Method for forming protective oxide-ceramic coating on surface of valve metals and alloys

Abstract

FIELD: protective coatings.SUBSTANCE: method for forming a protective oxide-ceramic coating on the surface of products made of valve metals and alloys by the plasma electrolytic oxidation method involves immersing the product as an electrode together with a counter electrode in a bath filled with an aqueous alkaline electrolyte, and supplying rectangular pulses of anodic and cathodic voltage to the electrodes using a pulsed power source. Anode and cathode voltage pulses are supplied with a current-free pause between them, the value of which is 4.8 times the duration of the anode pulse. The amplitude values of the cathode voltage pulses range from 85 to 149 V with a pulse repetition frequency from 1000 Hz to 4000 Hz, while the current pulse repetition frequency is additionally regulated.EFFECT: obtaining oxide-ceramic coatings with specified properties under reduced energy conditions, with lower values of cathode voltage.1 cl, 2 ex

Description

[0001] The invention relates to the field of applying protective coatings, in particular to plasma-electrolytic oxidation of products made of valve metals and alloys, and can be used to form oxide-ceramic coatings on the surface of products with increased wear and corrosion resistance, heat resistance, and dielectric strength.

[0002] A method is known for forming a protective ceramic coating on the surface of metal products, which includes immersing the product as an electrode together with a counter electrode in an aqueous electrolyte solution, applying alternating rectangular voltage pulses to the electrodes using a power source, wherein the duration of the anode voltage pulses is 5-20 μs, the magnitude of the anode voltage pulses is 400-1800 V and the magnitude of the cathode voltage pulses is 200-900 V (patent WO2008120046A1, date of publication 09.10.2008).

[0003] The disadvantages of the known solution include the use of 1800 V, which, due to its magnitude, falls into the high-voltage category (see, for example, URL: https: / / ru.wikipedia.org / wiki / Високое_нятие (date of access: 28.07.2025)). This creates additional risks for maintenance personnel and requires additional occupational safety measures.

[0004] A method and device for forming ceramic coatings on metals and alloys is known, described in patent WO 03 / 83181. The method allows for the formation of ceramic coatings on valve metals and alloys with a current pulse repetition rate of 500 to 10,000 Hz.

[0005] A drawback of the known solution is the shape of the anode and cathode current pulses applied to the electrodes, which have a sharp peak on the leading edge. This current pulse shape can lead to overcurrent of the power electronic switching elements in high-frequency converters and complicates the correct selection of these expensive components.

[0006] The closest in terms of the set of essential features - the prototype of the claimed invention - is a method for forming a protective oxide-ceramic coating on the surface of products made of valve metals and alloys by the method of plasma electrolytic oxidation (hereinafter - PEO), including immersing the product as an electrode together with the counter electrode in a bath filled with an aqueous alkaline electrolyte, and sequentially supplying rectangular pulses of anodic and cathodic voltage to the electrodes using a pulsed power source (patent RU 2681028, published 01.03.2019. Bulletin No. 7). According to the known solution, the magnitude of the cathode pulses is 150-400 V, depending on the nature of the materials being processed. In this case, the pulse repetition frequency, according to the known solution, is from 3300 Hz to 33000 Hz.

[0007] Among the drawbacks of the known solution is the authors' disregard for the need to regulate the current pulse repetition rate in frequency ranges greater than 50 Hz. However, based on the applicant's experience, the lack of regulation can lead to stress concentrators in the form of microporosity and cracks at the interface between the valve alloy and the forming coating.

[0008] The technical objective of the proposed invention is to simplify the technological process of forming an oxide-ceramic coating.

[0009] The technical result of implementing the proposed invention is the production of oxide-ceramic coatings with specified properties under reduced energy conditions (at lower values ​​of cathode voltage).

[0010] The solution to the technical problem is achieved due to the fact that in the proposed method for forming a protective oxide-ceramic coating on the surface of valve metals and alloys (hereinafter referred to as the method), which includes immersing the product as an electrode together with a counter-electrode in a bath filled with an aqueous alkaline electrolyte, and sequentially supplying rectangular pulses of anode and cathode voltage to the electrodes using a pulse power source, the amplitude values ​​of the voltage pulses of the cathode voltage are from 85 V to 149 V at a pulse repetition frequency of 1000 Hz to 4000 Hz.

[0011] The claimed ranges of cathode voltage amplitudes and pulse repetition rates were obtained by the applicant during experimental work. The process involved investigating the formation of oxide-ceramic coatings on the surfaces of aluminum, titanium, magnesium, zirconium, tantalum, niobium, and their alloys. Work was conducted to optimize a number of process parameters, including the ratio of the anode pulse duration to the dead-time interval. The applicant determined that reducing the cathode voltage amplitude requires a corresponding reduction in the dead-time interval to minimize process productivity losses. Compared to the parameter values ​​given in the technical solution adopted as the closest equivalent, it is proposed to reduce the dead-time interval to a range of 4.7-4.9 times the anode pulse duration.

[0012] Research conducted by the applicant substantiated the need to regulate the current pulse repetition rate for frequency ranges above 50 Hz. The process parameters selected and claimed as essential features assume operation at frequencies from 1000 Hz to 4000 Hz, which necessitates the introduction of such regulation as a mandatory requirement. Methods and means that can be used to regulate the current pulse repetition rate are known in the prior art and are not the subject of protection under this application.

[0013] Achieving the required coating thickness at reduced cathodic voltage amplitudes results in a slight increase in process time. However, this increase is offset by the benefits of the reduced voltage.

[0014] To compare the proposed method and the method described in the technical solution adopted as the closest analogue (hereinafter referred to as method “P”), the applicant conducted a series of experiments repeating the experiments described in the said solution.

[0015] Examples of the invention.

[0016] Disks of heat-resistant aluminum alloy AK4-1 T1 (2618 T6), 62 mm in diameter and 6.5 mm in thickness, were used as samples (5 each for Method "P" and 20 each for the claimed method). During oxidation, the disk, along with two stainless steel counter electrodes, was placed in a bath of alkaline silicate electrolyte with a pH of at least 10 (for other valve metals, the composition of the aqueous alkaline electrolyte solution will be different).

[0017] In the first series of experiments, rectangular voltage pulses with a pulse repetition frequency of 5700 Hz (method "P", 5 samples), 1000 Hz (the lower limit of the range of the claimed method, 5 samples) and 4000 Hz (the upper limit of the range of the claimed method, 5 samples) were applied to the electrodes. The duration of the anode pulses was 15 μs, the cathode pulses - 65 μs and the current-free pause between them - 72 μs. The amplitude of the voltage pulses was: anode 385 V and cathode 85 V. The effective current density was: in the anode circuit 14 A / dm 2 and in the cathode circuit 18-16 A / dm 2 The oxidation time was 24 minutes, and the thickness of the formed coating was 80 microns.

[0018] In the second series of experiments, rectangular voltage pulses with a pulse repetition frequency of 1000 Hz (the lower limit of the range of the claimed method, 5 samples) and 4000 Hz (the upper limit of the range of the claimed method, 5 samples) were applied to the electrodes. The duration of the anode pulses was 15 μs, the cathode pulses - 65 μs and the current-free pause between them - 72 μs. The amplitude of the voltage pulses was: anode 1200 V and cathode 149 V. The effective current density was: in the anode circuit 14 A / dm 2 and in the cathode circuit 18-16 A / dm 2 The oxidation time was 20 minutes, and the thickness of the formed coating was 78 microns.

[0019] In addition, the applicant carried out work to form coatings on the surface of products made of aluminum, titanium, magnesium, zirconium, tantalum, niobium and their alloys.

[0020] Research of the coatings formed by the proposed method showed that their technical characteristics are practically identical to the characteristics of the coatings obtained at higher amplitude values ​​of the cathodic voltage, and the increase in the duration of the process with a decrease in the amplitude values ​​of the cathodic voltage is insignificant.

[0021] The above allows us to state that the stated technical result has been achieved.

Claims

A method for forming a protective oxide-ceramic coating on the surface of articles made of valve metals and alloys by means of plasma electrolytic oxidation, which includes immersing the article as an electrode together with a counter-electrode in a bath filled with an aqueous alkaline electrolyte, and supplying rectangular pulses of anode and cathode voltage to the electrodes using a pulsed power source, characterized in that the anode and cathode voltage pulses are supplied with a dead time between them, wherein the value of the dead time is 4.8 times the duration of the anode pulse, and the amplitude values ​​of the cathode voltage pulses are from 85 to 149 V at a pulse repetition frequency of 1000 Hz to 4000 Hz, while the repetition frequency of the current pulses is additionally regulated.