RU2617088C1 - Method for producing anticorrosive wear-resistant coatings on magnesium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method comprises forming a micro-nano-porous ceramic-ike layer by plasma electrolytic oxidation (PEO) of the alloy surfaces in an electrolyte containing sodium silicate and sodium fluoride in the bipolar mode, the process is carried out potentiodynamically at a voltage increasing at a rate of 16-18 V/min to 260-270 V, and during anodic surface polarization and potentiostatically at a voltage of -(30-50) V during its cathodic polarization, followed by polymer film applying on the formed porous layer by submersing for 10-15 s in a polyvinylidene fluoride solution -(C₂H₂F₂)-_n in N-methil-2-pyrrolidone (C₅H₉NO)_n with thermal treatment of the applied polymer at 70-110°C for 3-5 hours.

EFFECT: improvement in strength properties, abrasion resistance and improved adhesion of the produced coatings while enhancing environmental safety and process simplification.

3 cl, 2 tbl, 3 ex, 5 dwg

Description

The invention relates to the production of protective coatings on metal surfaces, in particular, to methods for applying anti-corrosion wear-resistant coatings to wrought magnesium alloys, which are characterized by low specific gravity and high strength, as well as ease of machining and are promising structural materials for mechanical engineering, automotive, aerospace engineering, electrical and radio engineering, for the production of computer equipment and other industries.

However, the practical use of magnesium alloys is significantly limited by their low resistance to corrosion and wear due to the formation of a porous, unstable layer of magnesium oxide on the metal surface under atmospheric conditions.

In this regard, it is urgent to develop methods designed to protect the surface of products and structures made of magnesium alloys by applying a coating that prevents corrosion and at the same time has residually high mechanical properties.

A known method of applying a protective anti-corrosion composite coating with high hardness and resistance to abrasion on magnesium alloys (CN No. 102234803, publ. 2011.11.10), including the formation on the surface of the alloy of a porous layer of oxide ceramics by plasma electrolytic oxidation (PEO) in the anode mode, applying a pore-filling organic coating by spraying acrylonitrile-butadiene-styrene resin or varnish and chemical precipitation of the final layer containing nickel and phosphorus. Despite the rather large thickness of the resulting multilayer coating, its protective properties decrease over time, since the organic layer is subject to sufficiently rapid aging and destruction, as a result of which the strength and integrity of the entire multilayer coating are violated and aggressive ions gain access to the metal surface.

A known method of applying a composite nanostructured protective coating on the surface of a magnesium alloy (CN No. 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 improving the strength properties, and due to its aging with time and the appearance of concomitant microscopic defects, aggressive environmental ions gain access to the ceramic-like layer, and then through its pores to the metal surface. As a result, during operation, the protective properties of the coating decrease.

A known method of producing protective coatings on magnesium alloys (RU No. 2543580, publ. 2015.03.10), including the formation on the alloy surface of a ceramic-like layer by plasma-electrolytic oxidation in bipolar mode in an aqueous electrolyte containing sodium silicate and sodium fluoride, with a uniform increase in voltage from 0 to 250-270 V and the effective current density of 0.5-1.0 a / cm ² for a period of anodic polarization of the metal surface and at a constant voltage - (25-30) The polarization at the cathode, followed by coating on a normed oxide-ceramic layer of 8-hydroxyquinoline and the heat treatment by immersion. Obtained in a known manner, the coating exhibits high corrosion resistance, but does not have a sufficiently high hardness and abrasion resistance for use in structures operating under conditions requiring high mechanical properties.

Closest to the claimed is a method of applying to the surface of a magnesium alloy a superhydrophobic corrosion-resistant coating (CN No. 102817063, publ. 2012.12.12), comprising the formation of a ceramic-like layer with anticorrosive properties with a micro-nanostructure of plasma electrolytic oxidation, which serves as the basis for applying polymer superhydrophobic film that prevents contact between the corrosive medium and the metal surface, and PEO is carried out in an electrolyte containing sodium molybdate, a guide sodium oxide, sodium dichromate or potassium, triethanolamine, polyethylene glycol, at a voltage of 70-160 V, a polymeric superhydrophobic film obtained from an organic composition containing a triazine derivative and a fluorine-substituted groups merkaptanas through its electrodeposition under constant current density 0.1-10 mA / cm ² . The coating exhibits quite high anticorrosion and hydrophobic properties: a decrease in the corrosion current in a NaCl solution by three orders of magnitude, the absence of corrosion after 96 hours exposure to salt spray, and a contact angle of up to 169.2 °.

However, the mechanical properties of the coating obtained by a known method do not provide the possibility of its operation in conditions that impose high requirements on the adhesion of the coating, its strength properties, 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, which also creates certain restrictions for widespread use and is associated with additional costs. The use of electrodeposition to form a polymer film complicates the method (its hardware).

The objective of the invention is to develop an environmentally safe and suitable for large-scale use of a method for producing magnesium alloys of protective coatings with high adhesion, having both high corrosion resistance and high mechanical properties.

The technical result of the method consists in increasing the strength properties, resistance to abrasive wear and improving the adhesion of the resulting coatings while improving the environmental safety of the method and its simplification.

The specified technical result is achieved by the method of obtaining a corrosion-resistant and wear-resistant coating on magnesium alloys, including the formation of a porous ceramic-like layer on the surface of the alloy by plasma electrolytic oxidation (PEO), followed by applying a polymer film to the formed porous layer, in which, in contrast to the known PEO carried out in an electrolyte containing sodium silicate and sodium fluoride, and the process is conducted in bipolar mode, potentiodynamically at a voltage uniformly increasing from 30 to 260-270 in at a rate of 16-18 I / min, during the anodic polarization the sample and potentiostatically at a voltage of - (30-50) in it during cathodic polarization, the polymer film is applied by impregnation, immersion in a solution of polyvinylidene - (C ₂ H ₂ F ₂ ) - _n in N-methyl-2-pyrrolidone (C ₅ H ₉ NO) _n for 10-15 seconds, followed by heat treatment at 70-110 ° C for 3-5 hours.

In an advantageous embodiment of the method, plasma-electrolytic oxidation is carried out in an electrolyte of the following composition, g / l:

sodium silicate Na ₂ SiO ₃ 10-50 sodium fluoride NaF 1-8

Optimum coating performance is ensured by immersing three times in a solution of polyvinylidene fluoride - (C ₂ H ₂ F ₂ ) - _n in N-methyl-2-pyrrolidone (C ₅ H ₉ NO) _n , each of which is accompanied by heat treatment.

The method is as follows.

After standard preparation of the magnesium alloy sample, a ceramic-like layer with micro- and nanopores is formed on its surface using the plasma electrolytic oxidation (PEO) method.

The oxidation is carried out in an electrolyte containing, g / l: sodium silicate Na ₂ SiO ₃ 10-50 and sodium fluoride NaF 1-8, under conditions of plasma microdischarges in bipolar mode.

The optimal characteristics of the PEO coating are ensured during the formation of a ceramic-like layer of a dense structure with micro- and nanoscale pores, which has high adhesion and is characterized by high strength and wear resistance, during PEO under certain conditions at a given, experimentally established, rate of coating formation.

The rate of formation of the PEO coating depends on the voltage growth rate, while the optimal formation rate, which allows one to obtain coatings with the best characteristics and the lowest defectiveness, is maintained at a voltage growth rate during anode polarization in the range of 16-18 V / min.

An increase in the voltage growth rate above 18 V / s leads to an increase in the number and size of pores and the formation of solid, but not having high electrochemical properties, PEO coatings. A voltage growth rate of less than 16 V / min leads to the formation of loose PEO coatings, a significant decrease in their thickness and deterioration of mechanical properties due to a decrease in peak currents during oxidation and insufficient microdischarge energy.

To ensure the necessary (optimal) rate of formation of the PEO coating during anodic polarization of the alloy sample, the rms voltage U _rms during the oxidation process is increased to 260-270 V at a speed of 16-18 V / min, with a controlled increase in voltage starting from 30 V. The current density j = 0.15-0.25 A / cm ² . With cathodic polarization of the sample, the process is carried out potentiostatically at a voltage of (30-50) V. Considering the rate of coating formation and the duty cycle of the polarizing signal (50%) of the thyristor current source used, the oxidation time is 13-15 minutes.

According to x-ray phase analysis, the formed layer with a thickness of 16-18 μm contains MgO and Mg ₂ SiO ₄ .

Then, a polymer film of polyvinylylene fluoride (PVDF) is applied to the obtained PEO layer by impregnation. A sample of a magnesium alloy coated with a PEO layer is dipped into a prepared solution of PVDF polymer - (C ₂ H ₂ F ₂ ) - _n in a solvent N-methyl-2-pyrrolidone (C ₅ H ₉ NO) and incubated for 10-15 s . After drying at elevated (70-110 ° C) temperature, the polymer application procedure is repeated. An increase in the immersion ratio leads to a significant improvement in the indicators of wear resistance and corrosion resistance of the coating, as well as an increase in the adhesion of the polymer film, while triple immersion, each of which is accompanied by drying, is optimal.

Polyvinylidene fluoride is a partially fluorinated polymer, which differs among fluoropolymers in the highest strength properties, hardness, as well as the purity of the material and the absence of technological additives. In addition, PDF is characterized by high thermal stability, good resistance to chemical attack, ultraviolet and gamma radiation; excellent physical and mechanical properties and exhibits good surface quality with a low coefficient of friction and high resistance to abrasion.

Polymer PVDF film seals pores on the surface of the PEO layer and significantly reduces surface roughness, which helps to improve the electrochemical and antifriction properties of the coating as a whole.

Thus, the composite coating formed in accordance with the proposed method exhibits high protective properties.

The solution of PVDF in N-methyl-2-pyrrolidone is characterized by stability, the absence of undissolved particles and sedimentation, which is a significant advantage when applying a polymer film in comparison with other fluoropolymers (polytetrafluoroethylene).

In addition, with high quality, PVDF is one of the cheapest fluoropolymers.

The characteristics of the coatings obtained are clearly shown in FIG. 1-5.

In FIG. Figure 1 shows an X-ray diffraction pattern of a PEO layer formed on a deformable MA8 magnesium alloy.

In FIG. Figure 2 shows the curves of the dependence of the friction coefficient on the number of cycles of deposition of the polymer film (curve 1 for the PEO layer; 2 for the PEO layer with the PVDF film applied by single dipping; 3 - for the PEO layer with the PVDF film applied by three dipping).

In FIG. Figure 3 shows the results of scratch testing at a load of 20 N for coatings formed by three-fold deposition of PVDF on a PEO layer (F _n is the applied load; Pd is the indenter penetration depth under load; μ is the friction coefficient; Rd is the residual indenter penetration depth after unloading ; L _c2 is the load at which peeling of the coating sections from the substrate occurs; L _c3 is the critical load at which the indenter penetrates to the metal).

In FIG. 4 shows potentiodynamic curves (1 - samples without coating; 2 - PEO coating; 3 - PEO coating with a single PVDF film; 4 - with a triple PVDF film).

FIG. 5 - impedance spectra (1 - uncoated samples; 2 - PEO coating; 3 - PEO coating with a once-applied PVDF film; 4 - with a triple-applied PVDF film).

An analysis of the tribological (antifriction) parameters of the coating obtained by the proposed method shows that with a single deposition of a polymer film on the PEO layer, the wear resistance of the coating increases almost 2 times (Fig. 2, Table 1). Increasing the application rate up to 3 times leads to a significant increase in the number of abrasion cycles to metal (more than 25 times).

According to the data in table 1, the appearance of areas where peeling of the coating occurred is observed at a load L _c2 of 18.2 ± 0.2 N, the critical load L _c3 at which the indenter penetrates the metal is 18.8 ± 0 , 3 N, which is significantly higher than the critical load for the PEO coating (13.8 ± 0.2 N).

The data of table 2, as well as FIG. Figures 4 and 5 show that for a coating obtained by the proposed method with a three-fold deposition of PVDF polymer on a PEO layer, the corrosion current density decreases by more than 4 orders of magnitude compared to values for an uncoated alloy: from I _to = 3.3⋅ 10 ^-5 A / cm ² to 6.0 · 10 ^-9 A / cm ² , in comparison with the data of the prototype, which detects a decrease in the corrosion current in a NaCl solution with a concentration of 0.1 mol / L by 3 orders of magnitude.

The shift of the corrosion potential towards the anode values indicates a decrease in the defectiveness of the coatings.

Thus, the proposed method provides obtaining coatings with high adhesion on deformable magnesium alloys that exhibit high anticorrosion and tribological properties, and, due to the presence of a polymer layer, the difference in tribological, adhesive and electrochemical characteristics of coatings obtained on different alloys is practically insignificant, which finds experimental confirmation.

Examples of specific implementation of the method

Samples of rectangular shape with a size of 20 mm × 30 mm × 1.5 mm, made of wrought magnesium alloys MA8, MA14 and MA20, were subjected to processing.

A TER4-100 / 460N-2-2UHL4 reversing thyristor unit with a frequency of polarizing pulses of 300 Hz and a duty cycle of 50% was used as a current source.

To obtain a polymer solution - (C ₂ H ₂ F ₂ ) - _n . in N-methyl-2-pyrrolidone C ₅ H ₉ NO) the PVDF polymer powder (MTI Corporation, USA) was stirred in a solvent using an IKA C-MAG HS 7 magnetic stirrer (Germany) at a speed of 250 revolutions per minute for 12 hours.

The thickness of the coatings was measured at three points using an eddy current thickness gauge VT-201 (Russia) with a 3% measurement error.

The phase composition of the surface layers was determined on a D8 Advance X-ray diffractometer (Cu Kα radiation).

Tribological tests were carried out on an automated friction machine Tribometer (CSM Instruments, Switzerland). After tribological testing, the cross-sectional area of the wear track was estimated using a MetekSurtronic 25 precision contact profiler.

The adhesion properties of the surface layers were studied by sclerometry (a research method that allows you to determine the friction coefficient of the test material and the nature of the destruction of the surface layers, to assess the adhesion and the level of critical loads at which chevron cracks appear in the coating, etc.) on a Revetest Scratch Tester (CSM Instrument) , Switzerland) with a uniform increase in load from 1 to 20 N with a loading speed of 8.5 N / min and a track length of 5 mm. For scratch testing, a Rockwell diamond indenter with an apex angle of 120 ° and a radius of 200 μm was used.

The study of electrochemical parameters was carried out in a 3% NaCl solution by polarization characteristics, as well as by impedance spectroscopy using VersaSTAT MS (Princeton Applied Research, USA).

Example 1

A sample of magnesium alloy MA8 of the Mg-Mn-Ce system (wt.%: 1.5-2.5 Mn; 0.15-0.35 Ce; impurities up to 0.3; Mg - the rest) was oxidized in the electrolyte composition, g / l: Na ₂ SiO ₃ 10 and NaF 8 in the bipolar mode, while the rms voltage U _rms in the anode phase was increased from 30 to 270 V at a speed of 16 V / min at a current density of j = 0.15 A / cm ² , and in the cathode phase, the process was conducted potentiostatically at a voltage of -50 V. The duration of the phase and pause of the anode and cathode components was set to 10 ms. The oxidation time was 15 minutes.

The polymer film was applied by immersing 3 times in a solution of PVDF polymer powder in N-methyl-2-pyrrolidone C ₅ H ₉ NO), each lasting 10 s, each dipping being accompanied by drying at a temperature of 70 ° C. The time of each drying of the PVDF polymer on the surface of the sample did not exceed 5 hours.

The thickness of the resulting white coating with a pink tint and a transparent, hard film on the surface of the oxide layer was 18 μm.

An analysis of the tribological and adhesive properties showed that the coating of this sample withstood 25 times more revolutions (68250), in comparison with the oxide layer, and the critical load at which the indenter penetrated to the metal was 18.8 ± 0.3 N, which is approximately 1.5 times higher than for PEO coating (13.8 ± 0.2 N).

The study of the electrochemical properties of the sample showed a decrease in the corrosion current density by more than 4 orders of magnitude: from I _to = 3.3 × 10 ⁻⁵ A / cm ² (corrosion current density for an unprotected metal) to 6.0 × 10 ⁻⁹ A / cm ² .

Example 2

Sample of magnesium alloy MA14 of the Mg-Zn-Zr system wt.%: Zn 5.0-6.0; Zr 0.3-0.9; The rest Mg) was oxidized under the conditions of Example 1 in an electrolyte of the composition, g / l: Na ₂ SiO ₃ 50 and NaF 1, in the anode phase at a current density of j = 0.15 A / cm ² and an increase in the rms voltage from 30 to 260 V s at a speed of 18 V / min, and in the cathode phase at a voltage of -30 V. The oxidation time is 13 minutes.

The PVDF polymer film was applied under the conditions of example 1 with drying at a temperature of 90 ° C. Each drying time did not exceed 4 hours

The thickness of the resulting white with a pink tint coating with a transparent, hard film on the surface of the oxide layer is 16 μm.

The properties of the resulting coating are similar to the properties of the coating obtained in example 1.

Example 3

A sample of magnesium alloy MA20 of the Mg-Zn-Zr system (wt.%: Zn 1.25; Zr 0.18; Mg balance) was oxidized under the conditions of example 1 in the electrolyte of the composition, g / l: Na ₂ SiO ₃ 50 and NaF 8 in bipolar mode in the anode phase at a current density of j = 0.25 A / cm ² and an increase in the rms voltage value from 30 to 270 V at a speed of 16 V / min, and in the cathode phase it is potentiostatic at a voltage of -50 V. Oxidation time 15 min .

The polymer film was applied by 3-fold immersion, lasting 15 seconds each, followed by drying at a temperature. 110 ° C. The time of each drying of the PVDF polymer on the surface of the sample did not exceed 3 hours

The thickness of the resulting white with a pink tint coating with a transparent, hard film on the surface of the oxide layer of 17 μm.

The properties of the resulting coating are similar to the properties of the coating obtained in example 1.

Claims (4)

1. A method of obtaining an anti-corrosion and wear-resistant coating on the surface of a magnesium alloy, comprising forming a porous ceramic-like layer by plasma electrolytic oxidation (PEO) on the surface of the alloy, followed by applying a polymer film to the formed layer, characterized in that the PEO is carried out in an electrolyte containing sodium silicate and sodium fluoride, in bipolar mode, potentiodynamically at a voltage uniformly increasing from 30 to 260-270 V at a speed of 16-18 V / min, during the anodic polarization of the sample a and potentiostatically at a voltage of (30-50) V during its cathodic polarization, and the polymer film is applied by impregnation by immersion in a solution of polyvinylidene fluoride - (C ₂ H ₂ F ₂ ) - _n in N-methyl-2-pyrrolidone (C ₅ H ₉ NO) _n for 10-15 s followed by heat treatment at 70-110 ° C for 3-5 hours 2. The method according to p. 1, characterized in that the PEO is carried out in an electrolyte of the following composition, g / l: sodium silicate Na ₂ SiO ₃ 10-50 sodium fluoride NaF 1-8 3. The method according to p. 1, characterized in that the impregnation and heat treatment is carried out three times.

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