6. Metal surface treatment
Metal surface treatment processes include plating, surface penetration, surface oxidation passivation, and the like. Its purpose is to improve the corrosion resistance of metals and improve the mechanical energy of metals. Surface treatments are widely used in the valve field.
The valve connection screw is usually galvanized, chrome-plated, and oxidized (blue) to improve the resistance to atmospheric and medium corrosion. In addition to the above methods, other fasteners are treated with a surface treatment such as phosphating.
The sealing surface and the closing member with small diameter often adopt surface treatment such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥0.4mm.
The problem of valve stem anti-corrosion is a problem that people pay attention to, and people have accumulated rich production experience. It often uses surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatments should be suitable for different valve stem materials and working environment. The valve stems in contact with atmospheric, water vapor medium and asbestos packing can be hard chrome-plated and gas nitriding (stainless steel is not suitable for ion nitriding); The valve in the hydrogen sulfide atmosphere has better protection performance by electroplating high-phosphorus nickel plating; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chrome plating; 2Cr13 can resist ammonia corrosion after quenching and tempering, Gas nitriding carbon steel can also resist ammonia corrosion, and all phosphorous nickel coatings are not resistant to ammonia corrosion; the gas nitriding 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is used to make valve stems.
The small-diameter valve body and handwheel are also often chrome-plated to improve their corrosion resistance and to decorate the valve.
7. Thermal spraying
Thermal spraying is a type of process block for preparing coatings and has become one of the new technologies for surface protection of materials. It is a national key promotion project. It uses a high energy density heat source (gas combustion flame, arc, plasma arc, electric heat, gas explosion, etc.) to heat and melt the metal or non-metal material, and then sprays it onto the pretreated basic surface in an atomized form to form a sprayed layer, or at the same time, the basic surface is heated to re-melt the coating on the surface of the substrate to form a spray-welded layer.
Most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on a metal or non-metal substrate by using one or more thermal spray methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong the service life. Thermal spray coating with special function has special properties such as heat insulation, insulation (or isoelectric), grindable seal, self-lubricating, heat radiation, electromagnetic shielding. Parts can be repaired by thermal spraying.
8. Control the corrosive environment
As for so-called environment, there are broad and narrow senses. In broad sense, it refers to the environment around the valve installation and its internal circulation medium; in narrow sense, environment refers to the conditions around the valve installation. Most environments are uncontrollable and production processes are not subject to change. Only when there is no damage to the product or process, the method of controlling the environment, such as boiler water deoxidation and the pH of the alkali in the refining process, can be used. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. also belong to controlling corrosion environments.
The atmosphere is full of dust, water vapor, and smoke. Especially in the production environment, such as smoke and halogen, toxic gases and fine powder emitted from equipment, it will cause different degrees of corrosion on the valve. The operator should periodically clean and purge the valve and regularly refuel according to the regulations in the operating procedures. This is an effective measure to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is provided with a well, the surface of the valve is painted with paint, etc., which are methods for preventing corrosion of the valve by corrosive substances. Increased ambient temperatures and air pollution, especially in closed environments, can accelerate corrosion. Open factory should be used as much as possible or ventilation and cooling measures should be adopted to reduce environmental corrosion.
9. Improve processing technology and valve structure
The anti-corrosion protection of the valve is a problem that has been considered from the design. Undoubtedly, a valve product with reasonable structural design and correct process method has a good effect on slowing the corrosion of the valve. Therefore, the design and manufacturing department should improve the components that are unreasonable in structural design, incorrect in process methods, and prone to corrosion, to suit the requirements of various working conditions. The gap at the valve connection is a good environment for corrosion of the oxygen concentration cell. Therefore, the connection between the valve stem and the closing member should be as far as possible without the use of the inner and the threaded connection. The valve welding should be double-sided butt welding and continuous welding, for spot welding and lap welding are prone to corrosion. And the valve thread joint should be made of PTFE tape and pad. Not only can it have a good seal, but it can also resist corrosion. The medium which is not easy to flow through dead angle is easy to corrode the valve. Except when the valve is used, it does not need to be flipped and the deposited medium is discharged, the recessed structure should be avoided as much as possible, and the valve should be provided with drain holes as much as possible when manufacturing the valve parts. Different metal contacts will constitute a point couple, which promotes corrosion of the anode metal. When selecting materials, metal contacts with large metal potential gaps and no passivation film should be avoided. In the production and processing process, in particular to stress corrosion generated during welding and heat treatment, attention should be paid to improving the processing method. After welding, appropriate protective measures such as annealing treatment should be used. Improve the surface roughness of the valve stem and the surface roughness of other valve parts. The higher the surface roughness level, the stronger the corrosion resistance is. Improve the processing and structure of fillers and gaskets, use flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE gaskets to improve sealing performance and reduce the corrosion of the ground stem and flange sealing surface.
Metal surface treatment processes include plating, surface penetration, surface oxidation passivation, and the like. Its purpose is to improve the corrosion resistance of metals and improve the mechanical energy of metals. Surface treatments are widely used in the valve field.
The valve connection screw is usually galvanized, chrome-plated, and oxidized (blue) to improve the resistance to atmospheric and medium corrosion. In addition to the above methods, other fasteners are treated with a surface treatment such as phosphating.
The sealing surface and the closing member with small diameter often adopt surface treatment such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥0.4mm.
The problem of valve stem anti-corrosion is a problem that people pay attention to, and people have accumulated rich production experience. It often uses surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatments should be suitable for different valve stem materials and working environment. The valve stems in contact with atmospheric, water vapor medium and asbestos packing can be hard chrome-plated and gas nitriding (stainless steel is not suitable for ion nitriding); The valve in the hydrogen sulfide atmosphere has better protection performance by electroplating high-phosphorus nickel plating; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chrome plating; 2Cr13 can resist ammonia corrosion after quenching and tempering, Gas nitriding carbon steel can also resist ammonia corrosion, and all phosphorous nickel coatings are not resistant to ammonia corrosion; the gas nitriding 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is used to make valve stems.
The small-diameter valve body and handwheel are also often chrome-plated to improve their corrosion resistance and to decorate the valve.
7. Thermal spraying
Thermal spraying is a type of process block for preparing coatings and has become one of the new technologies for surface protection of materials. It is a national key promotion project. It uses a high energy density heat source (gas combustion flame, arc, plasma arc, electric heat, gas explosion, etc.) to heat and melt the metal or non-metal material, and then sprays it onto the pretreated basic surface in an atomized form to form a sprayed layer, or at the same time, the basic surface is heated to re-melt the coating on the surface of the substrate to form a spray-welded layer.
Most metals and their alloys, metal oxide ceramics, cermet composites, and hard metal compounds can be coated on a metal or non-metal substrate by using one or more thermal spray methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong the service life. Thermal spray coating with special function has special properties such as heat insulation, insulation (or isoelectric), grindable seal, self-lubricating, heat radiation, electromagnetic shielding. Parts can be repaired by thermal spraying.
8. Control the corrosive environment
As for so-called environment, there are broad and narrow senses. In broad sense, it refers to the environment around the valve installation and its internal circulation medium; in narrow sense, environment refers to the conditions around the valve installation. Most environments are uncontrollable and production processes are not subject to change. Only when there is no damage to the product or process, the method of controlling the environment, such as boiler water deoxidation and the pH of the alkali in the refining process, can be used. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. also belong to controlling corrosion environments.
The atmosphere is full of dust, water vapor, and smoke. Especially in the production environment, such as smoke and halogen, toxic gases and fine powder emitted from equipment, it will cause different degrees of corrosion on the valve. The operator should periodically clean and purge the valve and regularly refuel according to the regulations in the operating procedures. This is an effective measure to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is provided with a well, the surface of the valve is painted with paint, etc., which are methods for preventing corrosion of the valve by corrosive substances. Increased ambient temperatures and air pollution, especially in closed environments, can accelerate corrosion. Open factory should be used as much as possible or ventilation and cooling measures should be adopted to reduce environmental corrosion.
9. Improve processing technology and valve structure
The anti-corrosion protection of the valve is a problem that has been considered from the design. Undoubtedly, a valve product with reasonable structural design and correct process method has a good effect on slowing the corrosion of the valve. Therefore, the design and manufacturing department should improve the components that are unreasonable in structural design, incorrect in process methods, and prone to corrosion, to suit the requirements of various working conditions. The gap at the valve connection is a good environment for corrosion of the oxygen concentration cell. Therefore, the connection between the valve stem and the closing member should be as far as possible without the use of the inner and the threaded connection. The valve welding should be double-sided butt welding and continuous welding, for spot welding and lap welding are prone to corrosion. And the valve thread joint should be made of PTFE tape and pad. Not only can it have a good seal, but it can also resist corrosion. The medium which is not easy to flow through dead angle is easy to corrode the valve. Except when the valve is used, it does not need to be flipped and the deposited medium is discharged, the recessed structure should be avoided as much as possible, and the valve should be provided with drain holes as much as possible when manufacturing the valve parts. Different metal contacts will constitute a point couple, which promotes corrosion of the anode metal. When selecting materials, metal contacts with large metal potential gaps and no passivation film should be avoided. In the production and processing process, in particular to stress corrosion generated during welding and heat treatment, attention should be paid to improving the processing method. After welding, appropriate protective measures such as annealing treatment should be used. Improve the surface roughness of the valve stem and the surface roughness of other valve parts. The higher the surface roughness level, the stronger the corrosion resistance is. Improve the processing and structure of fillers and gaskets, use flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE gaskets to improve sealing performance and reduce the corrosion of the ground stem and flange sealing surface.
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