Scanning electrochemical microscopy for the investigation of corrosion inhibition of triazino-benzimidazole-2-thiones in hydrochloric acid solution

Article abstract of DOI:10.1007/s11164-015-2369-7

Aryl-triazino-benzimidazole-2-thiones were synthesized via three-component reaction between ammonium thiocyanate, benzoyl chlorides, and 2-aminobenzimidazole in acetone. The structure of the products was confirmed by NMR, FT‐IR, and mass spectrometer. The

Full text of DOI:10.1007/s11164-015-2369-7

Res Chem Intermed
DOI 10.1007/s11164-015-2369-7
Scanning electrochemical microscopy
for the investigation of corrosion inhibition of triazino-
benzimidazole-2-thiones in hydrochloric acid solution
N. Esmaeili1 J. Neshati1 I. Yavari2
•
•
Received: 16 June 2015 / Accepted: 19 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Aryl-triazino-benzimidazole-2-thiones were synthesized via three-com-
ponent reaction between ammonium thiocyanate, benzoyl chlorides, and 2-aminoben-
zimidazoleinacetone. Thestructure ofthe productswasconfirmedbyNMR, FT-IR, and
mass spectrometer. The corrosion behaviors of these compounds on carbon steel in 2 M
HClsolutionwereinvestigatedbyscanningelectrochemicalmicroscopy, operatedinthe
substrate-generation/tip-collection mode using Fe2?/Fe3? redox couple as redox
mediator. The tip current values and local ferrous ion concentrations were obtained. The
inhibition efficiency of the inhibitors was investigated by weight loss measurements.
When the inhibition efficiency was more than 85 %, scanning electrochemical micro-
scopy and weight loss measurements were in good agreement.
Keywords Scanning electrochemical microscopy Á Benzoimidazole Á Corrosion
inhibition
Introduction
The coupling of scanning probe techniques with electrochemistry were performed
by Bard in 1989 [1]. Scanning electrochemical microscopy (SECM) has become a
powerful technique in many areas of fundamental electrochemical research, due to
its high spatial resolution and sensitivity. This technique consists of a mobile
ultramicroelectrode (UME), which has developed into a versatile tool for the
&
J. Neshati
neshatij@ripi.ir
1
2
Industrial Protection Division, Research Institute of Petroleum Industry,
P.O. Box 14665-137, Tehran, Iran
Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
123

N. Esmaeili et al.
investigation of the topography and redox activity of solid–liquid, liquid–liquid, and
liquid–gas interfaces [2, 3]. Information about reactions that occur in the solution
space between the tip and sample can also be obtained, which has facilitated the use
of this technique in applications beyond the solid/liquid interface [4–6]. SECM has
been used in measuring, characterizing, and evaluating corroding systems [7].
Localized corrosion processes and electrochemical activity distributions on surfaces
can thus be investigated in real time with high spatial resolution. SECM has been
used to study corrosion and the localized corrosion phenomena at surfaces [8–12].
The substrate generation–tip collection (SG/TC) mode in SECM has been usually
exploited for such purposes [13–15]. In the SG/TC mode, a redox-active species is
generated at the sample and is amperometrically detected at the UME. Local fluxes
of ferrous ions at steel surfaces have been monitored by oxidation of Fe2? ions at
the tip [16–21].
In this work, four aryl-triazino-benzimidazole-2-thiones (1a–1d) were prepared
via multicomponent reactions [9, 22–27] from thiocynate, benzoyl chlorides, and
2-aminobenzimidazole. The applicability of SECM was investigated to characterize
the inhibiting effect of these compounds against the corrosion of carbon steel in 2 M
HCl solution. Direct SECM evidence for the corrosion process was obtained
through the amperometric measurement of Fe2? ions as the dissolution of the carbon
steel proceeds in acidic conditions. The ferrous ion concentrations were calculated
by using tip current (itip). In addition, the inhibition efficiency of the inhibitors was
studied using weight loss measurements. The adsorption mechanism was investi-
gated by calculating isotherm equations.
Materials and instruments
The reagents and solvents used in this study were obtained from Merck and used
without further purification. Melting points were recorded on a Buchi 535 apparatus.
IR spectra, Shimadzu IR-460 spectrometer; 1H- and 13C-NMR spectra, Bruker
DRX-300 Avance; EI-MS (70 eV): Finnigan MAT-8430 mass spectrometer in m/z;
SECM, Uniscan model 370, UK; potentiostat/Galvanostat, Zahner model MeX6,
Germany; were used.
General procedure for the preparation of aryl-triazino-benzimidazole-
2-thiones
Aryloylisothiocyanates 3 were prepared from ammonium thiocyanate (1.52 g,
20 mmol) and benzoyl chlorides (20 mmol) in acetone (30 mL) at room temper-
ature. Then, 2-amino-benzoamidazole (2.66 g, 20 mmol) was added. The reaction
mixture was refluxed for 2 h, solvent removed and the residue was purified by
recrystallization from EtOH to give product 1 (Fig. 1). The structures of products
123

Scanning electrochemical microscopy for the investigation…
N
NH2
O
O
S
acetone
r.t.
N
H
C
+
KSCN
Ar
N
4
Ar
Ar
Cl
reflux
3
2
Ar
HO
Ar
N
O
NH
NH
N
NH
NH
H
S
S
S
N
N
NH
N
N
N
5
6
1
1a: Ar = Ph
1b: Ar =Tol
1c: Ar = MeO-C6H4
1d: Ar = O2N-C6H4
Fig. 1 Synthesis of aryl-triazino-benzimidazole-2-thiones 1a–1d
1
1a–1d were deduced from their IR, H- and 13C-NMR spectroscopy, and mass
spectrometry.
4-Phenyl[1, 3, 5]triazino[1,2-a]benzimidazole-2(1H)-thione (1a) Yellow powder;
yield: (75 %); IR (KBr): 3434 (NH), 3100, 1622 1474; 1H-NMR (300 MHz,
DMSO-d6): 9.40 (s, 1H); 8.42 (d, 3J = 7.3 Hz, 2H); 7.40-7.60 (m, 7 H); 13C-NMR
(70 MHz, DMSO-d6): 178.7 (C=S); 150.4 (C); 142.0 (C); 134.3 (C); 132.0 (CH);
128.4 (2 CH); 128.2 (2 CH), 127.1 (CH); 126.6 (C); 122.1 (CH); 116.9 (2 CH);
112.1(C). MS: m/z (%) = 278 (M?, 100), 245 (5), 219 (56.7), 192 (5), 175 (53.3),
104 (6.7), 90 (8.3), 77 (10).
4-(4-Methylphenyl)[1, 3, 5]triazino[1,2-a]benzimidazole-2(1H)-thione (1b) Yel-
low powder; yield: (81 %); IR (KBr): 3356 (NH), 3063, 1658; 1256 (C=S);
1H-NMR (300 MHz, DMSO-d6): 9.35 (s, 1H); 8.01 (d, 3J = 8.2 Hz, 2 H); 7.21 (d,
3J = 8.2 Hz, 2 H); 7.17–7.11 (m, 4 CH); 13C-NMR (70 MHz, DMSO-d6): 177.1
(C=S); 167.6 (C); 162.1 (C) 155.4 (C); 148.1 (C); 142.4 (CH); 134.4 (C); 131.0
(CH); 129.4 (C); 129.0 (2 CH); 128.3 (2 CH); 121.61 (CH); 113.5 (CH); 21.1 (Me);
MS: m/z (%) = 292 (M?, 5), 251 (52), 223 (32), 194 (10), 105 (14), 119 (100), 91
(65), 65 (30).
(4-Methoxyphenyl)[1, 3, 5]triazino[1,2-a]benzimidazole-2(1H)-thione (1c) Yel-
low powder; yield: (83 %); IR (KBr): 3421 (NH), 3074, 2837; 1692, 1260 (C=S);
1H-NMR (300 MHz, DMSO-d6): 12.3 (s, 1H); 8.15 (d, 3J = 8.1 Hz, 2 H); 7.46 (d,
3J = 7.9 Hz, 2 H); 7.11–6.99 (m, 4 H); 3.82 (s, 3 H); 13C-NMR (70 MHz, DMSO-
d6): 182.2 (C=S); 166.9 (C); 163.1 (C); 162.5 (C); 148.4 (C); 135.0 (C); 130.9 (CH);
130.3 (2 CH), 125.9 (CH); 124.8 (C); 121.4 (CH); 113.7 (2CH); 113.6 (CH); 55.5
(MeO). MS: m/z (%) = 308 (M?, 31), 267 (78), 239 (20), 135 (100), 92 (15), 77 (18).
123

N. Esmaeili et al.
4-(4-Nitrophenyl)[1, 3, 5]triazino[1,2-a]benzimidazole-2(1H)-thione (1d) Yellow
powder; yield: (85 %); IR (KBr): 3353, (NH); 1680; 1554, 1339 (NO2); 1159
1
3
(C=S); H-NMR (300 MHz, DMSO-d6): 9.68 (s, 1 H); 8.54 (d, J = 8.7 Hz, 2 H);
8.28 (d, 3J = 9.18 Hz, 2 H); 8.13 (d, 3J = 8.7 Hz, 1 H); 7.61 (d, 3J = 8.8 Hz, 1 H);
7.53 (t, 3J = 7.3 Hz, 1 H); 7.44 (t, 3J = 7.4 Hz, 1 H); 13C-NMR (70 MHz, DMSO-
d6): 189.3 (C=S); 169.6 (C); 156.9 (C); 149.8 (C); 145.4 (C); 140.9 (C); 130.7 (2
CH), 130.2 (CH), 128.8 (CH); 124.8 (CH); 123.7 (2 CH); 113.51 (CH); MS: m/z
(%) = 323 (M?, 53), 282 (98), 254 (51), 160(53), 150 (100), 104 (41), 92 (24), 76
(35).
Electrochemical measurements
SECM photographs were obtained from the carbon steel surface by using a scanning
electrochemical microscope. The electrodes were a 25 lm platinum UME tip, an
Ag/AgCl/KCl (saturated) reference electrode, sample electrode (carbon steel), and a
platinum counter electrode, all set up in a cell made of poly(tetrafluoroethene).
Scans were conducted parallel to the sample surface in an area of
500 lm 9 500 lm in X and Y directions. Scan rate was 10 lm s-1. The
measurements were performed with the UME tip at a height of 20 lm over the
specimen surface. Fe2?/Fe3? redox couple was used as electrochemical mediator at
the tip. The gap (20 lm) between the UME and carbon steel was measured using the
scaled video camera system attached to the SECM. The potential of the tip was held
at ?0.6 V versus the reference electrode. The carbon steel samples with
composition 0.19 % C, 0.4 % Mn, 0.04 % P, 0.005 % S, and Fe for the balance
were used for weight loss studies of inhibitors.
Results and discussion
SECM experiments
The presence iron ions at the carbon steel surface under the UME tip was examined
by SECM. The tip distance over the sample was determined by approach curves.
These curves were drawn when the tip was moved towards the surface in a
controlled motion on the Z axis [28]. Approach curve in Fig. 2 shows the
concentration profile of Fe2? ions as a function of the distance from the carbon steel
surface in 2 M HCl. Approach curve was measured at the open-circuit potential.
According to Fig. 2, at a distance of\20 lm, the current reaches the current limit.
Therefore, the distance between tip and sample surface should be set \20 lm.
Corrosion occurs when the carbon steel is exposed to the acidic solution resulting
in the release of Fe2? ions in the anodic half-cell reaction based on Eq. (1). The
ferrous ions are detected at the UME through their oxidation to Fe3? ions by setting
the tip potential at ?0.60 V based on Eq. (2), as shown in Fig. 3.
123

Scanning electrochemical microscopy for the investigation…
Fig. 2 SECM approach curve
with tip potential ?0.60 V
versus Ag/AgCl/KCl (saturated)
of the carbon steel surface
measured in 2 M HCl solution at
the open-circuit potential
Fig. 3 Schematic
representation of the SECM tip
operating at ?0.6 V
Dissolution reaction on Sample: Fe ! Fe2þþ2eÀ
ð1Þ
ð2Þ
UME: Fe2þ ! Fe3þ þ eÀ
Figure 4 shows a cyclic voltammogram measured at the UME tip in a 2 M HCl
solution at the scan rate of 0.05 V s-1. It depicts the electrochemical reactions
associated with the dissolved Fe(II) ions in the electrolyte. The voltammetric wave
corresponds to the Fe(II)/Fe(III) electrode reaction as shown in Eq. (2). By setting
the tip potential to ?0.60 V versus Ag/AgCl/KCl (sat.), the oxidation of the Fe(II)
species dissolved in the electrolyte can be detected through their oxidation to Fe(III)
at the UME. Since no Fe(II) species were originally present in the electrolytic phase,
they can only originate from corrosion processes at the carbon steel metal directly
exposed to the 2 M HCl solution.
The cathodic process consumes H? and dissolved oxygen from the solution
phase. In acidic media, species are consumed or generated on sample through
possible reactions which can be summarized by the following equations [4, 13]. At
low pH and oxygen-poor solutions, it may be assumed that the cathodic reaction is
the reduction of protons (6).
123

N. Esmaeili et al.
Fig. 4 Cyclic voltammogram
measured at the SECM-tip in
2 M HCl of carbon steel sample
with scan rate of 0.05 V s-1
O2 þ 4Hþ þ 4eÀ ! 2H2O
O2 þ 2Hþ þ 2eÀ ! H2O2
ð3Þ
ð4Þ
ð5Þ
ð6Þ
2H2O2 ! 2H2O þ O2 ðdecomposition reactionÞ
2Hþ þ 2eÀ ! H2
The release of metal ions at sample surface and the consumption of Fe2? at the
tip can be monitored during operation in the generator–collector mode [29, 30].
Therefore, changes in the local concentration of ferrous ions were recorded by
scanning the microelectrode over the sample when the proper potential value (?0.6)
was set at the tip.
SECM plots in Fig. 5 were used to study the film formation of aryl-triazino-
benzimidazole-2-thiones 1a–1d on the carbon steel in 2 M HCl solution. Based on
Fig. 5a in the absence of an inhibitor, the current fluctuations were induced by
anodic dissolution occurring on the carbon steel sample surface. As shown in
Fig. 5b, the local defects in the corrosion of carbon steel in the presence 1a inhibitor
are considered the potential precursor sites for localized corrosion, when thin
surface films are formed during the adsorption of corrosion inhibitors on metals. The
resulting SECM maps showed that maximum of current density was 1.7 9 10-5
A
(17 lA) in blank solution. The current maximum observed for 1a–1d inhibitors
were 2.1 9 10-7, 3.8 9 10-8, 1.7 9 10-8 , and 2.3 9 10-7 A, respectively. The
corrosion inhibition were calculated based on the mean corrosion currents which are
represented and replaced by tip currents characterizing the oxidation of Fe(II)
species from corrosion reaction of carbon steel (Table 1). The inhibition efficiency
(IESECM) at each inhibitor concentration was calculated from the value of the
maximum current using the Eq. (7):
!
i
tipðmaxÞ À it0ipðmaxÞ
IESECM% ¼
 100
ð7Þ
itipðmaxÞ
where itipðmaxÞ and i0tipðmaxÞ are the maximum current density of the tip in the absence
and presence of the inhibitor, respectively.
123

Scanning electrochemical microscopy for the investigation…
Fig. 5 Images created by SECM measurements on a carbon steel sample in 100 mg L-1 of inhibitor in
2 M HCl solution. a Blank, b 1a, c 1b, d 1c, e 1d. Tip–substrate distance: 20 lm. Tip potential: ?0.6 V
versus Ag/AgCl/KCl (saturated) reference electrode. The figures represent an area of 500 lm 9 500 lm
in X and Y directions
Based on Eq. (8), the values given in Table 1 indicate an estimate of the local
ferrous ion concentration obtained from the steady-state current at the SECM tip
[13].
itip ¼ 4nroFDC;
ð8Þ
where n (n = 1) is the number of electrons transferred during the redox reaction, ro
is the radius of the UME, F is the Faraday constant (96,485 C mol-1), C is the local
ferrous volume concentration, and D is the diffusion coefficient of the redox species.
The diffusion coefficient of Fe2? in 2 M HCl solution (D = 3.3 9 10-6 cm2 s-1
)
was calculated by using the Randles–Sevcik equation. This steady-state current is
123

N. Esmaeili et al.
Table 1 The tip currents, local ferrous ion concentrations and IE% of compounds 1a–1d in 2 M HCl on
the carbon steel at the different concentration
Inhibitor
C (mg L-1
)
itip (A)
CFe(II) (mM)
IE%
From SECM
IE%
From weight
loss
Blank
–
1.0 9 10-5
4.02 9 10-6
1.21 9 10-6
4.85 9 10-7
1.18 9 10-7
2.79 9 10-6
4.90 9 10-7
1.3 9 10-7
2.46 9 10-8
9.8 9 10-7
4.25 9 10-7
1.45 9 10-8
9.8 9 10-9
2.35 9 10-6
8.60 9 10-7
4.80 9 10-7
1.17 9 10-7
3.14
–
–
1a
25
50
75
100
25
50
75
100
25
50
75
100
25
50
75
100
1.26
59.8
87.9
95.1
99.8
72.1
95.0
98.7
99.8
90.2
95.7
99.8
99.9
76.5
91.4
95.2
98.7
44.4
76.9
86.5
90.2
45.6
80.6
89.8
93.5
63.0
82.1
91.8
95.4
36.6
73.8
84.7
89.0
3.80 9 10-1
1.52 9 10-1
3.71 9 10-2
8.76 9 10-1
1.54 9 10-1
4.08 9 10-2
7.73 9 10-3
3.08 9 10-1
1.33 9 10-1
4.55 9 10-3
3.08 9 10-3
7.38 9 10-1
2.70 9 10-1
1.51 9 10-1
3.67 9 10-2
1b
1c
1d
explained with the diffusion layer being hemispherical in shape and extending out
into the solution. The amount of Fe?2 species diffusing to the carbon steel surface is
defined by the volume with this expanding hemisphere, rather than a plane pro-
jecting into the solution as for a planar electrode. The current at a small disk also
reaches steady state in a fraction of a second. These characteristics show that a UME
used as a scanning tip and moving in a solution can be treated as a steady-state
system. In addition, the small currents that characterize most experiments with
UME tips, resistive drops in the solution during the passage of current are generally
negligible [31].
Table 1 shows the tip currents, local ferrous ion concentrations and IE% of
compounds 1a–1d at difference concentrations in 2 M HCl. The local concentration
of Fe2? were observed 3.2 mM at 20 lm from the surface of carbon steel in the
absence of inhibitors. In the presence of inhibitors 1a–1d, the ferrous ion
concentration and tip current reduced. Increase in inhibitors concentration up to
100 mg L-1 caused iron ion concentrations and tip currents decreased (Table 1).
The compound 1a decreased the extent of acidic dissolution for the carbon steel, but
the current fluctuations appeared on the sample surface (Fig. 5b). The local ferrous
ion concentration and itip for compound 1a were 0.039 mM and 2.1 9 10-7 A,
respectively.
The adsorption of 1b–c molecules on the carbon steel surface is responsible
for reduction of corrosion current and formation of a uniform adsorption film
(Fig. 5c-d). The corrosion current was decreased by the compound 1e, but local
123

Scanning electrochemical microscopy for the investigation…
Fig. 6 SECM contour plot of
carbon steel in 100 mg L-1 of
inhibitor 1e in 2 M HCl.
Etip = ?0.60 V,
Esample = OCP, SECM Pt tip
radius: 25 lm. Scan at constant
height near the sample (20 lm)
defects appeared on the carbon steel surface (Fig. 5e). The contour plot in Fig. 6 has
been shown as a typical SECM scan of compound 1e over carbon steel at a constant
height of about 20 lm, as obtained from approach curve. The contour plot clearly
shows a localized production of ferrous ions above the defect of carbon steel. Based
on IESECM calculated from Eq. (7) the performance of inhibitors was reduced in the
following order: compound 1c [ compound 1b [ compound 1a [ compound 1d.
All the compounds exhibited more than 98 % anticorrosion activity at the
concentration of 100 mg L-1. These compounds have several functional adsorption
centers such as C=S groups, O and/or N heteroatoms, and aromatic rings. The
adsorption of the examined inhibitors can occur by formation of links between the
d-orbital of iron atoms and the lone pairs present on the N, O, and S atoms, as well
as the p-bonds of the aromatic ring. The difference in the protective action of the
aryl-triazino-benzimidazole-2-thiones may be explained by functional groups on the
aromatic ring. Among the aryl-triazino-benzimidazole-2-thiones investigated in this
study, 1c had the best performance. This can be attributed to the electron-donating
methoxy group. The presence of methoxy group enhances the possible interaction
between the p-electrons with the vacant d-orbitals of iron atoms. Although the nitro
group atom is electron-withdrawing, the presence of two oxygen atoms in its
structure increases the corrosion inhibition.
Weight loss measurements
The weight loss experiments were carried out in 2 M HCl solutions for a duration of
6 h based on ASTM G31-2004. The samples were immersed in triplicate, and an
average corrosion rate was calculated. The inhibition efficiency (IE) at each
inhibitor concentration was calculated from the value of weight loss using the
Eq. (9):
ꢀ
ꢁ
Wuninh À Winh
IE% ¼
 100
ð9Þ
Wuninh
where Winh and Wuninh are the weight loss of the metal in presence and absence
of the inhibitor, respectively. The results are listed in Table 1. It is obvious that
the IE increases with increasing inhibitor concentration. According to Table 1, at
123

N. Esmaeili et al.
100 mg L-1 of inhibitors, the IE increases up to more than 89 %, which can be
caused by the formation of a protective film on carbon steel surface. The electron-
donating groups (methyl and methoxy) of the benzene ring in para position
increased inhibition efficiency. The IE of the investigated inhibitors decreased in the
following order: compound 1c [ compound 1b [ compound 1a [ compound 1d.
The IE obtained from SECM showed good agreement with those obtained from
weight loss experiments. According to Table 1, it can be deduced that when inhi-
bition efficiency was over 85 %, the amount the SECM methodology obtained was
more than the weight loss method, and the difference of the two methods was about
10 %. The difference in results may be because the measurements of current at a
small disk (UME) are done in a fraction of a second, while in the weight loss
method, the average of corrosion is calculated. The corrosion current at the moment
can be more or less than the average rate of corrosion.
Adsorption isotherms
Adsorption isotherms were used for the study of adsorption ability of aryl-triazino-
benzimidazole-2-thiones 1a–1d on the carbon steel in 2 M HCl solutions. In order
to obtain the isotherm, the linear relation between degree of surface coverage and
inhibitor concentration (Cinh) must be found. Surface coverage (h) was determined
from weight loss measurements according to Eq. (10):
IE%
h ¼
ð10Þ
100
where IE is the inhibition efficiency from weight loss measurements.
Adsorption isotherms including Langmuir, Temkin, Frumkin, and Flory–Huggins
were fitted for aryl-triazino-benzimidazole-2-thiones 1a–1d. The best fit was
obtained with the Langmuir isotherm (Fig. 7). According to this isotherm [32],
Cinh
1
¼ Cinh
þ
ð11Þ
h
Kads
where, Cinh is the inhibitor concentration, Kads is the adsorption equilibrium con-
stant, and h is surface coverage given by Eq. (10). The constant of an adsorption,
Fig. 7 Longmuir adsorption
plots for carbon steel in 2 M
HCl containing inhibitors of 1a–
1d
123

Scanning electrochemical microscopy for the investigation…
Table 2 The values of Kads and
Inhibitor
Kads (104 M-1
)
DGꢀads (kJ mol-1
)
DGꢀads of 1a–1d for carbon steel
in 2 M HCl in 298 k
1a
1b
1c
1d
1.22
1.31
2.42
1.29
-33.27
-33.44
-34.96
-33.40
Kads, is related to the standard free energy of adsorption, DGꢀads, with the following
Eq. (12);
ꢀ
ꢁ
ꢀ
ꢁ
where, R is the universal gas constant and T is the absolute temperature.
1
ÀDGꢀads
Kads
¼
exp
ð12Þ
55:55
RT
Adsorption isotherm provides information on the adsorption process such as
surface coverage, adsorption equilibrium constant, and information on the
interaction between the organic compound and electrode surface. The negative
values of DGꢀads in Table 2 indicate that adsorption of the inhibitors 1a–1d on the
carbon steel surface is spontaneous. It is well known that values of DGꢀads lower than
-20 kJ mol-1, are consistent with the electrostatic interaction between charged
molecules and the charged metal surface (physisorption); those around
-40 kJ mol-1 involve charge sharing or charge transfer from inhibitor molecules
to the metal surface to form a coordinate type of metal bond (chemisorption) [33].
The calculated DGꢀads values of 1a–1d are -33.33 to -35.46 kJ mol-1, indicating
their adsorption mechanism on carbon steel are both physisorption and chemisorp-
tion (Table 2). The large negative value of DGꢀads for 1c shows its strong adsorption
on the carbon steel surface. This isotherm assumes that the adsorbed molecules
occupy only one site and there are no interactions with other adsorbed species [32].
The inhibition efficiency of a compound depends on several factors, including the
number of adsorption active centers, the mode of interaction with the metal surface,
and molecular size. The active centers in the structure of 1a–1d are sulfur atoms,
nitrogen atoms, oxygen atoms, and two aromatic rings (Fig. 1). The presence of two
methoxy groups in 1c enhances the possible interaction between the p-electrons
with the vacant d-orbitals of iron atoms.
Conclusion
In summary, the corrosion behavior of four aryl-triazino-benzimidazole-2-thiones
were investigated on carbon steel in 2 M HCl solution using SECM and weight loss
measurements. The results of SECM measurements and local ferrous ion
concentrations confirm that inhibitors reduce corrosion current and form a uniform
film on the carbon steel surface. The average currents obtained by SECM or local
ferrous ion concentrations were used for comparison of the IE of the studied aryl-
triazino-benzimidazole-2-thiones. The inhibition efficiency of aryl-triazino-
123

N. Esmaeili et al.
benzimidazole-2-thiones increases with an increase in concentration of inhibitors.
All the compounds exhibited more than 89 % inhibition activity at a concentration
of 100 mg L-1. When the inhibition efficiency was more than 85 %, SECM and
weight loss results were comparable. The adsorption of the compounds on tꢀhe
carbon steel surface obeys Langmuir’s adsorption isotherm. The calculated DGads
values for 1a–1d were -33.33 to -35.46 kJ mol-1. The DGaꢀds values revealed that
the corrosion inhibition of 1a–1d on the metal surface are due to the formation
physisorption and chemisorptions adsorption films.
Acknowledgments The authors are grateful for financial support of the Research Institute of Petroleum
Industry.
References
1. A.J. Bard, F.R. Fan, J. Kwak, O. Lev, Anal. Chem. 61, 132–138 (1989)
2. A.L. Barker, M. Gonsalves, J.V. Macpherson, C.J. Slevin, P.R. Unwin, Anal. Chim. Acta 385,
223–240 (1999)
3. S. Amemiya, A.J. Bard, F.-R.F. Fan, M.V. Mirkin, P.R. Unwin, Annu. Rev. Anal. Chem. 1, 95–131
(2008)
4. J.L. Fernandez, A.J. Bard, Anal. Chem. 75, 2967–2974 (2003)
5. P. Sun, F.O. Laforge, M.V. Mirkin, Phys. Chem. Chem. Phys. 9, 802–823 (2007)
6. P. Sun, Z. Liu, H. Yu, M.V. Mirkin, Langmuir 24, 9941–9944 (2008)
7. L. Niu, Y. Yin, W. Guo, M. Lu, R. Qin, S. Chen, J. Mater. Sci. 44, 4511–4521 (2009)
8. K. Fushimi, K.A. Lill, H. Habazaki, Electrochim. Acta 52, 4246–4253 (2007)
9. N. Esmaeili, J. Neshati, I. Yavari, J. Ind. Eng. Chem. 22, 159–163 (2015)
10. A. Davoodi, J. Pan, C. Leygraf, S. Norgren, Electrochim. Acta 52, 7697–7705 (2007)
´
´
11. S. Gonzalez, J.J. Santana, Y. Gonzalez-Garcıa, L. Fernandez-Merida, R.M. Souto, Corros. Sci. 53,
´
´
´
1910–1915 (2011)
12. S.S. Jamali, S.E. Moulton, D.E. Tallman, M. Forsyth, J. Weber, A. Mirabedini, G.G. Wallace, J.
Electroanal. Chem. 739, 211–217 (2015)
13. A.J. Bard, M.V. Mirkin, Scanning Electrochemical Microscopy, 2nd edn. (CRC Press, Taylor &
Francis Group, LLC, Boca Raton, London, 2012)
14. G. Wittstock, M. Burchardt, S.E. Pust, Y. Shen, C. Zhao, Angew. Chem. Int. Ed. 46, 1584–1617
(2007)
15. M.V. Mirkin, B.R. Horrocks, Anal. Chim. Acta 406, 119–146 (2000)
16. Y. Yin, L. Niu, M. Lu, W. Guo, S. Chen, Appl. Surf. Sci. 255, 9193–9199 (2009)
17. E. Volker, C.G.L. Inchauspe, E.J. Calvo, Electrochem. Commun. 8, 179–183 (2006)
18. B.T. Luong, A. Nishikata, T. Tsuru, B.T. Luong, A. Nishikata, T. Tsuru, Electrochemistry 71,
555–561 (2003)
19. J. Izquierdo, L. Nagy, J.J. Santana, G. Nagy, R.M. Souto, Electrochim. Acta 58, 707–716 (2011)
20. K. Fushimi, M. Seo, Electrochim. Acta 47, 121–127 (2001)
´
21. A.C. Bastos, A.M. Simoes, S. Gonzalez, Y. Gonzalez-Garcia, R.M. Souto, Electrochem. Commun. 6,
´
1212–1215 (2004)
22. J. Zhu, H. Bienayme (eds.), Multicomponent Reactions (Wiley, Weinheim, 2005)
23. P.T. Anastas, L.G. Heine, T.C. Williamson, Green Chemical Syntheses and Processes (American
Chemical Society, Washington, DC, 2000)
24. R.C. Cioc, E. Ruijter, R.V.A. Orru, Green Chem. 16, 2958–2975 (2014)
25. B. Ganem, Acc. Chem. Res. 42, 463–472 (2009)
26. L.H. Choudhury, T. Parvin, Tetrahedron 67, 8213–8570 (2011)
27. A. Bhatnagar, P.K. Sharma, N. Kumar, Int. J. Pharm. Tech. Res. 3, 268–282 (2011)
28. E.P. Sascha, M. Wiebke, W. Gunther, Z. Phys. Chem. 222, 1463–1517 (2008)
29. J. Izquierdo, J.J. Santana, S. Gonzalez, R.M. Souto, Electrochim. Acta 55, 8791–8800 (2010)
´
30. Y. Gonzalez-Garcıa, G.T. Burstein, S. Gonzalez, R.M. Souto, Electrochem. Commun. 6, 637–642
´
´
(2004)
123

Scanning electrochemical microscopy for the investigation…
31. A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications (Wiley, New
York, 2001)
32. G. Avci, Colloids Surf. A Physicochem. Eng. Asp. 317, 730–736 (2008)
33. E. Kamis, F. Belluci, R.M. Latanision, E.S.H. El-Ashry, Corrosion 47, 677–686 (1991)
123