Synthesis of New Pyridine Based 1,3,4-Oxadiazole ... - ACS Publications

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Synthesis of New Pyridine Based 1,3,4-Oxadiazole Derivatives and their Corrosion Inhibition Performance on Mild Steel in 0.5 M Hydrochloric Acid Doddahosuru M. Gurudatt and Kikkeri N. Mohana* Department of Studies in Chemistry, University of Mysore, Manasagangothri, Mysore 570 006, India S Supporting Information *

ABSTRACT: The influence of three newly synthesized oxadiazole derivatives on the corrosion inhibition of mild steel in 0.5 M HCl solution was studied using mass loss and electrochemical techniques. The corrosion rate decreased with increasing concentration of inhibitors and increased with increase in temperature of the medium. Adsorption of the all the three inhibitors obeyed the Langmuir isotherm model. Polarization curves indicated that the inhibitors are of mixed type. Electrochemical impedance spectroscopy measurements explained the mechanism of action of inhibitors. Various activation and adsorption thermodynamic parameters were calculated. The surface adsorbed film was characterized by scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX). The electronic properties of the inhibitors were obtained from AM1 semiempirical quantum chemical approach. Excellent correlation was found between theoretical and experimental results.

1. INTRODUCTION Mild steel is the most important engineering and construction material in the world. The corrosion of mild steel is of fundamental academic and industrial concern that has received a considerable amount of attention.1 Corrosion causes serious problems to industries and human beings, affecting both cost and productivity. Corrosion inhibitors are one of the most promising methods to overcome corrosion. The use of eco-friendly corrosion inhibitors is increasing day by day.2 Oxygen and/or nitrogen containing heterocyclic compounds with various substituents are considered to be effective corrosion inhibitors.3,4 Oxadiazole derivatives offer special affinity to inhibit corrosion of metals in acid solutions.5,6 These compounds rich in heteroatoms can be regarded as environmental friendly inhibitors because of their characteristics of strong chemical activity and low toxicity.7 Due to increasing environmental awareness and adverse effect of some chemicals, research activities in recent times are geared toward developing cheap and environmentally acceptable corrosion inhibitors.8 In view of the wide application of mild steel, several studies have been carried out on the corrosion inhibition of mild steel. Most of the well-known corrosion inhibitors are heterocyclic compounds having more heteroatoms in their aromatic or long carbon chain systems.9,10 It has been known that efficient inhibitors should possess plentiful π-electrons and unshared electron pairs on either nitrogen atoms or sulfur atoms of the inhibitors to the d-orbital of iron. The adsorption characteristics of organic molecules are also affected by sizes, electron density at the donor atoms, and orbital character of donating electrons.11,12 Organic compounds containing functional electronegative groups, π-electron in triple or conjugated double bonds and the presence of aromatic rings in their structure are the major adsorption centers and are usually good inhibitors.13 Recently, many workers have oriented their interest to the development of new corrosion inhibitors such as pyrazole,14,15 triazole,16,17 tetrazole,18,19 imidazole,20 and oxadiazole derivatives.21,22 The © 2014 American Chemical Society

selection of oxadiazole derivatives with various substituents as corrosion inhibitor is based on the presence of nitrogen and oxygen atoms in aromatic system, which facilitates electrophilic attack.23 Quantum chemical calculations have become an effective tool in the analysis and elucidation of many experimental observations. The semiempirical methods of calculation have been used to correlate of the obtained data with the inhibition efficiency.24,25 The present study aimed to synthesize the three new oxadiazole derivatives and investigate their efficiency as corrosion inhibitors for mild steel in 0.5 M hydrochloric acidic media using mass loss and electrochemical techniques. The experimental findings were discussed with various activation and adsorption thermodynamic parameters. The protective film formed on the metal surface was characterized by scanning electron microscopy (SEM). Further, to understand the relationship between molecular structure of these derivatives and their inhibitive action, quantum chemical parameters such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap (ΔE), and the dipole moments (μ) have been computed and discussed.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The chemical composition of mild steel which was used as a working electrode is given in Supporting Information Table S1. Prior to gravimetric and electrochemical measurements, the surface of the specimens was polished under running tap water using SiC emery paper up to 1200 grade, rinsed with distilled water, dried on a clean tissue Received: Revised: Accepted: Published: 2092

June 29, 2013 January 20, 2014 January 21, 2014 January 21, 2014 dx.doi.org/10.1021/ie402042d | Ind. Eng. Chem. Res. 2014, 53, 2092−2105

Industrial & Engineering Chemistry Research

Article

weight losses of the specimens were used to calculate the percent inhibition efficiency (η %). Then the tests were repeated with different concentrations of the inhibitor at varying temperatures. 2.4. Electrochemical Measurements. Polarization and electrochemical impedance spectroscopy (EIS) experiments were carried out using a CHI660D electrochemical workstation. A conventional three-electrode cell consisting of a saturated calomel reference electrode (SCE), a platinum auxiliary electrode, and the working electrode with 1 cm2 exposed areas were used. The specimens were pretreated similarly as done in the gravimetric measurements. The electrochemical tests were performed using the synthesized oxadiazole derivatives with various concentrations ranging from 0 to 300 ppm at 30 °C. Potentiodynamic polarization measurements were performed in the potential range from −850 to −150 mV with a scan rate of 0.4 mV s−1. EIS measurements were carried out at the open circuit potential (OCP), prior to the EIS measurement, a steadystate period of 30 min was observed, which proved sufficient for OCP to attain a stable value. The ac frequency range extended from 10 to 0.05 kHz with signal amplitude of ±10 mV. 2.5. Surface Analysis. The SEM analysis was performed using a JSM-5800 electron microscope with the working voltage of 20 kV and the working distance 24 mm. In SEM micrographs, the specimens were exposed to the 0.5 M HCl in the absence and presence of inhibitors under optimum conditions after a desired period of immersion. The SEM images were taken for polished mild steel specimen and specimen immersed in solution without and with inhibitors. Energy dispersive analysis of X-ray (EDAX) was performed using Ultra 55, field emission SEM-EDAX (Karl Zeiss). 2.6. Quantum Chemical Calculations. The molecular structures of 6-MMOPP, 5-MPOP, and 4-BPOMP were fully geometrically optimized by AM1 semiempirical method with Spartan 08 V1.2.0. Four main related parameters such as the energy of the highest occupied molecular orbital (EHUMO), the energy of the lowest unoccupied (ELUMO), energy gap (ΔE = ELUMO − EHOMO), and dipole moment (μ) were gained. MOPAC (Molecular Orbital Package) calculations were carried out for four different Hamiltonians including Parametric Model 3 (PM3), Austin Model 1 (AM1), Recief Model 1 (RM1), and Modified Neglect of Diatomic Overlap Model (MNDO). Mulliken charge populations of atoms in the inhibitors were also calculated.

paper, immersed in benzene for 5 s, dried and then immersed in acetone for 5 s, and dried with clean tissue paper. Finally, the specimens were kept in desiccators until use. At the end of the test, the specimens were carefully washed with benzene and acetone, dried and then weighed. Appropriate concentrations of acid were prepared by using double-distilled water. The concentration range of inhibitor employed was 50−300 ppm. 2.2. Synthesis of Inhibitors. The general strategy for the synthesis of oxadiazole derivatives is shown in Scheme 1. The Scheme 1. Scheme for the Synthesis of Pyridine Based 2,5Disubstituted 1,3,4-Oxadiazoles

required starting material, substituted aryl/heteroaryl carboxylic acid hydrazide was prepared in good yield. The synthesized acid hydrazide on cyclization with 6-methyl-pyridine-2-carboxylic acid using dry POCl3 as cyclizing agent results in 2,5-disubstitued 1,3,4-oxadiazole derivatives.26 The abbreviations, substituents (R1), molecular structures, and names of all the three pyridine based 2,5-disubstituted 1,3,4-oxadiazoles are given in Table 1. All the solvents and chemicals used were of analytical reagent grade and used as such. FTIR spectra were recorded using a Jasco FTIR 4100 double beam spectrometer. 1H NMR spectra were recorded on Bruker DRX-500 spectrometer at 400 MHz using DMSO-d6 as solvent and TMS as an internal standard. LC Mass spectra were recorded using Agilent-SC/AD/10-017 instrument. 2.3. Weight Loss Measurements. Mild steel specimens were immersed in the acid solutions for 3 h at different temperature. The temperature of the environment was maintained by thermostatically controlled water bath with accuracy of ±0.2 °C (Weiber limited, Chennai, India), under aerated condition. After 3 h of immersion the specimens were removed, rinsed in water and acetone, and dried in desiccators. The weight loss was recorded to the nearest 0.0001 g by using an analytical balance (Sartorius, precision ±0.1 mg). The average weight loss of three parallel specimens was obtained. Relative

Table 1. Abbreviations, Substituent (R1), Molecular Structures, and Names of All the Oxadiazole Derivatives

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dx.doi.org/10.1021/ie402042d | Ind. Eng. Chem. Res. 2014, 53, 2092−2105


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mass spectral studies. The IR (KBr) spectra of the synthesized compounds showed CN stretching band at around 1533− 1684 cm−1, COC absorption band at around 1239− 1282 cm−1, and the absence of absorption bands at 3350, 3300, 3250, and 1725 cm−1 due to NH, NH2, and COOH functions, respectively, clearly evidence the formation of 1,3,4-oxadiazole ring.26 1 H NMR (400.15 MHz, DMSO-d6) δ ppm: [6-MMOPP]: δ 1.78 (s, 6H,CH3−C(CN)−CH3), 2.59 (s, 3H, C−CH3), 7.53 (d, J = 10.84 Hz, 1H, het-H), 7.70 (d, J = 10.44 Hz, 1H, Ar-H), 7.82 (d, J = 11.60 Hz, 1H, Ar-H), 8.13 (t, J = 10.24 Hz, 1H, het-H), 8.24 (s, 1H, Ar-H), 8.40 (dd, J = 2.96, 10.86 Hz, 1H, het-H), 9.20 (d, J = 2.56 Hz, 1H, Ar-H). 1 H NMR (400.15 MHz, DMSO-d6) δ ppm: [5-MPOP] δ 2.71 (s, 3H, C−CH3), 7.53 (d, J = 10.84 Hz, 1H, Py-H), 7.65−7.69 (t, 1H, 6-methyl-Py-H), 8.39−8.42 (m, 1H, Py-H), 8.49−8.52 (m, 1H, Py-H), 8.82 (dd, J = 2.12, 6.50 Hz, 1H, 6-methyl-Py-H), 9.20 (d, J = 2.56 Hz, 1H, Py-H), 9.32 (d, J = 2.16 Hz, 1H, 6-methyl-Py-H). 1 H NMR (400.15 MHz, DMSO-d6) δ ppm: [4-BPOMP] δ 2.59 (s, 3H, C−CH3), 7.53 (d, J = 8.16 Hz, 1H, het-H), 7.86 (d, J = 8.40 Hz, 2H, Ar-H), 8.09 (d, J = 8.40 Hz, 2H, Ar-H), 8.38 (dd, J = 2.12, 8.12 Hz, 1H, het-H), 9.18 (s, 1H, het-H). Further evidence for the formation of 1,3,4-oxadiazole was also confirmed by recording the mass spectra which shows m/z 305.0 (M + 1), 239.8 (M + 1) for 6-MMOPP and 5-MPOP, respectively, and for 4-BPOMP 315.0 (M+), 317.0 (M + 2) was observed. The spectral data (IR, 1H NMR, and mass) of all the synthesized compounds were in full agreement with the proposed structures.26,27 3.2. Potentiodynamic Polarization. The potentiodynamic polarization curves obtained from the corrosion behavior of mild steel in 0.5 M HCl in the absence and presence of 6-MMOPP, 5-MPOP, and 4-BPOMP inhibitors are shown in Figure 1a−c. The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and Tafel slopes (i.e., cathodic (bc) and anodic (ba) obtained from the polarization measurements are listed in Table 2. The η % was calculated using the following equation:

Figure 1. Polarization curves for mild steel in 0.5 M HCl containing different concentration of (a) 6-MMOPP, (b) 5-MPOP, and (c) 4BPOMP.

3. RESULTS AND DISCUSSION 3.1. Characterization of Inhibitors. The synthesized inhibitors were purified and characterized by IR, 1H NMR, and

η % = 1 − ((Icorr)p /(Icorr)a ) × 100

(1)

Table 2. Potentiodynamic Polarization Parameters for the Corrosion of Mild Steel in 0.5 M HCl in the Absence and Presence of Different Concentrations of Synthesized 6-MMOPP, 5-MPOP, and 4-BPOM Inhibitors at 303 K inhibitor blank 6-MMOPP

5-MPOP

4-BPOMP

C (ppm)

Ecorr (mV)

icorr (mA cm−2)

ba (mV dec−1)

bc (mV dec−1)

η (%)

50 100 150 150 200 300 50 100 150 150 200 300 50 100 150 150 200 300

−496 −431 −446 −439 −436 −453 −452 −442 −456 −449 −489 −463 −461 −472 −446 −447 −496 −471 −463

0.2730 0.0824 0.0629 0.0439 0.0404 0.0349 0.0282 0.0770 0.0655 0.0588 0.0507 0.0413 0.0383 0.1280 0.0884 0.0779 0.0499 0.0408 0.0377

13.155 20.497 18.662 17.982 18.222 16.04 17.376 19.274 18.559 18.691 14.146 17.369 18.513 13.902 17.474 17.383 15.411 16.188 17.369

9.909 4.435 5.809 5.698 5.616 5.013 5.033 4.551 6.488 4.42 7.002 4.71 4.674 6.533 4.959 4.960 7.879 6.275 4.710

69.78 76.95 83.92 85.19 87.19 89.67 71.8 76.0 78.4 81.4 84.8 85.9 53.0 67.6 71.5 80.1 82.6 84.6

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Table 3. Impedance Parameters for the Corrosion of Mild Steel in 0.5 M HCl in the Presence of Different Concentration of the 6-MMOPP, 5-MPOP, and 4-BPOMP Inhibitors inhibitor blank 6-MMOPP

5-MPOP

4-BPOMP

C (ppm)

Rct (Ω cm2)

Cdl (μF cm−2)

η (%)

50 100 150 200 250 300 50 100 150 200 250 300 50 100 150 200 250 300

170.4 676.2 691.8 768.4 1079.0 1231.0 1295.0 544.8 711.8 839.4 1000.0 1081.0 1136.0 382.6 506.8 624.6 940.0 1090.0 1140.0

162 291 285 260 240 232 226 262 216 209 201 193 185 553 343 350 338 336 329

74.8 75.3 77.8 84.2 86.1 86.8 68.7 76.1 79.7 82.9 84.2 85.0 55.5 66.4 72.7 81.9 84.4 85.1

5-MPOP, and 4-BPOMP inhibiters act as mixed type of inhibitors; however, the anodic effect is much more pronounced. Among the synthesized oxadiazole inhibitor, 6-MMOPP shows highest inhibition efficiency. The values of ba and bc exhibited no significant changes, which suggested that the oxadiazole derivatives are mixed type of inhibitors and inhibit corrosion by blocking the active sites of the metal;31 i. e., they reduce anodic dissolution and also retard the hydrogen evolution reaction via blocking of the active reaction sites on the metal surface or can even screen the covered part of the electrode, therefore protecting it from the action of corrosive medium. The higher values of ba compared to bc suggest that the anode is more polarized when the external current is applied. The order of inhibition efficiency was 6- MMOPP > 4-BPOMP > 5-MPOP. 3.3. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy is a powerful tool in the investigation of the corrosion and adsorption phenomena. The Nyquist plots for mild steel in 0.5 M HCl in the absence and presence of 6-MMOPP, 5-MPOP, and 4-BPOMP are shown in Figure 2a−c. Nyquist impedance plots were analyzed by fitting the experimental data to a simple circuit model (Figure 3) that includes the solution resistance (Rs), charge transfer resistance (Rct), and double layer capacitance (Cdl). The values are presented in Table 3. The η % was calculated using the charge transfer resistance as follows:

Figure 2. Nyquist plots in the absence and presence of different concentrations of (a) 6-MMOPP, (b) 5-MPOP, and (c) 4-BPOMP in 0.5 M HCl.

Figure 3. Electrochemical equivalent circuit used to fit the impedance.

where, (Icorr)a and (Icorr)p are the corrosion current density (mA cm−2) in the absence and presence of the inhibitor, respectively. From the potentiodynamic polarization curves, it can be clearly seen that, as the concentration increases, both anodic and cathodic curves were shifted toward the lower current density. This phenomenon implies that the inhibitor could suppress anodic reaction of the metal dissolution as well as cathodic hydrogen evolution.28 Ferreira and others29,30 reported that, if the deviation in the Ecorr is greater than 85 mV in inhibited system with respect to uninhibited, the inhibitor could be recognized as cathodic or anodic type whereas the deviation in Ecorr less than 85 mV, it could be recognized as mixed type of inhibitor. In the present investigation, the maximum deviation range is less than 85 mV for all the three inhibitors, which reveals that 6-MMOPP,

η%=

1/(R ct)a − 1/(R ct)p 1/(R ct)a

× 100

(2)

where, (Rct)a and (Rct)p are charge transfer resistances in the absence and presence of inhibitor, respectively. From the Nquist plots (Figure 2a−c), it is observed that the diameters of the capacitive loop increases with increasing concentrations of the inhibitors which specify the increasing coverage of metal surface. Further, it is clear from Table 3 that, by increasing the concentrations of inhibitors, the Rct values increases. This is because, the addition of inhibitor increases the adsorption over the metal surface and results in the formation of a protective layer which may decrease the electron transfer between the metal surface and the corrosive medium.32 On the other hand, the values of Cdl decreased with increasing in 2095



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Figure 4. Bode plots for mild steel in 0.5 M HCl in the presence and absence of different concentration of 6-MMOPP, 5-MPOP, and 4-BPOMP inhibitors.

efficiency. Therefore, it is suggested that the inhibitors act by adsorption at the mild steel surface or solution interface and the change in Cdl values is caused by the displacement of water molecules by the adsorption of organic molecules on the metal surface, thus decreasing the extent of the metal dissolution.33 4-BPOMP was found to have a lower inhibition efficiency and larger Cdl value compare to that of 6MMOPP and 5-MPOP (Table 3). Therefore, good inhibition efficiency could be obtained at relatively low concentration of the inhibitors. According to the Helmholtz model, the capacitance of the double layer is inversely proportional to the thickness of a protective layer.34 Figure 4a−c shows the Bode plots recorded for the mild steel electrode immersed in 0.5 M HCl in absence and presence of inhibitor 6-MMOPP, 5-MPOP, and 4-BPOMP, respectively. A new phase angle shift in the higher frequency range and a change in the phase angle shift with increase in 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 change in the phase angle shift is obviously correlated with the progress of surface coverage by inhibitor molecules.35

Figure 5. Variation of CR as a function of temperature and concentration of 6-MMOPP, 5-MPOP, and 4-BPOMP.

inhibitor concentration indicating that the inhibition efficiency increases. The decrease in Cdl values can be attributed to a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer which leads to an increase in the inhibition 2096



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Figure 6. Arrhenius plots for the corrosion of mild steel in 0.5 M HCl in the absence and presence of different concentrations of (a) 6-MMOPP, (b) 5-MPOP, and (c) 4-BPOMP. Figure 7. Alternative Arrhenius plots for mild steel in 0.5 M HCl in the absence and presence of different concentrations of (a) 6-MMOPP, (b) 5-MPOP, and (c) 4-BPOMP.

3.4. Weight Loss Measurements. 3.4.1. Effect of Inhibitor Concentration. The weight loss measurements were carried out as a function of temperature (30−60 °C) and concentration (50−300 ppm) at 3 h of immersion time. The corrosion rate (CR) was calculated from the following equation: CR =

ΔW St

agreement with those obtained from the electrochemical methods. 3.4.2. Effect of Temperature. The effect of temperature on the inhibitive performance of the synthesized oxadiazole derivatives on mild steel in 0.5 M HCl were studied in the temperature range of 30−60 °C in the absence and presence of different concentrations of inhibitor during 3 h of immersion time. The CR gets increased with the rise in temperature in the uninhibited solution, but in the presence of inhibitor, CR gets highly reduced (Figure 5). Hence, inhibition efficiency decreases with the rise in temperature. It may be due to the fact that higher temperature accelerates hot-movement of the organic molecules and weakens the adsorption capacity of inhibitor on the metal surface.38,39 Thermodynamic parameters such as the activation energy Ea*, the entropy of activation ΔSa*, and the enthalpy of activation ΔHa* for the corrosion of mild steel in the absence and presence of different concentrations of 6-MMOPP, 5-MPOP, and 4-BPOMP were calculated using the following Arrhenius-type equation:

(3) −2

−1

where, ΔW is the weight loss (gm cm h ), S is the surface area of the specimen (cm2), and t is the immersion time (h). The corrosion inhibition efficiency η (%) were calculated according to eq 4 η(%) =

(C R )a − (C R )p (C R )a

× 100

(4)

where (CR)a and (CR)p are corrosion rates in the absence and presence of the inhibitor, respectively. Weight loss data of mild steel in 0.5 M HCl in the absence and presence of various concentrations of inhibitors are tabulated in Supporting Information Table S2. The corrosion rate of mild steel decreases with increase in inhibitors concentration. The inhibitor was found to attain the maximum inhibition efficiency at 300 ppm for all the studied inhibitors (Figure 5). This is due to the fact that adsorption and the degree of surface coverage of inhibitor on the mild steel increases with the inhibitor concentration; thus, the mild steel surface gets efficiently separated from the medium.36 The protective property of these compounds is probably due to the interaction between π-electrons and heteroatoms with positively charged steel surface.37 The results obtained from the weight loss measurements are in good

⎛ E* ⎞ C R = k exp⎜ − a ⎟ ⎝ RT ⎠

(5)

An alternative formulation of the Arrhenius equation is CR = 2097

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

(6)



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where, k is Arrhenius pre-exponential factor, h is Planck’s constant, N is Avogadro’s number, T is the absolute temperature, and R is the universal gas constant. Using eq 5 and from a plot of the ln CR versus 1/T (Figure 6a−c), the values of Ea* and k at various concentrations of 6-MMOPP, 5-MPOP, and 4-BPOMP were computed from slopes and intercepts, respectively. Further, using eq 6, plots of ln(CR/T) versus 1/T gave straight lines (Figure 7a−c) with a slope of (−ΔHa*/2.303R) and an intercept of [log(R/Nh) + ΔS*a /2.303R], from which the values of ΔH*a and ΔS*a were calculated and are listed in Supporting Information Table S3. The lower values of Ea* in the inhibited systems compared to that in the blank suggest chemisorptions mechanism;40 whereas, higher values of E*a indicate a physical adsorption mechanism.41 In the present study, the values of E*a in inhibited solution increase when compared to uninhibited acid solutions (Table S3). This supports physisorption of 6-MMOPP, 5-MPOP, and 4-BPOMP on a mild steel surface. The positive sign of activation enthalpy (ΔH*a ) reflects the endothermic nature of the steel dissolution process and that the dissolution of steel is difficult.42 Negative values of (ΔSa*) imply that the activated complex in the rate-determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex.43,44 3.4.3. Adsorption Isotherm. The dependence of the degree of surface coverage (θ) as a function of concentration (C) of the inhibitor was tested graphically by fitting it to various isotherms to find the best isotherm which describes this study. The Langmuir adsorption isotherm was found to be the best description for all three synthesized oxadiazole derivatives on mild steel. According to this isotherm, θ is related to the inhibitor concentration, C, and adsorption equilibrium constant Kads as C 1 = +C θ K ads

Figure 8. Langmuir isotherm for the adsorption of (a) 6-MMOPP, (b) 5-MPOP, and (c) 4-BPOMP on mild steel in 0.5 M HCl at different temperatures.

(7)

The plot of C/θ versus C gave a straight line (Figure 8a−c) with a slope of around unity confirming that the adsorption of oxadiazole derivatives on mild steel surface in 0.5 M HCl obeys the Langmuir adsorption isotherm. According to the Langmuir adsorption isotherm, there is no interaction between the adsorbed inhibitor molecules, and the energy of adsorption is independent of the degree of surface coverage (θ). The Langmuir isotherm assumes that the solid surface contains a fixed number of adsorption sites, and each site occupies one adsorbed species. The equilibrium adsorption constant, Kads, is related to the standard Gibb’s free energy of adsorption (ΔGads) with the following equation: K ads =

⎡ ΔG ⎤ 1 exp⎢ ads ⎥ 55.5 ⎣ RT ⎦

(8)

where 55.5 is the concentration of water in solution (mol L−1), R is the universal gas constant, and T is the absolute temperature. The calculated ΔGads values of the studied oxadiazoles are tabulated in Supporting Information Table S2. The enthalpy and entropy of adsorption (ΔHads and ΔSads) can be calculated using eq 9. ln K ads = ln

ΔHads ΔSads 1 − + 55.5 RT R

Figure 9. Plot of ΔGads vs absolute temperature of 6-MMOPP, 5MPOP, and 4-BPOMP.

The negative values of ΔGads suggest that the adsorption of inhibitor molecules onto steel surface is a spontaneous phenomenon. More negative values of ΔGads suggest the strong interaction of the inhibitor molecules with the metal surface.45 Usually values of ΔGads up to −20 kJ/mol are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption) while those negative values higher

(9)

The entropy of adsorption can be calculated based on the following thermodynamic equation. ΔGads = ΔHads − T ΔSads

(10) 2098



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Figure 10. SEM images of mild steel in 0.5 M HCl after 6 h immersion at 30 °C (a) before immersion (polished), (b) without inhibitor, (c) with 300 ppm of 6-MMOPP, (d) with 300 ppm of 5-MPOP, and (e) with 300 ppm of 4-BPOMP.

of bond (chemisorption).46 Using eq 10 and from a plot of ΔGads vs T (Figure 9), the values of ΔSads and ΔHads were computed from slopes and intercepts, respectively, and the results are presented in Supporting Information Table S4. The values of ΔSads and ΔHads give information about the mechanism of corrosion. The negative value of ΔHads indicates that adsorption process is exothermic. An exothermic adsorption process may be chemisorption or physisorption or mixture of both;47 whereas, the endothermic process is attributed to chemisorptions.48 In the exothermic adsorption process, physisorption can be distinguished from the chemisorption on the basis of ΔHads values. For the physisorption process the magnitude of ΔHads is around −40 kJ mol−1 or less negative while its value −100 kJ mol−1 or more negative for chemisorptions.49 In the present work, the calculated ΔGads values (Supporting Information Table S4) between −19.31 and −19.85 indicated that the adsorption mechanism of the synthesized oxadiazole derivatives on mild steel in 0.5 M HCl solution is physisorption.50 The negative value of ΔHads again conformed that these oxadiazole derivatives adsorb on the mild steel surface through physisorption. The value of ΔSads is negative for all

Table 4. Calculated Values of Semiempirical Parameters for 6-MMOPP, 5-MPOP, and 4-BPOMP using PM3, AM1, RM1, and MNDO Hamiltonians inhibitor 6-MMOPP

5-MPOP

4-BPOMP

semiempirical EHOMO ELUMO (ΔE = ELUMO − dipole method (kJ mol−1) (kJ mol−1) EHOMO) (kJ mol−1) (D) AM1 RM1 PM3 MNDO AM1 RM1 PM3 MNDO AM1 RM1 PM3 MNDO

−887.18 −890.94 −904.45 −888.45 −913.28 −920.23 −924.20 −912.35 −118.68 −103.80 −123.24 −121.68

−115.70 −102.48 −125.15 −119.82 −126.37 −115.82 −134.18 −126.26 −885.40 −888.89 −901.34 −880.82

771.48 788.46 779.30 768.63 786.91 804.41 790.02 786.09 766.72 785.09 778.10 759.14

2.12 2.28 1.93 2.20 4.86 5.39 4.97 4.86 3.53 3.92 3.58 3.54

than −40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type 2099



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Figure 11. (a) HOMO, (b) LUMO, (c) Mulliken charge density, and (d) optimized structure of 6-MMOPP.

(Figure 10c), 5-MPOP (Figure 10d), and 4-BPOMP (Figure 10e), the mild steel surface could be observed with a thin layer of the inhibitor molecules, giving protection against corrosion. The inhibited mild steel surface was smoother than the uninhibited surface indicating the presence of a protective layer of adsorbed inhibitors preventing acid attack. The formed surface film has higher stability and low permeability in aggressive solution than uninhibited mild steel surface. Hence, they show an enhanced surface properties, which seemed to provide corrosion protection to the mild steel beneath them by restricting the mass transfer of reactants and products between the bulk solution and the mild steel surface. EDAX spectra were recorded to characterize the surface composition of mild steel in the absence and presence of inhibitors. The elements and its atomic percentages were

the inhibitor implies that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex .51 3.5. Morphological Investigation. The SEM experiments were carried out in order to verify the adsorption of 6-MMOPP, 5-MPOP, and 4-BPOMP derivatives on mild steel surface. The SEM micrographs obtained for the mild steel surface in the absence and presence of optimum concentration (300 ppm) of the inhibitors in 0.5 M HCl at 3 h immersion time and 30 °C are shown in Figure 10a−e. The image of the polished mild steel is shown in Figure 10a. The mild steel surface in the absence of inhibitors exhibited a highly corroded surface with pits and cracks (Figure 10b). This is due to the attack of mild steel surface with aggressive acid medium. However, in the presence of 6-MMOPP 2100



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Figure 12. (a) HOMO, (b) LUMO, (c) Mulliken charge density, and (d) optimized structure of 5-MPOP.

4-BPOMP and their inhibition effect on the mild steel surface. Table 4 presents the calculated values of semiempirical parameters for the synthesized inhibitors using PM3, AM1, RM1, and MNDO Hamiltonians. The calculated quantum chemical parameters included the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the energy gap (ΔE), the total energy of the molecule (TE), and the dipole moment (μ). These quantum chemical parameters are obtained after geometric optimization with respect to all nuclear coordinates. The energy of HOMO (EHOMO) is related to the electron donating ability of the molecule and the values indicate that the molecule has a tendency to donate orbital electrons to appropriate acceptor molecules with low energy or empty the 3d orbital of Fe to form a coordinate bond.55,56 Thus, increasing values of EHOMO enable adsorption by influencing the transport process through the adsorbed layer. The energy of the LUMO (ELUMO) indicates the ability of the molecule to accept electrons. The lower the value of

tabulated in Supporting Information Table S5. The spectra in the absence and presence of three inhibitors were shown in Supporting Information Figure S6. EDAX spectra of mild steel in 0.5 M HCl revealed the presence of high atomic oxygen content in the absence of inhibitors (Table S5a); whereas in the presence of 300 ppm inhibitors (Table S5b−S5d), the percentage of atomic oxygen is relatively lower. This clearly indicates that the studied inhibitors protect the mild steel surface. 3.6. Quantum Chemical Calculations. Quantum chemical methods are useful in determining the molecular structure as well as elucidating the electronic structure and reactivity.52 Therefore, it has become a common practice to carry out quantum chemical calculations in the field of corrosion inhibition studies. The selection of effective and appropriate inhibitors for the corrosion of metals has been widely carried out based on empirical approach.53,54 In the present study, quantum chemical calculations were performed for investigating the correlation between the molecular structures of 6-MMOPP, 5-MPOP, and 2101



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Figure 13. (a) HOMO, (b) LUMO, (c) Mulliken charge density, and (d) optimized structure of 4-BPOMP.

ELUMO, the more probable it is that the molecule would accept electrons, so that a back-donating bond can be formed with its antibonding orbitals.57 A good corrosion inhibitor is usually those organic compounds which not only offer electrons to an unoccupied orbital of the metal but also accept free electrons from the metal.54,58 Similarly, low values of the energy gap (ΔE = ELUMO − EHOMO) yields good inhibition efficiencies, because the energy required to remove electron from the last occupied orbital will be low.55 Low values of the dipole moment (μ) favor the accumulation of inhibitor molecules on the metallic surface.57 In the present study, the value of dipole moment of 6MMOPP is lower than that of 5-MPOP and 4-BPOMP for all semiempirical parameters methods; thus the inhibitor 6MMOPP showing higher efficiency when compare to 5-MPOP and 4-BPOMP. Quantum chemical parameters for 6-MMOPP, 5-MPOP, and 4-BPOMP are tabulated in Table 4. The calculated quantum chemical calculation provides trends in the reactivity

and selectivity features of the studied compounds. The results show that 6-MMOPP is the most preferred compound as corrosion inhibitor. A comparison of the studied oxadiazoles compounds show that there are some significant quantum chemical parameter differences between them as a result of difference in electron density around the atoms. Figures 11a−d, 12a−d, and 13a−d show the HOMO density distribution, LUMO density distribution, the Mulliken charge population analysis, and optimized structures of 6-MMOPP, 5-MPOP, and 4-BPOMP molecules. The use of Mulliken population analysis has been used to find out the adsorption centers of the inhibitors.59 From the Mulliken charge population analysis, it can be seen that C0, C1, N2, C4, C5, C6, O8, N10, N11, C12, C13, C14, C15, C16, C17, C19, C20, C21, and N22 [in 6-MMOPP]; C0, C1, N2, C4, C5, C6, O8, N10, N11, C13, C14, N15, C16, C17 [in 5-MPOP]; and C0, C1, N2, C4, C5 O7, N9, N10, C11, C12, C13, C14, C15, and C16 [in 4-BPOMP] were the atoms with excess negative charges. In 2102



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all the three molecules N, O, and aromatic ring carbon atoms have the highest negative charge. This implies that the total electron density is located around these atoms. Therefore, the adsorption of 6-MMOPP, 5-MPOP, and 4-BPOMP inhibitors on mild steel would take place through these atoms. 3.7. Mechanism of Corrosion Inhibition. The inhibition efficiency of an organic compound depends on many factors including the electronic structure, number of adsorption centers, mode of interactions with metal surface, molecular size, and chemical properties of the inhibitor being adsorbed. It is wellknown that metal has an affinity toward nitrogen and sulfur- and oxygen-bearing ligands. The studied three inhibitors 6-MMOPP, 5-MPOP, and 4-BPOMP contain benzeneacetonitrile, pyridine, and bromobenzene groups, respectively. The unshared electron pairs on N, O, and Br are capable of forming interaction with mild steel, enhancing the adsorption of the inhibitors on the metal surface. The inhibitors are expected to get adsorbed through the lone pair of electrons on the N atoms of the oxadiazole ring as well as π-electron density on the aromatic rings by their interaction with the metal surface. 6-MMOPP and 5-MPOP have nearly the same size and the number of active centers, but 6-MMOPP shows higher inhibition efficiency than 5-MPOP due to higher delocalization π- electrons density at the benzene ring, and the least delocalization is found in 4-BPOMP. In 6-MMOPP, the carbon which is attached to the nitrile group is also attached to two electron donating methyl groups, so the electron population is more toward nitrile group. As a result, the net negative charge developed on the nitrile group resulting in the highest inhibition efficiency.60,61 The N−C−C skeleton is linear in nitrile, reflecting that the sp hybridization of the triply bonded carbon thus acts as anchoring site. The C−N distance is short at 1.16 Å consistent with a triple bond. It should also be emphasized that the large size and high molecular weight of 6-MMOPP can also contribute the greater inhibition efficiency.62

Article

ASSOCIATED CONTENT

S Supporting Information *

1. The chemical composition of the mild steel, the results of weight loss measurements, and activation adsorption parameters for mild steel in 0.5 M HCl without and with different concentrations of studied inhibitors at various temperatures. 2. Correlation of inhibition efficiencies between polarization and impedance. 3. EDAX spectral data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-821-2419654. E-mail address: drknmohana@gmail. com. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION 1. All the studied oxadiazole derivatives shown excellent inhibition property for the corrosion of mild steel in 0.5 M HCl solutions, and the inhibition efficiency increases with increasing concentration of the inhibitors. 2. The inhibition ability of these compounds follow the order 6-MMOPP > 4-BPOMP > 5-MPOP, and the inhibition efficiencies determined by polarization, EIS, and weight loss methods are in good agreement with each other. 3. The adsorption of all the studied molecules obeys the Langmuir isotherm model. The negative values of free energy of adsorption indicated that the adsorption of the oxadiazole molecule is spontaneous process. 4. The calculated ΔGads and ΔHads values indicated that the adsorption mechanism of the synthesized oxadiazole derivatives on mild steel in 0.5 M HCl solution is physisorption. 5. SEM analysis shows that the formed surface film has higher stability and low permeability in aggressive solution than uninhibited mild steel surface, and EDAX analysis indicates that the studied inhibitors protect the mild steel surface. Hence, they show enhanced surface properties. 6. Quantum chemical calculations are well-correlated with the experimental observation. 2103



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