RU2392359C1 - Method of coating obtainment - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves oxidation for 60-90 minutes at current density of 25- 35 A/dm², during which oxygen of 5-15°C is supplied to part detail through sprayer contacting cathode at the flow rate of 0.1-1.0 m³/minute per square metre of oxidised surface, and detail performs advancing and rotating movement at 10-30 mm distance from the sprayer. Before oxidation, electrolytes are heated to 70-80°C, stirred and cooled down to 15-20°C, and oxidation is performed first for 10 minutes in electrolyte containing 6-8 g/l of caustic potash and 40-50 g/l of boric acid, then for 15-20 minutes in electrolyte containing 6-8 g/l of caustic potash, 40-50 g/l of boric acid, 40-60 g/l of fine-dispersed corundum of 3-5 mcm fraction and 1-2 g/l of chrome oxide, then for 25-35 minutes in electrolyte containing 6-8 g/l of caustic potash, 40-50 g/l of boric acid, 4-60 g/l of fine-dispersed corundum of 5-7 mcm fraction and 1-2 g/l of chrome oxide, and further in electrolyte containing 6-8 g/l of caustic potash, 40-50 g/l of boric acid, 40-60 g/l of fine-dispersed corundum of 7-10 mcm fraction and 1-2 g/l of chrome oxide.

EFFECT: reduced power, enhanced productivity and increased thickness, hardness, breakdown voltage and electric resistance of coatings formed.

2 tbl, 2 dwg, 1 ex

Description

The invention relates to the field of surface treatment of parts, in particular to microarc oxidation of parts made of aluminum alloys, and can be used in mechanical engineering, instrumentation and other industries.

From the sources of patent information, there are known methods for producing coatings on aluminum and its alloys, including oxidation in a solution of caustic potassium (3 ... 5 g / l) and boric acid (20 ... 40 g / l) and subsequent heating [Patent RU 2136788. Method for producing coatings . Atroshchenko E.S., Chufistov O.E., Kazantsev I.A., Durnev V.A. - Bull. 08/2002; Patent RU 2166570. A method for producing coatings. Atroshchenko E.S., Chufistov O.E., Kazantsev I.A., Dryazgin A.V., Simtsov V.V. - Bull. 11/2003]. However, the values of thickness, hardness, and breakdown voltage of these coatings do not exceed 200 μm, 235000 MPa, and 3000 V.

Also known is a method of producing coatings on aluminum and its alloys in an electrolyte containing potassium hydroxide (4 ... 6 g / l), boric acid (20 ... 30 g / l) and fine aluminum particles (20 ... 25 g / l) [RU Patent 2291233. Electrolyte of microarc oxidation of aluminum and its alloys. Kuznetsov Yu.A., Batishchev A.N., Feryabkov A.V., Kulakov K.V., Tarasov K.V., Sevastyanov A.L. - Bull. 1/2007]. However, the values of thickness, hardness and breakdown voltage of coatings formed by this method are lower than coatings formed by the method protected by patents RU 2136788, 2166570.

The closest in technical essence to the proposed method is a method for producing coatings on parts made of aluminum alloys, including oxidation in a solution of caustic potassium (3 ... 5 g / l) and boric acid (20 ... 40 g / l) with an additional supply of oxygen to the oxidizable surface parts performing translational and rotational movements [Patent RU 2339745. A method for producing coatings. Chufistov O.E., Demin S.B., Chufistov E.A. - Bull. No. 33 dated November 27, 2008 (prototype)]. However, this method is more complex and gives a relatively small advantage in increasing the productivity of oxidation, increasing the thickness and breakdown voltage of the coatings.

The objective of the invention is to reduce the power spent on oxidation, increase its productivity, as well as a comprehensive increase in the physico-mechanical properties of the formed coatings.

The technical result of solving this problem is manifested in a decrease in the power spent on oxidation by 5 ... 7%, an increase in its productivity by 10% or more, an increase in the thickness of formed coatings by 15 ... 20%, hardness by 5 ... 8%, and breakdown voltage by 16 ... 22%, electrical resistance by 160 ... 240%.

The problem is solved in a method for producing coatings on parts of aluminum alloys, including oxidation in an aqueous solution of an electrolyte based on caustic potassium and boric acid for 60 ... 90 min at an electric current density of 25 ... 35 A / dm ² , during which the surface of the part under pressure through the spray contacting the cathode, oxygen is supplied at a temperature of 5 ... 15 ° C and a flow rate of 0.1 ... 1.0 ^m3 / min per square meter of oxidizable surface and part contacting to the anode, makes translational and rotational reduction, so that the oxidized surface is at a distance of 10 ... 30 mm from the atomizer, and before oxidation, the electrolyte solutions are heated to 70 ... 80 ° C, stirred and cooled to 15 ... 20 ° C, and the oxidation is carried out in four successive stages: first, during 10 minutes in a solution containing potassium hydroxide (6 ... 8 g / l) and boric acid (40 ... 50 g / l), then for 15 ... 20 minutes in a solution containing potassium hydroxide (6 ... 8 g / l), boric acid (40 ... 50 g / l), finely divided corundum with a size of the main fraction of 3 ... 5 μm (40 ... 60 g / l) and chromium oxide (1 ... 2 g / l), then in 25 ... 35 min in a solution containing potassium hydroxide (6 ... 8 g / l), boric acid (40 ... 50 g / l), finely divided corundum with a main fraction size of 5 ... 7 μm (40 ... 60 g / l) and chromium oxide (1 ... 2 g / l), then in a solution containing potassium hydroxide (6 ... 8 g / l), boric acid (40 ... 50 g / l), finely divided corundum with a main fraction size of 7 ... 10 μm (40 ... 60 g / l) and chromium oxide (1 ... 2 g / l).

The method is implemented as follows. In four baths, an aqueous electrolyte solution is prepared, the main components of which are potassium hydroxide (6 ... 8 g / l) and boric acid (40 ... 50 g / l). Then, chromium oxide (1 ... 2 g / l) and corundum (40 ... 60 g / l) are added to the second, third and fourth baths in the form of refined fine micropowder (GOST 3647-80), and corundum with the size of the main is added to the second bath fractions of 3 ... 5 microns, corundum with a size of the main fraction of 5 ... 7 microns is added to the third bath, corundum with the size of the main fraction of 7 ... 10 microns is added to the fourth bath.

Further, the electrolyte solutions in all baths are heated by passing an electric current through them to 70 ... 80 ° C, stirred until the boric acid is completely dissolved and cooled to temperatures of 15 ... 20 ° C.

An increased concentration of caustic potassium (6 ... 8 g / l) provides an increase in the conductivity of the solution and, consequently, a decrease in the power spent on oxidation. However, this concentration of potassium hydroxide also contributes to an increase in the intensity of the dissolving action of the solution on the formed coating. To prevent this, a higher concentration of boric acid (40 ... 50 g / l) is needed in the solution, which ensures passivation of the surfaces of parts in the solution. However, if the concentration of boric acid is 40 g / l or higher, it does not completely dissolve in the aqueous solution at temperatures of 15 ... 20 ° C and partially precipitates. During the MAO process, sediment particles can be introduced into the coating structure, which is undesirable, since it can lead to a violation of the phase composition and a decrease in the physicomechanical properties of the coatings. Therefore, the solutions are heated to 70 ... 80 ° C and stirred until the boric acid is completely dissolved, then it is cooled to temperatures of 15 ... 20 ° C, and boric acid does not precipitate for 8 ... 9 hours (work shift).

After preparing the solutions, the aluminum alloy part is fixed on the output link (shaft) of the translational and rotational motion drive, is contacted with the anode and sequentially oxidized in solutions of the first, second, third and fourth baths at a current density of 25 ... 35 A / dm ² . In the process of oxidation, the part performs translational and rotational movements, and oxygen is supplied under pressure at a temperature of 5 ... 15 ° C and a flow rate of 0.1 ... 1.0 m ³ / min through a pre-installed stainless steel atomizers in contact with the cathode per square meter of oxidizable surface. In this case, the part in the bath is always located so that its oxidized surface at any moment of movement is at a distance of 10 ... 30 mm from the nozzles.

Oxidation in the solution of the first bath is carried out for 10 minutes and allows you to get a dense oxide coating with a thickness of up to 60 microns, resulting from the interaction of oxygen released from the electrolyte and aluminum located in the outer layers of the workpiece. An additional supply of oxygen at a potential difference between the atomizer and the part ensures mixing, oxygen saturation, and a decrease in the temperature of the electrolyte near the oxidized surface. At the same time, the rate of formation of a new oxide increases simultaneously, the dissolving effect of the electrolyte on the coating decreases, and the coating thickness increases rapidly.

Subsequent oxidation in the solution of the second bath for 15 ... 20 min can significantly increase the thickness of the coating, which can reach 150 microns, and is carried out with the presence in the solution of corundum particles with a size of the main fraction of 3 ... 5 microns and chromium oxide.

The additional oxygen supply at the potential difference between the atomizer and the component, along with mixing, oxygen saturation and lowering the temperature of the solution near the oxidized surface, also provides directional transport of corundum particles that are polarized in the MAO process under the influence of ionic complexes formed in a solution of caustic potassium and boric acid with the addition of chromium oxide. This leads to the accumulation of polarized particles on the oxidized surface, experiencing the action of numerous microarc discharges.

During breakdown, local coating failure occurs (breakdown crater forms) with the appearance of a microarc discharge [5]. A corundum particle located near the crater falls into the microarc action zone and is included in the coating structure. At the same time, the rate of formation of a new oxide is simultaneously increased, fine particles of corundum are introduced into the growing oxide layer, and the dissolving effect of the electrolyte on the coating is reduced. As a result, the growth rate of the coating thickness increases significantly.

Further oxidation in the third bath makes it possible to obtain coatings up to 200 microns thick and is carried out with the presence of corundum particles in the solution with the size of the main fraction 5 ... 7 microns and chromium oxide. Moreover, due to an increase in the thickness of the coating, the potential difference increases, at which breakdowns of the coating occur, as a result of which microarc discharges become less frequent, but more powerful. Therefore, during breakdown, the size of the breakdown crater simultaneously increases, and the released energy increases [5], ensuring the inclusion of a larger particle with a size of 5 ... 7 microns in the coating structure.

The final oxidation in the fourth bath allows to obtain coatings up to 250 microns thick and is carried out with the presence of corundum in the solution with the size of the main fraction 7 ... 10 microns and chromium oxide. In this case, the size of the craters and the energy released during the discharges become sufficient to include larger particles with a size of 7 ... 10 microns in the coating structure.

The proposed method is illustrated by the schemes shown in FIG. 1, 2. Figure 1 shows the oxidation scheme: through pipeline 1, oxygen under pressure enters the atomizer 2, from which through calibrated holes it is directed to the surface of the oxidized part 3, mounted on the drive shaft of the translational and rotational movements 4.

In FIG. 2 shows a coating formation scheme: at the first stage, an initial thickness of coating 2 is formed on alloy 1 in a solution containing no corundum; In the subsequent 2nd, 3rd, and 4th stages, the final thickness of coating 2 in solutions containing corundum particles 3 is gradually formed on alloy 1.

Compared with the prototype [4], the proposed method simultaneously allows:

- reduce the power spent on oxidation, not less than 5%;

- increase the productivity of oxidation, not less than 10%;

- increase the thickness of the coatings by at least 15%;

- increase the hardness of the coating by at least 5%;

- increase the breakdown voltage of coatings by at least 16%;

- increase the electrical resistance of coatings by at least 160%.

Example. Parts (rods with a diameter of 20 mm) made of AMg3 alloy were divided into six groups - five parts each. Parts of all six groups were oxidized for 75 min at a current density on the surface of the part of 30 A / dm ² and an average electrolyte temperature of 15 ° C. At the same time, the processing of each batch had its own exceptional features, indicated in table 1.

When processing certain groups of parts, there were:

- * preparation of the solution for use, which includes heating the solution to 75 ° C, stirring it until the boric acid is completely dissolved and cooling to 15 ° C;

- * oxygen supply to the surface of the part, carried out at an oxygen temperature of 10 ° C and a flow rate of 0.5 m ³ / min per 1 m ^{2 of} oxidizable surface, located at a distance of 15 ... 20 mm from the atomizer.

Thus, the first group of parts was processed in full accordance with the prototype [4], the sixth group in full accordance with the proposed method, the remaining four groups in partial accordance with the proposed method.

In the process of oxidation of parts of all groups, the average values of the potential difference at the bath terminals were measured while maintaining a constant current density on the surface of the part — about 30 A / dm ² .

Table 1 Part group oxidation conditions Part Group No. Oxidation stages Electrolyte solution * Preparation of the solution for use * Oxygen supply on the surface of the part Stage number Duration min The content of caustic potassium, g / l The content of boric acid, g / l The content of chromium oxide, g / l Corundum content, g / l Corundum particle size, microns one one 75 four thirty - - - - applies 2 one 10 four thirty - - - not applicable applies 2 fifteen four thirty one fifty 3 ... 5 3 thirty four thirty one fifty 5 ... 7 four twenty four thirty one fifty 7 ... 10 3 one 75 7 45 one fifty 5 ... 7 applies applies four one 10 7 45 - - - not applicable applies 2 fifteen 7 45 one fifty 3 ... 5 3 thirty 7 45 one fifty 5 ... 7 four twenty 7 45 one fifty 7 ... 10 5 one 10 7 45 - - - applies not applicable 2 fifteen 7 45 one fifty 3 ... 5 3 thirty 7 45 one fifty 5 ... 7 four twenty 7 45 one fifty 7 ... 10 6 one 10 7 45 - - - applies applies 2 fifteen 7 45 one fifty 3 ... 5 3 thirty 7 45 one fifty 5 ... 7 four twenty 7 45 one fifty 7 ... 10

The measurement results showed that when oxidizing parts of the first and second groups in solutions containing potassium hydroxide (4 g / l) and boric acid (30 g / l), the average potential difference was 545 V. When oxidizing the details of all other groups, including the sixth group, in solutions containing caustic potassium (7 g / l) and boric acid (45 g / l), the average value of the potential difference was 515 V, therefore, in this case, in comparison with the proposed method, the prototype requires power for 6.8% more.

Then, by standard methods [6, 7], the thickness, hardness, breakdown voltage, and electrical resistance of the resulting coatings were measured. The test results presented in table 2 confirm that the proposed method provides higher performance and higher physical and mechanical properties of the coatings.

table 2 Coating Properties Part Group No. Thickness, microns Hardness, GPa Breakdown voltages, V Electric resistance, Ohm one 169.55 17.84 ... 22.44 2383,997 4.73 × 10 ¹⁴ 2 193.02 18.90 ... 23.11 2721,341 7.03 × 10 ¹⁴ 3 192.91 18.72 ... 23.02 2718,192 6.99 × 10 ¹⁴ four 192.74 18.55 ... 22.99 2711,899 6.98 × 10 ¹⁴ 5 185.09 18.17 ... 22.83 2576,454 6.71 × 10 ¹⁴ 6 200.18 19.74 ... 23.81 2826,587 10.59 × 10 ¹⁴

The combination of concentrations of caustic potassium (6 ... 8 g / l) and boric acid (40 ... 50 g / l) is the most rational, since this ensures good conductivity of the solution and a relatively passive state of the oxidized surface, therefore, oxidation requires low power and allows to obtain coatings with high physical and mechanical properties.

With a decrease in the concentration of caustic potassium, the conductivity of the solution decreases, and the power spent on oxidation increases. With an increase in the concentration of caustic potassium, the intensity of the solvent action of the solution on the coating increases, therefore, its physical and mechanical properties decrease. With a decrease in the concentration of boric acid, surface passivation worsens, microarc discharges occur at low breakdown voltages, and the energy released in this case is not enough to incorporate corundum particles into the coating, therefore its physical and mechanical properties are reduced. With an increase in the concentration of boric acid, it dissolves during heating, but during cooling it precipitates and can be included in the coating, disrupting its phase composition and reducing the physical and mechanical properties.

During the first stage of oxidation in a solution containing corundum, the growth of the coating slows down, since the energy released during breakdowns of a thin coating is not sufficient to include corundum particles in its structure, but these particles worsen the interaction conditions of the electrolyte of the oxidized surface.

During the second, third and fourth stages of oxidation in solutions containing corundum particles, the average size of which differs from 3 ... 5, 5 ... 7 and 7 ... 10 microns, respectively, the growth of the coating slows down. Particles of large sizes are poorly embedded in the structure, and particles of smaller sizes are well embedded in the structure of the coating, but because of their small volume, the amount of corundum included in the coating is reduced.

The concentration of chromium oxide 1 ... 2 g / l also provides the best combination of oxidation performance and physico-mechanical properties of coatings. At a higher concentration of chromium oxide, the oxidation performance does not increase, but the hardness and electrical resistance of the coatings decrease, since substances containing chromium appear in them. At a lower concentration of chromium oxide, the hardness and electrical resistance do not decrease, but the oxidation performance decreases (not less than 3 ... 4%).

With a duration of oxidation of more than 90 minutes, the porosity of the coating increases and its physical and mechanical properties decrease. When the oxidation duration is less than 60 minutes, there is not enough time for the full implementation of all four stages of oxidation.

Information sources

1. Patent RU 2136788. A method for producing coatings. Atroshchenko E.S., Chufistov O.E., Kazantsev I.A., Durnev V.A. - Bull. 08/2002.

2. Patent RU 2166570. A method for producing coatings. Atroshchenko E.S., Chufistov O.E., Kazantsev I.A., Dryazgin A.V., Simtsov V.V. - Bull. 11/2003.

3. Patent RU 2291233. Electrolyte of microarc oxidation of aluminum and its alloys. Kuznetsov Yu.A., Batishchev A.N., Feryabkov A.V., Kulakov K.V., Tarasov K.V., Sevostyanov A.L. - Bull. 1/2007.

4. Patent RU 2339745. A method for producing coatings. Chufistov O.E., Dyomin S.B., Chufistov E.A. - Bull. No. 33 dated November 27, 2008 (prototype).

5. Chernenko V.I., Snezhko L.A., Papanova I.I. Obtaining coatings by anodic-spark electrolysis. - L .: Chemistry, 1991 .-- 126 p.

6. Testing equipment: a reference book in 2 volumes / Ed. Klyueva V.V. - M.: Mechanical Engineering, 1982. - 528 p.

7. GOST 9450-76. Microhardness measurement by indentation of diamond tips.

Claims (1)

A method of producing coatings on parts made of aluminum alloys, including oxidation in an aqueous solution of an electrolyte based on caustic potassium and boric acid for 60-90 min at an electric current density of 25-35 A / dm ² , during which the surface of the part is pressurized through a spray in contact with the cathode, oxygen is supplied with a temperature of 5-15 ° C and a flow rate of 0.1-1.0 m ³ / min per square meter of oxidized surface, and the part in contact with the anode performs translational and rotational movements so that the oxidizable turn the value was at a distance of 10-30 mm from the atomizer, characterized in that before oxidation, the electrolyte solutions are heated to 70-80 ° C, stirred and cooled to 15-20 ° C, and the oxidation is carried out in four successive stages, first for 10 min in a solution containing potassium hydroxide 6-8 g / l and boric acid 40-50 g / l, then for 15-20 minutes in a solution containing potassium hydroxide 6-8 g / l, boric acid 40-50 g / l, finely divided corundum with the size of the main fraction 3-5 microns 40-60 g / l and chromium oxide 1-2 g / l, then for 25-35 minutes in solution, containing I eat potassium hydroxide 6-8 g / l, boric acid 40-50 g / l, finely divided corundum with a main fraction size of 5-7 microns 4-60 g / l and chromium oxide 1-2 g / l, and then in solution, containing potassium hydroxide 6-8 g / l, boric acid 40-50 g / l, finely divided corundum with a main fraction size of 7-10 μm 40-60 g / l and chromium oxide 1-2 g / l.

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