RU2620224C2 - Method of obtaining protective coating on magnesium and its alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes three stages: in the first step on the substrate forming a primary coating thickness of at least 15 microns, which is obtained substrate MAO in an aqueous electrolyte containing from 10 to 40 g/l of alkali metal metasilicate, 2 to 10 g/l fluorides of alkali metals and from 3 to 15 g/l of alkali metal hydroxides, for a pulse current with an amplitude of 400-450 V, 250-350 ms pulse duration, the second step is carried out first chemical bleed coating in an aqueous solution containing from 0.5 to 1.5 mol/l phosphoric acid, 0.5 to 1.5 m l/l hydrogen fluoride or fluoride, is then formed on the substrate with a secondary coating using an electrolyte and MAO mode of the first stage, after which the second chemical is carried bleed coating in an aqueous solution of the first etching, and in the third stage, an electrolyte and a first-stage MDO are used, with the formation of a final nanostructured non-metallic inorganic protective coating on the substrate.

EFFECT: increasing the adhesion strength of the coating to the substrate, corrosion resistance and resistance to external factors, primarily external mechanical influences.

4 cl, 2 dwg, 1 tbl

Description

The invention relates to the field of electrolytic coating of magnesium and its alloys using microarc oxidation in aqueous solutions of electrolytes and can be used to obtain protective corrosion-resistant coatings with increased mechanical strength in mechanical engineering, instrumentation and aerospace engineering.

A recent progressive trend in the field of protective surface treatment of products from magnesium and magnesium alloys is the transition from traditional anodizing to microarc oxidation (microplasma anodizing in solutions). This method allows the formation of sufficiently hard protective coatings with high adhesion to the substrate. Magnesium products with a protective coating of MDO are used in the aircraft and automotive industries, electrical and radio engineering, computer, space and defense equipment.

The actual problem of the method remains the insufficient resistance of the formed coatings to external mechanical loads. In the case of magnesium, even with a partial peeling of the protective layer, the likelihood of developing a focus of corrosion is high. One of the known methods for improving the physicomechanical properties of coatings is to prepare the surface of the metal on which they are formed. For valve metals, which include magnesium, such a method of surface preparation prior to standard electrochemical anodizing is its texturing.

The known method described in patent US4589972 A (publ. 05/20/1986), providing for the texturing of the metal surface with steam and its subsequent anodizing, which allows to achieve a high ability of the surface to absorb infrared radiation.

The operation of texturing the surface with steam is unsuitable for processing parts of complex shape. For the processing of magnesium products, this approach is generally inapplicable.

In the patent US5288372 A (publ. 22.02.1994) describes a method of processing a metal, mainly aluminum surface, including coating by anodizing in the first solution, its subsequent etching and coating by anodizing in another solution. The surface after texturing increases its roughness and has a reduced reflectivity, the effect of unifying the surface is achieved before applying the finish coating layer.

First, the specific modes and compositions of the solutions for coating presented in the examples of the implementation of the above method do not allow their use for the formation of coatings on magnesium. Secondly, due to standard electrochemical anodizing, the required roughness of the metal surface is not achieved, providing maximum mechanical adhesion to the coatings applied to it.

In patent RU2424381 C1 (published on July 20, 2011) a method for producing a wear-resistant coating on aluminum and its alloys using the microarc oxidation method is disclosed. The method involves applying an anodic oxide coating to an aluminum surface in two stages in an alkali metal silicate or aluminate solution at an alternating current density of 5-150 A / dm ² , in which an alkali metal silicate or aluminate solution with a concentration of 20-150 g / l in combination with sodium silicofluoride with a concentration of 2-20 g / l and holding the part in it for 20-40 minutes, and in the second stage, an alkaline solution of silicate or aluminate with a concentration of 2-10 g / l and an alkali concentration of 0.5- 4.0 g / l with a shutter speed of 1.5-2 hours, its last blowing partial bleed hydrofluoric acid and also applying a finishing layer by anodization in a solution. The coating obtained in the first stage is only partially etched — only the external “technological” layer is etched.

The implementation of the method does not imply controlled texturing of the surface of the substrate. The modes and solutions given in RU2424381 C1 do not allow the formation of protective coatings that are resistant to mechanical stress on the surface of magnesium and magnesium alloys.

Known patent CN103695983 A (publ. 02.04.2014), which describes a method for controlled production of a given nanoscale texture of an aluminum surface. The average diameter of the etched sections is 60-1080 μm. Texturing is achieved by applying a layer of anode oxide in acidic electrolytes and its subsequent chemical etching with a mixture of sulfuric acid and potassium dichromate.

The described method does not involve coating the substrate after texturing. The modes and solutions for texturing aluminum surfaces described in CN103695983 A do not allow texturing of magnesium and its alloys in a similar way. In addition, the use of traditional anodizing and a single cycle: coating application / etching allows you to vary the surface texture parameters in an extremely narrow range.

RU 2206642 C2 (published on June 20, 2003) discloses a method for modifying the surface of medical devices by exposing them to microarc discharges in an electrolyte solution and forming a ceramic coating during such processing. As a result of this effect, texturing of the metal surface occurs. The inventors indicate the possibility of applying a finely porous bioceramic coating on a textured surface.

The method is intended for processing predominantly titanium products and describes texturing options in solutions containing chloride and nitrate ions. The use of solution compositions given in RU2206642 C2, in which titanium is processed and bioceramic coatings are applied to its surface, is not suitable for conducting controlled texturing of magnesium and at the same time applying protective coatings on its surface with high resistance to external mechanical stresses due to the different chemical nature of this material (magnesium).

Known disclosed in patent US4620904 A1 (publ. 04.11.1986) is a method of producing a protective coating on a magnesium alloy, including electrochemical surface treatment under spark discharges in an aqueous electrolyte containing silicate and alkali metal hydroxide, as well as a fluorine-containing component (alkali metal fluoride or hydrofluoric acid). The treatment is carried out at a solution temperature ^of 20-40 C, a pH of 12-14 at a potential difference between the work piece and the counter magnesium 150-400 V for the time required for forming the desired coating thickness.

The coating according to this method is applied to a magnesium surface without any preliminary preparation, which does not allow to achieve resistance of the formed coating to external mechanical stresses.

As a prototype, the method disclosed in US5240589 A (publ. 08/31/1993) for a protective coating on a magnesium alloy, including a preliminary chemical treatment of a magnesium surface and subsequent electrochemical processing, was selected. The pretreatment is carried out at a temperature of ^about 40-100 C in ammonium fluoride solution to form the initial magnesium fluoride film on the surface. Subsequent electrochemical processing is carried out essentially as described in the previous analogue - US4620904 A. The electrochemical treatment is carried out in an aqueous solution containing alkali metal silicate and hydroxide, as well as a fluorine-containing component (alkali metal fluoride, and / or alkali metal fluorosilicate, and / or hydrofluoric acid) when the potential difference between the processed magnesium product and the counter electrode is at least 100 V, for the time required to form the coating of the desired thickness.

The disadvantages of the above analogue are:

- the use of preliminary chemical treatment of the magnesium surface, which is essentially a chemical etching in an acidic fluoride-containing solution, which does not lead to the formation of a noticeable texture on the magnesium surface;

- preliminary chemical treatment allows only to form a very thin (1-2 microns) fluoride film, which ensures the subsequent achievement of increased corrosion resistance of the finish coating, however, this does not solve the problem of improving the adhesion of the coating to the base material and obtain a coating that is resistant to external mechanical stresses .

The objective of the invention is to develop a method that allows you to form a protective nanostructured nonmetallic inorganic coating on the surface of magnesium and its alloys, which has increased adhesion to the base material and resistance to external factors (primarily to external mechanical stresses) using the microarc oxidation method (MAO) ), which is a microplasma surface treatment in electrolyte solutions.

 The problem is achieved in that, like the well-known, the proposed method for producing a protective coating on magnesium or its alloys is carried out by its microarc oxidation. New is that the method includes the following three stages of coating formation:

- at the first stage, a primary nanostructured nonmetallic inorganic coating with a thickness of at least 15 μm is formed on a substrate of magnesium or its alloy, which is obtained by microarc oxidation of the substrate in a first aqueous electrolyte solution containing at least water-soluble silicates, alkali metal fluorides and alkali;

- at the second stage, the first chemical etching of the primary coating obtained at the first stage is carried out in a second aqueous electrolyte solution containing at least phosphoric acid, then, successively applying techniques and modes similar to coating formation at the first stage, a secondary nanostructure is formed on the substrate a non-metallic inorganic coating, after which a second chemical etching of the coating is carried out in a second aqueous electrolyte solution;

- at the third stage, using techniques and modes similar to the formation of the coating at the first stage, form the finish nanostructured non-metallic inorganic coating on the substrate.

Thus, the production method includes the application steps and the etching steps of the coating, which alternate one after another, while the coating application (formation) process is performed 3 times: the first coating formation on the substrate and its subsequent first chemical etching; second coating formation, similar to coating formation in the first stage, and its subsequent second chemical etching; third (finish) coating formation, similar to coating formation in the first stage. At each of the three stages of coating formation, the process is carried out by microarc oxidation of a magnesium substrate in an aqueous electrolyte solution containing water-soluble silicates, alkali metal fluorides and alkali.

The coating is formed at each stage until a thickness of at least 15 μm is reached, preferably up to a thickness of 30 μm.

In some embodiment of the method according to the invention, the first electrolyte solution may be an aqueous solution containing from 10 to 40 g / l of water-soluble alkali metal metasilicates, from 2 to 10 g / l of water-soluble alkali metal fluorides and from 3 to 15 g / l of alkali metal hydroxides .

In a preferred embodiment of the method according to the invention, the first electrolyte solution contains 30 g / l of sodium nine-metasilicate, 4 g / l of sodium fluoride, 6 g / l of potassium hydroxide.

In any of the specific embodiments of the method, when forming a coating in the first, second and third stages, a pulse current of the following parameters can be used: the amplitude of the voltage pulses is 400-450 V, the pulse duration is 250-350 μs.

In each of the two procedures for chemical etching of coatings formed in the first and second stages, the process is carried out in an aqueous electrolyte solution by exposure to acid solutions, while the second aqueous electrolyte solution may be a mixture of phosphoric and hydrofluoric acids or a mixture of phosphoric acid and water-soluble fluorides and / or hydrofluorides.

In another preferred embodiment of the method of the invention, the second electrolyte solution is an aqueous solution containing from 0.5 to 1.5 mol / L of phosphoric acid and from 0.5 to 1.5 mol / L of hydrogen fluoride or water-soluble fluoride.

In the optimal version of the method, the duration of the first stage and the third stage (stage of forming the topcoat) is 20 minutes, and the duration of the second stage is 5 minutes. In an optimal embodiment of the method, the temperature of the second electrolyte solution is from 20 to 25 ° C.

The inventive sequence of techniques allows you to form a coating on a substrate made of magnesium or its alloy, with a textured metal-coating interface. Such a coating, due to better adhesion to the substrate material, is resistant to external factors (external mechanical influences).

The inventive method includes the alternation of coating methods by microarc oxidation (MAO) and chemical etching procedures, which allows stepwise texturing to achieve the desired texture of the metal-nanostructured non-metallic coating interface. When coating by the MAO method due to the multitude of microplasma discharges and associated processes, the metal – nanostructured nonmetallic coating is textured.

By texturing we mean a set of operations, as a result of which the structure of the metal surface changes and the required parameters of the surface profile arise, including its roughness.

By nanostructured nonmetallic coating in the present invention is meant a ceramic silicate-oxide coating formed on the surface of an article / sample of magnesium or magnesium alloy by the MAO method.

The choice of the composition of the first electrolyte solution, including water-soluble metasilicates, fluorides and alkali, is due to the possibility of forming nanostructured nonmetallic coatings in it with a high speed and a quick transition of the process to the microplasma mode, low cost and availability of reagents. In addition, the use of this electrolyte composition for applying the final coating layer allows one to obtain dense nanostructured nonmetallic coatings that provide reliable protection of magnesium products from corrosion and wear [US4620904, US5240589, US5264113, US5266412, US5470664, RU2357016]. The formation of a coating on the surface of magnesium in this electrolyte occurs in accordance with the following main reactions:

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The forming coating has microporosity and nanoporosity.

Subsequent etching obtained at the first stage of the proposed method of the primary nanostructured non-metallic coating provides a metal surface with a given texture. One of the possible variants of such a texture corresponds to the image in FIG. 2. In the process of etching, additional texturing also occurs due to more active penetration of the etching solution (second solution) through the pores of the coating. Repeatedly alternating the steps of applying and etching the coating allows us to achieve the required surface parameters, which allows us to consider the number of application – etching operations as a factor in controlling the surface texture of the magnesium material.

The choice of the composition of the second solution, including phosphoric acid, hydrofluoric acid and / or water-soluble fluorides, is due to the ability of this composition to dissolve the oxide-silicate coating formed in the first stage. The presence of fluorine ions in the composition of the second solution minimizes the effect directly on the magnesium base due to the formation of a passivating fluoride film.

Microplasma texturing (microplasma deposition of the coating followed by etching) contains a significant number of control factors, including both the composition of the electrolyte and the duration of exposure, and methods of organization, as well as electrical parameters.

At the third stage - the stage of applying the finish coating - the magnesium surface with the achieved specific texture is again coated, which allows, taking into account the additional stage of microplasma exposure, according to the claimed method, to form a two-layer material with a textured metal-ceramic coating interface.

The formed texture of the interface with the maximum roughness parameters Ra allows to achieve the maximum area and adhesion of the coating to the magnesium base material, which, in turn, determines the resistance of the coating to external mechanical stresses. On a textured surface, the distribution of tearing and tensile forces arising from external influences (mechanical or thermal) differs significantly from their distribution on a smooth layer interface [Mamaev AI, Mamaeva VA, Dorofeeva TI, Emelyanova E .YU. The mechanism of modeling the loads of deformation and fracture of layered non-metallic inorganic materials with nano- and micro-sized wave surface texturing // Bulletin of Higher Education. Physics. 2012.V. 55. No. 9/3. S. 78 - 86], which allows to avoid delamination of the coating.

The coatings obtained by the present method are characterized by resistance to external mechanical stress and increased adhesion to the base material.

The invention is illustrated by graphic materials and examples of its specific implementation.

In FIG. 1 shows a projection of the surface profile of a magnesium sample No. 4 (Table 1) with a maximum roughness Ra obtained by the present method after three stages of applying / etching the MAO coating, demonstrating the reality of obtaining a textured surface in accordance with the claimed method.

In FIG. 2 shows a micrograph of the surface of sample No. 4 ′ (Table 1) before mechanical load (a) and after mechanical load (b).

Examples

Samples (in the form of flat plates with a width of 7 mm and a thickness of 1 mm) made of a magnesium alloy of grade MA2-1 were polished to grade 10, roughness. Then, 1 M solution of Na ₃ PO ₄ at a temperature not lower than 60 ° ^C was carried out by cleaning the surface of sample of organic contaminants. After washing with distilled water, alcohol, and again with distilled water, the samples were dried in an oven at a temperature of 70 ° C.

Then, coatings were formed on the surfaces of the samples in the microarc mode. The sample served as an anode; a stainless steel bath with a water cooling jacket acted as a cathode. The composition of the coating solution: sodium fluoride (NaF) 4 g / l, potassium hydroxide (KOH) 4 g / l, sodium metasilicate (Na2SiO3⋅9H2O) 30 g / l, the rest is distilled water.

The samples were processed by passing a pulse current of the following parameters: amplitude of voltage pulses 400-450 V, pulse duration 200 μs, processing time 20 minutes. The coating thickness after the first application step was (20 ± 5) μm. The coatings were etched with a solution containing 1 mol / L orthophosphoric acid and 0.5 mol / L sodium fluoride for 5 minutes.

By stagewise deposition of the coating by the MAO method with its subsequent etching, surface texturing was achieved. Sample No. 1 was etched in a stripping solution (second solution) without first coating to determine the effect of chemical etching on the surface texture.

Roughness values and surface profiles after texturing were determined using a commercially available STIL Micro Measure 3D Station non-contact profilometer manufactured by STIL and Mountains Map Universal software (Version 2.0.13), which allows projection of surface profiles and determination of roughness parameters in accordance with ISO 25178 and ISO 4287. Before determining the roughness values of textured metal surfaces, the reaction products were removed by short-term immersion in 30% astvor nitric acid.

The number of cycles of application / etching of the coating and the achieved roughness parameters of the textured samples are shown in table 1.

Table 1 - Processing parameters and properties of the samples (for samples with a 2′-6 ′ coating, the expected values of Ra for the substrate at the interface are given)

Sample Number
(1-6) Number of MDO coating operations The number of operations to bleed the coating MDO Ra cp., Microns
Behavior at ultimate mechanical load before breaking Coating adhesion, MPa one 0 one 0.818 - 2 one one 1,123 - 3 2 2 4,180 - four 3 3 5,523 - 5 four four 3,977 - 6 5 5 2,883 - 2 ′ one 0 - peeling 3 ′ 2 one 1.23-4.18 weak peeling four' 3 2 4,180-5,523 without peeling 5' four 3 3,977-5,523 without peeling 6 ′ 5 four 2,883-3,977 weak peeling

The resulting coating samples with a textured phase boundary were subjected to tensile testing. The tests were carried out on an INSTRON 5948 test machine in accordance with ISO 21180–201. A load was applied to the samples at a constant tensile speed of 5 mm / min until a relative elongation of 15% was reached, after which the nature of the destruction of the coatings was evaluated.

The analysis of microphotographs of sample 4 ′ (Fig. 2) obtained by the claimed method indicates the presence of characteristic longitudinal cracks corresponding to the step of texturing and allowing to relax emerging mechanical stresses without peeling of the coating.

Thus, the results presented in the table indicate the achievement of optimal resistance of the coatings to mechanical stresses during two application – etching cycles before applying the topcoat. A similar result can be achieved by carrying out three application – etching cycles before applying the finish coating, however, from the point of view of labor costs, it is preferable to carry out exactly two application – etching cycles.

Five samples with the coating obtained by the present method (type 4 ′ samples - Table 1) were also tested for corrosion resistance in the salt spray chamber at a temperature of (35 ± 2) ° С according to GOST 9.308-85, method 1. The test results showed no corrosion damage after 400 hours of salt spray in the chamber for all five samples.

Thus, obtained by the present method, the coatings are resistant to external mechanical loads and are protective against the effects of a corrosive environment.

Claims (7)

1. A method of obtaining a protective coating on a substrate of magnesium or its alloy, including microarc oxidation (MAO) of a substrate of magnesium or its alloy, characterized in that the coating is obtained in the following three stages:  on the first of which a primary nanostructured nonmetallic inorganic coating with a thickness of at least 15 μm is formed on the substrate, which produces the MAO of the substrate in an aqueous electrolyte containing from 10 to 40 g / l of water-soluble alkali metal metasilicates, from 2 to 10 g / l of water-soluble alkali metal fluorides and from 3 to 15 g / l of alkali metal hydroxides, when using a pulse current with an amplitude of voltage pulses of 400-450 V and a pulse duration of 250-350 μs,  at the second stage, the first chemical etching of the primary coating obtained at the first stage is carried out in an aqueous solution containing from 0.5 to 1.5 mol / L of phosphoric acid, from 0.5 to 1.5 mol / L of hydrogen fluoride or water-soluble fluoride, then form a secondary nanostructured nonmetallic inorganic coating on the substrate using an electrolyte and an MAO mode of the first stage, after which a second chemical etching of the coating is carried out in an aqueous solution of the first chemical etching, and in the third stage, an aqueous electrolyte and the MAO mode of the first stage are used with the formation of a finish nanostructured nonmetallic inorganic protective coating on the substrate. 2. The method according to p. 1, characterized in that the formation of the coating at each of the above steps is carried out until a thickness of at least 15 microns is achieved. 3. The method according to p. 2, characterized in that the formation of the coating is carried out, preferably, to achieve a thickness of 30 microns. 4. The method according to p. 1, characterized in that the aqueous electrolyte contains 30 g / l of nine-sodium metasilicate, 4 g / l of sodium fluoride and 6 g / l of potassium hydroxide.

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