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Correlated ab Initio and Electroanalytical Study on Inhibition Behavior of 2‑Mercaptobenzothiazole and Its Thiole−Thione Tautomerism Effect for the Corrosion of Steel (API 5L X52) in Sulphuric Acid Solution Majid Gholami,† Iman Danaee,*,† Mohammad Hosein Maddahy,† and Mehdi RashvandAvei‡ †

Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran Department of Chemistry, K. N. Toosi University of Technology, Tehran, Iran

‡

S Supporting Information *

ABSTRACT: The inhibition effect of 2-mercaptobenzothiazole (MBT) on the corrosion of steel (API 5L X52) in 1 M H2SO4 was investigated by electrochemical and theoretical methods. The results of the investigation show that this compound has excellent inhibiting properties for steel corrosion in sulphuric acid and the inhibition efficiency increases with the increase in inhibitor concentration. The adsorption of the inhibitor on the surface of steel was found to obey a Langmuir adsorption isotherm. The effect of temperature on the corrosion behavior of steel without and with the inhibitors was studied, and the activation and thermodynamic parameters were calculated. Ab initio quantum chemical calculations at the density functional theory (DFT) level were performed to correlate electronic structure parameters of MBT and its different tautomers and conformers with its inhibition performance. The quantitative structure activity relationship (QSAR) approach was also used to correlate the quantum chemical parameters with the experimentally determined inhibition efficiencies.

1. INTRODUCTION Corrosion is a fundamental process playing an important role in economics and safety, particularly for metals and alloys. Steel has found wide application in a broad spectrum of industries and machinery.1 In most industrial processes, acidic solutions are commonly used for the pickling, industrial acid cleaning, acid descaling, oil well acidifying, etc.2,3 Unfortunately, iron and its alloys could corrode during these acidic applications particularly with the use of hydrochloric acid and sulphuric acid, which results in terrible waste of both resources and money.4 Using inhibitors is an important method for protecting materials against deterioration due to corrosion, especially in acidic media.1,5 The inhibitors are classified as anodic, cathodic, or mixed, depending on which reactions become inhibited during the corrosion process. A mixed inhibitor influences both the anodic and cathodic processes.6 The corrosion inhibition efficiency of organic compounds is related to their adsorption properties. Adsorption depends on the nature and the state of the metal surface, on the type of corrosive medium, and on the chemical structure of the inhibitor.7 Organic compounds bearing heteroatoms with high electron density such as phosphor, sulfur, nitrogen, oxygen, or those containing multiple bonds which are considered as adsorption centers are effective as corrosion inhibitor.8,9 The compounds contain both nitrogen and sulfur in their molecular structure have exhibited greater inhibition compared with those contain only one of these atoms.10 Some N-heterocyclic compounds containing N and S herteroatoms such as thiadiazole derivatives, thiazole derivatives, and benzothiazole were widely used in protection of steel in acid solutions.11−13 © 2013 American Chemical Society

With increasingly stringent environmental policies, it is very urgent to search for effective, safe, and environmentally friendly corrosion inhibitors. Traditionally, the large-scale and trial and error experimental methods such as the rotation specimen method and dynamic simulation for cooling water are mainly used to assess the performances of corrosion inhibitors.14 However, they are often expensive and time-consuming, and they cannot elucidate the inhibition mechanism. With the improvement of hardware and software in computational science, theoretical chemistry such as quantum chemical calculation and molecular dynamics simulation method has been used recently to explain the mechanism of corrosion inhibition.15−18 Since quantum chemistry calculation of the inhibition mechanism was started in 1971, researchers have focused on exploring the relationship between the corrosion inhibition efficiency and a number of quantum parameters, which will be helpful for obtaining a molecular design of newly effective corrosion inhibitors. In this regard, the treatment of mild steel corrosion in an acidic environment through organic compounds of low toxicity which do not contain heavy metals and organic phosphates has resulted in considerable savings to the oil and gas industry.19 Most of the efficient inhibitors used in industry are organic compounds which mainly contain oxygen, sulfur, and nitrogen atoms and multiple bonds in the molecule through which they are absorbed on metal surface.20 2-Mercaptobenzothiazole (in the following MBT), a N- and Scontaining heterocyclic compound, has been widely invesReceived: Revised: Accepted: Published: 14875

July 5, 2013 September 7, 2013 September 29, 2013 October 10, 2013 dx.doi.org/10.1021/ie402108g | Ind. Eng. Chem. Res. 2013, 52, 14875−14889

Industrial & Engineering Chemistry Research

Article

The electrochemical measurements were carried out using computer controlled Auto Lab potentiostat/galvanostat (PGSTAT 302N). The potentiodynamic polarization curves were scanned in a potential range −800 to −200 mV at a sweep rate of 1 mV s−1. All tests were carried out at constant temperatures by controlling the cell temperature using a water bath. The measurements were repeated three times for each condition to ensure the reliability and reproducibility of the data. Corrosion current Icorr were calculated from Tafel extrapolation methods. Electrochemical impedance spectroscopy (EIS) was done at OCP in the frequency range of 100 kHz10 mHz using a 10 mV peak-to-peak AC voltage excitation. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of homewritten least-squares software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance.25,26 The surface morphology of specimens after immersion in the acidic solutions with and without the optimal concentration of the inhibitor was performed using scanning electronic microscope SEM (Hitachi S4160). The energy dispersive X-ray analysis of steel surface was investigated using scanning electron microscopy (SEM, VEGA, TESCAN-LMU) equipped with an EDX probe. 2.3. Computational Procedure. Full geometry optimizations are accomplished in the gas phase without any symmetry constraints by means of the hybrid functional B3LYP27,28 and the 6-31+G* basis set, employing the Gaussian 98 code.29 The applied basis set is comprised of Pople’s well-known 6-31G* basis set30 and an extra plus due to the importance of diffuse functions.31,32 After geometry optimization, vibrational analysis is performed, and the resulting geometry is checked with respect to being true minima on the potential energy surface, as shown by the absence of imaginary frequencies.33 Statistical analyses are performed using the SPSS program version 15.0 for Windows. Nonlinear regression analyses are performed by unconstrained sum of squared residuals for loss function and estimation methods of Levenberg−Marquardt using the SPSS program version 15.0 for Windows.

tigated because this compound can form hydrophobic complexes with many metals such as iron, copper, cobalt, nickel, etc. and therefore is used as a corrosion inhibitor.21−23 Also, Feng et al.24 performed calculations of a self-assembled monolayer of MBT in order to investigate the adsorbate− surface interactions by computational method. The aim of the present work is to investigate the effects of different structures of MBT as alternative acid corrosion inhibitors for mild steel in 1 M H2SO4 solution using electrochemical techniques. Thermodynamic parameters such as enthalpy, entropy and Gibbs free energy were evaluated from experimental data. The relationships between the inhibition performances of the investigated inhibitor in 1 M H2SO4 and some quantum chemical parameters, such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), the energy gap between ELUMO and EHOMO (ΔELUMO−HOMO), dipole moments, molecular volume, electronegativity (χ), global chemical hardness (η), softness (S), the fraction of electrons transferred from the inhibitor to the iron surface (ΔN), and local reactivity analyzed by the condensed Fukui function have also been investigated by quantum chemical calculations. After all the calculations, quantitative structure activity relationship (QSAR) approach shall be used to validate the accuracy of the proposed theoretical methodology for evaluation of inhibition of this inhibitor.

2. EXPERIMENTAL DETAILS 2.1. Materials. The specimens used in electrochemical measurements were mechanically cut into 1 × 1 × 0.5 cm3, and inserted to polyester resin leaving only 1 cm2 of the surface area exposed to electrolyte. The electrical conductivity was provided by a copper wire. Tests were performed on a steel grade API X52 of the following chemical composition (wt %): C: 0.22, Si: 0.45, Mn: 1.4, P: 0.025, S: 0.015, and Fe (remainder). Before measurements, the surface of working electrode was mechanically abraded with 320, 400, 600, 800, 1000, and 1200 grades of emery paper, degreased with acetone, and rinsed by distilled water before each electrochemical experiment. The tests were performed in 1 M H2SO4 solution containing various concentrations of MBT, whose chemical structure is given in Figure 1. The concentration range of the inhibitor was

3. RESULTS AND DISCUSSION 3.1. Potentiodynamic Polarization Measurements. Potentiodynamic polarization curves for the steel electrode in 1 M H2SO4 solution in the absence and presence of various concentrations of MBT at 25 °C are shown in Figure 2. The values of related electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), cathodic and anodic Tafel slopes (βa and βc), polarization resistance (Rp), corrosion rate (CR) (mpy), and the degree of surface coverage (θ) were calculated from the related polarization curves and are given in Table 1. The degree of surface coverage, inhibition efficiency34 and corrosion rate35 were calculated using the equations:

Figure 1. Chemical structures of the studied organic compound.

varied from 1 × 10−4 to 1 × 10−3 M. All chemical materials were prepared from Merck. For each experiment, a freshly prepared solution was used. 2.2. Methods. Electrochemical measurements were carried out in a conventional three-electrode system. Ag/AgCl (3 M KCl) and platinum sheet were used as the reference electrode and counter electrode, respectively. The API X52 steel was the working electrode. The area of the working electrode exposed to the solution was 1 cm2. Before electrochemical measurement the specimens were immersed in a test solution at open circuit potential (EOCP) for 30 min to attain a stable state.

⎛ I ̇ − Icorr ⎞ θ = ⎜ corr ⎟ ̇ Icorr ⎝ ⎠

(1)

IE = (θ × 100)

(2)

CR = 14876

0.129IcorrEw Da

(3)

dx.doi.org/10.1021/ie402108g | Ind. Eng. Chem. Res. 2013, 52, 14875−14889


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3.2. Electrochemical Impedance Spectroscopy Measurements. Figure 3 shows the Nyquist diagrams for the

Figure 2. Anodic and cathodic polarization curves for steel in 1 M H2SO4 without and with various concentration of MBT at 25 °C: (1) blank, (2) 1 × 10−4, (3) 2 × 10−4, (4) 3 × 10−4, (5) 5 × 10−4, (6) 7 × 10−4, and (7) 1 × 10−3 M. Figure 3. Nyquist plots for steel in 1 M H2SO4 without and with various concentration of MBT at 25 °C: (1) blank, (2) 1 × 10−4, (3) 2 × 10−4, (4) 3 × 10−4, (5) 5 × 10−4, (6) 7 × 10−4, and (7) 1 × 10−3 M.

̇ where Icorr and Icorr are the corrosion current densities in uninhibited and inhibited acid solution, Ew is the equivalent weight (g), D is the density, and a is the area (cm2). As it can be seen from Figure 2, the addition of MBT to the corrosive solution decreases both anodic dissolution of iron and also retards cathodic hydrogen evolution reactions. The corrosion current density as well as corrosion rate of steel considerably was decrease in the presence of the inhibitor. These results are indicative of the adsorption of inhibitor molecules on the mild steel surface.4 There is no definite trend in the shift of Ecorr in the presence of MBT; therefore, MBT can be arranged as a mixed-type inhibitor, and the inhibition action is caused by the geometric blocking effect. Namely, the inhibition action comes from the reduction of the reaction area on the surface of the corroding metal.36,37 In addition, the influence is more pronounced in the cathodic polarization plots compared to that in the anodic polarization plots. It is clear that the addition of the inhibitor shifts the cathodic curves to a greater extent toward the lower current density when compared to the anodic curves.34 Polarization resistance (Rp) values were determined from the slope of the polarization curve using Stern−Geary equation:34,38,39 Rp =

corrosion of steel in 1 M H2SO4 in the absence and presence of various concentrations of MBT. These diagrams have a similar shape throughout all tested concentrations, indicating that almost no change in the corrosion mechanism occurs due to the inhibitor addition.41 The diameter of the capacitive loop in the presence of inhibitor is bigger than that in the absence of inhibitor. This increase was more and more pronounced with increasing inhibitor concentration, which indicated the adsorption of inhibitor molecules on the metal surface. As it can be seen from Figure 3, the Nyquist plots do not yield perfect semicircles as expected from the theory of EIS. The deviation from ideal semicircle is generally attributed to the frequency dispersion as well as to the inhomogeneities of surface and mass transport resistant.42,43 The equivalent circuit compatible with the Nyquist diagram recorded in the presence of inhibitor is depicted in Figure 4.

βa βc

1 2.303(βa + βc) Icorr

(4)

By increasing the inhibitor concentration the polarization resistance increases, indicating adsorption of the inhibitor on the metal surface to block the active sites.40

Figure 4. Equivalent circuits compatible with the experimental impedance data in Figure 3 for corrosion of steel electrode in different inhibitor concentrations.

Table 1. Potentiodynamic Polarization Parameters for the Corrosion of Steel in 1 M H2SO4 Solution in the Absence and Presence of Different Concentrations of MBT at 25 °C conc./M blank 1.0 × 2.0 × 3.0 × 5.0 × 7.0 × 1.0 ×

10−4 10−4 10−4 10−4 10−4 10−3

−Ecorr (±2)/mV

Icorr (±3)/μA cm−2

CR (±3)/mpy

βa (±1)/mV dec−1

−βc (±1)/mV dec−1

Rp (±5)/Ω cm2

θ (±0.02)

482 527 525 528 508 511 501

312 168 65 48 25 21 10

142.58 76.77 29.75 21.93 11.42 9.6 4.57

40.7 30.2 44.2 43.5 44.3 43.1 50.4

98.2 101 94.6 105 70.8 75.4 70.1

40.2 60 200 275 481 570 1305

0.46 0.78 0.84 0.92 0.94 0.97

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The simplest approach requires the theoretical transfer function Z(ω) to be represented by a parallel combination of a resistance Rct and a capacitance Cdl, both in series with another resistance Rs:44 1 Z(ω) = R s + 1/R ct + iωCdl (5) where ω is the frequency in rad/s, ω = 2πf, and f is frequency in Hz. To obtain a satisfactory impedance simulation of steel, it is necessary to replace the capacitor (Cdl) with a constant phase element (CPE) Qdl in the equivalent circuit. The most widely accepted explanation for the presence of CPE behavior and depressed semicircles on solid electrodes is microscopic roughness, causing an inhomogeneous distribution in the solution resistance as well as in the double layer capacitance.42,44 Constant phase element Qdl, Rs, and Rct can be corresponded to double layer capacitance, Qdl = Rn‑1Cndl solution resistance and charge transfer resistance, respectively. To corroborate the equivalent circuit, the experimental data are fitted to equivalent circuit and the circuit elements are obtained. Table 2 illustrates the equivalent circuit parameters for the

Figure 5. Anodic and cathodic polarization curves for steel in 1 M H2SO4 without inhibitor at different temperatures: (1) 25, (2) 45, (3) and 65 °C.

Table 2. Impedance Data for Steel in a 1 M H2SO4 Solution without and with Different Concentrations of MBT at 25 °C conc./M blank 1.0 × 2.0 × 3.0 × 5.0 × 7.0 × 1.0 ×

10−4 10−4 10−4 10−4 10−4 10−3

Rs (±0.2)/Ω

Rct (±5)/Ω

Qdl (±10−3)/F

n (±0.01)

1.8 2.1 2.2 2.2 2.2 2 2.1

29 65 88 486 618 861 1121

0.003 0.003 0.002 0.001 0.0008 0.0007 0.0006

0.88 0.88 0.9 0.89 0.91 0.9 0.89

impedance spectra of corrosion of steel in 1 M H2SO4 solution. The data indicate that increasing charge transfer resistance is associated with a decrease in the double layer capacitance. It has been reported that the adsorption of organic inhibitor on the metal surface is characterized by a decrease in Cdl.7 The decreased values of Cdl may be due to the replacement of water molecules at the electrode interface by organic inhibitor of lower dielectric constant through adsorption, suggests that MBT inhibitor acts by adsorption at the metal-solution interface. The increase in values of Rct and the decrease in values of Cdl with increasing the concentration also indicate that MBT acts as primary interface inhibitor and the charge transfer controls the corrosion of steel under the open circuit conditions.35 3.3. Effect of Temperature. The change of the corrosion rate with the temperature was studied in the absence and in the presence of MBT in 1 M H2SO4. For this purpose, potentiodynamic polarization measurements were performed at different temperatures from 25 to 65 °C in the absence and in the presence of different concentrations of MBT (Figures 5 and 6). Figures 5 and 6 show that raising the temperature has no significant effect on the corrosion potentials but leads to a higher corrosion current density (Icorr). With increasing temperature, the steel corrosion resistance decreased in both the presence and absence of inhibitor. For 45 and 65 °C, the electrochemical parameters were extracted and summarized in Table S1 and S2 (see the Supporting Information). The dependence of the corrosion rate on temperature can be expressed by the Arrhenius equation:45

Figure 6. Anodic and cathodic polarization curves for steel in 1 M H2SO4 with 1 × 10−3 M of MBT at different temperatures (1) 25, (2) 45, (3) and 65 °C.

⎛ −E ⎞ Icorr = λ exp⎜ a ⎟ ⎝ RT ⎠

(6)

where λ is a constant, Ea is the activation energy of the metal dissolution reaction, R is the gas constant, and T is the absolute temperature. Figure 7 shows the logarithm of Icorr against the reciprocal of temperature (1/T) in the absence and presence of MBT. The values of Ea were computed from the slope of the linear plots (Ea/R) and are listed in Table 3. It is seen that the values of Ea ranged from 71.8 to 76.5 kJ mol−1. These values are higher than the value of 66.7 kJ mol−1 obtained for the free acid solution indicating that the corrosion reaction of mild steel is inhibited by MBT, hence supports the phenomenon of physical adsorption.46 The increase in the activation energy in the presence of the additives signifies physical adsorption. Higher values of Ea in the presence of inhibitor can be correlated with increasing thickness of the double layer which enhances the Ea of the corrosion process.46 Enthalpy and entropy of activation (ΔHa, ΔSa) were calculated from the transition state theory:34,47 14878



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Figure 8. Typical Arrhenius plots of ln(Icorr/T) vs 1/T for steel in 1 M H2SO4 at different concentrations of MBT: ◆, blank; ×, 1 × 10−4 M; ▲, 2 × 10−4 M; ○, 3 × 10−4 M; ●, 5 × 10−4 M; +, 7 × 10−4 M; and ■, 1 × 10−3 M.

Figure 7. Typical Arrhenius plots of ln Icorr vs 1/T for steel in 1 M H2SO4 at different concentrations of MBT: ◆, blank; ×, 1 × 10−4 M; ▲, 2 × 10−4 M; ○, 3 × 10−4 M; ●, 5 × 10−4 M; +, 7 × 10−4 M, and ■, 1 × 10−3 M.

Icorr =

⎛ ΔSa ⎞ ⎛ −ΔHa ⎞ ⎛ RT ⎞ ⎜ ⎟ exp⎜ ⎟ exp⎜ ⎟ ⎝ Nh ⎠ ⎝ R ⎠ ⎝ RT ⎠

where Org(sol) and Org(ads) are the organic species dissolved in the aqueous solution and adsorbed onto the metallic surface and x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor. Attempts were made to fit experimental data to various isotherms including Frumkin, Langmuir, Temkin, Freundlich, Bockris-Swinkels, and Flory−Huggins isotherms. By far the results were best fitted by Langmuir adsorption isotherm equation:34,50

(7)

where h is the Plank constant (6.626176 × 10−34 J s) and N is Avogadro’s number (6.02252 × 1023 mol−1). Figure 8 shows the ln(Icorr/T) versus 1/T for steel dissolution in 1 M H2SO4 in the absence and presence of different concentrations of MBT. Straight lines are obtained with a slope of −ΔHa/R and an intercept of ln(R/Nh) + ΔSa/R.34 The values of Ea, A, ΔHa, and ΔSa are calculated and shown in Table 3. The positive values of ΔHa mean that the dissolution reaction is an endothermic process.34 Practically Ea and ΔHa are of the same order. The values of ΔSa in the absence and presence of the tested compounds were negative; this indicates that the activated complex in the rate determining step represents an association rather than dissociation step. This means that the activated molecules were in a higher order state than that at the initial state.7 3.4. Adsorption Isotherm and Thermodynamic Parameters. The efficiency of organic molecules as corrosion inhibitors mainly depends on their adsorption ability on the metal surface.4 There are some factors that influence the adsorption processes including the nature and charge of metal surface, the chemical of inhibitor, and the type of electrolyte.48 The adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasisubstitution process between the organic compound in the aqueous phase [Org(sol)] and water molecules at the electrode surface [H2O(ads)]1,49 Org(sol) + x H 2O(ads) ⇄ Org(ads) + x H 2O(sol)

θ = K adsC 1−θ Rearranging eq 9 gives:

(9)

C 1 = +C θ K ads

(10)

where C is the concentration of inhibitor and Kads is the adsorptive equilibrium constant. Plots of C/θ against C yield straight lines as shown in Figure 9. Both linear correlation coefficient (R2 > 0.99) and slope are very close to 1, indicating the adsorption of MBT on steel surface obeys the Langmuir adsorption isotherm. This isotherm assumes that the adsorbed molecules occupy only one site and there are no interactions with other adsorbed species.51 The value of equilibrium constant can be calculated from the reciprocal of the intercept of isotherm line. The high value of the adsorption equilibrium constant reflects the high adsorption ability of this inhibitor on steel surface. Further, it is related with the standard free energy of adsorption ΔGads according to the following equation:52 ΔGads = −RT ln(55.5K ads)

(8)

(11)

Table 3. Activation Parameters of the Dissolution of Steel in 1 M H2SO4 Solution in the Absence and Presence of MBT conc./M blank 1.0 × 2.0 × 3.0 × 5.0 × 7.0 × 1.0 ×

10−4 10−4 10−4 10−4 10−4 10−3

Ea (±1)/kJ mol−1

λ (±106)/A cm−2

ΔHa (±1)/kJ mol−1

ΔSa (±1)/J mol−1 K−1

Ea − Ha = RT/kJ mol−1

66.73 71.88 84.58 81.13 75.98 75.20 76.51

× × × × × × ×

64.10 69.25 81.95 78.50 73.35 72.60 73.90

−96.85 −84.62 −50.21 −64.04 −87.15 −91.30 −92.66

2.6 2.6 2.6 2.6 2.6 2.6 2.6

1.6 6.8 4.3 8.1 5.0 2.9 2.6

8

10 108 109 109 108 108 108

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Figure 9. Langmuir adsorption isotherm (C/θ vs C) of inhibitor in 1 M H2SO4 at different temperatures: , 25; ■, 45; and ▲, 65 °C.

Figure 10. ln(Kads) vs 1/T for adsorption MBT on the steel surface.

The values of Kads and ΔGads are listed in Table 4. The negative values of ΔGads indicate that the adsorption of

3.5. Molecular Structure and ab Initio Calculation. Spatial molecular and electronic structure of an inhibitor is efficient parameters to characterize its inhibition performance.61 In this regard, quantum chemical calculations have proved to be a powerful tool for studying corrosion inhibition mechanism and recently, corrosion publications have contained substantial quantum chemical calculations.15,62 Considering that MBT molecule can exist in two tautomeric forms: thiole form (2mercaptobenzothiazole; MBT; thio-enol) and thione form (benzothiazoline-2-thione; BTT; thio-keto). Both forms are likely to coexist in solutions,63 so we calculated both of them. Figure 11 shows the two tautomeric forms of MBT molecule. It

Table 4. Equilibrium Adsorption Parameters for Adsorption MBT on the Steel Surface in 1 M H2SO4 Solution temp/K

Kads (±102)/L mol−1

ΔGads (±1)/kJ mol−1

298 318 338

11111.11 1.0 × 104 1.0 × 103

−33.03 −34.97 −35.22

inhibitor molecule on steel surface is a spontaneous process. Generally, values of ΔGads up to −20 kJ mol−1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than −40 kJ mol−1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption).53−56 The values of standard free energy of adsorption are between these two. Therefore, it can be concluded that the adsorption of these inhibitors on the steel surface takes place through both chemical and physical adsorption namely mixed type.4,53,57,58The enthalpy and entropy of adsorption (ΔHads and ΔSads) can be calculated using the following equations:34,59 ΔGads = ΔHads − T ΔSads

(12)

ΔSads ΔHads + − ln(55.5) (13) RT R The plots of ln Kads versus 1/T for adsorption MBT as shown in Figure 10. The lines obtained represent a slope of (ΔHads/R) and intercept of [ln(55.5) + ΔSads/R]. The calculated values of ΔHads and ΔSads are −16.3 kJ mol−1 and 57 J mol−1 K−1, respectively. An endothermic adsorption process (ΔHads > 0) is due to chemisorption, while an exothermic adsorption process (ΔHads < 0) may be attributed to physisorption, chemisorption or a mixture of both.34 When the process of adsorption is exothermic, physisorption can be distinguished from chemisorptions according to the absolute value of ΔHads. The positive sign of ΔSads arise from substitution process, which can be attributed to the increase in the solvent entropy and more positive water desorption entropy. It also interpreted with increase of disorders due to the more water molecules which can be desorbed from the metal surface by one inhibitor molecule.60 ln K ads = −

Figure 11. Optimized geometries of thiole (MBT) and thione (BTT) forms of 2-mercaptobenzothiazole and two conformers of MBT.

can be seen in Figure 11 that BTT has one conformation, but its thiole tautomer (MBT) could be cis (S−H moiety eclipsing CN bond), trans (S−H group eclipsing C−S bond), or gauche (S−H out-of-plane). In this study, the gauche is excluded as a transition state due to the prediction of an imaginary wavenumber. In continue, we perform potential surface scans employing B3LYP method with the 6-31+G* basis set using the optimized structural parameters of trans-MBT (Figure 12). The dihedral 14880



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C1−N equilibrium distance in (1.29−1.37 Å) implies that there is a formation of a localized double bond, with the C1 center having an sp2 hybridization. These structural parameters suggest that the ring is relatively rich in electrons, which are available for their donation to another species. On the other hand, the region (S2−C1N) becomes delocalized when the protonation occurs at N, yielding similar structures which are similar in all the geometries (Figure 13, right panel). In the cis-MBT and trans-MBT structures, (Figures 11 and 13 and Table S3 (see the Supporting Information)), the bond angles of N−C1−S2 (116°), C1−S2−C2 (88°), S2−C2−C3 (109°), C2−C3−N (115°), and C3−N−C1 (111°) are approximately similar, meanwhile in the BTT structure, these angles are completely different from these values. Also, values of the dihedral angles of cis-MBT, trans-MBT, BTT, cisMBTH+, trans-MBTH+, and BTTH+ structures imply that these structures have a complete planarity and have a great capability of corrosion inhibition. Furthermore, the theoretical results show that for thione form, a more stable structure is obtained, compared to that of thiole form which follow the trend BTT > cis-MBT > trans-MBT (see Figure 11 and Table S4 (see the Supporting Information)). From the above results, it could be concluded that, different structures of MBT molecules are completely planar, which may result in relatively strong interaction between molecules and metal surface. However, different factors need to be considered for elucidating the orientation of organic molecules on the electrode surface. The atoms and groups of the molecules may interact with the electrode surface depend on the geometry of the inhibitor as well as the nature of their frontier molecular orbitals. Frontier molecular orbital (HOMO and LUMO) theory is useful in predicting the adsorption centers of the inhibitor responsible for the interaction with metal surface.62,66 The HOMO, HOMO-1, HOMO-2, and LUMO populations of cis-MBT, trans-MBT, and BTT structures and the cases of protonated forms are shown in Figures 14 and 15, respectively. It can be seen in Figures 14 and 15 that the frontier molecular orbitals distributions obtained for different structures have given very close results. For effective overlapping, the energy difference between the orbitals generally must be low, and the overall energy difference between the orbitals (HOMO and HOMO-1) is low enough for cis-MBT, trans-MBT, and BTT structures (Figure 14) and the cases of protonated forms (Figure 15), indicating their participation in the metal−ligand interaction. However, the energy differences between the HOMO-2 and HOMO-1 (for the mentioned structures) are slightly higher than those of HOMO and HOMO-1. Furthermore, it has been observed that HOMO-2 of BTT shows some delocalization regions, but in the case of the protonated moiety, those delocalization regions moved to HOMO-1 (Figure 15) confirming the involvement of HOMO-1 and HOMO-2 in the ligand−metal interaction. It can be concluded that the parameters of orbitals, such as HOMO, HOMO-1, and HOMO-2 are important for chemical reactivity over iron surface.67 To continue, the natural bond orbital (NBO) charge population analysis parameters of cis-MBT, trans-MBT, and BTT structures are depicted in Figure 16. Furthermore, the more negative the atomic charges of these molecules, the greater the chance the metal accepts the electron to its lowest unoccupied orbitals. According to Figure 16, for cis-MBT and trans-MBT, it is clearly indicated that the nitrogen atoms have excess electron densities compared to the other atoms.

Figure 12. Potential surface scan of 2-mercaptobenzothiazole (MBT), calculated via B3LYP/6-31+G*.

angle H−S−C−S of the trans thiole conformer is rotated by 30° increments around C−S bond while keeping other structural parameters rigid. From potential surface scans, the trans/cis S−H barrier to internal rotation is estimated to be 0.12/0.13 eV (2.73/2.90 kcal mol−1) which is in good agreement with the reported values.64 Moreover, in the current study, the potential barrier maximum is obtained at a C−S−H rotational angle of ∼90° against ∼120° for MBT.65 The structural and electronic properties of cis-MBT, transMBT, BTT, cis-MBTH+, trans-MBTH+, and BTTH+ structures are analyzed, and the results show that there is a strong effect on the chemical properties, specifically in the electron-donating capability to the metal. Figures 11 and 13 and Table S3 (see the Supporting Information) represent that the calculated bond distances of cis-MBT, trans-MBT and BTT and their protonated structures are in good agreement with the experimental values.24,64 For instance, the N−S2 bond distance is 1.78 Å in the neutral structures of the molecules, indicating there is a single bond formation between those atoms, and the

Figure 13. Optimized geometries of cis-MBTH+, trans-MBTH+, and BTTH+. 14881



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Figure 14. Molecular orbitals LUMO, HOMO, HOMO-1, and HOMO-2 of cis-MBT, trans-MBT, and BTT and their energies.

Figure 16. Natural bond orbital (NBO) charges population analysis of cis-MBT, trans-MBT, and BTT structures using DFT at the B3LYP/631+G* basis set level.

atom shows high significant difference, −0.587 and −0.103, respectively. However, both of them are more negative than that of the endocyclic sulfur atom, so the same conclusion can also be drawn that BTT is chemically adsorbed on the iron via the nitrogen atom and exocyclic sulfur atom. If the steric hindrance is considered for further analysis, it is more possible for the exocyclic sulfur atom than the nitrogen atom to bond to iron. Especially for the thione form, except for the thiazole ring, there is a hydrogen atom bonded to the nitrogen atom, which makes the nitrogen atom react to the iron surface more difficultly. So it is suggested that BTT molecule reacts to the iron surface primarily via exocyclic sulfur atom and then the nitrogen atom.24 In line with these theoretical procedures, density functional theory (DFT) studies68,69 have also been found to be successful in providing insights into the chemical reactivity and selectivity, in terms of global parameters, such as electronegativity (χ), hardness (η) and softness (S), and local ones such as the Fukui function ( f ( r ⃗)) and local softness (s( r ⃗)).70 Thus, for an Nelectron system with total electronic energy (E) and an external potential (v( r ⃗)); chemical potential (μ) known as the negative of electronegativity (χ), has been defined as the first derivative of the E with respect to N at v( r ⃗).71

Figure 15. Molecular orbitals LUMO, HOMO, HOMO-1, and HOMO-2 of cis-MBTH+, trans-MBTH+, and BTTH+ and their energies.

⎛ ∂E ⎞ χ = −μ = −⎜ ⎟ ⎝ ∂N ⎠v(r)

Therefore, the nitrogen atom is most likely to bond to iron. In contrast to the thiole form, in BTT, the exocylic sulfur atom has some negative atomic charges. That is why the S2 atom in these structures has a positive atomic charge because the electron of endocyclic sulfur atom can conjugated with the π electron of the ring, the density of its electron cloud will decrease, so the possibility of bonding to iron will reduced. For the thione form, the atomic charge of the nitrogen atom and the exocyclic sulfur

(14)

Hardness (η) has been defined within the DFT as the second derivative of the E with respect to N at v( r ⃗) property which measures both the stability and reactivity of a molecule.72,73 ⎛ ∂ 2E ⎞ ⎛ ∂μ ⎞ η = ⎜ 2⎟ = ⎜ ⎟ ⎝ ∂N ⎠ν ⎝ ∂N ⎠ν 14882

(15)



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where E is the electronic energy, N is the number of electrons, ν is the external potential due to the nuclei, and μ is the chemical potential. Following Janaks’ theorem74 and according to the valence state parabola model,75 this parameter can be approximated in terms of the energies of HOMO and LUMO molecular orbitals. From the quantum chemical point, the fraction of electronic charge (ΔN) transferred from the inhibitor to the metal is another important factor.76 When a bulk metal and an organic corrosion inhibitor are brought together, electron flow will occur from the atom of the lower χ value to the atom of the higher χ value until the chemical potentials become equal. Then, ΔN, the fraction of charge transferred, may be written as χM − χI ΔN = 2(ηM + ηI) (16)

It is evident from Table S4 (see the Supporting Information) that MBT molecule has the highest EHOMO in its thione form. This means that the electron donating ability of MBT is weaker in the thiole form. It is clear from Table S4 (see the Supporting Information) that in the case of protonated cis-MBT (cisMBTH+) exhibits the lowest ELUMO, making the cis-MBTH+ the most likely form for the interaction of mild steel with MBT molecule. The calculations in Table S4 (see the Supporting Information) further show that MBT in the thione form has the smallest ΔE value (4.448 eV) indicating that BTT is the most reactive form of this inhibitor that can easily adsorb on the metal surface causing higher protection. This agrees with the experimental results that MBT could have better inhibitive performance on mild steel surface in the thione form, i.e., through electrostatic interaction between the BTT and the vacant d orbital of mild steel (physisorption). Moreover, the adsorption of MBT on the steel surface using the thiole form also plays a part in the overall inhibiting process. This also agrees well with the value of ΔG0ads and ΔH0ads obtained experimentally. The molar volumes of the cis-MBT, trans-MBT, and BTT and their protonated structures follow the trend trans-MBT > trans-MBTH+> BTT > cis-MBTH+> BTTH+ > cis-MBT which is also tabulated in Table S4 (see the Supporting Information). trans-MBT has the highest molecular volume among these structures, which provides the largest coverage area between the molecular inhibitor and surface and the highest inhibition efficiency. Absolute hardness, η, and softness, S, are important properties to measure the molecular stability and reactivity. A hard molecule has a large energy gap and a soft molecule has a small energy gap. Soft molecules are more reactive than hard ones because they could easily offer electrons to an acceptor. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where S has the highest value and η the lowest value.85 The result from Table S4 (see the Supporting Information) shows that MBT in the thione form has the lowest energy gap, lowest hardness, and the highest softness. This confirms that MBT could have better inhibitive performance on mild steel surface in the thione form. Using a theoretical χM value of 7 eV mol−1 and ηM value of 0 eV mol−1 for the iron atom,76 ΔN, the fraction of electrons transferred from inhibitor to the iron molecule, is calculated and listed in Table S4 (see the Supporting Information). According to other reports,76,86 values of ΔN show an inhibition effect resulting from electrons donation. Agreeing with Lukovits’s study,86 if ΔN < 3.6, the inhibition efficiency increases with increasing electron-donating ability at the metal surface. Thus in the present study, MBT molecules in both the thiole and thione forms are donor of electrons and the mild steel surface is the acceptor of electrons. The electrophilicity index, ω, which measures the electrophilic power of a molecule, is calculated for both thiole and thione forms of MBT and their protonated structures. It has been reported that the higher the value of ω, the higher the capacity of the molecule to accept electrons.87 In this study, the cis-MBTH+ has the highest value of ω and by extension the highest capacity to accept electrons from the metal. This process increases the adsorption capacity of MBT on the steel surface. In a corroding system, it is important to note that the inhibitor acts as a Lewis base while the metal acts as a Lewis acid. It is important to consider the situation corresponding to a molecule that is going to receive a certain amount of charge at

where the subscripts M and I represent the metal and inhibitor, respectively. According to Pearson, operational and approximate definitions of the electronic chemical potential (μ) and the absolute hardness (η) of a chemical system are given by77 −μ =

η=

(I + A ) =χ 2

(17)

(I − A ) 2

(18)

where I is the ionization potential and A is the electron affinity. Since (I + A)/2 is the Mulliken electronegativity for atoms, the value of χ for any system is known as the absolute electronegativity. According to Koopman’s theorem, the frontier orbital energies are given by78 I = −E HOMO

(19)

A = −E LUMO

(20) 79

80

81

Recently, Parr et al., Gomez et al., Chattaraj et al., and Liu82 have introduced an electrophilicity index (ω) defined as

ω=

μ2 2η

(21)

S=

1 2η

(22)

Table S4 (see the Supporting Information) shows the calculated theoretical parameters which provide information about the reactive behavior of cis-MBT, trans-MBT, BTT, cisMBTH+, trans-MBTH+, and BTTH+ structures. EHOMO is often associated with the electron donating ability of a molecule; high values of EHOMO are likely to indicate the tendency of the molecule to donate electrons to appropriate acceptor molecules with lower energy MO. ELUMO, on the other hand, indicates the ability of the molecule to accept electrons.83 The binding ability of the inhibitor to the metal surface increases with increasing HOMO and decreasing LUMO energy values. Thus, the lower the value of ELUMO, the most probable it is that the molecule would accept electrons. Moreover, the gap between the HOMO and LUMO energy levels of the molecule is an important parameter that determines the reactivity of the inhibitor molecule toward the adsorption on the metallic surface. As ΔE decreases (most especially for the thione form), the reactivity of the molecule increases leading to increase in the inhibition efficiency of the molecule.84 14883



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is the atom in the molecule where the value of f + is the highest while the preferred site for electrophilic attack is the atom in the molecule where f − has the highest value. The calculated values of the Fukui functions for the non-hydrogen atoms are reported in Table S5 (see the Supporting Information). It is possible to observe from Table S5 (see the Supporting Information) that sulfur atoms (S1 and S2) are the most susceptible sites for electrophilic attacks. These sites in thione form present the highest values of f ‑ which are 0.412 for S1 and 0.180 for S2, respectively. The value of f + is highest on S1, S2, C1, C6, and N atoms indicating that these atoms are likely to be engaged in a nucleophilic attack on the inhibitor. Fukui indices are tabulated for the other structures in Table S5 (see the Supporting Information). Figure 17 (left panel) shows the NBO atomic charges calculated for MBT and its different forms. It has been reported

some center and is going to back-donate a certain amount of charge through the same center or another one.20 To describe the energy change associated with these two processes, the second order simple charge transfer formula is regarded as a two-parameter expression, in which the donation and backdonation processes are differentiated through the use of the values of the chemical potential for each case, while the hardness is fixed to the value of η = (μ+ − μ−)in both situations. Thus, when the molecule receives a certain amount of charge, ΔN+20,88 ΔE + = μ+ ΔN + +

1 η(ΔN +)2 2

(23)

While when the molecule back-donates a certain amount of charge, ΔN−, then ΔE− = μ−ΔN − +

1 η(ΔN −)2 2

(24)

If the total energy change is approximated by the sum of the contributions in eqs 23 and 24, and assuming that the amount of charge back-donated is equal to the amount of charge received, ΔN− = −ΔN+, then ΔE T = ΔE + + ΔE− = (μ+ − μ−)ΔN + + η(ΔN +)2 (25)

The most favorable situation corresponds to the case when the total energy change becomes a minimum with respect to ΔN+, which implies that ΔN+ = −(μ+ − μ‑)/2η and that ΔE T =

−(μ+ − μ−)2 η =− 4η 4

(26)

The calculations from Table S4 (see the Supporting Information) indicate that η > 0 and ΔET < 0 in both thiole and thione forms of 2-mercaptobenzothiazole and their protonated structures. This result implies that the charge transfer to the MBT molecule followed by back-donation from the molecule is energetically favorable. Similar observation has been reported.89 However, it is important to note that ΔET values obtained does not predict that a back-donation process is going to occur; it only establishes that if both processes occur (charge transfer to the molecule and back-donation from the molecule), the energy change is directly proportional to the hardness of the molecule. The Fukui functions indicate the regions on the inhibitor molecule on which nucleophilic and electrophilic reactions are likely to occur. The Fukui function90 is defined by ⎡ ∂ 2E ⎤ ⎡ ∂ ⎤ ⎡ ∂ρ(r ) ⎤ f (r ) = ⎢ ⎥=⎢ ⎥ =⎢ ⎥ ⎣ ∂ν(r )∂N ⎦ ⎣ ∂ν(r ) ⎦ N ⎣ ∂N ⎦ν

(27)

where ν(r) is the external potential, ρ(r) is total charge density, and N is the total number of electrons. Due to discontinuity in the ρ(r) versus N curve, one can define three different types of Fukui function, viz f k+ = [qk (N + 1) − qk (N )]

Figure 17. Electrostatic properties of (A) cis-MBT, (B) trans-MBT, (C) BTT, (D) cis-MBTH+, (E) trans-MBTH+, and (F) BTTH+: The NBO charges population and the side view of the dipoles are displayed on the left while the middle and right panels show the contour and isosurface representation of electrostatic potential, respectively.

(for nucleophilic attack) (28)

f k−

= [qk (N ) − qk (N − 1)]

that the more negative the atomic charges of the adsorbed center, the more easily the atom donates its electron to the unoccupied orbital of the metal.91 It is clear from Figure 17, that the nitrogen atom as well as some carbons atoms carries negative charge centers which could offer electrons to the mild steel surface to form a coordinate bond. It should be noted that

(for electrophilic attack) (29)

where qk(N), qk(N + 1), and qk(N − 1) are the electron population of the kth atom for N, N + 1, and N − 1 electron systems, respectively. The preferred site for nucleophilic attack 14884



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where IETheor is the inhibition efficiency, A and B are the regression coefficients determined by regression analysis, xi is a quantum chemical index characteristic for the molecule (i), and Ci denotes the experimental concentration i. Application of eq 31 for cis-MBT, trans-MBT, BTT, cis-MBTH+, trans-MBTH+, and BTTH+ structures yields eqs 32−37, respectively.

there are more negative charge centers in the thione form of MBT than in the thiole forms. The dipole moment (μ), which is defined as the first derivative of the energy with respect to an applied electric field, is mainly used to predict the direction of a corrosion inhibition process. The dipole moment is the measure of polarity in a bond and is related to the distribution of electrons in a molecule. Although literature is inconsistent on the use of μ as a predictor of the direction of a corrosion inhibition reaction, it is generally agreed that the adsorption of polar compounds possessing high dipole moments on the metal surface should lead to better inhibition efficiency.87 The dipole moment is another indicator of the electronic distribution within a molecule. Some authors state that the inhibition efficiency increases with increasing values of the dipole moment, which depends on the type and nature of molecules considered. However, there is a lack of agreement in the literature on the correlation between μ and %IE, as in some cases no significant relationship between these values has been identified.92 In this study, it could be noted that other than BTT, cis-MBTH+ exhibits higher values of dipole moment than the others. The direction of the dipole can be understood by considering the electrostatic potential (middle and right panels of Figure 17), which discerns electron density rich regions centered on the mentioned atoms. 3.6. Qunatitative Structure Activity Relation (QSAR) Study. In the results reported so far, quantitative structure and activity relationship (QSAR) has also been used to correlate the inhibition efficiency of the studied different conformers and tautomers of MBT. An attempt to correlate the quantum chemical parameters with the experimental inhibition efficiencies shows that there is no simple relation or no direct trend relationship can be derived with the inhibition performance of cis-MBT, trans-MBT, BTT, cis-MBTH+, trans-MBTH+, and BTTH+ inhibitors using only one method. Though a number of appropriate model equations have been reported by other investigators76,93−96 between the inhibition efficiency of various inhibitors used and some quantum chemical parameters, a composite index and a combination of more than one parameter97,98 have been used to perform QSAR which might affect the inhibition efficiency of the studied molecules. Consequently, a relation may exist between the composite index and the average corrosion inhibition efficiency for a particular inhibitor molecule. Therefore, for this study, parameters have been selected relevant to the activity of the molecules under investigation. The linear model approximates inhibition efficiency (IETheor %) as in the equation below:

IE Theor = AxiCi + B

IE Theor = [(4.06E HOMO − 14.89E LUMO + 9.52ΔE + 2.02ΔN + 2.54μ + 149.53V + 2.66)Ci] /[1 + (4.06E HOMO − 14.89E LUMO + 9.52ΔE + 2.02ΔN + 2.54μ + 149.53V + 2.66)Ci] × 100 IE Theor = [(2.60E HOMO − 7.31E LUMO + 5.43ΔE + 1.53ΔN + 2.86μ + 108.14V + 1.87)Ci] /[1 + (2.60E HOMO − 7.31E LUMO + 5.43ΔE + 1.53ΔN + 2.86μ + 108.14V + 1.87)Ci] × 100

(Axi + B)Ci 100 1 + (Axi + B)Ci

(33)

IE Theor = [(2.23E HOMO − 8.37E LUMO + 5.74ΔE + 1.76ΔN + 6.47μ + 119.80V + 2.07)Ci] /[1 + (2.23E HOMO − 8.37E LUMO + 5.74ΔE + 1.76ΔN + 6.47μ + 119.80V + 2.07)Ci] × 100

(34)

IE Theor = [( −11.02E HOMO − 6.07E LUMO + 5.95ΔE + 0.58ΔN + 4.47μ + 120.58V + 2.09)Ci] /[1 + ( −11.02E HOMO − 6.07E LUMO + 5.95ΔE + 0.58ΔN + 4.47μ + 120.58V + 2.09)Ci] × 100

(35)

IE Theor = [( −10.88E HOMO − 5.99E LUMO + 5.89ΔE + 0.58ΔN + 5.39μ + 116.45V + 2.08)Ci] /[1 + ( −10.88E HOMO − 5.99E LUMO + 5.89ΔE + 0.58ΔN + 5.39μ + 116.45V + 2.08)Ci] × 100

(36)

IE Theor = [( −10.88E HOMO − 5.99E LUMO + 5.89ΔE

(30)

+ 0.58ΔN + 5.39μ + 116.45V + 2.08)Ci]

where IETheor is the inhibition efficiency, A and B are constants obtained by regression analysis, xi is a quantum chemical index characteristic for the molecule, and Ci is the experiment’s concentration of the inhibitor. Such linear approach is not found to be satisfactory for correlating the present results. Consequently, the nonlinear model (NLM) proposed by Lukovits et al.86 and also used by Khaled98 for studying the interaction of corrosion inhibitors with metal surfaces in acidic solutions derived from the equation below 31 based on the Langmuir adsorption isotherm has been used IE Theor =

(32)

/[1 + ( −10.88E HOMO − 5.99E LUMO + 5.89ΔE + 0.58ΔN + 5.39μ + 116.45V + 2.08)Ci] × 100

(37)

From the results obtained, it can be seen that there is a good and acceptable coefficient correlation (R2 = 0.9588) between the experimental and calculated/estimated inhibition efficiencies of the studied different conformers and tautomers of MBT using the B3LYP/6-31+G* method as shown in Figure 18a. (all structures show same results). Figure 18b shows a plot of the theoretical and experimental inhibition efficiencies versus the

(31) 14885



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Figure 18. (A) Correlation of experimental and theoretical inhibition efficiencies of cis-MBT, trans-MBT, BTT, cis-MBTH+, trans-MBTH+, and BTTH+ structures using the B3LYP/6-31+G* method. (B) Plot of IEexp% and IETheor% versus 2-MBT concentrations.

concentrations of MBT for the studied different structures of this inhibitor, such as cis-MBT, trans-MBT, BTT, cis-MBTH+, trans-MBTH+, and BTTH+ which confirms the accuracy of the obtained results from Figure 18a. Also it is noticeable to mention that it is difficult to predicte compounds usefulness to be a good corrosion inhibitor through a inclusive approach or find some universal type of correlation. Other than a number of excluded parameters such as, effect of solvent molecules, surface nature, adsorption sites of the metal atoms or oxides sites or vacancies, competitive adsorption with other chemical species in the fluid phase and solubility should also be taken into consideration. However, the most vital component of the QSAR approach is the determination of the correct molecular descriptors. 3.7. Scanning Electron Microscopy Studies. The effect of inhibitor on the electrode surface was confirmed by SEM observations of the electrode surface. The SEM images of steel in 1 M H2SO4 solution in the absence and presence of 1 mM MBT after 6 h exposure are given in Figure 19. As it is shown in Figure 19a, the steel surface was strongly damaged in the absence of inhibitors due to metal dissolution in corrosive solution. Moreover, the surface is very rough. In contrast, in the presence of MBT, Figure 19b shows that there is much less damage on the steel surface, which clearly confirms the inhibition action. In accordance, it can be concluded that MBT can efficiently inhibits the corrosion of steel. The EDX analysis of steel surface obtained after immersing in acidic solution in presence of inhibitor is presented in Figure 19c. The corresponding elemental analysis are obtained as O, 1.22; Si, 0.22; S, 0.40; Mn, 0.71; and Fe, 97.45 wt % which was related to the steel surface. The inhibitor layer was very thin and EDX cannot detect it.

Figure 19. SEM images of steel exposed to 1 M H2SO4 solution in the (a) absence and (b) presence of 1 mM of MBT. (c) EDX analysis of steel surface obtained after immersing in acidic solution containing inhibitor.

4. CONCLUSIONS 1. 2-Mercaptobenzothiazole (MBT) acts as an excellent inhibitor for the corrosion of steel in 1 M H2SO4 solution especially in high concentration. Inhibition efficiency IE% increases with the inhibitor concentration, and the maximum IE % values reaches about 97% in the presence of 1 mM inhibitor. 2. The potentiodynamic polarization curves indicated that MBT behaves as mixed type corrosion inhibitor by inhibiting both anodic metal dissolution and cathodic hydrogen evolution reactions. 3. Impedance measurements indicate that, with increasing inhibitor concentration, the polarization resistance increased, while the double-layer capacitance decreased. 4. Corrosion current density is increased by increasing the temperature, but the rate of its increase is lower in the presence of MBT. 14886



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ylimino)methyl)phenol Schiff base on mild steel. Mater. Chem. Phys. 2011, 125, 796. (7) Danaee, I.; Ghasemi, O.; Rashed, G. R.; RashvandAvei, M.; Maddahy, M. H. Effect of hydroxyl group position on adsorption behavior and corrosion inhibition of hydroxybenzaldehyde Schiff bases: Electrochemical and quantum calculations. J. Mol. Struct. 2013, 1035, 247. (8) Singh, A. K. Inhibition of Mild Steel Corrosion in Hydrochloric Acid Solution by 3-(4-((Z)-Indolin-3-ylideneamino)phenylimino)indolin-2-one. Ind. Eng. Chem. Res. 2012, 51, 3215. (9) Zhao, J.; Zhang, N.; Qu, C.; Wu, X.; Zhang, J.; Zhang, X. Cigarette Butts and Their Application in Corrosion Inhibition for N80 Steel at 90 °C in a Hydrochloric Acid Solution. Ind. Eng. Chem. Res. 2010, 49, 3986. (10) Gerengi, H.; Sahin, H. I. Schinopsis lorentzii Extract As a Green Corrosion Inhibitor for Low Carbon Steel in 1 M HCl Solution. Ind. Eng. Chem. Res. 2012, 51, 780. (11) John, S.; Joseph, A. Electroanalytical and Theoretical Investigations of the Corrosion Inhibition Behavior of Bis-1,2,4Triazole Precursors EBATT and BBATT on Mild Steel in 0.1 N HNO3. Ind. Eng. Chem. Res. 2012, 51, 16633. (12) Solmaz, R. Investigation of the inhibition effect of 5-((E)-4phenylbuta-1,3-dienylideneamino)-1,3,4-thiadiazole-2-thiol Schiff base on mild steel corrosion in hydrochloric acid. Corros. Sci. 2010, 52, 3321. (13) Tao, Z.; Zhang, S.; Li, W.; Hou, B. Adsorption and Inhibitory Mechanism of 1H-1,2,4-Triazol-l-yl-methyl-2-(4-chlorophenoxy) Acetate on Corrosion of Mild Steel in Acidic Solution. Ind. Eng. Chem. Res. 2011, 50, 6082. (14) Hsieh, M. K.; Dzombak, D. A.; Vidic, R. D. Effect of Tolyltriazole on the Corrosion Protection of Copper against Ammonia and Disinfectants in Cooling Systems. Ind. Eng. Chem. Res. 2010, 49, 7313−7322. (15) Jafari, H.; Danaee, I.; Eskandari, H.; RashvandAvei, M. Electrochemical and Theoretical Studies of Adsorption and Corrosion Inhibition of N,N′-Bis(2-hydroxyethoxyacetophenone)-2,2-dimethyl1,2-propanediimine on Low Carbon Steel (API 5L Grade B) in Acidic Solution. Ind. Eng. Chem. Res. 2013, 52, 6617. (16) Chidiebere, M. A.; Ogukwe, C. E.; Oguzie, K. L.; Eneh, C. N.; Oguzie, E. E. Corrosion Inhibition and Adsorption Behavior of Punica granatum Extract on Mild Steel in Acidic Environments: Experimental and Theoretical Studies. Ind. Eng. Chem. Res. 2012, 51, 668. (17) Yurt, A.; Bereket, G. Combined Electrochemical and Quantum Chemical Study of Some Diamine Derivatives as Corrosion Inhibitors for Copper. Ind. Eng. Chem. Res. 2011, 50, 8073. (18) Fu, J. J.; Li, S. N.; Wang, Y.; Cao, L. H.; Lu, L. D. Computational and electrochemical studies of some amino acid compounds as corrosion inhibitors for mild steel in hydrochloric acid solution. J. Mater. Sci. 2010, 45, 6255−6265. (19) Ebenso, E. E.; Kabanda, M. K.; Murulana, L. C.; Singh, A. K.; Shukla, S. K. Electrochemical and Quantum Chemical Investigation of Some Azine and Thiazine Dyes as Potential Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Solution. Ind. Eng. Chem. Res. 2012, 51, 12940. (20) Gomez, B.; Likhanova, N. V.; Dominguez-Aguilar, M. A.; Martinez-Palou, R.; Vela, A.; Gasquez, J. L. Quantum Chemical Study of the Inhibitive Properties of 2-Pyridyl-Azoles. J. Phys. Chem. B 2006, 110, 8928−8934. (21) Contini, G.; Di Castro, V.; Stranges, S.; Richter, R.; Alagia, M. Gas-Phase Photoemission Study of 2-Mercaptobenzothiazole. J. Phys. Chem. A 2002, 106, 2833. (22) Goh, L. Y.; Weng, Z.; Leong, W. K.; Vittal, J. J. Organometallic Radical-Initiated Cleavage of the Metal Chelate and Thiazole Rings in a Cyclopentadienylchromium Complex. J. Am. Chem. Soc. 2002, 124, 8804−8805. (23) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. The influence of heterocyclic thiols on the electrodeposition of Cu on Au(111). J. Electoanal. Chem. 1998, 441, 109−129.

5. The adsorption of MBT molecules on the metal surface from 1 M H2SO4 solution obeys a Langmiur adsorption isotherm. The thermodynamic parameter of adsorption indicates that the adsorption of MBT involves both physical adsorption and chemical adsorption. 6. The high value of the adsorption equilibrium constant (Kads) suggested that MBT is strongly adsorbed on the steel surface. The negative signs of ΔGads and ΔHads indicate that the adsorption process is spontaneous and exothermic. 7. The results of SEM showed lower corrosion product content and a light oxidative surface with inhibitor concentration increasing. 8. The results obtained from the theoretical calculations show that the MBT molecule can exist in different forms in the solution. Among these different forms, some quantum chemical parameters and the local reactivity analyzed by the condensed Fukui function also reveal that the MBT molecule in the thione form shows a better inhibition behavior than when in the thiole form. The relationship between the efficiency of inhibition of mild steel corrosion in 1 M H2SO4 by different structures of MBT and the EHOMO, ELUMO, ELUMO − EHOMO, and ΔN values are calculated by DFT. The results of quantum-chemical calculations and the electroanalytical results are in conformity with each other.

■

ASSOCIATED CONTENT

S Supporting Information *

Potentiodynamic polarization parameters at 45 and 65 °C and corresponding tables to thermodynamic parameters obtained from statistical model and some global and local reactivity parameters based on the density functional theory. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (0098631) 4429937. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors acknowledge the financial support of the Office of Vice Chancellor of Research of their corresponding universities. REFERENCES

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