Experimental and theoretical approach to the corrosion inhibition of mild steel in acid medium by a newly synthesized pyrazole carbothioamide heterocycle

Article abstract of DOI:10.1016/j.molstruc.2019.127051

Heterocyclic compound such as 3,5-disubstitued pyrazole carbothioamide was synthesized by cyclocondensation reaction between chalcone derivative and thiosemicarbazide as nucleophile substrate in ethanolic sodium hydroxide solution. The structure of synthe

Full text of DOI:10.1016/j.molstruc.2019.127051

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Experimental and theoretical approach to the corrosion inhibition of mild steel in acid
medium by a newly synthesized pyrazole carbothioamide heterocycle
F. Boudjellal, H.B. Ouici, A. Guendouzi, O. Benali, A. Sehmi
PII:
S0022-2860(19)31151-2
DOI:
https://doi.org/10.1016/j.molstruc.2019.127051
MOLSTR 127051
Reference:
To appear in: Journal of Molecular Structure
Received Date: 16 May 2019
Revised Date: 15 August 2019
Accepted Date: 8 September 2019
Please cite this article as: F. Boudjellal, H.B. Ouici, A. Guendouzi, O. Benali, A. Sehmi, Experimental
and theoretical approach to the corrosion inhibition of mild steel in acid medium by a newly synthesized
pyrazole carbothioamide heterocycle, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/
j.molstruc.2019.127051.
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DDP, 30 °C
24 h immersion
MS + 1 M HCl
MS + 1 M HCl + DDP
DDP (HOMO)
DDP (LUMO)

Experimental and theoretical approach to the corrosion inhibition of mild
steel in acid medium by a newly synthesized pyrazole carbothioamide
heterocycle
F. Boudjellal1, H. B. Ouici1, A. Guendouzi1, O. Benali2, A. Sehmi1
1Department of Chemistry, Faculty of Science, University of Saida, Algeria.
2Department of Biology, Faculty of Science, University of Saida, Algeria
E-mail : ouici.houari@yahoo.fr
Abstract
Heterocyclic compound such as 3,5-disubstitued pyrazole carbothioamide was synthesized by
cyclocondensation reaction between chalcone derivative and thiosemicarbazide as nucleophile substrate in
ethanolic sodium hydroxide solution. The structure of synthesized compound namely, (E)-5-(4-
(dimethylamino)phenyl)-3-(4-(dimethylamino)styryl)-2,3-dihydro-1H-pyrazole-1-carbothioamide (DDP) was
confirmed by infrared FT-IR, 1H NMR and 13C NMR spectra. The inhibitory action was investigated using the
gravimetric and electrochemical methods. The results show the strong adsorption of target molecule on the
surface of mild steel. The adsorption of (DDP) molecules on the steel surface follows the Langmuir adsorption
isotherm and the calculated (ꢀꢀ°ꢁꢂ ) values of the synthesized inhibitor suggested that the adsorption of this
s
compound involves two types of interaction, chemisorption and physisorption. The polarization curves showed
that the (DDP) act as mixed type inhibitor. The SEM analysis reveals that the corrosion inhibition is due to the
formation of a protective film on the metal surface. DFT calculations were performed to illustrate the
relationship between the molecular structure of (DDP) and their inhibitory properties.
Keywords: Pyrazole, Mild steel, Corrosion, Inhibition, DFT.
1

1. Introduction:
The hydrochloric acid solutions are widely used in several industrial processes, such as pickling,
descaling, acid cleaning, oil well acidizing, etc [1]. Because of their aggressiveness, the use of corrosion
inhibitors is considered as the most effective method for the protection of many metals against acid attack and
reduces the dissolution of metals [2]. The organic compounds are known to be applicable as corrosion inhibitors
for steel in acidic environments. Heterocyclic compounds are organic molecules containing functional groups
with heteroatom’s, which considerate as efficient inhibitors against metal corrosion in acidic environments [3, 4].
These molecules can easily adsorbed on the metal surface and form a bond between their heteroatom’s and the
metal surface, thereby reducing the corrosion rate in acidic solutions [5–7]. Pyrazole derivatives are better
known, and more widely studied by researchers because of their many important chemical and biological
properties [8], they have been commonly used in medicinal chemistry [9-13]. In addition to these biological
activities, this family of organic compounds has nitrogen and sulfur heteroatom in their molecular structure as
well as pyrazole ring with benzene moieties which can create the inhibition properties in acidic medium and can
be strongly adsorbed onto the surface [14, 15]. In our previous study, we have exploited the inhibitory properties
of two compounds of pyrazole carbothioamide (DPC and DPCM), which show a high protective action for mild
steel in 1 M HCl solution [14]. Moreover, only a few studies using pyrazole carbothioamide derivatives as
corrosion inhibitor can be found. These compounds are considered as green corrosion inhibitors because they
have lower toxicity with high solubility in acidic environments compared to other organic compound families.
Also, this type of organic compounds can easily obtain with a good yield and high purity from chalcone
derivatives [14, 16]. The pyrazole entity is often found in many biologically active molecules [17, 18].
The aims of the present work is to synthesis and examine the inhibitory properties of new pyrazole
heterocyclic compound containing carbothioamide entity as substituent in their structure for the corrosion
inhibition of mild steel in 1 M HCl solution. Effect of concentration, temperature and immersion time on the
inhibitory efficiency of pyrazole has been studied systematically. Thermodynamic and kinetic parameters were
calculated to predict the inhibition mechanism. Furthermore, the adsorption of inhibitor on the steel surface was
discussed by using the electrochemical measurements. The surface morphology of steel samples without and
with (DDP) in the electrolytic solution was observed to confirm the presence of the film or the layer formed by
the adsorption of (DDP) molecules on mild steel surface using scanning electron microscopy (SEM). Finally, the
effect of the structural parameters on inhibitory efficiency has also been studied using (DFT) calculations.
The structure of target pyrazole namely, (E)-5-(4-(dimethylamino)phenyl)-3-(4-(dimethylamino)styryl)-2,3-
dihydro-1H-pyrazole-1-carbothioamide (DDP) is given in the Fig. 1.
2. Materials and methods
2. 1. Reagents and spectral analysis
All chemicals used in this study were supplied by Sigma-Aldrich and Merck. Melting points were
determined using a Böetius apparatus and are uncorrected. The infrared spectra were registered with a Vertex 70
Bruker spectrometer using potassium bromide disc technique and the results are expressed in wave number
(cm–1). The nuclear magnetic resonance (1H NMR and 13C NMR) spectra were registered on a Varian Gemini
1
300 BB spectrometer working at 300 MHz for H NMR and 75 MHz for 13C NMR, using DMSO-d6 as the
2

solvent. Chemical shifts are expressed in δ (ppm) using TMS as the internal standard. The heterocyclic
compound was synthesized in accordance with the method described in the literature [19].
2. 2. Mild steel material
Tests were performed on a freshly prepared sheet of DC06EK mild steel of composition (wt %) given in
the Table 1. Specimens used in all the experiments were mechanically cut into 20 mm × 15 mm × 2 mm
dimensions, and then abraded with SiC abrasive papers 200, 400, 600, 800 and 1000 grit respectively, washed in
absolute acetone, dried in room temperature and stored in moisture free desiccators.
2. 3. Inhibitor and solution
Fig. 1 shows molecular structure of the heterocyclic compound used in this study named as, (E)-5-(4-
(dimethylamino)phenyl)-3-(4-(dimethylamino)styryl)-2,3-dihydro-1H-pyrazole-1-carbothioamide (DDP). The
titled compound was synthesized from the reactions described in scheme 1 and 2. The electrolyte solution,
1 M HCl was prepared by dilution of analytical grade 37% HCl with bi-distilled water. The solution was used for
all experimental purposes. The concentration range of target inhibitor used in this study was 10-5 M, 5 × 10-5 M,
10-4 M and 5 × 10-4 M, respectively.
2. 4. Weight loss and electrochemical measurements
Weight loss measurements were carried out in solution of 1 M HCl acid in the absence and the presence
of (DDP) inhibitor on mild steel. Sheets with dimensions 20 mm × 15 mm × 2 mm were used. They were
polished successively with different grades of emery paper up 1200 grade. Each run was carried out in a glass
vessel containing 100 ml of the test solution. A clean weight mild steel sample was completely immersed at an
inclined position in the vessel. After 1 h of immersion in 1 M HCl without and with addition of inhibitor at
different temperatures (30, 40, 50 and 60°C), the specimen was withdrawn, rinsed with distilled water, washed
with acetone, dried and weighed. The weight loss was used to calculate the corrosion rate (ν) in mg/cm2.h from
the following equation [20]:
∆w
(1)
v =
S.t
Where ꢀw is the average weight loss of three mild steel sheets, S the total area of one steel specimen, and t is the
immersion time 1 h. With the calculated corrosion rate, the inhibition efficiency (%IE) was calculated as
follows:
v0 −v
(2)
%IE =
×100
v0
Where ν0 and ν are the corrosion rate of the mild steel coupons in the absence and presence of (DDP),
respectively.
Electrochemical experiments were carried out in a glass cell consisting of a platinum electrode (CE) and a
saturated calomel electrode (SCE) was used as a counter electrode and a reference electrode, respectively. The
mild steel as working electrode with reactive surface area 1 cm2 was used for electrochemical studies. The
potentiodynamic current–potential curves were recorded by changing the electrode potential automatically from
-750 mV to -350 mV with a scanning rate of 0.5 mV.s-1. The ac impedance measurements were performed at
corrosion potentials (Ecorr) over a frequency range of 10 kHz–40 mHz, with a signal amplitude perturbation of
10 mV. Nyquist plots were obtained.
3

2. 5. SEM surface morphology analysis
A JEOL (JSM-7001F) SEM was used to observe the surface morphology of the mild steel specimens
with and without (DDP) in 1 M HCl solution after 24 h of immersion. Mild steel samples used for SEM surface
analysis were cut into 20 mm × 15 mm × 2 mm dimensions. They were abraded with SiC abrasive papers 200,
400, 600, 800 and 1000 grit respectively, the specimens were immersed for 24 h in the solution of 1 M HCl
without and with optimum concentration 5 × 10-4 M of (DDP) inhibitor at 30 °C, and washed with acetone and
distilled water, dried in warm air.
2. 6. DFT quantum calculations
To investigate the correlation between the molecular structure of (DDP) and its inhibition properties, a
quantum chemical study has been performed. Geometric structures and electronic properties of (DDP) pyrazole
have been calculated by GAUSSIAN 09W software [21], using the Becke’s three-parameter hybrid density
functional B3LYP method with the standard 6-31G* basis set [22].
The interaction and binding energy (EInt, EBind) between the target inhibitor and the Fe (1 1 0) surface was
calculated for the minimum energy configuration with the basis set superposition error (BSSE) and evaluated
using the counterpoise method to eliminate basis functions overlap effects using equation (3) and (4) [21, 23]:
(3)
EInt = EInh - Fe - (EInh + EFe
)
EBind = −EInh
(4)
Where EInh–Fe is the total energy of the iron crystal together with the adsorbed inhibitor molecule, EInh and EFe are
the total energy of the free inhibitor molecular and the iron crystal, respectively.
3. Results and discussions
3. 1. Synthesis of pyrazole carbothioamide (DDP)
The target pyrazole (DDP) was synthesized in two steps. The synthetic pathway followed for the
preparation of the title compound was accomplished as shown in Scheme 1 and 2. The key intermediate such as
dibenzalacetone (4-DBA) employed in the synthesis of the target pyrazole (DDP) was prepared according to a
literature method [24]. Thus, dibenzalacetone (4-DBA) was synthesized by the Claisen-Schimidt reaction
between propanone (acetone) and 4-N,N-dimethylaminobenzaldehyde using ethanol as solvent in the presence of
sodium hydroxide as a catalysts. This compound was converted to the corresponding pyrazole carbothioamide
(DDP) by refluxing with thiosemicarbazide hydrochloride in ethanol as solvent. Whereas, the cyclocondensation
of thiosemicarbazide with dibenzalacetone (4-DBA) at reflux conditions yielded (DDP) after has been
neutralized with water ice to give the pyrazole in good yield. Finally, the synthesized compound was
recristallized from an ethanol/water (4:1) mixture. The purity of the synthesized compounds (4-DBA) and (DDP)
was confirmed by using thin layer chromatography. Moreover, the melting point technique was used to
determine the melting points of these compounds. The comparison of the spectroscopic data of the new
compounds (Scheme 1 and 2) with those of the previously reported analogues further confirmed the above
structure. The pyrazole (DDP) prepared was tested in the following of this work as a corrosion inhibitor of mild
steel in 1 M HCl medium.
4

3. 1. 1. Synthesis of dibenzalacetone derivative (4-DBA) using Claisen-Schimidt condensation
(1.5 g, 0.037 mol) of sodium hydroxide (NaOH) was added to 20 ml of water (H2O) and 50 ml of ethanol
(EtOH) mixture. (12.38 g, 0.033 mol) ml of 4-N,N-dimethylaminobenzaldehyde and 2.5 ml of propanone were
added. After 30 min of occasional stirring at room temperature, the product was filtered and the yellow (4-DBA)
1
was recristallized from ethylacetate (yield (75%), mp=192 °C). H NMR (DMSO, 300 MHz, δ (ppm)): 3.04 (s,
12H, 2N(CH3)2), 6.51-7.70 (m, Ha, Hb and 12H, Ar-H). 13C NMR (DMSO, 300 MHz, δ (ppm)): 190.32 (C1),
122.46 (C2), 144.62 (C3), 124.96 (C4), 130.53-132.0 (C5 and C9), 111.51-112.25 (C6 and C8), 40.24 (C10).
3. 1. 2. Synthesis of (E)-5-(4-(dimethylamino)phenyl)-3-(4-(dimethylamino)styryl)-2,3-dihydro-1H-
pyrazole-1-carbothioamide (DDP)
A mixture of dibenzalacetone (4-DBA) (3.20 g, 0.01 mol), thiosemicarbazide hydrochloride (1.30 g, 0.01 mol)
and sodium hydroxide (NaOH) (0.50 g, 0.0125 mol) was refluxed in 50 ml of ethanol for 6-8 hours. The
resultant mixture was concentrated, cooled and poured into crushed ice. The orange solid mass thus separated
out was dried and recrystallized from ethanol (yield (77%), mp=185 °C) [25-27]. 1H NMR (DMSO, 300 MHz, δ
(ppm)): 3.36 (s, 12H, 2N(CH3)2), 6.51-7.04 (dd, 1H, C2–Ha of pyrazole), 6.51-7.04 (dd, 1H, C2–Ha of pyrazole),
6.51-7.04 (dd, 1H, C3–Hb of pyrazole), 7.48-8.00 (m, 8H of Ar-H), 11.18 (s, 2H, NH2). 13C NMR (DMSO, 300
MHz, δ (ppm)): 152 (C1), 63.22 (C3), 177.44 (C4), 121-130 (C5, C6), 40.22 (C7).
1
The structure of compound (DDP) was confirmed by their FT-IR, H NMR and 13C NMR spectra. The FT-IR
absorptions due to the thione group (C=S) in (DDP) appeared at 1071.82 cm-1 (str) and 658.8 cm-1 [27]. The
amine (-NH2) group in (DDP) appeared at 3479.08-3344.21 cm-1, respectively. The absorption bands associated
1
with other functional groups present all appeared in the expected regions. The H NMR spectra of substitued
pyrazole in DMSO-d6 exhibited a multiplet in the aromatic region at 7.48-8.00 ppm corresponding to the
aromatic hydrogen’s protons (Ar-H). The formation of the pyrazole ring is confirmed by the presence of the
imine (C=N) function at 1574.9 cm-1 in FT-IR spectrum of (DDP). Furthermore, the proton (Ha) and proton (Hb)
1
of pyrazole ring will appear in the region of 6.51-7.04 ppm as a multiplet in H NMR spectrum of (DDP). The
signals obtained from 1H NMR spectra further confirmed the proposed structure of pyrazole; the primary amine
1
function (-NH2) of carbothioamide entity (-CSNH2) resonate at 11.18 ppm. It should be noted that the H NMR
and 13C NMR spectrums of pyrazole showed a characteristic signal at 3.36 ppm and 40.22 ppm respectively,
matched the six protons of methyl group of the substituent dimethylamino (-N(CH3)2). Finally, the presence of
thione group (C4=S) is confirmed by the presence of a signal at 177.44 ppm in the 13C NMR spectrum according
to the literature [28].
3. 2. Effect of DDP concentration on mild steel corrosion in 1 M HCl
Studying of the corrosion behavior of mild steel in 1 M HCl at 30 °C is represented in Fig. 2. As shown
from this figure, by increasing the concentration of this derivative, the corrosion rate (νcorr) of mild steel in 1 M
HCl was decreased. This means that the presence of (DDP) retards the corrosion of mild steel in 1 M HCl or in
other words, this heterocyclic derivative act as inhibitor.
The values of inhibition efficiency of investigated inhibitor are given in Table 2. From this table and
Fig. 2, we can observe, that the inhibition efficiency increases (%IE) with increasing the concentration of
pyrazole. The maximum (%IE) of 96.60% was achieved at (5 × 10-4 M) at 30 °C. The results indicate that (DDP)
5

is more efficient in the 1 M HCl solution. The high inhibitive performance of this type of heterocyclic compound
suggests a higher bonding of pyrazole ring to the surface, which posse’s higher number of lone pairs from
heteroatom’s (S and N) and (π) orbitals [29-31].
3. 3. Effect of temperature on inhibition efficiency of (DDP)
The influence of temperature on inhibition efficiency evolution was studied by weight loss measurements
at 30, 40, 50, and 60 °C containing different concentrations of (DDP) pyrazole (Table 3). Fig. 3 shows the
variation of inhibition efficiency with temperature. This indicates that inhibition efficiency (%IE) increases with
inhibitor concentration but decreases with temperature. Which can be attributed to that the higher temperatures
might cause desorption of inhibitor molecules from the steel surface [32].
The values of inhibition efficiency obtained from the weight loss for different temperatures in 1 M HCl
are given in Table 3. The results show that the maximum (%IE) was about (96.60%, 5 × 10-4 M) and
(96.30%, 5 × 10-4 M) at 30 °C and 60 °C, respectively, which indicated that (DDP) was a good inhibitor in 1 M
HCl at this concentration 5 × 10-4 M [33, 34]. Also, a decrease in inhibition efficiency values with rise in
temperature may be due to the physically adsorption (low energy bonds) of inhibitor molecules on the metal
surface under study [35].
The relationship between the corrosion rate (νcorr) of mild steel in acidic media and absolute temperature
(T) is often expressed by the Arrhenius equation [35]:
E
a
lnv = ln A-
(5)
RT
Where νcorr is the corrosion rate, Ea is the apparent activation energy, R is the molar gas constant (8.314
J/K.mol), T is the absolute temperature, and A is the frequency factor. The plots of ln(νcorr) against (1/T) for mild
steel corrosion in 1 M HCl in the absence and presence of different concentrations of (DDP) are given in Fig. 4.
Apparent activation energy values in the absence and presence of (DDP) at different temperatures were
calculated from ln(νcorr) vs. (1/T) plots are given in Table 4.
The data in Table 4 show that all the linear regression coefficients (R2) are very near to 1, which illustrate
that the linear relationship between ln(νcorr) and the inverse of the absolute temperature (1/T ) is good. Also, the
value of (Ea) in the presence of (DDP) is higher than that in the uninhibited 1 M HCl solution. It can be observed
that the activation energy parameters for corrosion process increases from 54.84 kJ/mol to 80.41 kJ/mol in the
absence and the presence of (DDP), respectively. This result shows that the addition of inhibitor significantly
reduced the dissolution of metal in the hydrochloric acid media [36]. According to the literature the higher (Ea)
in presence of inhibitor for mild steel in comparison with blank solution is due to the electrostatic interactions
(physisorption process) of molecules at electrode surface. In addition, the low value of the activation energy
(60.21 kJ/mol) at the optimum concentration (5 × 10-4 M) results in the stability of the formed complex (Metal-
DDP) at the interface metal/solution [37, 38].
6

3. 4. Effect of immersion time on surface activity of (DDP)
Effect of immersion time for 30 to 1440 minute on inhibition efficiency of (DDP, 5 × 10-4 M) on the
corrosion of mild steel in 1M HCl at 30 °C was studied using weight loss measurements. Table 5 shows the
variation of corrosion rates and inhibition efficiency (%IE) with time in the absence and presence of (DDP)
inhibitor. The Table 5 illustrates that the corrosion rate of mild steel is greatly decreased with immersion time in
the presence of (DDP) inhibitor, the corrosion rate (νcorr) value was about (0.03046 mg/cm2.h at 30 min) and
decreased towards (0.00840 mg/cm2.h and 0.00980 mg/cm2.h ) at (480 min and 1440 min), respectively. This
phenomenon may be associated to the progressive adsorption of (DDP) molecules on the steel surface, while in
the absence of (DDP) inhibitor the corrosion rate of mild steel was slightly decreased with immersion time from
(0.56 mg/cm2.h at 30 min) to (0.42 mg/cm2.h and 0.52 mg/cm2.h ) at (480 min and 1440 min), respectively. This
behavior may be due to the formation of a passive film of corrosion product Fe(OH)2 on the steel surface, which
decreases the corrosion rate of the steel. However, this film formed by Fe(OH)2 molecules can easily be
dissolved in the hydrochloric acid solution [36]. Also, results in Table 5 show that the effect of immersion time
on inhibition efficiency of (DDP) at 30 °C. In the presence of inhibitor, increasing time resulted in increasing
(%IE) in 30–1440 min and attains a maximal value 98% after 1440 min. In general, the inhibition efficiency is
slightly changed with time immersion time. This reveals that the adsorptive organic layer of (DDP) becomes
very compact and uniform at longer immersion time [39, 40].
3. 5. Thermodynamic parameters
It should be noted that the protection of metals against corrosion by using of organic compounds is
explained by their adsorption on the metal surface. The latter is well known in three forms: physical adsorption,
chemisorptions or mixed adsorption (physisorption with chemisorptions) [41]. The adsorption phenomenon can
be implemented by studying the adsorption isotherms.
In order to study the variation of the adsorbed amount with the inhibitor concentration, assuming the
increase of the inhibition is attributable to the adsorption of (DDP) on the mild steel surface and obeys Langmuir
adsorption isotherm mode, according to the following equation [42]:
Ci
1
=
+ Ci
(6)
θ
Kads
Where Ci is the concentration of inhibitor, Kads the adsorptive equilibrium constant and θ is the surface coverage
calculated as follows [43]:
v
0 −
v
(7)
θ
=
v
0
Where ν0 and ν the corrosion rate in the absence and presence of (DDP), respectively.
The linear regressions parameters between (Ci/θ) and (Ci) are given in Table 6. Straight lines of (Ci/θ) vs
(Ci) at different temperatures 30, 40, 50 and 60 °C are illustrated in Fig. 5 and Fig. 6. It is clear that all linear
correlation coefficients (R2) and slopes values are equal to (1), which indicates that the adsorption of (DDP) on
mild steel surface is perfectly follows Langmuir adsorption isotherm at all used temperatures [44-47]. The
adsorptive equilibrium constant (Kads) values are calculated from the reciprocal of the intercept of (Ci/θ) as (Ci).
It is related to the standard free energy of adsorption (
ꢀ
ꢀ°ꢁꢂ ) as shown the following equation [48]:
s
7

0




1
∆Gads
(8)
K
ads
=
exp -

55.5
RT

Where R is the gas constant (8.314 J/K.mol), T the absolute temperature Kads, the value 55.5 is the molar
concentration of water in solution [44]. The values of Kads and
ꢀ
ꢀ
°
of (DDP) at 30, 40, 50 and 60 °C are
ꢁꢂ
s
listed in Table 6.
The negative values of (ꢀ
ꢀ°ꢁꢂ ) confirmed that the adsorption of (DDP) molecule on steel surface is a
s
spontaneous process. Generally, values of (ꢀ
ꢀ°ꢁꢂ ) up to -20 kJ/mol are consistent with the electrostatic
s
interaction between the charged molecules and the charged metal (physical adsorption) while those more
negative than -40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal
surface to form a coordinate type of bond (chemisorption) [49-52]. In the present work, the parameters of
(ꢀꢀ°ꢁꢂ ) are around –43.00, -43.55, -44.65 and -43.74 kJ/mol for (DDP) at 30, 40, 50 and 60 °C, respectively.
s
The results suggested that the adsorption of heterocyclic pyrazole on surface involves the chemical adsorption
process. Indeed, the corrosion inhibition of mild steel in 1 M HCl in the presence of target pyrazole is mainly
attributed to physical adsorption; this finding is justified by the slight decrease in inhibition efficiency (%IE)
with increasing temperature. Table 6, also shows that (Kads) of (DDP) is higher than that of other molecules
(DPC and DPCM) of the same family, which confirmed that (DDP) is strongly adsorbed on steel surface under
study [53, 54]. The presence of heteroatom’s, conjugate system with (π) band, pyrazole ring as well as the
aromatic substituants in (DDP) structure promotes the sharing of electron (charge transfer) between inhibitor
molecules and the vacant iron (d) orbital to establish coordinate type bonds (chemisorption). The electrostatic
interactions (physisoprtion) can take place between the charged (DDP) molecules (DDP-H)+ and the chloride
ions (Cl¯ ) adsorbed at the interface of the electrode interface. From these results, the adsorption process of
(DDP) on the surface can be presented as follows (Fig. 7).
3. 6. Potenciodynamic polarization curves
Fig. 8 shows the potentiodynamic polarization curves for MS in 1 M HCl at different concentrations of
(DDP) at 30 °C. The addition of (DDP) inhibitor to the 1 M HCl solution leads to a decrease in the corrosion rate
and change dramatically the anodic and cathodic curves to lower values of current densities. This indicates that
the both cathodic and anodic reactions of MS electrode corrosion were affected by the presence of (DDP) in HCl
solution.
Electrochemical parameters of the kinetics corrosion process were calculated from extrapolation of the
linear Tafel lines to the corrosion potential and the results are given in Table 7. The inhibition efficiency (%IE)
was defined as:
I0 − I
%IE =
×100
(9)
I0
Where I0 and I represent corrosion current density values without and with (DDP) inhibitor, respectively,
determined by extrapolation of Tafel lines to the corrosion potential.
8

Table 7 shows that an increase in (DDP) concentration is resulted in increased inhibition efficiency to reach a
maximum value of 95.50% at 5 × 10-4 M. The results in Table 7 reveal that, increasing of (DDP) concentration
leads to the change in potential (Ecorr) values toward more positive potential and the values of corrosion current
density decreased indicating the surface activity of this type of pyrazole. In addition, we can see that the shift in
corrosion potential is around (52.3, 58.9, 89.3, and 115.2 mV) for (10-5, 5 x 10-5, 10-4 and 5 x 10-4 M),
respectively. Indeed, some authors reported that the shift in (Ecorr) is higher than 85 mV compared to the blank
solution, indicating that the inhibitor is regarded as a cathodic or anodic type inhibitor, while the displacement is
lower than 85 mV, the inhibitor is considered as a mixed type. In this study we can observe that the values of
shift in (Ecorr) at (10-4 and 5 x 10-4 M) are higher than 85 mV, but at the concentration of (10-5 and 5 x 10-5 M) the
values of shift in (Ecorr) are lower than 85 mV, which mean that the (DDP) can considered as a mixed type
inhibitor with predominant anodic effectiveness [55]. The values of both anodic and cathodic Tafel slopes are
slightly changed with the (DDP) inhibitor addition, indicating that the inhibitors retarded the mild steel
dissolution by simple blocking the anodic and cathodic sites of the corrosion reaction on the steel surface,
without changing the mechanism of corrosion process [56].
The corrosion of mild steel in HCl solution can be presented as follows [57, 58]: The anodic dissolution
reaction of iron in the presence of chloride anion in hydrochloric acid solution.
(10)
(11)
(12)
(13)
On the other hand, the cathodes H2 evolution can be represented as:
(14)
(15)
(16)
It is generally accepted that the mechanism of adsorption process involves the replacement of water molecules
adsorbed at the surface of the metal.
(17)
Where (n) is the size ratio, that is, the number of water molecules replaced by one organic inhibitor. The
inhibitor molecules may then combine with freshly generated Fe2+ to forming metal–inhibitor complexes,
according to Equation (18). Therefore, it is possible to suggest that the presence of chloride ion (Cl¯ ) on the steel
surface can facilitate the adsorption of the positive complex through to the electrostatic interactions [59].
(18)
3. 7. Electrochemical impedance spectroscopy (EIS)
The Nyquist and Bode Modulus plots for MS in 1 M HCl at 30 °C in the absence and presence of
various concentrations of pyrazole inhibitor are given in Fig. 9 and Fig. 10, respectively. These diagrams were
obtained after 1 h of immersion in 1 M HCl at 30 °C.
9

The impedance spectra in the Fig. 9 are presented as semicircle with diameter increases with the increase
of (DDP) concentration. This indicates that the corrosion process of mild steel in 1 M HCl is controlled by the
charge transfer phenomenon and the addition of (DDP) to electrolytic solution does not change the mechanism
of MS corrosion in the acid solution [60]. Also, the obtained impedance plots are not perfect semicircles which
are due to the frequency dispersion as a result of the heterogeneity of MS surface. Furthermore, it is clear, from
Fig. 9 that the adsorptions of (DDP) on the electrode surface increase the impedance response of MS in
comparison with the uninhibited HCl solution [61]. The Fig. 10 illustrates the Bode modulus plot,
we can see an increase of the impedance values at low frequencies and that the shape of the curve remains the
same without and with the addition of (DDP) to the corrosive solution. This result confirms the evolution of the
protective film adsorbed on the steel surface with the increase in the concentration of (DDP) inhibitor [62].
Electrochemical impedances spectra of MS in 1 M HCl are simulated by the equivalent circuit diagram presented
in the Fig. 11.
.
The electrochemical impedances parameters of charge transfer resistance (Rct), double layer capacitance
(Cdl) and inhibition efficiency (%IE) are given in Table 8. The (%IE) is calculated by charge transfer resistance
obtained from Nyquist plots, according to the equation (19) [63].
R
ct (inh) - R
%IE =
ct ×100
(19)
Rct (inh)
Where Rct and Rct(inh) are the charge transfer resistance values without and with (DDP), respectively.
Table. 8 shows that, charge transfer resistance (Rct) increases with the increasing of (DDP)
concentration in the HCl environment. On the other hand, the decreasing values of (Cdl) with the addition of
(DDP) compared to that in absence of the inhibitor may be attributed to a lower local dielectric constant, which
due the gradual replacement of water molecules by the adsorption of the (DDP) molecules at electrode interface
and the formation of organic layer increasing the thickness of the electrical double layer and reduces the surface
of mild steel exposed to acidic solution according to Helmholtz equation (20) [64].
ε°ε S
Cdl =
(20)
d
Where S electrode surface, εo permittivity of air, ε local dielectric constant and d film thickness
The Inhibition Efficiency (%IE) increases with the concentration of (DDP) to reaches 95.30 % (5 × 10-4
M) which confirm the effectiveness of this type of pyrazole derivative containing the carbothioamide (-CSNH2)
entity in their structure [17]. Also, the phase shift (n) values are given in Table 8. Decrease of (n) values by the
increase of inhibitor concentration compared to the uninhibited solution can be attributed to the increase of the
surface inhomogeneity as a result of the (DDP) molecules adsorption [33].
3. 7. Scanning electron microscopy (SEM)
The surface morphology of mild steel in 1 M HCl with and without (DDP) inhibitor was studied by
scanning electron microscopy in order to confirm that the corrosion inhibition of MS is due to the formation of
an organic layer of (DDP) molecules. Fig. 12 shows the SEM photos of MS surface in 1 M HCl solution after
immersion for 24 h in absence and presence of 5 × 10-4 M of (DDP) compound. It can be observed from
Fig. 12 (A) a uniform surface along with the presence of some cracks and small black holes, which may be due
10

to the defect of steel. The Fig. 12 (B) of the MS surface after immersion in uninhibited 1 M HCl shows an
aggressive attack of the corroding medium on the steel surface. On the other hand the presence of (DDP)
inhibitor in 1 M HCl solution leads to the formation of an organic layer on the surface of steel as shown in Fig.
12 (C). Consequently, the inhibition action of this type of molecule may be related to the formation of a stable
film at the interface MS/HCl, with prevents the aggressive attack of the medium on the surface of the steel under
study.
4. Quantum Chemical studies
4. 1. DFT calculation
The corrosion inhibition by means of organic compounds depend a several factors, including the structural
parameters, such as stereochemistry of the molecule, the electronic density, frontier molecular orbitals and others
witch play a decisive role on the mechanism of corrosion inhibition [65, 66]. In general, the organic molecules,
especially the heterocyclic compounds containing a particular function groups, heteroatom’s and aromatic
systems in their structure have high ability to act as corrosion inhibitors. In order to find a relationship between
the inhibition efficiency and structural properties of (DDP), some quantum parameters such as dipole moment
(µ), total energy (Etot), highest occupied molecular orbital (EHOMO), lowest unoccupied molecular orbital (ELUMO
)
energies, (ELUMO–EHOMO) energy gap (∆E =ELUMO–EHOMO) and molecular volume (V) were calculated using
B3LYP/6-31G* method. The optimized structure, (HOMO, LUMO) electronic distribution and data quantum’s
are given in the Fig. 13 and Table 9, respectively.
It is clear from this Fig. 13 that the HOMO and LUMO orbital’s of (DDP) gives rise to interactions
between the inhibitor and metal surface. This figure shows that the electronic density is concentrated on the
heteroatom’s of pyrazole ring and on the conjugate system of (DDP), which shows that these sites are the best
centers for the donor-acceptor process between the inhibitor and (d) orbital of iron under study [67]. The analysis
of data in Table 9 shows that the calculated EHOMO, ELUMO and ꢀE energies are -5.08 eV, -1.94 eV and 3.14 eV,
respectively. According to the FMO (frontier molecular orbital theory), less negative EHOMO energy and the
smaller energy gap (ꢀE =ELUMO–EHOMO) implies the tendency of the inhibitor molecules to share one or more
electrons to the vacant (d) orbital of the iron atoms [68]. Consequently, these results indicate that (DDP)
molecules can easily transfer electrons to the vacant (d) orbitals of the iron surface. In other words, the (DDP)
molecules have been strongly adsorbed on the steel surface involving the electron sharing to achieve better
corrosion inhibition activity compared to other heterocyclic compounds [14, 33, 60]. Also, increasing values of
dipole moment (µ=9.72D) and molecular volume (V=340.57 cm3/mol) may facilitate adsorption by influencing
the transport process through the adsorbed layer, and promote the electrostatic interaction between the metal
charge and the charged molecules of this compound [69]. These results show the performance of (DDP)
compared to (DPC, DPCM) molecules of the same pyrazole family [14].
4. 2. Fukui indice analysis
The fukui indices given in Table 10 have been calculated in order to give further insight into the
experimental study and to predict the best adsorption sites for nucleophilic and electrophilic exchanges [70]. The
data in Table 10 shows that the highest values of (fk+) for nucleophilic attack are on N4, S7 and C19 while the
highest value of (fk-) is on S7, which indicate that these positions are the preferred site for the adsorption of
11

(DDP) inhibitor on metal surface through donor-acceptor process (Fig. 14). The results obtained are in good
agreement with the optimized structure, HOMO and LUMO electronic distribution cited in the Fig. 13, this
allows us to calculate the interaction and binding energy for the best disposition of the (DDP) molecule on the
iron surface as shown in flowing figure (Fig. 14). The calculated interaction and binding energy values are listed
in Table 11.
The calculated interaction energy values of the (DDP-Fe) system in vacuum and HCl solution are
respectively; -21.11 and -27.01 kcal/mol. The higher and negative value of interaction energy is due to the strong
adsorption of (DDP) molecule on iron surface [71]. Also, the higher positive value of binding energy for target
inhibitor (27.01 kcal/mol) suggests that the protective organic film formed by the adsorption of (DDP) at the
interface Metal/HCl is stable and adherent to the acid attack. These results confirmed the complementary
between experimental data and theoretical quantum methods, which have proved that this type of molecules
family can be a good corrosion inhibitor of steel in HCl medium [72].
5. Conclusion
A
novel
pyrazole
heterocyclic
derivative
namely,
(E)-5-(4-(dimethylamino)phenyl)-3-(4-
(DDP) was synthesized by
(dimethylamino)styryl)-2,3-dihydro-1H-pyrazole-1-carbothioamide
cyclocondensation reaction of the corresponding dibenzalcetone (4-DBA) and thiosemicarbazide in sodium
hydroxide solution and ethanol solvent. The structures of this compound was determined by FT-IR, 1H NMR and
13C NMR spectra. The compound was tested as inhibitor for corrosion of mild steel in 1 M HCl medium. From
the results of different methods used in this study, we can draw the following conclusions:
1- (DDP) acts as a good inhibitor for the corrosion of MS in 1 M HCl. The (%IE) values increase with the
inhibitor concentration to attain 96.60% at 5 × 10-4 M. While slightly decrease with the temperature.
2- The adsorption of (DDP) on MS surface obeys Langmuir adsorption isotherm. The adsorption process is a
spontaneous and the decrease of (Kads) with temperature accompanied by a decrease in inhibition efficiency.
3- The kinetic (Ea) and thermodynamic (ꢀꢀ°ꢁꢂ ) parameters indicates that the adsorption of pyrazole on MS
s
surface involves both physical adsorption and chemical adsorption.
4- (DDP) pyrazole acts as a mixed type inhibitor in 1 M HCl, and SEM analysis shows that the inhibition
mechanism of target inhibitor is due to the formation of organic layer, which effectively protects steel from HCl
attack.
5- Optimized molecular structures obtained by quantum chemical calculations illustrate that the (DDP) may be
adsorbed on the steel surface involving donor-acceptor interactions between the π-electrons, N and S
heteroatom’s (pyrazole ring) and the vacant (d) orbitals of iron atoms. Also, the results show a good correlation
of dipole moment of (DDP) and it inhibition activity.
6- Interaction energy and binding energy shows that the (DDP) is strongly adsorbed into the iron surface.
Acknowledgement
The author’s thanks ministry of higher education and scientific research of Algeria (MESRS) and the HPC
ressources of UCI-UABT of the University Abou bekr Belkaïd of Tlemcen for financial support in this research.
12

References
[1] J. Cruz, R. Martınez, J. Genesca, E. Garcıa-Ochoa, Experimental and theoretical study of 1-(2-ethylamino)-2-
́
́
methylimidazoline as an inhibitor of carbon steel corrosion in acid media, Journal of Electroanalytical
Chemistry, 566 (2004) 111-121.
[2] M.A. Migahed, Electrochemical investigation of the corrosion behaviour of mild steel in 2M HCl solution in
presence of 1-dodecyl-4-methoxy pyridinium bromide, Materials Chemistry and Physics, 93 (2005) 48-53.
[3] I.I. Bergmann, Corrosion Inhibitors, Macemillan, New York, 1963.
[4] Y.I. Kuznetsov, Organic Inhibitors of Corrosion of Metals, Springer US, 1996.
[5] K.M. Ismail, Evaluation of cysteine as environmentally friendly corrosion inhibitor for copper in neutral and
acidic chloride solutions, Electrochimica Acta, 52 (2007) 7811-7819.
[6] M. Benabdellah, R. Touzani, A. Aouniti, A. Dafali, S. El Kadiri, B. Hammouti, M. Benkaddour, Inhibitive
action of some bipyrazolic compounds on the corrosion of steel in 1 M HCl: Part I: Electrochemical study,
Materials Chemistry and Physics, 105 (2007) 373-379.
[7] H. Ashassi-Sorkhabi, S. Moradi-Alavian, M.D. Esrafili, A. Kazempour, Hybrid sol-gel coatings based on
silanes-amino acids for corrosion protection of AZ91 magnesium alloy: Electrochemical and DFT insights,
Progress in Organic Coatings, 131 (2019) 191-202.
[8] F.M. Abdelrazek, P. Metz, N.H. Metwally, S.F. El‐Mahrouky, Synthesis and Molluscicidal Activity of New
Cinnoline and Pyrano [2, 3‐c] pyrazole Derivatives, Archiv der Pharmazie: An International Journal
Pharmaceutical and Medicinal Chemistry, 339 (2006) 456-460.
[9] M. Messali, M. Larouj, H. Lgaz, N. Rezki, F.F. Al-Blewi, M.R. Aouad, A. Chaouiki, R. Salghi, I.-M. Chung,
A new schiff base derivative as an effective corrosion inhibitor for mild steel in acidic media: Experimental and
computer simulations studies, Journal of Molecular Structure, 1168 (2018) 39-48.
[10] A. Singh, K.R. Ansari, Y. Lin, M.A. Quraishi, H. Lgaz, I.-M. Chung, Corrosion inhibition performance of
imidazolidine derivatives for J55 pipeline steel in acidic oilfield formation water: Electrochemical, surface and
theoretical studies, Journal of the Taiwan Institute of Chemical Engineers, 95 (2019) 341-356.
[11] A. Nazarov, N. Le Bozec, D. Thierry, Scanning Kelvin Probe assessment of steel corrosion protection by
marine paints containing Zn-rich primer, Progress in Organic Coatings, 125 (2018) 61-72.
[12] H. Idrissi, S. Ramadan, J. Maghnouj, R. Boulif, Modern concept of acoustic emission (AE) coupled with
electrochemical measurements for monitoring the elastomer-coated carbon steel damage in phosphoric acid
medium, Progress in Organic Coatings, 63 (2008) 382-388.
[13] E.E. Elemike, H.U. Nwankwo, D.C. Onwudiwe, Experimental and theoretical studies of (Z)-N-(2-
chlorobenzylidene) naphthalen-1-amine and (Z)-N-(3-nitrobenzylidene)naphthalen-1-amine, and their corrosion
inhibition properties, Journal of Molecular Structure, 1155 (2018) 123-132.
[14] H.B. Ouici, O. Benali, A. Guendouzi, Experimental and quantum chemical studies on the corrosion
inhibition effect of synthesized pyrazole derivatives on mild steel in hydrochloric acid, Research on Chemical
Intermediates, 42 (2016) 7085-7109.
[15] F. El-Taib Heakal, S.K. Attia, S.A. Rizk, M.A. Abou Essa, A.E. Elkholy, Synthesis, characterization and
computational chemical study of novel pyrazole derivatives as anticorrosion and antiscalant agents, Journal of
Molecular Structure, 1147 (2017) 714-724.
13

[16] R. A. Gupta, S. G. Kaskhedikar, Synthesis, antitubercular activity, and QSAR analysis of substituted
nitroaryl analogs: chalcone, pyrazole, isoxazole, and pyrimidines, Med Chem Res, 22 (2013) 3863–3880
[17] A.M. Farag, N. A. Kheder, K. M. Dawood, A. M. El Defrawy, A facile access and computational studies of
new 4,5-biprazole derivatives, Heterocycles, 94 (2017) 1245-1256;
[18] A. M. Farag, A. S. Mayhoub, S. E. Barakat, A. H. Bayomi, Synthesis of new N-phenylpyrazole derivatives
with potent antimicrobial activity, Bioorg. Med. Chem, 16 (2008) 4569-4578.
[19] B. Rezessy, Z. Zubovics, J. Kovács, G. Tóth, Synthesis and structure elucidation of new thiazolotriazepines,
Tetrahedron, 55 (1999) 5909-5922.
[20] M. Lebrini, M. Traisnel, M. Lagrenée, B. Mernari, F. Bentiss, Inhibitive properties, adsorption and a
theoretical study of 3, 5-bis (n-pyridyl)-4-amino-1, 2, 4-triazoles as corrosion inhibitors for mild steel in
perchloric acid, Corrosion science, 50 (2008) 473-479.
[21] Gaussian 09, Revision A.1, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P.
Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta,
F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E.
Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R.
Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J.
Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian,
Inc., Wallingford CT, 2009
[22] A.D. Becke, A new mixing of Hartree–Fock and local density‐functional theories, The Journal of chemical
physics, 98 (1993) 1372-1377.
[23] S.F. Boys, F.d. Bernardi, The calculation of small molecular interactions by the differences of separate total
energies. Some procedures with reduced errors, Molecular Physics, 19 (1970) 553-566.
[24] Y.R. Huang, J.A. Katzenellenbogen, Regioselective synthesis of 1, 3, 5-triaryl-4-alkylpyrazoles: novel
ligands for the estrogen receptor, Organic letters, 2 (2000) 2833-2836.
[25] S. Hassan, Synthesis, antibacterial and antifungal activity of some new pyrazoline and pyrazole derivatives,
Molecules, 18 (2013) 2683-2711.
[26] S. L. Zhu, Y. Wu, C.-J. Liu, C.-Y. Wei, J.-C. Tao, H.-M. Liu, Synthesis and in vitro cytotoxic activity
evaluation of novel heterocycle bridged carbothioamide type isosteviol derivatives as antitumor agents,
Bioorganic & Medicinal Chemistry Letters, 23 (2013) 1343-1346.
[27] T. A. Mohamed, A. M. Hassan, U. A. Soliman a,1, W. M. Zoghaib, J. Husband, M. M. Abdelall, Infrared,
Raman and NMR spectra, conformational stability, normal coordinate analysis and B3LYP calculations of 5-
amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbothioamide, J. Mol. Struct, 985 (2011) 277–291
[28] K. Kumara, N. Shivalingegowda, L.D. Mahadevaswamy, A.K. Kariyappa, N.K. Lokanath, Crystal structure
studies and Hirshfeld surface analysis of 5-(4-methoxyphenyl)-3-(thiophen-2-yl)-4,5-dihydro-1H-pyrazole-1-
carbothioamide, Chemical Data Collections, 9-10 (2017) 251-262.
[29] H. B. Ouici, M. Belkhouda, O. Benali, R. Salghi, L. Bammou, A. Zarrouk, B. Hammouti, Adsorption and
inhibition effect of 5-phenyl-1, 2, 4-triazole-3-thione on C38 steel corrosion in 1 M HCl, Research on Chemical
Intermediates, 41 (2015) 4617-4634.
14

[30] A. Singh, M. Talha, X. Xu, Z. Sun, Y. Lin, Heterocyclic Corrosion Inhibitors for J55 Steel in a Sweet
Corrosive Medium, ACS Omega, 2 (2017) 8177-8186.
[31] R. Solmaz, E. Altunbaş, G. Kardaş, Adsorption and corrosion inhibition effect of 2-((5-mercapto-1, 3, 4-
thiadiazol-2-ylimino) methyl) phenol Schiff base on mild steel, Materials Chemistry and Physics, 125 (2011)
796-801.
[32] H. Ouici, M. Tourabi, O. Benali, C. Selles, C. Jama, A. Zarrouk, F. Bentiss, Adsorption and corrosion
inhibition properties of 5-amino 1,3,4-thiadiazole-2-thiol on the mild steel in hydrochloric acid medium:
Thermodynamic, surface and electrochemical studies, Journal of Electroanalytical Chemistry, 803 (2017) 125-
134.
[33] H. B. Ouici, O. Benali, A. Guendouzi, Corrosion inhibition of mild steel in acidic media using newly
synthesized heterocyclic organic molecules: Correlation between inhibition efficiency and chemical structure, in,
AIP Publishing, 2015, pp. 020086.
[34] R. Solmaz, Investigation of the inhibition effect of 5-((E)-4-phenylbuta-1, 3-dienylideneamino)-1, 3, 4-
thiadiazole-2-thiol Schiff base on mild steel corrosion in hydrochloric acid, Corrosion Science, 52 (2010) 3321-
3330.
[35] E.E. Oguzie, C. Unaegbu, C.N. Ogukwe, B.N. Okolue, A.I. Onuchukwu, Inhibition of mild steel corrosion
in sulphuric acid using indigo dye and synergistic halide additives, Materials Chemistry and Physics, 84 (2004)
363-368.
[36] X. Li, S. Deng, H. Fu, G. Mu, Inhibition effect of 6-benzylaminopurine on the corrosion of cold rolled steel
in H2SO4 solution, Corrosion Science, 51 (2009) 620-634.
[37] M.M. Osman, R.A. El-Ghazawy, A.M. Al-Sabagh, Corrosion inhibitor of some surfactants derived from
maleic-oleic acid adduct on mild steel in 1 M H2SO4, Materials chemistry and physics, 80 (2003) 55-62.
[38] M. Elachouri, M.S. Hajji, M. Salem, S. Kertit, J. Aride, R. Coudert, E. Essassi, Some nonionic surfactants
as inhibitors of the corrosion of iron in acid chloride solutions, Corrosion, 52 (1996) 103-108.
[39] I.B. Obot, E.E. Ebenso, I.A. Akpan, Z.M. Gasem, A.S. Afolabi, Thermodynamic and density functional
theory investigation of sulphathiazole as green corrosion inhibitor at mild steel/hydrochloric acid interface, Int J
Electrochem Sci, 7 (2012) 1978-1996.
[40] S.L. Granese, B.M. Rosales, C. Oviedo, J.O. Zerbino, The inhibition action of heterocyclic nitrogen organic
compounds on Fe and steel in HCl media, Corrosion Science, 33 (1992) 1439-1453.
[41] E. Khamis, The effect of temperature on the acidic dissolution of steel in the presence of inhibitors,
Corrosion, 46 (1990) 476-484.
[42] X. Li, S. Deng, H. Fu, T. Li, Adsorption and inhibition effect of 6-benzylaminopurine on cold rolled steel in
1.0 M HCl, Electrochimica Acta, 54 (2009) 4089-4098.
[43] I. Sekine, Y. Hirakawa, Effect of 1-hydroxyethylidene-1, 1-diphosphonic acid on the corrosion of SS 41
steel in 0.3% sodium chloride solution, Corrosion, 42 (1986) 272-277.
[44] W.A. Badawy, K.M. Ismail, A.M. Fathi, Corrosion control of Cu–Ni alloys in neutral chloride solutions by
amino acids, Electrochimica Acta, 51 (2006) 4182-4189.
[45] A. Azim, L.A. Shalaby, H. Abbas, Mechanism of the corrosion inhibition of Zn Anode in NaOH by gelatine
and some inorganic anions, Corrosion Science, 14 (1974) 21-24.
15

[46] F. Bentiss, M. Bouanis, B. Mernari, M. Traisnel, M. Lagrenee, Effect of iodide ions on corrosion inhibition
of mild steel by 3, 5-bis (4-methylthiophenyl)-4H-1, 2, 4-triazole in sulfuric acid solution, Journal of Applied
Electrochemistry, 32 (2002) 671-678.
[47] X. Sheng, Y.-P. Ting, S.O. Pehkonen, Evaluation of an organic corrosion inhibitor on abiotic corrosion and
microbiologically influenced corrosion of mild steel, Industrial & Engineering Chemistry Research, 46 (2007)
7117-7125.
[48] E. Cano, J.L. Polo, A. La Iglesia, J.M. Bastidas, A study on the adsorption of benzotriazole on copper in
hydrochloric acid using the inflection point of the isotherm, Adsorption, 10 (2004) 219-225.
[49] F. Bentiss, M. Lebrini, M. Lagrenée, Thermodynamic characterization of metal dissolution and inhibitor
adsorption processes in mild steel/2, 5-bis (n-thienyl)-1, 3, 4-thiadiazoles/hydrochloric acid system, Corrosion
Science, 47 (2005) 2915-2931.
[50] W.H. Li, Q. He, S.-t. Zhang, C.-l. Pei, B.-r. Hou, Some new triazole derivatives as inhibitors for mild steel
corrosion in acidic medium, Journal of Applied Electrochemistry, 38 (2008) 289-295.
[51] F. Bensajjay, S. Alehyen, M. El Achouri, S. Kertit, Corrosion inhibition of steel by 1-phenyl 5-mercapto 1,
2, 3, 4-tetrazole in acidic environments (0.5 M H2SO4 and 1/3 M H3PO4), Anti-Corrosion Methods and
Materials, 50 (2003) 402-409.
[52] M. Şahin, S. Bilgic, H. Yılmaz, The inhibition effects of some cyclic nitrogen compounds on the corrosion
of the steel in NaCl mediums, Applied Surface Science, 195 (2002) 1-7.
[53] P. Singh, A. Singh, M.A. Quraishi, Thiopyrimidine derivatives as new and effective corrosion inhibitors for
mild steel in hydrochloric acid: electrochemical and quantum chemical studies, Journal of the Taiwan Institute of
Chemical Engineers, 60 (2016) 588-601.
[54] O. Benali, L. Larabi, M. Traisnel, L. Gengembre, Y. Harek, Electrochemical, theoretical and XPS studies of
2-mercapto-1-methylimidazole adsorption on carbon steel in 1 M HClO4, Applied surface science, 253 (2007)
6130-6139.
[55] X. Li, S. Deng, H. Fu, Three pyrazine derivatives as corrosion inhibitors for steel in 1.0 M H2SO4 solution,
Corrosion science, 53 (2011) 3241-3247.
[56] C. Cao, On electrochemical techniques for interface inhibitor research, corrosion science, 38 (1996) 2073-
2082.
[57] A. Yurt, A. Balaban, S.U. Kandemir, G. Bereket, B. Erk, Investigation on some Schiff bases as HCl
corrosion inhibitors for carbon steel, Mater. Chem. Phys. 85 (2004) 420.
[58] W. Li, Q. He, C. Pei, B. Hou, Experimental and theoretical investigation of the adsorption behaviour of new
triazole derivatives as inhibitors for mild steel corrosion in acid media, Electrochim. Acta 52 (2007) 6386.
[59] Q.B. Zhang, Y.X. Hua, Corrosion inhibition of mild steel by alkylimidazolium ionic liquids in hydrochloric
acid, Electrochim. Acta 54 (2009) 1881–1887.
[60] H.B. Ouici, O. Benali, Y. Harek, S.S. Al-Deyab, L. Larabi, B. Hammouti, Influence of the 2-Mercapto-1-
Methyl Imidazole (MMI) on the Corrosion Inhibition of Mild Steel in 5% HCl, Int. J. Electrochem. Sci, 7 (2012)
2304-2319.
[61] M. Lebrini, M. Lagrenée, H. Vezin, M. Traisnel, F. Bentiss, Experimental and theoretical study for
corrosion inhibition of mild steel in normal hydrochloric acid solution by some new macrocyclic polyether
compounds, Corrosion Science, 49 (2007) 2254-2269.
[62] A.K. Singh, S.K. Shukla, M. Singh, M.A. Quraishi, Mater. Chem. Phys. 129 (2011) 68–76.
16

[63] Q. Qu, Z. Hao, L. Li, W. Bai, Y. Liu, Z. Ding, Synthesis and evaluation of Tris-hydroxymethyl-(2-
hydroxybenzylidenamino)-methane as a corrosion inhibitor for cold rolled steel in hydrochloric acid, Corrosion
Science, 51 (2009) 569-574.
[64] A. E. Elkholy, F. Heakal, Electrochemical measurements and semi-empirical calculations for understanding
adsorption of novel cationic Gemini surfactant on carbon steel in H2SO4 solution, J. Mol. Struct, 1156 (2018)
473-482.
[65] R.N. Singh, A. Kumar, R.K. Tiwari, P. Rawat, A combined experimental and theoretical (DFT and AIM)
studies on synthesis, molecular structure, spectroscopic properties and multiple interactions analysis in a novel
Ethyl-4-[2-(thiocarbamoyl) hydrazinylidene]-3, 5-dimethyl-1H-pyrrole-2-carboxylate and its dimer,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 112 (2013) 182-190.
[66] H. Jafari, I. Danaee, H. Eskandari, M. RashvandAvei, Combined computational and experimental study on
the adsorption and inhibition effects of N2O2 schiff base on the corrosion of API 5L grade B steel in 1 mol/L
HCl, Journal of Materials Science & Technology, 30 (2014) 239-252.
[67] M. Messali, H. Lgaz, R. Dassanayake, R. Salghi, S. Jodeh, N. Abidi, O. Hamed, Guar gum as efficient non-
toxic inhibitor of carbon steel corrosion in phosphoric acid medium: Electrochemical, surface, DFT and MD
simulations studies, Journal of Molecular Structure, 1145 (2017) 43-54.
[68] L.M. Rodríguez-Valdez, W. Villamisar, M. Casales, J.G. Gonzalez-Rodriguez, A. Martínez-Villafañe, L.
Martinez, D. Glossman-Mitnik, Computational simulations of the molecular structure and corrosion properties of
amidoethyl, aminoethyl and hydroxyethyl imidazolines inhibitors, Corrosion science, 48 (2006) 4053-4064.
[69] K. Babic´-Samardzˇija, K.F. Khaled, N. Hackerman, Appl. Surf. Sci. 240, 327–340 (2005)
[70] H. Lgaz, O. Benali, R. Salghi, S. Jodeh, M. Larouj, O. Hamed, M. Messali, S. Samhan, M. Zougagh, H.
Oudda, Pyridinium derivatives as corrosion inhibitors for mild steel in 1M HCl: electrochemical, surface and
quantum chemical studies. Der Pharma Chem. 8 (2016) 172–190. Brought to you by| Google Googlebot-Web
Crawler SEO Said et al.: C11H19N2I| 3 phthalazinone derivatives and their antihypertensive activities, Eur. J.
Med. Chem, 39 (2004) 1089-1095.
[71] A.Y. Musa, A.A.H. Kadhum, A.B. Mohamad, M.S. Takriff, Experimental and theoretical study on the
inhibition performance of triazole compounds for mild steel corrosion, Corrosion Science, 52 (2010) 3331-3340.
[72] X. Li, S. Deng, H. Fu, Blue tetrazolium as a novel corrosion inhibitor for cold rolled steel in hydrochloric
acid solution, Corrosion Science, 52 (2010) 2786-2792.
17

Table 1. Chemical composition (wt %) of mild steel DC06EK.
C
Si
Mn
P
Al
Ti
Ni
Fe
Element
0.0004
0.007
0.190
0.0008
0.069-0.08
0.17
0.0089
Balance
wt %
Table 2. Calculated values of corrosion rate (νcorr) and inhibition efficiency (%IE) for mild steel corrosion in
1 M HCl in the absence and presence of (DDP) at 30 °C.
Inhibitor
Conc. (mol/l)
νcorr (mg/cm2.h)
%IE
---
Blank
10-5
5 × 10-5
10-4
0.50
0.074
0.036
0.030
0.017
85.20
92.80
94.00
96.60
DDP
5 × 10-4
Table 3. Calculated values of corrosion rate (νcorr) and inhibition efficiency (%IE) for mild steel corrosion in 1 M
HCl in the absence and presence of (DDP) at 30, 40, 50, 60 °C.
Concentration
Mol/l
νcorr (mg/cm2.h)
Inhibition efficiency (%IE)
30°C
40 °C
50 °C
60 °C
30°C
40 °C
50 °C
60 °C
---
---
---
---
Blank
0.70
1.50
3.50
0.5
10-5
5 × 10-5
10-4
0.100
0.068
0.051
0.026
0.350
0.180
0.120
0.081
1.010
0.653
0.380
0.130
85.20
92.80
94.00
96.60
85.71
90.28
92.71
96.30
76.66
88.00
92.00
94.60
71.14
81.34
89.14
96.30
0.074
0.036
0.030
0.017
DDP
5 × 10-4
Table 4. Activation parameters of the dissolution of mild steel in 1M HCl in the absence and presence of
different concentrations of (DDP).
Inhibitor
Concentration (mol/l)
A
R2
Ea (kJ/mol)
Blank
10-5
5 × 10-5
10-4
1.23 × 109
6.00 × 1011
2.22 × 1012
3.62 × 1010
3.75 × 108
0.9841
0.9755
0.9883
0.9854
0.9820
54.84
75.50
80.41
70.50
60.21
DDP
5 × 10-4

Table 5. Affect of immersion time on corrosion rate of mild steel in 1 M HCl in the absence and presence of
(DDP, 5 × 10-4 M) at T=30 °C.
Immersion
time (min)
νcorr (mg/cm2.h)
νcorr (mg/cm2.h)
DDP (5 × 10-4 M)
Inhibition
Blank
Efficiency (%IE)
30
60
0.56
0.5
0.03046
0.01740
0.02166
0.01200
0.00960
0.00840
0.00980
94.56
96.52
94.30
96.76
97.60
98.00
98.11
120
240
360
480
1440
0.38
0.37
0.40
0.42
0.52
Table 6. Thermodynamic parameters of the adsorption of (DDP) on mild steel surface in 1 M HCl 30, 40, 50
and 60 °C.
Inhibitor
T (°C)
30
R2
Slope
1.030
1.034
1.053
1.026
Kads
ꢀGads (kJ/mol)
-43.00
0.9999
0.9999
0.9999
0.9990
4.60 × 105
3.35 ×105
3.00 × 105
1.31 × 105
40
-43.55
DDP
50
-44.65
60
-43.74

Table 7. Potentiodynamic polarization parameters for the corrosion of MS in 1 M HCl without and with different
concentrations of (DDP) at 30 °C.
DDP
Ecorr
Icorr
Ba
-Bc
%IE
(mol / l)
(mV vs. SCE)
(uA /cm 2)
(mV/dec)
(mV/dec)
-583.3
-531.0
-524.4
-494.0
-468.1
290
39.4
30.0
26.5
13.1
73.1
71.6
52.7
68.2
71.0
95.6
149.5
121.3
219.3
137.5
…
Blank
10-5
86.41
89.65
90.86
95.50
5 x 10-5
10-4
5 x 10-4
Table 8. Impedance parameters and inhibition efficiency for the corrosion of mild steel in 1 M HCl with different
concentrations of (DDP) at 30 °C.
Conc.
Rct (Ω. cm2)
n
Cdl (µF/cm2)
%IE
42.11
347.60
571.70
776.30
893.40
0.89
0.84
0.78
0.86
0.85
755.80
81.48
69.60
64.74
126.8
….
Blank
10-5
87.88
92.63
94.60
95.30
5 × 10-5
10-4
5 × 10-4
Table 9. Calculated quantum chemical parameters of (DDP) pyrazole derivative.
Inh
µ (D)
Etot (Har)
EHOMO (eV)
ELUMO (eV)
ꢀE (eV)
V (cm3/mol)
9.72
-1526.83
-5.08
-1.94
3.14
340.57
DDP

Table 10. The Fukui indices (DDP) pyrazole estimated using POP=NPA analysis.
+
-
+
-
Atom k
fk
fk
Atom k
fk
fk
-0.01254
0.00644
-0.00491
0.11707
0.06665
0.01070
0.12452
0.02312
0.06110
-0.02739
-0.00734
0.00503
0.01589
0.00963
-0.00414
-0.00776
0.05164
0.02524
0.01542
-0.02174
0.16483
0.02042
0.03695
0.03681
-0.00361
0.02657
0.00956
0.03630
0.00261
0.00977
-0.00353
-0.00296
0.14144
-0.02612
0.03801
0.00970
0.07868
0.00371
0.06416
0.02993
-0.00836
-0.00839
-0.00057
0.08172
-0.01286
-0.01239
0.02520
0.02981
0.01517
0.02845
0.02368
0.02494
0.01205
0.07652
-0.01252
-0.01262
C 1
C 2
N 3
N 4
C 5
C 6
S 7
C 15
N 16
C 17
C 18
C 19
C 20
C 21
C 22
C 23
C 24
C 25
N 26
C 27
C 28
N 8
C 9
C 10
C 11
C 12
C 13
C 14
Table 11. Interaction and binding energy values for (DDP) using DFT//B3LYP/6-31G* calculations
Systems
Interaction energy (kcal/mol)
Binding energy (kcal/mol)
-21.11
-27.01
21.11
27.01
Fe-DDP/Vacuum
Fe-DDP/HCl solution

Fig. 1. Molecular structure of (DDP) inhibitor
Scheme 1. Synthesis of the intermediate dibenzalacetone (4-DBA)
7
O
NH2. HCl
N
7
S
NH
NaOH/EtOH
25°C, 30 min
5
Ha
N
N
H2N
2
1
6
Ha
3
N
7
N
Hb
N
S
4
NH2
7
Scheme 2. Synthesis of pyrazole carbothioamide derivative (DDP)

98
96
94
92
90
88
86
84
νcorr
0.5
0.4
0.3
0.2
0.1
0.0
%IE
ν
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
Conc. (mol/l)
Fig. 2. Relationship of corrosion rate (νcorr) and inhibition efficiency (%IE) with concentration of (DDP) in
1 M HCl at 30 °C.
0
2
4
6
8
10
10
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
8
6
4
30 °C
40 °C
50 °C
60 °C
2
0
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
Conc. (mol/l)
Fig. 3. Relationship between inhibition efficiency (%IE) and concentration of (DDP) in 1 M HCl at different
temperatures
0
2
4
6
8
10
10
1,5
1,0
0,5
8
0,0
-0,5
-1,0
-1,5
-2,0
-2,5
-3,0
-3,5
-4,0
-4,5
-5,0
6
r
r
o
c
v
n
l
4
Blank
10-5
M
5 x10-5 M
10-4 M
2
5 x10-4
M
0
0,00300 0,00305 0,00310 0,00315 0,00320 0,00325 0,00330
1/T (K-1)

Fig. 4. Arrhenius plot for mild steel corrosion in 1 M HCl in the absence and presence of different concentrations
of (DDP).
0
2
4
6
8
10
0,0006
0,0005
0,0004
0,0003
0,0002
0,0001
0,0000
10
30 °C
40 °C
8
6
Θ
/
C
4
2
0
0,0000
0,0001
0,0002
0,0003
0,0004
0,0005
Conc. (mol/l)
Fig. 5. Langmuir isotherm adsorption modes of (DDP) on the MS surface in 1 M HCl at 30 °C and 40 °C.
0
2
4
6
8
10
0,0006
0,0005
0,0004
0,0003
0,0002
0,0001
0,0000
10
50 °C
60 °C
8
6
Θ
/
C
4
2
0
0,0000
0,0001
0,0002
0,0003
0,0004
0,0005
Conc. (mol/l)
Fig. 6. Langmuir isotherm adsorption modes of (DDP) on the MS surface in 1 M HCl at 50 °C and 60 °C.

Fig. 7. Mechanism of corrosion inhibition of MS in the presence of (DDP) in 1 M HCl

2.5
2.0
-1
-2
-3
-4
-5
-6
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Blank
-
10 5
M
-
5 X 10 5
M
-
10 4
M
5 X 10 4 M
-
-700
-600
-500
-400
-300
-200
E (mV vs SCE)
Fig. 8. Polarization curves for MS in 1 M HCl with different concentrations of (DDP) at 30 °C.
0
2
4
6
8
10
10
0
2
4
6
8
10
10
30
20
10
0
Blank
Fit of Blank
Blank
8
10-5
M
]
²
800
600
400
200
0
6
m
-5
c
.
m
h
o
5 X 10
M
M
8
4
[
i
Z
-
10-4
M
2
5 X 10-4
0
60
0
10
20
30
40
50
Zr
[
ohm.cm²]
6
)
2
m
c
Ω
(
i
4
Z
-
2
0
0
200
400
600
800
2
Zr (Ω cm )
Fig. 9. Nyquist plots for MS in 1 M HCl solution in the absence and in the presence of different concentrations
of (DDP) at 30 °C

0
2
4
6
8
10
10
Blank
10-5
M
3
2
1
0
5 X 10-5
M
M
8
10-4
5 X 10-4
M
)
6
2
m
c
Ω
(
I
Z
I
4
g
o
l
2
0
0
1
2
3
4
logf (Hz)
Fig. 10. Bode Modulus plots for MS in 1 M HCl solution in the absence and in the presence of different
concentrations of (DDP) at 30 °C
Fig. 11. The equivalent circuit of the impedance spectra obtained for (DDP) pyrazole in 1 M HCl

A
B
C
Fig. 12. SEM analysis of MS surface: (A) before immersion; (B) after 24 h of immersion in 1 M HCl at 30 °C;
(C) after 24 h of immersion in 1 M HCl + 5 × 10-4 M (DDP) at 30 °C.