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bicarbonate media
Electrochimica Acta 47 (2002) 4531 /4541 www.elsevier.com/locate/electacta
Electrodissolution of cobalt in carbonate/bicarbonate media J.A. Caldero´n a,b, O.R. Mattos a,*,1, O.E. Barcia a,c,1, S.I. Co´rdoba de Torresi d,*,1, J.E. Pereira da Silva d a
Laborato´rio de Corrosa˜o Professor Manoel de Castro, PEMM/COPPE/UFRJ, Cx. Postal 68.505, CEP 21945-970, Rio de Janeiro, R.J., Brazil b Grupo de Corrosio´n y Proteccio´n, Universidad de Antioquia, P.O. Box. 1226, Medellı´n, Colombia c Dep. Fı´sico-Quı´mica, IQ/UFRJ, Rio de Janeiro, Brazil d Departamento de Quı´mica Fundamental, Instituto de Quı´mica, Universidade de Sa˜o Paulo, Sa˜o Paulo (SP), Brazil Received 25 June 2002; received in revised form 19 August 2002
Abstract The electrodissolution of cobalt in carbonate/bicarbonate solutions was studied at room temperature by steady state polarisation, interfacial pH measurements and Raman spectroscopy. The active dissolution of cobalt leads to an initial CoO film formation. The metal passivation occurs by a slow transformation of the CoO into a Co3O4 oxide. The influence of HCO3 and CO2 anions was 3 investigated. Two different parallel electrochemical processes were proposed to account for the anion role on the electrochemical steady state behaviour of cobalt in the studied solutions. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cobalt; Dissolution; Carbonate/bicarbonate; Raman spectroscopy
1. Introduction The electrochemical behaviour of cobalt in alkaline media has been studied in a large range of pH values, involving solutions of different nature and composition [1 /11]. Those studies have evidenced different dissolution and passivation steps, depending on the nature of the solution and on the technique used. Cobalt passivation involves several oxidation states with cobalt oxide and hydroxide formations as a function of the applied potential. However, the anion role has been not sufficiently explained and the composition and structure of the passive film formatted is still controversial. In strong alkaline solution and at low anodic polarisation it was suggested [1 /3] that cobalt dissolution results on the formation of Co(OH)2, whereas, at more positive potentials, Co3O4 or CoOOH species replace the previous one. The CoO film is responsible for the initial passivity. In weakly alkaline borate solutions was found a successive formation of CoO, duplex CoO/ Co3O4 and duplex Co3O4/Co2O3 films, corresponding to * Corresponding authors. Fax: /55-21-290-6626 E-mail address: [email protected] (O.R. Mattos). 1 ISE members.
different dissolution/passivation steps [5]. In carbonate/ bicarbonate solutions the hydrodynamic dependence of the dissolution process was reported [6]. In these media, the oxides species are similar to those found in borate [7]. Concerning the anion role, it is proposed that HCO3 ions are more aggressive to CoO film than Co2O3 and borate ions do not attack any anodic film [6,7]. The composition of the surface layer in the intermediate anodic potential was found to be dependant on the concentration ratio of OH and HCO3 ions in solution [8]. Recently, the structure and electrochemical proprieties of the duplex anodic layer of CoO/ Co3O4 yielded during potentiodynamic polarisation of cobalt in carbonate/bicarbonate media were studied by electroreduction technique [9 /11]. The above works have been performed using mainly potentiodynamic techniques and the steady state has yet been little explored. Moreover, most of works on cobalt dissolution in alkaline media have been focused on metal passivation and less attention has been paid to the first steps of dissolution. The present paper mainly concerns the electrochemical behaviour of cobalt during metal active dissolution region, from open-circuit potential until the first passivation step. Steady state polarisation curves, Raman spectroscopy and interfacial
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 5 4 2 - X
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pH are the techniques used and the electrolyte are carbonate/bicarbonate media.
2. Experimental 2.1. Materials For electrochemical measurements, a classical threeelectrode cell was used. A pure cobalt cylindrical bar (Johnson Matthey Chemicals Ltd) of 5 mm diameter (0.2 cm2, exposed area) embedded in epoxy resin was set as rotating disc electrode (RDE). Saturated sulphate electrode (SSE) was used as reference and a large area platinum grid as counter electrode. Before the anodic polarisation measurements, the working electrode was polished until 600 grade emery paper, immersed in a 0.1 M H2SO4 solution for 1 min, rinsed with doubledistilled water and finally cathodically polarised in the working solution for 2 min. The cathodic potential used was negative enough to cause hydrogen evolution in order to obtain a free oxide cobalt surface. Polarisations were carried out in potentiostatic mode and stationary currents were taken by chronoamperometry using an AUTOLAB PGSTAT 30. All potential values are given versus SSE. The electrolyte consisted of an x M KHCO3/y M K2CO3, (x /0.375, 0.75, 1.5, 2.25, 2.5; y/0.025, 0.05, 0.1, 0.15, 0.167) mixture with fixed x /y ratio in order to keep the pH constant at 8.9. The solutions were prepared from analytical grade chemicals and previously boiled double distilled water to remove dissolved CO2. Electrolytes were de-aerated by purging with purified nitrogen during the whole measurement. 2.2. Local pH measurements For the pH measurements at the cobalt/electrolyte interface was used as working electrode a cobalt electrodeposited 75 mesh Pt screen fixed at the end of a flat-bottomed pH electrode (Mettler Toledo, mod. Inlab 426). The pH electrode was connected to a pH meter, thus measuring the solution pH during the electrodissolution process. The assembled electrode was motionless and the solution agitated with the aid of a mechanical stirrer. At these conditions, pH and current were recorded simultaneously during potentiostatic anodic polarisation.
Fig. 1. Flow Cell for in situ Raman measurements.
microscope and a CCD detector. For in situ experiments an electrochemical flow-cell (volume /4.0 /103 dm3) was used (Fig. 1). In situ Raman experiments were performed with and without continuous solution renovation. In the first case, the solution flow was regulated to 103 dm3 s 1. A SSE was used as reference and a Pt spiral wire encrusted on the bottom of the cell was the counter electrode. With this set up a laminar flow was achieved.
3. Results 3.1. Polarisation Anodic steady state polarisation curves of cobalt at 0, 100 and 225 rpm in a 0.75 M KHCO3/0.05 M K2CO3 solution are shown in Fig. 2. It should be noted that a mixed control on the anodic process in the whole potential range exists. Different slopes at low and at high anodic potentials are also observed. The first one between the open-circuit potential (OCP) and ://0.98 V, another one between ://0.98 V and the current density peak potential (Ec.d.p.)://0.86 V. The presence of two slopes in the steady state polarisation curves suggests that two different electrochemical processes are likely to be taking place during cobalt anodic polarisation between the OCP and the Ec.d.p.. Additionally,
2.3. Raman measurements In situ and ex situ Raman experiments were performed during cobalt anodic polarisation using a Renishaw Raman Imaging System 3000, coupled with a He /Ne laser from Spectra Physics (Mod. 127, lo / 632.8 nm), equipped with an Olympus metallurgical
Fig. 2. Steady state polarisation curves of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9.
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stationary currents at high anodic potentials were more rapidly achieved (20 /30 min) than those at low anodic potentials, where 2 /3 h were necessary to obtain stationary current values. The peak potential (Ec.d.p.) is practically independent of the RDE rotation rate (V). After this potential the current gradually decreases due to the beginning of the first passivation process. During this passivation process, a dark and adherent film slowly covers the cobalt surface. The stationary current is reached after nearly 24 h. Steady state polarisation curves of Co in different carbonate/bicarbonate solution compositions at V /100 rpm are shown in Fig. 3. For all the studied solutions, the general shape of the polarisation curves is the same, also showing the two slopes already described. The solution composition was varied from a base solution concentration of carbonate/bicarbonate. Solutions of 0.5, 1.0, 2.0 and 3.33 times the base solution concentration were prepared; those were: 0.375 M KHCO3/0.025 M K2CO3, 0.75 M KHCO3/0.05 M K2CO3, 1.5 M KHCO3/0.1 M K2CO3, 2.5 M KHCO3/0.167 M K2CO3, respectively. The anodic current density increases with increasing solution concentration for the same polarisation potential. This effect is more remarkable on the second slope observed at high anodic polarisation than at low potentials. The variation of the current density versus the square root of the rotation rate is plotted in Fig. 4 for different potentials and solution concentrations. A non-linear behaviour was observed for I versus V 1/2 and I 1 versus V 1/2 relations. The same kind of behaviour was already reported for copper dissolution [12]. These relations could be analysed by the expression: It I0
Ik1
1 A1 V1=2
(1)
where, Ik and A are the classical terms obtained by the Koutecky /Levich expression. I0 is a non-diffusion current.
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Fig. 4. Variation of anodic current density vs. square root of the rotation rate in different carbonate/bicarbonate solutions and anodic potentials, pH 8.9. (a) 0.375 M KHCO3/0.025 M K2CO3, (b) 0.75 M KHCO3/0.05 M K2CO3, (c) 1.5 M KHCO3/0.1 M K2CO3. (j) /1.0 V, (m) /0.97 V, (') /0.90 V, (%) /0.87 V, ( */) fitting using Eq. (1). (V vs. SSE).
A non-linear fitting of the experiment current using Eq. (1) was made by the simplex method (full line in Fig. 4). The parameters I0, Ik and A , involved in the fitting were obtained and the results are given in Table 1 and Fig. 5. It is possible to note that the three parameters are influenced by the potential and solution concentration.
3.2. Interfacial pH measurements
Fig. 3. Steady state polarisation curves of Co in different bicarbonate/ carbonate solutions, pH 8.9, V /100 rpm. ( /j /) 0.375 M KHCO3/ 0.025 M K2CO3; ( /m /) 0.75 M KHCO3/0.05 M K2CO3; ( /' /) 1.5 M KHCO3/0.1 M K2CO3; ( /% /) 2.5 M KHCO3/0.167 M K2CO3.
Interfacial and bulk pH were monitored during anodic polarisation of electrodeposited cobalt in 0.75 M KHCO3/0.05 M K2CO3 solution. The steady state polarisation and local pH are shown in Fig. 6. Similar curves were obtained for the other carbonate/bicarbonate solution concentrations. The steady state polarisation curve of electrodeposited cobalt has the same behaviour as the pure cobalt cylindrical bar. This indicates that the deposit was homogeneous and the anodic electrochemical process during interfacial pH monitoring can be compared with that of RDE. A
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Table 1 Fitting results (from Eq. (1)) of the experimental current Ca
E (VSSE)
I0 (mA cm2)
Ik (mA cm 2)
A (mA cm 2 rpm 1/2)
0.5 (0.375 M KHCO30.025 M K2CO3)
1.0 0.97 0.90 0.87 1.0 0.90 0.87 1.0 0.97 0.90 0.87 1.0 0.90
0.03 0.07 0.05 0.03 0.096 0.129 0.095 0.157 0.263 0.34 0.30 0.234 0.44
0.437 0.693 1.708 2.552 0.823 3.644 5.083 1.061 1.911 5.17 6.72 1.293 8.49
0.0147 0.0174 0.0405 0.0496 0.018 0.076 0.100 0.048 0.060 0.168 0.242 0.063 0.290
1.0 (0.75 M KHCO30.05 M K2CO3)
2.0 (1.50 M KHCO30.10 M K2CO3)
3.0 (2.25 M KHCO30.15 M K2CO3) a
C is a factor used to multiply the concentration of the base solution 0.75 M KHCO30.05 M K2CO3).
Fig. 6. Steady state polarisation curve and local pH measurements of Co in 0.75 M KHCO3/0.05 M K2CO3. Bulk pH 8.9.
Fig. 5. Dependence of fitting parameters given in Eq. (1) on solution concentration. ‘‘C’’ is defined on Table 1. (j) /1.0 V, (m) /0.97 V, (') /0.90 V, (%) /0.87 V (V vs. SSE).
constant interfacial pH was observed during cobalt anodic polarisation from OCP to Ec.d.p.. 3.3. Raman spectroscopy In order to analyse the film formed at cobalt electrode surface during anodic polarisation in carbonate/bicarbonate medium, in situ and ex situ Raman spectroscopy
experiments were performed. In all cases, Raman spectra were taken after the stationary current was attained. Raman spectra of chemically pure compounds were also obtained. Fig. 7 shows the Raman spectra of KHCO3, K2CO3, CoCO3, and Co3O4. Three bands at 634, 675 and 1027 cm 1 are observed on the KHCO3 spectrum, the latter band at 1027 cm 1 being the highest and more important one. The K2CO3 sample exhibits only one sharp band at 1059 cm 1. In the CoCO3 spectrum the highest and more important band appears at 1083 cm 1, together with other bands at 303, 483, 619, 692 and 725 cm 1 of lower intensity. The Co3O4 compound exhibits a broad band in the range of about 450 /700 cm 1. At this region a sharp and high band at 481 cm 1 is remarkable, other bands on the same region at 522, 619, 671, and 690 cm 1 also appear. This spectrum is characteristic of spinel structure compounds like Mn3O4, Fe3O4 and Co3O4 [13]. In situ Raman spectra of cobalt in the 0.75 M KHCO3/0.05 M K2CO3 medium without solution renovation and for different anodic potentials are shown in Fig. 8. Only the bands at 1014 and 1063 cm 1 appear in all anodic polarisation conditions. Comparing with pure samples (Fig. 7), it can be seen that the bands at 1014 and 1063 cm 1 correspond to bicarbonate and
J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541
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Fig. 7. Raman spectra of pro analysis reagents. KHCO3, K2CO3 and Co3O4 from Merck and CoCO3 from Sigma-Aldrich.
carbonate compounds, respectively. The band intensities vary according to the anodic potential. An increase was observed in the intensity of the band at 1063 cm 1 with respect to the intensity of the band at 1014 cm 1 with ncreasing anodic polarisation. At more positive potentials, the relative intensity of the band pointed at 1063 cm 1 is very close to that pointed at 1014 cm1. These results suggest that during the electrochemical process there is a conversion of bicarbonate to carbonate species, giving rise to a carbonate concentration increase. No cobalt compounds were detected in any anodic potential condition. In situ Raman spectra of cobalt in the 0.75 M KHCO3/0.05 M K2CO3 medium, with continuous solution renovation are shown in Fig. 9, for different anodic potentials. Similar to Fig. 8, only bicarbonate and carbonate bands at 1012 and 1064 cm 1 are observed. However, no variation on the relative intensity of the bands was observed at any point on the whole range of anodic potential. A broad and not well-defined band at 600 cm 1 is detected but no bands related to cobalt species were observed in this experimental condition. It is interesting to remark that, despite the visually observed changes on the electrode surface appearance, no cobalt species produced during anodic polarisation were detected in the in situ Raman experiments. In order to characterise the grey film formed on the metal
surface, ex situ Raman experiments were needed. Fig. 10 shows in situ and ex situ Raman spectra of cobalt in 0.75 M KHCO3/0.05 M K2CO3 medium at the peak potential Ec.d.p. //0.86 V. When the stationary current was reached, in situ Raman spectra were taken. After that, the electrode was rapidly washed to remove possible remaining solution from the surface and then an ex situ Raman spectrum was taken. A broad band at 522 cm 1 in the ex situ Raman spectrum was observed. Different from that observed in the in situ spectrum, neither bicarbonate nor carbonate characteristic band appears. According to the literature [14,15], the band at 522 cm 1 corresponds to stoichiometric CoO. These results suggest that the bands observed in the in situ Raman spectra, shown in Figs. 8 and 9, correspond to the electrolytic solution Raman scattering and not to species formed on cobalt surface during anodic polarisation. Furthermore, in line with results shown in Fig. 6, the interfacial pH does not change during anodic polarisation, indicating that free carbonate and bicarbonate species remain constant in solution. Therefore, the carbonate species observed during anodic dissolution (Fig. 8) seems to be a non-free species produced during the electrochemical process. The Raman spectrum of the cobalt surface was only obtained after taking off the electrode from the cell and removing the remaining solution from its surface. Consequently, due to the strong solution Raman scattering, the metal
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Fig. 8. In situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9 without solution renovation, for different anodic potential (V vs. SSE).
surface can be only characterised by ex-situ Raman measurements. On the other hand, the difference between Figs. 8 and 9 in connection with the variation on the relative intensity of the bands (Fig. 8) occurs because the experiments were performed at different hydrodynamic setting. This clearly shows that hydrodynamic control is necessary to keep constant the experimental conditions without the influence of dissolution products throughout the electrochemical measurements. In general, this point is not taken into account during in situ Raman works involving electrodissolutions processes. Ex situ Raman spectra of the cobalt RDE in the 0.75 M KHCO3/0.05 M K2CO3 medium at different anodic polarisation potentials are shown in Fig. 11. It should be noted that at OCP (Fig. 11 (a)) no important bands in the region of interest, between 200 and 1200 cm 1, appear. Only a small shoulder at 524 cm 1 is present.
Increasing the anodic polarisation, the band at 524 cm 1 increases and becomes more important, as can be observed in spectra shown in Fig. 11 (b), (c) and (d). At anodic potentials close to the peak (Ec.d.p. //0.86 V), the band at 524 cm 1 is high and sharp (Fig. 11 (e)). It is clear that a continuous increase of crystallinity and thickness of the CoO film occurs as a consequence of the metal anodic polarisation. These results are consistent with most of works in cobalt dissolution in carbonate/ bicarbonate media [6 /11]. The Raman spectrum in Fig. 11 (f) was taken at the cobalt surface after 20 h of anodic polarisation at /0.82 V. This potential lies within the passivation region on the steady state polarisation curve (Fig. 2). The bands in the Raman spectrum of Fig. 11 (f) correspond to a new species formed at the cobalt surface. That new species comes out from the earlier CoO film. Comparing to Raman spectra of pure compounds in Fig. 9, it is evident that the bands
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Fig. 9. In situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9 with solution renovation, for different anodic potential (V vs. SSE). Flow solution: 10 3 dm3 s 1.
observed in Fig. 11 (f) are related to Co3O4. No bands corresponding to bicarbonate or carbonate compounds were observed in ex situ Raman spectra.
4. Discussion
Fig. 10. In situ and ex situ Raman spectra of Co in 0.75 M KHCO3/ 0.05 M K2CO3, pH 8.9, E//0.86 V vs. SSE.
The electrochemical results seen in Figs. 2 /5 show that carbonate and bicarbonate are active species that participate during the charge transfer and mass transport processes. However, as already seen by local-pH measurements, the interfacial pH is constant. This indicates a buffer effect of carbonate/bicarbonate system. In the bulk solution, the following equilibrium must be considered:
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Fig. 11. Ex situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9, V/100 rpm, for different anodic potentials: (a) OCP//1.090 V; (b) /1.030 V; (c) /1.00 V; (d) /0.940 V; (e) /0.880 V; (f) /0.820 V (V vs. SSE).
KHCO3 ? HCO 3 K
(2)
K2 CO3 ? CO2 3 2K 2 HCO 3 ?CO3 H
(3) (4)
The interfacial pH results shown in Fig. 6 can only be interpreted assuming that non-dissociated KHCO3 and K2CO3 are in excess at the electrode surface, that maintains the hydrogen concentration [H ]0 constant. From Eq. (4) it can be seen that the pH is entirely defined by the [CO2 3 ]/[HCO3 ] ratio. Consequently, if the pH in the interfacial region is constant, the ratio of these species is also constant and the mass transport cannot be directly associated with the existence of a carbonate/bicarbonate concentration gradient. In this case, it is necessary to assume that the mass transport
takes place by means of a cobalt complex involving a carbonate ion. The literature [6] proposes the complex as the diffusion species away from the Co(CO3)2 2 electrode. Therefore, for the present discussion, the following reactions are considered: k1
CoHCO 3 0(CoHCO3 )ads e k2
2 (CoHCO3 )ads HCO 3 ? Co(CO3 )2 2H e k2
(5) (6)
In which (CoHCO3)ads /u1 is an adsorbed species at the electrode surface following the Langmuir isotherm. The charge balance for Eqs. (5) and (6) is:
J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541
I1 F
K1 (1u1 )K2 u1 K2 [Co(CO3 )2 2 ]
(7)
where: K1 k1 [HCO 3 ]0 exp K2 k2 [HCO 3 ]0 exp
a1 F V RT a2 F
V
RT (1 a2 )F K2 k2 [H ]20 exp V RT
(8)
F
2K1 K 2K2 [Co(CO3 )2 2 ](1K)
(12)
(13)
(15)
Replacing Eq. (15) in Eq. (12) and considering the diffusion layer as d LV1=2 ; with L as a constant, we take: I11
(K1 K2 ) 2FK1 K2
K2 2FDK2
LV1=2
(CoO)ads 2H 2e
(19)
The oxide can be dissolved by [H ] as:
It I0 I1 or It I0
1
(20)
(21)
I11
where It is the total anodic current of the cobalt dissolution. Eq. (21) imposes that the surface coverage u1 concerning Eq. (5) must be independent from u2 /(CoO)ads in Eq. (19). The same was considered for copper dissolution in which two independent processes were proposed to share the electrode surface by the parameters g and (1/g) [12]. Taking into account this surface sharing, Eq. (16) must be re-written as: I11
2FD
0
(14)
dI1
k3 ; [(CO3 )2 ]
(11)
where d is the Nernst diffusion layer of the complex and D is its diffusion coefficient. Then: [Co(CO3 )2 2 ]0
The global process is:
where Co2 can be taken as an aqueous complex. In cobalt oxide formation described by Eqs. (17) and (18), it makes no difference to take (CO3)2 or HCO3 ions as the catalytic species, because the global process given by Eq. (19) is the same. Coupling between I1 and the non-diffusion current I0 produced by Eq. (19) must be considered as:
According to the above discussion, the mass transport can be written as: I1 2D[Co(CO3 )2 2 ]0 F d
(18)
k4
where: a2 FV k2 exp RT K a1 FV a2 FV k2 exp k1 exp RT RT
(17)
CoCO3 H2 O 0 CoOCO2 3 2H
(CoO)ads 2H 0 Co2 H2 O
Under steady state (du1)/(dt )/0, thus: I1
CoCO2 3 0 CoCO3 2e
(10)
The mass balance is: b du1 K1 (1u1 )K2 u1 K2 [Co(CO3 )2 2 ] dt
a catalytic role in the cobalt oxidation process:
CoH2 O (9)
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(K1 K2 ) K2 LV1=2 2(1 g)FK1 K2 2(1 g)FDK2
(22)
In the present work, I0 can be calculated independently from I1 on the basis of the mechanism described by Eqs. (17) /(20), as follows: Charge balance: I0 2F
gK3 (1u2 )
(23)
and, the mass balance: (16)
Eq. (16) can be compared to the second term of the right side of Eq. (1). To simulate the experimental results, it is necessary to introduce the non-diffusion current I0 into the total current expression. According to Raman spectroscopy results, only cobalt oxide was detected onto the electrode surface. In Ref. [6] it is proposed that this oxide is formed directly by an electrochemical reaction between Co and H2O. It could be considered that the reaction proposed by [6] gives rise to the I0 current. Nevertheless, as can be observed in Fig. 5, I0 is influenced by the solution concentration. In this case, is assumed that the CO2 or HCO3 ion plays 3
b du2 dt
K3 (1u2 )K4 u2
(24)
with: K3 k3 [(CO3 )2 ]0 exp
a3 F RT
V
K4 k4 [H ]20
(25) (26)
Considering the steady state (du2)/(dt )/0, so: 1u2
K4 K3 K4
Then, the current I0 is:
(27)
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I0 g
2FK3 K4
(28)
K3 K4
anodic current. Actually, the I0/I1 ratio will change according to polarisation conditions.
Replacing the Eqs. (22) and (28) in Eq. (21), we take: It g
5. Conclusions
2FK3 K4 K3 K4 1
(K1 K2 ) 2(1 g)FK1 K2
K2 2(1 g)FDK2
(29) LV
1=2
It follows from Eq. (1) that: Ik1
(K1 K2 ) 2(1 g)FK1 K2
A1
K2 2(1 g)FDK2
(30) L
(31)
and I0 is given by Eq. (28). The complete expressions for the terms in Eq. (1) are: aF 2Fk3 k4 [H ]20 [(CO3 )2 ]0 exp 3 V RT I0 g (32) a F 2 3 2 V k4 [H ]0 k3 [(CO3 ) ]0 exp RT F V (a1 a2 ) 2F (1 g)[HCO3 ]0 k1 k2 exp RT (33) Ik a1 F a2 F V k2 exp V k1 exp RT RT and: A
2FD(1 k2 [H ]20
g)[HCO 3 ]0 k2
FV L exp RT
(34)
As can be seen in Eq. (32), I0 is not influenced by the mass transport and is potential dependent, in agreement with the experimental results. The influence of the solution concentration on I0 is given by the relative importance of K3 versus K4. The current Ik and the mass transport coefficient A are both potential dependent and influenced by solution concentration. The above mechanism is able to account for the steady state experimental results and can further be used to explain the EHD and ac impedances in a forthcoming work [16]. The existence of two different slopes in the metal active dissolution region on the steady state polarisation curve (Fig. 2) could be explained by the predominance of one of the two different metal dissolution paths. Those paths are given by the electrochemical reactions described on Eqs. (5), (6) and (19) for specific polarisation potentials. Therefore, the currents I1 and I0 obtained by the reaction mechanism already discussed probably contribute with different weight to the total
Steady state polarisation curves of cobalt in carbonate/bicarbonate media present two slopes in the active dissolution region between the open-circuit potential and the potential of the first current density peak. The analysis of polarisation curves indicates an important influence of carbonate/bicarbonate concentration as well as of the mass transport on the metal electrodissolution process. The anodic behaviour of cobalt in this media can be explained by the coupling of two independent anodic currents, I0 and I1, as It /I0/I1, where I0 is a non-diffusion anodic current and I1 follows the Koutecky /Levich expression. The anion concentration and polarisation conditions influence both I0 and I1 currents. A kinetic model consistent with an empirical current expression used to fit the experimental results was proposed. Raman spectroscopy experiments clearly showed that, during cobalt anodic polarisation, a CoO film is generated in the active dissolution region. Metal passivation takes place by a slow transformation of the CoO precursor oxide into a Co3O4 passive film.
Acknowledgements J.A. Caldero´n and J.E.P. da Silva acknowledge CAPES and FAPESP (Proc. N8 98/15686-3), respectively, for the doctoral scholarships granted. Authors thank the CAPES, CNPq, FAPERJ and FINEP for financial support. We also thank the Laboratorio de Espectroscopia Molecular (IQ/USP) for Raman facilities as well as Dr. Bernard Tribollet (CNRS-Paris) for fruitful discussions.
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J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541 [11] C.A. Gervasi, F.E. Varela, J.R. Vilche, Mater. Sci. Forum 289 / 292 (1998) 1057. [12] E. D’Elia, O.E. Barcia, O.R. Mattos, N. Pe´be`re, B. Tribollet, J. Electrochem. Soc. 143 (1996) 961. [13] M.C. Bernard, A. Hugot-Le Goff, B.V. Thi, S. Cordoba de Torresi, J. Electrochem. Soc. 140 (1993) 3065.
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[14] F. Borgatti, L. Braicovich, N.B. Brookes, G. Ghiringhelli, A. Tagliafferri, J. Electron Spectrosc. Relat. Phenom. 101 /103 (1999) 467. [15] J. Nan, Y. Yang, Z. Lin, Electrochim. Acta 46 (2001) 1767. [16] J.A. Caldero´n, PhD Thesis, COPPE UFRJ, Rio de Janeiro, 2002.
Electrodissolution of cobalt in carbonate/bicarbonate media J.A. Caldero´n a,b, O.R. Mattos a,*,1, O.E. Barcia a,c,1, S.I. Co´rdoba de Torresi d,*,1, J.E. Pereira da Silva d a
Laborato´rio de Corrosa˜o Professor Manoel de Castro, PEMM/COPPE/UFRJ, Cx. Postal 68.505, CEP 21945-970, Rio de Janeiro, R.J., Brazil b Grupo de Corrosio´n y Proteccio´n, Universidad de Antioquia, P.O. Box. 1226, Medellı´n, Colombia c Dep. Fı´sico-Quı´mica, IQ/UFRJ, Rio de Janeiro, Brazil d Departamento de Quı´mica Fundamental, Instituto de Quı´mica, Universidade de Sa˜o Paulo, Sa˜o Paulo (SP), Brazil Received 25 June 2002; received in revised form 19 August 2002
Abstract The electrodissolution of cobalt in carbonate/bicarbonate solutions was studied at room temperature by steady state polarisation, interfacial pH measurements and Raman spectroscopy. The active dissolution of cobalt leads to an initial CoO film formation. The metal passivation occurs by a slow transformation of the CoO into a Co3O4 oxide. The influence of HCO3 and CO2 anions was 3 investigated. Two different parallel electrochemical processes were proposed to account for the anion role on the electrochemical steady state behaviour of cobalt in the studied solutions. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cobalt; Dissolution; Carbonate/bicarbonate; Raman spectroscopy
1. Introduction The electrochemical behaviour of cobalt in alkaline media has been studied in a large range of pH values, involving solutions of different nature and composition [1 /11]. Those studies have evidenced different dissolution and passivation steps, depending on the nature of the solution and on the technique used. Cobalt passivation involves several oxidation states with cobalt oxide and hydroxide formations as a function of the applied potential. However, the anion role has been not sufficiently explained and the composition and structure of the passive film formatted is still controversial. In strong alkaline solution and at low anodic polarisation it was suggested [1 /3] that cobalt dissolution results on the formation of Co(OH)2, whereas, at more positive potentials, Co3O4 or CoOOH species replace the previous one. The CoO film is responsible for the initial passivity. In weakly alkaline borate solutions was found a successive formation of CoO, duplex CoO/ Co3O4 and duplex Co3O4/Co2O3 films, corresponding to * Corresponding authors. Fax: /55-21-290-6626 E-mail address: [email protected] (O.R. Mattos). 1 ISE members.
different dissolution/passivation steps [5]. In carbonate/ bicarbonate solutions the hydrodynamic dependence of the dissolution process was reported [6]. In these media, the oxides species are similar to those found in borate [7]. Concerning the anion role, it is proposed that HCO3 ions are more aggressive to CoO film than Co2O3 and borate ions do not attack any anodic film [6,7]. The composition of the surface layer in the intermediate anodic potential was found to be dependant on the concentration ratio of OH and HCO3 ions in solution [8]. Recently, the structure and electrochemical proprieties of the duplex anodic layer of CoO/ Co3O4 yielded during potentiodynamic polarisation of cobalt in carbonate/bicarbonate media were studied by electroreduction technique [9 /11]. The above works have been performed using mainly potentiodynamic techniques and the steady state has yet been little explored. Moreover, most of works on cobalt dissolution in alkaline media have been focused on metal passivation and less attention has been paid to the first steps of dissolution. The present paper mainly concerns the electrochemical behaviour of cobalt during metal active dissolution region, from open-circuit potential until the first passivation step. Steady state polarisation curves, Raman spectroscopy and interfacial
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 5 4 2 - X
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pH are the techniques used and the electrolyte are carbonate/bicarbonate media.
2. Experimental 2.1. Materials For electrochemical measurements, a classical threeelectrode cell was used. A pure cobalt cylindrical bar (Johnson Matthey Chemicals Ltd) of 5 mm diameter (0.2 cm2, exposed area) embedded in epoxy resin was set as rotating disc electrode (RDE). Saturated sulphate electrode (SSE) was used as reference and a large area platinum grid as counter electrode. Before the anodic polarisation measurements, the working electrode was polished until 600 grade emery paper, immersed in a 0.1 M H2SO4 solution for 1 min, rinsed with doubledistilled water and finally cathodically polarised in the working solution for 2 min. The cathodic potential used was negative enough to cause hydrogen evolution in order to obtain a free oxide cobalt surface. Polarisations were carried out in potentiostatic mode and stationary currents were taken by chronoamperometry using an AUTOLAB PGSTAT 30. All potential values are given versus SSE. The electrolyte consisted of an x M KHCO3/y M K2CO3, (x /0.375, 0.75, 1.5, 2.25, 2.5; y/0.025, 0.05, 0.1, 0.15, 0.167) mixture with fixed x /y ratio in order to keep the pH constant at 8.9. The solutions were prepared from analytical grade chemicals and previously boiled double distilled water to remove dissolved CO2. Electrolytes were de-aerated by purging with purified nitrogen during the whole measurement. 2.2. Local pH measurements For the pH measurements at the cobalt/electrolyte interface was used as working electrode a cobalt electrodeposited 75 mesh Pt screen fixed at the end of a flat-bottomed pH electrode (Mettler Toledo, mod. Inlab 426). The pH electrode was connected to a pH meter, thus measuring the solution pH during the electrodissolution process. The assembled electrode was motionless and the solution agitated with the aid of a mechanical stirrer. At these conditions, pH and current were recorded simultaneously during potentiostatic anodic polarisation.
Fig. 1. Flow Cell for in situ Raman measurements.
microscope and a CCD detector. For in situ experiments an electrochemical flow-cell (volume /4.0 /103 dm3) was used (Fig. 1). In situ Raman experiments were performed with and without continuous solution renovation. In the first case, the solution flow was regulated to 103 dm3 s 1. A SSE was used as reference and a Pt spiral wire encrusted on the bottom of the cell was the counter electrode. With this set up a laminar flow was achieved.
3. Results 3.1. Polarisation Anodic steady state polarisation curves of cobalt at 0, 100 and 225 rpm in a 0.75 M KHCO3/0.05 M K2CO3 solution are shown in Fig. 2. It should be noted that a mixed control on the anodic process in the whole potential range exists. Different slopes at low and at high anodic potentials are also observed. The first one between the open-circuit potential (OCP) and ://0.98 V, another one between ://0.98 V and the current density peak potential (Ec.d.p.)://0.86 V. The presence of two slopes in the steady state polarisation curves suggests that two different electrochemical processes are likely to be taking place during cobalt anodic polarisation between the OCP and the Ec.d.p.. Additionally,
2.3. Raman measurements In situ and ex situ Raman experiments were performed during cobalt anodic polarisation using a Renishaw Raman Imaging System 3000, coupled with a He /Ne laser from Spectra Physics (Mod. 127, lo / 632.8 nm), equipped with an Olympus metallurgical
Fig. 2. Steady state polarisation curves of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9.
J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541
stationary currents at high anodic potentials were more rapidly achieved (20 /30 min) than those at low anodic potentials, where 2 /3 h were necessary to obtain stationary current values. The peak potential (Ec.d.p.) is practically independent of the RDE rotation rate (V). After this potential the current gradually decreases due to the beginning of the first passivation process. During this passivation process, a dark and adherent film slowly covers the cobalt surface. The stationary current is reached after nearly 24 h. Steady state polarisation curves of Co in different carbonate/bicarbonate solution compositions at V /100 rpm are shown in Fig. 3. For all the studied solutions, the general shape of the polarisation curves is the same, also showing the two slopes already described. The solution composition was varied from a base solution concentration of carbonate/bicarbonate. Solutions of 0.5, 1.0, 2.0 and 3.33 times the base solution concentration were prepared; those were: 0.375 M KHCO3/0.025 M K2CO3, 0.75 M KHCO3/0.05 M K2CO3, 1.5 M KHCO3/0.1 M K2CO3, 2.5 M KHCO3/0.167 M K2CO3, respectively. The anodic current density increases with increasing solution concentration for the same polarisation potential. This effect is more remarkable on the second slope observed at high anodic polarisation than at low potentials. The variation of the current density versus the square root of the rotation rate is plotted in Fig. 4 for different potentials and solution concentrations. A non-linear behaviour was observed for I versus V 1/2 and I 1 versus V 1/2 relations. The same kind of behaviour was already reported for copper dissolution [12]. These relations could be analysed by the expression: It I0
Ik1
1 A1 V1=2
(1)
where, Ik and A are the classical terms obtained by the Koutecky /Levich expression. I0 is a non-diffusion current.
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Fig. 4. Variation of anodic current density vs. square root of the rotation rate in different carbonate/bicarbonate solutions and anodic potentials, pH 8.9. (a) 0.375 M KHCO3/0.025 M K2CO3, (b) 0.75 M KHCO3/0.05 M K2CO3, (c) 1.5 M KHCO3/0.1 M K2CO3. (j) /1.0 V, (m) /0.97 V, (') /0.90 V, (%) /0.87 V, ( */) fitting using Eq. (1). (V vs. SSE).
A non-linear fitting of the experiment current using Eq. (1) was made by the simplex method (full line in Fig. 4). The parameters I0, Ik and A , involved in the fitting were obtained and the results are given in Table 1 and Fig. 5. It is possible to note that the three parameters are influenced by the potential and solution concentration.
3.2. Interfacial pH measurements
Fig. 3. Steady state polarisation curves of Co in different bicarbonate/ carbonate solutions, pH 8.9, V /100 rpm. ( /j /) 0.375 M KHCO3/ 0.025 M K2CO3; ( /m /) 0.75 M KHCO3/0.05 M K2CO3; ( /' /) 1.5 M KHCO3/0.1 M K2CO3; ( /% /) 2.5 M KHCO3/0.167 M K2CO3.
Interfacial and bulk pH were monitored during anodic polarisation of electrodeposited cobalt in 0.75 M KHCO3/0.05 M K2CO3 solution. The steady state polarisation and local pH are shown in Fig. 6. Similar curves were obtained for the other carbonate/bicarbonate solution concentrations. The steady state polarisation curve of electrodeposited cobalt has the same behaviour as the pure cobalt cylindrical bar. This indicates that the deposit was homogeneous and the anodic electrochemical process during interfacial pH monitoring can be compared with that of RDE. A
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Table 1 Fitting results (from Eq. (1)) of the experimental current Ca
E (VSSE)
I0 (mA cm2)
Ik (mA cm 2)
A (mA cm 2 rpm 1/2)
0.5 (0.375 M KHCO30.025 M K2CO3)
1.0 0.97 0.90 0.87 1.0 0.90 0.87 1.0 0.97 0.90 0.87 1.0 0.90
0.03 0.07 0.05 0.03 0.096 0.129 0.095 0.157 0.263 0.34 0.30 0.234 0.44
0.437 0.693 1.708 2.552 0.823 3.644 5.083 1.061 1.911 5.17 6.72 1.293 8.49
0.0147 0.0174 0.0405 0.0496 0.018 0.076 0.100 0.048 0.060 0.168 0.242 0.063 0.290
1.0 (0.75 M KHCO30.05 M K2CO3)
2.0 (1.50 M KHCO30.10 M K2CO3)
3.0 (2.25 M KHCO30.15 M K2CO3) a
C is a factor used to multiply the concentration of the base solution 0.75 M KHCO30.05 M K2CO3).
Fig. 6. Steady state polarisation curve and local pH measurements of Co in 0.75 M KHCO3/0.05 M K2CO3. Bulk pH 8.9.
Fig. 5. Dependence of fitting parameters given in Eq. (1) on solution concentration. ‘‘C’’ is defined on Table 1. (j) /1.0 V, (m) /0.97 V, (') /0.90 V, (%) /0.87 V (V vs. SSE).
constant interfacial pH was observed during cobalt anodic polarisation from OCP to Ec.d.p.. 3.3. Raman spectroscopy In order to analyse the film formed at cobalt electrode surface during anodic polarisation in carbonate/bicarbonate medium, in situ and ex situ Raman spectroscopy
experiments were performed. In all cases, Raman spectra were taken after the stationary current was attained. Raman spectra of chemically pure compounds were also obtained. Fig. 7 shows the Raman spectra of KHCO3, K2CO3, CoCO3, and Co3O4. Three bands at 634, 675 and 1027 cm 1 are observed on the KHCO3 spectrum, the latter band at 1027 cm 1 being the highest and more important one. The K2CO3 sample exhibits only one sharp band at 1059 cm 1. In the CoCO3 spectrum the highest and more important band appears at 1083 cm 1, together with other bands at 303, 483, 619, 692 and 725 cm 1 of lower intensity. The Co3O4 compound exhibits a broad band in the range of about 450 /700 cm 1. At this region a sharp and high band at 481 cm 1 is remarkable, other bands on the same region at 522, 619, 671, and 690 cm 1 also appear. This spectrum is characteristic of spinel structure compounds like Mn3O4, Fe3O4 and Co3O4 [13]. In situ Raman spectra of cobalt in the 0.75 M KHCO3/0.05 M K2CO3 medium without solution renovation and for different anodic potentials are shown in Fig. 8. Only the bands at 1014 and 1063 cm 1 appear in all anodic polarisation conditions. Comparing with pure samples (Fig. 7), it can be seen that the bands at 1014 and 1063 cm 1 correspond to bicarbonate and
J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541
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Fig. 7. Raman spectra of pro analysis reagents. KHCO3, K2CO3 and Co3O4 from Merck and CoCO3 from Sigma-Aldrich.
carbonate compounds, respectively. The band intensities vary according to the anodic potential. An increase was observed in the intensity of the band at 1063 cm 1 with respect to the intensity of the band at 1014 cm 1 with ncreasing anodic polarisation. At more positive potentials, the relative intensity of the band pointed at 1063 cm 1 is very close to that pointed at 1014 cm1. These results suggest that during the electrochemical process there is a conversion of bicarbonate to carbonate species, giving rise to a carbonate concentration increase. No cobalt compounds were detected in any anodic potential condition. In situ Raman spectra of cobalt in the 0.75 M KHCO3/0.05 M K2CO3 medium, with continuous solution renovation are shown in Fig. 9, for different anodic potentials. Similar to Fig. 8, only bicarbonate and carbonate bands at 1012 and 1064 cm 1 are observed. However, no variation on the relative intensity of the bands was observed at any point on the whole range of anodic potential. A broad and not well-defined band at 600 cm 1 is detected but no bands related to cobalt species were observed in this experimental condition. It is interesting to remark that, despite the visually observed changes on the electrode surface appearance, no cobalt species produced during anodic polarisation were detected in the in situ Raman experiments. In order to characterise the grey film formed on the metal
surface, ex situ Raman experiments were needed. Fig. 10 shows in situ and ex situ Raman spectra of cobalt in 0.75 M KHCO3/0.05 M K2CO3 medium at the peak potential Ec.d.p. //0.86 V. When the stationary current was reached, in situ Raman spectra were taken. After that, the electrode was rapidly washed to remove possible remaining solution from the surface and then an ex situ Raman spectrum was taken. A broad band at 522 cm 1 in the ex situ Raman spectrum was observed. Different from that observed in the in situ spectrum, neither bicarbonate nor carbonate characteristic band appears. According to the literature [14,15], the band at 522 cm 1 corresponds to stoichiometric CoO. These results suggest that the bands observed in the in situ Raman spectra, shown in Figs. 8 and 9, correspond to the electrolytic solution Raman scattering and not to species formed on cobalt surface during anodic polarisation. Furthermore, in line with results shown in Fig. 6, the interfacial pH does not change during anodic polarisation, indicating that free carbonate and bicarbonate species remain constant in solution. Therefore, the carbonate species observed during anodic dissolution (Fig. 8) seems to be a non-free species produced during the electrochemical process. The Raman spectrum of the cobalt surface was only obtained after taking off the electrode from the cell and removing the remaining solution from its surface. Consequently, due to the strong solution Raman scattering, the metal
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Fig. 8. In situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9 without solution renovation, for different anodic potential (V vs. SSE).
surface can be only characterised by ex-situ Raman measurements. On the other hand, the difference between Figs. 8 and 9 in connection with the variation on the relative intensity of the bands (Fig. 8) occurs because the experiments were performed at different hydrodynamic setting. This clearly shows that hydrodynamic control is necessary to keep constant the experimental conditions without the influence of dissolution products throughout the electrochemical measurements. In general, this point is not taken into account during in situ Raman works involving electrodissolutions processes. Ex situ Raman spectra of the cobalt RDE in the 0.75 M KHCO3/0.05 M K2CO3 medium at different anodic polarisation potentials are shown in Fig. 11. It should be noted that at OCP (Fig. 11 (a)) no important bands in the region of interest, between 200 and 1200 cm 1, appear. Only a small shoulder at 524 cm 1 is present.
Increasing the anodic polarisation, the band at 524 cm 1 increases and becomes more important, as can be observed in spectra shown in Fig. 11 (b), (c) and (d). At anodic potentials close to the peak (Ec.d.p. //0.86 V), the band at 524 cm 1 is high and sharp (Fig. 11 (e)). It is clear that a continuous increase of crystallinity and thickness of the CoO film occurs as a consequence of the metal anodic polarisation. These results are consistent with most of works in cobalt dissolution in carbonate/ bicarbonate media [6 /11]. The Raman spectrum in Fig. 11 (f) was taken at the cobalt surface after 20 h of anodic polarisation at /0.82 V. This potential lies within the passivation region on the steady state polarisation curve (Fig. 2). The bands in the Raman spectrum of Fig. 11 (f) correspond to a new species formed at the cobalt surface. That new species comes out from the earlier CoO film. Comparing to Raman spectra of pure compounds in Fig. 9, it is evident that the bands
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Fig. 9. In situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9 with solution renovation, for different anodic potential (V vs. SSE). Flow solution: 10 3 dm3 s 1.
observed in Fig. 11 (f) are related to Co3O4. No bands corresponding to bicarbonate or carbonate compounds were observed in ex situ Raman spectra.
4. Discussion
Fig. 10. In situ and ex situ Raman spectra of Co in 0.75 M KHCO3/ 0.05 M K2CO3, pH 8.9, E//0.86 V vs. SSE.
The electrochemical results seen in Figs. 2 /5 show that carbonate and bicarbonate are active species that participate during the charge transfer and mass transport processes. However, as already seen by local-pH measurements, the interfacial pH is constant. This indicates a buffer effect of carbonate/bicarbonate system. In the bulk solution, the following equilibrium must be considered:
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Fig. 11. Ex situ Raman spectra of Co in 0.75 M KHCO3/0.05 M K2CO3, pH 8.9, V/100 rpm, for different anodic potentials: (a) OCP//1.090 V; (b) /1.030 V; (c) /1.00 V; (d) /0.940 V; (e) /0.880 V; (f) /0.820 V (V vs. SSE).
KHCO3 ? HCO 3 K
(2)
K2 CO3 ? CO2 3 2K 2 HCO 3 ?CO3 H
(3) (4)
The interfacial pH results shown in Fig. 6 can only be interpreted assuming that non-dissociated KHCO3 and K2CO3 are in excess at the electrode surface, that maintains the hydrogen concentration [H ]0 constant. From Eq. (4) it can be seen that the pH is entirely defined by the [CO2 3 ]/[HCO3 ] ratio. Consequently, if the pH in the interfacial region is constant, the ratio of these species is also constant and the mass transport cannot be directly associated with the existence of a carbonate/bicarbonate concentration gradient. In this case, it is necessary to assume that the mass transport
takes place by means of a cobalt complex involving a carbonate ion. The literature [6] proposes the complex as the diffusion species away from the Co(CO3)2 2 electrode. Therefore, for the present discussion, the following reactions are considered: k1
CoHCO 3 0(CoHCO3 )ads e k2
2 (CoHCO3 )ads HCO 3 ? Co(CO3 )2 2H e k2
(5) (6)
In which (CoHCO3)ads /u1 is an adsorbed species at the electrode surface following the Langmuir isotherm. The charge balance for Eqs. (5) and (6) is:
J.A. Caldero´n et al. / Electrochimica Acta 47 (2002) 4531 /4541
I1 F
K1 (1u1 )K2 u1 K2 [Co(CO3 )2 2 ]
(7)
where: K1 k1 [HCO 3 ]0 exp K2 k2 [HCO 3 ]0 exp
a1 F V RT a2 F
V
RT (1 a2 )F K2 k2 [H ]20 exp V RT
(8)
F
2K1 K 2K2 [Co(CO3 )2 2 ](1K)
(12)
(13)
(15)
Replacing Eq. (15) in Eq. (12) and considering the diffusion layer as d LV1=2 ; with L as a constant, we take: I11
(K1 K2 ) 2FK1 K2
K2 2FDK2
LV1=2
(CoO)ads 2H 2e
(19)
The oxide can be dissolved by [H ] as:
It I0 I1 or It I0
1
(20)
(21)
I11
where It is the total anodic current of the cobalt dissolution. Eq. (21) imposes that the surface coverage u1 concerning Eq. (5) must be independent from u2 /(CoO)ads in Eq. (19). The same was considered for copper dissolution in which two independent processes were proposed to share the electrode surface by the parameters g and (1/g) [12]. Taking into account this surface sharing, Eq. (16) must be re-written as: I11
2FD
0
(14)
dI1
k3 ; [(CO3 )2 ]
(11)
where d is the Nernst diffusion layer of the complex and D is its diffusion coefficient. Then: [Co(CO3 )2 2 ]0
The global process is:
where Co2 can be taken as an aqueous complex. In cobalt oxide formation described by Eqs. (17) and (18), it makes no difference to take (CO3)2 or HCO3 ions as the catalytic species, because the global process given by Eq. (19) is the same. Coupling between I1 and the non-diffusion current I0 produced by Eq. (19) must be considered as:
According to the above discussion, the mass transport can be written as: I1 2D[Co(CO3 )2 2 ]0 F d
(18)
k4
where: a2 FV k2 exp RT K a1 FV a2 FV k2 exp k1 exp RT RT
(17)
CoCO3 H2 O 0 CoOCO2 3 2H
(CoO)ads 2H 0 Co2 H2 O
Under steady state (du1)/(dt )/0, thus: I1
CoCO2 3 0 CoCO3 2e
(10)
The mass balance is: b du1 K1 (1u1 )K2 u1 K2 [Co(CO3 )2 2 ] dt
a catalytic role in the cobalt oxidation process:
CoH2 O (9)
4539
(K1 K2 ) K2 LV1=2 2(1 g)FK1 K2 2(1 g)FDK2
(22)
In the present work, I0 can be calculated independently from I1 on the basis of the mechanism described by Eqs. (17) /(20), as follows: Charge balance: I0 2F
gK3 (1u2 )
(23)
and, the mass balance: (16)
Eq. (16) can be compared to the second term of the right side of Eq. (1). To simulate the experimental results, it is necessary to introduce the non-diffusion current I0 into the total current expression. According to Raman spectroscopy results, only cobalt oxide was detected onto the electrode surface. In Ref. [6] it is proposed that this oxide is formed directly by an electrochemical reaction between Co and H2O. It could be considered that the reaction proposed by [6] gives rise to the I0 current. Nevertheless, as can be observed in Fig. 5, I0 is influenced by the solution concentration. In this case, is assumed that the CO2 or HCO3 ion plays 3
b du2 dt
K3 (1u2 )K4 u2
(24)
with: K3 k3 [(CO3 )2 ]0 exp
a3 F RT
V
K4 k4 [H ]20
(25) (26)
Considering the steady state (du2)/(dt )/0, so: 1u2
K4 K3 K4
Then, the current I0 is:
(27)
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I0 g
2FK3 K4
(28)
K3 K4
anodic current. Actually, the I0/I1 ratio will change according to polarisation conditions.
Replacing the Eqs. (22) and (28) in Eq. (21), we take: It g
5. Conclusions
2FK3 K4 K3 K4 1
(K1 K2 ) 2(1 g)FK1 K2
K2 2(1 g)FDK2
(29) LV
1=2
It follows from Eq. (1) that: Ik1
(K1 K2 ) 2(1 g)FK1 K2
A1
K2 2(1 g)FDK2
(30) L
(31)
and I0 is given by Eq. (28). The complete expressions for the terms in Eq. (1) are: aF 2Fk3 k4 [H ]20 [(CO3 )2 ]0 exp 3 V RT I0 g (32) a F 2 3 2 V k4 [H ]0 k3 [(CO3 ) ]0 exp RT F V (a1 a2 ) 2F (1 g)[HCO3 ]0 k1 k2 exp RT (33) Ik a1 F a2 F V k2 exp V k1 exp RT RT and: A
2FD(1 k2 [H ]20
g)[HCO 3 ]0 k2
FV L exp RT
(34)
As can be seen in Eq. (32), I0 is not influenced by the mass transport and is potential dependent, in agreement with the experimental results. The influence of the solution concentration on I0 is given by the relative importance of K3 versus K4. The current Ik and the mass transport coefficient A are both potential dependent and influenced by solution concentration. The above mechanism is able to account for the steady state experimental results and can further be used to explain the EHD and ac impedances in a forthcoming work [16]. The existence of two different slopes in the metal active dissolution region on the steady state polarisation curve (Fig. 2) could be explained by the predominance of one of the two different metal dissolution paths. Those paths are given by the electrochemical reactions described on Eqs. (5), (6) and (19) for specific polarisation potentials. Therefore, the currents I1 and I0 obtained by the reaction mechanism already discussed probably contribute with different weight to the total
Steady state polarisation curves of cobalt in carbonate/bicarbonate media present two slopes in the active dissolution region between the open-circuit potential and the potential of the first current density peak. The analysis of polarisation curves indicates an important influence of carbonate/bicarbonate concentration as well as of the mass transport on the metal electrodissolution process. The anodic behaviour of cobalt in this media can be explained by the coupling of two independent anodic currents, I0 and I1, as It /I0/I1, where I0 is a non-diffusion anodic current and I1 follows the Koutecky /Levich expression. The anion concentration and polarisation conditions influence both I0 and I1 currents. A kinetic model consistent with an empirical current expression used to fit the experimental results was proposed. Raman spectroscopy experiments clearly showed that, during cobalt anodic polarisation, a CoO film is generated in the active dissolution region. Metal passivation takes place by a slow transformation of the CoO precursor oxide into a Co3O4 passive film.
Acknowledgements J.A. Caldero´n and J.E.P. da Silva acknowledge CAPES and FAPESP (Proc. N8 98/15686-3), respectively, for the doctoral scholarships granted. Authors thank the CAPES, CNPq, FAPERJ and FINEP for financial support. We also thank the Laboratorio de Espectroscopia Molecular (IQ/USP) for Raman facilities as well as Dr. Bernard Tribollet (CNRS-Paris) for fruitful discussions.
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