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ARTICLE

Newly synthesized quercetin derivatives as corrosion inhibitor for

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mild steel in 1 M HCl: combined experimental and theoretical

Received 00th January 20xx,
Accepted 00th January 20xx
investigation
DOI: 10.1039/x0xx00000x Dipankar Sukul,a** Aparesh Pal,a Sourav Kr. Saha,b,c Sanjoy Satpati,a Utpal Adhikari,a
www.rsc.org/ Priyabrata Banerjee,b,c*

To evaluate corrosion inhibition efficacy of derivatives of naturally available organics, mono and di- 4-(2-
hydroxyethyl)piperazin-1-yl)methyl derivatives of quercetin, a flavonoid, have been synthesized. Their potentialities as
anti-corrosive agents are assessed for mild steel in 1M HCl employing weight-loss technique as well as electrochemcial
methods. Comparing rate of corrosion in uninhibitied and inhibited solutions as a funtion of temperature, thermodynamic
parameters of adsorption of these derivatives on mild steel surface, and kinetic parameters of corrosion process are
evaluated. These parameters, together with information derived from electrochemical methods, are further used to
ascertain the mechanism of corrosion and mode of adsorption of inhibitors with intricate details. Density functional theory
(DFT) calculation is employed to explain relative corrosion inhibition propensity between the studied mono and di
quercetin derivatives. Molecular dynamics (MD) simulations are done for obtaining interaction energy between the
inhibitor molecules and metal surface. Results show both the derivatives, acting as mixed-type inhibitor, exert profound
anti-corrosive influence (around 95% inhibition efficiency at 1 mM concentration at 313K). Theoretical studies suggest
that trihydroxy chromone ring and dihydroxy phenyl ring of quercetin maintain a planar orientation with each other and
get adsorbed on metal surface (mostly chemisorption).

subsequently provide effective corrosion inhibition. Plant extracts

Introduction are rich in various classes of phytochemicals, among those
polysaccharides and polyphenols are mostly abundant. Flavonoids
Corrosion inhibition always remains a challenging task for scientists
are the largest family of polyphenolic compounds, whereas
from the consideration of safety, economics and environment.
quercetin is being the most abundant dietary flavonol. Literatures
Application of suitable corrosion inhibitor together with other
suggest that plant extracts rich in polyphenols provide good
methods of control, like cathodic protection, is one of the practical
corrosion inhibition, but exact contribution of polyphenols could
methods to combat the menace of corrosion. Inorganic inhibitors
never be accounted for due to presence of other phytochemicals,
based on heavy metal ions, which are otherwise highly efficient, are
like polysaccharides, alkaloids, glycosides (e.g. saponins) and
now being systematically replaced by organic inhibitors due to
others23-27. With this background, in this work we have intended to
concern of environmental hazard. Among organic inhibitors,
1-8 evaluate the efficacy of quercetin, the most important flavonoid, in
biocompatible green corrosion inhibitors, like polysaccharides ,
9-15 16-18 retarding the rate of corrosion of mild steel in 1M HCl.
amino acids and proteins , vitamins , and a variety of plant
19-22
extracts have received considerable attention recently. Quercetin is a plant-derived aglycone form of Rutin, a flavonoid
Heteroatoms such as O, N, and S present in bio-molecules, together glycoside28. Being a plant polyphenol, quercetin exerts high anti-
with unsaturation and aromatic moiety are mostly responsible for oxidant property28. Apple (with skin), raw onion, dry tea leaves and
adsorption of these bio-molecules on metal surface, which many others are rich in quercetin. But, at around room
temperature, quercetin is sparingly soluble in water29. To enhance
its solubility in aqueous solution, it has been derivatized into 6-((4-
(2-hydroxyethyl)piperazin-1-yl)methyl)-quercetin and 6,8-di-((4-(2-
hydroxyethyl)piperazin-1-yl)methyl)-quercetin (compound A and B,
respectively in Fig. 1). To best of our knowledge, these quercetin
derivatives have been prepared for the first time and their anti-
corrosive activities towards mild steel in 1M HCl are monitored
through electrochemical potentiodynamic polarization and

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OH
2.394 ppm (4H, methylene hydrogen atoms of piperazine ring);
OH 2.588 ppm to 2.619 ppm (4H, remaining methylene hydrogen atoms
N
of piperazine ring); centered at 3.482 ppm (6H, multiplate,
OH O N OH
OH methylene hydrogens attached to alcoholic OH group, methylene
OH O
hydrogen atoms attached to N atom outside the piperazine ring,

Physical Chemistry Chemical Physics Accepted Manuscript

OH
OH
N OH O OH benzylic H atoms); 3.884 ppm (4H, OH groups); 6.178 ppm (1H,
d a
N OH O H ); 6.9 ppm (1H, doublet, H ); centered at 7.647 ppm (1H, doublet
b c
N of doublet, H ); centered at 7.702 ppm (1H, doublet, H ) 9.392 ppm
N
(1H, broad singlet, -OH hydrogen); 12.532 ppm (1H, singlet, -OH
hydrogen atom).
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HO
OH
1
Compound A Compound B H NMR of Compound B (Fig S3)
Fig. 1 Chemical structures of studied Quercetin derivatives
2.338 ppm to 4.405 ppm (8H, multiplet, methylene hydrogen
atoms of one of the two piperazine rings), 3.448 ppm to 3.496 ppm
impedance spectroscopic techniques together with weight loss (6H, multiplet, four exocyclic methylene hydrogen atoms with
study. Detailed quantum chemical calculations and molecular respect to one piperazine ring and two methylene hydrogen atoms
dynamic simulation are done to corroborate the experimental of one hydroxymethyl group); 3.747 ppm to 3.758 ppm (8H, broad
observation. Scanning Electron microscopy (SEM) images of mild two line pattern, methylene hydrogen atoms of remaining
steel surfaces after exposure in the presence and absence of piperazine ring), 3.851 ppm to 3.882 ppm (6H, multiplet, remaining
inhibitors are compared to validate visually the experimentally set of four exocyclic methylene hydrogen atoms with respect to one
obtained result. piperazine ring and two methylene hydrogen atoms of one
a
hydroxymethyl group); doublet centered at 6.892 ppm (1H, H );
Experimental details b
quartet centered at 7.606 ppm (1H, H ), doublet centered at 7.734
c
ppm (1H, H ).
Synthesis and characterization of inhibitors
FTIR spectrum of quercetin (Fig S4 Q)
Synthesis of Compound A
Quercetin contains aromatic rings together with several phenolic
0.21 g of paraformaldehyde (75 mmol) (from Alfa-Aesar) was OH groups at different positions. This makes its FT-IR spectrum
-1
dissolved in 30 mL of methanol followed by the addition of 0.975 g complicated. Absorption peak centered at 3743 cm indicates the
of 2-piperazinyl ethanol (75 mmol) (from Alfa-Aesar). The mixture presence of –OH groups (O-H stretching) and concomitant
0 -1 -1
was allowed to warm at 60 C for 1.5 hrs. Quercetin dihydrate (2 g, appearance of absorption peaks at 1452 cm to 1381 cm and
-1 -1
82 mmol) (from Alfa-Aesar) dissolved in 30 mL of hot methanol was 1262 cm to 1012 cm is indicative of C-O-H bending and C-O
30 -1
added by a dropping funnel to the reaction mixture and the mixture stretching vibrations, respectively . A strong peak at 1664 cm is
was then allowed to reflux for another 2 h. The yellow solid for carbonyl group attached to aromatic ring. Aromatic C=C
-1
precipitate appeared after cooling the reaction mixture at room accounts for the appearance of peaks in the region 1664 cm to
-1 -1 -1
temperature was collected by filtration and purified by 1519 cm . Appearance of peaks at 941 cm to 822 cm can be
crystallization from hot methanol. Yield of compound A was 81%. explained taking out of plane C-H bending vibrations into
30
consideration .
Synthesis of Compound B
FTIR spectra of Compound A and B (Fig S4 A and B, respectively)
0.42 g of paraformaldehyde (150mmol) was dissolved in 40 mL of
methanol followed by the addition of 1.950 g of 2-piperazinyl No significant change in the nature of functional group occurred on
0
ethanol (150 mmol). The mixture was allowed to warm at 60 C for going to compound A from quercetin, except the appearance of
1.5 hrs. Quercetin dihydrate (2 g, 82 mmol) dissolved in 40 mL of quaternary amine moieties. Normally stretching vibrations for
hot methanol was added by a dropping funnel to the reaction aromatic C-N and aliphatic-N appear within the range of 1360 –
mixture and the mixture was then allowed to reflux for another 2 h. -1 -1
1310 cm and around 1410 cm respectively. These absorption
The yellow solid precipitate appeared after cooling the reaction peaks are merged with the peaks appeared for C-O-H bending
mixture at room temperature was collected by filtration and vibrations. Similar thing has happened to compound B. Because of
purified by crystallization from hot methanol (yield 83%). the similarities found in the FT-IR spectra of all these three
1
Quercetin and its derivatives A and B were characterized by H NMR compounds, it is very tedious to differentiate between quercetin,
using DMSO-d6 as the solvent (Figs S1-S3 in ESI), FTIR (Fig S4 in ESI), compound A and compound B solely on the basis of their FT-IR
and ESI Mass (Figs S5-S7 in ESI). spectra.
1
H NMR of Compound A (Fig S2) ESI-Mass

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ESI-mass data corresponding to the molecular ion peaks are found prepared by method already described, washed thoroughly with
+
for quercetin, compound A and B at m/z values [L-H ] of 303.0 (Fig distilled water and acetone, and finally dried under vacuum.
S5), 445.1 (Fig S6), 587.2 amu (Fig. S7), respectively. High Percentage inhibition efficiency,  (%) is calculated following the
abundance ratio of molecular ion peak confirms the formation of relation:
the respective quercetin derivatives.  
 % = × 

Physical Chemistry Chemical Physics Accepted Manuscript

(3)

Specimen and solution where, w0 and w are the weight loss of the metal coupons in acid
Cylindrical specimens were prepared by cutting commercially medium without and with inhibitor.
available mild steel rod (wt% composition: 0.22 C, 0.31 Si, 0.60 Mn,
0.04 P, 0.06 S and the remainder iron). Metal surfaces were then Surface morphology
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abraded using silicon carbide papers of 400-1600 grit and cleaned For visual inspection of the efficiency of inhibitor towards corrosion
thoroughly. HCl (1 M) solution was prepared using 35% HCl (GR inhibition of mild steel in acid medium, SEM images (Hitachi S-
grade, Merck India). 3000N microscope) of mild steel surface after immersion (for 6 h) in
1 M HCl in absence and presence of inhibitor are compared.
Electrochemical measurements
Computational details for quantum chemical calculation
Potentiodynamic polarization and electrochemical impedance
measurement were done using conventional three-electrode Density Functional Theory (DFT) using ORCA programme (version
system (model: Gill AC, ACM Instruments, UK) consisting of mild 2.7.0) is employed to extract various molecular parameters for
2
steel working electrode (WE) with exposed area of 0.25 cm , inhibitors in their respective energy optimized states. The geometry
platinum as counter electrode and saturated calomel electrode optimizations of all inhibitors are performed with B3LYP [28-30]
(SCE) as reference. Before electrochemical tests, the WE was kept in functional level of DFT employing triple-ζ quality basis sets TZV(P)
contact with 200 mL of 1 M HCl for 45 mins to achieve the steady with one set of polarization functions on the atoms like N, O and S.
state. For carbon and hydrogens like atoms, slightly smaller polarized
The potentiodynamic polarization plots were obtained within the split-valence SV(P) basis sets are used which are double-ζ quality in
range of potential ±250 mV from the respective rest potential at the valence region and have polarizing set of d functions on atoms
31,32
constant sweep rate of 0.5 mV/sec. Inhibition efficiency,  (%) is other than hydrogens . SCF calculations are converged tightly (1
−8 −7 −7
defined as: × 10 Eh in energy, 1×10 Eh in the density and 1 × 10 in
i −i (1) maximum element of the DIIS error vector). All theoretical
η P (%) = corr corr(inh) × 100
icorr calculations are done in aqueous phase, considering solvent as a
where, icorr and icorr(inh) are the values of corrosion current density continuum of uniform dielectric constant (ε) where solute is placed
(estimated by Tafel extrapolation method) of uninhibited and as a uniform series of inlocking atomic spheres31,32. In acidic
33
inhibited specimens, respectively. solution of 1M HCl, quercetin molecule remains in neutral form .
Electrochemical impedance (EIS) measurements were performed in Whereas, the attached piperazine ring is prone towards
the frequency range 100 kHz to 10 mHz with a.c. amplitude of ± 10 protonation in acidic solution. Considering these, we have
mV (rms) at the rest potential. Inhibition efficiency,  (%) is defined evaluated the energies and electronic distribution of the frontier
as: molecular orbitals (HOMO and LUMO) for quercetin derivatives in

  both neutral and protonated forms in aqueous system.
 % = × 100 (2)

Various intrinsic molecular parameters, like electron affinity (A),
0
where, Rp and Rp are the values of polarization resistance in ionization potential (I), electronegativity (χ, dictates the ability of a
presence and absence of the inhibitor. Rp represents a cumulative group of atoms to attract electrons towards itself), global hardness
effect of different resistances operating at the metal-electrolyte (η, measure of the resistance of an atom towards charge transfer)
interface, such as, charge transfer resistance, resistance due to and global softness (σ, susceptibility of inhibitor molecules towards
adsorbed corrosion products as well as inhibitors. All charge transfer), are calculated following equations34-36:
electrochemical tests were performed at room temperature of χ = (I+A)/2 (4)
around 303K. η = (I-A)/2 (5)
σ = 1/ η = 2/(I-A) (6)
Weight loss measurements I = -EHOMO (7)
A= -ELUMO (8)
Results derived from electrochemical techniques are verified with
The fraction of electrons transferred from the inhibitor molecule to
weight loss measurement. Further, effect of temperature on rate of
the metallic atom (ΔN) is calculated using the following relation [39-
corrosion in absence and presence of inhibitor is assessed following
41]:
this technique. Here, mild steel rectangular coupons (wt. %   
 = (9)
composition: 0.19 C, 0.21 Si, 0.21 Mn, 0.01 P, 0.01 S and the   

remainder iron) having dimension of 2.5 × 2.5 × 0.1 cm3 are

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Concept of ΔN is based on the assumption that between two the interaction energies between the studied quercetin derivatives
interacting systems of different electronegativities (here, the and Fe (1 1 0) surface. In this simulation process, interaction
metallic surface and an inhibitor molecule), the electron will flow between the studied molecules and iron surface is carried out in a
from the molecule of lower electronegativity to that of a higher simulation box of (39.47 Å × 39.47 Å × 77.23 Å) with periodic

Physical Chemistry Chemical Physics Accepted Manuscript

value, until the chemical potentials become the same. To calculate boundary condition to avoid any arbitrary boundary effect. Here,
ΔN, electronegativity of Fe (χFe) is replaced by the work function of ten layers of iron atoms are used to provide sufficient depth to
Fe (1 1 0) surface (most stable densely packed surface over other Fe overcome the issue related to cut-off radius. In this investigation, a
and the global hardness of iron,  is
11,31
surfaces),  = 4.82 eV three layered simulation box is created. The first layer contains Fe
34-36
taken as zero considering I = A for the metallic bulk . slab and the second layer is the solution slab which contains H2O
+ -
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(150 no.), together with H3O and Cl ions (15 no. each) as well as
inhibitor molecule and the remaining part of the box is the vacuum
+ -
Local reactivity analysis (Fukui indices) layer. Presence of H3O and Cl ions makes the MD simulation closer
to the real system. After construction of the simulation box, MD
The local reactivity of the molecules is analyzed through an simulation is carried out using the COMPASS (Condensed Phase
3
evaluation of the Fukui indices using Dmol module, Material Optimized Molecular Potentials for Atomistic Simulation Studies) ab
TM
studio version 6.1 by Accelrys Inc, San Diego, CA. All the initio force field. In general, the parameterization procedure is
calculation are performed applying B3LYP exchange correlation divided in two phases: (i) ab initio parameterization, (ii) empirical
function and the double numeric with polarization (DNP) basis set. optimization. The MD simulation is performed at 298.0 K using
Here Fukui functions are obtained through the finite difference canonical ensemble (NVT) with a time step of 1.0 fs and for a
approximation using Hirschfeld population analysis (HPA)37-40. By simulation time of 200 ps. All the bulk atoms in Fe (1 1 0) surface
invoking the HSAB principle in a local sense, the regions of a were kept frozen and all the concerned molecules are allowed to
molecule where the Fukui function is large are chemically softer interact with the metal surface freely during the entire simulation
than the regions where the Fukui function is small. Fukui function at process. The interaction energy (Einteraction) between the inhibitor
a point r in space around the molecule is introduced as the first molecule and the Fe (1 1 0) surface has been calculated by using the
31,32,41,42
derivative of electron density at that point with respect to number following equation :
of electrons N present in the molecule at a constant external
37-40
potential v . Einteraction = Etotal − ( Esurface +H O + H O + +Cl- + Einhibitor )
$%"
2 3 (15)
!" = # $
& (10)
' where the total energy of the simulation system is defined as Etotal,
+ -
As electron density is discontinuous with respect to number of energy of the iron surface together with H2O, H3O , Cl is classified
electron, a left hand and right hand side derivatives are introduced as -./01234546 5789: and that of the free inhibitor molecule as
as: Einhibitor.
$%" 
! " =# & (11)
$ '
$%" − Results
! " = # & (12)
$ ' Polarization measurements
+
where f can be used to probe the reactivity when electrons are Comparing the potentiodynamic polarization curves at
-
added to the system (attack of a nucleophile) and f to probe the different concentrations of mono and di-substituted quercetin
reactivity when electrons are extracted from the system (attack of derivatives (Fig. 2a and 2b, respectively), it is seen that both
an electrophile). the inhibitors act as mixed type. This is so as current densities
Taking the finite different approximation, a condensed form of corresponding to cathodic and anodic reactions decrease
these functions is proposed as37-40: distinctively from those for uninhibited specimen. Cathodic
and anodic Tafel slopes also do not reveal any systematic
!
) ≈ +)  +  − +)  (for nucleophilic attack) (13) variation with concentration, supporting the mixed type
10-13
!
) ≈ +)  − +)  −  (for electrophilic attack) (14) inhibitor characteristics . Further, corrosion potentials vary
within a very narrow range of ±10 mV only from that of
where, qk (N+1), qk (N) and qk (N-1) are the gross atomic population uninhibited one (Table 1). Table 1 further reveals that
(i.e. gross atomic charge) of the atom k in N+1, N and N-1 electron corrosion current densities for compound A and B at same
concentration values are very close. Inhibition efficiencies
systems, respectively.
calculated from corrosion current density values, as a result,
are also very comparable with slightly higher efficacy for
Molecular dynamics simulation
compound B.

Molecular dynamics (MD) simulation technique using Material

StudioTM software 6.1 (from Accelrys Inc.) is employed to evaluate

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Electrochemical impedance spectroscopy (EIS)

Physical Chemistry Chemical Physics Accepted Manuscript

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Fig. 2 Potentiodynamic polarization plot for mild steel in 1 M

HCl in presence of compound A (a) and B (b)
Fig. 3 Nyquist plots for mild steel in 1 M HCl in presence of
Table 1 Data from polarization studies for mild steel in 1M HCl compound A (a) and B (b)
in absence and presence of Quercetin derivative
Nyquist plots for different concentrations of compound A and B are
Inhibitor -Ecorr icorr βa -βc ηP% shown in figs. 3a and 3b, respectively. Corresponding Bode
conc. (mV/SCE) (µA cm-2) (mVdec-1) (mVdec-1)
(mM) impedance and phase angle plots for compound A and B are
depicted in figs. S8 and S9 (in ESI). For both compound A and B,
With compound A
increase in concentration results in enhancement in diameter of
0 507 1794 78.22 105.9
0.1 504 243 71.79 79.12 86.4 capacitive loop, reflecting increased resistivity towards charge-
0.25 510 213 69.74 77.22 88.1 transfer at metal-electrolyte interface. This justifies the adsorption
0.5 506 156 76.58 96.50 91.3 of inhibitors on the metal surface, which provides a non-conducting
1 507 119 76.89 80.46 93.4 layer on the metal surface, replacing the pre-adsorbed water
With compound B 10-13
molecules and ions present in the electrolytic solution . Further,
0.1 512 204 72.76 80.52 88.6
observed capacitive loops are not perfectly semi-circle, but
0.25 497 175 63.83 95.35 90.2
depressed under the real axis. This accounts for the surface
0.5 503 142 64.17 82.18 92.1
1 508 100 68.38 94.54 94.4 heterogeneity, which may be associated with un-even formation of
adsorbed layer on metal surface, involving both the inhibitor as well
43,44
Mixed type inhibition property is explained by enhanced as corrosion products . Obtained Nyquist and Bode plots
electron density at anodic sites on the metal surface by charge correspond to one-time constant only. This is evident from the
transfer from inhibitor towards metal, and depleted electron existence of only one capacitive loop in the Nyquist plots, one
density at cathodic sites on metal surface by virtue of retro- maximum in Bode phase angle plots, and one negative fluctuation
31,32
electron donation from metal to the inhibitor molecules. in the Bode impedance plots (Fig. 3, S8 and S9) . Considering all,
Presence of higher number of N atoms in compound B (and obtained Nyquist plots are fitted with an equivalent circuit depicted
hence more number of lone pair of electrons) than those of in Fig. 4. Rp stands for polarization resistance, significance of which
compound A should be responsible for relatively higher is already discussed. Rs corresponds to the resistance of the solution
inhibition efficiency exerted by the former. in-between the working and reference electrode. The constant
phase element (CPE) used in the equivalent circuit, instead of the

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Table 2 Data from EIS for mild steel in 1M HCl in absence and the variation of double layer capacitance (Cdl) with inhibitor
10-13,
presence of Quercetin derivatives concentration, it is recalculated using the following equation
31.32
:
Inh. Rp Q n Cdl ηz(%) χ2 1-n 1/n
Cdl= (Q.Rp ) (17)
Conc. (Ω (µΩ-1sn (µF ×103

Physical Chemistry Chemical Physics Accepted Manuscript

(mM) cm2) cm-2) All the fitted and calculated parameters are tabulated in table
cm-2)
2. Chi squared values (χ2) listed in table 2 is of the order of 10-3
With compound A
0 8.5 1090 0.82 174.1 4.45
indicating goodness of fitting the EIS plots, and thus validating
0.1 54.6 367 0.83 83.3 84.5 1.82 choice of the equivalent circuit. The Cdl values decrease with
0.25 72.5 240 0.83 57.7 88.3 2.57 increasing inhibitor concentration and this decrement is more
evident for compound B. This exemplifies enhanced
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0.5 90 201 0.82 53.9 90.6 3.23

1 118 231 0.84 53.1 92.8 2.99 hydrophobic nature
With compound B
0.1 60.1 234 0.81 60.9 85.9 4.56
0.25 76.5 204 0.81 56.4 88.9 4.13
0.5 99.1 198 0.82 54.3 91.4 6.11
1 129 134 0.82 38.9 93.4 3.93

double layer capacity (Cdl), accounts for the observed depressed Fig. 4 Equivalent circuit model used to fit the impedance spectra
semi-circle. CPE is related to its impedance as:

-1 -n
at the metal electrolyte interface, which is due to adsorption of
ZCPE = Q (iω) (16) organic inhibitor molecules on the metal surface replacing
surface adsorbed water molecules and other ions constituting
where, Q is a proportionality coefficient, ω is the angular frequency,
43,44 electrolytic solution. Like polarization experiment, inhibition
n is a measure of surface irregularity . CPE is reduced to the
efficiencies calculated from Rp values also reveal that propensity
classical lumped elements capacitor (C), resistance (R), and to reduce effect of corrosion is higher for compound B.
inductance (L) when n = 1, 0, and -1, respectively. To demonstrate

Table 3 Corrosion rate of mild steel in 1 M HCl in presence and absence of Quercetin derivatives at different temperatures
obtained from weight loss measurement

Temp. Inhibitor Corrosion rate (mg cm-2 h-1) ηW% Ө

(K) Conc. (mM)
Compound A Compound B Compound A Compound B Compound A Compound B
0 2.901
0.001 1.698 1.128 41.5 61.1 0.415 0.611
0.1 0.568 0.531 80.4 81.7 0.804 0.817

293 0.25 0.481 0.447 83.4 84.7 0.834 0.847

0.5 0.358 0.309 87.6 89.3 0.876 0.893
1 0.296 0.259 89.8 91.1 0.898 0.911
0 5.098
0.001 2.822 1.546 44.6 69.7 0.446 0.697

303 0.1 0.741 0.704 85.5 86.2 0.855 0.862

0.25 0.592 0.543 88.4 89.3 0.884 0.893
0.5 0.457 0.407 91.0 92.0 0.910 0.920
1 0.370 0.333 92.7 93.5 0.927 0.935
0 9.654
0.001 5.021 1.850 48.0 80.8 0.480 0.808
0.1 1.062 0.988 89.0 89.8 0.890 0.898

313 0.25 0.815 0.728 91.6 92.4 0.916 0.924

0.5 0.605 0.531 93.7 94.5 0.937 0.945
1 0.531 0.457 94.5 95.3 0.945 0.953
0 14.480
0.001 11.417 6.734 21.2 53.5 0.212 0.535
0.1 2.383 2.259 83.5 84.4 0.835 0.844

323 0.25 2.123 2.012 85.3 86.1 0.853 0.861

0.5 1.666 1.543 88.5 89.2 0.885 0.892
1 1.419 1.383 90.2 90.4 0.902 0.904

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Weight loss measurement

1.4 Compound A
Effect of temperature and immersion time
1.2
293 K
Weight loss measurement also corroborates the result obtained 1.0
303 K

Physical Chemistry Chemical Physics Accepted Manuscript

313 K
electrochemically, i.e. rate of corrosion of mild steel after 6h of

C/θ ( mM )
323 K
0.8
immersion in 1 M HCl at different temperatures decreases
considerably in presence of compound A and B with respect to 0.6

uninhibited specimen (table 3), and is more pronounced at higher 0.4

inhibitor concentration. It signifies inhibition of all forms of
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0.2
corrosion prevailing at the mild steel surface in contact with acid
solution (like, overall corrosion, galvanic corrosion as well as various 0.0

types of localized corrosion) due to adsorption of inhibitors on the 0.0 0.2 0.4 0.6 0.8 1.0
C ( mM )
metal surface. Further, increase in rate of corrosion with increase in
temperature justifies corrosion as activation controlled process.
Interestingly, inhibition efficiencies derived from weight loss data at 1.4 Compound B

various temperatures show that for both the compounds, it first 1.2
293 K
increases with temperature, reaches maximum at around 313K and 303 K
1.0 313 K
then starts to decrease. Quercetin derivatives exert anti-corrosive
323 K
effect towards mild steel for a considerable time of exposure (table 0.8

C/θ ( mM )
S1 in ESI). Upto 96 h at 303K, inhibition efficiency remains well
0.6
above 90% with 1mM concentration, suggesting the compactness
and acid resistivity of the inhibitor layer formed on mild steel 0.4

surface. 0.2

0.0
Adsorption isotherm study
0.0 0.2 0.4 0.6 0.8 1.0
C (mM)
To explain the nature of adsorption of quercetin derivatives on mild
Fig. 5 Langmuir Adsorption isotherms involving compound A (up)
steel surface in HCl medium, experimental data points from weight
and B (down)
loss measurement at different temperatures are fitted with
Langmuir adsorption isotherm, which is of the following form:
Fig. 5 shows fitting of experimental data points of the plot C/ ; vs. C
C 1 (18)
θ
= +C and correlation coefficient (r2) value of the order 0.999 with nearly
K ads
unit slope confirms applicability of Langmuir adsorption model
(Table 4). For both the inhibitors, ∆Gads0 values tend towards more
Here, C is the concentration of inhibitors used and ; be the degree
-1
of surface coverage at the corresponding inhibitor concentration, negative values with increase in temperature and exceed 40kJ mol
which is may be equated to at higher temperature range. For compound B, the value is slightly
more negative, reflecting better adsorption propensity over the
<= % other. Observed variation of ∆Gads 0 with temperature ascertains
;= (19)
>??
Kads is the adsorption constant and free energy of adsorption, ∆Gads
0 complex phenomena of adsorption, consisting of both
physisorption and chemisorptions11-13,45. Plotting ∆Gads
0 vs. T, we
is related to this by the relation:
? ?
get an idea of ∆ABCD and ∆EBCD as per the following equation:
0
∆ G ads = − RT ln( 55 .56 K ads ) (20) ? ? ?
∆FBCD = ∆ABCD − G∆EBCD (21)
55.56 being the molar concentration of water, R is the universal gas ?
Giving a linear fitting to the data points between 293-313K, ∆ABCD is
constant and T be the experimental temperature. -1
found to be positive (around 31 kJ mol ) (endothermic chemical

Table 4 Adsorption parameters in presence of Quercetin derivatives at different temperatures

Temp. (K) Slope R2 Kads×10-3 (mol-1) 0

∆ G ads (kJ mol-1 )

Comp. A Comp. B Comp. A Comp. B Comp. A Comp. B Comp. A Comp. B

293 1.1061 1.0904 0.9995 0.9995 78.802 85.397 -37.25 -37.45
303 1.0738 1.0649 0.9998 0.9998 117.371 127.226 -39.53 -39.73
313 1.0542 1.0458 0.9999 0.9999 179.211 194.175 -41.93 -42.14
323 1.1019 1.1013 0.9998 0.9998 97.182 128.535 -41.63 -42.38

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Table 5 Activation parameters in presence of various concentrations of Quercetin derivatives

Conc. (mM) λ × 10-6 (mg cm-2 h-1) E* (kJ mol-1) ∆H* (kJ mol-1) ∆S* (J K-1 mol-1)

Comp. A Comp. B Comp. A Comp. B Comp. A Comp. B Comp. A Comp. B

Uninhibited 137.4 43.05 40.49 -97.67

Physical Chemistry Chemical Physics Accepted Manuscript

0.001 969.03 44.51 49.33 43.08 46.79 40.54 -81.42 -107.03
0.10 1.54 1.55 36.40 36.57 33.85 34.01 -135.02 -134.97
0.25 1.70 1.74 37.15 37.42 34.59 34.86 -134.13 -134.00
0.50 1.88 3.08 38.11 39.67 35.55 37.11 -133.34 -129.25
1.00 2.66 5.48 39.40 41.55 36.86 38.99 -130.48 -124.38
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?
adsorption) with positive ∆EBCD value (around 230 J K-1 mol-1). But
?
fitting at higher temperature range (313-323K), ∆ABCD are seen to
-1 -1
be negative (-51 kJ mol for compound A and -34 kJ mol for
?
compound B). Associated ∆EBCD values become more and more less
-1 -1 -1 -1
positive (-30 J K mol for compound A and 24 J K mol for
compound B). Similar endothermic adsorption is seen for many
46,47
other organic inhibitor molecules . For example, adsorption of
hydrophobic surfactants on hydrophilic silica surface also exhibits
such phenomena47. This is associated with the dehydration process
during adsorption of inhibitor molecules on the metal surface.
Dehydration process is itself endothermic and also accounts for the
?
observed positive ∆EBCD value. At elevated temperature,
exothermic chemisorptions (as free energy of adsorption value
-1
exceeds 40 kJ mol value) becomes more dominating factor. This is
? ?
evident from the negative ∆ABCD values. In addition, ∆EBCD
becomes less positive and even negative. Thus, it is concluded that
at lower temperature range, overall adsorption is entropy
controlled, whereas at higher temperature it becomes enthalpy
driven.

Activation parameters

Kinetic-thermodynamic parameters are evaluated following the

variation of corrosion rate (CR) with temperature and employing
11-13,45,48
the Arrhenius equations :

N∗
HIJ KL = HIJ M − (22)
P.R?R S
S ∆Z ∗ ∆[ ∗
KL = WXY # & WXY # & (23) Fig. 6 Arrhenius plots for mild steel in 1 M HCl solution in
TU V S
absence and presence of compound A (up) and compound B
(down)
where E* is the activation energy of the corrosion process, λ is
the Arrhenius frequency factor (pre-exponential factor), R is the calculated E* and ∆H* for inhibited sample do not change much
universal gas constant, h is the plank’s constant, NA is Avogadro’s or decrease from those calculated for the uninhibited system11-
number, T is the absolute temperature, ∆S* is the entropy of 13,49
. Accordingly, obtained activation parameters support the
activation and ∆H* is enthalpy of activation. Slope and intercept inference drawn from adsorption parameters that
of the plot log CR vs 1/T provide E* and λ, respectively (Fig. 6, chemisorption is dominating over physisorption for these
Table 5). ΔH* and ΔS*, on the other hand, are evaluated from adsorbate-adsorbent pairs at higher inhibitor concentrations. At
the slope and intercept, respectively, of the plot of log (CR/T) vs these concentration levels, observed retardation in rate of
(1/T) (Fig. S10 in ESI, Table 5). Table 5 reveals that at very low corrosion can be explained in terms of substantial decrease in
inhibitor concentration, E* and ΔH* increase from those for Arrhenius frequency factor (λ). When inhibitor concentration is
uninhibited specimen. However, reverse trend is obtained with low and not able to cover the metal surface completely,
gradual increase in inhibitor concentration. In general, chemical physisorption dominates over chemisorption, providing higher
adsorption of the inhibitor molecules is predicted when the energetic barrier towards charge transfer. It is seen that for

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uninhibited sample, ∆S* is negative. This is associated with the plane (Fig. 8). Planar orientation of chromone and phenyl rings
mechanism of hydrogen evolution reaction (h.e.r.) in acidic makes them perfectly suitable for adsorption on the metal surface.
aqueous solution. Mechanism of h.e.r. involves slower Further, electronic distributions in HOMO and LUMO for both the
electrochemical discharge reaction (eq. 24) followed by either compounds A and B are distributed over these two rings (Fig. 8).
chemical recombination (at lower overpotential range, eq. 25) or

Physical Chemistry Chemical Physics Accepted Manuscript

This facilitates electron transfer from HOMO of the inhibitor
electrochemical desorption reaction (at higher overpotential molecules to vacant Fe 3d orbitals as well as retro-donation from
50,51
range, eq. 26) . filled 4s Fe orbital to the LUMO
31,32,41,42
. Transfer of electron from
+ -
inhibitor increases the electron density on the anodic sites of the
H3O + M + e →MHads + H2O (24) metal surface, thereby retards the rate of anodic metal dissolution
2MHads ↔ H2 + 2M (25)
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+ - reaction. On the other hand, retro-electron donation leads to

MHads + H3O e ↔ H2 + M + H2O (26)
deficiency of electron density on the cathodic sites for electronation
of hydrogen ions (cathodic hydrogen evolution reaction). Facile
Activated states of all these intermediate steps correspond to
two-way electro transfer thus accounts for the observed mixed type
decrease in randomness, which is reflected in the negative value
inhibition characteristics of the quercetin derivatives. In addition to
of ∆S*. In the presence of inhibitor molecule, ∆S* is found to be
even more negative from uninhibited sample (except for very adsorption on the metal surface, quercetin is also known for
2+ 52
low inhibitor concentration). Thus, the activated states of these complex formation with Fe ion present in the system . This may
2+
steps in presence of inhibitor become more orderly arranged. As result into a passive Fe -inhibitor layer formation on the metal
inhibitor blocks the cathodic reaction sites by an appreciable surface limiting further oxidation. Hydroxyethylpiperazine side
extent, adsorption of proton on metal surface and recombining chain, though not directly involved in charge transfer during
surface adsorbed hydrogen atoms require closer interaction, adsorption, may have a very important role to play on the overall
resulting in higher negative ∆S* value. adsorption process. Being an organic base, piperazine is very
susceptible to uptake proton from aqueous solution and remains in
Surface images

Optimised Geometry

Fig. 7 SEM images of mild steel surface in 1 M HCl solution in

absence (left) and presence of 1 mM compound B (right)

Comparing the surface electron microscopic (SEM) images of mild

steel surfaces after immersion in uninhibited and inhibited (1 mM
compound B) HCl solution for 6h, it is evident that quercetin
derivatives impart profound anti-corrosive effect. In presence of
inhibited solution, mild steel surface is very clean, devoid of any
HOMO
roughness or pit formation (Fig. 7). This supports the formation of
adsorption layer of quercetin derivatives wrapping mild steel
surface, which acts as an electrochemical barrier towards overall
corrosion process.

DFT calculation

Inhibitors in neutral state

LUMO
From the energy optimised molecular geometry for quercetin
Compound A Compound B
derivatives, it is observed that trihydroxy chromone ring and
dihydroxy phenyl ring maintain a planar orientation with each
Fig. 8 Optimised geometry and electron distribution in HOMO and
other, whereas hydroxyethylpiperazine side chain protrudes out of
LUMO for compound A and B in their neutral form

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protonated form. It is reported that pKa values for 1,4- This is facilitated by lower electronegativity value (χ) of compound B
dimethylpiperazine at ambient temperature of 298K are 8.4 and 3.8 than the other. Higher global softness (S) allows compound B to
53
in aqueous solution . On the other hand, mild steel in HCl bears a interact with other molecule more intricately. Dipole moment (µ) is
positive charge on its surface and Cl- ions together with water another important parameter which is responsible for better
54

Physical Chemistry Chemical Physics Accepted Manuscript

molecules remain adsorb on it . Protonated piperazine interacts electrostatic interaction between two interacting systems. Higher µ
-
with Cl ions electrostatically, thereby providing a close contact value of compound B suggests stronger electrostatic interaction
between the inhibitor molecules with metal surface. When inhibitor involving compound B and the charged metal surface11. Apart from
molecules approach sufficiently close to the metal surface, direct all these compound B with more number of substituent groups,
electron transfer involving HOMO and LUMO of quercetin have larger molar volume than compound A, and thus can provide
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derivatives with the metal surface become feasible. As a result, greater degree of surface coverage. Though values of various
quercetin molecules form an adsorption layer on the metal surface intrinsic molecular parameters are different for compounds A and
-
replacing the pre-adsorbed Cl and water molecules to certain B, these are not very far apart. This is manifested in the
extent by virtue of a combined effect of covalent and electrostatic corresponding inhibition efficiency values for both the compounds,
interaction. Such type of synergistic interaction is reported difference being very narrow.
11-13, 31,32
previously .
Local reactivity analysis
Comparing compound A and B, it is observed that increase in
To identify the atoms present in quercetin derivatives which are
number of hydroxyethylpiperazine side chain reduces the energy
gap between HOMO and LUMO. Generally, increase in HOMO locally reactive against nucleophilic and electrophilic attack, \] and
energy reflects easy of electron donation from this level, whereas \] for individual atoms present in both the derivatives are
decrease in LUMO energy is related with facile electron acceptance. computed following the method described in experimental section
and the values are tabulated in table S2 (in ESI). From the table it is
Thus, decrease in energy gap corresponds to favourable interaction
between a donor-acceptor system by both forward and retro- observed that O and C atoms involving trihydroxy chromone ring
donation of electrons. In this present case, it is observed that and attached dihydroxy phenyl ring of quercetin moiety possess
energies of both HOMO and LUMO increase, where extent of higher \] and \] values compared to atoms involving in the
increment for HOMO energy is relatively more than that of the substituent group. Thus, atoms of the quercetin ring are mostly
other. Thus, it may be concluded that though overall energy gap is responsible for interaction with metal surface through both the
decreasing going from compound A to B, effect of electron electron donation to metal (electrophilic attack at the respective
donation from inhibitor to metal is relatively more dominating than atom) and acceptance (nucleophilic attack at the respective
56,57
atom) . This is in complete agreement with the conclusion drawn
the retro-donation from the metal to inhibitor. This is also reflected from the analysis of electron distribution at HOMO and LUMO of
in the fraction of electron donation values (ΔN). According to Elnga the studied inhibitors. Among the atoms present in the substituent
et al., inhibition efficiency of corrosion inhibitors increases by virtue groups, only one N atom present in the piperazine ring, which is
of electron donation to the metal when ΔN is positive and its value closer to the quercetin ring, (N4 in compound A and N34 in
remains within 3.655. Thus, compound B has greater propensity compound B) possesses high value of \] , and thus offers some
towards electron donation to the metal compared to compound A.

Table 6 Calculated molecular parameters for Compound A and B from DFT study (parameters having their usual unit)
EHOMO ELUMO ∆E µ I=‒ A=‒
Inhibitors χ η σ ∆N
(eV) (eV) (eV) (Debye) EHOMO ELUMO

Neutral form

Compound A -5.7766 -2.1769 3.5997 3.70 5.7766 2.1769 3.9767 1.7998 0.5556 0.234

Compound B -5.6901 -2.1204 3.5697 5.45 5.6901 2.1204 3.9057 1.7848 0.5602 0.256

Mono-protonated form

Compound A -5.8950 -2.3522 3.5428 16.26 5.8950 2.3522 4.1236 1.7714 0.5645 0.197

Compound B -5.8674 -2.3056 3.5618 8.88 5.8674 2.3056 4.0865 1.7809 0.5615 0.206

Fully protonated form

Compound A -5.9376 -2.4128 3.5248 45.33 5.9376 2.4128 4.1752 1.7624 0.5674 0.183

Compound B -6.1563 -2.8040 3.3523 51.51 6.1563 2.8040 4.4801 1.6761 0.5966 0.101

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contribution towards electron donation to the metal surface. (a) (c)

Inhibitors in their protonated state

Physical Chemistry Chemical Physics Accepted Manuscript

As previously discussed, piperazine ring is highly susceptible to
accept proton from acidic solution. As more than one basic sites
are present in the inhibitor molecules, protonation may happen
upto various extent. Hence, it is important to investigate the
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optimized geometry, electron distribution in frontier molecular

orbitals and corresponding molecular parameters in different
protonated states to ascertain any possible effect of protonation (b) (d)

towards mode of adsorption of quercetin derivatives on mild

steel surface. For mono-protonated form, N4 of compound A
and N34 of compound B are selected for protonation, as these N
atoms are mostly reactive towards any possible attack by an
electrophile (table S2 in ESI). Electron distribution in HOMO and
LUMO and corresponding quantum chemical parameters for
both the compounds in their mono- and fully protonated states
(where all the N atoms present in piperazine rings of both
compound A and B are protonated) are presented in table 6 and Fig. 9 Equilibrium adsorption configurations of compound A (a
figs. S11-S12 (in ESI). In protonated states, trihydroxy chromone and b) and compound B (c and d) on Fe (1 1 0) surface obtained
ring and dihydroxy phenyl ring maintain planar orientation like by molecular dynamics simulation. Top: top view, Bottom: side
that in neutral form. Furthermore, electronic distribution in view.
HOMO and LUMO is also dispersed along this planar quercetin
MD simulation result towards adsorption of quercetin
moiety (figs. S11 and S12 in ESI). Thus, it may be concluded that
derivatives on Fe (1 1 0) surface in aqueous HCl medium
like the neutral form, mutual electronic interaction between
unambiguously demonstrates higher interaction energy
quercetin derivative and mild steel surface involves planar
between the Fe surface and compound B than that involving
quercetin group. Energies of HOMO and LUMO decrease with
compound A (table 7). This corroborates the experimentally
degree of protonation, and the effect is more pronounced for
obtained results. Further, distance between the C and O atoms
compound B. Energy gap between HOMO and LUMO for
present in quercetin ring and Fe surface for both the compounds
compound B in fully protonated form is much less than that of
is calculated and found well below 3.5 Å (Fig. 9 and S13 in ESI).
compound A in equivalent state. Furthermore, fully protonated
This suggests the formation of covalent type of bonding
compound B possesses higher dipole moment and global
between the inhibitor molecules and Fe surface involving the
softness. All these parameters indicate towards more effective 31,56,57
quercetin ring . However, N atoms present in attached side
adsorption of compound B over that of compound A in their
chain lay above the quercetin ring and the intermediate distance
protonated form. Thus, compound B is expected to deliver
(just higher than 3.5 Å) between these atoms and Fe plane rules
greater inhibitory action towards corrosion of mild steel in acidic
out the possibility of formation of strong covalent type bonding.
solution. In the mono-protonated state, fraction of electron
Close association of inhibitor molecules and metal surface is a
transfer for compound B is higher than that for compound A.
pre-requisite for effective adsorption, which will, in-turn,
But, in fully protonated state this trend is reversed. This is
provides high extent of inhibition efficiency for substantial time
attributed to the presence of higher positive charge in
of exposure in a corrosive environment. It is note-worthy that
compound B over the other in their fully protonated states.
under comparable simulation condition obtained interaction
Thus, it may be concluded that in mono-protonated form, effect
energy for benzimidazole based inhibitor molecules are within
of forward electron donation towards metal is the dominating
the range of 500-800 kJ/mol31,58. Thus, obtained interaction
factor during adsorption of inhibitor molecules and subsequent
energy in the range of 1100-1300 kJ/mol signifies that the
corrosion inhibition. In fully protonated state, as LUMO energy
flavonoid derivatives are in no way inferior to synthetic
of compound B is much lower than that of A, retro donation
benzimidazole based corrosion inhibitors. This also accounts for
from metal to inhibitor may play a bigger role in dictating overall
the comparable free energy of adsorption values for quercetin
adsorption process.
derivatives compared to benzimidazole derivatives.
MD simulation

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Table 7 Output obtained from MD simulation for adsorption of Acknowledgement

Quercetin derivatives on Fe (1 1 0) surface PB thanks Department of Science and Technology (DST), Govt. of
India for supporting research projects (SB/FT/CS-003/2012 and
-1
System Einteraction (kJ mol ) GAP-183112). SKS thanks DST, New Delhi, India for his DST

Physical Chemistry Chemical Physics Accepted Manuscript

+ -
‒1129.51 Inspire Fellowship. DS thanks DST-FIST (no. SR/FST/CSI-
Fe(1 1 0)+H2O+H3O +Cl +Compound A
+ -
267/2015) for infrastructural development of the department.
Fe(1 1 0)+H2O+H3O +Cl +Compound B ‒1390.97

Conflict of Interest
Conclusion There is no conflict of interest to declare.
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It is observed that biomolecules suitably derivatized has the

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