Article pubs.acs.org/IECR
Chemical and Physical Interactions of 1-Benzoyl-3,3-Disubstituted Thiourea Derivatives on Mild Steel Surface: Corrosion Inhibition in Acidic Media Mayakrishnan Gopiraman,†,‡ Nagamani Selvakumaran,† Devarayan Kesavan,† Ick Soo Kim,*,‡ and Ramasamy Karvembu*,†,§ †
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India Nano Fusion Technology Research Laboratory, Interdisciplinary Graduate School of Science and Technology, Shinshu University, Ueda, Nagano 386 8567, Japan § Centre of Excellence in Corrosion and Surface Engineering, National Institute of Technology, Tiruchirappalli 620 025, India ‡
S Supporting Information *
ABSTRACT: 1-Benzoyl-3,3-diphenylthiourea (1), 1-benzoyl-3,3-dibenzylthiourea (2), 1-benzoyl-3,3-diethylthiourea (3), 1benzoyl-3,3-dibutylthiourea (4), 1-benzoyl-3,3-bis(2-methylpropyl)thiourea (5), and 1-benzoyl-3,3-bis(propan-2-yl)thiourea (6) were synthesized and investigated as inhibitors for the corrosion of the surface of mild steel in 1.0 M HCl by chemical and electrochemical measurements. The inhibition efficiencies obtained from different methods were in good agreement with each other. Inhibitor 2 showed a higher inhibition efficiency according to all of the methods. The Tafel polarization method revealed the mixed-mode inhibition of inhibitors with predominant control of the anodic reaction. At all studied temperatures, the adsorption of the inhibitor molecules onto the steel surface was found to follow the Langmuir adsorption isotherm. The values of the Gibbs free energy of adsorption strongly supported spontaneous chemical and/or physical adsorption of inhibitor molecules. The adsorption mechanism for inhibition was supported by ultraviolet−visible (UV−vis), Fourier transform infrared (FTIR), Raman, and scanning electron microscopy−energy-dispersive X-ray (SEM−EDS) spectroscopic methods, and adsorption isotherm measurements. The crystalline/amorphous nature of the inhibitors adsorbed onto the mild steel surface was indicated by wide-angle X-ray diffraction (WAXD) analysis.
1. INTRODUCTION Mild steel, one of the most commonly used metals in daily life, corrodes in many environments under a variety of circumstances. Certainly, the processes used to clean metal structures in industry mainly use acids and alkalis, resulting in deterioration of the metal. The extensive use of steel for commercial processes is attributed to its unique properties such as strength, ease of fabrication, and low cost. Therefore, the problem of steel corrosion, particularly in acidic media, cannot always be avoided simply by replacing steel with a more resistant metal or alloy. Among several types of inhibitors, the predominant use of chemical inhibitors in industry can be understood from the number of patents filed during the past century.1,2 Chemical inhibitors are promising materials for protecting metals from different kinds of corrosion. Since Speller et al.3 first reported the use of an organic inhibitor with hydrochloric acid for cleaning scaled water pipes, several organic molecules have been investigated for use against the acid corrosion of steel.4−6 The anticorrosive action of an organic inhibitor is due to the fact that it limits the cathodic or anodic reaction or both the cathodic and anodic reactions.7 Structure is one of the key factors in whether an inhibitor exhibits efficient corrosion inhibition process. The presence of “special” elements such as oxygen, nitrogen, sulfur, or phosphorus plays a crucial role during corrosion inhibition by organic compounds.7 © 2012 American Chemical Society
Thiourea and its derivatives have long been studied for use against the corrosion of a wide range of metals in various corrosion environments.8−14 Although several reports are available on the role of thiourea derivatives in the inhibition of the corrosion of mild steel, such data have still not been compiled to support a better understanding of the behavior of these inhibitors under different circumstances. Hence, we have investigated the corrosion inhibition properties of six Nbenzoylthiourea derivatives containing aryl or alkyl substituents using mild steel in acidic medium. Specifically, the investigated derivatives were 1-benzoyl-3,3-diphenylthiourea (1), 1-benzoyl3,3-dibenzylthiourea (2), 1-benzoyl-3,3-diethylthiourea (3), 1benzoyl-3,3-dibutylthiourea (4), 1-benzoyl-3,3-bis(2methylpropyl)thiourea (5), and 1-benzoyl-3,3-bis(propan-2yl)thiourea (6). To determine the usefulness of these potential inhibitors, the effects of substituents (side chains), and the mechanisms of inhibition at different temperatures, the inhibition characteristics of 1−6 were evaluated at temperatures ranging from 300 ± 1 to 330 ± 1 K. The electrochemical behavior of the inhibitors was examined to reveal the mode of protection of mild steel from corrosion. The observation of the film Received: Revised: Accepted: Published: 7910
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ization curves were recorded at a sweep rate of 0.5 mV s−1. Inhibition efficiency [ηp (%)] was calculated from corrosion current density values using the equation
formation on surface of mild steel specimens during the inhibition process led us to analyze scraped samples by means of ultraviolet−visible (UV−vis), Fourier transform infrared (FTIR), and Raman spectroscopies. Adsorption isotherms were plotted from weight loss measurements at different temperatures to determine the mechanism of inhibition. Thermodynamic parameters were calculated to explain the modes of interaction (physical and chemical) of thiourea derivatives on the mild steel surface at different temperatures. In addition, the inhibited and uninhibited mild steel specimens were subjected to surface analyses [scanning electron microscopy−energydispersive X-ray (SEM−EDS)] to study the changes on the surface of mild steel in the presence and absence of inhibitors. Wide-angle X-ray diffraction (WAXD) analysis was employed to reveal the crystalline or amorphous nature of the inhibitors.
ηp (%) =
θ=
(M 0 − M ) × 100 (M 0 )
η (%) 100
(3)
where I′corr and Icorr are the corrosion current densities in the absence and presence of an inhibitor, respectively. Electrochemical impedance measurements were carried out in the range from 100 kHz to 10 MHz at an amplitude of 10 mV.15 The electrical equivalent circuit for the system is shown
2. EXPERIMENTAL SECTION 2.1. Specimens and Electrolyte. The chemical composition of the mild steel specimens used in the present study was 0.10% C, 0.34% Mn, 0.08% P, and 99.34% Fe. Mass loss measurements were carried out on specimens with dimensions of 3 cm × 2 cm × 0.03 cm. For electrochemical studies, steel specimens with an exposed area of 1 cm2 were used as the working electrodes. Prior to mass loss studies, the mild steel specimens were abraded thoroughly with emery papers of 400− 1200 grades, cleaned with doubly distilled water, rinsed with acetone, and dried in air. AR-grade HCl (Merck) and doubly distilled water were used to prepare 1.0 M HCl solution for all experiments. 2.2. Mass Loss Methods. Mass loss experiments were performed at 300, 310, 320, and 330 ± 1 K with different concentrations of inhibitors. The optimized immersion time was found to be 2 h. The results of the mass loss experiments are reported as the means of three runs, each with a fresh mild steel specimen and 100 mL of fresh acid solution. The inhibition efficiency [ηm (%)] and surface coverage (θ) were calculated using the equations ηm (%) =
(I ′corr − Icorr) × 100 (I ′corr )
Figure 1. Equivalent circuit model for electrochemical impedance measurements. In the given electrical equivalent circuit, Rs is the solution resistance, Rct is the charge-transfer resistance, and Cdl is the double layer capacitance.
in Figure 1. Inhibition efficiency [ηi (%)] was calculated from the charge-transfer resistance values according to the equation ηi (%) =
(R ct − R′ct ) × 100 R ct
(4)
where Rct and R′ct are the charge-transfer resistances in the absence and presence of an inhibitor, respectively. 2.4. FTIR, UV−Visible, and Raman Spectroscopic Studies. To investigate the interactions between the inhibitor molecules and the mild steel surface, mild steel scraps were prepared according to the literature procedure.16 In a typical experiment, a mild steel specimen was coated with an inhibitor by immersion in a 100 ppm solution of inhibitor containing 1.0 M HCl for 2 h. Then, the specimen was removed and dried in vacuo for 48 h, after which the surface film of the dried mild steel specimen was scratched with a knife and the resultant powder (scraped sample) was used for spectroscopic analysis. FTIR spectra were recorded using a Perkin-Elmer FTIR spectrophotometer, UV−vis spectra were recorded using a PG Instruments model T90+ spectrophotometer in dimethylformamide (DMF) solvent, and Raman spectra were recorded using a Hololab 5000 instrument. Additionally, the melting points of the scraped samples were also determined using a MEL-TEMP II melting point apparatus. 2.5. Wide-Angle X-ray Diffraction (WAXD). To confirm the crystalline properties of the inhibitors, the scraped samples obtained as described in the preceding subsection were subjected to wide-angle X-ray diffraction (WAXD) analysis using a Rotaflex RTP300 X-ray diffractometer (Rigaku Co., Tokyo, Japan). The X-ray diffractometer was operated at 50 kV and 200 mA. Nickel-filtered Cu Kα radiation was used for the measurements with an angular range of 5° < 2θ < 75° at room temperature. 2.6. Surface Analysis. The test specimens were immersed in 100 mL of 1.0 M HCl with an inhibitor concentration of 100 ppm for 2 h. After this time, the specimens were dried, and
(1)
(2)
where M0 and M are the mass loss of the mild steel in the absence and presence of inhibitors, respectively. 2.3. Electrochemical Experiments. Electrochemical experiments were carried out using a traditional three-electrode cell. The working electrode was mild steel with an exposed area of 1.0 cm2. Pt and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. Electrochemical measurements were performed using 604B CH electrochemical workstation. The data for the electrochemical impedance spectra and polarization curves were fitted using the software provided with the 604B system. Prior to electrochemical measurements, the working electrode was abraded mechanically, washed with acetone, rinsed several times with doubly distilled water, dried, and finally immersed in acid solution for 2 h. All tests were performed with freshly abraded electrode in test solution under continuous stirring. The linear Tafel segments of the anodic and cathodic curves were extrapolated to the corrosion potential to obtain the corrosion current densities. The anodic and cathodic polar7911
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Table 1. IUPAC Names, Molecular Structures, Abbreviations, and Molecular Weights of the Inhibitors
UV−vis (DMF), nm: λ 291, 358. FTIR (KBr disk), cm−1: νN−H, 3190; νCO, 1698; νCS, 1279. 5: Elemental analysis for C16H24N2OS, calcd (found), %: C, 65.71 (65.73); H, 8.27 (8.30); N, 9.58 (9.58); S, 10.96 (10.95). UV−vis (DMF), nm: λ 283, 361. FTIR (KBr disk), cm−1: νN−H, 3274; νCO, 1693; νCS, 1267. 6: Elemental analysis for C14H20N2OS, calcd (found), %: C, 63.60 (63.62); H, 7.62 (7.63); N, 10.60 (10.59); S, 12.13 (12.14). UV−vis (DMF), nm: λ 282, 418. FTIR (KBr disk), cm−1: νN−H, 3261; νCO, 1689; νCS, 1224. 3.2. Mass Loss Methods. 3.2.1. Effect of Inhibitor Concentration. Calculated values of inhibition efficiency [ηm (%)], surface coverage (θ), standard deviation (σ), and weight loss obtained from gravimetric measurements with the addition of different concentrations (0, 25, 50, 75, and 100 ppm) of 1 after 2 h of immersion in 1.0 M HCl solution at 300 K are reported in Table 2. The same parameters calculated for the other inhibitors (2−6) are summarized in the Supporting Information (Tables S1−S5). The values of inhibition efficiency and surface coverage increased with increasing concentration of thiourea derivative (1−6), which might be because increasing the concentration of inhibitor increased the availability of electron donors (O, S, and N) and aromatic rings. This finding supports the hypothesis that the mild steel surface can be covered effectively by inhibitor molecules at 100 ppm. On the other hand, increasing the inhibitor concentration decreased the weight loss. This is because, in acidic solutions, during the anodic process of corrosion, metal ions pass from the metal
their surface morphology was examined using a Hitachi-3000H scanning electron microscope.
3. RESULTS AND DISCUSSION 3.1. Inhibitors. The syntheses and X-ray crystal structures of four of the desired thiourea derivatives (2 and 4−6) were reported by one of our group members,17−20 and those of the other two thiourea derivatives (1 and 3) were reported elsewhere.21,22 The IUPAC names, chemical structures, abbreviations, and molecular weights of the thiourea derivatives used in this investigation are listed in Table 1. The purities of the compounds were checked by thin-layer chromatography (TLC), and the structures were confirmed by analytical and spectroscopic (UV−vis and FTIR) techniques. 1: Elemental analysis for C20H14N2OS, calcd (found), %: C, 73.30 (73.28); H, 5.59 (5.61); N, 7.77 (7.78); S, 8.90 (8.91). UV−vis (DMF), nm: λ 294, 381. FTIR (KBr disk), cm−1: νN−H, 3229; νCO, 1697; νCS, 1282. 2: Elemental analysis for C22H20N2OS, calcd (found), %: C, 72.26 (72.27); H, 4.85 (4.89); N, 8.43 (8.44); S, 4.81 (4.82). UV−vis (DMF), nm: λ 290, 362. FTIR (KBr disk), cm−1: νN−H, 3239; νCO, 1697; νCS, 1314. 3: Elemental analysis for C12H16N2OS, calcd (found), %: C, 60.99 (61.01); H, 6.82 (6.83); N, 11.85 (11.86); S, 13.57 (13.57). UV−vis (DMF), nm: λ 291, 356. FTIR (KBr disk), cm−1: νN−H, 3241; νCO, 1691; νCS, 1298. 4: Elemental analysis for C16H24N2OS, calcd (found), %: C, 65.71 (65.73); H, 8.27 (8.30); N, 9.58 (9.58); S, 10.96 (10.97). 7912
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decrease with increasing temperature. The decrease in inhibition efficiency up to the temperature of 320 ± 1 K reveals that the physical forces between the thiourea derivatives and the metal surface disappeared at elevated temperature, which suggests that the inhibitors are physically adsorbed on the mild steel surface.25,26 Hence, the thiourea derivatives exhibited maximum inhibition efficiency at 300 ± 1 K. The inhibition efficiency of all the tested inhibitors showed no significant difference when the temperature was increased from 320 ± 1 to 330 ± 1 K, which suggests that the inhibitors protect the metal surface effectively even at higher temperatures (320 and 330 K). Additionally, the values of mass loss and surface coverage favored the corrosion inhibition nature of the thiourea derivatives. 3.3. Electrochemical Impedance Measurements. Electrochemical impedance measurements were carried out to calculate the inhibition efficiencies of thiourea derivatives 1−6 against the acid corrosion of mild steel and also to compare the inhibition capabilities of the thiourea derivatives at different concentrations (0, 25, 50, 75, and 100 ppm) at 300 K. Figure 2 shows Nyquist plots for mild steel in 1.0 M HCl in the presence and absence of the thiourea derivatives, and the calculated values are presented in Table 3. The results show that the thiourea derivatives are effective inhibitors of the corrosion of mild steel in 1.0 M HCl even at low concentrations. As shown in Figure 2, the increase in the diameter of the Nyquist semicircles with increasing concentrations of thiourea derivatives suggests that the presence of the inhibitor molecules greatly but gradually influences the corrosion kinetics on the electrode surface.27 Moreover, the figure shows a depressed capacitive loop at the high-frequency range. This can be attributed to the charge-transfer reaction, the electric double layer, and the surface inhomogeneity of structural or interfacial origin, such as those found in adsorption processes.28 As can be seen from Table 3, the value of the charge-transfer resistance (Rct) increased with increasing inhibitor concentration, indicating considerable surface coverage by the inhibitor molecules through strong bonding to the surface.29,30 Moreover, the value of Cdl decreased with increasing inhibitor concentration, which is probably due to a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting strong adsorption of the inhibitors onto the surface of steel.31 In addition, Bode plots were prepared using the same experimental data as in the Nyquist format (see Figure S1 in the Supporting Information). A new phase angle shift in the higher-frequency range and a continuous increase in the phase angle shift with increasing concentration of inhibitors were observed. This phase angle shift resulted from the formation of an inhibitor film, which changed the electrode interfacial structure. The continuous increase in the phase angle shift is obviously correlated with the progress of surface coverage by the inhibitor molecules.27 The impedance results revealed that the inhibition efficiencies of the thiourea derivatives were different from each other. The maximum inhibition efficiency of 96.5% was exhibited by 2, and the minimum inhibition efficiency of 83.5% was exhibited by 3. The inhibition efficiency was found to follow the order 2 > 1 > 3 > 4 > 6 > 5. 3.4. Tafel Polarization Technique. The potentiodynamic polarization behavior of mild steel in 1.0 M HCl in the presence and absence of thiourea derivatives is shown in Figure 3. Various corrosion parameters such as corrosion potential
Table 2. Weight Losses, Inhibition Efficiencies, Surface Coverages, and Standard Deviations for Various Concentrations of 1 (Inhibitor) for the Corrosion of Mild Steel in 1.0 M HCl from Weight Loss Measurements at Different Temperatures concentration of inhibitor (ppm) blank 25 50 75 100 blank 25 50 75 100 blank 25 50 75 100 blank 25 50 75 100
weight loss (mg/h)
Temperature = 300 ± 38.79 12.77 8.22 6.16 2.67 Temperature = 310 ± 43.85 15.55 11.79 8.91 5.06 Temperature = 320 ± 49.78 18.30 13.64 10.39 6.71 Temperature = 330 ± 58.49 19.96 14.96 11.78 7.91
ηm (%)
θ
σ
− 67.1 78.0 84.1 93.1
− 0.67 0.78 0.84 0.93
− 0.09 0.04 0.06 0.07
− 64.5 73.1 79.7 88.5
− 0.65 0.73 0.80 0.88
− 0.02 0.05 0.08 0.06
− 63.2 72.6 79.1 86.5
− 0.63 0.73 0.79 0.87
0.04 0.06 0.02 0.07
− 66.0 74.7 79.9 86.8
− 0.66 0.75 0.80 0.87
− 0.04 0.06 0.07 0.04
1K
1K
1K
1K
surface into the solution, and cathodic corrosion results in the discharge of hydrogen ions to produce hydrogen gas or reduction of oxygen.23 However, in the presence of inhibitor, inhibitor molecules could affect either or both the anodic and cathodic processes. Thiourea derivatives 1−6 showed maximum inhibition efficiencies of 93.1%, 96.0%, 81.5%, 90.0%, 91.3%, and 86.7%, respectively, at 100 ppm. Inhibitor 2 provided the highest inhibition efficiency, whereas inhibitor 3 provided the lowest inhibition efficiency. The variation in the inhibition efficiency might be due to the substituents and molecular sizes of the inhibitors.23 The lone pairs of electrons on sulfur and oxygen are responsible for the coordination type of adsorption of inhibitors onto the mild steel surface (chemisorption), and aromatic rings are responsible for weak physical forces between inhibitors and the mild steel surface (physisorption).24 Accordingly, for 2, the higher inhibition efficiency was achieved because of the presence of additional aromatic rings. Although 1 and 2 contain the same number of aromatic rings, 2 exhibited a higher inhibition efficiency, which might be due to the higher molecular mass of 2 (360.46) compared to 1 (332.41). In addition, the protonated inhibitor species might also assist the physical interaction of the inhibitors with Cl−ads ions that are adsorbed on the mild steel surface.25 3.2.2. Effect of Temperature. The values of inhibition efficiency [ηm (%)], surface coverage (θ), and weight loss obtained from mass loss measurements at various temperatures (310−330 K) are summarized in Table 2 and Tables S1−S5 of the Supporting Information. The corrosion inhibition efficiency of thiourea derivatives on the mild steel surface was found to 7913
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Figure 2. Nyquist plots of EIS measurements of mild steel in 1.0 M HCl at 300 ± 1 K for different concentrations of thiourea derivatives.
that the adsorbed thiourea derivatives might form a surface film that acts as a physical barrier to control the diffusion of ions from the mild steel surface and consequently retard the corrosion process. The maximum inhibition efficiency of 96.5% was exhibited by 2, and the minimum inhibition efficiency of 83.9% was exhibited by 3, which correlates well with the mass loss and electrochemical impedance measurements. The results from the electrochemical measurements suggest that the inhibitors inhibit mild steel in 1.0 M HCl mainly by forming an adsorbed layer at the mild steel/solution interface through both chemical and physical interactions. Mainly three kinds of adsorption/interaction are possible between thiourea derivatives and the mild steel/solution interface: (i) chargetransfer-type interactions between unshared electron pairs (present in oxygen, sulfur, and nitrogen) or π electrons and the vacant, low-energy d orbitals of Fe surface atoms (chemisorption at anodic sites); (ii) electrostatic interactions between aromatic rings and the mild steel surface/solution
(Ecorr), corrosion current density (Icorr), anodic Tafel slope (ba), cathodic Tafel slope (bc), surface coverage (θ), and inhibition efficiency [ηp (%)] are reported in Table 4. The decrease of the Icorr values with increasing inhibitor concentration indicates the inhibiting nature of the thiourea derivatives. As can be seen from Figure 3 and Table 4, the values of ba and bc exhibited no significant changes, suggesting that the thiourea derivatives are mixed-type inhibitors and inhibit corrosion by blocking the active sites of the metal.32 On the other hand, according to Riggs,33 if the displacement in corrosion potential is more than ±85 mV with respect to the corrosion potential of the blank, the inhibitor can be considered as a distinctive cathodic or anodic inhibitor.33,34 In the present study, even though a shift toward the positive side was observed, the average displacement was not more than 85 mV, which suggests mixed-type behavior of the inhibitors.35−37 These observations suggest that the thiourea derivatives inhibit both the anodic and cathodic reactions and mainly inhibit the anodic reaction, which suggests 7914
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of adsorption (ΔHads), and entropies of adsorption (ΔSads) for the six inhibitors are presented in Table 5. These values were found from weight loss, Tafel, and EIS data. Many adsorption isotherms were attempted, but the best-fit straight line was obtained for a plot of C/θ (mol/L) vs C (mol/L) with a slope of around unity. The correlation coefficient (r2) was used to choose the isotherm type that best fit the experimental data. These plots suggest that the adsorption of thiourea derivatives on a metal surface follows the Langmuir adsorption isotherm (Figure 4), which can be represented by the equation
Table 3. EIS Parameters for the Corrosion of Mild Steel in 1.0 M HCl in the Presence and Absence of Thiourea Derivatives at 300 K concentration of inhibitor (ppm)
Rct (Ω m2)
Cdl (μF cm−2)
blank
11.94 Inhibitor 1 36.31 58.09 82.44 184.91 Inhibitor 2 80.61 150.17 235.44 348.79 Inhibitor 3 24.58 37.19 38.97 72.28 Inhibitor 4 30.02 71.89 78.61 121.02 Inhibitor 5 57.59 90.27 147.31 158.77 Inhibitor 6 37.59 57.66 82.86 100.19
261.08
25 50 75 100 25 50 75 100 25 50 75 100 25 50 75 100 25 50 75 100 25 50 75 100
ηi (%)
θ
−
−
28.88 11.81 5.97 1.16
67.1 79.5 85.5 93.5
0.67 0.80 0.86 0.94
5.31 1.73 0.79 0.34
85.2 92.1 94.9 96.6
0.86 0.92 0.95 0.97
64.10 31.53 25.68 8.17
51.4 67.9 69.4 83.5
0.51 0.68 0.69 0.84
43.37 8.76 6.73 2.63
60.0 83.4 84.8 90.1
0.60 0.83 0.85 0.90
13.79 4.95 1.94 1.61
79.3 86.8 91.9 92.5
0.79 0.87 0.92 0.93
28.37 11.89 5.76 3.71
68.2 79.3 85.6 88.1
0.68 0.79 0.86 0.88
C inh 1 = + C inh θ K ads
where Kads is the adsorption constant, Cinh is the concentration of inhibitor, and θ is the surface coverage. Once Kads is known, the free energy of adsorption (ΔGads) can be calculated as ΔGads = −RT ln(55.5K ads)
(7)
where T is the temperature and the constant value of 55.5 represents the concentration of water in solution in mol/dm3. The negative values for the Gibbs free energies of adsorption (Table 5) reveal the spontaneity of the adsorption process and the stability of the adsorbed layer on the mild steel surface,40 and the fact that the values increased in magnitude with temperature suggests that the inhibition process is exothermic in nature. According to Noor,41 if |ΔGads| ≤ 20 kJ mol−1, the interaction between the inhibitor and the charged metal surface is electrostatic in nature (physisorption), whereas if |ΔGads| ≥ 40 kJ mol−1, the interaction is of the charge-transfer type (chemisorption). In our study, ΔGads was around −34 kJ mol−1 for all six thiourea derivatives at 300 K and increased in magnitude (i.e., became more negative) with increasing temperature. This reveals that the adsorption of the thiourea derivatives on the metal surface occurred by both electrostatic (π electrons of aromatic rings) and charge-transfer (CS, C O, and −NH) processes at 300 K. As the temperature increased to 320 K, the inhibition efficiency of the thiourea derivatives decreased, which might be due to the disappearance of electrostatic interaction between the metal surface and the π electrons of the inhibitor molecules. However, at 330 K, the absolute value of ΔGads became greater than 40 kJ mol−1, which indicates that the coordination was through a charge-transfer process between the donor atoms (S, O, and N) of the thiourea derivatives and the mild steel surface. Other thermodynamic functions (ΔHads and ΔSads) can also be calculated from the relation
interface or between protonated inhibitor species and adsorbed Cl−ads ions (physisorption at cathodic sites); and (iii) a combination of the two (mixed type).38 The electrochemical measurements are well-correlated with the mass loss results. Furthermore, layer formation on mild steel by inhibitors is supported by spectroscopic and surface analyses, as well as adsorption isotherms. 3.5. Adsorption Isotherms. Precise elucidation of the mechanism of adsorption of an inhibitor on a metal surface is of fundamental importance for the design and development of corrosion inhibitors. The elucidation of the adsorption isotherm involves mainly the fitting of the data with various isotherms such as the Langmuir, Frumkin, Temkin, and Freundlich isotherms. Quasilattice models consider the surface of the metal in aqueous solution to be covered with water dipoles, and for the adsorption of organic molecules to occur, these water dipoles must be replaced by organic molecules as describe in the equation39
ΔGads = ΔHads − T ΔSads
(8)
where ΔHads and ΔSads are the enthalpy and entropy of adsorption, respectively. The calculated values of the free energy of adsorption (ΔGads) for each inhibitor were plotted as a function of temperature, and straight lines were obtained, as shown in Figure 5. The values of ΔHads and ΔSads were then determined from the intercepts and slopes of these plots and are provided in Table 5. The negative values of ΔHads reflect the exothermic nature of the adsorption process on mild steel. The negative values for the entropy of activation are in line with an associatively activated process.42 3.6. FTIR Spectra. To study the types of interactions (chemical and physical) of the thiourea derivatives with the mild steel surface in 1.0 M HCl, FTIR spectra were recorded
nH 2Omild steel + inhibitorsolution → inhibitormild steel + nH 2Osolution
(6)
(5)
where n is the number of water molecules removed from the metal surface for each molecule of inhibitor adsorbed. The correlation coefficients (r2), slopes, Gibbs free energies of adsorption (ΔGads), adsorption constants (Kads), enthalpies 7915
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Figure 3. Tafel polarization curves of mild steel in 1.0 M HCl at 300 ± 1 K for different concentrations of thiourea derivatives.
thiourea derivatives and the thiourea derivatives adsorbed on the mild steel surface, but it provided the same results. 3.7. UV−Visible Spectra. To study the type of interactions of the thiourea derivatives with the mild steel surface in 1.0 M HCl, the UV−visible spectra were recorded for inhibitors 1−6 and for the inhibitors adsorbed on the mild steel surface (Fe1−6). The inhibitors showed two main absorption bands around 280−300 and 340−420 nm, which can be assigned to π → π* and n → π* transitions, respectively (Figure 7). In the case of the inhibitors adsorbed on the mild steel surface, the bands around 340−420 nm (n → π*) completely disappeared, and a small downward shift was observed in the band around 280−300 nm. In addition, the d → d transition was observed for the inhibitors adsorbed on the mild steel surface in the range of 603−621 nm, which clearly supports the formation of a chemisorbed film by the thiourea derivatives (most probably due to O and S) on the iron surface.31 3.8. Raman Spectra. To confirm the coordination type of bond formation and the physical interactions of the thiourea derivatives with the mild steel surface, Raman spectra were recorded for the inhibitors and the inhibitors adsorbed on the
for inhibitors 1−6 and for the inhibitors adsorbed on mild steel or on scraped samples. Figure 6 shows that all of the characteristic bands corresponding to the functional groups (CS, CO, and N−H) were present in the FTIR spectra of the thiourea derivatives. The FTIR spectra of inhibitors adsorbed on the mild steel surface in 1.0 M HCl were compared with those of the free inhibitors. The stretching frequency due to CO in thiourea derivatives was shifted to lower frequency in the thiourea derivatives adsorbed onto the mild steel surface, which reveals that the CO group is involved in coordination-type bond formation with the mild steel surface.43,44 The characteristic peak due to CS disappeared in the thiourea derivatives adsorbed on the mild steel surface, which indicates that CS also bonds with the mild steel surface through a coordination mechanism. Moreover, broadening of the N−H and C−H peaks (Figure 6) reveals that the thiourea derivatives undergo chemical and/or physical interactions with the mild steel surface.45 Nevertheless, there are many broad bands in the FTIR spectra of the scraped samples, which provide no significant information. Hence, an attenuated-total-reflection (ATR) study was carried out for the 7916
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Table 4. Tafel Polarization Parameter Values for the Corrosion of Mild Steel in 1.0 M HCl in the Presence and Absence of Thiourea Derivatives at 300 K concentration of inhibitor (ppm)
Icorr (μA cm−2)
blank
773.35
25 50 75 100
246.04 155.39 101.97 47.84
25 50 75 100
101.80 59.99 48.55 26.89
25 50 75 100
367.89 246.34 231.90 124.92
25 50 75 100
305.71 115.48 109.65 67.43
25 50 75 100
158.49 102.59 59.99 53.68
25 50 75 100
246.68 158.97 101.84 80.15
Ecorr (mV vs SCE) −0.521 Inhibitor 1 −0.519 −0.518 −0.515 −0.461 Inhibitor 2 −0.515 −0.511 −0.518 −0.506 Inhibitor 3 −0.501 −0.494 −0.489 −0.498 Inhibitor 4 −0.424 −0.441 −0.439 −0.517 Inhibitor 5 −0.491 −0.525 −0.446 −0.509 Inhibitor 6 −0.509 −0.522 −0.515 −0.517
mild steel surface (Figure 8). In the case of the thiourea derivatives, predominant vibrational modes were observed around 400−1100 cm−l, in close agreement with Edsall’s report.46 The peaks in this range changed dramatically in the thiourea derivatives adsorbed on the mild steel surface, which reveals that the inhibitor molecules were chemisorbed on the mild steel.47 In addition, the peaks at 1690 and 1260 cm−1 due to the CO and CS groups, respectively, of the thiourea derivatives shifted dramatically or disappeared in the inhibitors adsorbed on mild steel, which reveals that the CO and CS groups were bonded to the mild steel surface. Furthermore, the thiourea derivatives showed strong Raman bands at 1460 and 1005 cm−1 and weak bands at 580 and 515 cm−1, which were assigned to asymmetrical and symmetrical C−N−C stretching vibrations, respectively.48 However, the peaks at 1460 and 1005 cm−1 due to the C−N−C groups in the inhibitors were dramatically changed in the scraped samples, which indicates that the C−N−C groups were adsorbed on the mild steel surface. Moreover, the shift in the peak due to CC (1500 cm−1) supports physical interactions between the phenyl rings and the mild steel surface. 3.9. Wide-Angle X-ray Diffraction (WAXD) Study. The thiourea derivatives and the thiourea derivatives adsorbed on the mild steel surface were scanned in the range of 5° < 2θ < 75° at a wavelength of 1.543 Å. Figure 9 shows the very high relative intensities of the peaks in the 5−30° range for the thiourea derivatives, suggesting a crystalline nature. For the
ηp (%)
θ
bc (mV/decade)
ba (mV/decade)
6.030
5.455
−
−
6.376 6.656 6.871 7.187
6.091 6.553 6.971 9.526
68.2 79.1 86.8 93.8
0.68 0.80 0.87 0.94
7.426 7.613 6.646 7.742
7.932 8.264 7.297 9.810
86.8 92.2 93.7 96.5
0.87 0.92 0.94 0.97
6.478 7.268 7.503 7.042
5.880 6.613 7.309 6.342
52.4 68.2 70.0 83.9
0.52 0.68 0.70 0.84
8.464 8.349 8.649 7.881
1.010 1.006 1.075 8.048
60.5 85.1 85.8 91.3
0.61 0.85 0.86 0.91
5.807 6.395 7.120 7.883
7.445 6.643 7.594 8.321
79.5 86.7 92.3 93.1
0.80 0.87 0.92 0.93
7.646 6.333 6.871 7.881
7.483 6.606 6.971 8.048
68.1 79.5 86.9 89.7
0.68 0.79 0.87 0.90
inhibitors adsorbed on mild steel, however, broad peaks were observed at 5−30°, supporting the formation of a chemisorbed film on the mild steel surface and the resulting amorphous nature of the inhibitors.49−51 3.10. Physical Appearance. The colors and physical appearances of the inhibitors and the inhibitors adsorbed on mild steel (see Figure S2 and Table S6 in the Supporting Information) were also compared. The change in color during the inhibition process supports the formation of a complex between the inhibitor molecules and the mild steel surface. Furthermore, the melting points of the scraped samples (see Table S6 in the Supporting Information) were relatively high, indicating that the inhibitor molecules might strongly hold up on the mild steel surface. 3.11. Surface Analysis. To confirm the interactions of the thiourea derivatives with the metal surface, SEM images and corresponding EDS spectra were recorded for the mild steel surface in 1.0 M HCl solution in the absence and presence of the thiourea derivatives. The SEM image of the mild steel surface (see Figure S3 in the Supporting Information) indicates the abraded characteristic surface and shows some scratches that arose during polishing. The SEM image of the mild steel surface after immersion in 1.0 M HCl for 2 h (see Figure S3 in the Supporting Information and Table 6) reveals that the surface was severely corroded because of the aggressive attack by 1.0 M HCl. However, the SEM images of the mild steel surface immersed for the same period of time in 1.0 M HCl 7917
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Table 5. Thermodynamic Parameters for the Adsorption of Thiourea Derivatives in 1.0 M HCl on the Mild Steel at Different Temperatures method
T (K)
r2
slope
weight loss
300 310 320 330 300 300
0.9965 0.9953 0.9972 0.9979 0.9985 0.9979
1.056 1.062 1.069 1.167 1.298 1.189
300 310 320 330 300 300
0.9999 0.9998 0.9999 0.9999 0.9999 0.9998
1.054 1.129 1.050 1.051 0.989 1.008
300 310 320 330 300 300
0.9998 0.9994 0.9986 0.9985 0.9961 0.9957
0.966 1.027 1.036 1.040 0.968 0.994
300 310 320 330 300 300
0.9965 0.9986 0.9988 0.9987 0.9969 0.9961
0.943 0.993 1.009 1.038 0.946 0.933
300 310 320 330 300 300
0.9996 0.9991 0.9941 0.9995 0.9998 0.9998
1.026 1.057 1.081 1.092 1.015 1.006
300 310 320 330 300 300
0.9999 0.9999 0.9999 0.9999 0.9999 0.9997
1.041 1.103 1.115 1.113 1.021 0.992
Tafel EIS weight loss
Tafel EIS weight loss
Tafel EIS weight loss
Tafel EIS weight loss
Tafel EIS weight loss
Tafel EIS
ΔGads (kJ/mol) Inhibitor −31.83 −36.57 −38.36 −46.84 −42.71 −36.54 Inhibitor −35.77 −36.50 −41.28 −46.39 −39.58 −46.70 Inhibitor −38.90 −37.37 −44.73 −52.32 −36.39 −42.21 Inhibitor −37.37 −38.33 −39.62 −41.32 −37.54 −37.57 Inhibitor −34.72 −34.89 −35.89 −37.41 −34.14 −34.05 Inhibitor −38.45 −39.70 −40.89 −42.36 −38.41 −38.19
Kads (kJ/mol)
ΔHads (kJ/mol)
ΔSads (J K−1 mol−1)
−109.08
−28.44
−75.43
−23.35
−106.67
−49.38
−22.31
−3.69
−71.57
−6.76
−3.48
−1.33
1 17667 26194 32944 37304 49218 41578 2 30520 25525 27793 39748 21120 24455 3 37416 35774 36137 34658 39149 40427 4 57920 51801 52999 62562 61892 62637 5 15103 12768 12992 14992 15903 15293 6 89349 88329 85142 91539 87730 80659
for the mild steel with 1.0 M HCl solution (see Figure S3 in the Supporting Information), a high chloride content was observed because of aggressive attack of 1.0 M HCl. The EDS spectra of mild steel immersed in 0.1 M HCl containing 100 ppm of inhibitor (1−6) indicate the presence of low chloride contents compared to that of the mild steel specimen in 1.0 M HCl. This clearly reveals that the inhibitors protected the mild steel surface from 1.0 M HCl solution.53 However, very low chloride contents on the mild steel surface in the presence of inhibitors (Table 6) might be due to the formation of corrosion product (FeCl2·nH2O) on the mild steel surface. Furthermore, the EDS spectrum of the mild steel surface (see Figure S3 in the Supporting Information) indicates no oxygen content, whereas the mild steel surface after immersion in 1.0 M HCl showed an oxygen content of 15.83% (Table 6), which revealed severe corrosion of the mild steel in 1.0 M HCl solution. On the other
solutions containing 100 ppm of one of the inhibitors (see Figure S3 in the Supporting Information) revealed the formation of a protective film by the inhibitors on the mild steel surface, which inhibited the corrosion significantly in acidic medium.52 Furthermore, even though the mild steel surface was covered by inhibitors, the SEM images of the mild steel specimens immersed in the inhibitor-containing solutions still showed the scratches formed during metal polishing. The imperfect covering of the mild steel surface by the thiourea derivatives might be due to the formation of corrosion products (FeO·nH2O and/or FeCl2·nH2O). EDS spectra were recorded (Table 6) mainly to determine the percentages of chloride and oxygen present in the mild steel surface in the presence and absence of thiourea derivatives. The EDS spectrum of mild steel revealed that the surface was chlorine-free in the absence of 1.0 M HCl solution; however, 7918
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Figure 4. Fitting of the corrosion data to the Langmuir adsorption isotherm at different temperatures.
hand, the EDS spectrum of the mild steel specimens immersed for 2 h in 1.0 M HCl solutions containing 100 ppm of inhibitor showed relatively low oxygen contents when compared to the
Figure 6. FTIR spectra of free and adsorbed thiourea derivatives. Figure 5. Gibbs free energy of adsorption of corrosion of mild steel in presence of thiourea inhibitors at different temperatures. 7919
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Figure 7. UV−visible spectra of free and adsorbed thiourea derivatives.
Figure 8. Raman spectra of free and adsorbed thiourea derivatives.
Figure 9. XRD patterns of free and adsorbed thiourea derivatives.
blank, which indicates the formation of an adsorption layer by the inhibitors on the mild steel, which effectively protected the metal from corrosion.53
UV−visible, and Raman spectra provides evidence for the chemisorption mechanism, and the effect of temperature provides evidence for the physisorption mechanism. The crystalline/amorphous natures of the inhibitors and the inhibitors adsorbed on the mild steel surface were investigated by WAXD. Additionally, SEM images provided information on protective film formation on the mild steel surface. Moreover, EDS spectra of mild steel in 1.0 M HCl in the presence and absence of inhibitors were recorded and showed that the mild steel corroded in 1.0 M HCl media had high chlorine and oxygen concentrations compared to those of the mild steel surfaces in the presence of inhibitors. In conclusion, 1-benzoyl3,3-disubstituted thiourea inhibitors are cost-effective and environmentally friendly materials for inhibiting the acid corrosion of mild steel.
4. CONCLUSIONS Thiourea derivatives 1−6 act as effective corrosion inhibitors for mild steel in 1.0 M HCl. The inhibition efficiencies of 1−6 increased with increasing concentration of inhibitor. Elevation of the temperature decreased the inhibition efficiency. The inhibition efficiency was found to follow the order 2 > 1 > 3 > 4 > 6 > 5. Differences in the inhibition efficiencies of the thiourea derivatives were correlated with the aromatic and aliphatic substituents of the inhibitors. The electrochemical polarization method revealed the mixed mode of inhibition of the inhibitors with predominant control of the anodic reaction. The adsorption of the inhibitor molecules on the mild steel surface was found to obey the Langmuir adsorption isotherm. Thermodynamic parameters revealed that the inhibitor molecules were adsorbed on the mild steel surface through both physisorption and chemisorption mechanisms. FTIR, 7920
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Table 6. EDS Analysis of Mild Steel and Mild Steel in 1.0 M HCl in the Absence and Presence of Thiourea Derivatives composition (%) medium
■
mild steel blank 1 in 1.0 M 2 in 1.0 M 3 in 1.0 M 4 in 1.0 M 5 in 1.0 M 6 in 1.0 M
HCl HCl HCl HCl HCl HCl
Fe
O
C
Cl
Mn
Si
S
99.31 76.73 89.03 90.22 87.98 89.39 89.60 90.71
− 15.83 7.06 5.40 7.20 5.95 5.51 4.59
0.16 − 2.96 3.41 3.55 3.12 3.71 3.45
− 7.12 0.29 0.21 0.46 0.42 0.51 0.21
0.34 0.32 0.20 0.22 0.30 0.22 0.19 0.24
0.19 − 0.16 0.21 0.24 0.39 0.14 0.21
− − 0.32 0.43 0.47 0.51 0.34 0.59
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ASSOCIATED CONTENT
S Supporting Information *
Bode plots, optical images, and SEM−EDS images. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +91 431 2503636. Fax: +91 0431 2500133. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Global COE program by the Ministry of Education, Culture, Sports, Science and Technology, Japan.
■
REFERENCES
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