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Corrosion is the destruction and deterioration of materials under the action of various environments. The corrosion of metals is mainly caused by chemical corrosion and pitting chemical corrosion, and the corrosion of non-metallic materials is generally caused by direct chemical and physical damage.
1. The form of valve corrosion
There are two forms of metal valve corrosion, namely uniform corrosion and localized corrosion. The rate of uniform corrosion can be evaluated by the average annual corrosion rate. Metal materials, graphite, glass, ceramics and concrete are divided into 4 grades according to the corrosion rate: the corrosion rate is less than 0.05mm/a is excellent; the corrosion rate is 0.05~0.5mm/a is good; the corrosion rate is 0.5~1.5 mm/a can still be used; corrosion rate greater than 1.5mm/a is not applicable, valve sealing surface, valve stem, diaphragm, small spring and other valve parts generally use first-class materials, valve body, bonnet, etc. are suitable for two Grade or grade three materials, for valves with high pressure, highly toxic, flammable, explosive, and radioactive media, use materials with little corrosiveness.
1. Uniform corrosion
Uniform corrosion is performed on the entire surface of the metal. Such as stainless steel, aluminum, titanium, etc., produce a protective film in an oxidizing environment, and the metal under the film will corrode uniformly. There is also a phenomenon, metal surface corrosion and peeling, this kind of corrosion is the most dangerous.
2. Localized corrosion
Localized corrosion occurs at the local location of the metal. Its forms include pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen Eclipse etc.
Pitting corrosion usually occurs on the metal of the passivation film or the protective film. Because of the defects on the metal surface, the active ions that can destroy the passivation film in the solution cause the passivation film to be partially damaged and extend into the metal to become etch holes. It is one of the most destructive and hidden corrosion forms of metals.
Crevice corrosion occurs in environments such as welding, riveting, gaskets or deposits, and it is a special form of pitting corrosion. The prevention method is to eliminate the gap.
Intergranular corrosion is from the surface along the grain boundary deep into the metal, causing the grain boundary to be reticularly corroded. In addition to impurities deposited on grain boundaries, intergranular corrosion is mainly caused by improper heat treatment and cold working. Austenitic stainless steel is prone to chromium-depleted areas on both sides of the welded seam and is corroded. Intergranular corrosion of austenitic stainless steel is the most common and most dangerous form of corrosion. The methods to prevent intergranular corrosion of austenitic stainless steel valves include: "Solid solution quenching" treatment, that is, heating to about 1100°C and water quenching, selecting austenitic stainless steel containing titanium and niobium, and carbon content below 0.03% , To reduce the production of chromium carbide.
Delamination corrosion occurs in a layered structure. The corrosion first develops vertically and inwardly, and then corrodes the parallel material on the surface. Under the expansion force of the corroded material, the surface is peeled off in layers.
Stress corrosion occurs when cracks occur under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion; eliminate or reduce the stress generated in welding and cold working through heat treatment, improve the unreasonable valve structure, avoid stress concentration, use electrochemical protection and spray anti-corrosion paint. Measures such as adding corrosion inhibitors and applying compressive stress.
Corrosion fatigue occurs in the joint action of alternating stress corrosion, causing the metal to crack. Heat treatment can be used to eliminate or reduce stress, surface shot peening and electroplating of zinc, chromium, nickel, etc., but it should be noted that the coating must not have tensile stress and hydrogen diffusion.
Selective corrosion occurs in materials with different compositions and impurities. In a certain environment, some elements are corroded and leached, and the remaining uncorroded elements are sponge-like. Commonly used are brass dezincification, copper alloy dealumination, and graphitization of cast iron.
Abrasion corrosion is a form of corrosion produced by the alternating action of fluid on metal wear and corrosion. It is a common type of corrosion in valves. This type of corrosion occurs mostly on the sealing surface. Prevention methods: use corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
Cavitation corrosion, also known as cavitation corrosion and cavitation corrosion, is a special form of wear corrosion. It is a bubble generated in the fluid, and a shock wave generated when it bursts. The pressure can be as high as 400 atmospheres, which can destroy the metal protective film and even tear the metal particles. Then, it is corroded to form a film. This process is repeated continuously to corrode the metal. The method to prevent cavitation corrosion can choose to use cavitation corrosion resistant materials, high-finish processing surface, elastic protective layer and cathodic protection.
Friction corrosion is the damage caused by vibration and sliding on the contact surface when two parts in contact with each other bear the load at the same time. Friction corrosion occurs at the bolt connection, the connection between the valve stem and the closure, and the ball bearing and the shaft. You can use lubricating grease to reduce friction, surface phosphating, use cemented carbide, and use spray tile treatment or cold working to improve surface hardness.
Corrosion is the damage caused by the diffusion of hydrogen atoms generated in chemical reactions into the metal. Its forms include hydrogen bubbling, hydrogen embrittlement and hydrogen corrosion.
Strong steel and steel containing non-metals are prone to hydrogen bubbling. When petroleum contains sulfides and hydrides, hydrogen bubbling is likely to occur. Use void-free killed steel instead of voided boiling steel, use rubber and plastic for protection, and add corrosion inhibitors to prevent bubbling.
The crystal lattice in strong steel is highly denatured. After hydrogen atoms enter, the strain of the 4-crystal lattice is greater, causing financial embrittlement. Alloy steels containing nickel and lead should be used, high-strength steels with high hydrogen embrittlement should be avoided, and hydrogen embrittlement should be avoided or reduced during welding, electroplating, and pickling. Under high temperature and high pressure, hydrogen enters the metal, and when combined with one element, it will chemically react and destroy it, which is called hydrogen corrosion. Austenitic stainless steel is fully resistant to high temperature hydrogen corrosion.
3. Non-metallic corrosion
Non-metal corrosion is the same size as metal corrosion. Most non-metal materials are non-electrical conductors and generally do not produce electrochemical corrosion, but pure chemical or physical corrosion. This is the main difference from metal corrosion. Non-metal corrosion is not necessarily weight loss but often weight gain. Weight loss is the main thing for metal corrosion. Many non-metal corrosions are caused by physical effects, while physical effects of metal corrosion are very rare; non-metal internal corrosion is a common phenomenon. The metal corrosion is mainly surface corrosion.
After the metal material comes into contact with the medium, the solution or gas will gradually diffuse into the inside of the material, causing a series of corrosion changes to the non-metallic material. Depending on the type and variety of the non-metallic material, the form of corrosion varies. The forms of corrosion include dissolution, swelling, bubbles, softening, decomposition, discoloration, deterioration, aging, hardening, and fracture. However, from a comprehensive point of view, the corrosion performance of non-metallic materials is much better than that of metallic materials, while the strength and temperature resistance of non-metallic materials are lower than that of metallic materials.
2. Anti-corrosion measures for metal valves
Electrochemical corrosion corrodes metals in various forms. It not only acts between two metals, but also produces potential differences due to poor solution solubility, poor oxygen solubility, and small differences in the internal structure of the metal, which intensifies corrosion. . Some metals themselves are not corrosion resistant, but they can produce a very good protective film after corrosion, that is, a passivation film, which can prevent the corrosion of the medium. It can be seen that in order to achieve the purpose of anti-corrosion of metal valves, one is to eliminate electrochemical corrosion; the other is when electrochemical corrosion cannot be eliminated; to produce a passivation film on the metal surface; the third is to use non-metallic materials without electrochemical corrosion Instead of metal materials. Several anti-corrosion methods are described below.
1. Choose corrosion-resistant materials according to the medium
In the section "Selection of Valves", we introduced the medium suitable for the common materials of the valve. It is just a general introduction. In the actual production, the corrosion of the medium is very complicated, even if a medium is used. The valve material is the same, the concentration, temperature, and pressure of the medium are different, and the corrosion of the medium to the material is also different. When the temperature of the medium increases by 10°C, the corrosion rate increases by about 1 to 3 times. The concentration of the medium has a great influence on the corrosion of the valve material. For example, if lead is in a small concentration of sulfuric acid, the corrosion is very small. When the concentration exceeds 96%, the corrosion rises sharply. Contrary to carbon steel, the corrosion is the most serious when the sulfuric acid concentration is about 50%. When the concentration increases to more than 6%, the corrosion drops sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but it is corrosive in medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, the corrosion is aggravated in more than 95% concentrated nitric acid.
It can be seen from the above cases that the correct selection of valve materials should be based on specific conditions, analyze various factors affecting corrosion, and select materials according to the relevant anti-corrosion manual.
2. Use non-metallic materials
Non-metallic corrosion resistance is excellent. As long as the valve temperature and pressure meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals. The valve body, bonnet, lining, sealing surface and other commonly used non-metallic materials are made. As for the gasket, the packing is mainly made of non-metallic materials. Use plastics such as polytetrafluoroethylene, chlorinated polyether, and rubber such as natural rubber, neoprene, nitrile rubber, etc. as the valve lining, while the valve body and the main body of the valve cover are made of general cast iron and carbon steel. That is to ensure the strength of the valve, but also to ensure that the valve is not corroded. The pinch valve is also designed according to the excellent corrosion resistance and excellent deformation properties of rubber. Nowadays, it is more and more appropriate to use nylon, polytetrafluoroethylene and other plastics, natural rubber and synthetic rubber to make various sealing surfaces and sealing rings, which are used on various valves. These non-metallic sealing surfaces are used as sealing surfaces. The material not only has good corrosion resistance, but also has good sealing performance, which is especially suitable for use in media with particles. Of course, their strength and heat resistance are low, and the scope of application is limited. The emergence of flexible graphite has brought non-metals into the high-temperature field, solved the long-term and difficult-to-solve leakage problems of fillers and gaskets, and is a good high-temperature lubricant.
3. Spray paint
Coating is the most widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark on valve products. Paint is also a non-metallic material. It is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., and covers the metal surface to isolate the medium and the atmosphere to achieve the purpose of anti-corrosion. Coatings are mainly used in environments that are not very corrosive, such as water, salt water, sea water, and the atmosphere. The inner cavity of the valve is often painted with anti-corrosion paint to prevent water, air and other media from corroding the valve. The paint is mixed with different colors to represent the materials used by Farn. The valve is sprayed with paint, usually once every six months to a year.
4. Add corrosion inhibitor
Adding a small amount of other special substances to the corrosive medium and corrosive substances can greatly slow down the rate of metal corrosion. This special substance is called a corrosion inhibitor.
The mechanism of the corrosion inhibitor to control corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used for media and fillers. Adding a corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel can become cremated in a large solubility range in oxygen-free sulfuric acid, and the corrosion is serious, but a small amount of copper sulfate or nitric acid is added. Waiting for the oxidant can make the stainless steel passivate, and a protective film is formed on the surface to prevent the corrosion of the medium. In the hydrochloric acid, if a small amount of oxidant is added, the corrosion to titanium can be reduced. Water is often used as the pressure test medium for valve pressure testing, which is likely to cause corrosion of the valve. Adding a small amount of sodium nitrite to the water can prevent water from corroding the valve. Asbestos fillers contain chlorides, which are very corrosive to the valve stem. If the method of washing with distilled water is used, the content of chlorides can be reduced. However, this method is difficult to implement and cannot be generalized. Ester is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of asbestos packing, the asbestos packing is filled with corrosion inhibitor and sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can form a passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem; the solvent can make the corrosion inhibitor slowly dissolve, and can play a lubricating effect; in asbestos Zinc powder is added as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can first combine with the chloride in asbestos to greatly reduce the chance of chloride contact with the valve stem metal, thereby achieving the purpose of anti-corrosion. If corrosion inhibitors such as red lead and calcium lead acid are added to the paint, spraying on the surface of the valve can prevent atmospheric corrosion.
5. Electrochemical protection
Electrochemical protection has two types: anode protection and cathodic protection. The so-called anode protection is to use the protective metal as the anode to introduce an external direct current to increase the anode potential 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 the passivation film. The corrosion of the metal cathode is drastically reduced. Anodic protection is suitable for metals that are easily passivated. The so-called cathodic protection is to use the protected metal as the cathode, and apply a direct current to reduce its potential in the negative direction. When it reaches a certain potential value, the corrosion current speed decreases and the metal is protected. In addition, cathodic protection can use a metal with a more negative electrode potential than the metal to be protected to protect the metal to be protected. If zinc is used to protect iron, zinc is corroded, and zinc is called sacrificial metal. In production practice, anodic protection is used less and cathodic protection is used more. Large valves and important valves adopt this cathodic protection method, which is an economical, simple and effective method. Adding zinc to asbestos filler to protect the valve stem is also cathodic protection.
6. Metal surface treatment
The metal surface treatment process is better than sleeping coating, surface penetration, surface oxidation passivation and so on. Its purpose is to improve the corrosion resistance of metals and improve the mechanical energy of metals. Surface treatment valves are widely used.
The valve connection screw is usually galvanized, chrome-plated, and oxidized (blued) to improve the resistance to atmospheric and medium corrosion. In addition to the above-mentioned treatments for other fasteners, surface treatments such as phosphating are also used according to the situation.
Sealing surfaces and closing parts with small diameters often use surface techniques such as nitriding and boronizing to improve their corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA, the nitriding layer is ≥0.4mm.
The anti-corrosion problem of valve stem is a problem that people pay attention to, and we have accumulated a wealth of production experience. Surface treatment processes such as nitriding, boronizing, chromium plating, nickel plating, etc. are often used to improve its corrosion resistance, corrosion resistance and abrasion resistance. Injury performance. Different surface treatments should be suitable for different stem materials and working environments. For stems in contact with the atmosphere and water vapor medium and asbestos fillers, hard chromium plating and gas nitriding processes can be used (Ion nitriding processes are not suitable for stainless steel); In the hydrogen sulfide atmosphere, the valve adopts electroplated high-phosphorus nickel coating, which has good protection performance; 38CrMoAlA adopts ion and gas nitriding and can also be corrosion resistant, but hard chromium coating is not suitable; 2Cr13 can resist ammonia corrosion after quenching and tempering. Carbon steel using gas nitriding is also resistant to ammonia corrosion, while all phosphorus-nickel coatings are not resistant to ammonia corrosion; after gas nitriding, 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is used to make valve stems.
Small-caliber valve bodies and hand wheels are also often chrome-plated to improve their corrosion resistance and 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 high energy density heat sources (gas combustion flame, electric arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metallic materials, and spray them on the pretreated basic surface in the form of atomization to form a spray coating. , Or heating the basic surface at the same time to make the coating melt again on the surface of the substrate to form the surface strengthening process of the spray welding layer. Most metals and their alloys, metal oxide ceramics, cermet composites and hard metal compounds can be coated with one or several thermal spraying methods to form a coating on a metal or non-metal substrate.
Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature resistance and other properties, and prolong its service life. Thermal spraying special functional coating, with special properties such as heat insulation, insulation (or abnormal electricity), abradable sealing, self-lubricating, thermal radiation, electromagnetic shielding, etc.; parts can be repaired by thermal spraying.
8. Control the corrosive environment
The so-called environment has two kinds of broad sense and narrow sense. The broad sense refers to the environment around the valve installation site and its internal circulation medium; the narrow sense refers to the conditions around the valve installation site. Most environments cannot be controlled, and production processes cannot be changed arbitrarily. Only in the case of no damage to products, processes, etc., environmental control methods can be used, such as boiler water deoxygenation, and alkali adjustment in the oil refining process to adjust the pH value. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. also belong to the control of the corrosion environment.
The atmosphere is full of dust, water vapor, and smoke. Especially in the production environment, such as smoke bittern, toxic gases and micro-powder emitted from equipment, will cause different degrees of corrosion to the valve. The operator should clean and purge the valve regularly and refuel it regularly in accordance with the regulations in the operating regulations. This is an effective measure to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is installed with a well, and the valve surface is sprayed with paint. These are all methods to prevent corrosive substances from corroding the valve. Rising ambient temperature and air pollution, especially for equipment and valves in a closed environment, will accelerate their corrosion. Open workshops or ventilation and cooling measures should be adopted as far as possible to slow down 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 beginning of the design, a valve product with a reasonable structure design and a correct process method. Undoubtedly, it has a good effect on slowing down the corrosion of the valve.
Therefore, the design and manufacturing department should make improvements to those parts that are unreasonable in structural design, incorrect in process methods, and prone to corrosion to make them suitable for the requirements of various working conditions.
The gap in the valve connection is a good environment for the corrosion of the oxygen concentration difference battery.
Therefore, the connection between the valve stem and the closing part should not be used in the form of threaded connection; double-sided butt welding and continuous welding should be used for valve welding. Spot welding and lap welding are prone to corrosion. The valve threaded connection is made of PTFE. Thin raw tape and pads. Not only can it have a good seal, but it can also corrode. Difficult-to-flow media can easily corrode the valve. In addition to not inverting the valve when using the valve and paying attention to the discharge of the deposited medium, when manufacturing valve parts, avoid recessed structures as much as possible, and set drain holes as far as possible.
Contacts of different metals will form point couples and promote the corrosion of anode metal. When selecting materials, it is inevitable that the metal potential difference is large and the metal contact cannot produce the passivation film. In the production and processing process, stress corrosion is produced during welding and heat treatment. Attention should be paid to improving the processing method. After welding, the corresponding protective measures such as annealing treatment should be adopted as much as possible. Improve the surface roughness of the valve stem and other valve parts. The higher the surface roughness level, the stronger the corrosion resistance. Improving the processing technology and structure of packing and gaskets, using flexible graphite and plastic packing, as well as flexible graphite adhesive gaskets and PTFE-coated gaskets, can improve sealing performance and reduce valve stem and flange sealing Corrosion of the surface.
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