versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. vol.56 no.2 Concepción 2011
J. Chil. Chem. Soc., 56, N° 2 (2011), págs.: 631-634
COBALT ELECTRODEPOSITION ONTO STAINLESS STEEL 304 FROM AMMONIACAL SOLUTIONS
NANCY RAMOS-LORA1, L.H. MENDOZA-HUIZAR1 AND C.H. RIOS REYES1
1Universidad Autónoma del Estado de Hidalgo. Centro de Investigaciones Químicas, Mineral de la Reforma, Hidalgo. C.P. 42186, México.e-mail: firstname.lastname@example.org
In this work we studied the cobalt electrodeposition onto a stainless steel 304 electrode from an aqueous solution containing 101 M of CoSO4 + 1M (NH4)2SO4 at natural pH of 4.5. Black cobalt deposits were obtained under our experimental conditions. The average diffusion coefficient calculated was 9.8X106 cm2s-1 while the AG for the formation of stable nucleus was 1.013X1020 J/nuclei. The critical cluster's size calculated was 0 which suggested that each active site is a critical nucleus on the SSE-304 surface. The Scanning Electron Microscopy images showed the formation of non-uniform cobalt particles.
Keywords: Black cobalt, SSE 304, electrodeposition, kinetic.
Often, a cobalt film is deposited onto a stainless steel electrode (SSE) to prevent its oxidation1. Several studies suggest that the cobalt films can be electrodeposited onto an SSE, if the deposition time and the cobalt concentration in solution can be controlled2,3,4. Thus, in such studies the thickness of the deposited film and the rate of mass deposition are the main parameters to analyze1,2,47. Cobalt electrodeposition onto SSE has been analyzed from sulfate and nitrate solutions3 while containing boric acid, ammonium citrate, tartaric acid and sodium chloride have been studied for cobalt plating baths4. Here, it is interesting to mention that the cobalt electrodeposition onto SSE from ammoniacal solutions has not been studied, in spite of the fact that it has been reported that the (NH4)+ ions in solution allow inducing the growth of smaller cobalt clusters and homogeneous deposits which might control the microstructure of the deposit8. Thus, even though the cobalt film deposited onto SSE has a great significance in the corrosion field1, the kinetic parameters related to this process are unclear yet. Only a work reports the nucleation parameters associated with cobalt electrodeposition onto SSE from nitrate solutions3. However, up to our knowledge, an analysis about the kinetics parameters that are controlling the cobalt electrodeposition onto SSE from ammoniacal solutions has not been analyzed yet. Thus, in the present work, we carried out an electrochemical and morphological study, in order to understand the Co electrodeposition process onto this system.
Cobalt electrodeposits onto stainless steel electrode 304 were carried out from an aqueous solution containing 0.1 M CoSO4 + 1.0 M (NH4)2SO4 at 25o C and pH=4.5. All solutions were prepared using analytic grade reagents with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N2 for 15 minutes before each experiment. The working electrode was an SSE-304 disc with 0.91 cm2 of area. The exposed surface was polished to a mirror finish with different grades of alumina down to 0.05 mm and ultrasonically cleaned before experiments. A graphite bar with an exposed area greater than the working electrode was used as counter electrode. A saturated silver electrode (Ag/AgCl) was used as the reference electrode, and all measured potentials are referred to this scale. The electrochemical experiments were carried out in a BAS potentiostat connected to a personal computer running the BAS100W software to allow the control of experiments and data acquisition. In order to verify the electrochemical behavior of the electrode in the electrodeposition bath, cyclic voltammetry was performed in the 0.600 to -1.000 V potential range. The kinetic mechanism of cobalt deposition onto SSE-304 was studied under potentiostatic conditions by the analysis of the experimental potentiostatic current density transients recorded through the potential step technique. The perturbation of the potential electrode always started at 0.600 V. The first potential step was imposed at different potentials detailed in this work. Microstructures of electrodeposits were examined by using a scanning electron microscope (SEM; JEOL6300) equipped with an energy-dispersive X-ray spectrometer (EDS).
RESULTS AND DISCUSSION
The electrodeposition technique is valuable because of its cost effectiveness, easy maintenance and quality deposits9. However, it needs a good knowledge of the nucleation and growth parameters to get deposits with reproducible properties10. Thus, it is important to determine the potentials where the deposit can be obtained. To find out the values where the cobalt electrodeposition starts, we carried out a voltammetric study at different scan rates (n). Figure 1 shows the voltammetric response, with a scan rate of 20 mV s1, obtained from SS-304/101 M of CoSO4 + 1M (NH4)2SO4 system. Observe at direct scan, the formation of a peak A at -0.860 V. During the inverse of the potential scan, it is possible to observe the crossover, EC1 (-0.790 V) which is typical of the formation of a new phase involving a nucleation process11. Sometimes, the second crossoverpotential EC2 (-0.360 V) may be associated to the thermodynamical potential of Mn+/M, only when EC2 is independent of the switch potential El and when El is less negative than the corresponding peak potential12. However, this was not the case for the EC2 analyzed in this work. In the anodic zone, it was possible to observe a principal peak C at around 0.260 V, preceded by a shoulder B (-0.182 V). Shoulder B has been associated with the dissolution of a hydrogen rich cobalt phase13. Also, it is shown the cobalt electrodeposition which starts at -0.630 V (Ecrist).
To determine the limiting control of the cobalt reduction process, the maximum current density (jp) value associated with peak A was plotted as a function of n1/2, Figure 2. Note, in this plot, a linear relationship which suggest a diffusion-controlled process according to the Berzins-Delahay's equation14,15.
Formation of new phases occurs through nucleation and growth mechanisms and the corresponding current transients can provide valuable information about the kinetics of electrodeposition. Figure 3 shows a set of current density transients recorded at different potentials by the potential step technique. These transients were obtained by applying an initial potential of 0.600 V on the surface of the SSE-304 electrode. At this potential value, the cobalt deposition had still not begun, see Figure 1. After the application of this initial potential, a step of negative potential (Ec) was varied on the surface of the electrode for 32 s. All transients showed a typical current maximum (/m) which is characteristic of a typical three dimensional nucleation process16, 17. The decayed current recorded after the current maximum was analyzed employing the Cottrell's equation18. However, Cottrell's equation could not predict the behavior obtained (not shown), suggesting the presence of an additional process to the cobalt electrodeposition process19.
To determine the nucleation and growth mechanism, as progressive or instantaneous, we compared the experimental transients, with their dimensionless curves by following the established by Sharifker et al.16, 17. These curves were plotted substituting the coordinates of the experimental local maximum (tm, /m), in
for progressive nucleation. Figure 4 shows a comparison of the theoretical dimensionless transients, generated by equations (1) and (2) with an experimental dimensionless current transient reported in Figure 3. Observe that at (t/tm < 1), it was not possible to classify the nucleation as instantaneous or progressive. Here, it must be reminded that the theoretical curves generated by equations (1) and (2) correspond to two extreme cases of the nucleation process and in some cases a classification it is not possible20. At (t /tm >1) there is a deviation from the predicted by equation (1) and (2). This behavior may be indicative of the presence of other contributions to the overall current during the Co deposition process additional to the 3D nucleation contribution19.
Analysis of the transients
Shoulder B, observed in Figure 1, indicates that the proton reduction process is present due to the existence of a hydrogen rich Co phase13. It has been proposed that when the proton reduction occurs simultaneously with the growth of Co centers under a diffusion-controlled process, the overall current density is given by21:
and all others parameters have their conventional meanings. Figure 5 shows a comparison of a reduction experimental current transient, with one theoretically generated by non-linear fitting of experimental data to Eq. (3). It can be observed, that the model expressed by this equation adequately accounted for the behavior of the experimental transient. The physical parameters obtained from the adjustments of Eq. (3) are reported in Table 1. The average diffusion coefficient calculated from the fittings was 9.8X10-6 cm2 s1. On the other hand, it is seen (Table 1) that an increment of the kpR, A and N0 is obtained when the potential applied is decreased. An increase in kpR values indicates that the reduction proton process is favored, suggesting a competition for the active sites on the surface by the H+ ions with the Co2+ cations.
Analysis of the kinetic parameters.
From the nucleation rate values reported in Table 1, it is possible to calculate the Gibbs free energy of nucleation employing the next equation22-24:
where T is the absolute temperature, K. The plot ln A vs n-2 plot, showed a linear relationship giving a slope of -3.6; the average AG calculated at different potentials was 1.013X10-20 J/nuclei. This energy corresponds to the AG value requirements for the formation of stable nucleus23, 24. The AG value obtained is slightly bigger than the value obtained for the electrocrystallization of Co on GCE and HOPG from sodium and ammonia sulfate solutions26, 27.
In the framework of the atomistic theory of electrolytic nucleation, it is possible to estimate the critical size of the Co nucleus ( nc ) from the potential dependence of A through the following equation28:
where αC is the transfer coefficient for Co reduction. The plot ln A vs E showed a linear tendency with a d(ln A)/d(h)= 22.05. Thus, the critical cluster's size calculated employing eq(12) and=0.5 was αc =0. This value suggests that each active site is a critical nucleus on the SSE-304 surface. Similar results have been obtained for the Co electrodeposition from sodium and ammonia sulfate solutions onto GCE and HOPG electrodes24, 25.
Figure 6 shows the SEM micrographs of cobalt deposits when a potential of -1350 mV was applied onto the SSE-304 surface during 192 s. From this figure it can be observed granular deposits, Figure 6a. An increase in magnification, Figure 6b-c, indicated the existence of non-uniform particles. It is interesting to note that the non-uniformity of the cobalt deposited may be related either the morphology of the metallic substrate or the hydrogen evolution. Also, it is important to consider that the potential of hydrogen evolution is very close to the cobalt electrodeposition potential causing a competition between both processes29. Additionally, the random formation of hydrogen bubbles on the electrode may cause a non uniform area available on SSE causing the formation of a non uniform deposit of cobalt. The EDS analysis of the particles shown in Figure 7 indicated that the chemical composition is cobalt.
We have studied the Co electrodeposition onto Stainless Steel electrode 304 from 10-1 M CoSO4, 1M (NH4)2SO4 aqueous solution by using the cyclic voltammetric and potentiostatic techniques. Black cobalt deposits were obtained in our experimental conditions. Nucleation parameters such as nucleation rate, density of active nucleation sites and saturation nucleus were determined from potentiostatic studies. An increase in kPR, A and N0 was obtained when the potential applied was decreased. The morphological analysis indicated the formation non-uniform particles with grain agglomeration.
N.R.L. is grateful for a graduate student fellowship from CONACYT. This work was done in partial fulfillment of N.R.L.'s Ph.D. requirements. We gratefully acknowledge financial support from CONACYT project APOY-COMPL-2008 No. 91261 and to the Universidad Autónoma del Estado de Hidalgo. Authors acknowledge Juan Hernández Ávila for the SEM technical assistance.
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(Received: September 29, 2009 - Accepted: April 26, 2011).