Experimental and Quantum Chemical Studies on the Corrosion

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Experimental and Quantum Chemical Studies on Corrosion Inhibition Performance of Benzimidazole Derivatives for Mild Steel in HCl Mahendra Yadav, Sumit Kumar, Rajesh Ranjan Sinha, and Debasis Behera Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400099q • Publication Date (Web): 15 Apr 2013 Downloaded from http://pubs.acs.org on April 15, 2013

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Industrial & Engineering Chemistry Research

Experimental and Quantum Chemical Studies on Corrosion Inhibition Performance of Benzimidazole Derivatives for Mild Steel in HCl Mahendra Yadav*, Debasis Behera, Sumit Kumar, Rajesh Ranjan Sinha

Department of Applied Chemistry, Indian school of Mines, Dhanbad 826004, India Email: [email protected]

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ABSTRACT In the present investigation three benzimidazole derivatives namely, 4-(phenyl)-5-[(2-methyl1H-benzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols

(Inh

I),

4-(4-methylphenyl)-5-[(2-

methyl-1H-benzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols (Inh II), 4-(4-methoxyphenyl)5-[(2-methyl-1H-benzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols (Inh III) were synthesized and investigated as an inhibitor for mild steel corrosion in 15% HCl solution by using weight loss, electrochemical polarization and electrochemical impedance spectroscopy (EIS) techniques. It was found that the inhibition efficiency of these inhibitors increases with increase in concentration. The effect of temperature on the corrosion rate was investigated and some thermodynamic parameters were calculated. Polarization studies show that all the studied inhibitors are of mixed type in nature. The adsorption of the inhibitors on the mild steel surface in the acid solution was found to obey Langmuir’s adsorption isotherm. Scanning electron microscopic (SEM) study was done on the inhibited and uninhibited mild steel samples to characterize the surface. Semiemperical AM1 method was employed for theoretical calculations and the obtained results were found to be consistent with the experimental findings. Keywords: Mild steel, Inhibition, Hydrochloric acid, EIS, SEM, Molecular modeling.


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1. INTRODUCTION Mild steel is frequently used as main construction material for down hole tubular, flow lines and transmission pipelines in petroleum industry. Acid solutions are commonly used for pickling, industrial acid cleaning, acid descaling and oil well acidifying1–5 processes. Acidization of petroleum oil wells is one of the important stimulation techniques for enhancing oil production. It is commonly brought about by forcing 15% to 28% hydrochloric acid as a solution into the well to open up near-bore channels in the formation and, hence, to increase the flow of oil. Hydrochloric acid is useful in removing carbonate, limestone and dolomites from the reservoirs rock. Also, an aqueous solution of hydrochloric and hydrofluoric acids can be used to dissolve quartz, sand and clay from the reservoir rocks5-7. Pickling involves chemical removal of oxides and scale from the surface of strip, sheet, plate, or semi-finished products of iron and steel by an aqueous solution of inorganic acids, such as sulfuric or hydrochloric acid. Hydrochloric acid pickling provides a faster and cleaner pickling with less acid consumption and reduced quantities of waste pickle liquor. The hydrochloric pickling baths are maintained at a temperature in the range of 65 to 80 °C and the acid concentration in the range of 5 to 50% hydrochloric acid. The acid cleaning generally refers to the use of acid solutions for final or near-final preparation of metal surfaces before plating, painting, or storage.Because of aggressiveness of acid solutions mild steel corrodes severely during these processes particularly with the use of hydrochloric acid, which results in terrible waste of both resources and money8. Addition of corrosion inhibitor is the procedure to mitigate the process of corrosion of metal against acid attack. Most of the well known corrosion inhibitors are organic compounds containing polar groups having nitrogen, sulphur and/or oxygen atoms and heterocyclic compounds with polar functional groups and conjugated double bonds

9-11

. These compounds can adsorb on the metal surface and block the

active sites on the surface, thereby reducing the corrosion rate. Most of the investigations are related

to

the

application

of

common

thiosemicarbazides12,13aminothiazoles14,

inhibitors

like

benzotriazoles15–17,

various

thioimidazole18,

derivatives

of

thiadiazole19,

mercapto-5-triazole 20 as potential inhibitors for mild steel in acid solutions. Keeping in view the above excellent behavior of the organic compounds containing nitrogen and sulphur as corrosion inhibitors, some derivatives of benzimidazole viz. 4-(phenyl)-5-[(2-methyl-1H-benzimidazol-13 ACS Paragon Plus Environment


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yl)methyl]-4H-1,2,4-triazole-3-thiols

(Inh

I),

4-(4-methylphenyl)-5-[(2-methyl-1H-

benzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols (Inh II),

4-(4-methoxyphenyl)-5-[(2-

methyl-1H-benzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols (Inh III) were synthesized and their corrosion inhibition property was studied by the weight loss and electrochemical techniques. 2. EXPERIMENTAL SECTION 2.1. Materials 2.1.1. Mild steel sample The corrosion studies were performed on mild steel samples with a composition (wt. %): C, 0.12; Mn, 0.11; Cu, 0.01; Si, 0.02; Sn, 0.01; P, 0.02; Ni, 0.02 and Fe balance. Mild Steel coupons having dimension 6.0 cm × 2.5 cm × 0.1 cm size were mechanically cut and abraded with different grade emery papers (120, 220, 400, 600, 800, 1500 and 2000 grade) for weight loss experiment. For electrochemical measurements mild steel coupons having dimension 1.0 cm × 1.0 cm× 0.1 cm were mechanically cut and abraded similarly to previous procedure, with an exposed area of 1 cm2 (rest covered with araldite resin) with 3 cm long stem. Prior to the experiment, specimens were washed with distilled water, degreased in acetone, dried and stored in vacuum desiccator. 2.1.2. Test Solutions Analytical reagent (AR) grade HCl was diluted with triple distilled water to obtain 15% HCl. The concentration range of inhibitors employed was varied from 20 to 200 ppm (mg L-1) and the volume of the electrolyte used was 250 mL. 2.1.3 Synthesis of corrosion inhibitors The N-(aryl)-2-[(2-methyl-1H-benzimidazol-1-yl)acetyl] hydrazinecarbothioamides 4a-c were prepared by condensing compound 3 with appropriate phenyl isothiocyanates. Cyclization of compounds 4a-c in 2 M NaOH solution under reflux gave the 4-(aryl)-5-[(2-methyl-1Hbenzimidazol-1-yl)methyl]-4H-1,2,4-triazole-3-thiols 5a-c

21

(Scheme 1). The purity of the

synthesized compounds were confirmed by Thin layer chromatography. The products were characterized by spectral and elemental analysis methods. Compounds 5a-c showed the –N–N– group band between 1253–1275 cm–1 and S–H group band between 2545 –2562 cm–1, which 4 ACS Paragon Plus Environment

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indicate the formation of the triazole ring. In the 1H-NMR spectra of compounds 5a-c, a sharp peak between δ 12.90–12.92 showed the presence of the C–SH proton and the –N–CH2 proton signal appeared between δ 6.00–6.18. The mass spectra of compounds 5a-c exhibited molecular ion peaks at m/z 321, 351, 337, 355, 444 and 336 together with their fragmentation peaks, which indicated the formation of required benzimidazole derivatives. The structure of Inh I, InhII and Inh III are shown in Figure 1.

2.2. Methods 2.2.1. Weight loss method Weight loss measurements were performed at 303 K by immersing accurately weighed mild steel test coupons in 250 mL of 15% HCl in absence and in the presence of 20, 50, 100, 150 and 200 ppm by weight of the inhibitors. The immersion time was optimized and optimized immersion time (6 h) was uniformly used for weight loss measurements. The test coupons were then removed from the electrolyte, washed thoroughly with distilled water, dried and weighed. Triplicate experiments were conducted for each concentration of the inhibitor for the reproducibility and the average of weight losses were taken to calculate the corrosion rate and inhibition efficiency of the inhibitors. The corrosion rate (CR), inhibition efficiency (η%) and surface coverage (θ) were determined by following equations22:

(

)

C R m m y -1 =

8 7 .6W A td

(1)

where W = weight loss, A = area of specimen in cm2 exposed in acidic solution, t = immersion time in hours, and d = density of mild steel ( g cm-3). θ =

C R0 − C Ri C R0

η (% ) =

(2)

C R 0 − CR i × 100 C R0

(3)

where CR0 and CRi are corrosion rate in absence and presence of inhibitors. This experiment was repeated at different temperature of 30, 40, 50 and 60 ˚C by using water circulated Ultra thermostat to determine the temperature dependence of the inhibition efficiency. 2.2.2. Electrochemical studies Electrochemical polarization measurements were carried out in a conventional three-electrode cell consisting of mild steel working electrode (WE), a platinum counter electrode (CE) and a 5 ACS Paragon Plus Environment


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saturated calomel electrode (SCE) as reference electrode, using CH electrochemical workstation at 25 ºC. Before starting the experiments, the working electrodes were immersed in the test solution for 30 min until a steady potential reached. The mild steel surface was exposed (1cm2) to various concentrations (50-200 ppm by weight) of different inhibitors in 250 mL of 15% HCl at 25 ºC. After establishment of the open circuit potential, dynamic polarization curves were obtained with a scan rate of 1 mVs-1 in the potential range from -700 to +300 mV. Corrosion current density (icorr) values were obtained by Tafel extrapolation method. All potentials were measured against SCE. The percentage inhibition efficiency (η %), was calculated using the equation η (%) =

0 icorr − icorr × 100 0 icorr

(4)

where, i0corr and icorr are the values of corrosion current density in absence and presence of inhibitors, respectively. Impedance measurements were carried out same electrochemical cell and electrochemical workstation as mentioned above in the frequency range from 10000 to 0.1 Hz using amplitude of 10 mV peak to peak with an ac signal at the open-circuit potential. The impedance data were obtained by using Nyquist plots. The polarization resistance (Rp) was calculated from the diameter of the semicircle in the Nyquist plot23. The polarization resistance (Rp) includes charge transfer resistance (Rct), diffuse layer resistance (Rd), the resistance of accumulated species at the metal/solution interface (Ra) and the resistance of film (in the presence of the inhibitor) at the metal surface (Rf) (Rp = Rct + Rd + Ra + Rf) . The inhibition efficiency (η %) was calculated from polarization resistance values obtained from impedance measurement according to the following relation23: η (%) =

Rp(inh) − Rp Rp(inh)

× 100

(5)

where Rp(inh) and Rp are charge transfer resistance in presence and absence of inhibitor respectively. The double-layer capacitance (Cdl) was calculated by using the following equation23: C dl =

1 2π f m ax R p

(6)

where fmax is the frequency at the maximum on the Nyquist plot. 6 ACS Paragon Plus Environment

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2.2.3. Scanning electron microscopic analysis The mild steel specimens of size 1.0 cm × 1.0 cm × 0.1 cm were abraded with a series of emery paper (grade 320-500-800-1200) and then washed with distilled water and acetone. After immersion in 15% HCl in the absence and the presence of optimum concentration of inhibitors (Inh I, Inh II, Inh III) at 25 0C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then the SEM images were recorded using JEOL JSM – 6380 LA analytical scanning electron microscope. 2.2.4. Quantum chemical study Quantum chemical calculations were carried out by using semi- empirical AM1 for calculating the physical properties of molecules from the program package HYPERCHEM 8.0. The geometry optimizations of three inhibitors were obtained by application of the unrestricted Hartre–Fock (UHF) method using AM1 parameterization. The coordinates of a molecular structure that represent the potential energy minimum were obtained using a Polak–Ribiere conjugate gradient method24. The iteration procedure was continued until root mean square energy gradient become lower than 0.1 kcal Å-1 mol-1. Theoretical parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), energy gap (∆E), dipole moment (µ), global hardness (η) and softness (σ) and the fraction of electrons transferred (∆N) were calculated. 3. RESULTS AND DISCUSSION

3.1. Weight loss measurements Corrosion inhibition efficiency (η %) offered by inhibitors (Inh I, Inh II and Inh III) have been evaluated by weight loss technique after 6 h of immersion at 303 K are listed in the Supporting Information (Table S1). From the Table S1 in the Supporting Information, it is apparent that inhibition efficiency increases with increase in the concentration of the inhibitors. This observation could be attributed to the increase in the amount of inhibitor molecules adsorbed on the metal surface, which separate the mild steel from the acid solution, resulting retardation of metal dissolution 25. The inhibition efficiency of inhibitors follow the order: Inh III > Inh II > Inh I. Corrosion inhibition studies were also carried out at different temperatures ranging from 7 ACS Paragon Plus Environment


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303 K to 333 K. Corrosion parameters namely corrosion rate (CR), surface coverage (θ) and inhibition efficiency (η %) of mild steel in 15% HCl in the presence and absence of inhibitor at different temperatures, obtained from weight loss measurements are are listed in the Supporting Information (Table S1). From the Table S1 in the Supporting Information, it is apparent that the maximum efficiency of 96.4% was obtained for 200 ppm concentration of Inh III at 333 K. The corrosion rate of mild steel in absence of inhibitors increased steeply when temperature is increased from 303K to 333K whereas in presence of inhibitors the corrosion rate increases slowly up to 323 K and A minor change in corrosion rate was observed between 323 K and 233 K for all the three studied inhibitors. The corrosion rates were much less in presence of inhibitors 26,27

as compared to the rates in presence of inhibitors at each temperature.

3.1.1. Thermodynamic and activation parameters Thermodynamic and activation parameters play an important role in understanding the inhibitive mechanism. The weight loss measurements were done in the temperature range of 303-333 K in the absence and presence of different concentrations of inhibitors (Inh I, Inh II and Inh III) in 15% HCl for mild steel. The apparent activation energy (Ea) for dissolution of mild steel in 15% HCl can be expressed by using Arrhenius equation. lo g C R =

− Ea + log A 2.3 03 R T

(7)

Where Ea is the apparent activation energy, R is the molar gas constant (8.314 J K-1mol-1), T is the absolute temperature (in K) and A is the Arrhenius pre-exponential factor. Figure 2 represents the Arrhenius plot of log CR against 1/T for the corrosion process of mild steel in 15% HCl solution in the absence and presence of inhibitors (Inh I, Inh II and Inh III) at concentrations ranging from 20 ppm to 200 ppm. From Figure 2, the slope (−Ea / R) of each individual line was determined and activation energy was calculated by using (Ea = (slope) × 2.303 × R) and the calculated value of Ea were summarized in the Supporting Information (Table S2). It is evident from the Table S2 in the Supporting Information, that the values of the apparent activation energy Ea for the inhibited solutions are lower than that for the uninhibited one, indicating a chemisorption process of adsorption 28. The energetic barrier is lower, facilitating the formation of Fe2+ ions which interact with the studied inhibitors to form a protective film at the surface of mild steel. Similar behaviour was reported in literature29 by some authors. The relationships


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between the temperature dependence of percentage η % of an inhibitor and the Ea can be classified into three groups according to temperature effects 30. (i) η % decreases with increase in temperature, Ea (inhibited solution) > Ea (uninhibited solution) (ii) η % increases with increase in temperature, Ea (inhibited solution) < Ea (uninhibited solution) (iii) η % does not change with temperature, Ea (inhibited solution) = Ea (uninhibited solution) In the present investigation all the three inhibitors follows the second group (ii) where Ea (inhibited solution) < Ea (uninhibited solution), and Ea decreases with inhibitor concentration, which further confirm η % increases with increase in temperature. The values of standard enthalpy of activation (∆H*) and standard entropy of activation (∆S*) can be calculated by using the equation: CR =

 ∆S *   ∆H *  RT exp  exp  −  Nh  R   RT 

(8)

where, h is Planck’s constant and N is the Avogadro number, respectively. A Plot of log (CR /T) against 1/T (Figure 3) gave straight lines with slope of (-∆H*/2.303R) and intercept of (log R/Nh + ∆S*/2.303R) from which the activation thermodynamic parameters (∆H* and ∆S*) were calculated and listed in the Supporting Information (Table S2). The positive sign of the enthalpy reflects the endothermic nature of the mild steel dissolution process31, 32. The negative value of ∆S*for all the three inhibitors indicates that activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disorder takes place during the course of transition from reactant to the activated complex 33. 3.1.2. Adsorption isotherm Basic information on the interaction between the organic inhibitors and the mild steel surface are obtained from various adsorption isotherms. The most commonly used adsorption isotherms are Langmuir, Temkin, and Frumkin isotherm. The surface coverage (θ) for different concentrations of inhibitors in 15% hydrochloric acid was tested graphically for fitting a suitable adsorption isotherm. Plotting Cinh/θ vs. Cinh yielded a straight line (Figure 4) with a correlation coefficient (r2) values 0.998, 0.997, 0.999 for Inh I, Inh II and Inh III respectively at 303K. This indicates that the adsorption of these inhibitors can be fitted to a Langmuir adsorption isotherm represented by the following equation. 9 ACS Paragon Plus Environment


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C in h

θ

=

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1 + C in h K ads

(9)

where, Cinh is the inhibitor concentration, Kads is the equilibrium constant for adsorptiondesorption process. The slope values for all the three inhibitors were also found very near to unity confirming the validity of Langmuir adsorption isotherms. From the intercept of Figure 4 the values of Kads were calculated. Large values of Kads obtained for all the three studied inhibitors imply more efficient adsorption and hence better corrosion inhibition efficiency. Using the values of Kads the values of ∆G◦ads was evaluated by using the following equation: ∆ G a0d s = − R T ln ( 5 5 .5 K a d s )

(10)

where R is the gas constant and T is the absolute temperature (K). The value of 55.5 is the concentration of water in solution in mol L-1. Calculated values of Kads and ∆G˚ads are listed in the Supporting Information (Table S3). It is apparent from the Table S3 in the Supporting Information that the Calculated values of ∆G˚ads lies between -37.8 to -45.1 kJ mol-1, -38.2 to 48.7 kJ mol-1, -42.7 to -49.1 kJ mol-1, for Inh I, Inh II and Inh III respectively in the temperature range 303 to 333 K. It is generally accepted that for the values of ∆G°ads upto – 20 kJ/mol, the type of adsorption were regarded as physisorption; the inhibition acts due to the electrostatic interactions between the charged molecules of the inhibitors and the charged metallic surfaces, while the values above - 40 kJ/mol were reported as chemisorption, which is due to the charge sharing or a transfer from the inhibitors molecules to the metal surface to form a covalent bond3436

.The values of ∆G°ads in our measurements suggested that the adsorption of these molecules

involves chemisorptions 37. 3.2. Potentiodynamic polarization measurements The anodic and cathodic polarization curves for the corrosion of mild steel in 15% HCl in the presence and absence of varying concentrations of inhibitors (Inh I, Inh II and Inh III) at 303K are shown in Figure 5. The polarization curves in Figure 5 exhibit cathodic polarization curves with well defined Tafel regions whereas, anodic branch of the polarization curves display a curve profile with arrests. This type of anodic polarization behaviour in acidic solution has been attributed to the deposition of the trace amount of corrosion products or impurities in the steel to form a non-passive surface film38. The corrosion current densities were calculated by extrapolation of linear parts of these curves to corresponding corrosion potential. The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), 10 ACS Paragon Plus Environment

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anodic Tafel slope (ba) and cathodic Tafel slope (bc) and percentage inhibition efficiency (η %) determined from polarisation curves are summarized in Table 1. The data in Table 1 clearly show that the current density decreases in the presence of inhibitors (Inh I, Inh II and Inh III); this indicated that inhibitors adsorbed on the metal surface and hence the inhibition efficiency increases with the increase in the inhibitor concentrations. It is apparent from the Figure 5 that the nature of the polarization curves remain the same in the absence and presence of inhibitors but the curves shifted towards lower current density in presence of inhibitors. It concluded that the inhibitors molecules retard the corrosion process without changing the mechanism of corrosion process in the medium of investigation. The presence of inhibitors cause small change in Ecorr value. This implies that the inhibitor acts as a mixed type inhibitor, affecting both anodic and cathodic reactions

39

. If the displacement in Ecorr is more than ±85 mV/SCE relating to

corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type 40. If the change in Ecorr is less than 85 mV, the corrosion inhibitor may be regarded as a mixed type. The maximum displacement in our study is less than 20 mV/SCE, which indicates that Inh I, Inh II and Inh III act as mixed type inhibitor. However the minor shift of Ecorr values towards positive direction on increasing the concentration of inhibitors suggesting the predominant anodic control over the reaction.

3.3. Electrochemical impedance spectroscopy Nyquist plot of mild steel in 15% HCl solution in absence and presence of different concentrations of Inh I, Inh II and Inh III at 303 K are shown in Figur 6. It follows from Figure 6 that a high frequency (HF) depressed semicircle was observed. The polarization resistance (Rp) and double layer capacitance (Cdl) obtained from the Nyquist plots and the calculated inhibition efficiency values (η %) are shown in Table 2. It is apparent from the Table 2 that the value of Rp increases on increasing the concentration of the inhibitors.The increase in Rp values is attributed to the formation of an insulating protective film at the metal/solution interface. It is also clear that the value of Cdl decreases on the addition of inhibitors, indicating a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting the inhibitor molecules function by the formation of the protective layer at the metal surface 41. This type of behavior can be generalized and explained by Helmholtz model42 given as: 11 ACS Paragon Plus Environment


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C dl =

εε 0 A

Page 12 of 35

(11)

d

where ε is the dielectric constant of the medium, ε0 is the permittivity of the free space; A is the effective surface area of the electrode, d is the thickness of the protective double layer formed by inhibitors. So, the changes in Rp and Cdl values were caused by the steady replacement of the water molecules by adsorption of inhibitor on mild steel surface, reducing the extent of metal dissolution43. The results obtained by electrochemical polarization, EIS measurements and weight loss measurements are in good agreement. The maximum inhibition efficiency (η %) of 90.5 % is shown by Inh III for 200 ppm concentration at 303 K. The results obtained for studied inhibitors are comparable to those obtained for previously reported similar type of inhibitors44-46.

3.4. Scanning electron microscopy The surface morphology of the mild steel samples in 15% HCl solution in the absence and in the presence of 200 ppm of Inh I, Inh II and Inh III are shown in Figure 7 (a, b c, d, e). The badly damaged surface obtained when the metal was kept immersed in 15% HCl solution for 6 h without inhibitor indicates significant corrosion. However, in presence of inhibitors the surface has remarkably improved with respect to its smoothness indicating considerable reduction of corrosion rate. This improvement in surface morphology is due to the formation of a good protective film of inhibitor on mild steel surface which is responsible for inhibition of corrosion.

3.5. Theoretical calculation In order to study the effect of molecular structure on the inhibition efficiency, quantum chemical calculations were performed by using semi-empirical AM1 method and all the calculations were carried out with the help of complete geometry optimization. Optimized structure, EHOMO and ELUMO are shown in Figure 8. The frontier molecular orbital energies ( EHOMO and ELUMO) are significant parameters for the prediction of the reactivity of a chemical species. The EHOMO is often associated with the electron donating ability of a molecule. The inhibition efficiency increases with the increasing EHOMO, values. High EHOMO values indicate that the molecule has a


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tendency to donate electrons to appropriate acceptor molecules with low energy empty molecular orbital. The lower value of ELUMO, suggesting that the the molecule easily accepts electrons from the donor molecules47, 48. It was reported previously by some researchers that smaller values of ∆E and higher values of dipole moment (µ) are responsible for enhancement of inhibition efficiency49,50.The quantum chemical parameters such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap (∆E= ELUMO- EHOMO), the dipole moment (µ), absolute electronagativity (χ), global hardness (η) and softness (σ) were calculated and summarized in Table 3. For the calculations of quantum chemical parameters the following equations were used 51-53 χ =−

η=

ELUMO + EHOMO 2

(12)

E LUMO − E HOMO 2

(13)

The inverse of the global hardness is designated as the softness, σ as follows:

σ=

1

(14)

η

where, hardness and softness are the properties to measure the molecular stability and reactivity. Hard molecule has large energy gap and a small gap exist in soft molecule. Soft molecules are more reactive than hard ones because they can offer electron to acceptors easily. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where σ which is a local property, has the highest value 51. In the recent literature54, it is reported by Kokaji that the work function (Φ) of metal surface is an appropriate measure of its electronagativity and should be used together with its vanishing absolute hardness to estimate the ∆N as given below: ∆N =

Φ − χ inh 2ηinh

(15)

According to the result obtained from the Table 3, the highest value of EHOMO (-8.6541 eV) and lowest values of ∆E (7.9472 eV) are found for Inh III. It can be affirmed that Inh III has more potency to get adsorbed on the mild steel surface than Inh I and Inh II. For our present 13 ACS Paragon Plus Environment


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investigation the inhibition efficiency increases with the increasing dipole moment of the inhibitors. In literature some degree of confusion exists when dealing with dipole momentum data interpretation where some authors reported positive55,56 and some reported negative57,58 relationship between µ and inhibition efficiency. From our present investigation Inh III is having highest value of σ (0.2516 eV) has the highest efficiency (Table ) which is good in agreement with experimental data. Generally ∆N shows inhibition efficiency resulted by electron transferred from the inhibitor molecule to the iron atom 51. According to Lukovits’s study

52

, if

the value of ∆N is < 3.6, the efficiency of inhibition increases with increasing electron donating ability of the inhibitor at the metal surface. An improvement in electronic releasing power was shown by replacing hydrogen atom by electron donating substituent (–CH3 and –OCH3 groups) which improves the inhibition efficiency. In this study, it can be seen from Table 3 that the ability to donate electrons to the metal surface (∆N) follows the order of Inh III > Inh II > Inh I, which is in good agreement with the order of inhibition efficiency of these inhibitors. 4. CONCLUSIONS (1) The synthesized bemzimidazole derivatives show good inhibition efficiencies for the corrosion of mild steel in 15% HCl solution and the inhibition efficiency increases on increasing the concentration of these inhibitors and with the increase in temperature. The order of inhibiting performance of the inhibitors is: Inh I < Inh II < Inh III. (2) The variation in the values of ba and bc (Tafel slopes) and the minor deviation of Ecorr with respect to E (corrosion potential of the blank) indicate that all the three tested inhibitors are mixed type in nature. (3) EIS measurements show that charge transfer resistance (Rp) increases and double layer capacitance (Cdl) decreases in presence of inhibitors, suggested the adsorption of the inhibitor molecules on the surface of mild steel. (4) It is suggested from the results obtained from SEM and Langmuir adsorption isotherm that the mechanism of corrosion inhibition is occurring mainly through adsorption process.


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(5) The experimental inhibition efficiencies of three inhibitors are closely related to the quantum chemical parameters EHOMO, ELUMO, ∆E and σ. 5. SUPPORTING INFORMATION Results of weight loss measurements, activation parameters and adsorption parameters for mild steel in 1 M HCl without and with different concentrations of studied inhibitors at various temperatures are given here. This material is available free of charge via the Internet at http://pubs.acs.org.



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(27) Fragoza-Mar L.; Olivares-Xometl, O.; Domínguez-Aguilar, M. A.; Flores, E. A.; ArellanesLozada, P.; Jimenez-Cruz F. Corrosion inhibitor activity of 1,3-diketone malonates for mild steel in aqueous hydrochloric acid solution. Corros. Sci. 2012, 61, 171–184. (28) Dehri, I.; Ozcan, M. The effect of temperature on the corrosion of mild steel in acidic media in the presence of some sulphur containing organic compounds. Mater. Chem. Phys. 2006, 98, 316-323. (29) Ashassi-Sorkhabi, H.; Shaabani, B.; Seifzadeh, D. Corrosion inhibition of mild steel by some schiff base compounds in hydrochloric acid. App. Surf. Sci. 2005, 239, 154-164. (30) Priya, A. R. S.; Muralidharam, V. S.; Subramania, A. Development of novel acidizing inhibitors for carbon steel corrosion in 15% boiling hydrochloric acid. Corrosion. 2008, 64, 541– 552. (31) Popova, A.; Sokolova, E.; Raicheva, S.; Chritov, M. AC and DC study of the temperature effect on mild steel corrosion in acid media in presence of benzimidazole derivatives. Corros. Sci. 2003, 45, 33–41. (32) Quartarone, G.; Moretti, G.; Tassan, A.; Zingales, A. Inhibition of mild steel corrosion in 1N sulphuric acid through indole. Mater. Corros. 1994, 45, 641-647. (33) Ramesh, S. V.; Adhikari, V. Inhibition of corrosion of mild steel in acid media by N0benzylidene-3-(quinolin-4-ylthio)propanohydrazide, Bull. Mater. Sci. 2007, 31, 699–711. (34) Talati, J. D.; Gandhi, D. K. N-Heterocyclic compounds as corrosion inhibitors for aluminium-copper alloy in Hydrochloric acid. Corros. Sci. 1983, 23, 1315-1332. (35) Szklarska-Smialowska, Z.; Mankowski, J. Crevice corrosion of stainless steels in sodium chloride solution. Corros. Sci. 1978, 18, 953-960.



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Caption for Figures Scheme 1. Synthetic route of inhibitors: (a) Inh I, (b) Inh II, (c) Inh III. Figure 1. Structure of inhibitors: Inh I, Inh II, Inh III.

Figure 2. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl (a): Inh III (b): Inh II (c): Inh I Figure 3. Transition state plot of log CR/T versus 1000/T for mild steel in 15% HCl at different concentration (a): Inh III (b): Inh II (c): Inh I Figure 4. Langmuir plots of (Cinh/θ) versus Cinh for (a): Inh III (b): Inh II (c): Inh I Figure 5. Potentiodynamic polarization curves for mild steel in 15% HCl in the presence and absence of inhibitor 303 K. (a): Inh I (b): Inh II (c): Inh III Figure 6. Nyquist plot for mild steel in 15% HCl acid containing various concentrations of (a) Inh I, (b) Inh II, (c) Inh III; (1) 0 ppm (blank) (2) 20 ppm (3) 50 ppm (4)100 ppm and (5) 150 ppm (6) 200 ppm at 303 K Figure 7. SEM image of mild steel in 15% HCl solution after 6 h immersion at 303K (a) before immersion (polished) (b) After immersion without inhibitor (c) in presence of 200 ppm of inhibitor Inh III (d) in presence of 200 ppm of inhibitor Inh II (e) in presence of 200 ppm of inhibitor Inh I. Figure 8. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) Inh I, (b) Inh II and (c) Inh III. [H, Grey; C, Cyan; N, Blue; O, Red; S, Yellow]



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Table 1. Electrochemical parameter and percentage Inhibition efficiency (η %) obtained from polarisation studies for mild steel in 15% HCl solution in the presence or absence of inhibitor at 303 K. inhibitor

Concentration (ppm)

Ecorr(mV)

ba (mV dec-1)

bc (mV dec-1)

Icorr (µAcm-2)

η%

20 50 100 150 200

-363 -352 -338 -343 -336 -334

95 75 84 87 86 88

135 104 96 93 87 85

573 172 148 128 90 54

69.8 74.1 77.5 84.3 90.5

Inh II

20 50 100 150 200

-348 -340 -337 -332 -327

79 71 68 65 64

127 93 84 78 74

197 176 142 95 86

65.5 69.2 75.2 83.5 85.1

Inh I

20 50 100 150 200

-343 -335 -330 -324 -312

68 65 66 67 64

102 91 83 78 76

217 195 171 121 103

62.1 66.1 70.1 78.8 82.1

Blank Inh III


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Table 2. Electrochemical impedance parameters and percentage inhibition efficiency for mild steel in 15% HCl in the absence and presence of inhibitor at different concentration at 303 K. inhibitor Blank Inh III

Rp (Ω cm2 ) 25 78 102 114 149 192

Concentration (ppm) 20 50 100 150 200

Cdl (µF cm2 ) 252 158 122 87 73 67

η%

68.3 75.5 78.2 83.3 87.1

Inh II

20 50 100 150 200

68 77 105 132 155

82 66 53 32 26

63.4 67.7 76.4 81.1 83.9

Inh I

20 50 100 150 200

63 74 80 107 134

71 58 41 24 20

60.5 66.1 68.5 76.5 81.1

Table 3.Quantum chemical parameters for different inhibitors.

Inh I

EHOMO (eV) -8.8459

ELUMO (eV) -0.7865

∆E (eV) 8.0594

µ (D) 3.2029

η (eV) 4.0297

σ (eV) 0.2462

∆N (e) 0.2709

Inh II

-8.7028

-0.7392

7.9636

3.4714

4.1194

0.2497

0.2896

Inh III

-8.6541

-0.7069

7.9472

5.0267

3.9735

0.2516

0.2918

Inhibitor



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Figures and Scheme

N

ClCH2COOC2H5

CH3

N

H

N

NH2NH2.H2O

N

CH3

CH3

K2CO3

N

Ethanol

N

CH2CONHNH2

CH2COOC2H5

Acetone

(3)

(2)

(1)

N

NCS

R

CH3 N H2 C

N N N

2NaOH

SH

N CH3 N CH2CONHNHCSNH

R (4a-4c)

(5a-5c)

R= H, CH3, OCH3

Scheme 1. Synthetic route of inhibitors: (a) Inh I, (b) Inh II, (c) Inh III.

N

N CH3

N

CH3

N

CH3

N H2 C

N N N

H2 C SH

N

N N N

H2 C SH

CH3 Inh I

Inh II

Figure 1: Structure of inhibitors: Inh I, Inh II, Inh III.


N N N

OCH3 Inh III

SH

R

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Figure 2. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl (a): Inh III (b): Inh II (c): Inh I



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Figure 3. Transition state plot of log CR/T versus 1000/T for mild steel in 15% HCl at different concentration (a): Inh III (b): Inh II (c): Inh I


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Figure 4. Langmuir plots of (Cinh/θ) versus Cinh for (a): Inh III (b): Inh II (c): Inh I



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Figure 5. Potentiodynamic polarization curves for mild steel in 15% HCl in the presence and absence of inhibitor 303K. (a): Inh I (b): Inh II (c): Inh III 30 ACS Paragon Plus Environment

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Figure 6. Nyquist plot for mild steel in 15% HCl acid containing various concentrations of (a) Inh I, (b) Inh II, (c) Inh III; (1) 0 ppm (blank) (2) 20 ppm (3) 50 ppm (4)100 ppm and (5) 150 ppm (6) 200 ppm at 303 K



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Figure 7. SEM image of mild steel in 15% HCl solution after 6 h immersion at 303K (a) before immersion (polished) (b) After immersion without inhibitor (c) in presence of 200 ppm of inhibitor Inh III (d) in presence of 200 ppm of inhibitor Inh II (e) in presence of 200 ppm of inhibitor Inh I.


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Figure 8. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) Inh I, (b) Inh II and (c) Inh III. [H, Grey; C, Cyan; N, Blue; O, Red; S, Yellow]


Experimental and Quantum Chemical Studies on the Corrosion

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