pubs.acs.org/Langmuir © 2009 American Chemical Society
Imidazole-Fe Interaction in an Aqueous Chloride Medium: Effect of Cathodic Reduction of the Native Oxide G. Bhargava,† T.A. Ramanarayanan, and S. L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009. †Current address: Headwaters Technology Innovation, Lawrenceville, NJ 08648. Received June 5, 2009. Revised Manuscript Received September 22, 2009 The effect of the reduction of the native surface oxide of Fe on the binding of imidazole (as a corrosion inhibitor) with Fe in an aqueous brine solution has been addressed here. The surface interactions and corrosion inhibition efficiency were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). It was shown that imidazole dissolved in brine bonds with the unreduced iron oxide surface via pyrrole-type nitrogen. However, surface interactions with Fe occur via both pyridine-type and pyrrole-type nitrogen atoms when imidazole is added to brine containing a cathodically reduced iron surface. The packing density of imidazole is found to be higher in the latter case with a corresponding increase in the corrosion inhibition efficiency.
Introduction Among the different methods of corrosion control of vessels, piping, and tubes used in aggressive environments, such as, for example, those encountered in the petrochemical industry, corrosion inhibition (CI) is one of the most cost-efficient,1 allowing the use of less expensive alloys. Refineries and petrochemical industries employ a variety of film-forming inhibitors to control wet corrosion (resulting from the presence of an aqueous phase). Although many effective corrosion inhibitors have been developed during the past many years to combat corrosion, the exact nature of inhibitor-metal interactions is still not well understood at the fundamental level. Very few studies have been reported that have probed the real nature of the inhibitor-iron bond.2-5 In many cases, it is also unclear whether the inhibitor is bonded to a metal surface, an oxide surface, or some other compound surface. Studies have also not addressed the differences in the inhibitor-metal interactions arising from the manner in which the inhibitor is delivered to the surface. In our previous study6 on imidazole dissolved in aqueous NaCl solution as a model inhibitor for iron corrosion, two possible orientations of imidazole molecules on Fe covered by iron oxide were proposed. The plane of the imidazole ring was found to be nearly parallel to the oxidized iron surface when added directly to aqueous NaCl solution, but it was nearly perpendicular to the iron oxide surface when predeposited onto the sample. To minimize the complexity, a simple corrosion system consisting of Fe exposed to 3% NaCl solution was selected. Fe is the *Corresponding author. E-mail:
[email protected]. (1) Olivares-Xometl, O.; Likhanova, N. V.; Dominguez-Aguilar, M. A.; Hallen, J. M.; Zamudio, L. S.; Arce, E. Appl. Surf. Sci. 2006, 252, 2139. (2) Edwards, A.; Osborne, C.; Webster, S.; Klenerman, D.; Joseph, M.; Ostovar, P.; Doyle, M. Corros. Sci. 1994, 36, 315. (3) Shiri, A.; Etman, M.; Dabosi, F. Electrochim. Acta 1996, 41, 429. (4) Khaled, K. F. Electrochim. Acta 2003, 48, 2493. (5) Babic-Samardzija, K.; Lupu, C.; Hackerman, N.; Barron, A. R.; Luttge, A. Langmuir 2005, 21, 12187. (6) Bhargava, G.; Ramanarayanan, T. A.; Gouzman, I.; Abelev, E.; Bernasek, S. L. Corrosion 2009, 65, 308. (7) Durnie, W.; De Marco, R.; Jefferson, A.; Kinsella, B. J. Electrochem. Soc. 1999, 146, 1751. (8) Revie, R. W. Uhlig’s Corrosion Handbook, 2nd ed.; John Wiley & Sons: New York, 2000.
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principal component of carbon steel, which is widely used in the oil and gas industry.7,8 Nitrogen-based heterocyclic organic molecules are an important class of corrosion inhibitors used in the petrochemical industry. The electron-donating character of nitrogen in the organic molecule is considered to influence the bonding of the molecule with iron. Oleic imidazolines have been used traditionally as CIs on steels in acidic environments.7,9-12 Imidazole and its derivatives have been widely studied as corrosion inhibitors on iron and steel.13-20 As such, imidazole (C3H4N2) has been selected as a model inhibitor molecule in this work. In our previous study,6 the bonding of imidazole with the surface oxide on iron has been discussed. There is no reason to suppose that imidazole interaction with an iron oxide surface would be identical to that with an iron surface. Therefore, the focus of the present study is to differentiate critically between imidazole-iron oxide and imidazole-iron surface interactions. An applied voltage of -1.5 V with respect to the standard calomel electrode in aqueous NaCl solution is used to achieve a reduction of surface iron oxide to metallic iron in these studies. The corrosion behavior is then examined over a 24 h period using electrochemical impedance spectroscopy (EIS) upon addition of imidazole to the solution. The results are compared with our previous results on Fe covered with iron oxide. High-resolution X-ray photoelectron spectroscopy (XPS) studies have been used to elucidate the surface chemistry differences in the two cases. (9) Lopez, D. A.; Schreiner, W. H.; de Sanchez, S. R.; Simison, S. N. Appl. Surf. Sci. 2004, 236, 77. (10) Ramachandran, S.; Tsai, B. L.; Blanco, M.; Chen, H.; Tang, Y.; Goddard, W. A. Langmuir 1996, 12, 6419. (11) Jovancicevic, V.; Ramachandran, S.; Prince, P. Corrosion 1999, 55, 449. (12) Cruz, J.; Martı´ nez, R.; Genesca, J.; Garcı´ a-Ochoa, E. J. Electroanal. Chem. 2004, 566, 111. (13) Ogretir, C.; Calis, S.; Bereket, G. J. Mol. Struct. 2003, 635, 229. (14) Bereket, G.; Hur, E.; Ogretir, C. J. Mol. Struct. 2002, 578, 79. (15) Abboud, Y.; Abourriche, A.; Saffaj, T.; Berrada, M.; Charrouf, M.; Bennamara, A.; Cherqaoui, A.; Takky, D. Appl. Surf. Sci. 2006, 252, 8178. (16) Raicheva, S. N.; Aleksiev, B. V.; Sokolova, E. I. Corros. Sci. 1993, 34, 343. (17) Subramanyam, N. C.; Mayanna, S. M. Corros. Sci. 1985, 25, 163. (18) Wahdan, M. H.; Gomma, G. K. Mater. Chem. Phys. 1997, 47, 176. (19) Benali, O.; Larabi, L.; Tabti, B.; Harek, Y. Anti-Corros. Met. Mater. 2005, 52, 280. (20) Popova, A.; Raicheva, S.; Sokolova, E.; Christov, M. Langmuir 1996, 12, 2083.
Published on Web 10/07/2009
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The difference in corrosion behavior is examined when imidazole is added directly to the corrosive medium with as-is Fe covered with oxide and with cathodically reduced Fe oxide samples. The performance of imidazole in the two cases has been examined using EIS over a period of 24 h until a steady state is reached. High-resolution XPS studies have been used to analyze the surface chemistry of the sample surface in the two situations. After gaining an understanding of the interaction of imidazole with iron in a neutral aqueous chloride medium as discussed in our previous work,6 the goal of the present study is to answer the specific question, will the imidazole binding characteristics change if the initial “native” oxide on the iron surface is reduced before imidazole addition to the electrolyte? Native oxide reduction can be achieved by applying an external voltage of approximat ely -1.5 V to the Fe surface and adding imidazole subsequently to the corrosive medium.
Experimental Approach Iron samples were cut using a diamond saw from a high-purity polycrystalline iron rod (99.98% purity, purchased from Electronic Space Products International (ESPI)). Disk-shaped samples of 15 mm diameter and 1.5 mm thickness were polished on one side with silicon carbide paper down to 1200 grit. This was followed by sonication in methylene chloride, isopropanol, and deionized water (18.2 MΩ cm, Milli-Q) for 10 min each. Finally, the samples were dried in high-purity nitrogen. Electrochemical impedance spectroscopy (EIS) was carried out in a standard three-electrode electrochemical cell to estimate the corrosion characteristics of Fe with and without the corrosion inhibitor. The reference electrode was a standard calomel electrode (SCE); the counter electrode consisted of two graphite rods connected by a wire, and the working electrode was the iron disk sealed inside a Teflon casing with ∼1 cm2 of the test surface exposed to the aqueous medium. All of the tests were conducted at room temperature in 3% NaCl solution. The working electrode was introduced after purging the electrolyte solution with argon for 30 min. Three sets of experiments were carried out in the three-electrode electrochemical cell. First, Fe was introduced into the electrolyte solution followed by EIS measurements for 24 h without the presence of imidazole. Then Fe was introduced into the electrolyte containing imidazole (corrosion inhibitor) at 500 ppm concentration by weight, followed by EIS measurements. Finally, a cathodic potential of -1.5 V (with respect to the SCE) was applied to the Fe working electrode for 20 min in the electrochemical cell containing the electrolyte solution. Imidazole (500 wt ppm) was then added to the electrolyte after 15 min of voltage application. The external voltage was turned off 5 min after imidazole addition. EIS measurements were then conducted in this third case. EIS measurements were carried out over a period of 24 h using an EG&G Princeton Applied Research model 273 A potentiostat/ galvanostat with an EG&G model 5210 lock-in amplifier. The instrument was controlled by a PC operated by Powersuite electrochemistry software from Princeton Applied Research. A single sinusoidal potential of 10 mV peak-to-peak was superimposed on the open-circuit potential over the frequency range of 10-1-105 Hz. The experimental frequency was scanned from the higher to the lower value. Nyquist plots were obtained for each experiment, and the polarization resistance, Rp, was determined. Once a constant Rp was attained, the iron sample was removed from the electrochemical cell, washed in ethanol, dried, and then transferred to a UHV system. XPS measurements were performed in the UHV chamber (base pressure ∼5 10-9 Torr) equipped with a monochromated Al KR X-ray source operated at 13 kV and 400 W and a Phoibos 150 hemispherical energy analyzer (SPECS). The spectrometer was calibrated to the position of the 3d5/2 line of sputtered-cleaned Ag with a binding energy of 368.25 eV.10 Measurements were carried out at a normal takeoff angle, 216 DOI: 10.1021/la9020355
Figure 1. Variation of current with time upon the application of an external voltage of -1.5 V. Imidazole was added 15 min after turning the voltage on. The applied voltage was maintained for an additional 5 min. θ=90°, between the sample surface and the direction of photoelectrons detected by the analyzer. A pass energy of 30 eV was used for high-sensitivity wide-range scans (survey), and a 10 eV pass energy was used to generate high-resolution spectra of Fe 2p, N 1s, C 1s, and O 1s for quantitative analysis.
Results EIS Studies. Electrochemical Reduction of Surface Oxide. The native surface oxide of iron was reduced by applying a voltage of -1.5 V to Fe with respect to SCE for 15 min in a 3% NaCl solution. The variation of current between the Fe surface (cathode in this case) and the graphite counter electrode as an anode is shown in Figure 1. The Figure shows a sharp decrease in current that then attains a constant value signifying the reduction of the metal oxide at a constant rate. Imidazole was added to the electrolyte solution at a concentration of 500 wt ppm after 15 min of voltage application. The addition of imidazole resulted in a further decrease in current attributable to the interaction of imidazole with the iron surface. The external voltage was turned off 5 min after imidazole addition. Once the voltage was switched off, EIS testing was carried out for 24 h. Subsequently, the samples were transferred, under argon cover, into a UHV chamber for XPS characterization. Thus any reoxidation of the iron surface was minimized. Electrochemical Impedance Spectroscopy: Polarization Resistance (Rp). EIS was carried out for 24 h for the baseline experiment, the imidazole-in-solution case, and reduced iron with imidazole. As seen in Figure 2 and listed in Table 1, the Rp values showed a significant increase from 100 Ω/cm2 for unprotected iron to 2700 Ω/cm2 for the in-solution case, indicating some corrosion inhibition. The Rp value further increased to 10 000 Ω/ cm2 for the experiments where imidazole was added subsequent to cathodic reduction. Hence, it can be concluded that imidazole provides significantly better corrosion inhibition on Fe in the absence of the native/surface oxide. Stated in another way, there is significantly greater chemisorption of imidazole on the reduced iron surface than on the native-oxide-covered surface. These findings were further substantiated by the XPS analysis. Bonding Studies by XPS. High-resolution scans of the Fe 2p, N 1s, O 1s and C 1s regions were obtained for the Fe sample. Fe 2p and N 1s have been presented here to compare the relative Langmuir 2010, 26(1), 215–219
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Article Table 1. Comparison of the Performance of Imidazole on Fe and Reduced Fea experiments O2-/Fe N/Fe Rp (Ω/cm2) uninhibited iron in solution 2.1 100 Fe in solution containing imidazole 1.0 0.1 2700 iron just after reduction in solution 0.6 reduced Fe exposed to solution 0.9 0.4 10 000 containing imidazole a Rp values were obtained for Fe samples after 24 h of corrosion. XPS was carried out after taking the Fe samples out of the electrochemical cell. O2-/Fe and N/Fe ratios were calculated after fitting the XPS data by analyzing the area under the peaks.
Figure 3. High-resolution spectrum of the Fe 2p region obtained for Fe (a) after 24 h of EIS in 3% NaCl solution without imidazole, (b) after 24 h of EIS in 3% NaCl solution with imidazole, (c) just after cathodic reduction with no EIS, and (d) after 24 h of EIS in 3% NaCl solution with imidazole added after cathodic reduction.
Figure 2. Nyquist plots at different time intervals for (a) the baseline, (b) imidazole added with iron surface in the as-is condition, and (c) imidazole added after the reduction of the iron surface.
performance of imidazole in the three sets of experiments. The peaks corresponding to different species present on the metal were assigned to the respective bonding states through the standard deconvolution fitting procedure using CASA XPS software (21) NIST XPS Standard Reference Database 20, version 3.4 (web version), 2003.
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provided by SPECS. The peak identification values are in agreement with the databases.21,22 A comparison of high-resolution scans of the Fe 2p region is shown in Figure 3. All of the spectra exhibit contributions from the Fe2þ and Fe3þ oxidation states of iron as shown in the Figure. The difference lies in the relative intensity of the Fe0 signal with respect to the higher oxidation peaks. The baseline curve (curve a) shows no Fe0 component, but with imidazole addition (curve b), there was an emergence of the Fe0 peak, indicating less surface oxidation. The Fe0 component was observed to be of much higher intensity when Fe was taken out of the electrochemical cell immediately after cathodic reduction (curve c). After 24 h of corrosion, this peak decreases but is still more intense than for the case where imidazole was added without cathodic reduction (curves b and d). This also suggests that imidazole inhibits oxide formation by slowing down the anodic dissolution of iron. The bonding characteristics of the two types of nitrogen in imidazole with iron are elucidated by the high-resolution scans of the N 1s region. The imidazole molecule has two nitrogen atoms. The nitrogen atom not bonded to the hydrogen atom has a localized lone pair of electrons that does not participate in the aromaticity of the molecule, whereas the other nitrogen has a lone pair that participates in the aromatic π-electron system (Figure 4). (22) Moulder, J. F.; .Stickle, W. F.; Sobol, P. E.; , Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer Corporation, Physical Electronics Division: Eden Prairie, MN, 1992.
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Figure 4. Molecular structure of (a) pyridine, (b) pyrrole, and (c) imidazole.
The former can be called pyridine-type nitrogen, and the latter can be called pyrrole-type nitrogen. The bonding characteristics of imidazole with Fe on a reduced surface can be better understood from the high-resolution scan of the N 1s region shown in Figure 5. The distinct difference in the surface composition of uninhibited iron when compared to the cases with imidazole is the absence of a nitrogen peak in the former case and its appearance in the latter. This difference suggests that the only source of nitrogen in the experiment is the two nitrogen atoms in the imidazole molecule. The two types of nitrogen atoms in imidazole were visible in the XP spectrum of the imidazole-in-solution case as shown in Figure 5b. In this case, peaks were observed at a binding energy of 400.50 eV depicting the C-NH-C bond1,23-25 and at a binding energy of 399.54 eV corresponding to the CdN-C-type bond.26 The former can be attributed to the pyrrole-type interaction (Npr), and the latter, to the pyridine-type (Npy) interaction. The Fe surface after undergoing cathodic reduction reacts with imidazole in such a way that it shows (Figure 5c) the emergence of an additional peak at a binding energy of 401.60 eV that can be assigned to the positively charged nitrogen (-Nþ).1,23,24,26 This -Nþ species originates from the pyridine-type nitrogen (Npy) with the lone pair transferred to acidic sites on the surface. The peak at 399.44 eV is assigned to CdN-C bonds 1, also arising from the pyridine-type (Npy) nitrogen. The third peak at 400.20 eV depicts the C-NH-C bond arising from the pyrrole-type (Npr) nitrogen. The intensity of the nitrogen signal increases by a factor of 5 when imidazole reacts with the reduced iron surface. This significant increase in nitrogen intensity indicates a greater packing of imidazole molecules on the reduced Fe surface. In support of this finding, O2-/Fe and N/Fe ratios were determined from the high-resolution spectra of the Fe 2p, O 1s, and N 1s regions. The ratios (O2-/Fe and N/Fe) were obtained by measuring the area under the peak for O2- (data not shown) and dividing it by the area under the total Fe peak. This ratio indicates the extent and oxidation state of Fe exposed to aqueous NaCl solution and is shown in Table 1. The trend shows that maximum oxidation corresponds to the aqueous NaCl solution without imidazole. The presence of imidazole decreases the oxide formation tendency. The addition of imidazole reduces oxide formation from 2.1 (uninhibited iron) to 1.0 in the in-solution case. For the reduced iron surface, the O2-/Fe ratio further decreased to 0.6, confirming the reduction of the surface native oxide. This ratio increased to 0.9 when the Fe sample was exposed to brine solution for 24 h in the presence of imidazole. Correspondingly, the N/Fe ratio increases from 0.1 to 0.4 when imidazole reacts with Fe in the presence and absence (although partial reduction was observed) of the native surface oxide, respectively. This 4-fold increase in the
(23) Martins, J. I.; Bazzaoui, M.; Reis, T. C.; Bazzaoui, E. A.; Martins, L. Synth. Met. 2002, 129, 221. (24) Prissanaroon, W.; Brack, N.; Pigram, P. J.; Liesegang, J.; Cardwell, T. J. Surf. Interface Anal. 2002, 33, 653. (25) Su, W. C.; Iroh, J. O. Electrochim. Acta 1999, 44, 3321. (26) Idla, K.; Talo, A; Niemi, H. E. M.; Forsen, O.; Ylasaari, S. Surf. Interface Anal. 1997, 25, 837.
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Figure 5. Comparison of high-resolution spectra of the N 1s region. (a) An iron sample exposed to 3% brine solution; (b) an iron sample exposed to imidazole in 3% brine solution; and (c) a cathodically reduced iron sample exposed to imidazole in 3% brine solution.
N/Fe ratio is consistent with the higher corrosion inhibition provided by imidazole (as reflected by higher Rp values).
Discussion The difference between the inhibitive properties of imidazole when interacting with as-is Fe and with reduced Fe can be understood from the XPS results. As previously stated, imidazole has two types of nitrogen atoms: pyridine type and pyrrole type. Assuming the concepts of Lewis acid and Lewis base to apply to surfaces,27 the chemisorption of pyridine to Fe conforms to the Lewis acid-base model of nitrogen lone pair donation to acidic sites on the surface. In that case, pyridine is adsorbed with the plane of the ring perpendicular to the metal surface with the nitrogen atom closest to the iron surface. However, pyrrole achieves the required six π electrons necessary for aromatic stabilization because of the contribution of two electrons from nitrogen. Because the six π electrons are delocalized over five annular atoms in the pyrrole molecule, it is classified as a π-electron-excessive aromatic system and is susceptible to electrophilic attack.28-30 Therefore, a likely orientation of pyrrole on iron is with the plane of the ring more parallel to the surface. From these arguments and the results presented previously, the plausible orientations of imidazole on the iron/iron oxide surface have been proposed. A schematic representation of the atomic arrangement (side view) of an iron surface with Fe3O4 with a pore is shown in Figure 6. In the case where imidazole was added to the corrosive medium (without electrochemical reduction), the absence of any -Nþ-type species suggests that the π-electron system of the ring interacts with Fe and the ring is more parallel to the surface as represented in Figure 6a. This indicates that imidazole interacts with the iron/iron oxide surface through the electron-rich heterocyclic ring rather than through nitrogen atoms individually. (27) Rozenfeld, I. L. Corrosion Inhibitors; McGraw-Hill: New York, 1981; p 10. (28) Stair, P. C. J. Am. Chem. Soc. 1982, 104, 4044. (29) Jones, R. A.; Bean, G. P. The Chemistry of Pyrroles; Academic Press: London, 1977. (30) Pan, F. M.; Stair, P. C.; Fleisch, T. H. Surf. Sci. 1986, 177, 1.
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Figure 6. Interaction of imidazole with a reduced Fe oxide surface. (a) Mixed pyridine-type (normal and angular) and pyrrole-type interactions were observed with a higher surface density of imidazole on the cathodically reduced surface, as compared to the case (b) without cathodic reduction.
However, when imidazole is allowed to react on Fe where the native oxide has been partially reduced, pyridine-type N also reacts with the metal such that imidazole is oriented in a mixed geometry with both parallel and perpendicular arrangements of molecules as shown in Figure 6b. As the variation of the O2-/Fe and N/Fe ratios suggests, more imidazole molecules are packed on the reduced Fe surface, providing significantly greater corrosion inhibition. The observed Rp values confirm the improvement in corrosion inhibition. The oxide formed during the electrochemical process is porous in nature and can be of a different composition from the native oxide originally present.31 Although Fe used in the study is polycrystalline, the oxide film shown in the Figure is an Fe3O4(111) surface because it is known to crystallize in a hexagonal close packing (hcp) of atoms.32 Upon electrochemical reduction, more of the Fe surface is exposed as shown in Figure 5b. Thus, the increase in the packing density of imidazole in the reduced case must be attributed to bonding on a pure Fe surface. (31) Qiao, M. H.; Tao, F.; Cao, Y.; Xu, G. Q. Surf. Sci. 2003, 544, 285. (32) Bhargava, G.; Gouzman, I.; Chun, C. M.; Ramanarayanan, T. A.; Bernasek, S. L. Appl. Surf. Sci. 2007, 253, 4322.
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Conclusions In this work, it was seen that the application of an external cathodic voltage leads to some reduction of the native/surface oxide on iron. Imidazole provides better corrosion inhibition on such a reduced surface than on the unreduced surface. Two possible orientations of imidazole molecules have been proposed in this study. The plane of the imidazole ring is more parallel to Fe when added directly to the aqueous NaCl solution. The reduction of the native surface oxide of Fe causes the orientation of molecules to change to a mixed type with both parallel and perpendicular arrangements, as inferred from the spectroscopic data and the corrosion inhibition effectiveness. Finally, dative bond formation via electron transfer from the pyridine-type N atom in imidazole was favored on a reduced iron surface. A mixed-type orientation of imidazole molecules was observed in this case with a higher surface density on the reduced Fe surface. Acknowledgment. This research was partially supported by the National Science Foundation, Division of Chemistry (CHE-0616457).
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