Solvent-free microwave assisted synthesis and corrosion inhibition study of a series of hydrazones derived from thiophene derivatives: Experimental, surface and theoretical study

Article abstract of DOI:10.1016/j.molliq.2019.03.126

A series of thiophene hydrazones namely acetyl thiophene benzohydrazide (ATBH), acetyl thiophene amino benzohydrazide (ATABH), propanoyl thiophene benzohydrazide (PTBH) and propanoyl thiophene amino benzohydrazide (PTABH) were synthesized by conventional

Full text of DOI:10.1016/j.molliq.2019.03.126

Accepted Manuscript
Solvent-free microwave assisted synthesis and corrosion
inhibition study of a series of hydrazones derived from thiophene
derivatives: Experimental, surface and theoretical study
A.K. Singh, S. Thakur, B. Pani, B. Chugh, H. Lgaz, Ill-Min
Chung, P. Chaubey, A.K. Pandey, J. Singh
PII:
S0167-7322(18)34856-6
DOI:
https://doi.org/10.1016/j.molliq.2019.03.126
MOLLIQ 10667
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
20 September 2018
30 December 2018
22 March 2019
Please cite this article as: A.K. Singh, S. Thakur, B. Pani, et al., Solvent-free microwave
assisted synthesis and corrosion inhibition study of a series of hydrazones derived from
thiophene derivatives: Experimental, surface and theoretical study, Journal of Molecular
Liquids, https://doi.org/10.1016/j.molliq.2019.03.126
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
Solvent-free microwave assisted synthesis and corrosion inhibition study of a
series of hydrazones derived from thiophene derivatives: Experimental,
Surface and Theoretical Study
1
, *
2
3
2
4 *
4*
A. K. Singh , S. Thakur , B. Pani , B. Chugh , H. Lgaz , Ill-Min Chung , P.
5
2
1
Chaubey , A. K. Pandey , J. Singh
1Department of Applied Science, Bharati Vidyapeeth College of Engineering, New Delhi,
India-110063
2
Department of Chemistry, Netaji Subhash Institute of Technology, University of Delhi,
New Delhi, India-110078
3
Department of Chemistry, Bhaskaracharya College of Applied Science, University of
Delhi, New Delhi, India-110078
4
Department of Crop Science, College of Sanghur Life Science, Konkuk University,
Seoul 05029, South Korea
5
SVKM’s Dr. Bhanuben Nanavati College of Pharmacy, Vile Parle (W), Mumbai,
INDIA-400056
*
Corresponding author
Tel. No. +919560285447
Email: ashish.singh.rs.apc@itbhu.ac.in
Abstract
A series of thiophene hydrazones namely acetyl thiophene benzohydrazide
(ATBH), acetyl thiophene amino benzohydrazide (ATABH), propanoyl thiophene
benzohydrazide (PTBH) and propanoyl thiophene amino benzohydrazide (PTABH) were
synthesized by conventional as well as by energy efficient microwave irradiation
exposure and investigated their potential to mitigate corrosion of mild steel in 0.5 M
1
H
2
SO
4
medium. The spectral and elemental analyses including FTIR, H NMR, and CHN
analysis were used to characterize synthesized hydrazones. The corrosion inhibition
potential of the synthesized compounds was investigated by weight loss and several well
known analytical tools such as open circuit potential (OCP), electrochemical impedance
spectroscopy (EIS) and potentiodynamic polarization (PDP). Metal corrodes by charge
1

ACCEPTED MANUSCRIPT
transfer mechanism across the interface of metal and acid solution and was found that
addition of all the inhibitors to the acid solution reduced the corrosion rate appreciably.
The inhibitor molecule showed a strong affinity for metal surface and adsorbed on its
surface to offer resistance to flow of charge transfer across the interface of mild steel and
acid solution. Langmuir adsorption isotherm was followed by adsorption of all the tested
compounds. The thin films resulted due to adsorption of compounds on the metal surface
were demonstrated by scanning electron microscope-energy dispersive X-ray (SEM-
EDX), FTIR and X-ray photoelectron spectroscopy (XPS). Atomic force microscopy of
mild steel surface was performed to examine the effect of inhibitors on its surface
property. The contact angle measurement was also performed to examine the wettability
of mild steel surface towards acid solutions. The experimental findings agreed well with
the theoretical results performed by density functional theory (DFT). The interaction of
inhibitors with mild steel, Fe (110) was investigated by MD simulations.
Key Words: Acid Corrosion, Energy Efficient Synthesis, X-ray photoelectron
spectroscopy, SEM-EDX, EIS, Miller indices
1
. Introduction
Generally, in nature, most of the metals are found in a chemically combined state
known as ore. These ores represent their low energy state (thermodynamically stable
state). The metals are extracted from these ores after applying a huge amount of energy.
Metals in the uncombined condition (extracted form) have a higher energy and are in
unstable state. They have their natural tendency to go back to the low energy state, i.e.,
combined state by recombining with the elements present in the environment which is the
main reason of corrosion [1]. The worldwide significance of metals has attracted
2

ACCEPTED MANUSCRIPT
attention of researchers to study about metallic corrosion. The impact of metallic
corrosion append economic loss, damage of metallic equipment, contamination of
products, accidents result in serious injury which may lead to loss of life.
Metals have become first choice to construct structures because of their
mechanical strength which makes them strong enough to withstand in different
environments. The problem of corrosion is not limited to iron only but almost all metals
have a tendency to corrode because of their high energy thermodynamic state. In fact,
there are a number of metals such as aluminum, zinc, chromium etc. corrode more readily
compared to iron but they have a tendency of passivation [2]. These metals could have
been the better choice to construct structures exposed directly to corrosive environment
nevertheless mild steel has been used in a number of industries [3, 4] due to its optimum
mechanical strength, malleability, weldability, and low cost.
Corrosion of metallic structures has always been a major issue because of huge
economic loss associated with it. It also imposes a huge plight for public safety due to the
collapse of buildings, bridges, etc. which result from corrosion. There are different
possible ways to overcome or moderate the problem of corrosion like surface coating,
inhibitors etc. The purpose of all the corrosion protection methods is to protect metals at
the place of use. The use of organic compounds as corrosion inhibitor [5-8] is considered
as simplest method due to its ease of use and cost-effectiveness. Thus, there is always a
need to investigate novel substances that can retard the aggressiveness of corrosion in
different environmental conditions/corrosive media.
Chemical synthesis has still been lively field as it could supply drugs and other
materials to our society. Therefore, Chemical Scientists develop newer and newer
3

ACCEPTED MANUSCRIPT
molecules with better properties to serve society in the best possible way. Throughout the
history of the discovery of new entity, researchers have faced a series of obstacles. This
could be too few chemical compounds to synthesize or it could be too many. Depending
on the state of scientific knowledge at the time, these shortages and excesses of ideas and
materials can result in bottlenecks in the discovery and development process of a new
entity. Synthesis of new chemical entities is important in overcoming the bottlenecks in
the discovery of new molecules. Chemical synthesis can bring about a number of changes
such as minor modification of existing molecular framework to make new material or to
modify existing route so that material can be synthesized more efficiently. The
conventional methods of synthesis where the reactants were heated by a conventional
source of heat energy have been very well documented and practiced so far. But these
methods have some limitations as sometimes they result into a low yield of product and
consume a huge amount of energy and time. Few new sources of heat energy were also
introduced from time to time such as isomental, oil bath, water bath, and hot plate but the
problems remain the same.
Recently, quite a few new methods viz. solid phase synthesis, microwave assisted
and ultrasound assisted synthesis were employed and found very promising as far as
economic and environmental problems are concerned. Microwave-assisted synthesis has
been growing fast as a popular method in the research scientist community. Microwave
directly transfers energy to the reactants [9] and not to the reaction vessel and due to its
rapid heating, the formation of undesired products is prevented. Thus, microwave-
assisted synthesis has been found a very fast and clean method of synthesis and provides
efficient conversion of reactants to product [10, 11].
4

ACCEPTED MANUSCRIPT
The corrosion inhibiting potential of a vast variety of organic compounds has
been tested in different corrosive media. Schiff bases have always been a subject of
intensive research as they show anti-malarial [12], antibacterial [13], antifungal [14],
anti-cancerous [15] and corrosion inhibition property [16]. Schiff bases, especially those
having thiophene nuclei as a part of their structure, can act as an initial unit to make a
transistor, light emitting diodes and organic solar cells by synthesizing charge
transporting molecules [17-19]. Thus, thiophene nucleus can be considered as a fast-
growing potential unit in the world of heterocyclic chemistry having very promising
characteristics. Due to the larger size of S-atom as compared to C, N and O atom, its lone
pair of electrons can be delocalized more easily in the heterocyclic ring. Variety of
biologically active complexes derived from Schiff bases with thiophene as their structural
units have been extensively studied [20, 21].
Generally, tedious experimental procedures have to be performed to predict the
corrosion inhibiting efficiency of organic compounds which may not be very cost
effective as well as time-consuming. Therefore, we need alternatives on the basis of
which we can predict their corrosion inhibition tendency without performing
experiments. Density functional theory has been proved very promising to predict
inhibition tendency of the molecule and to explore the inhibition mechanism.
In view of this, we have synthesized a series of thiophene hydrazones by the
microwave-assisted method and investigated their performance against mild steel
corrosion in the acid medium due to corrosion by different electrochemical as well as
weight loss method. The experimental results were well compared and found consistent
with the theoretical results.
5

ACCEPTED MANUSCRIPT
2
2
. Experimental
.1 Synthesis of thiophene hydrazones
All the thiophene hydrazones were synthesized according to the previously
discussed method [22]. The schematic representation of the synthesis of inhibitors and
their structures are given as Fig. 1. The physical parameters and spectral characterization
data are presented as Table 1.
An open capillary method was used to determine the melting points of all the
synthesized compounds. The elemental composition was determined by CHN analysis
performed by CHN analyzer. The CHN analyzer used was Exeter Analytical Inc. model
CE-440. Jeol AL 300 FTNMR and JASCO FT/IR-5300 spectrophotometer were used to
1
record the H NMR and FTIR spectra of all the four synthesized compounds. The thin
film of inhibitors formed due to their adsorption was also characterized by using JASCO
FT/IR-5300 spectrophotometer.
2
.2 Gravimetric analysis
Mild steel samples of 2.5 × 2.0 × 0.025 cm were used to carry out gravimetric
analysis. These samples were dipped for 3h in the electrolytic solution, i.e. 0.5 M H SO
The electrolytic solutions have different concentrations of inhibitors. After 3h, these
3
2
4
.
samples were weighed to calculate the weight loss during an experiment. The gravimetric
analyses were performed at four different temperatures i.e. 303 K, 313 K, 323 K, and 333
K to explain activation parameters as well as to throw the light about the effect of
temperature on the process.
2
.3 Electrochemical analyses
6

ACCEPTED MANUSCRIPT
Gamry potentiostat/galvanostat (model 300) was used to run all the electrochemical
measurements. The experimental set up consisted of three electrode cell- a saturated
calomel reference electrode, Pt counter and mild steel working electrode [23]. The
working temperature of all the experiments was kept constant at 303 K temperature. The
mild steel strip, i. e. working electrode was dipped for 30 minutes in electrolytic solution
before starting to run experiments. This 30 minute time was sufficient to stabilize the
corrosion potential. The frequency range was fixed between 100000-0.01 Hz to run
electrochemical impedance spectroscopy (EIS) under potentiostatic conditions. The EIS
experiments were run at 10 mV peak to peak amplitude with AC signal at Ecorr. The
impedance spectroscopy data were analyzed by Echem analyst software. The potential
range to run potentiodynamic polarization experiments was selected between 250mV
-1
(
SCE) and the scan rate was kept at 1 mV s . The open corrosion potential was allowed
was stabilized first and then all the electrochemical experiments were run.
2
2
.4 Morphological study
.4.1 Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX)
Scanning electron micrographs were scanned to get information about thin film
deposited on the electrode surface due to adsorption of all the inhibitors which retarded
corrosion of mild steel in an acidic electrolyte. Different electrolytic solutions were
-1
prepared by dissolving 400 mg L concentration of all the inhibitors in 0.5 M H
2
SO
4
separately. The working electrode samples were dipped in these solutions for 3h. Then
these samples were taken out, dried and scanned by TM 3000 microscope to get SEM
images and EDX. The TM 3000 microscope was fitted with EDX analyzer also. The
acceleration voltage and working distance were selected at 5000 V and 8.5 mm,
7

ACCEPTED MANUSCRIPT
respectively. The EDX spectra were recorded to analyze the film composition developed
due to adsorption of inhibitors. The mild steel coupons taken out from inhibited solutions
when dipped into 0.5 M H
2
SO showed protection from corrosion up to an appreciable
4
extent which supports the view of film formation on the metal surface due to adsorption
of inhibitors.
2
.4.2 Atomic force microscopy (AFM)
AFM has emerged as an ever-important primary technique to study surface
morphology. The surface morphology of electrodes immersed in the electrolytic solution
and the effect of the addition of inhibitors to the solution has been widely studied by
AFM in recent times [24]. The mild steel electrode samples were dipped in different
-1
electrolyte solution (0.5 M H
2
SO + 400 mg L of inhibitor) separately for 3 h; taken out
4
and then scanned topographical images in contact mode with atomic force microscope
supplied from Bruker, USA.
2
.4.3 FTIR spectroscopy
The thin film developed on the electrode surface due to adsorption of different
inhibitors were removed first and then analyzed by recording FTIR spectra using JASCO
FTIR-5300 spectrophotometer to confirm the thin film formation on the metal surface
due to adsorption of inhibitors.
2
.4.4 X-ray photoelectron spectroscopy (XPS)
The elemental composition of the film deposited on the surface and binding
energy of the steel sample was confirmed by recording XPS spectrum of the mild steel
specimen with ATBH. The XPS spectra were recorded by Omicron Multiprobe Surface
8

ACCEPTED MANUSCRIPT
-11
analysis System supplied from Germany. The pressure was kept 5 × 10 Torr to record
XPS and the radiation source used was Mg Kα with an energy of 1253.6 eV.
2
.5 Contact angle measurement
The hydrophilicity of the electrode surface to different electrolytic solutions was
investigated by measuring its contact angle to the electrolyte. The contact angle
measurement was done by Rame-Hart goniometer (Netcong) supplied from the US and
the method used for measurement was static sessile drop method.
2
.6 Computational studies
2
.6.1 Quantum chemical calculation
DFT and MD simulations
3
The DMol module associated with Materials Studio software (version 6.0) [25,
2
6] was used to perform all the quantum chemical calculations. In these calculations, the
generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE)
formula was used for the electronic exchange-correlation potential [3], and all-electron
calculations were performed with double-numeric basis set (DNP 4.4)[27]. The solvation
3
effects (aqueous phase) was included in DMol calculations by COSMO [28] controls.
The ionization potential (IP) and the electronic affinity (EA) were derived from the
energy associated with HOMO and LUMO [29]:
IP  EHOMO
EA  ELUMO
(1)
(2)
Using the values of IP and EA the electronegativity values and global hardness of
inhibitor molecule can be calculated with the help of following equations [30]:
9

ACCEPTED MANUSCRIPT
IP  EA



(3)
(4)
2
IP  EA

2
Using the Pearson method the number of transferred electrons (ΔN) can be calculated
according to equation (5) [31]:

 inh
N 
(5)
2
(  )
Fe
inh
The values of

obtained from DFT are 3.91, 4.82 and 3.88 eV for Fe (100), Fe
(110) and Fe (111) surfaces, , respectively [32]. Here, we use only Fe (110) surface and
the reason for this surface selection are its higher stabilization energy along with packed
surface [33].
Further, the local reactivity of tested compounds has been further explored and
detailed by computing Fukui functions [34]. With the help of finite difference
approximations Fukui functions can be determined as [35]:
+
k
f  q (N+1)  q (N)
(6)
(7)
k
k
-
k
f  q (N)  q (N 1)
k
k
Here, qk represents gross charge of k atom. The values of charge on anionic, cationic and
neutral species are represented byqk (N 1) qk (N 1)andqk (N), respectively. In this
study, Fukui functions have been expressed by using Mulliken population analysis
MPA).
A simulation box of dimension 24.82×24.82×35.69 Å having 491H
and 1 tested inhibitor molecule was used to perform MD simulations. All MD simulations
,
(
3
2
2
O, 9SO4
1
0

ACCEPTED MANUSCRIPT
were performed using Materials Studio® software at temperatures T = 303K maintained
constant by the Andersen thermostat, a time step of 1 fs, NVT ensemble and a simulation
time of 2000 ps to reach simulation system under an equilibrium state. The energy
minimization and MD calculations were achieved by using the COMPASS (Condensed-
phase Optimized Molecular Potential for Atomistic Simulation Studies) force field [36].
The calculation of interaction and binding energy using equations 8 and 9 [37] enabled us
to get information about the interactions between the tested inhibitors and Fe (110)
surface:
Einteraction  Etotal (Esurface+solution +Einhibitor
Ebinding  Einteraction
)
(8)
(9)
Where, Etotal represents the energy of the entire system, the Esurface+solution , denotes the
entire energy of Fe (110) and the electrolytic solution in the absence of inhibitor
molecules and Einhibitor denotes the whole energy of inhibitor molecules.
3
. Results and Discussion
3
.1 Synthesis
The synthesis of the inhibitors was achieved by carrying out reaction according to
the scheme presented as Fig. 1. Initially, the products were synthesized by refluxing
.015 moles of each A and B for 4-5 h in ethanol. Few drops of glacial acetic acid were
0
added as a catalyst. The products were separated as a precipitate after cooling the reaction
medium. The same reaction can be completed by exposure of microwave irradiation to a
mixture of reactant A and B in presence of acid clay. After 3-5 minutes (min) exposure of
microwave irradiation (MW) the product C separated in reasonably good yield (≈ 90%).
1
1

ACCEPTED MANUSCRIPT
Thus, exposure to microwave irradiation provided very fast and clean route compared to
conventional method of synthesis for the formation of these thiophene hydrazones.
3
3
.2 Weight loss analysis
.2.1 Relation of corrosion rate with inhibitor concentration
The 0.5 M H
seen by the corrosion rate (8.13 g cm h ). The corrosion rate progressively slowed
down when increasing amount of inhibitor added to 0.5 M H SO solution as seen in Fig.
. Thus, at high concentration of inhibitors, adsorption is more facilitated and thereby
corrosion of metal is checked.
.2.2. Temperature effect/ Activation parameters
To analyze the effect of altering temperature on the corrosion rate, weight loss
experiments were performed at four different temperatures in the temperature range 303-
33 K with temperature gap 10 K. The reaction kinetics of corrosion followed Arrhenius
equation and transition state equation as follow [38]:
2
SO solution showed very much aggressiveness against mild steel as
4
-2
-1
2
4
3
3
3

Ea
logC 
log
(10)
R
2
.303RT
*
*
RT
 S 
 H 
C 
exp
exp
(11)
R




Nh
R
RT




*
*
Where, CR is written for corrosion rate. All other terms such as Ea
,

,
R
, T, S
,
H
,
N and h have usual meaning of activation energy, Arrhenius factor, gas constant, absolute
temperature, entropy of activation, enthalpy of activation, Avogadro's number and
Planck's constant, respectively.
1
2

ACCEPTED MANUSCRIPT
The activation energy and Arrhenius pre-exponential factor can be calculated from slope
and intercept of the straight line obtained by plotting log C
R
against 1/T as seen in Fig.
SO
4
a. The values of activation energy and Arrhenius factor obtained for 0.5 M H
2
4
solution and for all the inhibitors are listed in Table 2. The negative slope of this plot
indicated adsorption of all the thiophene hydrazones and therefore the value of activation
energy increased by the presence of these inhibitors and consequently the energy barrier
to precede corrosion increased [39].
The values of the Arrhenius factor were also increased by the presence of compounds (as
seen in Table 2) while they should be decreased for impeding corrosion. Thus, the
decreased corrosion rate could be due to the dominant variation of activation energy [40].
The plot of log C
R
/T vs. 1/T presented as Fig. 4b. The slope and intercept of the
straight line obtained by plotting log C
R
vs. 1/T provided the value of enthalpy and
*
entropy of activation listed in Table 2. The value of enthalpy of activation, ΔH increased
due to the presence of all the inhibitors which indicated that the minimum energy barrier
to be crossed to take place corrosion is raised by the presence of all the inhibitors. Thus,
the mild steel dissolution became difficult by the presence of these inhibitors.
*
The large negative value of entropy of activation ( S ) indicated that this process
passed through an intermediate state or transition state whose entropy is lowered by this
value or in other words transition state represented association rather than dissociation
*
[
41]. The entropy of activation ( S ) increased positively by the addition of all the
inhibitors; maximum increment noticed for PTABH. The increased value of entropy of
activation reduced the probability of attaining intermediate state or transition state [42]
and hence the reduced corrosion rate.
1
3

ACCEPTED MANUSCRIPT
3
.2.3 Adsorption isotherm
The study about adsorption isotherm is one of the most important processes in
corrosion inhibition using organic compounds because it is the adsorption isotherm which
can enlighten about the mechanism of adsorption. Different adsorption isotherm models
were tested to explain the experimental data. The equation which can explain the entire
isotherm in general as [41]:
(
2aθ)
f (, x)exp
 K C
(12)
ads inh
where, Kads is adsorption constant, Cinh is inhibitor concentration,

is the fraction of
metal surface covered with inhibitor molecule and f (, x) is a physical model dependent
configurational factor.
Langmuir adsorption isotherm explained the experimental results in a perfect way
as its slope and linear regression coefficient was found close to unity. Langmuir
adsorption isotherm is one of the simplest models which assume homogeneous and
independent single layer adsorption as well as ideal solutions. This isotherm is based on
assumption that no interaction involved inhibitor molecules adsorbed on the electrode
surface. This isotherm can be presented by an equation as [43]:
C inh
1

 Cinh
(13)

Kads
The description of experimental data with Langmuir isotherm is presented in Fig. 4c. The
intercept of this plot provided the value of Kads which measures the adsorbing capacity of
inhibitor or strength of its adsorption. By means of adsorption equilibrium constant, Kads
,
o
the calculation of free energy of adsorption ( G ) can be done with the help of equation
ads
(14) as [44]:
1
4

ACCEPTED MANUSCRIPT
o
ads

G  RT ln(55.5K )
(14)
ads
o
Where, G is free energy of adsorption which gives an idea about the spontaneity of
ads
the adsorption process, T stands for absolute temperature, 55.5 represents molar strength
of water in solution and R is gas constant.
The values of adsorption parameters are presented in Table 3. A number of
previous studies [45, 46] reported that the value of standard free energy of adsorption
gives an idea about the spontaneity of the process as well as nature of interaction among
the inhibitor molecules adsorbed at the electrode surface. The value of G <20 kJ mol-1
o
ads
usually associated with pure electrostatic interaction i.e. physical adsorption while values
-1
>
40 kJ mol to chemical interaction through charge transfer or charge sharing [47]. In
present study, the value of standard free energy of adsorption obtained (at 303 K) for
ATBH, PTBH, ATABH and PTABH were found to be 28.4, 28.9, 29.2 and 29.8; ,
respectively which indicate that these inhibitors were adsorbed on the electrode surface
not only through electrostatic interaction or chemical interaction but they were adsorbed
through the mixing of both types of interactions. The increasing values of standard free
energy of adsorption for ATBH, PTBH, ATABH, and PTABH indicate their increasing
adsorption capacity and thus their order of efficiency. These molecules can exist in two
forms: as a neutral molecule and as a protonated molecule. The neutral molecule is
adsorbed chemically by transfer or sharing of electrons while charged protonated
molecules are adsorbed electrostatically by the interaction between oppositely charged
metal surface and the inhibitor molecules.
Further adsorption thermodynamic parameters such as enthalpy and entropy for
adsorption process were calculated to enlighten about adsorption of the inhibitor
1
5

ACCEPTED MANUSCRIPT
molecules. The gravimetric experiments were performed in the temperature range of 303-
-1
3
33 K separated by 10 K with 400 mgL concentration of all the tested inhibitors to
calculate the thermodynamic adsorption parameters. Van’t Hoff equation can be used to
calculate these parameters as [48]:
0
ads
0
ads
1
H
RT
S
ln K 


(14)
ads
5
5.5
R
0
ads
0
ads
Where, all the symbols Kads
,
H
,
S , R and T have their usual meaning.
The plot of ln Kads vs. 1/T provided a straight line and presented as Fig. 4d. The slope (=
0
ads
H

) and intercept of this line enabled to calculate enthalpy and entropy of
R
0
0
adsorption. The calculated values of H and S for all the inhibitors, ATBH, PTBH,
ads
ads
ATABH, and PTABH are given in Table 3. The exothermic nature of adsorption of
inhibitors was confirmed by the negative value of enthalpy of adsorption while the
positive value of entropy of adsorption indicated that the adsorptions of inhibitors were
mainly controlled by the entropy of adsorption. The positive entropy reflected the strong
interaction of adsorbate i.e. inhibitors with the adsorbent surface, i.e. mild steel. The
increment of entropy is attributed to solvent entropy [49], i.e. increased randomness of
water molecules. The adsorption of tested inhibitors is a quasi-substitution process
includes the adsorption of inhibitors present in the electrolytic solution to the electrode
surface at the cost of desorption of already adsorbed solvent molecules, i.e. water
molecules. Thus, the complete process includes a decrease in entropy of inhibitors and
the increase in entropy of solvent. The resultant entropy is the sum of the entropy of
1
6

ACCEPTED MANUSCRIPT
inhibitor and solvent and therefore positive entropy is the result of solvent entropy
increment.
3
3
.3 Electrochemical parameters
.3.1 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy (EIS) has been very informative to deduce the
information regarding the mechanism of corrosion inhibition. The performance of
inhibitor on the basis of EIS can be given by equation as:
i
ct
0
ct
R  R
P % 
100
(15)
EIS
i
R
ct
i
ct
0
ct
Where, the symbol
R
and
R
denote charge transfer resistance with organic compound
tested as a corrosion inhibitor and charge transfer resistance in mere H
2
SO acid solution,
4
respectively.
The impedance behavior of mild steel in the experimental solution with the
different amount of all the four organic compounds namely ATBH, PTBH, ATABH, and
PTABH were tested and the results presented as Table 4. The EIS spectra of mild steel
-1
(
MS) in tested acid solution with 400 mg L (all other results data are given in the Table
4
.) of tested compounds are presented as Fig. 5a-c. The Nyquist plots contain a single
semicircle only whose diameters reflect the value of Rct. The increasing diameter of
Nyquist plots by addition of increasing amount of each inhibitor separately indicated that
their corrosion mitigating potential increases with their amount in the acid solution [50,
5
1]. The order of ability to mitigate corrosion was found as ATBH < PTBH < ATABH <
PTABH. Fig. 5a shows Nyquist plots of MS with optimum concentration i.e. 400 mg L-1
together with that in 0.5 M H
2
SO solution. The shape of these capacitive loops is not real
4
1
7

ACCEPTED MANUSCRIPT
semicircle instead of a depressed one. This depression is the characteristic effect of
surface roughness or inhomogeneity of MS electrode and is only due to the frequency
dispersion effect of the electrode [52]. The performance of double layer capacitance is
not like ideal capacitance. Thus, the appropriate circuit, which is required to analyze the
impedance, results by fitting the experimentally obtained graphs involve constant phase
element (CPE) instead of Cdl together with surface inhomogeneity regulatory factor (n).
The equivalent circuit which is used to fit the experimental results in this study presented
as Fig. 4d. The EIS parameters obtained after fitting all the experimental data with the
proposed circuit is presented in Table 4.
The total impedance of CPE is calculated using equation (16) as [53]:

0
1
n
ZCPE  Y ( j)
(16)
represents CPE extent, n having a value of 0 < n < 1 is the phase shift, j
represents an imaginary number while ω is the angular frequency with a maximum value
Where, Y
0
of Z
i
. The electrochemical behavior of any system is principally decided by the value of
1
, CPE behaves
n. If n 1, the system behaves like an ideal capacitor, i.e. Y
0
= C; ifn 
2
1
1
like a Warburg impedance, i.e.Y0 
;
Y0 
when n  0; and
 L ifn  1
.
R
Y0
The Bode-phase plots of MS in acid solution presented as Fig. 4b &c. The Bode
plots showed that the lower frequency impedance modulation was increased by the
addition of all the four compounds. The maximum increment was noted for PTABH. As
shown in Fig. 4c, the maxima of phase angle plot is increasing regularly by the presence
of inhibitors namely ATBH, PTBH, ATABH, and PTABH. The increasing phase angle in
the presence of these inhibitors confirmed their capacitive behavior. The order of the
1
8

ACCEPTED MANUSCRIPT
increment is as follow: 0.5 M H
their effectiveness order against corrosion in 0.5 M H
.3.2 Polarization behavior study
Potentiostatic method has been basically succeeded by potentiodynamic
2
SO
4
< ATBH < PTBH < ATABH < PTABH. This is also
2
SO electrolyte solution.
4
3
polarization study due to its time-consuming nature. Tafel polarization is generally
carried out to get some information about the mechanism of electrode reactions, the role
of inhibitors, activation energy, and other energy aspects. The superiority of the
polarization method over other methods is closely related to the fact that it enables us to
measure instantaneous reaction rates at metal-solution interface from a single experiment.
The polarization curves of mild steel obtained after running the experiments in 0.5
-1
M H
2
SO solution with 400 mg L of all the four inhibitors, i.e. ATBH, PTBH, ATABH
4
and PTABH along with that in acid solution only are presented as Fig. 6. The data
obtained by extrapolating cathodic and anodic polarization curves at all the
concentrations of tested inhibitors are listed in Table 5.
The equation used to compare the corrosion inhibiting potential of tested
inhibitors as [54]:
0
corr
i
i
i
corr
P % 
100
(17)
PDP
0
i
corr
0
corr
i
Where,
i
and
i
stands for the value of corrosion potential for the bare acid solution
corr
and inhibited acid solution, respectively.
Fig.6 confirmed the reduction of corrosion current density by the presence of all
the tested inhibitors, i.e. ATBH, PTBH, ATABH, and PTABH. The corrosion current
density continued to reduce with the gradually increasing amount of inhibitors as seen in
1
9

ACCEPTED MANUSCRIPT
Table 5. The occurrence of almost parallel cathodic polarization curves indicates that the
mechanism of cathodic reactions has not changed significantly. Thus, these inhibitors
lessened the corrosion rate by covering the active sites (responsible for corrosion) present
on the electrode surface. Also, insignificant change (< 85 mV) of corrosion potential
(Ecorr) occurred by the addition of inhibitors compared with that of blank acid solution; it
indicated that these tested inhibitors classified as mixed type inhibitors. Similar findings
have also been reported for other organic inhibitors by other authors [55, 56]. These
inhibitors control both anodic as well as cathodic reactions of corrosion as indicated by
an alteration of anodic and cathodic Tafel slopes. The difference in anodic Tafel slope
might be due to a change of chemical to the electrochemical mechanism. However, the
shifting of corrosion potential in the cathodic region confirmed predominant cathodic
nature of tested inhibitors. The results obtained from different methods are consistently
similar to each other as presented in Table 6.
3
3
.4 Morphological study approach
.4.1 Scanning electron microscopy
The high definition SEM images of the surface of different mild steel samples
-1
which were immersed in acid solution only and with 400 mg L concentration of all the
compounds, ATBH, PTBH, ATABH, and PTABH were obtained from FE-SEM are
displayed as Fig. 7. The mild steel immersed in acid solution only got damaged and
scratched terribly while those immersed in the solution with inhibitor remained smooth
and protected appreciably by the use of all these inhibitors. However, the protection is
more noticeable with ATABH and PTABH as shown in Fig. 7d & e. This is according to
their corrosion inhibiting potential; as ATABH and PTABH both showed excellent action
2
0

ACCEPTED MANUSCRIPT
against acid corrosion. The order of efficiency of the compounds is as ATBH< PTBH<
ATABH< PTABH.
3
.4.2 Energy-dispersive X-ray spectroscopy
Energy-dispersive X-ray spectroscopic analysis was done to ascertain the
presence of the elements on the electrode surface after immersing in acid solutions with
and without inhibitors. The EDX spectrum of mild steel immersed in acid solution only
*
shows characteristic peaks of its constituent elements (Fig. 7a ) while those of mild steel
*
*
samples immersed in inhibitor solution (Fig. 7 b -e ) shows peaks of C, O, N and S also,
which indicates that the inhibitors molecules involved in bonding with mild steel
electrode surface through these atoms. This fact is further supported by the findings of
the IR and XPS spectra of the film removed from the electrode surface (discussed later in
section 3.4.4 and 3.4.5 respectively).
3
.4.3 Atomic force microscopic analysis
Atomic force microscopy was used to analyze surface features for the first time in
986. The advantage of using AFM for the surface study is to facilitate the analysis at the
1
micro- to nanoscale and even at the molecular level. AFM has emerged as an important
topography analyzing tool as it allows the measurement of surface roughness. The three
dimensional (3D) topographical images of electrodes recorded by AFM are presented as
Fig. 8a-e. The surface of mild steel exposed directly to the acid solution (Fig. 8a) got
damaged terribly as it shows rougher and porous surface (average surface roughness =
5
68 nm) compared to others which were immersed in inhibitor solutions. The surface of
mild steel electrodes dipped in different inhibitor solutions remained smooth and thus
showed protection from corrosion by the application of these inhibitors. The surface
2
1

ACCEPTED MANUSCRIPT
roughness of the electrode surface immersed in different inhibitor solutions was found to
be 326, 268, 228 and 181 nm for ATBH, PTBH, ATABH, and PTABH, respectively as
shown in Fig. 8b-e. The values of surface roughness were according to the corrosion
inhibiting potential of these inhibitors, i.e. ATBH < PTBH < ATABH < PTABH.
3
.4.4 FTIR spectroscopy
To understand the interaction of mild steel surface with inhibitors, IR spectra of
inhibitor film removed from the surface were run. The IR spectra of original inhibitor and
removed inhibitors film are presented as Fig. 2a & b, respectively. The prominent peaks
obtained in the recorded spectra of the original compounds ATBH, PTBH, ATABH and
PTABH due to the presence of various characteristic functional groups appeared in the
spectra of films removed from the electrode surface. However, these peaks shifted
marginally as compared to the IR spectra of original compounds. The presence and
shifting of these peaks in the spectra of films removed from the metal surface ensured the
interaction of inhibitors with the mild steel surface leading to the complex formation
which resulted into metal protection against aggressive corrosion.
3
.4.5 X-ray photoelectron spectroscopic study
To confirm the nature of organic film developed due to adsorption of inhibitors,
XPS analysis was done with the mild steel sample dipped in ATBH solution. The XPS
spectra of mild steel immersed in 0.5 M H SO solution with an optimum concentration
2
4
of ATBH is shown in Fig. 9 a-e. The XPS spectra of the film deposited on the electrode
surface (Fig. 9a) contains multiple peaks for different elements like S 2p, C 1s, N 1s, O
1
s and Fe 2p3/2 which confirms the adsorption of inhibitor ATBH on the electrode
2
2

ACCEPTED MANUSCRIPT
surface. The high-resolution spectra for different elements like S, C, N, O, and Fe are
presented as Fig. 9b, c, d, e & f, respectively.
The XPS spectrum of S consists of two peaks as shown in Fig. 9b. These two
peaks were observed at 168.5 and 169.75. The first peak at 168.5 is probably attributed to
S-Fe complex while the second peak at 169.75 assigned to adsorbed metal sulfate.
The deconvoluted C 1s spectra of acid immersed mild steel having ATBH
inhibitor (Fig. 9 c) shows three peaks; the first peak at 284.74 is assigned to C-C, C=C
and C-H part of aromatic ring, the second obtained at 286.26 is attributed to C=N and C-
S of thiophene ring and the third peak (288.22) obtained at slightly higher energy is either
+
+
attributed to carbon atom of C=N and C=O. This contribution of C=N for this peak is
resulted either from protonation of C=N bond and/or due to coordination of N
mild steel surface.
9
with the
The Fig. 9d presented N 1s spectrum consist of two peaks: first at 399.4 assigned
to C-N and =N- part of ATBH, i.e. unprotonated nitrogen atom. The second peak
observed at 400.5 attributed to coordination of N-atom with Fe to form a complex. This
coordination might have taken place between =N- part of ATBH and/or –N- part with Fe.
The O 1s spectrum of electrode surface immersed in acid-ATBH solution consists
2
-
of two peaks (Fig. 9e): the first one at 529.7 is attributed to O which is involved in the
metal oxide formation (Fe or Fe ) while the second one at 531.8 is assigned to
2
O
3
3
O
4
FeOOH or oxygen of adsorbed water molecules.
The Fe 2P3/2 spectrum exhibits three peaks at 711.52, and 715.63. The peak at
3
+
3+
7
7
10.8 assigned to Fe i.e. Fe oxides or hydrous iron oxides while the peak observed at
15.63 assigned to the satellite of Fe (III).
2
3

ACCEPTED MANUSCRIPT
Thus, the XPS analysis of mild steel immersed in inhibitor ATBH solution
confirms its adsorption on the electrode surface. Further, the formation of an insoluble
3
+
and stable layer of Fe products, i.e. Fe
across the layer which resulted into reduced corrosion rate.
.4.6 Study of wetting behavior of surface
2
O and FeOOH retarded the diffusion of ions
3
3
To understand the wetting behavior of the electrode surface with different inhibitor
solution, contact angles of the metal surface to the all the solutions were measured. The
result obtained is presented as Fig. 10. The contact angle progressively increases by the
presence of increasing amount of inhibitors in the solution and hence confirms decreased
affinity of the metal surface to the inhibitor solution as compared to the acid solution
only. The order of contact angle obtained for different inhibitors solution is ATBH <
PTBH < ATABH < PTABH which is the same as that of their corrosion inhibiting
potential.
3
3
3
.4.7 Quantum chemical calculation
.4.7.1DFT calculations
.4.7.1.1Global reactivity descriptors
Quantum chemical calculations (QCC) have been proposed as a promising tool to
explain and interpret observations in different experiments. In corrosion research, the
QCC was successfully used to explain and predict a large number of known molecular
parameters directly associated with the inhibitive performance of corrosion inhibitors
[
57]. In this investigation, theoretical calculations were used to get fundamental insights
into the effect of functional groups on the effectiveness of tested inhibitors. Theoretical
calculations were done using DFT in the liquid phase and the results are depicted in Fig.
2
4

ACCEPTED MANUSCRIPT
1
1 and Table 6. Highest occupied molecular orbital (HOMO) and Lowest unoccupied
molecular orbital (LUMO) (Fig.11) play an important role in determining the most
reactive regions in the inhibitor molecule. HOMO is referred to as the orbital that could
act as an electron donor while LUMO is the orbital that could act as the electron acceptor
[
58]. Here, it could be readily observed that the electron density distribution in the
HOMO of ATBH, PTBH, and PTABH is mostly concentrated on the thiophene moiety
and nitrogen atoms, though we observe some electron density distribution from other
atoms. Interestingly, the electron density distribution in the HOMO of ATABH has
moved towards aniline moiety. Compared to HOMO, the electron density distribution in
the LUMO of all inhibitors is localized almost on the entire molecular structure except in
inhibitor ATABH that is mainly concentrated on thiophene and N atoms. These electron
density distributions of the HOMO and LUMO surfaces suggest that possible interactions
have occurred mainly when the inhibitor molecules are in close contact with the steel
surface. Further, these results revealed the interactions between inhibitor molecules and
metal surface have occurred via donation and back-donation of the electrons as well as by
electrostatic interactions.
There have been a number of successful explanations regarding the metal-
inhibitor interactions from the molecular orbital theory viewpoint. Important properties of
inhibitor molecules can be clarified by elucidating the mechanism of metal-inhibitor
interactions on the basis of the HOMO and LUMO energy as well as energy gap [59].
EHOMO is a significant molecular characteristic that determines the nucleophilic ability of
the inhibitor molecule. There is now a general consensus that EHOMO could conclude the
possibility of a reaction by an electrophilic attack. In other words, a higher value of
2
5

ACCEPTED MANUSCRIPT
EHOMO would facilitate the electron-donating ability, so that the heteroatoms from
inhibitor molecule would easily transfer their electron to a metal surface [60]. ELUMO
indicates a higher tendency of the inhibitor molecule to receive electrons from the metal
surface [59]. On the other hand, the LUMO orbitals could form π-back bonds by
accepting electrons from the metal using π∗ orbitals. It can be inferred from Table 6 that
the inhibitor PTABH has the highest value of EHOMO, indicating the highest tendency of
donating electrons to the iron atoms. This strongly demonstrates that the assembly of both
amino and ethyl groups is primarily the reason for the enhanced donating ability and
therefore, increased inhibition performance. On the other hand, inhibitor ATABH has
certain differences compared to inhibitor PTABH which means that amino group acts
more effectively than an ethyl group, while inhibitor ATBH has the lowest donating
ability. The results listed in Table 7 suggest that ELUMO values do not show any trend
towards an increase or decrease in electron accepting ability with the inhibition
efficiency. The electron donating ability of inhibitors play the crucial role in the
adsorption extent, thus in the inhibition performances of tested compounds. However, it
remains clear that HOMO distribution provides insight knowledge to the role of the
tendency of electron accepting of each inhibitor and indicates that the inhibitors prefer to
form π-back bonds also and are not only an electron-donating agent. The energy
difference between the HOMO and the LUMO energies is known as a HOMO-LUMO
gap (∆E). ∆E is one of the key factors determining the reactivity of the inhibitor
molecule. The smaller the value of ∆E, the greater the reactivity of an inhibitor molecule
[
61]. The values of ∆E of all inhibitor molecules are close to each other, but PTABH has
a lower ∆E value, confirming its greater ability to inhibit the corrosion of iron.
2
6

ACCEPTED MANUSCRIPT
Additionally, it is noted that an inhibitor molecule transfers its electrons to metal if ∆N>
0
and vice versa if ∆N< 0 [62]. Therefore, according to this criterion, the results in Table
7
shows that all inhibitor molecules have the tendency to donate electrons to the metal
surface. Further inspection of the results revealed that the ∆N values follow the same
trend of the HOMO energy, indicating that all compounds exert their action preferably by
donating electrons to the metal surface.
Overall, it is clear that small changes in the substituents of the inhibitor molecule led to
an interesting reactivity profile. The experimental results show that the inhibition
efficiency for studying compounds follows the order; PTABH > ATABH > PTBH >
ATBH which implies that compound PTABH has the highest corrosion inhibition
tendency. In addition to the common reactive sites, the highest reactivity of the
compound PTABH is due to the presence of amino and ethyl groups on its molecular
structure and thus enhancing its interactive strength. In the case of compound ATABH,
the presence of amino group on its molecular structure makes it more reactive than
compounds PTBH and ATBH. It is well-known fact that functional groups containing
heteroatoms considerably increase the interactive ability of a corrosion inhibitor.
3
.4.7.1.2 2.2 Fukui functions
Local reactivity analysis could gather more detailed information about the most
reactive sites in each inhibitor molecule. The Fukui functions were calculated on the basis
of Mulliken population analysis [63]. In this concept, Fukui function expresses the
sensitivity of the atomic site of a chemical compound towards donation and acceptation
of an electron and this can be correctly evaluated by the electrophilic and nucleophilic
Fukui function values, respectively. Higher the value of Fukui function at a particular
2
7

ACCEPTED MANUSCRIPT
site, higher is its local reactivity [63]. The results are summarized in Table 8 from which

we notice that in all inhibitor molecules, the sulfur atom possesses high values of fk and

fk (except in PTBH), indicating that the thiophene moiety has generally greater
contribution in accepting and donating of electrons, while the aniline moiety is mainly
responsible on the electron donating ability of inhibitor PTBH. Other reactive centers are
distributed over the entire molecular structure of inhibitors. The results obtained for these
regions in nucleophilic and electrophilic attacks are in good agreement with the
distribution of HOMO and LUMO orbitals and support the high capability of tested
compounds to react with the surface of metal through donor-acceptor interactions
between their reactive sites and steel surface.
3
.4.7.1.3 Molecular Dynamic (MD) simulations
During the recent years, molecular dynamics simulations have proven to be a
versatile technique for the simulation of a large number of corrosion inhibition systems
enabling the correlation of the extent of adsorption with experimentally-observed
corrosion inhibition [64]. Of course, this leads to a deep molecular understanding of the
interactions between the inhibitor molecules and the metal surface. In the followings, we
perform the molecular dynamics calculation on the adsorption of the inhibitors on the
steel surface. In order to mimic the experiment, the corrosive particles along with water
molecules are implemented in the simulation system. The system reaches equilibrium
only if both energy and temperature reach balance [65]. Fig. 12 shows the most stable
low energy adsorption configurations of the inhibitors on Fe (110). From the results, one
can see a close contact between all inhibitors and the iron surface which is indicative of a
strong interaction between them. It is common knowledge that heteroatoms-containing
2
8

ACCEPTED MANUSCRIPT
functional groups pretentiously affect the adsorption ability of corrosion inhibitors, thus
their inhibitive performances. In our present case, indeed, this effect was clearly
observed. The strongest effect was exerted by the thiophene and aniline moieties as well
as nitrogen atoms.
The interaction energies and the binding energies of interaction in systems are
listed in Table 9. An interesting point in this data is that the values of the interaction
energy of all inhibitor molecules are close to each other which can be explained in two
ways: by a small difference in the molecular structure of compounds, and/or by the
presence of the thiophene moiety in all compounds, which can ensure a strong binding
with iron atoms as observed in Fukui functions results. A high value of interaction energy
for inhibitor PTABH reflects the higher stability of its adsorption [59]. Further, we note
that all inhibition systems are associated with strong, stable and spontaneous adsorption
on Fe(110) surface [66]. Also, the high magnitude of the binding energy indicates that the
inhibitor molecules adsorbed through more than one adsorption centers [67, 68]. Here
again, it is worth noting that the highest correlation was observed with experimental and
DFT results.
4
. Conclusions
Thiophene derived hydrazones have been synthesized conventionally as well as by
microwave-assisted method. Their corrosion inhibiting potential has also been
investigated. This study resulted into following conclusions:
1
. The microwave-assisted method of synthesis is found superior compared to
conventional one as it leads to a high yield of the products and also consumes much
less time.
2
9