LT4651B - Process and apparatus for coating metals - Google Patents

Abstract

The invention describes the process of coating metal consisting of aluminium, zirconium, titanium, hafnium and their alloys. During the process the metal to be coated is placed into a tub of electrolyte, filled with aqueous solution of alkaline metal hydroxide. The opposite electrode is dipped into the tub or filled with electrolyte; the surface of the metal that is being coated and the opposite electrode get a modified wave-like alternating electrical current provided by a high voltage source of at least 700V, this modified wave-like electrical current is increased from zero to its maximum level and is reduced to a level that is less than 40% of the maximum faster than a quarter of its full cycle, thus creating an electrical discharge, warm-up, fusion and thermal thickening of the hydroxide coat on the surface of the metal and at the same time forming and fusing a ceramic coating with this metal, and changing the composition of the electrolyte during the formation of the ceramic coating, adding alkaline metal oxygen acid salt.

Description

The invention relates to a ceramic coating method for metal, to products coated therewith and to a device for realizing this method.

Metals such as valves are characterized by electrolytic rectification and therefore the present invention is directed to a method and apparatus for coating aluminum, zirconium, titanium, hafnium and their alloys.

More particularly, the present invention is directed to electrolysis, which uses a high-voltage waveform in the form of a wave, which melts even during a very short period of time, and changes the composition of the electrolyte during electrolysis.

Aluminum, titanium and their alloys have excellent strength / weight ratios, making them widely used, for example, in aviation, as well as fast moving parts for internal combustion engines. Since these metals are nonetheless not resistant to wear, various coatings are used to improve their resistance to wear and erosion. Such coatings must be resistant to chemicals, in particular acids and alkalis, high temperatures, abrasion resistant and have dielectric properties. An inexpensive, widespread anodizing method is suitable for medium-load conditions, but ceramic coatings are required at high loads.

Many methods of electrolytic plating of metals using direct current and / or voltages of less than 600 V are known. Such methods are described, for example, in U.S. Pat. 3956080, 4082626 and 4659440. These patents and most recent discoveries describe processes for forming ceramic films by anodic discharge, which provide good results in coating adherence and corrosion resistance.

U.S. Pat. No. 5147515 discloses the use of an electrolytic bath with a water-soluble or aqueous solution of colloidal silicate and / or saline of oxygen acid in which ceramic particles are dispersed. Depending on the waveform, these patents state that the power source can be a DC current of any wave shape, ideally pulsed (rectangular), in the form of a saw blade. or dc waveform of dc. The peak shape of a wave has not been found to provide significant advantages in forming a dense, solid film.

The film formation rate in the eight examples of the present invention is shown below:

- An example Film thickness (microns) Coating time (minutes) Formation speed (microns / minute) - 1. 35 20th 1.75 2. 31st 20th 1.55 3. 38 30th 0.93 4. 27th 20th 1.35 5. 36 30th 1.20 6th 14th 30th 0.47 7th 15th 30th 0.50 8th 28th 30th 0.93 i to

Such slow film formation is not comparable to the present invention.

In addition, U.S. Patent No. 5147515 did not disclose whether it is possible to produce very thin coatings, e.g., 300 to 700 microns thick.

The recently disclosed coating formation method, also known as Kepla, is based on plasmochemical anodic oxidation. The cathode is a surface film of an organic electrolyte over which suspended particles that form an anode are suspended. Plasma is formed which forms a ceramic coating on the anode surface and heats the workpiece. The formation of an oxide film on the anode surface results in a film no larger than 10 microns and takes 8-10 minutes. The workpiece heats up because it is not immersed in the liquid; asymmetrical or small details are distorted. Another disadvantage of Kepla's combustion method is that the volatile electrolyte pollutes the environment.

Therefore, one object of the present invention is to overcome the known disadvantages of the ceramic coating method and to provide a method which produces a solid, well adherent and non-porous film.

Another object of the present invention is to provide coatings with a thickness of 300 microns and thicker in the medium term.

These and other objects are accomplished by the present invention, which provides a process for the ceramic coating of metals selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys. This method involves immersing the metal acting as an electrode in an electrolytic bath filled with an alkali metal hydroxide aqueous solution, immersing the opposite electrode in an electrolyte or electrolyte, passing a modified wave current from a high voltage source of at least 700 V to the surface of the metal to be coated. at the opposite electrode. The method differs in that this modified waveform current is increased from zero to maximum value faster than a quarter of a full alternating cycle, thereby causing a dielectric discharge, heating, melting and thermal compaction of the hydroxide film formed on the surface of this metal by forming and welding the ceramic. the coating with the metal surface and changing the composition of the electrolyte during the ceramic coating formation, the alkaline metal oxygen acid is added to the electrolyte upon this change.

Another object of the present invention is an apparatus for efficiently carrying out the above process. The present invention relates to a ceramic bath ceramic bath apparatus comprising a metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys, comprising an electrolytic bath filled with an alkali metal hydroxide aqueous solution, an electrode immersed in an electrolyte or , alternating current from at least 700 V, the source of said alternating current electrode, comprising at least one of these coated articles and electrolyte immersion of the article, thereby increasing the value of the current formed in the form of a waveform. from zero to its maximum value, and falls below 40% of its maximum value in less than a quarter of a full alternating cycle, of means for terminating the electrochemical circuit and controlling the addition of an alkaline metal oxygen salt to the bath during operation.

A distinctive feature of the process of the present invention is its ability to form solid coatings of 300 microns in about 90 minutes. Such rapid coating formation is achieved by changing the electrolyte composition during the coating process. The quality of the coating is not compromised by the rapid formation of a thick coating layer, as the modified shaped current instantly melts the layer adjacent to the metal product even after the film of the intended thickness is formed.

The invention will now be described in more detail with reference to the accompanying drawings, in which:

Fig. 1 is a preferred view of a pulsed current in wave form;

Fig. 2 shows the relationship between coating thickness and electrolysis time;

Fig. 3 is a schematic diagram of a coating unit in a bath; and

Fig. 4 is a schematic diagram of a conveyor coating machine in a bath.

It should be noted that the drawings are provided for purposes of illustration only, of the preferred embodiments of the present invention, and are not intended to illustrate the details necessary for a deeper understanding of the invention. The description of the process according to the drawings will be apparent to one skilled in the art, and several embodiments of the invention may be practiced.

The purpose of the process is to coat ceramics with aluminum, zirconium, titanium and hafnium. It is also possible to coat alloys of these metals, provided that the sum of the alloys does not exceed 20% of the total. The process parameters can be optimized by tailoring them to the specific coating metal and bearing in mind that the specific coating properties are important in each particular case.

The coated metal product is connected as an electrolytic bath electrode and immersed in the bath.

For aluminum coating, the bath is filled with water and an alkali metal hydroxide solution. If the coating is to be securely bonded to the metal, an electrolyte solution containing 0.5 to 2 g / l sodium hydroxide or potassium hydroxide is used. Fine particles of various materials are added to the electrolyte when needed to improve special coating properties such as reducing friction. Upon addition of such particles, the electrolyte is mixed to form a suspension. Similarly, colored coatings are obtained by the addition of fine pigment particles to the electrolyte.

The best counter electrode is a stainless steel bath filled with electrolyte. If it is desired to store the electrolyte in a leakproof container for safety reasons, an iron, nickel or stainless steel electrode is normally immersed in the bath.

A modified waveform of alternating current from at least 700 V high voltage source is then passed through the aluminum product and other electrode. Dielectric discharge, heating, melting and thermal compaction of the hydroxide film formed on the metal surface occurs by forming the ceramic coating and welding it to the metal surface. Micro-welding arc is visible during the coating process. The desired waveform pulsed current is obtained in a conventional manner using a capacitor battery connected in series between the 800-1000 V high voltage source and the coated metal product.

Fig. 1 shows the most appropriate current waveform. High voltage alternating current increases the duration of the microwave arc, which causes intense, local, temporary heating, melting, and welding to the product surface of the formed coating. Anodizing is performed during the first positive half-cycle, then the product is a positive electrode. After that, the already formed dielectric coating dielectrically decomposes, thereby generating microwaves. The arc duration increases nearly to the end of the first half-cycle. The arc burning is repeated during the second half-cycle, when the article becomes a negative electrode.

Fig. 2 is a graph showing the time / coating thickness dependence at a constant electrolyte composition. Curve 1 is typical of a process in which the electrolyte is pure potassium hydroxide. Curves 2-5 are typical of processes in which increasing concentrations of sodium tetrasilicate were used.

Curve 6 represents the process of the present invention. It has been found that the rate of coating formation increases by changing the electrolyte composition during the process. The composition of the electrolyte is altered by the addition of its salts containing alkali metal cations and amorphous oxalic acid anions. The amorphous element is selected from the group consisting of salts of B, Al, Si, Ge, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr, V, Ta, Cr, Mo, Mn and Fe added to the electrolyte , concentration ranging from 2 to 200 g / liter of solution. The preferred amorphous element is polysiloxane, the most preferred salt being sodium silicate.

As can be seen from the curve, changing the electrolyte composition can result in a coating of 200 microns in about 50 minutes and a film forming rate of about 4 microns per minute. Tests have shown that the quality of adhesion of a film so rapidly to the metal surface is not compromised.

It goes without saying that once salt is added to the electrolyte, the only practical way to reduce the salt concentration in the bath again is to add a large amount of new electrolyte to the next batch of metal products. This problem is solved by the device described below with reference j in FIG.

It was further found that it is possible to form non-porous coatings by gradually decreasing the current when the film thickness has almost reached the desired size. In practice, this is done by gradually reducing the capacitive impedance used to form the wave, thereby reducing current and ultimately stopping the process.

The term "modified", as used above, means that the waveform is different from the standard sinusoid, usually associated with an alternating current wave, and is modified, for example, as shown in Figure 1 to optimize the coating formation process.

Table 3 lists the different types of pavement for different needs. Samples of aluminum alloys coated with ceramics for a variety of design needs. Examples 3 and 4 were prepared as described above.

TABLE

Aluminum alloy, also known as "duralumin", is a 2014 grade alloy and is widely used in aviation due to its strength / weight ratio. For this reason, this alloy was selected for testing. Table 2 lists the resulting coating characteristics.

Corundum

Aluminum

Aluminum silicate

Porosity of coating / cm

Pore diameter in microns

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The present invention also provides a ceramic-coated article made in the manner described above. One example of such a product is the aluminum alloy piston of the internal combustion engine. The second example is an aluminum block of an internal combustion engine designed for minimum lubrication. The third example is a heat-resistant spacecraft shield that protects the spacecraft during entry into the atmosphere. The fourth example is electrical insulation, which acts as a heat sink for the electronic board.

Fig. 3 shows a single-charge ceramic coating unit 2 of articles 1 (first electrode) made of a metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys. The unit 2 has a 40 liter electrolyte bath 3 filled electrolyte 4 consisting of water and an alkali metal hydroxide solution. Bath 3 is made of stainless steel and forms the second electrode. The electrolyte is mixed with a mixing medium 5.

The first electrode comprises at least one coating article 1 and a means for immersion 4 in the electrolyte 6.

At least 700 V AC is powered by a 40000 volt step-by-step transformer 7 to supply 800, 900 or 1000 V,

Capacitor battery 8 has a total capacity of 375 FF and includes capacitors of 25, 50, 100 and 200 FF, Alternatively, it could be a rectifier and inverter circuit (not shown) or other described by Fink and Beaty in The Standard Handbook for Electronic Engineers, 12th edition, pp. 22-96, 22-97, measure,

The connectors 9 are for connecting an electrochemical circuit. Operator control panel 10 Located on the left-hand side of the bathtub 3, which is located behind the security door 11. Opening the security door 11 cuts the power supply.

During operation of the device 2, salt 12 from the supply hopper 13 is added to the bath

3. The hopper 13 supplies a salt 12 containing alkali metal cations and amorphous oxygen oxide anions. A suitable salt 12 is sodium tetrasilicate.

Fig. 4 illustrates a ceramic conveyor coating device 14 for articles 1. The first electrolyte bath 15 is filled with an electrolyte 4 consisting of water and an alkali metal hydroxide solution. The second electrolyte bath 16 is filled with an electrolyte 17 consisting of water, an alkali metal hydroxide solution and a small amount of salt 12. The third electrolyte bath 18 is filled with an electrolyte 19 consisting of water, an alkali metal hydroxide solution and a higher salt content than the electrolyte 17.

For convenience, baths 15, 16, 18 may be constructed as a single container 20 with two vertical partitions 21 acting as electrodes. The other electrode is at least one coated article 1 and a transport means 6 which immerses the article 1 sequentially in the electrolytes 4, 17, 19. The manually or manually controlled manipulator 22 transfers the article 1 from the first bath 15 to the second bath 16 and then to the third bath 16. bath 18.

In unit 14, the electrolyte in each bath is not changed during the process and can be reused. With multiple electrolytes of different composition, the coating rate is 2.5-4 microns per minute.

The electrical components are the same as those of the Machine described with reference j in FIG.

Those skilled in the art will appreciate that the invention is not limited to the examples given, and that it may be embodied in other specific forms without departing from the spirit of the invention. The presented embodiments of the invention are for purposes of illustration only, the scope of the invention being defined by the definition, not the foregoing description, the invention includes all modifications equivalent to the solutions set forth in the definition.

Claims (13)

DEFINITION OF INVENTION A method of forming a ceramic coating on a surface of a metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys, comprising immersing the metal as an electrode in an electrolyte bath filled with an aqueous alkali metal hydroxide solution or filling it with an electrolyte, passing a modified wave AC current from a 700 V high voltage source to the surface of the coated metal, characterized in that it increases the modified wave current from zero to its peak value and reduces it to a value less than 40% of its maximum values in less than a quarter of a complete alternating cycle, thereby causing a dielectric discharge, heating, melting and thermal compaction of the hydroxide film formed on the metal surface, forming and welding the ceramic coating with this metal, and changing the electrolyte composition of the ceramic coating during the formation by addition of an alkali metal oxalic acid salt. 2. A method according to claim 1, characterized in that it produces this modified waveform alternating current using a capacitor battery connected in series between the high voltage source and the metal. 3. A process according to claim 1 or 2, wherein sodium tetrasilicate is added. A process according to any one of the preceding claims, for attaching the coating to the metal as tightly as possible, characterized in that it uses an electrolyte consisting of an aqueous solution containing 0.5 to 2 g / l sodium hydroxide. A process according to any one of the preceding claims, for attaching the coating to the metal as tightly as possible, characterized in that it uses an electrolyte consisting of an aqueous solution containing 0.5-2 g / l potassium hydroxide. 6. A process according to any preceding claim, wherein the amorphous element is selected from the group consisting of B, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr , V, Nb, Ta, Cr, Mo, W, Mn and Fe, add 2 to 200 g / l of salt to the solution. 7. A process according to any one of the preceding claims, characterized in that fine particles of pigment material are added to obtain a coating of the desired color. A process according to any preceding claim for obtaining a non-porous coating, characterized in that it gradually reduces current and stops the coating formation process. 9. A ceramic-coated metal article, as defined in any of the preceding claims. 10. A device for the ceramic coating of articles of metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys, consisting of: electrolyte baths filled with an alkali metal hydroxide aqueous solution; an electrode immersed in an electrolyte or filled with an electrolyte; means for immersing another electrode consisting of at least one of these coated articles and immersing the article in the electrolyte; AC power supply from at least 700 V high voltage source: alternating current waveformers, characterized by having an alternating current source that increases the waveform current from zero to peak value and reduces it to a value less than 40% of its peak value, faster than a quarter of a full alternating cycle; connectors connecting the electrochemical circuit; and means for adding a controlled amount of an alkaline metal oxygen acid salt to the bath while the unit is operating. 11. The apparatus of claim 10, wherein the high voltage source is a 1000 V stepper transformer, and the alternating current waveform is a capacitor battery connected in series between the high voltage source and the metal product. 12. A device for the ceramic coating of articles of metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and their alloys, comprising: first electrolyte baths filled with an alkali metal hydroxide aqueous solution; a second electrolyte bath filled with an aqueous alkali metal hydroxide solution to which a small amount of an alkaline metal oxygen acid salt is added; a third electrolyte bath filled with an aqueous solution of an alkali metal hydroxide, to which is added an amount of an alkaline metal oxalic acid salt greater than the second bath; electrodes immersed in or filled with an electrolyte; means for transporting the articles from the first bath to the second bath and thereafter to the third bath; an alternating current source supplied from a 700 V high voltage source; alternating current waveformers, characterized by having a source of alternating current which increases the current of the waveform from zero to peak value and reduces it to a value less than 40% of its peak value, faster than a quarter of a full alternating cycle; and connectors connecting the electrochemical circuit in each bath. 13. The apparatus of claim 12, wherein the high voltage source is a 1000 V stepper transformer, and the alternating current waveform is a capacitor battery connected in series between the high voltage source and the metal product.