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Comparative Study of Inhibition Effects of Benzotriazole for Metals in Neutral Solutions As Observed with Surface-Enhanced Raman Spectroscopy P. G. Cao,† J. L. Yao,‡ J. W. Zheng,† R. A. Gu,*,† and Z. Q. Tian‡ Department of Chemistry, Suzhou University, Suzhou 215006, People’s Republic of China, and State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China Received April 19, 2001. In Final Form: September 12, 2001 Surface-enhanced Raman spectroscopy has been successfully extended to the study of corrosion inhibition of bare iron and nickel metals. The inhibition effects of benzotriazole (BTAH) for copper, iron, and nickel electrodes in 0.1 M KCl solution were investigated by using both polarization curves and in situ Raman techniques. The protective films formed on copper and iron surfaces, in the presence of BTAH, are characterized as [CuIBTA]n and [FeII(BTA)2]n, respectively. The formation of Fe-N coordinated bonds and the deprotonation of the triazole ring may occur while BTAH interacts with the iron surface. On the contrary, BTAH may interact with the nickel surface as neutral molecules in the whole potential range investigated resulting in a poor inhibition effect. The surface complex is characterized as [Ni-BTAH]. The potential dependence of the Raman spectra on copper and iron shows that the BTA- ion in the surface complex may rebind with H+ at more negative potentials and accordingly the inhibition efficiency of benzotriazole decreases.
Introduction One of the most important methods in corrosion protection of metals is the utilization of organic inhibitors. Inhibition of metal corrosion using organic chemicals has been the subject of numerous studies. Many in situ surface vibrational spectroscopic techniques, including infrared (IR), Raman, and sum frequency generation (SFG), have been applied in this field, particularly to probe the interaction at the metal/inhibitor interface.1-5 However, it is well-known that both IR and SFG techniques are subject to the poor ability of obtaining vibrational information in the low-frequency region.3,6 Another problem arising for IR is the strong adsorption of IR light by water in aqueous electrochemical systems. Fortunately, these problems can be well solved by using the Raman technique.7 In corrosion science, surface-enhanced Raman scattering (SERS) and resonance Raman scattering (RRS) techniques have been used individually or together to enhance the scattering signal from the electrochemical interface.8-13 Of these studies, copper and silver are the substrates investigated extensively due to their giant enhancing effect. Kester and co-workers5 first utilized SERS to study the structure of benzotriazole (BTAH) adsorbed on a copper surface. Direct evidence for BTACu interactions was obtained. SERS, however, has not * To whom correspondence should be addressed. Tel: 86+512+ 5112813,5112645.Fax: 86+512+5231918.E-mail:
[email protected]. † Suzhou University. ‡ Xiamen University. (1) Luo, H.; Guan, Y. C.; Han, K. N. Corrosion 1998, 54, 619. (2) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (3) Lipkowski, J.; Ross, P. N. In Adsorption of Molecules at Metal Electrodes; VCH: New York, 1992. (4) Musiani, M. M.; Mengoli, G.; Fleischmann, M.; Lowry, R. B. J. Electroanal. Chem. 1987, 217, 187. (5) Kester, J. J.; Furtak, T. E.; Bevelo, A. J. J. Electrochem. Soc. 1982, 129, 1716. (6) Beden, B. J. Electroanal. Chem. 1993, 345, 1. (7) Schrotter, W. H.; Klochner, H. W. In Raman Spectroscopy of Gases and Liquids; Weber, A., Ed.; Springer-Verlag: Berlin, 1979; Vol. 11, p 123.
been used widely in some transition metals such as iron which are of great importance in industry. Two methods have been employed in studying the corrosion and inhibition of these transition metals. One is to investigate the RRS effect of organic inhibitors, for example, 1,10phenanthroline on iron.14 The other is to induce the SERS effect to these metals, through either depositing silver particles as a signal amplifier on transition metals15-19 or depositing the latter on the former.20-25 The long-range electromagnetic mechanism is believed to contribute to these systems.16 Nevertheless, the existence of pinholes and the dissolution of the transition metals during the anodic oxidation reaction may sometimes lead to the misinterpretation of the Raman spectra because the adsorbate can be adsorbed onto the transition metal, the SERS-active metal, or even the edge between them. A more reliable method would be to obtain directly the (8) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (9) Otto, A.; Mrozek, I.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (10) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241. (11) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (12) Tian, Z. Q.; Gao, J. S.; Li, X. Q.; Ren, B.; Huang, Q. J.; Cai, W. B.; Liu, F. M.; Mao, B. W. J. Raman Spectrosc. 1998, 29, 703. (13) Melendres, C. A. In Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry; Gutirrez, C., Melendres, C. A., Eds.; Kluwer Academic Publishers: Boston, 1990; p 181. (14) Van Duyne, R. P. In Chemical and Biochemical Applications of Lasers; Moore, C. B., Ed.; Academic Press: New York, 1979; Vol. 4. (15) Gui, J.; Devine, T. M. J. Electrochem. Soc. 1991, 138, 376. (16) Oblonsky, L. J.; Devine, T. M.; Ager, J. W.; Perry, S. S.; Mao, X. L.; Russo, R. E. J. Electrochem. Soc. 1994, 141, 3312. (17) Dong, J.; Sheng, Z.; Xue, G. Chem. Phys. Lett. 1994, 231, 183. (18) Gao, Q.; Li, F.; Lu, Y. et al. Spectrosc. Lett. 1998, 31, 167. (19) Li, F.; Lu, Y.; Xue, G. Chem. Phys. Lett. 1997, 264, 376. (20) Mengoli, G.; Musiani, M.; Fleischmann, M.; Mao, B. W.; Tian, Z. Q. Electrochim. Acta 1987, 32, 1239. (21) Uehara, J.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1990, 137, 2677. (22) Aramaki, K.; Ohi, M.; Uehara, J. J. Electrochem. Soc. 1992, 139, 1525. (23) Gu, R. A.; Deng, Z. F.; Yao, J. L.; Sun, R. Spectrosc. Spectral Anal. 1997, 17, 15. (24) Feilchenfeld, H.; Gao, X.; Weaver, M. J. Chem. Phys. Lett. 1989, 161, 321. (25) Leung, L. W.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 5113.
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surface Raman spectra of inhibitors from bare transition metals. Recently, Tian and co-workers have successfully extended the SERS studies to some transition metals (Pt, Ni, Fe) by using a high-sensitivity confocal Raman system and proper electrochemical roughening procedures.26-31 This enables us to further study the corrosion and inhibition behavior of bare nickel and iron metals, which will lead to a better understanding of the inhibition mechanism of organic inhibitors on such transition metals. The aim of our investigations is to design SERS experiments which should be correlated with conventional polarization curve measurements for these transition metals to give a dynamic description of the interface as a function of the key variables: potential, solution pH (particularly the local pH in the vicinity of the electrode surface), and concentration of inhibitors and of the substrate. In this paper, we will investigate in detail the adsorption behavior of benzotriazole on metals such as Ni, Fe, and Cu. The inhibiting effect of BTAH on the corrosion of copper has been investigated extensively since the pioneering work of Cotton and co-workers.32 The formation of polymeric complexes with Cu(I) ions was presumed to be the reason BTAH has high efficiency for copper especially at high pH of the solution.5,33-35 Studies on iron have shown that BTAH is a good inhibitor too.36 However, the composition and structure of the inhibitive film are still not confirmed. So, it will be of great importance to further investigate the different interactions between BTAH and metals. In this paper, interest is mainly focused on the comparison of the inhibition effects of BTAH for copper, iron, and nickel in neutral chloride-containing solutions on the basis of the in situ Raman spectra. The inhibition mechanisms of BTAH for these metals will be discussed. Experimental Section The working electrodes were polycrystalline Fe (99.99%), Ni (99.99%), and Cu (99.99%) rods embedded in a Teflon sheath, with a geometric surface area of 0.1 cm2. All three electrodes were roughened prior to the SERS experiments. They were first mechanically polished successively with 0.5 µm aluminum powder down to 0.05 µm to a mirror finish followed by ultrasonic cleaning with Milli-Q water. A chemical etching method was used for the Cu electrode. That is, a polished and rinsed copper electrode was then immersed in 0.1 M nitric acid for about 5 min. The roughening procedure for iron is to immerse it into sulfuric acid followed by an oxidation-reduction cycle.37 A similar electrochemical oxidation-reduction cycle was also applied to nickel to obtain a properly roughened surface.30 All the potentials were quoted versus a saturated calomel electrode (SCE). All of the chemicals used were of analytical reagent grade as received, and the solutions were prepared using Milli-Q water. A three-compartment cell with a Pt counter electrode and a SCE electrode was used for all the potentiodynamic polarization (26) Gao, J. S.; Tian, Z. Q. Spectrochim. Acta 1997, A53, 1595. (27) Tian, Z. Q.; Ren, B.; Mao, B. W. J. Phys. Chem. 1997, B101, 1338. (28) Ren, B.; Huang, Q. J.; Cai, W. B.; Mao, B. W.; Liu, F. M.; Tian, Z. Q. J. Electroanal. Chem. 1996, 415, 175. (29) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9. (30) Huang, Q. J.; Yao, J. L.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 1997, 271, 101. (31) Cao, P. G.; Yao, J. L.; Ren, B.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 2000, 316, 1. (32) Dugdale, I.; Cotton, J. B. Corros. Sci. 1963, 3, 69. (33) Poling, G. W. Corros. Sci. 1970, 10, 359. (34) Rubim, J. C.; Gutz, I. G. R.; Sala, O.; Orville-Thomas, W. J. J. Mol. Struct. 1983, 100, 571. (35) Thierry, D.; Leygraf, C. J. Electrochem. Soc. 1985, 132, 1009. (36) Asakura, S.; Lu, C. C.; Nobe, K. J. Electrochem. Soc. 1974, 121, 1276. (37) Gu, R. A.; Cao, P. G.; Yao, J. L.; Ren, B.; Tian, Z. Q. J. Electroanal. Chem. 2001, 505, 95.
Figure 1. Potentiodynamic polarization curves for the copper (a), iron (b), and nickel (c) electrodes in 0.1 M KCl without and with the presence of 1.0 × 10-4 M BTAH. measurements, which were carried out in an aerated 0.1 M KCl solution with and without BTAH at room temperature (15 °C). After 20 min of immersion of the electrode in the solution, the anodic and cathodic polarization curves were recorded starting from the open-circuit potential, using a scan rate of 1 mV s-1. Several runs were made for each solution. Raman spectra were recorded using a confocal microprobe Raman system (LabRam I from Dilor Inc.). The excitation line was 632.8 nm from an internal He-Ne laser. The detailed description of the Raman system was given elsewhere.27
Results and Discussion Electrochemical Measurements. Figure 1a-c shows the typical polarization curves of the copper, iron, and nickel electrodes, respectively, in 0.1 M KCl with and without BTAH at 15 °C. Both the anodic metal dissolution and the cathodic hydrogen evolution reaction are suppressed by adding 1.0 × 10-4 M BTAH for copper and
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Figure 2. Normal Raman spectrum of BTAH in the solid state (a) and the SERS spectra of BTAH adsorbed onto copper (b), iron (c), and nickel (d) from 0.01 M BTAH + 0.1 M KCl solution.
iron. The cathodic process, however, is accelerated with the presence of BTAH for the nickel electrode. The currentplatform in the potential region between -0.3 and -0.4 V for copper is due to the reduction of a trace amount of dissolved oxygen or copper ion. The corrosion current densities (Icorr) were determined from the Tafel polarization curves by the Tafel extrapolation method. The inhibition efficiency (η) was then calculated by eq 1,
η ) 1 - Icorr(2)/Icorr(1)
(1)
where Icorr(2) and Icorr(1) refer to the current densities for the inhibited and uninhibited electrodes, respectively. The values of η calculated for the copper, iron, and nickel electrodes are 70.4%, 59.9%, and 31.9%, respectively, indicating a decrease in the inhibiting effects of BTAH for the three metals in the same order. The relatively low η compared to the reported values4,36 may be due to the shorter immersion time (20 min) of the electrodes in the inhibitor-containing solution which is insufficient for the formation of a protective layer at the surface. SERS and Normal Raman Spectra of BTAH. A normal Raman spectrum of solid BTAH in the frequency range 400-1800 cm-1 is shown in Figure 2a. The SERS spectra of BTAH adsorbed on the roughened copper, iron, and nickel electrodes at open circuit potentials (-0.28, -0.62, and -0.37 V, respectively) are also presented (Figure 2b-d). All spectra in this paper have been subtracted from the contribution of the background. The spectrum obtained from the copper surface is similar to those reported in the literature.4,38 Assignments of the major bands are listed in Table 1.5,34 In the surface Raman spectra, distinct frequencies observed below 1800 cm-1 are mainly assigned to the triazole portion of the molecule. It can be seen that the SERS intensity on iron is about one order smaller than that obtained on copper (Figure 2) but slightly stronger than that on nickel. This may suggest that the interaction between the BTAH molecule and metals decreased in the order Cu > Fe > Ni. This is (38) Tornkvist, C.; Zthierry, D.; Bergman, J.; Liedberg, B.; Leygraf, C. J. Electrochem. Soc. 1989, 136, 58.
also supported by the fact that the blue-shift in frequency of the in-plane trigonal breathing mode (from 1021 cm-1 in the solid Raman spectra to 1042, 1032, and 1025 cm-1 in the surface Raman spectra on Cu, Fe, and Ni, respectively) decreases in the same order. Comparison of the normal Raman and the SERS spectra of BTAH shows some other differences which are also a result of the surface interaction. The bands at 1095, 1125, and 1146 cm-1 assigned to NH in-plane deformation of BTAH decrease in relative intensity or even diminish after BTAH is adsorbed onto all three metal surfaces. The band at 1209 cm-1 attributed to the triazole ring breathing mode redshifts to 1183, 1192, and 1198 cm-1 for Cu, Fe, and Ni, respectively. From the above observations, characteristic triazole modes are enhanced much more than that of the benzene ring and are shifted greatly. The typical change is that the benzene ring breathing mode at 782 cm-1 and the triazole stretching mode at 1385 cm-1 reverse in intensity after BTAH is adsorbed onto the electrode surfaces. This suggests that the triazole portion of the BTAH molecule may be closer to the metal surface. It can also be found that the spectral features of both Fe and Ni are similar to that of Cu. The high similarity of the Raman spectra of BTAH on the Fe, Ni, and Cu surfaces means that BTAH may coordinate with these metal surfaces in a similar configuration. 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.34 The structure of the surface polymer film is represented in Figure 3a. Accordingly, it is reasonable to assume that BTAH is also bound to both Fe and Ni surfaces via the triazole nitrogen atoms to form a compact resistant film. The surface complex on iron characterized as [FeII(BTA)2]n is given tentatively in Figure 3b. The decrease in frequency of the triazole breathing mode of BTAH on Cu and Fe could now be explained. Since the BTAH molecule deprotonates by adsorption onto metals and the BTAanion has the C2v symmetry point group, the electron density of the triazole ring rearranges which weakens the vibration of the triazole breathing leading to a decrease in frequency. However, BTAH may exist as its neutral molecule in the surface complex for a nickel electrode which may be characterized as [Ni-BTAH], because the frequency of the in-plane trigonal breathing mode at 1025 cm-1 is quite near to that of the neutral molecule (1021 cm-1). The relatively low red-shift value in frequency of the triazole breathing mode (11 cm-1) compared with copper (26 cm-1) and iron (17 cm-1) supports this assumption. Potential Dependence of the SERS Spectra of BTAH on Copper. As has been mentioned above and studied by many research groups, BTAH can coordinate with copper by adsorption in a neutral solution to form a surface polymer film which suppresses the anodic dissolution of copper.33-35 Therefore, factors of the compactness and composition of the surface complex will inevitably influence the inhibition efficiency of BTAH. The SERS spectrum of BTAH at a roughened copper electrode in 0.1 M KCl as a function of applied potentials is demonstrated in Figure 4. The intensities of all the Raman bands increase as the potential is stepped negatively and reach the maximum at -1.1 V. The most interesting feature is that the in-plane trigonal breathing mode redshifts from 1041 cm-1 at -0.5 V to 1021 cm-1 at -1.1 V. The large downshift of this band may imply some changes in the film structure. Rubim et al.39 has investigated in detail the differences between the Raman spectra of
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Table 1. Proposed Assignments of the Raman Spectra of Benzotriazole in the Solid State and the SERS Spectra of Benzotriazole Adsorbed on the Copper, Iron, and Nickel Electrodes in 0.1 M KCl at Potentials Indicated copper
iron
nickel
assignment
solid BTAH
oca
-1.1 V
oc
-1.1 V
oc
-1.1 V
tzbring torsion tz ring bend bz ring breathing in-plane trigonal breathing NH in-plane bend tz ring breathing CH in-plane bend tz ring stretch bz ring stretch
538m c 630m 782vs 1021s 1146vw 1209m 1279m 1385s 1595m
559m 648w 789s 1042m 1142m 1183m 1294w 1388vs 1575m
558m 639w 789s 1021s 1142m 1188s 1293m 1388vs 1574m
557m 632w 790s 1032s
557m 636m 790s 1022s 1143m 1193s 1290m 1390vs 1574m
554w
558w
789w 1025m
787w 1021m 1129vw 1195s 1288w 1389s
1192s 1289m 1390vs 1574m
1198s 1287w 1387s
a oc: Potential of open circuit. b Abbreviations: bz, benzene; tz, triazole. c Wavenumbers (in cm-1) followed by relative intensities (vs, very strong; s, strong; m, medium; w, weak; vw, very weak).
Figure 3. The molecular structure of the surface complex: (a) [CuIBTA]]n and (b) [FeII(BTA)2]n.
coordination compounds of BTAH with Cu(I), Cu(II), and Zn(II). They attributed the bands at 1050 and 1020 cm-1 to the characteristic bands of [CuI(BTA)]n and [CuI(BTAH)]4, respectively. In combination with our measuring results, it is reasonable to assume that the initially formed complex [CuI(BTA)]n may change to [CuI(BTAH)]4 at more negative potentials. That is, the coordinated BTAanion may rebind with H+. This is also supported by the observations that the NH in-plane bending mode located at 1144 cm-1 appears and increases in intensity with negative potentials. As we know, the surface acid-base equilibrium (which differs markedly from the solution equilibrium) is potential dependent,4 so at more negative potentials, more H+ will be attracted onto the electrode surface and in turn facilitate the reprotonation of BTA-. Thus, the surface polymer film should decompose with the penetrating of H+ and form a compound as [CuI(BTAH)]4, which results in less inhibition efficiency for copper. Potential Dependence of the SERS Spectra of BTAH on Iron. Figure 5 shows the potential-dependent SERS spectra of BTAH at a roughened iron electrode in 0.1 M KCl. The potential is varied from positive to negative. Changes in relative intensity and band frequency with potential are quite similar to those observed on copper, suggesting some similar changes in the structure of the surface film occurring on the iron electrode. On scanning the potential from -0.5 to -1.1 V, the in-plane trigonal breathing mode shifts from 1034 to 1022 cm-1. The band’s (39) Da Costa, S. L.; Rubim, J. C.; Agostinho, S. M. J. Electroanal. Chem. 1987, 220, 259.
Figure 4. SERS spectra of benzotriazole obtained from 0.1 M KCl at a copper electrode at potentials indicated. Acquisition time, 10 s; accumulations, 2.
full width at half-maximum (fwhm) also decreases markedly. The NH bending mode of the triazole ring appears at 1143 cm-1, and its intensity increases with the negative shift of the potential. On the contrary, only slight changes of the frequencies of the modes relevant to the benzene ring are observed. For instance, the benzene ring stretching mode at 790 cm-1 hardly changes in the whole potential range. This may suggest that the structural changes are only relevant to the triazole portion of the BTAH molecule but not to the benzene ring. Similar to that on copper, the surface acidity is considered to have direct effect on the structure and composition of the surface complex at the iron electrode. We presume that the surface complex on iron may decompose as more H+ are attracted in the vicinity of the iron surface at more negative potentials. This may lead to a change in the initially formed surface complex [FeII(BTA)2]n to some coordination compound like [FeII(BTAH)2]Cl2 which causes less inhibition efficiency for iron. The identity of this compound needs additional experimental confirmation. Potential Dependence of the SERS Spectra of BTAH on Nickel. Figure 6 shows the potential-dependent SERS spectra of BTAH adsorbed on a roughened nickel electrode in the same solution as Figures 4 and 5. The
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Figure 5. SERS spectra of benzotriazole obtained from 0.1 M KCl at an iron electrode at potentials indicated. Acquisition time, 100 s; accumulations, 2.
potential is varied also from positive to negative. In comparison with the spectra obtained on copper and iron, the band intensity decreases and both the intensity and frequency of all the bands alter little with the variation of applied potentials. The difficulty in discerning clearly the NH in-plane bending mode (1129 cm-1, -1.1 V) might be due to the poor quality of the whole spectra compared with those of copper and iron. Despite this, it is reasonable to conclude that the composition and the structure may not change with potentials. The initially formed surface complex at the open-circuit potential has been characterized as [Ni-BTAH], which exists in the whole potential range investigated. Because of the rather lower strength in the interaction between BTAH and nickel, the inhibition efficiency of BTAH hence is comparatively low for nickel. Summary By use of a high-sensitivity confocal Raman system and proper surface roughening procedures, the inhibition effects and mechanisms of BTAH for copper, iron, and nickel have been investigated. The high inhibition efficiency calculated from the Tafel polarization curve measurements of BTAH for iron and copper and the relatively low η value for nickel have been well explained by the surface Raman spectra. BTAH is chemisorbed onto an iron electrode surface like copper in a neutral chloridecontaining solution by formation of Fe-N coordination bonds. The surface protective films are characterized as [CuIBTA]n and [FeII(BTA)2]n, respectively, which suppress the anodic dissolution of copper and iron. In the case of
Cao et al.
Figure 6. SERS spectra of benzotriazole obtained from 0.1 M KCl at a nickel electrode at potentials indicated. Acquisition time, 100 s; accumulations, 2.
nickel, although BTAH is also chemisorbed onto the surface, BTAH coordinates with nickel as a neutral molecule which results in a weak interaction between them. Thus, the surface complex film [Ni-BTAH] formed on the nickel surface is not so compact and the inhibition efficiency lowers. Potential-dependent surface Raman experiments on copper and iron present interesting changes of frequencies and intensities with applied potentials, suggesting a change in composition of the surface complex film. The coordinated BTA- may rebind with H+ at more negative potentials, and its interaction with metals accordingly weakens, which results in less inhibition efficiency of BTAH. On the nickel electrode, the BTAH molecule may exist in its neutral form in the whole potential range investigated, which leads to a relative low inhibition efficiency. Acknowledgment. This work was supported by the Natural Science Foundation of China and the Ministry of Education of Jiangsu Province and the financial support of 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. LA010575P
