WO2011149330A1

WO2011149330A1 - Method of electrodepositing nickel-cobalt alloy

Info

Publication number: WO2011149330A1
Application number: PCT/MY2011/000033
Authority: WO
Inventors: Jeffrey Basirun Wan; Ebadi Mehdi; Alias Yatimah; Hakimim Zainuddin Khairul; Shaharia Bhuyan Mohamad; Sulaiman Azhar
Original assignee: Mimos Berhad
Priority date: 2010-05-26
Filing date: 2011-04-19
Publication date: 2011-12-01
Also published as: MY176034A

Abstract

The present invention provides a method to fabricate Ni-Co alloy where in this process, electrodeposition of Ni-Co alloy is conducted in a non-aqueous electroplating bath consisting of a mixture of ionic liquid, 1-ethyl-3-methyllimidazolium chloride (EMIC) [20] and organic compound, ethylene glycol (EG) [22] as the solvent together with Ni and Co ions, with the application of a magnetic field parallel to the cathode [24] surface. With application of an external permanent magnetic field parallel to the cathode [24] surface, the rate of transport of Ni and Co ions to the cathode surface is increased, therefore increasing the current density of electrodeposition of Ni-Co alloys. As the magnetic force is stronger for the Co ion compared to Ni ion, there will be an increased transport of Co ion compared to the Ni ion to the cathode surface. This result in higher Co content in the Ni-Co alloy compared to Ni, even when the bath composition ratio of Co is lower than the bath composition of Ni.

Description

METHOD OF ELECTRODEPOSITING NICKEL-COBALT ALLOY

The present invention relates to Nickel-Cobalt alloy electrodeposition enhancement by using external magnetic field in a non-aqueous electroplating bath.

BACKGROUND ART

Alloys of nickel (Ni) and cobalt (Co) are typically used in areas where high temperature performance, particularly creep resistance, is required. These alloys are typically selected to be used as gas turbine components such as blades, turbine wheels and latter stage compressor disks, which are subjected to long term rotational stresses and high temperatures. The alloy is also commonly used for heat treating fabrications including furnaces, retorts and fixtures, for strength at temperature and resistance to oxidation, carburization, sulfi cation and nitriding.

Presently, Nickel-Cobalt (Ni-Co) alloy is fabricated via electroplating from an aqueous or non-aqueous solution consisting of Ni and Co ions. The electrodeposition current density magnitude depends on mass transport factors such as diffusion, convection and ionic migration of Ni and Co ions to the cathodic surface. Under normal conditions, the current density cannot be increased beyond a certain limit, due to the limitations of fixed mass transport.

Prior arts of conventional Ni-Co electroplating showed that the Co content in Ni-Co alloy is relatively low due to the low deposition preference of the Co ions in solution which can be explained from thermodynamics theory. A prior art listed a method of fabricating Ni-Co alloy via aqueous electrolytes for electrodeposition of the ions. The method consists of a process with composition for electrodeposition of hard, tarnish resistant; and shining Ni and Co containing coatings; using aqueous acidic baths containing metal ions and boric acid. As a shine promoter, an aliphatic aldehyde and an aromatic carbonyl compound, and optionally wettings agents are added. This electroplating process is an electrodeposition process conducted in an aqueous solution with the use of shine promoter as brightener and a stress reliever. The anode material is Ni and Co with electrodeposition using constant potential (chrono- amperometry) and NiCI₂and CoCI₂. Another prior art listed the method to fabricate Ni-Co alloy with phosporus in low stress electroplating which, in this process Ni-Co alloy deposition contains at least about 2% to 25% atomic volume of phosporus. The process solution contains Ni and optionally cobalt sulphate, hydrophosphorus acid or a salt thereof at a low pH, acidic condition. This electroplating process is an electrodeposition process conducted in an aqueous solution with the use of shine promoter as brightener and a stress reliever. The anode material is Ni and Co with electrodeposition using constant current density (chrono-potentiometry) and NiS0₄ and CoSCy Organic etching agents are also included in the process. The present invention is made in view of the prior arts described above to increase high deposition of Co content. The proposed process is conducted to increase the rate of transport of Ni and Co ions and resulting in higher Co content in the alloys; solving the low Co deposition issue in conventional electroplating. SUMMARY OF INVENTION

The present invention proposes a non-aqueous electroplating bath for the electrodeposition of Ni-Co alloys with the application of a magnetic field parallel to the cathode surface. This invention is with regards to the feasibility of electrodeposition of Ni-Co alloy from a non-aqueous bath consisting of a mixture of an ionic liquid, 1-ethyl-3- methyllimidazolium chloride (EMIC) and an organic compound, ethylene glycol (EG) as the solvent together with Ni and Co ions. With application of an external permanent magnetic field parallel to the cathode surface, the rate of transport of Ni and Co ions to the cathode surface is increased, therefore increasing the current density of electrodeposition of Ni-Co alloys. As the magnetic force is stronger for the Co ion compared to Ni ion, there will be an increased transport of Co ion compared to the Ni ion to the cathode surface. This will result in higher Co content in the Ni-Co alloy compared to Ni, even when the bath composition ratio of Co is lower than Ni. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a drawing showing the structure of 1-ethyl-3-methyllimidazolium chloride (EMIC). Fig. 2 is a drawing showing the structure of ethylene glycol (EG). Fig. 3 is a schematic drawing showing the cell apparatus for the electrodeposition of Ni- Co alloy with external magnetic field parallel to the cathode surface.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention is described in detail.

The invention involves a method to electrodeposit Ni-Co alloy using non-aqueous electroplating bath solution consisting of an ionic liquid and an organic solvent together with Ni and Co anions with an application of a parallel magnetic field to the electrodeposition surface. The presence of a magnetic field will result in higher Co content compared to the Ni content in the electrodeposited Ni-Co alloy.

In this invention, the non-aqueous bath consists of the following:

Solvent: 1-ethyl-3-methyllimidazolium chloride (EMIC) [20] and ethylene glycol

(EG) [22] in the ratio of EG:EMIC, 5:1 in volume

NiCI₂:0.25M or 14.7g Ni per liter

CoCI₂:0.05M or 2.9g Co per liter The bath composition of Co is five times less than the bath composition of Ni. Fig. 1 and Fig. 2 show the structure of EMIC [20] and EG [22] respectively.

The ionic liquid of the bath is but not limited to EMIC [20]. Other ionic liquid can be used based on the generic chemical formula of 1-Ri-3-R₂ imidazolium A^", where the ionic liquid in use can be any combination of R1 , R2 and anion A^".

The R1 and R2 that can be used are listed as follows:

Methyl (CH₃)

Ethyl (C₂H₅)

Propyl (C₃H₇)

Buthyl (C₄H₉)

Penthyl (C₆Hn)

Hexyl (C₆H₁₃)

Hepthyl (C₇H₁₅) Octyl (C₈H₁₇)

The A^" that can be used is listed as follows:

Chloride (CO

Trifluoromethanesulfonate (CF₃S0₃ ^~)

Bromide (Br^")

Thiocyanate (SCN )

Tetrafluoroborate (BF₄ ^~)

Trifluoroacetate (CF₃COO^~)

Hydrogen sulphate (HS0₄ )

Iodide (I )

Dicyanamide (N(CN)₂ ^")

Methylsulfate (CH₃OS0₃ )

Ethylsulfate (C₂H₅S0₄ )

Bis(Trifluoromethylsulfonyl)imide N(CF₃S0₂)₂ ^"

Tetracynaborate (B(CN)₄ ^"

The organic solvent used is di-alcohol but not limited to EG [22] (HO)CH₂CH₂(OH). Other di-alcohol that can be used is listed as follows:

Ethane-1 ,1-diol

Propane-1-3-diol (HO)CH₂CH₂CH₂(OH)

Propane-1-2-diol (HO)CH(OH)CH₂CH₂

Propane-1-1-diol (HO)₂CH₂CH₂CH₃ The volume proportion of organic solvent and ionic liquid ranges from 5:1 to 1 :2.

The electrodeposition temperature is 25°C - 60°C while the electrodeposition is using constant potential, chrono-amperometry from potentials of -0.8V to -1.2V vs. reference electrode. The electroplating condition is a magnetic field strength of 4-9 Tesla parallel to the cathode [24] surface.

Fig. 3 shows the cell for the electrodeposition of Ni-Co alloy from a solution of Ni and Co ions dissolved in a non-aqueous solution consisting of ionic liquid and an organic solvent. At least two magnet [26] bars with the strength of 4 to 9 Tesla is placed to create a magnetic field parallel to the cathode [24] surface and at least two anodes [28] is place in juxtaposition of the cathode [24].

The magnitude of electrodeposition current arises from the mass transport of Ni and Co ions in solution. Mass transport in an electrode process can be categorized coming from 3 sources which are diffusion, ionic migration and convection. These mass transports are responsible for the transport of Ni and Co ions to the cathode [24] surface.

With the use of an external magnetic field, being applied parallel to the electrode surface, there are five additional forces, apart from diffusion, ionic migration and convection which can increase the rate of mass transport of Ni and Co ions. The forces from an applied magnetic field parallel to the cathode [24] surface which act on these ions are:

Paramagnetic force (F_P);

Field gradient force (F_e);

Lorentz force (F_L);

Electrokinetic force (F_E); and

Magnetic damping force (F_D)

In the study for this invention, cyclic voltammetry of Ni and Co ions in the mixture of EMIC [20] / EG [22] is observed under with or without permanent parallel magnetic field condition. Current density is elevated with applied magnetic field to the cathode surface and it was noted that the electrodeposition current of Ni-Co alloy increases with the presence of magnetic field. Electrodeposition of Ni-Co alloy was also conducted at a constant potential (chrono- amperometry) of -0.90V vs. SCE reference electrode and -1.10V vs. SCE reference electrode and the results showed that there was increase in the percentage of Co atoms compare to Ni atoms despite the face that the Co concentration used in the electrodeposition bath was five times lower than the Ni concentration of 0.25M.

The increase of Co content in the Ni-Co alloy electrodeposited from a non-aqueous bath with the application of PMMF is the increase of paramagnetic force for the Co ion compared to the Ni ion which increases the mass transport of the Co ions. The paramagnetic force is given as: F_P= X_mB² c

2μ₀

and this force depends on molar susceptibility of the ion, where, B is the magnetic force field strength, Vc is the concentration of gradient of the paramagnetic ions in the diffusion layer and μ₀ is the magnetic permeability of vacuum. X_m is the molar susceptibility and X_m= Np₀p²/3kT, where N is the number of molecules per unit volume, μ is the magnetic moment of the atom or ion, μ₀ is the magnetic permeability of vacuum, k is the Boltzman constant and T is the absolute temperature.

When the magnetic moment of Ni and Co ions are studied, it was noted that the magnet moment of Co ion is higher than Ni ion. The greater moment of Co ion will account for the greater paramagnetic force being experienced by the Co ions in the solution with the presence of the PMMF compared with Ni ion. This resulted in greater mass transport for Co ions compared to Ni ion, which accounts for larger percentage of Co in the Ni-Co alloy electrodeposits despite the fact that Co is less noble a metal compared to Ni.

The standard reduction potentials of Co and Ni are: Co²⁺ + 2e ► Co E/V = -0.277

Ni²⁺ + 2e ► Ni E/V = -0.250

From thermodynamics data, the electrodeposition of Co occur less readily compared to the electrodeposition of Ni from their ions in solution. However, with the application of PMMF the rate of mass transport of Co ions increases more than the mass transport of Ni ions in solution, and therefore giving more current of Co electrodeposition and increased content of Co atoms in Ni-Co alloys, even though the concentration of Co ions is five times smaller than the concentration of Ni ions in the solution. Accordingly, the invention disclosed a method of electrodepositing Ni-Co alloy where in this process, electrodeposition of Ni-Co alloy is conducted in a non-aqueous electroplating bath consisting of a mixture of ionic liquid, 1-ethyl-3-methyllimidazolium chloride (EMIC) [20] and organic compound, ethylene glycol (EG) [22] as the solvent in a ratio of EG:EMIC, 5:1 in volume, together with Ni and Co ions from NiCI₂ and CoCI₂ solution, with the application of a magnetic field parallel to the cathode [24] surface.

With application of an external permanent magnetic field parallel to the cathode [24] surface, the rate of transport of Ni and Co ions to the cathode surface is increased, therefore increasing the current density of electrodeposition of Ni-Co alloys. As the magnetic force is stronger for the Co ion compared to Ni ion, there will be an increased transport of Co ion compared to the Ni ion to the cathode surface. This result in higher Co content in the Ni-Co alloy compared to Ni, even when the bath composition ratio of Co is five times lower than the bath composition of Ni.

It is the combination of the above features and its technical advantages give rise to the uniqueness of such invention. Although the descriptions above contain much specificity, these should not be construed as limiting the scope of the embodiment but as merely providing illustrations of some of the presently preferred embodiments.

Claims

1. A method of electrodepositing Nickel-Cobalt alloy comprising:

electrodepositing a non-aqueous electroplating bath in a cell containing a mixture of an ionic liquid and an organic compound as solvent, together with Ni and Co anions; characterized in that;

the ionic liquid is of chemical formula 1-R 3-R₂ imidazolium A^", where Ri and R₂ are alkyl groups and A^' is an anion; and

organic solvent is a di-alcohol.

2. A method according to claim 1 , wherein the volume proportion of organic solvent and ionic liquid ranges from 5:1 to 1 :2.

3. A method according to claim 1 , wherein the ionic liquid is 1-ethyl-3- methyllimidazolium chloride (EMIC) [20].

4. A method according to claim 1 , wherein the organic solvent is ethylene glycol (EG)

[22].

5. A method according to claim 1 , wherein the organic solvent is ethane-1 ,1-diol, propane-1-3-diol (HO)CH₂CH₂CH₂(OH), propane-1-2-diol (HO)CH(OH)CH₂CH₂ or propane-1-1-diol (HO)₂CH₂CH₂CH₃.

6. A method according to claim 1 , wherein the bath composition of Co is five times less than the bath of Ni.

7. A method according to claim 1 , wherein the preparation of Ni and Co anions in the non-aqueous electroplating bath comprises:

preparing NiCI₂of 0.25M or 14.7g Ni per liter;

preparing CoCI₂ of 0.05M or 2.9g Co per liter; and

dissolving NiCI₂ and CoCI₂ in solvent mixture with slightly elevated temperature of 50°C - 60°C.

8. A method according to claim 1 , wherein the cell for the electrodeposition comprises at least two magnet bars [26] to create a magnetic field parallel to the cathode [24] surface.

9. A method according to claim 1 , wherein the cell for the electrodeposition comprises at least two anodes [28] in juxtaposition of the cathode [24].

10. A method according to claim 8, wherein the magnetic field strength from magnet bars is 4 to 9 Tesla.

11. A non-aqueous electroplating bath for electrodepositing Nickel-Cobalt alloy comprising: 1-ethyl-3-methyllimidazolium chloride (EMIC) [20], ethylene glycol (EG) [22], NiCI₂and CoCI_2.