EP0823496B1

EP0823496B1 - Process for producing ceramic layer by plasma enhanced electrolysis and product thereof

Info

Publication number: EP0823496B1
Application number: EP95930367A
Authority: EP
Inventors: Qingshan High-tech. Dvping Zoom 14 KONG; Hongbo High-tech. Developing Zoom 14 ZUO; Donghui High-tech. Developing Zoom 14 MI
Original assignee: Harbin Huanya Micro - Arc Co Ltd; HARBIN HUANYA MICRO ARC CO Ltd
Current assignee: Sunny Asia Co Ltd Harbin
Priority date: 1995-04-18
Filing date: 1995-09-11
Publication date: 2002-04-03
Anticipated expiration: 2015-09-11
Also published as: EP0823496A1; JP2937484B2; CN1034522C; WO1996033300A1; AU3378895A; DE69526256D1; EP0823496A4; JPH10509772A; DE69526256T2; CN1115793A

Description

The present invention concerns surface chemical treatment, in particular surface chemical treatment for metal material.

With the development of modern industry and science technology, following on steel and aluminum, ceramic materials have become the third generation engineering materials because of their special performances, superiority of rich resources. However, because the whole ceramic materials are brittle, not easy to process, their wide application has been limited all the time. Creating ceramic layers on the surface of metals and alloys, cheap metal materials may be used instead of expensive metal materials with the prerequisite for guaranteeing the use performance, at the same time endow with the substrate metal material some special performances that can not be obtained by the other surface treatment methods and enlarge its suitable range. On the other hand, using easy-processing materials as the substrate to apply surface ceramic coating, can increase the shaping and processing performances for the ceramic materials, providing guarantee for making complicated shape spares with ceramic coating, and carving out a new way for widely using ceramic products.

Processes for providing metal oxide ceramic layer on metal substrate through chemical reaction and plasma chemical reaction by arc discharging in an electrolyte solution were known from EP 0 462 073, EP 0 280 886, CN 87 1 03694 and US 3,834,999. Following are some shortcomings of these processes.

1. After processing, the homogeneity of the ceramic layer is inferior, the aspect of the precision of surface dimensions and roughness presents problems, and it is very difficult to be amended and repaired.

2. Products are of dull coloring and pattern, and have a poor decorative effect.

3. The electrolyte is unstable, short in potlife and expensive.

The object of the invention is to overcome the above defects in the prior art and to provide a process for surface ceramic-coating on metal substrate in an electrolyte solution containing (NaPO₃)₆ and a product thereof.

Another object of the invention is to provide a kind of metal products coated by ceramic layer.

The subject matter of the present invention is a process for preparing a ceramic layer on a metal substrate comprising the steps of plasma arc discharging on the substrate metal as an anode under voltage of 100-400V and current density of 0.5-20A/dm² to electrochemically oxidize the anode in an electrolyte solution which essentially comprises 10-50 g/l of (NaPO₃)₆ and 5-20 g/l of H₃BO₃ and/or Na₂B₄O₇ ^·7H₂O, and also comprises at least one oxyacid-salt selected from the group consisting of Na₃PO₄, Na₂SO₄, Na₂SiO₃, NH₄VO₄, NaVO₃, Na₂CrO₄, Na₂WO₄, CoSO₄, NiSO₄, Fe₂(SO₄)₃, MnSO₄ ^·H₂O and Cr₂(SO₄)₃, and optionally comprises additives selected from the group consisting of Ca(Ac)₂, Zn(Ac)₂, Co(Ac)₂, Ni(Ac)₂, H₂SiF₆, KF and EDTA, thereby forming a ceramic structure layer on the surface of the substrate.

In the present invention, the growing course of the ceramic layer is: in electrochemical oxidizing electrolytic bath, the metal workpiece is as anode, adding direct current electrical field between the cathode and the anode, there will be the following anodic oxidation reactions on the surface of the workpiece. Al = Al³⁺ + 3e OH^- = 1/2 H₂O + 1/2 [O]+e 2Al + 3[O] = Al₂O₃

Form a dense oxide thin film, and its thickness is about 0.01-0.2µm, the layer is thinner which with electric insulation is called resistance layer. With the voltage increasing between the cathode and the anode, the electrical field becomes very high (about 10⁷ V/cm), the phenomenon of electricity puncture will happen. On the surface of anodic workpiece generates plasma arc discharge. The arc discharge produced at this place where there are defects, crevices, thin layers. The energy density is very high at this place (about 10⁴-10⁷ W/cm²). At the interface between anode surface and electrolyte, excites a series of chemical reactions, makes some matters of electrolyte separated and excited. Because of the enhanced affect by plasma arc discharge, besides the main reaction of electrochemical anodic oxidation, some electrolytes join the electrochemical reactions, sintered on the base surface, form a layer with ceramic construction.

The process of the present invention also comprises the steps of cleaning the substrate metal before anodic oxidization, and rinsing, then sealing the surface after anodic oxidation. Therefore, the process of the present invention comprises the following steps:

1. Cleaning

The surface of light alloys, such as Al, Ti, Nb, Zr etc, is covered by different kinds of oil during various manufactures and preservations. So, before anodic treatment, the oil must be cleaned up thoroughly. At this time, alkaline cleaning liquid, which is formulated as Na₃PO₄·12H₂O 50-60g/l, Na₂SiO₃ 10-15g/l, Na₂CO₃ 10-20g/l and surfactant 0.1-10ml/l, is preferred. It is required that the cleaning liquid must be forced to be stirred or to be sprayed to the surface of the workpiece in the bath, to make it clean up at the temperature of 40-60°C for 20-30 min. Then the workpiece is immersed in a potcher containing clean water at the temperature of 15-20°C and rinsed until there are no clean liquid aforementioned remained on the surface of the workpiece.

2. Oxidization

Products with different colors, pattern, designs and properties, which can be applied in various field, can be obtained by using various electrolyte solutions, controlling various operation current, voltage, temperature of the solutions, stirring strength and pattern. The voltage for arc discharging is usually between 100-400V, the current density is 0.5-20A/dm², and the temperature of the solution is 10-50°C.

<1> Process for forming white ceramic film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₃PO₄·12H₂O	10-30 g/l,
Na₂B₄O₇·7H₂O	5-20 g/l,
Ca(Ac)₂	0.1-5 g/l,
Na₂SiO₃	0.1-10 g/l,
Zn(Ac)₂	0.1-12 g/l,
Na₂SO₄	5-10 g/l,
H₃BO₃	5-20 g/l,

All above are chemical pure reagents or industrial products find made for special use. The electrolyte solution is combined with distilled water. The combining sequence is: first dissolving (NaPO₃)₆ thoroughly, then adding the other materials, and adjusting pH with H₃PO₄ to pH 4.0-10.5. After combining the solution, it is required to be laid aside still for more than 24 hours, and the temperature is controlled at 15-45°C. The solution is forced to be uniform by spraying and stirring. The operation current density i=0.5-5A/dm², voltage V=100-400V, and the duration for oxidization is 7-30 min. A white ceramic film thus can be obtained with the thickness of the film being 5-28µm.

<2> Process for forming blue ceramic film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
H₃BO₃	5-20 g/l,
EDTA	1-6 g/l,
Na₂SO₄	5-10 g/l,
Na₃PO₄·12H₂O	5-15 g/l,
CoSO₄	5-20 g/l,
NiSO₄	1-10 g/l,
Co(Ac)₂	10-20 g/l,

pH=4.0-6.0. The electrolyte solution is combined according to <1>. The solution is required to be laid aside still for more than 24 hours, the temperature is 10-30°C. Two kinds of products can be made. One is a homogeneous blue film, the other is a blue dotted film with different size dots which have decorative effect. The operation conditions are: controlling current density of anode i=0.5-10A/dm², voltage V=150-300V. To form a homogeneous blue film, it is required to force to stir the solution to make the solution uniform completely for 5-20 min. A blue dotted film with different size dots which have decorative effect can be formed by means of changing the stirring manner to the solution to decrease the numbers of the discharge centers on the surface of the workpiece, increase the current density of the discharge centers, thus cause some matters to discharge and produce different size pattern dots on the surface, and get excellent decorative effect. The detailed are controlling the current density of anode i=0.5-7A/dm², forcing the solution uniformly by stirring (as the operation condition aforementioned) for 5-10 min, and then laying the solution aside still 3-5 min., then suddenly stirring or moving the workpiece in the electrolyte bath. To achieve the aim of the concentration changed greatly, the control time is 1-5min., to form different dots patterns decorative films. The thickness of the film is 5-15µm, and a blue film from light blue to dark blue is formed. This operation process also can be used in producing products of other colors.

<3> Process for forming cream-colored film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₃PO₄·12H₂O	5-10 g/l,
Ni(Ac)₂	2-15 g/l,
H₃BO₃	5-10 g/l,
Na₂SO₄	5-10 g/l,
Na₂B₄O₇·7H₂O	5-10 g/l,
Fe₂(SO₄)₃	2-10 g/l,
EDTA	1-6 g/l,
MnSO₄·H₂O	2-10 g/l,

The method for combining is the same as in <1>. pH is 4-6, temperature of the solution is controlled between 15-45°C. The operation current density i=0.5-10A/dm², V=125-350V, oxidizing time is 5-20min., and the film thickness is 5-25µm. With the stirring manner used in <1>, a cream-colored film from light cream-color to dark cream-color is formed.

<4> Process for forming Dark pink film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₂B₄O₇·7H₂O	5-20 g/l,
Na₃PO₄·12H₂O	10-30 g/l,
Na₂SiO₃	0.5-10 g/l,
Zn(Ac)₂	0.1-12 g/l,
MnSO₄·H₂O	5-20 g/l,

The method for combining is the same as in <1>, and the operation current density i=0.5-10A/dm2, V=150-350V, and the temperature is 10-40°C. Two types of products can be obtained. A decorative film with various dots patterns or a homogeneous dark pink film can be obtained by using the method of <2>. The oxidizing time is 5-25 min, the thickness is 5-25µm. The film thus obtained is from light pink to dark pink.

<5> Process for forming coffee-colored film to dark film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₂B₄O₇·7H₂O	5-10 g/l,
NH₄VO₃	2-10 g/l,
NaVO₃	2-10 g/l,
Na₂SO₄	5-10 g/l,

The method for combining is the same as in <1>. pH=3-6, the operation current density i=0.5-5A/dm², V=150-350V, the oxidizing time is 5-20min., and temperature of solution is 10-35°C. Using various stirring manners described in <2>, the homogeneous color film or dots patterns film can be obtained. The color is from light coffer, coffee color to dark, the thickness is 5-15 µm.

<6> Process for forming milky-yellow to yellow film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₂B₄O₇·7H₂O	5-10 g/l,
NH₄VO₃	2-7 g/l,
NaVO₃	2-7 g/l,
Ni(Ac)₂	5-15 g/l,
MnSO₄	1-5 g/l,
H₃BO₃	5-10 g/l,

The method for combining is the same as in <1>. pH=3-7, the operation current density i=0.5-10A/dm², V=150-300V, the oxidizing time is 5-10min., and the temperature of the solution is 10-30°C. Using various stirring manners described in <2>, the homogeneous color film or dots patterns film can be obtained. The color is from milky yellow, yellow to dark yellow, the thickness is 5-20 µm.

<7> Process for forming imitative ancient bronze-colored film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₂B₄O₇·7H₂O	10-15 g/l,
Na₃PO₄·12H₂O	10-15 g/l,
NH₄VO₃	1-10 g/l,
Na₂CrO₄	2-10 g/l,

The method for combining is the same as in <1>. pH=6-10.5, the operation current density i=0.5-10A/dm², V=150-350V, the oxidizing time is 5-20min., and the temperature of the solution is 10-50°C. The color is from light yellow ancient bronze to dark of it, the thickness is 5-15 µm.

<8> Process for forming grey film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
Na₂B₄O₇·7H₂O	10-50 g/l,
Na₂SO₄	5-10 g/l,
Na₃PO₄·12H₂O	10-15 g/l,
CoSO₄	2-15 g/l,
Cr₂(SO₄)₂	2-15 g/l,
Co(Ac)₂	2-10 g/l,
Ni(Ac)₂	2-10 g/l,
NH₄VO₃	2-10 g/l,

The method for combining is the same as in <1>. The operation current density i= 0.5-10A/dm², V=125-350V, the oxidizing time is 5-20min., and the temperature of the solution is 20-50°C. Using various stirring manners as described in <2>, the homogeneous color film in light grey to dark grey or dots patterns film in grey can be obtained. The thickness is 5-20 µm.

<9> Process for forming hard ceramic film

Formulation of the electrolyte solution:

(NaPO₃)₆	10-50 g/l,
H₂SiF₆	2-20 ml/l,
KF	1-10 g/l,
Na₂B₄O₇·7H₂O	7-20 g/l,
Na₂WO₄	1-20 g/l,

The method for combining is the same as in <1>. pH=3-7, the operation current density i=1-15A/dm², V=100-200V, the oxidizing time is 5-25min., and the temperature of the solution is 10-30°C. The solution is forced to be uniform by stirring. The color of the film on the surface of alloys is dark grey. The thickness is 10-100 µm.

Besides the above two kinds of products, among these electrolyte solutions, any two kinds of them can be mixed together to produce different color film. For example, the white film oxidized in electrolyte <1> can be overlapped by blue dots in electrolyte <2> with the above stirring manner, and become another kind of product. The details are shown in the following examples.

3. Rinse

Temperature of water is 15-60°C. The requirement is cleaning up the workpiece until there are no electrolyte solution remaining on the surface of it.

4. Sealing process

After rinsing, the workpiece can be sealed by using the process of dip coating, pour coating, spray coating to improve the luster. Paints of water-soluble acrylic acid resin or water-soluble amino resin can be used, and according to the requirements of the paint, be baked at 150-250°C for 5-30min for the case that a water-soluble acrylic acid is used. After baking, the products must be inspected to be standard, and then packed.

In the present invention, the content of the material from the electrolyte in the layer by using the process of the present invention is higher than that of traditional anodic oxide film lay er, and there are no oxide hydrates of the substrate metal in the film.

Therefore, the products obtained by using the process of the present invention are composed of substrate metal and the ceramic layer on the surface of the substrate. In the described layer, the content of the substrate metal oxide is 70.0-95.0% by weight, the content of the other metal oxides, non-metal oxides, inorganic salts or their mixture is 5.0-3.0% by weight. Said ceramic layer is formed by using the process of anodic oxidation enhanced plasma arc discharge. The described other metal oxides, non-metal oxides, inorganic salts or their mixture come from the electrolyte solution.

In the present invention, the layer's appearance of the products is stacked in regular mosaic manner, and the rate of holes is very low, which is less than 0.5%. There are little macroscopic defect. Each composition is uniformly dispersed in the layer. Because the metal atoms on the substrate surface take part in the reaction directly, the layer and the substrate are combined closely, and have no obvious boundary.

The homogeneity of the ceramic layer of the products in the present invention is good, and the combination strength between the layer and the substrate is higher. The holes in the layer are little. Both ram resistance and corrosion resistance are good, and the colors are bright and there are many patterns, as well as the decorative effect is great. The process of the present invention is suitable for surface treatment of the substrate workpiece of any dimension, shape and construction.

The present invention is further illustrated in detail by the following examples.

Example 1

To an oxide bath (1.5L) with 1000ml distilled water, 35g of (NaPO₃)₆ was added and dissolved thoroughly. 10.5g of Na₂B₄O₇·H₂O, 10g of Na₃PO₄·12H₂O were added and the pH of the solution was adjusted by using H₃PO₄ to 4.5-5.0. Then, 5g of Ca(AC)₂ was added to the solution to obtain an electrolyte solution. The electrolyte solution was laid aside still for 24 hours and stand-by. A plate of LD31 aluminum with 50mm×100mm×5mm was cleaned with alkaline cleaning solution, then rinsed. The workpiece was put on hanging and connected to power source. The oxidization treatment began when the solution in the bath was stirred in the case of the workpiece as anode and a stainless steel plate as cathode. Keeping the current being constant 1A, the voltage was slowly raised to 160-180V, on the surface of the workpiece, there was the phenomenon of plasma arc discharge. When the voltage raised to 210-240V, the current decreased. The duration for oxidization was 10 min., then the oxidization was stopped by adjusting the current to 0, the voltage to 0 and shutting off the power source. The workpiece was taken out form the bath and a white film was thus obtained. After cleaning, the holes were closed, and the workpiece was dip coated in water-soluble acrylic acid resin, and then baked for 5 min. at 220°C, and then taken out to be an end product. The thickness of the film was measured as 10µm, microhardness (HV) was 310kg/mm² (5g), and wear resistance (judgment of the time of spraying sands) was 300 second. CASS test: class 9.

Example 2

To an oxide bath (1.5L) with 1000ml distilled water, 35g of (NaPO₃)₆ was added and dissolved thoroughly. 10g of H₃BO₃, 2g of CoSO₄ and 2g of EDTA were added to the solution to obtain an electrolyte solution. The electrolyte solution was laid aside still for 24 hours and stand-by. Using the method described in Example 1, a workpiece was oxidized and a product with white film was obtained. After cleaning, the workpiece was put into the oxide bath, and the solution was kept stationary, then power source was connected to begin oxidizing. The current was 0.7A/dm² for 1.5min, then the workpiece was moved (or the electrolyte solution was stirred) . On the surface, there was relative less amount of the arc discharge, and this course was kept for 1 min, then the oxidizing stopped by shutting off the power source. The workpiece was taken out form the bath and a blue dotted pattern on white substrate was thus obtained. After glazing as described in Example 1, the end product was thus obtained.

Example 3

To an oxide bath (7.2m×1.6m×2.3m) for industrial production with 20000 liter distilled water, 700kg of (NaPO₃)₆ was added and dissolved thoroughly by forced stirring. 140kg of Na₂B₄O₇·7H₂O, 100kg of NH₄VO₃ and 200kg of Na₂SO₄ were added to the solution to obtain an electrolyte solution. The electrolyte solution was laid aside still for 24 - 48 hours. A set of cooling water machines was adopted to keep the bath temperature at 15-35°C. A stainless steel plate was taken as a cathode. The total area of a group of building sections of aluminum alloys, in an amount of 15, was 2700 dm², with each of them was 180 dm². The sections were put into the cleaning bath, dipped for 25 min. After raised by a shop traveler, the sections were dropped water freely, then were put into a potcher. After raised, they were put into the second potcher and then to the oxide bath to begin electrifying and oxidizing. The current was 1A/dm², the voltage was raised slowly. Stirring the solution, cooling, when the voltage was raised to 150V, small arc light on the surface of the workpieces occurred. The operation conditions described ahead were kept for 10min, the end voltage was 230V. Then the workpieces were put into potcher to rinse again, and dipped into resin bath, and baked in an oven. After unloading from the hanging, products were packed. The color of the film was coffee-color, and the thickness was measured as 8-11µm. The appearance of the end products was homogeneous, and the microhardness (HV) was 260-480kg/mm² (0.049N), and wear resistance (judgment of the time of spraying sands) was 300-500 seconds.

Example 4

To an oxide bath (1.5L) with 1000ml distilled water, 25g of Na₃PO₄·12H₂O, 7g of Na₂B₄O₇·7H₂O and 10g of Na₂SiO₃ were added and dissolved thoroughly to obtain an electrolyte solution. The electrolyte solution was laid aside still for 24 hours and stand-by. A stainless steel plate was used as a cathode, and a piece of Ti alloy (model TAL) with 50mm×100mm×1mm was cleaned and put into the oxide bath to be oxidized. The current of anode was 3A, and the voltage began to be risen. When the voltage reached to 100V, small arc light on the surface of the product occurred. The oxidizing time was controlled to 15min, voltage 150V, then current was decreased and the power source be shut off. The workpiece was taken out and rinsed. After baking, the color of the workpiece was grey. The thickness of the film was measured as 15µm, CASS test: class 9.

Example 5

To an oxide bath with 100 liter distilled water, 3.5kg of (NaPO₃)₆ was added and dissolved thoroughly. 1000ml of H₂SiF₆, 1.5kg of Na₂B₄O₇·7H₂O, 0.5kg of Na₂WO₄·2H₂O and 0.2kg/l of KF were added and the obtained electrolyte solution was laid aside still for 24 hours and stand-by. A pure aluminum plate with 150mm×100mm×10mm was cleaned, put on hanging and put into the oxide bath to be oxidized. The current was 15A, the voltage was raised slowly. Time for oxidization treatment was 20min, the end voltage was 180V. The current was decreased and the power source was shut off. The workpiece was taken out from the bath, rinsed and baked until to be an end product. The thickness of the film was measured as 50-70µm, the time for wear resistance of spray sands was 720-800 second, microhardness was 900-1300HV (0.098N), and the combination strength with substrate was 25.6-35.0kg/mm².

Claims

A process for preparing a ceramic-coating on the surface of a substrate metal comprising the steps of plasma arc discharging on the substrate metal as an anode under voltage of 100-400V and current density of 0.5-20A/dm² to electrochemically oxidize the anode in an electrolyte solution which essentially comprises 10-50 g/l of (NaPO₃)₆ and 5-20 g/l of H₃BO₃ and/or Na₂B₄O₇·7H₂O, and also comprises at least one oxyacid-salt selected from the group consisting of Na₃PO₄, Na₂SO₄, Na₂SiO₃, NH₄VO₄, NaVO₃, Na₂CrO₄, Na₂WO₄, CoSO₄, NiSO₄, Fe₂(SO₄)₃, MnSO₄·H₂O and Cr₂(SO₄)₃, and optionally comprises additives selected from the group consisting of Ca(Ac)₂, Zn(Ac)₂, Co(Ac)₂, Ni(Ac)₂, H₂SiF₆, KF and EDTA, thereby forming a ceramic structure layer on the surface of the substrate.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₃PO₄·12H₂O 10-30 g/l, Na₂B₄O₇.7H₂O 5-20 g/l, Ca(Ac)₂ 0.1-5 g/l, Na₂SiO₃ 0.1-10 g/l, Zn(Ac)₂ 0.1-12 g/l, Na₂SO₄ 5-10 g/l, H₃BO₃ 5-20 g/l,

and the pH of the solution is 4.0-10.5, the arc discharge voltage is 100-400V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, H₃BO₃ 5-20 g /l, EDTA 1-6 g/l, Na₂SO₄ 5-10 g/l, Na₃PO₄·12H₂O 5-15 g/l, CoSO₄ 5-20 g/l, NiSO₄ 1-10 g/l, Co(Ac)₂ 10-20 g/l,

and the pH of the solution is 4.0-6, the arc discharge voltage is 150-300V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₃PO₄·12H₂O 5-10 g/l, Ni(Ac)₂ 2-15 g/l, H₃BO₃ 5-10 g/l, Na₂SO₄ 5-10 g/l, Na₂B₄O₇·7H₂O 5-10 g/l, Fe₂(SO₄)₃ 2-10 g/l, EDTA 1-6 g/l, MnSO₄·H₂O 2-10 g/l,

and the arc discharge voltage is 125-350V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₂B₄O₇·7H₂O 5-20 g/l, Na₃PO₄·12H₂O 10-30 g/l, Na₂SiO₃ 0.5-10 g/l, Zn(Ac)₂ 0.1-12 g/l, MnSO₄·H₂O 5-20 g/l,

and the arc discharge voltage is 150-350V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₂B₄O₇·7H₂O 5-10 g/l, NH₄VO₃ 2-10 g/l, NaVO₃ 2-10 g/l, Na₂SO₄ 5-10 g/l,

and the pH of the solution is 3-6, the arc discharge voltage is 150-350V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₂B₄O₇·7H₂O 5-10 g/l, NH₄VO₃ 2-7 g/l, NaVO₃ 2-7 g/l, Ni(Ac)₂ 5-15 g/l, MnSO₄ 1-5 g/l, H₃BO₃ 5-10 g/l,

and the pH of the solution is 3-7, the arc discharge voltage is 150-300V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₂B₄O₇.7H₂O 10-15 g/l, Na₃PO₄·12H₂O 10-15 g/l, NH₄VO₃ 1-10 g/l, Na₂CrO₄ 2-10 g/l,

and the arc discharge voltage is 150-350V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, Na₂B₄O₇·7H₂O 10-50 g/l, Na₂SO₄ 5-10 g/l, Na₃PO₄·12H₂O 10-15 g/l, CoSO₄ 2-15 g/l, Cr₂(SO₄)₂ 2-15 g/l, Co(Ac)₂ 2-10 g/l, Ni(Ac)₂ 2-10 g/l, NH₄VO₃ 2-10 g/l,

and the arc discharge voltage is 125-350V.
The process according to claim 1, wherein said electrolyte contains: (NaPO₃)₆ 10-50 g/l, H₂SiF₆ 2-20 ml/l, KF 1-10 g/l, Na₂B₄O₇·7H₂O 7-20 g/l, Na₂WO₄ 1-20 g/l,

and the arc discharge voltage is 100-200V.
The process according to one of the claims 1-10, wherein the electrolyte solution is forced to stir during oxidizing to make it uniform.
The process according to one of the claims 1-10, wherein the electrolyte solution is forced to stir uniformly, and then the stirring is stopped and suddenly the stirring is started again or the substrate is made to move in said solution.
Products prepared in a process according to one of claims 1-12.