RU2263164C1 - Method of application of protective coatings based on aluminum and its alloys - Google Patents

Abstract

FIELD: mechanical engineering and instrument-making industry; electrolytic treatment of articles made out of aluminum and its alloys.

SUBSTANCE: the invention is pertaining to electrolytic treatment of articles made out of aluminum and its alloys, in particular, to plasma- electrolytic oxidation, and may be used in various branches of mechanical engineering and instrument-making industry. The method includes plasma- electrolytic oxidation in the water electrolyte containing a boron-containing compound using a pulsating alternating anode-cathode current with a pulse duration from 0.0033 s up to 0.4 s or on using an alternating current of an industrial frequency at an effective current density of 5-100 A/dm². At that the ratio of the effective values of anode and cathode currents densities equals to 4:1-0.4:1, and the ratio of duration of the anode and cathode pulses is equal to 5:1 - 0.5:1. The technical result is a decrease of consumption of electrical power for realization of the method.

EFFECT: the invention ensures a decrease of consumption of electrical power for realization of the method.

7 cl, 16 ex

Description

The invention relates to the electrochemical processing of products from aluminum or its alloys, namely to plasma-electrolytic oxidation, and can be used in various industries of engineering and instrumentation.

A known method of producing ceramic coatings on products from aluminum and its alloys, described in the patent of the Russian Federation No. 2213166, publ. 09/27/03. The method includes microarc oxidation carried out in a pulsed anode mode at a current density of 100-300 A / dm ² or anode-cathode mode at a density of anode current of 100-300 A / dm ² , cathodic current of 50-120 A / dm ² , with a pulse duration current of 50-100 μs in an aqueous electrolyte containing alkali metal phosphates and fluorides, as well as boron-containing compounds (H ₃ BO ₃ and / or Na ₂ B ₄ O ₇ · 10H ₂ O). The resulting coating consists of two layers: the inner, firmly adhered to the base, a thickness of 5-10 microns and a functional outer layer 10-40 microns thick with a microhardness of 680-2250 kg / mm ² .

A disadvantage of the known technical solution is the complex composition (multicomponent) of the electrolyte, which in addition requires the observance of special safety measures, since the fluorides that are part of it are environmentally and harmful and pose a health hazard.

Closest to the claimed is a method for producing coatings on aluminum and its alloys [US Pat. RF №2136788, publ. 10.09.99], including microarc (plasma-electrolytic) oxidation of products lasting 45-100 min at a current density of 25-35 A / dm ² in an electrolyte containing 20-40 g / l of boric acid and 3-5 g / l of potassium hydroxide , followed by heating the products to a temperature of at least 500 ° C with a holding time of at least 100% of the oxidation time. The method provides an increase in hardness, wear resistance and modulus of normal elasticity of products from aluminum and its alloys.

A disadvantage of the known technical solution is the complexity of the method, due to the need to heat the oxidized surface to a temperature of at least 500 ° for 45-100 minutes, as well as the high cost of time and energy for its implementation. In addition, the implementation of the known method requires maintaining the temperature of the electrolyte in a certain range.

The objective of the proposed technical solution is to create an easy-to-implement method for producing protective coatings on aluminum and its alloys, which allows to reduce energy costs for its implementation by eliminating the operation of additional heat treatment of the resulting coating and lowering the effective energy costs during the oxidation operation and at the same time providing the possibility of obtaining high-quality coatings in a wide temperature range of the electrolyte.

The problem is achieved by the method of applying protective coatings to aluminum and its alloys, including plasma electrolytic (microarc) oxidation for a time sufficient to form a coating in an electrolyte containing a boron-containing compound, the oxidation being carried out using a pulsed alternating anode-cathode electric current with pulse duration from 0.0033 s to 0.4 s or on alternating electric current of industrial frequency at an effective current density of 5-100 A / dm ² , and the ratio of eff The objective values of the density of the anodic and cathodic currents are in the range 4: 1 ÷ 0.4: 1, and the ratio of the values of the duration of the anodic and cathodic pulses in the interval is 5: 1 ÷ 0.5: 1. Processing time is from 20 to 120 minutes.

The inventive method allows to achieve optimal results when processing in an aqueous electrolyte, including, g / l:

potassium hydroxide KOH - 1-4 and boric acid H ₃ BO ₃ - 5-12;

or potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O - 5-50;

or sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O - 10-60;

or potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O - 5-50 and potassium hydroxide KOH - 1-15;

or sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O - 10-60 and potassium hydroxide KOH - 1-15,

moreover, the electrolyte may additionally contain, g / l: sodium polyphosphate Na ₆ P ₆ O ₁₈ - 0.5-10 and / or sodium aluminate NaAlO ₂ - 0.5-10.

The introduction of KOH into the composition of the electrolyte increases the dissipating ability of the electrolyte, thereby providing more uniform coatings, which is important in the processing of complex products and products from aluminum alloys (for example, D16, D19), and also helps to reduce the energy intensity of the oxidation process.

In particular cases of using the invention, the addition of Na ₆ P ₆ O ₁₈ also provides more uniform coatings by increasing the dissipation capacity of the electrolyte, and the addition of NaAlO ₂ in the composition of the electrolyte improves the efficiency of coating formation, which reduces the oxidation time. In addition, the addition of sodium aluminate further increases the wear resistance of the formed coatings.

The method is as follows.

In an electrolytic bath is placed an aqueous solution of boron-containing electrolyte containing the appropriate components in the claimed amounts.

An oxidizable article of aluminum or its alloy is installed in a bath with an electrolyte. In the case of using alternating current of industrial frequency in a bath with an electrolyte, two identical products are suspended symmetrically and parallel, alternately playing the role of anode and cathode. In the case of anode-cathode pulse current, the second electrode is the inner surface of the wall of the bath.

Submit alternating current with an effective density of 5-100 A / DM ² for 20-120 minutes. The ratio of the effective density of the anodic and cathodic currents and the duration of the anodic and cathodic pulses in the case of a pulsed anodic-cathodic mode is provided by adjusting the computer-controlled reverse thyristor unit.

As a result of oxidation, a dense coating with a smooth or slightly rough surface with a thickness of 20-100 microns is formed. X-ray phase analysis shows that the resulting coatings mainly include high-temperature γ- (η) or α-phases of aluminum oxide. Due to the high heat generation during cathodic discharges, phase transitions of aluminum oxides Al ₂ O ₃ are carried out in the forming coating, leading to the formation of its high-temperature phases. In addition, the formation of these high-temperature phases is initiated by boron compounds that make up the electrolyte, which intensify the occurrence of solid-phase reactions at lower temperatures.

Coatings are two-layer. They include a surface, rather loose layer with a thickness of 5 to 40 μm, formed by "islands" of Al ₂ O ₃ that practically close to each other. The thickness of this layer depends on the alloy being processed, the composition and temperature of the electrolyte, and the treatment mode. This "technological" layer is easy to grind and grind. Directly below it is a continuous, very dense layer of high-temperature alumina (mixture of oxides), which ensures the wear resistance of the coating.

As a result, the inventive method, in contrast to the prototype, does not require additional heat treatment to obtain wear-resistant coatings.

In addition, the coatings obtained by the claimed method are lipophilic and have high oil absorption values, which helps to reduce their wear when using products with such coatings in the friction units.

Coatings have high corrosion resistance to atmospheric corrosion. Corrosion resistance of coatings contributes to the shift of the hydrophilic-hydrophobic balance of the surface of the coatings in the hydrophobic region.

The reduction in energy costs per unit area of the formed coating is due to the following reasons.

Firstly, the method eliminates the additional operation of thermal annealing, thereby providing significant energy savings.

Secondly, the presence of the cathode component of the current reduces the effective cost of electricity for oxidation in the anode-cathode mode due to the presence of the valve effect. At the same density of the anode and cathode current, the resistance of the formed coating in the cathode half-cycle is negligible. Thus, the voltage in the cathode half-cycle is ten times less than in the anode one (for example, at the end of the treatment, the cathode voltage is 30 V compared to 500 V of the anode voltage), while the energy consumption in relation to processing with anode polarization decreases by several time.

Thus, the technical result of the proposed method is to simplify the method of producing protective coatings on aluminum and its alloys, as well as reducing the cost of electricity for its implementation by eliminating the operation of additional heat treatment of the resulting coating and reducing the effective cost of electricity directly to the oxidation operation while ensuring high wear - and corrosion resistance of coatings obtained with its help.

The inventive method ensures the achievement of a technical result in a wide temperature range of the electrolyte (up to 80-90 ° C) due to the fact that the used electrolyte is non-aggressive with respect to aluminum and its oxides. An increase in temperature does not lead to the interaction of the electrolyte with the resulting coating and does not adversely affect the quality of the latter. The properties of the formed coatings vary slightly with increasing temperature of the electrolyte.

This serves as a guarantee of maintaining the quality of the coatings in case of unpredictable heating of the electrolyte during coating in the factory.

Examples of specific implementation of the method

A sample of aluminum or its alloy with a total area of 24 cm ² before coating was degreased for 5 min in an aqueous alkali solution (40%), washed with running water, then clarified in 15% nitric acid, washed again with running water.

The prepared sample was placed in a stainless steel bath with a water cooling jacket.

In the case of using the anode-cathode mode, the bath was one of the electrodes. In the case of using alternating current of industrial frequency, two identical samples served as electrodes, symmetrically hanging in the bath. The bath in this case was not involved as an electrode.

As the electrolyte, an aqueous solution of one of the claimed compounds was used, while the pH of the electrolyte was 9-12.

The current source was a TER-100 / 460N-2-2UKHLI thyristor converter, controlled by a special program with a computer and providing an anode-cathode pulse with amplitude and duration adjustable to the oxidized sample.

The final molding voltage (anode mode) was about 500 V; in the cathode mode, the final voltage did not exceed 30 V.

In the case of using alternating current with an industrial frequency of 50 Hz, the voltage supplied to the electrodes and, accordingly, the current density was regulated using a transformer.

After processing, the samples were washed with water and dried in air.

Resistance to mechanical stresses of the formed coatings on aluminum and its alloys was evaluated using a special setup simulating face friction. The coated sample made a reciprocating motion 30.7 times per minute in 1 cm increments under the end face of the tungsten wire with a diameter of 0.1 cm. The pressure on the sample was 8.7 MPa.

The destruction of the coating was judged by a sharp decrease in the electrical resistance of the contact end of the wire - coating - metal. If the coating did not deteriorate within 2 hours, the test was stopped and a conclusion was drawn about the high wear resistance of the resulting coating.

The corrosion resistance of the coating to an atmosphere containing chloride ions was evaluated by the drop method according to GOST 9302-88.

The oil consumption of the coatings was determined according to GOST 9.302-08. The method is based on measuring the amount of oil adsorbed by a coating. For coatings obtained according to examples 1-16, the oil absorption values are in the range of 2-8 · 10 ^-4 mg / mm ³ .

To determine the angle of wetting with water, the method of measuring the profile of a “lying” drop was used [Adamson A. Physical surface chemistry. M .: Mir, 1979, p.275]. The wetting angle for water for coatings obtained according to examples 1-16 is from 40 to 60 ° C.

Oil on the surface of all coatings obtained according to the above examples spontaneously spreads.

The microhardness of the coatings was determined on a DUH-W201 installation (Japan) according to the Vickers method.

The microhardness for coatings obtained according to examples 1-16 is 2500-3800 MPa, and for all aluminum alloys except duralumin, these values are obtained only for a continuous layer, after removing the upper "technological" layer. For duralumin alloys, the microhardness values are the same for both layers.

Example 1

A sample of an aluminum alloy AMg5 (% Mg 4.8-5.8; Mn 0.5-0.8; Ti 0.02-0.1, the rest Al) was oxidized in an electrolyte, which is an aqueous solution containing 8 g / l of boric acid H ₃ BO ₄ and 3 g / l of potassium hydroxide KOH (pH 10). The anodic-cathodic oxidation mode at an anode current density of I _a = 30 A / dm ² , cathodic current I _k = 30 A / dm ² and the anode and cathode pulses of 0.04 s. The oxidation time was 60 min at an electrolyte temperature of 10-20 ° C.

A light (almost white) coating with a thickness of 28 μm was not rubbed during the abrasion time exceeding 2 hours.

The droplet greening time characterizing the corrosion resistance of the coating was 32 min.

Example 2

A sample of aluminum alloy AMg3 (%, Mg 3.2-3.8; Mn 0.3-0.6; Si 0.5-0.8; other impurities 1; the rest Al) was oxidized in an electrolyte, which is an aqueous solution containing 8 g / l of boric acid H ₃ BO ₄ and 3 g / l of potassium hydroxide KOH (pH 10), at an alternating current of industrial frequency (50 Hz) at an effective current density of 15 A / dm ² for 120 min at an electrolyte temperature 70-80 ° C.

The color coating of baked milk with a thickness of 72-73 μm was not wiped during the abrasion time exceeding 2 hours.

The droplet greening time characterizing the corrosion resistance of the coating was 45 min.

Example 3

A sample of aluminum alloy D16 (%, Cu 3.8-3.9; Mg 1.2-1.8; Mn 0.3-0.9, the rest Al) was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 20 g / l (pH 11) in the anode-cathode mode with an effective density of the anode current I _a = 15 A / dm ² , cathodic current I _k = 15 A / dm ² and the duration of the anode and cathode pulses 0.04 from. The oxidation time was 90 min at an electrolyte temperature of 10-25 ° C.

The resulting coating with a thickness of 46 μm of a light (almost white with a gray tint) color was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 36 minutes.

Example 4

A sample of aluminum alloy A7 (technically pure aluminum - 99.7% Al) was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 40 g / l (pH 11) in the anode-cathode mode at an effective density the anode current I _a = 15 A / dm ² , the cathodic current I _k = 15 A / dm ² and the duration of the anode pulses 0.04 s, cathode pulses 0.01 s. The oxidation time was 120 min at an electrolyte temperature of 10-20 ° C.

The resulting coating with a thickness of 47 μm of a light (pale gray) color was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 40 minutes.

Example 5

Sample from aluminum alloy B96 (%, Zn 7.6-8.6; Mg 2.5-3.2; Cu-2.2-2.8; Ni 0.2-0.5; Cr 0.1- 0.25; impurities 1.0, the rest aluminum) was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 10 g / l (pH 10) in the anode-cathode mode at an effective anode current density of I _a = 60 A / dm ² , cathodic current I _k = 60 A / dm ² and the duration of the anode pulses 0.0033 s, cathode pulses 0.0033 s. The oxidation time was 20 min at an electrolyte temperature of 35-40 ° C.

The resulting coating with a thickness of 38 μm of a light color was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 30 minutes.

Example 6

A sample of an aluminum alloy AMg5 was oxidized in an aqueous solution of sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O with a concentration of 50 g / l (pH 11.5) in the anode-cathode mode at an effective current density I _a = 15 A / dm ² , the cathode current I _k = 10 A / dm ² and the duration of the anode pulses 0.04 s and the cathode pulses 0.008 s. The oxidation time was 120 min at an electrolyte temperature of 20-30 ° C.

A light coating with a thickness of 71 μm was not rubbed during an abrasion time exceeding 2 hours (while the technological layer was completely removed).

The droplet greening time was 43 min.

Example 7

A sample of aluminum alloy AMg5 was oxidized in an aqueous potassium tetraborate solution of Na ₂ B ₄ O ₇ · 4H ₂ O at a concentration of 15 g / l (pH = 10) in the anode-cathode regime with an effective density of the anode current I _a = 30 A / dm ^2, cathodic current I _k = 15 A / dm ² and the duration of the anode pulses 0.04 s and the cathode pulses 0.03 s. The oxidation time was 90 min at an electrolyte temperature of 20-30 ° C.

A light coating with a thickness of 59 μm was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 36 minutes.

Example 8

A sample of aluminum alloy AMg2 (%, Mg 1.8-2.8, Mn 0.2-0.6, ~ 1 impurity, the rest Al) was oxidized in an aqueous solution of sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O s concentration of 40 g / l (pH 11) in the anode-cathode mode with an effective density of the anode current I _a = 18 A / dm ² , cathodic current I _k = 12 A / dm ² and the duration of the anode and cathode pulses of 0.02 s. The oxidation time was 90 min at an electrolyte temperature of 30-40 ° C.

A light coating with a thickness of 50 μm was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 34 minutes.

Example 9

A sample of an aluminum alloy D19 (%, Cu 3.8-4.3; mg 1.7-2.3; Mn 0.5-1.0; impurities 1.35; the rest Al) was oxidized in an aqueous solution of potassium tetraborate K ₂ V ₄ O ₇ · 4H ₂ O with a concentration of 30 g / l and potassium hydroxide KOH with a concentration of 10 g / l (pH 12) in the anode-cathode mode with an effective density of the anode current I _a = 15 A / dm ² , cathode current I _k = 10 A / dm ² and the duration of the anode pulses 0.02 and the cathode pulses 0.04 s. The oxidation time was 40 min at an electrolyte temperature of 20-30 ° C.

A light gray coating with a thickness of 85 μm was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 42 minutes.

Example 10

A sample of aluminum alloy D16 was oxidized in an aqueous solution of sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O with a concentration of 20 g / L and potassium hydroxide KOH with a concentration of 2 g / L (pH 11) in the anode-cathode mode at an effective density of the anode current I _a = 20 A / dm ² , cathodic current I _k = 20 A / dm ² and the duration of the anode and cathode pulses 0.04 s. The oxidation time was 60 min at an electrolyte temperature of 20-30 ° C.

A light gray coating with a thickness of 45 μm was not rubbed during an abrasion time exceeding 2 hours.

The droplet greening time was 38 minutes.

Example 11

A sample of aluminum alloy D16 was oxidized in an aqueous solution of sodium tetraborate Na ₂ B ₄ O ₇ · 10H ₂ O with a concentration of 40 g / L and Na ₆ P ₆ O ₁₈ with a concentration of 5 g / L (pH 9.5) in the anode the cathode mode at an effective density of the anode current I _a = 15 A / dm ² , the cathode current I _k = 15 A / dm ² and the duration of the anode and cathode pulses of 0.01 s. The oxidation time was 90 min at an electrolyte temperature of 20-30 ° C.

The light gray coating with a thickness of 44 μm was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 46 minutes.

Example 12

A sample of aluminum alloy D19 was oxidized in an electrolyte, which is an aqueous solution containing 10 g / l of boric acid H ₃ BO ₄ , 1 g / l of potassium hydroxide KOH and 2 g / l of NaAlO ₂ (pH 9) in the anode-cathode mode at the effective density of the anode current I _a = 20 A / dm ² , the cathode current I _k = 10 A / dm ² and the duration of the anode and cathode pulses 0.005 s. The oxidation time was 120 min at an electrolyte temperature of 10-20 ° C.

A light gray coating 36 μm thick was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 32 minutes.

Example 13

A sample of an aluminum alloy AMg5 was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 20 g / l (pH 11) in the anode-cathode mode at an effective current density I _a = 15 A / dm ² , cathodic current I _k = 30 A / dm ² and the duration of the anode and cathode pulses of 0.03 s. The oxidation time was 120 min at an electrolyte temperature of 20-30 ° C.

A light coating with a thickness of 48 μm was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 34 minutes.

Example 14

A sample of aluminum alloy AMg5 was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 20 g / l (pH 11) in the anode-cathode mode with an effective density of the anode current I _a = 40 A / dm ² , cathodic current I _k = 10 A / dm ² and the duration of the anode and cathode pulses of 0.04 s. The oxidation time was 120 min at an electrolyte temperature of 20-30 ° C.

A light coating with a thickness of 82 μm was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 40 minutes.

Example 15

A sample of an aluminum alloy AMg5 was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 20 g / l (pH 11) in the anode-cathode mode at an effective current density I _a = 100 A / dm ² , cathodic current I _k = 100 A / dm ² and the duration of the anode and cathode pulses of 0.0033 s. The oxidation time was 20 min at an electrolyte temperature of 20-30 ° C.

A light coating with a thickness of 35 μm was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 33 minutes.

Example 16

A sample of aluminum alloy AMg5 was oxidized in an aqueous solution of potassium tetraborate K ₂ B ₄ O ₇ · 4H ₂ O with a concentration of 20 g / l (pH 11) in the anode-cathode mode with an effective density of the anode current I _a = 40 A / dm ² , cathodic current I _k = 80 A / dm ² and the duration of the anode pulses 0.005 s and the cathode pulses 0.0033 s. The oxidation time was 25 min at an electrolyte temperature of 20-30 ° C.

A light coating 32 μm thick was not rubbed during an abrasion time exceeding 2 hours. The droplet greening time was 44 minutes.

The proposed method allows to increase the service life of products operated under friction and increased mechanical wear, and, in addition, provides protection against atmospheric corrosion.

Claims (7)

1. The method of applying protective coatings to aluminum and its alloys, including plasma electrolytic oxidation for a time sufficient to form a coating in an aqueous electrolyte based on a boron-containing compound, characterized in that the oxidation is carried out on a pulsed alternating anode-cathode current with a pulse duration from 0.0033 to 0.4 with either AC power frequency at an effective current density of 5-100 a / dm ^2, the ratio of the effective values of density of the anodic and cathodic currents pa but 4: 1-0,4: 1 and the ratio of the length of the anode and cathode pulses is 5: 1-0.5: 1. 2. The method according to claim 1, characterized in that the processing is carried out in an electrolyte containing, g / l: KOH 1-4 H ₃ IN ₃ 5-12 3. The method according to claim 1, characterized in that the processing is carried out in an electrolyte containing, g / l: K ₂ B ₄ O ₇ · 4 H ₂ O 5-50 4. The method according to claim 1, characterized in that the processing is carried out in an electrolyte containing, g / l: Na ₂ B ₄ O ₇ · 10H ₂ O 10-60 5. The method according to claim 1, characterized in that the treatment is carried out in an electrolyte containing, g / l: K ₂ B ₄ O ₇ · 4 H ₂ O 5-50 KOH 1-15 6. The method according to claim 1, characterized in that the treatment is carried out in an electrolyte containing, g / l: Na ₂ B ₄ O ₇ · 10H ₂ O 10-60 KOH 1-15 7. The method according to any one of claims 1 to 6, characterized in that the electrolyte in which the processing is carried out further comprises, g / l: Na ₆ P ₆ O ₁₈ 0.5-10.0 and / or NaAlO ₂ 0.5-10.0