Investigation of the inhibition efficiencies of novel synthesized cobalt complexes of 1,3,4-thiadiazolethiosemicarbazone derivatives for the acidic corrosion of carbon steel

Article abstract of DOI:10.1016/j.molstruc.2019.127447

Two novel thiosemicarbazone linked with 1,3,4-thiadiazole ligands were synthesized through the condensation reaction of the acetyl group at position 5 of 1,3,4-thiadiazole derivatives with thiosemicarbazide in acidic ethanol solution in addition to their

Full text of DOI:10.1016/j.molstruc.2019.127447

Journal Pre-proof
Investigation of the inhibition efficiencies of novel synthesized cobalt complexes of
1,3,4-thiadiazolethiosemicarbazone derivatives for the acidic corrosion of carbon steel
Ahmed Fawzy, Thoraya A. Farghaly, Hoda A. El-Ghamry, Tahany M. Bawazeer
PII:
S0022-2860(19)31556-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127447
MOLSTR 127447
Reference:
To appear in: Journal of Molecular Structure
Received Date: 30 August 2019
Revised Date: 15 November 2019
Accepted Date: 18 November 2019
Please cite this article as: A. Fawzy, T.A. Farghaly, H.A. El-Ghamry, T.M. Bawazeer, Investigation of
the inhibition efficiencies of novel synthesized cobalt complexes of 1,3,4-thiadiazolethiosemicarbazone
derivatives for the acidic corrosion of carbon steel, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.127447.
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Graphical Abstract
Two novel 1,3,4-thiadiazolethiosemicarbazone ligands were synthesized in addition to
their Co(II) chelates which have been proved using various analysis tools. The
synthesized ligands and their complexes were also investigated as inhibitors for the
corrosion of carbon steel in 1.0 M HCl using different techniques. The obtained
results revealed that such compounds act as good inhibitors.
SEM
SEM
WL
PDP
EIS

Investigation of the inhibition efficiencies of novel synthesized cobalt
complexes of 1,3,4-thiadiazolethiosemicarbazone derivatives for the
acidic corrosion of carbon steel
Ahmed Fawzy1,2 , Thoraya A. Farghaly3*, Hoda A. El-Ghamry2,4, Tahany M. Bawazeer1
1 Chemistry Department, Faculty of Applied Science, Umm Al–Qura University, Makkah, Saudi Arabia
2 Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
3 Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
4 Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
_______________________________________________________________________
Abstract
Two novel thiosemicarbazone linked with 1,3,4-thiadiazole ligands were synthesized
through the condensation reaction of the acetyl group at position 5 of 1,3,4-thiadiazole
derivatives with thiosemicarbazide in acidic ethanol solution in addition to their Co(II)-
complexes. The divalent cobalt complexes of both ligands have been synthesized
through the reaction of cobalt acetate with each ligand leading to the formation of the
two complexes [(LH)Co(OAc)]0.5H2O and [(LCl)Co(OAc)(H2O)2]; where LH and LCl
are 1,3,4-thiadiazolethiosemicarbazone ligands. The two complexes have been proved to
have tetrahedral and octahedral geometry from the results of UV-Vis spectra and
magnetic moment. Analysis of the IR spectra of both ligands and complexes ascertained
that the ligands behaved as monobasic tridentate coordinating to the metal ion center
through azomethine-nitrogen atoms and thiadiazole ring in addition to deprotonated SH
group. The synthesized ligands and their complexes were also investigated as inhibitors
for the corrosion of carbon steel in 1.0 M HCl using weight loss, potentiodynamic
polarization, electrochemical impedance spectroscopy and scanning electron microscopy
techniques. The obtained results revealed that the inhibition efficiencies of the tested
derivatives were found to depend on the concentrations and structures of such
derivatives, and were decreased with raising temperature. At the same inhibitors
concentration, the inhibition efficiencies are slightly increased in the order: LH > LCl >
LH-Co > LCl-Co. The results derived from all applied techniques are harmonious with
each other
Keywords: Thiadiazoles, Cobalt complexes, Corrosion, Inhibition, Carbon steel
1

* Corresponding author: thoraya-f@hotmail.com
1. Introduction
Corrosion is considered as the most serious problem facing metals and their alloys,
which in turn causes various industrial problems, such as deterioration of buildings and
machinery, corrosion of boilers in factories and petroleum pipeline in oil refineries and
others, causing short life of these facilities and low operational efficiency of them [1].
The application of corrosion inhibitors is the best way to control corrosion of the metal
and alloy surfaces due to its facility of implementation and cost-effectiveness [1,2]. For
this reason, numerous organic compounds particularly including heteroatoms (N, O, S)
in the form of conjugated π-bond, polar groups, and aromatic rings, have been specified
as perfect corrosion inhibitors [3-6].
1,3,4-Thiadiazoles have been extensively investigated for their applications in
several fields as pharmaceutical, agrochemicals and industrial sectors [7-10]. For
instance, methazolamide, acetazolamide and sulfamethazole are marketdrugs having a
1,3,4-thiadiazole nucleus [11]. In addition, they have found to be good oxidation
inhibitors [12] and metal complexing agents [13] where 1,3,4-thiadiazoles and
thiosemicarbazones play important roles in the synthesis of valuable metal-complexes
[14-16] due to the presence of two nitrogen and sulfur donor atoms. A literature survey
[17, 18] and through our previous research work [19, 20] showed that
thiosemicarbazones have many biological applications and are also used as anti-
corrosion of mild steel and many minerals in the acidic medium. Furthermore, Bouraiou
and coworkers have recently proved that cobalt complexes exhibited corrosion inhibition
ability for steel in acidic medium [21].
Carbon steel is the most widely used steel alloy used in various industries
including petroleum, automotive, channels, plates, cold-rolled sheets and power
generation [22]. Due to its broad applications in industrial sectors, it predominantly
exists immersed with acidic medium, mainly HCl which is exceedingly utilized for acid
pressing for elimination of oxide level from it before undergoing next processing,
decaling, and acidization of oil wells [22]. Carbon steel is extremely oversensitive
towards corrosion in acidic media.
Motivation from all the above activities and extending our experience in the
preparation of effective complexes [23-29], we interested herein to synthesize two 1,3,4-
2

thiadiazolethiosemicarbazone ligands and their divalent cobalt complexes. Further, these
compounds were tested for their corrosion inhibition performance on the acidic
corrosion of carbon steel in 1.0 M HCl solution with implementation of various
techniques as weight loss (WL), potentiodynamic polarization (PDP), electrochemical
impedance spectroscopy (EIS) and scanning electron microscopy (SEM).
2. Experimental
2.1. Materials and Methods
All used reagents in this study were of very pure and obtained from BDH, Aldrich or
Sigma. The utilized solvents for recording all spectral investigations in this text were
bought with high purity from Aldrich. The elemental microanalyses [C, H, N] of the
synthesized derivatives were carried out at Cairo University, in the microanalytical
center on Perkin-Elmer 2400 CHN Elemental analyzer. The Infrared (IR) spectra were
performed on a 1430-Perkin-Elmer infrared-spectrophotometer ranging from 4000 to
200 cm-1 as sample-KBr discs. The NMR spectra were carried out in dimethyl sulfoxide
(DMSO-d6) on 1H-NMR (Varian operating at 400 MHz) and 13CNMR (Varian operating
at 100 MHz). A Shimadzu UV-Vis 240 spectrophotometer was used to record the
electronic absorption spectra. The solid complexes magnetic susceptibility was recorded
at ordinary temperature by using magnetic susceptibility instrument via the Gouy's
technique. TGA (thermogravimetric analysis) of the two prepared solid complexes were
recorded on the TG-50-Schimadzu thermogravimetric analyzer under 10°C /min heating
rate/nitrogen atmosphere with temperature ranging from 25 to 800 °C. The
electrochemical corrosion measurements, potentiodynamic polarization (PDP) and
electrochemical impedance spectroscopy (EIS), were conveyed out using a thermostated
PGSTAT30 potentiostat/galvanostat using a three-electrode electrochemical cell with
carbon steel as the working electrode (WE), saturated calomel electrode (SCE) as the
reference electrode (RE) and the platinum counter electrode (CE). The morphology of
carbon steel surface before and after addition of the synthesized compound was
investigated using scanning electron microscopy (SEM) using JEOL Scanning Electron
Microscope (SEM) model T-200. A carbon steel rod which was the working electrode
(its composition, wt. %: C = 0.11%, Si = 0.25%, S = 0.05%, P = 0.04 %, Mn = 0.45%
and the remainder is iron) and was contemporized before each experiment as reported
earlier [2,3], then it was placed immediately into 1.0 M HCl (corrosive medium, blank)
and/or the desired inhibitor concentration at open circuit potential (OCP) until a steady
3

state was reached. The 1.0 M HCl solution was prepared by diluting the analytical
reagent (37% HCl, AR grade) with double-distilled water.
2.2. Synthesis of 5-acetyl-3-aryl-2-imino-1,3,4-thiadiazole (3a and 3b)
Thiadiazole derivatives 3a and 3b have been prepared as previously described [30] as
follows: A solution of 0.01 mole = 1g of potassium thiocyanate 2 in 10 mL water has
been added, while stirring, to suspensions of hydrazonoyl chloride derivatives 1 (0.005
mol) in ethanol (50 mL). The mixture has been then stirred for 5 hours at room
temperature, during which dissolution took place and new solids have been formed. The
latter have been collected, washed was water, and methanol was used to recrystallized to
give products 3a and 3b.
2.2.1. 5-Acetyl-3-phenyl-2-imino-1,3,4-thiadiazole (3a)
Yellow solid, (0.19 g, 90 %), mp, 70-72 0C (EtOH), (Lit. mp. 74- 76 ºC) [30].
2.2.2. 5-Acetyl-3-(4-chlorophenyl)-2-imino-1,3,4-thiadiazole (3b)
Yellow solid, (0.2 g, 80 %), mp. 126-128 0C (EtOH), (Lit. mp 127 oC) [30].
2.3. Synthesis of compounds 5a and 5b (LH and LCl ligands)
A mixture of 0.01 mole of 5-acetyl-3-aryl-2-imino-1,3,4-thiadiazolederivatives (3a or
3b), 0.01mole of thiosemicarbazide and a few drops of conc. HCl in ethanol (30 mL)
have been refluxed for 5 h. The solid products, which formed after cooling, have been
collected and recrystallized from ethanol/dioxane to give the thiosemicarbazone
derivatives LH and LCl (Scheme 1).
1H NMR of LH (DMSO-d6) δ 2.30 (s) (CH3, 3H,), 7.62-8.14 (m) (Ar-H and NH, 6H),
8.15 (s) (NH, 1H), 8.40 (s) (NH, 1H), 10.40 (s) (NH, 1H).
1H NMR of LCl (CDCl3) δ 2.34 (s) (CH3, 3H), 6.40 (s) (NH, 1H) 7.10 (d) (Ar-H, J =
9Hz, 2H), 7.30 (d) (Ar-H, J = 9Hz, 2H,), 7.33 (s) (NH, 1H), 8.33 (s) (NH, 1H), 8.79 (s)
(NH, 1H); 13C NMR of LCl (DMSO-d6) δ 21 (CH3), 127, 130, 131, 134, 137, 140, 145,
172 (C=S).
4

Scheme 1. Synthetic pathway of ligands, LH (5a) and LCl (5b)
2.4. Synthesis of cobalt complexes (LH-Co and LCl-Co)
The two metal complexes LH-Co and LCl-Co have been synthesized by slow adding of
a 0.001 mol of cobalt acetate tetrahydrate (0.249 g of Co(CH3COO)2.4H2O) dissolved in
hot methanol to hot methanolic solution containing 0.292 g or 0.326 g of ligand LH or
LCl, respectively (0.001mol and 20 ml ). The formed mixtures have been subjected to
heat with reflux for 2 h at 65 °C during which colored products were separated out.
Using vacuum filtration to separate the colored precipitates from the hot solutions, then
methanol & ether were used to wash the precipitate and finally dried under vacuum.
3. Results and discussions
3.1. Stoichiometry and nature of the complexes
Molecular formulae of the synthesized ligands and their metal complexes along with
their empirical formulae, molecular weights, colors, melting points, molar conductance
values and analytical results are depicted in Table 1. Analysis of the synthesized organic
ligands (LH &LCl) and their divalent cobalt complexes (LH-Co and LCl-Co) by means
of elemental analysis tool showed that the structures of the synthesized compounds are
in good agreement with the predicted molecular formulae represented in Table 1. The
results supported the formation of the cobalt chelates in the molar ratio 1:1 (M:L).
5

Measurement of the molar conductance from 10-3M solution of the measured complexes
in DMF supported their non-electrolytic type (
=14.6 &–18.2 ꢀ-1cm2 mol-1 for LH-Co
Λ
and LCl-Co, respectively) [31,32]. The compounds were found to the highly stable in
air. They are easily soluble in most polar organic solvent but not soluble in non polar
solvents.
Table 1. Micro-analysis and physical characteristic of the synthesized compounds
Microanalysis,
Comp.
code
Molecular formula
Color
(Λm)[
m. p.
a]
Calc. ( Found) %
(Empirical formulae)
(Mol. Wt.)
(yield)
C%
45.19
H%
N%
M%[c]
C11H12N6S2
Orange
(292.38)
Deep reddish
brown
--
224[b]
4.14
28.74
--
LH
(89%) (45.23) (4.18) (28.62)
˃300 37.32 3.61 20.09
(64 %) (37.18) (3.89) (20.21) (14.32)
[(LH)Co(OAc)]0.5H2O
C13H15N6O2.5S2Co
14.6
14.09
LH-Co
(418.36)
Orange
326.83
Deep reddish
brown
C11H11N6S2Cl
--
180[b]
(92%) (40.51) (3.25) (25.86)
˃300 32.54 3.57 17.51
40.42
3.39
25.71
--
LCl
[(LCl)Co(OAc)(H2O)2]
C13H17N6O4S2ClCo
18.2
12.28
LCl-Co
(84 %) (32.47) (3.48) (17.49) (11.87)
(479.83)
a Molar conductivity of 10-3 M DMF solution ꢀ-1cm2 mol-. b decomposition temperature.
ccalculated from TG thermograms
3.2. EI-Mass spectroscopy
The mass spectra of the organic ligands LH & LCl and their complexes LH-Co & LCl-
Co were interpreted in order to ensure the composition and pureness of such compounds.
Figure 1 & 2 illustrate the mass spectra with some characteristic fragments of LH and its
cobalt complexes LCl-Co while the mass spectra of LCl and its cobalt complex LCl-Co
are interpreted in Figs S1 and S2. Mass spectra of LH, LCl, LH-Co and LCl-Co gives the
molecular ion peak at 293, 327, 419 and 480, respectively, corresponding to M++1 for
LH and LH-Co and the molecular weight of and LCl and LCl-Co (theoretical molecular
weights are 292.39, 326.83, 418.36 and 479.83 for LH, LCl, LH-Co and LCl-Co,
respectively) confirming the formation of the compounds as proposed in Table 1 and
Scheme 1.
6

H
N
H
S
HN
S
N
N
H
N
N
CH3
m/z = 292.38
N
N
LH
m/z = 105
H
N
H
S
N
N
H
m/z = 76
HN
CH3
S
N
m/z = 116
N
N
M++1
CH3
m/z = 217
Figure 1. The EI-mass spectra with fragmentation pattern for ligand LH
7

H3C
N
S
N
Co
O
NH2
S
N
0.5H2O
CH3
N
HN
C
Ph
O
m/z = 418.36
LH-Co
H3C
N
S
N
NH2
H3C
N
Co
N
N
S
H3C
m/z = 201
S
N
S
Co
N
N
N
NH2
S
Co
HN
N
Ph
m/z = 255
N
HN
Ph
m/z = 349.98
CoS
m/z = 91
M++1
Figure 2. The EI-mass spectra with fragmentation pattern for complex LH-Co
3.3.1H and 13C NMR spectra
Our target multi-dentate ligands LH and LCl were prepared via condensation reaction in
acidic medium from 5-acetyl-1,3,4-thiadiazole derivatives, 3, and thiosemicarbazide, 4,
1
as sketched in Scheme 1. The data extracted from H NMR of the two ligands (Fig. 1)
1
proved their structures. H NMR spectra of the two ligands LH and LCl showed the
presence of singlet signal for CH3 protons at 2.30 and 2.34 ppm, respectievely. Also,
8

they have four singlet signals at δ 7.62-8.14 (m, 6H, Ar-H and NH), 8.15, 8.40, 10.40
and one in the aromatic region (for LH) and at δ 6.40, 7.33, 8.33 and 8.79 ppm (for
LCl). The apperance of four different NH signals indicated the existance of H-bond
between one proton of NH2 and N=C groups which leds to the nonequevelant two
13
protons of the amino group of thiosemicarbazone moiety [20]. Also, C NMR of the
synthesized ligand LCl appeared all the nine characteristic signals for the non-
equivalent carbon atoms (Figure 3).
Figure 3. 1H NMR (a) and 13C NMR (b) spectra of ligand LCl
3.4. FT-IR spectra
Identification of the coordination sites in the organic ligand that participate in bond
formation with the central ion can easily achieved by accurate comparison of FT-IR
spectra of the ligand with that of metal chelates. Coordinated function groups peaks,
usually, undergo alteration either in their positions and/or their intensities. Some other
peaks disappear after complexation. The most important infrared bands showed by the
organic ligands and their cobalt chelates are represented in Table 2.
The spectra of the two ligands LH and LCl exhibited two bands at 3398 & 3264
cm-1 for LH and at 3411& 3222 cm-1 for LCl that assigned to asymmetric and
symmetric vibrations of NH2 groups while The bands appearing at 3152 and 3139 cm-1
was assigned to ν(NH) of the imino group [33]. These vibrations occurred, in the spectra
9

of complexes, almost at the same place indicating that these bands remained
uncoordinated in the metal complexes. Some of these bands underwent shift in their
position to higher or lower wavenumbers in spectra of metal complexes as a result of
their involvement in H-bonding [34].
The absorption band characteristic of the ν(SH) of thioimide group that
frequently appears in the range 2500–2700 cm-1 is missing in the spectrum of LCl ligand
which proves that this ligand is present in the thione form in the solid state and the
possibility of thioamide-thioimidoltautomerism (H–N–C=S, N=C–SH) is not possible
[35]. Such band appeared in the spectrum of L-H at 2630 cm-1 as a sharp signal showing
that this complex exists in the thiole form in the solid state.
The three bands appearing at in the spectrum of LH at 1605, 1288 & 889 cm-
1and appearing at in the spectrum of LCl at 1595, 1299 & 734 cm-1 assigned to ν(C=N),
ν(C=S) and δ(C=S),respectively. In the spectra of complexes LH-Co and LCl-Co, such
bands have shifted to higher or lower wavenumbers confirming the attachment of
azomethine nitrogen and thiolate sulfur to the metal center. The participation in the
coordination to the metal center [35]. The participation nitrogen atom in coordination to
Co center is further supported by the existence of non-ligand bands at 455 and 461 cm-1
in the spectra of LH-Co and LCl-Co, respectively, which assignable to ν(N-M). The
two bands appearing in the spectrum of LH-Co at 1564 and 1308 cm-1, and in the
spectrum of LCl-Co at 1545 and 1342 cm-1 assignable to νasy and νsym of the acetate
group, respectively. The difference between the two bands was calculated to be 256 and
203 cm−1 for LH-Co and LCl-Co, respectively, supporting the monodentate nature of
the acetato group [26].
Table 2. Assignments of diagnostic important bands in IR spectra for the
compounds under study.
10

Compound
LH
νNH2,νNH
νC-H sp3 νC=N νN-N δNH
νCS,
νM-N
3398, 3264, 3152
3430,3279, 3156
3411, 3222,3139
3429, 3231, 3183
2932
2940
2975
2930
1605 1074 1489 1288, 889
-
1564 1021 1498 1308, 845 455
1595 1197 1495 1299, 734
1601 1088 1486 1317, 725 461
LH-Co
LCl
-
LCl-Co
3.5.Thermogravimetric analysis
Thermal analysis is a helpful tool that is applied to assign the thermal behavior and
stability of metal chelates as well as understanding their coordination behavior through
introducing exemplary information about their thermal characteristics, intermediates, and
final products of their decomposition steps [23]. It is also very helpful to assign the
nature and amount of H2O and/or other solvent content and as well as the coordinated
anionic groups. Based on these facts, the synthesized metal chelates, LH-Co & LCl-Co,
were subjected to TG/DTG analysis. Detailed thermo-analytical results for the two
complexes LH-Co & LCl-Co are depicted in Table 3 and interpreted in Fig. 4.
Careful study of the complexes thermograms showed that complexes LH-Co & LCl-
°
Co decomposed within five successive steps within the temperature range 28-800 C.
The first step appeared at 28-93 and 28-156 °C for LH-Co & LCl-Co and assigned to the
loss of lattice and coordinated water molecules, respectively. Within the second stage
°
that appeared at 93-229 and 156-220 C, partial decomposition of the organic ligands
occurred with the loss of CH3 +NH for LH-Co and CH3 +Cl- for LCl-Co. The
coordinated acetate anion got lost within the third step which appeared at 229-323 and
°
220-325 C for LH-Co & LCl-Co, respectively. The last two stages in both complexes
assigned to the degradation of the organic ligands that appeared within the ranges 323-
°
°
500 and 500-800 C for LH-Co and 25-493 and 493-594 C for LCl-Co leading to the
formation of CoS as final decomposition product from which the percent of the metal
content has been calculated.
11

Figure 4.TG/DTG curves of the two complexes LH-Co & LCl-Co
Table 3. The decomposition stages, temperature ranges, theoretical, found weight loss
and DTG temperature of cobalt complexes.
Complex no.
(Mol. Wt.)
Temp.
range
(°C)
DTG
temp
Mass loss %
Calc. Exp.
Assignment
28-93
93-229 205
48
2.15
7.17
1. 70 -Loss of half lattice H2O molecule.
7.50 - Loss of oneCH3+ NH groups.
LH-Co
[(LH)Co(OAc)]0.5H2O 229-323 282 14.11 13.65 - Loss of one coordinated CH3COO-
(418.36)
group.
323-500 459 36.02 33.41 - Loss of C3HN3S moiety.
500-800 553
9.30
9.90 -Further decomposition of ligand leaving
CoS+4C.
14.32 14.09 -% of metal content.
7.5 7.82 -Loss of two coordinated H2O molecules.
LCl-Co
28-156
77
[(LCl)Co(OAc)(H2O)2] 156-220 199 10.52
9.92 - Loss of one Cl ion + CH3 group.
(479.83)
220-325 265 17.93 18.08 - Loss of one coordinated CH3COO-
group+ C=N group.
325-493 440 28.17 28.38 - Loss of C7H5NS moiety.
493-594 560 11.46 11.42 -Further decomposition of ligand leaving
CoS.
12.28 11.87 -% of metal content.
3.6.UV-Vis spectra and magnetic moment measurements
Electronic spectral analysis is considered the main tool that is applied to assign the type
of geometrical structure exhibited by the metal center in metal complexes. It is also very
useful to assign the type of electronic transitions exhibited by different functions groups
in both organic and inorganic compounds.
12

The UV-Vis spectra of the two ligands LH & LCl and their complexes were
measured in the solid state using nujol mull technique. The free ligands exhibited two
bands at 248 & 294 nm for LH and at 241 & 284 cm-1 for LCl that assigned to assigned
to π→π* transitions of aromatic benzene ring and azomethine groups, respectively. The
band that appeared at 357 and 335 nm in the spectra of LH and LCl, respectively,
assigned to n→π* transition of azomethine groups. In the spectra of cobalt complexes,
LH-Co and LCL-Co, such bands have shifted to longer wavelengths (appeared at 251,
306 and 360 nm in the spectrum of LH-Co and at 252, 300 and 346 nm in the spectrum
of LCl-Co) as a consequence of coordination of the azometnie nitrogen to the metal
center [36, 37].
LH-Co complex exhibited also one broad band centered at 668 nm due to
4
4A2(F)→ T1(F) transition characteristic for tetrahedral geometry [38]. Another broad
band centered at about 536 nm due to spin-orbital coupling. The tertrahedral geometry of
LH-Co has been further supported by calculating the ꢀeff value which found to be 4.08
B.M evidencing tetrahedral geometry [38].
The electronic spectrum of LCl-Co complex exhibited two bands at 559 and 683
4
4
cm-1 assignable to T1g(F)→4T1g(p) and T1g(F)→4A2g(F) transitions, respectively,
supporting octahedral configuration [39,40]. Such complex exhibited ꢀeff of 4.65 B.M
which is characteristic of high-spin octahedral complexes.
Based on the previous results the structures of the two complexes LH-Co and LCl-Co
can be illustrated as following (Scheme 2).
Scheme 2. Structures of the two complexes LH-Co and LCl-Co
3.7. Investigation of the corrosion inhibition of the prepared ligands and
complexes
3.7.1. Weight-loss measurements
13

The experiments of weight-loss (WL) were carried out on cylindrical carbon steel rods
with areas closed to 12 cm2 and were prepared for these measurements as reported earlier
[2,3]. The inhibition efficiencies (% IE) of the investigated inhibitors were calculated
from the values of corrosion rate (CR) of carbon steel (determined in mpy) using the
following equation [41]:
CR


inh 
1−
% IE = θ x 100 =
x 100
(1)


CR

Where, the values of corrosion rate are expressed as CR and CRinh in the obscurity and
existence of inhibitor, respectively.
WL measurements of carbon steel in 1.0 M HCl were performed in the absence and
existence of different concentrations of the synthesized ligands (LH, LCl) and their
complexes (LH-Co, LCl-Co) in the concentration ranging from 100 to 900 ppm at 25 °C.
For briefly, curves of weight-loss (WL) versus time obtained for the ligand LH and its
cobalt complex (LH-Co) are illustrated in Fig. 5. % IE and values of CR of carbon steel
of all investigated compounds are also estimated and tabulated in Table 4. The collected
data from Table 4 revealed that CR values reduced with increasing inhibitor
concentration. Therefore, the new synthesized derivatives can be considered as effective
corrosion inhibitors for carbon steel in HCl (1.0 M). Careful examination of the results
(tabulated in Table 4 and clarified in Fig. 6 (a & b) showed that at the same inhibitors
concentration, the inhibition capacities are little enhanced in the order: LH > LCl > LH-
Co > LCl-Co indicating that the possible steric effects and electronic density of donor
atoms of the test compound are considered as the main reason in the adsorption
operation.
14

Figure 5. Weight-loss (WL) versus immersion time of carbon steel corrosion in 1.0 M
HCl solution in the absence and presence of different concentrations of ligand LH (a)
and its complex LH-Co (b) at 25 °C.
Figure 6. Variation of inhibition efficiencies with the concentrations of the synthesized
ligands and their complexes: (a) LH & LH-Co, (b) LCl & LCl-Co, for the corrosion of
carbon steel corrosion in 1.0 M HCl solution at 25 °C.
3.7.2. Potentiodynamic polarization (PP) measurements
In both EIS and PP techniques, we calculated the percentage of IE of the test inhibitors as
published earlier [2,3]. The PP measurements for carbon steel in solution of HCl (1.0 M)
in the presence and absence of various concentrations of the thiadiazole-compounds are
carried out (only PP curves obtained for the ligand LCl and its cobalt complex (LCl-Co)
as representative examples are only presented here in Fig. 7) and all parameters related to
corrosion are evaluated and are tabulated also in Table 4. The extracted results showed
that the addition of fixed concentrations of thiadiazole compounds to the blank solution
turned both the cathodic and anodic branches of curves of the polarization towards lower
current density values, indicating the delay of the anodic and cathodic reactions, thereby
inhibiting iron corrosion. Since, the inhibitor exhibited anodic and cathodic inhibition
effects with slightly shift in Ecorr value to more anodic potentials, it could be deduced that
such new thiadiazole derivatives perform as mixed-type inhibitors [41-43]. Enhancement
of % IE values were found with increasing concentrations of inhibitors and the extent of
inhibition efficiencies of the compounds, at the same concentration, followed the order:
15

LH > LCl > LH-Co > LCl-Co, in consistent with the results obtained from WL
measurements.
Table 4. Corrosion parameters of carbon steel corrosion in 1.0 M HCl solution and the
values of % IE of the synthesized compounds obtained from the three employed
techniques (WL, PP and EIS) at 25 oC.
WL
% IE
PP
-βc
(mV/dec.) (mV/dec.)
EIS
% IE
Concn.
(ppm)
Inh.
-Ecorr
(mV(SCE)
βa
icorr
% IE
--
CR
241
Rct
(µA/cm2)
0
--
448
96
74
610
55
--
81
57
38
24
17
87
60
36
27
21
92
65
41
28
24
89
68
38
31
27
66
76
84
90
93
64
75
85
89
91
62
73
83
88
90
63
72
84
87
89
441
435
431
427
421
445
441
435
418
406
442
441
434
421
417
446
437
431
443
424
127
128
133
127
135
136
138
121
142
124
124
130
128
137
141
139
137
143
147
131
85
88
94
87
95
93
99
102
106
110
83
79
93
92
104
89
98
97
220
140
92
67
49
214
159
92
61
43
64
77
85
89
92
65
74
85
90
93
63
72
82
87
90
64
73
81
85
87
152
240
345
458
688
145
220
344
459
610
157
195
344
458
610
146
212
324
502
557
64
77
84
88
92
62
75
84
88
91
65
72
84
88
91
62
74
83
89
90
100
200
300
400
500
100
200
300
400
500
100
200
300
400
500
100
200
300
400
500
LH
LCl
226
171
110
79
LH-Co
61
219
165
116
92
LCl-Co
86
88
79
16

Figure 7. Potentiodynamic polarization curves of carbon steel corrosion in 1.0 M HCl
solution in the absence and presence of different concentrations of: (a) ligand LCl, and
(b) its complex LCl-Co at 25 °C.
3.7.3. Electrochemical impedance spectroscopy (EIS) measurements
The adsorption phenomena and the mechanisms in corrosion researches are explained by
using the commonly EIS technique. Nyquist plots of carbon steel corrosion in 1.0 M HCl
solution in the absence and presence of several concentrations of thiadiazole-compounds
are plotted but only Nyquist plots for the ligands LH and LCl are illustrated in Figure 6.
The values of Rct and % IE are also listed in Table 4. Figure 8 illustrated that, for all
examined compounds, the impedance spectra in absence and existence of inhibitors
exhibit single depressed capacitive loops, which reveals that the adsorption of the
examined inhibitors occurs by simple surface coverage and such compounds behave as
primary interface inhibitors, and the corrosion of carbon steel was mainly controlled by
the process of charge transfer and double layer behavior. Furthermore, it can be noticed
that the general shape of the capacitive loops is similar in the absence or presence of the
inhibitors with different concentrations suggesting that there is no change in the
mechanism of the corrosion process [44]. The obtained impedance spectra were analyzed
by fitting the experimental data to a simple equivalent circuit model, illustrated in Fig. 9,
which includes a double layer capacitance (Cdl) and a charge transfer resistance (Rct), and
the two elements are in series with a solution resistance (Rs). The diameter of the
capacitive semicircle of carbon steel in the corrosive medium (solution of 1.0 M HCl)
was lower than that of the inhibited solutions. Hence, the carbon steel electrode/HCl
system show higher impedance in the presence of inhibitors. This behaviour
demonstrates that the corrosion resistance of carbon steel in the presence of the examined
inhibitors occurs as a result of a decrease in the rate of carbon steel corrosion, and this
attitude enhanced with increasing inhibitor concentration. Also, increasing value of Rct
with increasing the inhibitor concentrations indicates that the examined compounds act
as inhibitors via adsorption at the carbon steel electrode/HCl solution interface leading to
a decrease in their electrical capacities as they displace water molecules and other ions
originally adsorbed on the metal surface [45]. Also, the increase of the value of Rct with
inhibitor concentrations implies that the number of the inhibitor molecules which
adsorbed on the surface of carbon steel increase resulting in the construction of
protective films on the electrode surface, and hence, become barriers to hinder the mass
17

and charge transfer, leading to increase the inhibition efficiencies. These results confirm
that the investigated compounds act as efficient inhibitors for the corrosion of carbon
steel in 1.0 M HCl solution. From the acquired outcomes of the three employed
techniques, it can be observed that the results of the inhibition efficiencies of the
inhibitors obtained from the three techniques are in good agreement with each others.
Figure 8. Nyquist plots for the corrosion of carbon steel corrosion in 1.0 M HCl solution
in the absence and presence of different concentrations of the ligands: (a) LH, and (b)
LCl at 25 °C.
Rct
Figure 9. The equivalent circuit model used to fit the experimental results.
3.6.4. Surface examinations
Scanning electron microscopy (SEM) micrographs of carbon steel samples in a free 1.0
M HCl (corrosive solution) and with 500 ppm of the examined compounds are
illustrated in Figure 10(a–f). Figure 10a and b manifest the pure carbon steel surface
before and after immersion in the corrosive solution for 12 h, respectively. Figure 10b
shows the very elevated roughness of carbon steel surface which became strongly
18

damaged along with the appearance of several cracks and pits which ascribed to the
aggressiveness of the examined acid medium. Figure 9 (c–f) shows SEM micrographs of
carbon steel surface after treatment with 500 ppm of the synthesized compounds: LH,
LCl, LH-Co and LCl-Co, respectively, supplemented to the corrosive solution. The
micrographs indicate that, the surfaces of carbon steel samples suffered remarked
changes from the previous case where both the elevated roughness and the strong
damage shown in the steel surface was disappeared and the surface was largely covered
with the inhibitor molecules on the whole surface. This can be attributed to the strong
adsorption of the inhibitors on carbon steel surface, causing a decrease in the contact
between carbon steel surface and the corrosive solution resulting in a good corrosion
inhibition and protection of the steel surface [44].
(a)
(b)
(d)
(f)
(c)
(e)
19

Figure 10. SEM micrographs of carbon steel surface: (a) before immersion in the
corrosive solution (1.0 M HCl), (b) after 12 h immersion in 1.0 M HCl, as well as (c),
(d), (e) and (f) after 12 h immersion in the corrosive media containing 500 ppm of the
synthesized compounds: LH, LCl, LH-Co and LCl-Co, respectively, at 25°C.
1.1.1. Adsorption considerations
An inhibition property was shown for thiadiazole ligands and their cobalt complexes via
their adsorption on the metal surface as well as some alternative adsorption isotherms
such as Langmuir, Frumkin, Freundlich, Temkin, etc. have been widely utilized to
investigate the mode of adsorption of the tested inhibitor molecules and then their
corrosion inhibition mechanism. In the present study, Langmuir adsorption isotherm,
which is a relation between the values of fractional surface coverage (Cinh/θ) for the
investigated inhibitors against inhibitor concentration (Cinh), was fitted and is shown in
Fig. 11. The values of correlation coefficient for the investigated compounds were near
to 1, which reveals that the inhibitor adsorption on the surface of carbon steel is well
fitted to the Langmuir adsorption isotherm [46,47], which is represented by the bellow
equation [48]:
Cinh
1
=
+ Cinh
θ
Kads
(2)
Where Kads is the equilibrium constant of adsorption.
4
3
2
1
0
4
3
2
1
0
(a)
(b)
LH
LCl
LCl-Co
LH-Co
0
1
2
3
4
0
1
2
3
4
103 Cinh (mol/l)
103 Cinh (mol/l)
20

Figure 11. Langmuir adsorption isotherms for the synthesized ligands and their
complexes: (a) LH, LH-Co and (b) LCl, LCl-Co adsorbed on the carbon steel surface
in 1.0 M HCl solution at 25 °C.
On comparison between the efficiency of the examined inhibitors (synthesized
ligands and their cobalt complexes) in the present investigation and the previous reports
of employment of metal ion complexes as corrosion inhibitors, e.g. Adam et. al. [43],
who studied the inhibition performance of three anionic oxide-vanadium Schiff base N-
salicylidene amino acid complexes on the corrosion of carbon steel in HCl solution. They
found that the synthesized complexes protect carbon steel form corrosion, yielding
maximum inhibition efficiency up to 94.7% in the presence of 1.0 mM inhibitor and all
complexes act as mixed-type inhibitors. Also, Abdel-Gaber, et. al. [47] studied the
corrosion inhibition of Schiff base (N,N’-bis-(salicylaldehyde)-1, 3-diaminopropane) and
its cobalt complex on the corrosion of steel in acidic medium. Their outcomes showed
that PDP measurement showed 73.41% inhibition efficiency in the presence of 35 mol/L.
However, EIS measurement displayed 84.3% and 83.2% inhibition efficiencies at the
same concentration and temperature. Abd El-Lateef, et. al. [44] synthesized four of metal
ion complexes of salicylidene anthranilate sodium salt ligand and studied their inhibition
action on the corrosion of carbon steel in 1.0 M HCl by various techniques. The results
obtained from PDP and EIS techniques displayed that, the examined ligand and their M-
complexes act as effective and mixed-type inhibitors and the maximum inhibition
efficiency was about 95%. Also, the adsorption of such compounds on carbon steel
surface could be exemplified by Langmuir isotherm.
4 . CONCLUSION
Two derivatives of 1,3,4-thiadiazolethiosemicarbazone, which synthesized by the
condensation reaction of 2,3-disubstituted-5-acetyl-1,3,4-thiadiazolederivatives with
thiosemicarbazide in acidic medium, have been used as chelating agent for the synthesis
of divalent cobalt complexes. The structures and purity of both ligands and their
complexes have been identified applying different analytical and spectral tools.
Analytical data indicated that the complexes have been formed in the molar ratio 1:1
(L:M) having the formulae [(LH)Co(OAc)]0.5H2O and [(LCl)Co(OAc)(H2O)2]; where
LH and LCl are 1,3,4-thiadiazolethiosemicarbazone ligands. When comparing the
21

results of infrared IR-spectra of ligands with those for metal complexes, we found that
ligands acted as monobasic tridentate connecting to the metal ion center through
nitrogen atoms of azomethine group & thiadiazole ring and deprotonated SH group. UV-
Vis spectra and magnetic moment data showed that the complexes have have tetrahedral
and octahedral geometry. The synthesized ligands and their complexes were also
investigated as inhibitors for the corrosion of carbon steel in HCl (1.0 M) at 25 °C using
various techniques as weight loss, potentiodynamic polarization, electrochemical
impedance spectroscopy and scanning electron microscopy. The results extracted from
these experimental results revealed that the inhibition efficiencies of the tested
derivatives was found to depend on their concentrations and structures. The inhibition
efficiencies of the synthesized compounds increased also with raising temperature. At
the same inhibitors concentration, the inhibition efficiencies are slightly increased in the
order: LH > LCl > LH-Co > LCl-Co. The results derived from all applied techniques
are harmonious with each other.
Conflict of Interest
The authors declare no conflict of interest
References
[1] R. F. Godec, M. G. Pavlovic, Corros. Sci., 2012, 58, 192-201.
[2] A. Fawzy, M. Abdallah, I. A. Zaafarany, S. A. Ahmed, I. I. Althagafi, J. Mol. Liq.
2018, 265, 276-291.
[3] A. Fawzy, I. A. Zaafarany, H. M. Ali, M. Abdallah, Int. J. Electrochem. Sci. 2018,
13, 4575-4600.
[4] M. Abdallah, O. A. Hazazi, A. Fawzy, S. El-Shafei, A. S. Fouda. Prot. Met. Phys.
Chem. Surf. 2014, 50, 659-666.
[5] O. A. Hazazi, A. Fawzy, M. R. Shaaban, M. I. Awad, Int. J. Electrochem. Sci. 2014,
9, 1378-1389.
[6] O. A. Hazazi, A. Fawzy, M. I. Awad, Chem. Sci. Rev. Lett. 2015, 4, 67-79.
[7] Y. Hu, C.-Y. Li, X.-M. Wang, Y.-H. Yang, H.-L. Zhu. Chem. Rev. 2014, 114, 5572-
5610.
[8] W. Liu, J. Gan, S. R. Yates, J. Agric. Food Chem. 2002, 50, 4003-4008.
22

[9] N.D. Jablonowski, A. Linden, S. Köppchen, B. Thiele, D. Hofmann, W. Mittelstaedt,
T. Pütz, P. Burauel, Environ. Pollut. 2012, 168, 29-36.
[10] M. Palomar-Pardavé, M. Romero-Romo, H. Herrera-Hernández, M. A. Abreu-
Quijano, N. V. Likhanova, J. Uruchurtu, J. M. JuárezGarcía, Corros. Sci. 2012, 54,
231-243.
[11] N. Siddiqui, P. Ahuja, W. Ahsan, S. N. Pandeya, M. S. Alam, J. Chem. Pharm. Res.
2009, 1(1), 19-30.
[12] Huo, S. J.; Zhu, Q.; Chu, C. S.; Fang, J. H. J. Phys. Chem. C 2012, 116, 20269-
20280.
[13] T. T. Al-Nahary, J. Saudi Chem. Soc., 2009, 13, 253-257.
[14] Z. Afrasiabi, E. Sinn, J. Chen, Y. Ma, A. L. Rheingold, L. N. Zakharov, N. Rath, S.
Padhye, Inorg. Chim. Acta, 2004, 357, 271-278.
[15] G. Pelosi. The Open Crystallography J., 2010, 3, 16-28.
[16] L. M. T. Frija, A. J. L. Pombeiro, M. N. Kopylovich. Coord. Chem. Rev., 2016, 308,
32-55.
[17] V. K. Sharma, S. Srivastava, J. Coord. Chem., 2008, 61, 178-191.
[18] W. Chen, H. Q. Luo, B. N. Li, Corros. Sci., 2011, 53 (10), 3356-2365.
[19] I. I. Althagafi, A. S. Abouzied, T. A. Farghaly, N. T. Al-Qurashi, M. Y. Alfaifi, M.
R. Shaaban, M. R. Abdel Aziz. J. Heterocyclic Chem., 2019, 56, 391-399.
[20] M. M. Alsharekh, I. I. Althagafi, M. R. Shaaban, T. A. Farghaly. Res. Chem. Inter.
2019, 45, 127-154.
[21] M. Bouchouit, M. E. Said, M. K. Ali, S. Bouacida, H. Merazig, N. K. Chaouche, A.
Chibani, B. Zouchoune, A. Belfaitah, A. Bouraiou. Polyhedron 2016, 119, 248-259.
[22] I. Ahamad, R. Prasad, M. A. Quraishi, Corros. Sci. 2010, 52, 3033-3041.
[23] A. M. Gouda, H. A. El-Ghamry, T. M. Bawazeer, T. A. Farghaly, A. N. Abdalla, A.
Aslam. Eur. J. Med. Chem. 2018, 145, 350-359.
[24] K. Y. El-Baradie, N. A. El-Wakiel, H. A. El-Ghamry, Appl. Organometal. Chem.
2015, 29, 117-125.
[25] N. A. El-Wakiel, Appl. Organometal. Chem. 2016, 30, 664-673.
[26] M. Gaber, S. K. Fathalla H. A. El-Ghamry, Appl. Organometal. Chem. 2019, 33,
e4707.
[27] K. M. Takroni, H. A. El-Ghamry, A. Fawzy, J. Inorg. Organomet. Polym. 2019,
doi.org/10.1007/s10904-019-01153-9
23

[28] N. El-Metwaly, T. A. Farghaly, I. Althagafi, M. G. Elghalban. J. Mol. Str. 2019,
1190, 86-101.
[29] R. El-Sharkawy, H.A. El-Ghamry, J. Mol. Liq. 2019, 282, 515-526.
[30] N. F. Eweiss; A. O. Osman; J. Heterocycl. Chem. 1980, 17, 1713-1717.
[31] W. J. Geary, Coord. Chem. Rev. 1971, 7, 81-122.
[32] H. A. El-Ghamry, S. K. Fathalla, M. Gaber, Appl. Organometal. Chem. 2018, 32,
e4136.
[33] H. A. El-Ghamry, M. Gaber , Farghaly T. A., Mini-rev. Med. Chem. 2019, doi:
10.2174/1389557519666190301143322.
[34] H. A. El-Ghamry, K. Sakai, S. Masaoka, K. El-Baradie, R. Issa, J. Coord. Chem.
2012, 65(2), 780-794.
[35] P. F. Rapheal, E. Manoj, M. R. PrathapachandraKurup. Polyhedron 2007, 26, 818-
828.
[36] M. Shakir, A. Abbasi, M. Azam, A.U. Khan, Spectrochim. Acta A 2011, 79, 1866-
1875.
[37] N. H. Yarkandi, H. A. El-Ghamry, M. Gaber, Mat. Sci. Eng. C 2017, 75, 1059-1067.
[38] X. Rana, L. Wang, Y. Lin, J. Hao, D. Cao, Appl. Organometal. Chem. 2010, 24,
741-748.
[39] A. B. P. Lever, Inorganic Electronic Spectroscopy, second ed. Elsevier, Amsterdam,
1984.
[40] M. Pfaller, L. Burmeister, M.A. Bartlett, M.G. Rinaldi, J. Clin. Microbiol. 1988, 26,
1437-1441.
[41] G. N. Mu, X. H. Li, Q. Qu, J. Zhou, Corros. Sci. 2006, 48, 445-459.
[42] M.M. Saleh, M.G. Mahmoud, H.M. Abd El-Lateef, Corros. Sci. 2019, 154, 70-79.
[43] M.S.S. Adam, H.M. Abd El-Lateef, K.A. Soliman, J. Mol. Liq. 2018, 250, 307-322.
[44] H.M. Abd El-Lateef, M.S.S. Adam, M.M. Khalaf, J. Taiwan Inst. Chem. Eng.
2018, 88, 286-304.
[45] C. Hsu, F. Mansfeld, Corrosion 2001, 57, 747-748.
[46] A.R. Sayed, M.M. Saleh, M.A. Al-Omair, H.M. Abd Al-Lateef, J. Mol. Struc.
2019, 1184, 452-461.
[47] Hany M. Abd El-Lateef, Kamal A. Soliman, Ahmed H. Tantawy, J. Mol. Liq. 2017,
232, 478-498.
[46] M. Christov, A. Popova, Corros. Sci. 2004, 46, 1613-1620.
24

[47] A. M. Abdel-Gaber, M. S. Masoud, E. A. Khalil, Corros. Sci. 2009, 51, 3021-3024.
25

Highlights
•
•
•
•
The
divalent
cobalt
complexes
of
the
two
novel
1,3,4-
thiadiazolethiosemicarbazone have been synthesized.
The structure of the two complexes have been proved to have tetrahedral and
octahedral geometry from their spectral data.
The synthesized ligands and their complexes were investigated as inhibitors for
the corrosion of carbon steel in HCl.
The obtained results revealed that the inhibition efficiencies of the tested
derivatives were found to depend on their concentrations and structures and were
increased with raising temperature.

All authors participated in the experimental work, analysis the data, writing and
revised the article

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: