RU2737838C1 - Method of producing protective composite coating on steel part - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to chemical surface treatment. Method involves successive application on surface of part of two layers of coating, each of which is formed by depositing aluminum powder in solution of inorganic binder by dyeing with subsequent thermo hardening of applied coating layer and machining of part with applied coating layer. Suspension of aluminum powder is used in solution of inorganic binder, prepared from following components, wt%: aluminum hydroxide 1.2-1.6, solution of 85% orthophosphoric acid 4.7-5.5, magnesium oxide 0.12-0.16, potassium permanganate 0.25-0.33, aluminum powder 45.0-55.0, sodium silicate or potassium silicate 0.25-0.37, distilled water - balance. Inorganic binder is distilled water to specific density of 1.20-1.30 g/cm³. Thermosetting is followed by additional treatment with corundum powder at pressure of 2 to 3 bar.

EFFECT: high protective capacity of the obtained coating from corrosion with simultaneous reduction of the thickness of the coating, hardening temperature of up to 200°C, simple process of applying protective coating, as well as avoiding use of substances of 1st class of hazard.

1 cl, 2 dwg, 2 tbl

Description

The invention relates to the field of chemical surface treatment and can be used to protect against corrosion of parts made of low-alloy carbon steels, operating in all climatic conditions and at temperatures up to 500 ° C.

There is a known method of obtaining protective coatings on a steel part, including the application of a suspension containing aluminum powder as a filler and phosphates and chromates as a binder. The annealing of the coating is carried out at a temperature above 250 ° C, followed by treatment of the coating with a 20-58% solution of sodium, potassium or lithium dihydrogen phosphate, and water or orthophosphoric acid is used as a solvent with its content not exceeding 90 wt. % (SU 1560621 A1, publ. 30.04.1990).

The disadvantage of this method is the impossibility of obtaining a corrosion-resistant coating without additional treatment of the coating with a solution of sodium, potassium or lithium dihydrogen phosphate, which implies the application of a protective coating from two different solutions, which complicates the technological process.

A known method of producing a protective coating based on an inorganic phosphate binder (US 2010 / 0288158A1, publ. 11/18/2010). The binder contains 85% phosphoric acid 332 ml, boron oxide 41.6 and magnesium carbonate 225 g. Aluminum powder and an alkaline corrosion inhibitor based on strontium chromate are used as fillers.

The suspension is applied in one or two layers. The thickness of each layer is about 20 microns. After application, the coating is dried for 15 to 30 minutes at 180 ° F (82 ° C) and then cured at 650 ° F (343 ° C) for 30-60 minutes.

To ensure the anodic nature of the protection of the sprayed coating after thermal curing, mechanical polishing with glass beads or sandblasting at low pressure is carried out, which increases the protective ability of the resulting coating.

The disadvantage of this method is the use of hexavalent chromium compounds as a corrosion inhibitor, in an amount from 2 to 15% of the total mass of the solution, which are carcinogenic substances of the first hazard class, which significantly worsens working conditions when applying protective coatings.

Known heat-protective coatings, consisting of inorganic binders based on phosphates, silicates or their mixtures, additionally containing ceramic particles based on zirconium dioxide and yttrium oxide (US 2013156958 A1, publ. 20.06.2013).

A feature of the structure of this heat-protective coating is the formation of a bimodal structure of alternating ceramic particles with a dispersion of 10-60 microns and fine particles from 0.1 to 5 microns. This structure can significantly reduce the thermal conductivity and porosity of the coating.

The coating is applied from an aqueous suspension by pneumatic spraying, followed by heat treatment.

The slurry is preferably applied in two coats or layers, each about 0.001 inches (25 microns) thick, then dried at 180 ° F (82 ° C) for 15 to 30 minutes and cured at 650 ° F (343 ° C) within 30-60 minutes after each layer.

The disadvantages of this method is the use of expensive fillers, as well as the absence in the composition of a metal filler that provides electrochemical protection of steels, which significantly reduces the protective ability of such coatings.

A known method of applying a two-layer protective coating containing inorganic silicates in the coating. (CA 2979238 A1, publ. 09/22/2016). The first layer is applied from an aqueous solution of potassium silicate, additionally doped with lithium silicate. Aluminum powder with a particle diameter of 4 to 15 microns is used as a filler. The solution for applying the second layer of the protective coating is a suspension of pigments based on Cr ₂ O ₃ and TiO ₂ oxides with a dispersion of 2.5-3.0 μm in a solution of aluminum dihydrogen phosphate A1 (H ₂ P0 ₄ ) with a molar ratio of components Al / PO ₄ ^3- from 1 / 1.24 to 1/3.

After application of each of the layers, dry for 15 minutes at 175 ° F (80 ° C) and bake in an air oven. The first layer, based on silicates, forms at 650 ° F (343 ° C). The second layer is heat treated at 1000 ° F (537 ° C) for 60 minutes. Dual coat thicknesses range from 1.4 to 1.7 mils (300 to 380 microns).

The disadvantages of this method are:

- application of a protective coating from two different solutions, which requires reconfiguring the equipment in the process of spraying the compositions;

- elevated heat treatment temperature (537 ° C), at which the final curing of the coating occurs, which requires the use of drying cabinets or ovens with chamber dimensions corresponding to the dimensions of the parts, and limits the scope of this coating for the protection of parts made of maraging steels, such as VKS-170 having a tempering temperature of 520 ° C.

- to ensure the protective ability of parts made of low-alloy steels under conditions of accelerated corrosion tests in a salt fog chamber for 2500 hours, it is necessary to use coatings with a thickness of 300 to 380 microns.

The closest analogue of the proposed invention is a method for producing a protective coating, proposed in US patent 6074464 A, publ. 13.06.2000.

The method of obtaining a protective coating includes applying a suspension of aluminum powder in a binder solution by painting to the surface to be protected, thermal curing of the coated part, and mechanical treatment of each coating layer. The suspension is made from the following components: 85% phosphoric acid 338 g, potassium permanganate 25 g and aluminum hydroxide (in a gel state) 117 g. Aluminum powder is used as a filler in a binder / filler ratio = 1 / 0.6.

Depending on the purpose and mode of operation, carbides can be added to the coating composition, which increase the wear resistance of the coating, sulfides, which help to reduce the coefficient of friction, color pigments, as well as pigments based on strontium chromate, which slow down corrosion processes during the operation of the part.

To improve surface wettability, ionic or non-ionic surfactants are included in the coating composition.

To reduce porosity, it is recommended to apply the coating in two layers. The first layer is applied from a composition containing an inorganic binder with an aluminum filler in a weight percentage of binder / filler = 60/40. The second layer is applied from a composition based on an inorganic binder with the addition of a surfactant and pigments that improve the corrosion resistance of the coating.

The composition is applied by dyeing, drying and curing at 650 ° F (343 ° C).

To ensure the sacrificial nature of the protection of the sprayed coating after thermal curing, mechanical polishing with glass beads or sandblasting at low pressure is carried out, which increases the protective ability of the resulting coating.

Accelerated corrosion test method according to ASTM B117 showed that steel corrosion products were absent after 500 h of exposure in a salt spray chamber.

The disadvantages of this method are:

- the temperature of coating formation is more than 300 ° С, which complicates the technological process of forming a protective coating;

- low content of aluminum filler in relation to the components of the binder (binder / filler ratio = 1 / 0.6), which reduces the protective properties of the coating due to a decrease in the electrical conductivity of the coating;

- the formation of a protective coating from various solutions, which reduces the manufacturability of the process and requires readjustment of equipment.

The technical objective of the present invention is the development of a composite coating for corrosion protection of parts made of low-alloy carbon steels, operating in all-climatic conditions and at temperatures up to 500 ° C, which does not contain substances of the first hazard class.

The technical result of the invention is to increase the protective ability of the resulting coating against corrosion, to reduce the curing temperature of the coating to 200 ° C, to simplify the technological process of preparing a composition for applying a protective composite coating and to form a protective coating with high adhesive properties.

The technical result is achieved by the proposed method of Obtaining a protective composite coating on a steel part, including the sequential application of two layers of coating on the surface of the part, each of which is formed by applying a suspension of aluminum powder in a solution of an inorganic binder by painting, thermal curing of the part with the applied coating layer, and mechanical treatment parts with a coating layer, according to the invention, a suspension of aluminum powder in a solution of an inorganic binder is prepared from the following components, wt. %: aluminum hydroxide - 1.2-1.6, solution of 85% phosphoric acid 4.7-5.5, magnesium oxide - 0.12-0.16, potassium permanganate 0.25-0.33, aluminum powder - 45.0-55.0, sodium silicate or potassium silicate - 0.25-0.37, distilled water - the rest, and the inorganic binder is brought with distilled water to a specific gravity of 1.201.30 g / cm ³ , while after thermosetting additional processing is carried out with corundum powder at a pressure of 2 to 3 bar.

Preferably, the thermal curing of each coating layer is carried out as follows: first, air drying is carried out, then heat treatment is carried out in an air oven at a temperature of 60-80 ° C for 10-20 minutes and then at a temperature of 180-220 ° C for 40-120 minutes.

The invention is illustrated by the figures.

Figure 1 shows the structure of a composite protective coating, where:

A - after thermal curing of the coating;

B - after coating treatment with F100 corundum powder at a pressure of 2 bar.

Figure 2 shows the appearance of specimens made of steel 30HGSA with a protective composite coating before and after corrosion tests in KST for 4500 h, where:

A - appearance of a composite coated specimen before testing

B - appearance of samples after accelerated corrosion tests

An increase in the protective ability of the resulting coating is achieved by increasing the content of aluminum filler in the inorganic binder to 45-55 wt% (which approximately corresponds to a ratio of 1/1), in contrast to the prototype, where a similar ratio is 1 / 0.6), as well as due to the fact that in addition to the outer layer of aluminum formed during the machining of the second layer, an inner layer of aluminum is added, formed during the machining of the first layer, which, like the outer one, is an anode material and performs tread properties with respect to the material of the part, but, being closer to the surface of the part, it more effectively provides electrochemical protection of the material of the part from atmospheric corrosion. In this case, the outer layer of aluminum, being in direct contact with an aggressive medium, becomes more anodic in relation to the first inner layer of aluminum, thereby protecting this inner layer of aluminum from corrosion electrochemically. Thus, the two-stage electrochemical protection of steel and aluminum parts significantly increases the protective ability of the coating.

Simplification of the technology of applying a protective coating is achieved due to the fact that both layers of the coating are formed from a solution of the same composition, which does not require reconfiguring the technological equipment.

A decrease in the curing temperature of the coating is achieved due to the empirically selected ratio of the components of the aqueous suspension. The addition of magnesium ions and silicate ions to the binder leads to an increase in the pH of the aqueous suspension, which contributes to a decrease in the temperature required to convert monosubstituted aluminum phosphates into insoluble di- and tri-substituted phosphates, that is, it contributes to a decrease in the curing temperature of the coating, as well as to an increase in adhesion.

The use of permanganate ions in the composition of the binder prevents the formation of hydrogen during the coating process, which allows the use of a composite coating for corrosion protection of high-strength steels.

The introduction of sodium or potassium inorganic silicates into the composition of the binder improves the adhesion properties of the resulting coatings due to the formation of a silica sol, which is an additional binder.

The proposed stepwise heat treatment mode allows the coating to be dried and calcined evenly over the entire surface and without the formation of bubbles.

The protective composite coating is applied as follows. A layer of suspension is applied to a steel part by any method (spraying, dipping, brush, roller, etc.) (the compositions of suspensions are shown in Table 1) and a stepwise heat treatment with an applied coating layer is carried out, and the part with an applied coating layer is machined. In the process of heat treatment, crystallization water is removed from the amorphous structure of the binder, with the formation of polymer chains of inorganic phosphates, providing adhesion of the aluminum filler particles to the steel substrate. Thus, a composite coating structure is formed containing a glassy amorphous binder phase and a metal phase consisting of spherical aluminum particles with a diameter of 1 to 10 microns. At the same time, possessing plasticity, aluminum particles, upon application of external pressure, become substantially compacted, while the porosity of the composite coating decreases. The compaction of particles promotes the appearance of electrical contact between the aluminum particles and the substrate, which provides electrochemical protection of the composite coating.

Preferably, the heat treatment is carried out as follows: first, it is dried in air, and then it is kept at a temperature of 60-80 ° C and the coating is finally thermo-cured at a temperature of 180-220 ° C with a hold at this temperature for 40-120 minutes.

The first layer heat-treated in this way is subjected to sandblasting with corundum powder with a fineness of 100-150 μm and a pressure of 2 to 3 bar until the electrical conductivity of the coating appears. The second layer is applied and treated in the same way.

To prevent blistering and peeling of the coating, it is recommended to carry out the first stage of heat treatment in an air oven at a temperature of 60-80 ° C for 10-20 minutes. At higher temperatures of the first stage, a sharp elimination of crystallization water from the coating structure is observed, which can lead to swelling and peeling of the coating.

The mechanical treatment of the first and second layers by polishing after their heat treatment is carried out until the appearance of electrical conductivity of 1.4-1.6 cm, providing electrochemical interaction of the coating and the substrate.

Examples of implementation.

The composition for applying a protective composite coating solution was prepared as follows. With continuous stirring and heating to 110 ° C, aluminum hydroxide is added to concentrated orthophosphoric acid. After dissolution, magnesium oxide is added in portions in the form of aqueous suspensions. The reaction is carried out until a clear solution is formed. After that, an aqueous solution of sodium or potassium silicate is prepared in a separate container and added in small portions to the reaction mixture with continuous stirring. After obtaining a homogeneous solution, the heating is stopped, and the binder is cooled to room temperature. Then the inorganic binder is brought with distilled water to a specific density of 1.20-1.30 g / cm ³ . Potassium permanganate is added to the resulting solution and stirred until it is completely dissolved.

To prepare an aqueous suspension for applying a protective coating, an aluminum powder with a particle size of 5 to 10 microns, preferably produced under the trademark ASD-4, is introduced into the resulting solution. The resulting mixture is subjected to ultrasonic treatment at a frequency of 35 kHz for 3-5 minutes. After the introduction of the aluminum filler, the suspension is stable for 5 hours, after which it must be disposed of.

The compositions of the prepared suspensions are shown in Table 1.

For corrosion tests, flat specimens with dimensions of 100 × 50 × 2 mm were made from steel 30KhGSA, high-strength maraging steel VKS-170ID, and structural steel 20. Steel specimens were preliminarily subjected to sandblasting. The composition for the formation of the composite coating was applied by spraying using a spray gun in two layers. After applying each layer in a Binder drying oven, a stepwise heat treatment was carried out: drying in air, and then stepwise thermal curing according to the modes indicated in Table 2.

After heat treatment of each layer, their surface was sandblasted at a pressure of 2 to 3 bar.

Corundum powder is used as an abrasive material, which has the necessary hardness to compact the composite coating, and is also safe for sandblasting in industrial conditions, in contrast to abrasive materials containing silicon, which cause silicosis.

For the treatment of the composite coating, it is preferable to use corundum powder grades F100 and F120. When using larger fractions such as F80, sandblasting should be carried out strictly at a pressure not exceeding 3 bar, in order to avoid erosive wear of the composite coating. The treatment of the protective composite coating with corundum powder leads to the compaction of the coating structure and the formation of an electrically conductive coating.

To select the mode of sandblasting of the protective composite coating, the electrical conductivity of the surface was measured according to GOST 27333.

When processing a protective composite coating at 2 bar, the electrical conductivity of the coating is 1.4 cm.

An increase in pressure during sandblasting to 3 bar made it possible to obtain a coating with an electrical conductivity of 1.6 cm, which indicates a more dense structure, a decrease in the porosity of the coating and, as a consequence, an increase in the protective ability of the protective composite coating.

At pressures below 2 bar, a sufficiently dense composite coating structure is not formed.

To determine the adhesion of a protective composite coating in accordance with GOST 31149, a two-layer protective composite coating with a total thickness of 50-60 μm was applied to flat steel samples with a size of 100 × 50 mm. A grid of parallel scratches was applied with a cutting tool with a distance between notches of 1 mm, followed by tearing off an adhesive tape with an adhesive strength of 2.4 N / cm.

The protective composite coating possesses high adhesion to low-alloy carbon steels, which is confirmed by the test results given in Table 2. No chips or flaking of the coating is observed. A slight decrease in adhesion properties is observed with a decrease in the curing temperature to 180 ° C.

Samples with coatings obtained by the proposed method and by the prototype method were tested by accelerated corrosion testing in a salt fog chamber according to GOST 9.308-85 at a temperature of 33-37 ° C with continuous spraying of a neutral 5% sodium chloride solution.

From the test results shown in Table 2, it can be seen that the proposed method for producing a protective composite coating provides a high protective ability of parts made of low-alloy steels and steels of the maraging class.

In figure 2, a darkening of the sample is observed, associated with the corrosion processes of the aluminum filler. Steel corrosion products, including in the area of coating damage, are not observed, which indicates a high protective ability of the composite coating and the anodic nature of the protection.

The coatings obtained according to the prototype have 3 times less protective ability.

Thus, the proposed invention greatly simplifies the technology of obtaining the coating and allows the coating of steel parts with a high protective ability.

Claims (2)

1. A method of obtaining a protective composite coating on a steel part, including the sequential application of two layers of coating on the surface of the part, each of which is formed by applying a suspension of aluminum powder in a solution of an inorganic binder by painting, followed by thermal curing of the applied coating layer and mechanical treatment of the part with the applied coating layer, characterized in that a suspension of aluminum powder in a solution of an inorganic binder is used, prepared from the following components, wt%: aluminum hydroxide 1.2-1.6, a solution of 85% phosphoric acid 4.7-5.5, magnesium oxide 0 , 12-0.16, potassium permanganate 0.25-0.33, aluminum powder 45.0-55.0, sodium silicate or potassium silicate 0.25-0.37, distilled water - the rest, and the inorganic binder is brought to distilled water to a specific gravity of 1.20-1.30 g / cm ³ , while after thermosetting, additional processing with corundum powder is carried out at a pressure of 2 up to 3 bar. 2. The method according to claim 1, characterized in that the thermosetting of each layer of the coating is carried out as follows: first, air drying is carried out, then heat treatment is carried out in an air oven at a temperature of 60-80 ° C for 10-20 minutes and then at a temperature 180-220 ° C for 40-120 minutes.

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