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Electrochemical and Surface-Enhanced Raman Spectroscopy Studies on Inhibition of Iron Corrosion by Benzotriazole Peigen Cao,† Renao Gu,*,† and Zhongqun Tian‡ Department of Chemistry, Suzhou University, Suzhou 215006, P. R. China, and State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, P. R. China Received January 23, 2002 The different initial interactions of benzotriazole (BTAH) with iron in both 0.5 M H2SO4 and simulated saline water (3.4% NaCl) have been investigated systematically by using a confocal microprobe Raman system, surface-enhanced Raman spectroscopy (SERS), and electrochemical potentiodynamic polarization curves. The SERS spectra of BTAH at the bare iron electrodes in both 0.5 M H2SO4 and simulated saline water have been obtained successfully for the first time. Electrochemical measurements show that the inhibition efficiency of BTAH is higher in saline water than that in acid solution, which is in accordance with the identification of the surface interaction on the basis of the SERS data. BTAH may be physisorbed onto the iron surface through its neutral molecule form or protonated BTAH2+ ions. While in saline water, the formation of the surface coordination compound characterized as Fe(II)-BTA may contribute to the outstanding spectral features. A cleavage of the NH bond is believed to occur while adsorption of BTAH onto iron proceeds. This more compact surface film results in a higher inhibition efficiency in comparison with that in an acid solution.
Introduction Prevention of corrosion of metal and nonmetal materials and products in various environments is of great importance in industry. Inhibition, by adding any material in small quantities that reduces the corrosion rate of a metal significantly, is one of the generally adopted preventive methods. The process of inhibition can be distinguished between interface and interphase types proposed by Lorenz and Mansfeld.1 The factors that influence the inhibition efficiency of an inhibitor mainly involve the nature of itself and the conditions of environments.2 In particular, the structure, composition, and nature of an inhibitor play an important role in many cases. For instance, in a given range the longer the hydrocarbon chain is, the higher inhibition efficiency can be observed on iron by some sulfides3 and mercaptans.4 The influences of environment include those of the aggressive conditions of temperature, pressure, and pH value of the corrosion media, etc. These factors make the corrosion system complicated. So the exact inhibition mechanisms are still unclear in many cases. Another important reason for this lack of understanding is a lack of knowledge of the metalinhibitor interface structure (i.e., the strength and type of adsorption/bonding, how the bond is altered with solution pH, applied potential, etc.) To get detailed and direct vibrational information reflecting the surface bond, adsorption type, and their * To whom all the correspondence should be addressed. Tel: 86+512+5112813. Fax: 86+512+5231918. E-mail: ragu@suda. edu.cn. † Suzhou University. ‡ Xiamen University. Tel: 86+592+2181906. E-mail: zqtian@ xmu.edu.cn. (1) Lorenz, W. J.; Mansfeld, F. Proceedings of the International Conference on Corrosion Inhibition; Hausler, R. H., Ed.; NACE: Houston, TX, 1987; p 7. (2) Trabanelli, G. Corrosion 1991, 47, 410. (3) Zucchi, F.; Trabanelli, G.; Gullini, G. Electrochim. Met. 1966, III, 407.
changes with outer factors, in situ techniques including spectroelectrochemical measurements are favorable. Unfortunately, up to now there have been few techniques which have the required sensitivity to directly investigate the interaction of inhibitors with a metal surface.5 Surface-enhanced Raman spectroscopy (SERS) provides in situ vibrational spectra with a high sensitivity for species adsorbed on metal electrodes such as silver, copper, and gold in an aqueous solution.6,7 This in situ technique can provide information about the chemical identity, structure, and bonding characteristics of species at the metal-solution interface. For instance, Kester and coworkers first utilized the SERS effect to study the structure of benzotriazole (BTAH) on copper.8 Direct evidence of BTA-Cu interactions has been obtained. However, numerous studies have been limited to copper and silver since only on these noble metals mentioned above can the giant enhancing effect (105-106) be observed. Few articles are documented on the inhibition of iron, nickel, and other transition metals using in situ SERS technique. Nevertheless, researchers have spared no effort extending the SERS technique to these transition metals during the last two decades. Several methods have been undertaken to obtain vibrational information about ironinhibitor interactions. For instance, the SERS spectra of a small number of inhibitors have been obtained from iron films electrodeposited onto a SERS-active silver substrate on the basis of the electromagnetic enhancement (4) Trabanelli, G.; Carassiti, V. Mechanism and Phenomenology of Organic Inhibitors. In Advances in Corrosion Science and Technology; Fontana, M. G., Staehle, R. W., Eds.; Plenum Press: New York, 1970; Vol. 1, p 147. (5) Melendres, C. A. Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry; Gutirrez, C., Melendres, C. A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; p 181. (6) Otto, A.; Mrozek, I.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (7) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241. (8) Kester, J. J.; Furtak, T. E.; Bevelo, A. J. J. Electrochem. Soc. 1982, 129, 1716.
10.1021/la025570m CCC: $22.00 © 2002 American Chemical Society Published on Web 08/21/2002
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mechanism.9-14 Aramaki and co-workers used this method to examine the adsorption on iron of sulfide,15 which was also investigated by Xue et al.,16 pyridine,14 propargyl alcohol,17 benzenethiol,18 aniline, and their derivatives.13 Devine et al. extended this method to study some inhibitors on stainless steel.19 Another method used is to deposit SERS-active metals onto iron,20-24 which has been undertaken by Devine’s group to study the structure of the passivation on iron and carbon steel.20,21 However, the former method is limited to metals that can be electrodeposited successfully; the latter also suffers the problem that the inhibitors would be adsorbed onto silver or even the edge of the two metals. So a more reliable method seems to obtain the surface Raman spectra directly from the bare iron or other transition metals. Recently, using a high-sensitivity confocal Raman system (LabRam I, Dilor Inc.) and a proper surfaceroughening procedure made this come to truth. For example, the SERS spectra of pyridine and pyrazine have been obtained from bare nickel and iron surfaces.25 The SERS spectra of thiocyanate have also been observed at a roughened iron electrode.26 In this work, the SERS spectra of benzotriazole adsorbed onto a roughened polycrystalline iron electrode were measured in simulated saline water (3.4% NaCl) and 0.5 M sulfuric acid. Polarization curves in the same systems were also measured by contrast, and the results were reviewed on the basis of the SERS data. Different adsorption types of BTAH on iron in the two systems were discussed. Experimental Section The working electrode was a polycrystalline iron rod (99.99%) embedded in a Teflon sheath with a geometric surface area of 0.1 cm2. Before the roughening pretreatment, the electrode was first mechanically polished successively with 0.5 µm aluminum powder down to 0.05 µm to give a mirrorlike surface. After polishing, the electrode was washed sequentially with ethanol and ultrasonic cleaning with Milli-Q water. The electrode was then rinsed several times with Milli-Q water. For Raman measurements, the roughening procedure was to apply an oxidationreduction cycle on iron, which produced nanoscale and atomic roughness for obtaining the SERS spectra. The detailed roughening method was described previously.27 All the potentials in this paper are quoted versus a saturated calomel electrode (SCE). (9) Leung, L. W.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113. (10) Feilchenfeld, H.; Gao, X.; Weaver, M. J. Chem. Phys. Lett. 1989, 161, 321. (11) Gu, R. A.; Deng, Z. F.; Yao, J. L.; Sun, R. Spectrosc. Spectral Anal. 1997, 17, 15. (12) Mengoli, G.; Musiani, M.; Fleischmann, M.; Mao, B. W.; Tian, Z. Q. Electrochim. Acta 1987, 32, 1239. (13) Uehara, J.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1990, 137, 2677. (14) Aramaki, K.; Ohi, M.; Uehara, J. J. Electrochem. Soc. 1992, 139, 1525. (15) Ohno, N.; Uehara, J.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 2512. (16) Li, T. F.; Lu, Y.; Xue, G. Appl. Spectrosc. 1997, 51, 804. (17) Aramaki, K.; Fujioka, E. Corrosion 1996, 52, 8. (18) Uehara, J.; Aramaki, K. J. Electrochem. Soc. 1991, 138, 3245. (19) Oblonsky, L. J.; Chesnut, G. R.; Devine, T. M. Corrosion 1995, 51, 891. (20) Gui, J.; Devine, T. M. J. Electrochem. Soc. 1991, 138, 376. (21) Oblonsky, L. J.; Devine, T. M.; Ager, J. W.; Perry, S. S.; Mao, X. L.; Russo, R. E. J. Electrochem. Soc. 1994, 141, 3312. (22) Dong, J.; Sheng, Z.; Xue, G. Chem. Phys. Lett. 1994, 231, 183. (23) Gao, Q.; Li, F.; Lu, Y.; et al. Spectrosc. Lett. 1998, 31, 167. (24) Li, F.; Lu, Y.; Xue, G. Chem. Phys. Lett. 1997, 264, 376. (25) Gu, R. A.; Cao, P. G.; Yao, J. L.; Ren, B.; Tian, Z. Q. J. Electroanal. Chem. 2001, 505, 95. (26) Cao, P. G.; Yao, J. L.; Zheng, J. W.; Ren, B.; Gu, R. A.; Tian, Z. Q. Proceedings of the XVIIth International Conference on Raman Spectroscopy; Aug 20-25, 2000, Beijing, P. R. China; p 290. (27) Cao, P. G.; Yao, J. L.; Ren, B.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 2000, 316, 1.
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Figure 1. Typical potentiodynamic polarization curves for iron electrode in 0.5 M H2SO4 containing various concentrations of BTAH. Scan rate: 1 mV/s. Solutions were prepared using Milli-Q water. All the chemicals were of analytical grade and were used as received without further purification. Electrochemical measurements were carried out using an EG&G Princeton Applied Research (PAR) model 273 potentiostat. Model 352 software was used to measure the potentiodynamic polarization curve. To obtain the corrosion current (Icorr) of iron, the weak polarization curve was measured by scanning over the range of Ecorr ( 30 mV at 1.0 mV/s. Before the anodic and cathodic polarization curves were recorded, the iron electrode was maintained at the corrosion potential for ca. 20 min. To avoid the influences caused by anodic polarization, the measurements were carried out in sequence of weak, cathodic, and anodic polarization in aerated solutions. The initial potential and the scan rate is Ecorr and 1.0 mV/s, respectively. Raman measurements were performed on a confocal microprobe Raman system (LabRam I from Dilor). The excitation line was 632.8 nm from an internal He-Ne laser with the power of 12 mW on the sample surface. The slit and pinhole used were 200 and 800 µm, respectively. With a holographic notch filter and a CCD detector, it has an extremely high detecting sensitivity. A 50× long working-length objective (8 mm) was used so that it was not necessary for the objective to be immersed in the solution. A detailed description of the Raman system has been described elsewhere.28
Results and Discussion Electrochemical Measurements. Figure 1 represents the potentiodynamic polarization curves of iron in 0.5 M H2SO4 without and with various concentrations of BTAH. The set of polarization curves is reproducible. A reduction in the corrosion rate of a different degree is observed in the presence of BTAH. It can be found from Figure 1 that BTAH suppresses both cathodic and anodic reactions of iron corrosion suggesting a mixed-type inhibitor of BTAH. The corresponding weak polarization curves are not presented here. The dynamic parameters of the corrosion potential (Ecorr) and the corrosion current (Icorr) calculated on the basis of the weak polarization data are listed in Table 1. It should be pointed out that the variation of Ecorr is less than 30 mV, suggesting a weak perturbation of inhibitor to the iron electrode surface. Inhibition efficiency η, which is defined by
η ) 1 - Icorr(2)/Icorr(1) where Icorr(1) and Icorr(2) refer to corrosion current densities in the absence and presence of BTAH, respectively, was then calculated, and they are listed also in Table 1. One (28) Tian, Z. Q.; Ren, B.; Mao, B. W. J. Phys. Chem. B 1997, 101, 1338.
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Figure 2. Typical potentiodynamic polarization curves of iron in 3.4% NaCl in the absence and presence of BTAH. Scan rate: 1 mV/s.
Figure 3. Variation of inhibition efficiency (η) for iron with the concentrations of BTAH in 0.5 M H2SO4 and 3.4% NaCl, respectively.
Table 1. Concentration Effect of BTAH on the Corrosion Potential (Ecorr), Corrosion Current Density (Icorr), and Inhibition Efficiency (η) in 0.5 M H2SO4 C (M)
Ecorr (mV)
Icorr (mA/cm2)
η (%)
1.024 × 10-5 3.072 × 10-5 6.144 × 10-5 1.024 × 10-4 3.072 × 10-4 6.144 × 10-4 1.024 × 10-3 3.072 × 10-3 6.144 × 10-3 1.024 × 10-2
-525 -523 -518 -522 -523 -522 -512 -503 -501 -499 -497
1.411 1.289 1.082 1.012 0.932 0.507 0.245 0.154 0.173 0.256 0.306
8.7 23.3 28.3 33.9 64.1 82.6 89.1 87.7 81.9 78.3
can see from Table 1 that the corrosion current decreases with the increase in concentration of BTAH, while the inhibition efficiency η increases and reaches a maximum value (89.1%) at the 1.024 × 10-3 M level. This phenomenon is consistent with results obtained for other azole compounds in acidic media and is attributed to the dissolution of the complex protective film.29 As we know, BTAH can be adsorbed on the iron surface by the formation of an iron-nitrogen coordinate bond, by a π-electron interaction between an aromatic ring and iron substrate, or by an electrostatic interaction between a negative charged iron surface and a protonated form of BTAH in acidic solution. Applying the potential negative to the potential of zero charge (pzc) and the existence of specific adsorbed anion on the surface both provide a negative charged surface. The adsorption type of BTAH in acid solution will be discussed in the latter part of this paper on the basis of Raman measurement results. Figure 2 presents a set of typical polarization curves of iron in simulated saline water (3.4% NaCl). BTAH acts to suppress the iron dissolution far more than it does the cathodic oxygen reduction indicating that it is an anodictype inhibitor. The results of the comparison of inhibition efficiency η of BTAH in both acidic and neutral NaCl solutions are shown in Figure 3. It is easy to find out that η in saline water is larger than that in acid solution and it is favorable at a low-concentration level of the inhibitor. Another typical change from Figure 2 is that the corrosion potential moves positively far more than it does in sulfuric acid. These results may suggest an existence of different inhibition mechanisms of BTAH in different systems. That (29) Gupta, P.; Chaudhary, R. S.; Namboodhiri, T. K. G.; Prakash, B. Br. Corros. J. 1982, 17, 136.
Figure 4. Normal Raman spectrum of benzotriazole in 0.1 M BTAH aqueous solution.
is to say, BTAH may just be physisorbed onto the iron surface by electrochemical static interaction in acidic solution. It is so weak as to give a relatively low inhibitive efficiency especially in the low-concentration region. While in saline water, a formation of surface film may occur during adsorption, which accordingly alters the corrosion potential greatly. This will be further confirmed in the following in situ Raman tests. Normal Raman spectra of BTAH and SERS Spectra in Sulfuric Acid. A normal Raman spectrum of BTAH in aqueous solution in the frequency range 2001800 cm-1 is represented in Figure 4. The tentative assignments of bands are listed in Table 2 together with the assignments of the surface Raman spectra on iron from saline water and sulfuric acid, respectively.8,30-33 Before discussing the results in detail, we will briefly review the previous work by other research groups on the inhibition of copper by BTAH. It is necessary to provide some background concerning the characterization of surface species formed during the corrosion of copper and its alloy exposed to corrosive media. The BTAH-Cu corrosion inhibitor system has been investigated by several research groups over the past two decades using the SERS spectra obtained with visible or near-infrared exciting radiation.8,30,34 The inhibition mech(30) Rubim, J. C.; Gutz, I. G. R.; Sala, O.; Orville-Thomas, W. J. J. Mol. Struct. 1983, 100, 571. (31) Hope, G. A.; Schweinsberg, D. P.; Fredericks, P. M. Spectrochim. Acta 1994, 50A, 2019. (32) Tornkvist, C.; Thierry, D.; Bergman, J.; Liedberg, B.; Leygraf, C. J. Electrochem. Soc. 1989, 136, 58. (33) Sylvia, L. F. A.; Rubim, J. C.; Agostinho, S. M. L. J. Electroanal. Chem. 1987, 259, 220. (34) Musiani, M. M.; Mengoli, G.; Fleischmann M.; Lowry, R. B. J. Electroanal. Chem. 1987, 217, 187.
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Table 2. Proposed Assignmentsa of the Raman Spectra of Benzotriazole in an Aqueous Solution and the SERS Spectra of Benzotriazole Adsorbed on Iron in Saline Water and 0.5 M H2SO4 at Potentials Indicated iron/0.5 M H2SO4
b
iron/3.4% NaCl
assgntb
BTAH/aq
Fe(II)-BTA complex
-0.5 V
-0.9 V
-0.5 V
-0.9 V
tz ring torsion tz ring bend bz ring breathing in plane trigonal breathing NH in plane bend NH in plane bend tz ring breathing CH in plane bend bz ring stretch tz ring stretch bz ring stretch
536m 627m 780vs 1014s 1126m 1148vw 1219m 1278m 1371vs 1385s 1595m
561m 640w 792s 1049s
548m 633w 786s 1020m 1124vs 1152m 1215m 1271w 1371vs 1383vs 1591m
545m 631w 785s 1018s 1124vs 1152m 1218w 1272m 1369vs 1382vs 1592m
560m 632w 790s 1035s
559m 635m 790s 1023s
1193s 1291m
1143m 1189s 1290m
1389vs 1574m
1388vs 1575m
1149m 1199s 1287m 1384vs 1574m
a Wavenumbers (in cm-1) followed by superscripted relative intensities (vs, very strong; s, strong; m, medium; w, weak; vw, very weak). Abbreviations: bz, benzene; tz, triazole. c oc: potential of open circuit.
Figure 5. Potential-dependent SERS spectra of BTAH adsorbed on iron electrode in 0.01 M BTAH + 0.5 M H2SO4. Laser line: 632.8 nm. Acquisition time: 100 s. Accumulations: 2.
anisms of BTAH for copper mainly involve two types. One is formation of polymeric complexes with cuprous ion, [CuI(BTA)]n, which is predominant in the neutral solution, and the other is the adsorption of BTAH either by its molecular form or by the protonated form BTAH2+ onto the Cu surface30,35-37 from acidic solutions. To be brief, more negative potential, low pH value, and low concentration of BTAH are favorable for the adsorption mechanism but unfavorable for the complex formation, which can be deduced clearly by the equilibrium below proposed by Aramaki and co-workers.38
n(BTAH)ads + nCu ) [Cu(BTA)]n + nH+ + neIn their study of the adsorption of BTAH and 4-OH-BTAH on copper, the transformation from “adsorption spectrum” to the “complex spectrum” was observed when increasing potentials to more positive values.38 What will occur on the iron surface? Figure 5 presented a set of potential-dependent SERS spectra of BTAH (35) Cotton, J. B.; Scholes, I. R. Br. Corros. J. 1967, 2, 1. (36) Poling, G. W. Corros. Sci. 1970, 10, 359. (37) Thierry, D.; Leygraf, C. J. Electrochem. Soc. 1985, 132, 1009. (38) Youda, R.; Nishihara, H.; Aramaki, K. Electrochim. Acta 1990, 35, 1011.
adsorbed on a roughened iron electrode in 0.5 M H2SO4. To the best of our knowledge, it might be the first time the SERS spectra of BTAH were obtained directly from a bare iron electrode surface, since the pioneering work by Knob and colleagues showing that BTAH is also a good inhibitor for iron.39 It is necessary to point out here that the success in obtaining the SERS spectra depended on both the high-sensitivity confocal Raman system and the proper surface-roughening procedure which induces a SERS effect. The calculated surface enhancement factor was of the order of 102-103 times.26 Because the shift of the maximum intensity potential on the Raman intensitypotential curve using the Fe/pyridine adsorption system is slight, i.e., less than 70 mV when the laser line changes from 632.8 to 514.5 nm,26 and it is also impossible to excite the plasmon resonance of iron in the visible light range, the lightning rod model proposed by Gersten and Nitzan40 hence was used to explain the SERS phenomenon on iron.26 Although the SERS effect from iron electrodes is still not unambiguous and more systematic investigation both experimentally and theoretically is required for deeper understanding, the application of the Raman technique to the corrosion inhibition of bare iron electrodes without inducing other SERS-active metals which may contaminate Fe surfaces is feasible now. In our work, the SERS spectrum obtained for BTAH in 0.5 M H2SO4 at an iron electrode (Figure 5) is highly resolved, and it is significantly different from the normal Raman spectrum. Some spectral features can be found by comparing Figures 4 and 5 carefully: (i) The bands assigned to the triazole portion of BTAH molecule (see Table 2) are enhanced in intensity more greatly than that assigned to the benzene ring. (ii) Most bands of BTAH change little in frequency after adsorption onto the iron electrode surface. (iii) The most striking feature in the SERS spectrum is the appearance of the greatly enhanced or possible new bands centered at 1124 and 1152 cm-1 (Figure 5, -0.5 V). The relative greater changes to the triazole ring indicate that there must be a significant change in this part of the molecule. The BTAH molecule may interact with the iron surface through the triazole ring portion rather than the benzene portion. A detailed analysis of the 1124 and 1152 cm-1 bands may help us establish the adsorption mode of BTAH in acidic solution. In the literature, various assignments in this region existed and there is still no consensus up to now. The assignments have been assigned variously to δ(C-H) (39) Asakura, S.; Lu, C. C.; Nobe, K. J. Electrochem. Soc. 1974, 121, 1276. (40) Gersten, J.; Nitzan, A. J. Chem. Phys. 1980, 73, 3023.
Inhibition of Iron Corrosion by Benzotriazole
Figure 6. Dependence of the relative intensity of the 979, 1124, and 1152 cm-1 bands at an iron electrode in 0.5 M H2SO4 on the applied potentials.
combination for the BTAH solid30 and also a triazole mode.36 The study by Rubim et al.30 on copper inhibition by BTAH in acidic KCl solution assigned a band at 1115 cm-1 to δ(N-H) which was caused by the formation of the surface complex [CuIClBTAH]4. In this work, the formation of the surface complex is not considered because of the lack of chloride ion. Further studies by Rubim et al.30 pointed out that the spectral feature in this region is vibration sensitive to the coordination to the triazole ring. As we know, there are mainly three forms of BTAH in an aqueous solution: BTA-; BTAH; BTAH2+. The concentration of BTAH2+ increases apparently with the lower pH value of the solution. This would indicate that the BTAH molecule might be in equilibrium with the protonated BTAH2+ in the electrochemical double layer region, which modifies the spectral features. Furthermore, observations of the lack of enhancement of other benzene C-H features in the SERS spectrum indicates that the bands at 1124 and 1152 cm-1 are not related to δ(C-H). For instance, the C-H in-plane bending mode at 1271 cm-1 is rather weak even at more negative potentials, as can be seen from Figure 5. The potential dependence of the SERS spectra provides interesting results for the two bands. When the potential is polarized at -0.3 V, no clear signal is observed from BTAH except for the band at 979 cm-1 which is assigned to the SO42- stretching mode. The application of -0.5 V to the electrode enables us to obtain a better-resolved spectrum. On scanning the potential to more negative values, all the band intensities related to the BTAH species increased slightly except for the 979 cm-1 band whose intensity decreases. The most notable change is found in the relative intensity of the two bands at 1124 and 1152 cm-1, while the frequencies of them change little with potential. As we know, a more negative potential may also influence the Raman band intensity in that the SERS activity changes with the applied potential. However, in that case all Raman transitions would be equally affected. The results given above clearly show that the applied voltage should only affect the interaction of adsorbate molecules with the iron surface. By using a spectrum separation method, we obtained the profile of intensitypotential for the three bands at 979, 1124, and 1152 cm-1 and this is illustrated in Figure 6. The maximum values appear at different applied potentials, suggesting the existence of some exchange reactions occurring on the surface. It has been postulated by Aramaki et al.41 that a band at 1140 cm-1 was due to the δ(N-H) in-plane bending mode using a deuteration method, which was further
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confirmed by Hope et al.31 The appearance and disappearance of this band was also used by Aramaki et al.38 to differentiate the “adsorption spectrum” and the “complex spectrum”. The assignment of δ(N-H) would support the chemical adsorption of BTAH with an iron surface by coordination of the iron through a triazole nitrogen lone pair. Thus, the shift of the band position δ(N-H) would be explained according to this interaction, but there should be only one band. The present SERS spectrum clearly shows two bands in this region. Since these spectral features are related to δ(N-H), there should be two similar δ(N-H) environments in the adsorbed species. Due to the protonation of BTAH in acidic solution, the BTAH molecule and BTAH2+ may coexist on the surface of iron. Hence we propose that the band at higher frequency may reflect the Fe-BTAH interaction, while the relative strong band is due to the adsorption of BTAH2+ through the electrostatic attraction by the negatively charged surfaces. Subsequently, the Raman intensity-potential profile depicted in Figure 6 can be explained as follows. It is well-known that at positive potentials in relation to the pzc the adsorption of anions at the metal-solution interface is favored. The pzc for iron in 0.5 M H2SO4 is reported at ca. -0.61 V (SCE).42 It is reasonable to assume that BTAH or BTAH2+ should not displace the position of the pzc significantly. So, at -0.3 V (see Figures 5 and 6) the iron electrode is positively charged and there is a large excess of sulfate anions at the interface. On the scanning of potential toward the pzc, the displacement of SO42- by the neutral molecule BTAH is favored, and one would expect a decrease in intensity of the band associated with SO42- symmetry stretching mode but an increase in intensity of the band at 1152 cm-1 attributed to the BTAH molecule. The adsorption quantity of BTAH reaches its maximum near pzc, as can be seen from Figure 6. This phenomenon is quite in accordance with the normal adsorption behavior of neutral organic molecules, which in turn further supports our assignment of the 1152 cm-1 band. When the electrode is polarized at potentials on the negative side of the pzc, the adsorption of BTAH2+ is favored apparently. As a result, another exchange reaction of BTAH by BTAH2+ then occurs at the interface. Negative potentials facilitate more BTAH2+ to enter the electrochemical double layer region, which causes an increase in intensity of the 1124 cm-1 band. On the other hand, the remaining 1152 cm-1 band even at -0.9 V where the 1124 cm-1 band reaches its maximum reveals a still existing weak interaction between BTAH and iron surface. Hence, the exchange reaction between BTAH and BTAH2+ is not thorough, and equilibrium should be established between them. It must be pointed out here that the influence of the solution cannot be ignored although the confocal lens was used in the optical path. So, the remaining band related to SO42- from the bulk solution at potentials negative to the pzc is still resolved. These results should be correlated to the electrochemical measurements. The relative low inhibiting ability of BTAH in 0.5 M H2SO4 in comparison with that in neutral solutions from the polarization curve measurements may be directly caused by the weak physisorption BTAH2+ on the basis of the SERS data above. Even the adsorption of BTAH is not strong enough to give a high inhibition efficiency in an acid solution. As we know, physical adsorption usually is reversible because of the weak interaction. In contrast, chemisorption has a higher heat of adsorption or activation energy than electrostatic (41) Youda, R.; Nishihara, H.; Aramaki, K. Corros. Sci. 1988, 28, 87. (42) Bockris, J. O’M.; Drasic, D. Electrochim. Acta 1962, 7, 293.
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Figure 8. Normal Raman spectrum of the Fe(II)-BTA complex and potential-dependent surface Raman spectra of BTAH on an iron electrode in saline water. Laser line: 632.8 nm. Acquisition time: 10 s (complex); 100 s (SERS). Accumulations: 3 (complex); 2 (SERS).
Figure 7. Schematic of adsorption mode of benzotriazole on iron electrode in sulfuric acid at potentials: (a) positive to potential of zero charge (pzc); (b) potential of pzc; (c) negative to pzc.
adsorption and, hence, usually is irreversible.43 From above, the protonated BTAH2+ cations can interact with the iron surface through electrostatic attraction depending on the polar and quantity of surface charge, which is represented in Figure 7. At potentials more positive than the pzc, the SO42- is predominant at the vicinity of the iron surface (see Figure 7a). On the scanning of potential toward the pzc, the displacement of SO42- by the neutral BTAH molecules and even the BTAH2+ cations occurs (see Figure 7b). At more negative potentials, the BTAH2+ cations dominates the iron electrode surface gradually (see Figure 7c), which is indicated by the increase of the intensity of the 1124 cm-1 band stretching mode. SERS Spectra of BTAH on Iron in Simulated Saline Water. A set of potential-dependent SERS spectra of BTAH at a roughened Fe electrode are represented in Figure 8. The tentative band assignments are also listed in Table 2. Comparison of the SERS spectra in saline water (43) Luo, H.; Guan, Y. C.; Han, K. N. Corrosion 1998, 54, 619.
with both the normal Raman (see Figure 4) and SERS spectra of BTAH in the sulfuric acid shows many differences. The SERS spectra obtained from saline water are quite different from the later two, especially in the band frequency. For instance, the triazole ring breathing mode red-shifts from 1219 cm-1 in the normal Raman spectrum to 1193 cm-1 in neutral saline water (at -0.5 V) but hardly changes in frequency after adsorption in 0.5 M H2SO4. It can also be found from Figure 8 that characteristic triazole modes are enhanced much more than that of benzene ring modes and are shifted greatly. A typical change is that the benzene ring breathing mode at 780 cm-1 and the triazole stretching mode at 1385 cm-1 in the normal Raman spectrum reverse in intensity after BTAH is adsorbed onto the electrode surface. The most striking spectral feature is that the δ(N-H) mode disappears after adsorption in the neutral solution, while it enhances greatly in the acid solution. These observations suggest that a great change in the triazole ring portion of BTAH should occur after interaction with the iron surface. The spectral features of BTAH obtained from the iron electrode surface in saline water are also quite in consistence with that occurring at a copper electrode.30 Rubim et al. proposed that the protective film on Cu in the presence of BTAH was [CuIBTA]n, in which BTAH coordinates with copper through two of the triazole nitrogen atoms to form a compact resistant film.30 Accordingly, it is reasonable to assume that BTAH is also bound to the Fe surface via the triazole nitrogen atoms to form a compact resistant film. For further confirmation, Figure 8 shows a normal Raman spectrum of the Fe(II)-BTA complex synthesized from our laboratory. For the inevitability of the oxidization of iron during the synthesizing process, the Raman bands of Fe-O oxides are also observed at ca. 310, 500, and 710 cm-1 in the complex spectrum. The tentative band assignments are listed in Table 2. The spectral features of BTAH on iron in saline water and those of the Fe(II)BTA complex are quite similar, as can be foretold by the
Inhibition of Iron Corrosion by Benzotriazole
Figure 9. Schematic of adsorption mode of benzotriazole on iron electrode in saline water.
above discussion, which supports our identification of the surface species. That is, the surface complex may be characterized as Fe(II)-BTA. In a neutral saline solution, BTAH interacts with the iron surface by the cleavage of the NH bond to form the surface complex. Thus, one can observe the disappearance of the NH in plane bending mode. Figure 9 describes a schematic adsorption mode of BTAH in neutral solutions, although this mode is tentative and should be modulated. A more systematic study both theoretically and experimentally is required for a complete understanding of the structure of the surface complex. In comparison with that in an acid solution (see Figure 7), one can easily find the two different interactions between BTAH and the iron surface. The former is rather a chemisorption mode, while the latter is more like a phsisorption one. Combining the electrochemical results, one can easily understand the existence of different inhibition efficiencies of BTAH in both sulfuric and saline solutions. Formation of a surface complex film provides a more compact barrier layer resulting in a high inhibitive action. Conclusions The investigation of the initial interaction of BTAH with iron electrode surfaces in both 0.5 M H2SO4 and simulated
Langmuir, Vol. 18, No. 20, 2002 7615
saline water has been studied by using a confocal microprobe Raman system, SERS techniques, and electrochemical polarization curve measurements. Main observations and coclusions have been summarized as follows: (1) Potentiodynamic polarization curve measurements show that BTAH is a mixed-type inhibitor for iron in 0.5 M H2SO4 and mainly an anodic-type one for iron in simulated saline water. Calculations of the inhibition efficiency of BTAH indicate a much higher value for the later, suggesting some different interactions of BTAH with the iron surfaces. (2) The SERS spectra at a bare iron electrode in 0.5 M H2SO4 has been observed successfully for the first time. Both the BTAH molecule and the BTAH2+ anions have been identified as the surface adsorbates. A tentative phsisorption mode is proposed to explain the relatively low inhibitive effect of BTAH for iron in acid solutions. (3) Potential dependent SERS spectra of BTAH adsorbed onto a bare iron electrode in simulated saline water have been acquired in situ. Great changes of the spectral features exist in comparison with those of the normal Raman and SERS spectra of BTAH in the acid solution. A chemisorption mode of BTAH for iron in neutral solutions is proposed to explain the high inhibition efficiency value. The surface species may be characterized as Fe(II)-BTA. A cleavage of NH bond occurs during the interaction of BTAH with the iron surface. In general, the SERS technique has been proved to be applied successfully in the corrosion science for the transition metals such as iron without the inducement of SERS-active metals (Ag, Au) as was used traditionally. Acknowledgment. We gratefully acknowledge support by the Natural Science Foundation of China, the Ministry of Education of Jiangsu Province, and the financial support of the State Key Laboratory for Physical Chemistry of Solid Surfaces of Xiamen University. The Raman spectroscopy experiments were carried out in Xiamen University. The authors are grateful for the kind help of the co-workers there. We also sincerely thank Professor B. W. Mao, Dr. B. Ren, and Dr. J. L. Yao for stimulating and helpful discussions. LA025570M
