Synthesis of thiourea derivatives and its evaluation as corrosion inhibitor for carbon steel

Article abstract of DOI:10.1080/10426507.2015.1035719

The inhibitory effect of thiourea-based compounds was evaluated using carbon steel body specimens in hydrochloric acid media. Thiourea derivatives, N-(pyrimidin-2-ylcarbamothioyl)benzamide and N-(6-methylpyridin-2-ylcarbamothioyl)benzamide, were synthesized using a simple route with good yields of approximately 70%. The inhibitory efficiencies were obtained by means of weight-loss experiments and electrochemical techniques (e.g., polarization curves and electrochemical impedance spectroscopy). The presence of a methyl functional group showed a better inhibitory efficiency compared with the derivate inhibitor without such modification. Analyzing Langmuir isotherms, ΔG⁰ads values indicate the chemical adsorption of thiourea-based compounds. The ECorr values obtained for the thiourea derivative with the methyl functionality was cathodically shifted by approximately -0.08 V, with an inhibition efficiency of 81%.

Full text of DOI:10.1080/10426507.2015.1035719

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Phosphorus, Sulfur, and Silicon and the Related
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Synthesis of Thiourea Derivatives and Its Evaluation as
Corrosion Inhibitor for Carbon Steel
Clara G. M. de Oliveiraa, Vitor W. Fariaa, Gabriel F. de Andradeb, Eliane D'Eliac, Murilo F.
Cabrald, Bruno A. Cotrimb, Gabriel O. Resendeb & Flávia C. de Souzaa
a Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus São
Gonçalo, 24425-005, São Gonçalo, RJ, Brazil
b Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus Rio de
Janeiro, 20270-021, Rio de Janeiro, RJ, Brazil
c Instituto de Química, UFRJ, Centro de Tecnologia - Cidade Universitária, 21941-909, Rio de
Janeiro, RJ, Brazil
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d Divisão de Metrologia de Materiais (DIMAT), Instituto Nacional de Metrologia, Qualidade e
Tecnologia, 25250-020 Duque de Caxias, RJ, Brazil
Accepted author version posted online: 09 Apr 2015.
To cite this article: Clara G. M. de Oliveira, Vitor W. Faria, Gabriel F. de Andrade, Eliane D'Elia, Murilo F. Cabral, Bruno A.
Cotrim, Gabriel O. Resende & Flávia C. de Souza (2015): Synthesis of Thiourea Derivatives and Its Evaluation as Corrosion
Inhibitor for Carbon Steel, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2015.1035719
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1035719
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SYNTHESIS OF THIOUREA DERIVATIVES AND ITS EVALUATION AS CORROSION
INHIBITOR FOR CARBON STEEL
Clara G. M. de Oliveira1, Vitor W. Faria1, Gabriel F. de Andrade2, Eliane D’Elia3, Murilo F.
Cabral4, Bruno A. Cotrim2, Gabriel O. Resende2, Flávia C. de Souza1,*
1Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus São Gonçalo,
24425-005, São Gonçalo, RJ, Brazil
2Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus Rio de Janeiro,
20270-021, Rio de Janeiro, RJ, Brazil
3Instituto de Química, UFRJ, Centro de Tecnologia - Cidade Universitária, 21941-909, Rio de
Janeiro, RJ, Brazil
4Divisão de Metrologia de Materiais (DIMAT), Instituto Nacional de Metrologia, Qualidade e
Tecnologia, 25250-020 Duque de Caxias, RJ, Brazil
*Correspondent author: flavia.souza@ifrj.edu.br
Abstract
The inhibitory effect of thiourea-based compounds was evaluated using carbon steel body
specimens in hydrochloric acid media. Thiourea derivatives, N-(pyrimidin-2-
ylcarbamothioyl)benzamide and N-(6-methylpyridin-2-ylcarbamothioyl)benzamide, were
synthesized using a simple route with good yields of approximately 70%. The inhibitory
efficiencies were obtained by means of weight-loss experiments and electrochemical techniques
(e.g. polarization curves and electrochemical impedance spectroscopy). The presence of a methyl
functional group showed a better inhibitory efficiency compared with the derivate inhibitor
without such modification. Analyzing Langmuir isotherms, G0 values indicate the chemical
ads
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adsorption of thiourea-based compounds. The ECorr values obtained for the thiourea derivative
with the methyl functionality was cathodically shifted by approximately −0.08 V, with an
inhibition efficiency of 81%.
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Introduction
In the oil industry, the use of corrosion inhibitors preserves the integrity of metal
components in facilities under severe corrosive conditions, which would normally lead to failure
or a reduction in the operating cycle of essential equipment such as risers, columns and well
structures, pipelines, distillation towers, and pressure vessels. In this respect, the petroleum
industry is by no means an exception, as it is a very important designer of pipe standards and
devotes its fundamental operations to the exploitation and transportation of crude oil, assorted
refine produce, and other basic processing fluids, often over great distances [1, 2].
Corrosion inhibition by organic compounds is usually related to their adsorption
properties on the metal surfaces. However, under-service conditions have transformed metal
surfaces from the beginning of testing and commissioning the pipelines into full-scale service.
Thus, adsorption occurs best, per force, on oxidized surfaces. This is one relevant aspect of the
environment in which inhibitors must work. In particular, the process of adsorption on metals
depends on different factors, such as the nature and the charge of the metal surface, the
temperature and pH of the surrounding media, and the molecular structure of the organic
compounds [3].
The use of chemical inhibitors to decrease the rate of corrosion has been the focus of
many efforts within the chemical process industry. The development of novel corrosion
inhibitors derived from neutral sources tailored to being nontoxic additions is considered to be of
fundamental relevance [3]. Compounds containing thiocarbonyl groups (e.g., thiourea) generally
show higher efficiency in inhibiting corrosion compared to similar chemical species that do not
contain the moiety. The efficiency of the inhibition may be explained by an adsorption
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mechanism that occurs on the steel surface, owing to the transfer of the coordination center from
the nitrogen to the sulfur atom, which is favored by the thiocarbonyl group, allowing a more
effective chemical interaction by coordinate covalent bonding, with the possibility of back-
bonding between the sulfur and metal surface atoms to occur.
Thiourea is an industrially and physiologically significant molecule. For example,
thiourea has widely been used as an electroplating additive, a corrosion inhibitor, a rubber
accelerator, and an extractant for precious metals [4]. Thiourea derivatives have been tested in
recent years as corrosion inhibitors for different metals and alloys such as mild steel [5, 6],
stainless steel [7], iron [8], and aluminum [9]. These compounds presented anticorrosion
activities in different acidic conditions, such as in the presence of hydrochloric acid [8], sulfuric
acid [10, 11], sodium nitrite [10], and formic acid [12].
The behavior of 1,3-dibenzylthiourea (DBTU) and 1,3-dibenzylurea (DBU) have been
investigated as carbon steel corrosion inhibitors in HCl solution by D’Elia and co-workers [5].
All electrochemical results and quantum chemical calculations have shown that DBTU acts as an
effective inhibitor on carbon steel in 1 mol L−1 HCl solution, whereas DBU is a poor corrosion
inhibitor. The sulfur atom may play an important role in the inhibition of thiourea derivatives for
carbon steel corrosion in acidic medium.
The objective of this work was to synthesize thiourea derivatives from benzoic acid and
to investigate the role of these substances (Figure 1) as corrosion inhibitors for carbon steel in
highly corrosive environments.
Results and Discussion
Synthesis of thiourea derivatives
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For the synthesis of the thiourea derivatives, benzoic acid was converted to its acyl
chloride (2), which was reacted with ammonium thiocyanate [13], generating an isothiocyanate
intermediate (3). The last step was the reaction of 3 with substituted anilines, generating the
thiourea derivative (1), as shown in Figure 2.
The products were obtained using a simple synthetic route without any further
purification, with yields ranging from 62 to 75%. Our synthetic procedure showed a similar
range of yields to those presented by Rauf et al. [14], who performed the same synthetic route by
means of microwave heating. Even though our procedure may be simpler than that used by Rauf
et al. [14], we could see good results in terms of the yield of the thiourea derivatives.
Weight-loss experiments
Electrochemical corrosion is a spontaneous process, which the metal is in contact with
electrolyte solution underlying by both anodic and cathodic reactions. These reactions can
happen in presence of water and room temperature. The general mechanism has considered that
the corrosion on mild steel in acid media can happen in several steps. Metallic iron from mild
steel reacts spontaneously with H+ forming ferric ion (Fe2+) and hydrogen gas, as follows [15]:

ads 
Fe H
FeH
(1)

FeH  e  FeH
(2)


ads 

ads 
FeH  H e  Fe H
(3)
(4)
ads 
2
Fe  Fe2  2e
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The results for weight-loss experiments are presented in Table 1; it is worth mentioning
that these tests were critical in order to determine the most effective corrosion inhibitor. The
weight-loss values showed in Table 1 have a relative standard deviation around 3 – 5%.
The corrosion rates of carbon steel coupons in 1 mol L−1 HCl, in the absence and
presence of the studied inhibitors, in the concentration range 1.0×10−4 1.5×10−3 mol L−1 after
−
immersion for 24 h at 25oC, as well as the efficiency values, are depicted in Table 1. The carbon
steel corrosion rate (Wcorr) was greatly reduced upon addition of the thiourea derivatives. This
behavior reflects the inhibitory effect of the thiourea derivatives towards carbon steel corrosion
in acidic solutions. In addition, the inhibition efficiency increased with increasing concentration
of thiourea derivative in this medium.
These data also suggest that the inhibition efficiency remains relatively high over long
periods of immersion (24 h) for high inhibitor concentrations. Inhibitor 1b showed the best
inhibition efficiency (near 100%) at high concentrations, particularly when compared with 1a. It
is likely that the distinct range of activity among the tested thiourea corrosion inhibitors is due to
steric and electronic effects. Compound 1a with the pyrimidine ring was less active than
compound 1b with a pyridine ring. Further studies, including that of molecular docking between
the inhibitors and the steel, should be performed in order to better understand the structure–
activity relationships for this class of corrosion inhibitor. Therefore, we chose this inhibitor (1b)
to carry out further corrosion assays.
The interaction between the inhibitors and the metal surface can be described by the
adsorption isotherm. During the corrosion inhibition of metals, solvent molecules may also be
adsorbed at the metal/solution interface. Thus, adsorption of organic inhibitor molecules, coming
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from an aqueous solution, can be considered as a quasi-replacement process between the organic
compounds in the aqueous phase [Org(sol)] and water molecules on the electrode surface
[H2O(ads)]:
Org(sol) + xH2O(ads) → Org(ads) + xH2O(sol)
(Reaction 1)
where x is the number of water molecules replaced by an organic inhibitor [12].
In order to obtain an adsorption isotherm, the linear relationship between the degree of
surface coating () and the concentration of the inhibitor (C) can be obtained. The Langmuir
isotherm is based on the assumption that all adsorption sites are equivalent and that binding of
particles occurs independently of any nearby occupied or unoccupied locations. According to
this isotherm,  is related to C by the following equation:
C/  C1/ K
(Equation 4)
where  is the adsorption equilibrium constant. Here, we have chosen to show only the
Langmuir isotherm for the inhibitor 1b, as it showed the best results in terms of the inhibition
efficiency (cf. Table 1). Figure 3 shows the relationship between C/ and the concentration of
inhibitor 1b. A linear relationship can be observed with good correlation coefficients (r),
confirming the validity of this approach (Table 2). The slope of the curve is close to unity,
suggesting that the inhibitor adsorbed molecules form a monolayer (i.e., a close-packed
recovery) on the surface of carbon steel, and as predicted by the Langmuir isotherm.
From the intercept of the straight line (Figure 3), K was calculated and related to the
standard Gibbs free energy, G0ads, according to the following equation:
0




1
G
ads 
K 
exp
(Equation 5)
55.55
RT

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where R is the universal gas constant, T is the absolute temperature of the system, and 55.55 mol
L−1 is the value of the concentration of water in solution expressed in mol L−1. Table 2 shows the
values of the slope and the correlation coefficient of the Langmuir isotherm, adsorption constant,
and standard free energy.
The obtained ΔG0 value was −33.4 kJ mol−1. Generally, values of ΔG0 up 20 kJ
ads
ads
mol−1 indicate physisorption, where electrostatic interactions attributed to inhibition act between
the charged molecules and the charged metal, whereas values equal to or smaller than 40 kJ
mol−1 are associated with chemisorption, which is the result of the sharing or transfer of electrons
from organic molecules of the inhibitor to the metal surface, forming a coordinate bond [16]. It is
difficult to distinguish between chemisorption and physisorption based only on these criteria,
especially when charged species are adsorbed. According to the obtained value, it can be
suggested that the adsorption of compound 1b is not merely physisorption or chemisorption but
obeying a comprehensive adsorption (chemisorption and physisorption).
EIS experiments were carried out in order to confirm the weight-loss data obtained for
inhibitor 1b. The electrochemical characterization included determination of Rct, the corrosion
rate (Wcorr), and the electrochemical mechanism. These data allow the analysis of the impedance
alternate currents (AC), which are based on the modeling of a corrosion process through an
electrical circuit.
Figure 4 illustrates the EIS diagrams obtained for carbon steel, in the absence and
presence of inhibitor 1b, at the OCP. Table 3 summarizes the impedance data from EIS
experiments carried out in both the absence and the presence of increasing inhibitor
concentrations. In inhibitor-free solutions, only one depressed capacitive loop was observed, and
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that loop can be attributed to the time constant of charge transfer and the double-layer
capacitance. Such a depression is characteristic of solid electrodes and is often ascribed to
dispersion effects, which have been attributed to roughness and inhomogeneity on the surface
during corrosion [17-19]. This behavior is unaffected by the presence of the inhibitor, indicating
the activation-controlled nature of the reaction during a one-charge-transfer process.
The intersection of this semicircle with the real axis at high frequencies produced a value
of approximately 1.45 cm2 for the ohmic resistance (Rs) of the solution. The Rct values were
calculated based on the difference in impedance values at lower and higher frequencies. The
double layer capacitance (Cdl) was calculated using the equation below:
1
Cdl 
(Equation 6)
2fmaxRct
where fmax is the frequency at which the imaginary component of the impedance is maximum. A
Cdl value of 636 F cm−2 was determined for the carbon steel electrode.
Table 3 shows an increase in Rct and a decrease in Cdl with increasing concentration of
inhibitor. These results may be attributable to the inhibitor adsorption on the metal/solution
interface.
Potentiodynamic polarization curves obtained with the carbon steel electrode, in the
presence and absence of inhibitor 1b, are presented in Figure 5. The respective electrochemical
parameters, such as OCP, Ecorr, jcorr, as well as the anodic and cathodic Tafel constants (βa and βc,
respectively, shown in Table 4) were calculated from the Tafel plots. It is worth to mention that
the scan rate equal to 1 mVs−1 can be considered fast due to the adsorption of corrosion inhibitor
can be released a slow process. However, the OCP after 1h was stabilized and the equilibrium at
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surface was reached. Then, such scan rate would not interfer in the Ecorr values observed in these
experiments.
By analyzing the potentiodynamic polarization curves, it could be observed that the
presence of the inhibitor caused a decrease in the corrosion current density, probably owing to
adsorption of the organic inhibitor onto the electrode surface. There is only a decrease in
cathodic current densities, which shows the inhibitory effect of this molecule. Therefore,
hydrogen evolution was inhibited, retarding the corrosion process. The calculated inhibition
efficiencies from jcorr values obtained in the absence and presence of the inhibitor ranged from 51
to 81% in the studied inhibitor concentration range. On the other hand, these results could
support the feasible Coulombic interactions between adsorbed cations, in this case, the
protonated form of the thiourea derivative 1b, and specifically adsorbed anions (chloride ions),
thereby increasing the ΔG0ads values, even if no chemical bond appears.
Table 4 indicates that, in presence of the inhibitor, both the OCP and Ecorr values
obtained from the Tafel curve were cathodically shifted in relation to the blank assay (55–62 mV
and 60–76 mV respectively), demonstrating that this inhibitor acts as an adsorption inhibitor with
predominantly cathodic characteristics [19, 20]
The cathodic Tafel slopes (c) did not change significantly upon the addition of the
inhibitor, showing that the absorbed inhibitor molecules did not affect the hydrogen evolution
reaction, that is, hydrogen evolution was probably decreased by the surface-blocking effect.
Regarding the anodic Tafel slope (a), we note a slight increase with the inhibitor concentration,
and this result suggests that the inhibitor adsorbed to the carbon steel could modify the metal
dissolution reaction.
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Experimental
Reagents
Benzoyl chloride, anhydrous acetone ammonium thiocyanate, p-analine, dimethyl
sulfoxide (DMSO), acetone, and hydrochloric acid (37%) were purchased from Sigma–Aldrich
(Saint Louis, MO). All reagents were of analytical grade and used without further purification.
The deionized water used throughout this work was obtained from a deionization system.
Apparatus and measurements
Proton nuclear magnetic resonance (1H NMR) spectra were recorded using a Bruker 500
1
MHz instrument. For the H NMR spectra, chemical shifts () are referenced from
tetramethylsilane (TMS; 0.00 ppm) and the coupling constants (J) are reported in Hz. FT-IR
spectra of thiourea and its derivatives were obtained at 25oC in a Bruker Alpha
spectrophotometer using the ATR module.
Electrochemical measurements were carried out using an Autolab potentiostat/galvanostat
(PGSTAT128 N Eco Chemie; Utrecht, Netherlands) coupled to a personal computer and
controlled with GPES 4.9 software. A conventional three-electrode cell (100 mL) was used for
all experiments. In all cases, ASTM 1020 carbon a steel specimen was used as a working
electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a Pt plate
(1.0 cm2) was used as the auxiliary electrode.
The supporting electrolyte was 1.0 mol L−1 HCl solution containing 5% (v/v) acetone,
and all electrochemical measurements were performed at 25oC. Electrochemical impedance
spectroscopy (EIS) data were obtained using a PC-controlled FRA2 (Eco Chemie, Utrecht,
Netherlands), coupled to the aforementioned potentiostat, scanning from 100 kHz to 10 mHz at a
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10 mV amplitude, with ten data points per frequency decade. In all experiments, the ASTM 1020
carbon steel electrode was allowed to reach its stable open-circuit potential (OCP), which
occurred after 1 h. This time was required due to the stabilization of OCP, considering blank and
samples recovered with corrosion inhibitor. Potentiodynamic anodic and cathodic polarization
curves were acquired using a scan rate of 1 mV s−1 from −300 to +300 mV in relation to the
stable open-circuit corrosion potential (Ecorr). Polarization curves were also obtained after EIS
measurements.
Synthesis of thiourea derivatives
Benzoyl chloride (5 mmol) dissolved in anhydrous acetone (5 mL) was added dropwise to a
solution of ammonium thiocyanate (5 mmol) dissolved in anhydrous acetone (5 mL). The
resulting solution was heated under reflux for 30 min, and precipitation was observed. The white
solid (ammonium chloride) was filtered and 2-aminopyrimidin or 2-amino-6-methylpyridine (5
mmol) dissolved in 5 mL of anhydrous acetone was added to the filtrate. The solution was stirred
under reflux for 2 h, and cold water was then added until total precipitation. The product was
filtered in Bücher funnel, yielding the desired thiourea derivatives in good yields (ca. 62–78%
1
yield). N-(pyrimidin-2-ylcarbamothioyl)benzamide (1a): H NMR (400 MHz, DMSO-d6) δ
7.30-7.33 (t, J=4.88 Hz, 1 H), 7.59–7.62 (t, J=7.76 Hz, 2 H), 7.68–7.71 (t, J=7.32 MHz, 1 H),
7.98–8.00 (d, J=7.28 MHz, 2 H), 8.78–8.80 (d, J=4.88 MHz, 2 H), 12.12 (s, 1 H), 13.57 (s, 1H).
FT-IR (KBr): 3336 (N-H st.), 3166, 3067 (C-H ar ), 2982, 1702 (C=O), 1556 (C=C ar),
1406, 1324, 1231 , 1164 (C=S), 1088, 902, 797, 697, 649, 521 cm−1. Compound 1a was obtained
as an orange solid with a 75 % yield.. N-(6-methylpyridin-2-ylcarbamothioyl)benzamide (1b):
1H NMR (400 MHz, CDCl3-d6) δ 2.53 (s, 3H), 7.02-7.04 (d, J= 7.52Hz, 1H), 7.53–7.56 (t, J=
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7.88 Hz, 2 H), 7.63–7.68 (m, 2 H), 7.90–7.92 (d, J=7.44 Hz, 2 H), 8.58-8.60 (d, J= 8,24 Hz, 1
H), 9.04 (s, 1 H), 12.98 (s, 1 H). FT-IR (KBr): 3119, 1678, 1598, 1520, 1253, 1169,
751, 711, 672 cm−1 Compound 1b was obtained as a yellowish solid with 69 % yield.
Supplementary information shows 1H NMR and FTIR data.
In order to follow each step of the synthesis of thiourea and its derivatives, thin layer
chromatography was carried out on aluminum sheets that were pre-coated with silica gel 60 (HF-
254, Merck) with a film thickness of 0.25 mm (data not shown).
Preparation of electrodes
ASTM 1020 carbon steel electrodes were cut and embedded as steel rods in epoxy resin,
exposing a surface area of 1.0 cm2 to the electrolyte. These electrodes were abraded with emery
paper of different granulometry (320, 400, 600, and 1000). Then, these electrodes were washed
with double-distilled water, degreased with acetone, and dried in air.
Weight-loss experiments
Carbon steel specimens with the same composition used in the electrochemical
measurements were mechanically cut into 3.0 cm × 1.0 cm × 1.0 cm sections and abraded with
emery paper of different granulometry (320, 400, 600, and 1000). Then, these pieces were
washed with double-distilled water, degreased with acetone, and dried in air.
The specimens were immersed in a 1.0 mol L−1 HCl solution containing 5% acetone, in
the absence and presence of corrosion inhibitors at different concentrations, for a period of 24 h.
Each measurement was carried out in duplicate. Afterwards, the specimens were removed, rinsed
with water and acetone, dried in warm air, and stored in a desiccator. The weight loss was
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determined by gravimetric testing, using an analytical balance with a precision of 0.1 mg. The
efficiency of inhibition (n%) was obtained using the following equation:
W0  W
(Equation 1)
n% 
100
W0
where W0 and W are the corrosion rates (g cm−2 h−1) in the absence and presence of the corrosion
inhibitor, respectively.
The ASTM 1020 carbon steel used for both the electrochemical and weight-loss
experiments shows the following nominal chemical composition (Table 5).
The compound with the best inhibitory efficiency was subjected to the electrochemical
testing of OCP, EIS measurements, and polarization curves. Moreover, this compound has the
best yielding during organic synthesis.
The n% value was also calculated from potentiodynamic polarization curves and EIS
diagrams from the following equations:
jcorr,0  j
n% 
corr 100
(Equation 2)
jcorr,0
where jcorr,0 is the corrosion current density in the absence of an inhibitor and jcorr is the corrosion
current density in the presence of an inhibitor, both obtained from Tafel plots.
Rct  R
n% 
ct,0 100
(Equation 3)
Rct
where Rct is the charge-transfer resistance in the presence of an inhibitor and Rct,0 is the charge-
transfer resistance in the absence of the inhibitor, both obtained from the EIS diagrams. To
estimate the corresponding values for Rct an extrapolation of the semi-circle was done.
Conclusions
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Through a simple, low-cost, and good yielding synthetic route, the production of two
thiourea organic derivatives was achieved, which show anti-corrosion activity in the range of 1.0
× 10−4 to 1.5 × 10−3 mol L−1 in weight-loss experiments for ASTM1020 carbon steel. Compound
1a, with a pyrimidine ring, was less active towards inhibitory action pared to compound 1b with
a pyridine ring. The adsorption process of N-(6-methylpyridin-2-ylcarbamothioyl)benzamide
followed a Langmuir isotherm, presenting a G0ads value of −33.4 kJ mol−1. The results obtained
from the polarization curves and electrochemical impedance diagrams demonstrate that
compound 1b acts as an inhibitor through an adsorption process, reducing only the cathodic
current densities.
These results support the feasible Coulombic interactions between the adsorbed
protonated form of thiourea derivative 1b and specifically adsorbed anions, which increase the
ΔG0ads value, even if no chemical bond appears.
Acknowledgments
C.G.M.O., G.F.A., B.A.C., G.O.R., and F.C.S. acknowledge PRFH/PETROBRAS for
scholarships and for the financial support. M.F.C. thanks Pronametro (#52600.022686/2014),
F.C.S., V.W.F. and E.D.E. thanks CNPq.
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Table 1. ASTM 1020 carbon steel weight loss and inhibition efficiencies in 1.0 mol L−1 HCl
with different concentrations of inhibitors 1a and 1b.
Inhibitor 1a
Inhibitor 1b
Concentration
(mol L−1)
Concentration
Wcorr
(g cm−2 h−1)
0.0361
n
(%)
-
Concentration
(ppm)
Wcorr
(g cm−2 h−1)
n
(%)
-
(ppm)
Blank
0.0349
0.0165
0.0081
0.0044
0.0011
1.0×10−4
2.0×10−4
5.0×10−4
1.5×10−3
26
52
0.0096
73
82
87
92
27
54
53
77
88
97
0.0067
129
387
0.0046
136
406
0.0030
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Table 2. Values of the adsorption constants and ΔG0ads obtained for the corrosion inhibitor 1b.
ΔG0ads (kJ mol−1)
Slope
r
K (L mol−1)
1.057
0.9955
1.29×104
−33.4
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Table 3. Values of frequency, charge-transfer resistance, and double-layer capacitance
obtained for the tested corrosion inhibitor (1b).
Inhibitor
concentration
(mol L-1)
Inhibitor
concentration
(ppm)
Rct
cm2)
Cdl
(F cm−2)
fmax
n
(Hz)
(%)
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1.74
11.2
11.2
11.2
11.2
144
397
580
634
741
636
35.7
24.4
22.4
19.1
-
1×10−4
5×10−4
1×10−3
2×10−3
27
64%
75%
77%
81%
136
271
543
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Table 4. Kinetic parameters obtained from Tafel plots for carbon steel in 1.0 mol L−1 HCl
solution containing 5% acetone in the presence and absence of inhibitor 1b.
Inhibitor
Inhibitor
jcorr
(A/cm2) (V vs SCE) (V vs SCE)
OCP
Ecorr
a
c
concentration concentration
n(%)
(V/dec) (V/dec)
(mol L-1)
(ppm)
Blank
5.51
2.73
1.74
1.05
1.60
−0.444
−0.504
−0.499
−0.506
−0.499
−0.427
−0.493
−0.489
−0.503
−0.487
0.007 0.008
0.008 0.011
0.008 0.009
0.012 0.010
0.011 0.010
-
1×10−4
5×10−4
1×10−3
2×10−3
27
51%
68%
81%
71%
136
271
543
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Table 5. Chemical composition of ASTM 1020 carbon steel.
C
Mn
P, max
S, max
wt.%
0.17−0.24 0.3−0.6
0.04
0.05
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Figure 1. Proposed structures for thiourea derivatives. 1a: N-(pyrimidin-2-
ylcarbamothioyl)benzamide, 1b: N-(6-methylpyridin-2-ylcarbamothioyl)benzamide.
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Figure 2. Synthesis of compound 1. (a) Thionyl chloride, reflux, 2 h; (b) substituted anilines or
amines, acetone, reflux, 2 h.
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Figure 3. Langmuir adsorption isotherm of N-(6-methylpyridin-2-ylcarbamothioyl)benzamide
(1b) on the carbon steel surface in 1.0 mol L−1 HCl solution.
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Figure 4. Nyquist plots for carbon steel in 1.0 mol L−1 HCl solution containing 5% acetone in
the presence and absence of inhibitor 1b.
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Figure 5. Polarization curves for carbon steel in 1.0 mol L−1 HCl solution containing 5% acetone
in the presence and absence of inhibitor 1b.
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