Acridine-based thiosemicarbazones as novel inhibitors of mild steel corrosion in 1 M HCl: Synthesis, electrochemical, DFT and Monte Carlo simulation studies

Article abstract of DOI:10.1039/c9ra04778f

Electrochemical, surface morphology, density functional theory and Monte Carlo simulation methods were employed in investigating the effects of (2E,2′E)-2,2′-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-diylidene)bis(N-phenylhydrazinecarbothioamide) (IAB-NP), (2E,2′E)-2,2′-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-diylidene)bis(N-(2,4-difluorophenyl)hydrazinecarbothioamide)IAB-ND) and (2E,2′E)-2,2′-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-diylidene)bis(N-(2-fluorophenyl) hydrazinecarbothioamide) (IAB-NF) on mild steel corrosion in 1 M HCl solution. From the studies, compounds IAB-NP, IAB-ND and IAB-NF inhibit mild steel corrosion in the acid and the protection efficiencies were found to increase with the increase in concentration of each compound. At the optimum inhibitor concentration of 1.5 × 10^-4 M, the inhibition efficiencies (%) of the compounds are in the order IAB-NF (90.48) > IAB-ND (87.48) > IAB-NP (85.28). Potentiodynamic polarization measurements revealed that all the compounds acted as mixed-type corrosion inhibitors. Experimental data for the adsorption of the studied molecules on a mild steel surface in 1 M HCl fitted into the Langmuir adsorption isotherm and the standard free energies of adsorption (ΔGoads) suggested both physisorption and chemisorption mechanisms. Scanning electron microscopy analyses confirmed the formation of a protective film on the mild steel surface by the inhibitor molecules, resulting in protection of the metal from corrosive electrolyte ions. The experimental findings were corroborated by both theoretical density functional theory and Monte Carlo simulation studies.

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Acridine-based thiosemicarbazones as novel
inhibitors of mild steel corrosion in 1 M HCl:
synthesis, electrochemical, DFT and Monte Carlo
simulation studies
Cite this: RSC Adv., 2019, 9, 29590
ab
abd
Ekemini D. Akpan,
Ibanga O. Isaac,c Lukman O. Olasunkanmi,
ab
ef
*
Eno E. Ebenso
and El-Sayed M. Sherif
Electrochemical, surface morphology, density functional theory and Monte Carlo simulation methods were
employed in investigating the effects of (2E,20E)-2,20-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-
1,8(2H,5H,9H,10H)-diylidene)bis(N-phenylhydrazinecarbothioamide) (IAB-NP), (2E,20E)-2,20-(3,3,6,6-tetramethyl-
9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-diylidene)bis(N-(2,4-difluorophenyl)hydrazinecarbothioa
mide)IAB-ND) and (2E,20E)-2,20-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-
diylidene)bis(N-(2-fluorophenyl) hydrazinecarbothioamide) (IAB-NF) on mild steel corrosion in 1 M HCl solution.
From the studies, compounds IAB-NP, IAB-ND and IAB-NF inhibit mild steel corrosion in the acid and the
protection efficiencies were found to increase with the increase in concentration of each compound. At the
optimum inhibitor concentration of 1.5 ꢀ 10ꢁ4 M, the inhibition efficiencies (%) of the compounds are in the
order IAB-NF (90.48) > IAB-ND (87.48) > IAB-NP (85.28). Potentiodynamic polarization measurements revealed
that all the compounds acted as mixed-type corrosion inhibitors. Experimental data for the adsorption of the
studied molecules on a mild steel surface in 1 M HCl fitted into the Langmuir adsorption isotherm and the
standard free energies of adsorption (DGoads) suggested both physisorption and chemisorption mechanisms.
Scanning electron microscopy analyses confirmed the formation of a protective film on the mild steel surface
by the inhibitor molecules, resulting in protection of the metal from corrosive electrolyte ions. The
experimental findings were corroborated by both theoretical density functional theory and Monte Carlo
simulation studies.
Received 25th June 2019
Accepted 13th September 2019
DOI: 10.1039/c9ra04778f
rsc.li/rsc-advances
metal.2,3 Susceptibility of metals to corrosion in nearly all
industrial environments has limited the advancement of new
technologies.4 Increasing the durability of construction mate-
rials and essential infrastructures such as rail tracks, bridges
and metallic structures including cars, storage tanks, corru-
gated iron, etc. by decreasing the rate of corrosion becomes
necessary. Because of wide applicability, high efficiency and
other numerous advantages, the use of inhibitors stands out as
the most efficient method in mitigating corrosion.5,6
Introduction
There is a continuous increase in the use of mild steel in the
construction and chemical industries and also for domestic
appliances because mild steel is relatively cheap and possesses
advantageous mechanical properties.1 Exposure of mild steel to
aqueous environments, especially concentrated acidic envi-
ronments results in the electro-dissolution and corrosion of the
Currently, there is a growing and continuous need to develop
active, appropriate and environmentally friendly inhibitors to
mitigate the electro-dissolution and corrosion of metals.7
Mostly studied and widely utilized corrosion inhibitors are
compounds containing heteroatoms, such as nitrogen, sulphur,
oxygen and p-electron systems. Moreover, previous studies have
revealed that these organic compounds inhibit corrosion by
adsorption on the surface of the metal.4,7–9 Organic molecules
used as corrosion inhibitors protect the metals from the
corrosive species in the immediate environment by forming
resistive layer on the metal surface, which ensures high resis-
tance to electron transfer reactions.10,11 Molecular orbitals and
aDepartment of Chemistry, Faculty of Natural and Agricultural Sciences, North-West
University, Private Bag X2046, Mmabatho 2735, South Africa. E-mail: Eno.Ebenso@
nwu.ac.za
bMaterial Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of
Natural and Agricultural Sciences, North-West University, Private Bag X2046,
Mmabatho 2735, South Africa
cDepartment of Chemistry, Faculty of Natural and Applied Sciences, Akwa Ibom State
University, Ikot Akpaden, Akwa Ibom State, Nigeria
dDepartment of Chemistry, Faculty of Science, Obafemi Awolowo University, Ile-Ife
220005, Nigeria
eCenter of Excellence for Research in Engineering Materials (CEREM), King Saud
University, P. O. Box 800, Al-Riyadh 11421, Saudi Arabia
fElectrochemistry and Corrosion Laboratory, Department of Physical Chemistry,
National Research Centre, El-Behoth St. 33, Dokki, Cairo 12622, Egypt
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electron densities around the donor atoms of an inhibitor functional groups such as –C]C–, –C]N–, –N]N–, –NH, –C]S
molecule strongly inuence the adsorption and corrosion and p-bonds will enhance the adsorption of the molecules on mild
inhibiting abilities of the inhibitor. Furthermore, steric factors steel surface. The compounds differ in the number of uorine (Fꢁ)
and the type of functional groups present in a compound used substituents on the phenyl rings, such that IAB-NP is unsubstituted,
as inhibitors are also very important.12–15
IAB-NF has one F-substituent at position 2 on the phenyl ring (2-F
Acridine derivatives, which are widely used as antibacterial substituted) and IAB-ND has two F-substituents at positions 2 and 4,
and antiprotozoal agents are the derivatives of tricyclic nitrogen i.e. it is 2,4-F substituted. The effect of F-substitution on the phenyl
heterocyclic compounds.16 Acridine derivatives are reported to ring on corrosion inhibition potentials of the compounds is there-
have unique physical and chemical properties, biological fore examined.
activities, and industrial applications.17 The use of acridine
derivatives as corrosion inhibitors have been reported in liter-
ature. The inuence of toluidine and acridine derivatives on the
Experimental and computational
corrosion of 63/37 brass in nitric acid solution was reported to
be excellent, with efficiency attributed to the elimination of
nitrous acid as well as the adsorption of onium type ions at the
metal surface.18 Diffusion and ionization steps associated with
the dissolution of a-brass in 1 M NaCl + 0.01 M HCl solution
had been found to be inhibited by alkyl acridinium halides. The
studies
Synthesis and characterization of acridine-based
thiosemicarbazones IAB-NP, IAB-ND and IAB-NF
The synthesis protocol for inhibitors IAB-NP, IAB-ND and IAB-
NF is as outlined in Scheme 1. Detailed procedures, purica-
tion methods, experimental yields and spectroscopy data,
compounds were also found to be effective in inhibiting copper
including proton nuclear magnetic resonance (1H NMR),
dissolution than brass.19
carbon-13 nuclear magnetic resonance (13C NMR), Fourier
Following the existing history of corrosion inhibition by acridine
transform infrared spectroscopy (FTIR), electron-spray ioniza-
derivatives, the present work reports the corrosion inhibition prop-
erties of some novel acridine-based thiosemicarbazones namely
(2E,20E)-2,20-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-
tion mass spectroscopy (ESI-MS) and heteronuclear multiple
bond correlation HMBC have been reported elsewhere.20
1,8(2H,5H,9H,10H)-diylidene)bis(N-phenylhydrazinecarbothioamide)
(IAB-NP), (2E,20E)-2,20-(3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetrahy-
Materials
droacridine-1,8(2H,5H,9H,10H)-diylidene)bis(N-(2,4-diuorophenyl)
Chemical composition of mild steel samples employed for
hydrazinecarbothioamide) (IAB-ND) and (2E,20E)-2,20-(3,3,6,6-tetra-
electrochemical and surface morphology studies was deter-
methyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8(2H,5H,9H,10H)-diyli-
mined prior to use; 0.076% C, 0.039% Ni, 0.015% P, 0.050% Cr,
dene)bis(N-(2-uorophenyl) hydrazinecarbothioamide) (IAB-NF) for
0.030% S and 99.456% Fe. The samples of steel used for elec-
mild steel in 1 M HCl. The effects of IAB-NP, IAB-ND and IAB-NF on
trochemical measurements were press-cut into 1 cm2 surface
mild steel corrosion in the acid were investigated using electro-
area. Before all measurements and analyses were done, mild
chemical and scanning electron microscopy techniques. Theoretical
steel surface was ground with different grade of emery papers
computational studies were carried out on the novel compounds
(600, 800 and 1200), washed with double distilled water,
using density functional theory (DFT) and Monte Carlo simulation
degreased with ethanol and acetone and dried at room
calculations. The compounds in this study were synthesized as
temperature and used immediately. The corrosive solution, 1 M
outlined in Scheme 1.20 It is anticipated that the presence of
HCl was prepared by carefully diluting appropriate amount of
32% analytical grade HCl (purchased from Merck South Africa)
with double distilled water.
Inhibitor solution
A stock solution of the respective inhibitors (concentration of
0.15 mM) were prepared by rst dissolution in 5 mL of acetone
(5% equivalent), and subsequently diluted to various concen-
trations; 0.13, 0.09, 0.07 and 0.05 mM.
Electrochemical measurements
The working electrode employed in these measurements was a mild
steel coupon with exposed area of 1 cm2. The other components of
the electrochemical cell include Ag/AgCl (3 M KCl) as the reference
electrode and a platinum rod as the counter electrode. All
measurements were carried out at 25 ꢂ 1 ꢃC on Metrohm AutoLab
Potentiostat/Galvanostat (PGSTAT302N) driven by AutoLab Nova 2.1
soware. To ensure a stable open circuit potential (OCP), the
working electrode was immersed in the test solution for 30 min
Scheme 1 Synthetic protocol for inhibitors IAB-NP, IAB-ND and IAB-
NF.20
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during which it undergoes free corrosion in the electrolyte before
each measurement.21
1
c ¼ ꢁ ðELUMO ꢁ EHOMO
Þ
(3)
(4)
2
The mild steel working electrode potential in the corrosive
electrolyte was swept between ꢁ250 mV and +250 mV relative to the
OCP at a scan rate of 1 mV sꢁ1. By extrapolating the linear regions of
the polarization curves, electrochemical parameters like corrosion
potential (Ecorr), corrosion current density (icorr), anodic Tafel slope
(ba) and cathodic Tafel slope (bc) were obtained and recorded. From
the derived corrosion densities, the protection abilities were calcu-
lated using eqn (1):
4Fe ꢁ cinh
DN ¼
2ðhFe þ hinh
Þ
where cinh and hinh are the electronegativity and hardness of the
inhibitor respectively, 4Fe(110) is the work function of the body-
centered cubic (bcc) crystal structure of Fe(110), and hFe is the
hardness of Fe. The parameter, 4Fe(110) has been reported to
have a value of 4.82 eV (ref. 28) and has been successfully used
in literature27,29 while was taken to be 0 eV molꢁ1
.
The work
30
icoorr ꢁ iciorr
% IEPDP
¼
ꢀ 100
(1)
function, 4 is a better descriptor of charge density of metal than
icoorr
electronegativity.29
where iicorr and icoorr represents the corrosion current densities
for inhibited and uninhibited systems, respectively, and %
IEPDP is the percentage inhibition efficiency.
Monte Carlo simulations details
The adsorption of isolated molecules of IAB-NP, IAB-ND and
IAB-NF on Fe(110) surface and their competitive adsorption in
the presence of water molecules on the same crystal surface
were investigated using the Monte Carlo simulation approach.
Fe(110) was used as a representative crystal surface for mild
steel, being essentially Fe. Moreover, Fe(110) cleaved plane is
a more appropriate surface than Fe(100) and Fe(111) based on
its favourable energetics and atomic density and this informed
its adoption in this study.31,32 The DFT optimized structures of
the inhibitor molecules were imported into the Materials Studio
6.0 and subjected to another geometry optimization with For-
cite calculations for further energy renement using the
Conjugate Gradient algorithm at convergence tolerance 2 ꢀ
An alternating current signal of 10 mV was allowed to pass
through the electrochemical unit at a range of 10ꢁ1 Hz to 105 Hz
frequency (at OCP) to record electrochemical impedance spec-
troscopy (EIS). The EIS spectra were tted into a suitable Ran-
dle's equivalent circuit and relevant parameters were recorded
including the charge transfer resistance. The percentage inhi-
bition efficiency (% IEEIS) was calculated using eqn (2):
Rct ꢁ Roct
% IEEIS
¼
ꢀ 100
(2)
Rct
where Roct and Rct are the charge transfer resistance without the
inhibitors and in the presence of the inhibitors, respectively.
For more accuracy and reproducibility of experimental data, the
electrochemical studies were performed triply at each tested
concentration of the tested inhibitors and mean values are
reported.
ꢁ1 ꢀꢁ1
10ꢁ5 kcal molꢁ1 (for the energy), 0.001 kcal mol
A
(for the
force), 10ꢁ5 A (for the displacement), and maximum of 500
iteration. H2O molecule was optimized at the same level of
theory. Fe crystal was imported from Materials Studio library,
cleaved into (110) plane, optimized using the Forcite module
with Universal Force Field before enlarged into a 10 ꢀ 10 super
cell and embedded in a vacuum slab.
ꢀ
Surface morphology studies
Scanning electron microscope (SEM) model JEOL JSM-6610 LV was
used for surface analysis of the mild steel coupon (1 cm2) allowed to
corrode without and in the presence of optimum concentration of
IAB-NF for 24 h.22 The coupon was removed from the corrosive
solution aer 24 h, gently washed with double distilled water,
acetone and air dried before analysis.
An optimized isolated inhibitor molecule was made to
ꢀ
interact with the Fe(110) surface at a maximum distance of 15 A
to the surface region of the Fe(110) crystal in a simulated
annealing task by invoking the adsorption locator module. The
iterative interactions were carried out at ultrane quality using
Smart algorithm and the energy parameters were calculated
with condensed-phase optimized molecular potentials for
atomistic simulation studies-27 (COMPASS27) force eld.
Competitive adsorption of the inhibitor molecules in the pres-
ence of 100 molecules of H2O was carried out in a similar
approach. The equilibrium congurations of inhibitor/Fe(110)
and inhibitor/100 H2O/Fe(110) were determined for each of
IAB-NP, IAB-ND and IAB-NF and their corresponding adsorp-
tion energies (Eads) were recorded.
Quantum chemical calculations details
The studied inhibitor molecules were subjected to quantum
chemical calculations using the density functional theory (DFT)
with the Becke-3-parameter (hybrid) combined with Lee–Yang–Parr
(correlation) functionals (B3LYP) and 6-31G basis set.23–25 The
calculations were performed with Gaussian 09.26 The initial and
optimized structures of the molecules and their frontier molecular
orbital iso-surfaces observed with the aid of GaussView 5.0. Frontier
molecular orbital energy parameters, that is, energy of the highest
occupied molecular orbital (EHOMO) and energy of the lowest
unoccupied molecular orbital (ELUMO) were derived for the most
Results and discussion
Potentiodynamic polarization study
stable congurations of the molecules. Global electronegativity (c) A potentiodynamic polarization measurement was carried out
and fraction of electron transferred (DN) from inhibitor molecule to for all studied inhibitors at various concentrations (0.05 mM to
the metal were calculated according to the equations:27
0.15 mM). The cathodic and anodic Tafel plots of metal exposed
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to a corrosive solution (1 M HCl) without the inhibitors and in optimum concentration of 0.15 mM follows: IAB-NF (90.89%) >
the presence of various concentrations of IAB-NP, IAB-ND and IAB-ND (85.59%) > IAB-NP (81.13%).
IAB-NF, respectively are represented in Fig. 1. Table 1 show
basic parameters (corrosion potential, Ecorr
, anodic and
Electrochemical impedance spectroscopy
cathodic Tafel slopes, ba and bc and corrosion current density,
icorr) which were extracted by extrapolating the linear region of
the cathodic and anodic branches of the Tafel curves. Further-
more, percentage of inhibition efficiency (% IEPDP) at each
concentration recorded were calculated using eqn (1). Careful
observation of Fig. 1 shows that evolution of hydrogen gas was
stunted and the anodic dissolution of the mild steel was
reduced, suggesting that the studied inhibitors repressed the
corrosion of the metal. A prominent suppression was obtained
in the cathodic branches of the inhibited Tafel plots compared
to that of the blank at all concentrations, suggesting the
inhibitors could be acting as cathodic inhibitors.33
However, Table 1 shows that the changes in Ecorr did not
follow any particular pattern, and there was no signicant
change in Ecorr of the inhibited system compared to that
recorded for the reaction without the inhibitors. The pattern
obtained for Ecorr in this study suggests that the mechanism of
corrosion did not change both in the inhibited and uninhibited
systems and shows that the studied compounds behaved as
mixed-type inhibitors.34,35 Generally, there is a shi towards
lower current densities for both the anodic and cathodic
branches of the Tafel plots (Fig. 1) in the presence of the studied
inhibitors compared to the blank. The anodic branch of Tafel
plots similar to what we obtained in this study has been
attributed to possible formation of nonpassive lm via the
deposition of impurities or small quantity of the products of
corrosion on the surface of the metal.36,37 Increase in concen-
tration of all studied inhibitors resulted in higher polarization,
showing a better inhibition efficiency. Thus, the order of inhi-
bition efficiency of the tested corrosion inhibitors at the
To gain more insights into the surface properties of the
corroding mild steel and the inhibitor as well as to monitor the
corrosion of mild steel against time, EIS measurements were
carried out. Table 2 contains electrochemical impedance
parameters, including solution resistance (Rs), charge transfer
resistance (Rp), phase shi (n), double layer capacitance (Cdl),
and the inhibition efficiencies (% EIEIS) (calculated according to
eqn (2)).
The Nyquist plots and Bode diagrams of mild steel in 1.0 M
HCl without the inhibitors and with the addition of varying
concentrations of IAB-NP, IAB-ND and IAB-NF are represented
in Fig. 2 and 3, respectively. As observed in the Nyquist plots,
single depressed semicircle with its centre under the real axis
indicates that the dissolution of mild steel and hydrogen
evolution in the studied corrosive electrolyte features single
charge transfer process.38 Larger diameter of semicircles asso-
ciated with inhibitor containing solutions in comparison to the
blank is an indication of possible formation of adsorbed lm of
the inhibitor molecules on the metal surface.39 The corrosion
mechanism of mild steel in 1 M HCl was not altered by the
addition of the inhibitor molecules as revealed by the similar
shape of the Nyquist plots in the absence and presence of the
inhibitors.40 The order of Rp values is inversely proportional to
the protection efficiencies of the inhibitors.41,42
The phase angle modulation with frequency and impedance
for corrosion of mild steel in 1 M HCl without the inhibitors and
in the presence of the respective inhibitors is represented by the
Bode plots (Fig. 3). Bode plots provides vital information about
the inductive, capacitive and resistive behaviours of the system
at different frequencies. The impedance spectra were analysed
using the electrical equivalent circuit represented in Fig. 4, and
a perfect t for experimental data was obtained and recorded in
Table 2. The pure double-layer capacitor (Cdl) was replaced in
the Randle's equivalent circuit with the constant phase element
(CPE) to ensure a more accurate t. Eqn (5) was employed in the
calculations of Cdl using the values of n and Y0, where the
magnitude of CPE and derivation parameters (ꢁ1 # n # 1)
depends on the surface morphology;43,44
ꢀ
ꢁ
1=n
Cdl
¼
YoR1pꢁn
(5)
were n represents the CPE exponent and Yo is the CPE constant,
respectively.
At intermediate frequencies, an increase in slope for all
studied inhibitors with increasing concentrations is observed
when looking at the linear portion of the Bode impedance
modulus plots (Fig. 3). This is a strong indication that inhibitor
molecules modify the surface of the mild steel as well as
changing the electrochemistry of the surface.22 The values of n
(CPE exponent) is linked to the nature of the interface between
the adsorbed inhibitors on the mild steel (which enables
protection from corrosion) and the corrosive media.45 The
values of n in this study (Table 2) are very close to unity, and
Fig. 1 (a–c) Tafel polarization plots for mild steel corrosion in the
absence and presence of various concentrations of IAB-NP, IAB-ND
and IAB-NF.
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