RU2816187C1 - Method of producing decorative coatings on articles from alloys of valve metals - Google Patents

Abstract

FIELD: various technological processes.

SUBSTANCE: method of producing decorative coatings on articles from alloys of valve metals relates to surface treatment and can be used in machine building and related industries. Method is promising for processing of products from alloys of aluminium, magnesium, titanium and other metals of valve group used in harsh climatic conditions, including at high temperature difference and high ambient humidity. According to the proposed method, first, articles from alloys of valve metals are subjected to jet-abrasive processing, which provides surface roughness of Ra 2.5–5.0, then treated with microarc oxidation in a solution of potassium hydroxide 8–10 g/l or caustic soda 7–9 g/l, having temperature of 35–45 °C, at AC density of 12–40 A/dm² for 4–8 minutes, then articles are heated to 330–1,000 °C with holding for 2.5–5 minutes, wherein heating temperature should be at least 15–20 °C is lower than the melting point of the alloy of the article, then, on the surface of the products, a paint material with the required decorative characteristics is applied in two steps — first, 20–25% of the paint material is applied at viscosity of 40–70 cSt, then 80–75% of paint material is applied at viscosity of 80–110 cSt, and then final drying is carried out.

EFFECT: jet-abrasive processing together with micro-arc oxidation and subsequent heating with observance of the above parameters provides a rational combination of roughness of surfaces of articles and porosity of oxide coatings formed on them; this, in turn, enables to achieve high adhesion of paint coatings to surfaces of articles, as well as high durability of paint coatings and the articles themselves.

1 cl, 1 dwg

Description

The invention relates to the field of surface treatment of products made of valve metal alloys, is intended to improve the decorative characteristics and increase the durability of these products and can be used in mechanical engineering and related industries.

Valve metals include aluminum, titanium, magnesium and other metals on which dense oxide films with unipolar conductivity are formed. Anodization and microarc oxidation methods make it possible to increase the thickness of these films by orders of magnitude and convert them into functional oxide coatings, which can also be obtained on valve metal alloys.

The adhesion of paints and varnishes to the surfaces of products made of valve metal alloys coated with dense natural oxide films is very low. Therefore, paint and varnish coatings applied to products made of valve metal alloys without special preliminary preparation are characterized by low durability and after a short time are destroyed and peeled off [1, 2]. However, despite these disadvantages, due to the widest range of colors, high degree of uniformity, low cost, ease of application and the possibility of restoration, many manufacturers and consumers give their preference to products with paint coatings, including products made from valve metal alloys.

To increase the adhesion of paints and varnishes, the surfaces of products made from valve metal alloys are first subjected to degreasing, etching, chroming, phosphating, anodizing and priming. But in most cases, even this does not ensure the necessary durability of paint and varnish coatings, especially on products operated in harsh climatic conditions [1, 2].

In connection with the above, the development of methods for producing durable paint and varnish coatings with high decorative characteristics and a wide range of colors on the surfaces of products made of aluminum alloys and other valve metals is of great interest for industrial practice.

From sources of patent information, a method for producing decorative paint and varnish coatings on products made of aluminum and its alloys is known [Patent RU 2061012. Method of producing decorative coatings on products made of aluminum and its alloys / Arkusha V.T., Mostovaya T.A., Shakhov A.N. . - 05/27/1996]. According to this method, first the surfaces of the products are degreased, and then a paint and varnish material of a certain composition is applied to them in one layer and dried for 10-15 minutes. However, this method does not make it possible to obtain paint coatings with a wide range of colors and cannot ensure their high adhesion and durability for products operated in wide temperature ranges and conditions of high humidity.

Also, from sources of patent information, a method for producing composite coatings on valve metal alloys is known [2527110. Method for producing composite coatings on valve metal alloys / Malyshev V.N., Volkhin A.M., Gantimirov B.M. - 08/27/2014]. According to this method, first, products made from valve metal alloys are subjected to microarc oxidation at a current density of 0.5-30 A/dm2 and a ratio of current values in the anodic and cathodic half-cycles of 1.1-1.2, and then the formed ceramic-like coatings are filled with ultra-high molecular weight polyethylene followed by melting of its surface layer and cooling. This method improves the tribological properties of products made of valve metals, but is not suitable for increasing their decorative properties, especially when providing a wide range of colors.

In addition, from sources of patent information, a method for producing heat-resistant insulating coatings on products made of aluminum alloys is known [Patent 2237758. Method for producing heat-resistant insulating coatings on products made from aluminum alloys / Novikov A.N., Kolomeichenko A.V., Pronin V.V. - 10.10.2004]. According to this method, products made of aluminum alloys are first subjected to microarc oxidation in a solution of caustic potassium 2-6 g/l and liquid glass 10-30 g/l to obtain a coating with a thickness of at least 100 microns, then heat treated for 60-90 minutes at 200-250°C, then impregnated in a fluoroplastic suspension and further dried at a temperature of 100-150°C. This method provides heat-resistant coatings with high electrical insulating properties, however, the implementation of this method requires a lot of time and energy and does not allow obtaining coatings with high decorative characteristics and a wide range of colors.

The closest in technical essence to the proposed method is the method previously proposed by the authors of the proposed invention for producing decorative coatings on products made from valve metal alloys [Patent RU 2803794. Method for producing decorative coatings on products from valve metal alloys / Chufistov O.E., Malyshev V.N. , Zolkin A.N., Chufistov E.A. -09/19/2023]. According to this method, products made from valve metal alloys are first subjected to micro-arc oxidation in a solution of caustic potassium 8-10 g/l or caustic soda 7-9 g/l at a temperature of 35-45°C and at an alternating current density of 10-35 A/dm2 for 5-15 minutes, subsequent heating to 330-1000°C with holding time for 3-10 minutes and further application of polymer paint and varnish material with a viscosity of 70-110 cSt. This method ensures the production of high-quality decorative coatings, but its implementation also requires significant time and energy costs.

The objective of the present invention is to develop a method for producing paint and varnish coatings on products made of valve metal alloys, characterized by high decorative properties, a wide range of colors, increased adhesion to product surfaces and durability, including in harsh climatic conditions, but with lower time and energy costs.

The technical result of solving the problem is manifested in a reduction in time and energy costs by no less than 10%, provided that high decorative properties and a wide range of colors of paint and varnish coatings are maintained, capable of operating in the temperature range - from -50 to +70 ° C and conditions of high humidity - up to 100% without destruction and peeling for a period of at least 12 months.

The problem is solved in a method for producing decorative coatings on products made of valve metal alloys, including micro-arc oxidation, subsequent heating and application of a coating paint layer, and, before micro-arc oxidation, the surfaces of the products are subjected to jet-abrasive treatment, ensuring a surface roughness of Ra 2.5-5, 0, then the products are treated with microarc oxidation in a solution of caustic potassium 8-10 g/l or caustic soda 7-9 g/l, having a temperature of 35-45 ° C, at an alternating current density of 12-40 A/dm2 for 4-8 min, then the products are heated to 330-1000°C with a holding time of 2.5-5 minutes, while the heating temperature should be at least 15-20°C below the melting point of the product alloy, then paint and varnish material with the required decorative characteristics in two steps - first, 20-25% of the paint and varnish material is applied with a viscosity of 40-70 cSt, then 80-75% of the paint and varnish material is applied with a viscosity of 80-110 cSt, and then it is finally dried.

The method is implemented as follows. First, the surfaces of products made from valve metal alloys are subjected to jet-abrasive processing, ensuring a surface roughness of Ra 2.5-5.0. To ensure such roughness, it is possible to carry out jet-abrasive processing with cooper slag powder with a main fraction size of 0.1-0.2 mm or with a powder with a main fraction size of 0.1-0.63 mm, which is more accessible. The air pressure at the nozzle outlet should be 0.6-0.7 MPa, the nozzle diameter should be 8-10 mm, and the treatment duration should be 4-6 seconds per 1 dm2 of product surface.

At the end of the jet-abrasive treatment, the surfaces of the products are blown with compressed air and washed with water until particles of cooper slag and substances are completely removed from the environment to prevent them from entering the solution for micro-arc oxidation.

Next, the surfaces of the products are subjected to microarc oxidation in an aqueous solution of caustic potassium 8-10 g/l or caustic soda 7-9 g/l in alternating current mode for 4-8 minutes. In this case, the average current density on the oxidized surfaces and the solution temperature are maintained in the ranges of 12-40 A/dm2 and 35-45°C, respectively. High solution temperatures are achieved by limiting its volume and (or) the intensity of its cooling.

Upon completion of micro-arc oxidation, the products are washed and immediately placed in a chamber of a thermal cabinet or oven, heated to 330-1000°C, and kept for 2.5-5 minutes. In this case, the temperature in the chamber must be at least 15-20°C lower than the melting point of the alloy of the product to exclude the possibility of its melting at the boundary with the oxide coating formed by microarc oxidation.

Then, if necessary, smooth out the tops of the protrusions of micro-irregularities of the surfaces, treating them with an abrasive elastic bar or an abrasive sponge with a fine grain of abrasive. In this case, it is enough to make two or three working strokes on each surface area in two directions perpendicular to each other at an average contact pressure of 500-1000 Pa. Upon completion of smoothing the tops of the protrusions of micro-irregularities, the product is put on shoes with compressed air.

Next, paint and varnish material based on alkyd, melamine-alkyd or pentaphthalic is applied to the surface of the products so that the average thickness of the resulting paint coating is 40-60 microns. In this case, paint and varnish material is applied to the surfaces of products at different viscosities in two stages - first 20-25% at a viscosity of 40-70 cSt, and then 80-75% at a viscosity of 80-110 cSt. To ensure the required viscosity values, the paint and varnish material is first divided into two volumes and diluted with a substance recommended by the manufacturer.

Then the paint and varnish material on the products is finally dried in compliance with the drying regime recommended by the manufacturer.

Jet-abrasive processing, which creates more developed surfaces with a roughness of Ra 2.5-5.0 in products made of valve metal alloys, further helps to increase the contact area and adhesion force of oxide coatings formed by microarc oxidation and paint and varnish coatings applied to them. With a lower roughness, the necessary increase in the contact area and adhesion force of oxide and paint and varnish coatings is not achieved, and with a higher roughness, the micro-irregularities become too large to be completely leveled by the subsequent application of a paint and varnish coating, the thickness of which, in order to avoid cracking during operation, should be 40- 60 microns.

Microarc oxidation in solutions of caustic potassium 8-10 g/l or caustic soda 7-9 g/l without adding liquid glass ensures the formation of oxide coatings with higher porosity of surface layers than coatings formed in solutions with a lower content of these caustic alkalis and significant content of liquid glass. A lower content of caustic alkalis does not ensure the achievement of the required porosity of coatings, and a higher content of alkalis contributes to too intense dissolution of coatings.

It is important to note that the above concentration of caustic alkalis for microarc oxidation is increased. This ensures an increase in the conductivity of solutions and a decrease in the energy intensity of microarc oxidation with a reduction in its cost by 2-6%.

A solution temperature of 35-45°C also enhances the dissolving effect of the solution on the coatings being formed, as a result of which the porosity of their surface layers increases and the degree of hydration increases with the formation of mono- and trihydroxides of valve metals. At temperatures below 35°C, denser oxide coatings are formed with a lower content of mono- and trihydroxides in the outer layers, and at temperatures above 45°C, coatings with relatively weak cohesion are obtained. At the same time, at such high temperatures, solutions noticeably evaporate, which leads to a deterioration in working conditions.

The duration of microarc oxidation of 4-8 minutes ensures the formation of coatings of relatively small thickness, but with porous and hydrated surface layers. With a shorter duration of micro-arc oxidation, coatings that are too thin and low-porous are obtained, and with a longer duration, the cost of micro-arc oxidation increases, and coatings with greater thickness and roughness are formed, which further increases the overall roughness of product surfaces, which is very difficult to level out by applying a subsequent paint coating.

Microarc oxidation with alternating current provides higher cohesion of coatings; with direct current, coatings are often formed with very weak coupling of particles to each other and the possibility of their chipping from the surface layers of the coatings. The current density on the oxidized surfaces is 12-40 A/dm2, taking into account the short duration of microarc oxidation treatment, provides a rational combination of the thickness of the coatings, the porosity of their surface layers, the roughness of their surfaces and the cost of microarc oxidation. With a longer duration of microarc oxidation, such a high current density could lead to the occurrence of arc breakdowns with local destruction of oxide coatings throughout the entire thickness, but with a duration of 4-8, the occurrence of arc breakdowns is practically excluded. At a lower current density, less thick, but more dense, smooth and even oxide coatings are formed, with which paint coatings do not provide sufficient adhesion strength. And with a higher current density, the energy intensity of microarc oxidation increases and arc breakdowns can also occur, leading to destruction of coatings that are difficult to level with a paint layer. After its application, noticeable funnel-shaped depressions form in places of such destruction.

Loading washed samples into a chamber of a thermal cabinet or oven, heated to 330-1000°C, ensures not only effective drying of the surfaces of oxide coatings from traces of washing water, but also promotes the loss of adsorbed moisture by the coatings with the transition of mono- and trihydroxides of valve metals into oxides with loss volume and, therefore, with an additional uniform increase in the porosity of the outer surfaces of the oxide coatings. It is important to note that such temperatures ensure rapid simultaneous transitions of trihydroxides to monohydroxides and monohydroxides to valve metal oxides. For example, in coatings formed by microarc oxidation on aluminum alloys, when rapidly heated to 480-500°C, at least three such transitions can occur simultaneously [7]. As a result of this, an accelerated change in the volume of individual structural components of the coatings occurs, leading to the opening of new pores and an additional uniform increase in the porosity of the surface layers of oxide coatings.

The duration of exposure in the chamber of a thermal cabinet or oven of 2.5-5 minutes ensures effective drying of coatings and the passage of transitions of mono- and trihydroxides of valve metals into their oxides with an increase in the porosity of the surface layers of coatings. With a shorter holding time, the transitions of mono- and trihydroxides into oxide do not have time to complete, and increasing the holding time beyond the specified interval leads to an increase in the duration and cost of processing, but does not contribute to further significant changes in the porosity of the surface layers of oxide coatings, since they have a small thickness.

Smoothing the tops of protrusions of microroughnesses of surfaces with oxide coatings is not a mandatory stage of the technology, but in some cases it is desirable to carry out it. It allows you to reduce the height of the largest protrusions on the surfaces of products due to partial removal of the oxide layer on their tops. After jet-abrasive processing of products made from valve metal alloys, protrusions with thin sharp peaks are formed on their surfaces. During the microarc oxidation process, the alloy on these peaks is completely converted to oxide, which is brittle and porous. Therefore, two or three working strokes with an abrasive elastic bar or abrasive sponge along the surfaces of products at a low average contact pressure (500-1000 Pa) in two perpendicular directions is quite sufficient for partial removal and chipping of the coating oxide from the tops of high protrusions. Upon completion of smoothing the tops of the protrusions, the surfaces of the products should be blown with compressed air to remove the separated parts of the oxide coatings.

The application of a paint and varnish material with an increase in its viscosity from 40-70 cSt to 80-110 cSt ensures the production of high-quality paint and varnish coatings with a continuous polymer chain of film on porous surfaces, formed by micro-arc oxidation on the rough surfaces of products, penetrating into the depressions and pores of the surface microrelief. These paint and varnish coatings are characterized by high adhesion to oxide coatings formed by microarc oxidation, as well as high durability and good decorative properties. The initial application of 20-25% of the paint and varnish material on the surface of products at a viscosity of 40-70 cSt ensures its penetration into the cavities of microroughnesses and pores of oxide coatings. And the subsequent application of 80-75% of the paint and varnish material with a viscosity of 80-110 cSt ensures a complete paint and varnish coating that levels out the roughness and porosity of product surfaces. With a lower viscosity, paint and varnish coatings have a reduced thickness, dullness, inherit the microrelief of coatings formed by micro-arc oxidation, due to which they are visible on the protrusions, and over time they begin to crack, but practically without peeling. At higher viscosity, the resulting paint and varnish coatings are characterized by unevenness and sagging, have low adhesion to oxide coatings and, over time, begin to peel off from them.

Compliance with the manufacturer's recommendations when drying paint and varnish material is necessary to guarantee the receipt of a high-quality, uniform paint and varnish coating without visible holes, pits and unacceptably large pores.

The method is illustrated by figure 1, which shows a fragment of the surface of product 1 after jet-abrasive processing (Fig. 1, a), after micro-arc oxidation with the formation of an oxide coating 2 and its heating (Fig. 1, b), after smoothing out the protrusions of irregularities (Fig. 1, c) and after applying paint coating 3 (Fig. 1, d).

Example 1. Body parts of locomotive speedometers made of aluminum alloys AMg3, D16, AK9 and AK12 were subjected to painting with alkyd and melamine-alkyd enamels under production conditions for several years, before which pre-treatment was carried out - phosphating, chromate plating, hard anodizing, priming. After the final drying of the enamels, laboratory tests of their adhesion to product surfaces were carried out using the lattice incision method. Then the parts were kept in a climate chamber for 90 days, including 30 days at a temperature of +70°C and a humidity of 60-98%, 30 days at a temperature of -50°C and a humidity of 50-70% and 30 days at a temperature of 25°C and humidity 95-98%. In all cases, after testing using the lattice incision method and subsequent exposure to a climate chamber, cracking and peeling of the enamels was observed.

After processing the same parts according to the method, which is the closest analogue (prototype) of the proposed method with microarc oxidation in solutions of caustic potassium 9 g/l and caustic soda 7 g/l for 9-11 minutes at a current density of 20-30 A/dm2 and solution temperatures of 35-40°C, with loading into the oven chamber heated to 485-500°C with exposure for 5-7 minutes, with spray painting with alkyd and melamine-alkyd enamels with a viscosity of 80-90 cSt, and drying according to According to the manufacturers' recommendations, no cracking or peeling of enamels was observed at all after laboratory tests using the lattice incision method and subsequent exposure in a climate chamber. Moreover, cracking and peeling of the enamels were not observed even after operating the speedometers for 12 months in different climatic conditions in the temperature and humidity ranges from -45 to +50°C and from 25 to 100%, respectively.

After processing exactly the same parts in full accordance with the proposed method with jet-abrasive treatment with cooper slag powder with a main fraction size of 0.1-0.63 mm at a pressure in the air stream of 0.7 MPa for 0.5 minutes, followed by micro-arc oxidation in solutions of caustic potassium 9 g/l and caustic soda 7 g/l for 5 minutes at a current density of 30 A/dm2 and solution temperatures of 35-40°C, heated to 480-490°C and held for 4 minutes, with painting from a spray gun with alkyd and melamine-alkyd enamels in two steps with an increase in viscosity from 40-70 to 80-110 cSt and drying according to the manufacturers' recommendations, no cracking or peeling of enamels after laboratory tests using the lattice cuts method and subsequent exposure in a climate chamber, as well as after No operation of the speedometers was observed for 12 months in different climatic conditions.

Example 2. Three experimental samples in the form of disks with a diameter of 35 mm and a thickness of 5 mm from technical titanium VT1-0 were subjected to various pre-treatments and subsequent painting with pentaphthalic-based enamel. The first disk, in full accordance with the proposed method, was subjected to jet-abrasive treatment with cooper slag powder with a main fraction size of 0.1-0.63 mm at a pressure in the air stream of 0.7 MPa for 0.1 minute (3 seconds per side), then micro-arc oxidation in a solution of caustic potassium 9 g/l at a current density of 30 A/dm2 and a solution temperature of 40°C, heating to 1000°C with exposure for 3 minutes, painting from a spray gun with melamine alkyd enamel in two steps with an increase in viscosity from 50- 52 to 88-90 cSt. The second and third discs were treated in a similar manner, but the second disc was not blasted and the third disc was not subjected to microarc oxidation and subsequent heating.

Next, laboratory tests were carried out using the lattice incision method, in which no enamel peeling was observed on the first and second disks, but on the third disk there was a slight peeling of the enamel along the edges of the incisions. Then the disks were kept in a climate chamber for 90 days, including 30 days at a temperature of +70°C and a humidity of 60-98%, 30 days at a temperature of -50°C and a humidity of 50-70% and 30 days at a temperature of +25°C and humidity 95-98%. After this, no enamel damage was observed in the first disc, slight enamel peeling along the edges of the incisions appeared in the second disc, and enamel peeling in the third disc became significant. After testing the first and second discs outdoors for 12 months, during which the temperature and humidity ranged from -27 to +36 ° C and from 29 to 92%, respectively, the second disc showed significant cracking and peeling of the enamel , no cracking or enamel peeling was observed in the first disc.

When implementing the proposed method in production practice, it was possible to reduce time and energy costs relative to the prototype by 30-35 and 15-20%, respectively, while maintaining the durability and decorative properties of paint and varnish coatings.

Thus, it can be argued that the proposed method successfully solves all the tasks and provides the possibility of obtaining high-quality decorative coatings on products made of valve metal alloys, and without prior priming the surfaces of the products.

Information sources

1. Mattsson E. Electrochemical corrosion. - M.: Metallurgy, 1991. - 156 p.

2. Denker I.I., Goldberg M.M. Protection of products made of aluminum and its alloys with paint and varnish coatings. - M.: Chemistry, 1975. - 176 p.

3. Patent RU 2061012. Method for producing decorative coating on products made of aluminum and its alloys / Arkusha V.T., Mostovaya T.A., Shakhov A.N. - 05/27/1996.

4. Patent RU 2527110. Method for producing composite coatings on valve metal alloys / Malyshev V.N., Volkhin A.M., Gantimirov B.M. - 08/27/2014.

5. Patent RU 2237758. Method for producing heat-resistant insulating coatings on products made of aluminum alloys / Novikov A.N., Kolomeichenko A.V., Pronin V.V. - 10.10.2004.

6. Patent RU 2803794. Method for producing decorative coatings on products made of valve metal alloys / Chufistov O.E., Malyshev V.N., Zolkin A.N., Chufistov E.A. -09.19.2023.

7. Tchufistov O.E., Tchufistov E.A. Effect study of MAO-coatings heat treatment on their structure, phase composition, physical and mechanical properties. Journal of Physics: Conference Series. 1399 (2019) 055097.

Claims (1)

A method for producing decorative coatings on products made from valve metal alloys, including micro-arc oxidation, subsequent heating and application of a coating paint layer, characterized in that before micro-arc oxidation, the surfaces of the products are subjected to jet-abrasive treatment, ensuring a surface roughness of Ra 2.5-5.0, then the products are treated with microarc oxidation in a solution of caustic potassium 8-10 g/l or caustic soda 7-9 g/l, having a temperature of 35-45 ° C, at an alternating current density of 12-40 A/dm ² for 4-8 minutes , then the products are heated to 330-1000 °C with a holding time of 2.5-5 minutes, while the heating temperature should be at least 15-20 °C below the melting point of the product alloy, then paint and varnish material with the necessary decorative elements is applied to the surface of the products characteristics in two steps - first, 20-25% of the paint and varnish material is applied with a viscosity of 40-70 cSt, then 80-75% of the paint and varnish material is applied with a viscosity of 80-110 cSt, and then it is finally dried.