Inhibitive Properties and Surface Morphology of a Group of

Langmuir 2005, 21, 12187-12196

12187

Inhibitive Properties and Surface Morphology of a Group of Heterocyclic Diazoles as Inhibitors for Acidic Iron Corrosion Ksenija Babic´-Samardzˇija,† Corina Lupu,† Norman Hackerman,*,† Andrew R. Barron,*,† and Andreas Luttge*,‡ Department of Chemistry, Rice University, Houston, Texas 77005, and Department of Earth Science, Rice University, Houston, Texas 77005 Received June 30, 2005. In Final Form: September 27, 2005 The heterocyclic diazoles 3-amino-1H-isoindole, indazole, imidazole, 4-bromoimidazole, 4-methylimidazole, pyrazole, 4-nitropyrazole, and 4-sulfopyrazole were investigated as corrosion inhibitors of iron in 1 M HCl using ac and dc techniques. The polarization curves showed a decrease in corrosion current for the inhibitorcontaining solution. Impedance spectra demonstrate that the charge-transfer resistance in the presence of these inhibitors was greater than in inhibitor-free solution, except for 4-nitropyrazole. The resistance increased with inhibitor concentration and with immersion time. The structural and electronic parameters of these diazoles were calculated using computational methodologies. The elemental composition and the speciation of the treated surfaces were investigated via XPS measurements, and morphological changes were monitored by vertical scanning interferometery.

Introduction In addition to their coordination ability in transition metal complexes,1 nitrogen heterocycles have been investigated as corrosion inhibitors in acidic media.2-12 Their performance as corrosion inhibitors is related to their structure and to the state of adsorption at the metal/ solution interface. Some aromatic diazoles are good inhibitors for copper,13 where the inhibition mechanism has been related to the formation of a Cu(I) complex structure on the surface which acts as a barrier to further corrosion. A surface ‘complex film’ between iron and some triazoles is believed to create a protecting layer, providing high inhibitor efficiency.14 In general, organic compounds can be adsorbed to the metal surface via negatively charged centers where the inhibitor property is believed to relate to the polar groups and/or π electrons.15 A partial transfer of electrons from the polar (donor) atoms directly to the metal surface atoms * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Earth Science. (1) Chetouani, A.; Aouniti, A.; Hammouti, B.; Benchat, N.; Benhadda, T.; Kertit, S. Corros. Sci. 2003, 45, 1675. (2) Gomma, G. K.; Wahdan, W. H. Mater. Chem. Phys. 1997, 47, 176. (3) Gomma, G. K. Mater. Chem. Phys. 1998, 56, 27. (4) Lee, W. J. Mater. Sci. Eng. A 2003, 348, 217. (5) Tang, L.; Mu, G.; Liu, G. Corros. Sci. 2003, 45, 2251. (6) Cruz, J.; Martinez, R.; Genesca, J.; Garcia-Ochoa, E. J. Electroanal. Chem. 2004, 566, 111. (7) Blajiev, O.; Hubin, A. Electrochim. Acta 2004, 49, 2761. (8) Es-Salah, K.; Keddam, M.; Rahmouni, K.; Srhiri, A.; Takenouti, H. Electrochim. Acta 2004, 49, 2771. (9) Popova, A. Christov, M. Raicheva, S.; Sokolova, E. Corros. Sci. 2004, 46, 1333. (10) Abd El-Maksoud, S. A. J. Electroanal. Chem. 2004, 565, 321. (11) Otmacic, H.; Telegdi, J.; Papp, K.; Stupnisek-Lisac, E. J. Appl. Electrochem. 2004, 34, 545. (12) Khaled, K. F.; Babic-Samardzija, K.; Hackerman, N. J. Appl. Electrochem. 2004, 34, 697. (13) Lakshminarayanan, V.; Kannan, R.; Rajagopalan, S. R. J. Electroanal. Chem. 1994, 364, 79. (14) Babic´-Samardzija, K.; Hackerman, N. J. Solid State Electrochem. 2005, 9, 483. (15) Trabanelli, G. Corrosion Mechanism: Corrosion Inhibitors; Mansfeld F., Ed.; Marcel Dekker, Inc: New York, 1987; p 131.

can be viewed as a coordinative type of bond. Thus, in the case of nitrogen heterocycles, donation from either the polar nitrogen or the π electrons would be expected. In acidic solutions, however, nitrogen-containing compounds are protonated, thus limiting potential interaction of the polar nitrogen donor groups with the metal surface. It has been reported that immersion of a metal surface in an acidic media results in aggregation and concentration of the acid anions at the metal surface.5 Chloride ions (from HCl) have a small degree of hydration and bring excess negative charges in the vicinity of the interface.5,16 Thus, positively charged ions such as protonated nitrogen heterocyclic compounds might adsorb onto the metallic surface via the negatively charged acid anions (e.g., Cl-). Therefore, there is potential that both physical adsorption and chemisorptions may be involved in the corrosion suppressing mechanism of nitrogen heterocycles. One of the aspects of recent corrosion inhibition studies is the use of molecular modeling to calculate electronic properties possibly relevant to explain the inhibiting action. Potential orientation of the molecule, favorable configurations, and steric and electronic effects would be useful for better understanding of inhibitor performance.17-20 Frontier orbital theory is used to predict the adsorption centers of the inhibitor molecule responsible for the interaction with surface metal atoms.12,17-20 Corrosion systems containing an organic inhibitor and metal act as Lewis base and acid, respectively; thus, their frontier orbitals are involved in these calculations.17 Other variables such as electron density on the donor atom,21 ionization potential,22 electron affinity,23 pKa,24 steric (16) Luo, H.; Guan, Y. C.; Han, K. N. Corrosion 1998, 54, 721. (17) Martinez, S.; Stagljar, I. J. Mol. Struct. (THEOCHEM) 2003, 640, 167. (18) O ¨ gretir, C.; Calis, S.; Bereket, G. J. Mol. Struct. (THEOCHEM) 2003, 635, 229. (19) Bereket, G.; Hu¨r, E.; O ¨ gretir, C. J. Mol. Struct. (THEOCHEM) 2002, 578, 79. (20) Awad, M. K.; Mahgoub, F. M.; El-Iskandarani, M. M. J. Mol. Struct. (THEOCHEM) 2000, 531, 105. (21) Ayers, R. C.; Hackerman, N. J. Electrochem. Soc. 1963, 110, 507.

10.1021/la051766l CCC: $30.25 © 2005 American Chemical Society Published on Web 11/22/2005

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Figure 1. The structure of heterocyclic diazoles.

effects,25 Hammett substituent constants,26 dipole moments, and the fraction of the electrons (∆N) transferred from the inhibitor molecule to the metal27 have been included in such correlations. These parameters might give useful information about the reactivity behavior, electronegativity, or atom charge. However, solution components and surface competition reactions are still neglected.12 The aim of the present paper is to present the inhibitive properties and surface morphology of a group of heterocyclic diazoles: 3-amino-1H-isoindole (AIN), indazole (IND), imidazole (IMZ), 4-bromoimidazole (BIM), 4-methylimidazole (MIM), pyrazole (PYR), 4-sulfopyrazole (SOP), and 4-nitropyrazole (NOP). The effect of these compounds on the corrosion of iron in 1 M HCl was studied. We used polarization resistance values, Tafel curve effects, and electrochemical impedance spectroscopy. These measurements were combined with surface analysis and quantification of the surface morphology using X-ray photoelectron spectroscopy (XPS) and vertical scanning interferometery (VSI), respectively. Molecular modeling was used in an attempt to relate structural and electronic characteristics of these molecules and their efficiencies as corrosion inhibitors. Experimental Section The structures of the heterocyclic diazole compounds along with the acronyms used throughout are shown in Figure 1. All nitrogen hetrocylic compounds (Aldrich Chemical Co.) were used without pretreatment, except for SOP28,29 and NOP,30 which were (22) Altsybiera, A. I.; Levin, S. Z.; Dorokhov, A. P. 3rd European Symposium of Corrosion Inhibitors; University of Ferrara: Ferrara, Italy, 1971. (23) Vosta, J.; Eliasek, J. Corros. Sci. 1971, 11, 223. (24) Funke, W.; Haman, K. Werkst. Korros. 1958, 9, 202. (25) Homer, L.; Meisel, K. Werkst. Korros. 1978, 29, 654. (26) Talati, J. D.; Modi, R. N. Corros. Sci. 1979, 19, 35. (27) Lukovits, I.; Kalman, E.; Zucchi, F. Corrosion 2001, 57, 3. (28) Rondestvedt, C. S.; Chang, P. K. J. Am. Chem. Soc. 1955, 77, 6352. (29) Mezei, G.; Raptis, R. G. New J. Chem. 2003, 27, 1399. (30) Maresca, K. P.; Rose, D. J.; Zubieta, J. Inorg. Chim. Acta 1997, 260, 83.

Babic´ -Samardzˇ ija et al. prepared according to literature methods. Fresh solutions of 1 M HCl (Fisher Scientific) were prepared before each experiment and contained the inhibitors in the concentration range of 1 × 10-4-1 × 10-2 M for each additive and for NOP up to 0.1 M. An iron rod, 5 mm in diameter (Puratronic 99.99%, Johnson Matthey) mounted in Teflon, was used as the working electrode for electrochemical studies. The counter electrode was a Pt wire (Premion 99.997%) and 99.9% platinum gauze (52 mesh, Johnson Matthey), separated from the working electrode by a sintered glass frit. The reference electrode was a saturated calomel electrode (SCE) to which our potentials are referred. The experiments were carried out under static conditions at 25 °C in aerated acid solutions. A fine Luggin capillary was placed close to the working electrode to minimize ohmic resistance. Before each experiment, the working electrode was rubbed with emery papers 180-4/0 grit (Buehler Ltd) and polished with 0.5 µm Al2O3 to give a mirror surface. The electrode was then washed in Millipore bi-distilled water in an ultrasound bath, rinsed with acetone and water, and dried at room temperature. Measurements were performed with a Gamry Instrument Potentiostat/Galvanostat/ZRA. This includes a Gamry Framework system based on the ESA400 and the VFP600 and Gamry applications that include DC105 corrosion and EIS300 electrochemical impedance spectroscopy measurements. A computer collected the data, and Echem Analyst 4.0 Software was used for plotting, graphing, and fitting data. Tafel curves were obtained by changing the electrode potential automatically from -250 to +250 mV versus open circuit potential (Eoc) at a scan rate of 1 mV‚s-1. Linear polarization resistance experiments were done at -25 to +25 mV versus Eoc at a scan rate of 0.125 mV‚s-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out under potentiostatic conditions in a frequency range of 100 kHz to 0.1 Hz with amplitude of 1 mV peak-to-peak using ac signals at the Eoc. Experiments were measured after 50 min immersion time for all EIS tests and 500 min for EIS-time experiments for each inhibitor at 10-2 M, respectively. The samples for XPS measurements were prepared by the same procedure as for the electrochemical experiments at corrosion potential in the 10-2 M inhibitor-containing solution with iron foil (0.25 mm thick, 99.99% Johnson Matthey) being used as the working electrode instead of the iron rod. The samples were dried under vacuum prior to placing in the XPS vacuum chamber. In addition, two more iron samples were prepared under NOP treatment using 10-2 and 10-4 M concentration. After the samples were immersed for 18 h, their surfaces were vigorously washed using distilled water and dried under vacuum before they were placed into the XPS chamber. The XPS measurements were performed using a PHI Quantera SXM scanning X-ray microscope instrument equipped with an Al KR monochromated source (hν ) 1486.6 eV) operated at a power of 100 W and having a beam diameter of 100 µm. All spectra were collected using a takeoff angle of 45° with respect to the sample surface plane. During the measurements, the working pressure in the chamber was kept below 1 × 10-8 Torr. The hemispherical energy analyzer was operated in the fixed retard ratio mode by using a 140 eV pass energy with 0.5 eV‚step-1 for survey spectra collection (0-1400 eV) and a 26 eV pass energy with a 0.1 eV‚step-1 for high-resolution scans of individual core levels. The analyzer was calibrated by measuring and correcting for the binding energies for the Au 4f7/2 (83.59 eV, fwhm ) 0.72 eV) from a separate gold sample acquired by using the same conditions as for our samples. The C 1s peak (284.8 eV) was used as internal reference for calibration of the binding energy scale (BE). The charging effect was minimal, so no electron bombardment was necessary. The high-resolution spectra were acquired for photoelectrons emitted from Fe 2p (10 sweeps), O 1s (3 sweeps), C 1s (3 sweeps), N 1s (25 sweeps), and Cl 2p (5 sweeps). The high-resolution spectra for each sample were acquired over 13 h. Multipak software 7.1 was used for all data processing. The high-resolution XPS spectra were analyzed by using a nonlinear least squares algorithm. A Shirley baseline correction and a Gaussian-Lorentzian function (∼80% Gaussian contribution) were applied for each spectrum. Atomic compositions were derived from the high-resolution scans; peak areas were derived after subtraction of the integrated baseline and corrected for sensitivity

Heterocyclic Diazoles as Inhibitors for Iron Corrosion

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Table 1. Electrochemical Parameters Obtained from Tafel Method Extrapolation of Iron in 1 M HCl and in the Presence of Different Concentrations of N-Heterocycles compound blank AIN

IND

IMZ

BIM

MIM

SOP

PYR

NOP

concn (M)

icorr (µA‚cm-2)

-Ecorr (mV)

βa (mV‚dec-1)

βc (mV‚dec-1)

CR (mm‚y-1)

IE (%)

10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 5 × 10-2 0.1

141.0 25.81 20.44 11.70 7.72 26.55 21.66 16.76 15.07 31.60 22.35 18.30 17.01 34.43 28.44 24.29 19.23 34.77 28.62 24.80 23.66 35.05 29.25 27.07 24.49 38.80 36.32 30.42 26.48 31.75 36.26 116.8 133.8 166.3 223.0

500.2 524.6 510.8 501.0 495.6 527.1 515.7 491.7 488.6 544.4 539.6 537.1 529.3 538.8 538.6 544.4 551.4 544.3 538.6 544.5 545.7 561.8 554.8 550.5 534.8 508.0 525.6 532.7 538.6 508.2 504.8 444.7 437.3 434.3 422.5

128.0 89.2 88.7 78.7 75.5 90.2 96.7 95.8 94.2 104.5 102.5 92.6 88.7 125.0 110.5 101.7 96.4 125.0 112.3 101.2 101.2 116.8 114.6 110.8 97.6 95.4 100.2 98.4 105.8 93.4 93.9 95.0 94.3 97.6 100.4

202.2 243.0 213.7 174.2 153.4 253.8 215.2 210.4 181.8 164.6 183.9 172.8 169.6 215.0 199.1 186.0 181.0 208.7 195.7 195.0 181.6 138.2 170.6 157.0 198.8 243.7 212.5 186.2 170.4 239.5 227.3 377.6 376.0 197.3 189.5

5.85 1.50 1.18 0.68 0.44 1.54 1.25 0.97 0.87 1.83 1.29 1.06 0.98 2.00 1.65 1.40 1.11 2.02 1.66 1.44 1.37 2.03 1.70 1.57 1.42 2.25 2.11 1.76 1.54 1.84 2.10 5.32 7.83 9.31 12.5

81.70 85.50 91.70 94.52 81.17 84.64 88.11 89.31 77.59 84.15 87.02 87.94 75.58 79.83 82.77 86.36 75.34 79.70 82.41 83.22 75.14 79.26 80.80 82.63 72.48 74.24 78.43 81.22 77.48 74.28 17.16 5.10 -

factors supplied in the software. The surface quantification results for iron were correlated with those obtained from solution analysis. The 1 M HCl acidic solutions resulted after inhibitor (AIN and NOP) treatment were analyzed via induced coupled plasma (ICP) in order to detect the soluble iron amount. After XPS measurements, the samples were taken to a whitelight vertical scanning interferometer (MicroXAM MP-8, ADE Phase Shift) equipped with two Mirau objectives. White-light VSI allows a vertical resolution of better than 2 nm level keeping a larger lateral surface in the sub-micrometer range, but it does not require vacuum conditions nor is it destructive and offers real-time data acquisition. Details about the principle and data acquisition of VSI may be found in the literature.31-35 Each sample was measured using a 10× and a 50× objective with a lateral resolution of 1.2 µm and 500 nm, respectively. The 50× objective provides a field of view of 165 µm × 124 µm. A series of 3D height maps was collected for each reaction time. Hyperchem version 7 (Hypercube, Inc.), a quantum-mechanical program, was used for molecular modeling. These molecular orbital calculations are based on the semiempirical self-consistent field method (SCF-MO). A full optimization of all geometrical variables without any symmetry constraints was performed at the restricted Hartree-Fock level (RHF) using the PM3 semiempirical SCF-MO method.

Results and Discussion Tafel and Linear Polarization Resistance (dc) Measurements. The corrosion current densities (icorr), (31) Lasaga, A. C.; Luttge, A. Science 2001, 291, 2400. (32) Luttge, A.; Bolton, E. W.; Lasaga, A. C. Am. J. Sci. 1999, 299, 652. (33) Luttge, A.; Winkler, U.; Lasaga, A. C. Geochim. Cosmochim. Acta 2003, 67, 1099. (34) Arvidson, R. S.; Ertan, E. I.; Amonette, J. E.; Luttge, A. Geochim. Cosmochim. Acta 2003, 67, 1623. (35) Lupu, C.; Arvidson, R. S.; Luttge, A.; Barron, A. R. Chem. Comm. 2005, 18, 2354.

corrosion potential (Ecorr), and βa and βc slopes were obtained by extrapolation of the cathodic and anodic regions of the Tafel plots. The corrosion rate (CR) was calculated from eq 1, where K is a constant that defines the units for the CR, and A, d, and Ew are sample area, density, and equivalent weight of iron, respectively. Inhibitor efficiency (IE) was calculated using eq 2, where i°corr and icorr correspond to current densities of uninhibited and inhibited solutions, respectively. All parameters of the diazoles are given in Table 1. Representative plots are shown in Figure 2 for iron in 1 M HCl without inhibitor and with 10-2 M inhibitor.

CR ) IE )

(

icorrKEw Ad

)

i°corr - icorr × 100 i°corr

(1) (2)

Table 1 shows that both icorr and CR decreased significantly with inhibitor concentration, except with the use of NOP. Current density decreases are evident even at the lowest inhibitor concentration with efficiencies of 7281%. At a concentration of 10-2 M, the inhibiting effectiveness of AIN reaches 94%. Cathodic and anodic slopes decrease with increased inhibitor concentration, suggesting that both anodic dissolution of iron and hydrogen evolution are suppressed (Figure 2). The highest inhibiting effect is seen with AIN and IND. In both cases, corrosion inhibition is under ‘mixed’ control with a slightly more pronounced anodic effect for AIN (Table 1 and Figure 2a). The inhibition effect of imidazole and its derivatives (IMZ, BIM, and MIM, Figure 2b) are

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Babic´ -Samardzˇ ija et al.

Figure 3. Correlation between Tafel corrosion rate and inhibitor concentration for the investigated diazoles AIN (- 9 -), IND (- 0 -), IMZ (- 2 -), BIM (- 3 -), MIM (- [ -), PYR (- × -), and SOP (- + -). Table 2. Polarization Resistance Data of Iron in 1 M HCl in the Absence and Presence of Different Concentrations of N-Heterocycles compound blank AIN

IND

IMZ

BIM

Figure 2. Anodic and cathodic Tafel polarization curves for iron in 1 M HCl and in the presence of 10-2 M (a) AIN and IND, (b) IMZ, BIM, and MIM, and (c) SOP, PYR, and NOP.

all in about the same efficiency range, and their Ecorr is shifted more cathodically. Interestingly, sulfo- and nitrogroup attached to the basic pyrazole (PYR) ring shows opposite effects. SOP and NOP are equally effective inhibitors at 10-4 M. However, SOP improves with concentration while NOP decrease (Table 1 and Figure 2c). Moreover, the corrosion potential is shifted anodically with NOP due to a large increase in βc. Of course, CR increased and inhibiting effect decreased. It is likely a higher concentration of NOP (5 × 10-2 and 0.1 M) would show it to be an accelerator. The effect of inhibitor concentration and Tafel corrosion rate are shown in Figure 3. All seven diazoles demonstrate almost the same decreasing CR trend with concentration. The order of their efficiency is indazole > imidazoles > pyrazoles. The lowest CR (Figure 3) and highest inhibition effect (Table 1) are evident for AIN. In contrast, the inhibitor efficiency of NOP decreased more than 70%. For the same concentration ranges, all the other compounds showed an 8-10% IE increase. Electrochemical corrosion parameters obtained from linear polarization resistance (Rp) measurements in 1 M HCl for these compounds are given in Table 2. Rp was used to calculate icorr for charge-transfer-controlled reaction kinetics for the case of small corrosion potential perturbation (eq 3),36 where βa and βc are anodic and cathodic slopes, respectively. Inhibition efficiency is

MIM

SOP

PYR

NOP

icorr -Ecorr IE Rp CR (Ω‚cm2) (µA‚cm-2) (mV) (mm‚y-1) (%)

concn (M) 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 5 × 10-2 0.1

184 1166 1340 1990 2966 911 952 1122 1253 889 949 1107 1202 855 925 1087 1174 834 900 1072 1105 788 855 932 1030 753 842 889 991 982 801 453 201 160 107

141.8 22.3 19.4 13.1 8.7 28.6 27.3 23.2 20.8 29.3 27.5 23.5 21.6 30.4 28.2 23.9 22.2 31.2 28.9 24.3 23.5 33.0 30.5 27.9 25.3 34.6 30.9 29.3 26.3 26.5 32.5 57.5 129.6 162.8 241.9

485.7 536.8 526.4 519.1 527.6 521.5 523.6 534.0 531.7 536.3 523.6 524.0 536.3 538.1 555.2 543.9 546.9 536.4 535.4 550.8 550.5 526.0 528.7 532.0 538.2 506.2 523.8 532.2 548.3 553.3 524.4 474.5 436.6 429.6 414.6

5.87 1.29 1.13 0.76 0.51 1.64 1.58 1.35 1.21 1.70 1.59 1.36 1.26 1.77 1.63 1.39 1.28 1.81 1.68 1.41 1.37 1.98 1.77 1.62 1.47 2.06 1.79 1.69 1.53 1.54 1.88 3.33 7.52 9.08 13.6

84.3 86.3 90.7 93.8 79.8 80.7 83.6 85.3 79.3 80.6 83.4 84.7 78.5 80.1 83.1 84.4 77.9 79.6 82.8 83.4 76.7 78.5 80.3 82.1 75.6 78.2 79.3 81.5 81.3 77.1 59.4 8.6 -

calculated from eq 4, where R°p and Rp represents polarization resistance for uninhibited and inhibited solution, respectively.

icorr )

(

βaβc

2.303(βa + βc)

(

IE ) 1 -

)

) () ×

R°p × 100 Rp

1 Rp

(3)

(4)

The slow scan through the potential range of (25 mV at open circuit potential gives a linear current response.


Langmuir, Vol. 21, No. 26, 2005 12191 Table 3. Impedance Data of Iron in 1 M HCl and in Presence of Different Concentration of N-Heterocycles compound blank AIN

IND

IMZ

BIM

MIM

SOP

PYR

NOP

Figure 4. Nyquist plots for iron in 1 M HCl and in the presence of 10-2 M AIN (a, 2), IND (a, 0), IMZ (b, 4), BIM (b, 1), MIM (b, 0), PYR (c, 2), SOP (c, 0), and NOP (c, 3) at 50 min immersion time in comparison with 1 M HCl in the absence of additive (a-c, 9).

This slope and intersection gives Rp and Ecorr values, respectively. Rp values increased with increasing inhibitor concentration, except for NOP where polarization resistance decreased (Table 2). Electrochemical parameters, i.e., icorr, CR, and IE, are in satisfactory correlation for both dc methods (Tables 1 and 2), with the same order of inhibitor efficiency obtained by both. EIS (ac) Measurements and Equivalent Circuit Analysis. Impedance measurements were carried out under potentiostatic conditions after 50 min of immersion, and Nyquist plots of uninhibited and inhibited solutions in 1 M HCl are shown in Figure 4. Values of charge-transfer resistance (Rct) were calculated from the difference in impedance at the lower and higher frequencies, Table 3. The Nyquist plots are significantly changed on addition of inhibitors (Figure 4), and except for NOP (Table 3 and Figure 4c), the impedance of the inhibited system increased with inhibitor concentration. The most pronounced effect and highest charge-transfer resistance is for AIN, while NOP actually shows a decrease. Figure 5a shows Rct increase with concentration for all inhibitors studied except NOP (Figure 5b). Double layer capacitance (Cdl) was calculated from Rct and the maximum imaginary component of the impedance ( - Z′′max) (eq 5). The value of the double-layer capacitance depends on many variables including electrode potential, temperature, ionic concentrations, types of ions, oxide layers, electrode roughness, impurity adsorption, etc. (36) Stern, M.; Geary, A. L. J. Electrochem. Soc. 1957, 104, 56.

a

concn (M) 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 5 × 10-2 0.1

Cdl coverage IE Rct Rct (θ) (%) (Ω‚cm2)b (Ω‚cm2)a (µF‚cm-2) 243.8 1226 2134 2834 2989 952.8 1015 1104 1154 905.1 983.6 1048 1123 862.2 966.9 999.6 1056 848.6 897.2 953.0 1015 632.9 865.5 933.1 993.4 614.3 756.7 812.0 895.5 861.1 773.7 366.8 198.4 100.6 92.8

95.8 13.0 10.9 8.2 7.8 16.7 15.7 14.4 13.8 17.6 16.2 15.2 14.2 18.5 16.5 15.9 10.3 18.8 17.7 16.7 10.5 25.1 18.4 17.7 11.0 25.9 21.0 19.6 17.8 18.5 9.6 4.3 2.5 0.5 0.4

0.80 0.89 0.91 0.92 0.74 0.76 0.78 0.79 0.73 0.75 0.77 0.78 0.72 0.75 0.76 0.77 0.71 0.73 0.74 0.76 0.62 0.72 0.74 0.76 0.60 0.68 0.70 0.73 0.72 0.69 0.34 -

80.1 88.6 91.4 91.8 74.4 75.9 77.9 78.9 73.0 75.2 76.7 78.3 71.7 74.8 75.6 76.9 71.3 72.8 74.4 76.0 61.5 71.8 73.9 75.5 60.3 67.8 69.9 72.8 71.7 68.5 33.5 -

576 3518

1715

1368

1328

1290

624

493

191

50 min immersion time. b 500 min immersion time.

Table 3 indicated that by increasing the concentration of diazoles Cdl values tend to decrease and the inhibition efficiency increases. The decrease in Cdl, which results from local dielectric constant decrease and/or an increase in the thickness of the electrical double layer, suggest that these molecules act by adsorption on the metal/ solution interface.37

f(-Z′′max) )

1 2πCdlRct

(5)

The degree of surface coverage for different concentrations of these compounds has been evaluated from EIS measurements too.38 Inhibition efficiency is calculated using eq 6, where R°ct and Rct are charge-transfer resistance values in uninhibited and inhibited solutions.

IE )

1/R°ct - 1/Rct × 100 1/R°ct

(6)

Experiments over longer reaction time show that the inhibition effect increases with immersion time at the highest inhibitor concentration, Table 3. At longer immersion time, they are more effective and, accordingly, corrosion inhibition efficiency increased.39 After a few hours of immersion, significant protection could be (37) McCafferty, E.; Hackerman, N. J. Electrochem. Soc. 1972, 119, 146. (38) Babic-Samardzija, K.; Khaled, K. F.; Hackerman, N. Appl. Surf. Sci. 2005, 240, 327. (39) Cicileo, G. P.; Rosales, B. M.; Varela, F. E.; Vilche, J. R. Corros. Sci. 1999, 41, 1359.

12192



Figure 7. Nyquist plot (a) and Bode-phase plots (b) of iron corrosion in 1 M HCl in the presence of AIN (10-4 M) at Ecorr. Figure 5. Correlation between charge-transfer resistance and inhibitor concentration in 1 M HCl for IND (a, - 9 -), IMZ (a, - 0 -), BIM (a, - 2 -), MIM (a, - 3 -), SOP (a, - [ -), PYR (a, - + -), AIN (b, - 9 -), and NOP (b, - 0 -).

Figure 6. Equivalent circuit model for the studied inhibitors.

achieved because of the increase in Rct (Table 3), suggesting an improvement in the quality of the film with time.40 Charge-transfer resistance values over longer reaction time for the group of indazoles and imidazoles shows enhanced inhibiting effect while for SOP and PYR it decreases. It is indicative that after prolonged immersion time the effect of NOP remains the same, showing its negative corrosion protection. The iron concentration of the solutions containing 10-2 M of the nitrogen heterocycles after longer EIS treatment was determined from induced couple plasma atomic absorption (ICPAA) analysis. Much higher amounts of iron (284 mg‚L-1) for NOP than for AIN (7.56 mg‚L-1) were found, suggesting dissolution of iron (i.e., more rapid corrosion) in the presence of NOP. These values correlate with the CR obtained by electrochemical methods (Tables 1-3). All impedance spectra were measured at the corrosion potential for each inhibitor and concentration. They are analyzed in terms of the equivalent circuit shown in Figure 6. Generally, the semicircular shape is indicative of a charge-transfer process occurring via a parallel capacitor and resistor, Figure 6. A constant phase element, CPE, is used to replace the capacitor.41-44 Here Rs, Rct, and CPE (40) Felhosi, I.; Kalman, E. Corros. Sci. 2005, 47, 695.

are solution resistance, charge-transfer resistance, and constant phase element, respectively. An example of the impedance spectra obtained at Ecorr on iron exposed in 1 M HCl with 10-4 M AIN is presented as Nyquist and Bode phase plots in Figure 7. Both simulated and measured spectra correlate in shape, so the suggested equivalent circuit simulates the reaction well. Table 4 shows a circuit element for all inhibitors studied and concentrations used. Solution resistance in all cases is small. The charge-transfer resistance increases, and the inhibiting power is higher (Tables 3 and 4). A large charge-transfer resistance is associated with a slower corroding system.44 Furthermore, better protection provided by an inhibitor can be associated with a decrease in capacitance (CPE) of the metal.45,46 It is clear that the largest values of Rct are shown by AIN followed by IND and the imidazoles, thus suggesting their enhanced inhibitor performance. With NOP, Rct, and CPE decreased notably, approaching the values for the uninhibited solution. The reason for that might be diffusion, along with desorption of NOP with partially involved adsorption of water47 or electrochemical reduction of nitro group.48 Molecular Structure and Adsorption. Inhibitor efficiency depends on the structure and the chemical properties of the inhibitor being adsorbed. The inhibitor layer has been related to the electronic structure of the molecule.39 The charge and orientation of the inhibitor molecule at the metal surface are important. Molecular modeling and frontier orbital theory might help in predicting the adsorption center of the inhibitor molecule (41) Juttner, K. Electrochim. Acta 1990, 35, 1501. (42) Pajkossy, T. J. Electroanal. Chem. 1994, 364, 111. (43) Fawcett, W. R.; Kovacova, Z.; Motheo, A.; Foss, C. J. Electroanal. Chem. 1992, 326, 91. (44) Khaled, K. F, Electrochim. Acta 2003, 48, 2493. (45) Hleli, S.; Abdelghani, A.; Tlili, A. Sensors 2003, 3, 472. (46) Koka, S.; Shi, A.; Ullett, J. Aerosp. Coat. Removal Forum 2004, 1. (47) Loveday, D.; Peterson, P.; Rodgers, B. JCT Coating Tech. 2004, 1, 88. (48) Tallec, A.; Hazard, R.; Suwinski, J.; Wagner, P. Pol. J. Chem. 2000, 74, 1177.


Langmuir, Vol. 21, No. 26, 2005 12193

Table 4. Circuit Elements Rs, Rct, CPE, and n Values Obtained after Applying the EC Model in Figure 6 compound blank AIN

IND

IMZ

BIM

MIM

SOP

PYR

NOP

concn (M)

Rs (Ω‚cm2)

Rct (Ω‚cm2)

CPE (µS)

n

1 M HCl 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 10-4 10-3 5 × 10-3 10-2 5 × 10-2 0.1

0.25 2.64 2.51 2.47 2.36 2.84 2.66 2.84 2.69 2.50 2.88 3.55 2.63 2.59 2.51 2.78 2.73 2.67 2.84 2.87 2.84 2.55 2.49 2.64 2.71 2.46 2.87 2.78 2.68 2.78 2.47 2.30 2.39 2.73 2.73

243 1238 2062 2650 2971 944 992 1087 1133 836 976 1002 1059 848 891 948 1058 840 888 971 999 621 775 897 982 584 757 779 865 834 698 318 195 91 83

10.51 48.80 25.34 16.03 14.77 60.55 56.79 55.35 46.15 66.73 54.52 53.31 43.63 67.29 51.00 72.36 64.09 64.26 63.60 65.88 56.79 81.87 69.55 67.06 66.94 85.96 72.61 64.20 60.90 74.84 55.09 29.68 10.01 20.17 17.10

0.71 0.78 0.80 0.83 0.83 0.76 0.76 0.77 0.77 0.77 0.76 0.74 0.75 0.73 0.74 0.74 0.75 0.75 0.76 0.76 0.76 0.73 0.74 0.75 0.75 0.72 0.75 0.75 0.76 0.75 0.76 0.81 0.70 0.88 0.87

responsible for principal interaction. For example, an aromatic ring in the molecule is almost always favorable for effective adsorption. On the basis of the results presented above, the best inhibiting properties found here came from bicyclic AIN and IND compounds. This superiority over IMZ, BIM, and MIM presumably stems from enhanced adsorption of the molecule on the iron surface; however, packing and orientation effects cannot be discounted. Our results also suggest that the exocyclic amino group on a pyrol enhances effectiveness, i.e., AIN is more effective than IND (see Tables 1-3). However, a study of a greater range of derivatives would be necessary for a definitive correlation of structure with substitution. With regard to the monocyclic additives, the relative position of the two nitrogen atoms appears to make the most significant difference in inhibition performance. Thus, the imidazole derivatives (IMZ, BIM, and MIM) are better inhibition agents than the pyrazole derivatives (PYR, SOP, and NOP). Both imidazoles and pyrazoles contain two nitrogen atoms: one “pyrolic” the other one “pyridinic” with a free electron pair available to react with H+; however, when two nitrogen atoms are adjacent in the ring, the inhibition efficiency is lower than when there is a carbon atom between them. Also, it is expected that the substituents will change the electronic properties (electron donation of the lone pair or π system) and therefore any interaction with iron. This is because complex formation can stem from the iron d sublevel as an electron acceptor and inhibitor donor atoms. Some of the key quantum chemical parameters were computed using the PM3 method. These are mainly the energies of the highest occupied (HOMO) and lowest

unoccupied (LUMO) molecular orbitals and dipole moment (µ). The difference between HOMO and LUMO levels is defined as the energy gap (∆), which is equal to the energy needed to excite an electron from a highest occupied into a lowest unoccupied MO.27 Generally, the correlation between ∆ values (8.508, 10.164, and 10.219) and inhibitor efficiencies (Tables 1-3) for indazole (IND), imidazole (IMZ), and pyrazole (PYR), respectively, shows that higher efficiency can be related to a lower energy difference, i.e., to those molecules more readily undergoing a chargetransfer interaction with the metal surface. This conclusion is supported by the eq 7 calculation of the number of transferred electrons (∆N),49 where χFe and χinh symbolize the absolute electronegativities of iron and the inhibitor molecule, respectively, while ηFe and ηinh represent the absolute hardness of iron and the inhibitor molecule, respectively. These quantities are related to electron affinity and ionization potential, directly connected to HOMO and LUMO energies.27,49 The number of electrons transferred as calculated for IND, IMZ, and PYR are 0.293, 0.257, and 0.237, respectively. Both energy difference and number of electrons transferred between iron and associated molecule will affect their inhibitor action. Similar results were obtained for a group of nitrogen compounds studied as corrosion inhibitors of carbon steel in sulfuric acid.50

∆N )

(χFe - χinh) 2(ηFe + ηinh)

(7)

Poor inhibitive properties and near acceleration, as found for NOP, are a consequence of its specific electronic structure. The induction constant of the nitro substituent (σ*) is positive, which corresponds to a negative induction effect and electron-accepting function.9 In addition, the dipole moment is very high (5.701 D, PM3 method) and the negative charge density in the molecule is high on the two oxygen atoms. MO calculations give dependable insight into the reason for inhibitor efficiencies found for three basic heterocycles, but they are limited and inconsistent in the case of other derivatives especially for NOP. This difficulty may stem from the disregard of implicit assumptions regarding the corrosion systems, specifically the competition for adsorption on the metal/solution interface.12 Surface Analysis. Further chemical analysis was undertaken with surfaces treated with two of the key inhibitors: AIN and NOP. On the basis of their electrochemical behavior, they represent the two extremes of inhibitor performance. AIN is the best inhibitor, and NOP changes from a mild inhibitor to an accelerator depending on its concentration. XPS analysis was used to investigate the composition of the organic thin film formed on the iron surface in acidic media by the organic inhibitors. VSI was used to investigate morphological changes associated with the chemical changes at the surface of the iron samples. Survey and high-resolution XPS spectra and VSI height maps were obtained for the untreated iron surface, exposed to the 1 M HCl, as well as iron surfaces treated with 10-2 M AIN or NOP after 1 and 18 h immersion in 1 M HCl. Table 5 presents the atomic concentrations obtained by processing the XPS spectra. There are no significant changes in the surface composition for the iron samples treated with 10-2 M AIN after 1 or 18 h immersion (see Table 5). This suggests that under AIN treatment the reaction step involved in the (49) Sastri, V. S.; Perumareddi, J. R. Corrosion 1997, 53, 617. (50) Bilkova, K.; Hackerman, N., unpublished work.

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Figure 8. 3D VSI plots taken using a 10× Mirau objective for unwashed iron electrode surfaces treated with AIN after (a) 1 and (b) 18 h immersion, and NOP after (c) 1 and (d) 18 h immersion (all scales in µm). Table 5. XPS Analysis (atomic %) of Iron Surfaces Exposed to 1 M HCl in the Presence of 10-2 M Additive sample

immersion time (h)

Fe (%)

C (%)

O (%)

N (%)

Cl (%)

blank AIN AIN NOP NOP NOP, washed

n/a 1 18 1 18 18

21.31 19.77 20.44 17.92 19.14 22.84

30.49 29.80 29.47 30.47 28.28 22.27

46.85 44.98 43.68 34.62 33.10 50.98

1.45 1.79 2.16 4.79 0.87

1.35 4.00 4.63 14.82 14.69 3.04

inhibition process occurs in the initial stages of the reaction. The 3D VSI plots shown in Figure 8a and b support this observation. The absolute surface roughness values of the samples exposed to AIN for 1 and 18 h are very similar (0.27 and 0.28 µm, respectively). There is a clear indication of iron dissolution in both cases that is supported by the ICP data that suggests a release of 7.56 mg‚L-1 Fe2+ ions in the solution after the AIN treatment. We propose that the induced surface roughness caused by iron dissolution is the catalyst for the reaction of the exposed surface with the AIN molecules and the formation of a thin organic film (absorbate) on the surface. The nitrogen content of the iron samples treated with 10-2 M NOP increases significantly between 1 and 18 h (Table 5), suggesting an increase in the amount of nitrogen heterocycle absorbed/reacted with the iron surface (see below). Figure 8c and d for these surfaces show dramatic topographical changes. The associated absolute surface roughness values (0.38 vs 5 µm) are consistent with the topological images. Thus, it would appear that exposing the surface to longer immersion in the NOP-containing solution dramatically increased corrosion with a concomitant increase in iron dissolution (284 mg‚L-1). Given the similarity of results for AIN and NOP for surfaces treated for 1 h but the disparity between the AIN and NOP for 18 h treatments, we have investigated the difference in chemical speciation on the surface under the latter’s conditions. Significant difficulties were found associated with the deconvolution of the Fe 2p peak shape with a single

symmetric Gaussian/Lorentzian function due to the strong multiplet splitting that results in an asymmetric peak shape.51-61 We therefore chose the Fe 3p region to obtain information of the iron chemical environment. The O 1s peaks were used as a corroboration of the Fe 3p peak assignments, while the N 1s was used to confirm the presence of the inhibitor on the surface. The Cl 2p and C 1s peak shapes seems to be essentially unaffected by the organic inhibitors; only changes in the intensities are observable. The high-resolution peaks for Fe 3p, O 1s, and N 1s for surfaces treated by 10-2 M AIN or NOP for 18 h are shown in Figure 9. The deconvolution of the Fe 3p spectra for the treated surfaces may be fitted to three main peaks (see Table 6). For AIN, two additional minor components (48.94 and 50.71 eV) appear to be associated with a reduced metallic surface (see below). The main peaks may be assigned as being due to iron in environments associated with FeOOH (i.e., oxyhydroxides), Fe2O3 (i.e., Fe3+ oxide), and Fe3O4 (i.e., Fe2+/Fe3+ mixed oxide). As may be seen for Figure 9 and Table 6, the difference between the AINand NOP-treated surfaces is that the latter has a significant increase in the amount of hydroxide species at the expense of Fe3+ oxides. The O 1s spectra show peaks due to Fe2+ oxides,62 Fe3+ oxides, hydroxides,51,62 and absorbed water and/or ClO-, with a significant increase in the hydroxide content of the NOP treated surfaces. The O 1s spectra support the observation that the NOP-treated samples have significantly more hydroxide than the AIN samples. As far as N 1s peaks are concerned, three components located at 398.90, 400.28, and 401.46 eV were used to fit the spectrum. For the AIN samples, the first component is assigned to the unprotonated N atom in the pyrazole ring,63-66 while the second component has the highest contribution and is mainly attributed to the nitrogen of the pyrazole ring (-NH-).66 The third component is a result of the protonation of the nitrogen of the pyrazole ring, which due to the increase of the positive charge on the nitrogen atom, a core-level chemical shift to higher binding energy is produced.67 Molecular AIN is expected to have a ratio of 1:1 for protonated/unprotonated nitrogen; however, from Figure 9e, it can be seen that the peaks for the protonated are larger than the unprotonated, suggesting that a fraction of the AINs are protonated at the pyrazole nitrogen. It is expected that an organic nitro compound (RNO2) should show a peak for the N 1s between 404 and 405 eV; however, (51) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (52) Gupta, R. P.; Sen, S. K. Phys. Rev. B 1974, 10, 71. (53) Wandelt, K. Surf. Sci. Rep. 1982, 2, 1. (54) Allen, G. C.; Tucker, P. M.; Wild, R. K. Philos. Mag. B 1982, 46, 411. (55) Mills, P.; Sullivan, J. L. J. Phys. D 1983, 16, 723. (56) Kurtz, R. L.; Henrich, V. E. Surf. Sci. 1983, 129, 345. (57) Allen, G. C.; Harris, S. J.; Jutson, J. A.; Dyke, J. M. Appl. Surf. Sci. 1989, 37, 111. (58) Kuivila, C. S.; Butt, J. B.; Stair, P. C. Appl. Surf. Sci. 1988, 32, 99. (59) Welsh, I. D.; Sherwood, P. M. A. Phys. Rev. B 1989, 40, 6386. (60) Mansour, A. N.; Brizzolara, R. A. Surf. Sci. Spe. 1998, 4, 345. (61) Fujii, T.; de Groot, F. M. F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. Phys. Rev. B 1999, 59, 3195. (62) Allen, G. C.; Curtis, M. T.; Hooper, A. J.; Tucker, P. M. J. Chem. Soc., Dalton Trans. 1974, 1526. (63) Nordberg, R.; Albridge, R. G.; Bergmark, T.; Ericson, U.; Hedman, J.; Nordling, C.; Siegbahn, K.; Lindberg, B. J. Arkiv Kemi 1968, 28, 257. (64) Lindberg, B. J.; Hedman, J. Chem. Scr. 1975, 7, 155. (65) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1969, 8, 2642. (66) Zhou, W. P.; Baunach, T.; Ivanova, V.; Kolb, D. M. Langmuir 2004, 20, 4590. (67) Schick, A. G.; Sun, Z. Langmuir 1994, 10, 3105.


Langmuir, Vol. 21, No. 26, 2005 12195

Figure 9. The XPS deconvoluted profiles for Fe 3p (a and b), O 1s (c and d), and N 1s (e and f) for iron surfaces treated with 10-2 M AIN (a, c, and e) and NOP (b, d, and f) in 1 M HCl, immersed for 18 h. Table 6. Binding Energies (eV), Their Assignment, and Relative Intensity for the Major Core Lines Observed for the Reacted Iron Electrode Surfaces binding compound element energy (eV) AIN

Fe 3p O 1s

N 1s NOP

Fe 3p O 1s

N 1s

53.98 55.35 57.15 529.98 530.40 531.83 533.14 398.89 400.28 401.68 54.14 55.23 57.38 529.98 530.40 531.83 533.50 399.42 400.28 401.46

assignment

relative abundance (%)

FeOOH Fe2O3 Fe3O4 FeO Fe2O3 FeOOH H2O/ClON N-H+ N-H+ FeOOH Fe2O3 Fe3O4 FeO Fe2O3 FeOOH H2O/ClON N-H+ (NO2) N-H+

5 78 17 27 33 35 5 40 53 7 34 50 16 18 20 58 4 31 55 14

no significant peak is observed in Figure 9f. Absent evidence for cleavage of the NOP molecule, we propose that the nitro group is shifted underneath the peak at 400.28 eV. The relative increase in this peak’s intensity is consistent with this proposal. The C 1s peaks for both samples show a series of peaks consistent with C-C, C-H,

CdC, C-N, CdN, and/ or C-OH type bonding on the surface.68,69,70 In summary, the XPS analysis of the AINand NOP-treated surfaces indicate absorption/reaction with the nitrogen heterocycles for both inhibitors but significant hydroxide content for the NOP treated sample. The N 1s spectrum for the AIN-treated sample suggest that protonation of the AIN occurs at the surface. The detection of significant soluble iron for the NOPtreated sample (see above) and the far rougher surface suggest that a soluble Fe-NOP species forms and may be readily removed from the surface, exposing fresh surface for further corrosion. The electrochemical results show that 10-4 M NOP acts as an inhibitor (∼77.5%), but at 10-2 M, it accelerates the process, suggesting that higher concentrations of NOP result in the formation of a moresoluble Fe-NOP complex. If this is true, then the use of higher concentrations of NOP should result in rougher surfaces (more corrosion) after washing the NOP-treated surface. Figure 10 shows the 3D VSI plots for iron treated surface with 10-2 and 10-4 M NOP after 18 h of immersion followed by vigorous washing before drying in a vacuum. As predicted, the surface treated with a higher concentration of NOP shows significant increase in the roughness (68) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer, Physical Electronics: Eden Prairie, MN, 1978; pp 38-76. (69) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers, The Scienta ESCA 300 Database; Wiley: Chichester, 1992. (70) Briggs, D.; Seah, M. P., Eds. Practical Surface Analysis; Wiley: Chichester, 1983.

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Figure 10. 3D VSI plots taken using a 50× Mirau objective for iron electrode surfaces treated with NOP after 18 h immersion time and surface washing in (a) 10-4 and (b) 10-2 M (all scales in µm).

Figure 11. The XPS deconvoluted profiles for Fe 3p, O 1s, and N 1s for iron surfaces treated with 10-2 M NOP and immersed for 18 h (a) before and (b) after washing with DI water.

(i.e., 0.9 µm for the 10-2 M NOP vs 0.7 µm 10-4 M NOP) consistent with the formation and removal of more of the Fe-NOP complex. The XPS analysis of the 10-2 M NOP-treated surface before and after washing (Table 5) shows a dramatic decrease in the nitrogen and chlorine content. The highresolution peaks for Fe 3p and O 1s for surfaces treated by 10-2 M NOP for 18 h and then washed are shown in Figure 11. The deconvolution of the Fe 3p spectra for the washed surface (Figure 11a) shows that in comparison with the unwashed surface (Figure 9b) there is a dramatic decrease in the iron-hydroxide content. In addition, there is exposure of metallic iron, as indicated by peaks at 52.18 and 49.03 eV. The O 1s spectrum (Figure 11b) also shows a decrease in the hydroxide concentration. These results,


in combination with the observation of significant iron content of the soluble fraction, suggest that the soluble species is an iron-hydroxide-chloride complex of NOP. Metal complexes of NOP derivatives have been crystallographically characterized and show that complexation occurs via the unprotonated nitrogen.71,72 Although not reported for NOP, there are many isolable Fe-OH and Fe-Cl complexes with nitrogen heterocycle ligands.73-75 We propose therefore, that NOP reacts with the iron surface in the presence of HCl to form a soluble [FeClx(OH)y(NOP)z]n complex that is readily removed from the metal surface. This removal of a soluble complex results in exposure of fresh surface and facilitates continued corrosion. Conclusions Tafel extrapolation, linear polarization resistance, and electrochemical impedance spectroscopy measurements have been used to study iron corrosion in 1 M HCl and the inhibiting effects of some diazole-type compounds. Analysis of the experimental data suggest that, except for NOP, all the heterocyclic compounds show fair to good inhibiting properties that increases with inhibitor concentration. The inhibitor efficiency in 1 M HCl increases according to the order: AIN > IND > IMZ > BIM > MIM > SOP > PYR . NOP. The results obtained via molecular modeling indicate that for a few of the compounds energy difference and number of electrons transferred between iron and inhibitor are possible indicators; however, this is not yet a reliable prediction. On the basis of XPS, ICPAA, and VSI analyses, we propose that a good inhibition agent should form an insoluble complex or surface species with low hydroxide content on the surface. This would follow the proposal that positively charged ions such as a protonated nitrogen heterocyclic compound might adsorb onto the metallic surface via the negatively charged acid anions (e.g., Cl-) rather than form a metal-inhibitor complex. While the exact nature of a good inhibition agent is still under investigation, we can predict what makes a poor inhibition agent. The formation of a soluble complex between the inhibition agent (e.g., NOP) and iron hydroxide species is clearly detrimental to the protection of the surface from continued corrosion. Acknowledgment. This article is dedicated to the memory of our colleague Richard E. Smalley (1943-2005). The authors are pleased to acknowledge the financial support provided by the Robert A. Welch Foundation, National Science Foundation, and Halliburton Energy Services. Professor Raphael G. Raptis, University of Puerto Rico, is especially acknowledged for his suggestions and for synthetic pyrazolate derivatives. LA051766L (71) Ardizzoia, G. A.; Brenna, S.; LaMonica, G.; Maspero, A.; Mashciocchi, N.; Moret, M. Inorg. Chem. 2002, 41, 610. (72) Cano, M.; Heras, J. V.; Gallego, M. L.; Peries, J.; Ruiz-Valero, C.; Pinilla, E.; Torres, M. R. Helv. Chim. Acta 2003, 86, 3194. (73) Boudalais, A. K.; Lalioti, N.; Spyroulias, G. A.; Raptopoulou, C. P.; Terzis, A.; Tangoulis, V.; Perlepes, S. S. J. Chem. Soc., Dalton Trans 2001, 955. (74) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 2666. (75) Chu, D.-Q.; Xu, J.-Q.; Cui, X.-B.; Duan, L.-M.; Wang, T.-G.; Tang, A.-Q.; Xing, Y.; Lin, Y.-H.; Jia, H.-Q. Mendeleev Commun. 2001, 66.

Inhibitive Properties and Surface Morphology of a Group of

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