]. By inspection of

lP

Table 8, O26 (AA-3), O38 (AA-1), and O39 (AA-2) atoms have the highest negative atomic charges; thus they are the preferred atoms for protonation. Interestingly, the analysis of quantum chemical parameters displayed in Table 7 reveals

na

that, in the same theoretical level, HOMO and LUMO energy values are higher than those of neutral inhibitors. It means that the electron-donating power increases, whereas the

ur

electron-accepting ability decreases. It is generally accepted that the HOMO energy of

Jo

protonated molecules usually decreases, which is not observed in our results. Knowing that HOMO distribution is not well distributed over the protonated oxygen atom, let us assume that HOMO energy may increase on protonation if protonation occurs on such atom which has very less contribution towards HOMO. On the other hand, the energy gap Egap values are slightly lower than those reported for neutral inhibitor molecules,

especially in the case of B3LYB levels, while increased Egap values are observed in HF 24

levels. These results confirm that in their protonated forms, inhibitor molecules are highly reactive species, thereby confirming the ability of protonated molecules to improve the inhibitive effect and thus the corrosion inhibition efficiency. Once again, these results reinforce the importance of electron-donating power in enhancing the inhibitive properties of inhibitors, and more importantly, highlight the significant contribution of protonated inhibitor molecules during adsorption. Furthermore, the number of transferred electrons (ΔN) between iron atoms and inhibitor

ro of

molecules is an effective tool to determine whether an inhibitor molecule can transfer its electrons or not. If ΔN > 0, the corrosion inhibitor transfers its electrons to metal and vice versa if ΔN < 0 [53–55]. The results in Tables 6 and 7 show that chalcone derivatives in

-p

both neutral and protonated forms possess a higher ability to donate electrons to MS surface. Further, it is crucial to note that ΔN values of protonated forms are considerably

re

higher than values reported for neutral molecules and are in the same order as HOMO

lP

energy values. This confirms, on the one hand, the increased electron-donating character of protonated chalcones and, on the other hand, the close correlation between the corrosion inhibition activity and the electron-donating ability.

na

All quantum chemical parameters supported the fact that the AA-3 inhibitor has a higher tendency to react with the metal surface as observed by the experimental results. This

ur

result is not very surprising given the fact that AA-3 has two methoxy groups together with

Jo

nonbonding electrons present on oxygen atoms and π-electrons in the heterocyclic ring system.

25

Table 6: Quantum chemical parameters for neutral molecules calculated at DFT/B3LYB/6-311++G (d, p) level in the aqueous phase. Parameters → Inhibitors ↓ HF/SDD level AA-1 AA-2 AA-3

EHOMO (eV)

ELUMO (eV)

∆Egap (eV)







(eV)

(eV)

-8.4983 -8.3095 -8.2752

1.1842 1.2364 1.3145

9.6826 9.5460 9.5898

4.8413 4.7730 4.7949

HF/6-31G level AA-1 AA-2 AA-3

-8.3555 -8.1549 -8.1204

1.5066 1.5624 1.6299

9.8622 9.7174 9.7503

HF/6-31G++ level AA-1 AA-2 AA-3

-8.4679 -8.2695 -8.2374

1.3279 1.3877 1.4514

B3LYP/SDD level AA-1 AA-2 AA-3

-6.3067 -6.0327 -5.8305

B3LYP/6-31G level AA-1 AA-2 AA-3 B3LYP/6-31G++ level AA-1 AA-2 AA-3

0.2065 0.2095 0.2085

3.6570 3.5365 3.4803

-1.1842 -1.2364 -1.3145

8.4983 8.3095 8.2752

0.1201 0.1344 0.1396

4.9311 4.8587 4.8751

0.2027 0.2058 0.2051

3.4244 3.2962 3.2452

-1.5066 -1.5624 -1.6299

8.3555 8.1549 8.1204

0.1415 0.1568 0.1615

9.7958 9.6573 9.6888

4.8979 4.8286 4.8444

0.2041 0.2070 0.2064

-2.3516 -2.2822 -2.1984

3.9551 3.7505 3.6321

1.9775 1.8752 1.8160

0.5056 0.5332 0.5506

-6.1486 -5.8637 -5.6689

-2.1042 -2.0256 -1.9507

4.0444 3.8381 3.7181

-6.3848 -6.0964 -5.9007

-2.3480 -2.2691 -2.1965

2.0184 1.9136 1.8521

Jo

ur

na

4.0368 3.8272 3.7042

-1.3279 -1.3877 -1.4514

8.4679 8.2695 8.2374

0.1276 0.1428 0.1472

4.3291 4.1574 4.0144

2.3516 2.2822 2.1984

6.3067 6.0327 5.8305

0.1240 0.1766 0.2217

0.4945 0.5210 0.5378

4.1264 3.9447 3.8098

2.1042 2.0256 1.9507

6.1486 5.8637 5.6689

0.1714 0.2280 0.2716

0.4954 0.5225 0.5399

4.3664 4.1828 4.0486

2.3480 2.2691 2.1965

6.3848 6.0964 5.9007

0.1123 0.1664 0.2082

-p

3.5699 3.4408 3.3929

re

lP

2.0222 1.9190 1.8590

26

ro of

IP (eV)

∆N110

(eV)

EA (eV)

Table 7: The computed quantum chemical parameters for protonated chalcones calculated

at DFT/B3LYB/6-311++G (d, p) level in the aqueous phase.







(eV)

(eV)

11.1591 11.1686 11.2600

5.5795 5.5843 5.6300

3.4865 3.4261 3.6553

11.3528 11.3615 11.4072

-7.9813 -8.0488 -7.8711

3.2204 3.1674 3.3867

B3LYP/SDD level AA-1 AA-2 AA-3

-4.2662 -4.1734 -4.1453

B3LYP/6-31G level AA-1 AA-2 AA-3 B3LYP/6-31G++ level AA-1 AA-2 AA-3

0.1792 0.1790 0.1776

2.5038 2.5839 2.3633

3.0757 3.0003 3.2667

8.0834 8.1683 7.9933

0.2075 0.2002 0.2181

5.6764 5.6807 5.7036

0.1761 0.1760 0.1753

2.1898 2.2546 2.0486

3.4865 3.4261 3.6550

7.8662 7.9353 7.7522

0.2316 0.2257 0.2429

11.2018 11.2162 11.2579

5.6009 5.6081 5.6289

0.1785 0.1783 0.1776

2.3804 2.4407 2.2422

3.2204 3.1674 3.3867

7.9813 8.0488 7.8711

0.2177 0.2121 0.2289

-1.0136 -0.9771 -0.9270

3.2525 3.1962 3.2182

1.6262 1.5981 1.6091

0.6148 0.6257 0.6214

2.6399 2.5752 2.5362

1.0136 0.9771 0.9270

4.2662 4.1734 4.1453

0.6702 0.7022 0.7096

-4.0607 -3.9600 -3.9331

-0.7317 -0.6908 -0.6506

3.3290 3.2691 3.2825

1.6645 1.6345 1.6412

-6.1500 -4.1965 -4.1151

-2.3453 -0.9328 -0.9159

ELUMO (eV)

∆Egap (eV)

-8.0834 -8.1683 -7.9933

3.0757 3.0003 3.2667

HF/6-31G level AA-1 AA-2 AA-3

-7.8662 -7.9353 -7.7522

HF/6-31G++ level AA-1 AA-2 AA-3

-p

EHOMO (eV)

0.6007 0.6117 0.6092

2.3962 2.3254 2.2918

0.7317 0.6908 0.6506

4.0607 3.9600 3.9331

0.7280 0.7630 0.7701

0.5256 0.6127 0.6251

4.2476 2.5646 2.5155

2.3453 0.9328 0.9159

6.1500 4.1965 4.1151

0.1504 0.6910 0.7203

re

lP

1.9023 1.6318 1.5996

Jo

ur

na

3.8046 3.2637 3.1992

ro of

IP (eV)

∆N110

(eV)

EA (eV)

Parameters → Inhibitors ↓ HF/SDD level AA-1 AA-2 AA-3

27

Table 8: Mulliken atomic charge distribution for compounds AA-1, AA-2, and AA-3 calculated at DFT/B3LYB/6-311++G (d, p) level in the aqueous phase. Atoms 1C 2C 3C 4C 5C 6C 10C 12C 14C 15C 16C 17C 18C 20C 22C 25O 26O 27C 30C 31O 32O 34O 35O 36C 40C

na ur Jo 28

AA-3 0.255076 -0.240426 0.053436 0.286847 -0.153859 -0.133982 -0.187494 -0.127551 0.168994 -0.135631 -0.040393 -0.102227 -0.212696 -0.163887 0.267324 -0.50488 -0.564913 -0.18789 0.471323 -0.326363 -0.395471 -0.551004 -0.522491 -0.291461 -0.289717

ro of

AA-2 -0.155714 0.001029 -0.067364 -0.188637 0.264773 -0.174693 -0.172815 -0.158775 0.175667 -0.130279 -0.105925 -0.042082 -0.161692 -0.209964 0.267082 -0.504017 -0.188224 0.471592 -0.325514 -0.396848 -0.516446 -0.293534 -0.564921

-p

Atoms 1C 2C 3C 4C 5C 6C 11C 13C 15C 16C 17C 18C 19C 21C 23C 26O 27C 30C 31O 32O 34O 35C 39O

re

AA-1 -0.167629 0.016439 -0.09144 -0.153163 0.055234 -0.155104 -0.172558 -0.157163 0.176479 -0.130471 -0.105042 -0.04154 -0.161596 -0.210081 0.267546 -0.503734 -0.188508 0.471723 -0.325393 -0.564768 -0.393595 -0.612034

lP

Atoms 1C 2C 3C 4C 5C 6C 11C 13C 15C 16C 17C 18C 19C 21C 23C 26O 27C 30C 31O 32O 34C 38O

3.5.2. Fukui indices and MEP of protonated inhibitor molecules

Considering the above results (DFT calculations), we can note that chalcone derivatives have a higher capacity to donate their electrons to the vacant d-orbitals of Fe atoms with protonated molecules playing the crucial role. To explore other properties that might be useful for examining factors underlying corrosion inhibition mechanism, we investigated Fukui functions and molecular electrostatic potential of chalcones. The local reactivity

ro of

descriptors may give more insights into the relationship between the inhibitive effect and local reactive sites, which go beyond the graphical HOMO and LUMO distribution. Fukui indices analysis was performed for all atoms of protonated compounds with the help of

-p

DMol3 program, and the results are represented in Fig. 6. Furthermore, the MEP for the three compounds in their protonated forms were extracted to support the theoretical

re

results.

lP

It is well known that the higher positive charge ( f  ) of an atomic site means that this site is associated to reactivity for a nucleophilic attack, while a higher negative charge ( f  ) is

na

related to reactivity for an electrophilic attack [56]. Overall, it can be seen from Fig. 6 that atomic sites C(11), C(15), and C(30) are the most nucleophilic and electrophilic sites for

ur

AA-1. In the case of AA-2, the most atoms, which are subject to electrophilic and nucleophilic attacks, are located on the C(11), C(15) and C(27). For the AA-3 molecule,

Jo

the most favorable sites for f  functions are positioned on C(6), C(10), C(12) and C(2), while C(1), C(3), C(4), C(6) and O(31) atoms, would be most preferred sites for electrophilic attack . Based on these results, we can note two further points, which put our discussion into perspective. The first point that could be directly inferred from Fukui function results is that, in three chalcone molecules, the phenyl ring and the doublebonded oxygen atom play an essential role in increasing the reactivity of molecules,

therefore donor-acceptor interactions. Second point is that AA-3 compound is characterized by widespread nucleophilic and electrophilic sites that can effectively participate in binding with the steel surface as compared to limited sites in the two other inhibitors, thus supporting its enhanced corrosion inhibition performance. To reinforce our discussion and to support the observations generated from f  and f  functions, we represented molecular electrostatic potential (MEP) maps (see Fig. S3) of three protonated inhibitor molecules. In MEP, the most negative and positive sites are

ro of

colored in red and blue, respectively, while a green color refers to zero MEP. The nucleophilic attacks are presented by red color, while electrophilic attacks are related to blue color [57]. So, electronegativity and reactivity of atoms decreases following the order:

-p

red > orange > yellow > green > blue [58,59]. As can be observed in Fig. S3, the negative regions (red) are concentrated on the atomic oxygen sites, while the positive centers (blue)

3.6. MD simulations details

lP

re

is distributed on the hydrogen and some carbon atoms of protonated molecules.

na

3.6.1. Interaction and binding energies

ur

Nowadays, MD simulation is a powerful theoretical approach commonly used for a better

Jo

understanding of corrosion inhibition systems. It provides information on the interaction modes, preferred orientation of inhibitors, and other parameters when the studied molecules interact with the iron metal [60]. In this study, MD simulations as computer modeling were carried out to investigate the adsorption of three chalcones onto the iron surface. The calculated value of interaction and binding energies of three molecules have been estimated through the formed interaction system when it reached an equilibrium state [61]. The temperature and energy fluctuation curves of the formed inhibition system

are shown in Fig. S4 (Supplementary material), and data from this Figure were analyzed to make sure that the system reached a steady state at the end of the simulation process. Figures 7 and 8 summarize the final adsorption configurations of protonated and neutral chalcone derivatives over the Fe surface in aqueous solution. It is apparent from the Figures that all chalcone molecules adopt near-flat orientation and are moved gradually near on Fe (110) surface in neutral as well as in protonated forms, which confirms the ability of tested compounds to protect the surface of the metal. Looking at Fig. 8,

ro of

protonated chalcone inhibitors moving totally to the surface and show more interaction and degree of adsorption with the iron surface as compared to neutral inhibitors, which are a little far away from the Fe(110) surface. Nevertheless, what we can see from this data is

-p

that the active centers of the phenyl ring and O atoms in addition to methoxy groups are the main responsible of the strong interactions between the studied inhibitors and Fe

re

atoms [62].

lP

From the experimental data, it is noticed that the AA-3 inhibitor has a higher tendency to inhibit the corrosion of the steel. Considering this, the simulation information is quite compelling, and it could explain what is going on. Interestingly, from the AA-3 top views, it

na

is quite observed that, the two methoxy groups present in AA-3 structure can strengthen the adsorption capacity of AA-3 compound and therefore its inhibition efficiency. Table 9

ur

presents the values of interaction and binding energies of three chalcone compounds in

Jo

neutral and protonated forms calculated under equilibrium conditions. The high binding energy for an inhibitor molecule reflects its better adsorption into the iron surface and its strong interaction with the Fe(110) surface [63,64]. Based on these numerical results, the binding energies calculated from the equation 11 follow the order of: Ebinding (AA-3) >

Ebinding (AA-2) > Ebinding (AA-1). While supporting the conclusions of this study with respect to the role of electron-donating functional groups, these results also provide evidence that

AA-3 and AA-2 inhibitors, which have methoxy groups in their molecular structure, exhibit a higher adsorption ability and therefore a good inhibitory action. Furthermore, the difference observed in the interaction ability of the studied chalcones in both forms (neutral and protonated molecules) is demonstrated by the interaction energy. Results in Table 9 reveal that the interaction energies values for protonated molecules are highly negative than those of neutral forms. Hence, it confirms the assumption that protonated inhibitors have the most significant contributions to the corrosion inhibition

ro of

process. Besides these findings, the results of this section (MD simulations) are also in line with DFT calculations and very well explain the inhibitory potency of the studied chalcone

-p

derivatives.

Table 9: Interaction and binding energies obtained during the adsorption of chalcone

re

derivatives on Fe (110) surface using MD simulation studies in the aqueous phase.

Fe + AA-2

Ebinding (kJ/mol)

EInteraction (kJ/mol)

Ebinding (kJ/mol)

-510.40

510.40

-569.87

569.87

-564.91

564.91

-694.13

694.13

-605.31

605.31

-713.32

713.32

ur

Fe + AA-3

EInteraction (kJ/mol)

na

Fe + AA-1

Protonated molecules

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Neutral molecules System

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3.6.2. Radial distribution function (RDF)

From MD simulations, it is evident that protonated chalcone derivatives have closer contact with the iron surface. Now, to assess the interaction modes between different interacting atoms and the metal surface, we introduce the RDF analysis. In corrosion inhibition studies, radial distribution functions are very useful tools to estimate the distance

between inhibitor’s atoms and metal surfaces. In our case, the RDF of O and C atoms were computed from MD simulations, and the results are set out in Fig. 9. Notably, it is well known that a small distance (1 Å ~ 3.5 Å) means that the interaction mode is a chemisorption type, while a distance greater than 3.5 Å is associated to physisorption [65]. From the Fig. 9, we can infer that the bonding lengths of Fe-C and FeO in all inhibitor molecules are smaller than 3.5 Å. What is interesting here is that oxygen atoms have significantly pronounced interactions with the iron surface. Thus, we can say

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that there is a high potential for chemical interactions during the adsorption process that take the main place through oxygen atoms, π-electrons in the heterocyclic ring, acetic acid, and methoxy functional groups. These findings corroborate previous experimental

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and theoretical results showing that the tested chalcone derivatives have the highest

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tendency to interact with the metal surface, thus preventing it from corrosion.

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3.7. SEM study

SEM analysis were applied to investigate the morphological variation of the surface of test

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coupons. Herein, with the help of SEM micrograph, we can compare the surface morphology of the MS samples in hydrochloric acid with and without 10-3 M of AA-3. Fig.

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10 displays the steel samples before and after exposure to the blank solution, and after

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addition of 10−3 M of AA-3 inhibitor for 24 h of immersion. As can be seen in Fig. 10a, the MS is smooth and unaffected except some scratches occurred due to the polishing process. While in Fig. 10b, the steel surface is highly deteriorated and corroded, which is due to its dissolution in the corrosive medium. In contrast, after the addition of inhibitor AA3 (Fig. 10c), the surface appears smooth with fewer surface defects. Thus, a protective film may be formed when the inhibitor interacts with the steel surface. Hence, the

examination of SEM images of mild steel demonstrates again the ability of AA-3 molecule to act as an excellent corrosion inhibitor against mild steel corrosion.

3.8. The practical relevance and comparison with other chalcones

The development of an efficient and green procedure in the synthesis of organic compounds has received great interest in modern heterocyclic chemistry. In this context,

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chalcones have attracted significant attention in recent years given their simple chemistry, ease of synthesis, a large number of replaceable hydrogens to yield a variety of derivatives [66,67]. Moreover, a survey in the literature reveals that compounds containing

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α,β-unsaturated keto groups showed good inhibitory activity against acid corrosion of metals [68–71]. However, despite this widespread relevance, very few experimental

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studies have focused on the corrosion inhibition properties of chalcones for different metals. In fact, our research group has developed and recently reported the synthesis and

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corrosion inhibition potential of three chalcone derivatives having an ester group [16]. Herein, our aim was to evaluate the effect that replacing the ester group by aryloxyacid

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group has on the adsorption performance of investigated chalcones. Table 10 compares the inhibition performance of AA-3 with selected chalcone derivatives used as corrosion

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inhibitors for steels in acidic medium.

Table 10. Quantitative comparison of the inhibition efficiency of AA-3 with those of other chalcones studied previously.

Inhibitor (2E,4Z)-5-(4(dimethylamino)phenyl)-1-

Steel/solution Concentration Inhibition Reference (M) efficiency -5 Stainless 2.5x10 89 [72] Steel Type

4′,4-dihydroxychalcone (P1)

[72]

2.5x10-5

87.4

[72]

1.1x10-5

89.8

[73]

10−4

60

[74]

5x10-3

88

[16]

5x10-3

93

[16]

5x10-3

95

[16]

1x10-3

96

Present work

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(E)-ethyl 2-(4-(3-(4fluorophenyl)acryloyl)phenoxy)acetate (AE-1) (E)-ethyl 2-(4-(3-(3,4dichlorophenyl)acryloyl)phenoxy)acetate (AE-2) (E)-ethyl 2-(4-(3-(2,5dimethoxyphenyl)acryloyl)phenoxy)acetate (AE-3) (E)-2-(4-(3-(2,5 dimethoxyphenyl)acryloyl) phenoxy)acetic acid (AA-3)

89.8

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1-(4-hydroxyphenyl)-3-phenylprop-2-en1- one

2.5x10-5

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1-(4-hydroxyphenyl)-3-(4-nitrophenyl)prop2-en-1-one (3)

304/1.0 M HCl Stainless Steel Type 304/1.0 M HCl Stainless Steel Type 304/1.0 M HCl Carbon steel/1.0 M HCl Steel/0.5 M sulphuric acid Mild Steel/1.0 M HCl Mild Steel/1.0 M HCl Mild Steel/1.0 M HCl Mild Steel/1.0 M HCl

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(thiophene-2-yl)penta-2,4dien-1-one (1) 1-(4-hydroxyphenyl)-3-phenylprop-2-en-1One (2)

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With regard to the effectiveness, two key observations can be drawn from Table 10. First, results show that AA-3 provides the highest inhibition efficiency with respect to those

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obtained by the other chalcones, despite the different experimental conditions. Second, and more interestingly, the data show substantial improvement in the inhibition effect of

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AA-3 versus AE-3. It is evident that AA-3 has superior corrosion inhibition potential at a low concentration (96% at 10-3 M) compared to AE-3 (95% at 5x10-3 M). Substitution by aryloxyacid group led to new chalcones, which are more effective at lower concentrations (Scheme 1). This finding is of paramount technical and economic importance in the development of effective corrosion inhibitors with a high industrial value. It potentially

provides a path to design cost-effective chalcone derivatives with ideal corrosion inhibition

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properties.

compound.

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Scheme 1. The effect of the aryloxyacid group on corrosion inhibition properties of AA-3

4. Conclusion

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In summary, the study aimed to assess the efficiency of newly synthesized chalcone derivatives as corrosion inhibitors for MS surface in HCl environment. Generally, the previous experimental and theoretical sections show that developed chalcones are good candidates for the use as corrosion inhibitors for mild steel. The main conclusions of this paper are summarized as follows:

1.

Chalcone derivatives have excellent inhibitive properties, and their performance increases with the increase in their concentration.

2.

Tafel curves have supported that the AAs act as mixed-type inhibitors.

3.

Impedance results showed that the addition of tested compounds to HCl solution increasing the polarization resistance and therefore, the η% value increased as follows: AA-3 > AA-2 > AA-1.

4.

The three chalcone derivatives were found to inhibit corrosion by spontaneous

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chemical and physical adsorption on the MS surface and were found to obey Langmuir adsorption model. 5.

SEM analysis of the inhibited coupons with AA-3 inhibitor has indicated the

6.

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remarkable adsorption capability over the metal surface.

DFT calculations, molecular dynamics simulations, and radial distribution function

Results from this study are discussed with respect to previous works in this area.

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7.

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for the three selected chalcones have further supported the experimental findings.

Results demonstrated that chalcones with aryloxyacid group instead of an ester group displayed the most potent corrosion inhibition activity. It not only improves

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inhibition efficiency but also significantly reduce the concentration needed to achieve a good performance. These results could facilitate rational design of

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specific corrosion inhibitors used in the protection of metals.

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Author Contributions Section:

A. Chaouiki, H. Lgaz, M. Chafiq: Performed the experiments, analyzed the data, and wrote the initial manuscript. I. H. Ali, R. Marzouki and I. Cretescu: Contributed data or analysis tools, performed the analysis, resources, review&editing. Shubhalaxmi and K. S. Bhat: Performed the synthesis of compounds, review&editing.

R. Salghi, H. Oudda and I-M. Chung: Conceived and designed the analysis, supervision, review&editing. All authors read and approved the manuscript.

Conflict of Interest: The authors declare that they have no conflict of interest.

Acknowledgments The authors extend their appreciation to the Deanship of Scientific Research at King

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Khalid University for funding this work through research groups program under grant number R.G.P.2/46/40.

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Figures captions:

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Fig. 1: Tafel curves of MS electrode in 1.0 M HCl with and without AAs at various concentrations at 303 K. Fig. 2: EIS curves of MS in 1.0 M HCl without and with various concentrations of AAs inhibitors at 303K. Fig. 3: Bode and phase angle plots of impedance spectra for mild steel in 1.0 M HCl containing 10-3 M of AA-1, AA-2, and AA-3. Fig. 4: Plots of the Langmuir adsorption isotherm of chalcone derivatives on Fe surface at 303 K. Fig. 5: The optimized molecular structure, HOMO and LUMO orbitals, MEPs of AAs at neutral form of inhibitors obtained by DFT/B3LYP/6-31 ++ G (d, p) level. Fig. 6: Electron donor and acceptor sites for AA-1, AA-2 and AA-3 molecules predicted from Fukui indices. Fig. 7: Equilibrium adsorption of the most stable configurations for adsorption of neutral AA-1, AA-2, and AA-3 on Fe (110) surface obtained by molecular dynamics simulations in the aqueous medium. Fig. 8: Equilibrium adsorption of the most stable configurations for adsorption of protonated AA-1, AA-2, and AA-3 on Fe (110) surface obtained by molecular dynamics simulations in the aqueous medium. Fig. 9: RDF curves of protonated chalcones compounds adsorbed on the surface of iron generated from MD simulation. Fig. 10: Surface morphology by SEM for MS samples in polished form (a) immersed in 1.0 M HCl (b), and in 1.0 M HCl +10-3 M of AA-3 (c) for 24h at 303K.

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