Inhibition of cobalt active dissolution by benzotriazole in slightly alkaline bicarbonate aqueous media

Electrochimica Acta 52 (2007) 4927–4941

Inhibition of cobalt active dissolution by benzotriazole in slightly alkaline bicarbonate aqueous media Danick Gallant a,b,1 , Michel P´ezolet a,2 , St´ephan Simard a,b,∗ a

b

D´epartement de Chimie, Universit´e Laval, Qu´ebec (Qu´ebec), Canada G1K 7P4 D´epartement de Biologie, Chimie et G´eographie, Universit´e du Qu´ebec a` Rimouski, 300, All´ee des Ursulines, Rimouski (Qu´ebec), Canada G5L 3A1

Received 30 November 2006; received in revised form 12 January 2007; accepted 20 January 2007 Available online 2 February 2007

Abstract The efficiency of benzotriazole as inhibiting agent for the corrosion of cobalt was probed at pH ranging from 8.3 to 10.2 in a sodium bicarbonate solution, chosen to simulate mild natural environments. From electrochemical, Raman spectroscopy, atomic force microscopy and ellipsometry experiments, we have demonstrated that benzotriazole markedly affects the electrodissolution reactions, which become modeled by the formation of a [Co(II)(BTA)2 ·H2 O]n film according to two different mechanisms. Surface-enhanced Raman spectroscopy has shown that the polarization of a cobalt electrode at cathodic potentials with respect to its potential of zero charge allows a mechanism of specific adsorption of the neutral form of benzotriazole to take place through a suspected metal-to-molecule electron transfer and which follows Frumkin’s adsorption isotherms. At the onset of the anodic dissolution, some experimental evidence suggests that these adsorbed neutral benzotriazole molecules deprotonate to yield a very thin [Co(II)(BTA)2 ·H2 O]n polymer-like and water-insoluble protective film, responsible for the inhibition of active dissolution processes occurring at slightly more anodic potentials. In the anodic dissolution region, deprotonated benzotriazole species present in the bulk solution favors the formation of a multilayered [Co(II)(BTA)2 ·H2 O]n film, which also contributes to the inhibition of any further cobalt dissolution usually observed at higher electrode potentials. © 2007 Elsevier Ltd. All rights reserved. Keywords: Benzotriazole; Cobalt; Corrosion inhibition; Frumkin’s adsorption isotherm; Raman

1. Introduction Cobalt-based alloys such as Stellites® have been used in nuclear, aerospace and gas-turbine industries because they offer unique combinations of properties, such as corrosion resistance [1,2]. However, it has often been demonstrated that when the degradation of such cobalt-based alloys occurs, the mechanism involved favors the preferential dissolution of the cobalt-rich phase [3–6]. Recently, the corrosion behavior of unalloyed metallic cobalt exposed to aqueous media containing species

∗ Corresponding author at: D´ epartement de Biologie, Chimie et G´eographie, Universit´e du Qu´ebec a` Rimouski, 300, All´ee des Ursulines, Rimouski (Qu´ebec), Canada G5L 3A1. Tel.: +1 418 723 1986x1488; fax: +1 418 724 1849. E-mail addresses: [email protected] (D. Gallant), [email protected] (M. P´ezolet), stephan [email protected] (S. Simard). 1 Tel.: +1 418 723 1986x1311; fax: +1 418 724 1849. 2 Tel.: +1 418 656 2481; fax: +1 418 656 7916.

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.01.057

currently found in natural environments has been investigated [7–10]. Since the demand for cobalt has continued to grow in tandem with the economics of industrialized nations in the last 15 years [11], studies concerning the control of cobalt corrosion in aqueous media today appear to be of great importance. To prevent the generalized corrosion of different metals and alloys operating in a wide variety of aqueous environments, the use of organic inhibitors has been found to be one of the most important and simple approaches to consider [12,13]. However, this avenue of research has not yet been extended to cobalt and cobalt-based alloys. The inhibitive action of benzotriazole was first recognized in 1947 for copper [14], and since then, it has become one of the most common and efficient water-soluble organic corrosion inhibitors in use [15–17]. Benzotriazole is conveniently abbreviated as BTAH, where the H refers to the labile 1-H atom attached to the N1 atom in the triazole group. However, despite all of the work devoted to understanding and elucidating the interaction mechanisms occurring between benzotriazole and metallic

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surfaces during the inhibition processes, the corrosion inhibition mode of benzotriazole is still debated and, up to now, no consensus has been reached [18]. Nevertheless, ever since the early pioneering work of Cotton and co-workers [19,20], two mechanisms have been proposed for the inhibition of copper corrosion which suggest that either benzotriazole is adsorbed onto the metal surface or it forms a [Cu(I)(BTA)]n polymeric complex with the cuprous ion [21,22], depending on the experimental conditions (solution composition and pH, temperature, benzotriazole concentration, applied potential, to name a few). Unfortunately, information concerning the use of benzotriazole as inhibiting agent for the corrosion of cobalt or cobalt-based alloys in aqueous media is scarce. In an early article, Brusic et al. [23] demonstrated that benzotriazole leads to a significant reduction of the corrosion rate (icorr ) of cobalt in a borate buffer solution, particularly in the presence of Cu2+ ions, which contribute to the formation of a thin Cu(I)-BTA film on the cobalt surface through a spontaneous reduction mechanism of copper ions. More recently, Schnyder et al. [24] have briefly shown, using Tafel plots, the efficiency of benzotriazole as corrosion inhibitor for WC–Co composite also exposed to a borate buffer solution. We have shown in previous studies that a slightly alkaline bicarbonate aqueous environment is particularly aggressive toward metallic cobalt polarized in the potential range of active dissolution [7–9]. In the present study, the effect of benzotriazole as a corrosion inhibitor has been investigated in the active dissolution region of an unalloyed cobalt electrode in order to simulate an anodic polarization by galvanic coupling. Electrochemical and impedance techniques, atomic force microscopy, normal and surface-enhanced Raman spectroscopy as well as ellipsometry were used to probe the potential of benzotriazole to reduce the magnitude of the electrodissolution processes of cobalt in slightly alkaline aqueous environments. As an ultimate goal, the results of this work would be extended, within the framework of future studies, to cobalt-based alloys since their protection is critical for many industrially oriented applications. 2. Experimental section 2.1. Electrochemical experiments The electrochemical experiments were carried out with a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE). The disk electrode was made of a cobalt cylindrical rod (99.95%) and had a surface area of 0.283 cm2 . The rotating gold-ring cobalt-disk electrode had a cobalt disk diameter of 4.5 mm while the gold ring had an inner diameter of 6 mm and an outer diameter of 8 mm. The collection efficiency of the RRDE (N = 0.382) was calculated according to Albery and Bruckenstein [25]. The disk and ring-disk electrodes were set in a Kel-F holder sealed with epoxy resin. The auxiliary electrode was a platinized platinum foil separated from the main compartment by a Nafion® membrane. The reference electrode was a saturated calomel electrode (SCE) connected to the cell by a bridge and a Luggin capillary. All potentials given below are referenced to this electrode. The volume of the electrochemical cell was 0.5 L,

so that the concentration of dissolved cobalt in the bulk can be neglected. Aqueous solutions were prepared using ACS grade chemicals and deionized water. Solutions were deaerated by bubbling a mixture of nitrogen and carbon dioxide. The pH values investigated were reached by the addition of small volumes of a concentrated NaOH solution, and maintained stable by bubbling different mixtures of N2 /CO2 . The electrode surface was successively ground with 600 and 1200 grit emery papers and mechanically polished with 5.0, 1.0 and 0.05 ␮m alumina suspensions, and rinsed with deionized water before each immersion. All experiments were carried out at a controlled temperature of 20 ◦ C. Electrochemical experiments were performed with a PAR 263-2A potentiostat and a Pine bipotentiostat model AFCBP1. The electrode rotator was a Pine analytical rotator model AFSAR. 2.2. Synthesis and characterization of [Co(II)(BTA)2 ·H2 O]n The preparation procedure of [Co(II)(BTA)2 ·H2 O]n (BTA− : deprotonated BTAH) was adapted from that of Brown and Aftergut for the synthesis of poly[bis(imidazolato)-Co(II)] [26]. A 10 mL CoCl2 ·6H2 O (0.198 g, 0.833 mmol) solution was added dropwise to an aqueous solution (500 mL) of benzotriazole (0.298 g, 2.5 mmol) and NaHCO3 (0.560 g, 6.67 mmol) (pH 8.3). The suspension was stirred at room temperature for 1 h, and the precipitate was then filtered out, washed with 1 L of water and dried under vacuum for 48 h. The compound is not soluble in common organic solvents (acetonitrile, acetone, DMF, DMSO, chlorinated solvent), but is soluble in oxidizing concentrated sulphuric acid. Elemental analysis of carbon, nitrogen and hydrogen was performed using a Perkin-Elmer model 2400 CHN Elemental Analyzer. Cobalt content was determined using a Perkin-Elmer model Plasma 40 Inductively Coupled Plasma (ICP) Emission Spectrometer. The vibrational spectroscopy characterization of the synthesized product was carried out with a Perkin-Elmer model 1600 series FTIR spectrometer using the KBr pellets technique. m.p. > 300 ◦ C. Anal. calcd. for [Co(II)(BTA)2 ·H2 O]n : Co, 18.8%; C, 46.0%; H, 2.6%; N, 26.8%. Found: Co, 18.8%; C, 46.3%; H, 2.1%; N, 26.5%. IR (KBr, cm−1 ): 743 m δCH outof-plane, 795 m δtriazole and δbenzene in-plane, 993 w δtriazole in-plane, 1271 m δCH in-plane, 1394 m triazole and benzene rings stretching, 1444 m triazole ring stretching, 1489, 1575 m benzene ring stretching, 3391 br H-bonded water [27]. The characteristic stretching bands of carbonate (∼1450 cm−1 ) and bicarbonate (∼1400 and 1650 cm−1 ) species are absent from the IR spectrum. 2.3. Normal Raman and SERS experiments The surface-enhanced Raman spectra of benzotriazole molecules potentiostatically adsorbed onto the cobalt disk electrode surface were obtained in situ through the optical window of a newly developed flow-through spectroelectrochemical threeelectrode cell, schematized in Fig. 1. The disk electrode was

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and the optical window was set at 2.5 mm in order to avoid any turbulent movement of the solution. Before each SERS experiment, the cobalt electrode was polished up to 0.05 ␮m alumina suspensions. The smooth cobalt electrode was then roughened ex situ (i.e., treated out of the solution in which the spectra will be acquired) during a single oxidation–reduction cycle (ORC) in a 0.5 M H2 SO4 solution between potentials −0.50 and −0.25 V, at a potential sweep rate of 5 mV s−1 (ω = 0 rpm). After careful rinsing with deionized water, the electrode was finally inserted into the spectroelectrochemical cell for the potential-controlled SER spectra acquisition. The SER spectra were obtained with a Horiba Jobin-Yvon model LabRam HR800 Raman spectrometer with a Peltier-cooled CCD detector operating at −70 ◦ C. Excitation was achieved using the 632.8 nm line of a Melles Griot HeNe 20 mW laser. The laser was focused on a d ≈ 2 ␮m spot using an Olympus BX40 microscope with a 50× long working distance objective (8 mm). The pinhole and slit sizes were 800 and 200 ␮m, respectively. The normal Raman spectra of selected materials were obtained ex situ using the same Raman apparatus, except for the spectrum of a very thin film that was recorded using the 514.54 nm line of an argon-ion laser at 4 mW. The 632.8 nm line was preferred for SERS analysis on cobalt [28,29] and is of a high enough energy for analysis of samples of semi-infinite thickness. A multimode atomic force microscope (AFM) model MMAFM-2 (Digital Instruments, Veeco Metrology Group) was used in the tapping mode (constant cantilever amplitude) to characterize the surface roughness and the increase in surface area resulting from the application of the ORC procedure. 2.4. Electrochemical impedance experiments

Fig. 1. (a) 3D schematic representation of the flow-through spectroelectrochemical three-electrode cell employed during surface-enhanced Raman spectroscopy experiments for potential-controlled in situ characterization of benzotriazole–cobalt interactions. Cell components: (A) cap; (B) solution input; (C) optical window; (D) glass; (E) cobalt disk working electrode; (F) Ag|AgCl|KClsat reference electrode; (G) platinum auxiliary electrode compartment; (H) solution output. (b) Lateral cross-section view.

made of a cobalt cylindrical rod (99.95%) set in a Kel-F holder and sealed with epoxy resin. The surface area of the disk was 0.126 cm2 . The auxiliary electrode was a platinized platinum electrode and the reference was an Ag|AgCl|KClsat electrode. However, all potentials given below are referenced to the saturated calomel electrode (SCE, +0.042 V versus Ag|AgCl|KClsat ). To reproduce in the spectroelectrochemical cell the mass transport processes occurring at a RDE surface rotating at a 1000-rpm frequency, a peristaltic pump allowing a controlled laminar flow (100 mL min−1 ) of the electrolyte solution between the metal–solution interface and the optical window was used. The thickness of the solution layer between the electrode surface

2.4.1. Determination of double layer capacitance Qa The usefulness of electrochemical impedance spectroscopy (EIS) to determine the increase in surface area resulting from the application of the ORC procedure was evaluated. To reach steady state conditions required to carry out EIS, a cobalt disk electrode having a geometrical area of 0.159 cm2 was immersed for 35 min (2100 s) in a 0.4 M NaHCO3 solution (pH 8.3) before impedance spectra acquisition. During the first 1200 s of immersion, the electrode was rotating at a frequency of 1000 rpm in open circuit conditions. Thereafter, the electrode was kept stationary and a steady state open circuit potential of −0.70 V versus SCE was reached at t = 2100 s. EIS measurements were thus performed at the open circuit potential using a Solartron SI 1287 electrochemical interface and a Solartron 1252A frequency response analyzer over a frequency range of 1000–0.25 Hz at 10 points/Hz decade using a 10 mV amplitude sinusoidal voltage. Analysis of the impedance data was made using ZView software. Considering the case of a system with uncompensated solution resistance and incorporating an oxidation reaction (cobalt oxidation) and a reduction process (hydrogen evolution reaction) as faradaic current components typically observed under polarization resistance open circuit conditions [30,31], the equivalent circuit presented in Fig. 2 was used to fit impedance experimental data acquired. Because a polycrystalline solid electrode typically shows deviation from the ideal capacitive behavior due to atomic

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Fig. 2. Electrical equivalent circuit used for fitting impedance data (Rs : solution resistance; Rp : polarization resistance; CPE: constant-phase element).

scale surface roughness and heterogeneities, anion adsorption, variation of coating composition and non-uniform potential and current distribution, the “constant-phase element” (CPE) was introduced in the equivalent electrical circuit as a substitute for capacitance to account for capacitance dispersion. Even though the physical meaning of the CPE is still not clear, it represents an extremely flexible fitting parameter [32,33]. According to Zoltowski [34], the impedance of the CPE is given by: ZCPE =

1 Qa (jω)α

(1)

√ j is the imaginary unit (j = −1), ω the angular frequency (2πf) and α is the CPE-power (0 ≤ α ≤ 1). One can find that the CPE is a pure capacitance for α = 1, an infinite Warburg impedance for α = 0.5 and a pure resistance for α = 0 [35,36]. Since the CPE-constant Qa (also considered as a double layer capacitance quantity [33]) is directly proportional to the electrode active area [34], it was employed to calculate the Qa,rough /Qa,smooth ratio that should represent the increase in surface area resulting from the application of the ORC procedure. 2.4.2. Determination of potential of zero charge The differential capacity at the interface of a cobalt electrode and a 0.4 M NaHCO3 solution (pH 8.3) was measured by surimposing an ac perturbation of 10 mV rms and 25 Hz on the 5 mV s−1 voltage ramp between potentials −1.05 and −0.40 V. Since at high electrolyte concentration the capacitive effect is determined predominantly by the inner part of the double layer, making the metal–solution interface better described by a capacitor [31,37], the differential capacity was calculated assuming a simple series RC equivalent circuit. The potential of zero charge (pzc) was determined from the position of the diffuse layer minimum on the C versus E curve [31,37–39]. 2.5. Single wavelength ellipsometry experiments Ellipsometric parameters Δ and ψ of electrodeposited films or corrosion products were recorded ex situ using a Horiba JobinYvon model UVISEL ellipsometer at a wavelength of 632.8 nm and an angle of incidence of 70◦ . In order to determine the optical constants and the thickness of the surface film, the Δ/ψ trajectory was calculated with a Fortran 77 program used to best fit the experimental Δ and ψ parameters. This program, initially developed by McCrackin [40] in 1969 and modified later by Tompkins [41], was updated and corrected. The improved version is available in Electrochimica Acta multimedia archives.

Fig. 3. Anodic linear sweep voltammetry scans for a cobalt electrode in a 0.4 M NaHCO3 solution at pH 8.9 containing 0 to 2 × 10−3 M benzotriazole; ω = 1000 rpm; dE/dt = 5 mV s−1 ; potential limits: −1.05 and +1.0 V.

3. Results and discussion 3.1. Adsorption study The linear sweep voltammetry (LSV) curves presented in Fig. 3 reveal that the addition of a low benzotriazole concentration (3 × 10−4 to 2 × 10−3 M) to a 0.4 M NaHCO3 solution at pH 8.9 markedly reduces the overall intensity of the anodic dissolution processes of cobalt. Since the intensity of the anodic current peak at −0.45 V decreases with an increasing benzotriazole concentration, it cannot be attributed to a benzotriazole oxidation process. Furthermore, it can be seen in Fig. 3 that each benzotriazole concentration investigated suppresses the oxidation reactions occurring in the potential region extending from −0.2 to +0.4 V. Additionally, a benzotriazole concentration equal to or higher than 5 × 10−3 M was found to entirely eliminate the electrodissolution of cobalt occurring in the potential range extending from −0.7 to +0.4 V (curves not shown). It is thus evident that benzotriazole molds both the kinetic and the thermodynamic parameters that control the cobalt electrodissolution processes. In a recent study [9], we have shown that the anodic dissolution region of a cobalt electrode polarized in aqueous bicarbonate media can be resolved into three main individual processes. However, as shown in Fig. 3, it seems that among these reactions, only the corrosion process occurring in the most cathodic portion of the dissolution region remains active when benzotriazole is added to a 0.4 M NaHCO3 solution. This process was previously identified as a diffusion-controlled production of the soluble complex species Co(CO3 )2 2− [8]. Therefore, to correctly evaluate the extent of the corrosion inhibition by benzotriazole, the magnitude of this specific dissolution component was resolved in a solution free of benzotriazole, according to the deconvolution procedure of LSV curves described in Ref. [9]. This data was thereafter used as the referential current density (see below jP1,free ) for the calculation of the inhibition efficiency when benzotriazole is added in the solution at pH 8.9. The same procedure was repeated for other pH investigated. From the

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θ =1−

jadd jP1,free

4931

(3)

where θ is the degree of coverage of the cobalt electrode surface by benzotriazole molecules, jadd the maximum current density recorded in the active dissolution region in the presence of an inhibitor, jP1,free the current density of the P1 component determined in the absence of an inhibitor and using the deconvolution procedure, f the interaction term parameter (f > 0 for lateral attraction interaction between the adsorbed benzotriazole molecules, and f < 0 for lateral repulsion interactions between the adsorbed benzotriazole molecules), c the inhibitor concentration (M), 55.5 the water concentration in solution (M), G◦ads the standard free energy of adsorption (kJ mol−1 ), R the gas constant (8.3145 J K−1 mol−1 ) and T is the absolute temperature (K) [18,45,46]. The Frumkin adsorption isotherm is a general expression since the limiting case for which f = 0 is representative of an interaction-free behavior between adsorbed species and defines the Langmuir isotherm [43,47]. Because the logarithmic term is very sensitive to inaccuracy in determining θ at the limiting values of the coverage [43], two intermediate θ values in the range 0.3–0.8 and their corresponding benzotriazole concentration were selected for the calculation of G◦ads and f parameters according to Eq. (2) [42]. From the G◦ads and f values determined, Eq. (2) can be reorganized under the form:  Fig. 4. (a) Deconvolution of the linear sweep voltammetry scan for a cobalt electrode in a 0.4 M NaHCO3 solution at pH 8.9. (b) P1 component resulting from the deconvolution in (a), and linear sweep voltammetry scans for a cobalt electrode in a 0.4 M NaHCO3 solution at pH 8.9 containing 3 × 10−4 to 2 × 10−3 M benzotriazole; ω = 1000 rpm; dE/dt = 5 mV s−1 ; potential limits: −1.05 and +1.0 V.

assumption that a dissolution process cannot be initiated as long as the intensity of the preceding one did not start to decrease, individual dissolution components (P1 + P2 + P3) and the overall fitting curve were obtained, as typically presented in Fig. 4 for solution pH 8.9. Components P2 and P3 were however useless since their corresponding corrosion reactions are no longer observable when benzotriazole is added to the corrosive aqueous environment. The formation of cobalt oxides CoO and Co3 O4 on the electrode surface [9,10] is then excluded from the corrosion behavior of cobalt in a bicarbonate solution containing benzotriazole. Assuming that the current density recorded in the active dissolution region is due to the oxidation of the metal’s sites unoccupied by adsorbed benzotriazole molecules, the inhibitive effect of benzotriazole in solutions of pH 8.3, 8.9, and 10.2 was tentatively explained in terms of thermodynamic adsorption parameters using the Frumkin isotherm. The Frumkin isotherm is commonly used to quantify the interactions occurring between a corrosion inhibitor and a metal [18,42–44] and it is expressed by the relationship:      − G◦ads θ 1 exp(−fθ) = c exp (2) 1−θ 55.5 RT

log c = log

θ 1−θ

 + Aθ + B

(4)

where A = −f/2.303 and B = ( G◦ads /2.303RT ) + log 55.5 Eq. (4) was used to plot the S-shaped Frumkin isotherm curves shown in Fig. 5a. The existence of adsorption interactions between benzotriazole molecules and the cobalt surface is thus confirmed since most of the experimental data fit nicely into the Frumkin isotherm plots. These adsorption isotherms demonstrate that as the pH is decreased from 10.2 to 8.3, the spontaneity of the benzotriazole adsorption process is favored since the free adsorption energy parameter undergoes a change from −23.9 to −27.7 kJ mol−1 . A similar pH effect has been observed for the adsorption of benzotriazole on a copper electrode in a 1 M sodium acetate solution [48]. Furthermore, the calculated G◦ads values are included in the −20 to −40 kJ mol−1 range which usually characterizes the chemisorption of benzotriazole on copper in a variety of aqueous media [45,48–52]. The interaction term parameter (f) strongly suggests that the attraction interactions between absorbed benzotriazole molecules are enhanced at a more alkaline pH. A positive value of f has also been associated to a vertical orientation of the inhibitor molecule on the electrode surface [18]. However, the interpretation of this parameter is delicate since it has been stated that in some cases, it is not related to the nature of the adsorption layer and is only a matching parameter [53]. Therefore, to avoid any useless speculation, the physical significance of the f parameter will not be discussed in the present study.

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HCO3 − + BTAH  H2 CO3 + BTA− K = 0.0141 (pKa BTAH = 8.2; pKa H2 CO3 = 6.35)

(6)

CO3 2− + BTAH  HCO3 − + BTA− K = 134.9 (pKb CO3 2− = 3.67; pKb BTA− = 5.8)

Fig. 5. (a) Frumkin’s adsorption isotherms for benzotriazole on cobalt in a 0.4 M NaHCO3 solution at pH 8.3 (), 8.9 (䊉) and 10.2 (), containing 1 × 10−5 to 3 × 10−2 M benzotriazole. (b) Relationship between Gibbs free energies of adsorption for benzotriazole on cobalt ( G◦ads ) and calculated [BTA− ]/[BTAH] ratios as a function of solution pH.

3.2. Nature of the inhibitive species To correlate the extent of the inhibition activity observed to the form of the inhibitive species interacting with the cobalt surface, the [BTA− ]/[BTAH] ratio was calculated with respect to the pH from 8.3 to 10.2. The existence of the protonated form of benzotriazole (i.e., [BTAH2 − ]) is unlikely in this pH range. As shown by Eq. (5), the BTAH molecule can be dissociated at the labile H atom position of the triazole ring to yield the BTA− anion, which is stabilized by resonance [16,49,54]. Since the concentration of hydroxide ion present in the investigated media is too low to control the BTAH dissociation, the equilibrium reactions presented below that consider HCO3 − and CO3 2− species as bases were rather used to establish the dissociation degree of BTAH.

(5)

(7)

Fig. 5b demonstrates that the free energy of adsorption follows the logarithm of the [BTA− ]/[BTAH] ratio. Hence, as can be seen, lowering the [BTA− ]/[BTAH] ratio enhances the spontaneous adsorption process of the benzotriazole molecules, strongly suggesting that the neutral form of benzotriazole (BTAH) mainly contributes to the inhibition of the active dissolution process of cobalt in aqueous bicarbonate media. The chemisorption of the neutral form of BTAH on a copper surface was postulated at negative potential values with respect to the potential of zero charge (PZC) of the metal [13,16]. In a 0.4 M NaHCO3 solution, adsorption of the neutral form of benzotriazole on cobalt should then be plausible at potential negative to −0.64 V versus SCE, the PZC of cobalt as positioned by the minimum on the C versus E curve presented in Fig. 6. The appropriate minimum point on the C versus E curve was determined considering an assumption of the Gouy–Chapman–Stern model which predicts that the capacitance at PZC diminishes when the electrolyte concentration is lowered [37,55]. Because of the expected thinness for the adsorbate film obtained during cathodic polarization, the surface-enhanced Raman spectroscopy (SERS) technique was necessary to investigate its nature. 3.3. SER spectra of adsorbed benzotriazole molecules on the cobalt electrode surface The SERS technique is well known for its high surface sensitivity which provides molecular level information from the metal–liquid interface for a metal immersed in an aqueous solution [56]. The large Raman signal enhancement it provides, measured for the first time on silver in 1974 by Fleischmann

Fig. 6. Differential capacity (C) recorded as a function of the electrode potential (E) in a 0.01–1.0 M NaHCO3 solution at pH 8.3 (10 mV rms sine wave modulated at 25 Hz; dE/dt = 5 mV s−1 ). Dashed line: the relevant anodic linear sweep voltammetry curve in 0.4 M NaHCO3 solution (j vs. E) (dE/dt = 5 mV s−1 ).

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et al. [57], has been most likely explained through the years on the basis of two mechanisms, namely the electromagnetic field enhancement and the chemical enhancement [58,59]. Since SERS is again most likely restricted to noble metals such as Ag, Au and Cu [29], obtaining a SER signal directly from a massive cobalt surface is unusual and somewhat challenging, and only a few studies conducted by Tian’s group have succeeded in this enterprise [28,29,60–63]. Nevertheless, a novel, simple, electrochemical etching procedure that consists of a single oxidation–reduction cycle (ORC) was recently developed in our laboratory (see Section 2.3 for details), allowing the observation of an amplified Raman signal resulting from the SERS effect. The metal roughening pretreatment is critical since it is expected to provide the appropriate particle size needed for the observation of the SERS effect [29,64,65]. Nanoscale features in the order of 10–200 nm, regardless of their shape, were usually reported to efficiently produce a surface-enhanced Raman signal [58,65–68]. To select the suitable parameters for the ORC procedure, AFM profiles of the roughened cobalt surfaces were recorded. As shown in Fig. 7b, using cathodic and anodic limits of −0.50 and −0.25 V during the ORC performed at 5 mV s−1 , respectively, surface features almost spherical in shape and having diameters ranging from 50 to 200 nm are obtained. The root-mean-square roughness (Rq ) of etched and mirror-like polished cobalt electrodes was calculated according to the relation [69,70]:  N 2 i=1 Zi Rq = (8) N where Zi is the current difference between the height of the element and the mean plane, and N is the number of points in the image. An average Rq value of 43 ± 5 nm (N = 6) was calculated for the roughened cobalt surface presented in Fig. 7b. Same order values of the Rq roughness parameter were previously determined for a SERS-active silver substrate [70]. Before performing the ORC procedure, the polished cobalt electrode (Fig. 7a) has some roughness with Rq = 2.3 ± 0.2 nm which is in the range expected for smooth surfaces [70]. Fig. 8a presents the normal Raman spectrum for a bulk solution of 0.4 M NaHCO3 at pH 10.2 and containing 5 × 10−3 M benzotriazole. An almost identical spectrum was recorded at the metal–solution interface of a smooth cobalt electrode polarized at E = −1.3 V (Fig. 8b). It is important to point out that a potential of −1.3 V is far more cathodic than the PZC of cobalt, and should therefore allow benzotriazole adsorption, as discussed above. However, no enhancement of the Raman bands located at 778 and 1374 cm−1 assigned to the benzene ring breathing mode [13,16,45,71–73] and benzene and/or triazole ring stretching vibrations [13,59,71,74], respectively, is observed on the spectrum recorded at the smooth cobalt–solution interface. The strong Raman bands at 1018 and 1066 cm−1 present in both spectra can be assigned to stretching vibrations of HCO3 − and CO3 2− species in solution, respectively [75]. Fig. 9 shows a set of potential-dependent SER spectra recorded at the interface between a cobalt electrode roughened according to the method described above, and a 0.4 M NaHCO3 contain-

Fig. 7. AFM images: (a) cobalt electrode mechanically polished up to 0.05 ␮m alumina suspensions and (b) cobalt electrode ex situ roughened during a single oxidation–reduction cycle (ORC) in a 0.5 M H2 SO4 solution between potentials −0.50 and −0.25 V, at a potential sweep rate of 5 mV s−1 (ω = 0 rpm).

ing 5 × 10−3 M benzotriazole solution at pH 10.2. These spectra clearly reveal the presence of benzotriazole molecules in the vicinity of the metal surface polarized at potentials between −1.3 and −0.8 V. As the potential was anodically moved within this range, the intensity of most of the Raman bands significantly decreased. At potentials slightly anodic to −0.8 V, the SER spectrum is that of the bulk solution, demonstrating that benzotriazole molecules have left the surface at such applied electrical potential. This observation is consistent with the fact that the adsorption of the neutral BTAH species is expected, as already mentioned, at potential negative with respect to the PZC of cobalt (−0.64 V versus SCE). Moreover, the position of the PZC justifies that anodic dissolution of cobalt in the presence of aggressive HCO3 − anionic species starts around −0.7 V. As far as a 5 × 10−3 M benzotriazole concentration allows a surface coverage ≥85% at the three pH studied, the spectral features reported for pH 10.2 have also been encountered at pH 8.3 and 8.9. A quick comparison of the spectra presented in Figs. 8 and 9

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Fig. 8. (a) Normal Raman spectrum for a 0.4 M NaHCO3 solution at pH 10.2 containing 5 × 10−3 M benzotriazole. (b) Raman spectrum for a smooth cobalt electrode surface (polished up to 0.05 ␮m) polarized at E = −1.3 V in a 0.4 M NaHCO3 solution at pH 10.2 containing 5 × 10−3 M benzotriazole.

reveals that the investigation of the adsorbate layer covering the cobalt electrode under cathodic polarization necessarily requires the SERS effect. Some major characteristic features assigned to a benzotriazole adsorption phenomenon are revealed in the spectra presented in Fig. 9. Firstly, upon adsorption, the characteristic bands due to the CCC in-plane bend [45], the ring breathing mode, and the CCC stretching vibrations of the benzene moiety of benzotriazole [45] are blue-shifted from 540 to 555 cm−1 , 780 to 788 cm−1 , and 1280 to 1283–1287 cm−1 , respectively (see Fig. 10a for Raman spectrum of solid benzotriazole). These frequency shifts suggest that the BTAH molecules are attached on the electrode surface. As seen in Fig. 9, the Raman band at ∼788 cm−1 due to benzotriazole adsorbed on cobalt increases in intensity relative to that of the band located at ∼780 cm−1 assigned to benzotriazole in solution as the potential becomes more negative. On the other hand, the bands assigned to the triazole ring such as the –N–N–N– stretching vibration located at 1210 cm−1 in solid benzotriazole [13,16,45,59,71,73,74,76], is red-shifted to 1191, 1181 and 1172 cm−1 as the electrode is sequentially polarized at −0.8, −1.1 and −1.3 V, respectively. The presence of this band is contrary to the hypothesis of a tria-

Fig. 9. SER spectra for benzotriazole adsorbed on a roughened cobalt electrode surface polarized at: (a) −0.8 V; (b) −1.1 V; (c) −1.3 V, in a 0.4 M NaHCO3 solution at pH 10.2 containing 5 × 10−3 M benzotriazole.

zole ring opening reaction upon benzotriazole reduction in acidic media [77–79]. The fact that the frequency of this band decreases linearly with the applied negative potential as shown in Fig. 11 suggests the attachment of the neutral benzotriazole molecule to the surface through an electron transfer from the cobalt electrode to the 6␲* antibonding molecular orbital (LUMO) of benzotria-

D. Gallant et al. / Electrochimica Acta 52 (2007) 4927–4941

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Fig. 11. Potential-dependent position of the –N–N–N– Raman stretching mode for benzotriazole adsorbed on cobalt.

Fig. 10. Ex situ normal Raman spectrum obtained with the 632.8 nm excitation line for (a) solid benzotriazole and (b) cobalt electrode surface following a linear sweep voltammetry scan performed in a 0.4 M NaHCO3 solution at pH 8.3 containing 1 × 10−4 M benzotriazole (ω = 1000 rpm; dE/dt = 5 mV s−1 ; potential limits: −1.05 and +1.0 V) (inset: AFM image of the surface); (c) [Co(II)(BTA)2 ·H2 O]n compound synthesized according to the procedure described in Section 2.2 (inset: suggested molecular structure of the compound).

zole which is predominantly N(2) py in character (N(2) being the middle N atom of the triazole ring and the y-axis is perpendicular to the BTAH molecular plane) [80]. Such a binding process involving an electron transfer is consistent with the chemisorption process suggested from the G◦ads values calculated using Frumkin’s adsorption isotherms. Furthermore, the binding process involving a transfer of electron density from the bonding orbitals of a metal to an antibonding orbital of a molecule is generally well accepted [81]. The filling of the 6␲* molecular orbital (MO) during a N(2) -coordination of benzotriazole on cobalt should result in a decrease in the bond order for the vibrational modes involving the N(2) atom, as observed in Fig. 11. Indeed, as the Co–N(2) bond is strengthened with negative-going potential, the frequency of the triazole ring breathing mode decreases. It is well known that adsorption can activate molecules by cleaving bonds or weakening them [82]. As mentioned above, there is however no indication here of bond cleaving resulting from adsorption. The shoulder located at 1144 cm−1 assigned to the NH in-plane bending mode [13,17,71,83] also suggests that the imino hydrogen of BTAH is retained upon benzotriazole adsorption. According to the electronic structure of benzotriazole as described by Jiang et al. [80], a charge transfer from cobalt to the 6␲* MO of benzotriazole should additionally correspond to an electron donation in the bonding MO of the benzene moiety of benzotriazole (i.e., 3␲, 4␲, 5␲) [80,84]. This electron donation explains the blue-shift observed for the Raman bands initially located at 540 cm−1 (CCC in-plane bend in benzene moiety) and ∼780 cm−1 (benzene ring breathing mode) in the spectrum of free benzotriazole in solution. The shift of these bands indicates that the benzene moiety does not interact directly with the cobalt surface. This finding, which is in agreement with previous results that suggest bonding via the N(2) atom, qualitatively indicates that the molecular plane of adsorbed benzotriazole molecules should be almost perpendicular to the surface [16]. This result is also in harmony with the description of interactions reported between benzotriazole and copper [16,27,85,86]. On the basis of these studies, a sp2 -d bonding is also expected between benzotriazole and cobalt. Taking in account the different light-collection geometries in the surface and bulk-phase Raman scattering [16], the “sur-

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face enhancement factor” (SEF) [63,87] was calculated using Raman bands located at ∼780 cm−1 (benzene ring breathing mode) and ∼1380 cm−1 (combination of the benzene and triazole rings breathing modes) on the spectrum presented in Fig. 9c, using the relation: SEF =

Isurf NA chσ Ibulk R

(9)

where Isurf and Ibulk are the Raman intensity of the molecule on the surface and in the bulk solution, respectively, NA is the Avogadro’s constant, c the concentration of the molecule in the bulk solution (5 × 10−3 M), h the effective thickness of laser illumination in the bulk phase (40 ␮m, as we have determined using a silicon wafer), σ the averaged surface area occupied ˚ 2 , considering a BTAH by a single adsorbed molecule (20 A molecule adsorbed perpendicularly to the surface, according to Refs. [16,18,57]), and R is the roughness factor of the surface, defined as the ratio of the real area of a roughened surface to that of a smooth surface (1.52, as determined from AFM images). Fig. 12 presents the Zimag versus Zreal Nyquist plots for the smooth and rough cobalt electrode (metal roughened according to the ORC procedure described in Section 2.3). Nyquist plots having a similar shape were reported for aluminum in sulphate solutions [32], copper in perchlorate solutions [31] and galvanized steel in concrete [30]. These systems were all studied using the circuit presented in Fig. 2. The impedance of this circuit is described by the following equation [88]: Z(ω) = Rs +

Rp 1 + Qa (jω)α

(10)

According to the fitting procedure of impedance data described in Section 2.4, double layer capacitance Qa of 16.9 and 26.7 ␮−1 sα were calculated for smooth and rough cobalt electrode, respectively. Hence, the Qa,rough /Qa,smooth ratio of 1.58 as obtained is very close to the roughness factor determined from AFM images (1.52), confirming that EIS can be used for fast

Fig. 12. Nyquist plots of measured and fitted data for smooth (polished up to 0.05 ␮m alumina suspensions) and rough (prepared according to the ORC procedure, see Section 2.3) cobalt electrodes in a 0.4 M NaHCO3 solution at pH 8.3 (inset: fitting parameters obtained from the electrical circuit in Fig. 2 and used to calculate Qa,smooth and Qa,rough ).

determination of the roughness factor. After the contribution of the bulk solution has been subtracted from the spectrum recorded at the metal–solution interface, a SEF of about two orders of magnitude (102 ) was estimated for both bands, a value which is consistent with SEF generally expected from transition metals [67,72]. In situ SER spectroscopy has thus been successfully employed to support the theory according to which neutral benzotriazole molecules chemisorb on the surface of a cobalt electrode polarized at negative potentials with respect to the PZC of cobalt. This chemisorption process is compatible with the moderately negative values of G◦ads calculated using the Frumkin adsorption isotherms [21]. The variation in the intensity of Raman bands as a function of applied potential suggests that benzotriazole molecules are progressively desorbed from the surface as the electrical potential approaches that of the zero charge of the metal. The occurrence of this desorption process will be discussed below in regard to corrosion inhibition considerations. 3.4. Electrodissolution mechanism of cobalt in solution containing benzotriazole To identify the mechanism by which benzotriazole acts as a corrosion inhibitor, it is of primary importance to identify the nature of the corrosion product electrogenerated during the dissolution process of cobalt in the presence of benzotriazole. As can be seen in Fig. 5a, most of the benzotriazole concentrations added to a 0.4 M NaHCO3 solution are efficient to inhibit to some extent the anodic electrodissolution of cobalt. However, an intriguing behavior is expected at very low benzotriazole concentrations (i.e., 1 × 10−5 and 1 × 10−4 M), which do not fit as well the calculated Frumkin isotherm curves. A main Co–BTAH interaction mechanism that could be quite different from that governing the dissolution inhibition at higher benzotriazole concentrations is thus anticipated at c(BTAH) = 1 × 10−4 M, especially at pH 8.3. Under these conditions, the deviation between the calculated isotherm curve and the experimental data is the largest. To probe the dissolution behavior as well as the type of metal–benzotriazole interaction prevailing under the latter experimental conditions, rotating ring-disk electrode (RRDE) experiments were carried out in a 0.4 M NaHCO3 solution containing 1 × 10−4 M benzotriazole at pH 8.3, 8.9 and 10.2. During the LSV scan performed at the cobalt disk electrode between potentials −1.05 and +1.0 V, the gold ring electrode was held polarized at −1.0 V. Table 1 reports the iring /idisk ratios determined in each solution at the point of maximum current density located in the active dissolution potential range. The comparison of calculated ratios with and without inhibitor at the three pH investigated shows that the relative amount of species electrogenerated at the cobalt disk, and thus reaching the gold ring electrode, diminishes as the pH approaches a near-neutral value. It is thus suggested that lowering the pH of a diluted benzotriazole solution favors the formation of an insoluble film on the cobalt surface during the electrodissolution process. This film should contain benzotriazole species since its addition in solu-

D. Gallant et al. / Electrochimica Acta 52 (2007) 4927–4941 Table 1 pH dependence of the iring /idisk ratio recorded using a rotating ring-disk electrode in the potential region of anodic dissolution of a cobalt electrode in a 0.4 M NaHCO3 solution containing 0 and 1 × 10−4 M benzotriazole c(BTAH) (M)

0 1 × 10−4

iring /idisk pH 8.3

pH 8.9

pH 10.2

0.796 0.175

0.471 0.331

0.389 0.382

(dE/dt)disk = 5 mV s−1 ; Ering = −1.0 V; ω = 1000 rpm.

tion is responsible for the decrease observed in the iring /idisk ratio. Interestingly, at the end of the experiment performed at pH 8.3, a uniformly tarnished, brown film was found on the cobalt electrode surface. The AFM image and the normal Raman spectrum (ex situ recorded) of this smooth film are presented in Fig. 10b. The comparison of this Raman spectrum with that of solid benzotriazole (Fig. 10a) clearly indicates that the uniformly electrogenerated corrosion product is mainly composed of benzotriazole. The benzene ring breathing mode, located at 782 cm−1 in solid benzotriazole, was blue-shifted to 790 cm−1 in the spectrum of the brown electrode surface, indicating that benzotriazole is bound to the cobalt through the N(2) atom, as discussed in Section 3.3 [13]. To identify the reaction that can occur between cobaltous species electrogenerated during anodic dissolution and benzotriazole species present in the bulk solution, a cobaltous salt was added to a solution containing benzotriazole and sodium bicarbonate at pH 8.3, according to the procedure described in Section 2.2. From the beginning of the reaction, a precipitate was massively formed. Elemental analysis as well as FTIR spectroscopy analysis performed on it have revealed the [Co(II)(BTA)2 ·H2 O]n polymer-like nature of the synthesized complex. According to Ndzie et al. [89], a monohydration of each Co(II)(BTA)2 unit is expected. The high thermal stability (m.p. > 300 ◦ C) of the compound is also consistent with this type of organometallic polymer-like structure. Because it is unlikely that we are observing a five membered cobalt centre [82], it is suggested that the chemisorbed water molecule is intercalated between triazole rings and maintained in place by H-bonding with both middle N atoms. The inset of Fig. 10c presents the suggested structure of the compound. Because it possesses two equivalent sites for coordination, benzotriazolato anion is acting as bridging ligand [26]. Fig. 10c also presents the normal Raman spectrum of the synthesized polymeric cobaltous complex. The broadness of the Raman bands observed on this spectrum is attributed to the polymeric network characterizing the compound. Since the Raman spectral features on the latter spectrum are similar to those observable on the Raman spectrum of the uniform surface film electrogenerated in a 0.4 M NaHCO3 + 1 × 10−4 M benzotriazole solution at pH 8.3 (Fig. 10b), it becomes reasonable to suggest that the electrodissolution process of cobalt in such a medium favors the formation of a [Co(II)(BTA)2 ·H2 O]n polymer-type insoluble complex as a corrosion product quite uniformly deposited on the electrode surface. Therefore, the decrease of the iring /idisk ratios in the presence of benzotriazole and presented in Table 1 can directly be assigned to the formation of a [Co(II)(BTA)2 ·H2 O]n insolu-

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ble film on the cobalt surface. On this basis, it is obvious that the electrogeneration of such a film is favored as the pH approaches a near-neutral value. At more alkaline pH, the iring /idisk ratio indicates that the involvement of benzotriazole in the electrodissolution process tends to vanish. It could now be of great interest to establish the sequence of the processes leading to the formation of the [Co(II)(BTA)2 ·H2 O]n complex on the electrode surface. At the beginning of the potential scan in the 0.4 M NaHCO3 + 1 × 10−4 M benzotriazole solution at pH 8.3, neutral benzotriazole molecules adsorb on the cobalt electrode surface, as suggested on the basis of SERS experiments. In the initial potential range of the LSV experiments performed to establish the Frumkin isotherms (i.e., −1.05 to ∼−0.8 V), the formation of a [Cox (BTA)y ·zH2 O]n -type compound on the electrode surface is unlikely since the production of cobaltous species is thermodynamically unfavorable [90–92]. Because an adsorption phenomenon involves an equilibrium state between surface and bulk species [93], a low benzotriazole concentration cannot provide a high coverage of the electrode surface, which thus remains partially unprotected. In the active dissolution region, these uncovered areas become highly susceptible to attack by the HCO3 − species, well known for their aggressiveness toward cobalt [8]. As a result, one could look for a massive production of the Co(CO3 )2 2− (aq) species, according to the two-step mechanism [8]: Step 1 :

Co + HCO3 − (aq) → CoCO3(s) + H+ (aq) + 2e− (11)

Step 2 :

CoCO3(s) + HCO3 − (aq)

→ Co(CO3 )2 2− (aq) + H+ (aq) Global reaction :

(12)

Co + 2HCO3 − (aq)

→ Co(CO3 )2 2− (aq) + 2H+ (aq) + 2 e−

(13)

It has just been shown above that in the presence of both BTA− and HCO3 − species, cobaltous ions preferentially bind BTA− species, leading to the formation of a [Co(II)(BTA)2 ·H2 O]n solid phase on the electrode surface. However, from the iring /idisk ratios presented in Table 1, it is evident that the formation of a [Co(II)(BTA)2 ·H2 O]n solid phase is excluded in a diluted benzotriazole solution at pH 10.2. Therefore, the production of the Co(CO3 )2 2− species dominates the dissolution behavior. In a solution of pH 10.2, most of the 1 × 10−4 M benzotriazole species in solution are found in their deprotonated form (BTA− ). Therefore, the surface coverage ensured by the neutral form of benzotriazole species is almost zero. As a consequence, the LSV curves recorded in the presence and absence of benzotriazole at a solution pH of 10.2 are similar, demonstrating that benzotriazole does not influence the corrosion behavior of cobalt under such conditions. However, due to the relatively high concentration of BTA− species present in the bulk solution at this pH value, the formation of a [Co(II)(BTA)2 ·H2 O]n polymeric film was expected in the anodic dissolution region. Instead, a massive production of the Co(CO3 )2 2− species was observed in the potential region associated with active

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Fig. 13. Calculated molar fractions of HCO3 − and CO3 2− species with respect to the solution pH.

dissolution phenomena. As represented in Fig. 13, the molar fraction of carbonate species (CO3 2− ) rapidly increases as the pH is moved from ∼9.2 to 10.2. Considering the (2−) charge of carbonate species, it becomes plausible to suggest their preferential ligation to cobaltous ions electrogenerated during the bicarbonate species attack as an explanation for the behavior observed. At more alkaline solution pH, the following dissolution mechanism thus predominantly takes place: Bicarbonate attack :

Co + HCO3 − (aq)

→ CoCO3(s) + H+ (aq) + 2e− Fast carbonate ligation :

constant pH. To investigate this critical parameter, ex situ single wavelength ellipsometry (SWE) measurements were carried out on the samples prepared in bicarbonate solutions containing a benzotriazole concentration ranging from 1 × 10−4 M (θ ∼ 0) to 3 × 10−2 M (θ ∼ 1). Benzotriazole concentrations were indeed selected in the whole range of surface coverage degrees previously calculated to plot Frumkin’s adsorption isotherms. Prior to its characterization with SWE, each sample was prepared during a single LSV experiment performed from −1.05 to +1.0 V in a solution at pH 8.3 containing 0.4 M NaHCO3 plus a given benzotriazole concentration. At the end of each LSV scan, the sample was thoroughly rinsed with deionized water and gently dried under a nitrogen gas flow. Ellipsometry analyses were then conducted at a wavelength of 632.8 nm and an angle of incidence of 70◦ . Fig. 14 presents the Δ–ψ experimental data recorded during the SWE experiments for the bare cobalt material and each sample prepared according to the method described above. For an ambient-film-substrate three-component system, the deviation of the Δ and ψ parameters’ values (which quantify the differential change in phase and amplitude of the electric vector upon reflection, respectively) from that of the ambient-substrate two-component film-free system is directly related to an increasing thickness or density of the film deposited on the substrate [41,94,95]. On the basis of this relationship, one can see in Fig. 14 that the thickness or packing of the brown film clearly increases as the benzotriazole concentration is decreased. The Δ/ψ trajectory associated with a film characterized by the com-

(14)

CoCO3(s) + CO3 2− (aq)

→ Co(CO3 )2 2− (aq)

(15)

In conclusion, in a solution at near-neutral pH containing a low concentration of benzotriazole, a water-insoluble [Co(II)(BTA)2 ·H2 O]n film is formed on the cobalt electrode surface during the electrodissolution process. Under such conditions, it was found that BTA− species efficiently compete with HCO3 − species for the ligation of cobaltous ions. As a consequence, the formation of the soluble Co(CO3 )2 2− complex becomes marginal. At a high solution pH, the trend is inverted since CO3 2− species easily bind cobaltous species. At this point, it is not obvious whether the [Co(II)(BTA)2 · H2 O]n solid phase electrogenerated is responsible for the more spontaneous dissolution inhibition observed at near-neutral solution pH. In the next two sections, the involvement in the cobalt electrodissolution inhibition of both the adsorbed benzotriazole molecules and the [Co(II)(BTA)2 ·H2 O]n surface film electrogenerated during the anodic dissolution process will be discussed. 3.5. Single wavelength ellipsometry investigation of the cobalt electrode surface The influence of the benzotriazole concentration on the growth of the [Co(II)(BTA)2 ·H2 O]n film was determined at a

Fig. 14. Δ/ψ trajectory defined by the complex refractive index N = 1.8–0.1j which best fits the ellipsometric parameters Δ and ψ ex situ recorded for bare cobalt material and samples for which photographs are presented at the bottom of this figure (λ = 632.8 nm; θ i = 70◦ ). Samples were electrogenerated during a linear sweep voltammetry scan in a 0.4 M NaHCO3 solution at pH 8.3 containing 1 × 10−4 to 3 × 10−2 M benzotriazole (ω = 1000 rpm; dE/dt = 5 mV s−1 ; potential limits: −1.05 and +1.0 V).

D. Gallant et al. / Electrochimica Acta 52 (2007) 4927–4941

plex refractive index N = 1.8–0.1j was found to best fit the experimental ellipsometric data. A refractive index of 1.8 has already been reported for the [CuBTA]n complex [96], while an extinction coefficient of 0.1 is in the range of the expected values (i.e., 0.0–0.1) for such a type of polymeric film [97]. Considering the complex refractive index 1.8–0.1j, and assuming that the entire changes in ellipsometric parameters Δ and ψ are caused by the surface film and not roughening, film thicknesses varying from 40 to 5 nm were calculated for the films electrogenerated in a solution containing a benzotriazole concentration ranging from 1 × 10−4 to 3 × 10−2 M, respectively. Therefore, in a highly diluted benzotriazole solution which poorly inhibits cobalt electrodissolution, the formation of a [Co(II)(BTA)2 ·H2 O]n film beyond the monolayer stage is favored. On the other hand, in a solution containing a higher benzotriazole concentration which efficiently protects cobalt from dissolution, a film of about 5 nm in thickness is instead found on the cobalt surface. As a result, it can be concluded that the [Co(II)(BTA)2 ·H2 O]n film cannot be regarded as a protective layer since a thick-packed film is representative of a low corrosion inhibition by benzotriazole. However, by acting as a physical barrier, the thick [Co(II)(BTA)2 ·H2 O]n layer present on the electrode surface blocks each of the electrodissolution processes of cobalt, which usually occur at potentials positive to −0.3 V. Therefore, the latter phase can only be regarded as a protective phase after it has been electrogenerated as a corrosion product in the active dissolution range. It then becomes necessary to describe a second protection mechanism involving benzotriazole, which preferentially takes place at high benzotriazole concentration and prior to the onset of the anodic dissolution reaction, and that would considerably limit the extent of the cobalt electrodissolution peak observed. 3.6. Description of a protection mechanism of cobalt against anodic dissolution: the production of [Co(II)(BTA)2 ·H2 O]n through deprotonation of adsorbed neutral BTAH molecules Prior to the initiation of the anodic dissolution reaction, the [Co(II)(BTA)2 ·H2 O]n phase is not present on the electrode surface because it is a product of the electrodissolution process, as discussed above. However, in Section 3.3, we demonstrated using SERS technique that neutral benzotriazole molecules chemisorb on a cobalt electrode surface polarized at cathodic potentials ranging from −1.3 to ∼−0.8 V. Moreover, our results suggest that benzotriazole molecules progressively leave the cobalt electrode surface as the potential is anodically moved toward the value at which the active dissolution initiates. In the past, it has been demonstrated that benzotriazole molecules adsorbed on copper are subjected to deprotonation at sufficiently anodic potentials, leading to the formation of the [Cu(I)BTA]n polymer-like complex [16]. Therefore, in the present case, it is likely that adsorbed neutral BTAH molecules that remain on the electrode surface at E ∼ −0.8 V deprotonate at the onset of the anodic dissolution reaction, to yield a [Co(II)(BTA)2 ·H2 O]n compact polymeric network according to the global oxidation

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mechanism: nCo + 2n(BTAH)ads + nH2 O → [Co(II)(BTA)2 ·H2 O]n + 2nH+ (aq) + 2ne−

(16)

Because of the expected thinness of the layer of adsorbed neutral BTAH molecules, the [Co(II)(BTA)2 ·H2 O]n film formed according to this mechanism must necessarily be organized as well in a very thin layer. This mechanism thus mainly explains the presence of the thin (∼5 nm) [Co(II)(BTA)2 ·H2 O]n compact film which completely protects cobalt from active dissolution, as observed in a solution containing a 3 × 10−2 M benzotriazole concentration. The [Co(II)(BTA)2 ·H2 O]n nature of this thin film was confirmed using normal ex situ Raman spectroscopy with a green laser line (514.54 nm) since the recorded spectrum, presented in Fig. 15, exhibits most of the spectral features characterizing this compound (see Fig. 10c). Because of the thinness of the film, the 514.54 nm line at 4 mW was preferred to resolve Raman bands of lower intensity. Therefore, as the benzotriazole concentration in solution is increased, the surface coverage by neutral BTAH molecules increases and the mechanism involving the deprotonation of adsorbed neutral benzotriazole molecules becomes progressively responsible for the [Co(II)(BTA)2 ·H2 O]n layer found on the cobalt electrode. The decrease in the magnitude of the electrodissolution rate is thus directly related to the surface coverage by neutral BTAH molecules that are subsequently transformed into a [Co(II)(BTA)2 ·H2 O]n thin film at the onset of anodic dissolution. The formation of such a film, being more or less discontinuous depending on the surface coverage, restrains the extent of the main cobalt electrodissolution reaction. From a quantitative point of view, the influence of both the solution pH and benzotriazole concentration on such an inhibition process was evaluated using Frumkin’s adsorption isotherms.

Fig. 15. Ex situ normal Raman spectrum obtained with the 514.54 nm excitation line for the sample presented in Fig. 14 and prepared in a solution containing a 3 × 10−2 M benzotriazole concentration.

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4. Conclusion This study, carried out with a combination of electrochemical and surface analysis techniques, has enabled us to elucidate the mechanisms governing interactions between benzotriazole and metallic cobalt in the context of a corrosion inhibition process. It has been shown that the inhibitive action of benzotriazole can be explained by the surface adsorption behavior of the neutral molecules described by the Frumkin adsorption isotherm, which is followed by the deprotonation of BTAH leading to the formation of a thin [Co(II)(BTA)2 ·H2 O]n water-insoluble and polymer-like compound on the electrode surface. This specific mechanism was found to be responsible for the inhibition of cobalt electrodissolution in the active potential range. In the potential region allowing cobalt active dissolution, a second mechanism has been required to explain the presence of a thick [Co(II)(BTA)2 ·H2 O]n film on the electrode surface. The latter involves a complexation phenomenon between electrogenerated cobaltous ions and deprotonated benzotriazole species present in the bulk solution. At near-neutral solution pH, because deprotonated benzotriazole species were found to compete efficiently with bicarbonate species for ligation of cobaltous cations, a thicker [Co(II)(BTA)2 ·H2 O]n film was found to be electrogenerated. Both mechanisms proposed above are thus leading to the electrogeneration of a similar corrosion product that efficiently protects cobalt from any further dissolution encountered in the absence of benzotriazole. The results presented in this study should be extended in future efforts to protect cobalt-based alloys. Acknowledgements This work was supported by a scholarship and a discovery grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would also like to express their gratitude to Prof. Franc¸ois J. Saucier for helpful discussions concerning the Fortran program, Marc Tougas (Lon´ don Scientific) for impedance facilities and Etienne B´elanger for spectroelectrochemical cell 3D schematic representation. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2007.01.057. References [1] V. Kuzucu, M. Ceylan, H. C ¸ elik, I. Aksoy, J. Mater. Process. Technol. 69 (1997) 257. [2] D.L. Klarstrom, J. Mater. Eng. Perform. 2 (1993) 523. [3] C. Maffiotte, M. Navas, M.L. Castano, A.M. Lancha, Surf. Interf. Anal. 30 (2000) 161. [4] W.H. Hocking, D.H. Lister, Surf. Interf. Anal. 11 (1988) 45. [5] N.S. McIntyre, D. Zetaruk, E.V. Murphy, Surf. Interf. Anal. 1 (1979) 105. [6] W.H. Hocking, F. Stanchell, E. McAlpine, D.H. Lister, Corros. Sci. 25 (1985) 531. [7] S. Simard, D. Gallant, Can. J. Chem. 82 (2004) 583. [8] D. Gallant, S. Simard, Corros. Sci. 47 (2005) 1810. [9] D. Gallant, M. P´ezolet, A. Jacques, S. Simard, Corros. Sci. 48 (2006) 2547.

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