Materials Chemistry and Physics

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1 Materials Chemistry and Physics 112 (2008) Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: Application of electrochemical frequency modulation for monitoring corrosion and corrosion inhibition of iron by some indole derivatives in molar hydrochloric acid K.F. Khaled Electrochemistry Research Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt article info abstract Article history: Received 27 April 2008 Received in revised form 14 May 2008 Accepted 23 May 2008 Keywords: EFM Corrosion inhibition EIS Molecular simulation Quantum chemical calculation The corrosion inhibition effect of four indole derivatives, namely indole (IND), benzotriazole (BTA), benzothiazole (BSA) and benzoimidazole (BIA), have been used as possible corrosion inhibitors for pure iron in 1 M HCl. In this study, electrochemical frequency modulation, EFM was used as an effective method for corrosion rate determination in corrosion inhibition studies. By using EFM measurements, corrosion current density was determined without prior knowledge of Tafel slopes. Corrosion rates obtained using EFM, were compared to that obtained from other chemical and electrochemical techniques. The results obtained from EFM, EIS, Tafel and weight loss measurements were in good agreement. Tafel polarization measurements show that indole derivatives are cathodic-type inhibitors. Molecular simulation studies were applied to optimize the adsorption structures of indole derivatives. The inhibitor/iron/solvent interfaces were simulated and the adsorption energies of these inhibitors were calculated. Quantum chemical calculations have been performed and several quantum chemical indices were calculated and correlated with the corresponding inhibition efficiencies Elsevier B.V. All rights reserved. 1. Introduction It is well known that iron and iron-based alloys are used most widely in industry. Consequently, great attention has been paid to studies on the corrosion of iron and its alloys. Acid solutions are extensively used in industry, the most important of which are acid pickling, industrial acid cleaning, acid-descaling and oil well acidizing. The commonly used acids are hydrochloric acid, sulfuric acid, nitric acid, etc. Since acids are aggressive, inhibitors are usually used to minimize the corrosive attack on metallic materials. Inhibitors are widely used in the corrosion protection of metals in several environments [1]. Electrochemical frequency modulation, EFM is used as a new technique for corrosion rate measurements [2 4]. In EFM,two ac voltage waveforms are summed and applied to an iron electrode. The frequencies of the two sinusoidal waveforms must share no common factors. Normally, 2 and 5 Hz are suitable frequencies for EFM. While simple in concept, EFM yields an impressive amount of information on the corrosion process including corrosion rate, Tafel constants and causality factors. The corrosion rate is calculated from the corrosion current density that is measured by EFM [3]. It address: khaled@asunet.shams.edu.eg. is worth emphasizing that the Tafel constants are not needed to measure the corrosion current density. For this reason, EFM enjoys a significant practical advantage compared to polarization techniques. EFM provides an independent measure of the anodic and cathodic Tafel constants. The causality factors are used to validate the data. They are similar to an internal check of the consistency of the measurement process. There are two causality factors 2 and 3; if the calculated causality factors are higher than 2 and 3, respectively, the quality of the measured data is not valid. Additionally, EFM may be less susceptible to errors in applied potential from ir effects than polarization measurements. Bogaerts et al. [3,4] proposed EFM as a novel technique for online corrosion monitoring. In this technique, current responses due to a potential perturbation by one or more sine waves are measured at more frequencies than the frequency of the applied signal, for example at zero, harmonic and intermodulation frequencies. This simple principle offers various possibilities for corrosion rate measurements. With this novel EFM technique, the corrosion rate can be determined from the corrosion system responses at the intermodulation frequencies. The EFM approach requires only a small polarizing signal, and measurements can be completed in a short period of time. EFM can be used as a rapid and non-destructive technique for corrosion rate measurements without prior knowledge of Tafel constants. EFM can be used successfully for corrosion rate measurements under various corrosion conditions, such as iron /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 1. Chemical structures for indole derivatives. in acidic environment without and with inhibitors and mild steel in a neutral environment. However, Mansfeld and co-workers [5] evaluate the EFM technique for several corrosion systems including active and passive systems and found that EFM measurements can be applied successfully only for a limited number of corrosion systems with fairly high corrosion rates and the technique should be used only with great caution for corrosion monitoring. Recently, Han et al. [6], set up a system for EFM measurement for the early corrosion rate of mild steel in see water system. It was shown that the corrosion rates determined with EFM technique were higher than the real values, especially at longer exposed times when diffusioncontrolled effect became obvious. But they were still at the same order of magnitude as those obtained with polarization curve analysis or weight loss method. In continuation of our previous work on evaluation for EFM as a new non-linear distortion technique as well as the development of new corrosion inhibitors for acidic solutions [2,7 10]. Corrosion inhibition data will be calculated using EFM technique and it will be compared with that obtained from traditional chemical and electrochemical techniques like weight loss, Tafel polarization and EIS measurements. In this work, EFM measurements were used to study the corrosion and corrosion inhibition of iron in 1 M HCl using some indole derivatives namely, indole (IND), benzotriazole (BTA), benzothiazole (BSA) and benzoimidazole (BIA), in order to evaluate the EFM as a new approach for online corrosion rate monitoring in iron/hcl systems, which might be helpful for online application of electrochemical techniques. Also, molecular simulation and quantum chemical calculations were used to study the adsorption of these compounds on iron surface. 2. Experimental Experiments were carried out using pure iron (Puratronic %) from Johnson Mattey Ltd., as the electrode material. Iron rods were mounted in Teflon with surface area of 0.28 cm 2 and are used for electrochemical measurements. Iron sheets with dimensions 5.0 cm 1.0 cm 0.1 cm with total surface area equals 11.2 cm 2 are used for weight loss measurements. The surface was abraded using emery papers of (180, 120, 0, 4/0) grit size, polished with Al 2O 3 (0.5 m particle size), cleaned in 18 M cm water in an ultrasonic bath, and subsequently rinsed in acetone and bidestiled water. A conventional electrolytic cell, as described elsewhere [11], was used for all experiments with a platinum counter electrode and a saturated calomel electrode (SCE) as a reference electrode. All reported potential values are on the SCE scale. Electrochemical experiments were carried out under static conditions at 25 ± 1 C in aerated solutions, with a fine Luggin capillary tip closely placed to minimize ohmic resistance. The structures of the heterocyclic compounds studied are presented in Fig. 1. All compounds investigated were obtained from Aldrich chemical co. They were put in 1 M HCl (Fisher Scientific) without pretreatment at concentration of 10 4 M, 10 3 M, M and 10 2 M. The electrode was immersed in these solutions for one hour before starting measurements. EFM measurements carried out using two frequencies of 2 and 5 Hz. The base frequency was 1 Hz. EIS measurements were carried out in a frequency range of 100 khz to 50 mhz with amplitude of 5 mv peak-to-peak using ac signals at respective corrosion potentials. Polarization curves were obtained by changing the electrode potential automatically from ( 250 mv SCE to +250 mv SCE) at the respective corrosion potentials with a scan rate of 1 mv s 1. Electrochemical measurements were performed with a Gamry Instrument Potentiostat/Galvanostat/ZRA. These include Gamry framework system based on the ESA400, Gamry applications that include EFM140 to perform EFM measurements, EIS300 for electrochemical impedance spectroscopy measurements and DC105 for dc corrosion measurements along with a computer for collecting the data. For weight loss measurements, the iron coupons were left hanged in the test solution for 6 h at 25 ± 1 C before recording the loss of their weights. The corrosion rate was calculated, in milligram per square centimeter per hour (mg cm 2 h 1 ), on the basis of the apparent surface area. The inhibition efficiencies calculations were based on the weight loss measurements at the end of the exposure period. The results of the weight loss experiments are the mean of three runs, each with a fresh iron coupon and fresh acid solution. For applying molecular modeling techniques to these compounds, the iron crystal was cleaved along with (0 0 1) plane, thus representing the iron surface. The liquid phase consisted of 400 water molecule and a single dissolved indole derivative. On the top of this aqueous layer, an additional layer of 200 water molecule serves as an upper limit for the aqueous layer acting like a wall with the same physical and chemical properties. To obtain this configuration we use Accelyrs molecular dynamics software with boundary conditions described elsewhere [12]. Molecular orbital calculations (MO) are based on the semi-empirical selfconsistent methods (SCF). A full optimization of all geometrical variables (bond lengths, bond angles and dihedral angles) without any symmetry constraint was performed at the restricted Hartree-Fock level (RHF). We used PM3, AM1, MONDO and MINDO/3 semi-empirical SCF-MO methods in the Hyperchem 8.03 program, implemented on an Intel Pentium IV, 3.6 GHz computer. 3. Results and discussion 3.1. Electrochemical frequency modulation (EFM) Electrochemical frequency modulation is a nondestructive corrosion measurement technique that can directly give values of the corrosion current without prior knowledge of Tafel constants. Like EIS, it is a small signal ac technique. Unlike EIS, however, two sine waves (at different frequencies) are applied to the cell simultaneously. Because current is a non-linear function of potential, the system responds in a non-linear way to the potential excitation. The current response contains not only the input frequencies, but also contains frequency components which are the sum, difference, and multiples of the two input frequencies. The two frequencies may not be chosen at random. They must both be small, integer multiples of a base frequency that determines the length of the experiment. Fig. 2 shows representative examples for the waveform when the two input frequencies are 2 and 5 Hz. The higher frequency must be at least two times the lower one. The higher frequency must also be sufficiently slow that the charging of the double layer does not contribute to the current response. Often, 10 Hz is a reasonable limit. Intermodulation spectra obtained from EFM measurements are presented in Fig. 3. Each spectrum is a current response as a function of frequency. The two large peaks, with amplitudes of about 200 microampere, are the response to the 2 and 5 Hz excitation frequencies. Those peaks between 1 microampere and 20 microampere are the harmonics, sums, and differences of the two-excitation frequencies. These peaks are used by the EFM140 software package to calculate the corrosion current and the Tafel constants. It is important to note that between the peaks the current response is very small. There is nearly no response

3 292 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 2. Wave form spectrum for the input frequencies 2 and 5 Hz for iron in 1 M HCl in absence and presence of various concentrations of indole derivatives. (<100 na) at 4.5 Hz, for example, the frequencies and amplitudes of the peaks are not coincidences. They are direct consequences of the EFM theory [3,4]. Corrosion kinetic parameters listed in Table 1 were calculated from EFM techniques using the following equations: i corr = i 2 ω 48(2iω i 3ω i2ω ) (1) i ω U 0 ˇc = 2 3 2i 3ω i ω i 2 2ω 2i 2ω Causality factor (2) = i ω 2 ±ω 1 i 2ω1 = 2.0 (4) Causality factor (3) = i 2ω 2 ±ω 1 i 3ω1 = 3.0 (5) (3) i ω U 0 ˇa = 2i 2ω i 3ω i ω i 2 2ω (2) where i is the instantaneous current density at the working electrode measured at frequency ω and U 0 is the amplitude of the sine wave distortion

4 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 3. Intermodulation spectrum for iron in 1 M HCl in absence and presence of various concentrations of indole derivatives. Table 1 shows the corrosion kinetic parameters such as inhibition efficiency (E EFM %), corrosion current density ( Acm 2 ), Tafel constants (ˇa, ˇc) and causality factors (CF-2, CF-3) at different concentration of indole derivatives in mol L 1 HCl at 25 ± 1 C. It is obvious from Table 1 that, the corrosion current densities decrease with increase in concentrations of these compounds. The inhibition efficiencies increase with increase in indole derivatives concentrations. The causality factors in Table 1 are very close to theoretical values which according to the EFM theory [3] should guarantee the validity of Tafel slopes and corrosion current densities. Inhibition efficiency (E EFM %) depicted in Table 1 calculated from the following equation. ( ) E EFM % = 1 i corr icorr (6) where i 0 corr and i corr are corrosion current densities in the absence and presence of the studied compounds, respectively. The great strength of the EFM is the causality factors which serve as an internal check on the validity of the EFM measurement [4].

5 294 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Table 1 Electrochemical kinetic parameters obtained by EFM technique for iron in absence and presence of various concentrations of indole derivatives in 1 M HCl at 25 ± 1 C Inhibitor Concentration (M) i corr ( Acm 2 ) ˇa (mv dec 1 ) ˇc (mv dec 1 ) E EFM% C.F-2 C.F-3 Benzoimidazole (BIA) Benzothiazole (BSA) Benzotriazole (BTA) Indole (IND) Fig. 4. Nyquist plots for iron in 1 M HCl in absence and presence of various concentrations of BIA. With the causality factors the experimental EFM data can be verified. The causality factors in Table 1 indicate that the measured data are of good quality. The standard values for CF-2 and CF-3 are 2.0 and 3.0, respectively. To evaluate the EFM technique as an effective Fig. 6. Nyquist plots for iron in 1 M HCl in absence and presence of various concentrations of BTA. corrosion monitoring technique, several traditional corrosion techniques implied to study the corrosion inhibition of iron by indole derivatives in 1 M HCl solutions. EIS, Tafel extrapolation and weight loss measurements are used to calculate the inhibition efficiency as well as other corrosion kinetic parameters. Fig. 5. Nyquist plots for iron in 1 M HCl in absence and presence of various concentrations of BSA. Fig. 7. Nyquist plots for iron in 1 M HCl in absence and presence of various concentrations of IND.

6 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 9. Anodic and cathodic Tafel polarization curves for iron in the absence and presence of various concentrations of compound BIA in 1 M HCl. Fig. 8. Equivalent circuit used to model impedance data in 1 M HCl solutions, OHP, outer Helmholtz plane; C dl, double layer capacitance; R s, solution resistance; R ct, charge-transfer resistance Electrochemical impedance spectroscopy Nyquist plots recorded for iron electrode in 1 M HCl solution and containing various concentrations of indole derivatives are shown in Figs As can be seen from Figs. 4 7, the Nyquist plots do not yield perfect semicircles as expected from the theory of EIS. The deviation from ideal semicircle was generally attributed to the frequency dispersion [13] as well as to the inhomogenities of the surface and mass transport resistant [7]. In the evaluation of Nyquist plots, the difference in real impedance at lower and higher frequencies is generally considered as charge-transfer resistance. The resistances between the metal and outer Hemlholtz plane (OHP) must be equal to the R ct. Figs. 4 7 clearly demonstrate that the shapes of the impedance plots for inhibited electrodes are not substantially different from those of uninhibited electrodes. The presence of indole derivatives increases the impedance (the semicircle diameter) but does not change other aspects of the corrosion behavior. The impedance spectra of inhibited solutions consist of one depressed semicircle with a considerable deviation from an ideal semicircle. Inhibitor molecules adsorb on the iron surface and modify the interface. The adsorption of inhibitor molecules on the metal surface decreases its electrical capacity because they displace the water molecules and other ions originally adsorbed on the metal surface [14]. This modification results in an increase of charge-transfer resistance. The R ct values increased with indole derivatives concentrations may suggest the formation of a protective layer on the iron electrode surface. This layer makes a barrier for mass and charge-transfer. The double layer can be represented by the electrical equivalent circuit diagrams to model metal/solution interface. The corresponding electrical equivalent circuit model for iron in uninhibited solution is given in Fig. 8 [7]. According to Fig. 8, the chargetransfer resistance, which corresponds to the diameter of Nyquist plot, determines the corrosion rate and represents the resistance between the metal/ohp (outer Helmholtz plane). Fig. 8 describes the potential distributions on the metal/solution interface and pro- Table 2 Circuit element R s, R ct, n and CPE values obtained using equivalent circuit in Fig. 8 for iron in 1 M HCl and different concentrations of indole derivatives in mol L 1 HCl at 25 ± 1 C Inhibitor Concentration (M) R s ( cm 2 ) R ct ( cm 2 ) n CPE/C dl ( Fcm 2 ) E imp (%) Benzoimidazole (BIA) Benzothiazole (BSA) Benzotriazole (BTA) Indole (IND)

7 296 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 10. Anodic and cathodic Tafel polarization curves for iron in the absence and presence of various concentrations of compound BSA in 1 M HCl. Fig. 12. Anodic and cathodic Tafel polarization curves for iron in the absence and presence of various concentrations of compound IND in 1 M HCl. Table 2 that the impedance of the inhibited system amplified with increasing the inhibitors concentration and CPE values decrease with increasing the inhibitors concentrations. This decrease in CPE results from a decrease in local dielectric constant and/or an increase in the thickness of the double layer, suggesting that indole derivatives inhibit the iron corrosion by adsorption at iron/acid interface [15]. It is well known that the capacitance is inversely proportional to the thickness of the double layer [16]. A low capacitance may result if water molecules at the electrode interface are largely replaced by organic inhibitor molecules through adsorption [16]. The larger inhibitor molecules also reduce the capacitance through the increase in the double layer thickness. The thickness of this protective layer increases with increase in inhibitor concentration. This process results in a noticeable decrease in CPE/C dl. This trend is in accordance with Helmholtz model, given by the following equation: Fig. 11. Anodic and cathodic Tafel polarization curves for iron in the absence and presence of various concentrations of compound BTA in 1 M HCl. posed electrical equivalent circuit diagram for the corrosion system blank solution. The values of impedance parameters determined from Nyquist plots, such as R ct and E imp %arelistedintable 2. It is apparent from C dl = εε 0A (7) d where d is the thickness of the protective layer, ε is the dielectric constant of the medium, ε 0 is the vacuum permittivity and A is the effective surface area of the electrode. The value of CPE/C dl is always smaller in the presence of the inhibitor than in its absence, as a result of the effective adsorption of the inhibitor. It is apparent Table 3 Electrochemical kinetic parameters obtained by Tafel polarization technique for iron in absence and presence of various concentrations of indole derivatives in mol L 1 HCl at 25 ± 1 C Inhibitor Concentration (M) j corr ( Acm 2 ) E corr (mv) ˇa (mv dec 1 ) ˇc (mv dec 1 ) E Tafel (%) Benzoimidazole (BIA) Benzothiazole (BSA) Benzotriazole (BTA) Indole (IND)

8 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Table 4 Corrosion rate in (mg cm 2 h 1 ), inhibition efficiency data obtained from weight loss measurements iron in absence and presence of various concentrations of indole derivatives in mol L 1 HCl at 25 ± 1 C Inhibitor Concentration (M) Weight loss (mg) Corrosion rate (mg cm 2 h 1 ) E w (%) Benzoimidazole (BIA) Benzothiazole (BSA) Benzotriazole (BTA) Indole (IND) that causal relationship exists between adsorption and inhibition. The inhibition efficiency was calculated using charge-transfer resistance as follows: ( ) E imp % = 1 R0 ct 100 (8) R ct words, the inhibitors decrease the surface area for corrosion without affecting the corrosion mechanism of iron in 1 M HCl solution, and only causes inactivation of a part of the surface with respect to the corrosive medium [18]. where R 0 ct and R ct are the charge-transfer resistances for uninhibited and inhibited solutions, respectively Tafel polarization measurements Polarization curves for iron in 1 M HCl in the absence and presence of BIA, BSA, BTA and IND of various concentrations at 25 ± 1 C are shown in Figs As can be seen the cathodic curves were more polarized than anodic curves where the cathodic reaction is remarkably affected by the inhibitors, whereas the anodic one is slightly shifted toward lower currents. Electrochemical kinetic parameters obtained by Tafel polarization technique for iron in absence and presence of various concentrations of indole derivatives in 1 M HCl at 25 ± 1 C are listed in Table 3. As can be seen in Table 3, higher ˇc values revealed that cathodic reduction rate was retarded [17]. This indicated that the indole derivatives influenced cathodic reaction more than anodic reaction and hence the addition of these compounds controls the rate of hydrogen evolution reaction on iron surface. BIA, BSA, BTA and IND are considered as cathodic inhibitors due to the negative shift in the corrosion potential and noticeable decrease of the cathodic current by adding them to 1 M HCl solution. The inhibition efficiency was evaluated from Tafel polarization measurements and listed in Table 3 using the following equation: ( ) E Tafel % = 1 j corr jcorr (9) where j 0 corr and j corr are the corrosion current densities for uninhibited and inhibited solutions, respectively. It is clear from Table 3 that, corrosion current density j corr decreased by addition of indole derivatives in 1 M HCl. The obtained inhibition efficiencies given in Table 3 show that indole derivatives act as effective inhibitors. It is obvious from Table 3 that the slopes of the anodic (ˇa) and cathodic (ˇc) Tafel lines remain almost unchanged upon addition of indole derivatives. Thus the adsorbed inhibitors act by simple blocking of active sites for both anodic and cathodic processes. In other Fig. 13. Comparison of inhibition efficiencies obtained with EFM, EIS, Tafel extrapolation and weight loss measurements for indole derivatives.

9 298 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 14. The amorphous cell containing the iron substrate, the solvent molecule and indole derivatives Weight loss measurements The weight loss of iron coupons in 1 M HCl without and with the addition of indole derivatives was used to calculate the inhibition efficiency E w % by using the following equation: E w % = w 0 w w (10) where w 0 and w are the weight loss of iron coupons in HCl solution without and with the addition of indole derivatives, respectively. Table 4 shows the values of inhibition efficiencies and corrosion rates obtained from weight loss method at different concentrations of BIA, BSA, BTA and IND in 1 M HCl at 25 ± 1 C. The amount of weight loss is found to decrease with increasing additive concentrations for these compounds. It is obvious from Table 4 that, indole derivatives inhibit the corrosion of iron in 1 M HCl solutions at all concentrations used in this study. The weight loss measurements revealed the excellent stability of the inhibitors in the acid medium. To assess the stability, the experiments were conducted by taking acid solution containing inhibitor which was kept for 10 days under air agitation and no change in inhibition efficiency values were observed. Fig. 13 represents comparisons of the inhibition efficiencies obtained for IND, BTA, BSA and BIA at concentrations (10 2 M, 10 3 M) using EFM, EIS, Tafel polarization and weight loss measurements. The calculated inhibition efficiency obtained from weight loss, Tafel polarization and EIS measurements are in good agreement with that obtained from EFM measurements. As can be seen in Fig. 13, the corrosion rates determined with EFM technique were high, but they were still at the same order of magnitude as those obtained with other conventional chemical and electrochemical techniques. These results are comparable with that mentioned in the literature [6] Molecular modeling and quantum chemical calculations Molecular simulation studies were performed to simulate the adsorption of the four indole derivatives on the iron surface in presence of solvent effects. Molecular structures of indole derivatives show that it is likely for these molecules to adsorb on iron surface by sharing the electrons of nitrogen atoms and/or -electron interactions of the aromatic rings [7]. Both interactions can make it possible for indole derivatives to form coordinated bond with iron. The adsorption progress of indole derivatives on iron surface is investigated by performing molecular mechanics (MM) using Table 5 Theoretical quantum chemical calculations using semi-emprical SCF-MO method (PM3) in Hyperchem 8.03 program as well as adsorption energy calculations Inhibitor Molecular area (Å 2 ) E HOMO (ev) E LUMO (ev) (D) Volume (Å 3 ) Adsorption energy (kj mol 1 ) Indole (IND) Benzotriazole (BTA) Benzothiazole (BSA) benzoimidazole (BIA)

10 K.F. Khaled / Materials Chemistry and Physics 112 (2008) Fig. 15. Structure of indole derivatives, molecular orbital plots and the charge density distribution. MS Modeling Software. As the three kinds of Fe surfaces (1 1 0, 1 0 0, 1 1 1), Fe (1 1 1) and Fe (1 0 0) surfaces have relatively open structures while Fe (1 1 0) is a density packed surface and has the most stabilization, so we choose Fe (1 1 0) surface to simulate the adsorption process [19]. The periodic boundary conditions (PBC) are applied to the simulation cell. The size of simulation box is 21.3 Å 21.3 Å 21.3 Å. The force field used in the current MM is COMPASS (condensed phase optimized molecular potentials for atomistic simulation studies) force field. All molecules are energy optimized, iron surface and solvent layers was constructed using the amorphous cell module, the whole system was energy optimized and the possibility of indoles adsorption on the iron surface were simulated as in Fig. 14. Fig. 14 shows the different steps of indole derivatives adsorption on iron surface with its different configurations. Through the simulation process, the configurations of the four indole derivatives changed greatly. The four indole derivatives move towards the iron surface which enhances the adsorption mechanism of their action as corrosion inhibitors. These molecules adsorbed on the iron surface through the heteroatoms (nitrogen and/or sulphur). It is found that through the simulation course, the benzene ring fluctuate up and down the iron surface while the rest of the molecule is attached to the iron surface. The mean adsorption energy obtained from the molecular simulation were calculated and tabulated in Table 5. As can be seen from Table 5, the adsorption energy for indole molecule was the highest and equals 28.6 kj mol 1. High values of adsorption energy indicates that the indole molecule will give the highest inhibition efficiency which is consistent with the inhibition efficiency obtained from the chemical and electrochemical techniques. The close contacts between indole (IND) and iron surface are shown in Fig. 14, indicate that the adsorption of indole occurred through the nitrogen atom and enhanced by the parallel benzene ring to the iron surface. The adsorption of these inhibitor molecules on the iron surface can be explained on the basis of the donor acceptor interaction between electrons of donor atoms N, S and aromatic rings of the inhibitors and the vacant d orbitals of iron surface atoms [20 22]. All investigated compounds are bicyclic containing a benzene ring fused to a five-membered heterocyclic ring without substituent (Fig. 1). So the influence of the chemical structure is limited to the molecular area in the adsorbed state, because it determines the area of the metal, shielded by the inhibitor. The approximate area, occupied by these molecules at planar adsorption has been calculated by the approximate method [23,24] and presented in Table 5. The approximate method is fast, and generally accurate within 10% for a given set of atomic radii. As in Table 5 the approximate area values are very close to each other and hence this parameter does not play a decisive role when comparing the inhibition properties of these compounds, the same results were given in calculating the molecular volume. This different behavior should be therefore prescribed

11 300 K.F. Khaled / Materials Chemistry and Physics 112 (2008) to deviations in the structural properties of the indole derivatives. The structural properties of indole derivatives can be obtained by means of quantum chemical calculations. The geometry of the indole derivatives, charge distribution densities as well as their molecular orbitals, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) are studied in an attempt to find the quantitative structure and activity relationship, QSAR and presented in Fig. 15 and Table 5. Fig. 15 shows the molecular orbital distribution of indole derivatives. Table 5 shows the calculated quantum chemical properties for indole derivatives, E HOMO (ev), dipole moment (D), approximate area, approximate volume and total energy. The adsorption energy increases with lower dipole moments. E HOMO is often associated with the electron donating ability of a molecule; whereas E LUMO indicates its ability to accept electrons. High values of E HOMO are likely to indicate a tendency of the molecule, to donate electrons to appropriate acceptor molecules with low energy and empty molecular orbital. Data obtained in Table 5 shows that some correlation was found between E HOMO and inhibition efficiency of indole derivatives. The less negative HOMO energy and the smaller energy gap (E LUMO E HOMO )are reflected in a stronger chemisorption and perhaps greater inhibitor efficiency [25]. Thus indole (IND) with its less negative E HOMO and lower dipole moment acts as the best corrosion inhibitor among the studied compounds. 4. Conclusions The following results can be drawn from this study: Electrochemical frequency modulation EFM can be used as a rapid and non-destructive technique for corrosion rate measurements without prior knowledge of Tafel slopes. Indole derivatives have shown good inhibitive effect for the corrosion of iron in 1 M HCl solutions. The results of EIS indicate that the value of CPE tends to decrease and both polarization resistance and inhibition efficiency tend to increase by increasing the inhibitor concentration. This result can be attributed to increase of the thickness of the electrical double layer. Tafel polarization studies have shown that the indole derivatives act as cathodic-type inhibitor. Data obtained from chemical and electrochemical measurements were in good agreement with the results obtained from EFM. Molecular modelling techniques can be used to simulate the adsorption from mol L 1 HCl solution of a single target molecule from indole derivatives on iron (1 0 0) surface. References [1] N. Hackerman, Langmuir 3 (1987) 922. [2] S.S. Abdel-Rehim, K.F. Khaled, N.S. Abdel-Shafi, Electrochim. Acta 51 (2006) [3] R.W. Bosch, J. Hubrecht, W.F. Bogaerts, B.C. Syrett, Corrosion 57 (2001) 60. [4] R.W. Bosch, W.F. Bogaerts, Corrosion 52 (1996) 204. [5] E. Kus, F. Mansfeld, Corros. Sci. 48 (2006) 965. [6] L. Han, S. Song, Corros. Sci. 50 (2008) [7] K.F. Khaled, Electrochim. Acta 48 (2003) [8] K.F. Khaled, N. Hackerman, Electrochim. Acta 48 (2003) [9] K.F. Khaled, N. Hackerman, Electrochim. Acta 49 (2003) 485. [10] K.F. Khaled, Appl. Surf. Sci. 230 (2004) 307. [11] K. Juttner, Electrochim. Acta 35 (1990) [12] K.F. Khaled, Electrochim. Acta 53 (2008) [13] M. El Achouri, S. Kertit, H.M. Gouttaya, B. Nciri, Y. Bensouda, L. Perez, M.R. Infante, K. Elkacemi, Prog. Org. Coat. 43 (2001) 267. [14] M. Lebrini, M. Lagrenee, H. Vezin, L. Gengembre, F. Bentiss, Corros. Sci. 47 (2005) 485. [15] M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, Corros. Sci. 44 (2002) 573. [16] P. Li, J.Y. Lin, K.L. Tan, J.Y. Lee, Electrochim. Acta 42 (1997) 605. [17] M.J. Sanghvi, S.K. Shukla, A.N. Misra, M.R. Padh, G.N. Meha, Bull. Electrochem. 13 (1999) 358. [18] M.A. Amin, S.S. Abd El-Rehim, E.E.F. El-Sherbini, R.S. Bayoumi, Electrochim. Acta 52 (2007) [19] S. Satoh, H. Fujimoto, H. Kobayashi, J. Phys. Chem. B 110 (2006) [20] N. Hackermann, E.S. Snavely, J.S. Payne, J. Electrochem. Soc. 113 (1967) 677. [21] T. Murakawa, S. Nagaura, N. Hackermann, Corros. Sci. 7 (1967) 79. [22] F. Bentiss, M. Traisnel, M. Lagrence, Appl. Surf. Sci. 161 (2000) 196. [23] W. Hasel, T.F. Hendrickson, W.C. Still, Tet. Comput. Methods 103 (1988) 103. [24] W.C. Still, A. Tempczyk, R.C. Hawley, T. Hendrickson, J. Am. Chem. Soc. 112 (1990) [25] K.B. Samardzija, K.F. Khaled, N. Hackerman, Appl. Surf. Sci. 240 (2005) 327.