Top Ten trick to confront "valve corrosion"

Metal valves can be said that engineering equipment is the most vulnerable parts of the key components of corrosion failure.Generally, the metal valve sealing surface, stem, diaphragm, spring and other valves generally use a material, body, bonnet, etc. applicable Secondary or tertiary materials for high pressure, highly toxic, flammable, explosive, radioactive medium valve, the choice of materials with little corrosivity.

Valve corrosion

Metal valves in the atmosphere or solution and other complex working conditions not only occur at any time the uniform corrosion of the metal surface, the local location of the metal are also prone to pitting, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue Corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction vibration corrosion, hydrogen corrosion and other local corrosion.

Corrosion protection of metal valves

1 Corrosion medium selection of corrosion-resistant materials

In production practice, medium corrosion is very complex, even in a medium used in the same valve material, the medium concentration, temperature, pressure, the media on the material corrosion is not the same medium temperature increased by 10 ℃ , Corrosion rate increased by about 1 to 3 times.Cement concentration on the valve material corrosion is very large, such as lead in a small concentration of sulfuric acid, corrosion is very small, when the concentration exceeds 96%, the sharp rise in corrosion.On the contrary carbon steel, When the concentration of sulfuric acid is about 50%, the corrosion is the most serious, when the concentration increases to more than 6%, but the sharp decline in corrosion, such as aluminum concentration of more than 80% concentrated nitric acid is very corrosive, but in the low concentration of nitric acid Corrosion in the serious but stainless steel, although strong corrosion resistance of dilute nitric acid, but more than 95% of the concentrated nitric acid corrosion but increased.

From the above examples we can see that the correct selection of valve materials should be based on the specific circumstances, analysis of a variety of corrosion factors, according to the anti-corrosion manual selection of materials.

2 non-metallic materials

Non-metallic corrosion resistance, as long as the valve temperature and pressure meet the requirements of non-metallic materials, not only to solve the corrosion problems, but also save precious metals. Valve body, bonnet, lining, sealing surface and other commonly used non-metallic materials , As gasket, filler mainly made of non-metallic materials.With polytetrafluoroethylene, chlorinated polyether and other plastics, and with natural rubber, neoprene, nitrile rubber and other rubber valve lining, and the valve body, The bonnet body is made of cast iron and carbon steel, which ensures the strength of the valve and protects the valve from corrosion. The pinch valve is also designed according to the excellent corrosion resistance and excellent deformability of the rubber. The use of nylon, PTFE and other plastics, with natural rubber and synthetic rubber to do a variety of sealing surfaces, seals, for all types of valves, these non-metallic materials used to seal the surface, not only resistance Corrosive, and good sealing performance, especially for use with granular media, of course, their strength and heat resistance are low, the scope of the application is limited.Flexible graphite, the non-metallic into the field of high temperature, solve The long-term difficult to solve the problem of gasket and gasket leakage, but also a good high-temperature lubricant.

3 metal surface treatment

Valve connection

Valves connecting bolts commonly used galvanized, chrome-plated, oxidation (blue) treatment to improve the resistance to atmospheric corrosion resistance, other fasteners in addition to using the above method of treatment, but also according to the situation with phosphating and other surface treatment.

Sealing surface and the caliber of the closure, often using nitriding, boriding and other surface technology to improve its corrosion resistance and wear resistance .38 CrMoAlA produced valve, nitriding layer ≥ 0.4mm.

Stem corrosion widely used nitriding, boronizing, chrome, nickel plating and other surface treatment technology to improve its corrosion resistance, corrosion resistance and abrasion resistance of the different amount of surface treatment should be suitable for different stem material and Working environment, in the atmosphere, steam media and asbestos packing contact with the valve stem, hard chromium plating can be used, gas nitriding process (stainless steel should not be used ion nitriding process); in the hydrogen sulfide atmosphere valve electroplating high-phosphorus 38CrMoAlA using ion and gas nitriding can also be corrosion resistant, but not suitable for hard chrome plating; 2Cr13 after quenching can withstand ammonia corrosion, the use of gas nitrided carbon steel is also resistant to ammonia corrosion, All phosphorus-nickel coating is not resistant to ammonia corrosion; after gas nitrided 38CrMoAlA material has excellent corrosion resistance and overall performance, use it to make stem more.

Small-caliber valve body and hand wheel are often chrome-plated to improve its corrosion resistance, decorative valves.

4 thermal spraying

Thermal spraying is one kind of process block for preparation of coatings, and has become one of the new technologies for surface protection of materials. It uses high energy density heat sources (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) After the non-metallic material is heated and melted, it is sprayed onto the pretreated basic surface in the form of atomization to form a sprayed layer or simultaneously heated on the basic surface to remelt the coating on the surface of the substrate to form the surface enhanced process of the sprayed 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 with one or several thermal spray methods.

Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature performance, extend the service life. Thermal spraying special functional coating, with insulation, insulation (or different), can be sealed, self-lubricating, Electromagnetic shielding and other special properties; the use of thermal spray repair parts.

5 spray paint

Coating is one of the most widely used anti-corrosion means, is an indispensable anti-corrosion materials and identification marks on the valve products.Coatings also belong to non-metallic materials, which are usually formulated from synthetic resin, rubber slurry, vegetable oil, solvents, etc. , Covering the metal surface, isolated media and the atmosphere, to achieve the purpose of preservation. Paint is mainly used for water, salt water, sea water, atmospheric corrosion is not too strong environment. Valve cavity commonly used anti-corrosion paint brushing to prevent water, air and other media Corrosion of the valve. Paints mixed with different colors to represent the materials used by Faen Valve spray paint, usually in six months to once a year.

6 Add corrosion inhibitor

Corrosion inhibitor corrosion control mechanism is that it promotes the polarization of the battery. Corrosion inhibitor is mainly used for the media and filler at the media to add corrosion inhibitors, equipment and valve corrosion can slow, such as chrome-nickel stainless steel in the Oxygen-containing sulfuric acid, a large range of solubility within the melting state, corrosion is more serious, but adding a small amount of copper sulfate or nitric acid and other oxidants, stainless steel can change the passive state, the surface layer of protective film to prevent the media of the dip Erosion, in hydrochloric acid, if you add a small amount of oxidant, can reduce the corrosion of titanium. Valve pressure test Commonly used water pressure test medium easily lead to corrosion of the valve, add a small amount of sodium nitrite in the water to prevent corrosion of water on the valve. Asbestos packing contains chloride, the valve stem corrosion, if the use of distilled water washing method can reduce the chloride content, but this method is difficult to implement, can not be generalized, the ester is suitable for special needs.

In order to protect the stem from asbestos packing corrosion, asbestos packing, the valve stem coated with corrosion inhibitor and sacrificial metal corrosion inhibitor sodium nitrite, sodium chromate can make the valve stem surface layer of passivation Film, to improve the corrosion resistance of the stem; solvent can slowly dissolve the corrosion inhibitor, and can play a role in lubrication; add asbestos in the sacrificial metal zinc powder, in fact, zinc is also a corrosion inhibitor, it can First with the asbestos in the chloride, chloride and valve stem metal contact opportunities greatly reduced, so as to achieve the purpose of preservation. If you join the coating of red lead, calcium lead and other corrosion inhibitors, spraying the valve surface to prevent Atmospheric corrosion.

7 electrochemical protection

There are two kinds of electrochemical protection anode protection and cathodic protection, such as the protection of iron with zinc, zinc is corrosive, zinc is called sacrificial metal.In production practice, the use of less anode protection, cathodic protection applications more. Large valves and important valves The use of this cathodic protection method is a simple and cost-effective method of adding zinc asbestos filler to protect the valve stem also belongs to the cathodic protection method.

8 control the corrosive environment

The so-called environment, both broad and narrow sense, the generalized environment refers to the installation of the valve around the environment and its internal circulation medium; Narrow environment refers to the valve installed around the conditions. Most of the environment beyond control, the production process can not be arbitrary Changes. Only in the case of no harm to the product, process, etc., can be used to control the environment, such as boiler water deoxygenation, alkali refining alkaline pH value adjustment process, etc. From this perspective, the addition of corrosion inhibition Agents, electrochemical protection, etc. also belong to control the corrosive environment.

The atmosphere is full of dust, water vapor, smoke, especially in the production environment, such as smoke halogen, toxic gases emitted from the device and the powder, will have different degrees of corrosion on the valve. Operators should be in accordance with the rules of operation, regular Cleaning, purging the valve, regular refueling, which is effective measures to control the environmental corrosion. Valve stem to install the protective cover, the valve set to the well, the valve surface spray paint, which are to prevent the erosion of the valve material erosion methods. Ambient temperature Elevated and air pollution, especially in closed environment equipment and valves, will accelerate its corrosion.Use open-ended plant or adopt ventilation, cooling measures to reduce environmental corrosion.

9 to improve the processing technology and valve structure

Corrosion protection valve is designed from the beginning to consider the issue of a structural design is reasonable, the correct method of valve products are undoubtedly slowing the corrosion of the valve is a good effect.

Therefore, the design and manufacturing departments to deal with those structural design is not reasonable, the process is not correct, easy to cause corrosion of parts, should be improved to make it suitable for a variety of operating conditions under different conditions for different types of valve corrosion, small Compiled a trick:

Valve corrosion

Austenitic stainless steel valve to prevent the emergence of intergranular corrosion methods are: "solution hardening" treatment, that is, heated to about 1100 ℃ water quenching, the choice of containing titanium and niobium, and carbon content of 0.03% or less of austenitic stainless steel , Reduce the production of chromium carbide.

Stress corrosion occurs in the simultaneous corrosion and tensile stress rupture. Prevent stress corrosion method; By heat treatment to eliminate or reduce the welding, cold processing stress, improve the irrational valve structure, to avoid stress concentration, the use of electrochemical protection, Spraying anti-corrosion coatings. Add corrosion inhibitors, compressive stress and other measures.

Wear and tear corrosion is the fluid on the metal wear and corrosion caused by the alternating role of a corrosion form is a common valve corrosion, the corrosion occurs in the sealing surface for more Prevention methods: Selection of corrosion-resistant, wear-resistant materials , Improve the structural design, the use of cathodic protection and so on.

Mo vibration corrosion is the mutual contact of the two components at the same time to withstand the load, the contact surface due to vibration and sliding damage caused by friction vibration occurs at the bolt connection, the valve stem and the closure of the connection between the ball bearing and the shaft and other parts. Lubricants can be applied to reduce friction, surface phosphating, the choice of cemented carbide, as well as spraying or cold processing to improve the surface hardness method of protection.

After welding to try to adopt the appropriate protective measures such as annealing to improve the stem surface roughness and other valve surface roughness, the higher the surface roughness level, the stronger the corrosion resistance Improve packing and gasket processing technology and structure , The use of flexible graphite and plastic packing, and flexible graphite adhesive pads and PTFE packing pads, can improve the sealing performance and reduce the corrosion of the valve stem and flange sealing surface.