View Article Online

View Journal

NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry

This article can be cited before page numbers have been issued, to do this please use: M. A. Quraishi, N.
K. Gupta, C. Verma, R. Salghi, H. Lgaz and A. K. Mukherjee, New J. Chem., 2017, DOI:
10.1039/C7NJ01431G.

Volume 40 Number 1 January 2016 Pages 1–846 This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry A journal for new directions in chemistry Accepted Manuscripts are published online shortly after
www.rsc.org/njc

acceptance, before technical editing, formatting and proof reading.

Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.

You can find more information about Accepted Manuscripts in the

author guidelines.

Please note that technical editing may introduce minor changes

to the text and/or graphics, which may alter content. The journal’s
ISSN 1144-0546 standard Terms & Conditions and the ethical guidelines, outlined
PAPER
in our author and reviewer resource centre, still apply. In no
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.

rsc.li/njc
 Page 1 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

New Phosphonate based corrosion inhibitors for mild steel in

hydrochloric acid useful for industrial pickling process:
Experimental and Theoretical approach

New Journal of Chemistry Accepted Manuscript

Neeraj Kumar Gupta1, Chandrabhan Verma1,3, R. Salghi4, H. Lgaz4,5, A.K. Mukherjee1, M.A.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Quraishi1,2*
1
Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi
-221005, India.
2
Center of Research Excellence in Corrosion, Research Institute, King Fahd University of
Petroleum & Minerals, Dhahran 31261, Saudi Arabia.
3
Department of Chemistry, Faculty of Agriculture, Science and Technology, North-West
University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa
4
Laboratory of Applied Chemistry and Environment, ENSA, Universite Ibn Zohr, PO Box 1136,
80000 Agadir,Morocco.
5
Laboratory of separation methods, Faculty of Science, Ibn Tofail University PO Box 242,
Kenitra, Morocco
*Corresponding author:

Ph.no. +91-9307025126; Fax: +91- 0542- 2368428

E-mail: maquraishi.apc@itbhu.ac.in; maquraishi@rediffmail.com

Abstract:

Present work deals with the synthesis and study of inhibition effect of three α-
aminophophonates namely, diethyl (((4-chlorophenyl)amino)(phenyl)methyl)phosphonate
(APCI-1), diethyl (((4-chlorophenyl)amino)(4-methoxyphenyl)methyl)phosphonate (APCI-2)
and diethyl (1-((4-chlorophenyl)amino)-3-phenylallyl)phosphonate (APCI-3) on mild steel
corrosion in 1 M hydrochloric acid solution using both experimental and theoretical methods.
Weight loss results showed that the inhibition performance of the studied compounds increases
with the concentration and the maximum inhibition efficiency were obtained at just 564 X10-6M
 New Journal of Chemistry Page 2 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

concentration. Among all the three tested inhibitors, APCI-3 showed the best result having
inhibition efficiency of 96.90%. The Potentiodynamic polarization study indicates that these α-
aminophosphonates act as mixed type inhibitors and predominantly functions as cathodic

New Journal of Chemistry Accepted Manuscript

inhibitor. Adsorption of the tested APCIs on the metallic surface obeyed the El-Awady
adsorption isotherm. The adsorption of these compounds on the metallic surface was also
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

supported by the scanning electron microscopy (SEM) and atomic force microscopy (AFM)
methods. A good insight about the inhibition mechanism of the tested compounds was derived
using the DFT based quantum chemical calculations for their neutral as well as protonated forms.
The orientation of inhibitors on the metallic surface and the interaction energies of these
molecules were obtained using molecular dynamic simulation studies. Both experimental and
theoretical studies suggested that the inhibition efficiency of the tested compounds followed the
order APCI-3>APCI-2>APCI-1 and well corroborated each other.

Keywords: α-Aminophophonates; Mild steel corrosion; SEM/AFM; DFT/ MD; El-Awady

adsorption isotherm.
1. Introduction
Corrosion is the most damaging and challenging problem all over the world. Each country
loses 3-5% of its GDP due to corrosion. According to NACE the annual global cost of corrosion
is approximately US $2.5 trillion, equating 3.4% of the global GDP. In India, the annual
corrosion cost is more than US $100-billion [1,2]. The cost of corrosion can be reduced by 15 to
35% by properly applying the existing methods of corrosion prevention.
The chemical synthesis using one pot multicomponent reactions (MCRs) for variety of
purposes are highly desirable owing to the growing interest of green chemistry in all branches of
science and technology [3,4]. Recently, MCRs have gained pronounce interest in the field of
synthetic organic and medicinal chemistry due to their several fascinating features, such as high
selectivity, convergence, atom economy, molecular complexity and versatility [5,6]. The MCRs
also associated with high yield, short reaction time, operation simplicity, mild operating
condition which attributed to their association with one step nature that results in facile
automation, small number of reaction steps, reduced production of waste, simple purification
method, fewer number of workup steps and enhance synthetic efficiency [7]. Therefore, the
MCRs provides a cheaper and eco friendly way to synthesize various compounds. Moreover, the
increasing ecological awareness demands green and sustainable solvents that have very low or
 Page 3 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

negligible adverse effect on the surrounding environment and living being. Towards utilization
of green and sustainable solvents, water has gained substantial attention due to its non-
flammable, non-toxic, uniquely redox-stable, non-hazardous, free availability and inexpensive

New Journal of Chemistry Accepted Manuscript

nature. It is general statement that “best solvent is no solvent”. Recently, chemical synthesis
using concept of multicomponent reactions in association with ultrasound (US) and microwave
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

have been considered as most powerful and interesting green and sustainable alternative
methods.
Literature survey reveals that previously phosphonates were introduced as scale inhibitors
in water treatment and later their good corrosion inhibition behavior was also recognized [8].
And their impact on the environment was reported to be negligible at the concentration level
used for corrosion inhibition [9]. However, authors mainly exploited the use of phosphonates as
corrosion inhibitor in protective metal phosphonate films [10] or in H2SO4 medium [11, 12] and
very few literatures are available on the use of phosphonates as corrosion inhibitor for mild steel
in HCl medium [13,14]. In view of these observations, it was thought worthwhile to synthesize
α-aminophophonates (APCIs) to investigate their effect on the corrosion of mild steel in acidic
medium. The APCIs undertaken in the present work were synthesized using one pot
multicomponent reaction under ultrasound irradiation and under solvent and catalyst free
condition. The criteria behind considering these compounds for the corrosion inhibition test was
based on the fact that they contain several heteroatoms (N and O) along with π- electrons of the
aromatic rings that can act as adsorption centers during the metal-inhibitor interaction [15].
Novelty of the present study lies on the fact that although there are variety of organic compounds
containing nitrogen, oxygen and sulfur which have been successively used as a corrosion
inhibitor, but the use of phosphorus containing compounds as corrosion inhibitor is relatively
scare. In view of the above fact and considering the green synthesis by MCRs and ultrasound
technique, we herein synthesized and studied the inhibition effect of three phosphorus containing
compounds for the mild steel in 1M HCl solution using weight loss, electrochemical (PDP, EIS),
surface (SEM, AFM), DFT based quantum chemical calculations and molecular dynamic (MD)
simulations methods. A good agreement was observed in the results of experimental and
theoretical measurements. The investigated APCIs exhibited higher corrosion inhibition
efficiency (96.9% at 564 x10-6M concentration) than that of hexamine, a commonly used
 New Journal of Chemistry Page 4 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

inhibitor in steel industries during pickling process which shows 90% efficiency at 714x10-6M
optimum concentration. Moreover hexamine is a toxic inhibitor so its use must be avoided.
2. Experimental:

New Journal of Chemistry Accepted Manuscript

2.1. Material
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

All the experiments including weight loss, electrochemical and surface analysis (SEM/AFM)
was performed on the mild steel specimens having following composition (wt%): C=0.076,
P=0.012, Mn=0.192, Si=0.026, Al=0.023, Cr=0.050 and Fe=99.621. All the mild steel specimens
were prepared as per the standard procedures already reported in our previous publications [2,7].
The test solution of 1 M HCl acid was prepared by diluting the analytical reagent (37% HCl, AR
grade) grade HCl by double distilled water.

2.2. Synthesis of inhibitor

All the three α-aminophosphonates (APCIs) were synthesized by solvent free and catalyst free
reaction by ultrasonic technique which can be represented according to the reaction scheme
shown in Fig.1 as described in literature [16]. In the reaction process 4-chloroaniline (1 mmol),
substituted benzaldehyde (1 mmol) and triethylphosphite (1.3 mmol) were poured in a beaker
and ultrasonicated for 20s. After completion of the reactions, the crude products were purified
through recrystallization that resulted yield about 99%.

Fig.1: Synthetic rout for investigated APCIs

 Page 5 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

The purity of the synthesized inhibitors was confirmed by the thin layer chromatography result
performed using Siliaplate TLC plates- Aluminum (Al) Silica. Synthesized compounds were
characterized by the infrared (IR) spectra recorded on KBr discs by using Perkin-Elmer

New Journal of Chemistry Accepted Manuscript

(Spectrum 100) Fourier transform (FT-IR) spectrophotometer. The IUPAC names, chemical
structures and IR data of synthesized APCIs are reported in Table 1.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Table 1: IUPAC name, molecular structure, molecular formula and analytical data of studied
APCIs.
S. Name of inhibitor Chemical structure Analytical data
No.
1 diethyl (((4- C17H21ClNO3P (mol. wt. 353.78);
chlorophenyl)amino)( FT-IR (KBr cm-1): 3470, 3183,
phenyl)methyl)phosp 2862, 2448, 2973, 2823, 1628,
honate (APCI-1) 1664, 1580, 1130, 1238, 974,
835, 786, 654.1H NMR (CDCl3,
TMS, 500 MHz) δ (ppm) 6.5-7.4
(8H, Ar-H and 1H, NH) 4.7 (1H,
CH), 3.6-4.1 (4H, P-O-CH2-),
1.1-1.3 (6H, P-O-CH2-CH3).
2 diethyl (((4- C18H23ClNO4P (mol. wt. 383.11);
chlorophenyl)amino)( IR (KBr cm-1): 3563, 3228,
4- 2986, 2854, 1670, 1594, 1474,
methoxyphenyl)meth 1339, 1232, 1160, 948, 827,
yl)phosphonate 792.1H NMR (CDCl3, TMS, 500
(APCI-2) MHz) δ (ppm) 6.5-7.3 (8H, Ar-H
and 1H, NH) 4.6 (1H, CH), 3.6-
4.1 (4H, P-O-CH2-), 3.7 (3H, O-
CH3) 1.1-1.3 (6H, P-O-CH2-
CH3).
3 diethyl (1-((4-
chlorophenyl)amino)- C19H23ClNO3P (mol. wt. 379.82);
3- IR (KBr cm-1): 3546, 3173,
phenylallyl)phosphon 2948, 2836, 2684, 1728, 1658,
ate (APCI-3)
1548, 1418, 1273, 1138, 926,
868, 675. 1H NMR (CDCl3, TMS,
500 MHz) δ (ppm) 6.6-7.4 (9H,
Ar-H and 1H, NH), 6.2-6.7 (2H,
-CH=CH-) 4.1-4.2 (4H, P-O-
CH2-), 3.7 (3H, O-CH3) 1.31-
1.34 (6H, P-O-CH2-CH3).
 New Journal of Chemistry Page 6 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

2.3. Gravimetric Measurements

Gravimetric technique is considered as one of the best method to study the inhibition
performance of the inhibitor due to its simplicity and reliability. In the present study, gravimetric

New Journal of Chemistry Accepted Manuscript

experiments have been used to calculate the corrosion inhibition performance of synthesized
APCIs by the previously described methods [17]. All the experiments were performed in
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

triplicate and the average value was taken for calculating corrosion rate for greater accuracy. The
corrosion rates CR (mg cm−2 h−1) was calculated by using the following equation:
CR = W / At (1)
where, W is the average weight loss occurred in the three parallel experiments, A is the total
surface area of one mild steel specimen, and t is the immersion time (3 h). From the calculated
corrosion rate, the inhibition efficiency η% and surface coverage area (θ) was calculated by
following equations:
η % = [(CR − CR(i) ) / CR ] × 100 (2)

θ = (CR − CR(i) ) / CR (3)

where CR and CR(i) represent the corrosion rates for mild steel in absence and presence of APCIs
in 1 M HCl solution, respectively.

2.4. Electrochemical measurements

All the Electrochemical studies were performed by the method already described in the
literature [18]. All electrochemical experiments were carried out by using three electrode cell
assembly connected to a Potentiostat/Galvanostat G300−45050 (Gamry Instruments,
Inc.,U.S.A.). The obtained data was analyzed by Echem Analyst 5.0 software package. Among
the three electrode system, mild steel with exposed area of 1 cm2 was used as a working
electrode, a platinum electrode as an auxiliary electrode, and a saturated calomel electrode (SCE)
as the reference electrode. Prior to each electrochemical experiment, the working electrode was
allowed to corrode freely for sufficient time so that it can achieve a steady open circuit potential
(OCP).
The electrochemical impedance studies (EIS) were carried out for the mild steel
specimens in the frequency range of 100 kHz to 0.01 Hz using an AC source with amplitude of
10 mV peak to peak. The polarization resistance was calculated by fitting the appropriate
 Page 7 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

electrical circuit on the obtained Nyquist plot. The corrosion inhibition performances of studied
APCIs were calculated from following equation:
η % = [( Rpi − Rp0 ) / Rpi ] × 100 (4)

New Journal of Chemistry Accepted Manuscript

Where Rip and R0p are the polarization resistance values in the presence and absence of APCIs in
1.0 M HCl solution, respectively.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Polarization measurements was carried out by changing the potential of working

electrode from -250 mV to +250 mV versus open circuit potential at a scan rate of 1 mV s−1.
Then the anodic and cathodic regions were used for extrapolating their linear segments to
calculate the corrosion potential value (Ecorr) and the corrosion current densities (icorr). The
calculated icorr value was used to calculate the inhibition efficiency of APCIs using following
relation [19]:
0 i 0
η % = [(icorr − icorr ) / icorr ] × 100 (5)
Where, i0corr and iicorr are the corrosion current densities in absence and presence of APCIs in 1 M
HCl solution.
2.5. SEM and AFM Measurements
SEM and AFM studies were conducted to get the insight into the change in surface
morphology of the corroding mild steel sample in 1 M HCl solution in the presence (at 564 x10-
6
M) and absence of APCIs. Mild steel coupons were immersed in 1 M HCl solution in the
absence, and presence of optimum concentration of APCIs for 3h at 350C, and then taken out
and cleaned with double distilled water and ethanol, dried at room temperature and finally
analyzed by SEM and AFM. The SEM study was performed by Ziess Evo 50XVP instrument
using accelerating voltage of 50 kV at 500X magnification. The AFM analysis of mild steel
corroded samples were performed by the NT-MDT multimode AFM, Russia, that is controlled
by Solver scanning probe microscope controller. A single beam cantilever with resonance
frequency between 240–255 kHz having spring constant of 11.5 Nm-1 with NOVA program was
used in semi contact mode to interpret the image. The scanning area for analysis was 10 µm×10
µm.
2.6. Quantum Chemical Calculations
Density functional theory (DFT) is an ab initio approach very popular for computing the ground
state electronic properties of molecules with high accuracy and in very less time as compared to
other computational methods [20]. In this present work all DFT calculations were performed by
 New Journal of Chemistry Page 8 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

using Gaussian 09 program package. The molecular structures of all the APCIs (neutral and
protonated forms) were geometrically optimized using the Becke three-parameter hybrid
functional together with the Lee–Yang–Paar correlation functional (B3LYP) and the 6-31+G

New Journal of Chemistry Accepted Manuscript

(d,p) orbital basis set for all atoms [21].
The electronic parameters for the most stable conformers of the molecules were used to
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

derive all the quantum chemical electronic parameters. The frontier molecular orbital (FMO)
energies, i.e. the highest occupied molecular orbital energy (EHOMO) and the lowest unoccupied
molecular energy (ELUMO) were calculated and used for the calculation of other important
parameters such as the energy gap (∆E), global hardness (η), softness (σ), global
electronegativity (χ), and the fraction of electrons transfer (∆N) from the inhibitor to the metal
atom by the following equations respectively [22,23]:
∆E = E LUMO − E HOMO (6)

η = ( E LUMO − EHOMO ) / 2 (7)

σ = 1/η (8)

χ = − ( ELUMO + E HOMO ) / 2 (9)

∆N = ( χ Fe − χinh ) / 2 (ηFe + ηinh ) (10)

Where χFe and ηinh represent the electronegativity of iron and hardness of inhibitor respectively.
The values of χFe and ηinh is taken as 7eV mol-1 and 0 eV mol-1 respectively considering the bulk
Fe atoms according to the Pearson's electronegativity scale [24].
2.7. Molecular Dynamics
Molecular Dynamic (MD) simulations of tested inhibitors were carried out in a
simulation box with periodic boundary conditions using Materials Studio 6.0 (from Accelrys
Inc.) [25]. The iron crystal was imported and cleaved along (110) plane and a slab of 5 Å was
employed. The Fe (110) surface was relaxed by minimizing its energy using smart minimiser
method. Then Fe (110) surface was enlarged to a (10 × 10) super cell in order to provide a large
surface to the inhibitors for interaction. A zero thickness vacuum slab was built. A supercell with
a size of a = b = 24.82 Å c = 25.14 Å, contains 500 H2O, 5H 3O + , 5Cl − and one molecule of tested
inhibitors was created. The simulation was performed in a simulation box (24.82×24.82×35.69
 Page 9 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

Å3) using the discover module having time step of 1 fs and simulation time of 500 ps carried out
at 303 K, NVT ensemble (constant number of atoms, constant-volume, constant-temperature)
and COMPASS force field [26]. In simulation system, the interactions between inhibitors and Fe

New Journal of Chemistry Accepted Manuscript

(110) surface can be understood by interaction and binding energies calculated using equation
(10) and (11) [27]:
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Einteraction = Etotal − ( Esurface+solution +Einhibitor+solution )+Esolution

(11)
EBinding = − Einteraction
(12)
Where Etotal represents the total energy of the entire system and Esurface+solution referred to the total

energy of Fe (1 1 0) surface and solution without the inhibitor and Einhibitor+solution represent the

total energy of inhibitor and solution; and Esolution is the total energy of the solution.
3. Result and Discussion

3.1.Weight loss experiment

3.1.1. Effect of APCIs concentration

The values of percentage inhibition efficiency (%η) of APCIs, corrosion rate (CR) for
mild steel specimen and the surface coverage (θ) by APCIs obtained from weight loss
experiments at different concentrations of inhibitors in 1 M HCl solution at 308K are reported in
Table 2.

Table 2: The weight loss parameters (±SD) derived for Mild Steel in 1 M HCl solution at
different concentrations of APCIs.

inhibitor Conc CR(mg cm−2 h−1) Surface coverage η%

(x10-6M) (θ)

Blank 0.0 7.66 --- -

141 3.20 0.583 58.6 ±0.35

APCI-1 282 1.60 0.791 79.1±0.35

423 0.86 0.887 88.7±0.75

564 0.56 0.926 92.6±0.43

 New Journal of Chemistry Page 10 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

141 2.66 0.52 65.2±0.43

APCI-2 282 1.30 0.830 83.0±0.87

423 0.66 0.913 91.3±0.75

New Journal of Chemistry Accepted Manuscript

564 0.40 0.948 94.8±0.43
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

141 2.26 0.704 70.4±0.75

APCI-3 282 0.80 0.896 89.6±0.87

423 0.43 0.943 94.3±0.75

564 0.23 0.969 96.9±0.43

Fig.2: Variation of corrosion inhibition efficiency with APCIs concentration.

The variation of inhibition efficiency with the concentration of APCIs is also shown in Fig.
2. It is clear from the data that APCIs inhibits the corrosion at all studied concentrations and
their η% increases with concentration and achieved maximum values of 92.60, 94.78 and
96.95% for APCI-1, APCI-2 and APCI-3, respectively at 564 x10-6M concentration. The
 Page 11 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

enhancement in η% with concentration is due to the increase in surface coverage by inhibitor

molecule and therefore isolating the metal surface from the acid solution [28]. However above
564 x10-6M concentration inhibition efficiency does not increase significantly suggesting that

New Journal of Chemistry Accepted Manuscript

probably the maximum surface coverage was obtained at this value. [29].

3.1.2. Temperature effect

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

The effect of temperature on the inhibition efficiency of a corrosion inhibitor is an important

factor to be considered before using a corrosion inhibitor in any industry. In order to study the
inhibition efficiency variation of studied APCIs with temperature and to calculate the activation
parameters, weight loss experiments were performed at different temperatures (308-338 K). The
variation of corrosion rate of mild steel in 1 M HCl solution with and without optimum
concentration of APCIs is given in Table 3 which clearly shows that the corrosion rate is
increasing with increase in temperature. This which might be due to the desorption or etching of
the inhibitor film present on the metal surface [30]. The desorption of inhibitor film causes
exposure of more metal surface to the HCl medium which resulted into higher corrosion rate
[31]. Moreover the elevated solution temperature sometime also causes molecular decomposition
which further decreases the corrosion inhibition performance [32].

Table 3: Variation of corrosion rate with temperature in absence and presence of optimum
concentration of APCIs

Temperature (K) Corrosion rate (CR) (mg cm-2 h-1)

Blank APCI-1 APCI-2 APCI-3

308 7.66 0.56 0.40 0.23

318 11.0 1.23 1.03 0.70

328 14.3 2.36 2.06 1.80

338 18.6 4.70 4.63 4.33

The apparent activation energy (Ea) of corrosion process and corrosion rate can be related by
Arrhenius equation [33] given by:
C R = A exp( − E a / RT ) (13)
 New Journal of Chemistry Page 12 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

Where, CR is the corrosion rate, R is the universal gas constant, T is the absolute temperature and
A is the pre-exponential factor. The value of Ea were calculated from the slope (Ea/2.303R) of
the Arrhenius graph plotted between log CR versus 1000/T for mild steel in 1 M HCl solution

New Journal of Chemistry Accepted Manuscript

with and without optimum concentration of APCIs which is shown in Fig. 3. The obtained values
of Ea are 24.48, 60.60, 69.65, 84.12 kJ mol-1 for blank solution, APCI-1, APCI-2, APCI-3
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

respectively which clearly shows that the value of Ea for uninhibited solution is less than that of
inhibited solution . The significant rise in the Ea value indicates the protective film formation by
inhibitor on the metal surface [34]. The adsorbed inhibitor isolates the metal surface from the
corrosive environment and thus increases the energy barrier for the charge and mass transfer
between metal and solution interface [28]. In the present study APCI-3 has the highest Ea value
which confirms the best inhibition performance of APCI-3 among three studied inhibitors.
However, the inhibition efficiency of all the three APCIs decreases with rise in temperature
which shows the physisorptive type of interaction among the adsorbed inhibitor film and the
metal surface [35].

Fig.3: Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 1M HCl solution in

absence and presence of APCIs.

 Page 13 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

3.1.3. Adsorption isotherm and thermodynamic parameters

It is well reported in literature that the adsorption of an organic molecule on the metal
surface is the most important step in the corrosion inhibition process [36]. To understand the

New Journal of Chemistry Accepted Manuscript

corrosion inhibition mechanism of inhibitors and to know the type of interaction between
inhibitor molecules and the metal surface the adsorption characteristics of the inhibitor was
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

studied by testing several adsorption isotherms including Langmuir, Temkin, Frumkin and and
thermodynamic/kinetic model of El-Awady isotherms.

Fig.4: El-Awady adsorption isotherm plot for mild steel in 1 M HCl solution in presence APCIs at 308K.

In this study, the values for degree of surface coverage (θ) were fitted to a series of
different stated adsorption isotherms but among them the Langmuir adsorption isotherm gave the
best fit with regression coefficient (R2) value very close to unity. This indicated a high degree of
fitness of the adsorption data to the Langmuir model. Langmuir adsorption isotherm expression
is generally represented by [17]

K ads C = θ / (1 − θ ) (14)
Where Kads is the adsorption- desorption equilibrium constant, θ is the surface coverage and C(inh)
is the APCIs concentration in mg L−1. The Langmuir isotherm plot gave a straight line when
 New Journal of Chemistry Page 14 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

plotted between log (θ/1-θ) and C(inh) but its slope deviated considerably (1.23 to 1.83) from the
unity which indicated that the isotherm could not be strictly applied (Table 4).

Table 4: Adsorption parameters calculated from El-Awady adsorption isotherm for mild steel in

New Journal of Chemistry Accepted Manuscript

presence of APCIs.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Inhibitor Slope (y) Intercept (log K)

Temperature(K) 308 318 328 338 308 318 328 338

APCI-1 1.58 1.47 1.43 1.44 6.23 5.68 5.38 5.15

APCI-2 1.63 1.51 1.39 1.32 6.52 5.90 5.29 4.79

APCI-3 1.83 1.66 1.34 1.23 7.45 6.53 5.19 4.54

In the derivation of Langmuir isotherm equation it is assumed that the adsorbed molecules do not
interact with each other, but this is not true in the present case. These adsorbed APCIs may
interact with each other by mutual repulsion or attraction and that might be the reason for the
deviation of slope from unity [37]. So, the experimental data was fitted for the modified form of
Langmuir isotherm known as El-Awady isotherm which is given by [38]
log(θ / 1 − θ ) = log K + y logC (15)
where, y is number of inhibitor molecules occupying one active site, θ is the degree of surface
coverage, C is the concentration, K is the constant related to the equilibrium constant of
adsorption process as Kads =K1/y. The values of y and log K calculated from El-Awady adsorption
isotherm plot (Fig.4) is given in Table 4. Values of 1/y less than one imply multilayer
adsorption, while 1/y greater than one suggests that a given inhibitor molecule occupies
more than one active site. In the present work the values of 1/y for all concentration were
found to be less than unity which indicates the presence of physisorption [39]. The calculated
value of Kads is reported in Table 5. In general, the value of Kads represents the strength of
adsorption of inhibitor molecule on the metal surface [40]. Higher the value of Kads, stronger is
the adsorption of inhibitor on metal surface. Stronger adsorption leads to the more surface
coverage by inhibitor on the metal surface and hence better inhibition efficiency [36]. Table 5
shows that the APCI-3 has the highest Kads value among all studied APCIs and thus has highest
 Page 15 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

inhibition efficiency. The Kads values can be related to the standard free energy of adsorption
(∆G0ads) according to the relation [41,42]:
ο
∆Gads = − RT ln(55.5 K ads ) (15)

New Journal of Chemistry Accepted Manuscript

where, R is the universal gas constant, T is the absolute temperature and value 55.5 represent the
molar concentration of water in acid solution. The calculated values for the ∆G0ads are listed in
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Table 5. The values of ∆G0ads at 308K were found to be -33.48, -33.89 and -34.19 kJ mol-1 for
APCI-1, APCI-2 and APCI-3 respectively. However, these values are negatively less than the
threshold value (-40 kJ/mol) expected for the chemisorption but greater than the (-20kJ/mol)
assigned for physisorption [43]. Hence the adsorption APCIs on the mild steel specimen in 1 M
HCl solution is a case of both physical as well as chemical adsorption [44].

Table 5: The values of Kads and ∆ G◦ads for mild steel in presence of APCIs in 1M HCl solution
at different temperature.

Inhibitor Kads (103 M-1) -∆G◦ads (kJ mol-1)

Temperature(K) 308 318 328 338 308 318 328 338

APCI-1 8.56 7.49 5.64 3.75 33.48 33.14 32.41 31.37

APCI-2 10.05 7.85 6.30 4.28 33.89 33.26 32.68 31.70

APCI-3 11.31 8.32 7.18 4.88 34.19 33.41 33.03 32.04

3.2. Electrochemical measurements

3.2.1. Polarization study

Potentiodynamic polarization study was performed in absence and presence of different

concentration of the APCIs in order to corroborate the findings of weight loss measurements.
Fig. 5 represents the polarization curves for inhibited and uninhibited mild steel specimens in 1M
HCl solution at 298K. Several polarization parameters such as anodic and cathodic Tafel slopes
(βa and βc), corrosion potential (Ecorr), and corrosion current density (icorr) were calculated by
extrapolating the linear portions of the cathodic and anodic polarization curves and listed in
Table 6 along with corresponding inhibition efficiencies (η%).
 New Journal of Chemistry Page 16 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig.5: Potentiodynamic polarization plots for mild steel in 1 M HCl solution in the absence and
presence of different concentration of APCIs.

From the result it is clear that on increasing the concentration of the inhibitor molecules
the corrosion current density decreases which suggest that the inhibition efficiency of the APCIs
increases with concentration. It can be also seen from the polarization curves that the inhibitor
molecules shifted the both anodic and cathodic domains of the polarization curves towards the
 Page 17 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

lower corrosion current density without changing the shape of the curves. This finding suggests
that studied inhibitor molecules inhibit mild steel corrosion in 1M hydrochloric acid solution by
forming a protective film on the metallic surface without changing the corrosion mechanism

New Journal of Chemistry Accepted Manuscript

[45].

Table 6: Tafel Polarization parameters (±SD) for mild steel in 1 M HCl solution in absence and
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

presence of different concentration of APCIs

Inhibitor Conc. Ecorr icorr βa -βc η(%) from Rp η(%)

(10-6 M) (mV/ (µAcm-2) (mV/dec) (mV/dec) icorr value (Ωcm2) from Rp

SCE) value

Blank ---- -445 1150±4.3 70.5±0.32 114.6±0.15 ---- 9.58 ----

APCI-1 141 -513 493±1.7 73.2±0.15 181.3±0.36 57.13±0.15 23.34 58.95

282 -510 246±2.6 64.3±0.36 124.3±0.28 78.60±0.23 42.19 77.29

423 -495 115±2.6 83.5±0.28 129.1±0.10 90.00±0.23 78.82 87.84

564 -522 81±1.0 71.2±0.15 158.7±0.10 92.93±0.08 121.3 92.10

APCI-2 141 -515 398±2.0 84.0±0.42 117±0.36 65.39±0.17 25.8 62.86

282 -518 198±1.7 56.1±0.14 68.1±0.14 82.78±0.15 62.54 84.68

423 -486 103±1.7 71.5±0.15 144.7±0.10 91.04±0.15 113.3 91.54

564 -517 61±1.0 103.5±0.28 121.3±0.28 94.69±0.08 186.1 94.24

APCI-3 141 -522 340±2.0 69.0±0.28 109.4±0.42 70.43±0.17 36.95 74.07

282 -526 132±1.0 107.1±0.36 167.5±0.20 88.52±0.08 91.79 89.56

423 -539 63±1.7 112.3±0.10 168.0±0.10 94.52±0.15 158.6 93.95

564 -516 32±1.7 85.7±0.15 161.9±0.10 97.21±0.15 299.2 96.79

Inspection of the results depicted in Table 6 revealed that presence of inhibitors in the
corroding solution at their optimum concentration causes substantial decrease in the values of
 New Journal of Chemistry Page 18 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

corrosion current density (icorr) and maximum decrease was obtained in the case of APCI-3. The
corrosion inhibition efficiency obtained from icorr values was compared with the values obtained
from polarization resistance and it was found that both follows the same trend and showed

New Journal of Chemistry Accepted Manuscript

maximum value for APCI-3. Results also showed that the presence of inhibitors in the corrosive
medium does not alter the value of Ecorr significantly as the maximum change in the values of
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Ecorr were 77 mV, 72 mV and 71 mV for APCI-1, APCI-2 and APCI-3, respectively that is less
that 85 mV which indicate that APCIs are behaved as mixed type inhibitors [46,47]. However,
from the results depicted in Table 6, it is clear that presence of inhibitors cause relatively more
change in the βc values as compared to the βa values. On the basis of ongoing discussion it can be
inferred that investigated compounds acted as mixed type corrosion inhibitor for mild steel in 1M
hydrochloric acid solution with some cathodic predominance [48].

3.2.2.Electrochemical impedance spectroscopic study

The Nyquist plots with equivalent circuit used in the present analysis for electrochemical
data interpretation and the Bode phase angle plots for inhibited and uninhibited metallic
specimens are depicted in Fig. 6, 7 and Fig. 8 respectively. All the calculated EIS parameters
such as solution resistance (Rs), polarization resistance (Rp), phase shift (n), double layer
capacitance (Cdl) and corresponding surface coverage (θ) and inhibition efficiency (η) are listed
in Table 7.
 Page 19 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig.6 : Nyquist plots for mild steel in 1 M HCl solution in the absence and presence of
different concentration of APCIs.

Fig.7: Equivalent circuit used to fit the EIS data for mild steel in 1 M HCl solution.

From Fig. 6 it is clear that Nyquist plots for uninhibited and inhibited mild steel
specimens gave the similar appearance which suggests that APCIs inhibit corrosion by
increasing the polarization resistance without changing the mechanism of corrosion [28]. The
Nyquist plots consist of depressed semicircles with the centers under the real axis, which is
commonly observed in the case of solid metal electrodes due to frequency dispersion of
interfacial impedance. This phenomenon is mainly due to the factors like surface roughness,
inhomogeneity of electrode surface, discontinuity in the electrode and the adsorption of inhibitor
 New Journal of Chemistry Page 20 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

and impurities [49]. It is important to mention that in the present investigation, polarization
resistance (Rp) was under taken for measurements rather than more commonly used charge
transfer resistance (Rct) because Rp represents the all type of resistances associated on the metal/

New Journal of Chemistry Accepted Manuscript

solution interfaces such as pore resistance (Rr), diffusion resistance (Rd), film resistance (Rf) and
charge transfer resistance (Rct) i.e. Rp =Rct + Rr + Rd + Rf, that would give more information about
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

the interface [50]. The result showed that the presence of inhibitor molecules at their optimum
concentration causes substantial increase in the values of Rp.

Table 7: Electrochemical impedance parameters (±SD) obtained from EIS measurements for
mild steel in 1 M HCl solution in absence and presence of optimum concentration of APCIs.

inhibito Conc. Rs Rp Cdl n η%

r (10-6M) (Ωcm2) (Ωcm2) (µFcm-2)

Blank 1.120 9.58±0.15 106.21 0.827±0.001 --

APCI-1 141 0.457 23.34±0.33 95.62 0.763±0.004 58.95±0.56

282 0.451 42.19±0.26 93.99 0.721±0.001 77.29±0.14

423 0.899 78.82±0.41 70.69 0.811±0.002 87.84±0.06

564 0.751 121.3±0.51 66.42 0.805±0.001 92.10±0.03

APCI-2 141 1.105 25.8±0.1 88.25 0.829±0.002 62.86±0.14

282 0.500 62.54±0.45 72.21 0.817±0.004 84.68±0.11

423 0.593 113.3±0.37 68.14 0.766±0.001 91.54±0.03

564 1.831 186.1±0.41 55.83 0.806±0.001 94.24±0.06

APCI-3 141 0.655 36.95±0.24 70.67 0.712±0.003 74.07±0.17

282 0.830 91.79±0.26 62.13 0.805±0.001 89.56±0.09

423 1.137 158.6±0.25 52.80 0.817±0.003 93.95±0.01

564 1.107 299.2±1.3 34.44 0.811±0.002 96.79±0.14

 Page 21 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

Results depicted in Table 7 showed that presence of inhibitors decreases the values of Cdl
as compared to the Cdl value of blank which is attributed to decrease in local dielectric constant
and/or an increase in the electrical double layer thickness [51]. Fig. 8 represents the Bode and

New Journal of Chemistry Accepted Manuscript

phase angle plots for the mild steel in 1M HCl in absence and presence of the optimum
concentration of APCIs. It is clear that the Bode phase angle plots have only one maximum i.e.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

one time constant at the intermediate frequencies. The broadening of this maximum indicates the
formation of inhibitor layer on mild steel surface. Moreover the increase in the phase angle
values in presence of APCIs reveals their inhibitive action [52].
 New Journal of Chemistry Page 22 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

Fig.8: Bode plots for mild steel in 1 M HCl solution in the absence and presence of APCIs at
different concentration.

3.3. Surface study

New Journal of Chemistry Accepted Manuscript

3.3.1. SEM analysis
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

The SEM study was performed on the mild steel specimens taken after 3h immersion
time in presence and absence of APCIs and the micrographs obtained are shown in Fig. 9(a-d).

Fig. 9(a-d): SEM image of mild steel surface after 3h immersion (a) without APCIs (b) with APCI-1 (c)
with APCI-2 (d) with APCI-3.
 Page 23 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

Fig. 9(a) represents the micrograph of mild steel specimen immerged in blank solution
whereas Fig. 9(b-d) represents mild steel in presence of APCIs. From Fig. 9(a) it can be seen that
the surface of mild steel specimen is severely corroded and damaged due to free acid attack.

New Journal of Chemistry Accepted Manuscript

However in Fig. 9(b-d) surface morphology of the specimens are significantly improved and the
surface is smoother and contains less number of pits. This study confirms the protective action of
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

mild steel in acid solution by APCIs.

3.3.2. Atomic force microscopy

The AFM study was performed to know the average roughness of mild steel specimen
surface in absence and presence of optimum concentration of APCIs after 3h immersion time.
Fig. 10(a) represent the 3D micrograph of mild steel surface in dipped in blank solution. The
observation of this micrograph shows that the surface of specimen is highly corroded and
contains many peaks and valley like areas due to the dissolution of metal in that region.
However, in presence of APCIs a relatively smoother and uniform surface morphology was
found in Fig. 10(b-d) due to the corrosion inhibition property of the inhibitors [21]. The average
surface roughness Ra (the average deviation of all points roughness profile from a mean line over
the evaluation length) of mild steel specimen was found to be 389 nm in absence of APCIs,
whereas in presence of APCI-1, APCI-2 and APCI-3 the Ra values were 141nm, 127nm and
102nm, respectively.
 Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.
New Journal of Chemistry
DOI: 10.1039/C7NJ01431G
View Article Online

New Journal of Chemistry Accepted Manuscript

Page 24 of 39
 Page 25 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig. 10(a-d): 3D AFM image of mild steel surface after 3h immersion (a) without APCI (b) with
APCI-1 (c) with APCI-2 (d) with APCI-3.

3.4.Quantum Chemical calculation

 New Journal of Chemistry Page 26 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig. 11: Optimized molecular structure of non-protonated APCI-1, APCI-2 and APCI-3.
 Page 27 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

Fig.12: The frontier molecular orbitals HOMO and LUMO of non-protonated APCI-1, APCI-2 and
APCI-3.

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig. 13: Optimized molecular structure of protonated APCI-1, APCI-2 and APCI-3.
 New Journal of Chemistry Page 28 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig. 14: The frontier molecular orbital HOMO and LUMO of protonated APCI-1, APCI-2 and
APCI-3.

Figs. 11-14 represent the fully optimized and frontier molecular electron distribution pictures
of neutral as well as protonated forms of studied inhibitors molecules. From the frontier
molecular electron distribution pictures for neutral form of the inhibitor molecules it can be seen
 Page 29 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

that HOMO and LUMO are mainly localized over the p-chlorophenyl ring and diethyl-
phosphonate moieties suggesting that only these part of molecules mainly involve in electron
sharing during metal-inhibitor interactions. In contrast, HOMO for protonated form of these

New Journal of Chemistry Accepted Manuscript

inhibitors mainly localized at phenyl ring of the benzaldehyde moiety, while LUMO is mainly
concentrated at p-chlorophenyl and diethyl-phosphonate moieties. The common quantum
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

chemical calculation parameters such as energy of highest occupied (EHOMO) and lowest
unoccupied (ELUMO) molecular orbitals, energy band gap (∆E), global electronegativity (χ),
hardness (η), softness (σ), fraction of electron transfer (∆N) and dipole moment (µ) derived for
neutral as well as protonated form of the investigated molecules are presented in Table 8.

Table 8: Quantum chemical parameters derived for neutral and protonated form of the
investigated APCIs

Form inhibitors EHOMO ELUMO ∆E η σ χ ∆N µ

(Hartree) (Hartree) (Hartree) (Debye)

APCI1 -0.1694 -0.0255 0.1439 0.0719 13.89 0.097 1.110 7.694

Neutral APCI2 -0.1689 -0.0267 0.1422 0.0711 14.06 0.097 1.120 8.044
APCI3 -0.1778 -0.0525 0.1101 0.0576 17.34 0.110 1.275 7.443
APCI1 -0.3781 -0.1686 0.2095 0.1047 9.54 0.273 -0.076 5.523
Protonate APCI2 -0.3610 -0.1692 0.1918 0.0959 10.31 0.264 -0.040 3.702
d APCI3 -0.3283 -0.1712 0.1571 0.0785 12.72 0.249 0.047 4.481

According to the concept of electron transfer (and chemical reactivity) a molecule with high
value of EHOMO and lower value of ELUMO would be associated with high chemical reactivity and
thereby associated high inhibition performance [53,54]. In our present investigation, values of
EHOMO increases on going from APCI-1 to APCI-3 suggesting that magnitude of electron transfer
from inhibitors to metal surface and therefore inhibition performance increases in the same order.
Similarly, value of ELUMO for any chemical species suggests its electron accepting tendency [53].
The values of ELUMO decreases (increases in negative) on going APCI-1 to APCI-3 indicating
that tendency of electron acceptation and thereby inhibition performance increases likewise [55].
During metal-inhibitor interactions, a lower value of ∆E (energy band gap) is consisted with high
 New Journal of Chemistry Page 30 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

inhibition efficiency [56]. In our case, values of ∆E well established the experimental order of
inhibition efficiency. In the present study values of EHOMO, ELUMO and ∆E for neutral as well as
protonated form of inhibitor molecules were consistent with the exponentially determined

New Journal of Chemistry Accepted Manuscript

inhibition efficiency order. Applying the Pearson electronegativity concept, it can be postulated
that a chemical species with high value of global electronegativity should be associated with
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

lower chemical reactivity and inhibition efficiency [57]. In our present case values of global
electronegativity did not follow any regular trend for neutral form of the inhibitor molecules.
However, for protonated form of the inhibition molecules, values of electronegativities were
accordance to the experimentally determined efficiency order. Besides, more commonly used
energies parameters (EHOMO, ELUMO and ∆E) chemical reactivity and the inhibition efficiency of
any chemical species can be predicted depending upon the value of its global softness (σ) and
hardness (η). Generally, a chemical species with higher value of softness and lower value of
hardness is comprised with strong metal-inhibitor interaction (high efficiency) [20]. In present
study values of softness followed the order: APCI-3 (17.34) > APCI-2 (14.06) > APCI-1 (13.89),
which is just accordance to the order of inhibition efficiency derived from weight loss and
electrochemical methods, while the values of global hardness follow just inverse order. The
values of global hardness and softness were well agreed with the experimental results for neural
as well as protonated forms. Relative inhibition performance of structurally similar molecules
can also be predicted based on values of their fraction of electron transfer which is a direct
measure of electron transfer from inhibitor to metal. Obviously, an organic inhibitor with higher
value of ∆N would be associated with higher inhibition efficiency as compared of the organic
molecule having lower value of ∆N [57]. In present study, values of ∆N are increasing on going
APCI-1 to APCI-3, indicating that magnitude of electron transfer (donation) followed the trend:
APCI-3 (1.275) > APCI-2 (1.120) > APCI-1 (1.110), which is consisted with the experimentally
determined inhibition efficiency. The values of dipole moment have been derived for neutral as
well as protonated form of inhibitor molecules and are listed in Table 8. Generally, value of
dipole moment is a measure of polarizability of any organic molecule on the metallic surface
during interactions between them. A molecule with high value of dipole moment is more
polarizable and covers larger surface area and thereby should act as better corrosion inhibition as
compared to the organic molecule with lower value of dipole moment. Both positive and
 Page 31 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

negative trends of inhibition efficiency have been reported with the values of dipole moment
[53]. In our present study values of dipole moments did not show any regular trend.

3.5. Molecular dynamics simulation

New Journal of Chemistry Accepted Manuscript

For the past years there has been a rapid rise in the use of MD simulations for understanding the
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

interaction between corrosion inhibitor-metals surface [58–60]. Thus in this study, MD

simulations were made to study the adsorption behavior of four phosphonate derivatives on Fe
(110) surface. For this purpose, a system containing 500 water molecules, 5Hଷ Oା , 5 Clି and one
molecule of tested APCIs was constructed to simulate the actual aggressive medium. After
500,000 steps, the system reaches equilibrium which explain that both the temperature and
energy reach balance[61]. From the final configurations of tested inhibitors (Fig. 15-16), it is
clear that the phosphonate molecules move to the Fe (110) surface to almost parallel or flat
disposition. In this instance, the interaction and binding energies of the adsorption of APCIs on
Fe (110) are calculated and summarized in Table 9.

Table 9. Selected energy parameters obtained from MD simulations for adsorption of APCIs on
Fe (110) surface

System ‫୍ܧ‬୬୲ୣ୰ୟୡ୲୧୭୬ ‫ܧ‬୆୧୬ୢ୧୬୥

(kJ/mol) (kJ/mol)
Fe + APCI-1 +500H2O + -743.67 743.67
5H 3O + + 5Cl −

Fe + APCI-2 +500H2O + -805.01 805.01

5H 3O + + 5Cl −

Fe + APCI-3 +500H2O + -877.11 877.11

5H 3O + + 5Cl −

The higher positive values of binding energies are attributed to an effective adsorption
and consequently the formation of adsorbed layer of phosphonate derivatives on Fe (110) surface
[62]. In the same case, the higher negative values of interaction energies indicate that there is a
strong interaction between tested inhibitors and metallic surface [63]. Based on these insights,
 New Journal of Chemistry Page 32 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

we can draw a conclusion that the phosphonates tested in this study can provide an effective
protection of steel.
APCI-1 APCI-2 APCI-3

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Fig. 15: Side views of the final adsorption of the APCIs on the Fe (110) surface in solution.

APCI-1 APCI-2 APCI-3

Fig. 16: Top views of the final adsorption of the APCIs on the Fe (110) surface in solution.

Fig. 17 presents the radial distribution function (RDF) curves of C, N, O, Cl and P of APCI-3
and Fe atoms. The radial distribution function (or pair correlation function) g(r) can be computed
through a structural analysis of the MD simulations results [64]. The RDF is widely used as
useful method to estimate the bond length. The peak within 3.5 Å, it’s an indication of small
bond length which indicates the chemisorption, while the peak outside 3.5 Å shows the physical
interactions [65].
 Page 33 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

2.4 5

(3.37, 1.95) Fe-C Fe-Cl

2.0
4
(3.37, 3.0)
1.6

New Journal of Chemistry Accepted Manuscript

3
g(r)

g(r)
1.2

2
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

0.8

1
0.4

0.0 0
0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20
x (Ångström) x (Ångström)

2.0 2.4
(4.35, 1.19) Fe-N (3.11, 1.55)
Fe-O
1.6 2.0

1.6
1.2
g(r)

g(r)

1.2

0.8
0.8

0.4
0.4

0.0 0.0
0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20
x (Ångström) x (Ångström)

3.0

(3.62, 1.73) Fe-P

2.5

2.0
g(r)

1.5

1.0

0.5

0.0
0 2 4 6 8 10 12 14 16 18 20
x (Ångström)

Fig. 17: Radial distribution functions of APCI-3 adsorbed on a Fe (110) surface.

 New Journal of Chemistry Page 34 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

From the equilibrium configuration of the APCI-3 molecule, it can be concluded that the most
significant interactions (chemisorption) with Fe atoms are having C, O and Cl with the peak
distance less than 3.5 whereas the interactions of Van der Waals force or Coulomb force can be

New Journal of Chemistry Accepted Manuscript

occurs between phosphorus and nitrogen atoms and Fe atoms [65].
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

4. Conclusions
The inhibition effect of three α-aminophophonates namely, diethyl (((4-
chlorophenyl)amino)(phenyl)methyl)phosphonate (APCI-1), diethyl (((4-chlorophenyl)amino)(4-
methoxyphenyl)methyl)phosphonate (APCI-2) and diethyl (1-((4-chlorophenyl)amino)-3-
phenylallyl)phosphonate (APCI-3) on mild steel corrosion in 1M hydrochloric acid solution was
studied using experimental and theoretical methods. From the obtained results following
conclusions were drawn:
1. All the studied compounds acted as good corrosion inhibitor and their inhibition
efficiency increases with the increase in concentration and maximum efficiency was
obtained at an optimum concentration of 564 x10-6M .
2. The inhibition efficiencies of the investigated compounds followed the order: APCI-3
(96.9%) > APCI-2 (94.8%) > APCI-1 (92.6%).
3. Potentiodynamic polarization study revealed that the studied inhibitors behaved as mixed
type inhibitor by decreasing both anodic and cathodic corrosion densities and maximum
decrease was observed by APCI-3.
4. EIS measurements showed that investigated inhibitor molecules inhibit corrosion by
adsorbing on the metal surface and forming the protective film over the metal/ electrolyte
interfaces.
5. Adsorption of the tested compounds over the metallic surface obeyed the El-Awady
adsorption isotherm.
6. The substantial improvement in the surface morphologies of the inhibited metallic
specimens found after SEM and AFM analysis suggested that the investigated molecules
adsorbed over the metal surface and protect it from corrosion by forming inhibitor film.
7. DFT based parameters revealed that tested compounds have strong tendency of
adsorption and thereby act as good corrosion inhibitors for mild steel corrosion in 1M
hydrochloric acid solution.
 Page 35 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

8. MD study reveals that all the investigated inhibitor molecules strongly adsorbed over the
metallic surface nearly by flat or parallel orientation and thereby protect the larger
surface area. The values of Einteraction were followed the order: APCI-3 (-877.11 kJ/mol) >

New Journal of Chemistry Accepted Manuscript

APCI-2 (-805.01 kJ/mol) > APCI-1 (-743.67 kJ/mol), which is in good agreement to the
experimental results.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Acknowledgment

Gupta and Verma, gratefully acknowledged Ministry of Human Resource Development

(MHRD), New Delhi (India) for support.

 New Journal of Chemistry Page 36 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

References

1. http://insights.globalspec.com/article/2340/annual-global-cost-of-corrosion-2-5-trillion.
2. C. Verma, E.E. Ebenso, M.A. Quraishi, J. Mol. Liq. 233 (2017) 403.

New Journal of Chemistry Accepted Manuscript

3. A. Domling, Chem. Rev. 106 (2006) 17.
4. D. Tejedor, F.G. Tellado, Chem. Soc. Rev. 36 (2007) 484.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

5. C. Beattie, M. North, P. Villuendas, Molecules 16 (2011) 3420.

6. M.M. Hooper, Brenton DeBoef, J. Chem. Educ. 86 (2009) 1077.
7. C. Verma, M.A. Quraishi, E.E. Ebenso, I.B. Obot, A. El Assyry, J. Mol. Liq. 219 (2016)
647.
8. H. S. Awad, S. Turgoose, Corrosion, 60 (2004) 1168.
9. R. Laamari, J. Benzakour, F. Berrekhis, A. Abouelfida, A. Derja, D. Villemin, Arab. J.
Chem. 4 (2011) 271.
10. K. D. Demadis, C. Mantzaridis, P. Lykoudis, Ind. Eng. Chem. Res. 45 (2006) 7795.
11. Y. Kharbach, A. Haoudi, M.K. Skalli, Y. Kandri Rodi, A. Aouniti, B. Hammouti, O.
Senhaji, A. Zarrouk, J. Mater. Environ. Sci. 6 (2015) 2906.
12. Bouklah, O. Krim, M. Messali, B. Hammouti, A. Elidrissi, I Warad, Der Pharma
Chemica, 3 (2011) 283.
13. M. Benabdellah, A. Dafali, B. Hammaouti, A. Aouniti, M. Rhomari, A. Raada, O.
Senhaji, J.J. Robin, Chem. Eng. Comm., 194 (2007) 1328.
14. M. Yadav, D. Sharma, S. Kumar, S. Kumar, I. Bahadur, E. E. Ebenso, Int. J.
Electrochem. Sci.,9 (2014) 6580.
15. N.A. Negm, E.A. Badr, I.A. Aiad, M.F. Zaki, M.M. Said, Corros. Sci. 65 (2012) 77.
16. B. Dar, A. Singh, A. Sahu, P. Patidar, A. Chakraborty, M. Sharma, B. Singh, Tetrahedron
Lett. 53 (2012) 5497.
17. N.K. Gupta, C. Verma, M.A. Quraishi, A.K. Mukherjee, J. Mol. Liq. 215 (2016) 47.
18. C. Verma, M.A. Quraishi, N.K. Gupta, Ain Shams Eng. J. (2016) xxx, xxx–xxx.
http://dx.doi.org/10.1016/j.asej.2016.07.003
19. M. Muralisankar, R. Sreedharan, S. Sujith, N.S.P. Bhuvanesh, A. Sreekanth, J. Alloys
Compd. 695 (2017) 171.
20. S. K. Saha, P. Banerjee, RSC Adv. 5 (2015) 71120.
21. C. Verma, M. A. Quraishi, L. O. Olasunkanmi, E.E. Ebenso, RSC Adv. 5 (2015) 85417.
 Page 37 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

22. C. Verma, M.A. Quraishi, A. Singh, J. Taiwan Inst. Chem. Eng. 000(2015)1.
23. I. Ahamad, R. Prasad, M.A. Quraishi, Corros. Sci. 52 (2010) 3033.
24. R. G. Pearson, Inorg. Chem. 27 (1988) 734.

New Journal of Chemistry Accepted Manuscript

25. Materials Studio, Revision 6.0, Accelrys Inc., San Diego, USA, 2013.
26. H. Sun, J. Phys. Chem. B. 102 (1998) 7338.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

27. Z. Zhang, N.C. Tian, X.D. Huang, W. Shang, L. Wu, RSC Adv. 6 (2016) 22250.
28. N.K. Gupta, M. A. Quraishi, C. Verma, A. K. Mukherjee, RSC Adv. 6 (2016) 102076.
29. C. Verma, M.A. Quraishi, Ain Shams Eng. J. 7 (2016) 1.
30. I. Ahamad, M.A. Quraishi, Corros. Sci. 52 (2010) 651.
31. J. Alijourani, K. Raeissi, M.A. Golozar, Corros. Sci. 51 (2009)1836.
32. C. Verma, M. A. Quraishi, K. Kluza, M. Makowska-Janusik, L.O. Olasunkanmi, E.E.
Ebenso, Sci. Rep. 7 (2017) 44432.
33. C. Verma, M.A. Quraishi, A. Singh, J. Taiwan Inst. Chem. Eng. 49 (2015) 229.
34. N.O. Eddy, H. Momoh-Yahaya, E.E. Oguzie, J. Adv. Res. 6 (2015) 203.
35. M.A. Deyab, J Ind. Eng. Chem. 22 (2015) 384.
36. P.B. Raja, A.K. Qureshi, A.A. Rahim, H. Osman, K. Awang, Corros. Sci. 69 (2013) 292.
37. R. Karthikaiselvi, S. Subhashini, J. Assoc. Arab Univ. Basic Appl. Sci. 16 (2014) 74.
38. A. A. EI-Awady, B. A. Abd-EI-Nabey, S. G. Aziz, J. Electrochem. Soc. 139 (1992) 2049.
39. A.F.S.A. Rahiman, S. Sethumanickam, Arab. J. Chem. 10 (2017) 3358.
40. N.A. Odewunmi, S.A. Umoren, Z.M. Gasem, J. Ind. Eng. Chem. 21 (2015) 239.
41. A. Singh, Y. Lin, W. Liu, S. Yu, J. Pan, C. Ren, D. Kuanhai, J. Ind. Eng. Chem. 20
(2014) 4276.
42. S. Issaadi, T. Douadi, A. Zouaoui, S. Chafaa, M.A. Khan, G. Bouet, Corros. Sci. 53
(2011) 1484.
43. N.A. Negm, E.A. Badr, I.A. Aiad, M.F. Zaki, M.M. Said, Corros. Sci. 65 (2012) 77.
44. M.M. Solomon, S.A. Umoren, A.U. Israel, E.E. Ebenso, J. Mater. Eng. Perform. 24
(2015) 4206.
45. El-Sayed, M. Sherif, J. Ind. Eng. Chem. 19 (2013) 1884.
46. I.B. Obot, S.A. Umoren, Z.M. Gasem, R. Suleiman, B. El Ali, J. Ind. Eng. Chem. 21
(2015) 1328.
 New Journal of Chemistry Page 38 of 39
View Article Online
DOI: 10.1039/C7NJ01431G

47. S. A. Umoren, I.B. Obot, A. Madhankumar, Z.M. Gasem, Carbohydr. Polym. 124 (2015)
280.
48. M.M. Solomon, S. A. Umoren, J. Colloid Interface Sci. 462 (2016) 29.

New Journal of Chemistry Accepted Manuscript

49. K. Ramya, R. Mohan, K.K. Anupama, A. Joseph, Mater. Chem. Phys. 149-150 (2015)
632.
Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

50. J. Haque, V. Srivastava, C. Verma, M.A. Quraishi, J. Mol. Liq. 225 (2017) 848.
51. M.A. Deyab, J. Ind. Eng. Chem. 22 (2015) 384.
52. G. Sı˘gırcık, T. Tüken, M. Erbil, App. Surf. Sci. 324 (2015) 232.
53. A. Ehsani, M.G. Mahjani, R. Moshrefi, H. Mostaanzadeh, J.S. Shayeh, RSC Adv. 4
(2014) 2003.
54. S.K. Saha, A. Dutta, P. Ghosh, D. Sukul, P. Banerjee, Phys. Chem. Chem. Phys. 17
(2015) 5679.
55. R. Tamilarasan, A. Sreekanth, RSC Adv. 3 (2013) 23681.
56. S. John, M. Kuruvilla, A. Joseph, RSC Adv. 3 (2013) 8929.
57. J. Haque, K.R. Ansari, V. Srivastava, M.A. Quraishi, I.B. Obot, J. Ind. Eng. Chem. 49
(2017) 176.
58. S. Kaya, L. Guo, C. Kaya, B. Tüzün, I.B. Obot, R. Touir, N. Islam, J. Taiwan Inst. Chem.
Eng. 65 (2016) 522.
59. L.H. Madkour, S. Kaya, C. Kaya, L. Guo, J. Taiwan Inst. Chem. Eng. 68 (2016) 461.
60. H. Lgaz, R. Salghi, S. Jodeh, B. Hammouti, J. Mol. Liq. 225 (2017) 271–280.
61. S.K. Saha, M. Murmu, N.C. Murmu, P. Banerjee, J. Mol. Liq. 224 (2016) 629.
62. L.O. Olasunkanmi, I.B. Obot, E.E. Ebenso, RSC Adv. 6 (2016) 86782.
63. A. Kokalj, Corros. Sci. 70 (2013) 294.
64. H. Lgaz, R. Salghi, S. Jodeh, B. Hammouti, J. Mol. Liq. 225 (2017) 271.
65. S.-W. Xie, Z. Liu, G.-C. Han, W. Li, J. Liu, Z. Chen, Comput. Theor. Chem. 1063 (2015)
50.
 Page 39 of 39 New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01431G

New Journal of Chemistry Accepted Manuscript

Published on 03 October 2017. Downloaded by University of Newcastle on 03/10/2017 14:39:46.

Phosphorus containing compounds have been evaluated by experimental and theoretical

techniques and more than 96% corrosion inhibition efficiency was observed at 200ppm
concentration