Treatment method of valve corrosion
Writer： admin Time：2021-03-26 20:55 Browse：℃
Treatment method and anticorrosion measures of valve corrosion
Corrosion is the damage and deterioration of materials under the action of various environments. The corrosion of metal is mainly caused by chemical corrosion and spot chemical corrosion. The corrosion of non-metallic materials is usually caused by direct chemical and physical effects.
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Corrosion pattern of valves
There are two kinds of corrosion modes of metal valve, namely uniform corrosion and local corrosion. The average corrosion rate can be used to evaluate the rate of uniform corrosion. Metal materials, graphite, glass, ceramics and concrete, are divided into four grades according to the corrosion rate: the ones with corrosion rate less than 0.05mm/a are excellent; those with corrosion rate of 0.05-0.5mm/a are good; those with corrosion rate of 0.5-1.5mm/a can be used; If the corrosion rate is greater than 1.5mm/a, the valve parts such as sealing surface, valve stem, diaphragm and small spring are generally made of grade I materials, and the valve body and bonnet are suitable for grade II or III materials. For valves with high pressure, highly toxic, inflammable, explosive and radioactive medium, the materials with small corrosion resistance are selected.
1. Uniform corrosion
Uniform corrosion is carried out on all surfaces of the metal. For example, stainless steel, aluminum, titanium and other products are produced in the oxidation environment, and the metal state under the film is corroded evenly. There is also a phenomenon where the metal surface is corroded and peeled off, which is dangerous.
2. Local corrosion
Local corrosion occurs in the local position of metal, and its forms include pore corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion, etc.
Pitting usually occurs on the metal of passivation film or protective film. It is because of the defects on the metal surface. The active ions of passivation film can be destroyed in the solution, which causes the passivation film to be damaged locally and extends into the metal interior to become the corrosion hole. It is one of the most destructive and hidden danger forms of metal.
Crevice corrosion occurs in welding, riveting, gasket or sediment environment, which is a special form of hole erosion. The prevention method is to eliminate the gap.
Intergranular corrosion is the corrosion of the grain boundary in a net form by penetrating the surface into the metal interior along the grain boundary. The intergranular corrosion is mainly caused by improper heat treatment and cold processing except for the deposition of impurities in grain boundaries. The welding joint of austenitic stainless steel is prone to corrosion due to the poor chromium zone. Intergranular corrosion of austenitic stainless steel is a common and dangerous corrosion form. The methods to prevent intergranular corrosion of austenitic stainless steel valve parts include: to carry out "solution quenching" treatment, namely, to heat to 1100 ℃ water quenching, and to select austenitic stainless steel containing titanium and niobium, and carbon content below 0.03%, so as to reduce the production of chromium carbide.
Delamination corrosion occurs in layered structure. Corrosion develops vertically and inward, then the material with parallel surface is corroded. Under the expansion force of corrosion, the surface is layered and flaking.
Stress corrosion occurs when corrosion and tensile stress are simultaneously acting. The method of preventing stress corrosion; eliminate or reduce the stress produced in welding and cold processing by heat treatment, improve the unreasonable valve structure, avoid stress concentration, and adopt electrochemical protection and spray anti-corrosion coating. The measures such as adding corrosion inhibitor and applying pressure stress are taken.
Corrosion fatigue occurs in the joint action of alternating stress corrosion, which causes metal to break. Heat treatment can be used to eliminate or reduce stress, surface shot peening and galvanizing, chromium, nickel, etc., but it is necessary to note that the coating should not have tensile stress and hydrogen diffusion.
Selective corrosion occurs in different components and impurities. In a certain environment, some elements are corroded and leached, and the remaining elements are sponge like. There are brass dezincification, copper alloy dealumination, graphite of cast iron, etc.
Wear corrosion is a kind of corrosion form produced by fluid alternating action on metal wear and corrosion, and it is a common corrosion of valves. This corrosion occurs on the sealing surface. Prevention methods: choose corrosion resistant and wear-resistant materials, improve the structure design, adopt cathodic protection, etc.
Cavitation corrosion, also known as cavitation and cavitation, is a special form of wear corrosion. It is a bubble in the fluid. The shock wave produced during the burst can reach 400 atmospheric pressure, which will destroy the metal protective film and even tear the metal particles. Then corrosion into film, this process is repeated, making metal corrosion. The methods to prevent cavitation corrosion can be made of cavitation resistant materials, high finish machining surface, elastic protective layer and cathodic protection.
Friction corrosion is the damage caused by vibration and sliding of contact surface, which is caused by the two parts contacting each other. Friction corrosion occurs at the bolt connection, the connection between the stem and the closing part, the ball bearing and the shaft. It can be protected by applying lubricating grease, reducing friction, phosphating surface, selecting cemented carbide, and improving surface hardness by spraying tile or cold processing.
Corrosion is the damage caused by the diffusion of hydrogen atoms in chemical reaction to the metal. The forms of corrosion are hydrogen bubble, hydrogen embrittlement and hydrogen corrosion.
Hydrogen bubbling is easy to occur in strong steel and non-metallic steel. Hydrogen bubble is easy to be produced when sulfide and hydrides are contained in oil. The cooling steel without holes is used instead of boiling steel with holes. Rubber and plastic are used to protect the steel from bubbling.
The lattice of strong steel is highly variable, and after hydrogen atom enters, the 4 lattice should be changed greatly, which will cause financial embrittlement. Alloy steel containing nickel and lead should be selected to avoid high strength steel with high hydrogen brittleness, and hydrogen embrittlement should be avoided or reduced in welding, electroplating and pickling. Hydrogen enters into metal at high temperature and high pressure, and it will react with a combination of elements and destroy it. It is called hydrogen corrosion. Austenitic stainless steel is completely resistant to high temperature hydrogen corrosion.
3. Non metallic corrosion
Nonmetal corrosion is the same as metal corrosion in size. Most nonmetal materials are non electric conductors, which generally do not produce electrochemical corrosion, but pure chemical or physical corrosion, which is the main difference from metal corrosion. For metal corrosion, weight loss is the main reason. Many of the non-metal corrosion is caused by physical action, but the physical action of metal corrosion is very rare. The internal corrosion of non-metal is a common phenomenon, while the surface corrosion is the main one.
After the metal material contacts with the medium, the solution or gas will gradually diffuse to the inside of the material, causing a series of corrosion changes of non-metal. According to the types and varieties of non-metal materials, the corrosion forms are different. The forms of corrosion include dissolution, swelling, bubble, softening, decomposition, discoloration, deterioration, aging, hardening and fracture. However, from a comprehensive point of view, non-metallic corrosion performance is much better than metal materials, but the strength and temperature resistance of non-metallic materials are lower than metal materials.
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Anticorrosion measures of metal valve
Electrochemical corrosion corrodes metals in various forms. It not only acts between two metals, but also produces potential difference due to poor solubility of solution, poor solubility of oxygen and tiny difference of internal structure of metal, which aggravates the corrosion. Some metal itself is not corrosion-resistant, but it can produce a very good protective film after corrosion, that is, passive film, which can prevent the corrosion of the medium. Therefore, in order to achieve the purpose of metal valve anti-corrosion, the first is to eliminate electrochemical corrosion; the second is when the electrochemical corrosion can not be eliminated; to make the metal surface produce a passive film; the third is to choose non-metallic materials without electrochemical corrosion instead of metal materials. Several anticorrosion methods are introduced below.
1. Select corrosion-resistant materials according to the medium
In the section of "selection of valves", we introduced the media suitable for common materials of valves, which is just a general introduction. In actual production, the corrosion of media is very complex. Even if the valve materials used in one medium are the same, the corrosion of media to materials is different with different concentration, temperature and pressure of media. The corrosion rate increases about 1 ~ 3 times when the medium temperature increases by 10 ℃. The medium concentration has a great influence on the corrosion of valve materials. For example, the corrosion of lead in sulfuric acid with low concentration is very small. When the concentration exceeds 96%, the corrosion rises sharply. On the contrary, the corrosion of carbon steel is serious when the concentration of sulfuric acid is about 50%, but it decreases sharply when the concentration of sulfuric acid is over 6%. For example, aluminum is highly corrosive in concentrated nitric acid with concentration more than 80%, but it is seriously corrosive in medium and low concentration nitric acid. Although the corrosion resistance of stainless steel to dilute nitric acid is very strong, the corrosion is aggravated in more than 95% concentrated nitric acid.
It can be seen from the above examples that the correct selection of valve materials should be based on the specific situation, analyze various factors affecting corrosion, and select materials according to the relevant anti-corrosion manual.
2. Non metallic materials are used
Nonmetal corrosion resistance is excellent, as long as the valve temperature and pressure meet the requirements of nonmetal materials, not only can solve the corrosion problem, but also can save precious metals. Valve body, valve cover, lining, sealing surface and other commonly used non-metallic materials, as for gasket, packing is mainly made of non-metallic materials. PTFE, chlorinated polyether and other plastics, as well as natural rubber, chloroprene rubber, NBR and other rubbers are used as valve lining, while the main body of valve body and valve cover is made of general cast iron and carbon steel. The strength and corrosion resistance of the valve are ensured. Pinch valve is also designed according to the excellent corrosion resistance and excellent denaturation of rubber. Now more and more people use nylon, polytetrafluoroethylene and other plastics, natural rubber and synthetic rubber to make all kinds of sealing surfaces and rings, which are used on all kinds of valves. These non-metallic materials used as sealing surfaces have good corrosion resistance and sealing performance, especially suitable for medium with particles. Of course, their strength and heat resistance are low, and their application scope is limited. The appearance of flexible graphite makes non-metal enter the field of high temperature, solves the long-term difficult problem of packing and gasket leakage, and is a good high temperature lubricant.
3. Painting
Coating is a widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark in valve products. Coatings are also non-metallic materials, which are usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc. they cover the metal surface, isolate the medium and atmosphere, and achieve the purpose of anti-corrosion. The coating is mainly used in water, salt water, sea water, atmosphere and other environment with less strong corrosion. The inner cavity of the valve is usually painted with anti-corrosion paint to prevent the corrosion of the valve by water, air and other media. There are different colors in the paint to indicate the material Farne uses. Valve spray paint, generally in half a year to a year.
4. Add corrosion inhibitor
Adding a small amount of other special substances into the corrosive medium and corrosives can greatly slow down the metal corrosion. This kind of special substance is called corrosion inhibitor.
The mechanism of corrosion inhibitor controlling corrosion is that it promotes the polarization of battery. Corrosion inhibitors are mainly used in media and fillers. Adding corrosion inhibitor in the medium can slow down the corrosion of equipment and valves. For example, chromium nickel stainless steel in sulfuric acid without oxygen can be cremated in a large range of solubility, and the corrosion is serious. However, adding a small amount of oxidant such as copper sulfate or nitric acid can make the stainless steel change into a passive state, and form a protective film on the surface to prevent the corrosion of the medium, It can reduce the corrosion of titanium. Water is often used as the medium of valve pressure test, which is easy to cause corrosion of the valve. Adding a small amount of sodium nitrite in water can prevent water corrosion of the valve. Asbestos packing contains chloride, which causes great corrosion to the valve stem. The content of chloride can be reduced by using distilled water washing method. However, this method is difficult to implement and can not be popularized. Ester is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of asbestos packing, the corrosion inhibitor and sacrificial metal are coated on the valve stem in the asbestos packing. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can form a passive film on the surface of the valve stem and improve the corrosion resistance of the valve stem; Zinc powder is added to asbestos as sacrificial metal. In fact, zinc is also a kind of corrosion inhibitor. It can first combine with chloride in asbestos, which greatly reduces the contact chance between chloride and valve rod metal, so as to achieve the purpose of anti-corrosion. If red lead, calcium plumbate and other corrosion inhibitors are added into the coating, spraying on the valve surface can prevent atmospheric corrosion.
5. Electrochemical protection
Electrochemical protection includes anode protection and cathode protection. The so-called anode protection is to introduce external direct current to protect the metal anode, so that the anode potential increases in the positive direction. When it increases to a certain value, a dense protective film is formed on the surface of the metal anode, which is called passive film. At this time, the corrosion of the metal cathode decreases sharply. Anodic protection is suitable for metals that are easily passivated. The so-called cathodic protection is to use the metal to be protected as the cathode and apply direct current to reduce its potential in the negative direction, so that when it reaches a certain potential value, the corrosion current speed will be reduced and the metal will be protected. In addition, cathodic protection can be used to protect the protected metal by the metal whose electrode potential is more negative than that of the protected metal. If zinc is used to protect iron, zinc is corroded. Zinc is called sacrificial metal. In the production practice, anode protection is less used and cathode protection is more used. It is an economical, simple and effective method to adopt this cathodic protection method for large valves and important valves. Adding zinc into asbestos packing to protect valve stem also belongs to cathodic protection method.
6. Metal surface treatment
The metal surface treatment process is better than sleeping coating, surface penetration, surface oxidation passivation, etc. Its purpose is to improve the corrosion resistance and mechanical energy of metal. Surface treatment is widely used on valves.
Valve connecting bolts are usually treated with zinc plating, chromium plating and oxidation (bluing) to improve the ability of atmospheric and medium corrosion resistance. In addition to the above methods, other fasteners are also treated with phosphating and other surface treatments according to the situation.
Nitriding and boronizing are often used to improve the corrosion resistance and wear resistance of sealing surface and closure parts with small diameter. For the valve disc made of 38CrMoAlA, the nitriding layer is more than or equal to 0.4mm.
The anti-corrosion of valve stem is a problem that people pay attention to, and have accumulated rich production experience. Surface treatment processes such as nitriding, boronizing, chromium plating and nickel plating are often used to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatment should be suitable for different material and working environment of valve stem. Hard chromium plating and gas nitriding process can be used for valve stem in contact with asbestos packing in atmosphere and water vapor medium (ion nitriding process is not suitable for stainless steel); high phosphorus nickel plating has better protection performance for valve in hydrogen sulfide atmosphere; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but hard chromium coating is not suitable; 2Cr13 can resist ammonia corrosion after quenching and tempering, carbon steel with gas nitriding can also resist ammonia corrosion, while all phosphorus nickel coatings are not resistant to ammonia corrosion; 38CrMoAlA material after gas nitriding has excellent corrosion resistance and comprehensive performance, so it is more suitable to make valve stem with it.
Small diameter valve body and hand wheel are often chrome plated to improve their corrosion resistance and decorate the valve.
7. Thermal spraying
Thermal spraying is a kind of technology for coating preparation, which has become one of the new technologies for material surface protection. It is a national key promotion project. It uses high energy density heat source (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metal materials, spray them on the pretreated basic surface in the form of atomization to form spray coating, or heat the basic surface at the same time to make the coating melt again on the substrate surface to form spray welding layer. Most metals and their alloys, metal oxide ceramics, metal ceramic composites and hard metal compounds can be coated on metal or nonmetal substrates by one or more thermal spraying methods.
Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and prolong the service life. Thermal spraying special functional coating, with heat insulation, insulation (or abnormal electricity), grindable sealing, self lubrication, thermal radiation, electromagnetic shielding and other special properties; the use of thermal spraying can repair parts.
8. Control of corrosive environment
The so-called environment, there are two broad and narrow sense, broad environment refers to the environment around the valve installation and its internal circulation medium; narrow environment refers to the conditions around the valve installation. Most of the environment can not be controlled, and the production process can not be arbitrarily changed. Only when there is no damage to the product and process, can the environmental control methods be used, such as boiler water deoxidation, pH value adjustment in refining process, etc. From this point of view, the addition of corrosion inhibitor and electrochemical protection are also controlled corrosion environment.
The atmosphere is full of dust, water vapor, smoke, especially in the production environment, such as bittern, poisonous gas and micro powder emitted by equipment, which will cause different degrees of corrosion to the valve. The operators should clean and purge the valve regularly and add oil regularly according to the regulations in the operation procedures, which is an effective measure to control environmental corrosion. Installation of protective cover for valve stem, installation of ground well for ground valve and painting on valve surface are all methods to prevent corrosive substances from eroding the valve. The rise of ambient temperature and air pollution, especially for the equipment and valves in closed environment, will accelerate their corrosion. Open workshop or ventilation and cooling measures should be adopted as far as possible to reduce environmental corrosion.
9. Improve the processing technology and valve structure
The anti-corrosion protection of valve is considered from the beginning of design. It is a valve product with reasonable structure design and correct process method. Undoubtedly, it has a good effect on reducing the corrosion of valve.
Therefore, the design and manufacturing departments should improve those parts which are unreasonable in structure design, incorrect in process method and easy to cause corrosion, so as to make them suitable for various working conditions.
The gap at the valve joint is a good environment for oxygen concentration battery corrosion.
Therefore, the connection between the valve stem and the closing piece should not be in the form of internal and threaded connection as far as possible; the valve should be welded by double-sided butt welding and continuous welding, and the spot welding and lap welding are easy to cause corrosion. The threaded connection of the valve should be made of polytetrafluoroethylene tape and gasket. It not only has good seal, but also can corrode. The dead angle is not easy to flow medium, which is easy to corrode the valve. In addition to not upside down when using the valve and paying attention to the discharge of the deposited medium, when manufacturing the valve parts, try to avoid the dent free structure, and set the drain hole as far as possible.
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