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Effects of constant magnetic field on the electrodeposition reactions and cobalt–tungsten alloy structure
Materials Chemistry and Physics 141 (2013) 370e377
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Effects of constant magnetic field on the electrodeposition reactions and cobaltetungsten alloy structure ski* Marek Zielin Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, University of Lodz, Tamka 12, 91-403 Lodz, Poland
h i g h l i g h t s The effect of magnetic field on the electrodeposition of CoeW alloys was studied. Fractures on the sample surface disappeared under the exposure to magnetic field. The Co3W, Co7W6, a-Co and W phases were identified in CoeW alloys. The content of particular phases changed under the exposure to magnetic field. Changes under the exposure to magnetic field are due to magnetohydrodynamic effect.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2012 Received in revised form 19 March 2013 Accepted 14 May 2013
The paper presents a study of the effect of constant magnetic field (CMF) on the basic processes of cobalt etungsten alloys electrodeposition. To the author’s knowledge, such an investigation has been performed for the first time for cobaltetungsten alloys. The applied research methods included scanning electron microscopy (SEM), energy dispersive X-ray (EDX) microanalysis, energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). SEM images of cobaltetungsten alloys revealed numerous fractures on the samples surface, formed as a result of residual stress during alloy deposition without CMF. In CMF such fractures disappeared. The cobalt content increased, with a simultaneous decrease of the tungsten content, under CMF condition. The XRD study of cobaltetungsten alloys allowed to identify phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. Under CMF conditions some crystal planes were deflected at angles ranging from 10 to 20 . The exposure to CMF caused also an increase of the volume fraction (by about 9% by volume) of the dominant phase (Co3W and Co7W6) in the alloy. The reason for these changes was the fact that the Lorentz force, generated in CMF, caused the magnetohydrodynamic (MHD) effect. This induced movement of the electrolyte. The Nernst diffusion layer was depleted, whereas a new NaviereStokes hydrodynamic layer appeared, which determined the velocity of electroactive molecules flow to the working electrode under CMF conditions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Magnetic materials Electrochemical techniques Electron microscopy Energy dispersive analysis of X-rays
1. Introduction The demand for alloy coatings with specific properties has increased in the recent years. Such coatings are characterized by much better resistance to corrosion than single-metal ones. The properties of alloys associated with their chemical composition and structure, such as microhardness, residual stress, plasticity and electric conductivity, can be adjusted by appropriate composition of the galvanic bath and appropriate selection of
* Tel.: þ48 42 6355788; fax: þ48 42 6355796. E-mail addresses: [email protected], [email protected]. 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.025
technological parameters. Cathodic electrodeposition of alloys based on elements belonging to the ferrous metals group (Fe, Co, Ni) is still an important problem in electrochemistry. Codeposition of the mentioned elements with metals such as chromium, molybdenum and tungsten, or non-metals such as phosphorus and boron is an interesting issue. They are elements which cannot be deposited from aqueous solutions alone, but only in the presence of ferrous metals by the so-called induced codeposition. The cobaltetungsten alloy is also an alloy of this type. It is characterized by high hardness and corrosion resistance. The higher the tungsten content in the alloy, the less plastic it is and has the higher electrical resistance. Cobaltetungsten alloys with tungsten content to 30% by weight are ferromagnetic. Abdel
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin
Hamid [1] studied the optimal conditions for cobaltetungsten alloy electrodeposition process, such as temperature, current density, concentration of metal ions or surfactants. Then, he investigated the morphology of alloy surfaces, their microhardness and resistance to corrosion. He observed that adding tungsten to cobalt resulted in smoothening of the coating. With tungsten content ranging from 3 to 10% by weight, the alloy had a fine-grained structure, whereas when tungsten content increased to 15% by weight, the structure of the alloy was fibrous with even finer grains. At high current densities, the percentage content of tungsten decreased, whereas increased bath temperature resulted in higher content of tungsten in the alloy. Capel et al. [2] studied the mechanical strength and corrosion resistance of cobaltetungsten alloys. They observed that heating the alloys increased their mechanical strength, but decreased their corrosion resistance and tribological properties. The mechanical strength of cobaltetungsten alloys was also studied by Weston et al. [3]. They checked the tear and wear rate of cobalt, chromium and cobaltetungsten alloy coatings according to the loads applied. They concluded that cobaltetungsten alloys demonstrate very good mechanical strength and hardness. Su and Huang [4] investigated electrodeposited cobaltetungsten alloy coatings using X-ray diffraction (XRD), scanning electron microscopy (SEM) and a Vickers hardness tester. They observed a significant influence of the current density and Na2WO4 concentration in bath on the microstructure, morphology and hardness of the coatings. Moreover, the tribological properties of the coatings were found to be greatly dependent on their grain size, microhardness, surface morphologies and composition. Chang et al. [5] demonstrated that higher tungsten content in cobaltetungsten alloys improved their corrosion resistance. Shobba et al. [6] studied the corrosion resistance of cobalte tungsten alloys exposed to sulfuric acid (VI) and potassium hydroxide. The study was carried out in the context of applicability of these alloys as anode materials. Electrodeposition of cobalte tungsten alloys with an addition of organic compounds was carried out by Wei et al. [7]. They investigated the effect of such compounds on magnetic properties of cobaltetungsten alloys, their structure and surface morphology. The addition of methyl methacrylate resulted in homogeneity of the alloy, whereas saccharine eliminated fissures on the alloy surface. Coumarin caused formation of an alloy with non-homogeneous morphology. Moreover, it changed the magnetic properties, causing an increase of saturation magnetization and a decrease of coercion force on the hysteresis curve. Also Szpunar et al. [8] studied magnetic properties of cobaltetungsten alloys. They found that increased tungsten content in the alloy caused a decrease of saturation magnetization on the hysteresis curve. The effects of magnetic field on electrodeposition of various alloys as well as their physical and chemical properties were also investigated. It had been observed earlier that magnetic field may affect electrochemical processes [9e11]. The obtained data indicated that such changes are due to magnetohydrodynamic (MHD) effect. The MHD effect is based on the Lorentz force, inducing movements of the electrolyte and increasing or decreasing transport of the electroactive molecules to the electrode. Koza et al. [12] studied the effect of constant magnetic field (CMF) with magnetic induction value ranging from zero do 1 T, in various configurations in relation to the electrode surface. They deposited electrochemically cobalt, iron and cobalteiron alloy. They observed that CMF applied parallel to the electrode surface increased the limiting current density and the deposition rate. This was due to the MHD effect, induced by maximal Lorentz force generated in the hydrodynamic layer. It led to additional convection, increasing mass transport of ions to the electrode, which resulted in the deposition
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of more densely packed, less coarse alloys with finer grains. Magnetic field applied perpendicular to the electrode surface caused no significant changes in the limiting current density and the deposition rate. Coey and Hinds [13] confirmed that CMF increased significantly copper electrodeposition rate. Then, they observed increased transport of cationic mass, both diamagnetic (Agþ, Zn2þ, Bi3þ) and paramagnetic (Cu2þ, Ni2þ) under CMF conditions. Pane et al. [14] conducted cobaltenickel alloy electrodeposition in CMF, in the presence of ferrite nanomolecules. The use of CMF favored an increase of percentage ferrite content in the alloy, especially at low current densities. The effect of CMF on changes of pH values in the vicinity of the electrode during electrodeposition of cobalt, iron and cobalteiron alloys was also investigated [15]. It was found that under CMF conditions pH values decrease as a result of increased transport of hydrogen ions to the electrode surface. Li et al. [16] prepared Fe-contained polyaniline in the absence and presence of an applied magnetic field of 600 mT. Using the electron paramagnetic resonance (EPR) technique, they showed that there were unpaired electrons in the Fe-contained polyaniline synthesized in the presence of the magnetic field. Hu and Chen [17] carried out three identical reactions in a mixture of nickel nitrate hexahydrate, nicotinic acid and sodium azide in the absence and presence of applied magnetic fields of 150 mT and 300 mT [Ni1.5(N3)(nic)2(Hnic)]n was obtained without the applied magnetic field, while the final product totally changed into [Ni2(nic)4(H2O)] as a magnetic field of 300 mT was used in the reaction. To explain this result, the authors concluded that the magnetic field changes the reaction pathway due to the reduction of activation energy, which is indicated by the slight changes of the crystal microstructure, such as bond angle and length. In this paper, the effect of CMF on electrodeposition of cobalte tungsten alloys has been studied. To the author’s knowledge, such an investigation has been performed for the first time for cobalte tungsten alloys. The results obtained show that the application of CMF causes changes in the kinetics of alloy deposition reactions, changes in the chemical composition of the alloys and their surface morphology, as well as changes in the crystallographic parameters of the alloys.
Fig. 1. Electrochemical cup with three-electrode measurement system, placed between the N and S pole pieces of a laboratory electromagnet, (a) with the magnetic induction B perpendicular to the current density j, and (b) with the magnetic induction B parallel to the current density j. F denotes the Lorentz force.
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Fig. 2. SEM images of the central portions of cobaltetungsten alloy samples, obtained with no exposure to magnetic field and in CMF with magnetic induction value B ¼ 1200 mT (B t j configuration).
2. Experimental Cobaltetungsten alloys were prepared by electrodeposition using three-electrode system. The three-electrode electrochemical cup, in which the alloys were deposited, consisted of a working electrode (gold, disc-shaped) with 0.1 cm2 surface area, an auxiliary electrode (platinum, mesh) and a reference electrode (saturated, calomel). The galvanic solution prepared to obtain cobaltetungsten alloy contained 0.1 M cobalt sulfate (CoSO4$7H2O), 0.1 M sodium tungstate (Na2WO4$2H2O), 0.6 M sodium citrate (Na3C6H5O7$2H2O), 0.05 M EDTA (C10H14O8N2Na2$2H2O) and 0.1 M sulfuric acid (H2SO4). The alloys were deposited at potential 1,25 VSCE (as related to the saturated calomel electrode) for 3500 s. The cobaltetungsten alloy electrodeposition potential was determined on the basis of the dependence of current on potential. The cobaltetungsten alloys were deposited without and in CMF produced by the N and S pole pieces of an ER 2525 laboratory electromagnet. The magnetic induction B, used in the study within the value range from zero to 1.2 T, was directed either parallel to the surface of the working electrode (i.e. perpendicular to the current density j direction, Bkj configuration), or perpendicular to the surface of the working electrode (i.e. parallel to the current density j direction, Bkj configuration), as presented in Fig. 1. The morphological structure of the alloys was studied by SEM using a Philips XL 30 instrument. Images were recorded in the backscattered electron or secondary electron mode at 10 keV or 20 keV primary beam energy. The chemical composition of the alloys was determined by energy dispersive X-ray (EDX) microanalysis using an Oxford Instruments ISIS system and also by energydispersive X-ray spectroscopy (EDS) using a DEPR spectrometer. The crystallographic structure of the alloys was studied by XRD using a Bruker D8 Discover X-ray diffractometer with a Cu Ka radiation. 3. Results and discussion Electrochemically obtained cobaltetungsten alloys were subjected to investigations by a variety of methods. The morphology of the alloys was studied using SEM. The cobaltetungsten alloys were obtained electrochemically either with no exposure to magnetic field or under CMF conditions. The following four types of cobaltetungsten alloy samples were used in the SEM investigations:
sample sample sample sample
1: 2: 3: 4:
B B B B
¼ ¼ ¼ ¼
0; 200 mT, Bkj; 600 mT, Bkj; 1200 mT, Bkj.
Fig. 3. SEM images of the peripheral portions of cobaltetungsten alloy samples, obtained with no exposure to magnetic field and in CMF with magnetic induction value B ¼ 1200 mT (B tj configuration).
The SEM studies showed the presence of numerous fractures in the central portion of the samples surface, formed as a result of residual stress during alloy deposition without CMF (Fig. 2). With CMF value B ¼ 1200 mT and Bkj configuration, such fractures disappeared. This effect has already been mentioned in another paper [18] concerning research on cobaltemolybdenumetungsten alloys and on cobalt [19,20]. The mentioned effect was also observed in the peripheral parts of the cobaltetungsten samples surface, as confirmed by the SEM images shown in Fig. 3. The magnetic field applied during the deposition of alloy layers from the electrolyte, reduces the layers internal stress. The tensile stress in CoW films obtained with applied magnetic field is lower than in those deposited in the absence of a magnetic field. When a magnetic field is applied parallel to the electrode surface, the deposits look more homogeneous than those deposited without field. It is also visible that the deposits obtained in the parallel magnetic field are more densely packed and possess smaller porosity. On the basis of the data recorded with EDX microanalysis in the scanning electron microscope (SEM), the percentage contents of cobalt and tungsten in the investigated cobaltetungsten alloys were determined. They were deposited with no exposure to magnetic field and in CMF with magnetic induction values B ¼ 200, 600 and 1200 mT. The obtained results are presented in Table 1. It can be observed that the effect of CMF involves an increase of cobalt content and a decrease of tungsten content. The Co:W ratio has changed from 2:1 to 4:1. The chemical composition of the obtained cobaltetungsten alloys was also investigated using the EDS method. For these studies, the cobaltetungsten alloy samples were divided into representative areas to get reliable chemical composition results. The obtained data are presented in Table 2. The Co:W weight ratio changed from about 2:1 without exposure to magnetic field, to about 3:1 under CMF conditions. The approximately 20% oxygen content in the overall composition of CoW (Table 2) comes from oxides. The layers of metals or alloys are particularly susceptible to rapid oxidation and the formation of oxides. The formation of oxides on metals or alloys is a normal and acceptable phenomenon in practice. The Table 1 Percentage content of the components in cobaltetungsten alloys obtained without and in CMF with different values of magnetic induction B, determined by EDX microanalysis in the SEM. B (mT)
0
200
600
1200
% wt. Co % wt. W % other components
51.4 25.4 23.2
52.0 23.1 24.9
53.6 17.7 28.7
54.9 13.5 31.6
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin Table 2 Chemical composition determined with the use of EDS, for cobaltetungsten alloys obtained without and in CMF with magnetic induction B ¼ 1200 mT and Bkj configuration. Element
% wt.
O Na W Co
B¼0
B ¼ 1200 mT
18.4 0.6 28.7 52.3
19.4 1.3 20.9 58.5
Fig. 4. X-ray diffractograms obtained for four types of cobaltetungsten alloy samples: 1 e black (B ¼ 0), 2 e red (B ¼ 200 mT, Bkj), 4 e blue (B ¼ 1200 mT, Bkj), 5 e green (B ¼ 1200 mT, B k j). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
concurrent process of incorporation of oxides into the alloy coating layer is also frequent. Tungsten has a high affinity to oxygen [DG0298 ¼ 182.5 kcal mol1 (for W/WO3)] and therefore it forms oxide compounds easily. As a consequence, such metals are either deposited with a high overpotential or not deposited at all. They can be deposited electrolytically only together with other metals,
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e.g. from the ferrous metals group (Co, Ni, Fe) by the so-called induced codeposition of metals. In contrast, cobalt has a low affinity to oxygen [DG0298 ¼ 51.0 kcal mol1 (for Co/CoO)] and can be deposited easily both alone and with other metals, e.g. with tungsten. The phase structure and crystalline lattice parameters of the investigated cobaltetungsten alloys were determined by the XRD method. The following four types of cobaltetungsten alloy samples were used in the XRD studies:
sample sample sample sample
1: 2: 4: 5:
B B B B
¼ ¼ ¼ ¼
0; 200 mT, Bkj; 1200 mT, Bkj; 1200 mT, Bkj.
Samples 4 and 5 were deposited in CMF with the same highest magnetic induction value B ¼ 1200 mT, but in different configurations: Bkj and Bkj, respectively. Sample 5 was in fact prepared for the purpose of comparison of the obtained results with those for sample 4. The phases were identified on the basis of the experimental data recorded using the XRD technique in asymmetric geometry 2qscan, with EVA software [21] as well as the data obtained from ICDD crystallographic database. The analysis of dominant phase texture (phase volume fractions) in the alloy coatings was based on experimental so-called pole figures with application of thin layer correction. The orientation distribution function (ODF), calculated according to the discrete method, was used in texture analysis [22] utilizing LaboTex software package [23]. After carrying out the phase analysis of cobaltetungsten alloys, the appropriate diffractograms were obtained, shown in Fig. 4. On the basis of the X-ray diffractograms of cobaltetungsten alloys, as well as the data obtained from ICDD crystallographic database, the following phases were identified: phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. Decreased intensity of the reflected beams indicated that under CMF conditions some crystal planes were deflected at a specific angle during the alloy formation process. The higher the value of magnetic induction, the more significant deflection of some crystal planes took place. However, the crystallographic system of cobaltetungsten alloys did not change as a result of the
Table 3 Phases, phase volume fractions and crystalline lattice parameters of cobaltetungsten alloys. Sample no.
Conditions
Alloy (metal)
Crystallographic system
Crystal lattice parameters ( A) 0.0005
Volume fraction (% vol.) 5
1
B¼0
Co3W
Hexagonal
69
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co
Cubic Cubic
a ¼ b ¼ 5.0947 c ¼ 4.2329 a ¼ b ¼ 4.7371 c ¼ 25.5952 a ¼ b ¼ c ¼ 3.4071 a ¼ b ¼ c ¼ 3.1985 a ¼ b ¼ 5.1075 c ¼ 4.2276 a ¼ b ¼ 4.7351 c ¼ 25.3820 a ¼ b ¼ c ¼ 3.4014 a ¼ b ¼ c ¼ 3.2011 a ¼ b ¼ 5.0947 c ¼ 4.2242 a ¼ b ¼ 4.7391 c ¼ 25.6140 a ¼ b ¼ c ¼ 3.4085 a ¼ b ¼ c ¼ 3.2065 a ¼ b ¼ 5.0714 c ¼ 4.2312 a ¼ b ¼ 4.7607 c ¼ 25.4761 a ¼ b ¼ c ¼ 3.4057 a ¼ b ¼ c ¼ 3.2068
2
4
5
B ¼ 200 mT Bkj
B ¼ 1200 mT Bkj
B ¼ 1200 mT Bkj
W
14 17 66
22 12 78
8 14 60
35 5
374
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exposure to CMF. The identified phases, their volume fractions and the crystalline lattice parameters of the investigated cobaltetungsten alloys are presented in Table 3. The deviations of the peaks from the nominal position of the diffraction line for particular phases can result from the presence of internal stresses, being often comparatively high in the coating layers. Many variants of diffractometry recordings were performed to find the optimal geometrical conditions making it possible to record reflections from CoW coating layers of 20 mm in thickness. The gold substrate was 2 mm thick. Fig. 4 presents the final diffractograms. The most pronounced diffraction reflections in the recorded diffractograms originate from gold (the substrate), with the lattice parameter a ¼ 4.0780 A. The dominant phase was Co3W in a hexagonal system and Co7W6 in a trigonal system. The lattice parameter c was taken into account in the conducted measurements as a characteristic parameter for the studied case. As a result of the exposure to CMF, cobaltetungsten alloy crystals were more flattened, which is confirmed by the decreasing lattice parameter c of the dominant phase, the SEM images (Figs. 2 and 3) and earlier studies [18]. Describing the crystallographic texture (spatial orientation of the crystallites), it can be stated that Co3W (hexagonal crystallographic system) and Co7W6 (trigonal crystallographic system) is just the dominant phase in the investigated alloys. The spatial orientation of crystallites of the (Co3W and Co7W6) phases in the studied coating layers showed a tendency toward the formation of {0 0 0 1} type (basal) lattice planes, slightly deflected (by 15e20 ) in relation to the sample surface. The precise orientation of crystallites should in fact be described as {0 1 1 3} type plane, parallel to the sample surface. The above-mentioned tendency was most pronounced in sample 1 (B ¼ 0) and weaker in samples 2, 4 and 5, i.e. in those exposed to constant magnetic field (Fig. 5). The inverse pole figures with the normal direction as related to the surface of the studied coating layers (Fig. 6) indicated that, in addition to the orientations mentioned above, only in sample 4 [where the highest magnetic induction B of constant magnetic field (B ¼ 1200 mT) was used with Bkj] a small fraction of the (Co3W and Co7W6) phases was oriented in such a way that there were {3 2 5 1} type planes parallel to the coating layer surface.
As a result of the exposure to CMF, the spatial orientation of the planes and the dominant phase volume fraction in the alloy changed. The content of the phases (Co3W and Co7W6) increased by about 9% vol. in CMF with magnetic induction B ¼ 1200 mT and Bkj configuration, with a simultaneous decrease of the cobalt and tungsten content. The described processes and changes due to the effect of CMF, resulting from the presented study, can be explained as follows. The most important effect of CMF on electrochemical processes is the MHD effect. The driving force of that effect is the generated Lorentz force F:
F ¼ jB
(1)
where j is the current density and B is the magnetic induction. In the case of configuration Bkj, this is a macroscopic effect. It causes additional convection, resulting in an increase of mass transport toward the working electrode. Consequently, a laminar flow of electrolyte near the working electrode surface occurs, reducing the diffusion layer thickness and causing an increase of potential-generating ions concentration. It causes a change of grain size in the deposited alloy, affects its texture and formation of various phases. Such effects are caused by additional convection, as well as by the magnetic properties of ions. CMF present in the medium determines the alloy thickness increase in the direction imposed by magnetization of the deposited alloy material. Generally speaking, the solution mass transport from and to the electrode can be effected by migration, diffusion and convection mechanisms. The diffusion is an important phenomenon always occurring in the process of electrolysis. The second Fick’s law describes the concentration changes in time [Eqs. (2) and (3)]:
vC ¼ DV2 C vt
(2)
where C is the concentration of electroactive ions in the body of the electrolyte, t is time and D is the electrolyte diffusivity,
Fig. 5. Pole figures for the basal planes of {0 0 0 1} type (top), prismatic ones of {1 1 2 0} type (center) and pyramidal ones of {1 1 2 2} type (bottom) of the hexagonal phase Co3W and trigonal phase Co7W6 in cobaltetungsten alloy samples 1, 2, 4 and 5.
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Fig. 6. Inverse pole figures for normal orientation in relation to the sample surface, for the Co3W phase and Co7W6 phase in cobaltetungsten alloy samples 1, 2, 4 and 5.
vC vt
¼ z
v2 C vz2
(3)
where z is the perpendicular direction from the electrode surface (cf. Fig. 7). In the transition state for the ongoing processes, concentration changes in time should be taken into consideration. The mass transport equation can be expressed as follows:
vC ¼ uCV2 f þ DV2 C vVC vt
(4)
where u is the ion mobility, 4 is the internal potential (electrical) phase and v is the bulk flow velocity. Eq. (4) is the NaviereStokes equation, describing the motion of fluid substances. Eq. (4) and hydrodynamic continuity Eq. (5) are the basic differential equations describing convective mass transport:
vr þ VðrvÞ ¼ 0 vt
(5)
where r is the fluid specific density. Ionic mass transfer can be extended by convection and consequently one can define the phenomenon as convective diffusion. There is a solution layer of thickness dD (Nernst diffusion layer) between the working electrode surface and the electrolyte mass, the mass transfer through which occurs only as a result of diffusion and migration. The layer thickness dD is dependent on hydrodynamic conditions and viscosity of the solution. If the solution is not stirred, diffusion layer growth with time occurs under potentiostatic conditions:
dD ¼
pffiffiffiffiffiffiffiffiffi pDt
(6)
where t is the mass transfer time. If there are differences in electrolyte density, e.g. as a result of additional energy supply, a liquid flow along the surface of a vertically positioned electrode, directed from its lower to upper edge, will be observed (Fig. 7). This is natural convection, which inhibits the growth of the diffusion layer dD. Thus, Eq. (4) for steadystate convection diffusion can be expressed as follows:
vC ¼ DV2 C vVC ¼ 0 vt
(7)
There is a gradient of concentrations (C Cel) in the aforementioned diffusion layer dD; thus, the mass diffusion transfer Jdiff can be written as:
Jdiff ¼ DðC Cel Þ dD
(8)
where Cel is the concentration of electroactive ions near the working electrode surface. The rate of reduction of electroactive ions on the working electrode surface is higher than the rate of reagents supply, thus mass transport in the diffusion layer can be expressed as follows:
Cel ¼ 0jz¼0 and Cel ¼ Cjz/N
(9)
The mass diffusion stream Jdiff direction is perpendicular to the electrode surface. The current density j can be described with the formula [24]:
j ¼ ðDnFF CÞ dD
Fig. 7. Growth inhibition of the Nernst diffusion layer dD as a result of the differences in electrolyte density emerging as a result of additional energy supply.
(10)
where n is the number of electrons involved in the Faradaic process and FF ¼ 96,487 C mol1 is the Faraday’s number. The force F generated as a result of the exposure to CMF caused electrolyte movements. The Nernst diffusion layer dD was depleted, while a new NaviereStokes hydrodynamic layer dH appeared (Fig. 8). The presented considerations concerned obviously the flat
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electrochemical system was observed when they were applied perpendicular to the electrode surface. The investigation of magneto-electrolysis is of significance from both the industrial and fundamental points of view. It may open up a novel method of modifying the microstructure, in particular the particle content, by controlling the magnetic fields with no need to adjust additives and electrolyte composition. 4. Conclusions
Fig. 8. Reduction in the Nernst diffusion layer thickness dD near the working electrode surface under the influence of CMF, and formation of the NaviereStokes hydrodynamic layer dH.
surface of the working electrode, and the CMF was parallel to the working electrode surface. The theoretical model of cathode current density j dependence on magnetic induction B was presented by Fahidy [25] and Aogaki et al. [26]: 1=4
j ¼ 4:3 103 n3=2 Awe Dv1=4 C 4=3 B1=3
(11)
where Awe is the area of the electrode. On the other hand, a quantitative model of the cathode current density j dependence on the magnetic induction B was presented by Lioubashevski et al. in their paper [24]. The dependence was described as follows: 1=6
jy0:63p1=6 Awe r1=3 D8=9 v2=9 ðnFF CÞ4=3 B1=3
(12)
As a result of the exposure to CMF [18], the thickness of Nernst diffusion layer dD decreased and amounted to:
dD y1:59 rRv2=3 D1=3
1=3
ðnFF CBÞ1=3
(13)
where R is the radius of the working electrode. It was also calculated that the velocity of electroactive molecules v increases with increasing magnetic induction B [18]:
. vy nFF CD4=3 Br1=3 dH 0:62h4=3
(14)
where h is the dynamic viscosity. With increasing magnetic induction B, the thickness of Naviere Stokes hydrodynamic layer dH, which determines the electrolyte flow under the exposure to CMF, also increases. A primary effect of magnetic fields on the electrochemical reaction is the MHD effect acting in the bulk electrolyte (Bkj configuration), which reduces the diffusion layer thickness and thus, the limiting current densities and deposition rates are increased with a superimposed magnetic field. The nucleation process is affected by the magnetic field when it is applied parallel to the electrode surface (maximal Lorentz force). This influence is attributed to the MHD effect. No significant effect of magnetic fields on the
The effect of CMF on the electrodeposition process of cobalte tungsten alloys has been investigated. The results obtained show that the application of CMF causes changes in the kinetics of alloy deposition reactions, changes in the chemical composition of the alloys and their surface morphology, as well as changes in the crystallographic parameters of the alloys. In the process of cobaltetungsten alloy electrodeposition, the Lorentz force was generated as a result of the exposure to CMF. This force induced MHD effects in the solution, thus causing electrolyte movements, with the resultant depletion of the Nernst diffusion layer and appearance of a new NaviereStokes hydrodynamic layer, which determined the velocity of electroactive molecules flow to the working electrode. The magnetic field influence on the electrochemical reaction is mainly related to convective phenomena. During alloy deposition without CMF, numerous fractures formed as a result of residual stress were observed in the electrodeposited cobaltetungsten alloy samples. Under CMF conditions, such fractures disappeared. Due to the increased mass transport in the presence of the applied magnetic field, higher nucleation density occurs which consequently leads to grains with smaller size compared to those obtained without applied field. The process conducted in CMF resulted also in an increase of the cobalt content, with a simultaneous decrease of the tungsten content. The Co:W weight ratio changed from 2:1 without magnetic field, to 3:1 or 4:1 under CMF conditions. The following phases were identified in the alloys: phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. The dominant phase in the studied alloys was Co3W with crystallographic planes of {0 0 0 1} type (basal). Under CMF conditions some crystal planes were deflected at a specific angle (ranging from 10 to 20 , depending on the value of magnetic induction) during the alloy formation process. The investigation showed that the orientation of crystallites should be described in this case mainly as a plane family of {0 1 1 3} type, and some planes as {3 2 5 1} type. The exposure to CMF changed also the volume fraction of the dominant phase in the alloy. An increase of the (Co3W and Co7W6) phases by about 9% by volume was observed in CMF, with a simultaneous decrease of the cobalt and tungsten content. It has been clearly demonstrated that the superimposition of magnetic fields influences significantly the resulting layer properties. A pronounced impact on the layer morphology has been observed. The layers deposited under the influence of the parallelto-electrode magnetic field are denser and more homogeneous than those obtained without a magnetic field. Acknowledgments This work was supported by the Lodz University. References [1] Z. Abdel Hamid, Mater. Lett. 57 (2003) 2558. [2] H. Capel, P.H. Shipway, S.I. Harris, Wear 255 (2003) 917. [3] D.P. Weston, P.H. Shipway, S.I. Harris, M.K. Cheng, Wear 267 (2009) 934.
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
F.-h. Su, P. Huang, Mater. Chem. Phys. 134 (2012) 350. J. Chang, J. Chou, R. Hsich, J. Lee, Mater. Chem. Phys. 118 (2009) 314. T. Shobba, S.M. Mayanna, C.A.C. Sequeira, J. Power Sources 108 (2002) 261. G. Wei, H. Ge, X. Zhu, Q. Wu, J. Yu, B. Wang, Appl. Surf. Sci. 253 (2007) 7461. B. Szpunar, M. Aus, C. Cheung, U. Erb, G. Palumbo, J. Szpunar, J. Magn. Magn. Mater. 187 (1998) 325. T.Z. Fahidy, J. Appl. Electrochem. 13 (1983) 553. R.A. Tacken, L.J.J. Janssen, J. Appl. Electrochem. 25 (1995) 1. H. Matsushima, T. Nohira, J. Mogi, Y. Ito, Surf. Coat. Technol. 179 (2004) 245. J.A. Koza, M. Uhlemann, A. Gebert, L. Schultz, Electrochim. Acta 53 (2008) 5344. J.M.D. Coey, G. Hinds, J. Alloys Compd. 326 (2001) 238. S. Pane, E. Gomez, E. Valles, J. Electroanal. Chem. 615 (2008) 117. J.A. Koza, M. Uhlemann, A. Gebert, L. Schultz, J. Electroanal. Chem. 617 (2008) 194.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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S. Li, C. Zhu, L. Tang, J. Kan, Mater. Chem. Phys. 124 (2010) 168. L. Hu, Q. Chen, Mater. Chem. Phys. 133 (2012) 541. ski, E. Mie˛ kos, J. Appl. Electrochem. 38 (2008) 1771. M. Zielin A. Krause, C. Hamann, M. Uhlemann, A. Gebert, L. Schultz, J. Magn. Magn. Mater. 290e291 (2005) 261. ski, J. Balcerski, M. Cichomski, J. Grobelny, W. Szmaja, W. Koz1owski, K. Polan ski, E. Mie˛ ko&sacute, Mater. Chem. Phys. 132 (2012) 1060. M. Zielin Software, Bruker AXS (2005). K. Pawlik, Phys. Status Solidi 134 (1986) 477. LaboTex, The Texture Analysis Software, by LaboSoft s.c. O. Lioubashevski, E. Katz, J. Willner, J. Phys. Chem. B 108 (2004) 5778. T.Z. Fahidy, Electrochim. Acta 18 (1973) 607. R. Aogaki, K. Fueki, T. Mukaibo, Denki Kagaku Shuki Taikai Koen Yoshishu (Japan) 43 (1975) 504.
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Effects of constant magnetic field on the electrodeposition reactions and cobaltetungsten alloy structure ski* Marek Zielin Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, University of Lodz, Tamka 12, 91-403 Lodz, Poland
h i g h l i g h t s The effect of magnetic field on the electrodeposition of CoeW alloys was studied. Fractures on the sample surface disappeared under the exposure to magnetic field. The Co3W, Co7W6, a-Co and W phases were identified in CoeW alloys. The content of particular phases changed under the exposure to magnetic field. Changes under the exposure to magnetic field are due to magnetohydrodynamic effect.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2012 Received in revised form 19 March 2013 Accepted 14 May 2013
The paper presents a study of the effect of constant magnetic field (CMF) on the basic processes of cobalt etungsten alloys electrodeposition. To the author’s knowledge, such an investigation has been performed for the first time for cobaltetungsten alloys. The applied research methods included scanning electron microscopy (SEM), energy dispersive X-ray (EDX) microanalysis, energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). SEM images of cobaltetungsten alloys revealed numerous fractures on the samples surface, formed as a result of residual stress during alloy deposition without CMF. In CMF such fractures disappeared. The cobalt content increased, with a simultaneous decrease of the tungsten content, under CMF condition. The XRD study of cobaltetungsten alloys allowed to identify phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. Under CMF conditions some crystal planes were deflected at angles ranging from 10 to 20 . The exposure to CMF caused also an increase of the volume fraction (by about 9% by volume) of the dominant phase (Co3W and Co7W6) in the alloy. The reason for these changes was the fact that the Lorentz force, generated in CMF, caused the magnetohydrodynamic (MHD) effect. This induced movement of the electrolyte. The Nernst diffusion layer was depleted, whereas a new NaviereStokes hydrodynamic layer appeared, which determined the velocity of electroactive molecules flow to the working electrode under CMF conditions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Magnetic materials Electrochemical techniques Electron microscopy Energy dispersive analysis of X-rays
1. Introduction The demand for alloy coatings with specific properties has increased in the recent years. Such coatings are characterized by much better resistance to corrosion than single-metal ones. The properties of alloys associated with their chemical composition and structure, such as microhardness, residual stress, plasticity and electric conductivity, can be adjusted by appropriate composition of the galvanic bath and appropriate selection of
* Tel.: þ48 42 6355788; fax: þ48 42 6355796. E-mail addresses: [email protected], [email protected]. 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.025
technological parameters. Cathodic electrodeposition of alloys based on elements belonging to the ferrous metals group (Fe, Co, Ni) is still an important problem in electrochemistry. Codeposition of the mentioned elements with metals such as chromium, molybdenum and tungsten, or non-metals such as phosphorus and boron is an interesting issue. They are elements which cannot be deposited from aqueous solutions alone, but only in the presence of ferrous metals by the so-called induced codeposition. The cobaltetungsten alloy is also an alloy of this type. It is characterized by high hardness and corrosion resistance. The higher the tungsten content in the alloy, the less plastic it is and has the higher electrical resistance. Cobaltetungsten alloys with tungsten content to 30% by weight are ferromagnetic. Abdel
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin
Hamid [1] studied the optimal conditions for cobaltetungsten alloy electrodeposition process, such as temperature, current density, concentration of metal ions or surfactants. Then, he investigated the morphology of alloy surfaces, their microhardness and resistance to corrosion. He observed that adding tungsten to cobalt resulted in smoothening of the coating. With tungsten content ranging from 3 to 10% by weight, the alloy had a fine-grained structure, whereas when tungsten content increased to 15% by weight, the structure of the alloy was fibrous with even finer grains. At high current densities, the percentage content of tungsten decreased, whereas increased bath temperature resulted in higher content of tungsten in the alloy. Capel et al. [2] studied the mechanical strength and corrosion resistance of cobaltetungsten alloys. They observed that heating the alloys increased their mechanical strength, but decreased their corrosion resistance and tribological properties. The mechanical strength of cobaltetungsten alloys was also studied by Weston et al. [3]. They checked the tear and wear rate of cobalt, chromium and cobaltetungsten alloy coatings according to the loads applied. They concluded that cobaltetungsten alloys demonstrate very good mechanical strength and hardness. Su and Huang [4] investigated electrodeposited cobaltetungsten alloy coatings using X-ray diffraction (XRD), scanning electron microscopy (SEM) and a Vickers hardness tester. They observed a significant influence of the current density and Na2WO4 concentration in bath on the microstructure, morphology and hardness of the coatings. Moreover, the tribological properties of the coatings were found to be greatly dependent on their grain size, microhardness, surface morphologies and composition. Chang et al. [5] demonstrated that higher tungsten content in cobaltetungsten alloys improved their corrosion resistance. Shobba et al. [6] studied the corrosion resistance of cobalte tungsten alloys exposed to sulfuric acid (VI) and potassium hydroxide. The study was carried out in the context of applicability of these alloys as anode materials. Electrodeposition of cobalte tungsten alloys with an addition of organic compounds was carried out by Wei et al. [7]. They investigated the effect of such compounds on magnetic properties of cobaltetungsten alloys, their structure and surface morphology. The addition of methyl methacrylate resulted in homogeneity of the alloy, whereas saccharine eliminated fissures on the alloy surface. Coumarin caused formation of an alloy with non-homogeneous morphology. Moreover, it changed the magnetic properties, causing an increase of saturation magnetization and a decrease of coercion force on the hysteresis curve. Also Szpunar et al. [8] studied magnetic properties of cobaltetungsten alloys. They found that increased tungsten content in the alloy caused a decrease of saturation magnetization on the hysteresis curve. The effects of magnetic field on electrodeposition of various alloys as well as their physical and chemical properties were also investigated. It had been observed earlier that magnetic field may affect electrochemical processes [9e11]. The obtained data indicated that such changes are due to magnetohydrodynamic (MHD) effect. The MHD effect is based on the Lorentz force, inducing movements of the electrolyte and increasing or decreasing transport of the electroactive molecules to the electrode. Koza et al. [12] studied the effect of constant magnetic field (CMF) with magnetic induction value ranging from zero do 1 T, in various configurations in relation to the electrode surface. They deposited electrochemically cobalt, iron and cobalteiron alloy. They observed that CMF applied parallel to the electrode surface increased the limiting current density and the deposition rate. This was due to the MHD effect, induced by maximal Lorentz force generated in the hydrodynamic layer. It led to additional convection, increasing mass transport of ions to the electrode, which resulted in the deposition
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of more densely packed, less coarse alloys with finer grains. Magnetic field applied perpendicular to the electrode surface caused no significant changes in the limiting current density and the deposition rate. Coey and Hinds [13] confirmed that CMF increased significantly copper electrodeposition rate. Then, they observed increased transport of cationic mass, both diamagnetic (Agþ, Zn2þ, Bi3þ) and paramagnetic (Cu2þ, Ni2þ) under CMF conditions. Pane et al. [14] conducted cobaltenickel alloy electrodeposition in CMF, in the presence of ferrite nanomolecules. The use of CMF favored an increase of percentage ferrite content in the alloy, especially at low current densities. The effect of CMF on changes of pH values in the vicinity of the electrode during electrodeposition of cobalt, iron and cobalteiron alloys was also investigated [15]. It was found that under CMF conditions pH values decrease as a result of increased transport of hydrogen ions to the electrode surface. Li et al. [16] prepared Fe-contained polyaniline in the absence and presence of an applied magnetic field of 600 mT. Using the electron paramagnetic resonance (EPR) technique, they showed that there were unpaired electrons in the Fe-contained polyaniline synthesized in the presence of the magnetic field. Hu and Chen [17] carried out three identical reactions in a mixture of nickel nitrate hexahydrate, nicotinic acid and sodium azide in the absence and presence of applied magnetic fields of 150 mT and 300 mT [Ni1.5(N3)(nic)2(Hnic)]n was obtained without the applied magnetic field, while the final product totally changed into [Ni2(nic)4(H2O)] as a magnetic field of 300 mT was used in the reaction. To explain this result, the authors concluded that the magnetic field changes the reaction pathway due to the reduction of activation energy, which is indicated by the slight changes of the crystal microstructure, such as bond angle and length. In this paper, the effect of CMF on electrodeposition of cobalte tungsten alloys has been studied. To the author’s knowledge, such an investigation has been performed for the first time for cobalte tungsten alloys. The results obtained show that the application of CMF causes changes in the kinetics of alloy deposition reactions, changes in the chemical composition of the alloys and their surface morphology, as well as changes in the crystallographic parameters of the alloys.
Fig. 1. Electrochemical cup with three-electrode measurement system, placed between the N and S pole pieces of a laboratory electromagnet, (a) with the magnetic induction B perpendicular to the current density j, and (b) with the magnetic induction B parallel to the current density j. F denotes the Lorentz force.
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Fig. 2. SEM images of the central portions of cobaltetungsten alloy samples, obtained with no exposure to magnetic field and in CMF with magnetic induction value B ¼ 1200 mT (B t j configuration).
2. Experimental Cobaltetungsten alloys were prepared by electrodeposition using three-electrode system. The three-electrode electrochemical cup, in which the alloys were deposited, consisted of a working electrode (gold, disc-shaped) with 0.1 cm2 surface area, an auxiliary electrode (platinum, mesh) and a reference electrode (saturated, calomel). The galvanic solution prepared to obtain cobaltetungsten alloy contained 0.1 M cobalt sulfate (CoSO4$7H2O), 0.1 M sodium tungstate (Na2WO4$2H2O), 0.6 M sodium citrate (Na3C6H5O7$2H2O), 0.05 M EDTA (C10H14O8N2Na2$2H2O) and 0.1 M sulfuric acid (H2SO4). The alloys were deposited at potential 1,25 VSCE (as related to the saturated calomel electrode) for 3500 s. The cobaltetungsten alloy electrodeposition potential was determined on the basis of the dependence of current on potential. The cobaltetungsten alloys were deposited without and in CMF produced by the N and S pole pieces of an ER 2525 laboratory electromagnet. The magnetic induction B, used in the study within the value range from zero to 1.2 T, was directed either parallel to the surface of the working electrode (i.e. perpendicular to the current density j direction, Bkj configuration), or perpendicular to the surface of the working electrode (i.e. parallel to the current density j direction, Bkj configuration), as presented in Fig. 1. The morphological structure of the alloys was studied by SEM using a Philips XL 30 instrument. Images were recorded in the backscattered electron or secondary electron mode at 10 keV or 20 keV primary beam energy. The chemical composition of the alloys was determined by energy dispersive X-ray (EDX) microanalysis using an Oxford Instruments ISIS system and also by energydispersive X-ray spectroscopy (EDS) using a DEPR spectrometer. The crystallographic structure of the alloys was studied by XRD using a Bruker D8 Discover X-ray diffractometer with a Cu Ka radiation. 3. Results and discussion Electrochemically obtained cobaltetungsten alloys were subjected to investigations by a variety of methods. The morphology of the alloys was studied using SEM. The cobaltetungsten alloys were obtained electrochemically either with no exposure to magnetic field or under CMF conditions. The following four types of cobaltetungsten alloy samples were used in the SEM investigations:
sample sample sample sample
1: 2: 3: 4:
B B B B
¼ ¼ ¼ ¼
0; 200 mT, Bkj; 600 mT, Bkj; 1200 mT, Bkj.
Fig. 3. SEM images of the peripheral portions of cobaltetungsten alloy samples, obtained with no exposure to magnetic field and in CMF with magnetic induction value B ¼ 1200 mT (B tj configuration).
The SEM studies showed the presence of numerous fractures in the central portion of the samples surface, formed as a result of residual stress during alloy deposition without CMF (Fig. 2). With CMF value B ¼ 1200 mT and Bkj configuration, such fractures disappeared. This effect has already been mentioned in another paper [18] concerning research on cobaltemolybdenumetungsten alloys and on cobalt [19,20]. The mentioned effect was also observed in the peripheral parts of the cobaltetungsten samples surface, as confirmed by the SEM images shown in Fig. 3. The magnetic field applied during the deposition of alloy layers from the electrolyte, reduces the layers internal stress. The tensile stress in CoW films obtained with applied magnetic field is lower than in those deposited in the absence of a magnetic field. When a magnetic field is applied parallel to the electrode surface, the deposits look more homogeneous than those deposited without field. It is also visible that the deposits obtained in the parallel magnetic field are more densely packed and possess smaller porosity. On the basis of the data recorded with EDX microanalysis in the scanning electron microscope (SEM), the percentage contents of cobalt and tungsten in the investigated cobaltetungsten alloys were determined. They were deposited with no exposure to magnetic field and in CMF with magnetic induction values B ¼ 200, 600 and 1200 mT. The obtained results are presented in Table 1. It can be observed that the effect of CMF involves an increase of cobalt content and a decrease of tungsten content. The Co:W ratio has changed from 2:1 to 4:1. The chemical composition of the obtained cobaltetungsten alloys was also investigated using the EDS method. For these studies, the cobaltetungsten alloy samples were divided into representative areas to get reliable chemical composition results. The obtained data are presented in Table 2. The Co:W weight ratio changed from about 2:1 without exposure to magnetic field, to about 3:1 under CMF conditions. The approximately 20% oxygen content in the overall composition of CoW (Table 2) comes from oxides. The layers of metals or alloys are particularly susceptible to rapid oxidation and the formation of oxides. The formation of oxides on metals or alloys is a normal and acceptable phenomenon in practice. The Table 1 Percentage content of the components in cobaltetungsten alloys obtained without and in CMF with different values of magnetic induction B, determined by EDX microanalysis in the SEM. B (mT)
0
200
600
1200
% wt. Co % wt. W % other components
51.4 25.4 23.2
52.0 23.1 24.9
53.6 17.7 28.7
54.9 13.5 31.6
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin Table 2 Chemical composition determined with the use of EDS, for cobaltetungsten alloys obtained without and in CMF with magnetic induction B ¼ 1200 mT and Bkj configuration. Element
% wt.
O Na W Co
B¼0
B ¼ 1200 mT
18.4 0.6 28.7 52.3
19.4 1.3 20.9 58.5
Fig. 4. X-ray diffractograms obtained for four types of cobaltetungsten alloy samples: 1 e black (B ¼ 0), 2 e red (B ¼ 200 mT, Bkj), 4 e blue (B ¼ 1200 mT, Bkj), 5 e green (B ¼ 1200 mT, B k j). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
concurrent process of incorporation of oxides into the alloy coating layer is also frequent. Tungsten has a high affinity to oxygen [DG0298 ¼ 182.5 kcal mol1 (for W/WO3)] and therefore it forms oxide compounds easily. As a consequence, such metals are either deposited with a high overpotential or not deposited at all. They can be deposited electrolytically only together with other metals,
373
e.g. from the ferrous metals group (Co, Ni, Fe) by the so-called induced codeposition of metals. In contrast, cobalt has a low affinity to oxygen [DG0298 ¼ 51.0 kcal mol1 (for Co/CoO)] and can be deposited easily both alone and with other metals, e.g. with tungsten. The phase structure and crystalline lattice parameters of the investigated cobaltetungsten alloys were determined by the XRD method. The following four types of cobaltetungsten alloy samples were used in the XRD studies:
sample sample sample sample
1: 2: 4: 5:
B B B B
¼ ¼ ¼ ¼
0; 200 mT, Bkj; 1200 mT, Bkj; 1200 mT, Bkj.
Samples 4 and 5 were deposited in CMF with the same highest magnetic induction value B ¼ 1200 mT, but in different configurations: Bkj and Bkj, respectively. Sample 5 was in fact prepared for the purpose of comparison of the obtained results with those for sample 4. The phases were identified on the basis of the experimental data recorded using the XRD technique in asymmetric geometry 2qscan, with EVA software [21] as well as the data obtained from ICDD crystallographic database. The analysis of dominant phase texture (phase volume fractions) in the alloy coatings was based on experimental so-called pole figures with application of thin layer correction. The orientation distribution function (ODF), calculated according to the discrete method, was used in texture analysis [22] utilizing LaboTex software package [23]. After carrying out the phase analysis of cobaltetungsten alloys, the appropriate diffractograms were obtained, shown in Fig. 4. On the basis of the X-ray diffractograms of cobaltetungsten alloys, as well as the data obtained from ICDD crystallographic database, the following phases were identified: phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. Decreased intensity of the reflected beams indicated that under CMF conditions some crystal planes were deflected at a specific angle during the alloy formation process. The higher the value of magnetic induction, the more significant deflection of some crystal planes took place. However, the crystallographic system of cobaltetungsten alloys did not change as a result of the
Table 3 Phases, phase volume fractions and crystalline lattice parameters of cobaltetungsten alloys. Sample no.
Conditions
Alloy (metal)
Crystallographic system
Crystal lattice parameters ( A) 0.0005
Volume fraction (% vol.) 5
1
B¼0
Co3W
Hexagonal
69
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co W Co3W
Cubic Cubic Hexagonal
Co7W6
Trigonal (orthorhombic)
a-Co
Cubic Cubic
a ¼ b ¼ 5.0947 c ¼ 4.2329 a ¼ b ¼ 4.7371 c ¼ 25.5952 a ¼ b ¼ c ¼ 3.4071 a ¼ b ¼ c ¼ 3.1985 a ¼ b ¼ 5.1075 c ¼ 4.2276 a ¼ b ¼ 4.7351 c ¼ 25.3820 a ¼ b ¼ c ¼ 3.4014 a ¼ b ¼ c ¼ 3.2011 a ¼ b ¼ 5.0947 c ¼ 4.2242 a ¼ b ¼ 4.7391 c ¼ 25.6140 a ¼ b ¼ c ¼ 3.4085 a ¼ b ¼ c ¼ 3.2065 a ¼ b ¼ 5.0714 c ¼ 4.2312 a ¼ b ¼ 4.7607 c ¼ 25.4761 a ¼ b ¼ c ¼ 3.4057 a ¼ b ¼ c ¼ 3.2068
2
4
5
B ¼ 200 mT Bkj
B ¼ 1200 mT Bkj
B ¼ 1200 mT Bkj
W
14 17 66
22 12 78
8 14 60
35 5
374
ski / Materials Chemistry and Physics 141 (2013) 370e377 M. Zielin
exposure to CMF. The identified phases, their volume fractions and the crystalline lattice parameters of the investigated cobaltetungsten alloys are presented in Table 3. The deviations of the peaks from the nominal position of the diffraction line for particular phases can result from the presence of internal stresses, being often comparatively high in the coating layers. Many variants of diffractometry recordings were performed to find the optimal geometrical conditions making it possible to record reflections from CoW coating layers of 20 mm in thickness. The gold substrate was 2 mm thick. Fig. 4 presents the final diffractograms. The most pronounced diffraction reflections in the recorded diffractograms originate from gold (the substrate), with the lattice parameter a ¼ 4.0780 A. The dominant phase was Co3W in a hexagonal system and Co7W6 in a trigonal system. The lattice parameter c was taken into account in the conducted measurements as a characteristic parameter for the studied case. As a result of the exposure to CMF, cobaltetungsten alloy crystals were more flattened, which is confirmed by the decreasing lattice parameter c of the dominant phase, the SEM images (Figs. 2 and 3) and earlier studies [18]. Describing the crystallographic texture (spatial orientation of the crystallites), it can be stated that Co3W (hexagonal crystallographic system) and Co7W6 (trigonal crystallographic system) is just the dominant phase in the investigated alloys. The spatial orientation of crystallites of the (Co3W and Co7W6) phases in the studied coating layers showed a tendency toward the formation of {0 0 0 1} type (basal) lattice planes, slightly deflected (by 15e20 ) in relation to the sample surface. The precise orientation of crystallites should in fact be described as {0 1 1 3} type plane, parallel to the sample surface. The above-mentioned tendency was most pronounced in sample 1 (B ¼ 0) and weaker in samples 2, 4 and 5, i.e. in those exposed to constant magnetic field (Fig. 5). The inverse pole figures with the normal direction as related to the surface of the studied coating layers (Fig. 6) indicated that, in addition to the orientations mentioned above, only in sample 4 [where the highest magnetic induction B of constant magnetic field (B ¼ 1200 mT) was used with Bkj] a small fraction of the (Co3W and Co7W6) phases was oriented in such a way that there were {3 2 5 1} type planes parallel to the coating layer surface.
As a result of the exposure to CMF, the spatial orientation of the planes and the dominant phase volume fraction in the alloy changed. The content of the phases (Co3W and Co7W6) increased by about 9% vol. in CMF with magnetic induction B ¼ 1200 mT and Bkj configuration, with a simultaneous decrease of the cobalt and tungsten content. The described processes and changes due to the effect of CMF, resulting from the presented study, can be explained as follows. The most important effect of CMF on electrochemical processes is the MHD effect. The driving force of that effect is the generated Lorentz force F:
F ¼ jB
(1)
where j is the current density and B is the magnetic induction. In the case of configuration Bkj, this is a macroscopic effect. It causes additional convection, resulting in an increase of mass transport toward the working electrode. Consequently, a laminar flow of electrolyte near the working electrode surface occurs, reducing the diffusion layer thickness and causing an increase of potential-generating ions concentration. It causes a change of grain size in the deposited alloy, affects its texture and formation of various phases. Such effects are caused by additional convection, as well as by the magnetic properties of ions. CMF present in the medium determines the alloy thickness increase in the direction imposed by magnetization of the deposited alloy material. Generally speaking, the solution mass transport from and to the electrode can be effected by migration, diffusion and convection mechanisms. The diffusion is an important phenomenon always occurring in the process of electrolysis. The second Fick’s law describes the concentration changes in time [Eqs. (2) and (3)]:
vC ¼ DV2 C vt
(2)
where C is the concentration of electroactive ions in the body of the electrolyte, t is time and D is the electrolyte diffusivity,
Fig. 5. Pole figures for the basal planes of {0 0 0 1} type (top), prismatic ones of {1 1 2 0} type (center) and pyramidal ones of {1 1 2 2} type (bottom) of the hexagonal phase Co3W and trigonal phase Co7W6 in cobaltetungsten alloy samples 1, 2, 4 and 5.
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Fig. 6. Inverse pole figures for normal orientation in relation to the sample surface, for the Co3W phase and Co7W6 phase in cobaltetungsten alloy samples 1, 2, 4 and 5.
vC vt
¼ z
v2 C vz2
(3)
where z is the perpendicular direction from the electrode surface (cf. Fig. 7). In the transition state for the ongoing processes, concentration changes in time should be taken into consideration. The mass transport equation can be expressed as follows:
vC ¼ uCV2 f þ DV2 C vVC vt
(4)
where u is the ion mobility, 4 is the internal potential (electrical) phase and v is the bulk flow velocity. Eq. (4) is the NaviereStokes equation, describing the motion of fluid substances. Eq. (4) and hydrodynamic continuity Eq. (5) are the basic differential equations describing convective mass transport:
vr þ VðrvÞ ¼ 0 vt
(5)
where r is the fluid specific density. Ionic mass transfer can be extended by convection and consequently one can define the phenomenon as convective diffusion. There is a solution layer of thickness dD (Nernst diffusion layer) between the working electrode surface and the electrolyte mass, the mass transfer through which occurs only as a result of diffusion and migration. The layer thickness dD is dependent on hydrodynamic conditions and viscosity of the solution. If the solution is not stirred, diffusion layer growth with time occurs under potentiostatic conditions:
dD ¼
pffiffiffiffiffiffiffiffiffi pDt
(6)
where t is the mass transfer time. If there are differences in electrolyte density, e.g. as a result of additional energy supply, a liquid flow along the surface of a vertically positioned electrode, directed from its lower to upper edge, will be observed (Fig. 7). This is natural convection, which inhibits the growth of the diffusion layer dD. Thus, Eq. (4) for steadystate convection diffusion can be expressed as follows:
vC ¼ DV2 C vVC ¼ 0 vt
(7)
There is a gradient of concentrations (C Cel) in the aforementioned diffusion layer dD; thus, the mass diffusion transfer Jdiff can be written as:
Jdiff ¼ DðC Cel Þ dD
(8)
where Cel is the concentration of electroactive ions near the working electrode surface. The rate of reduction of electroactive ions on the working electrode surface is higher than the rate of reagents supply, thus mass transport in the diffusion layer can be expressed as follows:
Cel ¼ 0jz¼0 and Cel ¼ Cjz/N
(9)
The mass diffusion stream Jdiff direction is perpendicular to the electrode surface. The current density j can be described with the formula [24]:
j ¼ ðDnFF CÞ dD
Fig. 7. Growth inhibition of the Nernst diffusion layer dD as a result of the differences in electrolyte density emerging as a result of additional energy supply.
(10)
where n is the number of electrons involved in the Faradaic process and FF ¼ 96,487 C mol1 is the Faraday’s number. The force F generated as a result of the exposure to CMF caused electrolyte movements. The Nernst diffusion layer dD was depleted, while a new NaviereStokes hydrodynamic layer dH appeared (Fig. 8). The presented considerations concerned obviously the flat
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electrochemical system was observed when they were applied perpendicular to the electrode surface. The investigation of magneto-electrolysis is of significance from both the industrial and fundamental points of view. It may open up a novel method of modifying the microstructure, in particular the particle content, by controlling the magnetic fields with no need to adjust additives and electrolyte composition. 4. Conclusions
Fig. 8. Reduction in the Nernst diffusion layer thickness dD near the working electrode surface under the influence of CMF, and formation of the NaviereStokes hydrodynamic layer dH.
surface of the working electrode, and the CMF was parallel to the working electrode surface. The theoretical model of cathode current density j dependence on magnetic induction B was presented by Fahidy [25] and Aogaki et al. [26]: 1=4
j ¼ 4:3 103 n3=2 Awe Dv1=4 C 4=3 B1=3
(11)
where Awe is the area of the electrode. On the other hand, a quantitative model of the cathode current density j dependence on the magnetic induction B was presented by Lioubashevski et al. in their paper [24]. The dependence was described as follows: 1=6
jy0:63p1=6 Awe r1=3 D8=9 v2=9 ðnFF CÞ4=3 B1=3
(12)
As a result of the exposure to CMF [18], the thickness of Nernst diffusion layer dD decreased and amounted to:
dD y1:59 rRv2=3 D1=3
1=3
ðnFF CBÞ1=3
(13)
where R is the radius of the working electrode. It was also calculated that the velocity of electroactive molecules v increases with increasing magnetic induction B [18]:
. vy nFF CD4=3 Br1=3 dH 0:62h4=3
(14)
where h is the dynamic viscosity. With increasing magnetic induction B, the thickness of Naviere Stokes hydrodynamic layer dH, which determines the electrolyte flow under the exposure to CMF, also increases. A primary effect of magnetic fields on the electrochemical reaction is the MHD effect acting in the bulk electrolyte (Bkj configuration), which reduces the diffusion layer thickness and thus, the limiting current densities and deposition rates are increased with a superimposed magnetic field. The nucleation process is affected by the magnetic field when it is applied parallel to the electrode surface (maximal Lorentz force). This influence is attributed to the MHD effect. No significant effect of magnetic fields on the
The effect of CMF on the electrodeposition process of cobalte tungsten alloys has been investigated. The results obtained show that the application of CMF causes changes in the kinetics of alloy deposition reactions, changes in the chemical composition of the alloys and their surface morphology, as well as changes in the crystallographic parameters of the alloys. In the process of cobaltetungsten alloy electrodeposition, the Lorentz force was generated as a result of the exposure to CMF. This force induced MHD effects in the solution, thus causing electrolyte movements, with the resultant depletion of the Nernst diffusion layer and appearance of a new NaviereStokes hydrodynamic layer, which determined the velocity of electroactive molecules flow to the working electrode. The magnetic field influence on the electrochemical reaction is mainly related to convective phenomena. During alloy deposition without CMF, numerous fractures formed as a result of residual stress were observed in the electrodeposited cobaltetungsten alloy samples. Under CMF conditions, such fractures disappeared. Due to the increased mass transport in the presence of the applied magnetic field, higher nucleation density occurs which consequently leads to grains with smaller size compared to those obtained without applied field. The process conducted in CMF resulted also in an increase of the cobalt content, with a simultaneous decrease of the tungsten content. The Co:W weight ratio changed from 2:1 without magnetic field, to 3:1 or 4:1 under CMF conditions. The following phases were identified in the alloys: phase Co3W in a hexagonal system, phase Co7W6 in a trigonal (orthorhombic) system, phase a-Co in a regular system and phase W in a regular system. The dominant phase in the studied alloys was Co3W with crystallographic planes of {0 0 0 1} type (basal). Under CMF conditions some crystal planes were deflected at a specific angle (ranging from 10 to 20 , depending on the value of magnetic induction) during the alloy formation process. The investigation showed that the orientation of crystallites should be described in this case mainly as a plane family of {0 1 1 3} type, and some planes as {3 2 5 1} type. The exposure to CMF changed also the volume fraction of the dominant phase in the alloy. An increase of the (Co3W and Co7W6) phases by about 9% by volume was observed in CMF, with a simultaneous decrease of the cobalt and tungsten content. It has been clearly demonstrated that the superimposition of magnetic fields influences significantly the resulting layer properties. A pronounced impact on the layer morphology has been observed. The layers deposited under the influence of the parallelto-electrode magnetic field are denser and more homogeneous than those obtained without a magnetic field. Acknowledgments This work was supported by the Lodz University. References [1] Z. Abdel Hamid, Mater. Lett. 57 (2003) 2558. [2] H. Capel, P.H. Shipway, S.I. Harris, Wear 255 (2003) 917. [3] D.P. Weston, P.H. Shipway, S.I. Harris, M.K. Cheng, Wear 267 (2009) 934.
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