Theoretically and experimentally exploring the corrosion inhibition of N80 steel by pyrazol derivatives in simulated acidizing environment

Article abstract of DOI:10.1016/j.molstruc.2020.127685

Acidizing is the important process used in petroleum industry for oil well stimulation. Here, we are exploring the green synthesis of 4,4’-((4-methoxyphenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (PZ-1) and 4,4’-((4-nitrophenyl)methylene)bis(3-m

Full text of DOI:10.1016/j.molstruc.2020.127685

Journal Pre-proof
Theoretically and experimentally exploring the corrosion inhibition of N80 steel by
pyrazol derivatives in simulated acidizing environment
Ambrish Singh, K.R. Ansari, M.A. Quraishi, Savas Kaya
PII:
S0022-2860(20)30008-9
DOI:
https://doi.org/10.1016/j.molstruc.2020.127685
MOLSTR 127685
Reference:
To appear in: Journal of Molecular Structure
Received Date: 11 November 2019
Revised Date: 2 January 2020
Accepted Date: 3 January 2020
Please cite this article as: A. Singh, K.R. Ansari, M.A. Quraishi, S. Kaya, Theoretically and
experimentally exploring the corrosion inhibition of N80 steel by pyrazol derivatives in simulated
acidizing environment, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.127685.
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2020 Published by Elsevier B.V.

Dr. Ambrish Singh: Conceptualization, Methodology, Visualization, Investigation
Dr. K. R. Ansari: Data curation, Writing- Original draft preparation
Dr. Savas Kaya: Software, Validation
Prof. M. A. Quraishi: Supervision, Writing- Reviewing and Editing

Theoretically and experimentally exploring the corrosion inhibition of N80
steel by Pyrazol derivatives in simulated acidizing environment
a
b*
b
c
Ambrish Singh , K. R. Ansari , M. A. Quraishi , Savas Kaya
a
School of Materials Science and Engineering, Southwest Petroleum University, Chengdu-610500, Sichuan,
China.
b
Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and
Minerals, Dhahran 31261, Saudi Arabia.
c
Faculty of Science, Department of Chemistry, Cumhuriyet University, 58140, Sivas, Turkey
*
Corresponding author
E-mail: kashif.ansari@kfupm.edu.sa
Abstract
Acidizing is the important process used in petroleum industry for oil well stimulation. Here, we
are exploring the green synthesis of 4,4'-((4-methoxyphenyl)methylene)bis(3-methyl-1-phenyl-
1
5
H-pyrazol-5-ol) (PZ-1) and 4,4'-((4-nitrophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-
-ol) (PZ-2) under ultrasonic irradiation and its potential application for N80 steel corrosion
mitigation in 15% HCl. The corrosion inhibition investigation was performed by open circuit
potential (OCP), electrochemical impedance spectroscopy (EIS), linear polarization (LPR),
potentiodynamic polarization (PDP), electrochemical frequency modulation (EFM), and
electrochemical frequency modulation trend (EFMT), weight loss, scanning electron microscopy
(SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), density
functional theory (DFT) and molecular dynamics (MD). The EIS and PDP results suggests an
increased impedance in presence of PZs and mixed nature of inhibitor action respectively. The
protection efficiency (ɳ%) are 98.4% (PZ-1) and 94.3% (PZ-2). The results of EFMT provides
the average decrease in corrosion rate and corrosion current density (icorr). The surface analysis
1

suggests a protective layer of PZs. DFT analysis reveals that PZ-1 with lower value of ∆E acts as
a better inhibitor compared to PZ-2. MD study supports a stronger binding and interaction ability
of PZ-1 than PZ-2.
Keywords: N80 steel; Pyrazol; DFT; Corrosion; EFMT
1
. Introduction
The application of N80 steel in petroleum industries as construction material for in transportation
and casing pipes due to its cheap cost and easy availability [1]. Acidizing of an oil well is one of
the most significant method to enhance oil recovery and usually carried out using 15-28%
hydrochloric acid (HCl). The presence of the aggressive acidic environment leads to easy
deterioration of the N80 steel surface. These deteriorated surfaces can give rise to accidents,
failures, economic and human losses. Therefore, the corrosion of N80 steel in the oil well can
possess serious threats causing complete stop to a working oil well. The protection of N80 steel
from aggressive solution was reduced using inhibitors [2, 3].
Various organic inhibitors have been tested for N80 steel in acidic medium [4-7]. However, the
problem with the available commercial inhibitors or synthesized inhibitors is that they are
usually useful in high concentrations and toxicity is always a concern. Use of formulations to
enhance the inhibition performance also increases the overall concentration. Therefore, it is very
important to develop and test effective non-toxic inhibitors that can be used even at high acid
concentration. The presence of heteroatoms and the benzene ring is an added advantage to any
inhibitor, as they help in metal-solution interactions via adsorption. However, the detail
mechanism of action is still unknown and choice of inhibitors is based on the number of
heteroatoms, chemical, physical and macroscopic properties[8, 9]. The inhibitor namely 4,4'-((4-
methoxyphenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (PZ-1) and 4,4'-((4-
2

nitrophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (PZ-2) was synthesized under
ultrasonic irradiation following the green principles to get the non-toxic inhibitor which can be
used at higher acid concentrations to mitigate corrosion of N80 steel.
2
2
. Experimental procedures
.1. Inhibitor
In a round bottom (RB) flask 4 mmol of ethyl acetoacetate and 4 mmol of phenyl
hydrazine in a mixture of water/ethanol (10 mL/10 mL) ratio was taken and ultrasonically
irradiated for 5 min. After this aldehyde (4-methoxy benzaldehyde and 4-nitro benzaldehyde) (2
mmol) was added into the RB and further irradiated for another 15-20 min. The scheme of
synthesis is shown in Fig. 1 [10].
Appearance of precipitate represents the formation product. The compound was
1
13
crystallized by ethanol. Molecular structure of inhibitor, melting points, H NMR, and C NMR
1
13
data are tabulated in Table S1. The H NMR and C NMR spectra are shown in supplementary
file (Fig. S1and S2) respectively.
4
,4'-((4-methoxyphenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (PZ-1)
1
Melting point: 176-177 °C); H NMR (300 MHz, DMSO-d ): δ 2.29 (s, 6H), 3.68 (s, 3H), 4.87
6
(s, 1H), 6.81 (d, 2H), 7.10 (d, 2H), 7.13-7.18 (m, 2H), 7.42 (t, 4H), 7.62-7.75 (m, 4H), 13.90 (s,
1
H).
1
3
6
C NMR (75 MHz, DMSO-d ): δ 11.6, 32.4, 55.0, 113.5, 114.3, 118.3, 120.5, 125.5, 128.1,
1
28.7, 128.8, 128.9, 134.0, 136.8, 146.2, 157.5.
4
,4'-((4-nitrophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (PZ-2)
1
Melting point: 230-232 °C); H NMR (300 MHz, DMSO-d ): δ 2.35 (s, 6H), 5.13 (s, 1H), 7.26
6
(t, 2H), 7.45 (t, 4H), 7.52 (d, 2H), 7.71 (d, 4H), 8.18 (d, 2H), 13.86 (s, 1H, -OH);
3

1
3
C NMR (75 MHz, DMSO-d
6
): δ 12.1, 33.7, 104.4, 121.1, 123.8, 126.2, 129.1, 129.4, 137.7,
1
46.4, 146.8, 150.8.
2
.2. Materials and specimens
The working electrodes of N80 steel were sealed using epoxy resin and let to dry overnight.
2
Table 1 represents the composition of N80 steel. The electrode of 1 cm area was abraded using
SiC emery papers of different grades prior to all experiments. The newly abraded N80 steel
electrodes were thoroughly rinsed using pure water, and then degreased using acetone followed
by absolute ethyl alcohol solution in ultrasonic bath. Finally, the working electrode was dried at
ambient temperature prior to experiment. Reagent grade hydrochloric acid (HCl) is used for the
preparation of 15% HCl [11].
2
.3. Weight loss measurements
In this experiment, inhibitor concentration used was in the range of 25 ppm-200 ppm. All
the experiments were performed according to the American Society for Testing Material G1-03
12, 13]. The tests were done in triplicate and the mean values are reported.
[
The calculation of corrosion rate (CRcorr) was done using equation 1:
4
8
.76×10 ×∆m
C =
(1)
R
A×t ×
ρ
3
where C
R
, ꢀm, ρ, A and t represents the corrosion rate (mm/y), weight loss (g), density (g/cm ),
2
area (cm ) and exposure duration (h) respectively. The inhibition efficiency (ηWL%) and surface
coverage (θ) were calculated as follows [13]:
C −C
R
R(i)
ηWL % =
×100
(2)
(3)
CR
C −C
R
R(i)
θ
=
CR
4

2
.4. Electrochemical Experiments
The electrochemical experiments were performed by three-electrode Gamry workstation.
The electrochemical setup consists of reference electrode (Ag/AgCl), counter electrode (graphite
2
rod) and working electrode (N80 steel with an exposed area of 1 cm ). The electrochemical tests
were performed in the 15% HCl solution. Before starting the experiment an open circuit potential
(
OCP) was achieved by immersing the working electrode in the corrosive solution for 1 hour
14]. All experiments were reproduced, and repeated for three times at room temperature.
The applied frequency is in the range of 100 kHz to 10 mHz, with an amplitude of 10
[
mv/dec for EIS [15]. In PDP a potential range of -250 mV to +250 mV with the scanning rate of
mV/s was used for cathodic and anodic reaction area scanning [16]. Using the bellows
1
equations the inhibition efficiency were calculated:
Rct(inh) − Rct
ηEIS
=
×100
(4)
(5)
Rct(inh)
icorr −icorr(inh)
ηPDP
=
×100
icorr
where Rct(inh) and R are the resistance of charge transfer with and without PZs respectively. i
ct
corr
and icorr(inh) are the current density without and with PZs respectively.
Two frequencies i.e. 2 and 5 Hz were used for EFM measurements.. EFMT
measurements were conducted to monitor the changes in corrosion rate and icorr values for 1 hour.
Both EFM and EFTM were analyzed at an amplitude of 10 mV and base frequency of 0.1 Hz.
2
.5. Morphological study
For all surface morphology analysis, N80 steel coupons were immersed for 24 h in the
corrosive solution without and with the addition of PZs at 308 K.
5

Ziess Evo 50 XVP instrument was used for SEM analysis, at the accelerating voltage of 5
kV and magnification of 5x.
AFM was analyzed by Dimension Icon Brock instrument. The 2D/3D images were
developed by Nanoscope analysis software.
The metal samples after immersion of 6 hours in 15% HCl with and without inhibitors
were exposed to XPS (Model: XSAM 800, Japan) instrument to sense the peaks formed during
the corrosion process. Spectrum together with Fe 2p, O 1s, N 1s and C 1s regions, were recorded.
XPS Peak-Fit 4.1 software was employed to fit the experimental data and then final regions were
obtained by Shirley subtraction method.
2
.6. Quantum chemical study
The optimization of the ground states of the synthesized inhibitors ware calculated at
DFT/GGA level calculation and applying DNP basis set and BOP functional. All calculation
were done using Materials Studio software package (version 6.0) [17]. The aqueous phase effect
was control by COSMO [18].
2
.7. MD simulations
The most stable and packed iron surface i.e. Fe (110) having a 5 Å slab was selected for
3
MD simulation. The simulation box with the dimension of 24.82×24.82×35.69 Å was selected
-
+
that contains 491H O, 9Cl , 9H O and one PZs molecule. The calculation were done using
2
3
BIOVIA Materials Studio® software [17]. All MD simulations were done at 333 K temperature
and maintained constant by the Andersen thermostat, a time step of 0.1 fs, NVT (fixed atom
num- ber, system volume and temperature) ensemble and a simulation time of 20 0 0 ps to reach
simulation system under an equilibrium state. The Condensed-phase Optimized Molecular
6

Potential for Atomistic Simulation Studies (COMPASS) force field was used for the
optimization of inhibitor molecules [18].
3
3
3
. Results and discussion
. 1. Quantum Chemical Studies
.1.1. Density functional theory (DFT)
Molecular level adsorption of PZs molecules was investigated using quantum chemical
calculations. The optimized molecular structures of PZs, highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Fig. 2.
The electron donation and electron acceptation tendency of molecules are described by highest
occupied molecular orbital (EHOMO) and energy of lowest unoccupied molecular orbital (ELUMO
)
[
19,20]. The energy gap i.e. ꢀE= (ELUMO-EHOMO) that provides the adsorption reactiveness of
PZs molecules over the N80 steel surface [21]. The calculated parameters are tabulated in Table
. According to Table 7, PZ-1 has higher value of EHOMO as well as lower value of ELUMO, which
7
reveals its higher tendency to donate and accept electrons respectively. Because of this reason,
PZ-1 is more effective corrosion inhibitor than PZ-2. The presence of electron donating
functional group i.e. OCH3 in PZ-1 causes to increase its electron donation ability while in PZ-2
electron withdrawing NO2 causes to reduce the electron donating ability. In addition to this, the
adsorption ability of PZ-1 is better than PZ-2, because of the lower value of ꢀE. The obtained
results from this study supports the results obtained by experiments
3
.1.2. Molecular Dynamics (MD)
Theoretical prediction methods including molecular dynamics simulations cannot yet
replace traditional experimental assays, but there is no doubt that they can provide useful
information in corrosion research [22]. Based on this information and in an attempt to further
7

probe the relationship between the effectiveness of inhibitors and their molecular structures, we
propose that the MD simulations can be used to achieve better results [23,24]. The equilibrium
configurations adsorption of PZs are shown in Fig. 3a, b respectively.
Understanding the factors that enhance the corrosion inhibition efficiency of an organic
compound is paramount to design and efficiently apply any preventive control. In our
simulations, we observed that all tested compounds adsorbed in a horizontal manner over the Fe
(110) surface. In this situation, it is possible that the interaction between inhibitor molecules and
iron surface could provide the means necessary for corrosion mitigation. This can be achieved in
several ways, such as via electron donation, electron acceptation, back-donation, etc.
Heteroatoms greatly enhance the adsorption ability of the corrosion inhibitors. The exposed part
of iron surface can be successfully reduced by the covering of the entire molecular structure of
inhibitors, thus preventing the metal surface from the corrosion attack which is the case in our
study. Importantly, the adsorption profile of both neutral and protonated forms of PZs are almost
having the same conformation that indicates their strong influence on the inhibition process.
The binding (Ebinding) and interaction (Einteraction) energies of PZs molecules are tabulated
in Table 8. According to Table 8, binding energy of PZ-1 is higher than PZ-2. This suggests the
stronger adsorption ability of PZ-1 than PZ-2 over the N80 steel surface [25,26]. This finding is
in accordance with those of the experimental results.
3
.2. X-Ray photoelectron spectroscopy (XPS)
The analyses of N80 steel surface by XPS are shown in Fig. 4. Measurements were performed on
steel coupons exposed to solution without and with inhibitors. Binding energies were revised
using the C 1s peak at 284.8 eV. Fig. 4a shows the C 1s spectrum for all samples that can be
deconvoluted in two components. The first component situated at 284.8 eV has the major
8

influence and is endorsed to the C-H of aromatic bonds, C-C, or C=C [36,27]. The carbon atom
of the C–O bond may be ascribed by the peak at 289.0 eV [28]. After immersion in PZs, the
deconvoluted C 1s spectrum for steel depicts three components demonstrating three dissimilar
carbon interactions, as shown in Fig. 4b, 4c. The presence of C-N bonds in PZs are confirmed by
third peak at 286.3 eV and that may arise due to the presence of amino groups or C-O bond in
the inhibitors [29]. Fe 2p3/2 peak is differentiated into oxide and hydroxide forms with 711.2 eV
and 724.9 eV binding energies respectively along with the elemental peak at 717.8 eV. The peak
2 3
at 711.2 eV confirms the presence of complexes of ferric ions such as Fe O , FeOOH. The
+
3
presence of hydroxides of Fe is confirmed by second peak at 724.9 eV. The appearance of peak
at a 717.8 eV is accredited to Fe(III) satellite forms of iron (Fig. 4d, 4e, and 4f) [30].
Fig.e 4g, 4h represents the O 1s peak determined into oxide and hydroxide with values of 530.2
and 531.8 eV respectively. The first peak at 530.2 eV may arise due to the presence of oxygen
2
−
3+
(
O ) bonded to iron (Fe ) like FeO, Fe O and/or Fe O . The appearance of second peak at
2 3 3 4
−
5
31.8 eV corresponds to OH , and it could be related to oxides of hydrous iron, like FeOOH
[31]. Adsorption of PZs of N80 steel surface can be elucidated by the presence of the N1s peak
which is absent in blank sample. The peak is at about 399.9 eV as shown in figure 4i, 4j [32].
The performed XPS studies give sufficient suggestion of PZs adsorption over the N80 steel
surface.
3
.3. Weight loss experiment
The N80 steel samples were immersed in 15% HCl at 308 K temperature for 6 h without
and with PZs. Fig. 5a,b, represents the variation of inhibition efficiency (ɳ %) and corrosion
WL
R
rate (C ) at different PZs concentration. The inspection of figure demonstrate that the values of
inhibition efficiency and corrosion rates were increased and decreased respectively with
9

increased in the concentrations of PZs. This phenomenon is due to the adsorbing nature of PZs
molecules over the metal surface.
3
3
.4. Electrochemical analysis
.4.1. Electrochemical impedance spectroscopy (EIS)
EIS study was carried out without and with different concentration of PZs in acidic
solution at 308K temperature. Open circuit potential (OCP) for PZ-1 and PZ-2 was carried out
after an immersion time of 30 minutes for 1000 seconds and are represented in Fig. S3a, b.
The observed potential after 1000 seconds show a stable nature. In the Nyquist plots (Fig.
6
a, b), a depressed semi-circle and an inductive loop towards inward is observed with different
concentrations of PZs. Although, the semicircular shape of the Nyquist plots for N80 steel is
similar due to similar reaction mechanism in acidic solution for different concentration of the
PZs. The semi-circle is usually attributed to resistance of charge transfer [33]. This is because
after corrosion the metal surface becomes inhomogeneous and rough. Although the addition of
PZs causes to increase the size of the depressed semi-circle and this increment is more
pronounced at the higher PZs concentration. This phenomenon suggest that the charge transfer
resistance (Rct) values are higher in presence of PZs which provides a better protection of the
N80 steel surface [34, 35].
In the bode plots (Fig. S4a, b), the increment of slope values occurs with the addition of
PZs (Table 2). This corresponds towards the inhibition action of PZs on N80 steel surfaces. At
the intermediate frequency in the phase angle plots, the values of the phase angle increases as the
PZs concentration increases (Fig. S4a, b). This is an indication of the formation of protective
barrier that isolate the N80 steel surface from the aggressive solution [36]. Thus, in other words
adsorption of the PZs molecules over the metal plays the significant part, which ultimately
1
0

increased its corrosion resistance property. The magnitude of the phase angle and slope values
are tabulated in Table 2.
The fitted experimental results of the impedance parameters are given in Table 3.
The equivalent circuits used for fitting is shown in (Fig. 7a, b). The equivalent circuit model contains CPE,
charge transfer resistance (R ) and Inductance (L). The frequency dependent distribution of the current density
ct
along the N80 steel surface can be compensated by using the CPE in place of pure capacitor [37, 38]. The
CPE impedance (ZCPE) was calculated using the below equation:
α
−1
]
ZCPE = Y [ j
ω
(9)
0
where j represent an imaginary number ( j = −1 ), Y
o
is the admittance and it is the constant for
CPE, ɷ is the angular frequency in rad/s and α, is the phase shift [39]. The adsorption of
inhibitor molecules at the metal solution interface causes the modification of the capacitance
values. Thus, it become important to calculate the values of double layer capacitance (Cdl
because the representation of Cdl by CPE and Y is not accurate when the values of α is less than
. The accurate relationship between the Cdl and CPE can be calculated using the equation [40]:
)
o
1
(
α-1)/α

1
1 
1
o
/α
C = Y
+
(6)
dl



Rs
Rct

According to the Table 3, the addition of PZs causes the increment in the values of the Rct
and diminution of C . This can be ascribed to the molecular adsorption of PZs over the metal
dl
surface and decreases direct contact between metal and aggressive solution [41].
3
.4.2. Electrochemical Frequency Modulation (EFM)
The EFM in an importance technique that gives the corrosion current values without calculating
Tafel constants [42]. The intermodulation and wave form spectra are shown in Fig. S5a-c.
1
1

The EFM parameters are tabulated in Table 4. The adsorption of the PZs molecules over the
metal surface causes to reduce the corrosion current with increasing the inhibitor concentrations.
The closeness of the casuality factors (Table 4) with that of the theoretically obtained values
supports the accuracy of the EFM technique [43]. The evaluation of the inhibition efficiency
(ηEFM%) was done as per equation 7:
inh
corr
blank

i



ηEFM % = 1−
×100
(7)

i

corr
blank
inh
where i
and i
are the corrosion current without and with PZs respectively. According
corr
corr
to Table 4, the casuality factors values are of noble eminence and close to the standard values i.e.
and 3.
Although electrochemical frequency modulation is a nice technique for measuring
2
instantaneous corrosion rate but it provides data for a short period of time. However, a new
technique known as electrochemical frequency modulation trend (EFMT) can monitor the
inhibitor and metal interaction and provide the corrosion rate for longer time period i.e. 1 hour.
The EFMT shows the higher corrosion rate and icorr values for N80 steel in absence of PZs while
it is much lower in presence of PZs as shown in Fig. 8a-c. Table 5 shows the average decrease in
corrosion rate and corrosion current (icorr) after the addition of PZs with respect to time. In the
due course of time, the inhibitors form a protective barrier over the metal surface that hindered
the corrosive solution to interact with the metal. This vindicates the good inhibition efficiency of
the PZs inhibitor.
3
.4.3. Potentiodynamic polarization (PDP)
The obtained PDP curve without and with different concentration of PZs in acidic solution at
room temperature are shown in Fig. 9a, b. The analyses of curve generate some parameters that
1
2

are presented in Table 6. The decrease in the icorr values (Table 6) with the addition of PZs is due
to the adsorption of the PZs molecules over the metal surface, which make the metal to act an
insulator for the aggressive solution. In addition to this, with increasing the concentration of PZs
the Ecorr values are shifting towards the cathodic region (more negative) as compared to the blank
(without PZs). This clearly shows that the PZs has predominant influence on the cathodic
reaction than the anodic [44,45]. Although, both the Tafel curve branches i.e. anodic and
cathodic are migrating in the direction of smaller current density. This suggests towards the
retardation of metal dissolution (anodic) and hydrogen evolution (cathodic). Thus, the action of
inhibitor could be classified in the category of mixed type [46]. The magnitude of both Tafel
slopes i.e. cathodic (βc) and anodic (βc) are almost same without and with PZs, which suggests
that the adding PZs does not causes any corrosion reaction modification [47]. Although, the
values of cathodic Tafel constants shows more displacement as compared to the anodic Tafel
constant. This further supports predominance nature of cathodic reaction [48].
3
3
.5. Surface analysis
.5.1. Scanning electron microscopy (SEM)
The morphological texture of N80 steel without and with the addition of PZs (200 ppm)
are depicted in Fig. S6a-c.
Surface property of the steel without adding PZs is very rough with the presence of large amount
of corrosion products because of the uncontrolled dissolution and corrosion (Fig. S6a). Although,
introduction of PZs molecules to the aggressive medium makes steel surface very smooth (Fig.
S6b, c). This observation suggests that in presence of inhibitor (PZs) the rate of corrosion
process is reduced due to the adsorption and formation of a protective layer of PZs molecules
over metal surface [35,31].
1
3

3
.5.2. Atomic force microscope (AFM)
Surface microstructural images of N80 steel are depicted in Fig. S7a-c. The severe damage of
steel surface without adding PZs are shown in Fig. S7a. However, the roughness property of the
steel surface was relatively reduced with the addition of the PZs molecules (Fig. S7b, c). This
anticorrosive property of the inhibitor is certainly due to their adsorption onto the metal surface.
4
. Conclusions
The corrosion inhibition performance of PZs increases as the concentration of PZs increases and
achieved maximum values of 98.4% (PZ-1) and 94.3% (PZ-2) at 200 mg/L. The results of EIS
reveals that Rct values increase as PZs concentration increases. PDP studies suggest that PZs are
mixed type inhibitor with predominantly cathodic in nature. EFM and EFMT results indicated
the good inhibitive action of PZs in the acidic solution. SEM and AFM analysis reveals that less
corrosion occurs with the addition of PZs, which is due to the formation of inhibitor film. XPS
result suggests that PZs molecules forms a film over the metal surface. DFT analysis reveals that
PZ-1 with lower value of ∆E acts as a better inhibitor compared to PZ-2. MD study supports a
stronger binding and interaction ability of PZ-1 than PZ-2.
Acknowledgment
KRA and MAQ thankfully acknowledge the financial support provided by the King Fahd
University of Petroleum and Minerals (KFUPM), Kingdom of Saudi Arabia under the Deanship
of Scientific Research (DSR) project number DF181007. KRA is thankful to KFUPM for the
Post-Doctoral Fellowship. AS is thankful to the Sichuan 1000 Talent Fund, financial assistance
provided by the Youth Scientific and Innovation Research Team for Advanced Surface
Functional Materials, Southwest Petroleum University number-2018CXTD06 and open fund
project number-X151517KCL42.
1
4

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2
1
7

Table 1: Composition of N80 steel in wt. (%).
N80 steel
C
Si
Mn
P
S
Cr
Fe
0
.31 0.19 0.92 0.010 0.008 0.2
98.362
Table 2: The slopes of the Bode impedance magnitude plots at intermediate frequencies (S) and
the maximum phase angles (α) for N80 steel in 15% HCl without and with different
concentration of the inhibitors.
PZ-1
PZ-2
C (ppm)
-S
α°
-S
α°
1
2
5
1
2
5% HCl
5 ppm
0.456
0.554
0.581
0.644
0.711
42.3
55.7
66.3
67.8
69.1
0.456
0.481
0.503
0.545
0.604
42.3
65.2
65.6
66.2
66.9
0 ppm
00 ppm
00 ppm

Table 3: Electrochemical impedance parameters of N80 steel in 15% HCl at different
concentration of PZs.
Cinh
Rs
(ꢀ)
Rct
(ꢀ cm )
Y0
(ꢁF/cm )
α
L
Cdl
ηEIS
(%)
2
2
2
2
(
mg/L)
5% HCl
PZ-1
(H cm ) µF/cm
1
8.2
9.7±0.329
398
0.528
--
256.7
--
2
5
1
2
5
7.8
7.6
5.9
7.5
45.9±0.817 241
163.8±0.524 123
194.2±0.736 78
615.3±0.697 64
0.767
0.779
0.872
0.880
18
26
56
31
99.1
55.4
44.7
27.6
79.0±0.244
94.0±0.097
95.0±0.203
98.4±0.184
0
00
00
PZ-2
2
5
1
2
5
9.43 61.7±0.555 225
8.63 103.4±0.282 142
8.99 119.6±1.049 109
7.11 171.3±0.880 105
0.711
0.722
0.846
0.860
3
103.2
74.7
66.5
53.7
84.2±0.113
91.0±0.023
92.0±0.383
94.3±0.299
0
29
37
11
00
00

Table 4: Electrochemical frequency modulation parameters for N80 Steel in 15% M HCl in
absence and presence of optimum concentration of inhibitors at 308K.
Solution
icorr
µA/cm )
βa
- βc
CF-2
CF-3
ηEFM
(%)
--
2
(
(mV/dec)
(mV/dec)
132.0
1
5% HCl
966.1±1.239 87.0
59.2±0.736 108.0
84.9±0.817 50.4
1.940
2.051
1.977
2.786
2.901
2.839
PZ-1
PZ-2
162.2
95.9
94.0±0.251
91.2±0.211
Table 5: Electrochemical frequency modulation trend parameters for N80 Steel in 15% M HCl
in absence and presence of optimum concentration of inhibitors at 308K after 1 hour.
Solution
icorr
µA/cm )
Corrosion rate
(mpy)
ηEFMT
(%)
-
2
(
1
5% HCl
37.07±0.788 16.06
10.6±0.410 04.6
14.9±1.239 06.4
PZ-1
PZ-2
71.4±0.189
60.0±0.225

Table 6: Potentiodynamic polarization parameters of N80 steel in 15% HCl at different
concentration of PZs.
Inhibitor
mg/L)
5% HCl
Ecorr
(mV/SCE)
-435±0.816 129.7±1.020 115
icorr
(ꢁA/cm )
βa
- βc
(mV/dec)
86
ηPDP
(%)
--
2
(
1
(mV/dec)
PZ-1
2
5
1
2
5
-406±0.816 49.4±0.997 90
-410±0.408 18.3±0.697 55
-422±0.408 11.8±0.524 87
122
79
62.0±0.011
86.0±0.161
91.0±0.248
95.0±0.309
0
00
00
92
-427±0.235 7.1±0.402
54
33
PZ-2
2
5
1
2
5
-411±0.816 52.7±0.555 98
-436±1.699 37.6±0.410 83
-442±0.432 21.2±0.736 79
-415±0.736 12.9±0.920 62
95
59.3±0.232
71.0±0.305
84.0±0.142
90.0±0.050
0
135
107
96
00
00
Table 7. Orbital energy parameters obtained from DFT calculation.
Neutral form
Protonated form
System EHOMO
ELUMO
∆E
EHOMO
ELUMO
∆E
(kJ/mol) (kJ/mol)
(kJ/mol)
(kJ/mol)
PZ-1
PZ-2
-4.906
-6.33
-1.305
-2.358
3.601
3.972
-6.760
-8.748
-3.014
-4.122
3.746
4.626

Table 8. Selected energy parameters obtained from MD simulations for adsorption of inhibitors
on Fe (110) surface.
Neutral form
̿ΗΜ΢ΓΠΏΑ΢ΗΝΜ
kJ/mol)
Protonated form
System
̿ΐΗΜΒΗΜΕ
̿ΗΜ΢ΓΠΏΑ΢ΗΝΜ
̿ΐΗΜΒΗΜΕ
(kJ/mol)
598.91
(
(kJ/mol)
- 598.91
- 258.59
(
kJ/mol)
631.22
301.15
Fe + PZ-1
Fe + PZ-2
- 631.22
- 301.15
258.59

Fig. 1: General route of synthesis

Optimized
(b)
HOMO
(c)
LUMO
Figure 2: DFT images for (a) Optimized structures of PZ-1 and PZ-2 (b) HOMO structures for
PZ-1 and PZ-2, and (c) LUMO structures for PZ-1 and PZ-2.

(a)
(b)

Figure 3: Molecular dynamic simulation images for (a) PZ-1 and PZ-2 in neutral form (b) PZ-1
and PZ-2 in protonated form.

2
6000
4000
2000
0000
8000
6000
4000
C
C
C1S
4000
2
2
2
2000
1
1
1
0
275
280
285
290
295
275
280
285
290
295
300
Binding energy (eV)
Binding energy (ev)
C
20000
4000
3000
2000
1000
0
Fe
19000
18000
17000
16000
15000
14000
3000
1
2
75
280
285
290
295
300
700
710
720
730
740
Binding Energy(eV)
Binding energy (ev)
41000
2
4000
3000
2000
1000
0000
9000
Fe
Fe
4
0000
2
39000
3
8000
7000
6000
35000
4000
3000
2
3
2
3
2
3
1
3
6
90
700
710
720
730
740
700
710
720
730
740
Binding energy (ev)
Binding energy (ev)
(b)
(a)