Electrochemical investigation of Substituted Pyranopyrazoles

Article pubs.acs.org/IECR

Electrochemical investigation of Substituted Pyranopyrazoles Adsorption on Mild Steel in Acid Solution Dileep Kumar Yadav and M. A. Quraishi* Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India ABSTRACT: Four substituted pyranopyrazole derivatives (PPZs) were synthesized and their effects on the electrochemical behavior of mild steel in 1 M HCl were investigated using gravimetric measurements, Tafel extrapolation method, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). Tafel polarization measurements revealed that these compounds effectively suppressed both the anodic and cathodic processes of mild steel corrosion in acid solution and acted as mixed-type inhibitors. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) examinations of electrode surface confirmed the existence of adherent layer of inhibitor on electrode surface. The UV−visible absorption spectrum of inhibitor solution containing mild steel indicated the formation of Fe−PPZ complex. Quantum chemical calculations have been used to evaluate the structural, electronic, and reactivity parameters of the selected pyranopyrazole derivatives in relation to their inhibition action.

1. INTRODUCTION In an effort to mitigate the corrosion of mild steel in acid solutions, the strategy is to isolate the metal from corrosive agents. During past few years, a variety of N-heterocyclic compounds have been reported to be effective as corrosion inhibitors for mild steel in acid solutions.1−4 Pyranopyrazoles are an important type of N-heterocyclic compounds and are used as pharmaceutical ingredients and biodegradable agrochemicals;5 this makes the evaluation of their inhibitive properties more significant in the context of the current priority to produce environmentally benign chemical inhibitors. The choices of these compounds are based on the consideration that these compounds contain a better π electron conjugation and heteroatoms (N, O) enhancing a better coordination and adsorption property. Inhibition occurs via adsorption of these heterocyclic molecules on metal surface following some known adsorption isotherms with the polar groups acting as adsorptive centers. The resulting adsorbed film of inhibitor acts as a barrier that isolates the metal from corrosive agents in acid medium. Recently, various experimental and theoretical techniques have been developed to study the structural properties of inhibitor molecules and their activity toward metal surface, but the quantum chemical calculations based on density function theory (DFT) method have become an attractive theoretical method because it gives exact basic vital parameters for even huge complex molecules. Thus, DFT has become an important tool for connecting some traditional empirical concepts with quantum mechanics.6 Therefore, DFT is a very powerful technique to probe inhibitor/surface interaction and to analyze experimental data. Various substituted heterocyclic compounds have been recently studied in considerable detail as effective corrosion inhibitors for mild steel in acidic media by our research group.7−10 Previously, some pyrazoles have been reported as corrosion inhibitors for steel in hydrochloric acid solution.11,12 However, to the best of our knowledge, no reports are available © 2012 American Chemical Society

mentioning the application of pyranopyrazole derivatives as corrosion inhibitors. The present study was undertaken to investigate the corrosion inhibition of mild steel in 1 M HCl by four pyranopyrazole derivatives. The study was conducted by potentiodyanamic polarization, linear polarization and electrochemical impedance spectroscopy. Quantum chemical calculations have been performed using DFT, and various quantum chemical indices were calculated and correlated with the inhibitive effect of PPZs. SEM and EDX spectra of mild steel surface were recorded in order to examine the changes in surface morphology of mild steel covered with a thin protective film of inhibitor. UV−visible spectroscopic investigations of the testing solutions before and after corrosion experiments were made and discussed.

2. EXPERIMENTAL SECTION 2.1. Inhibitors. The pyranopyrazole derivatives were synthesized according to scheme given in Figure 1.5 The purity of the products was confirmed by thin-layer chromatography with a mixture of ethyl acetate/n-hexane (2:8) using the SiliaPlate TLC Plates−Aluminum (Al) Silica. However, the products were further purified by recrystallization from ethanol. The melting points of compounds were determined in open capillaries and matched with literature.5 Infrared (IR) spectra were recorded on KBr discs by using a Perkin-Elmer (Spectrum100) Fourier transform (FT-IR) spectrophotometer. The chemical structure, abbreviations, IUPAC name, IR data, and melting points of synthesized compounds are given in Table 1. 2.2. Electrodes and Solutions. Corrosion tests were performed on mild steel specimens of following composition (wt %): C = 0.076, Mn = 0.192, P = 0.012, Si = 0.026, Cr = Received: Revised: Accepted: Published: 8194

January 25, 2012 May 22, 2012 May 25, 2012 May 25, 2012 dx.doi.org/10.1021/ie3002155 | Ind. Eng. Chem. Res. 2012, 51, 8194−8210

Industrial & Engineering Chemistry Research

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measurements, linear polarization measurements, and electrochemical impedance spectroscopy. Gravimetric experiments have been performed according to the standard methods.13 Triplicate experiments have been done and the corrosion rates CR (mg cm−2 h−1) were calculated from the following equation:14

CR =

W At

(1)

where W is the average weight loss of three parallel mild steel specimens, A is the total area of one mild specimen, and t is immersion time (3 h). With the calculated corrosion rate, the inhibition efficiency η% was calculated as follows:14 η% =

C R − C R(i) CR

× 100

(2)

and surface coverage (θ) values were calculated by eq 3:

Figure 1. Synthetic route of pyranopyrazoles.

θ=

0.050, Al = 0.023, and remainder Fe. Mild steel specimens used in gravimetric and electrochemical experiments were mechanically cut into 2.5 × 2 × 0.025 and 8 × 1 × 0.025 cm dimensions, and then abraded with SiC abrasive papers of grades 800, 1000, and 1200, respectively, washed with acetone, ultrasonically cleaned in absolute ethanol, dried at ambient temperature, and stored in a moisture-free desiccator before use in corrosion studies. The aggressive solution, 1 M HCl was prepared by dilution of analytical grade 37% HCl with doubledistilled water. 2.3. Gravimetric Measurements. The gravimetric (weight loss) method is used to optimize the concentration of inhibitors. The simplicity and reliability of the gravimetric method provide a baseline method to other well established techniques of corrosion monitoring such as Tafel polarization

C R − C R(i) CR

(3)

where CR and CR(i) are the values of the corrosion rates (mg cm−2 h−1) of mild steel in absence and presence of inhibitors, respectively. 2.4. Electrochemical Measurements. The electrochemical experiments were carried out using a conventional threeelectrode cell assembly at ambient temperature. A mild steel specimen of 1 cm2 area exposed was used as working electrode, a high purity platinum foil of 1 cm2 dimension was used as counter electrode, and a saturated calomel electrode (SCE), i.e. (Cl−|(4M) Hg2Cl2 (s)| Hg (l)|Pt) via Luggin capillary probe, was used as reference electrode. The tip of the Luggin capillary was kept close to that of the working electrode. All the experiments were performed in absence and presence of

Table 1. Molecular Structure and Analytical Data of Studied Inhibitors

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dx.doi.org/10.1021/ie3002155 | Ind. Eng. Chem. Res. 2012, 51, 8194−8210


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FeOHads → FeOH+ + e− (rate determining step)

different concentrations of inhibitors used in gravimetric measurements. All electrochemical measurements were carried out using a Gamry Potentiostat/Galvanostat (model G-300) connected with a personal computer with EIS software (Gamry Instruments Inc., USA). The electrochemical experiments data were collected and analyzed by electrochemical software, Gamry Echem Analyst 5.0. All the experiments were carried out after immersion of mild steel for 30 min in 1 M HCl in absence and presence of different concentrations of inhibitors. Prior to polarization and EIS experiments the electrode was allowed to corrode freely and its OCP was recorded as a function of time for 200 s. After this time a steady-state OCP corresponding to corrosion potential (Ecorr) of the working electrode was obtained. The above-mentioned procedure was repeated for each concentration of the investigated inhibitors. Impedance measurements were carried out using AC signals of amplitude 10 mV peak to peak at the open circuit potential in the frequency range 100 kHz to 0.01 Hz. The linear polarization study was carried out from cathodic potential of −0.020 V vs OCP to an anodic potential of +0.020 V vs OCP at a scan rate of 0.125 mV s−1. Potentiodynamic current−potential curves were obtained by changing the electrode potential automatically from −250 to +250 mVSCE versus open circuit potential at a scan rate of 1 mVs−1. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain corrosion current densities (Icorr). 2.5. Surface Characterization. The mild steel specimens of size 2.5 × 2 × 0.025 cm were immersed in 1 M HCl in absence and presence of optimum concentration (200 mg L−1) of PPZs for 3 h. Thereafter, the mild steel specimens were taken out, washed with distilled water, degreased with acetone, ultrasonically cleaned with absolute ethanol, dried at ambient temperature, and mechanically cut into 1 cm2 sizes for SEM and EDX investigations. Superficial observations were carried out at an accelerating voltage of 5 kV and 5K× magnification on a Ziess Evo 50 XVP instrument. Chemical composition of the corrosion products was recorded by an EDX detector coupled to the SEM. 2.6. UV−visible Spectroscopy. The 1 M HCl solution containing 200 mg L−1 of PPZ-4 before and after 48 h of mild steel immersion at 308 K was subjected to UV−visible absorption detection using a Hitachi U-2900 double-beam spectrophotometer. 2.7. Theoretical Study. All the calculations were performed with Gaussian 03, E 0.01.15 The molecular structures of the neutral species were fully and geometrically optimized using the functional hybrid B3LYP (Becke, three-parameter, Lee−Yang−Parr exchange-correlation function) density function theory (DFT) formalism with electron basis set 6-31G (d, p) for all atoms. The quantum chemical parameters obtained were EHOMO, ELUMO, ΔE (ELUMO − EHOMO), μ, total energy, and Mulliken charge on heteroatoms (N, O).

FeOH+ + H+ ↔ Fe+2 + H 2O

The cathodic hydrogen evolution steps are as follows: Fe + H+ ↔ (FeH+)ads

(FeH+)ads + e− ↔ (FeH)ads (FeH)ads + H+ + e− → Fe + H 2

The adsorbed intermediates accounting for the retardation of Fe anodic dissolution in the presence of inhibitor have been given as: Fe + H 2O ↔ FeH 2Oads FeH 2Oads + INH ↔ FeOH−ads + H+ + INH

FeH 2Oads + INH → Fe − INHads + H 2O FeOH−ads → FeOHads + e− (rate determining step) Fe − INHads → Fe − INH+ads + e−

FeOHads + Fe − INH+ads ↔ Fe − INHads + FeOH+ FeOH+ + H+ ↔ Fe+2 + H 2O

The above mechanism clearly indicates that inhibitor molecules (INH) replace some adsorbed H2O to give the intermediate Fe-INHads and reduce the amount of FeOH−ads (in rate determining step) and consequently retard the dissolution of Fe. The variation of corrosion rate (CR) with inhibitor concentration is listed in Table 2. It is observed that all inhibitors showed very good inhibition efficiency at their optimum concentration (200 mg L−1), which may be attributed to larger coverage of metal surface with inhibitor molecules. No significant change in inhibition efficiency was observed above this concentration. Table 2. Parameters Obtained from Gravimetric Measurement for Mild Steel in 1 M HCl Containing Different Concentrations of PPZs at 308 K inhibitor blank PPZ-1

PPZ-2

3. RESULTS AND DISCUSSION 3.1. Gravimetric Measurements. The mechanism of inhibition of mild steel in hydrochloric acid solution in the presence of inhibitor molecules gives some knowledge of interaction between the adsorbed protective film of the inhibitor and the metal surface. According to the mechanism, the dissolution of iron in acid solution depends primarily on the adsorbed intermediate species as follows:6

PPZ-3

PPZ-4

Fe + H 2O ↔ FeOHads + H+ + e− 8196

concentration (mg/L)

corrosion rate (mg cm−2 h−1)

surface coverage (θ)

η (%)

0.0 50 100 150 200 50 100 150 200 50 100 150 200 50 100 150 200

7.00 1.40 1.10 1.02 0.84 2.00 1.79 1.72 1.39 0.89 0.61 0.54 0.50 0.69 0.51 0.32 0.11

0.80 0.84 0.85 0.88 0.73 0.74 0.75 0.80 0.87 0.91 0.92 0.92 0.90 0.92 0.95 0.98

80.0 84.2 85.4 88.0 71.4 74.4 75.9 80.1 87.2 91.2 92.3 92.8 90.1 92.7 95.4 98.4



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Figure 2. Chrono-potentiometric (zero current) curves for mild steel in 1 M HCl without and with different concentrations of PPZs at 308 K.

3.2. Electrochemical Measurements. 3.2.1. Open Circuit Potential versus Time. The open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no potential or current is being applied to the cell. Prior to running potentiodynamic polarization and EIS, it is necessary to maintain the stability of the OCP. Moreover, determining the variation of the OCP of the working electrode versus time is significant in defining domains of corrosion, partial, or complete inhibition, and in determining inhibitor threshold concentrations. Figure 2a−d depict the variation of the OCP of the steel electrode with time in 1 M HCl solution in absence and presence of different concentrations of PPZs at 308 K. In absence of inhibitors, that the steady-state values of OCP are more negative than the immersion potential (Eocp at t = 0) suggest that before the steady-state condition is achieved the preimmersion, air-formed oxide film on the electrode has to dissolve. This steady-state potential, achieved quickly (after 30 min of immersion), corresponds to the free corrosion of bare metal.16 It is obvious from Figure 2a−d that addition of studied concentrations of PPZs to 1 M HCl solution shifts the steadystate potential (Ecorr) to more negative values without changing the general features of the E−t curves, indicating that they catalyze the oxide film dissolution. These results may be interpreted on the basis of formation of stable Fe (II) complexes with the N containing ligands. Moreover, increase in the concentration of PPZs diminishes this effect as a result of its adsorption on mild steel surface.17 The shift of the OCP can be explained in terms of formation of a protective layer of inhibitors on the metal surface. 3.2.2. DC Techniques: Tafel and LPR. Tafel polarization methods involve changing the potential of the working electrode and monitoring the current that is produced as a function of time or potential. Tafel polarization curves for mild steel in 1 M HCl at different concentrations of PPZs are shown in Figure 4a−d. It can be observed that the presence of

inhibitor causes a prominent decrease in corrosion rate, i.e. shifts the anodic curves to more positive potentials and cathodic curves to more negative potentials, and to the lower values of corrosion current densities. It is obvious that cathodic Tafel curves are more linear than anodic Tafel curves over the applied potential range. The curvature of anodic Tafel curves may be attributed to the deposition of corrosion products or impurities on the mild steel (e.g., Fe3C) to form a nonpassive surface film.18 Figure 3a illustrates how to estimate the anodic and cathodic Tafel slopes for inhibitor-free 1 M HCl solution and also applied to inhibited systems. It has been shown in Figure 3a that in the Tafel extrapolation method, the use of both the anodic and cathodic Tafel regions is undoubtedly preferred over the use of only one Tafel region.18 Linear sections of the cathodic and anodic Tafel curves were extrapolated to get the accurate evaluation by Tafel extrapolation up to their intersection at the point where corrosion current density (Icorr) and corrosion potential (Ecorr) were obtained. In this example, the Tafel slopes were estimated to be βa = 70 mV/dec and βc = 114 mV/dec The rest of the values for Icorr, βa, and βc (together with the Ecorr) were also evaluated for studied concentrations of all PPZs using the respective polarization curves based on the same calculations presented in Figure 3a. The polarization parameters Ecorr, Icorr, anodic and cathodic Tafel slopes (βa, βc) are listed in Table 3. Inhibition efficiency values, η (%) were calculated from equation ⎛ Icorr(i) ⎞ η% = ⎜1 − ⎟ × 100 Icorr ⎠ ⎝

(4)

where Icorr(i) is the corrosion current density at particular concentration of PPZs, and Icorr is the corrosion current density in the absence of PPZs in the solution. The Tafel polarization plots presented in Figure 4a−d reveal that the addition of PPZs shifts the cathodic and anodic 8197



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It is well-known that the spontaneous dissolution of mild steel in acid medium gives Fe2+ ions, and this reaction is accompanied by hydrogen evolution reaction, which is a cathodic reaction (Figure 3b). The corrosion of iron or steel in free acidic solutions is controlled by the hydrogen evolution reaction.17 In presence of adsorbed PPZs molecules, the proceeding of electrochemical reactions is retarded on the surface of mild steel due to covered surface sites. The studied PPZs affect the rate of corrosion through the variation of degree of surface coverage; this in turn results in reducing the H+ concentration and corresponding hydrogen evolution over potential.19 The parallel cathodic Tafel curves in Figure 4 suggested that the hydrogen evolution was activation-controlled and the reduction mechanism was not affected by the presence of PPZs. The values of βc changed in inhibited system, which indicated the influence of the inhibitor molecules on the kinetics of hydrogen evolution.19 The anodic curves of the electrode in hydrochloric acid solutions containing PPZs shifted to the direction of current reduction which suggested that the PPZs could also suppress the anodic reaction. If the change in Ecorr value was more than 85 mV, a chemical compound could be recognized as an anodic or a cathodic type inhibitor.14 The largest displacement of Ecorr was about 54.0 mV toward cathodic direction (Figure 4 and Table 3). Based on these results, PPZs are considered as mixed-type inhibitors. In other words, the addition of PPZs to 1 M HCl solutions reduces the anodic dissolution of mild steel and also retards the cathodic hydrogen evolution reaction. As it can be seen from the polarization results (Table 3), the Icorr values, in all cases, decrease in the presence of all four PPZs, and this decrease enhances with increase in concentrations of PPZs. It is obvious that, under the same conditions, the corrosion current decreases in the order of PPZ-2 > PPZ-1 > PPZ-3 > PPZ-4. This sequence reflects the increased ability of PPZ-4 to inhibit mild steel corrosion in hydrochloric acid solutions as compared to PPZ-1, PPZ-2, and PPZ-3. It is important to note that in anodic domain, the potential above −350 mV vs SCE, the presence of PPZs did not change the current vs potential characteristics (Figure 4), which suggests that the inhibition mode of PPZs depends on electrode

Figure 3. (a) Tafel extrapolation method of cathodic and anodic polarization curves for mild steel in 1 M HCl at 308 K HCl. (b) Schematic depiction of cathodic and anodic mechanism of mild steel dissolution in 1 M HCl.

branches toward lower currents, probably as a consequence of the blocking effect of the adsorbed inhibitor molecules. No significant changes were observed in Ecorr values in presence of inhibitors. It is well-known that the spontaneous dissolution of iron in acids can be described by the anodic dissolution reaction Fe = Fe+2 + 2e−

(5)

accompanied by the corresponding cathodic hydrogen evolution reaction. 2H+ + 2e− = H 2

(6)

Table 3. Polarization Parameters for Mild Steel in 1 M HCl Solution Containing Different Concentrations of PPZs at 308 K Tafel polarization −1

linear polarization

inhibitor

Cinh (mgL )

Icorr (μA/cm)

Ecorr (V/SCE)

βa (mV/dec)

βc (mV/dec)

blank PPZ-1

0.0 50 100 150 200 50 100 150 200 50 100 150 200 50 100 150 200

892 189 149 99 79 265 201 196 152 139 95 82 77 126 89 48 33

−444 −495 −498 −464 −465 −454 −494 −492 −471 −484 −480 −472 −474 −481 −462 −453 −477

70 133 129 72 78 64 119 126 77 102 121 80 50 65 48 36 75

114 191 208 148 153 122 221 220 142 164 192 131 106 110 101 70 125

PPZ-2

PPZ-3

PPZ-4

8198

θ 0.78 0.83 0.88 0.91 0.70 0.77 0.78 0.82 0.84 0.89 0.91 0.92 0.85 0.90 0.94 0.96

η (%)

Rp (Ω cm2)

θ

η (%)

78.8 83.7 88.8 91.1 70.4 77.4 78.0 82.1 84.4 89.3 91.8 92.3 85.8 90.1 94.6 96.3

10.1 49.3 76.1 99.4 103.3 38.2 46.1 49.11 58.2 92.1 93.3 100.1 139.4 72.95 124.4 298.5 604.4

0.79 0.86 0.89 0.89 0.73 0.78 0.79 0.90 0.87 0.88 0.89 0.92 0.86 0.91 0.96 0.98

79.5 86.7 89.1 89.9 73.7 78.0 79.4 82.6 87.7 88.1 89.9 92.7 86.0 91.8 96.5 98.3



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Figure 4. Tafel curves for mild steel in 1 M HCl without and with different concentrations of PPZs at 308 K.

Figure 5. Nyquist plots for mild steel in 1 M HCl without and with different concentrations of PPZs at 308 K.

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should be noticed that impedance spectra did not present perfect semicircles and the depression of large semicircles (i.e., rather than perfect semicircles) in the complex impedance plane of the Nyquist plots, with the center under the real axis, appeared. Deviation of this kind often referred to frequency dispersion and surface heterogeneity which results from surface roughness, impurities, dislocations, grain boundaries, inhibitor adsorption, and formation of porous layers.24 From Figure 5 it can be seen that Nyquist plots show a single semicircle capacitive loop in the higher frequency (HF) zone and a very small inductive loop (in some cases) in the lower frequency (LF) zone. The HF capacitive loop can be attributed to the charge transfer reaction and time constant of the electric double layer and to the surface inhomogeneity of interfacial origin, such as those found in adsorption processes on metal surface, and the LF inductive loop may be attributed to the relaxation process of intermediates (covered the reaction surface, i.e. species such as Cl−ads and H+ads) and adsorption−desorption process of inhibitive molecules on working electrode.25 In other words, the inductive behavior at low frequency is probably due to the consequence of the layer stabilization byproducts of the corrosion reaction on the electrode surface ([FeOHads] and [FeHads]) involving inhibitor molecules and their reactive products or it may also be attributed to the redissolution of passivated surface.26 When there is nonideal frequency response, then it is common practice to use distributed circuit, and the electrochemical system (mild steel/1 M HCl) is expected to be characterized by distributed capacitance. This is verified by the deformation of the capacitance semicircle whose center lies under the real axis. The distributed capacitance is modeled by constant phase element (CPE), and the corresponding structure model of the interface is shown in Figure 6b which is the model used to fit the experimental impedance data.23 The equivalent circuit model consists of a CPE, charge transfer resistance (Rct), and solution resistance (Rs). The double layer usually behaves as a constant phase element (CPE) rather than a pure capacitor. The CPE is substituted for the capacitor to fit the semicircle more

potential. In our case, where the corrosion inhibition depends on the potential of electrode, the observed inhibition phenomenon would be accompanied with the formation of a bidimensional layer of adsorbed species on mild steel surface.20 The behavior of PPZs at potential higher than −350 mV vs SCE could be associated with the significant dissolution of mild steel, suggesting that desorption rate of inhibitors is more than its adsorption.20 However, PPZs influence anodic reaction at potentials lower than −350 mV vs SCE. This discussion clearly shows that the corrosion inhibition of mild steel is under cathodic and anodic control. In LPR measurements, polarization resistance of the material is defined as the plot of the potential−current density (ΔE/ΔI) curve which is approximately linear and the linear polarization resistance can be found from the slope of the graph. Stern and Geary originally described a region in the vicinity of corrosion potential where a linear dependence of potential on applied current existed for a corroding electrode. They derived an LPR equation relating the slope of this linear region to the corrosion rate and Tafel slopes as21 Rp =

B ΔE = Icorr ΔI

(7)

where Rp is polarization resistance, ΔE/ΔI is the slope of the linear polarization curve, which is mainly controlled by the corrosion current density, Icorr which is relatively insensitive to the anodic (βa) and cathodic (βc) Tafel slopes. B is a proportionality constant, which can be calculated according to following equation:

B=

βa × βc 2.3(βa + βc)

(8)

The appropriate values of βa and βc were used in the calculation of polarization resistance (Rp). The inhibition efficiency was calculated using polarization resistance as follows:22 ⎛ Rp ⎞ ⎟ × 100 η% = ⎜⎜1 − R p(i) ⎟⎠ ⎝

(9)

where Rp and Rp(i) are the polarization resistances of uninhibited and inhibited systems respectively. Table 3 indicates that with the increase in concentration of PPZs, the values of polarization resistance increased, which suggests that mild steel corrosion retards in the inhibited system relative to the uninhibited system. The highest Rp values were obtained at 200 mg/L concentration for all PPZs. 3.2.3. AC Technique: EIS. EIS studies the system response to the application of a periodic small amplitude ac signal, and analysis of the system response contains information about the interface, its structure, and the reactions taking place there. The impedance response of mild steel in 1 M HCl solution was significantly changed after the addition of all PPZs, and the impedance of the inhibited system increased with inhibitor concentration. Figure 5 represents complex plane impedance plots (Nyquist plots) at the respective corrosion potential for mild steel in 1 M HCl with nil (inset diagram) and different concentrations of PPZs at 308 K. From Figure 5 it can be observed that the diameter of the semicircles increases with increase in concentrations of PPZs, indicating that corrosion is mainly a charge transfer process.23 Furthermore, at 200 mg L−1 concentration of PPZs larger diameter semicircles were obtained than at the other three lower concentrations. It

Figure 6. (a) Bode (log f vs log |Z|) and phase angle (log f vs α) plots of impendence spectra for mild steel in 1 M HCl at 200 mg L−1 of PPZs at 308 K. (b) Equivalent circuit model used to fit the EIS data. 8200



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Figure 7. Simulated and experimentally generated EIS plots: (a) Bode phase angle and (b) Nyquist plot.

Table 4. Electrochemical Impedance Parameters and the Corresponding Inhibition Efficiencies of Mild Steel in 1 M HCl in the Absence and Presence of Different Concentrations of PPZs at 308 K inhibitor

Cinh (mg L−1)

Rs (Ω)

Rct (Ω cm2)

n

Y0 (μF cm−2)

Cdl (μF cm−2)

θ

η (%)

blank PPZ-1

0.0 50 100 150 200 50 100 150 200 50 100 150 200 50 100 150 200

1.02 0.81 0.60 0.69 0.73 0.64 0.64 0.67 0.67 0.76 0.95 0.65 0.85 0.71 0.83 0.85 1.12

9.10 54.5 57.0 68.7 71.1 32.8 39.5 41.0 44.2 76.4 106 110.2 128 72.0 123 297 603

0.82 0.83 0.79 0.81 0.80 0.79 0.79 0.80 0.79 0.81 0.82 0.81 0.85 0.83 0.84 0.83 0.84

251 142 139 114 109 195 156 99 93 102 99 89 46 168 85 77 74

106 59 57 34 31 70 62 43 40 63 41 39 31 39 36 26 20

0.83 0.83 0.86 0.87 0.72 0.76 0.77 0.79 0.88 0.91 0.91 0.93 0.87 0.92 0.96 0.98

83.2 83.9 86.7 87.2 72.2 76.9 77.8 79.4 88.0 91.4 91.9 93.2 87.3 92.6 96.9 98.8

PPZ-2

PPZ-3

PPZ-4

YCPE = Y0(jω)n

accurately. The admittance, YCPE, and impedance, ZCPE, of a CPE are expressed as27

(10)

and 8201



ZCPE

⎛1⎞ = ⎜ ⎟[(jω)n ]−1 ⎝ Y0 ⎠

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where ε0 is the permittivity of free space (8.854 × 10−12 Fm1−), ε is the local dielectric constant of medium,and S is the surface area of the electrode. Equation 14 suggests that Cdl is inversely proportional to the thickness of protective layer d. Figure 6a depicts the Bode impedance magnitude and phase angle plots recorded for mild steel electrode immersed in 1 M HCl in the absence and presence of 200 mg L−1 PPZs at its open circuit potential. The impedance plots at higher frequency limit (100 kHz) correspond to the ohmic resistance of the films of corrosion product and the solution between the working electrode and reference electrode. The log |Z| values and phase angle fall to zero at high frequency. This is the response to resistive behavior of mild steel electrode which corresponds to solution resistance enclosed between the reference electrode and working electrode.14 In intermediate frequency region, a linear relationship between log |Z| vs log f with a slope near −1 and the phase angle approaching −80° has been observed. This is a characteristic response of capacitive behavior at intermediate frequencies. An ideal capacitive behavior would be the result if a slope value attained −1 and a phase angle value attained −90°.31 The slopes of Bode impedance magnitude plots at intermediate frequencies, S, and the maximum phase angles, α, showed deviation from the values of −1 and 90°, respectively. These deviations were considered to be the deviation from the ideal capacitive behavior at intermediate frequencies. The gradual approach of −S and α to the ideal capacitive values in the beginning of the immersion may be related to slowing of the rate of dissolution with time. For the same reason, the faster attainment of steady state of −S and α and their higher values in inhibited solutions than in uninhibited solution (Table 5) reflect the inhibitive action of

(11)

where Y0 is the amplitude comparable to a capacitance (with a μF cm−2), j is the square root of −1, ω is angular frequency (ω = 2π f max) at which the imaginary part of the impedance (−Zim) is maximal and f max is AC frequency at maximum, n is the phase shift, which can be used as a gauge of the heterogeneity or roughness of the mild steel surface. The CPE can be expressed by the values of n if resistance (n = 0, Y0 = R), capacitance (n = 1, Y0 = C), inductance (n = −1, Y0 = L), and Warburg impedance (n = 0.5, Y0 = W).14 The double layer capacitance (Cdl) values can be calculated from CPE parameter values Y and n using the following equation:28 Cdl =

Yωn − 1 sin(n(π /2))

(12)

The simulated and experimentally generated Nyquist and Bode phase angle plots are shown in Figure 7a and b. The quantitative results of impedance measurements are given in Table 4. Careful inspection of the results given in Table 4 indicates that the values of Rs are very small compared to Rct in presence of PPZs than in its absence; this in turn leads to an increase in inhibition efficiency. The inhibition efficiency (η %) using Rct values was calculated from the equation ⎛ R ⎞ η% = ⎜⎜1 − ct ⎟⎟ × 100 R ct(i) ⎠ ⎝

(13)

Table 5. Slopes of the Bode Impedance Magnitude Plots at Intermediate Frequencies (S) and the Maximum Phase Angles (α) for Mild Steel in 1 M HCl solution at 200 mg L−1 of PPZs at 308 K

where Rct(i) and Rct are the charge transfer resistance in presence and absence of PPZs, respectively. The Rct values increase with increase in concentration of PPZs which approaches maximum at optimum concentration (200 mg L−1). The most pronounced effect and the highest Rct value 603 Ω cm2 was obtained for PPZ-4 at 200 mg L−1. In addition, the values of proportional factor Y0 of CPE vary regularly with the concentrations of PPZs. The change in values of Rct and Y0 can be related to the gradual removal of water molecules by PPZs molecules on the electrode surface and consequently leads to a decrease in the number of active sites necessary for the corrosion reaction.14 The increased Rct values were attributed to insulated adsorption layer formation of inhibitors. The values of phase shift, n (ranges from 0.79 to 0.85), did not change significantly and its ongoing stability showed the charge transfer controlled dissolution mechanism of mild steel in 1 M HCl with and without inhibitors.29 Moreover, the values of Cdl decrease in presence of PPZs than in its absence and the lowest value was obtained for PPZ-4, i.e. 20 μFcm−2 at 200 mg L−1. The decrease in Cdl can result from the decrease of the local dielectric constant or increase of thickness of electrical double layer, which suggest the substitution of H2O molecules (with higher dielectric constant) with inhibitor molecules (with lower dielectric constant) leading to the formation of a thin protective film on the electrode surface.29 However, the more the adsorption of inhibitor molecules on the metal surface, the more the thickness of the barrier layer is increased according to the expression of the Helmholtz model30

Cdl =

εε0 S d

inhibitor

−S

−α°

blank PPZ-1 PPZ-2 PPZ-3 PPZ-4

0.50 0.77 0.74 0.78 0.79

40.90 70.24 65.78 74.51 75.59

the PPZs in the mild steel dissolution process.32 The Bode phase angle plots show single maximum (one time constant) at intermediate frequencies, broadening of this maximum in presence of PPZs accounting for the formation of a protective layer on the electrode surface.32 It is obvious from the results that the inhibition efficiencies calculated from electrochemical impedance are in agreement with the efficiencies calculated from gravimetric and polarization measurements. 3.3. Adsorption Considerations. Basic information of the interaction of inhibitor molecules to the metal surface could be provided from the adsorption isotherms. To obtain the isotherm, the linear relation between the values of θ and the inhibitor concentration (C inh) must be found. Several adsorption isotherms were applied to fit the surface coverage (θ) values at different concentrations of inhibitors. All of these isotherms are of the general form33 f (θ , x)exp( −2aθ ) = K adsC

(14) 8202

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Figure 8. Temkin (a) and Frumkin (b) isotherms for adsorption of PPZs on mild steel surface (gravimetric data). Langmuir isotherms for adsorption of PPZs on mild steel surface from (c) gravimetric, (d) Tafel polarization, (e) LPR, and (f) EIS data.

where f (θ, x) is the configuration factor which depends upon the physical model and assumption underlying the derivatives of the isotherm, θ is the surface coverage, C is the inhibitor concentration, x is the size ratio, a is the molecular interaction parameter, and Kads is the equilibrium constant of adsorption process. The plot of surface coverage (θ) as a function of logarithm of inhibitor concentration (log C) was evaluated. The correlation coefficients (R2) were used to determine the best fits. Figure 8a shows a correlation between surface coverage (θ) and inhibitor concentration in the corrosive medium which can be represented by the Temkin adsorption isotherm.34

K adsC = e fθ

The obtained plots were not linear for Temkin and Frumkin adsorption isotherms with correlation coefficients (R2) ranges from 0.755 to 0.911 (Figure 8a) and 0.804−0.922 (Figure 8b), respectively. Assumptions of Langmuir are that the concentration of adsorbate in the bulk of electrolyte (C) is related to the degree of surface coverage (θ) according to the equation given below:34 K adsC =

θ e fθ 1−θ

(18)

By plotting values of log (θ/1 − θ) versus values of log C, straight line graphs were obtained (Figure 8c−f), which suggested the adsorption of PPZs on metal surface obeyed Langmuir adsorption isotherm. The obtained plots of the PPZs were almost linear with correlation coefficient (R2) ranging from 0.981 to 0.989 for gravimetric (Figure 8c), 0.984−0.997 for Tafel polarization (Figure 8d), 0.987−0.990 for LPR (Figure 8e), and 0.982−0.997 for EIS (Figure 8f). Comparing the degree of linearity of Temkin, Frumkin and Langmuir

(16)

Figure 8b shows a correlation between surface coverage (θ) and log (θ/1 − θ)C, which can be represented by the Frumkin adsorption isotherm.34 K adsC =

θ 1−θ

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adsorption isotherms as measured by values of R2 show the best applicability of Langmuir adsorption isotherm. The fit of experimental data of above isotherms suggests that the addition of PPZs inhibits the corrosion reaction of mild steel by adsorption process. On the other hand, the equilibrium constant of adsorption (Kads) is also related to standard energy of adsorption (ΔG°ads) by the equation35 K ads =

⎛ −ΔG°ads ⎞ 1 ⎟ exp⎜ 55.5 ⎝ RT ⎠

percentage atomic content of mild steel samples obtained from EDX analysis is listed in Table 7. The results of EDX survey spectra are displayed in Figure 10. Figure 10a and b present EDX spectra of abraded and uninhibited mild steel specimens, respectively. The abraded mild steel specimen shows some characteristic peaks of the elements (C, Fe, Mn, O, Si, and Cr) constituting the mild steel sample. In EDX spectrum of uninhibited mild steel specimen, the peak of O is absent which confirms the dissolution of air-formed oxide film and free corrosion of bare metal. However, for inhibited solutions (Figure 10c−f), the EDX spectra showed an additional line characteristic for the existence of N (due to the N atoms of the PPZs). The spectra of Figure 10c−f show that the Fe peaks are considerably suppressed relative to the abraded and uninhibited mild steel sample. The suppression of Fe lines occurs because of the overlying inhibitor film on mild steel surface. The EDX analysis confirms the result of OCP (dissolution of air-formed oxide film and corrosion of bare metal) and also polarization measurements which suggest that the surface film inhibits the metal dissolution, and hence retards the hydrogen evolution. Therefore, EDX examinations of the electrode surface support the results obtained from gravimetric and electrochemical measurements. 3.5. UV−visible Spectroscopy. A strong evidence for the formation of metal complex was obtained using UV−visible spectroscopic analysis. UV−visible absorption spectra obtained from 1 M HCl solution containing 200 mg L−1 of PPZ-4 before and after 48 h of mild steel immersion at 308 K are shown in Figure 11. The electronic absorption spectra of PPZ-4 before the mild steel immersion display two bands in the UV-region (224 and 288 nm). These bands may arise due to π−π* and n−π* transitions with a considerable charge transfer character. After 48 h of mild steel immersion it is clear that there is an increase in the absorbance of this band which indicates the formation of a complex between two species in solution (Fe2+ and PPZ-4). Obot et al.6 have reported the shift in the value of absorbance indicates the complexation between two species in solution. However, there was no significant change in the shape of the spectra before and after mild steel immersion. These experimental findings provide strong evidence for the complex formed between Fe2+ and PPZ-4 in 1 M HCl solution. UV− visible investigation confirms the formation of a protective film of inhibitor on metal surface. 3.6. Theoretical Investigation. Computational methods have a strong impact toward the design and development of organic corrosion inhibitors. Recently, density function theory (DFT) has been used to analyze the characteristics of the inhibitor/surface mechanism and to describe the structural nature of the inhibitor on the corrosion process. Furthermore, DFT is considered to be a very useful technique to probe the inhibitor/surface interaction as well as to analyze the experimental data.37 Thus in our present investigation, DFT method was employed to give some insight into the inhibition action of PPZs molecules on the mild steel surface. The quantum chemical parameters such as charges on heteroatoms (N, O), EHOMO, ELUMO, the energy gap ΔE (ELUMO − EHOMO), and dipole moment (μ) were obtained for all the four neutral PPZs molecules to predict their activity toward metal surface. These quantum chemical parameters were generated after geometric optimization with respect to all nuclear coordinates. Figure 12 shows the optimized geometry of all PPZs. Frontier orbital density distribution is useful in predicting adsorption

(19)

where R is the gas constant and T is the absolute temperature. The value of 55.5 is the concentration of water in solution in mol L−1. The values of Kads and ΔG°ads are listed in Table 6. Table 6. Values of Kads and ΔG◦ads of PPZs for Mild Steel in 1 M HCl at 308 K inhibitor

Kads (104 M−1)

ΔG◦ads (kJ mol−1)

PPZ-1 PPZ-2 PPZ-3 PPZ-4

2.23 1.48 6.23 8.21

−36.94 −34.89 −40.96 −42.26

Generally, the energy values of ΔG°ads around −20 kJ mol−1 or less negative are associated with an electrostatic interaction between charged inhibitor molecules and charged metal surface, i.e. physisorption; those of −40 kJ mol−1 or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type bond, i.e. chemisorptions.36 The values of ΔG°ads in our measurements range nearly from −34 to −43 kJ mol−1 (Table 6). It can be suggested that the adsorption of these derivatives particularly involves chemical adsorption (i.e., chemisorption) of inhibitor molecules on metal surface. The higher values of Kads and ΔG°ads refer to higher adsorption and higher inhibiting effect with increasing exposure time. 3.4. Surface Investigation. 3.4.1. SEM Analysis. SEM micrographs obtained for mild steel surface after specimens were immersed for 3 h in 1 M HCl solutions in absence and presence of optimum concentration (200 mg L−1) of PPZs are presented in Figure 9a−f. A comparison can be drawn with the morphology of the micrographs. Figure 9a is the micrograph of abraded mild steel surface; abrading scratches are clearly visible on the surface. The resulting morphology of mild steel surface in uninhibited solution is shown in Figure 9b, which reveals a very rough surface with cracks and pits due to rapid corrosion attack; it can be concluded that mild steel surface was greatly damaged in absence of PPZs. Figure 9c−f are the morphologies that resulted after testing in 1 M HCl in the presence of PPZs; the results proved that there is less damage on steel surfaces and the inhibitor can protect mild steel surface effectively in 1 M HCl. The morphology of mild steel surface in presence of PPZ-4 is relatively smooth and less corroded (Figure 9f), which might be attributed to strong adsorption of the inhibitor on metal surface to suppress the corrosion. 3.4.2. EDX Analysis. The goal of this section was to confirm the results obtained from the gravimetric and electrochemical measurements that a protective film of PPZs is formed on the mild steel surface. To achieve our goal, the EDX examinations of the mild steel surfaces were performed in 1 M HCl solution in absence and presence of PPZs. EDX survey spectra were used to determine the elements present on mild steel surface before and after exposure to the inhibitor solution. The 8204



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Figure 9. SEM micrographs of mild steel surfaces: (a) abraded, (b) uninhibited 1 M HCl, (c) PPZ-1, (d) PPZ-2, (e) PPZ-3, and (f) PPZ-4.

be due to presence of N and O atoms with π-electrons in the inhibitor molecule. The calculated molecular parameters are listed in Table 8. The use of Mulliken charge analysis to estimate the adsorption centers of inhibitors has been widely reported and it is general consensus by several authors that the more negatively charged a

centers of the all PPZs molecules responsible for the interaction with metal surface atoms. Figure 13 shows the HOMO and the LUMO density distribution of all four PPZs. It can be seen from Figure 13 that PPZs have slightly different HOMO and LUMO distributions. HOMO density distributions were principally localized on pyrazole and pyran ring, which might 8205



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Table 7. Percentage Atomic Contents of Elements Obtained from EDX Spectra inhibitor polished mild steel blank PPZ-1 PPZ-2 PPZ-3 PPZ-4

Fe

C

Si

Mn

Cr

70.3

28.0

0.23

0.34

0.44

63.9 65.2 66.3 66.9 66.0

36.1 28.8 28.7 28.5 28.1

N

O 0.69

0.24 2.90 3.30 3.11 2.99

3.10 2.70 2.11 2.91

heteroatom (listed in Table 8), the more is its ability to adsorb on the metal surface.38 It is well documented in literature that the higher values of EHOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor state of molecule with low energy. Moreover, the gap between LUMO and HOMO energy levels of the molecule ΔE was another important factor that should be considered.38 A molecule with lower ΔE is more polarizable and is generally

Figure 11. UV−visible spectra of the solution containing 200 mg L−1 PPZ-4 + 1 M HCl before (red) and after (blue) 48 h of mild steel immersion.

associated with a high chemical reactivity. Quantum chemical parameters listed in Table 8 reveal that PPZ-4 has high highest

Figure 10. EDX spectra of mild steel surfaces: (a) abraded, (b) uninhibited 1 M HCl, (c) PPZ-1, (d) PPZ-2, (e) PPZ-3. and (f) PPZ-4. 8206



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Figure 12. Optimized structures of PPZs (Ball and Stick Model).

EHOMO and lowest ΔE, which is obvious from its highest inhibition efficiency among PPZs. According to literature, there is a lack of agreement on the correlation between the dipole moment and inhibition mechanism. It was reported that higher values of dipole moment probably increase the adsorption between the chemical compound and metal surface.39 PPZ-4 has the highest dipole moment in comparison to the other three inhibitors which suggests its strong adsorption and greater inhibition on mild surface. It can be concluded that the quantum chemical investigation provides good support to experimental results. 3.7. Adsorption and Inhibition Mechanism. The adsorption mechanism is influenced by nature and charge on metal surface and chemical structure of inhibitors. The charge on metal surface is due to electric field which emerges at the metal/electrolyte interface. It is well-known that steels are positively charged with respect to the potential of zero charge (PZC) in acid solutions.40 The PPZ molecule may adsorb on the metal/acid solution interface by (Figure 14) (I) electrostatic interaction of protonated inhibitor molecules with already adsorbed chloride ions (physisorption), (II) unshared electron pairs of heteroatoms and vacant d-orbital of Fe surface atoms (chemisorption), or (III) interaction of d-electron of iron surface atom to the vacant orbital of inhibitor molecule (retro donation).

In aqueous HCl solution, the inhibitor molecules may adsorb through protonated heteroatoms (N, O) and already adsorbed Cl− ions on mild steel surface. Initially, the protonated form of PPZ molecules in acid medium start competing with H+ ions for electrons on mild steel surface. After release of H2 gas, the cationic form of inhibitors returns to its neutral form and heteroatoms with free lone pair electrons promote chemical adsorption. The accumulation of electrons on mild steel surface render it more negative and to relieve the metal from extra negative charge the electron from the d-orbital of Fe might be transferred to the vacant π* (antibonding) orbital of inhibitor molecules (reterodonation) and hence strengthen adsorption on metal surface.41 Experimental and theoretical data reveal that η% for four PPZs are in the order PPZ-4 > PPZ-3 > PPZ-1 > PPZ-2. This order of efficiency is best explained in terms of the presence of substituents in the phenyl moiety. The greater inhibition effect of PPZ-4 might be due to presence of a ringactivating methoxy group in their phenyl moiety. The weaker performance of PPZ-2 as inhibitor is due to ring-deactivating nitro group, which decreases the electron density of phenyl moiety.

4. CONCLUSION PPZs act as very good inhibitors for mild steel in 1 M HCl solution and the extent of inhibition was concentration dependent. Inhibition efficiency values increase with inhibitor 8207



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Figure 13. Frontier molecular orbital density distributions of PPZs: (a) PPZ-1 (left, HOMO; right, LUMO), (b) PPZ-2 (left, HOMO; right, LUMO), (c) PPZ-3 (left, HOMO; right, LUMO), and (d) PPZ-4 (left, HOMO; right, LUMO).

Table 8. Calculated Quantum Chemical Parameters of the PPZs Mulliken charges of heteroatoms inhibitor

Na

Nb

Nc

Nd

PPZ-1 PPZ-2 PPZ-3 PPZ-4

−0.213 −0.207 −0.214 −0.214

−0.601 −0.598 −0.601 −0.601

−0.811 −0.808 −0.812 −0.812

−0.217 −0.208 −0.218 −0.217

Ne −0.093

Of −0.518 −0.514 −0.518 −0.518

Og

Oh

−0.287

−0.293

−0.554

dipole (μ) (D)

EHOMO (Hartree)

ELUMO (Hartree)

ΔE (Hartree)

3.6903 3.4096 4.3919 9.7635

−0.2142 −0.2217 −0.2099 −0.2009

−0.0077 −0.0125 −0.0122 −0.1265

0.2065 0.2092 0.1977 0.0744

increase and Cdl values decrease with increasing inhibitor concentration. The adsorption model obeys Langmuir adsorption isotherm. The Kads and ΔG°ads together with low

concentration. Polarization curves proved that PPZs were mixed type inhibitors, which can suppress anodic and cathodic reactions at the same time. EIS plots indicated that Rct values 8208



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(7) Ahamad, I.; Quraishi, M. A. Mebendazole: New and efficient corrosion inhibitor for mild steel in acid medium. Corros. Sci. 2010, 52, 651. (8) Ahamad, I.; Prasad, R.; Quraishi, M. A. Adsorption and inhibitive properties of some new Mannich bases of isatin derivatives on corrosion of mild steel in acidic media. Corros. Sci. 2010, 52, 1472. (9) Singh, A. K.; Quraishi, M. A. Investigation of adsorption of isoniazid derivatives at mild steel/hydrochloric acid interface: Electrochemical and weight loss methods. Mater. Chem. Phys. 2010, 123, 666. (10) Yadav, D. K.; Maiti, B.; Quraishi, M. A. Electrochemical and quantum chemical studies of 3,4-dihydropyrimidin-2(1H)-ones as corrosion inhibitors for mild steel in hydrochloric acid solution. Corros. Sci. 2010, 52, 3586. (11) Abdel-Rehim, S. S.; Khaled, K. F.; Al-Mobarak, N. A. Corrosion inhibition of iron in hydrochloric acid using pyrazole. Arabian J. Chem. 2011, 4, 333. (12) Hameed, R. S. A.; Al-Shafey, H. I.; Abul Magd, A. S.; Shehata, H. A. Pyrazole Derivatives as Corrosion Inhibitor for C- Steel in Hydrochloric Acid Medium. J. Mater. Environ. Sci. 2012, 3 (2), 294. (13) ASTM G-31-72, Standard Practice for Laboratory Immersion Corrosion Testing of Metals; ASTM: Philadelphia, PA, 1990. (14) Yadav, D. K.; Quraishi, M. A.; Maiti, B. Inhibition effect of some benzylidenes on mild steel in 1 M HCl: An experimental and theoretical correlation. Corros. Sci. 2012, 55, 254. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr. J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratman, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, Piskorz, A. P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford CT, 2007. (16) Rammelt, U.; Koehler, S.; Reinhard, G. Electrochemical characterisation of the ability of dicarboxylic acid salts to the corrosion inhibition of mild steel in aqueous solutions. Corros. Sci. 2011, 53, 3515. (17) Hassan, H. H.; Abdelghani, E.; Amin, M. A. Inhibition of mild steel corrosion in hydrochloric acid solution by triazole derivatives Part I. Polarization and EIS studies. Electrochim. Acta 2007, 52, 6359. (18) Khaled, K. F.; Amin, M. A. Corrosion monitoring of mild steel in sulphuric acid solutions in presence of some thiazole derivatives − Molecular dynamics. Chem. Electrochem. Stud. 2009, 51, 1964. (19) Negm, N. A.; Kandile, N. G.; Aiad, I. A.; Mohammad, M. A. New eco-friendly cationic surfactants: Synthesis, characterization and applicability as corrosion inhibitors for carbon steel in 1N HCl. Colloids Surf., A 2011, 391, 224. (20) Yuce, A. O.; Kardas, G. Adsorption and inhibition effect of 2thiohydantoin on mild steel corrosion in 0.1 M HCl. Corros. Sci. 2012, 58, 86. (21) Mahgoub, F. M.; Al-Nowaiser, F. M.; Al-Sudairi, A. M. Effect of Temperature on the Inhibition of the Acid Corrosion of Steel by Benzimidazole Derivatives. Prot. Met. Phys. Chem. Surf. 2011, 47, 381. (22) Zhang, F.; Pan, J.; Claesson, P. M. Electrochemical and AFM studies of mussel adhesive protein (Mefp-1) as corrosion inhibitor for carbon steel. Electrochim. Acta 2011, 56, 1636. (23) Tan, K. W.; Kassim, M. J. A correlation study on the phenolic profiles and corrosion inhibition properties of mangrove tannins

Figure 14. Schematic depiction of adsorption modes of pyranopyrazole molecule on mild steel surface in 1 M HCl.

corrosion rates in presence of PPZs suggested that they were strongly adsorbed on mild steel surface. UV−visible spectrophotometric studies clearly reveal the formation of Fe−PPZ complex which may be responsible for the observed inhibition. Theoretical calculations provide good support to experimental results.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: maquraishi@rediffmail.com or maquraishi.apc@itbhu. ac.in. Tel.: +91 930 7025126. Fax: +91 542 2368428. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge All India Council for Technical Education (AICTE) for the financial assistance and Department of Science and Technology (DST), New Delhi, India for facilitation of our study.

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REFERENCES

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Electrochemical investigation of Substituted Pyranopyrazoles

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