Influence of the Adsorption of N Species on the Anodic Dissolution of Ni

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Influence of the Adsorption of N Species on the Anodic Dissolution of Ni A. G. Munõz,*,† G. Benitez,‡ M. E. Vela,‡ and R. C. Salvarezza‡ Instituto de Ingenierıá Electroquı´mica y Corrosio´ n (INIEC), Departamento de Ingenierıá Quı´mica, Universidad Nacional del Sur, Avenida Alem 1253, 8000 Bahia Blanca, Argentina, and Instituto de Investigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata-CONICET, Sucursal 4 Casilla de Correo 16, 1900 La Plata, Argentina Received October 14, 2003. In Final Form: December 13, 2003 The dissolution and passivation of Ni in nitrite-containing acid solutions are investigated by Auger spectroscopy, atomic force microscopy, and conventional electrochemical techniques. The dissolution/ passivation of the Ni surface is consistent with a competition between adsorbed OH- and nitrogen-containing species with a potential-dependent surface coverage. Nitrogen-containing species hinder the passivation of the Ni surface, shifting the formation of the complex nickel hydroxide/oxide film to more positive potential values. The dynamics of the dissolving interface, followed by atomic force microscopy, reflect first the competition of adsorbed species, leading to the development of protrusions and cavities, and finally the formation of the passive film that promotes surface smoothening by a preferential dissolution of the protrusion tips under ohmic control.

1. Introduction Dissolution and passivation of metals continue to be a subject of considerable scientific interest not only in the field of material science but also, because of its technological impact, in the field of surface science as a model system for the study of complex surface chemical reactions. It is generally accepted that dissolution and passivation of many metals and alloys proceed through the formation of intermediate MeOH adsorbed species that later lead to MeOHz-1 species in solution or Me(OH)z species at the surface that hinder or completely block further dissolution.1 It is also well established that the addition of a small concentration of anions can alter dramatically the dissolution and passivation processes. In fact, electrochemical results have provided indirect evidences about the key role of anion adsorption on metal dissolution and passivation. In contrast, practically a limited number of studies have provided direct evidence on the competition between OH- and anions at the metal surface by using surface-sensitive techniques.2 The particular selection of NO2- as the competing anion for this paper may be of promising technological interest because of its inhibitory effects on the corrosion of some metals.3 Dissolution and passivation of Ni in phosphate solutions containing nitrite are another subject of recent interest. In fact, the addition of NO2- and NO3- to baths is a common practice in order to accelerate the surface reactions in the phosphating of Zn-coated steel.4,5 Besides the precipitation * Author to whom correspondence should be addressed. E-mail: [email protected]. † Universidad Nacional del Sur. ‡ Universidad Nacional de La Plata-CONICET. (1) Sato, N. In Passivity of Metals; Frankenthal, R. R., Kruge, J., Eds.; The Electrochemical Society: Princeton, NY, 1978; p 29. (2) McIntyre, N. S.; Chan, T. C. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Johh Wiley & Sons: New York, 1990; Vol. 1, p 485. (3) Szklarska-Smialowska, Z. Pitting Corrosion of Metals; NACE: Houston, TX, 1986; Chapter 12. (4) Zimmermann, D.; Munõz, A. G.; Schultze, J. W. Electrochim. Acta 2003, 48, 3267. (5) Rausch, W. Die Phosphatierung von Metallen; Leuze Verlag: Salgau, Germany, 1988.

of a phosphate layer, the cementation of Ni is the most important reaction in defining the final corrosion properties of pores left in the coating.4,6 An enhanced corrosion resistance of Zn coatings was also attained by a cementation process carried out in sulfate solutions containing Ni2+.7 Investigations about the cementation of Ni on corroding Zn during phosphating were already made by means of simultaneous measurements of the rest potential and surface electrode capacity.4 There, a decrease of the capacity together with a rapid increase of the potential was the main feature observed during the first stages of phosphating in nitrite-containing solutions. This fact was related with the cementation and subsequent passivation of the Ni-plated surface of pores. Thus, a study of the Ni passivation in the presence of nitrite is essential to understand the properties of cementated films in this type of bath. In previous papers,8,9 the influence of nitrite on the electrochemical response of Ni in acid phosphate and sulfate solutions was already analyzed. The results were interpreted in terms of the adsorption of NO2-, which may conduct to an inhibition of the dissolution and passivation processes; both of them involve the presence of adsorbed intermediates in their mechanisms. Nevertheless, the role played by other accompanying anions, such as HxPO4x-3, with reference to their possible adsorption, is not known yet. In this paper, we have studied the competition of OHand nitrogen-containing species at the Ni surface by using Auger electron spectroscopy combined with electrochemical techniques. We have also followed the dynamics of the interface during the dissolution and passivation of the nickel surface by using contact-mode atomic force microscopy (AFM). Results from this study reveal the key role of adsorbed nitrite anions on the Ni dissolution and (6) Klusmann, E.; Ko¨nig, U.; Schultze, J. W. Werkst. Korros. 1995, 46, 83. (7) . Velichenko, A. B.; Portillo, J.; Sarret, M.; Mu¨ller, C. Electrochim. Acta 1999, 44, 3377. (8) Munõz, A. G.; Schultze, J. W. Electrochim. Acta 2004, 49, 293. (9) Munõz, A. G.; Salinas, D. R. J. Electroanal. Chem. 2003, 547, 115.

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passivation. The effect of nitrite is reflected in the nature and composition of the adsorbed layer, topography of the dissolving surface, and electrochemical behavior of the electrolyte/metal interface. 2. Experimental Section 2.1. Electrochemical Setup. Ni rods (99.98%) embedded in a Teflon holder with an exposed area of 0.502 cm2 were used as working electrodes. They were polished with 1000 emery paper and 0.3 µm alumina and finally sonicated in pure water. Electrochemical experiments were performed in a conventional cell, using a Pt sheet and a saturated sulfate electrode (sse; E0 ) 0.64 V versus a standard hydrogen electrode) as the counter and reference electrodes, respectively. Solutions were prepared from analytical-grade chemicals and pure water (Millipore-Q). A 1 M NaH2PO4 solution of pH 3 was used as the base electrolyte. Potentials are referred to the sse. The concentration of NO2- was varied by adding NaNO2 to the phosphate solution. All experiments were carried out under a saturated N2 atmosphere and at room temperature. Sweep voltammetries were performed using a linear-voltage sweep generator EG&G PAR model 175 and a potentiostatgalvanostat EG&G PAR model 173. Impedance measurements were performed with a Solartron electrochemical interface 1286 and a Solartron frequency response analyzer 1250 coupled to a Hewlett-Packard computer 9216. Spectra were run in a frequency region from 105 to 0.01 Hz with 10 mV of amplitude. 2.2. Auger Spectroscopy. Spectra were taken using a singlepass cylindrical-mirror analyzer (CMA; Physical Electronics). The normalized Auger signal was calculated using the elemental sensitivity factor given by Payling10

∑ (I /s )

Ai ) (Ii/si)/

j

j

j

(1)

where Ai is the normalized Auger signal, Ii, the peak to peak height, and si, the elemental sensitivity factor for the element i. The summation in the denominator runs over all of the elements present in the sample. 2.3. AFM Studies. Surface topography changes of the Ni samples were followed by using ex situ AFM imaging with Nanoscope III equipment (Digital Instrument, Santa Barbara, CA). Silicon-nitride tips were used under the contact mode by applying different forces in the range 70 nN < f < 250 nN.

3. Results 3.1. Electrochemical Characterization of the System. The electrochemical response of the system offers a first approximation to the dissolution and passivation processes occurring with and without the presence of nitrite (Figure 1). The presence of two anodic peaks gives evidence of different passivation processes, from which the first one would correspond to that generated by adsorption and precipitation of Ni(OH)2, and the second, to the formation of an inner NiO layer.11,12 Accordingly, the adsorption of N species seems to prevail over that of passivating Ni(OH)2, because higher dissolution currents are attained before passivation may occur (first anodic peak). Furthermore, the potential shift toward more positive values for the formation of the inner NiO layer (second anodic peak) also suggests a kinetic hindrance introduced by adsorbed N species. Additionally, the reaction system is complicated by the local alkalization, because this shift was only observed under electrode rotation in a solution with pH 5.8 but not with pH 3.8 (10) Payling, R. J. Electron Spectrosc. Relat. Phenom. 1985, 36 (1), 99-104. (11) Real, S. G.; Vilche, J. R.; Arvıá, A. J. Corros. Sci. 1980, 20, 586. (12) Barbosa, M. R.; Real, S. G.; Vilche, J. R.; Arvıá, A. J. J. Electrochem. Soc. 1988, 135, 1077.

Figure 1. Potentiodynamic cyclic scans performed on Ni in phosphate solutions with and without the presence of nitrite. Es,c ) -0.85 V, and Es,a ) 0.2 V. v ) 0.005 V s-1.

The adsorption of N species was already discussed in other papers performed on Pt,13,14 wherein the adsorption of nitrite and related reduction products (NO and N2O) was postulated to be involved in the reduction mechanism. However, these assumptions were formulated only in light of electrochemical evidence. Although information from the literature about nitrite reduction is scarce, it is reasonable to propose the following mechanism in accordance with the reported studies on Pt:13-16

NO2- T NO2 ads-

(2)

NO2 ads- + 2H+ + e f NOads + H2O

(3)

2NOads + 2e + 2H+ f N2O + H2O

(4)

The electrochemical aspects of the reduction of nitrite were already presented in detail in a previous paper.8 From that, it is known that Tafel’s relationship near the corrosion potential (Figure 2) arises from the reduction of adsorbed nitrite as the controlling step (eq 3). Then, the current plateau at E < -0.7 V is closely related to the limitation introduced by the flux of H+ toward the surface as expected from steps 3 and 4. The hydrogen evolution reaction here does not seem to exert an important influence, as shown by the polarization performed without nitrite. The electrochemical polarizations show interesting evidence for the adsorption competition between intermediates involved in the reduction of NO2- and Ni dissolution, that is, NxO and NiOH. It is denoted by different corrosion potentials depending on whether the starting potential was anodic or cathodic (Figure 2). Further electrochemical evidence for adsorption dynamics can be found in the current transients obtained after applying different potential steps in a nitritecontaining solution (Figure 3). Here, the rapid decrease of the reduction rate at values more negative than the corrosion potential is a consequence of local alkalization. As the corrosion potential is approached, the transients (13) Horańyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1985, 188, 265. (14) Horanyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1982, 140, 347. (15) Petrii, O. A.; Safonova, T. Ya. J. Electroanal. Chem. 1992, 331, 897. (16) Schmid, G.; Lobeck, M. A.; Keiser, H. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 189.

Anodic Dissolution of Ni

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Figure 2. Potentiodynamic scans performed on Ni in phosphate solutions with nitrite, Ei (‚‚‚) ) -0.58 V and Ei (s) ) -1.0 V, and without nitrite, Ei (- - -) ) -1.2 V. v ) 5 × 10-4 V s-1.

Impedance spectroscopy helped in obtaining a closer inspection of the changes occurring at the metal/solution interface during Ni dissolution and passivation. Nyquist’s plots obtained in steady-state conditions (stabilization time > 30 min) after successive displacements of the potential from -1.0 V toward more positive values (Figure 4) show important changes on both the double-layer structure and electrochemical reaction system. From the values of steady-state current at each potential and the behavior of polarization resistance suggested at the lowest frequencies, it was possible to construct a polarization curve (dotted line in Figure 4a). It must be pointed out that the potential dependence of the steady-state current is a little different from that shown in Figure 1. For instance, the first voltammetric peak at -0.5 V is overlapped by the reduction reaction as a consequence of the long-time passivation effects. At E < -0.8 V, a unique semicircle was observed, indicating that the reduction reaction is kinetically controlled. The first change in the reaction system is indicated by a linear behavior arising at low frequencies for -0.7 and -0.6 V. This type of response obeys the presence of a Warburg element, coming from the diffusional contribution, in series with the charge-transfer resistance RCT (see the details in Figure 4c). The diffusion impedance (ZW) can be analyzed by the following expression, derived from the solution of the diffusion equation under appropriate boundary conditions:21

ZW(jω) ) [RT/(n2F2x2CD1/2)](ω-1/2 - jω-1/2) (7) where C and D are the concentration and diffusion coefficient of the diffusing species, respectively, and ω is the angular frequency (ω ) 2πf). Then, at sufficiently low frequencies, the expression for the total real impedance is reduced to Figure 3. Potentiostatic current transients performed on Ni in phosphate solutions containing 5 × 10-3 M NO2-. Ei ) ECORR(τ)0) ) -0.54 V.

Z′(ωf0) ) Rel + RCT + [RT/(n2F2x2CD1/2)]ω-1/2 (8)

where adsorbed NiOH acts as a limiting intermediate. Now, we are able to explain these types of transients as follows: At the first instance of polarization, the coverage of adsorbed N species predominates over that of NiOH, and a net cathodic current is observed. Afterward, a continuous increase of the surface pH not only reduces the contribution of the cathodic reaction but also favors the formation of NiOH, and the anodic current increases (eqs 5 and 6).

The good linear relationship of real impedance (Z′) with ω-1/2 at low frequencies supports the presence of a diffusion process (Figure 5). Also, the extrapolations to ω-1/2 ) 0 give values of Rel + RCT similar to those found by extrapolation from the corresponding high-frequency Nyquist’s loops. In reference to the diffusion species, it should be Ni2+, because this response just appears to reach the region of active dissolution. Thus, assuming a diffusion coefficient D ) 5 × 10-6 cm2 s-1 and n ) 2,9 an increase in the surface [Ni2+] from 6.0 × 10-5 M at -0.7 V to 2.4 × 10-4 M at -0.6 V can be estimated by eq 8 and the slopes of Z′ versus ω-1/2 plots. The presence of the inductive effects observed at E ) -0.4 V is usually interpreted in terms of the relaxation effects brought about by the adsorbed intermediates in the reaction mechanism. Furthermore, the negative polarization resistance observed at -0.3 V suggests the beginning of a potential-controlled surface blockage by a growing passive layer.22 The potential dependence of the surface capacity and charge-transfer resistance (ctr) (Figure 6), obtained by fitting the experimental data of the first capacitive loop to an RC circuit,23 offers important information about

(17) Plieth, W. F. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1978; Vol. 8, p 321. (18) Piatti, R. C. V.; Arvıá, A. J.; Podesta´, J. J. Electrochim. Acta 1969, 14, 541. (19) Burstein, G. T.; Wright, G. A. Electrochim. Acta 1975, 20, 95. (20) Itagaki, M.; Nakazawa, H.; Watanabe, K.; Noda, K. Corros. Sci. 1997, 39, 901.

(21) Raistrick, I. D.; Macdonald, J. R.; Franceschetti, D. R. In Impedance Spectroscopy; Macdonald, J. R., Ed.; John Wiley & Sons: New York, 1987; Chapter 2, p 73. (22) Jouanneau, A.; Keddam, M.; Petit, M. C. Electrochim. Acta 1976, 21, 287. (23) Boukamp, B. A. Equivalent Circuit; University of Twente: Twente, The Netherlands, 1988/89.

show an initially increasing cathodic current, which then evolves toward anodic values in order to become stationary. This fact does not seem to be simply related to the change of surface pH, as could be inferred from a monotonic decrease of the cathodic current. To continue with our analysis, the dissolution mechanism of Ni must first be examined. There is a general agreement that it can be represented by17-20

Ni + H2O T NiOHads + H+ + e

(5)

NiOHads f NiOH+ + e

(6)

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Figure 4. Steady-state current-potential dependence (a) and Nyquist’s plots (b-f) obtained on Ni at different polarization potentials starting from the most negative one in phosphate solution containing 5 × 10-3 M NO2-. ∆E ) 10 mV. Numbers are in hertz. Arrows indicate the scan direction.

Figure 5. Dependence of the impedance real part with ω-1/2 at low frequencies for the spectra taken at -0.7 and -0.6 V, shown in Figure 4.

Figure 6. Potential dependence of double-layer capacity and ctr obtained by fitting the first semicircle of the impedance diagrams, shown in Figure 3. [NO2-] ) 5 × 10-3 M.

adsorption phenomena. In fact, the capacity minimum observed at -0.8 V indicates the neighborhood of the potential of zero charge (pzc). A value of -0.97 V was

reported in a sodium sulfate solution of pH 5.24 Accordingly, the increase of the ctr near this point can be explained in terms of a progressive occupancy of adsorption sites by OH-, restraining the access of NO2- for its reduction. Toward more positive potentials, the rise of capacity and the fall of ctr are surely due to a roughness increase generated by metal dissolution. Finally, the formation of a passive layer is denoted at E > -0.5 V by a new decrease of capacity. 3.2. Auger Spectroscopy. The use of surface-sensitive techniques is almost one of the most important points in this paper, because it allows the connecting of electrochemical results with surface chemistry in a more direct manner. Auger spectra were taken after 300 s of polarization in a 5 × 10-3 nitrite phosphate solution, and as expected, the signal attributed to the transition KL2,3L2,3 of nitrogen can be clearly noted at -1.1 and -0.7 V. However, it disappears at 0 V, where the passivation has been completed (Figure 7a). Additionally, a shoulder at the high-energy side of the Ni MVV transition was found at this later potential (Figure 7b), with it being a characteristic feature of transition-metal oxides.25 The carbon signal, on the other hand, surely comes from air contamination, whereas traces of P were not found (at least in the potentials under study), indicating the absence of chemisorbed HxPO4x-3. The analysis of the signal relation N/O taken from the different spectra reveals important information concerning the relative coverage of different adsorbed species. In this sense, the potential dependence of the normalized signals of N and O (Figure 8a) shows a maximum of N/O near the unity close to -0.7 V (Figure 8b). On the other hand, the carbon signal remains practically constant at potentials more negative than -0.4 V, i.e., before the formation of NiO. Therefore, a fixed contribution to the O signal from CO2 contamination should be considered. (24) Arvıá, A. J.; Posadas, D. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. 3, p 211. (25) Weissmann, R.; Mu¨ller, K. Surf. Sci. Rep. 1981, 1 (5), 251-309.


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Figure 7. (a) Auger spectra obtained after 300 s of polarization of Ni at different potentials in a phosphate solution containing 5 × 10-3 M NO2-. (b) Details showing the signal alterations of the Ni MVV transition as commonly found in transition-metal oxides.

Figure 8. (a) Potential dependence of the N- and O-normalized signals obtained from Auger spectra after 300 s of polarization in a phosphate solution with 5 × 10-3 M NO2-. (b) Potential dependence of the N/O relation and capacity of the interface according to the impedance response.

It must also be pointed out that different grains of the polycrystalline sample are expected to have distinct reactivities for the nitrite-reduction and Ni-dissolution reaction. Thus, it is not surprising to observe some data dispersion, represented by the error bars, on analyzing the different points of the sample. The concentration of N maintains a constant value in the cathodic region (E < -0.6 V). Thus, a variation of the N/O relation could be ascribed to a modification of the relative coverage of different adsorbed N species, viz., NO2and NO. Toward the anodic region (E > -0.6 V), the continuous increase of the O signal together with a decrease of the N signal is clear evidence for the preponderance of OH- adsorption and the beginning of oxide formation. This fact just takes place at potentials more positive than that corresponding to the pzc, where the minimum in capacity is observed (Figure 8b). 3.3. Surface Morphology. The effects of adsorbed N species on the interface dynamics during the dissolution and passivation of Ni were analyzed by following the changes in the root-mean-square (rms) roughness derived from ex situ AFM images. Changes in the surface morphology, when performing a potentiodynamic polarization (Figure 1), are shown by images obtained after taking out the electrode from the solution at different controlled potentials (Figure 9). The initial surface shows a slight corrugated morphology (rms ) 2.3 nm), whereon some polishing marks are still visible.

When images a and c in Figure 9 are compared, it can be seen that the presence of nitrite leads to a rougher surface in the active dissolution region because it is reflected by the higher rise of the rms roughness (Figure 10). However, the rms roughness decreases rapidly with the progress of the anodic scan. On the contrary, in the absence of nitrite, it remains constant up to -0.3 V. At the most anodic potentials (+0.2 V), the surface shows a more flattened aspect with the presence of some emerging grains, which, despite similar rms roughness values, appear to be more extended in the absence of nitrite (Figure 9, images b and d). With the potential increase, the adsorption of passivating Ni(OH)2 will gradually cover the surface. Thus, metal dissolution is restrained to continuously decreasing localized nonpassivated zones. In the presence of nitrite, however, the nonpassive condition may be maintained longer by the strong adsorption of N species. Thus, greater metal amounts are dissolved at localized, but homogeneously distributed, nonpassivated sites, and a more irregular surface topography arises (Figure 9a). At the end of the first anodic peak, a continuous thick hydroxide layer (as a second phase) is likely to be formed on the surface. This, in turn, establishes an ohmic control, which conducts to a rapid dissolution of the metal protrusions. Despite this, some points are strongly passivated by the adsorption of Ni(OH)2, and nondissolved metal protrusions still remain. This process occurs

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Figure 9. Ex situ (5 µm × 5 µm) AFM surface plots obtained after performing potentiodynamic linear scans with different anodic limits (Ea) in a phosphate solution with 5 × 10-3 M NO2- (a and b) and without nitrite (c and d). Cathodic starting potential, Es,c ) -0.85 V, and v ) 0.005 V s-1.

Figure 10. Potential dependence of the rms roughness obtained from analysis of AFM images of 5 µm × 5 µm windows after potentiodynamic linear scans in a phosphate solution with 5 × 10-3 M NO2-. Es,c ) -0.85 V.

simultaneously with the beginning of the NiO formation (second anodic peak), conducting to the final irreversible passivity. Finally, the surface morphology at the passive region is the result of the distribution of the active dissolution sites and dissolution rates on them. The influence of nitrite in the active dissolution region (E < -0.5 V) was further investigated by analyzing the evolution of surface morphology for similar dissolution charges (Figure 11). In the presence of nitrite, the initial granulated surface morphology (rms roughness ) 12 nm) transforms gradually into a rougher one (rms roughness ) 27 nm) but with a lesser amount of greater grains. In the absence of nitrite, however, the rms roughness increases from 11 to 16 nm, and the initial small surface grains evolve to more extended islands. 4. Discussion The complex reduction of nitrite on Ni introduces adsorbed N species that compete with those involved in

Ni dissolution for adsorption sites. This fact is manifested not only by different electrochemical behaviors, determined by different electronic and chemical surface properties, but also by a particular evolution of morphology. The presence of adsorbed N species was inferred from electrochemical experiments. In fact, the adsorption of nitrite was taken as the starting point for explaining the increase of the surface capacity and the increment of the active dissolution defined by the first anodic peak in potentiodynamic polarizations.8 Also, a Frumkin’s type of isotherm was suggested from the increment of doublelayer capacity at constant potential with the increasing nitrite concentration in acid sulfate solutions.9 However, despite that, direct evidence for it was still missing. Some spectroscopic studies dealing with the reduction of NO3were performed on Pt, where it was suggested that NO2is coadsorbed O down with 1-fold or 2-fold coordination26 together with NO. Furthermore, the presence of a low coverage of adsorbed NO as a reduction product was detected.27 The necessary direct evidence for the adsorption of N species on Ni was covered by Auger measurements presented in this paper. Thus, the searched connection of surface experiments with previously conceived ideas from electrochemical experiments can be established. The proximity to the pzc indicated by the minimum surface capacity at -0.8 V plays a decisive role in adsorption phenomena (Figure 6). Certainly, it is expected that an electrochemical adsorption competition of negatively charged species, such as NO2- and OH-, will be established on exceeding this point in the anodic direction. Here, the adsorption of OH- is the preceding step for the formation of NiOHads, an intermediate in the anodic dissolution reaction scheme (eqs 5 and 6). This fact is also (26) da Cunha, M. C. P. M.; Weber, M.; Nart, F. C. J. Electroanal. Chem. 1996, 414, 163. (27) Go´mez, R.; Rodes, A.; Ortis, J. M.; Feliu, J. M.; Pe´rez, J. M. Surf. Sci. 1995, 342, L1104.


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Figure 11. Ex situ (2 µm × 2 µm) AFM surface plots obtained after different polarization times in phosphate solutions with 5 × 10-3 M NO2- at (a) τ ) 200 s and (b) τ ) 600 s and without nitrite at (c) τ ) 600 s and (d) τ ) 1800 s. Details are the current transients for the first 300 s of polarization.

pointed out by the appearance of a net anodic current at E > -0.9 V in the polarization without nitrite (Figure 2). Then, the predominance of one of the adsorbing species depends on the established surface conditions (E and pH). Accordingly, an increase in the hydroxyl coverage might partially explain the variation of surface oxygen upon approaching -0.8 V (Figure 8b). However, another explanation may also be addressed in terms of the variation of the coverage fraction of adsorbed NO2- and NO. At the most negative potentials, the reduction of nitrite becomes autolimited by the rapid increase of the surface pH. As a consequence, the consumption of adsorbed NO2- decreases, incrementing its coverage fraction. Near the corrosion potential, the evolution of current from cathodic to anodic values in the potential steps (Figure 3a) gives evidence for the dynamics of adsorption competition between N species and NiOH. Initially, the reduction of adsorbed NO2- and NO dominates, but then the subsequent rise of the pH favors the formation of NiOHads, leading to the increase of the metal dissolution rate. In a similar way, the appearance of a maximum ctr at the capacity minimum (Figure 6) is also evidence for the increase of OH- coverage and the promoted dissolution reaction (eq 5). This effect is also pointed out by the appearance of a Warburg response at low frequencies in Nyquist’s plots (Figure 4). The adsorption competition between N species and passivating Ni(OH)2 is also reflected by higher rms roughness values in the active dissolution region (Figures 9 and 10). Other researchers22 suggested that the adsorption of OH- on the dipole Ni-OH or Ni-HSO4 produces a blocking effect and leads to primary or secondary passivity, respectively. Then, these passivating states are destroyed by the adsorption of a third anion or OH-, respectively. According to these ideas, the depassivating effect promoted by nitrite might be thought of in terms of its adsorption on the surface Ni(OH)2 complex. On the other hand, the displacement of the first anodic peak toward more anodic potentials indicates an inhibiting

effect on the active dissolution of Ni. According to the kinetic scheme represented by eqs 5 and 6, the preferential adsorption of N species would diminish the anodic dissolution rate, so that more anodic potentials are then necessary to acquire the formation of a Ni(OH)2 film (appearance of the first voltammetric peak in Figure 1). The random nature of the passivation/dissolution process promoted by the competitive adsorption of OHand N species on the precursor site (NiOHads) leads to an increasing surface roughness when polarizing within the active dissolution potential region (Figure 11). The appearance of flat regions in the microscopic dimensions with some high protrusions is also a result of the progressive surface coverage by N species and the consequent time-dependent depassivation. On the other hand, the passivation process predominates in the absence of nitrite, and the dissolution proceeds in a more localized manner, conducting to the formation of the extended cavities separating nondissolved islands. Others addressed similar conclusions by studying the active dissolution of Ni(111) occurring with simultaneous surface passivation, where a significant increase of the surface roughness was also observed.28 They explained the pronounced differences in the local dissolution rate in terms of a local pinning caused by passivating adsorbates. Therefore, a local disruption of the passivating adlayer leads to a highly active surface, in a manner similar to that occurring in the pitting of an oxide-covered surface. It would occur on the more favorable sites, decreasing the adsorption probability of passivating Ni(OH)2. Within the passive region, the formation of second phases introduces a new restraint for dissolution. There is a general agreement that the first and second voltammetric peaks (Figure 1) correspond to the formation of Ni(OH)2 and NiO layers, respectively. In addition, ellipsometric studies performed in acid phosphate solutions (28) Scherer, J.; Ocko, B. M.; Magnussen, O. M. Electrochim. Acta 2003, 48, 1169.

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were interpreted in terms of a film composed of a base layer of partially dehydrated Ni(OH)2 and an upper, potential-dependent film at E > 0 V.29 This double-layer structure was recently extensively analyzed by in situ scanning tunneling microscopy on Ni(111) in sulfuric acid30,31 and consists of a crystalline, inner NiO(111) layer and porous, amorphous hydroxide on top. Therefore, it is expected that the adsorbed N species are gradually released by the formation of the passive layer, as also evidenced by the fall of the N signal in the Auger experiments (Figure 8a). The appearance of a negative resistance is also an important feature observed in the passive region only after the addition of nitrite8 (Figure 4f). Thus, this result seems to indicate that the coverage by NiO is controlled by the potential-dependent desorption of N species. The generation of a passivating film also has several consequences on the surface dynamics. In effect, the metal protrusions are more rapidly dissolved because of the ohmic control established by the formation of a thin hydrous Ni(OH)2.12 This explains the rapid decrease of the surface roughness observed when the first voltammetric peak is anodically exceeded (Figure 10).

Mun˜ oz et al.

tigations allows for a closer insight into the surface dynamics during the dissolution/passivation of metals. In this paper, Ni in acid phosphate solutions containing nitrite was specifically chosen because of its great technological interest. In this system, the competition for adsorption sites between OH- and N species involved in the different steps of the nitrite reduction is defined by the potential and near surface solution conditions. The adsorption of N species interferes with the surface dissolution/passivation dynamics of Ni by blocking the adsorption sites. The coverage of N species, predominating on the cathodic side of the pzc, decays gradually upon anodically exceeding this point by a preferential adsorption of OH-. This competition of the adsorbed species together with passivation inhibition brought about by the adsorption of N species leads to the development of rougher surfaces with the presence of protrusions and cavities. Then, the potential-dependent formation of an oxide/ hydroxide passive film releases strongly adsorbed N species from the surface at potentials more positive than the pzc and introduces an ohmic control, which conducts to a surface smoothening.

5. Conclusions The advantageous combination of surface-sensitive techniques with electrochemical and topography inves(29) Chao, C. Y.; Szklarska-Smialowska, Z.; Macdonald, D. D. J. Electroanal. Chem. 1982, 131, 289. (30) Zulli, D.; Maurice, V.; Marcus, P. J. Electrochem. Soc. 2000, 147, 1393. (31) Maurice, V.; Talah, H.; Marcus, P. Surf. Sci. 1993, 284, L431.

Acknowledgment. The Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) and the ANPCyT (PICT99-5030) are gratefully acknowledged for the financial support. LA0359225

Influence of the Adsorption of N Species on the Anodic Dissolution of Ni

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