RU2547372C2 - Production of coatings on surfaces of metals and alloys - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to electrochemical processing of metal surfaces and can be used for local surface processing of structures, for example, of titanium allots in machine building, medicine, aircraft engineering, etc. Method of making the protective pore-less coating by micro arc oxidation on sheet surface comprises cleaning and degreasing of sheet surface. Then, the device composed of vinyl plastic case with O-ring and stainless steel gauze in conical tube are placed on local section of said sheet. Sodium hydro phosphate-based electrolyte is continuously fed in said case through closed circuit onto the sheet-anode at forced pressure of 0.4-0.5 atm. Now, it is fed to cathode at maximum voltage of 190 V and current density of 0.5 A/dm² for 10 minutes.

EFFECT: higher corrosion resistance.

1 tbl, 1 dwg

Description

The invention relates to methods for the electrochemical treatment of metals and alloys to obtain protective corrosion-resistant oxide coatings on the surface of parts.

Known: "The method of microarc obtaining protective films on the surface of metals and alloys" RF patent No. 2061107, MKI C25D 11/06, publ. 05/27/96, "Method for applying an electrolytic coating to the surface of metals and alloys" patent No. 21202086, MKI C25D 11/06, 05/27/98, "Method for applying a corrosion-resistant oxide layer with a locally reduced thickness on the surface of a metal part" , German patent No. 4442792, IPC C25D 11/06, publ. 05/04/1998

The disadvantages of these processing methods include the complexity of processing in bathtubs with electrolyte of large-sized parts, as well as the inability to process the surface of the parts directly in operating conditions.

Also known is the "Electrolytic method of applying protective coatings on the surface of metals and alloys", patent RU 2294987, IPC C25D 11/02, publ. September 15, 2005

The disadvantages of this method are the uneven distribution of the coating material on the treated surface and high energy consumption, since microarc oxidation is carried out at a voltage of 200-250 V and a current density of 1.0-3.0 A / dm ² .

Also known is the "Method of producing coatings on parts of metals and alloys in the compression microarc oxidation mode and device for its implementation" patent RU 2324014, IPC C25D 11/02, publ. December 20, 2007

The disadvantages of this method are: the difficulty of processing the part when it is completely immersed in the electrolyte solution in a hermetically sealed container, the presence of an additional device for creating a vacuum.

The closest in technical essence and taken as a prototype is the "Method for protective coatings on the surface of metals and alloys" patent RU 2194804, IPC, C25D 11/02, publ. 12/20/2002.

In this method, a device is used that provides local oxidation that moves across the entire surface of the part. The method includes microarc oxidation using a device equipped with an anode and cathode, and a porous screen through which a liquid electrolyte is supplied.

The disadvantages of this method are the presence of porosity in the protective oxide coating and the inability to obtain a large thickness of the coating. In the known method, the electrolyte is first supplied to the cathode and merged through the foam to the anode. In this case, the electrolyte is saturated with hydrogen gas due to the main cathodic reaction of water electrolysis: H + e → H.

Hydrogen, being one of the strongest reducing agents, reduces metal oxides and forms a porous, loose oxide film MeO + H ₂ → Me + H ₂ O or (MeO ₂ + H ₂ → MeO + H ₂ O). Pores reduce the effective thickness and corrosion resistance of coatings. In addition, the electrolyte must be removed, i.e. used once.

The technical result of the invention is to increase the corrosion resistance of oxide coatings by reducing porosity and increasing the effective thickness of the oxide coating.

The technical result is achieved due to the fact that in the method of producing protective coatings on the surface of metals and alloys, including microarc oxidation using a device equipped with an anode and a cathode through which a liquid electrolyte is supplied, in accordance with the invention, the electrolyte is first supplied to the anode under pressure 0, 4-0.5 atm, and then to the cathode. A pressure of 0.4-0.5 atm provides pressure for the movement of an electrolyte stream that carries away hydrogen bubbles in the direction opposite to the anode and mechanical compaction of the formed coatings.

Pressure above 0.5 atm is impractical, since a strong pressure of the electrolyte will prevent the production of thicker coatings and reduce corrosion resistance. A pressure below 0.4 atm will not ensure the movement of the electrolyte flow, while hydrogen will not be carried away with the flow, there will be pores in the coating that reduce corrosion resistance.

The cathode is made in the form of a mesh metal screen, which eliminates the ingress of electrolytic hydrogen on the treated surface and does not change the ion-gas composition of the electrolyte in this zone. From the zone of the electrolyte medium with the anode component being treated, atomic and molecular hydrogen in the form of gas bubbles under pressure with an electrolyte stream is drawn into the mesh electrode-cathode and flows through the return hoses to the electrolyte tank. Due to the removal of hydrogen, there is a decrease in the reduction ability of the electrolyte and the formation of oxide films. At the same time, thicker and less porous oxide films are formed on the treated surface: (Me + O → MeO ₂ ).

Due to the mechanical compaction of initially pliable surface oxide formations with intensive movement of the electrolyte from the anode to the cathode under a pressure of 0.4-0.5 atm, it becomes possible to form pore-free oxide coatings of greater thickness.

Scheme microarc oxidation according to the claimed method is presented in the drawing. The workpiece 1 (anode) is tightly attached to the casing 3 of vinyl plastic or other electrical insulating material. The tight contact of the vinyl plastic body with the workpiece is ensured by a rubber o-ring. The electrolyte 4 is fed to the anode 1 through a flexible hose 5 under pressure from an electric pump 7. The electric pump provides continuous movement of the electrolyte in a closed loop: from a tank 8 with electrolyte 4 through a flexible hose 5 to the workpiece anode 1, then to the cathode 2, through a conical pipe 9 and hose 6 to tank 8.

The method is as follows.

The surface of the part to be microarc oxidized is cleaned and degreased. For example, a BT1-0 titanium alloy sheet with a thickness of 4 mm and an area of 1 m ^{2 was used} .

A sheet of vinyl plastic is placed on the sheet closely adjacent to the surface of the part.

As a mesh screen-cathode 2, a design in the form of a conical pipe 9 with a stainless steel metal mesh with 0.5 mm cells with an area of 1 dm ^{2 was used} . The mesh cell size is not regulated.

A tank 8 with an electrolyte 4 with a volume of 50 l is placed under the workpiece 1. The electrolyte is used with known compositions based on Na ₃ PO ₄ · H ₂ O. An electric pump with a capacity of 0.1 m ³ / h ensures continuous intensive movement of the electrolyte.

The electrolyte is fed into the casing through flexible rubber hoses to the workpiece-anode under a forced pressure of 0.4-0.5 atm. The part does not sink into the tank. The process proceeds on a local area of the treated surface at a maximum voltage of 190 V and a current density of 0.5 A / dm. ²

To obtain a given thickness of the oxide layer, the processing process is maintained for a certain estimated time. With a processing time of 10 minutes, the coating thickness will be 10 μm.

The coating thickness is determined using a multifunctional electromagnetic thickness gauge "Constant K 52", the roughness of the coatings is measured with a portable profilometer. Porosity was evaluated as a percentage when weighing samples. The results are presented in table 1.

The results of the properties of oxide coatings obtained using the proposed method.

Table 1 Way Coating Properties Porosity,% Coating thickness, microns The roughness parameter, Ra, microns Microhardness, HV kgf / mm ² The claimed 5,0 10 0.32 780 Famous 35.0 0.5-1.0 1.25 230

The absence of porosity and the large thickness of the oxide layer increase the corrosion resistance of the coating.

The developed technology allows to obtain oxide coatings with a thickness of up to 10-15 microns, firmly bonded to the base, in addition, it is environmentally friendly, does not require special environmental measures. The collection and storage of electrolyte in an open container promotes natural degassing, which allows its multiple use in a closed cycle.

Claims (1)

A method of obtaining a protective non-porous coating by microarc oxidation on the surface of a sheet of titanium alloy, characterized in that it includes cleaning and degreasing the surface of the sheet, installing in the local area of the sheet a device in the form of a vinyl plastic housing with a sealing ring and a cathode in the form of a stainless steel metal mesh in a conical pipe and a continuous supply of an electrolyte based on sodium hydrogen phosphate into the casing in a closed circuit to the processed anode sheet under forced pressure m 0.4-0.5 atm, and then to the cathode at a maximum voltage of 190 V and a current density of 0.5 A / dm ² for 10 minutes

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