J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. Corrosion inhibition of α-brass in HNO3 by indole and 2-oxyindole A.S. Fouda*, K. Shalabi, H. Elmogazy Chemistry Department, Faculty of Science, Mansoura University, Mansoura-35516, Egypt, email:asfouda@mans.edu.eg, Fax: +2 050 2246254 Tel: +2 050 2365730 Received 7 Dec 2013, Revised 16 July 2014, Accepted 17 July 2014 email: asfouda@hotmail.com, Fax: +20502246254, Tel: +20502365730 Abstract The influence of indole and 2-oxyindole on the corrosion rate of α-brass in 1M HNO3 was investigated using weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) techniques. It was found that the investigated compounds behave as inhibitors. The inhibition efficiency increases with increasing the inhibitor concentration, but decreases with increasing the temperature. The adsorption of these compounds on the α-brass surface follows Langmuir’s adsorption isotherm. The electrochemical results indicated that all the investigated compounds act as mixed-type inhibitors. Some thermodynamic parameters for corrosion processes were determined and discussed. The results obtained from chemical and electrochemical techniques were in good agreement. Keywords:Corrosioninhibition; α-brass; HNO3; Indole; oxyindole, EIS 1. Introduction Brass has been widely used as tubing material for condensers and heat exchangers in various cooling water systems [1-7]. Brass is susceptible to a corrosion process known as dezincification and this tendency increases withincreasing zinc content of the brass [8, 9]. During the past decade, many techniques have been usedto minimize the dezincification and corrosion of brasses.Particularly, heterocyclic organic compounds containing nitrogen, sulphur and/or oxygen atoms are often used to protect metals from corrosion, e.g. aminopyrazole [1012], amino-thiazole, triazole and thiols [13-14], phenylhydrazone derivatives [15], amino acids [16], cysteine and Glycine [17] found many applications in corrosion inhibition of copper alloys. A number of studies have recently appeared in the literature[17-22] on the topic of the corrosion inhibition of α-brass in acidic medium. But little work appears to have been done on the inhibition of α-brass alloy in HNO3using indole derivatives. The aim of this paper is to investigate the inhibition efficiency of indole and 2-oxyindole towards the corrosion of commercial α-brass in 1M HNO3solutions using weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. 2. Experimental 2.1 Materials The experiments were performed with local commercial α-brass (Helwan Company of Non-Ferrous Industries, Egypt) with the following composition (weight %) Cu 67.28, Pb 0.029, Fe 0.002, Zn 32.689 2.2 Solutions The aggressive solutions, 1M HNO3 were prepared by dilution of analytical grade (67.5%) HNO 3 with bidistilled water.All the investigated compounds were obtained from Aldrich chemical company. All chemicals and reagents were of analytical grade. The measurements were performed in 0.5 M HCl without and with the presence of the investigated compounds in the concentration range from1x10-6 to 1x10-4 M.The names and molecular structures of the investigated compounds are shown below. 2.3Weight lose measurements Seven parallel α-brass sheets of 2× 2 × 0.6 cm were abraded with emery paper (grade 320–1200) and then washed with bidistilled water and acetone. After accurate weighing, the specimens were immersed in a 100 ml beaker, which contained 100 ml of HNO3 with and without addition of different concentrations of the investigated compounds.All the aggressive acid solutions were open to air. After 3 h, the specimens were taken out, washed, dried, and weighed accurately. The average weight loss of the seven parallel α-brass sheets could be obtained. The inhibition efficiency (IE%) and the degree of surface coverage, θ, of investigated compounds for the corrosion of α-brass in HNO3were calculated from Eq. (1) [23]: 1691 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. IE % = θ x 100 = [1- (W / W°)) ] ×100 (1) where W° and W are the values of the average weight losses without and with addition of the inhibitor, respectively. inhibitors Structure Molecular weight Chemical formula Indole 117.15 C8H7N 2-Oxyindole 133.15 C8H7NO 2.4 Electrochemical measurements Electrochemical experiments were performed using a typical three-compartment glass cell consisted of the α-brass specimen as working electrode (1 cm2), saturated calomel electrode (SCE) as a reference electrode and a platinum foil (1 cm 2) as a counter electrode. The reference electrode was connected to a Luggin capillary and the tip of the Luggin capillary is made very close to the surface of the working electrode to minimize IR drop. All the measurements were done in solutions open to atmosphere under unstirred conditions. All potential values were reported versus SCE. Prior to every experiment, the electrode was abraded with successive different grades of emery paper, degreased with acetone and washed with bi-distilled water and finally dried. Tafel polarization curves were obtained by changing the electrode potential automatically from (-0.5 to 0.5 V vs. SCE) at open circuit potential with a scan rate of 1 mVs-1. Stern-Geary method [24] used for the determination of corrosion current is performed by extrapolation of anodic and cathodic Tafel lines to a point which gives log i corr and the corresponding corrosion potential (Ecorr) for inhibitor free acid and for each concentration of inhibitors. Then icorr was used for calculation of inhibition efficiency (IE %) and surface coverage (θ) from Eq. (2): IE % = θ x 100 = [1- (icorr(inh) / icorr(free)) ] ×100 (2) whereicorr(free) and icorr(inh) are the corrosion current densities in the absence and presence of inhibitor, respectively. Impedance measurements were carried out in frequency range from 100 kHz to 0.1Hz with amplitude of 5 mV peak-to-peak using ac signals at open circuit potential. The experimental impedance was analyzed and interpreted based on the equivalent circuit. The main parameters deduced from the analysis of Nyquist diagram are the resistance of charge transfer R ctand the capacitance of double layer Cdl. The inhibition efficiencies (IE %) and the surface coverage (θ) obtained from the impedance measurements are defined by Eq. (3): IE % = θ x 100 = [1- (R°ct /Rct)]x100 (3) whereRoct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively. Electrochemical frequency modulation, EFM, was carried out using two frequencies 2 and 5 Hz. The base frequency was 0.1 Hz, so the wave form repeats after 1 s. 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. The Intermodulation spectra contain current responses assigned for harmonical and intermodulation current peaks. The larger peaks were used to calculate the corrosion current density (i corr), the Tafel slopes (βc and βa) and the causality factors CF2& CF3 [25, 26]. The electrode potential was allowed to stabilize for 30 min before starting the measurements. All the experiments were conducted at 25 ± 1°C. Measurements were performed using Gamry Instrument Potentiostat/ Galvanostat/ ZRA (PCI4-G750). This includes a Gamry framework system v6.03 Gamry applications include DC105 software for DC corrosion measurements, EIS300 software for electrochemical impedance spectroscopy measurements and EFM 140 for electrochemical frequency modulationmeasurements along with a computer for collecting data. Echem analyst v6.03 software was used for plotting, graphing, and fitting data. 2.5 Quantum calculation All the quantum chemical study has been carried out using PM3 semi-empirical method, available by Spartan version 10.1. Molecular orbital calculation was based on semi-empirical method. This method has been used with full geometry optimization. 3. Results and Discussion 3.1. Chemical Method (Weight-loss measurements) The weight loss-time curves of α-brass with the addition of indole in 1M HNO3 at various concentrations is shown in Fig. 1. The curves of Fig. 1shows that the weight loss values of α-brass in 1M HNO3 solutions containing the indole decrease as the concentration of the inhibitor increases; i.e., the corrosion inhibition strengthens with the inhibitor concentration, this is appear in the Table 1. 1692 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. This trend may result from the fact that the adsorption of inhibitor on the α-brass increases with the inhibitor concentration, thus the α-brass surface is efficiently separated from the medium by the formation of a film on its surface [27]. weight loss, mg cm -2 30 20 blank -5 3x10 M -5 5x10 M -5 7x10 M -5 9x10 M -5 12x10 M -5 15x10 M 10 0 60 120 180 Time, min Figure 1.Weight loss- time curves for the corrosion of α-brass in 1M HNO3in the absenceand presence of different concentrations of indole at 25°C. Obtained values of IE% are given in Table (1), the order of decreasing inhibition efficiency of the investigated compounds is as follows:2-oxyindole> indole. Table 1. Variation of inhibition efficiency (IE %) of investigated compounds with their molar concentrations at 25oC from weight loss measurements at 120 min immersion of α-brass in 1 M HNO3 Conc., M 3x10-5 5 x10-5 7 x10-5 9 x10-5 12 x10-5 15 x10-5 Indole 2-Oxyindole θ IE% θ IE% 0.497 0.569 0.681 0.770 0.857 0.893 49.7 56.9 68.1 77.0 85.7 89.3 0.556 0.618 0.718 0.789 0.883 0.907 55.6 61.8 71.8 78.9 88.3 90.7 3.2 Adsorption Isotherm It is generally assumed that the adsorption of the inhibitors on the metal surface is the essential step in the inhibition mechanism [28]. To calculate the surface coverage θ it was assumed that the inhibitor efficiency is due mainly to the blocking effect of the adsorbed species and hence IE %= 100 x θ (29). In order to gain insight into the mode of adsorption of the extract on α-brass surface, the surface coverage values from weight loss measurements were theoretically fitted into different adsorption isotherms and the values of correlation coefficient (R2) were used to determine the best-fit isotherm. Fig. 2shows the plot C/θ vs. C which is typical of Langmuir adsorption isotherm. Perfectly linear plot was obtained with regression constant (R2) > 0.99 and slope about unity. The Langmuir isotherm is given as [30]: C/θ = 1/Kads + C (4) where C is the inhibitor concentration and Kads is the equilibrium constant of adsorption process and is related to the standard free energy of adsorption ΔG˚ads by Eq. (5): Kads= 1/55.5 exp (-ΔG°ads/RT) (5) The value of 55.5 is the concentration of water in solution expressed in mole per liter, R is the universal gas constant and T is the absolute temperature . 1693 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. The deviation of the slope from unity as observed from this study could be interpreted that there are interactions between adsorbate species on the metal surface as well as changes in adsorption heat with increasing surface coverage [31, 32], factors that were ignored in the derivation of Langmuir isotherm. The calculated ΔG˚ads values were also given in Table 2. The negative values of ΔG˚ads ensure the spontaneity of the adsorption process and the stability of the adsorbed layer on the α-brass surface. It is well known that values of ΔG˚ads of the order of -40 kJ mol-1 or higher involve charge sharing or transfer from the inhibitor molecules to metal surface to form coordinate type of bond (chemisorption); those of order of -20 kJ mol-1 or lower indicate a physisorption [33, 34]. The calculated ΔG˚ads values (Table 2) are more negative than 20 kJ mol-1 indicate, therefore, that the adsorption mechanism of the investigated compounds on α-brass in 1 M HNO3 solution is typical strong physisorption and the investigated compounds are approximately constant for αbrass. 0.00018 0.00016 2 indole R =0.9972 2 2-oxy indole R =0.9974 0.00014 C/ 0.00012 0.00010 0.00008 0.00006 0.00004 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016 C, M Figure 2.Curve fitting of corrosion data for α-brass in 1M HNO3in the presence ofdifferent concentrations of indole to Langmuir adsorption isotherm at 25°C Table 2. Equilibrium constant (Kads) and adsorption free energy (ΔG˚ads) of investigated compounds adsorbed on α-brass surface in 1M HNO3 at 25˚C Langmuir isotherm Indole 2-Oxyindole Kads, M-1 -∆G°ads, kJ Kads, M-1 - ∆G°ads, kJ mole-1 -1 mole 0.7059 37.6 0.9421 38.4 3.3 Effect of Temperature The effect of temperature on the rate of corrosion of α-brass in 1MHNO3containing different concentration of the investigated compounds was tested by weight loss measurements over a temperature range from 25 to 50°C. The results revealed that, the rate of corrosion increases as the temperature increases and decreases as the concentration of these compounds increases for all compound used. The activation energy (E a*) of the corrosion process was calculated using Arrhenius equation : k=A exp (-Ea*/ RT) (6) where k is the rate of corrosion, A is the Arrhenius constant, R is the gas constant and T is the absolute temperature. Figure 3 represents the Arrhenius plot in the presence and absence of 15x10-5M of investigated compounds. Ea* values determined from the slopes of these linear plots are shown in Table 3. The linear regression (R2) is close to 1 which indicates that the corrosion of α-brass in 1M HNO3 solution can be elucidated using the kinetic model. Table 3 showed that the value of Ea* for inhibited solution is higher than that for uninhibited solution, suggesting that dissolution of α-brass is slow in the presence of inhibitor and can be interpreted as due to physical adsorption [35]. It is known from Eq. 6 that the higher Ea* values lead to the lower corrosion rate. This is due to the formation of a film on the α-brass surface serving as an energy barrier for the α-brass corrosion [36]. 1694 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. 2 1 M HNO3 R =0.923 2 Indole R =0.925 2 2-Oxyindole R =0.921 -0.5 -2 log k, mg cm min -1 0.0 -1.0 -1.5 3.05 3.10 3.15 3.20 3.25 1000/T, K 3.30 3.35 3.40 -1 Figure 3.log k – 1/T curves for α-brass dissolution in 1 M HNO3in the absence and presence of 5x10-5 M of indole Enthalpy and entropy of activation (ΔH*, ΔS*) of the corrosion process were calculated from the transition state theory (Table 3): k = (RT/ Nh) exp (ΔS*/R) exp (-ΔH*/RT) (7) where h is Planck’s constant and N is Avogadro's number .A plot of log (k/ T) vs. 1/ T for α-brass in 1M HNO3at 15x10-5M of investigated compounds, gives straight lines as shown in Fig. 4. The positive signs of ΔH* reflect the endothermic nature of the steel dissolution process. Large and negative values of ΔS* imply that the activated complex in the rate-determining step represents an association rather than dissociation step, meaning that decrease in disordering takes place on going from reactants to the activated complex [37, 38]. 1 M HNO3 2 R =0.913 2 Indole R =0.919 2 2-Oxyindole R =0.913 -1 log( k/T) ,mgcm min K -1 -2.5 -2 -3.0 -3.5 -4.0 3.05 3.10 3.15 3.20 3.25 1000/T ,K 3.30 3.35 3.40 -1 Figure 4.log k/T – 1/T curves for α-brass dissolution in 1 M HNO3in the absence and presence of 5x10-5 M of indole Table 3. Activation parameters of the corrosion of α-brass in 1 M HNO3 in the absence and presence 15x10-5 M of inhibitor Inhibitors 1 M HNO3 Indole Oxyindole ∆H*, kJ mol-1 53.0 62.4 63.3 Ea*, kJ mol-1 55.5 64.8 66.2 1695 -∆S*, J K-1 mol-1 75.3 63.6 64.5 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. 3.4 PotentiodynamicPolarization Measurements Figure 5 shows the anodic and cathodic Tafel polarization curves for α-brass in 1M HNO3in the absence and presence of varying concentrations of indole at 25°C, respectively. From Fig. 5, it is clear that both anodic metal dissolution and cathodic H2 reduction reactions were inhibited when investigated inhibitors were added to 1 M HNO3 and this inhibition was more pronounced with increasing inhibitor concentration. Tafel lines are shifted to more negative and more positive potentials with respect to the blank curve by increasing the concentration of the investigated inhibitors. This behavior indicates that the undertaken additives act as mixed-type inhibitors [39,40]. 0.6 E, mv( vs. SCE) 0.4 Blank -5 3x10 M -5 5x10 M -5 7x10 M -5 9x10 M -5 12x10 M -5 15x10 M 0.2 0.0 -0.2 -0.4 -0.6 1E-6 1E-5 1E-4 1E-3 log i, A cm 0.01 0.1 -2 Figure 5.Potentiodynamic polarization curves for corrosion of α-brass in 1 M HNO3in the absence and presence of different concentrations of indole at 25 ºC The results in Table 4 show that the increase in inhibitor concentration leads to decrease the corrosion current density (icorr), but the Tafel slopes (βa ‚ βc)‚are approximately constant indicating that the retardation of the two reactions (cathodic hydrogen reduction and anodic metal dissolution) were affected without changing the dissolution mechanism [41-43].The order of decreasing inhibition efficiency of the investigated compounds is as follows: 2-oxyindole > indole 3.5 Electrochemical Impedance Spectroscopy (EIS) The effect of inhibitor concentration on the impedance behavior of α-brass in 1M HNO3solution at 25 ºC is presented in Fig. 6 (a,b). The curves show a similar type of Nyquist plots (Fig.6a) for α-brass in the presence of various concentrations of indole. The existence of single semi-circle showed the single charge transfer process during dissolution which is unaffected by the presence of inhibitor molecules. Deviations from perfect circular shape are often referred to the frequency dispersion of interfacial impedance, which arises due to surface roughness, impurities, dislocations, grain boundaries, adsorption of inhibitors, and formation of porous layers and in homogenates of the electrode surface [44, 45]. Inspections of the data reveal that each impedance diagram consists of a large capacitive loop with one capacitive time constant in the Bode–phase plots (Fig.6b). The electrical equivalent circuit model is shown in Fig. 7. It used to analyze the obtained impedance data. The model consists of the solution resistance (Rs), the charge-transfer resistance of the interfacial corrosion reaction (Rct) and the Constant phase element (CPE). Excellent fit with this model was obtained with our experimental data. The values of the interfacial capacitance Cdl can be calculated from CPE parameter values Y0 and n using the expression [46]: Cdl = Y0 (ωmax) n-1 (4) where Y0 is the magnitude of the CPE, ωmax is the angular frequency at which the imaginary component of the impedance reaches its maximum values and n is the deviation parameter of the CPE: -1 ≤ n ≤ 1. 1696 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. Inhibitor Table 4. Effect of concentration of the investigated compounds on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βa & βc), inhibition efficiency (IE%), degree of surface coverage (θ) and corrosion rate(C.R.) for the corrosion of α-brass in 1 M HNO3 at 25oC Conc., M 2-oxyindole Indole 1 M HNO3 icorr, µA cm-2 Ecorr, mV vs SCE βa, mVdec-1 βc, mVdec-1 C.R, mpy Ө IE% 883 54 20 31 435.8 -- -- -5 3x10 5x10-5 7x10-5 452 448 417 16 29 26 133 60. 99 166 174 164 223.1 221.0 205.9 0.488 0.493 0.528 48.8 49.3 52.8 9x10-5 192 25 42 159 94.5 0.783 78.3 12x10 -5 186 28 72 137 92.0 0.789 78.9 15x10 -5 155 3 66 177 76.5 0.825 82.5 3x10-5 358 19 55 88 176.8 0.595 59.5 -5 5x10 7x10-5 9x10-5 120 91.3 79 1 6 -53 52 42 61 58 71 139 59.3 45.0 38.9 0.864 0.897 0.911 86.4 89.7 91.1 12x10-5 15x10-5 64.4 25.8 17 -32 22 48 29 50 31.8 12.7 0.927 0.971 92.7 97.1 EIS data (Table 5) show that the Rct values increases and the Cdl values decreases with increasing the inhibitor concentrations. This is due to the gradual replacement of water molecules by the adsorption of the inhibitor molecules on the metal surface, decreasing the extent of dissolution reaction. The higher (Rct) values, are generally associated with slower corroding system [47, 48].The order of decreasing inhibition efficiency of the investigated compounds is as follows: 2-oxyindole > indole. a Blank -5 3x10 M -5 5x10 M -5 7x10 M -5 9x10 M -5 12x10 M -5 15x10 M Zimg, ohm cm 2 100 50 0 -50 0 50 100 150 200 Zreal, Ohm cm 250 300 350 400 2 Figure 6a.The Nyquist plots for corrosion of α-brass in 1 M HNO3in the absence and presence of different concentrations of indole at 25°C 3.6 Electrochemical frequency modulation measurements (EFM) The EFM 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 ac signal. It is generally accepted that in most cases, the corrosion rates determined with the EFM technique, are much higher than the values determined with other techniques exhibiting low corrosion rates [49].The modulation frequencies that are used in the EFM technique are in the capacitive region of the impedance spectra. However, results of the paper showed good agreement of corrosion rates obtained with the Tafel extrapolation method and are presented in Figs. 8a-8f are examples of α-brass immersed in 1 M HNO3 solutions in presence of different concentrations of 2-oxyindole.some devoid of investigated compounds and others containing different concentrations of investigated compounds at 25°C. Each spectrum is a current response as a function of frequency. 1697 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. Blank -5 3x10 M -5 5x10 M -5 7x10 M -5 9x10 M -5 12x10 M -5 15x10 M b 60  10 0 Zphz, logZmod ,ohm cm 2 100 1 -60 0.01 0.1 1 10 100 1000 10000 100000 1000000 log F,Hz Figure 6b. The Bode plots for corrosion of α-brass in 1 M HNO3 in the absence and presence of different concentrations of indole at 25°C Figure 7.Electrical equivalent circuit model used to fit the results of impedance Table 5. EIS data of α-brass in 1 M HNO3 and in the absence and presence of different concentrations of investigated inhibitors at 25 ºC 2-Oxyindole indole inhibitors Concentrations, M 1 M HNO3 3x10-5 5x10-5 7x10-5 9x10-5 12x10-5 15x10-5 3x10-5 5x10-5 7x10-5 9x10-5 12x10-5 15x10-5 Rct, Ω cm2 Cdl, μF cm-2 Ө IE% 42.08 129.90 168.00 194.30 207.70 282.90 389.40 152.60 276.30 351.00 412.70 549.30 854.20 754 307 255 200 179 139 111 312 308 254 119 60 24.1 ------0.676 0.750 0.783 0.797 0.851 0.892 0.724 0.848 0.880 0.898 0.923 0.951 -----67.6 75.0 78.3 79.7 85.1 89.2 72.4 84.8 88.0 89.8 92.3 95.1 The calculated corrosion kinetic parameters at different concentrations of the investigated compounds 1 M HNO3at 25 °C (icorr, βa, βc, CF-2, CF-3 and % IE) are given in Table 6. From Table 6, the corrosion current densities decrease by increasing the concentration of investigated compounds and the efficiency of inhibition increases by increasing investigated compounds concentrations. The causality factors in Table 6 are very close to theoretical values which according to EFM theory [26, 50] should guarantee the validity of Tafel slopes and corrosion current densities. Values of causality factors in Table 3 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. The deviation of causality factors from their ideal values might be due to the perturbation amplitude was too small or that the resolution of the frequency spectrum is not high enough. Another possible explanation is that the inhibitor is not performing very well. The obtained results showed good agreement of corrosion kinetic parameters obtained with the EFM, Tafel extrapolation and EIS methods. 1698 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. blank 1E-3 a current (A) 1E-4 1E-5 1E-6 1E-7 1E-8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 frequency (HZ) -5 -5 3x10 M 5x10 M 1E-3 1E-3 c b 1E-4 current (A) current (A) 1E-4 1E-5 1E-6 1E-7 1E-8 1E-5 1E-6 1E-7 1E-8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 frequency (HZ) -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -5 9x10 M 1E-3 1E-3 d e 1E-4 current (A) current (A) 1E-4 1E-5 1E-6 1E-7 1E-8 1E-5 1E-6 1E-7 1E-8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 frequency (HZ) -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 frequency (HZ) -5 1.6 -5 15x10 M 12x10 M 1E-3 1E-3 g 1E-4 f current (A) 1E-4 current (A) 1.6 frequency (HZ) -5 7x10 M 1E-5 1E-6 1E-7 1E-5 1E-6 1E-7 1E-8 1E-8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 frequency (HZ) frequency (HZ) Figures. 8a- 8g. Intermodulation spectrum for α-brass in 1 M HNO3 solutions without and with various concentrations (3x10-5-15x10-5M) of 2-oxyindole at 25°C. The calculated corrosion kinetic parameters at different concentrations of the investigated compounds 1 M HNO3at 25 °C (icorr, βa, βc, CF-2, CF-3 and % IE) are given in Table 6. From Table 6, the corrosion current densities decrease by increasing the concentration of investigated compounds and the efficiency of inhibition increases by increasing investigated compounds concentrations. The causality factors in Table 6 are very close to theoretical values which according to EFM theory [26, 50] should guarantee the validity of Tafel slopes and corrosion current densities. Values of causality factors in Table 3 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. The deviation of causality factors from their ideal values might be due to the perturbation amplitude was too small or that the resolution of the frequency spectrum is not high enough. Another possible explanation is that the inhibitor is not performing very well. The obtained results showed good agreement of corrosion kinetic parameters obtained with the EFM, Tafel extrapolation and EIS methods. 3.7 Quantum chemical parameters of investigated compounds The EHOMO indicates the ability of the molecule to donate electrons to an appropriated acceptor with empty molecular orbitals, whereas the ELUMO indicates its ability to accept electrons. The lower the value of E LUMO, the more ability of the molecule is to accept electrons [51]. The higher the value of EHOMO of the inhibitor, the easier is its ability to offer electrons to the unoccupied d-orbital of metal surface, and the greater is its inhibition efficiency. As is seen from Table 7, there are only small differences (less than 0.08 eV) between the values of EHOMO for the different molecules, which indicate that these molecules have very similar capacities of charge donation to the metallic surface. It was found that the variation of the calculated LUMO energies among all investigated inhibitors is rule less, and the inhibition efficiency is misrelated to the changes of the ELUMOTable 7. The HOMO–LUMO energy gap, ΔE approach, which is an important stability index, is applied to develop theoretical models for explaining the structure and conformation barriers in many molecular systems. The smaller the value of ΔE, the more is the probable inhibition efficiency the compound has [52-54]. The dipole moment μ, 1699 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. electric field, was used to discuss and rationalize the structure [55, 56]. It was shown from (Table 7) that 2oxyindole molecule has the smallest HOMO–LUMO gap compared with the other molecules. Accordingly, it could be expected that 2-oxyindole molecule has more inclination to adsorb on the metal surface than the other molecules. 2-oxyindole indole Table 6. Electrochemical kinetic parameters obtained by EFM technique for α-brass in the absence and presence of different concentrations of investigated compounds in 1 M HNO3 at 25°C̊ Conc., icorr, βa, βc, C R, Inh CF-2 CF-3 θ IE% -2 -1 -1 M µA cm mVdec mV dec mpy blank 846.4 55.7 111.6 417.7 1.90 3.10 -5 3x10 384.7 55.2 122.4 189.8 1.86 3.34 0.546 54.6 5x10-5 258.7 34.5 37.9 127.7 1.85 2.88 0.694 69.4 7x10-5 217.6 40.8 53.1 107.4 2.01 3.20 0.743 74.3 -5 9x10 177.0 24.2 24.8 87.4 1.37 3.19 0.791 79.1 12x10-5 142.1 47.1 100.8 70.1 1.88 3.37 0.832 83.2 15x10-5 101.9 44.8 141.5 50.3 1.88 2.60 0.880 88.0 -5 3x10 280.5 57.3 115.8 138.4 1.88 3.42 0.669 66.9 5x10-5 228.9 18.9 31.2 112.9 2.25 2.82 0.730 73.0 7x10-5 199.5 57.2 117.6 98.5 1.95 2.73 0.764 76.4 9x10-5 57.2 58.5 94.8 28.2 2.10 2.76 0.932 93.2 -5 12x10 43.0 54.1 129.9 21.2 1.94 2.93 0.949 94.9 15x10-5 23.0 67.9 118.2 11.3 1.98 2.95 0.973 97.3 PM3 indole oxyindole2 HOMO LUMO Mulliken atomic charges Figure 9.The optimized molecular structures, HOMO, LUMO and Mulliken atomic charges of the inhibitor molecules using PM3 1700 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. The use of Mulliken population analysis to estimate the adsorption centers of inhibitors has been widely reported and it is mostly used for the calculation of the charge distribution over the whole skeleton of the molecule [57]. There is a general consensus by several authors that the more negatively charged hetero atom is, the more is its ability to adsorb on the metal surface through a donor–acceptor type reaction [58-60]. Variation in the inhibition efficiency of the inhibitors depends on the presence of electronegative O- and N- atoms as substituent in their molecular structure. The calculated Mulliken charges of selected atoms are presented in Fig. 9. Table7. The calculated quantum chemical parameters for investigated compounds by using PM3 E HOMO ( eV) ELUMO (eV) ΔE (eV) Area (Å2) indole -13.97 -5.33 8.640 151.17 2-oxyindole -14.05 -7.30 6.750 153.97 3.6 Mechanism of Corrosion Inhibition From the observations drawn from the different methods, corrosion inhibition of α-brass in 1M HNO3solutions by the investigated inhibitors as indicated from weight loss, potentiodynamic polarization and EIS techniques were found to depend on the concentration and the nature of the inhibitor. The order of inhibition efficiency is as follows: 2-oxyindole > indole These compounds can beadsorbed in a flat orientation through the nitrogen of the pyrrole ring and oxygen of carbonyl group in case of 2-oxyindole. It wasconcluded that the mode of adsorption depends on the affinityof the metal toward the π-electron clouds of the ring system [61]. Metals, such as Cu and Fe, which have a greateraffinity toward aromatic moieties, were found to adsorb benzene rings in a flat orientation. Thus, it is reasonable to assume that the tested inhibitors are adsorbed in a flat orientation through the N- and O-atoms and π-electrons of the benzene ring. 2-oxyindole has the highest percentage inhibition efficiency. This being due to the presence of an extra carbonylgroup and higher molecular size while indole comes after 2-oxyindole in inhibition efficiency due to the absence of carbonyl group and lower molecular size. Conclusions 1. Indole and 2-oxyindole have proved to be corrosion inhibitors for corrosion of α-brass in 1M HNO3 solution 2. The inhibition efficiency increases with increase in the concentration of these inhibitors but decreases with an increase in temperature 3. The inhibition of α-brass alloy in 1 M HNO3 solution was found to obey Langmuir adsorption isotherm. 4. The thermodynamic values obtained from this study indicate that the presence of the inhibitors increases the activation energy and the negative values of ∆G°ads indicate spontaneous adsorption of the inhibitors on the surface of α-brass 5. The % IE obtained from electrochemical impedance spectroscopy and potentiodynamic polarization measurements show good agreement with those obtained from weight loss experiments. References 1. Li S.L., Ma H.Y., Lei S.B.,Yu R., Chen S.H., Liu D.X., Corrosion 54 (1998) 947. 2. North, R.F., Pryar M.J., Corros. Sci. 10 (1970) 297. 3. Elmorsi, M.A., EL-Sheikh, M.Y., Bastweesy A.M., Ghoneim M.M., Bull. Electrochem. 7 (1991) 158. 4. Osman, M.M., Mater. Chem. Phys. 71 (2001) 12. 5. Quraishi M.A., Farooqi I.H., Saini P.A., Br. Corros. J., 35 (2000) 78. 6. Shih H.C., Tzou R.J., J. Electrochem. Soc., 138 (1991) 958. 7. Quartarone G., Moretti G., Bellami T., Corrosion, 54 (1998) 606. 8. Gad-Allah A.G., Abou-Romia M.M., Badawy M.W., Rehan H.H., J. Appl. Electrochem., 21 (1991) 829. 9. Mitra A.K., R&D Journal, NTPC 2 (1996) 52. 10. Elmorsi M.A., Hassanein A.M., Corros. Sci., 41 (1999) 2337. 11. Ammeloot F., Fiaud C., Sutter E.M.M., Electrochim. Acta, 43 (1997) 3565. 12. Gad Allah A.G., Badawy M.W., Rehan H.H., Abou-Romia M.M., J. Appl. Electrochem., 19 (1989) 982. 13. Trabanelli G., Brunoro G., Monticelliand C., Foganolo M., Corros. Sci., 25 (1985) 1019. 14. Sylvia da Costa L.F.A., Agostinho M.L., Nobe, K., J. Electrochem. Soc., 140 (1993) 3483. 15. Abdallah M., El-Agez M., Fouda A.S., Int.J.Electrochem.Sci., 4(2009)336. 1701 J. Mater. Environ. Sci. 5 (6) (2014) 1691-1702 ISSN : 2028-2508 CODEN: JMESCN Fouda et al. 16. Fouda A.S., Nazeer A.A., Ashour, E.A., Zastita Materijala, 52 (2011) 21. 17. Nazeer A.A., Fouda A.S., Ashour E.A., J. Mater. Environ. Sci., 2 (2011)24. 18. Mihit M., Belkhaouda M., Bazzi L., Salghi R., El Issami S., Ait Addi E., Port. Electrochim. Acta, 25 (2007) 471. 19. Mihit M., Salghi R.,El Issami, S., Bazzi L., Hammouti B., Ait Addi El., Kertit, S., Pigment & Resin Technology, 35 (2006)151. 20. Rehan H. H., Al-Moubarak N. A., Al-Rafai H. A., Port. Electrochim. Acta, 21 (2003) 99. 21. Fouda A.S., Mahfouz H., J. Chil. Chem. Soc., 54 (2009)302. 22. Fouda A.S., El-Attar K.M., J. Mater. Eng. Perf., 21 (2012) 2352. 23. Mu G.N., Zhao T.P., Liu M., Gu T., Corrosion, 52 (1996) 853. 24. Stern M., Geary A. L., J. Electrochem. Soc., 104 (1957) 56. 25. Abdel-Rehim S.S., Khaled K.F., Abd-Elshafi N.S., Electrochim. Acta, 51 (2006) 3269. 26. Bosch R.W., Hubrecht J., Bogaerts W.F., Syrett B.C., Corrosion, 57 (2001) 60. 27. Aljourani J., Raeissi K., Golozar M.A., Corros. Sci., 51 (2009) 1836. 28. Dinnappa R. K., Mayanna S. M., J. Appl. Elcreochem., 11 (1982) 111. 29. Patel N., Rawat A., Jauhari S., Mehta G., European J. Chem., 1 (2010) 129. 30. Langmuir I., J. Am. Chem. Soc., 39 (1947) 1848. 31. Bhat J. I., Alva V. D. P., J. Korean. Chem. Soc., 55 (2011) 835. 32. Oguzie E. E., Okolue B. N., Ebenso E. E., Onuoha G. M., Onuchukwu A. I., Mater. Chem. Phys., 87 (2004) 394. 33. Aramaki K., Hackerman N., J. Electrochem. Soc., 116 (1969) 568. 34. Tang L., Li X., Li, L., Mu G., Liu G., Mater. Chem. Phys., 97 (2006) 301. 35. Lipkowski J., Ross P.N., Adsorption of Molecules at Metal Electrodes, VCH, New York, (1992) 36. Da Costa S. L. F. A., Agostinho S. M. L., Corros. Sci., 45 (1989) 472. 37. El-Sherbiny E.F., Mater. Chem. Phys., 60 (1999) 286. 38. Fouda A.S., Al-Sarawy A.A., El-Katori E.E., Desalination, 201 (2006) 1. 39. Aljourani J., Raeissi K., Golozar M.A., Corros. Sci., 51 (2009)1836. 40. Amar H., Tounsi A., Makayssi A., Derja A., Benzakour J., Outzourhit, A., Corros. Sci., 49 (2007) 2936. 41. Migahed M.A., Azzam E.M.S., Morsy S.M.I., Corros. Sci., 51 (2009) 1636. 42. Moussa M.N.H., El-Far A.A., El-Shafei A.A., Mater. Chem. Phys., 105 (2007) 105. 43. Benabdellah M.,Touzan R., Aouniti A., Dafali A.S., El-Kadiri S., Hammouti B., Benkaddour M., Mater. Chem. Phys., 105 (2007) 373. 44. Bayol E., Kayakirilmaz K., Erbil M., Mater. Chem.Phys., 104 (2007) 74. 45. Benalli O., Larabi L., Traisnel M., Gengembra L., Harek Y., Appl. Surf. Sci., 253 (2007) 6130. 46. Hsu C.S., Mansbitfeld F., Corrosion, 57 (2001) 747. 47. Epelboin I., Keddam M.,Takenouti H., J. Appl. Electrochem., 2 (1972) 71. 48. Mayer B.J.C., Tuttner K., lorenz W. J., Electrochim. Acta, 28 (1983) 171. 49. Kus E., Mansfeld F., Corros. Sci., 48 (2006) 965. 50. Gamry Echem Analyst Manual, 2003. 51. Gao G., Liang C., Electrochim. Acta, 52 (2007) 4554. 52. Feng Y., Chen S., Guo Q., Zhang Y., Liu G., J. Electroanal. Chem., 602 (2007) 115. 53. Gece G., Bilgic S., Corros. Sci., 51 (2009) 1876. 54. Martınez S., Mater. Chem. Phys., 77 (2002) 97. 55. Ozcan M., Dehri I., Erbil M., Appl. Surf. Sci., 236 (2004) 155. 56. Roque J.M., Pandiyan T., Cruz J., Garcia-Ochoa E., Corros. Sci., 50 (2008) 614. 57. Kandemirli, F., Sagdinc, S., Corros. Sci. 49 (2007) 2118. 58. Bereket G., Ogretic C., Ozsahim C., J. Mol. Struct., (THEOCHEM) 663 (2003) 39. 59. Li W., He Q., Pei C., Hou B., Electrochim. Acta, 52 (2007) 6386. 60. Rajenran S., J. Electrochem. Soc., 54 (2005) 61. 61. Chauhan L.R., Gunasekaran G., Corros. Sci., 49 (2007) 1143. (2014) ; http://www.jmaterenvironsci.com 1702