New phosphonate based corrosion inhibitors for mild steel in hydrochloric acid useful for industrial pickling processes: Experimental and theoretical approach

Article abstract of DOI:10.1039/c7nj01431g

The present work deals with the synthesis and study of the inhibition effect of three α-aminophosphonates, 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 was obtained at just 564 × 10^-6 M concentration. Among all the three tested inhibitors, APCI-3 showed the best result having an inhibition efficiency of 96.90%. The potentiodynamic polarization study indicates that these α-aminophosphonates act as mixed type inhibitors and predominantly function as a cathodic 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 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 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.

Full text of DOI:10.1039/c7nj01431g

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New Phosphonate based corrosion inhibitors for mild steel in
hydrochloric acid useful for industrial pickling process:
Experimental and Theoretical approach
Neeraj Kumar Gupta1, Chandrabhan Verma1,3, R. Salghi4, H. Lgaz4,5, A.K. Mukherjee1, M.A.
Quraishi1,2*
1Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi
ꢀ221005, India.
2Center of Research Excellence in Corrosion, Research Institute, King Fahd University of
Petroleum & Minerals, Dhahran 31261, Saudi Arabia.
3Department of Chemistry, Faculty of Agriculture, Science and Technology, NorthꢀWest
University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa
4Laboratory of Applied Chemistry and Environment, ENSA, Universite Ibn Zohr, PO Box 1136,
80000 Agadir,Morocco.
5Laboratory of separation methods, Faculty of Science, Ibn Tofail University PO Box 242,
Kenitra, Morocco
*Corresponding author:
Ph.no. +91ꢀ9307025126; Fax: +91ꢀ 0542ꢀ 2368428
Eꢀmail: maquraishi.apc@itbhu.ac.in; maquraishi@rediffmail.com
Abstract:
Present work deals with the synthesis and study of inhibition effect of three αꢀ
aminophophonates namely, diethyl (((4ꢀchlorophenyl)amino)(phenyl)methyl)phosphonate
(APCIꢀ1), diethyl (((4ꢀchlorophenyl)amino)(4ꢀmethoxyphenyl)methyl)phosphonate (APCIꢀ2)
and diethyl (1ꢀ((4ꢀchlorophenyl)amino)ꢀ3ꢀphenylallyl)phosphonate (APCIꢀ3) 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 X10ꢀ6M

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concentration. Among all the three tested inhibitors, APCIꢀ3 showed the best result having
inhibition efficiency of 96.90%. The Potentiodynamic polarization study indicates that these αꢀ
aminophosphonates act as mixed type inhibitors and predominantly functions as cathodic
inhibitor. Adsorption of the tested APCIs on the metallic surface obeyed the ElꢀAwady
adsorption isotherm. The adsorption of these compounds on the metallic surface was also
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 APCIꢀ3>APCIꢀ2>APCIꢀ1 and well corroborated each other.
Keywords: αꢀAminophophonates; Mild steel corrosion; SEM/AFM; DFT/ MD; ElꢀAwady
adsorption isotherm.
1. Introduction
Corrosion is the most damaging and challenging problem all over the world. Each country
loses 3ꢀ5% 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 $100ꢀbillion [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

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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 nonꢀ
flammable, nonꢀtoxic, uniquely redoxꢀstable, nonꢀhazardous, free availability and inexpensive
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
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
αꢀaminophophonates (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 πꢀ electrons of the
aromatic rings that can act as adsorption centers during the metalꢀinhibitor 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 x10ꢀ6M concentration) than that of hexamine, a commonly used

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inhibitor in steel industries during pickling process which shows 90% efficiency at 714x10ꢀ6M
optimum concentration. Moreover hexamine is a toxic inhibitor so its use must be avoided.
2. Experimental:
2.1. Material
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 αꢀaminophosphonates (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 4ꢀchloroaniline (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

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The purity of the synthesized inhibitors was confirmed by the thin layer chromatography result
performed using Siliaplate TLC platesꢀ Aluminum (Al) Silica. Synthesized compounds were
characterized by the infrared (IR) spectra recorded on KBr discs by using PerkinꢀElmer
(Spectrum 100) Fourier transform (FTꢀIR) spectrophotometer. The IUPAC names, chemical
structures and IR data of synthesized APCIs are reported in Table 1.
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 (((4ꢀ
C17H21ClNO3P (mol. wt. 353.78);
FTꢀIR (KBr cmꢀ1): 3470, 3183,
2862, 2448, 2973, 2823, 1628,
1664, 1580, 1130, 1238, 974,
835, 786, 654.1H NMR (CDCl3,
TMS, 500 MHz) δ (ppm) 6.5ꢀ7.4
(8H, ArꢀH and 1H, NH) 4.7 (1H,
CH), 3.6ꢀ4.1 (4H, PꢀOꢀCH2ꢀ),
1.1ꢀ1.3 (6H, PꢀOꢀCH2ꢀCH3).
chlorophenyl)amino)(
phenyl)methyl)phosp
honate (APCIꢀ1)
2
diethyl (((4ꢀ
chlorophenyl)amino)(
4ꢀ
methoxyphenyl)meth
yl)phosphonate
(APCIꢀ2)
C18H23ClNO4P (mol. wt. 383.11);
IR (KBr cmꢀ1): 3563, 3228,
2986, 2854, 1670, 1594, 1474,
1339, 1232, 1160, 948, 827,
792.1H NMR (CDCl3, TMS, 500
MHz) δ (ppm) 6.5ꢀ7.3 (8H, ArꢀH
and 1H, NH) 4.6 (1H, CH), 3.6ꢀ
4.1 (4H, PꢀOꢀCH2ꢀ), 3.7 (3H, Oꢀ
CH3) 1.1ꢀ1.3 (6H, PꢀOꢀCH2ꢀ
CH3).
diethyl (1ꢀ((4ꢀ
chlorophenyl)amino)ꢀ
3ꢀ
phenylallyl)phosphon
ate (APCIꢀ3)
3
C19H23ClNO3P (mol. wt. 379.82);
IR (KBr cmꢀ1): 3546, 3173,
2948, 2836, 2684, 1728, 1658,
1548, 1418, 1273, 1138, 926,
868, 675. 1H NMR (CDCl3, TMS,
500 MHz) δ (ppm) 6.6ꢀ7.4 (9H,
ArꢀH and 1H, NH), 6.2ꢀ6.7 (2H,
ꢀCH=CHꢀ) 4.1ꢀ4.2 (4H, PꢀOꢀ
CH2ꢀ), 3.7 (3H, OꢀCH3) 1.31ꢀ
1.34 (6H, PꢀOꢀCH2ꢀCH3).

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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
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
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
θ
% = [(CR − CR(i) ) / CR ]×100
= (CR − CR(i) ) / CR
(2)
(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

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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)
Where Rip and R0 are the polarization resistance values in the presence and absence of APCIs in
p
1.0 M HCl solution, respectively.
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]:
η
% = [(ic0orr − iciorr ) / ic0orr ]×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 x10ꢀ
6M) 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 NTꢀMDT 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 Nmꢀ1 with NOVA program was
used in semi contact mode to interpret the image. The scanning area for analysis was 10 ꢁm×10
ꢁm.
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

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using Gaussian 09 program package. The molecular structures of all the APCIs (neutral and
protonated forms) were geometrically optimized using the Becke threeꢀparameter hybrid
functional together with the Lee–Yang–Paar correlation functional (B3LYP) and the 6ꢀ31+G
(d,p) orbital basis set for all atoms [21].
The electronic parameters for the most stable conformers of the molecules were used to
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 (ꢂE), global hardness (η), softness (σ), global
electronegativity (χ), and the fraction of electrons transfer (ꢂN) from the inhibitor to the metal
atom by the following equations respectively [22,23]:
(6)
(7)
ꢀE = ELUMO − EHOMO
η
σ
χ
= ELUMO − EHOMO / 2
(
)
=1/
η
(8)
= − E
+ EHOMO / 2
(9)
(
)
LUMO
ꢀN = (χFe
−
χinh ) / 2
η
(
+
ηinh
(10)
)
Fe
Where χFe and ηinh represent the electronegativity of iron and hardness of inhibitor respectively.
The values of χFe and ηinh is taken as 7eV molꢀ1 and 0 eV molꢀ1 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,5H3O+
,
5Cl− and one molecule of tested
inhibitors was created. The simulation was performed in a simulation box (24.82×24.82×35.69

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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, constantꢀvolume, constantꢀtemperature)
and COMPASS force field [26]. In simulation system, the interactions between inhibitors and Fe
(110) surface can be understood by interaction and binding energies calculated using equation
(10) and (11) [27]:
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
(x10ꢀ6M)
0.0
CR(mg cm−2 h−1)
Surface coverage
η%
(θ)
Blank
7.66
3.20
1.60
0.86
0.56
ꢀꢀꢀ
ꢀ
141
0.583
0.791
0.887
0.926
58.6 ±0.35
79.1±0.35
88.7±0.75
92.6±0.43
APCIꢀ1
282
423
564

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141
282
423
564
141
282
423
564
2.66
1.30
0.66
0.40
2.26
0.80
0.43
0.23
0.52
65.2±0.43
83.0±0.87
91.3±0.75
94.8±0.43
70.4±0.75
89.6±0.87
94.3±0.75
96.9±0.43
APCIꢀ2
APCIꢀ3
0.830
0.913
0.948
0.704
0.896
0.943
0.969
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 APCIꢀ1, APCIꢀ2 and APCIꢀ3, respectively at 564 x10ꢀ6M concentration. The

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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 x10ꢀ6M concentration inhibition efficiency does not increase significantly suggesting that
probably the maximum surface coverage was obtained at this value. [29].
3.1.2. Temperature effect
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 (308ꢀ338 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 cmꢀ2 hꢀ1)
Blank
7.66
11.0
14.3
18.6
APCIꢀ1
0.56
APCIꢀ2
0.40
APCIꢀ3
0.23
308
318
328
338
1.23
1.03
0.70
2.36
2.06
1.80
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:
CR = A exp(−Ea / RT )
(13)

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Where, CR is the corrosion rate, R is the universal gas constant, T is the absolute temperature and
A is the preꢀexponential 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
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 molꢀ1 for blank solution, APCIꢀ1, APCIꢀ2, APCIꢀ3
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 APCIꢀ3 has the highest Ea value
which confirms the best inhibition performance of APCIꢀ3 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.

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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
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
studied by testing several adsorption isotherms including Langmuir, Temkin, Frumkin and and
thermodynamic/kinetic model of ElꢀAwady isotherms.
Fig.4: ElꢀAwady 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]
KadsC =θ / (1−θ)
(14)
Where Kads is the adsorptionꢀ 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

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plotted between log (θ/1ꢀθ) 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 ElꢀAwady adsorption isotherm for mild steel in
presence of APCIs.
Inhibitor
Slope (y)
Intercept (log K)
Temperature(K) 308
318 328 338
1.47 1.43 1.44
1.51 1.39 1.32
1.66 1.34 1.23
308
318
328
338
APCIꢀ1
APCIꢀ2
APCIꢀ3
1.58
1.63
1.83
6.23
6.52
7.45
5.68
5.90
6.53
5.38 5.15
5.29 4.79
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 ElꢀAwady 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 ElꢀAwady 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 APCIꢀ3 has the highest Kads value among all studied APCIs and thus has highest

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inhibition efficiency. The Kads values can be related to the standard free energy of adsorption
(ꢂG0ads) according to the relation [41,42]:
ꢀ
Gaοds = −RT ln(55.5Kads )
(15)
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 ꢂG0 are listed in
ads
Table 5. The values of ꢂG0 at 308K were found to be ꢀ33.48, ꢀ33.89 and ꢀ34.19 kJ molꢀ1 for
ads
APCIꢀ1, APCIꢀ2 and APCIꢀ3 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 Mꢀ1)
ꢀ∆G◦ads (kJ molꢀ1)
318 328 338
Temperature(K) 308
318 328 338
7.49 5.64 3.75
308
APCIꢀ1
APCIꢀ2
APCIꢀ3
8.56
33.48 33.14 32.41 31.37
33.89 33.26 32.68 31.70
34.19 33.41 33.03 32.04
10.05 7.85 6.30 4.28
11.31 8.32 7.18 4.88
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%).

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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

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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
[45].
Table 6: Tafel Polarization parameters (±SD) for mild steel in 1 M HCl solution in absence and
presence of different concentration of APCIs
Inhibitor Conc.
Ecorr
icorr
ꢀβc
η(%) from
icorr value
Rp
η(%)
βa
(10ꢀ6 M) (mV/ (ꢁAcmꢀ2)
SCE)
(mV/dec)
(ꢃcm2) from Rp
(mV/dec)
value
Blank
ꢀꢀꢀꢀ
141
ꢀ445 1150±4.3 70.5±0.32
114.6±0.15 ꢀꢀꢀꢀ
9.58
23.34
42.19
78.82
121.3
25.8
ꢀꢀꢀꢀ
58.95
APCIꢀ1
ꢀ513 493±1.7
ꢀ510 246±2.6
ꢀ495 115±2.6
ꢀ522 81±1.0
ꢀ515 398±2.0
ꢀ518 198±1.7
ꢀ486 103±1.7
ꢀ517 61±1.0
ꢀ522 340±2.0
ꢀ526 132±1.0
ꢀ539 63±1.7
ꢀ516 32±1.7
73.2±0.15
64.3±0.36
83.5±0.28
71.2±0.15
84.0±0.42
56.1±0.14
71.5±0.15
181.3±0.36 57.13±0.15
124.3±0.28 78.60±0.23
129.1±0.10 90.00±0.23
158.7±0.10 92.93±0.08
282
423
564
141
282
423
564
141
282
423
564
77.29
87.84
92.10
62.86
84.68
91.54
94.24
74.07
89.56
93.95
96.79
APCIꢀ2
APCIꢀ3
117±0.36
68.1±0.14
65.39±0.17
82.78±0.15
62.54
113.3
186.1
36.95
91.79
158.6
299.2
144.7±0.10 91.04±0.15
103.5±0.28 121.3±0.28 94.69±0.08
69.0±0.28 109.4±0.42 70.43±0.17
107.1±0.36 167.5±0.20 88.52±0.08
112.3±0.10 168.0±0.10 94.52±0.15
85.7±0.15
161.9±0.10 97.21±0.15
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

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corrosion current density (icorr) and maximum decrease was obtained in the case of APCIꢀ3. 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
maximum value for APCIꢀ3. 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
Ecorr were 77 mV, 72 mV and 71 mV for APCIꢀ1, APCIꢀ2 and APCIꢀ3, 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.

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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

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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/
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
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
r
Conc.
Rs
Rp
Cdl
n
η%
(10ꢀ6M)
(ꢃcm2)
1.120
0.457
0.451
0.899
0.751
1.105
0.500
0.593
1.831
0.655
0.830
1.137
1.107
(ꢃcm2)
(ꢁFcmꢀ2)
Blank
APCIꢀ1
9.58±0.15
23.34±0.33
42.19±0.26
78.82±0.41
121.3±0.51
25.8±0.1
106.21 0.827±0.001
ꢀꢀ
141
282
423
564
141
282
423
564
141
282
423
564
95.62
93.99
70.69
66.42
88.25
72.21
68.14
55.83
70.67
62.13
52.80
34.44
0.763±0.004 58.95±0.56
0.721±0.001 77.29±0.14
0.811±0.002 87.84±0.06
0.805±0.001 92.10±0.03
0.829±0.002 62.86±0.14
0.817±0.004 84.68±0.11
0.766±0.001 91.54±0.03
0.806±0.001 94.24±0.06
0.712±0.003 74.07±0.17
0.805±0.001 89.56±0.09
0.817±0.003 93.95±0.01
0.811±0.002 96.79±0.14
APCIꢀ2
APCIꢀ3
62.54±0.45
113.3±0.37
186.1±0.41
36.95±0.24
91.79±0.26
158.6±0.25
299.2±1.3

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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
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.
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].

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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
3.3.1. SEM analysis
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(aꢀd).
Fig. 9(aꢀd): SEM image of mild steel surface after 3h immersion (a) without APCIs (b) with APCIꢀ1 (c)
with APCIꢀ2 (d) with APCIꢀ3.

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Fig. 9(a) represents the micrograph of mild steel specimen immerged in blank solution
whereas Fig. 9(bꢀd) 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.
However in Fig. 9(bꢀd) 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
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(bꢀd) 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 APCIꢀ1, APCIꢀ2 and APCIꢀ3 the Ra values were 141nm, 127nm and
102nm, respectively.

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Fig. 10(aꢀd): 3D AFM image of mild steel surface after 3h immersion (a) without APCI (b) with
APCIꢀ1 (c) with APCIꢀ2 (d) with APCIꢀ3.
3.4.Quantum Chemical calculation

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Fig. 11: Optimized molecular structure of nonꢀprotonated APCIꢀ1, APCIꢀ2 and APCIꢀ3.

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Fig.12: The frontier molecular orbitals HOMO and LUMO of nonꢀprotonated APCIꢀ1, APCIꢀ2 and
APCIꢀ3.
Fig. 13: Optimized molecular structure of protonated APCIꢀ1, APCIꢀ2 and APCIꢀ3.

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Fig. 14: The frontier molecular orbital HOMO and LUMO of protonated APCIꢀ1, APCIꢀ2 and
APCIꢀ3.
Figs. 11ꢀ14 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

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that HOMO and LUMO are mainly localized over the pꢀchlorophenyl ring and diethylꢀ
phosphonate moieties suggesting that only these part of molecules mainly involve in electron
sharing during metalꢀinhibitor interactions. In contrast, HOMO for protonated form of these
inhibitors mainly localized at phenyl ring of the benzaldehyde moiety, while LUMO is mainly
concentrated at pꢀchlorophenyl and diethylꢀphosphonate moieties. The common quantum
chemical calculation parameters such as energy of highest occupied (EHOMO) and lowest
unoccupied (ELUMO) molecular orbitals, energy band gap (ꢂE), global electronegativity (χ),
hardness (η), softness (σ), fraction of electron transfer (ꢂN) and dipole moment (ꢁ) 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
inhibitors
σ
χ
ꢂN
µ
EHOMO
ELUMO
ꢁE
η
(Debye)
(Hartree)
(Hartree)
(Hartree)
APCI1
APCI2
APCI3
APCI1
APCI2
APCI3
ꢀ0.1694
ꢀ0.1689
ꢀ0.1778
ꢀ0.3781
ꢀ0.3610
ꢀ0.3283
ꢀ0.0255
ꢀ0.0267
ꢀ0.0525
ꢀ0.1686
ꢀ0.1692
ꢀ0.1712
0.1439
0.1422
0.1101
0.2095
0.1918
0.1571
0.0719
0.0711
0.0576
0.1047
0.0959
0.0785
13.89
14.06
17.34
9.54
0.097
0.097
0.110
0.273
0.264
0.249
1.110
1.120
1.275
ꢀ0.076
ꢀ0.040
0.047
7.694
8.044
7.443
5.523
3.702
4.481
Neutral
Protonate
d
10.31
12.72
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 APCIꢀ1 to APCIꢀ3 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 APCIꢀ1 to APCIꢀ3 indicating
that tendency of electron acceptation and thereby inhibition performance increases likewise [55].
During metalꢀinhibitor interactions, a lower value of ꢂE (energy band gap) is consisted with high