What are the valve anti-corrosion measures?
Corrosion is the destruction and deterioration of materials under the action of various environments. Metal corrosion is mainly caused by chemical corrosion and electrochemical corrosion, and the corrosion of non-metallic materials is generally caused by direct chemical and physical effects. What are the corrosion forms of valves? What anti-corrosion measures are there?
Table of Contents
- 1 Forms of valve corrosion
Anti-corrosion measures for metal valves
- 2.1 1. Select corrosion resistant materials according to the medium
- 2.2 2. Use non-metallic materials
- 2.3 3. Spray paint
- 2.4 4. Add corrosion inhibitor
- 2.5 5. Electrochemical protection
- 2.6 6. Metal surface treatment
- 2.7 7. Thermal spraying
- 2.8 8. Control the corrosive environment
- 2.9 9. Improve processing technology and valve style
There are two forms of metal valve corrosion, namely uniform corrosion and local 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: those with corrosion rate less than 0.05mm/a are excellent; if the corrosion rate is 0.05~0.5mm/a, it is good; If the corrosion rate is 0.5~1.5mm/a, it can still be used; If the corrosion rate is greater than 1.5mm/a, it is not applicable, valve sealing surface, valve stem, diaphragm, Little Spring and other valve parts generally use Grade I materials, valve body, valve cover and other suitable Grade II or grade III materials, used for high pressure, highly toxic, flammable, explosive, radioactive medium valve, the material with little corrosion is selected.
1. Uniform corrosion
Uniform corrosion is performed on all surfaces of the metal. Such as stainless steel, aluminum, titanium and other protective films produced in the oxidation environment, the metal state corrosion under the film is uniform. There is also a phenomenon that the metal surface is corroded and peeled off, which is the most dangerous.
2. Local corrosion
Local corrosion occurs at the local position of the metal. Its forms include pore corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, abrasion corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion, etc.
Pitting corrosion usually occurs on the metal of the passivation film or protective film, because there are defects on the metal surface, which can destroy the active ions of the passivation film in the solution, causing local damage of the passivation film and extending into the metal, as a corrosion hole, it is one of the most destructive and hidden corrosion forms of metals.
Crevice corrosion occurs in the environment such as welding, riveting, gasket or sediment, which is a special form of pore corrosion. The prevention method is to eliminate gaps.
Intergranular corrosion is to penetrate deep into the metal from the surface along the grain boundary, making the grain boundary appear network corrosion. The intergranular corrosion is mainly caused by improper heat treatment and cold processing except for the accumulation of impurities by grain boundary precipitation. Both sides of the welding seam of austenitic stainless steel are prone to corrosion due to poor chromium zone. Intergranular corrosion of austenitic stainless steel is the common and most dangerous corrosion form. The methods to prevent intergranular corrosion of austenite stainless steel valve parts include: “Solid solution quenching” treatment, that is, heating to about 1100℃ for water quenching, selecting titanium and niobium, however, the production of chromium carbide is reduced by the austenitic stainless steel with carbon content below 0.03%.
Delaminating corrosion occurs in layered structure. Corrosion develops vertically inward first, and then corrodes substances with parallel surfaces. Under the expansion force of corrosive substances, the surface is layered and exfoliated.
Stress corrosion occurs under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion; Eliminate or reduce the stress generated during welding and cold working through heat treatment, improve unreasonable valve structure, avoid stress concentration, and adopt Electrochemical protection and spray brush anti-corrosion coating. Add corrosion inhibitor, apply compressive stress and other measures.
Corrosion fatigue occurs at the joint action of alternating stress corrosion, causing metal rupture. Heat treatment can be carried out to eliminate or reduce stress, surface shot blasting treatment, galvanized, chromium, nickel, etc., but attention should be paid to the coating without 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 spongy. Common examples include brass zinc removal, copper alloy aluminum removal, cast iron graphitization, etc.
Wear corrosion is a corrosion form produced by the alternating action of metal wear and corrosion in fluid pair, and is a common corrosion of valves. This corrosion occurs mostly on the sealing surface. Prevention method: Select corrosion-resistant and anti-abrasive materials, improve structural design, and adopt cathodic protection, etc.
Cavitation corrosion, also known as cavitation and cavitation, is a special form of abrasion corrosion. It is the bubble generated in the fluid and the shock wave generated when it bursts. The pressure can be as high as 400 atmospheric pressure, which damages the metal protective film and even tears the metal particles. Then the corrosion film is formed, and this process is repeated constantly, causing the metal to corrode. The method to prevent cavitation corrosion can be selected as bubble corrosion resistant material, processing surface with high smoothness, elastic protective layer and cathodic protection, etc.
Friction vibration corrosion is the damage caused by vibration and sliding of 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 closing piece, and between the ball bearing and the shaft. Lubricating grease can be applied to reduce friction, alramenting, hard alloy can be selected, and surface hardness can be improved by spray tile treatment or cold processing.
Corrosion is the damage caused by the diffusion of hydrogen atoms generated in the chemical reflection into the metal. Its forms include hydrogen bubbling, hydrogen embrittlement and hydrogen erosion.
Strong steel and non-metallic steel are prone to hydrogen bubbling. Hydrogen bubbling is easy to occur when Petroleum contains sulfide and hydride. Instead of boiling steel with holes, calm steel without holes is used, rubber and plastic are used for protection, and corrosion inhibitor is added to prevent bubbling.
The lattice in strong steel is highly variable. After the hydrogen atom enters, the lattice strain is larger, causing financial embrittlement. Alloy Steel containing nickel and lead should be selected, high strength steel with high hydrogen embrittlement should be avoided, and hydrogen embrittlement phenomenon should be avoided or reduced during welding, electroplating and pickling. Hydrogen enters the metal under high temperature and high pressure, and it will be destroyed by chemical reaction with a combination of elements, which is called hydrogen corrosion. Austenitic stainless steel is completely resistant to hydrogen corrosion.
3. Non-metallic corrosion
Non-metallic corrosion is the same as metal corrosion. Most non-metallic materials are non-electrical conductors and generally do not produce electrochemical corrosion, but pure chemical or physical corrosion, which is the main difference from metal corrosion. Non-metallic corrosion is not necessarily weight loss but often weight gain. For metal corrosion, weight loss is the main. Non-metallic corrosion, many of which are caused by physical effects, while physical effects of metal corrosion are rare; non-metallic internal corrosion is a common phenomenon, while metal corrosion is mainly surface corrosion.
After the metal material comes into contact with the medium, the solution or gas will gradually spread to the inside of the material, causing a series of corrosion changes of non-metals. According to the types and varieties of non-metallic materials, the corrosion forms are different. Corrosion forms include dissolution, swelling, bubbles, softening, decomposition, discoloration, deterioration, aging, hardening, fracture and other phenomena. However, from a comprehensive point of view, non-metallic corrosive performance is much better than that of metal materials, while the strength and temperature resistance of non-metallic materials are lower than that of metal materials.
Electrochemical corrosion corrodes metals in various forms. It not only acts on the two metals, but also produces potential difference due to the poor solubility of the solution, the poor solubility of oxygen, and the slight difference in the internal organization of the metal, the corrosion is intensified. Some metals themselves are not corrosion resistant, but they can produce very good protective film after corrosion, namely passivation film, which can prevent medium corrosion. It can be seen from this that to achieve the purpose of corrosion prevention of metal valves, the first is to eliminate electrochemical corrosion; The second is that when electrochemical corrosion cannot be eliminated; Passivation film should be generated on the metal surface; the third is to use non-metallic materials without electrochemical corrosion instead of metal materials. The following describes several anti-corrosion methods.
1. Select corrosion resistant materials according to the medium
In actual production, the corrosion of the medium used in common materials of valves is very complicated. Even if the valve materials used in a medium are the same, the concentration, temperature and pressure of the medium are different, the corrosion of materials by media is also different. The corrosion rate increases by 1~3 times for every 10℃ increase of medium temperature. The medium concentration has a great influence on the corrosion of valve materials. For example, lead is in sulfuric acid with small concentration, and the corrosion is very small. When the concentration exceeds 96%, the corrosion rises sharply.
On the contrary, the corrosion of carbon steel is the most serious when the concentration of sulfuric acid is about 50%. When the concentration increases to more than 6%, the corrosion decreases sharply instead. For example, aluminum is highly corrosive in concentrated nitric acid with a concentration of more than 80%, but the corrosion is serious in medium and low concentration nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, corrosion in concentrated nitric acid above 95% is aggravated instead.
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 relevant anti-corrosion manuals.
2. Use non-metallic materials
Non-metallic corrosion resistance is excellent. As long as the temperature and pressure of the valve meet the requirements of non-metallic materials, it can not only solve the corrosion problem, but also save precious metals. The valve body, valve cover, lining, sealing surface and other common non-metallic materials are made. As for the gasket, the packing is mainly made of non-metallic materials.
The valve is lined with plastics such as polytetrafluoroethylene, chlorinated polyether and natural rubber, neoprene rubber, nitrile rubber and so on, while the body of the valve body and valve cover are made of general cast iron and carbon steel. That is, the strength of the valve is ensured, and the valve is not corroded. Pinch valve is also designed according to the excellent corrosion resistance and excellent denaturation energy of rubber.
Now, more and more pairs of plastics such as nylon and polytetrafluoroethylene are used, natural rubber and synthetic rubber are used as various sealing surfaces and sealing rings for various valves, which are used as non-metallic materials for sealing surfaces, not only good corrosion resistance, but also good sealing performance, especially suitable for use in granular media. Of course, their strength and heat resistance are both low, and their application scope is limited. The emergence of flexible graphite has made non-metals enter the field of high temperature, solved the leakage problem of packing and gasket which is difficult to solve for a long time, and is a good high temperature lubricant.
3. Spray paint
Coating is the most widely used anti-corrosion method, and it is also an indispensable anti-corrosion material and identification mark in valve products. Paint is also a non-metallic material, which is usually made up of synthetic resin, rubber slurry, vegetable oil, solvent and so on, covering the metal surface, isolating medium and graceful, achieving the purpose of corrosion prevention.
The coating is mainly used in environments with less corrosion such as water, brine, seawater, graceful. The inner cavity of the valve is usually painted with anticorrosive paint to prevent water, air and other media from corroding the valve. The paint is mixed with different colors to indicate the materials used by faen. Valve spray paint, usually in half a year to once a year.
4. Add corrosion inhibitor
Adding a small amount of other special substances into the corrosion medium and corrosion substance can greatly slow down the corrosion rate of metal. This special substance is called corrosion inhibitor.
The mechanism of corrosion inhibitor controlling corrosion is that it promotes the polarization of the battery. Corrosion Inhibitor is mainly used in medium and filler. Adding corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel becomes cremation state within a large solubility range in oxygen-free sulfuric acid, with serious corrosion, however, adding a small amount of oxidant such as copper sulfate or nitric acid can transform the stainless steel into passive state, and a protective film is formed on the surface to prevent the corrosion of the medium. If a small amount of oxidant is added to hydrochloric acid, the corrosion of titanium can be reduced.
Water is often used as the medium for pressure test of the valve, which is easy to cause the corrosion of the valve. Adding a small amount of sodium nitrite in the water can prevent the corrosion of the valve by water. Asbestos filler contains chloride, which corrodes the valve stem greatly. If the washing method of steamed reed water is adopted, the chloride content can be reduced. However, this method is difficult to implement and cannot be popularized generally, esters are suitable for special needs.
In order to protect the stem and prevent the corrosion of asbestos filler, the stem is coated with corrosion inhibitor and sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can generate a layer of passivation film on the surface of the valve stem to improve the corrosion resistance of the valve stem; The solvent can slowly dissolve the corrosion inhibitor and play a lubricating role; Adding zinc powder to asbestos as sacrificial metal, in fact, zinc is also a corrosion inhibitor, which can combine with chloride in asbestos first, greatly reducing the contact opportunity between chloride and stem metal, thus achieving the purpose of corrosion prevention. If red pill, lead acid calcium and other corrosion inhibitors are added to the coating, spraying on the surface of the valve can prevent graceful corrosion.
5. Electrochemical protection
Electrochemical protection includes anode protection and cathode protection. The so-called anode protection is to use the protective metal as the anode to introduce an applied direct current to increase the anode potential in a positive direction. When it increases to a certain value, a dense protective film is generated on the surface of the metal anode, that is, the passivation film, at this time, the corrosion of the metal cathode decreases sharply.
Anode protection is suitable for easily passivated metals. The so-called cathodic protection is to use the protected metal as the cathode and add direct current to reduce its potential in the negative direction. When it reaches a certain potential value, the corrosion current speed decreases, metal is protected. In addition, cathodic protection can protect the protected metal by using a metal whose electrode potential is more negative than that of the protected metal. If zinc is used to protect iron, zinc is corroded, and zinc is called sacrificial metal. In production practice, anode protection is used less and cathode protection is used more. Large valves and important valves adopt this cathode protection law, which is an economical, simple and effective method. Zinc is added to asbestos packing to protect the valve stem.
6. Metal surface treatment
Metal surface treatment process is better than sleeping coating, surface penetration, surface oxidation passivation, etc. Its purpose is to improve the corrosion resistance of metal and improve the mechanical energy of metal. Surface-treated valves are widely used.
Galvanized, chrome plating and oxidation (bluing) are commonly used to improve the ability of atmospheric and medium corrosion resistance. Other fasteners are not only treated by the above methods, but also treated by phosphating and other surface treatments according to the situation.
The sealing surface and closing parts with small caliber often adopt surface processes such as nitriding and boronizing to improve their corrosion resistance and wear resistance. Valve disc made of 38crmoala, nitrided layer ≥ 0.4mm.
The anti-corrosion problem of valve stem has been paid attention to by people and has accumulated rich production experience. Surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating are often adopted to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatments should be suitable for different stem materials and working environment. The stem in contact with asbestos filler in the atmosphere and water vapor medium can adopt electroplating hard chrome, gas nitridation process (stainless steel should not adopt ion nitridation process); The valve in hydrogen sulfide graceful environment adopts electroplated high phosphorus nickel coating, which has better protective performance; 38crmoala adopts ion and gas nitridation, which can also resist corrosion, however, hard chromium coating should not be used; 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel nitrided with gas can also resist ammonia corrosion, while all phosphorus nickel coatings are not resistant to ammonia corrosion; after gas nitridation, 38crmoala material has excellent corrosion resistance and comprehensive performance, and it is used to make many valve stems.
Small-caliber valve bodies and handwheels are often chrome plated to improve their corrosion resistance and decorate valves.
7. Thermal spraying
Thermal spraying is a kind of process block for preparing coating 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 (gaseous combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metal materials, spray to the basic surface after pretreatment in the form of atomization to form a spraying layer, or heat the basic surface at the same time to melt the coating on the surface of the substrate again to form a surface strengthening process of spray welding layer. Most metals and their alloys, metal oxide ceramics, metal ceramic composites and hard metal compounds can use one or more thermal spraying methods to form coatings on metal or non-metal substrates.
Thermal spraying can improve the surface corrosion resistance, abrasion resistance, high temperature resistance and other properties, and prolong the service life. Thermal spraying special functional coating has special properties such as heat insulation, insulation (or different electricity), grinding and sealing, self-lubrication, thermal radiation, electromagnetic shielding, etc. Thermal spraying can repair parts.
8. Control the corrosive environment
The so-called environment is broad and narrow. 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 environments cannot be controlled and the production process cannot be changed at will. Only in the case of no damage to products, processes, etc., environmental control methods can be adopted, such as deaeration of boiler water, adjustment of ph value by domestic alkali in refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitor and electrochemical protection also belong to the controlled corrosion environment.
The graceful is full of dust, water vapor and smoke, especially in the production environment, such as halogen, toxic gas and fine powder emitted by equipment, will cause different degrees of corrosion to the valve. Operators should clean and purge the valve regularly and refuel regularly according to the regulations in the operating procedures, which is an effective measure to control environmental corrosion. The installation of protective cover for valve stem, the setting of ground well for ground valve, and the spraying of paint on the surface of valve are all methods to prevent corrosive substances from eroding the valve. Rising ambient temperature and air pollution, especially for equipment and valves in a closed environment, will accelerate their corrosion. Open workshop should be adopted as far as possible or ventilation and cooling measures should be adopted to mitigate environmental corrosion.
9. Improve processing technology and valve style
The anti-corrosion protection of the valve is a problem that has been considered since the design, a valve product with reasonable structural design and correct technological method. Undoubtedly, it has a good effect on slowing down the corrosion of the valve.
Therefore, the design and manufacturing department should improve the components that are not reasonable in structural design, incorrect in technological methods and easy to cause corrosion to suit the requirements under various working conditions.
The gap at the valve connection is a good environment for oxygen concentration battery corrosion.
Therefore, the connection between the valve stem and the closing part should not adopt the connection form of lining and thread as far as possible; Double-sided butt welding and continuous welding are applied for valve welding, and spot welding and lap welding are prone to corrosion, valve thread connection, using PTFE raw tape and pad. It can not only have good sealing, but also corrode. Dead angle is not easy to flow medium, easy to corrode the valve, in addition to the use of the valve is not upside down and pay attention to discharge the deposition medium, when manufacturing valve parts, should try to avoid no sag structure, drain holes should be set for the valve as far as possible.
Different metal contacts will form point pairs to promote anode metal corrosion. When selecting materials, it is inevitable that metal contacts with large metal potential differences cannot produce passivation films. In the manufacturing and processing process, special preparation is stress corrosion generated during welding and heat treatment. Attention should be paid to improving the processing method, and corresponding protective measures such as annealing treatment should be adopted as much as possible after welding. To 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 gasket, using flexible graphite and plastic filler, as well as flexible graphite sticking gasket and ptfe packing gasket can improve the sealing performance, reduce the corrosion of the sealing surface of the stem and flange.
Source: Network Arrangement – China Valve Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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