KR20000076300A - Process and apparatus for coating metals - Google Patents

Abstract

The present invention provides a method of forming a ceramic coating on a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals.

This method immerses a metal as an electrode in an electrolytic bath consisting of an aqueous solution of an alkali metal hydroxide, provides a counter electrode immersed in or containing electrolyte, and deforms the shape wave alternating current from a high voltage source of at least 700 V. Passing through the surface of the metal to be coated and the opposite electrode, wherein the modified shape wave current rises from zero to its maximum height and falls below 40% of its maximum height within one quarter of the total alternating cycle Causing dielectric breakdown, heating, melting and thermal densification of the hydroxide film formed on the surface of the metal film to form and weld a ceramic coating on the metal, and changing the composition of the electrolyte while the ceramic coating is being formed, the alteration of It consists of achieving by adding acid salts. The present invention also provides an apparatus for carrying out this method.

Description

Metal coating method and apparatus {PROCESS AND APPARATUS FOR COATING METALS}

Figure 1 shows a shape wave pulse of a preferred form,

2 is a graph showing the relationship between coating thickness and electrolysis time,

3 is a schematic view of a batch coating apparatus,

4 is a schematic view of a sequential coating apparatus.

The present invention relates to a ceramic coating method of a valve metal, a product coated by the method and an apparatus for carrying out the method.

The valve metal represents electrolytic rectification, and the present invention therefore relates to providing aluminum, zirconium, titanium, hafnium and coating methods and apparatus for coating these alloys.

In particular, the present invention relates to an electrolytic process which uses shape wave high voltage alternating current to achieve melting even during the coating of a thick layer, which thick layer is achieved in a short time by changing the electrolyte composition during the course of the process.

Aluminum, titanium and their alloys have desirable strength and weight ratios that make these metals suitable for many applications, such as high speed moving parts in aircraft and internal combustion engines. However, since these metals do not exhibit particularly good wear properties, coatings are often used to improve wear and corrosion resistance. The coatings applied appear to achieve additional design requirements such as resistance to chemicals (particularly acids and alkalis), tolerance of exposure to high temperatures, reduced wear and the provision of dielectric properties. Low cost, widely used anodizing methods achieve these goals considerably for proper use, but ceramic coatings are required for stringent use requirements.

Many electrolytic coating methods for these metals using direct current and / or voltages up to 600 V are known. Such a method is described, for example, in US Pat. Nos. 3,560,80,4082626 and 4,465,40. These patents and most recently published methods describe methods of forming ceramic films using anode spark discharge techniques, and methods of achieving good results with respect to coating corrosion resistance and adhesion. However, such a method has two major drawbacks: low film hardness and slow film formation.

In U.S. Pat. The voltage is increased to 50-200V during film formation, and may finally exceed 1000V. For waveforms, the patent states that the output from the power supply may be a direct current with any waveform, but it may be desirable to have a pulse shape (rectangle waveform), sawtooth waveform, or DC half waveform. Such terms do not mean that sharp peaked waveforms make the best contribution to providing a dense hard film.

The film formation rate reported in the eight examples provided in the patent can be calculated as follows.

Such slow film formation rate is not sufficiently compared with the film formation rate of the present invention.

There is also no indication in US Pat. No. 5475515 whether the method of this patent can produce very thick coatings, for example in the range of 300-700 microns.

Recently developed coating methods, known as kepla-coating methods, are based on plasma chemical anodization. The negative electrode is a surface film of an organic electrolyte, and the components to be coated thereon are suspended to form a positive electrode. Plasma is formed on the anode causing the production of a ceramic coating to heat the workpiece. Due to the formation of an oxide film on the anode, this method produces a film with a thickness of about 10 microns or less and terminates in 8-10 minutes. Workpiece heating occurs because the work piece is not surrounded by liquid, and asymmetric or slender work pieces may be subject to torsional deformation. Another disadvantage of the Kepla-coating process is that high electrolyte evaporation poses environmental concerns.

Accordingly, an object of the present invention is to eliminate the disadvantages of the known ceramic coating method and to provide a method for producing a hard film having strong adhesion and minimum porosity.

Another object of the present invention is to provide a method for producing a coating up to a thickness of 300 microns or more within a suitable time interval.

The above object of the present invention is achieved by providing a method of forming a ceramic coating on a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, the method comprising an aqueous solution of an alkali metal hydroxide. The surface of the metal and the reverse of which the electrode is to be immersed in the electrolytic bath, and the counter electrode is immersed in or contains an electrolyte solution, and the shape wave alternating current is to be coated from a high voltage source of at least 700V. Consisting of passing through an electrode, wherein the modified shaped wave current rises from zero to its maximum height within one-quarter of the total alternating cycle, thereby permitting dielectric breakdown and heating of the hydroxide film formed on the surface of the metal. Ceramic on the metal, melting, thermal densification Welding to form a biting and sikimyeo changing the composition of said electrolyte while said ceramic coating is formed, the change is achieved by the addition of hydroxy acid salt of an alkali metal.

It is another object of the present invention to provide an apparatus for performing the method in a cost effective manner. Accordingly, the present invention provides a batch ceramic coating apparatus for a product made of a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, the apparatus comprising an aqueous solution of alkali metal hydroxide. Electrolytic bath consisting of, an electrode immersed in or containing an electrolyte solution, at least one of the products to be coated and other electrodes consisting of means for suspending the product in the electrolyte, alternating current from a high voltage source of at least 700V Current source, AC waveform forming means in which the shape wave current rises from zero to its maximum height and falls below 40% of its maximum height within one quarter of the total alternating cycle, the connector element for completing the electrochemical circuit, and Controlled oxyacid salts of alkali metals while the device is in operation Means for adding a feed amount to the bath.

A distinguishing feature of the method of the present invention is that it is suitable for producing hard coatings on the order of 300 microns in a suitable time frame of about 90 minutes. This rapid coating rate is achieved by changing the composition of the electrolyte while the coating process is running. The quality of the coating is not compromised by the rapid formation of thick coatings since the current formed by the deformation achieves instantaneous melting of the layer adjacent to the metal workpiece even after the film is formed to the above-described thickness.

The present invention will be described in detail with reference to the accompanying drawings so that the present invention can be more fully understood.

The method of the present invention is used to form ceramic coatings on aluminum, zirconium, titanium, and hafnium. The process of the present invention is also suitable for alloying the metals if all of the alloying elements do not constitute more than about 20%. Method parameters may be optimized to suit the individual metals to be coated and the properties of the coating that are considered important for the particular application.

The metal workpiece to be coated is connected as an electrode of the electrolytic bath and immersed in the electrolytic bath.

For aluminum coatings, the electrolytic bath consists of a solution of water and alkali metal hydroxides. For baths required to optimize the coating to provide maximum adhesion between the metal and its coating, the electrolyte consists of an aqueous solution containing 0.5-2 g of sodium or potassium hydroxide per liter. Various materials of fine particles are added when required to improve special properties such as low friction properties of coatings. If such particles are added, the electrolyte is stirred to keep the particles in suspension. Similarly, colored coatings are prepared by adding fine particles of colored material.

A preferred counter electrode for this method is a stainless steel bath containing electrolyte solution. For example, if it is desirable to keep the electrolyte in a nonconductive container for safety reasons, ferrum, nickel or stainless steel electrodes are inserted into the bath in a conventional manner.

For aluminum workpieces, a strain wave alternating current from a high voltage source of at least 700V (typically 800V) is passed between the metal workpiece and the other electrode. As a result, dielectric breakdown, heating, melting, and thermal densification of the hydroxide film formed on the surface of the metal occur to form a ceramic coating on the surface of the metal and weld.

Arc micro welding can be seen during coating. A convenient and cost effective way to obtain the required shape wave electric pulse current is by the use of a capacitor bank connected in series between a high voltage source of 800-1000V and the metal workpiece being coated.

Figure 1 shows the waveform of the preferred shape of the current. The effect of using alternating current with high voltage is to prolong the life of the microarc causing intense, local and transient heating, as a result of which welding and melting of the coating are formed on the immersed metal workpiece. Anodic oxidation is achieved during the first positive half-cycle and the metal workpiece is an anode. The already formed dielectric coating then loses its dielectricity, whereby the occurrence of microarc begins. The arc life extends almost to the end of the first half-cycle. The burning of the arc is repeated during the second half-cycle when the workpiece becomes the cathode.

Figure 2 shows the time / coating thickness relationship for how the electrolyte composition remains constant (specified lines 1-5). Line 1 relates to how the electrolyte is pure potassium hydroxide, and line 2-5 relates to the method in which increasing concentrations of sodium tetrasilicate are used.

Line 6 relates to the method of the present invention. It has been found that even faster coatings are possible by changing the composition of the electrolyte while the ceramic coating is being formed. The alteration achieved consists of adding to the electrolyte a salt containing a cation of an alkali metal and an oxyacid anion of an amorphous element.

The amorphous element is composed of B, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn and Fe The salt is added at a concentration of 2-200 g per liter of solution. Preferred amorphous element is silicon and preferred addition salt is sodium tetrasilicate.

As can be seen from the graph, the change in electrolyte composition during the operation enables the production of a 200 micron thick coating in about 50 minutes and exhibits a film formation rate of 4 microns per minute. Testing has shown that rapid film formation is achieved without compromising the quality of film deposition on the metal workpiece.

Once the added salt is mixed into the electrolyte, the only practical way to reduce the salt concentration again to coat the next batch of metal product is to add a significant amount of fresh electrolyte. However, this problem is solved in the following apparatus in connection with FIG.

It was also found that a void-free coating can be made by gradually reducing the current flow when the film is almost at its desired thickness. In practice, this is achieved by progressively decreasing the capacitance used to form the waveform, thus weakening the current until the process is stopped.

As can be seen from the above, the term “modified” as used herein relates to something other than a standard sine wave whose waveform is typically associated with an alternating wave, instead it is shown, for example, as shown in FIG. 1 to optimize the coating effect. It is about being deformed together.

Table 1 shows the different types of coatings for different requirements. Embodiments are a list of ceramic coated aluminum alloys to achieve various design requirements. Examples 3 and 4 were prepared by the technique described above.

Table 1

The aluminum alloy known as "Duralumin" has an alloy designation of 2014, and because of its strength / weight ratio, it has found widespread use in aircraft construction. Thus this alloy was chosen for the test coating.

Table 2 shows the properties of the coatings achieved and the results obtained.

TABLE 2

The present invention also provides a ceramic coated metal product produced by the above-described method. One example of such a product is an aluminum alloy piston for an internal combustion engine. A second example of such a product is an aluminum engine block for an internal combustion engine intended to operate with minimal lubrication, and a third example is a spacecraft protection tile designed to withstand reentry into the atmosphere. There is an electrical insulator.

3 shows a batch ceramic coating apparatus 10 of a product 12 (first electrode) made of a valve metal selected from a group consisting of aluminum, zirconium, titanium, hafnium and alloys thereof. The apparatus 10 has a 40-l electrolytic bath 14 consisting of an electrolyte solution 16 of a solution of water and alkali metal hydroxide. The bath 14 is made of stainless steel and forms a second electrode. A stirring means 15 is provided for stirring the electrolyte.

The first electrode consists of at least one of the products 12 to be coated and the conducting means 18 for suspending the product in the electrolyte solution 16.

At least 700V of alternating current is a 40000V-amp step-up transformer 20 designed to supply up to 800, 900 or 1000V.

The capacitor bank 22 consists of a capacitor having a capacitance of 375 kHz and a nominal capacitance of 25, 50, 100 and 200 kHz. Alternatively, such means are of the type shown in rectifier and converter circuits (not shown), or in Pink and Betty's book "Standard Handbook for Electricians, 12th Edition 22-96, pages 22-97." Other means of

In addition, a connector element 24 is provided to complete the electrochemical circuit. On the left side of the bathtub 14 is an operator control panel 26, which is enclosed behind the safety door 28. Opening the safety door 28 cuts off power.

The salt containing feed hopper 30 with the solenoid actuated supply valve 32 provides a means for adding salt 34 to the bath 14 while the device 10 is in operation.

The hopper 30 supports a portion of the salt 34 containing a cation of an alkali metal and an oxyacid anion of an amorphous element. Suitable salt 34 is sodium tetrasilicate.

4 shows a sequential ceramic coating device 36 of a product 12. The first electrolytic bath 38 contains an electrolyte solution 16 composed of a solution of water and alkali metal hydroxide. The second electrolytic bath 40 contains an electrolyte solution 42 composed of water, a solution of alkali metal hydroxide and a low concentration of salt 34. The third electrolytic bath 44 contains an electrolyte solution 46 composed of water, a solution of alkali metal hydroxide, and a salt of higher concentration in the electrolyte 42.

For convenience, the baths 38, 40, 44 may consist of a single stainless steel container 48 forming an electrode provided with two vertical dividers 50. The other electrode consists of at least one of the products 12 to be coated and conductive means 18 for suspending the product 12 in the electrolyte solution 16, 42, 46. Manual or automatic maneuver 52 allows the product 12 to be transported from the first bath 38 to the second bath 40 and then to the third bath 44.

In the device 36, the electrolyte in each bath remains unchanged during operation and may therefore be used repeatedly. The use of several electrolytes, each having a different composition, allows for coating at a rate of about 2.5-4 microns per minute.

The electrical components used are the same as described above with reference to FIG.

Although the present invention has been described in detail by the embodiments described above, the present invention is not limited thereto, and the present invention can be variously modified without departing from the spirit and scope of the present invention.

Claims (13)

In a method of forming a ceramic coating on a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, the metal is immersed as an electrode in an electrolytic bath composed of an aqueous solution of alkali metal hydroxide. ; Providing a counter electrode immersed in or containing the electrolyte solution; Pass the modified shape wave alternating current from a high voltage source of at least 700 V through the surface of the metal to be coated and the counter electrode so that the modified shape wave current rises from zero to its maximum height and not more than a quarter of the total alternating cycle Forming and welding a ceramic coating on the metal, causing dielectric breakdown, heating, melting and thermal densification of the hydroxide film formed on the surface of the metal, thereby lowering to 40% or less of its maximum height within the metal; Modifying the composition of the electrolyte while the ceramic coating is being formed, the alteration being effected by the addition of an oxyacid salt of an alkali metal. The method of claim 1, wherein the modified shape wave alternating current is obtained by use of a capacitor bank connected in series between the high voltage source and the metal. The method according to claim 1 or 2, wherein the added salt is sodium tetrasilicate. The method of claim 1, wherein the electrolyte is optimized to provide maximum adhesion between the metal and the coating, wherein the electrolyte consists of an aqueous solution containing 0.5-2 g of sodium hydroxide per liter. Way. 5. The method of claim 1, wherein the electrolyte is optimized to provide maximum adhesion between the metal and the coating, wherein the electrolyte consists of an aqueous solution containing 0.5-2 g of potassium hydroxide per liter. Way. The method according to any one of claims 1 to 5, wherein the amorphous element is B, Al, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, P, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn and Fe, wherein said salt is added at a concentration of 2-200 g per liter of solution. The method according to any one of claims 1 to 6, characterized in that microparticles of coloring material are added to produce the coating of the required color. 8. A method according to any one of the preceding claims, characterized in that it is arranged to produce a void-free coating, but the coating is stopped by gradually reducing the current flow. A ceramic coated metal article produced by the method as claimed in claim 1. A batch ceramic coating apparatus for a product made of a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, comprising: an electrolytic bath composed of an aqueous solution of alkali metal hydroxide; An electrode immersed in or containing electrolyte solution; Another electrode consisting of at least one of said products to be coated and means for suspending said product in said electrolyte solution; AC source from high voltage source of at least 700V; Means for forming an AC waveform in which the shape wave current rises from zero to its maximum height and falls below 40% of its maximum height within one quarter of the total alternating cycle; Connector elements for completing electrochemical circuits; And means for adding a controlled supply of oxyacid salt of alkali metal to the bath while the device is in operation. 11. The apparatus of claim 10, wherein the high voltage source is a boost transformer up to 1000V and the means for forming the AC waveform is a capacitor bank connected in series between the high voltage source and the valve metal. A sequential ceramic coating apparatus for a product made of a valve metal selected from the group consisting of aluminum, zirconium, titanium, hafnium and alloys of these metals, the apparatus comprising: a first electrolytic bath composed of an aqueous solution of an alkali metal hydroxide; A second electrolytic bath composed of an aqueous solution of an alkali metal hydroxide and a low concentration of an oxyacid salt of an alkali metal; A third electrolytic bath composed of an aqueous solution of an alkali metal hydroxide and a concentration higher than that present in the second bath of the oxyacid salt of the alkali metal; An electrode immersed in or containing electrolyte solution; An electrode composed of at least one of said products to be coated and means for suspending said product in said electrolyte solution; Means for transferring the product from the first bath to the second bath and from the third bath; AC commissioner of at least 700V; Means for forming an AC waveform in which the shape wave current rises from zero to its maximum height and falls below 40% of its maximum height within one quarter of the total alternating cycle; And a connector element for completing the electrochemical circuit in each bath. 13. The apparatus of claim 12, wherein the high voltage source is a boost transformer up to 1000V and the means for forming the AC waveform is a capacitor bank connected in series between the high voltage source and the valve metal.