Analysis of the initial stages of electrocrystallization of Fe, Co and Fe-Co alloys in chloride solutions

Journal of the Brazilian Chemical Society

Mascaro, Lucia H.¹  Universidade Federal do Paraná, Curitiba, Brazil¹  Bento, Fábio R.¹ 

关键词: nucleation;    electrodeposition;    iron alloys;    electrocrystallization;   

DOI  :  10.1590/S0103-50532002000400015

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Electrodeposition is one of the best methods for producing alloys, depositing films with structures different from those obtained by the evaporation method. Many alloys are electrodeposited commercially from plating baths containing a mixture of metal ions in aqueous electrolyte.1-6 These alloys are important due to the many possible industrial and technological applications they possess. Electrodeposited thin films of binary alloys of iron group (Fe, Co and Ni) are of interest due to the various applications in electronics and microtechnology . The best known example is permalloy, a FeNi alloy used in soft magnetic read/write heads.7,8 The cobalt alloys also present interesting magnetic properties making them candidates for disc storage media.7 The electrodeposition of iron-group alloys belongs to Brenner's anomalous codeposition category,9 which is characterized by the unusual preferential electrodeposition of less noble metals. The co-deposition of Fe-Co alloys has been less investigated if compared to Fe-Ni alloys. These alloys have been studied in sulfate,10-17 citrate,18 acetate19 solutions at different pHs and morphology, composition and properties of alloys obtained depend on experimental conditions. Despite of the multiple aspects considered in these studies, the mechanism of the nucleation and electrocrystallization processes of Fe, Co and Fe-Co alloys in aqueous media have received much less attention. The Co electrodeposition in aqueous solution, for example, is accompanied by simultaneous hydrogen evolution reaction (HER),20,21 which makes analysis of experimental current transients of cobalt electrodeposition very difficult. Correia et al.22 and Gómez et al.23 analyzed the electrocrystallization of Co on carbon and gold substrates, from chloride baths and a progressive nucleation mechanism was found. Soto et al.24, 25 studied Co electrodeposition, using different pHs and concluded that distinct mechanisms of nucleation are involved during early stages of Co deposition. At pH 9.4 was observed three different kinds of parallel nucleation processes were observed as well as a 2D progressive and instantaneous nucleation and 3D progressive nucleation. At pH 4.4 the nucleation process is 3D progressive. Potential step experiments made by Bertazolli e Sousa26 in sulfate solution, pH 6, showed that Co deposition is formed by a mechanism of progressive nucleation followed by 3-D growth. Bertazolli and Pletcher13 also studied Fe-Co alloy nucleation and similar behavior to that of pure Co was found. Thus, the purpose of the work reported here was to study the electrocrystallization of Fe, Co and Fe-Co alloys onto Pt electrode in chloride medium. Potentiostatic steps were used to investigate the nucleation and growth processes.  Experimental The electrochemical measurements were performed with a conventional three-electrode glass cell. The working electrode was built from a 5mm-diameter platinum disk. The disk was sealed onto a glass tube and the exposed surface (0.202 cm2) was polished to a mirror finishing with a 1 mm alumina powder. The auxiliary electrode was a platinum foil with a surface area of 2 cm2 . A saturated calomel electrode (SCE) was used as a reference electrode and was connected to the main cell trough a Luggin capillary. Iron and cobalt were deposited from 0.1 and 0.01 mol L-1 CoCl2 and FeCl2 solutions containing 1.0 mol L-1 KCl ( pH 2.0 adjusted by HCl). Fe-Co alloys were deposited from 0.1 mol L-1 Fe(II) + 0.1 mol L-1 Co (II) (ratio 1:1 ), 0.01 mol L-1 Fe(II) + 0.1 mol L-1 Co (II) (ratio 1:10 ) and 0.1 mol L-1 Fe(II) + 0.01 mol L-1 Co (II) (ratio 10:1 ) in 1.0 mol L-1 KCl solutions. All the solutions were prepared from analytical grade reagents, using ultrapure Millipore-Q water (Millipore-Q system). Metal deposition onto platinum was studied by means of the potential step and cyclic voltammetry technique. The cyclic voltammetry were initially carried out between 0.3 and -1.1 V at 0.2 V s-1, starting from 0.3 V and negative followed positive scan. Potentials were controlled with an EG&G PARC mod. 273A potentiostat/galvanostat linked to a microcomputer PC AT 386 with EG&G PARC M270 software for experimental control and data acquisition.  Results and Discussions Electrodeposition of individual metals (Fe and Co)Figure 1 shows typical voltammetric curves obtained for the Pt electrode in a 0.1 mol L-1 FeSO4 and 0.1 mol L-1 CoCl2, 1.0 mol L-1 KCl solution at pH 2.0. The scan potential rate was 20 mVs-1. The voltammograms obtained shows the presence of cathodic and anodic peaks associated deposition and dissolution of metals. Fe deposition occurs at more negative potentials than Co deposition because Co is a nobler metal. The voltammogram obtained for Fe is similar to the literature in the same solution. 27   Co curve shows one cathodic and one anodic peaks. Bertazolli and Sousa26 showed that, at pH 2.8, the stripping peak corresponded to the dissolution of the hydrogen rich phase. Therefore, the peaks in Figure 1 (dashed line) can be associated with the redox couple Co(H2O)62+/Co(0) or H+/H reactions. 25 The voltammograms in Figure 1 show a crossover between cathodic branches current observed for Fe and Co deposition, which is characteristic of nucleation process. 28 In order to determine the electrocrystallization and kinetic characteristics of the early stage of phase formation of Fe and Co in 1.0 mol L-1 KCl a more convenient potentiostatic method was employed. In this method the nucleation processes were analyzed by selecting an initial potential where no deposition of Fe or Co could be detected in the CVs curves and a final potential less positive located in the region of cathodic peak. Thus potentials jumps were performed from 0.0 V to more negative potentials. A family of current transients obtained at different potentials is shown in Figure 2. The final potential chosen for Co deposition was between ¾0.86 and ¾0.90 V where nothing hydrogen ion reduction was expected. For Fe deposition the potential range was between ¾1.0 and ¾1.08 V.  It can be seen in Figure 2, that current increases rapidly to a maximum (Imax and tmax) and then decreases gradually with time. Moreover Imax, increases and tmax decrease with E. The initial decreasing current corresponding to the charging of the double layer followed by the rising current, is due either to the growth of a new phase and/or to an increase of the number nuclei. During this stage of deposit growth, the nuclei develop diffusion zones around themselves. As these zones overlap the hemispherical mass-transfer give way to a linear mass-transfer resulting in an effectively planar surface. The current then falls that corresponding to linear diffusion. This is the limiting diffusion current for the deposition process. These curves show a typical response of a three-dimensional (3D) multiple nucleation with diffusion controlled growth.29 Kinetic information about electrocrystallization process could then be obtained by analyzing the rising portion and the maximum of the experimental current transients.Analysis of the rising portion of the current transientsIn this

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