RU2467098C1 - Method of plasma-electrolytic removal of coatings from titanium nitrides or those of compounds of titanium with metals - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to removal of protective coatings from part surfaces of titanium alloys and may be used in reconditioning blades of steam turbines, compressor plant vanes and gas turbine engine vanes. Method comprises dipping the part in electrolyte, making vapor-gas envelopment around part surface and igniting the discharge by feeding electric potential to said part. Note here that potential of 320-340 V is applied to machined part while 2-7%-water solution pf the mix of NH₄F and KF is used as electrolyte at the following ratio, wt %: NH₄F - 16 - 26, KF making the rest. Note here that complete removal is carried out at 0.2 A/dm² to 0.8 A/dm² and 70°C - 90°C.

EFFECT: higher quality and efficiency.

10 cl, 1 ex

Description

The invention relates to a technology for electrolyte-plasma removal of protective coatings from titanium nitride from the surface of parts made of titanium alloys and can be used to restore parts of turbomachines, in particular working and guide vanes of steam turbines, blades of gas pumping units and compressors of gas turbine engines.

The blades of the compressor of a gas turbine engine (GTE) and gas turbine installation (GTU), as well as steam turbines, are exposed to significant dynamic and static loads during operation, as well as corrosion and erosion destruction. Based on the performance requirements for the manufacture of gas turbine compressor blades, titanium alloys are used, which, compared to technical titanium, have higher strength, including at high temperatures, while maintaining a sufficiently high ductility and corrosion resistance (for example, titanium alloys grades VT5, VT6, VT8, VT18U, VT3-1, VT22, etc.). In addition, to ensure enhanced performance properties, titanium nitride protective coatings are applied to the surface of the blade pen. However, the presence of such a coating on the blades of turbomachines causes problems in removing the coating with technological or operational defects during repeated application, including during repair.

Traditionally, protective ion-plasma coatings, in particular titanium nitride coatings, are removed by chemical etching.

Known solution for the etching of titanium alloys (USSR AS No. 1194907, C23F 1/26, 1983), containing, wt.%: Hydrochloric acid 10.0-15.0; ammonium fluoride 2.0-4.0; sodium chloride 0.05-1.0; sodium acetate 0.05-0.5; water - the rest.

Stripping using a known solution leads to a deterioration in the quality of the surface layer, an inhomogeneous surface is formed, and, moreover, this method is characterized by a low rate of coating removal.

Also known is a solution for removing coatings from titanium nitride and carbonitride (RF Patent No. 2081207, C23F 1/44, 1/26, 1997), containing, g / l: hydrofluoric acid - 70-100; hydrochloric acid - 40-80; phosphoric acid - 100-290; water - the rest, as well as a composition for cleaning surfaces of steel and titanium alloys from complex nitride coatings (RF Patent No. 2151821, C23G 1/02, 2000), containing, g / l: nitric acid - 28.0-32, 0; potassium tetrafluoroborate - 26.0-30.0; sodium polyphosphate - 0.5 · 10 ^-3 -1.5 · 10 ^-3 ; water - the rest.

However, the known methods for removing coatings from titanium nitride do not allow processing parts from titanium or a titanium alloy without significant etching of the material of the surface layer. In addition, the known methods use etching solutions, the composition of which is not environmentally friendly.

Due to the fact that the blades of turbines for compressors and steam turbines made of titanium and titanium alloys are highly sensitive to stress concentrators, the occurrence of defects during removal of coatings from them is unacceptable, since during operation defects formed on the surface of the blades lead to their destruction. Therefore, the development of coating removal methods, allowing to obtain high-quality surfaces of turbomachine parts with high environmental friendliness and performance of the coating removal process, is a very urgent task.

The most promising methods for processing turbomachine blades are electrochemical methods (Griliches S.Ya. Electrochemical and chemical polishing: Theory and practice. Influence on the properties of metals. L .: Mashinostroyenie, 1987), while electrolytic-plasma processing methods are of the greatest interest. details (for example, Patent GDR (DD) No. 238074 (A1), IPC C25F 3/16, publ. 06.08.86, as well as Patent RB No. 1132, IPC C25F 3/16, 1996, Bull. No. 3).

A known method of processing metal surfaces, including anode processing in an electrolyte (Patent RB No. 1132, IPC C25F 3/16, 1996, Bull. No. 3), as well as a method of electrochemical processing (US Patent N 5028304, IPC B23H 3/08, C25F 3 / 16, C25F 5/00, publ. 02.07.91).

However, the known methods of electrochemical processing do not allow the removal of nitride coatings from the surface of parts made of titanium and titanium alloys.

Closest to the claimed technical solution is a method of electrolyte-plasma surface treatment of a part made of titanium or titanium alloys, comprising immersing the part in an electrolyte, forming a vapor-gas shell around the workpiece surface and igniting a discharge between the workpiece and the electrolyte by applying an electric potential to the workpiece (Patent RF №2373306, IPC C25F 3/16. Method for multi-stage electrolyte-plasma polishing of products from titanium and titanium alloys. Bull №32, 20 09).

However, the known method (RF Patent No. 2373306, IPC C25F 3/16) is multi-stage, which leads, on the one hand, to an increase in the complexity of the part processing process, a decrease in the quality and reliability of the processing process due to the need to provide more process parameters and their ratios, and also to increase its complexity. In addition, the purpose of this method is not to remove coatings, but to polish surfaces.

The problem to which the invention is directed, is to develop a process for removing nitride coatings, which allows to obtain high-quality surfaces of parts of turbomachines with high performance of the process of removing coatings from the surface of parts from titanium and titanium alloys.

The technical result of the present invention is to improve the quality of removal of nitride coatings from parts of titanium and titanium alloys while increasing the productivity of the processing process.

The problem is solved in that the method of electrolyte-plasma removal of coatings from titanium nitride or nitrides of titanium compounds with metals from the surface of parts made of titanium and titanium alloys, comprising immersing the part in an electrolyte, forming a vapor-gas shell around the workpiece surface and igniting a discharge between the workpiece and electrolyte by supplying an electric potential to a workpiece, characterized in that an electric potential of 320 is applied to the workpiece B to 340 B, and a 2-7% aqueous solution of a mixture of NH ₄ F and KF is used as the electrolyte, when their content, wt.%: NH ₄ F is from 16% to 26%, KF is the rest, and removal of the coating is the current value from 0.2 A / dm ² to 0.8 A / dm ² , at a temperature of 70 ° C to 90 ° C until the coating is completely removed, while the following options are possible: removal of the coating from parts is carried out from an area of 1 cm ² up to 4000 cm ² , and as a removable coating from titanium nitrides and nitrides of titanium compounds with metals, a TiN, TiN ₂ or Ti-Me-N coating is used, where Me is Zr, Al, W, Mo, AlZr, AlZrMo or combination, N - az t; as parts use the blades of a turbomachine; surfactants are additionally introduced into the electrolyte composition in a concentration, wt.%: 0.4-0.8; TiF ₄ is additionally introduced into the electrolyte composition, wt.%: from 0.3 to 0.8; Before removing the coating from the parts, in the same modes as the modes of removing the coating from the parts, similar coatings are removed from auxiliary elements from titanium or a titanium alloy with a washout of the precipitate formed in the electrolyte, and removal of the coating from auxiliary elements is carried out until the process of coating removal is stabilized.

The inventive method of electrolyte-plasma removal of coatings from titanium nitride or nitrides of titanium compounds with metals from the surface of parts made of titanium and titanium alloys is as follows. A workpiece made of titanium or a titanium alloy coated with titanium nitride or nitrides of compounds of titanium with metals is immersed in a bath with an aqueous electrolyte solution, a positive electric potential is applied to the product, and a negative potential is applied to the electrolyte, as a result of which a discharge arises between the workpiece and the electrolyte . The process of electrolyte-plasma removal of the coating is carried out at an electric potential from 320 V to 340 V, and a 2-7% aqueous solution of a mixture of KF and NH ₄ F is used as the electrolyte, with their content, wt.%: 80% KF and 20% NH ₄ F. Removal of the coating, depending on the parameters of the part (with a polishing area of 1 cm ² to 4000 cm ² ) and a given surface microgeometry, are carried out with a current value from 0.2 A / dm ² to 0.5 A / dm ² , at temperature from 70 ° C to 90 ° C, until complete removal of the coating. The workpiece may be a turbomachine blade. To improve the quality of coating removal, surfactants can be added to the electrolyte composition in a concentration, wt.%: From 0.4 to 0.8% and / or TiF ₄ , wt.%: From 0.3 to 0.8 % Before removing the coating from the parts, in the same modes as the modes of removing the coating from the parts, similar coatings are removed from auxiliary elements from titanium or a titanium alloy with a washout of the precipitate formed in the electrolyte, and removal of the coating from auxiliary elements is carried out until the process of coating removal is stabilized. Removing the coating is carried out in an electrolyte medium while maintaining a vapor-gas shell around the part. As a bath, a container made of a material resistant to electrolyte is used. The pH of the electrolyte is in the range of 4-9. The electrolyte temperature is in the range of 70-90 ° C.

When implementing the method, the following processes occur. Under the influence of flowing currents, the surface of the part is heated and a vapor-gas shell forms around it. Excessive heat arising from the heating of the part and the electrolyte is removed through the cooling system. At the same time, the set process temperature is maintained. Under the action of electric voltage (electric potential between the part and the electrolyte), a discharge appears in the vapor-gas shell, which is an ionized electrolytic plasma, which ensures the occurrence of intense chemical and electrochemical reactions between the treated part and the medium of the gas-vapor shell.

When a positive potential is applied to the part, during the course of these reactions, the surface of the part is anodized with simultaneous chemical etching of the formed oxide. Moreover, with anodic polarization, the vapor-gas layer consists of electrolyte vapors, anions, and gaseous oxygen. Since etching occurs mainly on microroughnesses, where a thin oxide layer is formed, and anodizing processes continue, as a result of the combined action of these factors, the coating is removed from the workpiece surface.

When an electrolyte is treated with a 2-7% aqueous solution of a mixture of KF and NH ₄ F, with their content, wt.%: NH ₄ F from 16% to 26%, KF - the rest, the surface of the part is covered with a layer of easily soluble plaque of fluoride compounds. At voltages from 320 V to 340 V, the discharge temperature is high enough to conduct a stable coating removal process.

When processing complex parts made of titanium and titanium alloys (for example, turbomachine blades), it is advisable to introduce surface-active substances (surfactants) into the electrolyte. The introduction of a surfactant reduces the surface tension coefficient of the solution, which improves the state of the gas-vapor layer at the gas-liquid interface. However, significant concentrations of surfactants should not be created, since this can lead to the formation of unwanted indelible films on the surface of the product. In addition, an increase in the concentration of surfactants can lead to the opposite effect, i.e. an increase in the surface tension coefficient of the solution. The concentration of the main components of the electrolyte is a fairly variable value. Moreover, the lower limit of their concentration is determined by the need to ensure the quantitative dominance of fluorine ions over oxygen ions both in the film formed on the surface of the product and in the vapor-gas shell. The upper limit of the concentration of the electrolyte solution is limited by an increase in the number of toxic gaseous products formed during processing (F ^- , NH ₃ ). To minimize joule-loss, the electrolyte must have sufficient electrical conductivity. When selecting an electrolyte concentration from a range from 2 to 7% aqueous solution of a mixture of KF and NH ₄ F (with wt.%: NH ₄ F from 16% to 26%, KF - the rest), as well as additional additives (surfactants in concentration of 0.4-0.8% or 0.3-0.8% TiF ₄ ) it is also necessary to take into account the possibility of its continued use without additional adjustment of the composition.

Example. Coatings were removed from titanium nitride with thicknesses from 7 μm to 20 μm from parts made of titanium alloys of grades VT-1, VT3-1, VT5, VT6, VT8, VT8m. The processed samples were immersed in a bath with an aqueous solution of electrolyte and positive voltage was applied to the part, and negative voltage was applied to the electrolyte. Removal of coatings was carried out in an electrolyte based on an aqueous solution, which included: from 2 to 7% aqueous solution of a mixture of KF and NH ₄ F (at wt.%: 80% KF and 20% NH ₄ F), as well as additional additives (surfactants in a concentration of 0.4-0.8% or 0.3-0.8% TiF ₄ ). During processing, circulating cooling of the electrolyte was performed (the average process temperature was maintained in the range of 70 ° ... 90 ° C).

Processing conditions for the proposed method: electrical potential (voltage) from 320 V to 340 V; electrolyte - 2-7% aqueous solution of a mixture of KF and NH ₄ F, when their content, wt.%: NH ₄ F from 16% to 26%, KF - the rest; additives to the electrolyte - surfactants in a concentration of 0.4-0.8% or 0.3-0.8% TiF ₄ ; the current value is from 0.2 A / dm ² to 0.8 A / dm ² , at a temperature of 70 ° C to 90 ° C, the processing time is until the coating is completely removed, with a processing area of 1 cm ² to 4000 cm ² . Before removing the coating from the parts, in the same modes (electric potential from 320 V to 340 V) as the modes of removing the coating from the parts, similar coatings from auxiliary elements were removed from titanium or a titanium alloy with a washout of the precipitate formed in the electrolyte, and removal of the coating from auxiliary elements was carried out until stabilization of the coating removal process. In addition, electrolytes were studied: from 2% to 7% (1% - unsatisfactory result (N.R.); 2%; 3%; 5%; 7%; 8% - (N.R.) aqueous solution of KF mixture and NH ₄ F, at their content, wt.%: NH ₄ F from 16% to 26% (14% - (НР); 16%; 20%; 24%; 26%; 28% - (Н .Р.)), KF - the rest. As a removable coating from titanium nitrides and nitrides of titanium compounds with metals, parts with coatings were used: TiN, TiN ₂ or Ti-Me-N, where Me is Zr, Al, W, Mo, AlZr, AlZrMo or a combination thereof, N is nitrogen.

Compared with the known method for removing nitride coatings (RF Patent No. 2151821, C23G 1/02, 2000), the productivity of the process according to the proposed method for removing nitride coatings increased, on average, by 7-9 times, and the average surface roughness from Ra 0 , 65 ... 0.45 microns for the proposed method are improved to Ra 0.09 ... 0.05 microns.

Thus, the studies showed that the application of the proposed method of electrolyte-plasma removal of coatings from titanium nitride or nitrides of titanium compounds with metals from the surface of parts made of titanium and titanium alloys can be increased, compared with the known method of removing nitride coatings (RF Patent No. 2151821, C23G 1/02, 2000), the quality of processing parts from titanium alloys VT-1, VT3-1, VT5, VT6, VT8 and VT8m with the above nitride coatings, while increasing the productivity of processing parts.

Using the method of electrolyte-plasma removal of coatings from titanium nitride or nitrides of titanium compounds with metals from the surface of parts made of titanium and titanium alloys, which includes the following essential features: immersion of the part in electrolyte; forming a vapor-gas shell around the workpiece surface and igniting a discharge between the workpiece and the electrolyte by applying an electric potential to the workpiece; application to the workpiece of electric potential from 320 V to 340 V; use as a electrolyte of a 2-7% aqueous solution of a mixture of NH ₄ F and KF, when their content, wt.%: NH ₄ F from 16% to 26%, KF - the rest; removal of coating at a current value of 0.2 A / dm ² to 0.8 A / dm ² , at a temperature of 70 ° C to 90 ° C until complete removal of the coating; removal of coating from parts from an area of 1 cm ² to 4000 cm ² ; use as a removable coating of titanium nitrides and nitrides titanium compounds with metals coating TiN, TiN ₂ or Ti-Me-N, where Me is Zr, Al, W, Mo, AlZr, AlZrMo or a combination thereof, N is nitrogen; the use of turbomachine blades as parts; the introduction of an additional electrolyte surfactants in a concentration, wt.%: 0.4-0.8; the introduction of an additional electrolyte of TiF ₄ , wt.%: from 0.3 to 0.8; before removing the coating from the parts, in the same modes as the modes of removing the coating from the parts, removing the coating from titanium nitride from auxiliary elements from titanium or a titanium alloy with washing out the precipitate formed in the electrolyte, when removing the coating from auxiliary elements to stabilize the process of removing the coating , allows to achieve a technical result of the proposed method is to improve the quality of removal of nitride coatings from parts of titanium and titanium alloys while increasing the productivity of the processing process.

Claims (10)

1. A method of electrolyte-plasma removal of coatings from titanium nitrides or nitrides of titanium compounds with metals from the surface of titanium and titanium alloy parts, comprising immersing the part in an electrolyte, forming a vapor-gas shell around the workpiece surface and igniting a discharge between the workpiece and the electrolyte by applying a workpiece of electric potential, characterized in that an electric potential of 320 V to 340 V is applied to the workpiece, and as an electrolyte using 2-7% aqueous solution of a mixture of NH ₄ F and KF at their content, weight%:. NH ₄ F 16 to 26, KF - else, the removal of the coating until complete removal is conducted at a current value of 0.2 A / dm ² to 0.8 A / dm ² and a temperature of 70 ° C to 90 ° C. 2. The method according to claim 1, characterized in that the coating is removed from the part from an area of 1 cm to 4000 cm, while the coating is removed from titanium nitrides or nitrides of titanium compounds with metals in the form of TiN, TiN ₂ or Ti-Me- N, where Me is Zr, Al, W, Mo, or a combination thereof, N is nitrogen. 3. The method according to claim 1, characterized in that the blades of a turbomachine are used as parts. 4. The method according to claim 2, characterized in that the blades of a turbomachine are used as parts. 5. The method according to any one of claims 1 to 4, characterized in that surfactants are additionally added to the electrolyte composition in a concentration, wt.%: From 0.4 to 0.8. 6. The method according to any one of claims 1 to ₄ , characterized in that the composition of the electrolyte is additionally introduced TiF ₄ , wt.%: From 0.3 to 0.8. 7. The method according to claim 5, characterized in that the composition of the electrolyte is additionally introduced TiF ₄ , wt.%: From 0.3 to 0.8. 8. The method according to any one of claims 1 to 4 and 7, characterized in that before removing the coatings from the parts according to the mode of removing coatings from the parts from titanium or a titanium alloy, auxiliary elements from titanium or a titanium alloy are processed to wash off the precipitate formed in electrolyte, moreover, the removal of the coating from auxiliary elements leads to the stabilization of the removal process. 9. The method according to claim 5, characterized in that before removing coatings from the parts according to the mode of removing coatings from parts from titanium or a titanium alloy, auxiliary elements from titanium or a titanium alloy are treated with a washout of the precipitate formed in the electrolyte to form a precipitate, moreover, removing the coating from auxiliary elements lead to stabilization of the coating removal process. 10. The method according to claim 6, characterized in that before removing coatings from parts according to the modes of removing coatings from parts from titanium or a titanium alloy, auxiliary elements from titanium or a titanium alloy are processed to wash off the precipitate formed into an electrolyte, and processing of auxiliary elements lead to stabilization of the coating removal process.