4-(m-tolylamino) Pyridine-3-sulfonamide (TSM) as ... - ACS Publications

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4‑Chloro-2-((furan-2-ylmethyl) amino)-5-sulfamoylbenzoic Acid (FSM) and N‑(Isopropylcarbamoyl)-4‑(m‑tolylamino) Pyridine-3sulfonamide (TSM) as Potential Inhibitors for Mild Steel Corrosion in 1 N H2SO4 Medium. Part I Hari Kumar Sappani† and Sambantham Karthikeyan*,‡ †

Materials Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, Tamilnadu, India Surface Engineering Research lab, CNBT, VIT University, Vellore 632014, Tamilnadu, India

‡

S Supporting Information *

ABSTRACT: 4-Chloro-2-((furan-2-ylmethyl)amino)-5-sulfamoylbenzoic acid (FSM) and N-(isopropylcarbamoyl)-4-(mtolylamino)pyridine-3-sulfonamide (TSM) drugs were tested as inhibitors for mild steel corrosion in 1 N sulphuric acid solution by weight loss studies and electrochemical methods. The weight loss measurements were carried at different temperatures ranging from 303 to 333 K. In all the studies, Inhibition efficiency increases with increase in concentrations of both FSM and TSM but decreases with rise in the temperature. Potentiodynamic polarization measurements confirmed that the inhibitive action of FSM and TSM is of mixed type. The activation parameters and thermodynamic values responsible for the adsorption were discussed. The adherent film of FSM and TSM formed on metal surface obeyed Langmuir adsorption isotherm. Fourier transform infrared spectroscopy and Ultraviolet−visible absorption spectral analysis were used to validate the mode of inhibition reaction to form complex between metal and inhibitor. The formation of protective layer on the mild steel surface was also confirmed by scanning electron microscopy (SEM) results.

1. INTRODUCTION Excessive corrosion attacks occur on mild steel surfaces deployed in service in aqueous aggressive environments. The dissolution of metals in such environment can be significantly suppressed by the addition of few compounds to the environment that adsorb on metal decrease its dissolution. Several organic molecules containing N, O, and S were reported as corrosion retarding agents for mild steel in acid solutions.1−8 The nature of the adsorption of inhibitor depends on the type of metal, structure of the molecule, and the strength of the electrolyte. The corrosion of mild steel may proceed through various mechanisms and manifest in different forms in any given environment. Accordingly, in order to be considered effective, an inhibitor may require performing through various functions. The adsorption of inhibitors is by the attractive forces arose between the inhibitor molecules and the metal surface. The electrostatic force in-between the charged metal surface and inhibitors is physisorption. The process of forming coordinate bond between the unshared electrons of the inhibitors and the metal surface is known as chemisorption. The adsorption will be favored by the presence of heteroatoms (S, N, and O) and their lone-pair of electrons and aromatic rings in the inhibitors.9−14 Various organic compounds are available as corrosion inhibitors. The selection of inhibitor for different processes is screened by various reasons by considering its availability and nontoxicity. This is in combination with the specific characteristics of much corrosion retarding substance, which partially demands the mixed mode inhibitors obtain high corrosion inhibition. Considering costs of most common corrosion © 2014 American Chemical Society

inhibitors, recently researchers focused on identifying new groups of inexpensive and effective inhibitors to address future environmental and safety needs. Hence, the identification of drugs as effective corrosion inhibitors for metals in acidic medium has paid good attention.15−25 The aim of the present investigation is to explore the use of 4-chloro-2-((furan-2-ylmethyl)amino)-5-sulfamoylbenzoic acid (FSM) and N-(isopropylcarbamoyl)-4-(m-tolylamino)pyridine3-sulfonamide (TSM) as effective inhibitors for mild steel corrosion in 1 N H2SO4 medium using weight loss and electrochemical studies. These compounds are used as diuretics in the treatment of hypertension and congestive heart failure. No report is evident for the utility of these drugs as inhibitors for mild steel corrosion in 1 N H2SO4 medium.

2. EXPERIMENTAL SECTION 2.1. Materials. Mild steel samples with an elemental composition were used as reported earlier.26 Analytical grade H2SO4 and double distilled water were used to prepare 1 N H2SO4 solutions. Two compounds viz, 4-chloro-2-((furan-2ylmethyl) amino)-5-sulfamoylbenzoic acid (FSM) and N(isopropylcarbamoyl)-4-(m-tolylamino) pyridine-3-sulfonamide (TSM) were purchased from Indian pharma limited. The structures of the antibiotics are given in Figure 1. Prior to the measurements the mild steel samples were abraded with Received: Revised: Accepted: Published: 3415

June 21, 2013 February 3, 2014 February 10, 2014 February 10, 2014 dx.doi.org/10.1021/ie401956y | Ind. Eng. Chem. Res. 2014, 53, 3415−3425

Industrial & Engineering Chemistry Research

Article

concentration of FSM and TSM for the corrosion of steel coupons at room temperature by employing Jasco Ultraviolet− near infrared (UV−NIR) spectrometer. 2.7. Fourier Transform-Infrared Spectroscopy. This spectrum was recorded using Shimadzu IR Affinity-1 model instrument in the frequency ranging from 4000 to 400 cm−1. FT-IR spectra of FSM and TSM powders and the scraps of inhibitors layered over metal surface were recorded by using standard procedure. 2.8. Theoretical Screening of Inhibitors. This screening was carried out by using Gaussian 03 software module. The molecular structures of the inhibitors were concluded by adapting standard quantum chemical calculations. Density functional theory (DFT)/B3LYP method (Becke’s threeparameter hybrid Hartree−Fock (HF)/DFT exchange functional (LYP)) with the basis set 6-31G (d,p) was used for calculations. Energies of highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), the energy gap between ELUMO and EHOMO (ΔE = ELUMO − EHOMO), and dipole moment (μ) were the parameters obtained from the theoretical studies.

Figure 1. Molecular structures of (A) FSM and (B) TSM.

different emery papers of 200, 400, 600, and 800 grade sheets, washed with double-distilled water, degreased with acetone and dried at room temperature. The range of the concentrations of inhibitors used for the inhibition is from 2 × 10−4 M to 14 × 10−4 M. Weight loss measurements, polarization studies, and impedance measurements were carried out in triplicate and average of inhibition efficiencies was taken. 2.2. Weight Loss Experiments. Weight loss experiments were carried out by following the previously described procedure.27 Weight loss experiments were done with steel coupons of sizes 4 cm × 1 cm × 0.2 cm. These experiments were performed at four different temperatures for 3 h by immersing the specimens in 1 N H2SO4 without and with various concentrations of inhibitors. After 3 h of immersion the samples were removed from the test solution, washed with distilled water, and then dried and weighed. The efficiency of inhibitors (IE), surface coverage (θ), and the rate of corrosion was calculated by standard equations: w − w1 IE (%) = 0 × 100 w0 (1) w − w1 θ= 0 w0

3. RESULTS AND DISCUSSION 3.1. Weight Loss Measurements. The efficiency of inhibitors at several concentrations of FSM and TSM by Table 1. Weight Loss Parameters of Mild Steel Immersed in 1 N H2SO4 in the Absence and Presence of Different Concentrations of FSM and TSM at 303 K

(2)

where w0 weight loss in the absence of inhibitor and w1 weight loss in the presence of inhibitor. 534W CR = mpy (3) DAT 2.3. Electrochemical Measurements. Electrochemical experiments were carried out using three electrode cell assembly at room temperature. The working electrode was a mild steel specimen of 1 cm2 dimension. Saturated calomel electrode (SCE) was used as a reference electrode and a platinum wire as a counter electrode. The polarization and impedance studies have been done using EG&G Princeton Applied Research Model (7310) Potentiostat. Open circuit potential (OCP) was obtained by immersing the working electrode in test solution for 30 min. Tafel polarization curves were recorded by changing the electrode potential from −250 to +250 mV versus open circuit potential with a scan rate of 1 mV s−1. Corrosion current (Icorr) and corrosion potential (Ecorr) were obtained by extrapolating anodic and cathodic curves of Tafel plots. Electrochemical impedance analyses were performed in a frequency range from 0.01 to 10000 Hz with optimal signal amplitude of 10 mV. 2.4. Scanning Electron Microscopy. Mild steel specimens (size 4 cm × 1 cm × 0.25 cm) were abraded with different emery papers and then washed with distilled water and acetone. After immersing in 1 N H2SO4 without and with addition of 14 × 10−4 M FSM and TSM for 180 min, SEM images were recorded. 2.6. Ultraviolet−Visible Spectroscopy Studies. This spectrum was recorded in test chemical consisting maximum

inhibitor concn. (M)

weight loss (g)

blank

0.0802

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

0.0231 0.0187 0.0161 0.0139

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

0.0099 0.0065 0.0054 0.0047

inhibition efficiency (%)

corrosion rate (mg cm−2 h−1)

surface coverage (θ)

6.68 FSM 71.19 76.68 79.92 82.66 TSM 87.65 91.89 93.26 94.13

1.93 1.56 1.34 1.16

0.7119 0.7668 0.7992 0.8266

0.82 0.54 0.45 0.39

0.8765 0.9189 0.9326 0.9413

exposing specimen to 1 N H2SO4 at 303 K is given in Table 1. It is observed from table that as concentration of FSM and TSM increased the performance of inhibition and the corresponding rate of corrosion diminished. This trend is due to the enhancement in adsorption of drugs on the surface of steel. At 14 × 10−4 M the FSM and TSM gave 82.66 and 94.13 percentage of inhibition efficiencies, respectively, at 303 K. There is no significant change in corrosion rates and inhibition efficiencies on further increase in concentrations of FSM and TSM. To study the effect of temperature on corrosion inhibition properties, the mild steel specimens are immersed in the acid solution bearing different concentrations of FSM and TSM at 303, 313, 323, and 333 K. The trend in the inhibition efficiencies of samples immersed in 1N H2SO4 with various concentrations of inhibitors at various temperatures is given in Figure 2. The resulted inhibition efficacies at various temperatures are given in Supporting Information Table S1 for FSM 3416

dx.doi.org/10.1021/ie401956y | Ind. Eng. Chem. Res. 2014, 53, 3415−3425


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Figure 2. Inhibition efficiency variations at various concentrations of (A) FSM and (B) TSM at different temperatures in 1 N H2SO4.

Figure 3. Tafel plots for mild steel in the presence of various concentrations of (A) FSM and (B) TSM in 1 N H2SO4.

Table 2. Potentiodynamic Polarization Parameters for Mild Steel Immersed in 1 N H2SO4 without and with Various Concentrations of FSM and TSM Ecorr (mV vs SCE)

Icorr (μAcm−2)

blank

−365.45

2075.78

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

−362.92 −371.32 −369.08 −359.29

701.28 581.45 505.96 426.91

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

−367.43 −328.27 −338.04 −296.18

405.59 299.99 219.48 175.44


βa (mV dec−1) 248.7 FSM 191.2 271.6 260.3 261.8 TSM 228.2 238.0 241.5 218.6

βc (mV dec−1)

inhibitor efficency (%)


170.5 161.0 168.8 174.6

66.21 71.98 75.62 79.43

0.6621 0.7198 0.7562 0.7943

208.5 135.6 167.4 113.3

80.60 85.54 89.42 91.54

0.8060 0.8554 0.8942 0.9154

183.8

respectively and to lower corrosion current densities. It is seen that cathodic curves are more linear than anodic curves over the applied potential range. The linear cathodic curves are due to the deposition of corrosion products on the metal surface.29,30 Values of Ecorr, Icorr, and anodic and cathodic Tafel slopes (βa, βc) are given in Table 2. Inhibition efficiencies are determined by using the equation

and TSM. From the experimental outcome, it is observed that the inhibition efficiencies rise with increase in concentrations of FSM and TSM and decreased with a rise in temperature. This trend is observed because at high temperatures the steel corrosion enhances.28 3.2. Potential−Current Curve Studies. Figure 3 shows the potentiodynamic polarization curves for mild steel immersed in 1 N H 2 SO 4 without and with various concentrations of FSM and TSM. It is clear that the addition of FSM and TSM reduced the corrosion rate. This reduction in corrosion is further supported by the shift in anodic and cathodic curves to more positive and negative potentials

IE (%) = 3417

Icorr − Icorr(i) Icorr

× 100

(4)



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Figure 4. Nyquist plots for mild in different concentrations of (A) FSM and (B) TSM in 1 N H2SO4.

The inhibitors FSM and TSM bring down the electrochemical reaction. FSM and TSM reduce the corrosion rate variations in the surface coverage leading to the decrease in concentration of H+ and its hydrogen evolution over potential.31 The influence of inhibitors on the kinetics of hydrogen evolution reaction is indicated by the cathodic Tafel slopes (βc).32 Due to the increase in the energy barrier for proton discharge, a decrease in gas evolution occurs.33 The presence of FSM and TSM in test solution shifted the anodic Tafel slopes toward the negative direction indicating compounds inhibit the Fe oxidation. According to the literature,34 it has been reported that (i) if the shift in Ecorr is 85 mV, with repect to Ecorr, the inhibitor behave as either cathodic or anodic. In this investigation, the shift in Ecorr is less than 69 mV for TSM and is even less for FSM, suggesting that both FSM and TSM act as mixed type of inhibitors.35 The equal shift in Tafel slopes also confirmed the mixed mode of inhibition. It is evidenced from polarization result that the presence of FSM and TSM has brought down the Icorr values than that of blank. It is noted from table that the decrease in corrosion current of TSM is greater than that of FSM indicating that the inhibitive effect of TSM is greater than FSM. 3.3. AC Impedance Spectroscopy. The AC impedance spectra obtained during the dissolution of mild steel in 1 N H2SO4 in the absence and presence of inhibitors are discussed in this section. In this experiment, the AC voltage is superimposed on the rest potential on mild steel surface during the corrosion reaction. The rest potential or the mixed potential is one at which two different electrochemical reactions occur simulta-

Table 3. Nyquist Parameters for Mild Steel in 1 N H2SO4 without and with Various Concentrations of FSM and TSM Rct [Ω cm2]

Cdl [μF cm−2]

blank

14.93

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

51.41 63.39 78.3 84.1

2 × 10−4 6 × 10−4 10 × 10−4 14 × 10−4

81.41 118.17 172.7 212.2

244.2 FSM 168.7 120.1 95.70 75.69 TSM 210.8 149.0 136.6 80.08


IE (%)


70.95 76.55 80.93 82.24

0.7095 0.7655 0.8093 0.8224

81.66 87.36 91.35 92.96

0.8166 0.8736 0.9135 0.9296

Figure 5. Electrical equivalent circuit.

where Icorr and Icorr(i) are the corrosion current densities in the absence and presence of inhibitors.

Table 4. Activation Parameters of Mild Steel in 1 N H2SO4 without and with Various Concentrations of FSM and TSM inhibitor FSM

TSM

concn. (10−4 M) blank 2 6 10 14 2 6 10 14

Ea (kJ mol−1)

λ (mg cm−2)

ΔH* (kJ mol−1)

ΔS* (J mol−1 K−1)

46 48 50 55 94 76 80 83 75

× × × × × × × × ×

44 41 43 50 51 89 89 86 73

−81 −94 −89 −70 −69 50 45 35 −9

0.83 0.02 0.03 1.02 0.42 2.73 6.27 1.61 3.76 3418

10

10 1010 1010 1010 1010 1013 1013 1014 1012



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Figure 6. Langmuir’s adsorption isotherm plots for (A) FSM and (B) TSM. (C) Plot of ln Kads vs 1/T.

Figure 7. SEM images of mild steel immersed in (A) 1 N H2SO4, (B) 14 × 10−4 M FSM, and (C) 14 × 10−4 M TSM.

Also, any loop on the tail-end of the plot is attributed to the contribution of the Warburg impedance. In most of the impedance plots, the semicircles are not complete, particularly at the low frequency end, and they have been estimated by extrapolation of a best fitted half circle for the experimental values. Such type of behavior has been reported for stainless steel corrosion in 1.8 M aerated H2SO4, and it is accounted as due to the slow corrosion process.36,37 In the present case, the situation is similar, as it corresponds to a highly resistive condition for the above charge transfer reactions reflected in their high Rt values. The decreased Rt value is claimed to reflect on the inhibition effect on the additives during corrosion process.

neously on the mild steel surface. Thus, the overall reaction for mild steel corrosion in acid medium is as follows: Fe + 2H+ → Fe 2 + + H 2

(5)

Using the Nyquist plots the charge transfer resistance values of the above reaction is calculated as the x-intercept of the semicircle where the x-axis represents the real part of the impedance. Perfect semicircles are observed, in case where the electrochemical reaction of interest is under charge transfer control. Where the reactions are partially under charge transfer and mass transport control, there is a drag noted in the semicircular plot. When the reaction is under diffusion control, a rising portion is noted in the low frequency end of the plot. 3419



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Figure 8. EDAX spectra of mild steel in (A) 1 N H2SO4, (B) 14 × 10−4 M FSM, and (C) 14 × 10−4 M TSM.

Figure 9. UV−visible spectra of solution containing 14 × 10−4 M (A) FSM (B) TSM before and after 3 h immersion of mild steel in 1 N H2SO4.

Figure 10. FT-IR spectra of (A) FSM and (B) TSM and their adsorbed layers on mild steel surface.

surface. Such anions substantially change both anodic and cathodic over potential and catalyze the electrochemical reactions. TSM and FSM may fall in to this category. Impedance spectroscopy gives information about the surface properties and kinetics of the electrode process simultaneously for the investigated systems. Mechanistic information of the corrosion process obtained from the impedance plots. Corrosion processes were widely examined by using this method. Nyquist plots for mild steel in 1N H2SO4 solution in the presence and absence of FSM and TSM are given in Figure 4. Charge transfer resistance (Rct) and double layer capacitance (Cdl) were obtained from the Nyquist plots. The following

Table 5. Theoretical Parameters of FSM and TSM Calculated with DFT Method inhibitor

EHOMO (ev)

ELUMO (ev)

ΔE (ev)

μ (Debye)

FSM TSM

−0.23 −0.27

−0.18 −0.26

0.05 0.01

4.33 12.73

It was reported that certain organic compounds, due to moments or distortion moments, induce some electric field, which results in the tendency for adsorption of the compounds on the metal and promotes electron exchange reactions thereon by penetrating the dissolving barrier films on the electrode 3420



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Figure 11. Optimized structures, HOMO and LUMO, of FSM and TSM.

3.4. Activation Parameters. The influence of temperature on rate of corrosion was studied by using Arrhenius and transition state equations39 adapting the values of weight loss studies at various concentrations of inhibitors in different temperatures

equation is used to calculate the inhibition efficiencies from Rct values, IE (%) =

R ct(i) − R ct(b) R ct(i)

× 100 (6)

where Rct(i) and Rct(b) are the charge transfer resistances with and without the inhibitors. The parameters calculated from nyquist plots are tabulated in Table 3. Randle’s equivalent circuit, shown in Figure 5, is used for the study. Rct is the charge transfer resistance, CPE is the constant phase equivalent, and Rs is the solution resistance is shown in this circuit. High-frequency (HF) depressed charge-transfer semicircles are observed in Figure 4. From HF semicircle, Rct and Cdl have analyzed. The delayed mechanism owing to adsorption of SO42− ions and hydrogen ions over steel is the reason for the formation of semicircle in the Nyquist plot. The increase in charge transfer resistance values and the corresponding decrease in double layer capacitance values, with increase in concentration of FSM and TSM results in higher inhibition efficiency. The increase in inhibition efficiencies is due to the increase in surface coverage on the mild steel surface by the inhibitors. Double layer capacitance (Cdl) is related to the thickness of the protective layer (d) by the following equation:38 Cdl =

εε0A d

−Ea + log λ 2.303RT

(8)

⎛ ΔS* ⎞ ⎛ ΔH * ⎞ RT ⎟ exp⎜ − ⎟ exp⎜ ⎝ ⎠ ⎝ RT ⎠ Nh R

(9)

log(CR ) =

CR =

where λ is the pre-exponential factor, Ea is apparent activation energy, ΔS* the apparent entropy of activation, ΔH* the apparent enthalpy of activation, h Planck’s constant, and N is Avogadro’s number. The results are given in Supporting Information Figure S1. From this linear regression, energy of activation and the preexponential parameters are given in Table 4. The rise in energy of activation will decrease the adsorption of inhibitors on the metal with the enhancement of corrosion rate and the operating conditions.40,41 The E a value increases with the increase of TSM concentration due to physisorption of TSM in the initial stage. On adding higher concentrations of TSM, the Ea value is continuously reduced because of chemical adsorption of the inhibitor.42 From eq 8 it is clear that Ea and λ influenced the rate of corrosion. The effect of λ on metal dissolution is lower than Ea. Both Ea and λ enhanced with concentrations of FSM and reduced in presence of higher TSM concentration. The main root cause for degrading the rate of corrosion of steel in 1 N H2SO4 in the presence of FSM is the higher values of activation

(7)

where ε0 is the free space permittivity, ε is the dielectric constant of the medium, and A is the surface area of the electrode. The increase in thickness of the double layer was confirmed by the decrease in double layer capacitance values, which in turn justify that the both inhibitors reduce the metal dissolution by effective adsorption. 3421



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optimum concentration of (14 × 10−4) of FSM and TSM. Part A of Figure 7 shows high damage and pits on the surface whereas parts B and C show less damage on the surface due to the adsorption of inhibitors. The image of mild steel immersed in the presence of TSM (Figure 7B) is comparably smooth and mildly affected in comparison with surface of specimen immersed in the presence of FSM (Figure 7C) leading to effective formation of TSM films on metal in acid medium. 3.6.1. EDX Analysis. The EDX results of mild steel taken in 1 N H2SO4 with and without the addition of both inhibitors are given in Figure 8. It is evident that Fe percentage in the absence of inhibitors in acid medium was 64%. In the presence of optimum concentrations of inhibitors the percentage of Fe contents was of 66% and 70%. The increments in percentage of Fe values in the presence of inhibitors confirming that the dissolution of metal has been reduced by virtue of adsorption of FSM and TSM on mild steel. 3.7. Mechanism of Inhibition. Initially FSM and TSM molecules get adsorbed on metal surface. The adsorption process is determined by structure of inhibitor, concentration of corrosive medium and electric field at the interface of metal− electrolyte. This leads to the calculation of potential zero charge (PZC).54 In H2SO4 medium, the FSM and TSM molecules can exist as cationic species, which may be adsorbed on the cathodic sites of the mild steel and bring down the evolution of hydrogen

energy. The increased Ea value is the determining factor to lower corrosion rate for TSM, at lower concentrations. Supporting Information Figure S2 indicates a plot of log (CR/T) vs 1/T. Straight lines were obtained, and slopes and intercepts are calculated and tabulated in Table 4. The thermodynamic parameters (ΔH* and ΔS*) reveals that the dissolution reaction of mild steel in 1N H2SO4 in the presence of FSM and TSM are higher than in the absence of inhibitors. The positive sign of the enthalpies reflect the endothermic nature of the mild steel dissolution process indicating that dissolution of mild steel is difficult.43 The values of entropy of activation (ΔS*) given in Table 4 clearly indicate that entropy of activation increased positively in the presence of FSM than in the absence of inhibitor. The rise of entropy of activation could be due to the enhanced randomness of molecules.44 Hence, increased ΔS* values for TSM confirms the higher entropy of inhibitor. At higher concentrations of TSM the decrease ΔS* is attributed to the decrease in disorderliness of the TSM molecules on the surface leading to uniform film formation on metal surface. 3.5. Adsorption Isotherm. Adsorption isotherms give information about the interaction of inhibitor molecules with the mild steel surface.45 The higher ability of the inhibitors to get adsorbed on the mild steel surface makes them better inhibitors. The coverage of surface (θ) values for different molarities of FSM and TSM was tested diagrammatically. A plot Cinh/θ versus Cinh (Figure 6A and B) obeyed Langmuir adsorption isotherm, which is expressed by the following equations: C inh 1 = + C inh θ K ads

K ads =

(10)

1 θ C inh 1 − θ

(12)

[ΔG0ads]

The free energy of adsorption and adsorption constant [Kads] at different temperatures were calculated and given in Supporting Information Table S2. The momentum of the adsorption reaction of inhibitors is confirmed by the negative data of ΔG0ads.46−48 The free energy of adsorption [ΔG0ads] in the present study lies in the range from −30 to −36 kJ mol−1, indicating that the physical adsorption is not a dominator. Some other interactions dominate physisorption.49−53 The performance of TSM is better than FSM, which is evidenced from ΔG0ads and Kads. The heat of adsorption (ΔH0ads) and entropy of adsorption (ΔS0ads) were analyzed (Figure 6C and Supporting Information Table S3) by Van’t Hoff equation50 ln K ads =

0 −ΔHads + constant RT

(14)

TSM + 2H+ ↔ [TSMH]2 +

(15)

The protonated forms of FSM and TSM could be attached to mild steel surface by the electrostatic interaction between protonated inhibitors and SO42− since the mild steel has positive charge in the acidic medium. This is further explained based on the assumption that, in the presence of H2SO4 the negatively charged SO42− may attach to positively charged mild steel surface. When FSM and TSM adsorb on mild steel surface electrostatic interaction takes place by partial transfer of electrons from the polar atoms (S, N, and O atoms and the delocalized π-electrons around the aromatic rings) of FSM and TSM to the mild steel surface. Molecular adsorption will also play a major role in adsorption process in addition to electrostatic interaction (physisorption) of FSM and TSM molecules on the mild steel surface. Since the metal surface is having higher negative charge due to the existence of high electron density, the transference of electrons take place from metal surface to unoccupied antibonding orbital’s of inhibitors. This favors high adsorption of drug molecules on metal.55 The above results confirmed that TSM performs better than FSM. The inferior inhibition of FSM is due to the electron withdrawing chlorine in its structure. Because of this, the complex formation between Fe and FSM is weakened, which leads to reduced inhibition of FSM. This is not happening in the case of TSM. TSM molecules easily form complexes with Fe that favors its impressive inhibition efficiency. These results were studied using UV−visible spectra in the presence and absence of inhibitor. 3.8. UV−Visible Spectroscopy. UV−visible spectral results for inhibitors are shown in Figure 9. The appearance of bands at regions 232 and 272 nm for FSM and at 230 and 268 nm for TSM might be due to π−π* and n−π* transitions.

(11)

0 ΔGads = −2.303RT log[55.5K ads]

FSM + 2H+ ↔ [FSMH]2 +

(13)

Adsorption in this study was an exothermic process, which was further evidenced by the reduction in entropy values, which could directly indicate in respect of FSM and TSM from the calculated values. The positive value of (ΔS0ads) confirms the effective adsorption of FSM and TSM over the metal. 3.6. Scanning Electron Microscopy. The SEM photos of metal surfaces are shown in Figure 7A−C. Figure 7A indicates plain metal in the absence of inhibitor. Parts B and C of Figure 7 show surfaces of mild steel specimens immersed in the 3422



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4. CONCLUSIONS

The appearance of bands at 272 nm for FSM and 268 nm for TSM indicated the formation of complexes between Fe and inhibitors.56 The shift of absorbance band for TSM is greater than FSM confirming that TSM adsorbs better on metal surface than FSM. Also, the acid medium without inhibitors has not shown any appreciable change in absorption values. Thus, the formation of inhibitor layers on metal is evident. 3.9. Infrared Spectroscopy. The comparison of IR spectra of FSM and TSM powders with that of powders adsorbed on the mild steel are given in Figure 10. The FT-IR spectrum of pure FSM powder is shown in Figure 10A(b). The band at 2897 cm−1 corresponds to the C−H stretching vibration. The absorption peak at 1282 cm−1 is attributed to C−N stretching vibrations. The peak visible at 738 cm−1 corresponds to C−S asymmetric stretching. The peak at 1430 cm−1 justifies to C C stretching vibrations. The peak at 1629 cm−1 is due to CO stretching vibrations. The peak value at 3380 cm−1 corresponds to N−H stretching vibrations. The peak at 1062 cm−1 is attributed to C−O stretching vibrations. The FT-IR spectrum of FSM film formed on steel is shown in Figure 10A(a). Here, the N−H stretching vibrations shifted from 3380 cm−1 to 3447 cm−1. The peaks at 2897 cm−1, 1282 cm−1, and 1062 cm−1 were due to C−H stretching, C−N stretching, and C−O stretching disappeared for the FSM adsorbed on the mild steel surface, which confirmed the interaction of C−H, C−N, and C−O groups with the metal. A shift in vibrational mode of N−H in presence of FSM was noted. This could be the reason for FeFSM complex formation. The FT-IR spectrum of pure TSM powder is shown in Figure 10B(b). The peak observed at 3365 cm−1 corresponds to N−H stretching vibrations. The band at 2897 cm−1 is attributed to the C−H stretching vibration. The peak visualized at 1645 cm−1 is due to CO stretching vibrations. The peak visible at 1410 cm−1 corresponds to CC stretching vibrations. The peak appeared at 1062 cm−1 is attributed to C−O stretching vibrations. The FT-IR spectrum of TSM adsorbed on the mild steel is shown in Figure 10B(a). The stretching of N−H vibrations altered from 3365 cm−1 to 3456 cm−1. The peaks shown at 2897 cm−1 and 1062 cm−1 were due to C−H stretching and C−O stretching disappeared for the TSM adsorbed on the mild steel surface, which established the interaction of C−H and C−O groups with the metal. The N− H vibrational shift from low frequency values to high frequency value may is due to the formation of Fe−TSM complex. 3.10. Quantum Chemical Investigation. From this investigation,57−62 the following parameters are considered as important to us. (i) The electron donating ability of inhibitor molecules is determined from EHOMO values. (ii) The transfer of electrons between filled d-orbital of mild steel and inhibitor molecules is calculated from ELUMO values. (iii) The effective adsorption of inhibitors on metal surface is well understood from low energy gap values. (iv) The distribution of charges after the adsorption of inhibitor on metal surface is studied from their increased dipole moment value. The results are presented in Table 5. It is also noted that this investigation has validated the performance of inhibitors, which is screened by chemical and electrochemical methods. Figure 11 shows the optimized HOMO and LUMO structures for FSM and TSM.

(i) The inhibitors brought down the dissolution of metal in 1 N H2SO4 medium. (ii) The mode of inhibition by FSM and TSM was under mixed control. (iii) The thermodynamic parameters evidently indicated the good adsorption of FSM and TSM and adsorption of inhibitors followed Langmuir adsorption isotherm. (iv) UV−visible technique revealed the Fe−inhibitor complex formation. (v) Nyquist plots established that the inhibitors reduced the mild steel corrosion through their effective adsorption of inhibitive layer, which is further evidenced from SEM and FT-IR. (vi) The delocalization of electrons of inhibitors significantly played in bringing down steel dissolution in 1N H2SO4. Among the compound studied, TSM performs superior than FSM. This is further substantiated from quantum chemical indices.

■

ASSOCIATED CONTENT

S Supporting Information *

Results of weight loss method, Arrhenius plots, Gibbs free energy parameters and thermodynamic parameters for mild steel in 1N H2SO4 without and with different concentrations of FSM and TSM 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +91-4162205708, +91-9585587561. Notes

The authors declare no competing financial interest.

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REFERENCES

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