Understanding Corrosion Inhibition of Mild Steel in Acid Medium by Some Furan Derivatives: A Comprehensive Overview

Transcription

1 C116 Understanding Corrosion Inhibition of Mild Steel in Acid Medium by Some Furan Derivatives: A Comprehensive Overview K. F. Khaled z /21/157 3 /C116/9/$28. The Electrochemical Society Electrochemistry Research Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo 11566, Egypt and Materials and Corrosion Laboratory, Chemistry Department, Faculty of Science, Taif University, Taif, Al-Haweiah, 888, Kingdom of Saudi Arabia The inhibition performance of some furan derivatives, namely, methyl 2-furoate, ethyl 2-furoate, and amyl 2-furoate on mild steel in normal hydrochloric acid medium 1 M HCl at 25 C, was studied by weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy EIS, and electrochemical frequency modulation measurements EFM. Furan derivatives inhibit the acidic corrosion even at very low concentrations, reaching a value of inhibition efficiency up to 93% at a concentration of 2 mm. The results obtained from the different corrosion evaluation techniques are in good agreement. Polarization curves indicate that the studied furan derivatives are mixed-type inhibitors. Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through an appropriate equivalent circuit model. A constant phase element has been used. The adsorption of furan derivatives on the steel surface in 1 M HCl solution was found to obey Langmuir s adsorption isotherm with a negative value of the free energy of adsorption G ads. Molecular dynamics simulations were carried out to establish the inhibition mechanism. 21 The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted July 24, 29; revised manuscript received October 26, 29. Published January 12, 21. Corrosion inhibition of mild steel is a matter of theoretical as well as practical importance. 1 Acids are widely used in industries such as pickling, cleaning, descaling, etc. Because of their aggressiveness, inhibitors are used to reduce the rate of dissolution of metals. Compounds containing nitrogen, sulfur, and oxygen have been reported as inhibitors. 2-4 The most efficient organic inhibitors are organic compounds having -bonds in their structures. The efficiency of an organic compound to act as an inhibitor is mainly dependent on its ability to get adsorbed on the metal surface which requires replacement of water molecule at a corroding interface as Org sol + nh 2 O ads Org ads + nh 2 O sol z khaledrice23@yahoo.com 1 An experimental inquiry into the kinetics of mild steel dissolution and corrosion inhibition has been carried out with a view to clarify certain debatable aspects about the mechanism of corrosion inhibition. Previous work in our laboratory has led us to a chemisorption step as a possible mechanism of inhibition which accounts for the persistence, specificity, and structural effects of the inhibitors. 5,6 It has been shown that systematic and orderly changes in the molecular structure of the organic compounds can be correlated to inhibition efficiency IE. 7-9 This correlation is based on certain physicochemical properties of the inhibitor molecule such as functional groups, steric factors, aromaticity, electron density at the donor atoms, and orbital character of the donating electrons Because adsorption is the key to the explanation of inhibition mechanism by organic molecules, the primary phenomenon of adsorption is as important as the subsequent phenomenon of inhibition. The relationship between adsorption and inhibition has been inadequately explained in the literature One reason for this is the meagerness of available experimental techniques for in situ measurement of adsorption at electrodes. Evaluation of adsorption from corrosion current measurements is indirect and with so many assumptions as to make the conclusions questionable. Donahue et al. proposed adsorption involving surface complex formation between the inhibitor and one of the intermediates in the anodic dissolution of mild steel. 16 Although heterocyclic compounds have been evaluated and used extensively as corrosion inhibitors of mild steel, furan and its derivatives have not been given sufficient attention for such applications. Vaidyanathan and Hackerman 14 tested such furan derivatives as furfurylamine, furoic acid tetrahydrofurfurylamine, and tetrahydrofurfuryl alcohol for their inhibitive action on anodic dissolution, cathodic partial processes, and corrosion currents. They found that inhibitive efficiency of these compounds could be explained on the basis of structure-dependent electron donor properties. Mitra 17 tested furan, furfuryl amine, and furfuryl alcohol and correlated their IEs with some quantum chemical values, namely, the energy of the highest occupied molecular orbital and the lowest free molecular orbital. This study aims to use some furan derivatives as possible corrosion inhibitors for mild steel in 1 M HCl solutions. Measurements were conducted using several corrosion monitoring techniques, such as weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy EIS. Electrochemical frequency modulation EFM, which is a recent nondestructive corrosion measurement technique that can directly give values of corrosion current without prior knowledge of Tafel constants, is also presented here with the aim to make an accurate determination of the corrosion rate. It is also the purpose of this paper to elucidate the adsorption behavior of some furan derivatives at the mild steel surface using molecular dynamics simulations. The present work also involves an extensive investigation of many furan derivatives, which have been chosen to evaluate the effects of systematic changes in the molecular structure and the polar function of the compound. This is achieved by changing the substituent with respect to the oxygen polar function. Also, an attempt is made to discuss the inhibitive action of the tested compounds according to the interface and interphase modes of inhibition. Experimental The structures of the furan compounds studied are presented below All the investigated compounds were obtained from Aldrich Chemical Co.; they were put in 1. M HCl Fisher Scientific without pretreatment at concentrations of 5, 1, 15, and 2 mm. All electrochemical measurements were performed in a typical three-compartment glass cell that consisted of the mild steel rod C =.14 wt %, S =.5 wt %, Si =.1 wt %, Mn =.9 wt %,

2 C117 and Fe = balance, its surface area was.28 cm 2 as a working electrode WE prepared using emery papers of different grit sizes up to 4/ grit, polished with Al 2 O 3.5 m particle size, the platinum mesh as a counter electrode, and a saturated calomel electrode SCE as the reference electrode. Solutions were prepared from bidistilled water. The electrode potential was allowed to stabilize for 6 min before starting the measurements. All experiments were conducted at 25 C. The electrolyte solution was made from analytical reagent grade HCl. The electrodes were arranged in such a way that a onedimensional potential field existed over the WE surface in solution. To get an impression about the process that occurred at the iron/acid interface, Tafel curves were obtained by changing the electrode potential automatically from 25 to + 25 mv SCE vs open-circuit potential with a scan rate of 1 mv/s. Also, EIS measurements were carried out in a frequency range of 1 4 mhz with an amplitude of 1 mv peak to peak using ac signals at open-circuit potential. EFM was carried out using frequencies of 2 and 5 Hz. The base frequency was 1 Hz, so the waveform 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, 1 Hz is a reasonable limit. In this study, we use a perturbation signal with an amplitude of 1 mv for both perturbation frequencies of 2 and 5 Hz. The choice for the frequencies of 2 and 5 Hz was based on three arguments. 18 First, the harmonics and intermodulation frequencies should not influence each other. Second, the frequency should be as low as possible to avoid influence of the capacitive behavior of the electrochemical double layer. Third, the frequency should be as large as possible to reduce time needed to perform a measurement. While these arguments do not allow the selection of perfect frequencies, the chosen frequencies were considered as reasonable compromise. Measurements were performed with a Gamry potentiostat/ galvanostat/zra. This includes a Gamry framework system based on the ESA4, Gamry applications that include DC15 for dc corrosion measurements, EIS3 for EIS measurements, and EFM14 to calculate the corrosion current and the Tafel constants along with a computer for collecting the data. Echem Analyst 4. software was used for plotting, graphing, and fitting data. Gravimetric experiments were carried out in a double glass cell equipped with a thermostated cooling condenser. The solution volume was 1 ml. The iron specimens used have a rectangular form length = 2 cm, width = 1 cm, and thickness =.6 cm with an exposed surface area of 4.36 cm 2. The duration of tests was 6 h at 25 1 C. Duplicate experiments were performed in each case, and the two values obtained are very similar so the mean value of the weight loss is reported. The weight loss allowed the calculation of the mean corrosion rate in g cm 2 h 1. Computational Details The Discover molecular dynamics module in Materials Studio 4.3 software from Accelrys Inc. 19 allows selecting a thermodynamic ensemble and the associated parameters, defining simulation time and temperature and pressuring and initiating a dynamics calculation. The molecular dynamics simulation procedures have been described in detail elsewhere. 2,21 Results and Discussion EIS measurements. Effect of exposure time in 1 M HCl solution. The effect of the exposure time of a mild steel WE in 1 M HCl on the EIS spectra was studied. The results obtained provided the determination of the WE exposure time. EIS was recorded after exposures of 3, 6, 12, and 24 min. The results are presented in Fig. 1. It is seen that the diameter of the impedance semicircle depends on the duration of the WE exposure in the acid solution. Figure 2 illustrates the variation in the polarization resistance with the exposure time t. It passes through a maximum at about -Z"/ Ω cm min 6 min 12 min 24 min Z'/ Ω cm 2 Figure 1. Impedance spectra recorded at different times of immersion of the mild steel WE in 1 M HCl. 6 min exposure. is determined from the real part of the complex plane impedance diagram Nyquist plot because it is defined by = lim Z re 2 where Z re is the real part of the Nyquist plots, is the frequency in radians, and Z re is the real part of faradaic impedance. 22 Therefore, at tending to, the imaginary part of the impedance Z img, while the real one Z re + R s, where R s is the ohmic resistance of the solution and the electrical leads. The fitted results derived using the corresponding equivalent circuit, which is described later, are presented in Table I. Inspection of Fig. 1 and 2 shows that the initial dissolution of the WE depends to a great extent on the electrode s preliminary treatment. At the beginning of the experiment, the dissolution process is the greatest on the sharp edges of the WE, then the corrosion rate decreases with time due to the formation of the corrosion products and the depletion of the sharp edges that were found in the beginning. When all of these processes have taken place, the dissolution rate is predominantly determined by the metal s nature and the experimental conditions. 23 Then the preliminary treatment s effect should not essentially affect the process any longer. The polarization resistance decreases as the time of immersion increases after 1 h Fig. 2. The corrosion products, which result from the dissolution of / Ω cm Time / min. 3 min. 6 min. 12 min. 24 min. Figure 2. Dependence of polarization resistance on the immersion time in 1 M HCl.

3 C118 Table I. Fitted parameters for iron in 1 M HCl after different immersion times at 25 C derived using equivalent circuit in Fig. 5a. Immersion time min R s CPE/Y F cm n -Z"/ Ω cm mm ETF 1 mm ETF 15 mm ETF 2 mm ETF -Z"/ Ω cm mMMEF 1 mm MEF 15 mm MEF 2 mm MEF Z'/ Ω cm 2 Figure 3. Measured and simulated complex plane impedance plots of mild steel corrosion in 1 M HCl solutions at E corr in the absence and presence of MEF at 25 C Z'/ Ω cm 2 Figure 4. Measured and simulated complex plane impedance plots of mild steel corrosion in 1 M HCl solutions at E corr in the absence and presence of ETF at 25 C. mild steel in HCl, start to form on the electrode surface as soon as the electrode immersed in the corrosive media. Their amount is insignificant, and besides, they have no protective properties. However, they could affect to a certain extent the impedance investigations at longer exposure time of the WE. Thus it was found experimentally that 1 h exposure under the conditions stated above was the compromise duration in view of minimization of the effect of the electrode s preliminary treatment and of the corrosion products. Effect of concentrations of furan derivatives. The effect of the concentration of the studied furan derivatives was studied from 5 to 2 mm in 1 M HCl solutions at 25 C 1. Organic compounds are known to yield unreliable and irreproducible results for concentrations higher than 2 mm. 24,25 For the same reasons, the present work tests the compounds only up to a concentration of 2 mm, employing concentration intervals of 5 mm. Several factors determine the appropriate inhibitor concentration in corrosion inhibition studies. Solubility of the inhibitor in the corrosive medium is one of the key factors in determining the optimum concentrations. No significant increase in the IE is obtained for the studied inhibitors above 2 mm. In the presence of methyl 2-furoate MEF and ethyl 2-furoate ETF in the whole concentration range, the electrochemical impedance spectra in Nyquist plots are characterized by one semicircle, which its center lies under the real axis Fig. 3 and 4. The quantitative analysis of the electrochemical impedance spectra was studied based on a physical model of the corrosion process with hydrogen depolarization and with charge-transfer controlling step. The simplest model includes the polarization resistance in parallel to the constant phase element CPE connected with the solution resistance R s. The solid electrode is inhomogeneous both on a microscopic and a macroscopic scale, and corrosion is a uniform process with fluctuating active and inactive domains where anodic and cathodic reactions take place at the corroding surface. The size and distribution of these domains depend on the degree of surface inhomogeneities. Inhomogeneities may also arise from adsorption phenomena and formation of porous and nonporous layers by passivation on coating For this reason the frequency distributed CPE was used instead of the capacitance of the double layer C dl at the mild steel/ solution interface. More generally, the CPE behavior could be treated as a space fractality, i.e., as manifestation of a selfsimilarity in the frequency domain. 29 The CPE impedance is given by 26,27,3,31 1 Z CPE = i n 3 A where A is a proportionality coefficient and n has the meaning of the phase shift, which value can be considered as a measure of the surface inhomogeneity. 26,3,32,33 The transfer function is thus represented by an equivalent circuit, having only one time constant Fig. 5a. Parallel to the double layer capacitance modeled by a CPE is the polarization resistance, R s being the electrolyte resistance. The fitted parameter values are presented in Tables II and III, while the calculated curves, using this parameter set, are presented in Fig. 3 and 4 as solid lines. Figure 5. Color online Equivalent circuits used to model impedance data in 1 M HCl solutions.

4 C119 Table II. Fitted parameters for iron in 1 M HCl in the absence and presence of various concentrations of MEF at 25 C derived using equivalent circuit in Fig. 5a. Conc R s CPE/Y F cm 2 n IE i MEF 5mM mm mm mm In amyl 2-furoate AMF, an additional time constant becomes visible, very well expressed in the Nyquist presentation Fig. 6. In this case, it is possible to separate the processes of the double layer charging and of the adsorption pseudocapacitance. It is clear that for AMF, the structural model of Fig. 5a is inadequate and a two-timeconstant model Fig. 5b is used to describe the electrochemical impedance spectra. The fitted parameters are presented in Table IV and the calculated curves are presented in Fig. 6 as solid lines. This model is valid for all concentrations of AMF. It can be seen that the values of are highly increased when the description is changed from the one-constant model to the two-constant one. The values of polarization resistance in AMF are composed of two components: the charge-transfer resistance and the inhibitor film resistance R f. Tafel polarization measurements. Polarization measurements have been carried out to gain knowledge concerning the kinetics of the anodic and cathodic reactions. Polarization curves of the mild steel in 1 M HCl solutions without and with addition of different concentrations of furan derivatives are shown in Figs The anodic and cathodic current-potential curves are extrapolated up to their intersection at a point where corrosion current density j corr and corrosion potential E corr are obtained. 34 The values of associated electrochemical parameters, i.e., corrosion potential E corr, corrosion current density j corr, cathodic Tafel slopes c, anodic Tafel solpes a, and percentage IE IE p values, were calculated from polarization curves and listed in Table V. The IE IE p was calculated from corrosion current density measurements according to the relation given below IE p = j corr j corr j corr 1 4 where j corr and j corr are uninhibited and inhibited corrosion current densities, respectively. They are determined by extrapolation of Tafel lines to the respective corrosion potentials. It is shown from Figs. 7-9 that increasing the furan derivative concentrations reduces both the cathodic and the anodic currents, and there is no definite trend in the shift of E corr values. These results indicate that all furan derivatives act as mixed-type inhibitors. 35 Based on the marked decrease in the cathodic and anodic current densities upon introducing the inhibitor in the aggressive solution, furan derivatives can be considered as a mixed-type inhibitor, meaning that the addition of furan derivatives reduces the anodic dissolution and also retards the cathodic hydrogen evolution reaction. From Tafel curves in Fig. 7-9, it is clear that both anodic metal dissolution and cathodic reduction reactions were inhibited when the furan derivatives were added to the acid solution and this inhibition was more pronounced with increasing inhibitor concentration. The cathodic polarization curves give rise to parallel Tafel lines with the nearly constant cathodic Tafel slopes c, indicating that the addition of inhibitor to the aggressive solution does not modify the proton reduction mechanism and this reaction is activation controlled. 36 The inhibitor is first adsorbed onto iron surface and then impedes by merely blocking the active sites of iron surface. In this way, the surface area available for H + ions is decreased, while the actual reaction mechanism remains unaffected. 36 EFM. Figure 1 shows a plot of current density j frequency f that was obtained by fast Fourier transform from the experimental current j time t data determined with EFM on iron after 1 h immersion in 1 M HCl solutions. The data in Fig. 1 were analyzed assuming that the cathodic reaction was under activation control. The EFM data were calculated for the studied system and presented in Table VI. The electrochemical parameters presented in Table VI include corrosion current density j corr, Tafel slopes a, c, and causality factors CF-2 and CF-3, which act as an internal check for the validity of EFM data. Inspections of these data conclude that the values of causality factors obtained under different experimental conditions are approximately equal to the theoretical values of 2 and 3, indicating that the measured data are of high quality As can be seen from Table VI, the corrosion current densities decrease by increasing the concentrations of furan derivatives. The IEs E EFM calculated from Eq. 5 increase by increasing furan derivative concentrations IE EFM = j corr j corr j corr 1 5 where j corr and j corr are the corrosion current densities in the absence and presence of furan derivatives, respectively. As can be seen from Table VI, as the inhibitor concentration increases, the inhibition increases up to 86% with the addition of 2 mm from AFM inhibitor. Table III. Fitted parameters for iron in 1. M HCl in the absence and presence of various concentrations of ETF at 25 C derived using equivalent circuit in Fig. 5a. Conc R s CPE/Y F cm 2 n IE i ETF 5mM mm mm mm

5 C Z"/ Ω cm mMAMF 1 mm AMF 15 mm AMF 2 mm AMF E/V SCE mMMEF 1 mm MEF 15 mm MEF 2 mm MEF Z'/ Ω cm 2 Figure 6. Measured and simulated complex plane impedance plots of mild steel corrosion in 1 M HCl solutions at E corr in the absence and presence of AMF at 25 C. 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 j/ µa cm -2 Figure 7. Anodic and cathodic Tafel polarization curves for mild steel in the absence and presence of various concentrations of MEF in 1 M HCl at 25 C. Weight loss measurements. Table VII shows the values of IE and corrosion rate W corr obtained from weight loss measurements at different concentrations of furan derivatives at 25 C. From the weight loss values, the IE w of each concentration were calculated using the following equations IE w = w w w where w and w are the corrosion rates in the absence and presence of inhibitors, respectively. It has been found that IE of all of these compounds increases by increasing their concentrations. The maximum IE for each compound was achieved at 2 mm and a further increase in concentration did not cause any appreciable change in the performance of the inhibitors data not included. In Fig. 11 comparison figures, the corrosion rates obtained with four different techniques including potentiodynamic polarization curve, EIS, EFM, and weight loss are compared. In general, the corrosion rates determined with the EFM technique are much higher than the values determined with other techniques, exhibiting low corrosion rates. 42 The results from EFM, EIS, and polarization curve are integral averages of the data in the experimental period of 1 h. As electrochemical techniques, the EFM technique gives a higher corrosion rate than analysis of the potentiodynamic polarization curve, EIS, and weight loss measurements because the entire frequency range of the EFM measurement cannot be low enough to be in the dc limit region of the impedance spectrum. The corrosion rates obtained with the electrochemical techniques are both lower than that with weight loss method. The results obtained with the four techniques are at the same order of magnitude and show the same trend, which is acceptable. The corrosion rates determined with the EFM technique for the mild steel/hcl system are reasonable and significant. Meanwhile, the EFM technique proves that it can be implemented to measure corrosion rates in a simple, inexpensive way. Adsorption isotherm. The corrosion inhibition process is based on the adsorption of the furan derivative molecules on the metal surface. It is essential to know the mode of adsorption and the adsorption isotherm that fits the experimental results. The most frequently used adsorption isotherms are Langmuir, Temkin, and Frumkin with the general formula f,x exp 2a = KC 7 where is the surface coverage, x is the number of water molecules replaced by one inhibitor molecule, a is the lateral interaction between adsorbed molecules, K is the adsorption equilibrium constant, and C is the inhibitor concentration. Plots of the data for each isotherm showed that all of the investigated compounds agreed with the Langmuir isotherm Fig. 12 that is given by 43 C = 1 K + C 8 To calculate the surface coverage, it was assumed that the IE is due mainly to the blocking effect of the adsorbed species and hence IE = In this work, the surface coverage was calculated from the relation IE EFM = 1 using the IE calculated from the EFM technique. The value of K is related to the standard free energy of adsorption G ads by the following equation 45 Table IV. Fitted parameters for iron in 1 M HCl in the absence and presence of various concentrations of AMF at 25 C derived using equivalent circuit in Fig. 5b. R ct R f C f F cm 2 R s CPE/Y F cm 2 n IE i AMF 5mM mm mm mm

6 C121 E/V SCE mMETF 1 mm ETF 15 mm ETF 2 mm ETF 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 j/ µa cm -2 Figure 8. Anodic and cathodic Tafel polarization curves for mild steel in the absence and presence of various concentrations of ETF in 1 M HCl at 25 C. G ads = RT ln 55.5K ads 9 where R is the gas constant and T is the absolute temperature. The value of 55.5 is the molar concentration of water in solution expressed in mol l 1. E/V SCE mm AMF 1 mm AMF 15 mm AMF 2 mm AMF 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 j/ µa cm -2 Figure 9. Anodic and cathodic Tafel polarization curves for mild steel in the absence and presence of various concentrations of AMF in 1 M HCl at 25 C. The values of adsorption constant, slope, and linear correlation coefficient R 2 can be obtained from the regressions between C/ and C, and the results are listed in Table VIII. The result shows that all the linear correlation coefficients and all the slopes are close to 1 and confirm that the adsorption of furan derivatives in 1 M HCl follows the Langmuir adsorption isotherm. The thermodynamic parameters for adsorption process obtained from Langmuir adsorption isotherms for the studied furan derivatives are given in Table VIII. The negative values of G ads 21.1, 21.6, and 23.2 for MEF, ETF, and AMF, respectively and the higher values of K reveal the spontaneity of the adsorption process and they are characteristic of strong interaction and stability of the adsorbed layer with the steel surface. The absolute value of the standard free energy of adsorption G ads of the investigated furan derivatives follows the order AMF ETF MEF. Generally, the energy values of 2 kj mol 1 or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface physisorption ; those of 4kJmol 1 or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond chemisorption. 46 The calculated G ads value, being closer to 2 kj mol 1, between the threshold values for physical adsorption and chemical adsorption indicates that adsorption of furan derivatives on the steel surface involves the two types of interaction. 47 There are many limitations on the application of Langmuir adsorption isotherm for the analysis of inhibition phenomena and related data. Some of these limitations are discussed below. Simple adsorption is applicable only under the following conditions. 1. The surface of the metal is homogeneous. 2. The adsorbate is specifically adsorbed, and each adsorbed species occupies only a single site of the surface. 3. There is no surface diffusion of the adsorbed compound. 4. The standard free energy of adsorption is independent of the degree of coverage. The Langmuir adsorption isotherm is somewhat idealized notwithstanding the fact that the adsorption may be highly selective with electrode reaction and may be potential-dependent, and the adsorbates may be competitive with water molecules. Adsorption isotherms are nonetheless useful if the uninhibited corrosion current is, in fact, proportional to the total number of sites that can possibly Table V. Electrochemical parameters calculated from polarization measurements on the mild steel electrode in 1 M HCl solutions without and with various concentrations of the three furan derivatives at 25 C. Inhibitor Inhib. M a c j corr A cm 2 E corr mv SCE IE p MEF 5mM mm mm mm ETF 5mM mm mm mm AMF 5mM mm mm mm

7 C122 Figure 1. Color online Intermodulation spectrum for mild steel in 1 M HCl in the absence and presence of various concentrations of furan derivatives at 25 C. adsorb inhibition. This, however, is not necessarily so, as it is known that a substantial corrosion rate is often observed at the saturation coverage of the inhibitor. In this paper, such discussions cannot be supported with detailed experimental data, which would be beyond the scope of this work. Such analysis would involve a detailed study employing transient experiments, in addition to the present dc and ac study. Moreover, it could require some spectroscopic measurements, X-ray analysis, or a combination of these techniques. Thus, it should be stressed at this stage that the following discussion is mainly qualitative and, to a significant degree, speculative. The tested furan derivatives showed a good capacity to inhibit the corrosion of mild steel for the tested range of the concentration. They inhibited the corrosion reactions of the substrate active sites by covering them with one or more of the interface adsorbing modes mentioned previously to form a protective layer of these molecules. The coverage of inhibitor particles may come out by adsorption chemisorption, with electron donating or sharing of the polar function of the inhibitors, rather than van der Waals forces physisorption, which are known to be reversible and do not yield an effective interface inhibition. It is believed that even if a complexation takes place between these compounds and the corrosion products or intermediates, it will be with weak formation forces and hardly affecting bonding forces, holding the surface atoms in the metal lattice and thus, the possibility of catalyzed reactions is very limited. Understanding the adsorption phenomena is of key importance in corrosion problems. Monte Carlo simulations help in finding the most stable adsorption sites on metal surfaces through finding the low energy adsorption sites on both periodic and nonperiodic substrates or to investigate the preferential adsorption of mixtures of adsorbate components. Adsorption energy as well as several outputs and descriptors calculated by the Monte Carlo simulation are presented in Table IX. The parameters presented in Table IX include total energy, in kj mol 1, of the substrate adsorbate configuration. The total energy is defined as the sum of the energies of the adsorbate components, the rigid adsorption energy, and the deformation energy. In this study, the substrate energy iron surface is taken as zero. In addition, the adsorption energy, in kj mol 1, reports the energy released or required when the relaxed adsorbate components furan derivatives are adsorbed on the substrate. The adsorption energy is defined as the sum of the rigid adsorption energy and the deformation energy for the adsorbate components. The rigid adsorption energy reports the energy, in kj mol 1, released or required when the un-

8 C123 Table VI. Electrochemical parameters calculated from EFM measurements on mild steel electrode in 1 M HCl solutions without and with various concentrations of the three furan derivatives at 25 C. Concentration M j corr A cm 2 a c CF-2 CF-3 IE EFM MEF 5mM mm mm mm ETF 5mM mm mm mm AMF 5mM mm mm mm relaxed adsorbate components i.e., before the geometry optimization step are adsorbed on the substrate. The deformation energy reports the energy, in kj mol 1, released when the adsorbed adsorbate components are relaxed on the substrate surface. Table I also shows de ads /dni, which reports the energy, in kj mol 1, of substrate adsorbate configurations where one of the adsorbate components has been removed. As can be seen from Table IX, AMF gives the maximum adsorption energy in a negative value found during the simulation process. Table VII. Corrosion rate (in g cm 2 h 1 ) and IE values obtained from weight loss measurements for mild steel in 1 M HCl solutions without and with various concentrations of furan derivatives at 25 C. Inhibition Efficiency /% EFM EIS Polarization Weight loss Inhibitor Concentration M Corrosion rate mg cm 2 h 1 IE w 4.95 MEF 5mM mm mm mm MEF ETF AMF Furan Derivatives Figure 11. Comparison of IE obtained in the presence of 2 mm of the furan derivatives by using EFM, EIS, Tafel polarization, and weight loss measurements at 25 C. ETF 5mM mm mm mm AMF 5mM mm mm mm C/θ (M) MEF ETF AMF.15 Table VIII. Thermodynamic parameters for the adsorption of furan derivatives in 1 M HCl on the mild steel at 25 C..1 Inhibitor Slope K G ads M 1 R 2 MEF ETF AMF C (M) Figure 12. Langmuir adsorption plots for mild steel in 1 M HCl in various concentrations of furan derivatives at 25 C.

9 C124 Table IX. Outputs and descriptors calculated by the Monte Carlo simulation for adsorption of furan derivatives on iron (1). Inhibitor Total energy Adsorption energy Rigid adsorption energy Deformation energy de ads /d Ni MEF ETF AMF High values of adsorption energy obtained in AMF molecules explain its highest IE. From the molecular structure of iron, it is evident that the unoccupied d-orbital exhibits a tendency to obtain the electron. The furan derivative, which is discussed in the present work, has many lone-pair electrons containing atoms on oxygen atoms, making it possible to provide electrons to the unoccupied orbitals of iron, to form a stable coordination type of bond. Therefore, the studied molecules are likely to adsorb on the iron surface to form stable adlayers and protect iron from corrosion. Conclusions The IEs of the three inhibitors obtained from weight loss, potentiodynamic polarization, and impedance methods well agree with those obtained from the EFM technique. Potentiodynamic polarization studies have shown that furan derivatives act as mixed-type inhibitors, and their inhibition mechanism is adsorption. Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through appropriate equivalent circuit model; a CPE has been used. The adsorption of furan derivatives on the steel surface in 1 M HCl solution was found to obey Langmuir s adsorption isotherm with a negative value of the free energy of adsorption G ads. Monte Carlo simulation technique incorporating molecular mechanics and molecular dynamics can be used to simulate the adsorption of furan derivatives on the iron 11 surface in 1 M HCl. References 1. S. A. Ali, M. T. Saeed, and S. V. Rahman, Corros. Sci., 45, K. C. Emregul and O. Atakol, Corros. Sci., 82, M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, and F. Bentiss, Corros. Sci., 44, M. Elayyachi, A. El Idrissi, and B. Hammouti, Corros. Sci., 48, K. Babic-Samardzija, K. F. Khaled, and N. Hackerman, Anti-Corros. Methods Mater., 52, K. F. Khaled, K. Babic-Samardzija, and N. Hackerman, Electrochim. Acta, 5, K. F. Khaled and N. Hackerman, Electrochim. Acta, 49, K. F. Khaled and N. Hackerman, Electrochim. Acta, 48, K. F. Khaled, Electrochim. Acta, 48, K. Aramaki and N. Hackerman, J. Electrochem. Soc., 15, K. Aramaki and N. Hackerman, J. Electrochem. Soc., 16, N. Hackerman and K. M. Hurd, in Corrosion Inhibition and Molecular Structure, First International Congress on Metallic Corrosion, Butterworth, London, p A. C. Makrides and N. Hackerman, Ind. Eng. Chem., 46, H. Vaidyanathan and N. Hackerman, Corros. Sci., 1, K. F. Khaled, Electrochim. Acta, 53, F. M. Donahue, A. Akiyama, and K. Nose, J. Electrochem. Soc., 114, A. Mitra, Indian J. Chem., 15A, R. W. Bosch, J. Hubrecht, W. F. Bogaerts, and B. C. Syrett, Corrosion (Houston), 57, J. Barriga, B. Coto, and B. Fernandez, Tribol. Int., 4, K. F. Khaled and M. A. Amin, Corros. Sci., 51, K. F. Khaled and M. A. Amin, Corros. Sci., 51, W. J. Lorenz and F. Mansfeld, Corros. Sci., 21, A. Popova, S. Raicheva, E. Sokolova, and M. Christov, Langmuir, 12, B. Donnelly, T. C. Downie, R. Grzeskowiak, H. R. Hamburg, and D. Short, Corros. Sci., 18, G. Trabanelli, in NACE International Corrosion Conference, on Corrosion Inhibition, Dallas, Texas, p Z. B. Stoynov, B. M. Grafov, B. Savova-Stoynova, and V. V. Elkin, Electrochemical Impedance, Nauka, Moscow K. Juttner, Electrochim. Acta, 1, J. Pang, A. Briceno, and S. Chander, J. Electrochem. Soc., 137, Z. Stoynov, Electrochim. Acta, 35, F. B. Growcock and R. J. Jasinski, J. Electrochem. Soc., 136, J. R. Macdonald, J. Electroanal. Chem. Interfacial Electrochem., 223, D. A. Lopez, S. N. Simison, and S. R. Sanchez, Electrochim. Acta, 48, U. Rammelt and G. Reinhard, Corros. Sci., 27, F. Bentiss, M. Traisnel, H. Vezin, and M. Lagrenée, Ind. Eng. Chem. Res., 39, H. Amar, A. Tounsi, A. Makayssi, A. Derja, J. Benzakour, and A. Outzourhit, Corros. Sci., 49, R. Solmaz, G. Kardaş, B. Yazıcı, and M. Erbil, Colloids Surf., A, 312, S. S. Abdel-Rehim, K. F. Khaled, and N. S. Abd-Elshafi, Electrochim. Acta, 51, K. F. Khaled, Mater. Chem. Phys., 112, K. F. Khaled, J. Appl. Electrochem., 39, K. F. Khaled, Mater. Chem. Phys., 112, P. Li, J. Y. Lin, K. L. Tan, and J. Y. Lee, Electrochim. Acta, 42, E. Kus and F. Mansfeld, Corros. Sci., 48, M. Christov and A. Popova, Corros. Sci., 46, E. Machnikova, K. H. Whitmire, and N. Hackerman, Electrochim. Acta, 53, M. Z. A. Rafiquee, N. Saxena, S. Khan, and M. A. Quraishi, Mater. Chem. Phys., 17, E. Kamis, F. Bellucci, R. M. Latanision, and E. S. H. El-Ashry, Corrosion (Houston), 47, M. Behpour, S. M. Ghoreishi, N. Soltani, M. Salavati-Niasari, M. Hamadanian, and A. Gandomi, Corros. Sci., 5,