WO1999034035A2

WO1999034035A2 - Method for microplasma oxidation of valve metals and their alloys, and device therefor

Info

Publication number: WO1999034035A2
Application number: PCT/RU1998/000448
Authority: WO
Inventors: Aleksandr Vladimirovich Timoshenko; Aleksandr Grigorievich Rakoch
Original assignee: R-Amtech International, Inc
Priority date: 1997-12-30
Filing date: 1998-12-30
Publication date: 1999-07-08
Also published as: US6264817B1; RU2124588C1; WO1999034035A3

Abstract

The invention concerns the field of microplasma-electrochemical processing of the surface of metallic objects, and especially the methods and devices for microplasma oxidation of valve metals and their alloys. The invention can be applied in areas like mechanical engineering, aircraft construction, in the petrochemical and oil industry and many other branches of industry. The main features of the invention: the component is immersed into the electrolyte with a specific speed and an initial polarizing current intensity is applied, which is high enough to generate on the surface of the treated component, which is immersed in the electrolyte, moving microplasma discharges. The component is held until the formation of a coating of a specific thickness. Hereby, the lowering phase of the voltage, at which a coating is forming, is done by lowering the voltage to a value which corresponds with the beginning of the extinction of the microplasma discharges and then maintaining it until the complete extinction of the isolated wandering microplasma discharges. Then the component is taken out of the electrolyte and is cooled. The method is realized with a device, containing a tank with a cooling agent, in which the electrolytic bath is located, a control block and a mechanism to vertically and horizontally move the treated component with the possibility to move with this mechanism the given component out of the electrolytic bath in the tank with the cooling agent.

Description

Method for microplasma oxidation of valve metals and their alloys, and device therefor

The invention concerns the field of microplasma-electroche- mical processing of the surface of metallic objects, and especially the methods and devices for microplasma oxidation of valve metals and their alloys. The invention can be applied in areas like mechanical engineering, aircraft construction, in the petrochemical and oil industry and many other branches of industry. One special area for its application is the manufacturing of components, the surfaces of which operate under conditions of friction, e.g. slide bearing bushes, transition pieces, valves of pneumatic devices, turbine blades, pistons and cylinders of engines etc.

The components which operate under conditions of friction or abrasion are traditionally made of antifrictional alloys (cast iron, bronze) or to the surfaces of the components, made of structural alloys, chrome- or nickel-base metallic or compound coatings are applied. In the latter case, this has a hardening effect on the surface, however, like with the use of antifrictional alloys, the abrasion resistance parameters stay low because of the insufficient hardness of the friction surfaces. This leads to a quick abrasion of the expensive components and makes it necessary to periodically change them during their exploitation.

Well known is an electrochemical method to generate a hard and abrasion-resistant coating (1) . This method consists in applying a chrome layer of a certain thickness to the surface of a component which operates under conditions of abrasion. The method is characterized by the use of an agressive and toxic electrolyte (chromic anhydride) , a high current density (up to 60 A/dm²) and it is crucial for the conditions under which the technological process itself is being conducted as well as for the quality of the preliminary processing of the surface. The slightest deviations lead to a weak cohesion of the coating with the surface of the component to which the coating is applied and as a result of this, to the exfoliation during exploitation.

Well known is a method for microplasma oxidation (2) of valve metals and their alloys, mainly aluminum and titanium. For this method an aqueous solution of electrolytes, containing phosphate, borate and tungsten alkali metall is used. In the beginning of the processing of the surface, a voltage is applied (up to 360 V) , during which a coating is beginning to form. During this process the current density is maintained constant (0,1 A/cm²) . The given voltage and current parameters are maintained for a period of 1 to 3 minutes and the voltage is then decreased to zero over a 1-lhalf minute period. The presented method is characterized by a series of restrictions in terms of the result that is achieved; these restrictions are the following:

- it is practically impossible to generate thick and abrasion-resistant coatings;

- there are considerable energy expenditures during the process of applying the coatings to the relatively large surfaces.

The above-mentioned insufficiencies restrict a wider application of the technique.

The most similar method in terms of the underlying technology is an electrochemical microarc technique of applying silicate coatings to aluminum components (3) . With this technique, the components, which are to be treated, are stepwise - in 4 to 7 cycles - immersed in an electrolytic bath with a sodium silicate, polyphosphate and arza ite-base electrolyte. Here, in the beginning of the process, when the components are being immersed in the electrolytic bath, an initial current density in the range of 5 to 25 A/dm² is applied to only 5 to 10 % of their total surface area and maintained constant during the following stepwise immersion. The main insufficiencies of this method are the following:

1. The complexity of the process, as it is necessary to organize the stepwise immersing and the controlling of the surface area of the components which are immersed in the electrolyte, and also to control and regulate the required current density level;

2. The coatings which are generated have a relatively low abrasion resistance, due to the chemical nature of the used electrolyte as well as the technological operations being conducted;

3. The method can only be used for the application of coatings to aluminum components. A change in the nature of the metal and of the chemical composition does not allow to generate high-quality coatings in terms of abrasion resistance and corrosion resistance parameters.

These insufficiencies are preventing a wider distribution of the method. This invention is solving the technical task of generating abrasion-resistant coatings of a specific thickness on the surfaces of components which are made of valve metals and their alloys with components of different chemical nature. It is also improving the technological effectiveness of the coating technique and is reducing the energy expenditures for this process while raising the quality of the coating. Apart from a high abrasion resistance of the components treated by the method, the presented method for microplasma oxidation also makes it possible to achieve a high corrosion resistance, what allows a substantial extension of the operational life of chemical reactors, pumps and units and components of devices which are operating in aggressive milieus. The above-mentioned technical result is achieved by the fact that the well-known method for microplasma oxidation of valve metals and their alloys comprises the following steps:

- immersing the component in the electrolyte;

- applying an initial polarizing current in the electric circuit, which is high enough that on the surface of the treated component, immersed in the electrolyte, moving microplasma discharges are forming.

- holding the component till the formation of a coating of a specific thickness;

- removing the forming voltage; - taking out the component

- rinsing the component with water

1) The immersing phase of the component in the electrolyte is done with a constant speed V, dm²/min, which is determined by the relation:

V=A-exp(B-N) (1)

with:

N - power output of the power supply, (Volt- Ampere) ;

A,B - coefficients, depending on the nature of the metal or the chemical composition of the alloy which is exposed to the microplasma oxidation;

2) The lowering phase of the voltage, at which a coating is forming, is done by lowering the voltage to a value, which corresponds with the beginning of the extinction of the microplasma discharges, and then maintaining the voltage up to the moment of complete extinction of the isolated moving microplasma discharges.

Experiments studying the influence of the immersion speed of the component in the electrolyte on the energy expenditure during the coating process of the objects and on the abrasion resistance of their surfaces have shown that their optimal values are in a sufficiently low immersion speed range, with the immersion speed being determined by the values of the coefficients A and B in equation (1) .

Thus for the microplasma oxidation of deformable aluminum alloys, the dependency of the immersion speed of the components in the electrolyte (V, dm²/min) on the strength of the power supply (N) can be described by the equation (1) , where A can have values ranging from 0.21 to 0.29, B has a value ranging from 2.0 ^• 10^"5 to 2.1 ^• 10^"5 (in the following the dimensions of the parameters A and B are omitted) . For the microplasma oxidation of casting aluminum alloys, containing up to 8% of silicon, this dependency can accordingly be described in form of equation (1) , where A has a value ranging from 0.07 to 0.09, however B a value ranging from 2.1 ^• 10^"5 to 2.2 ^• 10^"5; for titanium alloys, containing up to 10% of alloy elements: A is ranging from 0.41 to 0.42 B is ranging from 1.7 ^• 10^"5 to 1.8 ^• 10^"5 for zirconium and hafnium alloys, containing up to 4% of alloy elements :

A is ranging from 0.38 to 0.4 B has the value 1.8 ^• 10^~5; for aluminized steel : A is ranging from 0.19 to 0.28, B is ranging from 1.9 ^• 10^"5 to 2.25 ^• 10^"5. A considerable number of experiments made it possible to determine that the coefficient A is changing in a range of (0.05-0.5) dm²/min; the coefficient B, however, is changing in a range (1.5-2.5) ^• 10^"5/Volt ^• Ampere.

During the immersion, the surface of the component wetted by electrolyte is increasing and as a result of this, the polarizing current density and the voltage applied between the component and the electrolytic bath are decreasing. By regulating the immersion speed of the component, which means by regulating the speed with which the surface of the component is wetted, it is possible to keep the value of the polarizing current density within the limits, in which the microplasma oxidation process can take place, which is providing abrasion- resistant coatings.

Exceeding a specific immersion speed value the microarc oxidation process can come to a complete standstill with the coating which has already been formed, dissolving. If the immersion speed value of the component is too small, isolated arcs of high energy capacity can be observed, what leads to the local destruction of the coating and as a result of this, to a low abrasion-resistance and low protection of the coated component against corrosion.

Since during the formation of the coating small pores are forming in it, the healing of the pores is necessary to increase the corrosion resistance of the coating. In this con- text, it is necessary that the microplasma oxidation process takes place (is contained) only in these pores; that means that the formation of chemical compounds (mainly oxides) takes place only in the pores. Practically, their whole healing is accompanied by the self-extinction of the microplasma oxidation process.

If the voltage is decreased to a value corresponding with the beginning of the extinction of the microplasma discharges, in the pores of the coating after a while isolated discharges begin to ignite, resulting in the healing of the pores, when this state is continued for a specific period of time. A contrasting analysis of the proposed invention with the prototype shows that the presented method is different from the known one in terms of the immersion speed of the components, the regime of decreasing the forming voltage and maintaining it from the beginning of the extinguishing to the complete disappearing of isolated microplasma discharges. All the above-listed factors guarantee the solution of the set task.

1. Obtaining abrasion-resistant coatings of a specific thickness, not only on the surfaces of aluminum components, but also other valve metals and their alloys with elements of different chemical nature;

2. Raising the technological effectiveness of the coating method and the energy expenditure for this process .

The researched references have shown that all the above-stated factors are not known. Thus, the presented factors satisfy the requirement of "novelty" of the invention, a precondition for patenting it . Taking into account the fact that the immersion speed for the different alloys, the levels of decreasing the forming voltage and maintaining it until the complete extinction of the microplasma discharges, were gathered experimentally, originating from the earlier mentioned demands on the microplasma oxidation process and the quality of the generated coatings, then the above-mentioned factors satisfy the requirements concerning the "inventive step". Since the electrolyte consists of known components and the presented method involves rr | ] φ φ Ω Hi Ξ rt rt ft⁾ ft⁾ H3 Φ cr Ω Ω Ω μ- 0 cr H3 rt 3 J Hi 3 -S <5 3 ft⁾ 3 ^■^1 rt Hi rr €

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- an increased power consumption, due to the necessity of pumping the electrolyte from the working tank in the reserve tank and back,

- the difficulty of maintaining the given regime of simultan- ous oxidation of a huge number of small components.

The above-listed insufficiencies are preventing a wider distribution of those devices.

The object of the present invention is to lower the energy consumption during the coating process, to improve the compactness of the device, and also to raise the quality of the generated oxide coatings while expanding the range of metals used for the coating.

The above-indicated object is achieved by the fact that the known device for generating coatings with the microarc oxidation process additionally comprises a mechanism to vertically and horizontally move the component (components) with a control block, and that the electrolytic bath is positioned within the cooling tank with a coaxial shift in relation to the axis of the tank. Hereby the capacity of the tank is at least three times higher than the capacity of the electrolytic bath.

The researched references have shown that all the above-stated factors are not known. Thus, the presented factors satisfy the requirement of "novelty" of the invention, a precondition for patenting it. Because the device consists of known components, the above-indicated factors satisfy the requirement concerning "industrial applicability" and because the geometrical characteristics and relations of the parts of the device were gathered experimentally, the above-mentioned factors satisfy the requirements concerning the "inventive step". Figure 1 shows a sketch of the device for the microplasma oxidation of valve metals and their alloys. The device consists of a control block for the mechanism moving the component 1, a mechanism 2 to vertically and horizontally move the component with a holding device, an electrolytic bath 3 with an electrolyte, the treated component 4, a tank 5 with a cooling agent (e.g. circulating water) to cool the electrolyte and rinse the treated component 4, an electromotor 6, power supply 7 with a control desk, a mixer 8 to stir the electrolyte, which is connected with the electromotor 6. The electrolytic bath 3 can be positioned in the tank 5 with a shift in relation to the axis of the tank 5 and the capacity of the tank is at least three times higher than the capacity of the electrolytic bath 3. In this case, the cooling agent which is in the tank 5 is also performing the function of a rinsing agent.

The techique for operating the given device has been realized in the following way.

To generate an abrasion- and corrosion-resistant coating a plane disc of casting aluminum alloy (Al 22) containing up to 15% of alloy components and with a total surface area of 32 dm² has been used. The component has been fixed in the holding device which is tightly connected with the mechanism 2 to vertically and horizontally move the component. In the control block for the mechanism moving the component 1, the instruction has been given to vertically immerse the component 4 in the electrolyte, which has been poured in the electrolytic bath 3 with a specific speed which preliminarily has been calculated according to the equation V=A-exp(B-N) (1) . In this case, the immersion speed for casting aluminum alloy amounted to 0.26 dm²/min. The output power of the power supply amounted to 60000 Volt -Ampere. The used electrolyte, in this case, was composed in the following way (mass%) :

1) NaOH - 0.3

2) Na[AlOH]₄ - 0.5

3) remelted monosubstituted natrium phosphate - 0.5

4) aqueous extract of raw material of plant origin, won by a mass ratio of raw material and extract of less than 0.01 - 12.0

5) water - the rest

It should be mentioned that experiments have also been conducted for a series of electrolytes of different composition which can be found in the cited references . After giving the instruction to lower the component 4 and the beginning of its immersion into the electrolyte, the power supply 7 is switched on and a polarizing current intensity of 120 A is applied, which is changing according to equation (1) according to of the immersion degree of the component 4 in the electrolyte. The electromotor 6 is switched on, starting the mixer 8, stirring the electrolyte.

The voltage providing the initial applied polarizing current intensity is high enough to generate microplasma discharges. According to the immersion scale of the component 4, the surface area wetted by electrolyte is increasing, the zone of microplasma discharges is scanned on the immersion surface of the component 4. During the above-indicated wetting speed of the surface of the component, the voltage is kept at a level which is high enough to maintain the burning of the discharges on the overall wetted surface (approximately 550-600 V) , up until the complete immersion of the component 4 in the electrolyte .

After the immersion of the component 4 in the electrolyte, the component is held (in this position) over a period of 35 to 45 minutes, during which the coating is applied to the surface of the component. Hereby, on the whole surface of the component 4 moving microarcs are burning, and then the forming voltage is lowered to a value which conforms with the beginning of the extinction of the microplasma discharges (e.g. up to 380 to 430 V) and the appearance of isolated wandering microplasma discharges. The ignition of the isolated discharges is restricted to the pores of the coating of the component 4. Then the voltage is maintained until the complete extinction of the isolated wandering microplasma discharges over a period of 10 to 14 minutes. Only after this operation the power supply 7 is switched off. It should be mentioned that the positioning of the electrolytic bath 3 in the tank 5 with the cooling agent (e.g. circulating water) is contributing to its cooling, which means to the improvement of the thermal conditions of its functioning. In the control block 1 for the mechanism moving the component Ω 0 rt ft ⁾ O TJ tr 0 H ,-, Ω ft ft⁾ >-3 Pi φ 0 Ω 3 ft⁾ Φ Ω 0 LQ μ- < rt ft rt rt ft ft⁾ Hi O rr rr CT 0 Φ Hi o μ> 0 rr μ- rr μ- H-¹ Hi Φ ft⁾ μ- 3 0 Hi Φ CO rt φ rr rr rr ft⁾ 0

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Φ TJ O ft⁾ rt rt rr rt ft ft μ- o Hi rr Φ 0 O CO Φ 3 Ω rr Φ Pi Φ 3 3 3 ft μ- ft⁾ H Φ Φ 0 Hj cr rt Ω ft> 3 Hi Φ ft ft⁾ O μ- d 3 μ- ft⁾ ft⁾ h-¹ Pi μ- 3 ft rt rt Φ rt

0 3 ft ft⁾ rt Hi ft) Φ 3 to ft⁾ ? rt 3 ft⁾ Ξ Φ Ω H-¹ Hf 0 3 rt Hi rt 0 rt LQ μ- 3 rr to LQ LO 3 rt ft⁾ Φ rt Ω Ω i μ- LQ rt μ- Hi Hi Pi ≤ 0 LQ ^ μ- rf- 0 rr 3 φ Φ μ- o LQ Ω rr 3 rt ft⁾ φ 0 0 3 ~-^ rt Φ rt ft) 0 CO 3 μ- TJ --» co ft⁾ < 1

Φ rt LQ rt 0 < o Φ o Φ TJ φ 3 P s: LQ 3^J P. rr rt 3 rt φ 3 Φ Pi rt μ- Ω < φ

H{ rr to Φ TJ Hi φ 3 μ- Hi rt 0 rt 3 Φ Φ Φ ft⁾ CO LQ H{ Φ 0 rr ft⁾ o ft Φ H, ' φ Hj Hi 3 X 0 TJ Φ 3 Hi μ- rr 0 tr rt P. rt σ to ft) < d Φ Ω μ- Hi rt

Px¹ μ- D Φ ft < Hi 0 X LQ 0 Ω 3 <. ft⁾ Hi CO ^ rr Φ μ- Ω rt μ- rt rt H{ 3 rt μ-

3 Ω 3 CO rr Hi 3 ft⁾ Hh d ft LQ 0 o 0 Φ Ω Hi H-^< μ- rr μ- Ω μ- μ- μ- LQ μ- Ω

0 0 ft⁾ Φ φ 0 ft φ 3 Ω Hi rr to Ω H-¹ rt 0 • μ- 3 ft⁾ 0 φ 0 3 3 3 Ω ft⁾

£ CO ft⁾ 3 3 H 1 cr 3 μ- 0 to H{ Φ rt cr d d rr ft⁾ rt Hi 3 • Hi CO LQ CO ft⁾ ft⁾ μⁱ

3 rt Pi rt LQ 3 φ rt 3 3 φ rr ft⁾ Φ TJ rt Φ Φ rt r ^ rt ft⁾ CO rt φ CO to ≤ Φ φ d ft⁾ ft⁾ rt 3 3 Ω P⁾ LQ Φ Hi μ- < μ- Φ rr Ω (t Hi ft⁾ ^

3 μ- rt Pi 3 3 0 μ- rr ft⁾ 3 rt o CO Φ 3 μ- 0 rt μ- 3 Hi 0 Φ rt ft⁾ rr d CO rt ^

Φ rr φ ft⁾ d 3 μ- Ω ~—* ft⁾ Ω 3 μ- Ω LQ Φ Hi Φ φ Ω t. rr rt 0 φ P⁾ 3 Hi . rt O rt to rt tr cr Pi - 3 φ CO cr 3 Hi Ω rt « μ- φ μ- μ- rr c rt Φ ft⁾ rt J rt 3 0 0 μ- Φ ^ Φ Ω Φ •<; *» μ- μ- O ft μ- ft⁾ rt 3 hh o HJ rt ft rr d rr ft⁾ Hi d 3 Φ rt rr Ω TJ rr - Pi CQ ft⁾ ft⁾ O O rr Ω 3 rt

Pi ft⁾ rt rr φ Φ rt φ 0 3 Hi LQ 3 it Φ O J 2 ft⁾ Pi rt ft 3 3 rr 0 Φ

CD ≤ 3 rr 0 Pi Hi 0 ft⁾ Ω CO rr Hi o 3 ft⁾ Hi <! rt 3 μ- μ- ? rt 0 Hi rt rr LQ Φ Pi H< J φ d φ 3 Φ 3 Hi rt to 1 Φ rr μ- Φ Ω 3 μ- H-¹ rr CO rr ft⁾ ft⁾ φ Ω Φ 0 rt 3 J . ft⁾ Φ μ- μ- ft⁾ to LO Φ 3 O LQ LΠ CO μ- μ- μ- φ Φ

3 rt Z φ rr rt

Pi μ- o CO ■ ft⁾ rt 3 μ- μ- • cr d TJ • • • lP co 3

Pi 0 Φ to ft⁾ d Φ Ω Φ Φ ft⁾ < O rr Φ φ 3 3 Φ rt rt ft rr LQ d Q μ- Ω

0 Hi ft Hi rt Pi 3 rr 0 d μ- Pi • μ- LQ Φ rr Φ Hi ft⁾ ft⁾ > μ- μ- C rt 0

Pi Hi μ- P⁾ μ- It H{ Φ Φ H{ Ω 3 rr 3 μ- to rt to Hh <i Pi ft⁾ ft⁾ 3

Φ rt CD Ω Ω 3 CO 0 0 Φ rt rt Φ P? tr LQ d Φ Ω rt ft Φ • rt LQ μ- J

Φ ft⁾ φ rr LQ Hi TJ 0 to Ω ft⁾ Pi 3 ^ rr 3 Pi Px 0 rr cr Φ 3 Φ φ 3 0 μ- 3 3 μ- rr *< Φ d H-¹ LQ φ Φ co ft Ω φ 3 P⁾ Hi μ- Φ H{ > H, 3 3

Ω TJ P⁾ Φ ft⁾ ft⁾ rt rt 3 ft⁾ Hi ft⁾ Φ μ- to Pi o μ- rr rt Φ 3 Ω Φ rt Hi • rt rt Φ

Φ H μ-> < HJ < rr Φ Pi r Hi 3 3 to μ- o Φ Φ CO ft ^■P? 3 ft 0 ft • rr 3 to φ μ- φ Φ Φ Φ 0 ft⁾ TJ Hi • <! 3 Hi CO rr 3 cr Φ Φ rt to 0 3 rr 3 d Ω μ- Hi μ- LQ rt μ- 1 Φ Φ 0 Hi 3

£ LQ ft⁾ Ω ft Φ 3 0 1 \ r 1 O rt 3 il-. ω to -< LQ 3 h-¹ Φ to -

References :

1. Vansovskaya, G.A. : "Galvanitcheskie pokrytiya" (Galvanic coatings), Moskva, Mashinostroenie, 1984, p.78.

2. Avtorskoe svidetelstvo SSSR 1783004, cl.C 25 D 11/02, published in 1992. (SU 1783004)

3. Patent of the Russian Federation 2065895, cl.C 25 D 11/04, published in 1996.

4. Chernenko, V.I. and others: "Poluchenie pokrytij anodno- iskrovym elektrolizerom" (Generating coatings with an anodic spark electrolytic bath), Leningrad, Khimiya, 1991, p. 85-90.

5. Patent of the Russian Federation 2010040, cl.C 25 D 11/02, published in 1994.

Claims

CLAIMS .

1. Method for microplasma oxidation of valve metals and their alloys, including the following steps:

- immersion of the component in the electrolyte,

- application of an initial polarizing current power in the electric circuit, which is high enough to generate on the surface of the treated component, which is immersed in the electrolyte, moving microplasma discharges,

- holding of the component until the formation of a coating of a specific thickness,

- removal of the forming voltage and taking out the component and then rinsing the component, characterized in that the immersion of the component in the electrolyte is done with a constant speed which is determined by the relation:

V=A-exp(B-N)

with:

V - immersion speed of the component, dm²/min;

N - output power of the power supply, N- (0,05-3) -10⁵

(Volt- Ampere) ; A - ( 0.05 -0.5) dm²/min; B - (1.5-2.5) ΓÇó 10^"5 (1/Volt- Ampere) ,

and after the formation of the coating is completed, the voltage in the electric current is lowered until isolated wandering microplasma discharges are appearing on the treated surface; and the treated surface of the component, which is immersed in the electrolyte, is held until the complete extinction of the isolated wandering microplasma discharges.

2. Device for microplasma oxidation of valve metals and their alloys, which includes a power supply connected with an electrolytic bath and a means to change the wetting level of the treated component, characterized in that it is equipped with a tank with a cooling agent, in which the electrolytic bath is located, and with a control block with which the means for changing the wetting level of the treated component is connected, whereby said means is realized in form of a mechanism to vertically and horizontally move the treated component with the possibility to move with it the given component out of the electrolytic bath in the tank with the cooling agent.

3. The device according to claim 2, characterized in that the capacity of the tank with the cooling agent is at least three times higher than the capacity of the electrolytic bath.