Experimental and quantum chemical studies of synthesized triazine derivatives as an efficient corrosion inhibitor for N80 steel in acidic medium

Article abstract of DOI:10.1016/j.molliq.2015.09.019

Corrosion inhibition of N80 steel in a 15% HCl solution was studied using two synthesized triazine derivatives, namely: N²-(4-(2-amino-6-(4-methoxyphenyl)pyrimidine-4-yl)phenyl)-N⁴,N⁶-diphenyl-1,3,5-triazine-2,4,6-triamine

Full text of DOI:10.1016/j.molliq.2015.09.019

Journal of Molecular Liquids 212 (2015) 151–167
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Experimental and quantum chemical studies of synthesized triazine
derivatives as an efficient corrosion inhibitor for N80 steel in
acidic medium
a
a
b,c
M. Yadav a, , S. Kumar , N. Tiwari , I. Bahadur
, E.E. Ebenso
⁎
b,c,⁎⁎
a
Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India
Department of Chemistry, School of Mathematical and Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046,
b
Mmabatho 2735, South Africa
c
Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), 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:
Received 3 July 2015
Received in revised form 4 September 2015
Accepted 9 September 2015
Available online xxxx
Corrosion inhibition of N80 steel in a 15% HCl solution was studied using two synthesized triazine derivatives,
namely: N2-(4-(2-amino-6-(4-methoxyphenyl)pyrimidine-4-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine-2,4,6-
triamine (APTT) and N2-(4-(5-(4-methoxyphenyl)isoxazol-3-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine-2,4,6-
triamine (MITT) using gravimetric measurement, potentiodynamic polarization and electrochemical impedance
spectroscopy (EIS) techniques. The inhibition efficiency was found to increase with increasing inhibitor concen-
trations and decreases with increasing temperatures. Some thermodynamic and kinetic parameters were calcu-
lated and discussed. The adsorption of inhibitor on N80 steel surface, obeyed the Langmuir adsorption isotherm.
Polarization studies showed that both inhibitors behave as mixed-type inhibitors. Scanning electron microscopy
(SEM), energy dispersive X-ray spectroscopy (EDX), FTIR, UV–visible spectroscopy and atomic force microscopy
(AFM) were performed for surface analysis of the uninhibited and inhibited N80 steel samples. The density func-
tional theory (DFT) was employed for theoretical calculations and the results obtained were found to be consis-
tent with the experimental findings.
Keywords:
N80 steel
Hydrochloric acid
Corrosion inhibition
EIS
SEM
DFT
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
corrosion, especially in acidic medium. Organic compounds containing
electronegative groups, triple bonds or conjugated double bonds are
The importance of carbon steel protection against corrosion in acidic
solutions has increased due to the fact that iron materials, which are
more susceptible to be attacked in aggressive media, are the commonly
exposed metals in industrial environments. Acid solutions widely used
in various industrial processes such as oil well acidification, petrochem-
ical processes, acid pickling, acid cleaning, and acid de-scaling, general-
ly, leads to serious metallic corrosion [1–3]. Acidification of petroleum
oil well for enhancing oil production is commonly carried out by forcing
a solution of 15 to 28% hydrochloric acid into the well through N80 steel
tubing. During this process N80 tubing get adversely affected by corro-
sion and therefore, to reduce the aggressive attack of the acid on tubing
and casing materials (N80 steel), inhibitors are added to the acid solu-
tion during the acidifying process [4–10]. The use of inhibitors is one
of the most practical and simple method to protect metals against
the most efficient organic inhibitors because of their nucleophilicity
and the capability to bind the metal surface by means of π-interactions
[11]. The presence of heteroatoms such as nitrogen, sulfur, phosphorus,
and oxygen, together with aromatic rings in the structure enhances the
adsorption capability of the molecules on the metal surface and improve
the corrosion inhibition efficiency of these compounds [12–16]. Besides
the number of hetero-atoms, planarity and the projected surface area
of these organic molecules play an important role in their inhibition
efficiency [17].
Some derivatives of triazine [18–20], isoxazole [21,22] and pyrimi-
dine [23–27] have been reported as excellent inhibitors for metals and
alloys in hydrochloric acid solution, and exhibit different inhibition
efficiency as a result difference in substituent groups and substituent
positions on the aromatic rings. To the best of our knowledge, screened
literature shows that the structural parameters, effect of temperature
and corrosion inhibition of the studied compounds in the present
work have not been reported. In the present study two triazine
derivatives were synthesized and investigated as corrosion inhibitors.
The inhibitor [APTT] has triazene and pyrimidine whereas the inhibitor
[MITT] has triazene and isoxazole as two anchoring sites for coordination
⁎
Corresponding author.
⁎⁎ Correspondence to: I. Bahadur, Department of Chemistry, School of Mathematical and
Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University
(Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa.
E-mail addresses: yadavdrmahendra@yahoo.co.in (M. Yadav),
bahadur.indra@gmail.com (I. Bahadur).
http://dx.doi.org/10.1016/j.molliq.2015.09.019
0167-7322/© 2015 Elsevier B.V. All rights reserved.

152
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
with the metal, therefore, the effect of the substituent on the corrosion
inhibitor properties of N80 steel in hydrochloric acid solution is further
discussed.
alcohol to get the APTT compound. 1-(4-(4,6-Bis(phenylamino)-
1,3,5-triazine-2-ylamino)phenyl)-3-(4-methoxyphenyl)prop-2-en-
1-one (0.01 mol) was dissolved in alcohol (25 mL) and hydroxylamine
hydrochloride (0.01 mol) was added to it. Then a solution of KOH (5 mL
of 40%) was added to the reaction mixture and refluxed for 6 h. The
reaction mixture was then cooled, poured into crushed ice and product
separated out was filtered, washed with water, dried and recrystallized
from alcohol to get the MITT compound. The purity of the synthesized
compounds was checked using thin layer chromatography (TLC) and
structure was confirmed by physico-chemical and spectroscopic data
as given below:
In continuation of our research into developing corrosion inhibitors
[4–7] with high efficiency, the present work focuses on synthesized
condensation products of triazine, pyrimidine and isoxazole, namely:
N2-(4-(2-amino-6-(4-methoxyphenyl)pyrimidine-4-yl)phenyl)-N4,N6-
diphenyl-1,3,5-triazine-2,4,6-triamine (APTT) and N2-(4-(5-(4-
methoxyphenyl)isoxazol-3-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine-
2,4,6-triamine (MITT) as corrosion inhibitors for N80 steel in acidic
medium using weight loss measurement, potentiodynamic polariza-
tion, AC impedance, FTIR, UV–visible spectroscopy, SEM, EDS, AFM
and quantum chemical calculations.
APTT
Yield (72%), m.p. = 152 °C. Analytical data: calculated %: C, 69.44; H,
4.88; N, 22.78; found %: C; 68.82; H, 4.85; N, 22.68.
IR (ν/cm−1): 3280 (NH), 1520 (C_N), 790 (C\\N).
1H NMR (CDCl3): δ/ppm = 5.22 (s, 2H, NH2, pyrimidine), 6.88–8.22
(m, 18 Ar–H and 3 NH), 3.86 (s, 3H, OCH3).
2. Experimental
2.1. Synthesis of inhibitors
The compounds APTT and MITT were synthesized as reported in lit-
erature [28] and as shown in Scheme 1. 1-(4-(4,6-Bis(phenylamino)-
1,3,5-triazine-2-ylamino)phenyl)-3-(4-methoxyphenyl)prop-2-en-1-
one (0.01 mol) was dissolved in alcohol (25 mL) and guanidine nitrate
(0.01 mol) was added to it. Then a solution of KOH (5 mL of 40%) was
added to the reaction mixture and refluxed for 10 h. The reaction mix-
ture was then cooled, poured into crushed ice and the product separated
out was filtered, washed with water, dried and recrystallized from
MITT
Yield (72%), m.p. = 152 °C. Analytical data: calculated %: C, 72.80; H,
4.89; N, 19.18; found %: C, 72.46; H, 4.85; N, 19.12.
IR (ν/cm−1): 3360 (NH), 1510 (C_N), 810 (C\\N).
1H NMR (CDCl3): δ/ppm = 6.92 (s, 1H, isoxazol), 7.12–8.15 (m, 18
Ar–H and 3 NH).
Scheme 1. Synthetic route of inhibitors APTT and MITT.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
153
2.2. N80 steel sample
the 15% HCl solution (150 mL) at 303 K. Corrosion current density
(icorr) and corrosion potential (Ecorr) values were obtained by the Tafel
extrapolation method. All potentials were measured against SCE. The
percentage inhibition efficiency (η%), was calculated using Eq. (4):
Corrosion studies were performed on N80 steel samples having com-
position: C, 0.31; Mn, 0.92; Si, 0.19; S, 0.008; P, 0.010; Cr, 0.20 and Fe bal-
ance. N80 steel coupons having dimension of 6.0 cm × 2.5 cm × 0.1 cm
were mechanically cut and abraded with different grade emery papers
(120, 220, 400, 600, 800, 1500 and 2000) grade for the weight loss exper-
iment. For electrochemical measurements, N80 steel coupons having
dimension 1.0 cm × 1.0 cm × 0.1 cm were mechanically cut and abraded
with an exposed area of 1 cm2 (rest covered with araldite resin) with a
3 cm long stem. Prior to the experiment, specimens were washed with
distilled water, degreased in acetone, dried and stored in a vacuum
desiccator.
i0corr−icorr
ηð%Þ ¼
ꢀ 100
ð4Þ
i0corr
where, i0corr and icorr are the values of corrosion current density in the
absence and presence of inhibitors, respectively.
2.4.3. Electrochemical impedance spectroscopy studies
Impedance measurements were carried out using the same electro-
chemical cell and electrochemical workstation as mentioned for polari-
zation measurements in frequency ranging from 100 kHz to 10 mHz,
using an amplitude of 10 mV peak to peak with an AC signal at the
open-circuit potential. The impedance data were obtained by using
the Nyquist and Bode plots. The charge transfer resistance (Rct) was
obtained by fitting the experimental data of the Nyquist plots in an
appropriate equivalent circuit. The inhibition efficiency (η%) was
calculated from the charge transfer resistance values obtained from
impedance measurement according to Eq. (5):
2.3. Test solution
Analytical reagent grade HCl was diluted with double distilled water
to obtain the 15% HCl solution. The concentration of inhibitors employed
varied from (20 to 150) ppm (mg L−1), and the volume of electrolyte
used was 250 mL for weight loss measurements and 150 mL for electro-
chemical studies.
2.4. Methods
Rct ðinhÞ−Rct
Rct ðinhÞ
ηð%Þ ¼
ꢀ 100
ð5Þ
2.4.1. Weight loss method
Weight loss measurements were performed at different tempera-
tures (303 to 333) K by immersing accurately weighed N80 steel test
coupons in 250 mL of the 15% HCl solution in the absence and presence
of (20, 50, 75, 100 and 150) ppm (mg L−1) of the inhibitors. The immer-
sion time was optimized (6 h) and was uniformly used for weight loss
measurements. The test coupons were removed from the electrolyte
after 6 h immersion time, washed thoroughly with distilled water,
dried and weighed. Triplicate experiments were conducted for each
concentration of the inhibitors for reproducibility and the average of
weight losses was taken to calculate the corrosion rate and inhibition
efficiency of the inhibitors. The corrosion rate (CR), inhibition efficiency
(η%) and surface coverage (θ) were calculated using Eqs. (1)–(3) [29]:
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 Eq. (6):
ꢀ
ꢁ
1=n
1−n
Cdl
¼
Y0Rct
ð6Þ
where Y0 is the CPE constant and n is the CPE exponent. The value of n
represents the deviation from the ideal behavior and it lies between 0
and 1.
À
Á
87:6W
Atd
CR mmy−1
¼
ð1Þ
2.4.4. UV–visible spectra
The UV–visible absorption spectra of various solutions before
and after immersion of the metal specimen for 6 h in the presence
of inhibitors were recorded using a Shimadzu model UV-160A
spectrophotometer.
where, W = weight loss (mg), A = area of specimen (cm2) exposed in
acidic solution, t = exposure time (h), and d = density of N80 steel
(g cm−3).
2.4.5. FTIR spectrum analysis
CR0−CRi
CR0
θ ¼
ð2Þ
ð3Þ
The FTIR spectrum of the pure compound and film formed on the
surface of N80 steel specimen was recorded on a Perkin Elmer FTIR
(Spectrum-2000) spectrophotometer.
CR0−CRi
CR0
ηð%Þ ¼
ꢀ 100
2.4.6. Scanning electron microscopic and energy dispersive X-ray
spectroscopy analysis
where, CR0 and CRi are corrosion rate in the absence and presence of
inhibitors.
The N80 steel specimens of size 1.0 cm × 1.0 cm × 0.1 cm were abrad-
ed with a series of emery paper (320–500–800–1200) grades and then
washed with distilled water and acetone. After immersion in the 15%
HCl solution in the absence and presence of optimum concentration of
inhibitors APTT and MITT at 303 K for 6 h, the specimen was cleaned
with distilled water, dried with a cold air blaster, and then the EDX and
SEM images were recorded using a Traktor TN-2000 energy dispersive
spectrometer and a JEOL JSM-6380 LA analytical scanning electron
microscope in vacuum mode by instrument operated at 10 kV.
2.4.2. Potentiodynamic polarization studies
Potentiodynamic polarization measurements were carried out in a
conventional three-electrode cell consisting of N80 steel working elec-
trode, a platinum counter electrode and a saturated calomel electrode
(SCE) as reference electrode, using CH electrochemical workstation
(Model No: CHI 760D, manufactured by CH Instruments, Austin, USA)
at 303 K. Before starting the experiments, the working electrodes
were immersed in the test solution until a steady potential was reached.
After establishment of the open circuit potential, potentiodynamic
polarization curves were obtained with a scan rate of 0.1 mV s−1 in
the potential ranging from (−700 to −300) mV. Potentiodynamic
polarization studies were performed in the absence and presence of
various concentrations (20–150) ppm by weight of both inhibitors in
2.4.7. Atomic force microscopy (AFM)
The morphology of the uninhibited and inhibited N80 steel surface
was investigated using atomic force microscopy. For AFM analysis the
N80 steel specimens of size 1 cm × 1 cm × 0.1 cm were immersed in
the test solution in the absence and presence of inhibitors for 6 h at

154
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
room temperature. Then the specimens were taken out from the solu-
tion, cleaned with distilled water, dried, and used for AFM. The AFM
analyses were carried out using a Nanosurf Easyscan 2 instrument.
The inhibition efficiency of APTT was found to be better than that of
MITT at all concentrations and temperatures.
3.1.2. Effect of temperature
2.4.8. Quantum chemical study
Corrosion inhibition studies were carried out at different tempera-
tures ranging from (303 to 333) K. Corrosion parameters namely: corro-
sion rate (CR), surface coverage (θ) and inhibition efficiency (η%) for N80
steel in the 15% HCl solution in the absence and presence of different
concentrations (20 to 150 ppm) of inhibitor at different temperatures
(303 to 333 K), obtained from weight loss measurements are given in
Table 2. It is clear in Table 2 that the corrosion rate increases with the
increase in temperature in the presence and absence of the inhibitors
according to the Arrhenius equation. In addition, increasing the temper-
ature can accelerate the diffusion of species involved in the electrochem-
ical reactions. As a result, increasing temperatures would increase the
corrosion rate of N80 steel. The corrosion rate of N80 steel in the absence
of inhibitors increased steeply from (303 to 333) K whereas the corro-
sion rate increases slowly in the presence of inhibitors. The inhibition
efficiency decreases with an increasing temperature from (303 to 333)
K due to the increase in the rate of desorption of adsorbed inhibitor
molecules on the surface of N80 steel [34].
Complete geometrical optimizations of the investigated molecules
were performed using density functional theory (DFT) with Becke's
three parameter exchange functional along with the Lee–Yang–Parr
nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set as
implemented in the Gaussian 03 program package [30,31]. Theoretical
parameters such as the energies of the highest occupied and lowest
unoccupied molecular orbitals (EHOMO and ELUMO, respectively), energy
gap (ΔE), dipole moment (μ), absolute electronegativity (χ), global
hardness (η), softness (σ), binding energy, molecular surface area and
fraction of electrons transferred from the inhibitor molecule to the
metal surface (ΔN) were calculated and Mulliken charges on HOMO
and LUMO orbitals were obtained for each synthesized inhibitor.
3. Results and discussion
3.1. Weight loss measurements
Corrosion inhibition studies for N80 steel in the 15% HCl solution in
the absence and presence of different concentrations of inhibitors (APTT
and MITT) at different temperatures (303 to 333) K for an immersion
period of 6 h were performed using the weight loss measurement.
3.1.3. Thermodynamic and activation parameters
The apparent activation energy (Ea) for dissolution of N80 steel in
the 15% HCl solution was calculated by using the Arrhenius (Eq. (7)):
−Ea
2:303RT
logCR ¼
þ log A
ð7Þ
3.1.1. Effect of inhibitor concentration
The corrosion inhibition efficiencies (η) of the inhibitors APTT and
MITT after 6 h of immersion at 303 K as evaluated by the weight loss
technique are given in Table 1. In Table 1, it can be seen that inhibition
efficiency increases with the increase in concentration of both inhibi-
tors. It is further evident in Table 1 that both inhibitors are good
inhibitors even at concentrations as low as 20 ppm. The inhibition
efficiency of APTT and MITT at a higher concentration (150 ppm) was
found to be 94.1% and 92.1%, respectively, while lower concentration
(20 ppm) was 77.2% and 75.0%, respectively at 303 K. The corrosion
inhibition efficiency offered by APTT and MITT at lower (20 ppm) as
well as higher (150 ppm) concentrations was found to be better as
compared to the inhibition efficiency of most of the pyridine derivatives
reported in literature [23–27]. The increase in inhibition efficiency with
increasing concentrations of inhibitor was due to the increase in the
surface coverage, resulting in retardation of metal dissolution [32,33].
where Ea is the apparent activation energy, R is the molar gas constant
(8.314 J K−1 mol−1), T is the absolute temperature (K) and A is the
Arrhenius pre-exponential factor. Fig. 1 presents the Arrhenius plot of
log CR against 1/T for the corrosion of N80 steel in the 15% HCl solution
in the absence and presence of inhibitors APTT and MITT at concentra-
tions ranging from (20 to 150) ppm. In Fig. 1, the slope of each individ-
ual line was determined, and the activation energy was calculated using
the expression Ea = (slope) × 2.303R. The calculated values of Ea are
summarized in Table 3. It is evident in Table 3 that the values of the
apparent activation energy for the inhibited solutions were higher
than those for the uninhibited solution, indicating that the dissolution
of N80 steel was decreased by making a barrier by the adsorption of
the inhibitors on the metal surface [35,36].
Table 1
Weight loss parameter, electrochemical parameter and percentage inhibition efficiency (η%) for corrosion of N80 steel in the 15% HCl solution in the presence or absence of different con-
centrations of inhibitors at 303 K.
Inhibitor
Weight loss data
Tafel extrapolation data
EIS data
Conc.
(ppm)
CR
θ
η%
Ecorr
(mV vs SCE)
βa
−βc
icorr
η%
Rct
Cdl
η%
(mm y−1
)
(mV dec−1
)
(mV dec−1
)
(μA m−2
)
(Ω cm2)
(μF cm2)
APTT
Blank
20
50
75
20.20
4.60
3.16
2.08
1.47
1.23
–
–
−495
−501
−503
−506
−509
−511
93
118
103
121
98
142
147
178
167
158
152
568
122
80
67
46
–
20
90
134
187
232
286
252
113
79
57
43
–
0.77
0.84
0.89
0.92
0.94
77.2
84.3
89.7
92.7
94.1
78.4
85.8
88.1
91.8
94.6
77.7
85.0
89.3
91.3
93.0
100
150
102
30
29
MITT
20
50
75
100
150
5.04
3.59
2.63
1.95
1.59
0.75
0.82
0.86
0.90
0.92
75.0
82.2
86.9
90.3
92.1
−492
−493
−496
−498
−502
89
114
101
95
165
156
147
160
139
143.1
90.8
75.5
57.9
42.0
74.8
84.0
86.7
89.8
92.6
81
119
168
202
250
128.3
97.6
68.2
51.7
37.3
75.6
83.2
88.1
90.4
92.0
108
Experimental errors 3%.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
155
Table 2
Corrosion parameters obtained from weight loss measurements of N80 steel in the 15% HCl solution in the presence and absence of inhibitors at different temperatures.
Inhibitor with conc.
303 K
313 K
323 K
333 K
Inhibitor with conc.(ppm)
303 K
313 K
323 K
333 K
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
APTT
Blank
20
50
75
20.20
4.60
3.16
2.08
1.47
1.23
–
–
34.55
8.56
6.08
4.18
3.05
2.54
–
–
55.47
15.20
11.12
7.98
6.05
5.21
–
–
92.69
28.58
21.57
16.23
12.93
11.37
–
–
0.772
0.843
0.896
0.927
0.940
77.22
84.32
89.68
92.70
94.08
0.752
0.823
0.878
0.911
0.926
75.20
82.39
87.88
91.17
92.63
0.725
0.799
0.856
0.890
0.906
72.58
79.94
85.60
89.08
90.60
0.691
0.767
0.825
0.860
0.877
69.16
76.72
82.48
86.05
87.73
100
150
MITT
20
50
75
100
150
5.04
3.59
2.63
1.95
1.59
0.750
0.822
0.869
0.903
0.921
75.02
82.22
86.97
90.34
92.13
9.15
6.62
5.20
3.69
3.15
0.735
0.808
0.849
0.893
0.908
73.50
80.82
84.93
89.31
90.88
16.60
12.13
9.77
7.44
6.07
0.700
0.781
0.823
0.865
0.890
70.07
78.12
82.38
86.58
89.04
30.44
23.24
19.27
15.27
12.89
0.671
0.749
0.792
0.835
0.860
67.15
74.94
79.21
83.52
86.09
Experimental errors 3%.
⁎
The values of standard enthalpy of activation (ΔH ) and standard
A plot of log (CR/T) against 1/T (Fig. 2) gave straight lines with a
⁎
⁎ ⁎
slope of −ΔH /2.303R and an intercept of [log(R/Nh) + (ΔS /2.303R)],
entropy of activation (ΔS ) were calculated by using Eq. (8):
⁎
from which the value of activation thermodynamic parameters ΔH
⁎
ꢂ
ꢃ
ꢂ
ꢃ
and ΔS were calculated, as listed in Table 3. The positive sign of the
enthalpy reflects the endothermic nature of the N80 steel dissolution
RT
Nh
ΔSꢁ
R
ΔHꢁ
RT
CR ¼
exp
exp
−
ð8Þ
⁎
process. The negative value of ΔS for both the inhibitors indicates
that the formation of the activated complex in the rate determining
step represents an association rather than a dissociation step, meaning
where, h is Planck's constant and N is Avogadro's number, respectively.
Fig. 1. Arrhenius plots for N80 steel corrosion in the 15% HCl solution of (a) APTT and (b) MITT.

156
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Table 3
isotherms. The most commonly used adsorption isotherms are the
Langmuir, Temkin, and Frumkin isotherms. The surface coverage (θ)
for different concentrations of inhibitors in the 15% HCl solution was test-
ed graphically for experimental fit to suitable adsorption isotherm. Plot-
ting Cinh/θ vs. Cinh at different temperatures yielded straight lines (Fig. 3),
the correlation coefficient (R2) and slope values for each line are given in
Table 4. The correlation coefficient and slope values in Table 4 are near to
unity, indicating that the adsorption of these inhibitors on N80 steel sur-
face obey the Langmuir adsorption isotherm represented by Eq. (9):
Activation parameter for N80 steel in the 15% HCl solution in the absence and presence of
inhibitors obtained from weight loss measurements.
Inhibitor Concentration Ea (kJ mol−1
)
ΔH (kJ mol
)
ΔS (J mol−1 K−1
)
−1
⁎
⁎
(ppm)
Blank
APTT
–
20
50
75
100
150
20
50
75
42.34
50.80
53.42
57.15
60.48
62.63
50.27
52.09
55.44
57.66
58.16
39.70
48.16
50.78
54.51
57.83
59.98
47.63
49.45
52.80
55.02
55.52
−89.20
−73.69
−68.14
−59.33
−51.29
−45.93
−74.84
−71.58
−63.16
−58.56
−58.37
MITT
Cinh
θ
1
Kads
¼
þ Cinh
ð9Þ
100
150
where, Cinh is the inhibitor concentration and Kads is the equilibrium
constant for the adsorption–desorption process. From the intercepts of
Fig. 3, the values of Kads were calculated. Large values of Kads obtained
for both studied inhibitors imply more efficient adsorption and hence
better corrosion inhibition efficiency. Using the values of Kads, the values
of ΔGa°ds were evaluated by using Eq. (10):
Experimental errors 3%.
that a decrease in disorder takes place during the course of the transi-
tion from reactants to the activated complex [37].
ΔGꢂads ¼ −RT lnð55:5Kads
Þ
ð10Þ
3.2. Adsorption isotherm
Basic information on the interactions between the organic inhibitors
and the N80 steel surface are obtained from various adsorption
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. The
Fig. 2. Transition state plot for N80 steel in the 15% HCl solution at different concentrations of (a) APTT and (b) MITT.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
157
Fig. 3. Langmuir plots for (a) APTT and (b) MITT.
calculated values of Kads and ΔG°ads are given in Table 4. In general, the
values of ΔGa°ds up to −20 kJ mol−1 are compatible with the electrostatic
interaction between the charged inhibitor molecules and the charged
metal surface (physisorption) and those which are more negative than
−40 kJ mol−1 involve charge sharing or charge transfer from the inhib-
itor molecules to the metal surface [38] (chemisorptions). The calculated
temperatures (303 to 333) K, being closer to −40 kJ mol−1, which
were between the threshold values for physical adsorption and chemical
adsorption, indicating that the adsorption process of inhibitors at the N80
steel surface involve both the physical as well as the chemical adsorption.
Quraishi and Shukla [39] studied 4-Substituted anilinomethylpropionate
as corrosion inhibitors for mild steel in hydrochloric acid solution. The
Gibbs free energy of adsorption for these molecules was reported to be
around −38 kJ mol−1. They concluded that the adsorption mechanism
of these molecules on steel involved two types of interactions, chemi-
sorptions and physisorptions. A similar conclusion was also reported
by Ozcan [40], who studied the use of cystine as a corrosion inhibitor on
mild steel in sulfuric acid. Thus, the calculated values of ΔG°ads (Table 4)
for both inhibitors suggest that the adsorption of these inhibitors at the
surface of the N80 steel is not pure physisorption or chemisorption but
it is combination of physisorption as well as chemisorption.
ΔG° values for APTT and MITT were found in the range of (−38.6
ads
to −41.6) and (−38.7 to −41.5) kJ mol−1, respectively, at different
Table 4
Adsorption parameters for APTT and MITT calculated from the Langmuir adsorption
isotherm for N80 steel in the 15% HCl solution at (303–333) K.
Inhibitor
Temperature
(K)
Kads
ΔGa°ds
Slope
R2
(M−1
)
(kJ mol−1
)
8.2 × 104
7.5 × 104
6.8 × 104
6.1 × 104
8.5 × 104
7.8 × 104
6.7 × 104
6.1 × 104
−38.6
−39.7
−40.7
−41.6
−38.7
−39.7
−40.6
−41.5
8.12
−62.66
−62.66
−62.66
−62.66
−60.58
−60.58
−60.58
−60.58
3.3. Electrochemical studies
APTT
303 K
313 K
323 K
333 K
303 K
313 K
323 K
333 K
3.3.1. Polarization studies
The potentiodynamic polarization curves for the N80 steel in the 15%
HCl solution in the absence and presence of different concentrations of
inhibitors, APTT and MITT are shown in Fig. 4(a) and (b) at 303 K. It
can be seen in Fig. 4(a) and (b), that the nature of the polarization
curves remains the same in the absence and presence of inhibitors but
MITT
9.62
Experimental errors 3%.

158
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Fig. 4. Potentiodynamic polarization curves for N80 steel in the 15% HCl solution in the presence and absence of inhibitors (a) APTT and (b) MITT at 303 K.
the curves shifted towards lower current density in the presence of
inhibitors, indicating that the inhibitor molecules retard the corrosion
process. The corrosion current densities and corrosion potentials were
calculated by extrapolation of linear parts of anodic and cathodic curves
to the point of intersections. The corrosion parameters such as corrosion
potential (Ecorr), anodic Tafel slope (βa), cathodic Tafel slope (βc), corro-
sion current density (icorr) and percentage inhibition efficiency (η%)
obtained from these curves are given in Table 1. The results revealed
that increasing concentrations of both inhibitors resulted in a decrease
in corrosion current densities and an increase in inhibition efficiency
(η%), suggesting that the adsorption of inhibitor molecules at the sur-
face of the N80 steel forms a protective film on the N80 steel surface
[41]. The efficiencies of the inhibitors are in the order: APTT N MITT at
303 K. The presence of inhibitors causes a minor change in Ecorr values
with respect to the Ecorr value in the absence of inhibitors. This implies
that the inhibitors act as a mixed type inhibitor, affecting both anodic
and cathodic reactions [42]. If the displacement in Ecorr is more than
85 mV relating to corrosion potential of the blank, the inhibitor can
be considered as a cathodic or anodic type [43]. If the change in Ecorr is
less than 85 mV, the corrosion inhibitor may be regarded as a mixed
type. The maximum displacement in our study is 16 mV, which indi-
cates that APTT and MITT act as mixed type inhibitors.
3.3.2. EIS studies
The Nyquist plots for N80 steel obtained at the interface in the 15%
HCl solution with and without the different concentrations of APTT
and MITT at 303 K are shown in Fig. 5(a) and (b). The existence of a
single semicircle with its center below in Nyquist plots (Fig. 5a,
b) for both inhibitors indicates the presence of a single charge transfer
process during metal dissolution which is unaffected by the presence
of inhibitor molecules. The Nyquist plots in the absence and presence
of inhibitors are characterized by one capacitive loop. The capacitive
loops are not perfect semicircles, because of non-homogeneity and
roughness of the N80 steel surface [44].
The EIS spectra of all tests were analyzed using the equivalent circuit
shown in Fig. 6, which is a parallel combination of the charge transfer
resistance (Rct) and the constant phase element (CPE), both in series
with the solution resistance (Rs). This type of electrochemical equiva-
lent circuit was reported previously to model the iron/acid interface
[45]. Constant phase element (CPE) is introduced instead of pure double
layer capacitance to give more accurate fit as the double layer at the
interface does not behave as an ideal capacitor.
The electrochemical parameters obtained from the fitting of the
equivalent circuit are given in Table 1. The data shown in Table 1
reveal that the value of Rct increases with the addition of inhibitors as

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
159
Fig. 5. Nyquist plot for N80 steel in the 15% HCl solution containing various concentrations of (a) APTT, (b) MITT, (1) 0 ppm, (2) 20 ppm, (3) 50 ppm, (4) 75 ppm and (5) 100 ppm at 303 K.
compared to the blank solution, the increase in Rct values is attributed to
the formation of an insulating protective film at the metal/solution
interface. The CPE value decreases upon increasing concentrations of
both inhibitors, indicating the adsorption of the inhibitor molecules on
the surface of N80 steel.
The single peak obtained in Bode plots (Fig. 7) for both inhibitors
indicates that the electrochemical impedance measurements were
fit well in one time constant equivalent model (Randle's cell model)
with constant phase element (CPE). Moreover, there is only one phase
maximum in Bode plot (Fig. 7a, b) for both inhibitors, which indicates
only one relaxation process and which would be the charge transfer
process, taking place at the metal–electrolyte interface. Fig. 7(a) and
(b) show that the impedance value in the presence of both inhibitors
is larger than that in the 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 decrease on increasing the concentra-
tion of inhibitors.
Electrochemical results (η%) are in good agreement with the results
(η%) obtained by the weight loss experiment, but a minor difference
in results have been observed, which can be attributed to the further
chemically (weight loss measurement) determined corrosion rate
which is independent of potential, whereas electrochemical measure-
ments depend on operational potential [46].
3.4. UV–visible spectroscopy
UV–visible spectroscopy provides strong evidence for the formation
of a metal complex. We obtained UV–visible absorption spectra for an
optimum concentration of inhibitors at 303 K before and after 6 h
immersion of N80 steel specimen as shown in Fig. 8. The electronic
absorption spectrum of inhibitors before the N80 steel immersion
⁎
⁎
shows bands in the UV–visible region due to π–π and n–π transitions
Fig. 6. Equivalent circuit applied for fitting of the impedance spectra.
with a considerable charge transfer character. After 6 h of immersion of

160
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Fig. 7. Bode plots for N80 steel in the 15% HCl solution in the absence and presence of different concentrations of inhibitors (a) APTT and (b) MITT.
N80 steel in the presence of inhibitors, the observed change in the
position of absorption maximum or change in the values of absorbance
indicate the formation of a complex between the inhibitors and iron in
solution. However, there was no any significant change in the shape of
the spectra. These experimental findings provide strong evidence
for the complex formed between Fe2+ and inhibitors in the 15% HCl
solution. UV–visible observation confirms the formation of protective
film of metal–inhibitor complex on the metal surface.
presence of the\\NH, C_N and C\\N groups, respectively. The shift in
peak position of \\NH, C_N and C\\N in FTIR spectra of inhibited
metal surface product of both inhibitors as compared to their pure
compound spectra, indicating the involvement of these groups in the
adsorption process of inhibitors with the metal. The presence of the
other bands of the inhibitor with minor shift in reflectance spectra of
the exposed specimen was also reported which indicates the interac-
tions of the inhibitors molecule with the N80 steel.
3.5. Analysis of FTIR spectra
3.6. Scanning electron microscopy
The FTIR spectra of pure inhibitors and the inhibited metal surface
product were recorded and shown in Fig. 9(a)–(e). The pure APTT
shows the IR bands around 3280, 1520 and 790 cm−1 due to presence
of the \\NH, C_N and C\\N groups, respectively. The pure MITT
shows the IR band around 3360, 1510 and 810 cm−1 due to presence
of the \\NH, C_N and C\\N groups, respectively. The FTIR spectra
of the inhibited metal surface product in APTT and MITT show bands
at 3310, 1540, 810 and 3380, 1540, 840 cm−1, respectively due to
The surface morphology of the N80 steel samples in the 15% HCl
solution in the absence and presence of 150 ppm of APTT and MITT is
shown in Fig. 10(a), (b), (c) and (d). Fig. 10(a) is SEM of the N80 steel
sample before immersion in the 15% HCl solution. The badly damaged
surface (Fig. 10(b)) obtained when the metal was kept immersed in
the 15% HCl solution for 6 h without inhibitor indicates significant
corrosion. However, in the presence of inhibitors (Figs. 10(c) and (d))
the surface has remarkably improved with respect to its smoothness

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
161
Fig. 8. UV–visible absorption spectra of various solutions before and after immersion of the N80 steel specimen for 6 h in the presence of inhibitors.
indicating considerable reduction of corrosion rate. This improvement
in surface morphology is due to the formation of a good protective
film of inhibitor on N80 steel surface which is responsible for inhibition
of corrosion.
3.8. Atomic force microscopy
Surface morphology of the polished N80 steel and N80 steel in
the 15% HCl solution in the absence and presence of inhibitors was
investigated through atomic force microscopy (AFM) and the results
are shown in Fig. 12(a)–(d). The average roughness of polished N80
steel (Fig. 12a) and N80 steel in the 15% HCl solution without inhibitor
(Fig. 12(b)) was found as 25 and 650 nm. It is clearly shown in
Fig. 12(b) that the N80 steel sample is getting cracks due to the acid
attack on the N80 steel surface. However, in the presence of the
optimum concentration (150 ppm) of APTT and MITT as shown in
Fig. 12(c) and (d), the average roughness was reduced to 75 and
95 nm, respectively. The lower value of calculated roughness for APTT
reveals that APTT protects the N80 steel surface more efficiently than
did MITT in the 15% HCl solution.
3.7. Energy dispersive X-ray spectroscopy
The results of EDX spectra are shown in Fig. 11(a), (b), (c) and (d).
Fig. 11(a) and (b) represent the EDX spectra of abraded and uninhibited
N80 steel specimen and Fig. 11(c) and (d) depict inhibited N80 steel
specimens. The abraded N80 steel specimen shows characteristic
peaks of elements (C, Mn, Cr, Fe) constituting the N80 steel sample.
The EDX spectra of uninhibited (Fig. 11(b)) N80 steel show a peak
corresponding to Cl in addition to the abraded sample peaks. The EDX
spectra of inhibited N80 steel contains the peaks corresponding to all
the elements present in the inhibitor molecules indicating the adsorp-
tion of inhibitor molecules at the surface of N80 steel. In addition to
that, EDX of inhibited spectra shows that the Fe peaks are considerably
suppressed as compared to abraded and uninhibited N80 steel sample.
The suppression of Fe lines might be due to the overlying inhibitor
film. This indicated that the N80 steel surface was covered with a
protective film of inhibitor molecules.
3.9. Theoretical calculations
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 structures, EHOMO and ELUMO are

162
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Fig. 9. FTIR spectrum of the pure inhibitors and film formed on the surface of N80 steel specimen.
shown in Fig. 13. The frontier molecular orbital energies (EHOMO and
ELUMO) are significant parameters for the prediction of the reactivity of
a chemical species. EHOMO is often associated with the electron donating
ability of a molecule. The inhibition efficiency increases with the increas-
ing EHOMO values. High values of EHOMO indicate that the molecule has a
tendency to donate electrons to the appropriate acceptor molecules
with low energy empty molecular orbital. The lower values of ELUMO
suggested that the molecule easily accepts electrons from the donor mol-
ecules [47]. It was reported previously by some researchers [48] that
smaller values of ΔE and higher values of dipole moment (μ) are respon-
sible for enhancement of inhibition efficiency. According to HSAB theory
hard acids prefer to co-ordinate to hard bases and soft acid to soft bases.
Fe is considered soft acid and will co-ordinate to a molecule having max-
imum softness and small energy gap (ΔE = ELUMO − EHOMO). Thus, the
inhibitor APTT having higher value of softness adsorbed strongly at the
surface of N80 steel and shows maximum inhibition efficiency.
surface area (SE), global hardness (η) and softness (σ) were calculated
and summarized in Table 5. For the calculations of quantum chemical
parameters Eqs. (11)–(13) were used [49]:
ELUMO þ EHOMO
χ ¼ −
ð11Þ
ð12Þ
2
ELUMO−EHOMO
η ¼
:
2
The inverse of the global hardness is designated as the softness, σ, as
follows:
1
η
σ ¼
ð13Þ
where, hardness and softness measure the stability and reactivity of a
molecule. Soft molecules are considered to be more reactive than hard
ones because they can offer electron to acceptors easily. For the simplest
transfer of electrons, adsorption could occur at the part of the molecule
where σ, which is a local property, has the highest value [49].
The quantum chemical parameters such as the energy of the highest
occupied molecular orbital (EHOMO), the energy of the lowest unoccu-
pied molecular orbital (ELUMO), energy gap (ΔE), the dipole moment
(μ), absolute electronegativity (χ), binding energy (BE), molecular

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
163
Fig. 10. SEM image of N80 steel in the 15% HCl solution after 6 h immersion at 303 K (a) before immersion (polished), (b) after immersion without an inhibitor, (c) in the presence of
inhibitor APTT and (d) in the presence of inhibitor MITT.
Kokalj [50] recently reported that the work function (Φ) of a metal
surface is an appropriate measure of its electronegativity and should
be used, together with its vanishing absolute hardness, to estimate the
fraction of electrons transferred (ΔN) as:
more basic nature of the pyrimidine ring nitrogen to the isoxazole ring
oxygen, which improved the inhibition efficiency of APTT. The high
negative values of binding energy obtained for both inhibitors
(Table 5) indicate the stability of the adsorbed inhibitor molecules
which cannot be split or broken apart from the metal surface easily.
Molecular surface area (a geometrical quantity) determines various
properties of inhibitor molecules, as they interact through their metal
surface residue. It has been found that the large surface area values
obtained for both studied inhibitors (Table 5) attributed to the unifor-
mity of coverage of the surface of the N80 steel. The higher value of
binding energy and larger molecular size of APTT (Table 5) indicate its
higher inhibition efficiency.
Φ−χinh
2ηinh
χ
Fe−χinh
À
Á
ΔN ¼
¼
:
ð14Þ
2 ηFe þ ηinh
To calculate the fraction of electrons transferred, a theoretical value
for the electronegativity of bulk iron was used, χFe ≈ 7 eV mol−1, and
global hardness of, γFe = 0 eV mol−1 was used.
In Table 5 it is clear that a higher value of EHOMO (−8.6888 eV), σ
(0.2500 eV−1), ΔN (0.2889) and lowest values of ΔE (7.9991 eV) are
found for APTT, indicating that APTT has more potency to get adsorbed
on the N80 steel surface resulting in greater inhibition tendency than
MITT. Dipole moment values of APTT and MITT are 4.8150 D and
3.7188 D, respectively, which clearly suggest that inhibitors are polar
compounds and can easily donate electrons forming strong dπ–pπ
bonding [51]. In the present study the inhibition efficiency increases
with the increasing dipole moment of the inhibitors, which could be at-
tributed as higher polarity compounds which will facilitate electrostatic
interaction between the electric field due to the charged metal surface
and electric moments of the inhibitors and contributes to their better
adsorption by influencing the transport process through the adsorbed
layer [52]. Generally, ΔN shows inhibition efficiency resulting from elec-
trons transferred from the inhibitor molecule to the iron atom. Accord-
ing to Lukovits et al. [53] if the value of ΔN is less than 3.6, the efficiency
of inhibition increases with increasing electron-donating ability of the
inhibitor at the metal surface. The structure of APTT and MITT is almost
similar, the only difference is that APTT contains a pyrimidine ring
whereas MITT contains an isoxazole ring. The electronic-releasing
power of APTT was found to be better than that of MITT due to the
Literature reveals that the use of Mulliken population and HOMO
population analyses can be used for the determination of possible
adsorption centers of the inhibitors [54–56]. The generalized interpreta-
tion given by several authors is that the higher the magnitude and the
number of the negatively charged heteroatom present in an inhibitor
molecule, the higher its ability to be adsorbed on the metal surface via
the donor–acceptor type bond and the more negatively charged regions
with major distribution of the HOMO are also interpreted as possible
centers of adsorption. Mulliken charges according to the numeration
of the corresponding atoms are shown in Fig. 14(a) and (b). It is evident
in Fig. 14(a) and (b) that both inhibitors had a considerable excess of
negative charge around the nitrogen, and some carbon atoms, indicat-
ing that these are the coordinating sites of the inhibitors.
3.10. Mechanism of inhibition
Corrosion inhibition of N80 steel in the hydrochloric acid solution by
APTT and MITT can be explained on the basis of molecular adsorption.
These compounds inhibit corrosion by controlling both anodic as
well as cathodic reactions. In acidic solutions these inhibitors exist as

164
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Fig. 11. EDX spectra of mild steel specimens (a) polished and (b) after immersion without inhibitor, (c) with 100 ppm APTT and (d) with 100 ppm MITT.
Fig. 12. AFM micrographs of the mild steel surface: (a) polished mild steel, (b) blank in 15% HCl solution, (c) with 100 ppm APTT and (d) with 100 ppm MITT.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
165
Fig. 13. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) APTT and (b) MITT.
protonated species. In both inhibitors nitrogen atoms present in the
molecules can be easily protonated in the acidic solution and converted
into quaternary compounds. These protonated species adsorbed on the
cathodic sites of the N80 steel and decrease the evolution of hydrogen.
The adsorption on the anodic site occurs through π-electrons of aromatic
rings and lone pair of electrons of the nitrogen and oxygen atoms which
decrease the anodic dissolution of N80 steel.
inhibiting performances of the inhibitors follow the order:
APTT N MITT.
The variation in the values of βa and βc (Tafel slopes) and the minor
deviation of Ecorr with respect to Ecorr of the blank, indicate that both
verified inhibitors are of the mixed type in nature.
EIS measurements show that charge transfer resistance (Rct)
increases and double layer capacitance (Cdl) decreases in the
presence of inhibitors, which suggest the adsorption of the inhibitor
molecules on the surface of N80 steel.
4. Conclusions
It is suggested from the results obtained from SEM, AFM and the
Langmuir adsorption isotherm that the mechanism of corrosion
inhibition is occurring mainly through the adsorption process.
Quantum chemical results were in good agreement with experimen-
tal results of both inhibitors.
The synthesized triazine derivatives show good inhibition efficien-
cies for the corrosion of N80 steel in the 15% HCl solution and the
inhibition efficiency increases with the increase in the concentration
of inhibitors and decreases with the increase in temperature. The
Table 5
Quantum chemical parameters for different inhibitors.
Inhibitor
EHOMO
(eV)
ELUMO
(eV)
ΔE
(eV)
μ
(D)
η(eV)
σ
(eV)
ΔN
χ
S.A.
B.E.
(Å2)
(kcal mol−1
)
APTT
MITT
−8.7888
−8.7616
−0.4897
−0.5986
8.2991
8.1630
1.8150
3.7188
4.1495
4.0815
0.2410
0.2450
0.2844
0.2841
−4.6392
−4.6801
714.5
752.3
−7734
−8732
Experimental errors 3%.

166
M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167
Fig. 14. The Mulliken charge density of (a) APTT and (b) MITT.
[3] M. Yadav, S. Kumar, I. Bahadur, D. Ramjugernath, Int. J. Electrochem. Sci. 9 (2014)
Acknowledgments
6529–6550.
[4] M. Yadav, D. Behera, S. Kumar, R.R. Sinha, Ind. Eng. Chem. Res. 52 (2013)
The authors acknowledge funding from North-West University
and Department of Science and Technology and the National Research
Foundation (DST/NRF) South Africa grant funded (Grant UID: 92333)
for the postdoctoral scholarship of Dr. I. Bahadur.
6318–6328.
[5] M. Yadav, D. Behera, U. Sharma, Corros. Eng. Sci. Technol. 48 (2013) 19–27.
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