Application of new isonicotinamides as a corrosion inhibitor on mild steel in acidic medium: Electrochemical, SEM, EDX, AFM and DFT investigations

Article abstract of DOI:10.1016/j.molliq.2015.09.047

Corrosion inhibition effect of 2-(2-(2-phenyl)-3-(isonicotinamido)-4-oxothiazolidin-5-yl) acetic acid (PITA) and N-(2-phenyl-4-oxothiazolidin-3-yl)-isonicotinamide (POTI) on mild steel corrosion in 15% HCl solution was studied by using weight loss, electrochemical polarization and electrochemical impedance spectroscopy (EIS) techniques. It was found that the inhibition efficiency of both inhibitors increases with increase in concentration. The effect of temperature on the corrosion rate was investigated and some thermodynamic parameters were calculated. Polarization studies show that both studied inhibitors are of mixed type in nature. The results show that both POTI and PITA are good inhibitors, and the adsorption of each inhibitor on mild steel surface obeys Langmuir adsorption isotherm. Scanning electron microscopic (SEM), energy dispersive X-ray spectroscopic (EDX) and atomic force microscopy (AFM) studies were used to characterize the surface morphology of uninhibited and inhibited mild steel specimens. The morphological study indicated adsorption of inhibitor molecules on the surface of the mild steel. The density functional theory (DFT) was employed for theoretical calculations. The results obtained from experimental measurements and those from theoretical calculations are in good agreement.

Full text of DOI:10.1016/j.molliq.2015.09.047

Journal of Molecular Liquids 212 (2015) 686–698
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Application of new isonicotinamides as a corrosion inhibitor on mild steel
in acidic medium: Electrochemical, SEM, EDX, AFM and
DFT investigations
a
a
b,
b
M. Yadav a, , R.R. Sinha , T.K. Sarkar , I. Bahadur , E.E. Ebenso
⁎
⁎
a
Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India
Department of Chemistry, Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus),
b
Private Bag X2046, Mmabatho 2735, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Corrosion inhibition effect of 2-(2-(2-phenyl)-3-(isonicotinamido)-4-oxothiazolidin-5-yl) acetic acid (PITA) and
N-(2-phenyl-4-oxothiazolidin-3-yl)-isonicotinamide (POTI) on mild steel corrosion in 15% HCl solution was
studied by using weight loss, electrochemical polarization and electrochemical impedance spectroscopy (EIS)
techniques. It was found that the inhibition efficiency of both inhibitors increases with increase in concentration.
The effect of temperature on the corrosion rate was investigated and some thermodynamic parameters were cal-
culated. Polarization studies show that both studied inhibitors are of mixed type in nature. The results show that
both POTI and PITA are good inhibitors, and the adsorption of each inhibitor on mild steel surface obeys Langmuir
adsorption isotherm. Scanning electron microscopic (SEM), energy dispersive X-ray spectroscopic (EDX) and
atomic force microscopy (AFM) studies were used to characterize the surface morphology of uninhibited and
inhibited mild steel specimens. The morphological study indicated adsorption of inhibitor molecules on the sur-
face of the mild steel. The density functional theory (DFT) was employed for theoretical calculations. The results
obtained from experimental measurements and those from theoretical calculations are in good agreement.
© 2015 Published by Elsevier B.V.
Received 11 August 2015
Accepted 27 September 2015
Available online xxxx
Keywords:
Mild steel
Corrosion inhibition
Isonicotinamides
EIS
SEM
AFM
DFT
1. Introduction
Acid solutions are extensively used in a variety of industrial processes
such as oil well acidification, acid pickling and acid cleaning [15], which
Heterocyclic organic compounds containing sulfur, phosphorus, oxy-
gen, nitrogen and aromatic rings are the most effective and efficient inhib-
itors for the metals in acidic medium [1–9]. Some of the heterocyclic
compounds are of medicinal importance and have been successfully test-
ed with various diseases and gained much importance as well. Nitrogen
heterocycles have received attention towards pharmaceutical chemistry
due to their diverse medicinal potential [10]. Amide bonds play a critical
role in the field of chemistry and biology in determining structure and dy-
namics in small molecules as well as in large molecules such as 2-(2-(2-
phenyl)-3-(isonicotinamido)-4-oxothiazolidin-5-yl) acetic acid (PITA)
and N-(2-phenyl-4-oxothiazolidin-3-yl)-isonicotinamide (POTI) which
are considered to be heterocyclic compounds. An important aspect of
the amide linkage is the partial double bond character that results from
donation of lone-pair electron density from amide nitrogen to the carbon-
yl carbon. Due to this a significant high barrier C―N bond rotation is
found to be in amine based compounds. Hence, the modified barrier pos-
sesses a number of structural and electronic properties which led to sig-
nificant importance of theoretical and experimental interest [11–14].
generally lead to serious metallic corrosion. To minimize the effect of cor-
rosion during these processes corrosion inhibitor is added to the acid so-
lution. Use of corrosion inhibitor is one of the most economical and
practical methods for minimizing and controlling corrosion process [16,
17]. The application of organic compounds containing nitrogen, oxygen
and sulfur as corrosion inhibitor in acid medium is reported in literature
[18–21]. Due to their industrial applications of corrosion inhibitors, re-
searchers are looking for efficient and environment friendly inhibitors.
Most of the corrosion inhibitors inhibit corrosion due to adsorption of in-
hibitor molecules on the metal surface. The nature of the inhibitor, surface
state of the metal and excess charge of the metal surface will affect the ad-
sorption behavior of inhibitor molecule on the metal surface [22].
Keeping in view the importance of corrosion inhibitors in various in-
dustries, the present work focused on synthesis of 2-(2-(2-phenyl)-3-
(isonicotinamido)-4-oxothiazolidin-5-yl) acetic acid (PITA) and N-(2-
phenyl-4-oxothiazolidin-3-yl)-isonicotinamide (POTI) in the laboratory
and evaluation of corrosion inhibition performance of these inhibitors
on mild steel in 15% HCl solution by weight loss measurement, potenti-
odynamic polarization, electrochemical impedance measurement as
well using quantum chemical calculations. To the best of our knowledge
there is no any information available in open literature on studied com-
pounds as a corrosion inhibitor for mild steel in acidic medium. The
⁎
Corresponding authors.
E-mail addresses: yadavdrmahendra@yahoo.co.in, bahadur.indra@gmail.com
(I. Bahadur).
http://dx.doi.org/10.1016/j.molliq.2015.09.047
0167-7322/© 2015 Published by Elsevier B.V.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
687
present work is a continuation of our systematic studies on corrosion in-
hibition of mild steel in acidic environment by synthesized organic com-
pounds [23–25].
(0.01 mol) or thiomalic acid (0.01 mol) on an oil-bath at 120–125°C
for 12 h. The reaction mixture was cooled and treated with 10% sodium
bicarbonate solution. The product was isolated and recrystallized from
methanol–dioxane (4:1). The structures of the compounds were sup-
ported by their IR, 1H-NMR and elemental analysis data and their purity
was confirmed by thin layer chromatography (TLC). The IR and 1HNMR
spectra of PITA and POTI are shown in Fig. 1 and Fig. 2, respectively.
The analytical and spectral data of the synthesized compounds are
given below:
2. Experimental
2.1. Mild steel samples
Mild steel specimen which contained C = 0.14 wt.%, Mn = 0.98 wt.%,
Si = 0.033 wt.%, S = 0.018 wt.%, P = 0.026 wt.%, Cr-0.02 wt.% and remain-
der iron was used for corrosion studies. Mild steel coupons having dimen-
sion 6.0 cm × 2.5 cm × 0.1 cm and 1.0 cm × 1.0 cm × 0.1 cm were
mechanically cut, abraded using emery papers of different grit sizes up
to 1200 grit, polished with Al2O3 (1 μm and then 0.3 μm particle size)
for weight loss measurement and electrochemical measurement studies
respectively. The specimens were cleaned with distillate water and
degreased with acetone, dried and stored in desiccator.
• PITA.
Yield: 76%; elemental analysis: C, 57.14; H, 4.20; N, 11.76; S, 8.96.
Found: C, 56.95; H, 4.16; N, 11.68; S, 8.92.
IR (cm−1): 3310 (NH), 1710 (C_O, thiazolidinone), 1570, 1470
(C_CAr), 1680 (C_O). 1 H-NMR (300 MHz, CDCl3) δ: 9.1 (2 H, pyri-
dine), 7.9 (2 H, pyridine), 8.1 (1 H, NH), 11.0 (1 H, COOH), 7.3 (5 H, phe-
nyl), 3.8 (1 H, thiazolidine), 5.8 (1 H, thiazolidine), 2.9 (1 H, CH2), 2.7
(1 H, CH2).
2.2. Test solution
• POTI.
The test solutions (15% HCl) were prepared by dilution of analytical
grade 37% HCl (Ramkem) with double distilled water. The concentra-
tions of the studied inhibitors ranged from 100 ppm to 400 ppm
(mg L−1) and the volume of test solution used for weight loss measure-
ment and electrochemical studies was 500 mL and 150 mL, respectively.
Yield: 72%; elemental analysis: C, 60.20; H, 4.35; N, 14.05; S, 11.07.
Found: C, 59.94; H, 4.32; N, 14.16; S, 10.58.
IR (cm−1): 3320 (NH), 1720 (C_O, thiazolidinone), 1580, 1470
(C_CAr), 1690 (C_O).
1 H-NMR (300 MHz, CDCl3) δ: 9.2 (2 H, pyridine), 7.8 (2 H, pyridine),
8.0 (1 H, NH), 7.1 (5 H, phenyl), 3.3 (2 H, thiazolidine), 5.9 (1 H,
thiazolidine).
2.3. Synthesis of corrosion inhibitors
The compounds 2-(2-(2-phenyl)-3-(isonicotinamido)-4-oxothiazolidin-
5-yl) acetic acid (PITA) and N-(2-phenyl-4-oxothiazolidin-3-yl)-
isonicotinamide (POTI) were synthesized by the reported procedure
[26] as shown in Scheme 1. To an equimolar methanolic solution of
isonicotinic acid hydrazide (0.1 mol) and substituted benzaldehyde
(0.1 mol), a few drops of glacial acetic acid were added. The mixture
was then refluxed on water bath for 5–6 h. It was then allowed to
cool, poured onto crushed ice and recrystallised from methanol. The
resulting compound was N′-benzylidene isonicotinohydrazides. The
compounds POTI and PITA were synthesized by heating mixture of N′-
benzylidene isonicotinohydrazides (0.01 mol) and thioglycolic acid
2.4. Weight loss method
Gravimetric experiments were performed according to the standard
methods [27]. The corrosion rate (CR), inhibition efficiency (η%) and
surface coverage (θ) was determined by following equations:
8:76 ꢀ 104 ꢀ W
D ꢀ A ꢀ t
À
Á
CR mmy−1
¼
ð1Þ
Scheme 1. Synthetic route and structure of 2-(2-(2-phenyl)-3-(isonicotinamido)-4-oxothiazolidin-5-yl) acetic acid (PITA) and N-(2-phenyl-4-oxothiazolidin-3-yl)-isonicotinamide
(POTI).

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M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
Fig. 1. IR spectra of PITA and POTI.
where, W = weight loss (g), A = area of specimen (cm2) exposed in
acidic solution, t = exposure time (h), and D = density of mild steel
(g cm−3);
2.5. Electrochemical method
The electrochemical studies were conducted in a conventional three-
electrode cell consisting of mild steel sample of 1 cm2 exposed area as
working electrode, a platinum counter electrode and a saturated calomel
electrode (SCE) as reference electrode, using CH electrochemical worksta-
tion (Model No: CHI 760D, manufactured by CH Instruments, Austin,
USA) at 303 K. Potentiodynamic polarization curves were obtained in
the potential range from −250 to +250 mV vs. SCE at OCP at a scan
rate of 1 mVs−1. The linear Tafel segments of anodic and cathodic curves
were extrapolated to obtain corrosion current densities (icorr). The per-
centage inhibition efficiency (η%), was calculated using the equation:
CR0−CRi
CR0
θ ¼
ð2Þ
ð3Þ
CR0−CRi
CR0
ηð%Þ ¼
ꢀ 100
where, CR0 and CRi are corrosion rate in the absence and presence of in-
hibitors. This experiment was repeated at different temperatures of 30,
40, 50 and 60 °C by using water circulated Ultra thermostat to deter-
mine the temperature dependence of the inhibition efficiency.
i0corr−icorr
i0corr
ηð%Þ ¼
ꢀ 100
ð4Þ
where, i0corr and icorr are the values of corrosion current densities in the
absence and presence of inhibitor, respectively.
Electrochemical impedance measurements were carried out using
AC signals of amplitude 10 mV peak to peak at the open circuit potential
in the frequency range of 100 kHz to 10 mHz. All impedance data were
fitted to appropriate equivalent circuits using ZSimpWin.3.21 software.
The inhibition efficiency (η%) was calculated from charge transfer resis-
tance values obtained from impedance measurements using the follow-
ing relation:
Rct ðinhÞ−Rct
Rct ðinhÞ
ηð%Þ ¼
ꢀ 100
ð5Þ
where Rct(inh) and Rct are charge transfer resistance in the presence and
absence of inhibitor respectively. The values of double layer capacitance
(Cdl) were calculated from charge transfer resistance and CPE parame-
ters (Y0 and n) using the expression [28]:
ꢀ
ꢁ
1=n
1−n
Cdl
¼
Y0Rct
ð6Þ
where Y0 is CPE constant and n is CPE exponent. The value of n repre-
sents the deviation from the ideal behavior and it lies between 0 and 1.
Fig. 2. 1H NMR spectra of PITA and POTI.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
689
Table 1
Corrosion parameters of mild steel in 15% HCl solution in the presence and absence of inhibitor at different temperatures, obtained from weight loss measurements.
Conc. (ppm)
303 K
313 K
323 K
333 K
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
Blank
28.2
–
–
58.1
–
–
98.9
–
–
144.5
–
–
PITA
100
200
300
400
5.80
3.22
1.29
1.07
0.794
0.885
0.954
0.962
79.41
88.58
95.41
96.18
13.35
7.83
3.12
2.82
0.770
0.865
0.946
0.951
77.02
86.51
94.62
95.13
26.59
16.88
8.87
0.731
0.829
0.910
0.928
73.11
82.93
91.03
92.87
45.83
31.67
18.91
15.83
0.682
0.780
0.869
0.890
68.28
78.08
86.91
89.04
7.04
POTI
100
200
300
400
7.55
5.06
1.93
1.59
0.732
0.820
0.931
0.943
73.21
82.05
93.15
94.34
16.83
11.51
4.77
0.710
0.801
0.917
0.932
71.02
80.18
91.78
93.24
32.45
22.92
12.83
9.36
0.671
0.768
0.870
0.905
67.18
76.82
87.02
90.53
55.73
41.09
24.43
21.34
0.614
0.715
0.830
0.852
61.43
71.56
83.08
85.23
3.92
2.6. Scanning electron microscopic and energy dispersive spectroscopy
analysis
2.7. Atomic force microscopy
The AFM images of polished, uninhibited and inhibited mild steel
samples were carried out using a Nanosurf Easyscan2 instrument,
Model: NT-MDT, Russia; Solver Pro-47.
For surface morphological study of the uninhibited and inhibited
mild steel samples, SEM and EDX images were recorded using the in-
strument HITACHI S3400N.
2.8. Quantum chemical study
Complete geometrical optimizations of the investigated molecules
are performed using density functional theory (DFT) with the Beck's
three parameter exchange functional along with the Lee–Yang–Parr
nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set is
implemented in Gaussian 03 program package [29,30]. Theoretical pa-
rameters such as the energies of the highest occupied and lowest unoc-
cupied molecular orbital (EHOMO and ELUMO), energy gap (ΔE) and dipole
moment (μ) were determined.
3. Results and discussion
3.1. Weight loss measurements
3.1.1. Effect of inhibitor concentration and temperature
Corrosion parameters namely, corrosion rate (CR), surface coverage
(θ) and inhibition efficiency (η %) of mild steel in 15% HCl solution in the
absence and presence of different concentrations (100–400 ppm) of in-
hibitor at different temperatures (303 K–333 K), obtained from weight
loss measurements are shown in Table 1. From Table 1, it is apparent
that inhibition efficiency increased with increasing the concentration
of the inhibitors. The inhibition efficiency of PITA and POTI at 400 ppm
was found to be 96.18% and 94.34% respectively, at 303 K (Table 1).
The increase in inhibition efficiency with increasing concentration of in-
hibitors is due to increase in the surface coverage, resulting retardation
of metal dissolution [25].
Table 2
Activation parameter for mild steel in 15% HCl solution in the absence and presence of in-
hibitor obtained from weight loss measurements.
Ea (kJmol−1
)
ΔH (kJ/mol)
ΔS (Jmol−1K−1
⁎
)
⁎
Inhibitor
Concentration
(ppm)
Blank
PITA
–
45.78
57.98
64.14
72.95
73.80
55.96
58.62
72.33
72.72
43.13
55.34
61.50
71.53
73.80
53.34
56.00
70.09
69.71
−74.45
−47.51
−32.18
−4.18
100
200
300
400
100
200
300
400
−2.80
POTI
−52.05
−46.69
−10.12
−9.55
Fig. 3. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl solution
(a) PITA (b) POTI.

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M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
It is also clear from the Table 1 that the inhibition efficiency de-
energy was calculated using the expression Ea = −(slope) × 2.303R.
The calculated values of Ea are summarized in Table 2. It is evident
from Table 2 that the values of the apparent activation energy for the
inhibited solutions were higher than that for the uninhibited solution,
indicating that the dissolution of mild steel was decreased due to forma-
tion of a barrier by the adsorption of the inhibitors on metal surface [32].
The values of standard enthalpy of activation (ΔH*) and standard en-
tropy of activation (ΔS*) were calculated by using the transition state
equation:
creased with increasing temperature from 303 K to 333 K. Such type
of behavior can be described on the basis that the increase in tempera-
ture leads to a shift of the equilibrium constant towards desorption of
the inhibitors molecules at the surface of mild steel [31]. The inhibition
efficiency of PITA is greater than POTI at all concentrations and
temperatures.
3.1.2. Thermodynamic and activation parameters
To evaluate the adsorption and thermodynamic activation parame-
ters of corrosion processes of mild steel in15% HCl solution, weight
loss measurements were carried out in the temperature range 303–
333 K in the absence and presence of inhibitors after 6 h of immersion
time. The values of the apparent activation energy (Ea) were calculated
using the Arrhenius equation:
ꢂ
ꢃ
ꢂ
ꢃ
RT
Nh
ΔSꢁ
R
ΔHꢁ
RT
CR ¼
exp
exp
−
ð8Þ
where, h is Planck's constant and N is the Avogadro number.
A plot of log (CR/T) against 1/T (Fig. 4 a, b) gave straight lines with a
slope of −ΔH*/2.303R and an intercept of [log(R/Nh) + ΔS*/(2.303R)],
from which the activation thermodynamic parameters ΔH* and ΔS*
were calculated, as listed in Table 2. The values of Ea and ΔH* are close
to each other, as expected from the concept of transition-state theory,
and follow the same pattern of variation with different concentrations
of the inhibitor. The negative value of ΔS* for both inhibitors indicates
that the formation of the activated complex in the rate determining
step represents an association rather than a dissociation step, meaning
that a decrease in disorder takes place during the course of the transi-
tion from reactants to activated complex [33].
−Ea
2:303RT
logCR ¼
þ logA
ð7Þ
where Ea is the apparent activation energy, R is the molar gas constant
(8.314 J K−1mol−1), T is the absolute temperature (K) and A is the Ar-
rhenius pre-exponential factor. Fig. 3 (a, b) presents the Arrhenius
plot of log CR against 1/T for the corrosion of mild steel in 15% HCl solu-
tion in the absence and presence of inhibitors PITA and POTI at concen-
trations ranging from 100 to 300 ppm. From Fig. 3 (a, b), the activation
Fig. 4. Transition state plot of log CR/T versus 1000/T for mild steel in 15% HCl solution at
different concentrations of (a) PITA (b) POTI.
Fig. 5. Langmuir plots of (Cinh/θ) versus Cinh for (a) PITA (b) POTI.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
691
Table 3
Adsorption parameters for PITA and POTI for mild steel in 15% HCl solution at a temperature range of 303–333 K.
Inhibitor
Temperature (K)
Kads (M−1
)
ΔG°ads (kJ mol−1
)
Slope
R2
ΔH°ads (kJ mol−1
)
ΔS°ads (J mol−1K−1
)
303
313
323
333
303
313
323
333
5.2 × 105
2.4 × 105
1.3 × 105
0.6 × 105
4.4 × 105
2.1 × 105
−37.5
−36.8
−36.2
−35.4
−37.1
−36.3
−35.6
−34.7
0.998
0.996
0.994
0.992
0.997
0.994
0.993
0.991
0.994
0.996
0.995
0.991
0.992
0.993
0.996
0.997
−64.47
−61.04
−89
−79
PITA
POTI
1.03 × 105
0.5 × 105
3.1.3. Adsorption isotherm
adsorption, indicating that the adsorption process of these inhibitors
at mild steel surface involves both the physical as well as chemical ad-
The interaction between the inhibitor and mild steel surface can be
very well understood in terms of the adsorption isotherm. Attempts
were made to fit experimental data to various isotherms including
Frumkin, Langmuir, Temkin, and Freundlich. The results were best fitted
by the Langmuir adsorption isotherm:
sorption. It is apparent from Table 3 that, the negative value of ΔG0
ads
decreased on increasing the temperatures, suggesting that the adsorp-
tion of inhibitor molecules on mild steel surface is not favorable with in-
creasing experimental temperature, indicating that physisorption has
the major contribution while chemisorption has the minor contribution
in the adsorption process.
The nature of adsorption (physisorption or chemisorptions or both)
of inhibitor on the surface of mild steel can be determined on the basis
of the values of enthalpy of adsorption (ΔH0ads) for the inhibitors. The
enthalpy and entropy changes for the adsorption of inhibitors (PITA
Cinh
θ
1
Kads
¼
þ Cinh
ð9Þ
where, Cinh is the inhibitor concentration, Kads is the equilibrium con-
stant for adsorption–desorption process. Plotting Cinh/θ vs. Cinh yielded
a straight line [Fig. 5] with a correlation coefficient (R2) and slope values
given in Table 3 at different temperatures. The R2 and slope values in
Table 3 are near to unity indicating that the adsorption of these inhibi-
tors obeys the Langmuir adsorption isotherm.
The values of Kads were calculated from the intercept of Fig. 5. Large
values of Kads were obtained for both studied inhibitors suggesting more
efficient adsorption and hence better corrosion inhibition efficiency.
Using the values of Kads, the values of ΔG°ads were obtained by using
the following equation:
ΔG0ads ¼ −RT lnð55:5Kads
Þ
ð10Þ
where R is the gas constant and T is the absolute temperature (K). The
value of 55.5 is the concentration of water in solution in mol L−1. Calcu-
lated values of Kads and ΔG°ads are listed in Table 3. The negative values
of ΔG°ads reveal the spontaneity of adsorption process. In general, values
of ΔG°ads up to −20 kJmol−1 are compatible with physisorption and
those which are more negative than −40 kJmol−1 involve chemisorp-
tions [34]. The calculated ΔG°ads values for PITA and POTI were found
in the range of −35.4 to −37.5 and −34.7 to −37.1 kJmol−1, respec-
tively, at different temperatures (303–333 K), these values were be-
tween the threshold values for physical adsorption and chemical
Fig. 7. Potentiodynamic polarization curves for mild steel in 15% HCl solution in the pres-
ence and absence of inhibitor at 303 K. (a) PITA (b) POTI.
Fig. 6. The relationship between ΔG0ads and temperature.

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M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
Table 4
Electrochemical parameter for mild steel in 15% HCl solution in the presence or absence of inhibitor at 303 K.
Conc.
(ppm)
Tafel extrapolation data
EIS data
n
Ecorr (V vs SCE)
−495
β
a (mV dec−1
)
-βc (mV dec−1
)
icorr (μAcm−2
)
η %
Rs (Ω cm2)
Rct (Ω cm2)
Y0 (μFcm−2
)
Cdl (μF cm2)
η %
Blank
103
182
10.0
–
1.02
2.7
483
0.852
168
–
PITA
100
200
300
−431
−437
−439
111
92
103
176
184
169
1.94
1.29
0.52
80.6
87.1
94.8
0.89
0.68
0.93
15.1
22.0
45.4
159
108
43
0.876
0.892
0.927
67.8
51.9
26.3
82.00
87.77
94.00
POTI
100
200
300
−435
−438
−442
87
103
94
181
172
158
2.70
1.67
1.24
74.1
83.3
93.2
0.95
0.83
0.77
11.0
17.2
35.0
219
137
60
0.868
0.884
0.927
87.5
61.8
32.8
75.45
84.12
92.22
and POTI) on mild steel surface were determined from the basic ther-
modynamic equation [35]:
ΔG0 ¼ ΔH0ads−TΔS0
ð11Þ
ads
ads
where ΔH0ads and ΔS0ads are the standard enthalpy and entropy changes
of adsorption process, respectively. The plot of ΔG0ads versus T was lin-
ear (Fig. 6) with the slope equal to −ΔS0ads and intercept of ΔH0
.
ads
The calculated values of ΔH0
and ΔS0
are listed in Table 3. The
ads
ads
ΔHads values are negative for both studied inhibitors, which suggests
that the adsorption of inhibitor's molecules on mild steel surface is an
exothermic process. It has been reported in literature that an
Fig. 9. Equivalent circuit applied for fitting of the impedance spectra.
Fig. 8. Nyquist plot for mild steel in 15% HCl solution containing various concentrations of
Fig. 10. Bode plots for mild steel in a 15% HCl solution in the absence and presence of dif-
(a) PITA (b) POTI at 303 K.
ferent concentrations of inhibitors (a) PITA (b) POTI.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
693
endothermic adsorption process (ΔHads N 0) is due to chemisorption
while an exothermic adsorption process (ΔHads b 0) may be attributed
to physisorption, chemisorption or a mixture of both [36]. In an exother-
mic process, physisorption is distinguished from chemisorption by con-
sidering the absolute value of ΔHads, it is lower than 40 kJ mol−1 for the
physisorption process whereas for chemisorption process approaches
100 kJ mol−1. In the present study, Hads values for PITA and POTI are
−64.47 kJ mol−1 and −61.04 kJ mol−1 respectively, which are larger
than the common physical adsorption heat, but smaller than the com-
mon chemical adsorption heat, emphasizing that mixed type (both
physical and chemical) adsorption takes place. The similar results
were also reported by other authors [37–39].
corresponding corrosion current density (icorr). This indicates that
both inhibitors functioned as mixed-type inhibitors in 15% HCl solution
[42].
The reduction in the value of corrosion current density in the pres-
ence of inhibitor is more pronounced with the increasing inhibitor con-
centrations, which results in increase in inhibition efficiency. The
inhibitor molecules are first adsorbed on the mild steel surface, blocking
the available reaction sites, and decrease the corrosion current density.
3.2.2. Electrochemical impedance spectroscopy (EIS) studies
Electrochemical impedance measurements were undertaken to pro-
vide information on the kinetics of the electrochemical processes at the
mild steel/acid interface and how this is modified by the presence of in-
hibitor. Nyquist plots for mild steel corrosion in 15% HCl solution in the
absence and presence of different concentrations (100–300 ppm) of the
inhibitors PITA and POTI are given in Fig. 8 (a, b), respectively. The
Nyquist plots show single semicircles for all systems over the frequency
range studied, corresponding to one time constant. The impedance
spectra were analyzed by fitting to the equivalent circuit model
shown in Fig. 9, which has been used previously to adequately model
the mild steel/acid interface [43]. In this equivalent circuit, the solution
resistance is shorted by a constant phase element (CPE) that is placed in
parallel to the charge transfer resistance. The CPE is used in place of a ca-
pacitor to compensate deviations from ideal dielectric behavior arising
from the inhomogeneous nature of the electrode surfaces [44]. The im-
pedance of the CPE is given by
The negative value of ΔS0ads obtained for adsorption of both the in-
hibitors as shown in Table 3 suggests that before the adsorption of
inhibitor's molecules on the mild steel surface, inhibitor molecules
might freely move in the bulk solution, but with the progress in the ad-
sorption, inhibitor molecules were orderly adsorbed on the mild steel
surface, as a result a decrease in entropy is observed [40]. From the ther-
modynamic principles, since the adsorption is an exothermic process, it
must be accompanied by a decrease of entropy [41].
3.2. Electrochemical measurements
3.2.1. Potentiodynamic polarization studies
Potentiodynamic polarization experiments were undertaken to dis-
tinguish the effect of both inhibitors on the anodic dissolution of mild
steel and cathodic hydrogen ion reduction. Typical potentiodynamic po-
larization curves for mild steel in 15% HCl in the absence and presence of
different concentration of inhibitors are shown in Fig. 7 (a, b), while the
electrochemical parameters derived from the polarization curves are
summarized in Table 4. Addition of the inhibitors (PITA and POTI) is
seen to affect the anodic as well as the cathodic partial reactions, shifting
the corrosion potential (Ecorr) slightly towards more positive (anodic)
values and reducing the anodic and cathodic current densities and the
−n
ZCPE ¼ Y0−−1ðjωÞ
ð12Þ
where Y0 and n stand for the CPE constant and exponent, respectively,
j = (−1)1/2 is an imaginary number, and ω is the angular frequency
in rad s−1 (ω = 2πf), where f is the frequency in Hz. The corresponding
electrochemical parameters are presented in Table 4 and reveal that the
both inhibitors increased the magnitude of Rct, with corresponding
Fig. 11. SEM image of mild steel in 15% HCl solution after 6 h immersion at 303 K (a) before immersion (polished), (b) after immersion without inhibitor (c) with 300 ppm PITA (d) with
300 ppm POTI.

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M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
Fig. 12. EDX spectra of mild steel specimens (a) polished, (b) after immersion without inhibitor (c) with 300 ppm PITA (d) with 300 ppm POTI.
decrease in the double layer capacitance (Cdl). The increase in Rct values
in inhibited systems, which corresponds to an increase in the diameter
of the Nyquist semicircle, confirms the corrosion inhibiting effect of
both inhibitors. The observed decrease in Cdl values, which normally re-
sults from a decrease in the dielectric constant and/or an increase in the
double-layer thickness, can be attributed to the adsorption of inhibitor
molecules (with lower dielectric constant compared to the displaced
adsorbed water molecules) onto the metal/electrolyte interface, there-
by protecting the metal from corrosive attack [45].
The Bode phase angle plots (Fig. 10 a, b) show single maximum (one
time constant) at intermediate frequencies, broadening of this maxi-
mum in the presence of inhibitors accounts for the formation of a pro-
tective layer on the electrode surface. Moreover, there is only one
phase maximum in Bode plot (Fig. 10 a, b) for both inhibitors, which in-
dicates only one relaxation process, which would be the charge transfer
process, taking place at the metal–electrolyte interface.
the different test solutions for 6 h at 303 K. SEM images of polished sam-
ple, uninhibited sample and inhibited samples are shown in Fig. 11 (a–
d). The morphology of the polished mild steel specimen (Fig. 11 a) is
very smooth and shows no corrosion while mild steel specimen dipped
in 15% HCl solution in the absence of inhibitor (Fig. 11 b) is very rough
and the surface is damaged due to metal dissolution. However, the pres-
ence of 300 ppm of inhibitor (PITA, POTI) suppresses the rate of corro-
sion and surface damage has been diminished considerably (Figs. 11 c,
d) as compared to the blank solution (Fig. 11 b) suggesting formation
of a protective inhibitor film at the mild steel surface.
3.4. Energy dispersive spectroscopy
Energy dispersive X-ray analysis (EDX) technique was employed in
order to get information about the composition of the surface of the
mild steel sample in the absence and presence of inhibitors in 15% HCl
solution. The results of EDX spectra are shown in Fig. 12 (a–d). The per-
centage atomic content of various elements of the polished, uninhibited
and inhibited mild steel surface determined by EDX is shown in Table 5.
The percentage atomic content of Fe for mild steel immersed in 15% HCl
Fig. 10 (a, b) shows that the impedance value in the presence of both
inhibitors is larger than in absence of inhibitors and the value of imped-
ance increases on increasing the concentration of both studied inhibi-
tors. These mean that the corrosion rate is reduced in the presence of
the inhibitors and continued to decreasing on increasing the concentra-
tion of inhibitors.
Table 5
Electrochemical results (η %) are in good agreement with the results
(η %) obtained by weight loss experiment.
Percentage atomic contents of elements obtained from EDX spectra.
Inhibitors
Fe
C
S
Cr
Mn
Cl
N
O
3.3. Scanning electron microscopy
Polished mild steel
Mild steel in blank HCl 83.12 15.68
85.26 12.46
–
–
0.86 0.46
0.67 0.28 2.29
–
–
–
–
6.36
Mild steel in PITA
Mild steel in POTI
69.24 19.26 1.72 0.56
71.23 19.36 1.68 0.54
–
–
0.31 3.42 13.28
0.32 3.42 10.62
Morphological studies of the surfaces of mild steel specimens in un-
inhibited and inhibited acid were analyzed by SEM after immersion in

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
695
Fig. 13. Atomic force micrographs of mild steel surface (a) polished mild steel, (b) mild steel in 15% HCl solution and (c) in the presence of inhibitor POTI (d) PITA.
Fig. 14. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for inhibitors in gaseous state (a) PITA (b) POTI [atom legend: white = H; gray = C; blue = N;
red = O].

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M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
Table 6
that PITA protects the mild steel surface more efficiently than POTI in
15% HCl solution.
Quantum chemical parameters for PITA and POTI in gaseous state.
Inhibitor
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
μ (D)
3.6. Quantum chemical analysis
PITA
POTI
−4.443
−4.648
−1.684
−1.636
2.759
3.012
2.32
2.40
In order to study the effect of molecular structure on the inhibition
efficiency, quantum chemical calculations were performed by using
DFT and all the calculations were carried out with the help of complete
geometry optimization. The optimized geometry, EHOMO and ELUMO of
PITA and POTI in gaseous and aqueous media are shown in Figs. 14(a,
b) and 15(a, b) respectively. As per the frontier molecular orbital theory,
frontier molecular orbital's HOMO and LUMO are involved in the course
of adsorption of the inhibitor molecules [46]. In general the higher the
value of HOMO, the higher would be the electron donating capacity of
the inhibitor to vacant d-orbital of the metal. And the lower the value
of LUMO, the greater would be the electron accepting ability of the in-
hibitor from the filled metal orbitals. But the most important parameter
is ΔE, which is the energy difference between LUMO and HOMO. The
lower the value of ΔE the easier would be the release of electron and
the stronger would the adsorption [47]. Thus to become a good corro-
sion inhibitor, easy donation of electron as well as easy acceptance of
electron in their vacant orbital's is necessary [48]. The quantum chemi-
cal parameters for PITA and POTI in gaseous and aqueous media are rep-
resented in Tables 6 & 7 respectively.See Fig. 15
Table 7
Quantum chemical parameters for PITA and POTI in aqueous media.
Inhibitor
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
μ (D)
PITA
POTI
−4.124
−4.442
−1.862
−1.746
2.262
2.696
2.86
2.62
solution is 83.12%, and those for mild steel dipped in an optimum con-
centration (300 ppm) of POTI and PITA are 71.23% and 69.24%, respec-
tively. From Fig. 12 (a–d) it is apparent that the spectra of inhibited
samples show suppressed Fe peaks, when compared with the polished
and uninhibited mild steel sample. This suppression of Fe lines is due
to the inhibitory film formed on the mild steel surface. The EDX spectra
of inhibited mild steel contain the peaks corresponding to all the ele-
ments present in the inhibitor molecules indicating the adsorption of in-
hibitor molecules at the surface of mild steel.
3.6.1. Gaseous PITA and POTI
3.5. Atomic force microscopy
In gaseous state inhibitor molecules act as neutral molecules. The in-
spection of Table 6 reveals that the EHOMO values of PITA are greater than
POTI, thus PITA has more electron donating capacity than POTI. Also
ELUMO value of PITA is lower than POTI, which makes PITA to accept
more electron than POTI from filled d-orbital of mild steel and causes
its stronger adsorption. The ΔE value of PITA is lower than POTI, which
causes PITA to release electron easily and intern to strengthen its ad-
sorption. So, overall the adsorption order can be given as follows:
PITA N POTI. The adsorption process is occurring through transfer of
lone pair of electrons of the inhibitor molecules to the vacant d-orbital
of the metal causing chemisorptions nature of adsorption.
The three-dimensional AFM images of polished, uninhibited and
inhibited mild steel samples are shown in Fig. 13 (a–d). The average
roughness of polished mild steel sample (Fig. 13 a) and mild steel sam-
ple in 15% HCl solution without inhibitor (Fig. 13 b) were found as 25
and 650 nm. It is clearly shown in Fig. 13 (b) that mild steel sample is
badly damaged due to the acid attack on surface. However, in the pres-
ence of optimum concentration (300 ppm) of POTI and PITA as shown in
Fig. 13 (c, d), the average roughness were reduced to 160 and 150 nm,
respectively. The lower value of roughness for PITA than POTI reveals
Fig. 15. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for inhibitors in aqueous media for PITA and POTI [atom legend: white = H; Gray = C; blue = N;
red = O].

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 686–698
697
Fig. 16. The schematic illustration of different modes of adsorption on metal/acid interface.
3.6.2. Aqueous media PITA and POTI
➢ In the presence of both inhibitors, charge transfer resistance in-
creases and double layer capacitance decreases due to adsorption
of the inhibitor molecules on the surface of mild steel.
➢ The potentiodynamic polarization curves showed that both inhibi-
tors act as mixed type inhibitors.
➢ The SEM, EDX and AFM investigations indicated that the inhibitor
molecules are adsorbed on the mild steel surface and protected the
mild steel against corrosion.
In aqueous media there is the possibility of the inhibitor molecules
to undergo protonation through negatively charged nitrogen atom.
The protonated inhibitor molecules then adsorb over the mild steel sur-
face. The quantum chemical parameters of the inhibitors in aqueous
media are reported in Table 7. Observing Table 7, we could say that in
aqueous media EHOMO values for both the inhibitors (PITA and POTI)
are higher than in gaseous state, this indicates that the protonation of
inhibitors occurs and protonated inhibitors have greater tendency to
donate electrons, and thus their adsorption capacity over mild steel sur-
face is more. Also ΔE values in protonated inhibitors are less as com-
pared to the neutral one, which suggests that protonated inhibitors
have more reaction capacity than the neutral one. So, overall in the
aqueous media protonated inhibitors have more interaction ability
than neutral inhibitors. The values of magnetic moment (μ) for both in-
hibitors in aqueous media are higher than in gaseous state indicating
the higher electrostatic force of attraction due to protonation of
inhibitor molecules resulting physisorption nature of adsorption. This
statement supports that both chemisorption and physisorption mecha-
nisms have taken place over mild steel surface and this also supports the
statement said in Section 3.1.3. So, pictorially the adsorption of both in-
hibitors on mild steel surface can be given (Fig. 16).
Acknowledgments
The authors acknowledge funding from North-West University and
Department of Science and Technology and the National Research Foun-
dation (DST/NRF) South Africa grant (Grant UID: 92333) for postdoctor-
al scholarship of Dr. I. Bahadur.
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