WO2004094699A1

WO2004094699A1 - Nano invar alloys and a process of producing the same

Info

Publication number: WO2004094699A1
Application number: PCT/KR2004/000516
Authority: WO
Inventors: Yong-Bum Park
Original assignee: Nano Invar Co. Ltd.
Priority date: 2003-04-24
Filing date: 2004-03-12
Publication date: 2004-11-04
Also published as: JP2006524292A; US20060037671A1; KR100505002B1; KR20040092613A

Abstract

The present invention relates to an electrolyte for producing a novel Fe-Ni alloy having an Ni content in a range of 33 to 42 wt%, specifically a nanocrystalline invar alloy having a grain size of 5 to 15 nm, by electroplating, and preparation conditions thereof. The electrolyte comprises, on the basis of 1 L of water, 32 to 53 g of ferrous sulfate or ferrous chloride, a mixture thereof; 97 g of nickel sulfate, nickel chloride, nickel sulfamate or a mixture thereof; 20 to 30 g of boric acid; 1 to 3 g of sodium saccharin; 0.1 to 0.3 g of sodium lauryl sulfate; and 20 to 40 g of sodium chloride. The Fe-Ni alloy sheet of the present invention exhibits excellent mechanical property compared to the conventional Fe-Ni alloy and a new property, i.e., a negative coefficient of thermal expansion at a given temperature range.

Description

NANO INVAR ALLOYS AND A PROCESS OF PRODUCING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrolyte for producing a novel Fe-Ni alloy having an Ni content in a range of 33 to 42 wt%. specifically a nanocrystalline invar alloy having a grain size of 5 to 15 nm, by electroplating, and preparation conditions thereof.

2. Description of the related Art

Fe-Ni alloys exhibit various properties according to the Ni content, and low thermal expansion properties are exhibited when the Ni content is in a range of 20% to 50% by weight (see D. R. Rancourt, S. Chehab and G. Lamarche, J. Mag. Mag. Mater. 78 (1989) 129.). Specifically, an alloy consisting of 64% Fe and 36% Ni, which is referred to as "an invar alloy", has a coefficient of thermal expansion of about zero. The invar alloy has, since its discovery in 1897 by Guillaume (see C. E. Guillaume, C.R. Acad. Sci. Paris 124 (1897) 176.), been commercially used for various practical applications as a typical low thermal expansion alloy.

Such a typical low thermal expansion invar alloy (Fe-36%Ni) is used in a variety of applications, such as a standard measurement apparatus, an internal combustion engine piston, bimetal, a temperature controller, a liquefied gas storage device, an IC lead frame, a shadow mask, which is an essential component of a cathode ray tube (CRT) for a color monitor of a TV or PC, other electronic devices, or the like.

Also, a shadow mask made of invar alloys is expected to be used not only in field emission displays (FEDs) for flat monitors, which have recently been developed, but also in lead frames for mounting integrated circuit (IC) chips.

There may be circumstances where alloys need to be shrinkable as the temperature at which the alloys are used, increases. In such case, development of alloys having a negative coefficient of thermal expansion in an operating temperature range is very highly demanded. Various processes have been employed to produce the Fe-Ni alloy sheets, and cold rolling has been typically used for that purpose. When conducting the cold rolling, vacuum melting, forging, hot rolling, normalizing, primary cold rolling, intermediate annealing, secondary cold rolling, final annealing under a reduction atmosphere and so on should be performed. In order to produce a thin invar alloy sheet having a thickness of 0.1 mm or less, it is necessary to carry out a multi-stage rolling process, as disclosed in U.S. Patent No. 494834, which is, however, complex, and makes it difficult to obtain homogenous product. Also, this process undesirably requires a high production cost. Furthermore, there are several problems that large-scale equipment, such as a vacuum melting furnace, forging facility, a hot roller or a multi-stage roller, is required, and a heating process for shaping as requested by final product is quite difficult to perform etc. Further, coefficients of thermal expansion are undesirably sensitive to impurities involving in the process and a change in the processing conditions (see Metals Handbook, 9th ed. Vol. 3, ASM (1980) 889.).

To overcome the limitations of the conventional preparation methods, vigorous research into preparation methods of Fe-Ni alloys by electroplating electroforming) has been carried out in recent years. However, according to the electroplating, since selecting a proper electrolyte or establishing proper processing conditions, such as a temperature or current density, are quite complicated, the use of electroplating for producing desired Fe-Ni alloys has not been successful.

SUMMARY OF THE INVENTION

Therefore, there is an increasing demand for providing proper electrolytes and processing conditions for producing nano invar alloys. In particular, since a sheet to be plated should have a width of at least 300 mm (30 cm) for commercial use, it is necessary to find out appropriate conditions for electroplating under such circumstances.

It is an object of the present invention to provide an electrolyte for producing a nano invar alloy sheet having a nano-scale grain size by electroplating or electroforming, and processing conditions thereof.

It is another object of the present invention to provide an Fe-Ni alloy having a negative coefficient of thermal expansion at a given temperature range.

It is still another object of the present invention to provide an Fe-Ni alloy having excellent mechanical properties compared to the conventional invar alloy.

It is yet another object of the present invention to provide a method for producing an Fe-Ni alloy having a negative coefficient of thermal expansion at a given temperature range.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of an electroplating apparatus for producing a nano invar alloy sheet according to the present invention;

FIG. 2 illustrates a change in the coefficient of thermal expansion depending on the composition ratio of a nano invar alloy according to the present invention;

FIG. 3 is a {lll} pole figure of texture after annealing a conventional invar alloy;

FIG. 4A is a {100} pole figure of texture of the nano invar alloy according to the present invention; and

FIG. 4B is a {111} pole figure of texture after annealing the nano invar alloy according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, there is provided an Fe-Ni alloy containing 33% to 38% by weight of Ni, produced by electroplating, using a solution as an electrolyte, on the basis of 1 liter (L) of water, comprising 32 to 53 g of ferrous sulfate (FeSθ₄ 7H₂0), ferrous chloride (FeClj 4H₂0) or a mixture thereof; 97 g of nickel sulfate (NiSU4 6H₂0), nickel chloride (NiCl₂- 6H₂0), nickel sulfamate (Ni(NH₂S0₃)₂) or a mixture thereof; 20 to 30 g of boric acid (H₃B0₃); 1 to 3 g of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate (Cι₂H₂₅0₄SNa); and 20 to 40 g of sodium chloride (NaCl), under the conditions that a pH of the electrolyte is in a range of 2 to 3, a current density is in a range of 50 to 100 mA/cm², and a temperature of the electrolyte is in a range of 45 to 60 °C .

A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of an electroplating apparatus for producing a nano invar alloy sheet according to the present invention.

In FIG. 1, electroplating was conducted such that an electrolyte 3 according to the present invention was put in an electroplating bath 9, and a circulation pump 5 was actuated to allow the electrolyte 3 to flow between a cathode 1 and an anode 2, spaced 10 mm apart from each other, at a flow rate of 0.1 to 2.0 m/sec. Here, reference numeral 6 denotes a filter, numeral 7 denotes a nozzle, and numeral 8 denotes a circulation pipe. When a 20 μm thick Fe-Ni alloy was electrodeposited on a cathode sheet, a current supply device 4 was stopped operating, and a resulting electroplated sheet was isolated from a cathode surface. According to an aspect of the present invention, the inclination 10 of an anode sheet depends on the flow rate.

The electrolyte proposed in the present invention is a solution having a composition comprising ferrous sulfate (FeSθ₄ 7H₂0) or ferrous chloride

(FeClj 4H₂0); nickel sulfate (NiSO₄ 6H₂0), nickel chloride (NiCl^ 6H₂0) or nickel sulfamate (Ni(NH₂S0₃)₂); 20 to 30 g/1 of boric acid (H₃B0₃); 1 to 3 g/1 of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g/1 of sodium lauryl sulfate (Cι₂H₂₅0₄SNa); and 20 to 40 g/1 of sodium chloride (NaCl). More desirable effects of the electrolyte are achieved by comprising 22 to 25 g/1 of boric acid (H₃B0₃), 2.0 to 2.4 g/1 of sodium saccharin (C₇H₄N0₃SNa), 0.1 to 0.2 g/1 of sodium lauryl sulfate

(Cι₂H₂₅0₄SNa) and 30 to 32 g/1 of sodium chloride (NaCl). Boric acid is added as a pH buffering agent, sodium saccharin is added as a stress relaxing agent for the electroplated product, sodium chloride is added for the purpose of enhancing the conductivity of the electrolyte, and sodium lauryl sulfate is added as a surfactant.

During electroplating, the pH of the electrolyte is maintained in a range of 2 to 3, the current density is in a range of 50 to 100 mA/cm², and the temperature of the electrolyte is in a range of 45 to 60 °C .

The Fe component and Ni component are released in the ionic form from the electrolyte and are electrodeposited on a cathode sheet in the form of Fe-Ni alloy having a thickness of 1 to 200 μm during electroplating. Tables 1 through 6 show examples of electrolytes for producing nano invar alloy sheets of the present invention by electroplating.

Table 1: Using solution containing ferrous sulfate (FeSθ₄ 7H₂0) and nickel sulfate (NiS0 6H₂0)

<On^' the basis of 1 liter of distilled water>

Table 2: Using solution containing ferrous sulfate (FeSθ₄ 7H₂O) and nickel chloride (NiCl₂ 6H₂0)

Table 3: Using solution containing ferrous chloride (FeCl^ 4H₂0) and nickel sulfate (NiS0₄ 6H₂0)

Table 4: Using solution containing ferrous chloride (FeClj 4H₂0) and nickel chloride (NiCl₂ 6H₂0)

Table 5: Using solution containing ferrous sulfate (FeSθ₄ 7H₂0) and nickel sulfamate (Ni(NH₂SQ₃)₂)

Table 6: Using solution containing ferrous chloride (FeCl^ 4H₂0) and nickel sulfamate (Ni(NH₂S0₃)₂)

Table 1 shows preparation results of Fe-Ni alloys having desired compositions according to Examples 1 through 3 using electrolytes containing ferrous sulfate (FeSU₄7H₂0) and nickel sulfate (NiSθ₄ 6H₂) as main components, with keeping the amounts of nickel sulfate at 97 g/1 and varying the amounts of ferrous sulfate in a range of 43 to 53 g/1.

Table 2 shows a preparation result of an Fe-Ni alloy having a desired composition according to Example 4 using an electrolyte containing ferrous sulfate (FeSU₄ 7H₂0) and nickel chloride (N1CI₂ 6H₂0) as main components, with keeping the amount of nickel sulfate at 97g/l and using 50 g/1 of ferrous sulfate.

Table 3 shows preparation results of Fe-Ni alloys having desired compositions according to Examples 5 and 6 using electrolytes containing ferrous chloride (FeCli 4H₂0) and nickel sulfate (NiSU₄ 6H₂0) as main components, with keeping the amounts of nickel sulfate at 97 g/1 and varying the amounts of ferrous chloride in a range of 42 to 44 g/1.

Table 4 shows preparation results of Fe-Ni alloys having desired compositions according to Examples 7 and 9 using electrolytes containing ferrous chloride (FeClj 4H₂0) and nickel chloride (NiClj 6H₂0) as main components, with keeping the amounts of nickel chloride at 97 g/1 and varying the amounts of ferrous chloride in a range of 44 to 50 g/1.

Table 5 shows preparation results of Fe-Ni alloys having desired compositions according to Examples 10 and 11 using electrolytes containing ferrous sulfate (FeSC>4 7H₂0) and nickel sulfamate (Ni(NH₂S0₃)₂) as main components, with keeping the amounts of nickel sulfamate at 97 g/1 and varying the amounts of ferrous sulfate in a range of 35 to 37 g/1.

Table 6 shows preparation results of Fe-Ni alloys having desired compositions according to Examples 12 and 13 using electrolytes containing ferrous chloride (FeCtø 4H₂0) and nickel sulfamate (Ni(NH₂S0₃)₂) as main components, with keeping the amounts of nickel sulfamate at 97 g/1 and varying the amounts of ferrous chloride in a range of 32 to 34 g/1.

The Fe-Ni alloys produced using the electrolytes having the compositions listed above by electroplating, have properties shown in Table 8 below, irrespective of kinds of electrolytes used in Tables 1 through 6. When comparing the conventional invar alloys shown in Table 7 with the nano invar alloys according to the present invention in view of their properties, it is confirmed that the nano invar alloys according to the present invention have better material characteristics than the conventional invar alloys. The comparison results are shown in Table 9.

Table 7: Physical properties of conventional invar alloys

Density, g/cm3 8.1

Tensile strength, MPa 450-585

Yield strength, MPa 275-415

Elastic limit, MPa 140-205

Elongation, % 30-45

Reduction in area, % 55-70

Brinell hardness 160

Modulus of elasticity, GPa(10⁶ psi) 150(21.4)

Thermoelastic coefficient, mm/m k 500

Specific heat, at 25-100 ^°C, J kg- ^°C 515

Thermal conductivit, at 20-100 °C, W/m k 11 Electrical resistivity, ml m 750-850

Table 8: Physical properties of nano invar alloys according to the present invention

Hardness, GPa 5.4

Tensile strength, MPa 1,045

Yield strength, MPa 805

Modulus of elasticity, GPa 85-120

Table 9: Comparison between conventional invar alloys and nano invar alloys according to the present invention in view of the properties

In other words, the nano invar alloy according to the present invention is at least two times higher than the conventional invar alloy in view of hardness, tensile strength, and yield strength. In detail, the yield strength of the nano invar alloys according to the present invention is 805 MPa, which is much higher than that of the conventional invar alloy, that is, 275 to 415 MPa. Therefore, the nano invar alloys according to the present invention can be advantageously applied in the fields where there is a demand for providing high strength.

Table 10: Coefficients of thermal expansion of conventional invar alloy

Table 11: Coefficients of thermal expansion of nano invar alloy (Fe - 36wt% Ni) according to the present invention

Table 10 shows average coefficients of thermal expansion of the conventional invar alloy depending on temperature ranges. As shown in Table 10, the conventional invar alloy has an average coefficient of thermal expansion of about 1.66 m/m K in a temperature range of 17 to 100 °C, and the coefficient of thermal expansion thereof increases as the temperature becomes higher. On the other hand, the nano invar alloy (Fe - 36 wt% Ni) according to the present invention exhibits a coefficient of thermal expansion of about 1.58 μm/m K in a temperature range of 20 to 100 ^°C , the coefficient of thermal expansion of 0 in a temperature range of 140 to 150 ^°C, and when the temperature increases to 150 °C or higher, the coefficient of thermal expansion thereof becomes a negative value. When the temperature is in a range of 20 to 200 ^°C, the average coefficient of thermal expansion of the nano invar alloy according to the present invention is -1.78 μm/m K Such thermal expansion behaviors are commonly exhibited when the Ni content of the nano invar alloy according to the present invention is in a range of 33 to 38 wt%.

FIG. 2 shows a change in the coefficient of thermal expansion of the nano invar alloy according to the present invention depending on its composition ratio, confirming the facts described hereinbefore. As shown in the drawing, in case that percentages of the Ni content, by weight, are 33% and 38%, the coefficients of thermal expansion for both cases are negative values at a given temperature or higher. Therefore, since the nano invar alloy according to the present invention has a negative coefficient of thermal expansion, applications where such properties are demanded may be newly possible applications of the present invention.

FIG. 3 is a {111} pole figure of the texture after annealing conventional invar alloy, FIG. 4A is a {100} pole figure of the texture of the nano invar alloy according to the present invention, and FIG. 4B is a {111} pole figure of the texture after annealing the nano invar alloy according to the present invention.

As is apparent from the drawings, when the conventional invar alloy is annealed, a growth texture of {001}<100> type is dominantly indicated. On the contrary, in case of the nano invar alloy according to the present invention, in a plated state, a {100}//ND fiber type is dominantly indicated, and when annealed, a growth of {111}//ND fiber texture type is indicated.

According to X-ray diffraction, the Fe-Ni alloy of the present invention has a nanocrystalline structure having a grain size of 5 to 15 nm. The results confirmed that the grain size of the invar alloy composition having the Ni content of 36% is very small to be in a range of 5 to 7 nm. Such a nanocrystalline structure presumably accounts for high yield strength of the invar alloy.

According to the present invention, since Fe-Ni alloys having low thermal expansion properties is produced by a single-step electroplating process, the production cost can be greatly reduced. Particularly, since the Fe-Ni alloys according to the present invention have a nanocrystalline structure, they exhibit excellent mechanical properties, thereby creating a new range in industrial uses.

Claims

WHAT IS CLAIMED IS:

1. An Fe-Ni alloy containing 33% to 38% by weight of Ni, produced by electroplating, using a solution as an electrolyte, on the basis of 1 liter (L) of water, comprising 32 to 53 g of ferrous sulfate (FeSθ4 7H₂θ)_> ferrous chloride (FeCk 4H₂0) or a mixture thereof; 97 g of nickel sulfate (NiS0₄- 6H₂0), nickel chloride (NiCl^ 6H₂0), nickel sulfamate (Ni(NH₂S0₃)₂) or a mixture thereof; 20 to 30 g of boric acid (H₃B0₃); 1 to 3 g of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate (Cι₂H₂₅0₄SNa); and 20 to 40 g of sodium chloride (NaCl), under the conditions of a pH of the electrolyte being in a range of 2 to 3, a current density being in a range of 50 to 100 mA/cm², and a temperature of the electrolyte being in a range of 45 to 60 °C .

2. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises:

43 to 53 g of ferrous sulfate (FeS0₄ 7H₂0);

97 g of nickel sulfate (NiS0₄ 6H₂0);

20 to 30 g of boric acid (H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate (C₁₂H₂₅0₄SNa); and

20 to 40 g of sodium chloride (NaCl).

3. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises: 50 g of ferrous sulfate (FeS0₄ 7H₂0);

97 g of nickel chloride (NiCl^ 6H₂0);

20 to 30 g of boric acid(H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

0.1 to 0.3 g of sodium lauryl sulfate (Cι₂H₂₅0₄SNa); and 20 to 40 g of sodium chloride (NaCl).

4. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises:

42 to 44 g of ferrous chloride (FeC 4H₂0); 97 g of nickel sulfate (NiS0₄ 6H₂0);

20 to 30 g of boric acid (H₃B0₃); 1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

0.1 to 0.3 g of sodium lauryl sulfate (C₁₂H₂₅0₄SNa); and 20 to 40 g of sodium chloride (NaCl).

5. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises:

44 to 50 g of ferrous chloride (FeCl₄ 4H₂0); 97 g of nickel chloride (NiCl₂ 6H₂0); 20 to 30 g of boric acid (H₃B0₃); 1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate

and

20 to 40 g of sodium chloride (NaCl).

6. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises: 35 to 37 g of ferrous sulfate (FeS0₄ 7H₂0);

97 g of nickel sulfamate (Ni(NH₂SO₃)₂);

20 to 30 g of boric acid (H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0 SNa);

0.1 to 0.3 g of sodium lauryl sulfate (Cχ₂H₂₅0 SNa); and 20 to 40 g of sodium chloride (NaCl).

7. The Fe-Ni alloy of claim 1, wherein the electrolyte, on the basis of 1 L of water, comprises:

32 to 34 g of ferrous chloride (FeCli 4H₂0); 97 g of nickel sulfamate (Ni(NH₂SO₃)₂);

20 to 30 g of boric acid (H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

0.1 to 0.3 g of sodium lauryl sulfate (Cχ₂H₂₅0 SNa); and

20 to 40 g of sodium chloride (NaCl).

8. The Fe-Ni alloy of any one of claims 1 through 7, wherein the Fe-Ni alloy has a thickness in a range of 1 to 200 μm.

9. The Fe-Ni alloy of any one of claims 1 through 7, wherein the Fe-Ni alloy has a grain size in a range of 5 to 15 nm.

10. The Fe-Ni alloy of any one of claims 1 through 7, wherein the Fe-Ni alloy has a negative coefficient of thermal expansion at a predetermined temperature or higher.

11. The Fe-Ni alloy of any one of claims 1 through 8, wherein the Fe-Ni alloy has a composition ratio of 64 t% Fe and 36 wt% Ni.

12. A method of producing an Fe-Ni alloy containing 33% to 38% by weight of Ni, comprising carrying out electroplating, using a solution as an electrolyte, on the basis of 1 liter (L) of water, comprising 32 to 53 g of ferrous sulfate (FeSθ4 7H₂0), ferrous chloride (FeCl₂- 4H₂0) or a mixture thereof; 97 g of nickel sulfate (NiS0₄- 6H₂0), nickel chloride (NiCl^ 6H₂0), nickel sulfamate (Ni(NH₂S0₃)₂) or a mixture thereof; 20 to 30 g of boric acid (H₃B0₃); 1 to 3 g of sodium saccharin (C₇H N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate (Cι₂H₂₅0 SNa); and 20 to 40 g of sodium chloride (NaCl), under the conditions of a pH of the electrolyte being in a range of 2 to 3, a current density being in a range of 50 to 100 mA/cm², and a temperature of the electrolyte being in a range of 45 to 60 TJ .

13. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises:

43 to 53 g of ferrous sulfate (FeSO₄ 7H₂0);

97 g of nickel sulfate (NiS0 6H₂0);

20 to 30 g of boric acid (H₃B0₃);

20 to 40 g of sodium chloride (NaCl).

14. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises: 50 g of ferrous sulfate (FeS0₄ 7H₂0);

97 g of nickel chloride (NiCl^ 6H₂0);

20 to 30 g of boric acid(H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

15. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises:

42 to 44 g of ferrous chloride (FeCl₄ 4H₂0); 97 g of nickel sulfate (NiS0₄ 6H₂0); 20 to 30 g of boric acid (H₃B0₃); ^• 1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

16. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises:

44 to 50 g of ferrous chloride (FeCl 4H₂0); 97 g of nickel chloride (NiCfc 6H₂0); 20 to 30 g of boric acid (H₃B0₃); 1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa); 0.1 to 0.3 g of sodium lauryl sulfate (C₁₂H₂₅0 SNa); and

20 to 40 g of sodium chloride (NaCl).

17. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises: 35 to 37 g of ferrous sulfate (FeSU4 7H₂0);

97 g of nickel sulfamate (Ni(NH₂S0₃)₂);

20 to 30 g of boric acid (H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

0.1 to 0.3 g of sodium lauryl sulfate (C₁₂H₂₅0 SNa); and 20 to 40 g of sodium chloride (NaCl).

18. The method of claim 12, wherein the electrolyte, on the basis of 1 L of water, comprises:

32 to 34 g of ferrous chloride (FeCl^ 4H₂0); 97 g of nickel sulfamate (Ni(NH₂S0₃)₂);

20 to 30 g of boric acid (H₃B0₃);

1.0 to 3.0 g of sodium saccharin (C₇H₄N0₃SNa);

0.1 to 0.3 g of sodium lauryl sulfate (C₁₂H₂₅0₄SNa); and

20 to 40 g of sodium chloride (NaCl).