Study of adsorption mechanism of chalcone derivatives on mild steel-sulfuric acid interface

Article abstract of DOI:10.1016/j.molliq.2020.113890

Chalcone derivatives namely: (E)-3-(4-methoxyphenyl)-1-(p-tolyl)prop-2-en-1-one (MTP), (E)-1,3-di-p-tolylprop-2-en-1-one (DTP), and (E)-3-(4-(diethylamino)phenyl)-1-(p-tolyl)prop-2-en-1-one (DEP) were synthesized and characterized by nuclear magnetic reso

Full text of DOI:10.1016/j.molliq.2020.113890

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Study of adsorption mechanism of chalcone derivatives on mild
steel-sulfuric acid interface
Banty Kumar, Hemlata Vashisht, Madhusudan Goyal, Ajai
Kumar, Fouad Benhiba, Ashok K. Prasad, Sudershan Kumar,
Indra Bahadur, Abdelkader Zarrouk
PII:
S0167-7322(20)32627-1
DOI:
https://doi.org/10.1016/j.molliq.2020.113890
MOLLIQ 113890
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
26 April 2020
12 July 2020
20 July 2020
Please cite this article as: B. Kumar, H. Vashisht, M. Goyal, et al., Study of adsorption
mechanism of chalcone derivatives on mild steel-sulfuric acid interface, Journal of
Molecular Liquids (2020), https://doi.org/10.1016/j.molliq.2020.113890
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Study of Adsorption Mechanism of Chalcone Derivatives on Mild
Steel-Sulfuric Acid Interface
Banty Kumara, Hemlata Vashishtb, Madhusudan Goyalc, Ajai Kumard,e, Fouad Benhibaf,g
Ashok K Prasada, Sudershan Kumar*,e, Indra Bahadur*,h, Abdelkader Zarroukg
aDepartment of Chemistry, University of Delhi, Delhi-110007
bDepartment of Chemistry, Kirori Mal College, University of Delhi, Delhi-110007
cDepartment of Chemistry, Pt. J.L.N. Government College Faridabad-121002, Department of
Higher Education-Haryana, India
dDepartment of Chemistry, Shri Venkateshwara University, Gajraula, UP-244236
eDepartment of Chemistry, Hindu College, University of Delhi, Delhi-110007
fDepartment of Chemistry, Faculty of Sciences, Ibn Tofail University, PO Box 133, 14000,
Kenitra, Morocco.
gDepartment of Chemistry, Faculty of Sciences, Mohammed V University, Av. Ibn Battouta,
P.O. Box. 1014 Agdal-Rabat, Morocco
hDepartment of Chemistry, Faculty of Natural and Agricultural Sciences, North-West
University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa
⁎ Corresponding authors: sudershankumar@hindu.du.ac.in, bahadur.indra@nwu.ac.za
Abstract
Chalcone derivatives namely: (E)-3-(4-methoxyphenyl)-1-(p-tolyl)prop-2-en-1-one (MTP),
(E)-1,3-di-p-tolylprop-2-en-1-one
(DTP),
and
(E)-3-(4-(diethylamino)phenyl)-1-(p-
tolyl)prop-2-en-1-one (DEP) were synthesized and characterized by nuclear magnetic
resonance (NMR), Mass spectroscopy (MS) and Infrared spectroscopy (IR). The corrosion
mitigation of mild steel (MS) in 0.5 M H2SO4 by these compounds has been investigated by
weight loss, electrochemical technique, surface analysis and theoretical studies. The
investigations were performed at three optimum concentrations (10–4, 10–5, and 10–6 M) at
three temperatures (298, 313 and 328 K) The weight loss Potentiodyanamic polarization
(PDP) and Electrochemical impedance spectroscopy (EIS) measurements showed that DEP has
the maximum inhibition efficiency (93.3% for highest concentration i.e 10–4 M) at 298 K
among all three inhibitors. The inhibition efficiencies vary directly and inversely with respect to
concentration and temperature respectively for all the additives. Potentiodynamic polarization
(PDP) measurements revealed that the chalcones are mixed-type corrosion inhibitors and
obeyed the Langmuir adsorption isotherms. Electrochemical impedance spectroscopy (EIS)
technique further proposes that the inhibitor molecules mitigate corrosion process by adsorbing

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at the metal/electrolyte interfaces. Further the adsorption of chalcones at the metallic surface
was supplemented by SEM and AFM studies where the decreased surface roughness of the
inhibited mild steel specimens was observed as compared to the uninhibited specimen.
Several thermodynamic parameters ( Ga0ds Ha0ds and Sa0ds ), DFT and MD theoretical
,
parameters were calculated to explain the adsorption behaviour of these compounds on
specimen surface.
Keywords: Corrosion inhibitor; chalcone; quantum chemical calculations; adsorption
isotherm.
1 Introduction
Metal corrosion is very old problem as far as human civilization is concerned and has been a
matter of investigation since ancient times [1-4]. It is always desirable to understand its
proper mechanism along with the solutions that prevent it at a tolerable level. With
modernization, use of metal structures increased enormously, the need of corrosion
prevention at the level of structural design was strongly felt. Enormous utilization of metals has
increased the loss share due to corrosion failure in various countries and could be 3-5% of
their GDP [5, 6]. Due to the mixed properties like flexibility high mechanical strength and low
cost, mild steel is among the most commonly used materials for building industrial structures
[7]. Sulphuric acid is an important mineral acid which is used in various industries with
different uses such as acid pickling, acid bath, descaling etc., therefore the effect of corrosion
inhibitors mainly in this medium remains the matter of investigation. Corrosion inhibitor is
among the most widely used practices to minimize the corrosion induced by mineral acids
[8]. Several type of corrosion inhibitors are used but one based on organic compounds are the
most operative, effective and economically viable due to their high adsorption nature and
synthetic route with cheaper reactants [9-14]. According to literature, hetero atoms (P, S, O
and N) and electron rich centres containing molecules act as good corrosion inhibitors [15-
18]. However, most of these organic inhibitors are found to be toxic and less eco-friendly and
have restricted use due to volatile nature and less solubility in polar solvents. Maximum
surface coverage, economic accessibility and environment friendly nature are the common
factors for selecting corrosion inhibitor [19-22].
Fascinating pharmacological properties are reported by Chalcones, flavonoids compounds
[23-24]. However, many research groups have reported chalcones derivatives as corrosion
inhibitor for different metals in acidic media, but most of the studies are performed in
hydrochloric acid medium [25-28]. This is first time that corrosion inhibition abilities of

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newly synthesised chalcone compounds with very low concentrations were tested in
sulphuric acid medium. The mitigating mechanism of few chalcone derivatives in acidic
medium have been reported by Li et al. [29,30]. However, the exact inhibition mechanism of
many chalcone derivatives is still unknown and a further deep analysis is required. Our
research group try to explore the possible inhibitory mode of action of newly synthesised
chalcones compound on the mild steel surface in sulfuric acid.
The inhibitive effects of MTP, DTP and DEP have been examined using weight loss,
electrochemical techniques and morphological studies. Quantum methods and computational
modelling method are also used to correlate theoretical understanding with experimental data.
2 Experimental section
2.1 Materials and methods
Chemicals were purchased from Sigma Aldrich and used without any purification. All
Solvents were purified and dried according to standard procedures.
The IR spectra were recorded on a Perkin-Elmer model 2000 FT-IR spectrometer by making
KBr disk for solid samples. For IR spectra, 5mg of the compound is taken. The 1H NMR and
the 13C NMR spectra were recorded on Jeol Delta 400 MHz and 100.6 MHz spectrometer,
respectively using tetramethylsilane (TMS) as internal standard and CDCl3 as solvent. For the
determination of the 1H NMR and 13C NMR spectra, the 5 mg, and 15 mg respectively of
compounds are taken and dissolved in CDCl3 to obtain a clear solution. The chemical shift
values are on δ scale and the coupling constant (J) are in Hz. HRMS analysis was carried out
using Agilent G6530AA LC Q-TOF mass spectrometer. Analytical TLCs were performed on
pre-coated Merck silica-gel 60F254 plates and the visualization of the developed plates was
performed by UV light.
Mild steel (MS) specimen having chemical composition (wt%) of C (1.62), Si (0.030), P
(0.020), Mn (1.00) and the balance was Fe was used to perform the corrosion inhibition
experiments. For Weight loss technique, MS coupons of 1cm× 1cm× 1cm were used while
for electrochemical techniques coupons having dimension of 1cm x 1cm x 3cm whose one
end was soldered with a copper wire and covered with epoxy resin araldite except one face
with surface area of 1 cm2 was used as the working electrode (WE). Prior to a new testing in
every technique, the MS specimen was polished with emery papers (grade 150 to 2000),
cleaned with acetone (CH3COCH3), rinsed with distilled water to scrap off impurities. The

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structure of investigating compounds (E)-3-(4-methoxyphenyl)-1-(p-tolyl) prop-2-en-1-one
(MTP), (E)-1,3-di-p-tolylprop-2-en-1-one (DTP) and (E)-3-(4-(diethylamino)phenyl)-1-(p-
tolyl)prop-2-en-1-one (DEP) are shown in Fig. 1 (a-c, respectively). Stock solutions of 1×10–4
M concentration of additives in 0.5 M H2SO4 solution were prepared by the calculated amount
of the solid compounds MTP, DTP and DEP. The lower concentrations, viz. 1×10–5 M and
1×10–6 M of these compounds were prepared by diluting the stock solution i.e. 1×10–4 M.
2.2 Synthesis and characterization
2.2.1 Synthesis of (E)-3-(4-methoxyphenyl)-1-(p-tolyl)prop-2-en-1-one (MTP), (E)-1,3-di-
p-tolylprop-2-en-1-one (DTP), (E)-3-(4-(diethylamino)phenyl)-1-(p-tolyl)prop-2-en-1-
one (DEP)
Chalcones were synthesized by previously reported procedure as given in Fig. 2, through base
catalyzed condensation of substituted acetophenones and substituted benzaldehydes [31]. To a
solution of acetophenone (0.01 mol) in ethanol (40ml), benzaldehyde (0.01 mol) derivative was
added. Then solution of aqueous KOH (60 %, 10 ml) was added with continuous stirring for 30
minutes. The mixing was continued for 2-3 hours at room temperature. After completion of
reaction which was confirmed by thin layer chromatography, ice cold HCl (25 ml, 10 %) was
added. The solid was obtained which was separated by filtration followed by washing with ice
cold water till neutralization. Finallyproduct was recrystallized with ethanol. All three chalcone
derivatives were characterized by NMR, IR and Mass Spectroscopy. NMR spectra of all
compounds are shown in Fig. 3 (a-c) and 13C NMR, IR and Mass Spectra of these compounds
are separately given in SI file.
MTP: Mol. Formula: C17H16O2; Mol. Wt. 252.1150; Yellow solid; Yield= 94 %; FT-IR
(CHCl3, cm-1): 3003, 1652, 810, 737, 672; 1H NMR (400 MHz, CDCl3) δ 7.92 (d, 2H), 7.77
(d, 1H), 7.59 (d, 2H), 7.41 (d, 1H), 7.30-7.25 (m, 2H), 6.93 (d, 2H ), 3.84 (s, 3H,), 2.42 (s,
13
3H) ppm; C NMR (100 MHz, CDCl3): δ 190.15, 161.67, 144.32, 143.44, 136.00, 130.24,
129.35, 128.65, 127.83, 119.90, 114.48, 55.48, 21.73 ppm; EM m/z (%) : 253 (M+1).
DTP: Mol. Formula: C17H16O; Mol. Wt. 236.1201; Yellow solid; Yield= 95 %; FT-IR
(CHCl3, cm-1): 3025, 1660, 807, 759, 735; 1H NMR (400 MHz, CDCl3) δ 7.93 (d, 2H), 7.78
(d, 1H), 7.55-7.47 (m, 3H), 7.29 (d, 2H), 7.22 (d, 2H), 2.43 (s, 3H), 2.39 (s, 3H) ppm; 13C
NMR (100 MHz, CDCl3): δ 190.24, 144.60, 143.59, 141.03, 135.88, 132.37, 129.77, 129.38,

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128.71, 128.52, 121.19, 21.76, 21.61 ppm; EM m/z (%): 237 (M+1).
DEP: Mol. Formula: C20H23NO; Mol. Wt. 293.1780; Yellow solid; Yield= 90 %; FT-IR
(CHCl3, cm-1): 3029, 1649, 806, 746, 683; 1H NMR (400 MHz, CDCl3) δ 7.91(d, 2H), 7.77
(d, 1H), 7.52 (d, 2H), 7.32-7.25 (m, 3H), 6.65 (d, 2H), 3.40 (q, 4H,), 2.42 (s, 3H), 1.19 (t, 6H)
ppm; 13C NMR (100 MHz, CDCl3): δ 190.30, 149.71, 145.58, 142.82, 136.65, 130.76,
129.21, 128.51, 122.04, 116.44, 111.37, 44.52, 21.65, 12.73 ppm; EM m/z (%): 294 (M+1).
2.3 Weight loss technique
The gravimetric study was evaluated at 298, 313 and 328 ± 2 K for all three studied
concentrations of chalcone compounds with average dipping time of 6 hours. For each set of
experiment, a freshly polished coupon with area 1cm2 was immersed into 100 mL of test
solution, same procedure was repeated in duplicate to get reproducible results. The corrosion
rate CR is calculated by dividing the weight loss, W (mg) of specimen by the product of Area,
A (cm2) of specimen and exposure time, t (hours) i.e CR=W/At [32]. The inhibition efficiency
IEWL (%) was determined by using the formula:
푪 −푪
푹
푰푬푾푳(%) =
푹(풊) × ퟏퟎퟎ
(1)
푪
푹
Where CR and CR (i) are the corrosion rate without and with the addition of the inhibitor in
acid solution respectively. The surface coverage (θ) of chalcones molecules on metal surface
in all techniques was calculated by using inhibition efficiency values according to the
following equation:
θ = IEWL( %)/100
(2)
Where IE is the inhibition efficiency of a studied additive.
2.4 Electrochemical methods
Three electrodes cell assembly consist of working, auxillary and reference, which was
assembled in a luggin capillary, cell assembly were used for electrochemical measurements.
The cell assembly was kept in a water thermostat for a fixed time to achieve an open circuit
potential (OCP). The measurements were done by using an electrochemical workstation CHI
760C (CH Instruments, Inc, USA). The Tafel graphs potential (E) – logarithm of current (log

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I) were obtained by this technique. The plots were recorded in the potential range –0.9 and +
0.0 V at a scan rate 1 mVs-1. By the extrapolation of these Tafel plots various corrosion
parameters were obtained such as corrosion potential (Ecorr in mV), corrosion current (Icorr in
mA/cm2), two slope values in mV/decade (βa and βc, respectively) and tabulated in Table 2.
Inhibition Efficiency (IEGPS %) was computed with concern surface coverage (θ) using the
relationships, in equations 3 & 4 [33]:
(3)
Icorr  Icorr(inh)
IEGPS (%) 
x100
ꢀ
Icorr
Ө푮푷푺 = ퟏ − ( 푰
)
(4)
푪풐풓풓 (풊풏풉)
푰
풄풐풓풓
Where Icorr and Icorr(inh) correspond to the corrosion current density (values in mA/cm2) in the
absence and presence of inhibitors in corrosive solution, respectively.
The Impedance studies were carried out just after the polarization measurements on the same
electrode system without any further surface modification by using the 10 mV sine wave as
the excitation AC signal at the OCP in the frequency range from 100 kHz to 0.1Hz. The
impedance descriptor charge transfer resistance (Rct values in Ωcm2) was assessed from the
semicircle in the Nyquist graphs. The inhibition efficiency for impedance (IEEIS %) was
determined from Rct according to equation (5) below [34]:
푹
−푹
풄풕(풂풄풊풅)
풄풕(풊풏풉)
푰푬푬푰푺 (%) =
× ퟏퟎퟎ (5)
푹
풄풕(풊풏풉)
The double layer capacitance, Cdl values were calculated according to the equation 6, [15]:
ퟏ
풇(−풁풎풂풙) =
(6)
ퟐ흅푪
푹
풅풍 풄풕
where f(-Zmax) is the maximum frequency on the imaginary impedance axis, and its values are
also listed in Table 3. Since the data is made to fit with the corresponding impedance values of
an equivalent circuit. The process was performed using the software ZSimpWin Version 3.21.
2.5 Morphological study (SEM and AFM)
The specimen coupons were immersed into 50 ml of inhibitors solution (MTP, DTP and DEP)
of higher (10–4 M) concentration prepared in 0.5 M H2SO4 for 24 h at room temperature.
Study was also done only in the presence of 0.5 M H2SO4. The coupons were withdrawn,
washed with deionised water, kept in a desiccator, and after that the SEM images were

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obtained with the JEOL JSM-6610 SEM instrument. The 3-Dimensional AFM analysis was
performed with a Nanosurf Easyscan2 instrument, model no. BT-02218.
2.6 Theoretical studies
The Quantum Chemical Calculations have been carried out by using the Hyperchem 8.0
software and structures optimization by semi-empirical AM1 Method to substantiate the
above information at the electronic level. Molecular dynamics (MD) simulation was applied
to evaluate the interaction and adsorption performance of selected chalcone derivatives on the
surface of Fe (110) using the Forcite module which is implemented in Materials Studio8 [35,
36].
3 Results and discussion
3.1 Effect of concentration on inhibition efficiency
The variation of corrosion inhibition efficiencies for all studied concentrations of inhibitors at
three temperatures are given in Table 1 and plots are given in Fig. 4(a), (b) and (c). It is
observed that percentage of IE increases with increase in the concentration of inhibitor.
Maximum IE% i.e.∼89% was shown by DEP inhibitor as compared to DTP (86%) and MTP
(85%) inhibitors at highest concentration (10-4 M). Hence DEP showed slightly
betterinhibition efficiency as compared to other two inhibitors and overall order of inhibition
is as follows: DEP > DTP > MTP. The high value of surface coverage (θ) leads to the fact
that adsorption of Chalcone molecules on the active site of mild steel surface is one of the
possible mechanisms of inhibition.
3.2 Galvanostatic polarization (GP) study
Polarization graphs for MS with (10–6 to 10–4 M) concentrations of MTP, DTP and DEP
inhibitors and without inhibitor were recorded at 298 K to 328 K and representative plots at
298 K depicted in Fig. 5 (a), (b) and (c) respectively. The various corrosion parameters were
obtained with the help of this study are tabulated in Table 2. The relative deflection of the
Ecorr values after the addition of an inhibitor as compared to the blank acid is largely gives the
indication about the anodic, cathodic or mixed-types behaviour of inhibitors. The swing of
Ecorr >85 mV towards either of the side is attributed blocking of anodic or cathodic site
accordingly by the inhibitor, while a change in Ecorr <85 mV implies that the additive is
mixed-type inhibitor, it means, it discontinues both mild steel at anodic half-cell and

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reduction of H+ ions at cathodic half-cell of corrosion circuit respectively [37]. The values of
corrosion potential obtained from this study are within the range of ±85 mV with respect to that
of the acid, both anodic and cathodic reaction are affected simultaneously and no particular
shift was spotted. This indicates that the all the used inhibitors are act as mixed-type of
inhibitors. The values of βc and βa, have been found to be irregular which indicates that
inhibition of corrosion of MS in acidic medium is not only due to adsorption [38] but there is
also involvement of anions and due to synergistic effect. This adsorption process of inhibitor is
also indicated by the decrease in current density values. With an increase of an inhibitor
concentration inhibition efficiency was also increases. It happens due to the presence of
conjugated π-electrons of chalcone which overlaps with the vacant d-orbitals of the metal
surface and excess of electron density donate back to the vacant π* orbitals (p-orbital) of
inhibitors resulting in a synergistic effect. A comparison of IEGPS (%) values of MTP, DTP
and DEP as shown in Table 2 reveals that DEP exhibits slight better inhibition of mild steel
corrosion than MTP, DTP throughout the studied concentration and temperature range.
Generally aromatic compounds have been observed better corrosion mitigation properties
than O-containing compounds due to the presence of conjugated π-electron rings which can
donate electron-density to vacant d-orbitals of Fe [39-42].
3.3. Electrochemical impedance spectroscopy
The corrosion behaviour of MS in 0.5 M H2SO4 solution in the absence and presence of
studied concentrations of all three additives at room temperature was further explored using
EIS electrochemical technique. The Nyquist plots for MS in 0.5 M H2SO4 without and with
various concentrations of the studied Chalcone compounds are shown in Figures 6 (a), (b)
and (c). The Nyquist plots show decrease in the radius of semicircles due to non-ideal
conduct of electrochemical interface on MS in the aggressive corrosive medium [43]. The
diameter of Nyquist plots increases with increase in the amount of inhibitors added in
corrosive soultion [44, 45]. This implys that the additives form defensive layer on the MS
surface thus rising the impedance of MS interface to the corrosion process. The EIS spectra
were fitted into the equivalent circuit shown in Figures 7 (a), (b) and (c), and the
electrochemical parameters obtained from the fitting and simulation analysis are listed in

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Table 3. The %IE The double layer capacitance, Cdl values were calculated according to the
equation 5 and 6 respectively and their values are also listed in Table 3. The Rct value may be
infered as the resistance created by inhibitor to the charge transfer ahead of the oxidation of
the MS. It is noticeable from the parameters given in Table 3 that with the increase in the
concentration of inhibitors the Rct values increases due to the formation of protective
defensive layer of inhibitor molecules on the MS surface thereby reducing the movement of
charged species across the interface. The Cdl values in the presence of the inhibitors are
generally lower than that of the blank acid system. This suggests a decrease in local dielectric
constant or an increase in the thickness of capacitive layer, which is attributed to the
adsorption of inhibitor molecules on the MS surface [46]. The %IE increases with increasing
concentration of the inhibitors but the trend of inhibitive performances obtained from the EIS
data cannot be generalized.
3.4 Adsorption isotherm
Various adsorption isotherms [47] were plotted to investigate the exact mechanism of
adsorption of inhibitors. From these isotherms various thermodynamic-kinetic parameters such
as Ga0ds
,
Ha0ds and Sa0ds were calculated.
By plotting the various isotherms it was obtained that the value of correlation coefficients (r2) for
the plots of Cinh/θ versus Cinh (Figure 9 (a), (b) &(c)) equal to or nearly 1 which directs that
Langmuir adsorption (LA) isotherms are followed by a specific adsorption method at an
applicable temperature for all inhibitors viz. MTP, DTP and DEP. The Langmuir adsorption
isotherm for an isotherm is denoted by following equation (Eq. 7) [48]:
ퟏ
푪풊풏풉/휃 =
+ 푪풊풏풉
(7)
푲
풂풅풔
The extent of adsorption of inhibitor with respect to total available site is called degree of
surface coverage, and is represented by θ, which can be obtained by dividing the inhibition
efficiency by 100. The value of θ lies between 0 and 1, i.e., 0< θ <1. In equation 7, Kads
indicates the adsorption equilibrium constant whose values were obtained from the intercepts
of the above plots. Gibbs Free Energy of Adsorption ( Ga0ds ) values were calculated from the

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equation 8;
풐
(
)
∆푮풂풅풔 = −푹푻풍풏 ퟓퟓ. ퟓ푲풂풅풔
(8)
Where; R and T denote the universal Gas Constant having value 8.314 JK–1mol–1 and the
absolute temperature in Kelvin respectively.
Physisorption ( Ga0ds ~ -20 kJ/mol) or chemisorption (Ga0ds ~ -40 kJ/mol), adsorption
behaviour of inhibitor molecules on metal surface is indicated by Ga0ds values having negative
sign which explain the spontaneity of the process. All inhibitors MTP, DTP and DEP are
showing chemisorption (values ranges from -40 to -50 kJ.mol-1) to mitigate corrosion on the
MS surface.
With the help of Van’t Hoff equation (Eq. 9) other thermodynamic parameters like enthalpy (
Ha0ds ) and entropy ( Sa0ds ) of adsorption were obtained.
−∆푯°
풍풏푲풂풅풔
=
풂풅풔 + 풄풐풏풔풕.
(9)
푹푻
Enthalpy of adsorption were evaluated by taking the slope ( Ha0ds /R) of the graph plotted
between natural logarithm of Kads against the reciprocal of T. The entropy of adsorption Sa0ds
further calculated by putting the values of Ga0ds andHa0ds in the following equations:
∆푮ퟎ풂풅풔 =−∆푯ퟎ풂풅풔 − 푻∆푺ퟎ풂풅풔
(10)
(11)
°
°
∆푯 −∆푮
∆푺°풂풅풔 =
풂풅풔
풂풅풔
푻
Generally, enthalpy values less than -40kJ/mol, is an indication of physiorption mechanism
whereas for chemisorption behaviour, the values are around -100kJ/mol and negative sign
shows the exothermic nature of this process. Calculated values of enthalpies of adsorption are -
78 kJ/mol, -62 kJ/mol and -63 kJ/mol for MTP, DTP and DEP, respectively, therefore all
inhibitors obey physiorption on the specimen. This may be explained due to electrostatic
attraction between positively charged oxidised metal surface and negatively charged species
formed due to the inhibitor molecules. The negative values of Sa0ds leads to the fact that the
intermediate species is formed due to adsorption than desorption process, which signifies that
no disordering occur in the reaction path [49]. The thermodynamic parameters such asGa0ds
,

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Ha0ds and Sa0ds obtained from Langmuir adsorption isotherm for MTP, DTP and DEP are
reported in Table 5. Further Arrhenius relationship was used to calculate the activation
energy (Eact) related with current rate [50, 51]:
−푬
풂풄풕
풍풐품 푰풄풐풓풓 = 풍풐품푨 − (ퟐ.ퟑퟎퟑ푹푻
)
(12)
The Arrhenius graphs of log of current density against 1000/T and plot of activation energy
versus concentrations of inhibitors MTP, DTP and DEP, respectively are plotted. The
activation energy has been calculated by the slopes of plotted graphs and the values are
tabulated in Table 5. The two modes via which an inhibitor protects the surface of the metal are
either by “blocking the geometry” effect that is by blocking the area available for reaction, or
by adjusting the activation energy of anodic and cathodic process on the uninhibited surface
during the inhibited corrosion reaction. Table 5 shows that Eact values are higher after the
addition of MTP, DTP and DEP inhibitors than in corrosive 0.5 M H2SO4 solution only shows
the reduced corrosion rates in the existence of inhibitors in corrosive medium.
3.5 Scanning electron microscopy (SEM)
Two dimensional SEM images of properly grinded metal coupons surface, and the surface of
specimens recovered from corrosive solution before and after the addition of highest
concentration i.e. 1×10–4 M of MTP, DTP and DEP, respectively,are shown in Figure 10 (a–
e). The well grinded plane specimen surface (Figure 10a) shows that it is homogeneous and is
free from roughness and pits whereas in the presence of acidic solution (Figure 10b) MS
coupon reveals destruction of uniform surface having pits and holes which is due to MS
dissolution and formation of plenty of corrosion products. However, a protective layer of
inhibitors with confined roughness on MS surface appeared which shows a significant
reduction in the addition of inhibitor in corrosive medium. Comparatively smoother surface
was depicted in the addition of DEP {Figure 10 (e)} than to seen in the presence of MTP and
DTP {Figure 10 (c-d)}. The corrosion inhibition is not only due to the conjugated pi-electrons
of phenyl rings, and electron donating tendency of heteroatom; here oxygen but also due to the
occurrence of synergistic ion also assists. Further it is concluded that inhibition effect of DEP
is higher than the other studied additives.

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3.6 Atomic force microscopy (AFM)
Three dimensional images of polished MS, in the absence and presence of inhibitor in studied
corrosive medium are shown in Figure 11 (a–e). The MS surface morphology was tested by
assessing roughness of specimen (RMS) and these values are tabulated in Table 6. The RMS
values of plane sample (Figure 11 a) and sample in acidic corrosive solution (Figure 11b)
were find out to be as 112.8 and 648.2nm, respectively. Noticeably showing that the
roughness of the 0.5 N H2SO4 exposed metal surface is very high and it shows an uneven
surface having rough lots due to extensive corrosion. Further it is observed that in the
presence of MTP, DTP and DEP mitigation of corrosion take place. The images depicted in
Figures 11 (c-e) shows that the inhibitors are forming protective layers on the MS surface
with adsorption as limited corrosion products formed after addition of 10-4 M inhibitors [52,
53]. The values of RMS are presented in Table 6 for MTP, DTP and DEP (184.3, 167.6 nm
and 141.2 nm) at 10-4 M, respectively, the trends obtained give the confirmationof increased
corrosion inhibition abilities of these inhibitor, and the slightly better protection of DEP in
comparison to MTP and DTP.
3.7 Quantum chemical (QC) calculation
In the light of experimental results for the selection of good inhibitor for MS, the quantum
chemical calculations have been carried out by using the Hyperchem 8.0 software and
structures optimization by semi-empirical AM1 Method to substantiate the above information
at the electronic level. Optimized geometries of inhibitors (Ball and stick model), mulliken
charges, EHOMO, the energy of highest occupied molecular orbital (ELUMO), the energy of
lowest unoccupied molecular orbital are shown in Figures 12-14, the gap energy (ΔE), the
dipole moment (µ) were calculated for the MTP, DTP and DEP inhibitors, respectively and
tabulated in Table 7. ELUMO tells the susceptibility of the inhibitor molecules towards hitting
by nucleophiles and frequently related to electron affinity (A). Smaller the ELUMO values are
the more will be the electron gaining tendencies of inhibitors. EHOMO is linked with electron
contributing capability of inhibitor molecules and the negative of EHOMO related to ionization
potential (I). The little energy difference between ELUMO and EHOMO i.e. ΔE makes the
possibility of transferring of the electrons from the inhibitor molecules to the atoms of the
iron very probable [54, 55]. The potential inhibition efficiencies increase if the MTP, DTP
and DEP have higher EHOMO energies and lower ELUMO energies and small ΔE as reported in
Table 7.
The calculated entire atomic Mullikan’s charges for all additives MTP, DTP and DEP

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molecules are depicted in Figure 12 (b), 10 (b) and Figure 14(b). By careful examination of
Figs., we reveal that the values of the electronegative charges of the oxygen, carbon atoms of
phenyl rings and ions which are present in the solution act as an active sites, whichjustify the
relationship between the donating electrons and inhibition efficiency [57,58]. The total energy
with negative sign (-70019.1, -62639.4 and -78482.6kcal/mol) clearly indicate that chalcone
compounds (MTP, DTP and DEP, respectively) are thermally stable and comparatively less
prone to break.
For the protonation of the molecules studied, the results obtained by the MarvinSketch
software show that the compound DEP is the only molecule that is favourable for protonation
in the nitrogen atom in the acidic medium. As shown in Table 7 that the protonation of the
DEP results in an increase of molecular reactivity, this is very clear, and ΔE, ELUMO and total
energy values are decreased. The effect of protonation has a negative influence on the
electrons donor power by the studied molecules towards the iron surface, therefore, EHOMO
value are decreased.
3.8 MD simulations
Molecular dynamics (MD) simulation was applied to evaluate the interaction and adsorption
performance of selected chalcone derivatives on the surface of Fe (110) using the Forcite
module [59]. The MD simulation was executed in a 32.270* 32.270* 34.134Å3 simulation
box with periodic boundary conditions, which was composed of the chemical species (with
inhibitory monomer + 20H3O+ + 10SO42- + 500H2O) and slab of Fe (110). The COMPASS
force field has been used [60]. All systems surveyed were carried out in the presence
Andersen thermostat at 298 K, NVT ensemble, with a simulation time of 400 ps and a time
step of 1.0 fs [61].
The results of the simulation are represented in Figure 12 and reported in Table 8. The
superior and lateral views displayed in Figure 15 show that the three chalcones derivatives
adhere to the metal surface through any part of the molecular structure, covering a large area
of iron. This result confirms that the molecules studied carry several electron donor and
acceptor sites, which facilitates their adsorption on the surface of the metal substrate. This
behaviour protects the metal surface against corrosion in the H2SO4 medium.
The two energy descriptors, i.e. interaction energy (E interaction) and binding energy (E binding),

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are calculated using the two equations below [62]:
푬풊풏풕풆풓풂풄풕풊풐풏 = 푬풕풐풕풂풍 − (푬풔풖풓풇풂풄풆+풔풐풍풖풕풊풐풏 + 푬풊풏풉풊풃풊풕풐풓
)
(13)
(14)
푬풃풊풏풅풊풏품 = −푬풊풏풕풆풓풂풄풕풊풐풏
The values of these descriptors are shown in Table 8, where it is noted that the negative
values of the E reflect the spontaneity of the adsorption process [63]. Generally, the
interaction
minimum value of E interaction reflects a higher interaction between the inhibitory molecule and
the metal surface, therefore the DEP molecule is well verified this finding [64]. This shows
that this molecule is considered to be a more efficient inhibitor against corrosion of the steel
being investigated. This inhibitory potential is possibly due to the existence of the mesomeric
donor electron group (N(CH3)2). The adsorption of an inhibitor molecule on the metal surface
was evaluated by the E binding such that the high value of this descriptor indicates much larger
adsorption [65] The increasing order of E binding is MTP<DTP<DEP, indicating that the DEP
inhibitor is a good and very effective candidate to best protect the metal surface.
Organic molecules carrying hetero atoms are able to protonate in acidic media [66]. In our
situation, the DEP molecule is the only one among the studied chalcone derivatives that
protone in the nitrogen atom in the chosen medium. Using MD simulation, we can assess the
degree of reactivity of protonated DEP with the atoms of the first layer of iron. Figure 15
represents the best adsorption configuration of protonated DEP on the Fe (110) surface. It is
clear from the side and top views that the entire structure of the DEP molecule adsorbs on the
surface of the iron, i.e. there is no change in the adsorption state compared to the neutral
form. This means that protonation does not change the conformation and configuration of the
DEP molecule, which shows that this molecule retains its molecular reactivity in relation to
the iron surface. Simulation results show that the E
value (-696.721/mol) is closer to
interaction

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that obtained for the neutral form of the adsorbed DEP molecule (-700.871 kJ/mol). This
indicates protonation does not affect the interaction behaviour of DEP/Fe (110) surface.
4. Mechanism of corrosion inhibition
By correlating literature with the results obtained from all techniques we can conclude that
the MTP, DTP and DEP act as mixed type of inhibitor by mitigating both anodic as well
cathodic mechanisms for MS corrosion in H2SO4 medium. This is an established fact that
organic compounds impede the metal corrosion due to adsorption on the surface. The
adsorption phenomena are mostly interpreted on the structure of the inhibitor molecule,
charge on the metal surface and the formation of metal solution interface. Therefore, the
expected mechanism for the adsorption of chalcones derivatives on the MS surface can be
proposed as follows: (a) the physisorption contacts between the protonated chalcone
2-
molecules and SO4 ions adsorbed on MS surface (electrostatic), (b) the chemisorption
behaviour of lone electron pair of heteroatoms (N and O) vacant d-orbitals of iron atoms, (c)
donor-acceptor exchanges of the π -electrons of aromatic ring of inhibitor to empty d orbital
of iron atoms.
Experimental outcomes give clear idea that the chalcone compounds act as effective against
the mild steel corrosion in 0.5 M H2SO4. This can be explained because chalcones possess
double bonds, aromatic rings and heteroatoms and these electron rich centres donate their
electron density to vacant d-orbitals of iron. Another possible interaction which can be denied
is interaction of protonated chalcones with the steel surface since MS carries positive charge
in acid solution so similar charges of protonated inhibitor and substrate repel each other.
Low degree of hydration due to the large size of sulfate ions, so number of water molecules
attached will be less, therefore electrostatic attractions of negatively charged end of water
dipoles attached with sulfate ions with positively charged MS surface is limited. Thus, overall
adsorption is due the combined effect of sulfate ions and inhibitor molecules. In addition,
these chalcones can adsorb on the mild steel via chemisorption, donating electrons from
nitrogen and oxygen atoms to iron surface by replacing water molecules.
5 Conclusions
Chalcone derivatives namely MTP, DTP and DEP are assessed as new and competent
inhibitors against the mild steel corrosion in 0.5 M H2SO4 solution through electrochemical,
morphological and quantum chemical analysis. The results obtained from these techniques
direct to the following outcomes:

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 MTP, DTP and DEP show that inhibition efficiency is directly proportional to the
concentration.
 MTP, DTP and DEP act effectively as a mixed type inhibitor.
 Weight loss outcomes are in a good agreement with electrochemical measurements.
 Langmuir adsorption (LA) isotherm is obeyed by all inhibitors. The interaction of hetero
atoms and π-electrons of phenyl rings with the mild steel surface is the main cause of
adsorption.
 SEM and AFM techniques showed the retardation of corrosion with addition of inhibitor.
Experimental and theoretical results complement each other to indicate that DEP is the best
inhibitor in the series studied.
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Figures
Figure1. Molecular structure of studied chalcones (a)(E)-3-(4-methoxyphenyl)-1-(p-
tolyl)prop-2-en-1-one (MTP)(b)(E)-1,3-di-p-tolylprop-2-en-1-one (DTP) and (c)(E)-3-(4-
(diethylamino)phenyl)-1-(p-tolyl)prop-2-en-1-one (DEP).
Figure 2. Schematic diagrams for the synthetic route of the chalcones derivatives.

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(a)
(b)

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(c)
Figure 3.1H NMR of chalcone derivatives (a) MTP (b) DTP and (c) DEP.
298 K
85
80
75
70
65
60
55
50
313 K
328 K
(a)
0.00000 0.00002 0.00004 0.00006 0.00008 0.00010
C (M)

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88
86
84
82
80
78
76
74
72
70
68
66
64
62
60
58
56
54
52
(b)
298 K
313 K
328 K
0.00000 0.00002 0.00004 0.00006 0.00008 0.00010
C (M)
(c)
90
85
80
75
70
65
60
55
50
298 K
313 K
328 K
0.00000 0.00002 0.00004 0.00006 0.00008 0.00010
C (M)
Figure 4. Effect of inhibitor concentration on corrosion inhibition efficiency for (a) MTP (b)
DTP and (c) DEP and at 298 K
0
(a)
-1
-2
-3
Tafel at 298 K
-4
10-4
10-5
10-6
M
M
M
-5
-6
0.5 M H2SO4
-7
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
E vs SCE (V)

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(b)
0
-1
-2
-3
-4
-5
-6
-7
At 298K Temp
10-4
10-5
10-6
M
M
M
0.5 M H2SO4
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
E vs SCE (V)
0
-1
-2
-3
-4
-5
-6
-7
(c)
At 298K Temp
10-4
10-5
10-6
M
M
M
0.5 M H2SO4
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
E vs SCE (V)
Figure 5. Galvanostatic polarization curves for MS in 0.5 M H2SO4 without and with various
concentrations: (…0.5 M H2SO4), (....10-4 M),(…10-5 M) and (...10-6 M) of (a) MTP, (b) DTP
and (c) DEP at temperatures 298 K.

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(a)
30
25
20
15
10
5
MTP at 298K Temp
0.5M H SO
2
4
-4
-5
10
10
10
M
M
-6
M
0
-5
-10
0
10
20
30
40
50
60
Z' (ohm.cm2)
(b)
80
70
60
50
40
30
20
10
0
DTP at 298K Temp
0.5M H SO
2
4
-4
-5
-6
10
10
10
M
M
M
-10
0
20
40
60
80
100
120
140
Z' (ohm.cm2)

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(c)
120
100
80
60
40
20
0
DEP At 298K Temp
0.5M H SO
2
4
-4
-5
-6
10
10
10
M
M
M
0
50
100
150
200
250
300
Z' (ohm.cm2)
Figure 6. Nquist Diagrams for MS in 0.5 M H2SO4 without and with various concentrations:
(…0.5 M H2SO4), (....10-4 M),(…10-5 M)and (...10-6 M) of (a) MTP, (b) DTP and (c) DEP at
temperatures 298 K.
(a)

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(b)
(c)
Figure 7. Nyquist Fitting of Experimental Data (MS in 0.5 M H2SO4 in the presence of 10-2
M of (a) MTP, (b) DTP and (c) DEP at temperatures 298 K.

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Figure 8. The Common Equivalent Circuit Corresponding to Experimental Data (MS in 0.5 M
H2SO4 in the presence of 10-2 M of (a) MTP, (b) DTP and (c) DEP
298K
(a)
313K
328K
C (M)
298K
(b)
313K
328K
C (M)

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298K
313K
328K
(c)
C (M)
Figure 9. Representative Langmuir adsorption isotherms for MS in 0.5 M H2SO4 containing
various concentrations of (a) MTP (b) DTP and (c) DEP at 298-328 K.
(a)
(b)
(d)
(c)