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

With the development of modern industry and science technology,following on steel and aluminum, the 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, we may use cheap metal materials 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.

At present, surface ceramic coating for the surface of metal and alloy can be carried out by using both plasma spraying and solid powder applying in addition to laser melt-coating, i.e., sintering on the metal surface by using additional powder material or part of powder material itself under high-density energy so as to obtain ceramic coating.

Technique for plasma spraying and laser melt-coating generally use solid powder as metal and carbonate, oxygenate and boronate compound powder. After high energy sintering, ceramic layer forms on surface of substrate. Its hard particle mostly consists in the carbonate, oxygenate and boronate compound of metal (transitional group), in an amount of about 40-90%, and about 10-60% of solid component. The surface ceramic layer produced by plasma spraying and laser melt-coating has a topography of irregular mosaic stacking with higher porosity and less defectives in macrostate. Each component exhibits un-uniform distribution in the coating layer, and the binding between each coating layer are of mechanical with clear boundary. Following are some shortcomings in these methods.

• 1. After processing, the homogeneity of the ceramic layer is bad, the aspect of the precision of surface dimensions and roughness exists problems, and it is very difficult to be amended and repaired.
• 2. The connecting between the layer and substrate are of mechanical. There are large amount of pore and internal stress between ceramic layer. And bond strength, shock resistance and corrosion resistance are not ideal.
• 3. The requirement on size to processable components is high. And it is very difficult to process internal pore and irregular component.
• 4. It is likely to be over-heated by using plasma spraying and laser melt-coating and decrease its property.

On the other hand, a common process for metal or alloy (such as aluminum alloy) surface treatment is anodic oxidation. By using Aluminum as an example, the process is anodic oxidizing the work piece which is anode in acidic electrolyte (such as sulfuric acid solution). As sulfuric acid process as an example, there are following reaction on aluminum anode and in the interface electrolyte:

        Al = Al³⁺ + 3e     (1)



        3SO₄^2- + 3H₂O = 2H₂SO₄ + 3[O] + 6e     (2)



        3[O] + 2Al = Al₂O₃     (3)



        Al₂O₃ + H₂O = Al₂O₃ ·H₂O     (4)



        Al₂O₃ ·H₂O + 3H₂SO₄ = Al₂(SO₄)₃ + 4H₂O     (5)



        Al₂O₃ + 3 H₂SO₄ = Al₂(SO₄)₃ + 3H₂O     (6)

From reaction (1), (2), (3) and (4), aluminum oxide is formed on the anode surface by combining the primary oxygen and the anode (work piece), and the aluminum oxide provides an internal oxide film which is of high density and high hardness with its thickness being 0.01-0.05µm. Such oxide film is not easy to be permeated by eletrolyte so to be called as a barrier. The outer oxide which contacts to the electrolyte consists with aluminum oxide and monohydrate aluminum oxide. Such oxide film has large amount of pore, and make electrolyte easily permeate their between. So, upon energizing, it is continuous to increase the amount of micropore and deepen and thicken the micropore, so as called film-forming. From reaction (5) and (6), because it contacts to and permeates the electrolyte, the aforementioned layer of aluminum oxide and monohydrate aluminum oxide contains to dissolve and reduce its thickness, so as called film-dissolving. Therefore, there are both film-forming and film-dissolving during the anodic oxidation of an aluminum and its alloy. So, it is necessary to control the formulation of the anodic oxidation solution and operation condition properly so that the rate of the film-forming is higher than that of the film-dissolving, and the oxide film with microporosity thus can be obtained on the surface of the anode (work piece). Such coating layer obtained by using conventional process shows a topography of capillary porous structure, in which components distributes uniformly in the layer and the binding between layer and substrate is high. The chemical components of the film general are Al₂O₃ and Al₂O₃·H₂O in the amount of 95.0-99.0, and other impurity in the amount of 1.0-5.0%. Such conventional oxidation technique has following shortcomings:

• 1. The porosity is high in layer. The abrasion resistance and hardness are low.
• 2. The layer is in dull coloring, has no ceramic property and texture and in poor decorative effect.

The object of the invention is to overcome the above defects in the prior art, and provide a process for surface ceramic-coating on metal substrate and a product thereof.

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

The present invention is a process for electrochemical anodic oxidation on a metal substrate as anode through plasma are discharging and sintering the electrolyte on the surface of the substrate so as to form a ceramic structure layer.

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 following anodic oxidation reaction on the surface of the workpiece.

        Al = Al³⁺ + 3e



        OH^- = 1/2H₂O + 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 these place where there are defects, crevices, thin layers. The energy density is very high at these 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 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 following steps:

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 clean 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 clean liquid must be forced to be stirred or to be sprayed to the surface of 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.

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.

(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. 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.

(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,

(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 as same as 〈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.

(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 as same as 〈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. The decorative film with various dots patterns or 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 in light pink to dark pink.

(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 as same as 〈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.

(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 as same as 〈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.

(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 as same as 〈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.

(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 as same as 〈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 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.

NaOH

1-5 g/l,

(NaPO₃)₆

10-30 g/l,

Na₂SiO₃

1-20 g/l,

KMnO₄

1-10 g/l,

Na₂WO₄

1-5 g/l,

The method for combining is as same as 〈1〉. pH=5-11, the operation current density i=0.5-10A/dm², V=100-250V, the oxidizing time is 5-20min. Using stirring manners described in 〈1〉, a film in light to dark milk-coffee-colored can be obtained. The thickness is 5-25 µm.

(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 as same as 〈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 show in the following examples.

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.

After rinsing, the workpiece can be sealed by using the process of dip coating, pour coating, spray coating and etc. to improve the luster. Paints of water-soluble acrylic acid resin or water-soluble amino resin, and etc. can be used, and according to the requirements of the paint, be baked at 150-250°C for 5-30min in the case when the 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, and there are no oxide hydrate 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 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 details by the following examples.

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 lad 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 was 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 was judgement the time of spraying sands was 300 second. CASS test: class 9.

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.

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 was put into the cleaning bath, dipped for 25 min. After raised by a shop traveler, the sections were dropped water freely, then was 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 was 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 was 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 was judgement the time of spraying sands was 300-500 second.

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.

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.

The above examples are given only as examples, they can not limit the present invention in any extend. Various modification and improvement may be made by persons skilled in the art without departing from the scope of the invention.