Design and synthesis of a novel corrosion inhibitor embedded with quaternary ammonium, amide and amine motifs for protection of carbon steel in 1 M HCl

Article abstract of DOI:10.1016/j.molliq.2020.113917

Methyl 11-bromoundecanoate [Br (CH₂)₁₀CO₂Me] (1) on treatment with tripropylamine gave quaternary salt [Pr₃N⁺(CH₂)₁₀CO₂Me]Br^? (2) which on treatment with diethylenetriamine afforded [Pr₃N⁺(CH₂)₁₀CONH(CH₂)₂NH(CH₂)₂NH₂] Br^? (3) containing inhibitive motifs of ammonium, amide and amine motifs embedded in a single frame. The precursor salt 2 and its derivative 3 were successfully synthesized in excellent yields and characterized using different spectroscopic techniques. For the first time, a detailed study on the corrosion inhibition behavior of corrosion inhibitors 2 and 3 for mild steel in 1 M HCl was carried out using electrochemical measurements and comprehensive computational analysis. Both the studied inhibitors showed excellent aqueous solubility. The high inhibition efficiency of 91percent and 93percent at a concentration of 200 mg L^?1 was obtained for corrosion inhibitors 2 and 3, respectively. The adsorption of the corrosion inhibitors exhibited the Langmuir isotherm with a mixture of physical and chemical modes of adsorption. The impedance studies showed a rise in the polarization resistance with increasing concentration of the inhibitors. Polarization measurements demonstrated that the inhibitors displayed a mixed-mode of inhibition with primarily cathodic nature. Surface analytical studies supported the inhibitor adsorption and a protective film formation on the carbon steel surface, which improved the surface smoothness of the steel surface. The DFT based quantum chemical calculations supported the experimentally obtained results and showed that the inhibitor 3 displays superior inhibition in comparison to the inhibitor 2. The Monte Carlo simulations revealed higher adsorption energy for the inhibitor 3 compared to 2.

Full text of DOI:10.1016/j.molliq.2020.113917

Journal Pre-proof
Design and synthesis of a novel corrosion inhibitor embedded
with quaternary ammonium, amide and amine motifs for
protection of carbon steel in 1M HCl
Dheeraj Singh Chauhan, Mumtaz A. Quraishi, Mohammad A.
Jafar Mazumder, Shaikh A. Ali, Norah A. Aljeaban, Bader G.
Alharbi
PII:
S0167-7322(20)32403-X
DOI:
https://doi.org/10.1016/j.molliq.2020.113917
MOLLIQ 113917
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
17 April 2020
13 June 2020
23 July 2020
Please cite this article as: D.S. Chauhan, M.A. Quraishi, M.A. Jafar Mazumder, et al.,
Design and synthesis of a novel corrosion inhibitor embedded with quaternary ammonium,
amide and amine motifs for protection of carbon steel in 1M HCl, Journal of Molecular
Liquids (2020), https://doi.org/10.1016/j.molliq.2020.113917
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2020 Published by Elsevier.

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Design and synthesis of a novel corrosion inhibitor embedded with quaternary
ammonium, amide and amine motifs for protection of carbon steel in 1M HCl
1
1
2
Dheeraj Singh Chauhan , Mumtaz A. Quraishi , Mohammad A. Jafar Mazumder* ,
2
3
3
Shaikh A. Ali , Norah A. Aljeaban , Bader G. Alharbi
1
Center of Research Excellence in Corrosion, King Fahd University of Petroleum & Minerals,
Dhahran 31261, Saudi Arabia.
2
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi
Arabia.
3
EXPEC ARC, Saudi Aramco, Dhahran 31311, Saudi Arabia.
*
Corresponding author
E-mail address: jafar@kfupm.edu.sa
Tel.: (966)13 860 7836; Fax: (966) 13 860 4277
Home Page: http://faculty.kfupm.edu.sa/CHEM/jafar/
1

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Abstract
Methyl 11-bromoundecanoate [Br(CH ) CO Me] (1) on treatment with tripropylamine gave
2
10
2
+
-
quaternary salt [Pr N (CH ) CO Me]Br (2) which on treatment with diethylenetriamine
3
2 10
2
+
-
afforded [Pr N (CH ) CONH(CH ) NH(CH ) NH ] Br (3) containing inhibitive motifs of
3
2 10
2 2
2 2
2
ammonium, amide and amine motifs embedded in a single frame. The precursor salt 2 and its
derivative 3 were successfully synthesized in excellent yields and characterized using different
spectroscopic techniques. For the first time, a detailed study on the corrosion inhibition behavior
of corrosion inhibitors 2 and 3 for mild steel in 1M HCl was carried out using electrochemical
measurements and comprehensive computational analysis. Both the studied inhibitors showed
excellent aqueous solubility. The high inhibition efficiency of 91% and 93% at a concentration
-1
of 200 mg L was obtained for corrosion inhibitors 2 and 3, respectively. The adsorption of the
corrosion inhibitors exhibited the Langmuir isotherm with a mixture of physical and chemical
modes of adsorption. The impedance studies showed a rise in the polarization resistance with
increasing concentration of the inhibitors. Polarization measurements demonstrated that the
inhibitors displayed a mixed-mode of inhibition with primarily cathodic nature. Surface
analytical studies supported the inhibitor adsorption and a protective film formation on the
carbon steel surface, which improved the surface smoothness of the steel surface. The DFT based
quantum chemical calculations supported the experimentally obtained results and showed that
the inhibitor 3 displays superior inhibition in comparison to the inhibitor 2. The Monte Carlo
simulations revealed higher adsorption energy for the inhibitor 3 compared to 2.
Keywords: Amidoamine; Quaternary ammonium salt; Corrosion inhibitor; Impedance; DFT
study.
2

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1
. Introduction
Alloy steels are commonly used in the industrial processes of acidizing, descaling, and
acid pickling on account of their low cost and good mechanical properties [1-3]. However, these
processes involve the use of concentrated mineral acids, namely hydrochloric and sulfuric acids,
which cause severe corrosion of the steel structures [3-5]. To counter the huge losses to the
economy and to avoid potential structural failure, which could harm human life, the acidic
solutions are mixed with corrosion inhibitors [6-8]. The corrosion inhibitors undergo adsorption
on the metallic surfaces and develop a protective film, which reduces the corrosion damage [9].
Most of the corrosion inhibitors are organic molecules containing an abundance of heteroatoms
(
N, S, O),  bonds, and phenyl rings. The organic compound based corrosion inhibitors are
mostly heterocyclic molecules having an abundance of functional groups and phenyl rings [2,
0]. These structures, although they are expected to aid in the capability of adsorption on the
1
metal surfaces and can donate/ accept electrons; however, the poor solubility of these organic
compounds, high cost of synthesis, and toxicity limit their applicability in the corrosion
inhibition.
In this context, the quaternary ammonium salt-based corrosion inhibitors present a potent
and environment benign category of corrosion inhibitors [11-14]. Nowadays, research and
development in the area of quaternary ammonium salt-based corrosion inhibitors are gaining
attention due to the attractive structural features of this class of compounds. The quaternary
ammonium salts exhibit high solubility, are environmentally benign, and show effective
adsorption behavior due to their charged structures [15-17]. These molecules considerably
decrease the extent of corrosion damage occurring at both the cathodic as well as the anodic
active corrosion sites. It is noteworthy to mention that in the case of heterocyclic corrosion
inhibitor compounds, in addition to the active corrosion inhibitor constituent, it is required to add
another synergistic agent to improve the inhibition efficiency and surfactant molecules to aid in
the dissolution [18, 19]. On the other hand, the quaternary ammonium salts possess a
combination of a cation and an anion, which can be expected to provide synergistic improvement
in the corrosion inhibition.
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Continuing our research work in the area of the organic corrosion inhibitors, we designed
and synthesized an inhibitor molecule 3 containing three inhibitor motifs of quaternary
ammonium, amide, and amine in a single frame (Scheme 1). The molecule of type 3 synthesized
using cheap and readily available commercial precursors and tested for the first time as a
corrosion inhibitor. The corrosion evaluation was carried out in 1M HCl solutions by the
electrochemical measurements, which were supported by the surface analysis. The experimental
results were supplemented with the computational studies carried out using the density functional
theory (DFT) based on quantum chemical calculations. The two quaternary ammonium salts
appear as potent candidates to be used in the acid pickling industry as corrosion inhibitor
formulations.
2
. Experimental
2
.1 Materials
1
1-Bromoundecanoic acid (99%), Tripropylamine ( 98%), and diethylenetriamine
(
DETA) (99.5%) were purchased from Sigma Aldrich. Sodium bicarbonate (NaHCO ), sodium
3
chloride (NaCl) and sodium sulfate (Na SO ) were obtained from BDH chemical Ltd. Methanol,
,
2
4
acetonitrile, diethyl ether, and acetone were procured from Fluka Chemie AG. Hydrochloric acid
37%) and sulfuric acid (98%) were purchased from Fisher Scientific. All chemicals used in this
(
study were reagent grade and used without further purifications. Milli-Q system was utilized to
produce deionized water. Before use, all glassware was cleaned using de-ionized water and
dried.
2
.2 Characterization of the corrosion inhibitors
1
H NMR spectra were acquired in CDCl using Bruker 400 MHz (Rheinstetten,
3
-
Germany) instrument. FT-IR (Perkin Elmer 16F PC FTIR) instrument (spectral resolution, 4 cm
2
;
number of scans: 32) was utilized to acquire the FTIR spectra. Carlo-Erba elemental analyzer
(Model: NA-1500) conducted the elemental analysis. Melting points are recorded in a calibrated
Electrothermal-IA9100- Digital Melting Point Apparatus using heating rates of 1 C/min near
the melting points. Thermogravimetric analysis (TGA) was performed by a thermogravimetric
analyzer from TA instruments (SDT Q600, USA) under N (flow rate 50 mL/min) using
2
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Platinum/Platinum–Rhodium using step-up temperature 10 °C/min at the temperature range 20–
4
00 °C.
2
.3 Synthesis of the corrosion inhibitors
The synthetic process of precursors and corrosion inhibitors is illustrated in Scheme 1.
2
.3.1 Synthesis of methyl 11-bromoundecanoate (1)
The precursor compound 11-bromoundecanoic acid methyl ester (1) was synthesized
following the modified procedure published elsewhere in the literature [20]. Briefly, 11-bromo-
undecanoic acid (25 g, 94.2 mmol) was dissolved in 250 mL of methanol. 5 mL H SO was
2
4
added slowly to the reaction mixture while stirring. The system was refluxed at 80 C for 18 h in
an oil bath with continuous stirring. After completion of the reaction, the crude reaction mixture
was cool, and excess methanol was removed by N purging in a hot water bath at 60 C. 150 mL
2
of de-ionized water was added to the reaction mixture. The organic layer was extracted by
diethyl ether (250 mL), followed by 125 mL of 5% NaHCO , 125 mL of water, and 125 mL of
3
1
5% NaCl. Finally, the organic layer was dried with Na SO , and then diethyl ether was
2 4
evaporated by purging N to obtain 11-bromo- undecanoic acid methyl ester, 23.9 g (91%).
2
Elemental composition of C H BrO is as follows: calculated; C, 51.6; H, 8.3; found; C,
1
2
23
2
1
5
1.3; H, 8.5. H NMR: 훿 (CDCl ) 1.30 (10H), 1.41 (2H), 1.63 (2H), 1.86 (2H), 2.32 (2H), 3.40
H 3
(2H), 3.68 (3H), FTIR: max (neat): 3460, 2923, 2858, 1741, 1149, 1366, 1171, 1111, 1009, 869,
-1
7
27, and 645 cm .
2
.3.2 Synthesis of quaternary salt 2 [11-methoxy-11-oxo-N,N,N tripropylundecan-1-aminium
bromide]
The precursor compound 1 (20.2 g, 72.2 mmol) was dissolved in 50 mL of acetonitrile,
and then 20.7 g (145 mmol) of tripropylamine was added slowly to the reaction mixture while
stirring. The reaction mixture was refluxed at 90 C for 20 h in an oil bath fitted with a long neck
condenser with continuous stirring. After the time elapsed, the reaction mixture was cool, and
excess acetonitrile was removed by N purging in a hot water bath at 60 C. The resultant crude
2
reaction mixture was then diluted with 5 mL of acetone, followed by the addition of diethyl ether
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(25 ml) results in the formation of precipitates. The precipitate was separated and was washed
twice with acetone and diethyl ether. Then acetone was evaporated by purging N and store in a
2
,
refrigerator to obtain a corrosion inhibitor 2 as crystal solid. Yield is 25.3 g (83%).
Melting point: 62-65 C. Elemental composition of C H BrNO is as follows:
2
1
44
2
1
calculated; C, 59.7; H, 10.5; found; C, 59.9; H, 10.4. H NMR: 훿 (CDCl ) 0.95 (9H), 1.16- 1.25
H
3
(12H), 1.47-1.68 (10H), 2.21 (2H), 3.25 (8H), 3.55 (3H), FTIR: max (neat): 2942, 2863 (CH3,
CH stretching), 1733 (C=O stretching), 1632, 1471 (O-CH stretching), 1181 (C-O stretching),
2
3
-1
9
74, 856, 757 and 569 cm .
2
.3.3 Synthesis of inhibitor molecule 3 [11-((2-((2-aminoethyl)amino)ethyl)amino)-11- oxo-
N,N,N tripropylundecan-1-aminium bromide]
The new inhibitor compound 3 was synthesized following a modified procedure for
amide synthesis [21]. The compound 2 (2.94 g, 6.91 mmol), DETA (3.03 g, 29.4 mmol), and
methanol (50 mL) were added into a 200 mL round bottom flask. Then reflux the reaction
mixture at 80 C for 20 h, followed by the evaporation of methanol, the reaction mixture was
stirred for an additional 20 h at 90 C. After completion of the reaction, the reaction mixture was
cool to room temperature, and 5 mL methanol was added to dissolve the reaction mixture. The
organic compound was then extracted using diethyl ether (3 × 50 mL). The extraction process
using methanol and diethyl ether was repeated twice. Finally, methanol was evaporated to obtain
3
and dried under vacuum at 60 C for 4 h. The yield was 3.24 g (95%).
Melting point: 135-139 C. Elemental composition of C H BrN O is as follows:
2
4
53
4
1
calculated; C, 58.4; H, 10.8; found; C, 60.1; H, 10.5. H NMR: 훿 (CDCl ) 0.99-1.04 (9H), 1.16-
H
3
1
.32 (12H), 1.56-1.77 (12H), 2.10-2.25 (3H), 2.63-2.78 (6H), 3.26-3.44 (10H), 6.98 (NH), FTIR:
max (neat): 3595 (NH ; amide, NH stretching), 2919, 2858 (aliphatic CH stretching), 1670
2
2
(amide, C=O stretching), 1551(amide, N-H stretching), 1465, 1311, 1125, 965, 858, 756, and
-1
5
79 cm .
2
.4 Corrosion Studies
2
.4.1 Mild steel sample
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2
A carbon steel working electrode having a surface area of 1 cm was employed for the
electrochemical tests. The elemental composition of the carbon steel sample is as follow (wt.%):
Fe 99.3; C 0.218; Si 0.0198; Mn 0.188; P 0.0091; S 0.0246; Ni 0.0256; Al 0.0456; Cu 0.0376; Cr
0
.0189; Mo 0.0089. Different grades of SiC papers were used to abrade the steel samples for
obtaining a smooth surface finish followed by acetone degreasing and washing with distilled
water. The electrochemical corrosion evaluation was carried out using a Gamry Reference 600
potentiostat equipped with EChem Analyst 6.33 suite.
2
.4.2 Electrochemical corrosion evaluation
The electrochemical measurements were undertaken using a standard three
electrochemical assembly employing a saturated calomel reference electrode (SCE) and a
cylindrical graphite counter electrode, respectively. The test electrolyte of 1M HCl was freshly
prepared before each test using analytical grade HCl, and the measurements were taken in
triplicate. The electrochemical testing was carried out in the stagnant and aerated condition. Prior
to the electrochemical measurements, the open circuit potential (EOCP) of the cell was recorded to
achieve a steady-state value. Based on the stable EOCP, the electrochemical impedance
spectroscopy (EIS) testing was undertaken in the frequency window of 100 kHz to 10 mHz at an
AC amplitude of 10 mV. The potentiodynamic polarization (PDP) studies were performed in the
-1
range of +250 mV by scanning the potential of the working electrode at a sweep rate of 1 mVs .
All the electrochemical measurements were carried out in triplicate to check the reproducibility
of the experimental data. Surface studies of the steel samples following immersion in the acid
electrolyte without and containing the inhibitors were carried out using scanning electron
microscope Quattro ESEM-FEG from ThermoFisherTM Scientific.
2
.5 Computational analysis
The analysis of the pKa was performed using the ChemAxon Chemicalize suite in the pH
range of 0-14 [22, 23]. Quantum chemical calculations using the density functional theory (DFT)
approach were utilized to obtain the reactivity indices of the two corrosion inhibitors for
estimating the adsorption capability of the inhibitors on the steel surface. For this purpose, the
computations were undertaken using Gaussian 09 software suite [24] employing the 6-31G (d,p)
basis set and B3LYP correlation functional. The electron density distributions of the frontier
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molecular orbitals (FMO) were computed, and the different reactivity parameters were
determined. The inhibitor adsorption on the Fe surface was further investigated computationally
by Monte Carlo simulations. For this purpose, the surface of the Fe (110) plane was chosen as
described vide infra. The surface was cleaved with a thickness of 5 Å and enlarged to 10 × 10
supercells, which was followed by the creation of a vacuum slab, which had a thickness of 30 Å
over it. The calculations were performed using the Adsorption Locator module of Biovia
Materials Studio 7.0 [25, 26].
3
. Results and discussion
3
.1 Synthesis and characterization of corrosion inhibitors
The synthetic steps of the precursor compound 1 and corrosion inhibitors 2 and 3 are
portrayed in Scheme 1. Briefly, 11-bromoundecanoic acid was treated with methanol in the
presence of H SO afforded precursor compound 1 in excellent yield. The methyl ester 1 was
2
4
refluxed with tripropylamine to give 2, which was then treated with DETA to generate 3 in a
1
9
5% yield. The H NMR and FTIR were utilized to confirm the structural composition of the
1
corrosion inhibitors 2 and 3. The H NMR and FTIR spectra are shown in Figs. 1 and 2,
respectively. The corrosion inhibitors were synthesized from cheap and commercially available
precursors. For an approximation, the cost of the synthesized inhibitor 3, assuming an average
yield of 90%, is estimated to be US$ 13 per kg. Both the molecules, being quaternary ammonium
salts, were readily soluble in the aqueous environment. When the corrosion inhibitor dissolved in
the 1M HCl solution for the corrosion evaluation, clear solutions were obtained without any
suspension or turbidity.
The TGA curves for the corrosion inhibitors 2 and 3 (dried under vacuum at 55 °C for 12
h) were examined at a temperature range 20-400 C to study the thermal stability of the corrosion
inhibitors. The TGA curves are depicted in Fig. 3. There is no weight loss occur up to 200 C,
which suggested that both corrosion inhibitors are thermally stable up to ≈ 200 °C.
3
.2 Open circuit potential vs. time
Prior to conducting the electrochemical testing, it is a prerequisite to obtaining a stable
open circuit potential (OCP) of the working electrode. The OCP refers to the potential difference
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between the working and the reference electrodes in the condition of zero net current flow (in
other words: Open circuit condition). The determination of a stable OCP vs. time is significant in
understanding the nature of corrosion and corrosion inhibition. The OCP-time curves of the two
inhibitors 2 and 3 are displayed in Fig. 4 (a, b) in the absence and the presence of the varying
concentrations. From the results, it is seen that the addition of the two corrosion inhibitors to the
electrolyte solution, produces a noticeable shift in the OCP towards more negative potentials
compared to that of the blank, which indicates that the inhibitor action is cathodic. The shift in
the potential can be understood to occur as a result of the development of a protective inhibitor
film on the metallic substrate [27, 28].
3
.3 Electrochemical impedance measurements
The electrochemical impedance spectroscopy (EIS) affords a non-destructive analytical
measure for understanding the electrochemical phenomena taking place at the surface of a
metallic sample immersed in the electrolytic solutions [29, 30]. The EIS measurements were
performed in the present study for the carbon steel sample immersed in 1M HCl without and
containing the different concentrations of the inhibitors 2 and 3. The results of the EIS analysis
as the Nyquist plots are shown in Fig. 5 (a and b). The Nyquist plots represent the characteristic
depressed semicircles with their centers lying under the real x-axis suggesting that the
electrochemical process is charge-transfer controlled [30-32]. This type of behavior is commonly
attributed to the frequency distributed electrochemical response of a metallic sample undergoing
corrosive attack under the influence of an acidic electrolyte. The electrochemical data were fitted
to the equivalent circuits shown in Fig. 5 (c and d) to obtain the different electrochemical
parameters, which are listed in Table 1.
The equivalent circuit is a modified Randel’s circuit containing the R , which represents the
s
uncompensated resistance of the electrolytic solution, and the R , which represents the charge
ct
transfer resistance. Herein, we have used a constant phase element (CPE) rather than an ideal
double-layer capacitor (C ), to accurately model the frequency distribution response [33-35]. In
dl
the case of the inhibited samples, the Nyquist plots show an additional capacitive semicircular
loop suggesting a significant contribution from the R (resistance of the film) and the CPE (the
f
f
corresponding constant phase element). Therefore, in this case, we have used an equivalent
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2
circuit model different from that of the blank carbon steel sample. The goodness of fit (

)
-3
values was of the order of 10 , supported the accuracy of the fitness of the proposed circuit
diagrams. The impedance of the CPE can be represented as [36]:

o
1
n
ZCPE  Y ( j)
(1)
Where, ZCPE is the impedance of CPE, Yo is a coefficient of proportionality,
frequency, and is a measure of the phase shift. The corrosion inhibition efficiency from the
EIS measurements was calculated as follows [37, 38]:

is the angular
n
i
p
b
p
R -R
η % =
×100
(2)
EIS
i
p
R
b
i
p
In the above equation, Rp and
R
respectively represent the polarization resistance of
the blank and the inhibited samples. Wherein, the Rp (polarization resistance) is equal to the Rct
for the blank steel sample as described previously [39, 40]. In the case of the inhibited samples,
the Rp is the collective contribution of the resistance from the Rct and Rf [39, 40] From the data
.
in Table 1, it is revealed that administering the increasing dosages of the inhibitors results in a
successive rise in the inhibition efficiency. The n values can also act as a gauge of the
homogeneity of the metallic surface. These values show an increase with the increase in the
inhibitor concentration, which supports and improvement in the homogeneity of steel surface
upon the adsorption of the corrosion inhibitor. The corrosion inhibitors adsorb at the metallic
surface via the replacement of the pre-adsorbed water molecules. The adsorption of the inhibitor
molecules results in a successive rise of the resistance at the electrochemical interface. The
increase in the interfacial resistance creates a barrier against the charge and the mass transfer
processes proceeding at the metal-solution interface and provides protection from corrosion.
Both the inhibitor showed a remarkable inhibition performance with an inhibition efficiency
-1
more than 90% at a modest dosage of 200 mg L . The corrosion inhibitor compound 3 showed a
greater inhibition performance compared to that of the 2. Both the inhibitor molecules are
composed of quaternary ammonium ions containing the three-propyl side chains in addition to a
large 11-membered hydrocarbon chain terminating in an ester group in case of inhibitor 2 and a
diethylenetriamine group in case of inhibitor 3. The presence of extra nitrogen atoms in the form
1
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of amine groups is attributed to provide a superior adsorption and inhibition performance to the
inhibitor 3 compared to the inhibitor 2.
3
.4 Potentiodynamic polarization studies
The potentiodynamic polarization experiments (PDP) are carried out in the corrosion
testing of inhibitor compounds for estimating the electrochemical kinetic parameters of the
inhibitor adsorption at the metal/solution interface [41]. The PDP technique was run for the
carbon steel sample without and with the inhibitors 2 and 3 at varying concentrations, and the
resulting Tafel plots are displayed in Fig. 6 (a and b). The linear branches of the anodic and the
cathode PDP curves were extrapolated to meet at their intersection point from where the various
electrochemical parameters including the Tafel slopes for the anodic/ cathodic processes ( and
a
c) corrosion current density (i ) and the corrosion potential (E ) were computed and are
corr
corr
given in Table 2. The corrosion current density (i ) data were used to calculate the inhibition
corr
efficiency as follows [42]:
0
corr
i
i
i
corr
ηPDP % 
100
(3)
0
i
corr
From the data shown in Table 2, it is seen that the icorr values for the carbon steel surface
in the presence of the inhibitors are significantly lower compared to that in the blank 1M HCl
solution. This supports that the addition of the corrosion inhibitors to the acid medium resulted in
the adsorption of the inhibitors on the steel surface and the formation of a protective film. As
expected, the inhibition efficiency rose with the introduction of successively higher
concentrations of the inhibitor molecules, and the inhibitor 3 showed better inhibition
performance compared to that of the 2. The anodic and the cathodic current curves show almost
parallel lines with no significant change in the slope values. This indicates that the inhibitor
adsorption on the steel surface does not cause any substantial change in the mechanism of
corrosion [43]. Furthermore, beyond a potential value of -300 mV, when the potential is swept
towards higher positive values, there is a change in the pattern of current flow, and a significant
rise in the rate of metal dissolution could be observed. This can be attributed to the desorption of
the inhibitor from the electrode surface and this value can be ascribed as the desorption potential
[
43].
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It can also be observed that the dosage of the inhibitors to the corrosive electrolyte
produced a lowering in both the anodic as well as the cathodic currents of the Tafel curves. This
suggests that the inhibitors effectively mitigate both the anodic metal electro-dissolution as well
as the cathodic evolution of the hydrogen gas. Moreover, with the successive rise in the inhibitor
concentration, a noticeable shift in the corrosion potentials was observed towards a negative
direction. However, the magnitude of the shift in the Ecorr is not prominent enough to designate
the inhibitors as a purely cathodic type. It suggests a mixed type of inhibition behavior with
cathodic predominance [44-46].
3
.5 Elucidation of adsorption isotherm
The interaction between an organic corrosion inhibitor with a metallic substrate is
strongly dependent upon its capability to adsorb on the metal surface. The structural aspects of
the inhibitor molecules, the composition of the corrosive electrolyte, nature, and composition of
the metallic substrate and the studied temperature are all crucial factors influencing the inhibitor
adsorption. The experimental data were fitted to several adsorption isotherms viz. Langmuir,
Temkin, and Freundlich as given below [47]:
Temkin:
exp (ƒ.θ) = K .C
(4)
(5)
(6)
ads
Freundlich: θ = K .C
ads
Langmuir:
(θ/1-θ) = K .C
ads
The most suitable fit of the data was incurred in the case of the Langmuir adsorption
isotherm, where Kads denotes the equilibrium constant for adsorption, C is the inhibitor
concentration, and ƒ is energetic inhomogeneity. Equation (6) can be rearranged in the form of a
plot of C vs. C/as:
C/θ = 1/K +C
(7)
ads
The plots of Langmuir isotherm for the 2 and 3 inhibitors calculated using the data of EIS
and PDP measurements are given in Fig. 7 (a and b). The obtained slope and the correlation
coefficients are close to unity, thereby validating the Langmuir isotherm. The obtained data of
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0
ads
the Kads were used to calculate the standard free energy of adsorption G as shown below [48,
4
9]:
0
ads
ΔG = -RTln(1000K )
(8)
ads
0
ads
The values of Kads and G for the inhibitors 2 and 3 were calculated and listed in Table 3. The
0
ads
-1
obtained values of the G are in the range of -20 to -40 kJ mol , which indicates a mixed-
mode of chemical and physical adsorption mechanisms [48, 49]. The negative values of the
0
ads
G
illustrated that the adsorption of the inhibitor on the carbon steel substrate is a spontaneous
process.
3
.6 Temperature effect on the inhibitor adsorption
The process of acid treatment carried out in the industries usually require the involvement
of high temperatures to achieve effective cleaning of the metallic substrates. Temperature is a
significant factor influencing the kinetics of inhibitor adsorption-desorption [8]. An elevation in
the temperature of the acidic solution can potentially weaken the adsorption-desorption equilibria
of the inhibitors on the metallic substrate. It may weaken the strength of the inhibitor adsorption
[
8]. Therefore, it becomes necessary to investigate the influence of the effect of temperature on
the adsorption-desorption behavior of a corrosion inhibitor on a given metallic substrate in the
acidic electrolyte. Accordingly, the impact of the temperature was investigated using the EIS and
the PDP measurements at a higher temperature of 333 K [50]. Using the optimum inhibitor
-1
dosages of 200 mg L , the blank and the inhibited steel samples were subjected to the EIS and
PDP measurements, and the results are shown in Fig. 8.
In Fig. 8a, the Nyquist plots for the carbon steel substrate without and containing the 100
-1
mg L concentration of the two corrosion inhibitors are displayed. The equivalent circuit
diagrams for the blank and the inhibitor-containing samples are the same as those given in Fig. 5
(
d). The addition of the inhibitors resulted in a considerable increase in the R values suggesting
p
the adsorption and the inhibition behavior of the two corrosion inhibitors. The corresponding
data on the electrochemical parameters and the inhibition efficacy are listed in Table 4. In the
next step, the corrosion inhibition behavior of the two molecules was investigated using the PDP
technique at the elevated temperature, and the results are displayed in Fig. 8(b), and
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corresponding electrochemical parameters are given in Table 4 [8]. Similar to the observations at
room temperature, the inhibitors primarily affect the cathodic branch of the polarization curves
and also shift the Ecorr more towards the cathodic direction, supporting the cathodic
predominance of the inhibition behavior.
It can be noted that the increase in the R values is considerably lower compared to that
p
observed in Fig. 8(a) for the identical concentrations of the inhibitors at the room temperature.
Similarly, in the PDP analysis, the corrosion current densities, although shown significant
lowering in the presence of the inhibitors, however, still the corrosion current densities are quite
large. This behavior could be attributed to the desorption of the inhibitors at the elevated
temperature. Although the inhibitors still exhibit an appreciable corrosion inhibition efficiency
close to 70% at the higher temperature. The corrosion rates (C ) were obtained from the
R
extrapolation of the linear Tafel segments and then used to determine the energy of activation
(Ea) according to the following equation (9) [51]:
CRb
Ea  1 1 
log


(9)



CRa
2.303R T T2
1

where the symbols CRa and CRb symbolize the corrosion rates obtained at the two different
-1
temperatures (T and T ). The E in the presence of inhibitors 2 and 3 were higher (56.1 kJ mol
1
2
a
-1
-1
and 60.7 kJ mol respectively) in comparison to the absence (25.2 kJ mol ) of inhibitors. This
supports the formation of a protective inhibitor film on the surface of carbon steel, which acts as
a protective barrier and increases the E value, which is required to be overcome for the
a
corrosion reaction to take place [37, 50].
3
.7 Surface analysis
The morphological investigation of the carbon steel samples was conducted following the
-1
immersion in the corrosive electrolyte of 1M HCl without and with 200 mg L of inhibitors 2
and 3 for 24 h at room temperature. The scanning electron microscopy (SEM) images recorded
of the metallic substrate are displayed in Fig. 9. The blank steel surface (Fig. 9a) shows a large
number of furrows and striations caused by the corrosive attack of the 1M HCl solution.
Extensive damage to the surface can be visualized due to the corrosion caused by the acid
solution. On the other hand, the steel samples inhibited by the introduction of the corrosion
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inhibitors 2 and 3 show a considerably smooth surface (Fig. 9 b and c). This suggests that the
adsorption of the inhibitors on the metallic samples protected from the corrosive attack of the
acid solution.
3
.8 Computational studies
3
.8.1 Analysis of pKa
An organic corrosion inhibitor, when added to an acidic corrosive electrolyte, can
undergo protonation [31, 42]. Therefore, it becomes necessary to identify the sites present in an
organic corrosion inhibitor molecule that are amenable to get protonated. For this purpose, the
analysis of pKa was performed in the pH range of 0-14 for both the molecules 2 and 3. The
molecule 2 does not undergo any further protonation whereas molecule 3 shows protonation at
the two N-atoms of the diethylene-triamine chain, as shown in the protonated form of 3 in Fig.
1
0. The neutral forms of both the inhibitors and the protonated form of inhibitor 3 were chosen
for the DFT study (vide infra).
3
.8.2 Theoretical estimation of the reactivity indices
A computational appraisal of the capability of organic molecules for the adsorption on
the metallic substrates can be achieved using DFT based computational studies. The frontier
molecular orbital energy levels can suggest the possibility of interaction between the two
reacting species. The high level of the energy of the highest molecular orbital (EHOMO) signifies a
greater inclination towards electron donation. On the contrary, a smaller value of the energy of
the lowest unoccupied molecular orbital (ELUMO) can signify the greater tendency to accept the
electrons in a given molecule. Therefore, a higher EHOMO and a lower ELUMO value in a corrosion
inhibitor molecule signify a greater tendency to undergo interaction with a given metallic
surface. The difference between the EHOMO and ELUMO of a molecule is termed as the molecular
orbital energy gap (E), and it is itself considered as a significant reactivity parameter. A lower
energy gap represents a greater tendency of reaction between two reacting species. Further, the
electronegativity (χ), and the chemical hardness (η) of a corrosion inhibitor are as follows [52]:
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1
2
χ = -

ELUMO +EHOMO

(10)
(11)
(12)
1
2
η = -

ELUMO -EHOMO

1
and, σ =
η
Where
σ
represents the global softness. Following the Hard and Soft Acids and Bases Principle,
a hard molecule is characterized by a large energy gap. On the inverse, a smaller energy gap
describes a soft or more reactive molecule [53]. The difference in the electronegativities of two
reacting species provides the driving force for the electron transfer to take place between two
molecules, which needs to overwhelm the energy difference posed by the global hardness [54].
The electrophilicity

and its reciprocal, the nucleophilicity index

are given by [55, 56]:
2
χ
1
ω =
=
(13)
4
η
ε
The electrophilicity index describes the propensity to accept the electrons in a given
inhibitor molecule [52, 56]. Therefore, a greater value of the electrophilicity index and hence, a
smaller nucleophilicity denote a higher tendency to accept electrons in a given molecule.
Fig. 11 shows the optimized geometries and the frontier molecular electron density
distributions of molecules 2 and 3 and the protonated form of 3, and the corresponding data are
-
presented in Table 5. It can be seen that the major sites for the HOMO surface are the Br ions,
which suggests that these are the major sites for electron donation. On the other hand, in both the
molecules, the three propyl chains attached to the charged nitrogen together represent the
LUMO, signifying that the molecules tend towards electron acceptance at these sites. This is a
major observation, which shows that in both the molecules, the inhibitor 2 containing the –
COOCH group and the inhibitor 3 containing the diethylenetriamine group, the electron
3
acceptance process is predominating. This is supported further with the values of the ELUMO in
the inhibitors, which is negative in both cases. However, in the case of inhibitor 3, the value is
more significant negative compared to that of inhibitor 2, suggesting that inhibitor 3 shows a
greater propensity to adsorb on the metal surface. Taking a look at the other parameters, it shows
that the hardness is lower, and correspondingly, the global softness is higher in the case of 3
1
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compared to 2. Besides, the electrophilicity index is greater and the nucleophilicity is lower in 3
compared to 2, which together represents its greater tendency to accept the electrons from the
metallic substrate. A look of the data shown in Table 5 reveals that the protonated form of the
inhibitor 3 shows the considerably high negative values of the EHOMO as well as the ELUMO. This
is explained based on a decrease in the tendency to donate electrons to the metal surface,
however, with a considerable rise in the inclination of the electron acceptance tendency from the
metallic substrate. The E value after protonation is also lower, signifying a high reactivity in
the protonated form. The inhibitor also shows a lower hardness and a higher value of softness,
which support its greater reactivity in the protonated form. The electronegativity index has risen
very greatly in the protonated form, and the nucleophilicity index is lowered accordingly, which
again supports the tendency of electron acceptance in the protonated form.
3
.8.3 Monte Carlo simulations
Monte Carlo simulations are carried out in the corrosion inhibition studies to figure out
the most probable orientation of a corrosion inhibitor on a given metal surface. In the present
case, the corroding metal surface is the carbon steel substrate [57]. Generally, Fe (110), Fe (111)
and Fe (110) planes are most commonly studied. Among the three planes, the Fe (110) surface is
most widely used because it covers > 60% of the entire Fe crystal surface area [58]. Thus, in the
present study, we selected this surface for modeling the inhibitor adsorption.
Fig. 12 (a, b) displays the adsorption orientations of the inhibitors 2 and 3 on the surface
of the Fe (110) plane, and the corresponding energy parameters are provided in Table 6. A
noteworthy observation is the alignment of the inhibitors on the Fe plane. Both the inhibitors
undergo flat adsorption on the Fe surface. This can allow the coverage of the large surface
coverage area of the Fe surface and results in a high inhibition efficiency. The adsorption energy
describes the total amount of energy released during the adsorption of the relaxed adsorbate
(
inhibitor) on the surface of the adsorbent (metal). A higher value of the adsorption energy in the
negative direction suggests a strong and stable interaction between the metal and the inhibitor
26]. In the present case, the molecule 3 exhibits higher adsorption energy compared to that of
[
the 2. This trend supports the trend of the molecular orbital energy gap (E) calculated above in
the DFT based quantum chemical calculations.
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Here it is to be noted that in the case of inhibitor 3, a diethylenetriamine group is
presently attached to the alkyl chain compared to an –COOCH in the case of 2. The presence of
3
the –COOCH functionality can provide an effective adsorption behavior to molecule 2. On the
3
other hand, in the case of molecule 3, attached to the –C=O group is a diethylenetriamine
(
DETA) side chain instead. Earlier, we have contributed to the DETA functionalized graphene
oxide (GO) as an effective corrosion inhibitor for steel surface in 1M HCl as well as in 15% HCl
36, 59]. In Fig. 12 (d), it is well evident that the inhibitor 3 lies almost in a flat orientation,
[
which allows the three N atoms of the DETA chain to come into contact with the Fe surface. The
presence of the DETA group can impart the resulting inhibitor water solubility as well as good
inhibition behavior due to the presence of the multiple adsorption centers in the form of the three
nitrogen atoms, as observed in our earlier report [36].
3
.9 Mechanism of adsorption
The adsorption and corrosion inhibition behavior of two quaternary ammonium salts
(QAS) compounds 2 and 3 were studied using electrochemical impedance measurements and
potentiodynamic polarization studies. The increase in the inhibition efficiency with inhibitor
concentration observed in the EIS studies supports the inhibitor adsorption on the steel surface in
the acidic electrolyte. According to this observation, the adsorption of the QAS-based inhibitors
on the steel surface in the acid solution can be represented by equation (14):
(14)
The above equilibrium suggests that the inhibitors can displace the pre-adsorbed water
molecules from the steel surface and adsorb at the metal/ solution interface. The polarization
measurements reveal that the addition of the inhibitors to the 1M HCl resulted in a drift in
corrosion potential towards the cathodic direction with a mixed type of behavior [60, 61]. The
inhibitors are quaternary ammonium ions containing a positively charged nitrogen atom
connected to four alkyl chains. The positively charged inhibitors can interact with the cathodic
corrosion active sites present on the steel surface via columbic attractions [8]. The bromide ions
can interact with the anodic sites and then also create an electrostatic bridge type of interaction
with the positively charged inhibitor molecules [8]. The mixed nature of the adsorption as
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revealed in the PDP measurements, suggests that the inhibitor minimizes both the anodic and the
cathodic processes of metal dissolution and the hydrogen evolution.
The surface analysis carried out via the SEM studies supported the inhibitor adsorption
by showing a considerably smooth morphology of the carbon steel in the presence of the
corrosion inhibitors. The DFT based calculations revealed that the significant sites of electron
donation in the two inhibitors are the bromide ions. This supports the above hypothesis of the
bridge-type linkage formation between the inhibitors and the metal surface. The Monte Carlo
simulations further illustrated that among the two inhibitors molecule 3, having a
diethylenetriamine group attached to the hydrocarbon chain shows the better capability of
adsorption on the metallic surface. Fig. 13 displays the schematic of the mechanism of
adsorption and inhibition performance of inhibitor 3 on the steel surface.
4
. Conclusions
Two quaternary ammonium salts namely-11-methoxy-11-oxo-N,N,N tripropylundecan-1-
aminium bromide (2), 11-((2-((2-aminoethyl)amino)ethyl)amino)-11- oxo- N,N,N
tripropylundecan-1-aminium bromide (3) were synthesized using low-cost and commercially
available precursors and characterized using different spectroscopic analysis. The molecules
were evaluated as new corrosion inhibitors for carbon steel surface in 1M HCl solution using the
electrochemical measurements and surface analysis complemented by computational studies.
Following major conclusions could be drawn from the present investigation:
1
. Both the molecules 2 and 3 exhibited excellent water solubility and were readily soluble
in the corrosive electrolyte of 1M HCl.
2
. The EIS studies supported the inhibitor adsorption on the metal substrate, revealing a rise
in the polarization resistance with an increase in the dosage of the inhibitors. The
inhibitor 3 showed a better adsorption behavior compared to that of inhibitor 2 with
-1

93% inhibition efficiency at a dosage of 200 mg L .
3
. The PDP studies revealed that the corrosion currents successively reduced with the
addition of the increased dosages of the inhibitors to the acid solution. Both the inhibitors
showed a mixed type of inhibition behavior with a cathodic predominance.
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4
. The inhibitor adsorption on the steel surface followed the Langmuir adsorption isotherm
with a mixed physical and chemical adsorption. The addition of the inhibitors to the
corrosive solution increased the energy of activation, indicating that the inhibitor
adsorption on the metallic surface increased the energy barrier for the corrosion process.
. The adsorption of the inhibitors on the metallic surface resulted in a considerably smooth
surface morphology in comparison to the blank steel surface supporting the adsorption of
the inhibitors.
5
6
. DFT analysis showed that the inhibitor 3 exhibited a smaller energy gap compared to the
inhibitor 2 supporting its greater inclination towards the adsorption on the steel surface.
The molecule 3 undergoes protonation in the acid solution and the protonated form
reveals a greater tendency to accept electrons.
7
. Monte Carlo simulation studies supported the superior adsorption behavior of molecule 3
compared to molecule 2 with higher adsorption energy.
Acknowledgment
The use of research facilities provided by King Fahd University of Petroleum & Minerals
and the financial assistance of Saudi Aramco, Dhahran, Saudi Arabia, through project # CHEM
414 is gratefully acknowledged. DSC would like to thank the KFUPM for the Post-Doctoral
2
fellowship.
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Figure Captions:
Scheme 1 Synthesis of and the corrosion inhibitors 2 and 3.
1
Fig. 1: H NMR spectra of corrosion inhibitors 2 and 3 in CDCl .
3
Fig. 2: FTIR spectra of synthesized corrosion inhibitors 2 and 3.
Fig. 3: TGA curves of synthesized corrosion inhibitors 2 and 3.
Fig. 4: OCP-time curves obtained for the corrosion inhibitors (a) 2 and (b) 3 on the carbon steel
surface in 1M HCl at room temperature.
Fig. 5: Nyquist plots obtained for the adsorption of corrosion inhibitors (a) 2 and (b) 3 on the
surface of carbon steel in 1M HCl at room temperature. Equivalent circuit diagrams used
for fitting the electrochemical impedance data for (c) blank and (d) inhibited mild steel
samples.
Fig. 6: Potentiodynamic polarization curves obtained for the adsorption of corrosion inhibitors
(a) 2 and (b) 3 on the surface of carbon steel in 1M HCl at room temperature.
Fig. 7 Langmuir isotherm plots obtained via (a) EIS and (b) PDP measurements for the
adsorption of corrosion inhibitors 2 and 3 on the carbon steel surface in 1M HCl solution
at room temperature.
-1
Fig. 8: (a) Nyquist plots and (b) polarization curves obtained for the adsorption of 100 mg L of
corrosion inhibitors 2 and 3 on the surface of carbon steel in 1M HCl at 333 K.
Fig. 9: Surface morphology (SEM) of the carbon steel samples recorded after immersion in 1M
-
1
HCl in (a) blank, and in the presence of 200 mg L corrosion inhibitors (b) 2 and (c) 3.
Fig. 10 Analysis of microspecies distribution of molecule 3 at varying pH.
Fig. 11: Optimized molecular structures of molecules (a) 2, (b) 3, (c) protonated form of 3, and
the corresponding HOMO/ LUMO electron density distributions.
Fig. 12: Top (a, c) and side (b, d) views of the most probable configurations for the adsorption of
the corrosion inhibitors 2 (a, b) and 3 (c, d) on the surface of Fe (110).
Fig. 13: Schematic of the mechanism of adsorption and inhibition behavior of corrosion inhibitor
3
on the carbon steel surface.
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Table 1: Electrochemical impedance parameters of carbon steel in 1M HCl at various
concentrations of corrosion inhibitors 2 and 3.
2
Sample Conc.
Rs
CPEf
Rf
(Ω cm )
CPEdl
Rct
(Ω cm )
ηEIS
(%)

1
×
-1
2
2
(mgL ) (Ω)
-3
0
-
3
-3
Y × 10
n1
Y × 10
n2
0
0

1 n
2
1 n
2

s cm
 s cm
Blank
25
0.663 --
--
--
81.61
0.73 8.96
6.026 --
2
3
1.231 1.214
0.968 1.014
0.888 0.748
0.919 0.648
0.68 18.62
0.69 27.00
0.71 33.36
0.74 48.91
75.53
60.19
46.05
37.52
0.54 15.11
0.55 24.90
0.58 46.78
0.61 60.69
1.326 73.4 +0.9
2.618 82.7 +0.6
1.131 88.8 +0.2
1.091 91.4 +0.4
5
1
2
0
00
00
25
0.488 1.158
1.077 0.733
1.601 0.658
1.045 0.523
0.73 20.82
0.74 37.72
0.75 49.59
0.77 63.77
66.58
53.19
45.84
33.17
0.51 15.82
0.53 29.21
0.55 38.10
0.63 62.56
0.432 75.5 +0.3
0.405 86.1 +0.7
0.697 89.7 +0.8
1.225 92.9 +0.6
5
1
2
0
00
00
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Table 2: Potentiodynamic polarization parameters of carbon steel in 1M HCl at different
concentrations of corrosion inhibitors 2 and 3.
Sample
Conc.
-Ecorr
(mV/SCE)
icorr
(μA cm )
a
c
ηPDP
-1 (%)
-
1
-2
(mgL )
-1
(mV dec ) (mV dec )
Blank
25
423
1249
83.0
139.4
--
2
3
490
488
489
501
348
312
206
144
87.7
89.2
88.6
92.2
132.2
130.4
130.0
133.8
72.05 +0.3
74.91 +0.2
83.44 +0.4
88.42 +0.5
5
1
2
0
00
00
25
459
475
489
493
377
297
199
108
79.3
81.6
89.9
87.8
145.4
124.7
123.5
142.2
69.69 +0.7
76.13 +0.6
84.00 +0.4
91.31 +0.2
5
1
2
0
00
00
2
9