Electrodissolution of cobalt in carbonate/bicarbonate media

Electrodissolution of cobalt in carbonate/bicarbonate media

J.A. Calderon a,b, O.R. Mattos a,*,1, O.E. Barcia a,c,1, S.I. Cordoba de Torresi d,*,1,J.E. Pereira da Silva d

a Laboratorio de Corrosao Professor Manoel de Castro, PEMM/COPPE/UFRJ, Cx. Postal 68.505, CEP 21945-970, Rio de Janeiro, R.J., Brazilb Grupo de Corrosion y Proteccion, Universidad de Antioquia, P.O. Box. 1226, Medellın, Colombia

c Dep. Fısico-Quımica, IQ/UFRJ, Rio de Janeiro, Brazild Departamento de Quımica Fundamental, Instituto de Quımica, Universidade de Sao Paulo, Sao 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 CO3

2� anions was

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 dissolu-

tion and passivation steps, depending on the nature of

the solution and on the technique used. Cobalt passiva-

tion 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 polar-

isation 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

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 depen-

dant on the concentration ratio of OH� and HCO3�

ions in solution [8]. Recently, the structure and electro-

chemical 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 po-

tential until the first passivation step. Steady state

polarisation curves, Raman spectroscopy and interfacial

* Corresponding authors. Fax: �/55-21-290-6626

E-mail address: [email protected] (O.R. Mattos).1 ISE members.

Electrochimica Acta 47 (2002) 4531�/4541

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0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

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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 three-

electrode 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 sulphateelectrode (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 double-

distilled water and finally cathodically polarised in the

working solution for 2 min. The cathodic potential used

was negative enough to cause hydrogen evolution inorder 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 tokeep 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 aidof a mechanical stirrer. At these conditions, pH and

current were recorded simultaneously during potentio-

static anodic polarisation.

2.3. Raman measurements

In situ and ex situ Raman experiments were per-

formed during cobalt anodic polarisation using aRenishaw Raman Imaging System 3000, coupled with

a He�/Ne laser from Spectra Physics (Mod. 127, lo�/

632.8 nm), equipped with an Olympus metallurgical

microscope and a CCD detector. For in situ experiments

an electrochemical flow-cell (volume�/4.0�/10�3 dm3)

was used (Fig. 1). In situ Raman experiments were

performed with and without continuous solution reno-

vation. In the first case, the solution flow was regulated

to 10�3 dm3 s�1. A SSE was used as reference and a Pt

spiral wire encrusted on the bottom of the cell was thecounter 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 presenceof two slopes in the steady state polarisation curves

suggests that two different electrochemical processes are

likely to be taking place during cobalt anodic polarisa-

tion between the OCP and the Ec.d.p.. Additionally,

Fig. 1. Flow Cell for in situ Raman measurements.

Fig. 2. Steady state polarisation curves of Co in 0.75 M KHCO3�/0.05

M K2CO3, pH 8.9.

J.A. Calderon et al. / Electrochimica Acta 47 (2002) 4531�/45414532

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.) ispractically 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�/100rpm 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 concentra-

tion were prepared; those were: 0.375 M KHCO3�/0.025

M K2CO3, 0.75 M KHCO3�/0.05 M K2CO3, 1.5 MKHCO3�/0.1 M K2CO3, 2.5 M KHCO3�/0.167 M

K2CO3, respectively. The anodic current density in-

creases with increasing solution concentration for the

same polarisation potential. This effect is more remark-

able 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 differentpotentials 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�1

I�1k � A�1V�1=2

(1)

where, Ik and A are the classical terms obtained by the

Koutecky�/Levich expression. I0 is a non-diffusion

current.

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

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/bicarbo-

nate solution concentrations. The steady state polarisa-

tion 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

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.

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).

J.A. Calderon et al. / Electrochimica Acta 47 (2002) 4531�/4541 4533

constant interfacial pH was observed during cobaltanodic 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/bicar-

bonate 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

Table 1

Fitting results (from Eq. (1)) of the experimental current

C a E (VSSE) I0 (mA cm�2) Ik (mA cm�2) A (mA cm�2 rpm�1/2)

0.5 (0.375 M KHCO3�0.025 M K2CO3) �1.0 0.03 0.437 0.0147

�0.97 0.07 0.693 0.0174

�0.90 0.05 1.708 0.0405

0.87 0.03 2.552 0.0496

1.0 (0.75 M KHCO3�0.05 M K2CO3) �1.0 0.096 0.823 0.018

�0.90 0.129 3.644 0.076

0.87 0.095 5.083 0.100

2.0 (1.50 M KHCO3�0.10 M K2CO3) �1.0 0.157 1.061 0.048

�0.97 0.263 1.911 0.060

�0.90 0.34 5.17 0.168

0.87 0.30 6.72 0.242

3.0 (2.25 M KHCO3�0.15 M K2CO3) �1.0 0.234 1.293 0.063

�0.90 0.44 8.49 0.290

a C is a factor used to multiply the concentration of the base solution 0.75 M KHCO3�0.05 M K2CO3).

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).

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.


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 poten-

tials, the relative intensity of the band pointed at 1063

cm�1 is very close to that pointed at 1014 cm�1. These

results suggest that during the electrochemical process

there is a conversion of bicarbonate to carbonate

species, giving rise to a carbonate concentration in-

crease. 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 inten-

sity 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 con-

dition.

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 polar-

isation. Furthermore, in line with results shown in Fig.

6, the interfacial pH does not change during anodic

polarisation, indicating that free carbonate and bicar-

bonate species remain constant in solution. Therefore,

the carbonate species observed during anodic dissolu-

tion (Fig. 8) seems to be a non-free species produced

during the electrochemical process. The Raman spec-

trum 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

Fig. 7. Raman spectra of pro analysis reagents. KHCO3, K2CO3 and Co3O4 from Merck and CoCO3 from Sigma-Aldrich.


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 hydro-

dynamic control is necessary to keep constant the

experimental conditions without the influence of dis-

solution products throughout the electrochemical mea-

surements. In general, this point is not taken into

account during in situ Raman works involving electro-

dissolutions 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

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).


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

The electrochemical results seen in Figs. 2�/5 show

that carbonate and bicarbonate are active species that

participate during the charge transfer and mass trans-

port processes. However, as already seen by local-pH

measurements, the interfacial pH is constant. Thisindicates a buffer effect of carbonate/bicarbonate sys-

tem. In the bulk solution, the following equilibrium

must be considered:

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.

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.


KHCO3?HCO�3 �K� (2)

K2CO3?CO2�3 �2K� (3)

HCO�3 ?CO2�

3 �H� (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 [CO32�]/[HCO3

�] ratio. Consequently, if

the pH in the interfacial region is constant, the ratio of

these species is also constant and the mass transportcannot 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

Co(CO3)22� as the diffusion species away from the

electrode. Therefore, for the present discussion, the

following reactions are considered:

Co�HCO�3 0

k1(CoHCO3)ads�e� (5)

(CoHCO3)ads�HCO�3 ?

k2

k�2

Co(CO3)2�2 �2H��e� (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:

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).


I1

F�K1(1�u1)�K2u1�K�2[Co(CO3)2�

2 ] (7)

where:

K1�k1[HCO�3 ]0 exp

�a1F

RTV

�(8)

K2�k2[HCO�3 ]0 exp

�a2F

RTV

�(9)

K�2�k�2[H�]20 exp

��

(1 � a2)F

RTV

�(10)

The mass balance is:

b du1

dt�K1(1�u1)�K2u1�K�2[Co(CO3)2�

2 ] (11)

Under steady state (du1)/(dt )�/0, thus:

I1

F�2K1K�2K�2[Co(CO3)2�

2 ](1�K) (12)

where:

K�k2 exp

�a2FV

RT

�

k1 exp

�a1FV

RT

�� k2 exp

�a2FV

RT

� (13)

According to the above discussion, the mass transport

can be written as:

I1

F�

2D[Co(CO3)2�2 ]0

d(14)

where d is the Nernst diffusion layer of the complex and

D is its diffusion coefficient. Then:

[Co(CO3)2�2 ]0�

dI1

2FD(15)

Replacing Eq. (15) in Eq. (12) and considering the

diffusion layer as d�LV�1=2; with L as a constant, we

take:

I�11 �

(K1 � K2)

2FK1K2

�K�2

2FDK2

LV�1=2 (16)

Eq. (16) can be compared to the second term of theright 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 riseto 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 CO32� or HCO3

� ion plays

a catalytic role in the cobalt oxidation process:

Co�CO2�3 0 CoCO3�2e� (17)

CoCO3�H2O 0 CoO�CO2�3 �2H� (18)

The global process is:

Co�H2O 0k3; [(CO3)2�]

(CoO)ads�2H��2e� (19)

The oxide can be dissolved by [H�] as:

(CoO)ads�2H� 0k4

Co2��H2O (20)

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) mustbe considered as:

It�I0�I1 or It�I0�1

I�11

(21)

where It is the total anodic current of the cobalt

dissolution.

Eq. (21) imposes that the surface coverage u1 con-

cerning 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 para-

meters g and (1�/g) [12]. Taking into account thissurface sharing, Eq. (16) must be re-written as:

I�11 �

(K1 � K2)

2(1 � g)FK1K2

�K�2

2(1 � g)FDK2

LV�1=2 (22)

In the present work, I0 can be calculated indepen-

dently from I1 on the basis of the mechanism described

by Eqs. (17)�/(20), as follows:

Charge balance:

I0

2F�gK3(1�u2) (23)

and, the mass balance:

b du2

dt�K3(1�u2)�K4u2 (24)

with:

K3�k3[(CO3)2�]0 exp

�a3F

RTV

�(25)

K4�k4[H�]20 (26)

Considering the steady state (du2)/(dt )�/0, so:

1�u2�K4

K3 � K4

(27)

Then, the current I0 is:


I0�g2FK3K4

K3 � K4

(28)

Replacing the Eqs. (22) and (28) in Eq. (21), we take:

It�g2FK3K4

K3 � K4

�1

(K1 � K2)

2(1 � g)FK1K2

�K�2

2(1 � g)FDK2

LV�1=2

(29)

It follows from Eq. (1) that:

I�1k �

(K1 � K2)

2(1 � g)FK1K2

(30)

A�1�K�2

2(1 � g)FDK2

L (31)

and I0 is given by Eq. (28).

The complete expressions for the terms in Eq. (1) are:

I0�g

2Fk3k4[H�]20[(CO3)2�]0 exp

�a3F

RTV

�

k3[(CO3)2�]0 exp

�a3F

RTV

�� k4[H�]2

0

(32)

Ik�2F (1 � g)[HCO�

3 ]0k1k2 exp

�F

RTV (a1 � a2)

�

k1 exp

�a1F

RTV

�� k2 exp

�a2F

RTV

� (33)

and:

A�2FD(1 � g)[HCO�

3 ]0k2

k�2[H�]20 exp

��

FV

RT

�L

(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 me-

chanism is able to account for the steady state experi-mental 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 polarisa-tion potentials. Therefore, the currents I1 and I0

obtained by the reaction mechanism already discussed

probably contribute with different weight to the total

anodic current. Actually, the I0/I1 ratio will change

according to polarisation conditions.

5. Conclusions

Steady state polarisation curves of cobalt in carbon-

ate/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 electrodisso-

lution process. The anodic behaviour of cobalt in thismedia 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 concentra-

tion 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 passi-

vation takes place by a slow transformation of the CoO

precursor oxide into a Co3O4 passive film.

Acknowledgements

J.A. Calderon and J.E.P. da Silva acknowledge

CAPES and FAPESP (Proc. N8 98/15686-3), respec-

tively, 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 facil-

ities as well as Dr. Bernard Tribollet (CNRS-Paris) for

fruitful discussions.

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Documents

Electrodissolution of cobalt in carbonate/bicarbonate media