RU2614917C1 - Method for protective composite coatings production on magnesium alloy - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method comprises plasma electrolytic oxidation (PEO) of the alloy surface in silicate-fluoride electrolyte in a bipolar mode, in two steps. During the first 200-240 s of alloy surface anodic polarization, process is performed galvanostatically at a current density of 0.5-0.7 A/cm², until the anode voltage reaches 250-270 V, and during cathodic polarization, - potentiostatically at a cathode voltage of -(30-40) V. Then, during 600-700 s, oxidation is continued at an anode voltage, decreasing to 200-210 V, and cathode voltage - to -(8-10) V. A fluoropolymer layer is applied to the formed substrate surface by dipping into a tetrafluoroethylene telomers solution in acetone, followed by coating drying and heat-treating the at 250-275°C for at least one hour. Fluoropolymer coating is repeated 2-3 times.

EFFECT: increased method technology at increased service life and improved corrosion resistance, anti-friction and hydrophobic properties of coatings.

3 cl, 2 dwg, 3 tbl, 4 ex

Description

The invention relates to the production of protective fluoropolymer-containing composite coatings with a high anticorrosive, antifriction and hydrophobic properties, as well as a long service life on a deformable magnesium alloy MA8 and can be used in aircraft and automotive industries, electrical and radio engineering, computer, space and defense equipment.

A known method of producing protective coatings on magnesium alloys (RU 2357016, publ. 2009.05.27) using plasma electrolytic oxidation (PEO) in an electrolyte containing 12-30 g / l sodium silicate and 5-10 g / l sodium fluoride, bipolar mode with the ratio of the periods of the anodic and cathodic polarization and an increase in voltage values from 0 to 250-300 V at a speed of 0.25-0.28 V / s and a current density of 0.5-1.0 A / cm ² with anodic polarization of the product and a constant voltage value of 25-30 V at cathodic polarization of the product for 8-20 minutes. The anticorrosion properties of the coating obtained in a known manner deteriorate during operation in a corrosive environment, since aggressive ions penetrate the through pores of the coating, which eventually leads to the appearance of microdefects in it and an increase in the corrosion rate. The known method does not provide a sufficiently long service life of anti-corrosion coatings.

A known method of applying a composite nanostructured protective coating on the surface of a magnesium alloy (CN 103668393, publ. 2014.03.26), including microarc oxidation with the formation of a porous ceramic-like layer and spraying the surface of the formed water-based paint layer. The mechanical characteristics of the coating obtained in a known manner, including hardness and wear resistance, are determined by the quality of the ceramic-like layer. Water-based paint does not make a significant contribution to increasing the strength properties and hydrophobicity of the coating, and over time, due to its aging and the appearance of accompanying microscopic defects, aggressive environmental ions gain access to the ceramic-like layer and through its pores to the metal surface. Thus, during operation, the contact surface area of the metal with the aggressive medium increases, the protective properties of the coating are reduced.

A known method of applying a superhydrophobic anticorrosion coating to the surface of a magnesium alloy (CN 102817063, publ. 2012.12.12), comprising forming using a plasma electrolytic oxidation (PEO) a micro-nanostructure ceramic-like layer with anticorrosive properties, which serves as the basis for applying a polymer superhydrophobic film, preventing contact between the corrosive medium and the metal surface, and PEO is carried out in an electrolyte containing sodium molybdate, sodium hydroxide, sodium dichromate Whether potassium, triethanolamine, polyethylene glycol, when the voltage values of 70-160 V, a polymeric superhydrophobic film obtained from an organic composition containing a triazine derivative and a fluorine-substituted mercaptan groups, through its electrodeposition under constant current density 0.1-10 mA / cm ^2. The coating exhibits quite high anticorrosive and hydrophobic properties: a decrease in the corrosion current in the NaCl solution by three orders of magnitude, the absence of corrosion after 96 hours in a salt spray chamber, and a contact angle of up to 169.2 °. However, the mechanical properties of the coating obtained in a known manner do not provide the possibility of its operation in conditions that impose high demands on the hardness of the coating and resistance to abrasion. In addition, the presence of hexavalent chromium compounds and organic components in the oxidation electrolyte composition requires special measures to protect personnel and the environment, including waste disposal.

A known method of producing protective coatings on magnesium alloys (RU 2543580, publ. 2015.03.10), including the formation of a ceramic-like layer by plasma electrolytic oxidation (PEO) of the alloy surface in an aqueous electrolyte containing sodium silicate and sodium fluoride, in bipolar mode with uniform increase voltage from 0 to 250-270 V and an effective current density of 0.5-1.0 A / cm ² during the period of anodic polarization and a constant voltage value of (25-30) V during the period of cathodic polarization of the surface, followed by 8 -about sihinolina formed on the oxide-ceramic layer and heat treatment. Obtained in a known manner, the coating exhibits high corrosion resistance, but does not have sufficiently high characteristics (hardness, antifriction properties and resistance to abrasion) for use under conditions that impose high demands on mechanical properties. The hydrophobicity of the known coating is also low.

A known method of treating the surface of a magnesium alloy (CN 10146943, publ. 2009.07.01), comprising a PEO process in a uniformly distributed particles of polytetrafluoroethylene (PTFE) electrolyte with the simultaneous deposition of particles of PTFE on the surface of a magnesium alloy and obtaining a compact composite self-lubricating coating with excellent anti-corrosion and hydrophobic properties. The method can be used to process various parts of magnesium alloys with the application of protective self-lubricating coatings. However, the fluoropolymer coating applied from the suspension may have defects and inhomogeneities, in addition, the residues of emulsifiers / stabilizers contained in the suspension, getting into the coating, worsen its properties. To obtain a defect-free coating, it is necessary to apply several successive layers. In addition, in the process of coating formation, thermal destruction of PTFE particles is possible under the influence of plasma microdischarges in the channel of which the temperature can exceed 3000 ° С (according to other sources, it reaches 20,000 ° С), since PTFE retains its properties only up to 400 ° С (the beginning of destruction Teflon is already observed in the temperature range 260-327 ° C). This, in turn, can significantly reduce the performance of the coating.

Closest to the claimed is a method of electrochemical oxidation of the surface of aluminum, magnesium, aluminum alloys and magnesium alloys with a uniform polymer coating (DE 4124730, publ. 1993.01.28), comprising intercalating a fluoropolymer or its precursor into pores and capillaries formed by anodizing the surface layer in particles, the size of which does not exceed the diameter of the surface pores and capillaries and is 1-50 nm (mainly 1-10 nm), and subsequent heat treatment with melting and compaction of fluoride Limer or full or partial polymerization of its precursor directly in the pores of the coating. The fluoropolymer or its precursor (mainly fluorinated monomer or oligomer) is applied to the anodized surface for no more than 24 hours after anodizing. The obtained fluoropolymer coating is characterized as firmly bonded to the substrate, homogeneous, with low corrosion, exhibiting non-stick properties, strength and wear resistance.

However, anodizing does not provide the formation of a substrate with a highly developed surface and a branched pore system, which is a prerequisite for applying a fluoropolymer layer having a high adhesion of sufficient thickness. As a result, the corrosion resistance, antifriction and hydrophobic properties of the coating obtained in a known manner do not reach high values, its protective properties and service life in some cases of application are insufficient. In addition, anodizing requires careful multi-stage preparation of the metal surface, which includes a number of operations (rough cleaning; grinding; polishing; degreasing, etching; washing), the presence of which, along with tough time frames (24 hours), which determine the conditions for the formation of a high-quality fluoropolymer-containing layer, reduces the manufacturability of the known method.

The objective of the invention is to provide a technologically advanced method of forming a multifunctional composite fluoropolymer-containing coating on a magnesium alloy with high protective properties and a long service life.

The technical result of the proposed method is to increase its manufacturability while improving corrosion resistance, antifriction and hydrophobic properties, as well as the service life of the formed coating.

The specified technical result is achieved by a method of producing protective composite coatings on a magnesium alloy, including electrochemical oxidation of the alloy surface to produce a porous substrate, applying a fluoropolymer precursor to the substrate and forming a fluoropolymer layer from the deposited precursor, as well as heat treatment, in which, unlike the known one, electrochemical oxidation the surface is performed by the PEO in an electrolyte containing in g / l: sodium silicate Na ₂ SiO ₃ -5H ₂ O 15-30 and sodium fluoride NaF 5-10, in bipolar th mode, wherein in the first process is carried out with 200-240 galvanostatically during anodic polarization at a current density of 0.5-0.7 A / cm ² to the anode voltage V and 250-270 potentiostatically during cathodic polarization, while maintaining the voltage at the cathode - (30-40) V, after which oxidation is continued for 600-700 s, reducing the anode voltage to 200-210 V and increasing the cathode to - (8-10) V, while the fluoropolymer layer is formed from a solution of tetrafluoroethylene telomers in acetone, the resulting coating is dried in air at room temperature and subjected to rmoobrabotke at 250-275 ° C for at least one hour.

In an optimal embodiment of the method, a solution of tetrafluoroethylene telomers on a PEO-formed substrate is applied by short-term dipping.

Also in an optimal embodiment of the method, the operation of applying the fluoropolymer is repeated 2-3 times.

The application of a layer of a polymer film on the developed surface of substrates obtained by electrochemical oxidation, in the general case, significantly improves their protective characteristics. In particular, the use of polytetrafluoroethylene (PTFE), which has a set of valuable properties (high chemical and atmospheric resistance, low friction coefficient, hydrophobicity, incombustibility, and prolonged lack of aging), allows one to obtain multifunctional coatings with high protective properties.

However, the use of polytetrafluoroethylene is limited by its insolubility in almost all known solvents, poor adhesion, the difficulty of applying sufficiently thin and thickness-controlled coatings, due to its high molecular weight, as well as the molecular and supramolecular structure of the teflon and fluoroplast-4 manufactured by the industry. The synthesis of low molecular weight fluorine-containing polymers by the telomerization method (the type of polymerization occurring in the presence of telogen chain carriers, which may be alcohols, aldehydes, hydrofluorocarbons, iodides, dibromochloroethanes, etc.) allows the production of low molecular weight soluble oligomers (telomeres ≤10-20.

The method is as follows.

The prepared parts (plates) of the wrought magnesium alloy MA8 (Mn 1.5-2.5 wt.%; Ce 0.15-0.35 wt.%; Mg - the rest) are subjected to electrochemical modification by plasma electrolytic oxidation (PEO) in silicate fluoride electrolyte in bipolar mode. The PEO operation is carried out in two stages: the first stage, lasting 200-240 s, is carried out galvanostatically during the anodic polarization of the magnesium alloy plate at a current density of 0.5-0.7 A / cm ² up to an anode voltage of 250-270 V, while during cathodic polarization, the cathodic voltage is kept constant (potentiostatically maintained) - (30-40). At the second stage, oxidation is continued for 600–700 s, reducing the anode voltage to 200–210 V (the rate of voltage decrease is approximately 0.06–0.12 V / s) and increasing the cathode to –– (8–10) V (the rate of voltage increase is about 0.03-0.06 V / s).

The conditions for PEO, namely, high temperatures in the discharge channels in the immediate vicinity of the treated surface and specific plasma-chemical reactions in the discharge zone, provide the formation of a ceramic-like layer containing high-temperature silicates phases, the structure of which is characterized by the presence of narrow (micro- and nanoscale) branched non-through pores. According to x-ray phase analysis, the coating contains MgF ₂ , MgO and Mg ₂ SiO ₄ . The formed PEO layer with a thickness of 14-18 μm, which has high adhesion to the metal surface and exhibits high values of microhardness, wear resistance and corrosion resistance, is the basis for creating composite coatings containing a fluoropolymer.

An organofluorine material for creating the indicated composite coatings is a solution of tetrafluoroethylene (TFE) telomeres in acetone obtained by radiation-chemical synthesis, according to which the free-radical telomerization reaction is initiated by ionizing γ-radiation, which allows the telomerization process to be carried out without introducing it into the system special substances initiating the reaction, polluting the final product. In this case, the initiator of the reaction is the solvent radicals formed under the action of radiation, and the solvent molecules act as a chain transmitter (telogen).

The TFE telomere used in the solution corresponds to the gross formula R ₁ - (CF ₂ = CF ₂ ) _n -R ₂ , where the functional end groups are R ₁ , R ₂ - Н, СН ₃ , СН ₂ ССОН ₃ , and the number n (TFE length block) does not exceed 15.

The liquid phase state of the synthesized telomeric tetrafluoroethylene allows the use of solution coating technologies to produce a thin film of uniform thickness.

The fluoropolymer layer is applied mainly by immersion of PEO-coated samples in a telomeric TFE solution with an exposure in the solution of no more than 10-15 s, followed by drying in air for about 5-20 minutes at 22-25 ° C until the acetone evaporates and the coating is heat treated with a fluorine-organic product precipitate embedded in the pores and formed on the surface for at least 1 hour at 250-275 ° C. In an advantageous embodiment of the method, the application of the fluoropolymer is repeated at least two times.

The positive effect of the multiple TFE telomere deposition on the continuity of the fluoropolymer film on the surface of the PEO layer and the decrease in porosity of the formed composite coating is confirmed by the analysis of images obtained by scanning electron microscopy (SEM). The surface topography becomes smoother.

In FIG. 1 shows images made at various magnifications: a — PEO coatings; as well as composite coatings obtained by the proposed method as a result of heat treatment at 250 ° C: b - after a single, c - after a double, d - after a three-time deposition of telomere TFE.

The data characterizing the anticorrosive properties of the obtained composite coatings, which are shown in Table 1, show a significant decrease in the density of corrosion currents and the refinement of the corrosion potential. In particular, when the polymer is applied three times, the corrosion current density I decreases by more than 2 orders of magnitude compared to the initial PEO layer.

Note:

hereinafter KP-1x - a composite coating with a single coating, KP-2x - with a double coating, KP-3x - with a triple coating of a fluoropolymer.

(полного сопротивления переменному току) при нанесении покрытия предлагаемым способом увеличивается до 10⁷ Ом⋅см², т.е. почти на 3 порядка по сравнению с ПЭО-покрытием, не подвергнутым обработке теломером ТФЭ, что объясняется заполнением пор покрытия расплавленным под воздействием температуры фторполимером, равномерно распределенным по поверхности.Impedance module

(impedance to alternating current) when applying the coating by the proposed method increases to 10 ⁷ Ohm⋅cm ² , i.e. almost 3 orders of magnitude compared to a PEO coating not subjected to TFE telomere treatment, which is explained by filling of the coating pores with a fluoropolymer molten under the influence of temperature uniformly distributed over the surface.

Obtained by the proposed method, a composite fluoropolymer-containing coating exhibits high resistance to aggressive media and greater stability compared to a coating without a polymer film. The appearance of the first hydrogen bubbles on its surface was observed 540 s after applying a drop of hydrochloric acid, which significantly exceeds the time for a PEO coating (95 s) and for a magnesium alloy without coating (0 s).

The data characterizing the hydrophobic properties of the obtained coatings are shown in table 2 of the surface.

(3% NaCl solution used as test fluid).

Values of the wetting angle (contact angle) in excess of 150 ° indicate superhydrophobicity of the resulting coatings.

A study of the tribological characteristics of the obtained composite coatings shows that the presence of a fluoropolymer film acting as a dry lubricant in the contact zone allows one to increase the number of cycles during which complete abrasion of the heteroxide layer occurs by at least 2 times. Three times applied fluoropolymer layers, thermally treated at 250 ° C, begin to collapse only after 120,000 cycles (Fig. 2). As the abrasion of the coating, an increase in the friction coefficient is observed from μ = 0.03 with dry sliding on the surface of the undisturbed fluoropolymer film to μ = 0.55 and higher with complete abrasion of the coating. In this case, the beginning of the abrasion of the PEO layer is characterized by a sharp increase in the coefficient of friction.

Data on the assessment of the wear resistance of composite coatings are given in table 3 (measurement error of not more than 5%). The incorporation of a fluoropolymer into the pores of the PEO layer reduces coating wear by at least an order of magnitude. The maximum (more than 3 orders of magnitude) reduction in wear is ensured by the coating obtained by applying the TFE telomere three times and heat treatment at 250 ° C. As noted above, the number of abrasion cycles of the resulting coating increases to 120,000, which is more than 53 times their number for PEO coating.

Thus, the composite fluoropolymer-containing coatings obtained by the proposed method exhibit high corrosion resistance, as well as antifriction properties, which provide high wear resistance of the coatings. In addition, these coatings have hydrophobic / superhydrophobic properties, which contributes to a significant expansion of the field of practical application of this material.

Examples of specific implementation of the method

Rectangular plates 30 mm × 15 mm × 1 mm in size made of magnesium alloy MA8 (mass%: Mn 1.3-2.2; Ce 0.15-0.35; impurities up to 0.3) were used as samples ; Mg rest). Before oxidation, to standardize the surface, the samples were treated with sanding paper of various grain sizes (600, 800, 1200), washed with distilled water, and degreased with alcohol.

The process of forming PEO coatings was carried out on a facility equipped with an automated control and monitoring system coupled to a computer with appropriate software using a reverse thyristor unit current source with a frequency of polarizing pulses of 300 Hz and a duty cycle of 50%.

A solution of telomeres of TFE with a number average molecular weight of 550 in acetone of the Cherflon ^® trademark obtained at the cryochemistry and radiation chemistry laboratory of the Institute of Problems of Chemical Physics of the Russian Academy of Sciences, where a method of radiation-chemical synthesis of telomeres was used, was used as the starting organofluorine material.

The electrochemical properties of the coatings were studied using the VersaSTATMC electrochemical system (Princeton Applied Research, USA). The measurements were carried out in a three-electrode cell at room temperature in a 3% NaCl solution. The working area of the sample was 1 cm ² . To establish the potential of free corrosion (E _to ) before starting the electrochemical measurements, the samples were kept in solution for 15 minutes. The control of the experiment was carried out using the software VersaSTATMC (Princeton Applied Research, USA).

, Германия) путем измерения и анализа краевого угла капли, наносимой на исследуемую поверхность при помощи микрошприца. Измерения проводили методом цифровой обработки видеоизображения сидящей капли.The hydrophobicity of the obtained coatings was evaluated on a DSA100 device (

, Germany) by measuring and analyzing the contact angle of a drop applied to the test surface using a microsyringe. The measurements were carried out by digitally processing the video image of a sitting drop.

The wear resistance of the protective coatings according to the “ball-disk” scheme was studied on a Revetest-RST tribometer (CSM Instruments, Switzerland) using an Al ₂ O ₃ corundum ball with a diameter of 10 mm at constant linear velocity and load (50 mm / s and 10 N, respectively) with the subsequent determination of the depth of the trace and the calculation of the degree of wear of the surface layers. The diameter of the track was 10 mm.

The surface morphology of the obtained coatings was studied using an EVO 40 scanning electron microscope (Carl Zeiss, Germany).

The resistance of coatings in aggressive environments was determined by measuring the time period of the action of a drop of 36.5% HCl (chemically pure) before the appearance of hydrogen bubbles.

Example 1

The prepared sample of magnesium alloy was subjected to plasma electrolytic oxidation in an electrolyte of the following composition, g / l:

sodium silicate Na ₂ SiO ₃ ⋅ 5H ₂ O fifteen sodium fluoride NaF 10

for 240 s at a constant current density of 0.5 A / cm ² with increasing the voltage at the anode to 250 V and the voltage at the cathode held in the potentiostatic mode at a value of -30 V. Then oxidation was continued, reducing the anode voltage to 200 V at a speed 0.08 V / s and at the same time increasing the cathode voltage to -8 V at a speed of 0.04 V / s.

The washed and dried plate coated with a finely porous PEO coating was slowly immersed in a telomeric TFE solution, kept in solution for 15 s and immediately removed. Then it was dried in air at room temperature (up to 25 ° С) for 15-20 minutes. After complete evaporation of the acetone, the coating was heat treated at 250 ° C for 60 minutes. Re-immersed the plate in a telomeric solution, dried and subjected to heat treatment.

The time of exposure to an HCl drop before the appearance of hydrogen bubbles is 500 s. Other characteristics of the resulting coating are shown in tables 1-3 (250 ° C KP-2x).

Example 2

The prepared sample was processed according to example 1 in an electrolyte containing, g / l: sodium silicate Na ₂ SiO ₃ ⋅ 5H ₂ O 30, sodium fluoride NaF 10. At the first stage PEO: current density 0.7 A / cm ² , up to a voltage of the anode was increased to 270 V, the cathode was kept equal to -40 V, the oxidation time was 240 s. At the second stage of PEO: the voltage at the anode was reduced to 200 V, at the cathode it was increased to -8 V, the oxidation time was 660 s, while the rate of decrease in the anode voltage was 0.10 V / s, the rate of increase in the cathode voltage was 0.05 V / from.

Dipping the plate in a telomeric TFE solution, drying and heat treatment of the dried plate at 250 ° C was carried out three times.

The exposure time to the resulting HC1 drop before the appearance of hydrogen bubbles is 540 s. Other characteristics of the coating are given in tables 1-3 (250 ° C, KP-3x).

Example 3

The prepared sample was processed according to example 1 in an electrolyte containing, g / l: sodium silicate Na ₂ SiO ₃ ⋅ 5H ₂ O 30, sodium fluoride NaF 10. At the first stage PEO: current density 0.7 A / cm ² , the final voltage 270 V anode, constant -40 V at the cathode, oxidation time 200 s. At the second stage of PEO: the voltage decreases at the anode to 200 V, at the cathode it rises to -10 V, the oxidation time is 690 s, while the rate of decrease in the anode voltage is 0.10 V / s, the rate of increase in the cathode is 0.04 V / s .

Dipping the plate in a telomeric TFE solution, drying and heat treatment of the dried plate at 275 ° C was carried out twice.

The time of exposure of the resulting coating to a drop of HCl before the appearance of hydrogen bubbles is 480 s. Other characteristics of the coating are given in tables 1-3 (275 ° C, KP-2x).

Example 4

The prepared sample was processed according to example 1 in an electrolyte containing, g / l: sodium silicate Na ₂ SiO ₃ ⋅ 5H ₂ O 30, sodium fluoride NaF 5. At the first stage of PEO: current density 0.7 A / cm ² , voltage at the anode increased to 270 V, constant voltage -40 V at the cathode, oxidation time 240 s. At the second stage of PEO: the voltage at the anode decreases to 210 V, at the cathode it rises to -10 V, the oxidation time is 600 s, while the rate of decrease of the anode voltage is 0.10 V / s, the rate of increase of the cathode is 0.05 V / s .

Dipping the plate in a telomeric TFE solution, drying and heat treatment of the dried plate at 275 ° C was carried out three times.

The time of exposure to the resulting coating of a drop of HCl before the appearance of hydrogen bubbles is 500 s. Other characteristics of the coating are given in tables 1-3 (275 ° C, KP-3x).

Claims (3)

1. A method of producing protective composite coatings on a magnesium alloy, including electrochemical oxidation of the alloy surface to produce a porous substrate, applying a fluoropolymer precursor to the substrate with the formation of a fluoropolymer layer from it and heat treatment, characterized in that the electrochemical oxidation is carried out by plasma electrolytic oxidation (PEO) in an electrolyte containing in g / l: sodium silicate Na ₂ SiO ₃ ⋅5H ₂ O 15-30 and sodium fluoride NaF 5-10, in the bipolar mode, wherein during the first 200-240 with a ho e alloy surface anodic polarization process is carried out in galvanostatic mode at a current density of 0.5-0.7 A / cm ² to the anode voltage of 250-270 V, and during cathodic polarization in a potentiostatic mode with cathode voltage - (30-40 ) B, after which the oxidation is continued for 600-700 s, reducing the anode voltage to 200-210 V, and the cathode voltage to - (8-10) V, and the fluoropolymer layer is formed from a solution of tetrafluoroethylene telomers in acetone, the resulting coating is dried on air at room temperature and subjected to heat treatment at 250-275 ° C for at least one hour. 2. The method according to p. 1, characterized in that the solution of tetrafluoroethylene telomers on the substrate formed by the PEO method is applied by short-term dipping. 3. The method according to p. 1 or 2, characterized in that the operation of applying the fluoropolymer is repeated 2-3 times.

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