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Evaluation of Lavandula mairei extract as green inhibitor for mild

steel corrosion in 1 M HCl solution. Experimental and theoretical
approach

A. Berrissoul, A. Ouarhach, F. Benhiba, A. Romane, A. Zarrouk,

A. Guenbour, B. Dikici, A. Dafali

PII: S0167-7322(20)31674-3
DOI: https://doi.org/10.1016/j.molliq.2020.113493
Reference: MOLLIQ 113493

To appear in: Journal of Molecular Liquids

Received date: 19 March 2020

Revised date: 1 May 2020
Accepted date: 30 May 2020

Please cite this article as: A. Berrissoul, A. Ouarhach, F. Benhiba, et al., Evaluation of
Lavandula mairei extract as green inhibitor for mild steel corrosion in 1 M HCl solution.
Experimental and theoretical approach, Journal of Molecular Liquids (2020),
https://doi.org/10.1016/j.molliq.2020.113493

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© 2020 Published by Elsevier.

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Evaluation of Lavandula mairei extract as green inhibitor

for mild steel corrosion in 1 M HCl solution. Experimental
and theoretical approach.

A. Berrissoul1, A. Ouarhach2, F. Benhiba3,4, A. Romane2, A. Zarrouk4, A.

Guenbour4, B. Dikici5, A. Dafali1,*
1
Mohamed the first University, Faculty of Sciences Oujda, Chemistry Department, Oujda/ Morocco.
2
Cadi Ayad University, Faculty of Sciences Semlalia, Chemistry Department, Marrakesh / Morocco.
3
Laboratory of Separation Processes (LSP), Faculty of Sciences, IbnTofail University, Kenitra,
Morocco.
4
Laboratory of Materials, Nanotechnology and Environment, Faculty of Sciences, Mohammed V
University, P.O. Box. 1014, Rabat, Morocco.

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5
Ataturk University, Faculty of Engineering, Metallurgical and Materials Engineering Department,
Erzurum /Turkey.

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* Corresponding author. Tel.: +212 677 931 116. Fax.: +212 536 500 603.
E-mail address: dafali2@yahoo.fr (A. Dafali).
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Abstract

Corrosion inhibition of mild steel sample in hydrochloric acid solution was examined via

weight loss and electrochemical measurements, Scanning Electron Microscopy (SEM) and X-

ray Photoelectron Spectroscopy (XPS) techniques. The use of Lavandula mairei Humbert

extract (LM) as an eco-friendly inhibitor of mild steel (MS) corrosion in 1M HCl solution has

significantly decreased MS corrosion rate. The corrosion inhibition efficiency increased with

the increase of LM concentration up to 92% obtained at 303 K for a 0.4 g L-1 concentration.

The adsorption of LM inhibitor on the mild steel surface followed the Langmuir adsorption

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isotherm. Anodic Tafel slope digression and continuous CR lessening with the LM extract

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concentration increase, has validated the limited dissolution of mild steel. Theoretical
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approaches based on chemical quantum calculations and molecular dynamics simulation
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clearly explains the mode of adsorption of the majority molecule on the metal surface.
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Keywords: Lavandula Mairei extract; Mild Steel corrosion; Electrochemical techniques;

SEM/XPS; Theoretical approach.
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1. Introduction

Industrial activities using metallic equipment are usually exposed to the corrosion

phenomenon. This may certainly cause a sudden failure that leads to the economic losses and

could affect the product quality. Cleaning of heat exchangers process, acid pickling surface

treatment and oil industry are some of the processes that use highly acid solutions particularly

HCl. Metallic components are then threatened and their life service may significantly be

reduced. For this reason, researchers are always looking for some efficient solution to

decrease the corrosion rate of metallic components particularly mild steel in acidic medium. A

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lot of corrosion protection methods are available such as surface coating and zinc-plating

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[1.2]. Concerning closed systems, the corrosion phenomenon could mainly be controlled
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through chemical inhibitor. Non-toxicity and ecofriendly aspect are always required by the
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environmental concerns. Otherwise, Sulphur, nitrogen, and oxygen atoms are generally the

main elements that exist in the functional groups of organic compounds. A larger part of
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organic compounds is not only expensive but ruinous to the environment as well. For this
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reason, green corrosion inhibitors are increasingly used. One of the important sources of

ecofriendly inhibitors are plants [3] and fruit peel extracts [4]. It has been shown that the
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extracts from leaves, barks, seeds, fruits and roots of plants contains mixtures of organic
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compounds with N, S, and O atoms that are effective compounds of metal corrosion inhibition

in aggressive electrolytes[5-9]. Morus alba pendula leaves extract has been used as green

corrosion inhibitor of mild steel in 1M HCl solution [10]. The charge transfer resistance of

corrosion reactions has increased from 152 to 1970 Ω cm2 at 298 K, respectively with 0 g/L

and 0.4 g/L as inhibitor concentration. The inhibition efficiency of Parsley (Petroselium

Sativum) leaves extract (PSL) on mild steel in one molar HCl solution has achieved 92.39% at

298 K for a 5 g/L[11]. Polarization showed that PSL acts as a mixed type inhibitor. Alkaloids

extract from Retama monosperma stems (AERST) has also been tested as corrosion inhibitor
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of mild steel in1M HCl at 303 K. Adsorption of AERST on the steel surface followed

Langmuir’s isotherm and the maximal inhibition efficiency was 83% with 400 ppm of the

extract [12]. Genus Lavandula belongs to the Lamiaceae family and was used by the Romans.

As well as this genus represented in Moroccan flora by 9 species and subspecies of which 5

are endemic to Morocco [13]. Lavandula oil knowns to have a various biological proprieties

such as antioxidant activity, insecticidal activity and anti-inflammatory effect [14-17].

The aim of the present work is to study the inhibitive effect of the ethanolic extract of the

Lavandula mairei Humbert (LM) on the corrosion of mild steel in 1M HCl solution by using

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potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods.

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The adsorption isotherm was envisaged in this study as well. The steel surface was examined
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by scanning electron microscopy (SEM), and the surface composition was determined by X-
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ray photoelectron spectroscopy (XPS) technique. Fine interpretation of experimental data

requires the use of DFT calculations of electronic adsorbate structures and MD simulation, to
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confirm or refute the hypotheses put forward. This is why; we decided to present a work that
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combines the theoretical and experimental aspects.

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2. Experimental
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2.1. Materials

2.1.1. Preparation of specimens

99.21% Fe; 0.21% C; 0.38% Si; 0.09% P; 0.05% Mn; 0.01% Al; 0.05% S is the mild steel

used in the following corrosion techniques. Before each experiment, the specimens were

polished with emery paper (360, 400, 600, 800, 1000, 1200 grades), rinsed with distilled

water and dried. The samples’ dimensions are 1 × 1× 0.2 cm3.

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2.1.2. Electrolyte

One molar hydrochloric solution was prepared using an analytical reagent grade 37 % HCl.

LM extract was dissolved in 1M HCl solution to obtain a concentration range of 0.05 g/L to

0.4 g/L.

2.1.3. Preparation of plant extract

Lavandula mairei Humbert was collected in May 2016 in the Ouarzazat region, Morocco. The

plant was identified and a voucher specimen (8448) was deposited at the herbarium of the

Faculty of Sciences Samlalia, Cadi Ayyad University. A quantity of 40 g of the aerial’s parts

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powder was macerated with 200 mL of ethanol for 24 h at room temperature using a magnetic

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stirrer. The extract was then shacked, filtered and evaporated in a rotating evaporator under
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reduced pressure until dryness[18]. The extract was stored in sealed glass vials at 277 to 278
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K prior to analysis. Each extraction was performed in triplicate. The yield of the ethanol

extract of Lavandula mairei Humbert was 7.25% of the dried plant. The extract was analysed
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by spectrometric, chromatographic and classical analytical techniques, as described in the

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literature [19-21]. The obtained constituent percentages of the Lavandula mairei Humbert

extract (LM) are summarized in Table 1. This extract is characterized by a strong presence of
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Quercetine-3-glucuronide (72.8%), as major compound. The others main components

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detected in this extract are Rosmarinic acid (9.3%) followed by Chicoric acid (8.7%) and

Acacetin-acetyl glucoside (4.7%).

Table 1

List of compounds identified in Lavandula mairei Humbert

Molecular weight Retention time (Rt) Pourcentages of

Compounds (min)
(g mol-1) compound (%)
Rosmarinic acid 360 14.90 9.3
Quercetine-3-glucuronide 478 16.18 72.8
chicoric acid 474 16.65 8.7
Acacetin-acetyl glucoside 488 19.15 4.7
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2.1.4. Identification of the major compound of the extract

HPLC-UV-ELSD-MS was used for the analysis of the ethanolic extract. At a concentration of

10 mg/mL, was separated on a SunFire C18 column. A gradient of 5% to 100% B in 45 min

was applied, followed by 100% B for 5 min. The flow rate was 0.5 mL/min. Spectra were

recorded with UV scanning from 190 – 600 nm. It should be noted that the ethanolic extract

contains several products with low percentages belonging generally to the families of

tetracarboxylic acid, flavone, and flavonoid. While Quercetin-3-glucuronide is the main

product of the alcoholic extract of LM.

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Fig. 1. Quercetin -3-glucuronide major compound of LM ethanolic extract.

2.2. Methods
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2.2.1. Weight loss method

The specimens were immersed for 6 hours in the test solutions that were maintained at
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constant temperature (303 K) via a temperature-controlled water bath, for all tests; 0.05, 0.1,

0.2 and 0.4 g/L were the inhibitor concentrations used as electrolytes for weight loss

measurements. Mild steel specimens were removed, rinsed with distilled water, air dried and

then weighted. The weight loss was calculated by subtracting the weight of the sample before

and after immersion. The inhibition efficiency w(%) calculated from the corrosion rate (CR)

is given by the following equations:

Weight loss  mg 
CR  (1)
Area  cm2   time  h 
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 CR 
 w  %   1   100 (2)
 CR0 

where: CR0 and CR are mild steel corrosion rates (mg cm-2 h-1) in the absence and presence of

the extract, respectively. The surface coverage (θ) was defined as follows (Eq. (3)):

 CR 
  1   (3)
 CR0 

2.2.2. Electrochemical measurements

A temperature-controlled cell was used with three electrodes: mild steel specimen embedded

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in a holder so that the exposed surface is 0.94 cm² as working electrode (WE), platinum

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electrode as auxiliary electrode, and saturated calomel electrode (SCE) as reference. WE was
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polished with emery papers, rinsed and dried before each electrochemical experiment. Mild
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steel cylindrical electrode was exposed to 100 mL of the test solution at free potential. The

open circuit potential (OCP) was recorded as a function of time up to 30 min. As a result, Ecor,
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which corresponds to a steady-state OCP, was obtained. All electrochemical experiments

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were performed using PGZ100 potentiostat/galvanostat monitored by a computer via Volta

Master 4 software. Polarization measurements (Tafel) were scanned from cathodic to the
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anodic direction, in a potential range of -800 mV to -200 mV with a scan rate of 0.5 mV S-1.
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EIS measurements were carried out in the frequency range 100 KHz–10 mHz using AC

signals of amplitude 10 mV at OCP. Using polarization curves and electrochemical diagrams,

the percentage of inhibitory efficiency was obtained as follows: Eqs. 4&5:

 icor
0
 icor 
Tafel  %    0   100 (4)
 icor 

 R p  R p0 
 EIS  %     100 (5)
 R
 p 
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0
where icor and icor are, respectively, current densities before and after addition of the inhibitor.

Rp and R p0 are polarization resistance in the presence and absence of inhibitor respectively.

2.2.3. Surface characterization

Mild steel specimens were dipped in the test solutions with and without the addition of 0.4

g/L of LM extract. After 24 hours, the samples were removed and dried. The morphologies of

the treated and non-treated mild steel exposed surfaces have been examined by a JEOL JSM-

IT100 scanning electron microscopy device and by energy dispersive X-ray spectroscopy

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(EDX) under high vacuum at an accelerating voltage of 20.00 kV.

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2.2.4. X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) spectra were obtained via XPS SPECS-Flex mode
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with the monochromatized Al–Kα X-ray source (hν = 1486.71 eV) and an X-ray beam of
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around 3 mm. 40 eV was the value of the pass energy used while operating the analyzer and
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an analyzer lens with medium area. Charging effects have been compensated, in the time of

analysis, using charge compensation. The C 1s (285.0 eV) binding energy (BE) was used as
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an internal reference. XPS spectra were deconvoluted using a non-linear least-squares

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algorithm with a Shirley baseline and a Gaussian–Lorentzian combination. Casa-XPS

software was mainly applied to complete all spectra deconvolution. XPS analyses were
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applied on the protected and non-protected steel surfaces with LM extract. The Mild steel

specimen was pretreated trough the same gravimetric test method. After 24 h immersion

period, the mild steel sheet was dried and then analyzed.

2.2.5. UV-visible spectrophotometric analysis

UV–Visible absorption spectrophotometric method was carried out on 1M HCl solution with

addition of 0.4 g/L of the LM extract before immersion and after immersion of mild steel

samples during 6 hours (the same as gravimetric analysis immersion time). UV–Visible Jasco
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V-730 model spectrophotometer, monitored by Spectra Manager software, was used in the

present analysis.

2.2.6. Quantum chemical studies

In order to elucidate the mode of action of the LM major compound on the steel surface, the

chemical reactivity of this molecule was studied using the density functional theory (DFT) at

the B3LYB function level with the 6-311G(d, p) basis set [22]. The chemical quantum

calculations were run using Gaussian 09 software [23]. Chemical quantum descriptors are

used to evaluate the overall reactivity of LM in order to justify the inhibitory performance of

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this molecule against corrosion of the studied steel [24]. These descriptors of reactivity are

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Total energy (TE), EHOMO, ELUMO, ∆Egap, dipole moment (μ) and ΔN110 [25-28].
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The molecular dynamics (MD) simulation was carried out via the Forcite module inserted in
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Materials Studio 8.0 program developed by BIOVIA Inc [29,30]. It was implemented in a

simulation box (27.3027.3033.13Å3) with periodic boundary conditions. In this model,

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there were 6 layers of iron representing a (11×11) unit cell. The vacuum layer that filled with
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500 H2O, 5 H3O+, 5 Cl- and the optimized structure of the LM molecule. The adsorption

system was optimized via COMPASS force field [31], then MD simulations was performed
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under the 303, 313 and 333 K, NVT ensemble with Andersen thermostat[32] and simulation
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time of 400 ps with 1 fs time step.

3. Result and discussion

3.1. Impedance spectroscopy

Figure 2 exhibits Nyquist diagrams of mild steel in 1M HCl solution at OCP with and without

various concentrations of LM ethanol extract at 303 K. The AC impedance spectroscopy

response of MS in LM ethanol extract (inhibited) and inhibitor-free solutions are graphically

represented as Bode (modulus, and phase angle) diagrams (Fig. 3). The semi-circular shape of
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impedance spectra suggests that corrosion process of mild steel reveals a capacitive attitude,

in one molar HCl test solution[33]. All Nyquist plots include one depressed capacitive loop;

hence, corrosion process of MS in such experimental conditions, is predominated by charge

transfer process. For the uninhibited (1M HCl) and inhibited (LM ethanol extract) cases, the

deviation from the perfect semi-circular shape, known as the frequency dispersion of the

interfacial impedance, might result from roughness as well as other forms of interfacial

phenomena [34]. Indeed, the slope values of the Bode-modulus plots (Fig. 3) for intermediate

frequency values were not equal to -1 (ideal capacitor) but tends to -1. This can attributed to

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the frequency dispersion of interfacial impedance. The Addition of the LM extract to the

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solution doesn’t change the dissolution mechanism of mild steel since the capacitive loops’
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shapes were the same. The diameter of the semi-circles increases with augmentation in
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inhibitor concentration, which leads to the inhibition efficiency rise, as it is shown in Table 2.
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200
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Blank
0.05 g/L
0.01 g/L
150
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0.02 g/L
0.04 g/L
-Zim [ cm²]

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100

50

0
0 50 100 150 200 250
Zre [ cm²]

Fig. 2. Nyquist plots of mild steel in 1M HCl with and without LM extract concentrations.
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2,5 0.05 g/L
0.10 g/L
60
0.20 g/L
2,0 0.40 g/L

-Phase angle (°)

40
LogIZI [ cm²]

1,5

20

1,0

0,5

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0,0

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-2 -1 0 1 2 3 4 5

Log f (Hz)
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Fig. 3. Bode (Log f vs. log |Z|) and phase angle (Log f vs. a) plots of impedance spectra for
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mild steel in 1M HCl in the absence and presence of LM extract.
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An excellent fit using the model described was found with our impedance data, For instance,
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the representative Nyquist diagram of fitted and experimental data are given in Fig. 4.
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-80
Experimental curve Experimental curve
Fit result 2.5 Fit result
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-Phase angle (Degre)

2.0
Log |Z| ( cm2)
-Zim [ cm2]

-40
100
1.5
-20

50 1.0
0

0.5 20
0
0 50 100 150 200 250 -2 -1 0 1 2 3 4 5

Zre [ cm2] Log (f) (Hz)

Fig. 4. Experimental and fitted Nyquist diagram for the interface of MS in 1M HCl solution
+0.4 g/L of LM extract at 303 K.
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Electrical equivalent circuit (EEC) has been utilized to evaluate the EIS data (Figure 5). The

used EEC consists of an solution resistance (Rs) in series with a constant phase element (CPE)

in parallel to the polarization resistance (Rp), where Rp comprises the charge transfer

resistance (Rct) and the resistance of the inhibitor (LM extract) film (Rf) (Rp = Rct + Rf ) [35].

Rs CPE

Rp

Fig. 5. The electrical circuit consistent with the experimental impedance data.

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Element Freedom Value Error Error %

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Rs Fixed(X) 0 N/A N/A
The ideal capacity of CPE-T
the double layerFixed(X) 0 dl) was substituted
capacitance (C N/A by CPE inN/A
the ECC
CPE-P Fixed(X) 1 N/A N/A
obviate the deviation Rp
resulting from Fixed(X)
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the frequency 0dispersion [36].N/A N/Aof CPE
The impedance
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Data File: as follows [37]:
(ZCPE) and Cdl can be described
Circuit Model File:
Mode: Run Simulation / Freq. Range (0.001 - 1000000)
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 i 
1 n
ZCPE  Q Maximum Iterations: 100 (6)
Optimization Iterations: 0
Type of Fitting: Complex
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Where Q named the Type of

CPE Weighting:
constant, n is a CPE Calc-Modulus
exponent determining the phase shift which

can be utilized as a gauge of roughness or heterogeneity of the surface ( 0 n 1 ), i2 = -1

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defined as an imaginary number and ω is the angular frequency (ω = 2πf, where f is the
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frequency). However, the double layer capacitances, Cdl, for a circuit comprising a CPE were

determined by utilizing the next formula [38]:

Cdl   QR1pn 
1/ n
(7)

In addition, the calculated values of Cdl are used to determine the relaxation time (τ) by

applying the next equation [39]:

  Cdl  R p (8)
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Table 2
Mild steel impedance parameters for in1M HCl solution with and without different
concentration of LM extract.
Conc. Rs Rp 104  Q n Cdl 1032  z
(g/L) ( cm²) ( cm²)
-1 n
(Ω S cm ) -2
(Fcm ) -2 (mS) (%)
Blank 2.00  0.05 018.17  0.43 4.03  0.08 0.815  0.002 132.0 0.89 02.40
0.05 3.87  0.02 077.78  0.52 2.37  0.05 0.830  0.002 104.6 0.97 08.13 76.6
0.10 2.42  0.02 106.90  0.83 1.75  0.05 0.829  0.004 077.0 2.32 08.23 83.0
0.20 2.23  0.01 151.72  1.23 1.42  0.06 0.822  0.003 061.8 1.14 09.38 88.1
0.40 2.99  0.04 215.80  2.23 1.13  0.05 0.819  0.006 051.7 2.11 11.16 91.6

The accuracy of the EIS measurements can be verified from the strong agreement between the

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experimental plots and fitting lines (Fig. 4), as well as the values of goodness of fit in Table 2.

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Indeed, the goodness of fit chi-squared (χ2) values are of the order of 10-3 (Table 2),

suggesting the validity of the proposed circuit (in theory the lower value of χ2 shows that the
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fitted data agrees well with the experimental data). It is observed that the Q values decrease
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when the concentration of LM ethanol extract increases. This result suggests that the Q values
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are mainly influenced by the concentration of the LM ethanol extract. The highest Rp (215.8

Ω cm2) for LM ethanol extract have been obtained at 0.4 g/L. Moreover, the value of 
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(relaxation time constant) increases with concentration of inhibitor as well, thus justifying a
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time of adsorption process, which is becoming increasingly important as a function of

concentration [40]. The biological molecules of phytochemical components of the LM ethanol

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extract are adsorbed on the exposed MS surface and block therefore the active sites, which are

available for corrosive dissolution whereby causing an increment in the Rp values which is

correlated with inhibitive performance protection. However, a substantial decrease in Cdl with

LM ethanol extract concentration was observed, may be owing to the decrease in the

dielectric constant through replacement of the pre-adsorbed water molecules [41]. Diminution

in dielectric constant intensifies the adsorption ability of the LM ethanol extract molecules.

This can be explained by in greater coverage and better protection of the metal against

dissolution, confirmed by the increase of the inhibition performance with the LM ethanol
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extract concentration as presented in Table 2. It is worth pointing that the rise in the deviation

parameter (n) values after the inhibitor addition, which can be related to an increase of surface

inhomogeneities caused by adsorption of the LM ethanol extract components on MS surface.

Furthermore, the value of τ increases with the LM ethanol extract content, meaning a slow

adsorption process [42].

3.2. Polarization curves

Mild steel polarization curves are exhibited in Figure 6, with and without the addition of LM

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extract. The study of these polarization measurements allows having information related to

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cathodic and anodic reactions. Electrochemical parameters including the potential of
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corrosion (Ecorr), corrosion current density (icorr), anodic Tafel slope (βa) and cathodic Tafel
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slope (βc) have been obtained via the extrapolation method [43] and are given in Table 3.

Equation (3) has been used to calculate the inhibition efficiency (Tafel (%)) from polarization
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measurements.
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2
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1
Log (i) (mA/cm²)
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Blank
-1
0.05 g/L
0.10 g/L
0.20 g/L
-2 0.40 g/L

-3
-800 -700 -600 -500 -400 -300 -200

E (mV/SCE)

Fig. 6. Polarization curves for mild steel in 1M HCl containing different concentrations of
LM ethanol extract at 303 K.
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Table 3
Polarization data of mild steel in 1M HCl containing different concentrations of the LM
ethanol extract at 303 K.
Conc. - Ecorr icorr βa -βc Tafel
(g/L) (mVSCE) (mA cm-2) (mV/dec) (mV/dec) (%)
Blank 389.3 1.509 146.3 219.3 ──
0.05 413.8 0.454 100.9 185.0 69.9
0.10 407.0 0.297 75.1 162.1 80.3

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0.20 405.5 0.232 76.0 190.0 84.6
0.40 376.0 0.154 72.5 216.8 89.8

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From Figure 6, it is clear that both anodic and cathodic curves move to the lower current
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densities with increasing the concentration of the LM ethanol extract and there is no definite
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trend in the shift of Ecorr values. Generally, this slight shift in corrosion potential with
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concentration is not considered (less than 25 mV) [33]; therefore, it can be confirmed that the

inhibitor LM acts as mixed-type inhibitors. It is remarked also that the cathodic curves have
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given rise to parallel Tafel lines, meaning that the evolution of the hydrogen is activation-

controlled, and the addition of inhibitor does not affect the mechanism of the hydrogen
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evolution reaction [43]. Moreover, the decrease of anodic current density means that the
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addition of LM reduced the anodic dissolution. An important decrease in corrosion current

density (icorr) generates an increase in inhibition efficiency with increase in concentration of

LM ethanol extract and which reaches its highest value (89.8%) at 0.4 g/L. This increase of

efficiency is due to both anodic and cathodic inhibition performance blocking active sites on

the steel surface by adsorption of LM ethanol extract.

3.3. Weight loss measurements

3.3.1. Concentration effect

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Weight loss measurements were carried out in 1M HCl solution with and without the addition

of different concentration of the LM ethanol extract. Mild steel samples were immersed in the

test solution for 6 hours at 303 K. The data on Table 4 indicates that the inhibition efficiency

has achieved a maximal value of 89% corresponding to 0.4 g/L as inhibitor concentration.

This result might be associated to LM compound adsorption on the mild steel exposed

surface. The formed layer isolates the metallic exposed area from aggressive ions in the test

solution. LM extract majority chemical compounds substitute H2O molecules existing on

mild steel surface so that they can create a protective film against HCl attack. It might be

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regarded as a quasi-substitution process[44].

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Table 4 -p
Corrosion rates data of mild steel in 1M HCl with addition of various concentrations of LM
ethanol extract.
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Conc. Corrosion rate (CR) Inhibition efficiency
(g/L) (mg cm-2 h-1) (W %)
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Blank 0.90 ──
0.05 0.28 68.9
0.10 0.18 79.9
0.20 0.16 82.2
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0.40 0.10 88.9

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3.3.2. Effect of temperature and thermodynamic activation parameters

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Temperature effect was examined in a test solution that contains 0.4 g/L of LM extract. It was

considered as the optimal concentration that corresponds to the maximal W %. The tests were

carried out on MS samples that were polished, rinsed with distilled water and dried before

each experiment. Values on Table 5 demonstrate an augmentation of MS corrosion rate with

temperature rising, which indicates that high temperature has lowered MS corrosion

resistance even if LM extract was added. Which means that its inhibition efficiency has

considerably decreased (from 86.5% to 58.8%). This could be related to kinetic energy
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augmentation and decrease in the adsorption ability of a LM major compound on MS exposed

surface at elevated temperature.

Table 5
Corrosion rates data of mild steel in 1M HCl with addition of various concentrations of LM
ethanol extract at different temperature.
Temperature Blank LM extract
(K) CR (mg/cm² h) CR (mg/cm² h) ηw (%)

of
303 0.89 0.12 86.5
313 1.36 0.30 77.9

ro
323 1.80 0.56 68.9
333 2.50 1.03 58.8
-p
In order to calculate activation parameters, Arrhenius formula has been used as follows [45]:
re
 E 
CR  A exp  a  (9)
 RT 
lP

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

First (Eq. 9), Ea is the activation energy of metal dissolution, A is the Arrhenius pre-
ur

exponential factor, R is the universal gas constant, and T is the absolute temperature (In
Jo

international unit). Second (Eq. 10), N is Avogadro’s number, h is Plank's constant, ΔHa is the

enthalpy of activation. and ΔSa is the entropy of activation. Figure 7 has given the opportunity

to calculate activation energy of MS dissolution using linear fitting tool. Thus, slopes values

have been utilized to determine Ea of mild steel dissolution for both uninhibited and inhibited

electrolytes, the values of the activation parameters have been collated in Table 6. The values

of Ea are respectively 28.38 and 59.47 kJ mol-1 in both blank solution and the LM extract.

Thus, activation energy has increased indicating that MS dissolution has decreased thanks to

the creation of a protective layer on the exposed metallic area[46]. Moreover, adsorption of

the inhibitor decreases which means that its desorption has been lowered [47].
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0.4 Blank
cm2) 0.4 g/L of LM

0.0
Ln CR (mg/h

-0.4

of
-0.8

3.00 3.05 3.10 3.15

ro
3.20 3.25 3.30
-p
1000/T (K-1)
re
Fig.7. Arrhenius plots of Ln CR vs. 1000/T for MS in one molar hydrochloric acid with and
lP

without addition of 0.4 g/L of LM extract.

-2.0
na

Blank
0.4 g/L of LM
Ln (CR/T) (mg/h cm-2)

-2.4
ur
Jo

-2.8

-3.2

-3.6
3.00 3.05 3.10 3.15 3.20 3.25 3.30

1000/T (K-1)

Fig.8. Arrhenius plots of Ln CR vs. 1000/T for MS in one molar hydrochloric acid with and
without addition of 0.4 g/L of LM extract.
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Table 6
Activation parameters Ea, ∆Ha and ∆Sa for MS dissolution in 1.0 M HCl with and without
addition of LM extract at 0.4 g/L.

Medium Ea ∆Ha ∆Sa Ea∆Ha

1 1
( kJ mol 1 ) ( kJ mol 1 ) ( J mol K ) ( kJ mol 1 )
1M HCl 28.38 25.74 -160.68 2.64
LM extract 59.47 56.83 -74.58 2.64

of
ro
The parameters ∆Ha and ∆Sa were determined using transition state formula (Equation 10 and

Figure 8). Indeed, the slope of Ln (CR/T) vs 1000/T straight line permits to determine ∆Ha
-p
values that significantly increases in presence of inhibitor, as it is shown in Table 6. The value
re
of ∆Ha for uninhibited solution is equal to 25.74 kJ mol-1 and reached 56.83 kJ mol-1 for
lP

inhibited solution indicating that MS dissolution process nature is endothermic and this

process becoming slower with the addition of LM extract. Otherwise, the entropy of
na

activation (∆Sa) has moved toward positive values comparing the results of the blank an
ur

inhibited studied electrolyte. The present result reveals that disorder has been raised from
Jo

reactants to the activation complex which has shown a dissociation instead of an

association[48].

3.4. UV-Visible analysis

Figure 9 shows UV-visible spectra before and after immersion of metallic samples in 1M HCl

solution containing 0.4 g/L that have been used in weight loss tests (6 hours was the

immersion time). The absorption spectra of LM extract studied before immersion of metal

show two absorption bands of short wavelength. The first band at 280 nm which can be

attributed to the electronic transitions π-π* of the aromatic ring of the major compound. A
 Journal Pre-proof

second band at

330 nm characterizes the electronic transitions n-π* of carbonyl group and enol form present

in the major compound (glucuronide) of LM extract. The UV-visible spectrum of the solution

containing 0.4 g/L of the inhibitor, after immersion of metallic samples, shows the existence

of a shift in the absorbance of these same bands without there being a significant difference in

the shape of the spectra, which indicating a possibility of interaction between major molecules

of extract and steel (physisorption) [49,50]. Previous work have reported that the

displacement of the wavelength with variation in absorbance is due to the formation of a

of
complex between inhibitor and metal ion in solution [51]. This prompts us to conclude in this

ro
case that there are possibility of the formation of a complex between major molecule of the
-p
extract and the iron ion and this contribute to the inhibitive action.
re
lP

LM extract
1.2
Fe + LM extract
na
Absorbance

0.8
ur

0.4
Jo

0.0
200 250 300 350 400 450

Wavelength (nm)

Fig. 9. UV-visible spectrum of inhibited solution before and after mild steel weight loss tests.

3.5. Surface and isotherm adsorption studies

3.5.1. SEM analysis

Figure 10 shows the SEM images of mild steel surface after 24 hours of immersion in the test

solutions in the absence and presence of 0.4 g/L of LM ethanol extract. Figure 10b shows that

the specimen surface was strongly damaged due to mild steel excessive dissolution in the
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presence of H  and Cl  ions. The addition of LM ethanol extract (0.4g/L) to the hydrochloric

solution, corrosion damages have been reduced as it is shown in Figure 10c; The metallic

surface looks smooth and less damaged as a result of the formation of a protective film on

mild steel surface[52]. EDX spectra displayed elements present on the metallic surface.

Figure 10a reveals the presence of Fe and C and the absence of oxygen element. 24 hours has

been passed as dipping time in the test solution (1M HCl) and iron percentage was reduced

due to its dissolution. The oxygen percentage has been significantly increased due to the

apparition of iron oxides. The LM ethanol extract has contributed to the decrease of oxygen

of
and chlorine percentages and the increase in Fe on the metallic surface. This findings suggest

ro
the creation of a protective layer which has decreased mild steel dissolution in 1M HCl[53].
-p
re
lP
na
ur
Jo
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(a) (b)

of
ro
-p
re
lP

EDX Fe C O Cl EDX Fe C O Cl

Wt.% 95.08 3.11 - - Wt.% 86.12 3.49 4.99 2.25

na
ur
Jo

(c)

EDX Fe C O Cl

Wt.% 93.09 3.26 0.45 0.68

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Fig. 10. Surface morphology and EDX results of mild steel before (a) and after immersion in
1M HCl in the absence (b) and presence of 0.4g/L LM ethanol extract (c).

3.5.2. Adsoption isotherm

Since the inhibition action of the LM ethanol extract is related to the adsorption process,

coverage surface θ is calculated directly related to the inhibition efficiency (W %) via the

of
following equation:

ro
w  % 
 (11)
100 -p
To evaluate the mechanism of LM extract adsorption on the MS surface in 1M HCl solution,
re
the isotherm of adsorption was defined. Figure 11 shows that the adsorption mechanism
lP

follows the Langmuir model. Inspection of Figure 6 reveals that a straight line was obtained

fitting Langmuir adsorption isotherm and correlation coefficient is closer to 1 (R2 = 0.9988).
na

C 1
 C (12)
 K ads
ur

C: concentration of inhibitor,  : surface coverage and Kads is the equilibrium constant of

Jo

adsorption. From the intercept of Langmuir line, Kads value calculated: Kads = 50.05 L/g.
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0.5

0.4

C/ (g/L) 0.3

0.2

0.1

0.0
0.0 0.1 0.2 0.3 0.4

of
C (g/L)

ro
Fig. 11. Langmuir’s adsorption isotherm for MS in 1M HCl containing LM extract at 303 K.
-p
The value of Kads was compared to other calculated values obtained in literature [54,55] and it
re
has been concluded that it was really high to those found in these work. So, since Kads is high,
lP

the adsorption of the extract molecules is strong. Concerning the slope parameter, it was

nearly equal to 1 (1.08); which could imply that each active site of the metallic exposed area
na

was filled by the inhibitor molecules[56]. Otherwise, the free adsorption energy was
ur

calculated using Eq.13 as follow[57]:


Gads   RT  Ln  C  K ads 
Jo

(13)

R: gas constant, T: temperature value, and C: water concentration (106 mg/L).


Founded on the literature, Gads values ≈ -20 kJ mol-1 or less negative, the adsorption is

supposed as physisorption; while those ≈ -40 kJ mol-1 or more negative, the adsorption is

typical of chemisorption [58]. Alas, the molecular weight of the investigated extract (LM


extract) is not recognized, and therefore the determination of the Gads value in this case is

not possible. This conclusion is in agreement with those described previously in the case of

some plant extracts used as corrosion inhibitors [59-60].

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3.5.2. XPS analysis

The combination of these techniques with others like XPS, allows a better understanding of

surface phenomena. Surface investigation of mild steel was executed via XPS to study the

composition of the adsorbed layer on the exposed area in the electrolyte. This technique

showed the formation of inhibitor layer containing the LM extract, with the major compound,

basing on the XPS spectra plot. The obtained XPS spectra of the MS exposed area after

immersion in one molar HCl solution containing 0.4g/L of LM is presented in Fig.12 this

figure shows the resolution XPS spectra found for extract LM, deposited on the surface of a

of
metal in solution (C 1s, O 1s, Fe 2p and Cl 2p). XPS spectra show complex forms that have

ro
been associated to the adequate species via a deconvolution fitting procedure using a non-
-p
linear least-squares algorithm with a Shirley baseline and a Gaussian–Lorentzian
re
combination. The inhibited interface chemical composition, described by XPS spectra of the

C 1s, O 1s, Cl 2p and Fe 2p core levels obtained from substrate on the mild steel surface
lP

(major compound of LM extract added). The binding energies (BE, eV) as well as their
na

quantifications (%) are depicted in Table 7.

ur

12000
Experimental result
(a) C1s Baseline
Jo

C-C / C=C / C-H

10000 C=O/C-O-H
Envelope
Intensity

8000

6000

4000

2000
300 295 290 285 280 275 270

Binding Energy (eV)

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20000
(b) O1s Experimental result
Baseline
Fe2O3/Fe3O4
16000 C-O-H/ C=O adsorbed H2O
Intensity Envelope

12000

8000

4000

of
0
540 535 530 525 520 515

ro
Binding Energy (eV)
-p
re
10000
(c) Cl 2p Experimental result
lP

Baseline
9000 Cl 2p3/2
Cl 2p1/2
Envelope
8000
na
Intensity

7000
ur

6000
Jo

5000

4000

208 206 204 202 200 198 196 194 192 190 188

Binding Energy (eV)

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18000
(d) Fe 2p

15000
Intensity

12000

9000
Experimental result
Baseline
Fe metal
6000 Fe2O3/Fe3O4/FeO
Fe 2p ½

of
Envelope
3000

ro
745 740 735 730 725 720 715 710 705
Binding Energy (eV)
-p
Fig. 12. The XPS deconvoluted profiles of C 1s (a), O 1s (b),Cl 2p (c) and Fe 2p(d) for LM
re
treated mild steel.
lP
na

Table 7
Binding energies (eV), relative intensity and their assignment for the major core lines
ur

observed for mild steel surface after immersion in HCl solution containing a LM extract
(Predominant product).
Jo

C 1s O 1s Cl 2p Fe 2p
Energy Attribution Energy Attribution Energy Attribution Energy Attribution
(eV) (eV) (eV) (eV)
C–C / C=C C-O-H/ C=O
284.91 532.01 198.56 Cl 2p3/2 724.08 Fe 2p1/2
/ C–H adsorbed H2O
C=O/ 710.17 Fe2O3/Fe3O4/FeO
286.58 530.27 Fe2O3/Fe3O4 200.01 Cl 2p1/2
C-O-H 706.94 Fe metal

C 1s spectrum deconvolution could be squared into two elements, showing two different

carbon profiles, defined by 284.91 and 286.56 eV binding energies (Figure 12a, Table 7). On

one hand, the first component, has the contribution (55.02 %), could refers to contaminant
 Journal Pre-proof

hydrocarbons and to C-C, C=C and C-H bonds in the predominant molecule of extract [40].

On the other hand, the second constituent refers basically to C=O bonds and enol-group C-O-

H [61].

After immersing MS specimen in one molar hydrochloric acid solution that contains 0.4 g/L

inhibitor, O 1s spectrum could correspond to two components (Fig. 12b, Table 7). The first

one, at approx. 532.01 eV, is ascribed to oxygen of adsorbed water [62], which remained on

the surface after drying the sample and might refer to(C=O)carbonyl groups present in major

compound [63,64], demonstrating that the inhibitor molecules are adsorbed on the steel

of
surface. The second component, observed at 530.27 eV, is attributed to O2and could be

ro
related to oxygen atoms bonded to Fe3+ in the Fe2O3 and/or Fe3O4 oxides [40]. Thus, it could
-p
lead to the apparition of O-Fe bond which is based on interactions, that are characterized by
re
donor acceptor aspects, between sp2 electron pairs existing on the oxygen of carbonyl group

and the vacant d orbitals of Fe. On the other hand, separating contribution of organic and
lP

inorganic oxygen respectively in O of carbonyl groups, in coordinate bond O-Fe and O in

na

FeOOH, in H2Oads from O 1s signal, seems not achievable [64].

Fe 2p spectrum of the MS sample surface after being dipped in 1M HCl solution that contains
ur

0.4g/L of extract LM represents clear peaks at 711 eV (Fe 2p3/2) and 724.08 eV (Fe 2p1/2)
Jo

values of binding energy (BE). Fe 2p3/2 XPS spectrum deconvolution reposes in three

different components (Fig. 12d, Table 7). Lower bending energy peak, with BE value around

706.94 eV, is associated to metallic Fe, even though, 710.17 eV BE value refers to Fe3+ [40],

approving that iron oxide species exist including Fe2O3, Fe3O4 and FeO as it has been

revealed in the O 1s spectrum. Another peak should normally appear in this spectrum (around

714 eV) corresponding to the existence of FeCl3 over steel the sample surface deriving from

the acid solution [65]. A confirmation of this presence is given by the high resolution

spectrum of the Cl 2p core-level (Fig. 12c), present at the steel surface, showing two peaks
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(Cl 2p3/2 at 198.56 eV at and Cl 2p1/2 at 200.01 eV (Table 7)) and which may be due to the

presence of ferric chloride salt.

A compound containing a pair of doubly bonded carbon atoms adjacent to a hydroxyl (−OH)

group, C=C-OH is normally in rapid equilibrium with an enol tautomer, which contains a

carbonyl group (C=O). Generally, the keto form predominates at equilibrium for this kind of

molecule and the enol form is important for reactional mechanism. On the other hand, a

deprotonated may occur in the interconversion of the two forms, giving rise to an enolate

anion and leading a strong nucleophile [65]. On the other hand, the inhibitor (predominant

of
molecule) could be adsorbed by interactions due to donor-acceptor ability, between the

ro
unshared electrons pairs of the oxygen atoms and the electrons of the aromatic ring. A
-p
bond might also be created with vacant 3d orbitals of the sample surface (chemisorption), thus
re
acting as a Lewis acid, generating the creation of protective chemisorbed film [40].

Ultimately, the previous XPS results have demonstrated the presence of chemical interactions
lP

of MS sample exposed surface and the major molecule of extract LM. As a matter of fact,
na

C=O and C-O-H group that have been revealed above the steel surface imply that the

investigated inhibitor was adsorbed via chemisorption aspect on the exposed surface in
ur

hydrochloric acid solution and bears out the obtained thermodynamic results. Thus, LM
Jo

extract could be considered as an adequate inhibitor for mild steel corrosion in the

hydrochloric acid medium.

3.5. Quantum chemical calculations

3.6.1. Electronic property behavior

In an attempt to correlate the inhibitory efficacy found in the experimental part with the

electronic properties of the Quercetin-3-glucuronide major compound of LM ethanolic

extract, we then assessed the reactivity of this compound in both neutral and protonated
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forms. The majority molecule LM has several oxygen atoms capable of protonation, therefore,

to determine the atom or atoms that are likely to bind the proton H+, we used the

MarvinSketch software which allows us to give all the distributions of protonated forms

versus pH [66]. Figure 13 illustrates that the oxygen atom of the carboxylic acid O45 of the

LM molecule is the most favorable site for protonation with a percentage of 99.73% and

stable at pH close to zero. The possibility of the presence of this form in the 1M HCl acid

medium is more important, giving insight into the mode of action of the LM molecule on the

metal surface.

of
ro
120
-p
(-9)% (-7)% (-6)%
Macrospecies distribution (%)

(-5)% (-4)% (-3)%

99.73% (-2)% (-1)% (0)%
re
100 6.37
H O H

H
lP

80 O H
7.90
H O
2.65 O O
O
60 H O H
na

13.29
H O H O
H 11.77
H H O
40 O O H
14.76 H H
12.39
H H O 8.56
ur

H
20
Jo

0 2 4 6 8 10 12 14
pH

Fig. 13. Profile distribution of protonated forms vs. pH using MarvinSketch software

The optimization of the structure of the neutral and protonated forms of the molecule under

study and their electron density distributions of FMOs (frontier molecular orbitals) (HOMO

and LUMO) are depicted in Figure 14. It is apparent that these molecular orbitals are

distributed over the surface of the 2-(3, 4-dihydroxyphenyl)-5, 7-dihydroxy-4H-chromen-4-

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one skeleton. This segment containing the aromatic rings is responsible for the reactivity of

the major molecule LM, i.e. more important for the interatomic interactions carried out

between the atoms of this molecule and the iron atoms of the metal surface. Hence, this

characteristic reinforces the adsorption of our molecule [67]. The protonated form has altered

the electron density distribution of FMOs, with the 2-(34-dihydroxyphenyl)-5,7-dihydroxy-

4H-chromen-4-one skeleton retaining its donor property, but losing electron acceptance, the

last property is located on the surface motif of 3,4,5,6-tetrahydroxytetrahydro-2H-pyran-2-

carboxylic acid. Thus, and as shown in Figure 14, the protonation results in an elongation in

of
the bond bound to the center of the protonation C44-O45. This indicates that there has been a

ro
modification in the geometry. These observations lead to the conclusion that protonation
-p
disturbs the electronic distribution of the molecule under test and in particular the electron
re
acceptor character, i.e. this behavior is favored in this procedure.
lP
na
ur
Jo
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of
ro
-p
re
lP
na

Fig. 14. Optimization of the structure and FMOs of the neutral and protonated forms of
ur

Quercetin -3-glucuronide major compound of LM ethanolic extract.

Jo

In general, DFT correlates the efficiency of inhibition with quantum chemical descriptors

[68]. Table 8 brings together the values obtained for the main descriptors of the neutral and

protonated forms of Quercetin -3-glucuronide major compound of LM ethanolic extract. As is

evident, EHOMO is consistently high, while ELOMO is low. Usually, it is agreed that EHOMO

represents the electron donating capacity of an inhibitor, whereas, ELOMO indicates the

electron accepting capacity of an inhibitor [69]. The protonation of the LM molecule has a

considerable influence on these two descriptors whose electron donating power is decreased,

while this protonated form becomes more available to receive electrons. Thus, and as
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indicated in Figure 13, the protonation causes an elongation in the bond bound to the center of

the protonation of C44-O45. This indicates that there has been a change in geometry. The

minimum ΔEgap value and the maximum dipole moment value of neutral and protonated LM

reflect that the reactivity of this molecule is very high, indicating a good inhibitor [43]. This

reactivity becomes more important in the case of the protonated form. In addition, the electron

donating power is measured by the number of electrons transferred (ΔN110), the results

combined in Table 8 show that the ΔN110 value of the neutral molecule LM is in the Lokovic

range [70]. This indicates that this molecule easily releases its electron. After protonation, this

of
property is lost, indicating that the protonated form has a character to receive electrons. The

ro
chemical reactivity of LM is also evaluated by the total energy and the values of this
-p
descriptor are listed in Table 8. It is observed that the protonated form has a less negative
re
value of TE, resulting in a better chemical reactivity [71].
lP

Table 8
na

Quantum chemical descriptor of the neutral and protonated forms of Quercetin -3-glucuronide
major compound of LM ethanolic extract.
ur

Molecules EHOMO ELUMO ΔEgap μ ΔN110 TE

(eV) (eV) (eV) (D) (eV)
Jo

LM -5.609 -1.565 4.044 5.820 0.305 -48692.812

LM (+1) -7.793 -5.871 1.922 13.915 -1.047 -48701.247

In order to determine the local selectivity of the majority LM inhibitor molecule and its

protonated form, Fukui functions were calculated using Dmol3 at the level of the BOP/DNP

theory implemented in the Materials studio software version 8 [72]. From this computation,

we can identify the reactive sites that are responsible for the electrophilic and nucleophilic

attacks of the molecule under study in neutral and protonated forms. According to the

literature [71,73], a molecule that has high values of the function centers of Fukui, is
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considered chemically more reactive. For electrophilic and nucleophilic attacks, the

condensed Fukui functions are calculated according to the following equation [74]:

fi   qNi 1 and fi   qNi  qNi 1 (14)

Where qNi  neutral  , qNi 1  cationic  , and qNi 1  anionic  are the atomic charges of atom i atom

in the molecule and N refers to the number of electrons.

A further additional descriptor has been used to describe an instinctive and simple method of

chemical reactivity, the "dual descriptor" [75,76]. This was calculated as follows:

of
fi 2  fi   fi  (15)

ro
The values of all atomic sites of the Fukui functions ( f i  , f i  and fi 2 ) are given in Table 9.
-p
re
lP
na
ur
Jo

Table 9
Calculated Fukui Functions of the LM and LM (+1).
Atoms LM LM (+)

fi fi  fi 2 
fi fi  fi 2
C1 0.025 0.036 0.011 0.021 0.012 -0.009
C2 0.027 0.033 0.006 0.021 0.011 -0.010
C3 0.017 0.033 0.016 0.015 0.001 -0.014
C4 0.008 0.010 0.002 0.009 0.002 -0.007
C5 0.006 0.011 0.005 0.004 0.003 -0.001
C6 0.025 0.027 0.002 0.019 0.011 -0.008
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C7 0.025 0.069 0.044 0.018 0.002 -0.016

C8 0.061 0.037 -0.024 0.053 0.004 -0.049
C9 0.038 0.064 0.026 0.014 0.015 -0.001
O10 0.022 0.028 0.006 0.008 0.012 0.004
O11 0.052 0.011 -0.041 0.015 0.020 0.005
O12 0.079 0.091 0.012 0.041 0.026 -0.015
O13 0.021 0.027 0.006 0.018 0.005 -0.013
O14 0.030 0.036 0.006 0.028 0.016 -0.012
C19 0.031 0.017 -0.014 0.052 0.000 -0.052
C20 0.022 0.032 0.010 0.026 0.006 -0.020
C21 0.036 0.032 -0.004 0.045 0.003 -0.042
C22 0.035 0.023 -0.012 0.047 0.011 -0.036
C24 0.033 0.030 -0.003 0.039 0.008 -0.031
C26 0.048 0.046 -0.002 0.053 0.016 -0.037
C28 0.007 0.002 -0.005 0.005 0.005 0.000

of
C29 0.001 0.001 0.000 0.000 0.004 0.004
C31 0.003 0.002 -0.001 0.003 0.009 0.006

ro
C33 0.002 0.000 -0.002 0.005 0.026 0.021
C34 0.002 0.002 0.000 0.003 0.020 0.017
O38 0.002 0.001 -0.001 0.000 0.018 0.018
-p
O40 0.009 0.008 -0.001 0.009 0.029 0.020
O42 0.011 0.010 -0.001 0.008 0.014 0.006
re
C44 0.001 0.000 -0.001 0.016 0.235 0.219
O45 0.010 0.009 -0.001 0.012 0.035 0.023
O47 0.002 0.007 0.005 0.001 0.009 0.008
lP

O48 0.064 0.044 -0.020 0.080 0.021 -0.059

O50 0.037 0.024 -0.013 0.055 0.014 -0.041
O52 0.001 0.003 0.002 0.022 0.207 0.185
na

LM
ur

0.04 Acceptor centers

Jo

0.02
2
fi

0.00

-0.02
C7 C8 C9 O11 O 12 O14 C19 O48 O50

-0.04 Donor centers

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LM (+1)
0.20
Acceptor centers
0.15

0.10
2
fi
0.05

0.00

Donor centers
-0.05
C8 C19 C33 C44 O45 O48 O50 O52)

of
Fig. 15. Graphical illustration of the second-order Fukui function ( fi 2 ) for selected atoms in
LM and LM (+1).

ro
-p
For the neutral molecule, the results grouped in Table 9 show that the high values of f i  are
re
located on the C8, O11 and O48 atoms which are responsible for the electrophilic attacks

(electron donors). While the high f i  values of the C7, C9 and O12 atoms are capable of
lP

receiving electrons (nucleophilic attacks). For the protonated molecule, there is an appearance
na

of new, more electron-donating atoms such as C8, C19, C21, C26, O48 and O50, indicating

that the protonation influences the local electron-donating selectivity. On the other hand, the
ur

electron acceptor atoms are represented by the C44 and O52 atoms with the most intense
Jo

values of f i  , which shows that the acceptor power of the protonated molecule is more

privileged.

These data are also confirmed by Figure 15 for the highest electron acceptor sites. Table 9 and

Figure 15 reveals that the atoms C44 and O52, respectively, with fi 2 > 0, are the highest

electron

acceptor centers. While the atoms that have the highest electron donating regions are C8, C19,

O48, and O50, which are designated with fi 2 < 0. These results are in accordance with our

previous findings on the electron density distribution of FMOs.

 Journal Pre-proof

3.6.2. MD simulation

To obtain further details on the interaction between the target molecule and the iron surface, a

molecular dynamics (MD) simulation was undertaken [77]. As observed in Figure 16, LM has

a large capacity to recover a large percentage of the constructed surface area of Fe (110) in the

acidic medium at 303 K. This behavior is possibly due to the aromatic rings and the high

number of oxygen atoms, which cause a large protection of the metal surface. The dynamic

process was realized and the system as a whole reached equilibrium until the temperature and

energy of the system were in equilibrium.

of
ro
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re
lP
na
ur
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Fig. 16. Top (c) and side ((a) and (b)) views of the equilibrium configuration of LM adsorbed
on Fe (110) surface.
 Journal Pre-proof

The interaction energy (Einteraction) for molecule interaction with the metal surface can give as

follow [78]:

Einteraction  Etotal  ( Esurface solution  Einhibitor )

(14)

The Einteraction is calculated when the crystal systems are in equilibrium configurations. As

pictured in figure 17, the obtained Einteraction values are -197.749, -187.553 and -178.282

kJ/mol when the simulated temperatures are 303, 313, 323 and 333 K, respectively. The

of
concluded results in Figure 17 show that at all temperature values studied, the interaction

energies are negative, reflecting the adsorption process [79]. In general, a more negative

ro
Einteraction value implies a stronger interaction force between the most relevant atoms of the
-p
LM molecule and the iron atoms of the metal surface. It appears that Einteraction values increase
re
with increasing temperature, indicating that temperature affects the inhibitory performance of
lP

the molecule under study.

na
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 Journal Pre-proof

Fig. 17. Variation of interaction energy with the temperature (303, 313, 323 and 333 K) for
adsorbed LM molecule on Fe (110).

The rigidity of the adsorbed layer on the metal surface depends essentially on the nature of the

bonds detected between the LM molecule and the surface of the iron metal [80]. To reveal

this, we employed the radial distribution function (RDF) method based on the structural

analysis of the MD simulation results [81]. Figure 18 shows that the values recorded on the

first peak for each bond length of LM at simulated temperature 303 K are situated in the

of
chemisorption interval (1-3.5 Å) for the Fe-O11 and Fe-O47 bond lengths, while the Fe-O10

value is in the physisorption range (> 3.5 Å). These data show that both adsorption processes

ro
are present, confirming the results obtained experimentally.
-p
re
lP
na
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Fig. 18. RDF method of LM on Fe (110) surface in acid medium solution at 303 K obtained
via MD simulation.
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of
4. Conclusion

ro
This study gave us the opportunity to draw the following conclusions:
-p
 Lavandula mairei Humbert ethanolic extract was extracted and investigated as a new
re
corrosion inhibitor for mild steel in 1 M HCl solution. The obtained results using the
lP

three electrochemical techniques have depicted that LM ethanol extract acted as a

good corrosion inhibitor. Its inhibitory effect rises with augmentation of concentration
na

and reaches 92 % as a maximal inhibition efficiency at 0.4 g/L. Tafel curves have

shown that the LM ethanol extract acts as a mixed-type inhibitor. The adsorption
ur

mode of LM ethanol extract on metal surface comply with the Langmuir adsorption
Jo

isotherm.

 SEM and EDX analysis allowed us to observe that the inhibition properties of this

extract are associated to the creation of a protective film above the exposed sample

surface.

 Surface analyze XPS shows chemical interactions between the major molecule of LM

ethanol extract and mild steel exposed area. C=O and C-O-H group existence on the

sample surface indicates that the evaluated inhibitor was chemisorbed on the metallic

sample surface.
 Journal Pre-proof

 Quantum chemical parameters obtained from DFT method, showed a good correlation

between experimental data and the electronic properties of the inhibitor major

compound.

 MD simulation shows that LM has a high capacity to occupy a large percentage of the

surface area of Fe (110) in the acidic medium at 303 K, and the increase in

temperature has a negative influence on the adsorption configurations by increasing

the interaction energy.

of
Acknowledgement
ro
-p
The authors acknowledge the CNRST/Rabat/Morocco (Centre National de la Recherche
re
Scientifique et Technique) for the financial support.
lP

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Dear Editor-in-Chief,

This statement is to certify that all Authors of the article “Evaluation of Lavandula
mairei extract as green inhibitor for mild steel corrosion in 1 M HCl solution. Experimental
and theoretical approach” have been seen and approved the manuscript being submitted.
We warrant that the article is the Auhor’s original work. We warrant that the article has not
received prior publication and is not under consideration for publication elsewhere. No

of
conflict of interest exists, or if such conflict exists, the exact nature must be declared. On
behalf of all Co-Authors, the corresponding Author shall bear full responsibility for the
submission.
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All Authors agree that author list is correct in its content and order and that no
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modification to the author list can be made without the formal approval of the Editor-in-
Chief, and all Authors accept that the Editor-in-Chief’s decisions over acceptance or
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rejection.
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Thank you so much

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Yours sincerely
Jo

Ali DAFALI
Laboratory of Applied Chemistry and Environment
Department of Chemistry
Faculty of Science - University Mohammed Premier
B.P. 717, 60 000 Oujda - Moroco
e-mail: dafali2@yahoo.fr
Mob: +212 67793 1116
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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:

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Graphical Abstract

200

Blank
0.05 g/L
0.01 g/L
150
0.02 g/L
0.04 g/L

-Zim [ cm²]
100

50

0
0 50 100 150 200 250
Zre [ cm²]

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Highlights

 Lavandulamairei ethanol extract (LM) was used as a green corrosion

inhibitor for mild steel.

 SEM, EDX and XPSwere applied on the steel surface to investigate the

protective layer composition.

 Theoretical study confirmed the adsorption of extract molecules on the steel

surface.

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