Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/026—Anodisation with spark discharge
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/024—Anodisation under pulsed or modulated current or potential
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/26—Anodisation of refractory metals or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/30—Anodisation of magnesium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• the present invention relates to the field of protective-coating deposition and particularly to the plasma electrolytic oxidation (PEO) of articles made of valve metals and alloys.
• PEO plasma electrolytic oxidation
• oxide ceramic coatings having an improved wear resistance, corrosion resistance, thermal resistance, and dielectric strength are formed on the surface of these articles for relatively short time.
• the method of forming the coatings according to the present invention may be used in the mechanical-engineering, automotive, aerospace and radio-electronic industries, and medicine, both in one-off production and in batch production.
• PEO methods involving high-frequency pulse electrolysis modes and a range of high polarization voltages is a new development in the PEO technology.
• WO 03/83181 discloses a method and apparatus for forming ceramic coatings on metals and alloys.
• the method allows the ceramic coatings to be formed on valve metals and alloys by using a current pulse rate from 500 to 10000 Hz.
• the drawback of the method is a waveform of anodic and cathodic current pulses supplied to electrodes, which has a spike at a leading edge. Such a waveform of the current pulses may lead to the occurrence of overcurrent in crucial power electronic elements of high-frequency transducers and makes it hard to choose these expansive elements correctly.
• US 20160186352 discloses a non-metallic coating and a method of its production. According to the method, bipolar voltage and current pulses having a trapezoidal waveform and a pulse rate 0.1-20 kHz are supplied to electrodes. An amplitude of anodic voltage pulses is maintained in a potentiostatic mode, while an amplitude of cathodic current pulses is maintained in a galvanostatic mode.
• the drawback of the method is that it involves using the voltage and current pulses with the trapezoidal waveform, which are less energy-efficient than pulses with a square waveform, thereby adversely impacting the performance of the PEO process.
• the power of the cathodic pulses may increase so much that it leads to the degradation of the already-formed coating.
• the main drawback of the inventions disclosed in US 20160186352 and WO 2008120046 is an absence of sufficiently long dead time within the period after the propagation of the anodic voltage and current pulses and before the cathodic voltage and current pulses. Such dead time is required to correct and restore concentrating and thermal conditions in near-electrode electrolyte layers.
• the main objective of the invention is to form, by using the method disclosed herein, an oxide ceramic coating having physical-mechanical and protective properties higher than those of the coatings formed by using the prior art PEO methods.
• the improvement of the physical-mechanical properties leads to improving practical protective characteristics of coatings, such as an abrasive and erosive wear resistance, a resistance to vibration and cavitation loads, a corrosion resistance.
• the improvement of the practical protective characteristics of the coatings allows increasing operational characteristics of articles with the coatings, as well as significantly extending the range of use thereof.
• Another objective of the invention is to provide the possibility of performing the PEO process with high speeds of oxide-ceramic coating formation by using short high-power (voltage and current) pulses, while avoiding undesired microplasma-to-arc discharge transitions and coating “loosening”.
• the oxidation process in intensive electrical modes allows not only increasing the process performance, but also obtaining the enhanced melted oxide ceramic coatings.
• the invention provides a method of forming a protective oxide ceramic coating on the surface of articles made of valve metals (aluminum, titanium, magnesium, zirconium, tantalum, niobium, beryllium) and alloys thereof, in which an article as an electrode is submerged together with a counter electrode in a bath filled with an aqueous alkaline electrolyte, and bipolar voltage pulses providing the implementation of the process in the PEO mode are supplied to the electrodes.
• valve metals aluminum, titanium, magnesium, zirconium, tantalum, niobium, beryllium
• PEO process conditions significantly impact both the process performance and coating quality, i.e. the physical-mechanical characteristics of the oxide ceramic coating.
• the invention describes a high-voltage high-frequency anode-cathode PEO method, which is a new promising development in the PEO methods.
• Processes of electroplasma-chemical reactions significantly speed up under the conditions of an extremely small pulse duration, a high pulse rate and high voltage pulse amplitudes.
• an ion migration speed rapidly increases in breakdown areas. All of this causes a substantial increase in the speed of oxide-ceramic coating formation.
• This invention involves modes in which an anodic voltage pulse is followed by quite long dead time and then a cathodic current pulse.
• Such dead time is required to equalize the concentration of an electrolyte in a near-electrode space and to absorb the heat resulted from discharges and electroplasma-chemical reactions by a metal substrate and the electrolyte. These processes occur due to the convection, diffusion and interaction of electrolyte ions to each other.
• the dead time has a minimum duration calculated based on the following ratio of the durations of the anodic pulse and the dead time: 1:5. It is the lowest required time for the relaxation and stabilization of a microplasma-breakdown process. A significant increase in the dead time will lead to a reduction of the PEO process performance.
• the above-indicated optimal duration of the period i.e. 30-300 microseconds, corresponds to the pulse rate 3.3-33 kHz, with the duration of the anodic pulses being 3-30 microseconds.
• the high speeds of oxide-ceramic coating formation are provided by using short high-power (voltage and current) pulses, assuming the avoidance of undesired microplasma-to-arc discharge transitions and coating “loosening”.
• the largest thickness of the coatings for relatively short oxidation time is achieved at high amplitude values of pulse voltages.
• Optimal thicknesses of the protective ceramic coatings are 20-100 microns, depending on a task to be solved and a material to be processed. These thicknesses are achieved for the oxidation time 5-20 minutes.
• the processing is performed at the following voltages: anodic voltages 600-1200 V and cathodic voltages 150-450 V, depending on the nature of materials to be processed.
• the amplitude of the anodic voltage is 900-1200 V
• the amplitude of the cathodic voltage is 250-450 V.
• the amplitude of the anodic voltage is 600-800 V
• the amplitude of the cathodic voltage is 150-250 V.
• High pulse voltages lead to an increase in the penetration depth of mictoplasma breakdowns (almost up to the metal substrate), which causes the formation of the coatings with a uniform composition and structure across the whole coating thickness.
• a voltage level is closely related to the effective current density.
• the main electrolysis parameters controlled in the PEO process are represented by the value of the effective current density (or average current) in the anodic and cathodic circuits.
• the current density impacts the number of microplasma discharges and, consequently, the process performance and the melting level of the coatings.
• the oxidation process is carried out at the effective current density 5-20 A/dm2 in the anodic period and 6-25 A/dm2 in the cathodic period, depending on the nature of the material to be processed.
• the effective current densities lower than the optimal values the hardness of the coatings is decreased, as well as oxidation performance is reduced.
• sizes of crystals in the coatings are increased and a coating porosity gets high, for which reason a coating strength and density is reduced.
• the technical result of the present invention consists in forming, by using the above-described intensive electrolysis modes, fully melted, homogeneous oxide ceramic coatings having a uniform thickness and unique physical-mechanical properties: a high hardness, elastic coefficient, adhesive and cohesive strength, and density.
• the oxidation process is carried out in a pulsed potentiostatic or pulsed galvanostatic mode within the anodic period, and within the cathodic period—in a pulsed potentiodynamic mode with a uniform rate of increase of the amplitude of the cathodic voltage pulses 1-3 V/min or in a pulsed galvanodynamic mode with a uniform rate of decrease of the amplitude of the cathodic current pulses 0.2-0.5 A/min.
• the slow rate of increase of the amplitude values of the cathodic voltage pulses is compensated by the slow rate of decrease of the amplitude values of the cathodic current pulses and, correspondingly, an average cathodic current.
• the PEO process is carried out in the pulsed galvanostatic mode within the anodic period and in the pulsed galvanodynamic mode within the cathodic period in order to increase coating quality reproducibility.
• the PEO process is carried out in the pulsed potentiostatic mode within the anodic period and in the pulsed potentiodynamic mode within the cathodic period.
• the invention is illustrated by the following exemplary embodiment of the method.
• Square voltage pulses with pulse rate 5.7 kHz were suppled to the electrodes.
• the duration of the anodic pulses was 15 microsec, the duration of the cathodic pulses was 65 microsec, and the duration of the dead time between the anodic and cathodic pulses was 95 microsec.
• the amplitude of the anodic voltage pulses was 1200 V, and the amplitude of the cathodic voltage pulses was 250-280 V.
• the effective current density was 14 A/dm2 in the anodic circuit and 18-16 A/dm2 in the cathodic circuit.
• the oxidation time was 19 min, and the thickness of the formed coating was 80 microns.
• the study of the oxide ceramic coatings on the samples was carried out by using the state-of-the-art measurement equipment.
• the hardness and elastic coefficient of the coatings were measured on microspecimens by using a nano-hardness tester (available from CSM Instruments) with load 20 mH.
• the hardness was 25-30 GPa and the elastic coefficient was 330-350 GPa across the whole section of the coating (from the external layer to the metal substrate).
• the adhesive and cohesive strength of the coatings was measured by using adhesion tester Revetest (available from CSM Instruments). The results of scratch testing were used to calculate the adhesive and cohesive strength of the coatings—it was equal to 300-320 MPa.
• the porosity of the coatings was determined on the microspecimens by using scanning-electron microscope S-3400N (available from Hitachi) with image resolution 3 nm. Pore sizes (diameters) in the coating were within in a range of 90 to 200 nm.
• the wear resistance of the coatings was estimated on a tribometer (available from CSM Instruments) at sliding friction according to the “ball-disk” scheme (a sliding distance was 2500 m). The average wear of the samples was 0.7*10-7 mm3/H/m.
• the method disclosed herein was used to produce stages (impellers and diffusers) of multistage electric-centrifugal submersible crude-oil pumps from B95 aluminum alloy with a protective nanostructured oxide ceramic coating.
• the pumps equipped with these new light stages were subjected to downhole testing under the conditions of pumping over a corrosive abrasive-carrying oily mixture.
• the results of the downhole testing have shown a threefold extension of lifetime compared to pumps with standard stages made of nickel cast iron—Ni-Resist.

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• 2018
  • 2018-01-17 RU RU2018101685A patent/RU2681028C2/ru active
• 2019
  • 2019-02-13 DE DE112019000447.6T patent/DE112019000447T5/de not_active Withdrawn
  • 2019-02-13 US US16/769,000 patent/US20210108327A1/en not_active Abandoned
  • 2019-02-13 WO PCT/RU2019/000089 patent/WO2019143270A2/ru active Application Filing

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