Protective Properties of Dodecanethiol Layers on Copper Surfaces

O. Azzaroni, M. E. Vela, M. Fonticelli, G. Benítez, P. Carro, B. Blum, and R. C. Salvarezza. The Journal of Physical Chemistry B 2003 107 (48), 13446...
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Langmuir 2001, 17, 1483-1487

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Protective Properties of Dodecanethiol Layers on Copper Surfaces: The Effect of Chloride Anions in Aqueous Environments O. Azzaroni, M. Cipollone, M. E. Vela, and R. C. Salvarezza* Instituto de Investigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET-CIC, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina Received June 19, 2000 The protective properties of dodecanethiol layers against copper corrosion in a Borax buffer containing chloride anions are investigated by using electrochemical techniques complemented with scanning tunneling microscopy. Results show that copper is protected against corrosion provided that the copper electrode does not reach the potential region of Cu(II) oxide formation and the concentration of chloride ions in the environment remains low. These findings should be taken into account for the use of alkanethiol layers for the corrosion protection of copper.

Introduction Alkanethiol layers adsorbed on solid surfaces have attracted in recent years a considerable scientific interest in the area of materials science.1 These films are promising because they are able to modify wetting and wear properties of solid surfaces, they can anchor different functional groups to metal surfaces in order to build chemical and biochemical sensors, they can be used to develop nanodevices for electronics,2 and they are also useful in the prevention of corrosion of metals. These types of films, at the monolayer level, have been extensively studied on gold, on silver, and to a lesser extent on copper using different techniques such as scanning tunneling microscopy (STM), atomic force microscopy, helium diffraction, X-ray photoelectron spectroscopy (XPS), thermal desorption, electrodesorption, etc.1,3-7 It has been reported that 1-3 nm thick barriers of thiols can block electron transfer and hinder the transport of water, oxygen, and aggressive ions to the metal surface, extending its immunity region hundreds of millivolts. Therefore, alkanethiol-covered metal systems result in novel materials with unusual corrosion resistance. Today, basic studies on the application of alkanethiol layers to modify the corrosion behavior of metals are restricted to a few systems of practical interest: iron/steels8 and copper.9,10 In the former case, however, the preparation of alkanethiol layers is difficult because of the interference of iron oxide that results in mixtures of thiols and oxide films. Conversely, copper is a good candidate for basic studies because thiol adsorption from pure thiol or thiol-containing toluene * To whom correspondence should be addressed. E-mail: [email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (3) Kondoh, H.; Kodama, C.; Sumida, H.; Nozoye, H. J. Chem. Phys. 1999, 111, 1175. (4) Rieley, H.; Kendall, G. K. Langmuir 1999, 15, 8867. (5) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (6) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506. (7) Imanishi, A.; Isawa, K.; Matsui, F.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282. (8) Stratmann, M. Adv. Mater. 1990, 29, 191. (9) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (10) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436.

solutions produces high-quality alkanethiol layers.11 Recently, a correlation has been found between the crystallinity of alkanethiol layers and their protective properties for copper surfaces in contact with aqueous solutions.12 It has been suggested that these layers can be used to prevent the corrosion of copper in air. In fact, alkanethiol layers inhibit the oxygen transport to the copper surface, block oxide formation,9 and hinder oxygen reduction.10 However, even in the atmosphere, oxide formation and metal dissolution involves the reaction with water and anions rather than oxygen. Corrosion of metals is strongly accelerated by the presence of chloride ions. Thus, the enhanced electrodissolution of alkanethiol-covered copper in hydrochloric acid has been reported.13 In many cases, when passive oxide films are formed on the metal surface, chloride ions promote the breakdown of the oxide films, leading to pitting corrosion.14 Pitting is one of the most dangerous forms of corrosion because the metal dissolves at a high rate, on the order of amperes per square centimeters, at certain sites whereas the rest of the surface remains covered by oxide dissolving at a lower rate, on the order of a few microamperes per square centimeters. The breakdown of the oxide films by chloride ions takes place when the potential of the metal surface (E) reaches a critical value called breakdown potential (Eb).14 Therefore, for E > Eb, the copper electrodissolution current through the oxide film shows a sudden increase due to the passive (oxide covered)/pitting stage transition.15 At present, the formation of copper oxides and the effect of chloride ions on the corrosion behavior of alkanethiolcovered metals in neutral and alkaline media have not been studied, although it is a crucial point that should be elucidated for a possible use of these layers in corrosion protection. In this work we report results on the oxide formation, dissolution, and pitting corrosion of dodecanethiol-covered (11) Ron, H.; Cohen, H.; Matlis, S.; Rappaport, M.; Rubinstein, I. J. Phys. Chem. B 1998, 102, 9861. (12) Jennings, J. K.; Munro, J. C.; Laibinis, P. Adv. Mater. 1999, 11, 1000. (13) Schrerer, J.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. Langmuir 1997, 13, 704. (14) Galvele, J. R. In Passivity of Metals; Frankenthal, R. P., Kruger, J., Eds.; The Electrochemical Society: Pennington, NJ, 1978; p 285. (15) Chialvo, M. R. G.; Salvarezza, R. C.; Vazquez Moll, D.; Arvia, A. J. Electrochim. Acta 1985, 30, 1501.

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copper in Borax buffer (pH 9) containing x M KCl (0.1 e x e 1). Alkanethiol forms nanometer thick layers on facecubic-centered metals.16 Using electrochemical techniques and STM, we found that copper is protected against corrosion by dodecanethiol layers provided that the electrode potential does not reach the potential region related to Cu(II) oxide formation and the concentration of chloride anions in the electrolyte remains lower than 0.1 M. These findings should be taken into account for the use of these kinds of alkanethiol layers in the corrosion protection of copper. The paper is organized as follows. In the first part we present results on the electrochemical behavior of copper in Borax buffer containing chloride anions and review results from the literature on this system. In the second part we analyze the electrochemical behavior of dodecanethiol-covered copper in the same electrolyte. Finally, in the third part, we compare the corrosion behavior of copper and dodecanethiol-covered copper under opencircuit conditions, in the presence of oxygen, by using electrochemical and STM data. Experimental Section Polycrystalline copper samples were mechanically polished with fine-grained emery paper and finally with alumina paste of 1 µm particle size. Then, the copper samples were treated as described in ref 17. Briefly, the samples were etched in HNO3, rinsed with Milli-Q water, and finally rinsed with ethanol. Immediately afterward, the copper samples were immersed in pure dodecanethiol for a time ta (1 h e ta e 30 days). Note that the exposure time to the atmosphere before immersion in dodecanethiol was less than a few seconds. Under this condition copper oxide formation is negligible.17 After careful rinsing with toluene, dodecanethiol-covered copper samples were used as working electrodes in a test solution consisting of 0.1 M Na2B4O7 (pH 9) + x M KCl (0.1 e x e 1). Electrochemical runs were made in a conventional glass-made, three-electrode electrochemical cell. A large-area Pt foil and saturated calomel were used as counter and reference electrodes, respectively. Potentials in the text are referred to the saturated calomel electrode (SCE). Current density (j) vs potential (E) curves were recorded at potential scan rates (v) in the range of 1 mV s-1e v e 50 mV s-1. Before the j vs E curves were performed, the test solution was saturated with nitrogen to avoid the interference of oxygen reduction. Potentiostatic measurements were also made at different E and x values for t ) 4 h to estimate the current density related to copper dissolution. STM images were taken using a Nanoscope III microscope (Digital Instruments, Santa Barbara, CA) after immersion of the copper samples at an open circuit in oxygencontaining Borax buffer + x M KCl for 4 days. To characterize the corrosion of the copper surfaces by STM, at least 50 images on different regions of the electrode surfaces were taken.

Results and Discussion 1. Electrochemical Behavior of DodecanethiolFree Copper. The j vs E curves for dodecanethiol-free copper electrodes recorded at v ) 1 mV s-1 from -0.4 V to Eb in the Borax buffer containing different x values are shown in Figure 1a. The results agree with those earlier reported for copper electrodes in the same electrolyte.15,18 Thus, for copper surfaces in contact with the Borax buffer + 0.1 M KCl, the j vs E curve shows two well-defined current peaks (AI, AII) and a broad current peak (AII′) at -0.2, -0.1, and +0.3 V, respectively. A detailed examination of the potential region preceding peak AI has revealed the presence of a small anodic current peak15 correspond(16) Kondo, T.; Yanagida, M.; Shimazu, K.; Uosaki, K. Langmuir 1998, 14, 5656. (17) Feng, Y.; Teo, W. K.; Siow, K.-S.; Gao, Z.; Tan, K.-L.; Hsieh, A.-K. J. Electrochem. Soc. 1997, 144, 55. (18) Strehblow, H. H.; Titze, B. Electrochim. Acta 1980, 25, 339.

Figure 1. (a) Current density (j) vs potential (E) curves recorded at v ) 1 mV s-1 for a dodecanethiol-free copper electrode in Borax buffer (pH 9) + 0.1 M KCl (solid line) and Borax buffer (pH 9) + 1 M KCl (dashed line). The electrochemical reactions associated with peaks AI, AII, and AII′ are described in the text. (b) j vs E curve recorded at v ) 1 mV s-1 for a dodecanethiolcovered copper electrode (ta ) 16 h) in Borax buffer (pH 9) + 0.1 M KCl (solid line) and Borax buffer (pH 9) + 1 M KCl (dashed line). The inset shows a voltammogram (v ) 50 mV s-1) recorded in region I of Borax buffer (pH9) + 0.1 M KCl showing the copper oxide formation and reduction at defects of the dodecanethiol film.

ing to the formation of a Cu(OH)ad monolayer19 through the reaction

Cu + H2O ) Cu(OH)ad + H+ + e

(1)

Earlier XPS18 and electrochemical data15,18 and recent surface-enhanced Raman spectroscopy data20 have revealed that the Cu(OH)ad layer is transformed into a Cu2O film at peak AI while a combined Cu2O/CuO/Cu(OH)2 electroformation on the Cu2O film takes place at peak AII.20 Besides, ring-disk electrochemical measurements15 in this potential region have shown that Cu dissolution as Cu(I) and Cu(II) species through this complex oxide film also contributes to the anodic current involved in peaks AI and AII. The broad peak AII′ corresponds to the growth of Cu(OH)2 outer layers.15 These processes can be schematically represented by

Cu(OH)ad w Cu2O/CuO/Cu(OH)2

(2)

Therefore, the oxide film on copper consists of inner Cu2O and outer CuO/Cu(OH)2 layers.17 In fact, when E is held in the potential region of peak AII′ (E ) 0.5 V) for t ) 2 min and then scanned in the negative direction (Figure 2a), peaks CII, CI, and CI′ are observed. While peak CII corresponds to the electroreduction of CuO to Cu2O, peaks CI and CI′ correspond to the electroreduction of Cu2O to Cu.15,18,20 Peak multiplicity in the voltammetric reduction of Cu2O to Cu has been reported previously in the literature.21,22 Thus, peak CI′ has been assigned to the electroreduction of an “aged” or “less hydrous” Cu2O film (19) Maurice, V.; Strehblow, H.-H.; Marcus, P. Surf. Sci. 2000, 458, 185. (20) Chan, H.; Takoudis, C.; Weaver, M. J. J. Phys. Chem. B 1999, 103, 357 and references therein.

Dodecanethiol Layers on Copper Surfaces

Figure 2. (a) j vs E curve recorded at v ) 50 mV s-1 for a dodecanethiol-free copper electrode after anodization at E ) 0.5 V for t ) 2 min in Borax buffer (pH 9) + 0.1 M KCl. (b) j vs E curve recorded at v ) 50 mV s-1 for a dodecanethiol-covered copper electrode after anodization at E ) 0.5 V (region II) for t ) 2 min in Borax buffer (pH 9) + 0.1 M KCl. Table 1. Electrodissolution Current Densities (jc) Recorded after 4 h of Anodization for Dodecanethiol-Covered (ta ) 16 h) and Dodecanethiol-Free Copper Electrodes in Borax Buffer Solutions at Different E and x Values jc/(µA cm-2) x/M

E/V

dodecanethiol-free copper electrode

dodecanethiol-covered copper electrode

0.1 0.1 1.0

0.0 0.5 0.0

10 6 20

0.25 5 3.24 (t ) 0 h), 10 (t ) 4 h)

to Cu.21,22 At more positive potentials than that corresponding to peak AII′, a nearly constant current, related to copper oxide growth and generalized copper dissolution through the complex oxide film, is observed until the breakdown potential is reached at Eb ) 0.8 V. For E > Eb pits grow into the copper electrode. Therefore, qt, the overall charge for E < Eb, is a measure of the nonlocalized corrosion of copper, i.e., oxide formation and generalized copper dissolution through the oxide film. On the other hand, the value of Eb indicates the resistance of the oxide film to pitting corrosion.14 To estimate the current related to generalized copper dissolution (jc), copper electrodes were anodized for 4 h at different E values in the Borax buffer + 0.1 M KCl. Ringdisk electrochemical measurements at constant E15 have shown that when the copper electrode is held at a constant E, the initial anodic current density is mainly related to oxide growth, a relatively fast process, whereas for t > 2 min, the current density approaches a nearly constant value corresponding to jc. Results shown in Table 1 indicate that for E ) 0.0 and 0.5 V nonnegligible values of jc are (21) de Chialvo, M. R. G.; Marchiano, S. L.; Arvia, A. J. J. Appl. Electrochem. 1984, 14, 165. (22) Go´mez Becerra, J.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1988, 33, 613.

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observed, i.e., generalized corrosion of the copper surface takes place. The increase in x from 0.1 to 1 M (Figure 1a, dashed line) results in a marked increase in the overall current due to the enhanced dissolution of copper as CuCl2complexes.15 Accordingly, the jc value read at E ) 0.0 V is 2 times greater than that measured for x ) 0.1 M at the same E value (Table 1). Besides, Eb shifts toward more negative potential values; i.e., pitting corrosion is also favored at high chloride concentrations. 2. Electrochemical Behavior of DodecanethiolCovered Copper. The j vs E curves (Figure 1b) made using dodecanethiol-covered copper electrodes (ta ) 16 h) show a behavior completely different from those shown in Figure 1a. For x ) 0.1 M (Figure 1b, solid line), two different potential regions can be observed. Region I exhibits a negligible current and corresponds to the region where the dodecanethiol film protects the copper surface.13 Region II, which has not been reported previously, comprises a single broad peak located at more positive potentials than those observed for oxide formation and copper dissolution on dodecanethiol-free copper. When the potential of the copper electrode is held for t ) 2 min in the potential region of region II (E ) 0.5 V) and then scanned in the negative direction, cathodic peaks CII, CI, and CI′ corresponding to the electroreduction of CuO and Cu2O are also observed (Figure 2b). Besides, in this potential region (E ) 0.5 V), the jc value for the dodecanethiol-covered copper reaches a value close to that measured for a dodecanethiol-free copper electrode (Table 1). Therefore, we conclude that in region II the dodecanethiol film loses part of its protective properties and oxide formation and copper dissolution take place. We turn now to region I where the dodecanethiol film protects the copper surface. In fact, in region I (E ) 0.0 V) the jc value for dodecanethiol-covered copper electrodes (ta ) 16 h) is 40 times smaller than that recorded for dodecanethiol-free copper electrodes at the same E and x values (Table 1). A detailed analysis of this region by voltammetry (Figure 1b, inset) also shows the cathodic peak (CI) related to the electroreduction of Cu2O formed at AI.15,18 Note that peak AI is not observed in Figure 1b because oxide formation is a relatively fast process and the amount of oxide formed on the dodecanethiol-covered copper is too small. Thus, peak AI is more clearly observed at v ) 50 mV s -1 (Figure 1b, inset) than at v ) 1 mV s-1 (Figure 1b). The charge density involved in AI/CI peaks is only a fraction (f) of that expected for a complete Cu2O monolayer. Therefore, f reflects the amount of defects at the dodecanethiol film where the copper surface is in contact with water, allowing the formation of Cu2O. The increase in ta from 1 to 16 h results in a decrease in f from 1.7 × 10-2 to 4.8 × 10-3. Finally, for ta ) 1 month, we cannot detect Cu2O formation at the dodecanethiol-covered Cu surface. Note that for ta ) 1 h the formation of a dodecanethiol monolayer has been reported,9 while for ta > 1 h a dodecanethiol multilayer could be formed. In fact, for ta ) 30 days, repetitive scanning of a 40 × 40 nm2 section of the dodecanethiol-covered copper surface results in the formation of a window because of the removal of dodecanethiol molecules by the STM tip (Figure 3a). The cross section of the window (Figure 3b) reveals that the dodecanethiol film is 4 nm in thickness; i.e., a multilayer is formed for long ta values. Therefore, as ta increases from hours to days, a thicker and less defective hydrophobic film is formed, hindering the transport of water to the copper surface. Results for region II also indicate that the peak potential (Ep) and qt depend on ta (Figure 4). Thus, in the range 0

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Figure 3. (a) 80 × 80 nm2 STM image showing a 40 × 40 nm2 window (at the center) produced by repetitive scanning of a dodecanethiol-covered copper surface (ta ) 30 days) with the STM tip. (b) Cross section of the window showing the presence of a dodecanethiol multilayer. Figure 5. j vs E curves recorded at v ) 1 mV s-1 for dodecanethiol-covered copper electrodes (ta ) 30 days) in the Borax buffer (pH 9) containing different molar concentrations of KCl (x). The inset shows the dependence of the breakdown potential (Eb) on x for dodecanethiol-covered (b) and dodecanethiol-free (0) copper electrodes.

2Cu(SC12H25)ad + CuO + H2O ) (S-C12H25)2 + 3Cu + 2OH- (3)

Figure 4. j vs E curves recorded at v ) 1 mV s-1 for dodecanethiol-covered copper electrodes at different ta values in the Borax buffer (pH 9) + 0.1 M KCl. The insets a and b show the peak potential (Ep) and charge density (qt) dependence on ta, respectively.

h < ta< 50 h, Ep is shifted toward more positive potentials as ta is increased, remaining constant for ta > 50 h (Figure 4, inset a). Besides, the value of qt decreases markedly with ta (Figure 4, inset b). Therefore, the copper oxide formation, generalized copper dissolution, and pitting corrosion are hindered as the adsorption time increases. The origin of the loss in the protective properties of dodecanethiol layers observed in region II is not clear. It has been reported that self-assembled alkanethiol monolayers on Au are oxidized to sulfonates,23 carboxylates, and sulfates24 at potential values in the range of 0.4 V < E < 1.1 V depending on the pH value and potential scan rates. However, the dependence of qt on ta indicates that the number of defects, and accordingly the number of copper oxide islands, controls the amount of copper corrosion. Note that at potential values corresponding to region II the copper oxide islands consist also of CuO (Figure 2b). It has been reported that thiols can reduce CuO to metallic copper.25 Therefore, we propose that adsorbed dodecanethiol molecules are oxidized to disulfides at the edges of the CuO islands according to the chemical reaction (23) Hagenstro¨m, H.; Schneewiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (24) Yang, D.-F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158. (25) Keller, H.; Simak, P.; Schrepp, W. Thin Solid Films 1994, 244, 799.

The bare copper produced by reaction (3) is immediately transformed into the complex oxide layer at the island edges through reactions (1) and (2). For ta ) 16 h and x ) 0.1 M, the breakdown of the complex oxide layer takes place in the same potential range as that found for dodecanethiol-free copper surfaces. This means that the oxide resistance to breakdown induced by the aggressive chloride anions has not been altered. Chloride is the most important aggressive anion for metals in aqueous environments and in the atmosphere. It has been found that chloride ions cannot reach dodecanethiolcovered silver surfaces, with the alkanethiol layers remaining unaltered.26 We analyze the effect of chloride anions on dodecanethiol-covered copper by increasing x at constant ta. Our results show that the increase in x produces significant changes in the corrosion behavior of dodecanethiol-covered copper electrodes. In fact, for ta ) 16 h, the increase in x from 0.1 to 1 M (Figure 1b, dashed line) results in the shift of Eb in the negative direction so that pitting of copper ocurrs in region I. In this case, pitting takes place at defects of the dodecanethiol multilayer where copper oxides are present. Besides, when the dodecanethiol-covered copper electrode is anodized in region I (E ) 0.0 V < Eb) in the Borax buffer + 1 M KCl, jc is initially small but increases continuously with time (Table 1). Thus, for x ) 1 M, the dodecanethiol film fails to protect the copper surface against generalized or pitting corrosion. Note that, even for ta ) 1 month, the adsorption time that produces the most protective dodecanethiol film, the copper surface “sees” the presence of chloride ions (Figure 5). Thus, copper is pitted at more negative potential values as x is increased from 0.1 to 1 M (Figure 5, inset). This means that chloride ions are able to reach uncovered regions where copper oxides are formed, then promoting the breakdown of the oxide film at these sites, and accordingly, pitting corrosion. The fact that Eb shifts in the negative direction with x in a way similar to that found for dodecanethiol-free copper electrodes (Figure 5, inset) indicates that no significant changes in the mechanism of oxide breakdown take place at oxide islands. (26) Schoenfisch, M. H.; Pemberton, J. E. Langmuir 1999, 15, 509.

Dodecanethiol Layers on Copper Surfaces

Figure 6. Typical STM images (10 × 10 µm2) of different copper surfaces in contact with the Borax buffer + x M KCl at an open circuit. The legend in the upper part indicates the root-meansquare (rms) roughness as a measure of copper corrosion. (a) Copper after polishing. (b) Dodecanethiol-covered copper (ta ) 4 days) after immersion in oxygen-containing Borax buffer + 0.1 M KCl for 4 days. (c) Dodecanethiol-free copper after immersion in oxygen-containing Borax buffer + 0.1 M KCl for 4 days. (d) Copper after polishing. (e) Dodecanethiol-covered copper (ta ) 4 days) after immersion in oxygen-containing Borax buffer + 1 M KCl for 4 days. (f) Dodecanethiol-free copper after immersion in oxygen-containing Borax buffer + 1 M KCl for 4 days. Tunneling conditions: It ) 1.5 nA, Ubias ) 300 mV, scan rate ) 1 Hz

3. Corrosion Behavior of Dodecanethiol-Covered and Dodecanethiol-Free Copper in Nondeaerated Borax Buffer Containing Chloride Anions under Open-Circuit Conditions. Our results show that Eb and jc depend on x. Therefore, defects at the adsorbed dodecanethiol film such as noncovered holes and grain boundaries should be preferential paths for the chloride anion transport to the copper oxide islands. We have shown that water and chloride ions are able to affect the corrosion resistance of dodecanethiol-covered copper. Increasing the adsorption time can drastically reduce the transport of water to the surface by the formation of a denser and less defective dodecanethiol multilayer. In this way, for x e 0.1 M, oxide formation, copper dissolution through the passive film, and pitting corrosion can be markedly

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reduced for potentials near 0 V, a potential value spontaneously reached for copper in oxygen containing aqueous electrolytes. In fact, STM images taken for dodecanethiol-covered copper surfaces (ta ) 4 days) immersed at an open circuit for 4 days in a nondeareated, i.e., oxygen containing, test solution with x ) 0.1 (Figure 6b) show no differences in relation to the initial copper surface (Figure 6a). On the other hand, in the same environment the dodecanethiol-free copper surface under similar conditions shows evidences of generalized corrosion (Figure 6c) due to copper dissolution as CuCl2- complexes. Note that the open circuit potential for both dodecanethiolcovered and dodecanethiol-free copper electrodes reaches potential values lying in region I. These results from a local technique such as STM agree with those obtained from our electrodissolution current (jc) measurements in region I, showing that jc is 40 times smaller when the copper surface is covered by the dodecanethiol layer (Table 1). On the other hand, the dodecanethiol-covered copper surface (ta ) 4 days) in contact with a test solution with x ) 1 at an open circuit exhibits evidences of corrosion (Figure 6e) when it is compared to the initial copper surface (Figure 6d). This means that the protective dodecanethiol layers have failed in this electrolyte containing a high chloride ion concentration in agreement with the jc measurements (Table 1). Corrosion initiates at defects of the dodecanethiol layers where copper oxide islands are present. However, the amount of copper dissolution is much smaller than that found for an uncovered copper surface in the same electrolyte, which exhibits strong generalized corrosion (Figure 6f). In summary, our electrochemical and STM results agree with previously reported data9,12,17,27-31 on the ability of alkanethiols to hinder copper oxide formation and copper dissolution in electrolyte solutions. However, we have demonstrated that this ability depends on the electrode potential and the concentration of aggressive anions. In fact, copper is protected against corrosion by a dodecanethiol multilayer provided that the electrode potential does not reach region II and the concentration of chloride ions in the environment remains low. These findings should be taken into account when alkanethiol layers are used in the corrosion protection of copper. Acknowledgment. The authors thank Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (Argentina) PICT 99-5030 and CONICET (Argentina) for the financial support of this work. LA000852C (27) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130. (28) Haneda, R.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 1856. (29) Haneda, R.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 2786. (30) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 3696. (31) Jennings, G. K.; Laibinis, P. E. Colloids Surf. A 1996, 116, 105.