Synthesis of Thiourea Derivatives and Its Evaluation

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Apr 12, 2015 - To cite this article: Clara G. M. de Oliveira, Vitor W. Faria, Gabriel F. de ... Cotrim, Gabriel O. Resende & Flávia C. de Souza (2015): Synthesis of ...
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Phosphorus, Sulfur, and Silicon and the Related Elements Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpss20

Synthesis of Thiourea Derivatives and Its Evaluation as Corrosion Inhibitor for Carbon Steel a

a

b

c

Clara G. M. de Oliveira , Vitor W. Faria , Gabriel F. de Andrade , Eliane D'Elia , Murilo F. d

b

b

a

Cabral , Bruno A. Cotrim , Gabriel O. Resende & Flávia C. de Souza 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

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Instituto de Química, UFRJ, Centro de Tecnologia - Cidade Universitária, 21941-909, Rio de Janeiro, RJ, Brazil 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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ACCEPTED MANUSCRIPT 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,* 1

Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus São Gonçalo,

24425-005, São Gonçalo, RJ, Brazil Downloaded by [University of Sherbrooke] at 15:32 12 April 2015

2

Instituto Federal de Educação Ciência e Tecnologia do Rio de Janeiro - Campus Rio de Janeiro,

20270-021, Rio de Janeiro, RJ, Brazil 3

Instituto de Química, UFRJ, Centro de Tecnologia - Cidade Universitária, 21941-909, Rio de

Janeiro, RJ, Brazil 4

Divisão de Metrologia de Materiais (DIMAT), Instituto Nacional de Metrologia, Qualidade e

Tecnologia, 25250-020 Duque de Caxias, RJ, Brazil *

Correspondent author: [email protected]

Abstract The inhibitory effect of thiourea-based compounds was evaluated using carbon steel body specimens

in

hydrochloric

ylcarbamothioyl)benzamide

and

acid

media.

Thiourea

derivatives,

N-(pyrimidin-2-

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, G0ads values indicate the chemical

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ACCEPTED MANUSCRIPT 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

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inhibition efficiency of 81%.

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ACCEPTED MANUSCRIPT 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

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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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ACCEPTED MANUSCRIPT 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 backbonding between the sulfur and metal surface atoms to occur. Thiourea is an industrially and physiologically significant molecule. For example,

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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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ACCEPTED MANUSCRIPT 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

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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]:

 FeHads 



Fe  H

 FeHads 



(1)

 e   FeHads 

(2)

 FeHads   H  e  Fe  H2

(3)

Fe  Fe2  2e

(4)

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ACCEPTED MANUSCRIPT 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

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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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ACCEPTED MANUSCRIPT 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].

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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:

K

 G 0ads  1 exp   55.55  RT 

(Equation 5)

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ACCEPTED MANUSCRIPT 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 ΔG0ads value was −33.4 kJ mol−1. Generally, values of ΔG0ads up 20 kJ

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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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ACCEPTED MANUSCRIPT 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.

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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:

Cdl 

1

(Equation 6)

2 f max R ct

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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ACCEPTED MANUSCRIPT 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

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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 E corr 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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ACCEPTED MANUSCRIPT 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.

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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 MHz instrument. For the

1

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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ACCEPTED MANUSCRIPT 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

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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% yield). N-(pyrimidin-2-ylcarbamothioyl)benzamide (1a): 1H 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): 1

H 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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ACCEPTED MANUSCRIPT 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

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chromatography was carried out on aluminum sheets that were pre-coated with silica gel 60 (HF254, 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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ACCEPTED MANUSCRIPT 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:

n% 

W0  W 100 W0

(Equation 1)

where W0 and W are the corrosion rates (g cm−2 h−1) in the absence and presence of the corrosion inhibitor, respectively. Downloaded by [University of Sherbrooke] at 15:32 12 April 2015

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:

n% 

jcorr,0  jcorr jcorr,0

100

(Equation 2)

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.

n% 

R ct  R ct,0 R ct

100

(Equation 3)

where Rct is the charge-transfer resistance in the presence of an inhibitor and Rct,0 is the chargetransfer 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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ACCEPTED MANUSCRIPT 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

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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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3452. 17.

Yuce, A.O.; Kardas, G.; Corros. Sci., 2012, 58, 86-94.

18.

da Rocha, J.C.; Gomes, J.A.C.P.; D'Elia, E.; Corros. Sci., 2010, 52, 2341-2348.

19.

Umoren, S.A.; Obot, I.B.; Obi-Egbedi, N.O.; J. Mater. Sci., 2009, 44, 274-279.

20.

Torres, V.V.; Amado, R.S.; de Sá, C.F.; Fernandez, T.L.; Riehl, C.A.S.; Torres, A.G.; D’Elia, E.; Corros. Sci., 2011, 53, 2385-2392.

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ACCEPTED MANUSCRIPT 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 Concentration

Concentration

Wcorr

n

Concentration

Wcorr

n

(mol L−1)

(ppm)

(g cm−2 h−1)

(%)

(ppm)

(g cm−2 h−1)

(%)

0.0361

-

0.0349

-

Blank

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Inhibitor 1b

1.0×10−4

26

0.0096

73

27

0.0165

53

2.0×10−4

52

0.0067

82

54

0.0081

77

5.0×10−4

129

0.0046

87

136

0.0044

88

1.5×10−3

387

0.0030

92

406

0.0011

97

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ACCEPTED MANUSCRIPT Table 2. Values of the adsorption constants and ΔG0ads obtained for the corrosion inhibitor 1b. r

1.057

0.9955

K (L mol−1) ΔG0ads (kJ mol−1) 1.29×104

−33.4

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Slope

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ACCEPTED MANUSCRIPT Table 3. Values of frequency, charge-transfer resistance, and double-layer capacitance obtained for the tested corrosion inhibitor (1b). Inhibitor

Inhibitor

concentration

concentration

-1

(mol L )

(ppm)

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Blank

fmax

Rct

Cdl

n

(Hz)

cm2)

(F cm−2)

(%)

1.74

144

636

-

1×10−4

27

11.2

397

35.7

64%

5×10−4

136

11.2

580

24.4

75%

1×10−3

271

11.2

634

22.4

77%

2×10−3

543

11.2

741

19.1

81%

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ACCEPTED MANUSCRIPT 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

concentration concentration (mol L-1)

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−4

1×10 5×10−4 1×10−3 2×10−3

(ppm) Blank 27 136 271 543

jcorr

OCP

Ecorr

a

 c

(A/cm2) (V vs SCE) (V vs SCE) (V/dec) (V/dec) 5.51 2.73 1.74 1.05 1.60

−0.444 −0.504 −0.499 −0.506 −0.499

21

−0.427 −0.493 −0.489 −0.503 −0.487

0.007 0.008 0.008 0.012 0.011

0.008 0.011 0.009 0.010 0.010

n(%)

51% 68% 81% 71%

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ACCEPTED MANUSCRIPT Table 5. Chemical composition of ASTM 1020 carbon steel. C

0.17−0.24 0.3−0.6

P, max

S, max

0.04

0.05

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wt.%

Mn

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Figure

1.

Proposed

structures

for

thiourea

derivatives.

1a:

N-(pyrimidin-2-

ylcarbamothioyl)benzamide, 1b: N-(6-methylpyridin-2-ylcarbamothioyl)benzamide.

23

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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.

25

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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.

26

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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.

27

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