First, the form of valve corrosion
There are two forms of corrosion in metal valves, 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 size of the corrosion rate: the corrosion rate is less than 0.05mm/a is excellent; The corrosion rate is good if it is 0.05~0.5mm/a; The corrosion rate is still acceptable when the corrosion rate is 0.5~1.5mm/a; If the corrosion rate is greater than 1.5mm/a, it is not applicable, the sealing surface, valve stem, diaphragm, small spring and other valve parts of the valve are generally made of primary materials, and the valve body, valve cover and other secondary or tertiary materials are applicable, and the valves used for high-pressure, highly toxic, flammable, explosive and radioactive media are selected with very little corrosive materials.
1. Uniform corrosion
Uniform corrosion is carried out on the entire surface of the metal. For example, stainless steel, aluminum, titanium, etc. produce a layer of protective film in the oxidizing environment, and the metal state under the film is uniformly corroded. There is also a phenomenon of corrosion and spalling of metal surfaces, and this corrosion is the most dangerous.
2. Local rot
Local corrosion occurs in the local position of the metal, and its forms include porosity, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion, etc.
Pitting corrosion usually occurs on the metal of the passivation film or protective film, which is due to the defects on the metal surface, and the active ions in the solution can destroy the passivation film, so that the passivation film is partially damaged, extending into the metal inside, and becoming an etched hole, which is one of the most destructive and hidden forms of corrosion of the metal.
Crevice corrosion occurs in environments such as welding, riveting, gasketing, or under sediment, and is a special form of porosity. The way to prevent it is to eliminate the gaps.
Intergranular corrosion is the penetration of the metal from the surface along the grain boundary, causing the grain boundary to corrode in a reticulated manner. In addition to the precipitation and accumulation of impurities at grain boundaries, intergranular corrosion is mainly caused by improper heat treatment and cold working. Both sides of the weld seam of austenitic stainless steel are prone to chromium-poor zones and corrosion. Intergranular corrosion of austenitic stainless steels is the most common and dangerous form of corrosion. The methods to prevent intergranular corrosion of austenitic stainless steel valves are: "solution quenching" treatment, that is, water quenching by heating to about 1100 °C, and selecting austenitic stainless steel containing titanium and niobium and carbon content below 0.03% to reduce the generation of chromium carbide.
Delamination corrosion occurs in a layered structure, where the corrosion first develops vertically inward, and then corrodes the material parallel to the surface, and under the swelling force of the corrosive, the surface is laminated and peeled off.
Stress corrosion occurs when corrosion and tensile stresses occur at the same time. methods of preventing stress corrosion; Through heat treatment, the stress generated in welding and cold working is eliminated or reduced, the irrational valve structure is improved, the stress concentration is avoided, and electrochemical protection and anti-corrosion coating are sprayed. Measures such as adding corrosion inhibitors, applying compressive stress, etc.
Corrosion fatigue occurs at the site of alternating stress corrosion, which fractures the metal. Heat treatment can be carried out to eliminate or reduce stress, surface shot peening and electroplating of galvanized, chromium, nickel, etc., but it should be noted that the coating should not have tensile stress and hydrogen diffusion.
Selective corrosion occurs in materials of different compositions and impurities, and in a certain environment, some elements are leached by corrosion, and the remaining elements that are not corroded are spongy. Common ones include dezincification of brass, aluminization of copper alloys, graphitization of cast iron, etc.
Abrasive corrosion is a form of corrosion caused by the alternating effect of fluid wear and corrosion on metals, and it is a common corrosion of valves, which mostly occurs on the sealing surface. Prevention methods: select corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
Cavitation corrosion, also known as cavitation and cavitation, is a special form of wear corrosion. It is a bubble produced in the fluid, and the shock wave generated when bursting, the pressure can be as high as 400 atmospheres, causing the metal protective film to break and even tear the metal particles. It then corrodes to form a film, and this process is repeated repeatedly, causing the metal to corrode. To prevent cavitation corrosion, you can choose cavitation corrosion-resistant materials, processing surfaces with high finish, elastic protective layer and cathodic protection.
Frictional corrosion is the damage caused by vibration and sliding of two parts in contact with each other at the same time. Friction corrosion occurs at bolted joints, at the junction between the stem and the shut-off, between the ball bearing and the shaft, etc. It can be protected by grease, friction reduction, surface phosphating, selection of cemented carbide, and tile spraying or cold working to improve the surface hardness.
Corrosion is the destruction caused by the diffusion of hydrogen atoms produced in a chemical reaction into the interior of the metal, in the form of hydrogen bubbling, hydrogen embrittlement, and hydrogen erosion.
Strong steels and steels containing non-metals are susceptible to hydrogen bubbling. When petroleum contains sulfides and hydrides, hydrogen bubbling is easy to occur. Cavitation-free sedative steel is used instead of cavitation-free boiling steel, protected by rubber and plastic, and corrosion inhibitors are added to prevent bubbling.
The lattice in strong steel is highly denatured, and after the hydrogen atom enters, the 4-lattice should change to a larger size, causing financial embrittlement. Alloy steels containing nickel and lead should be selected, high-strength steels with high hydrogen embrittlement should be avoided, and hydrogen embrittlement should be avoided or reduced in welding, electroplating, and pickling. At high temperature and pressure, hydrogen enters the metal and is destroyed by a chemical reaction with a combination of elements, which is called hydrogen erosion. Austenitic stainless steels are fully resistant to high-temperature hydrochloride.
3. Non-metallic corrosion
Non-metallic corrosion is the same size as metal corrosion, and the vast majority of non-metallic 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. Non-metallic corrosion is not necessarily weightless but often weight-gaining, weightlessness is the main for metal corrosion, non-metallic corrosion, many are caused by physical action, and metal corrosion physical action is very rare; Non-metallic internal corrosion is a common phenomenon, while metal corrosion is mainly surface corrosion.
After the metal material is in contact with the medium, the solution or gas will gradually diffuse into the interior of the material, causing a series of corrosion changes in the non-metal, and the corrosion form is different according to the different types and varieties of non-metallic materials. The forms of corrosion include dissolution, swelling, bubbles, softening, decomposition, discoloration, deterioration, aging, hardening, fracture and other phenomena. However, from a comprehensive point of view, the non-metallic corrosion performance 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.
Second, the anti-corrosion measures of metal valves
Electrochemical corrosion corrodes metals in various forms, and it not only acts between the two metals, but also produces potential differences due to the poor solubility of the solution, the poor solubility of oxygen, and the slight differences in the internal structure of the metal, which aggravates the corrosion. Some metals themselves are not corrosion-resistant, but it 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 second is when electrochemical corrosion cannot be eliminated; It should be able to produce a passivation film on the metal surface; The third is to choose non-metallic materials without electrochemical corrosion instead of metal materials. Here are a few ways to prevent corrosion.
1. Corrosion-resistant materials are selected according to the medium
In the section of "valve selection", we introduce the medium applicable to the common materials of the valve, but it is just a general introduction, in the actual production, the corrosion of the medium is very complex, even if the valve material used in a medium is the same, the concentration, temperature and pressure of the medium are different, and the corrosion of the medium to the material is not the same. For every 10°C increase in medium temperature, the corrosion rate increases by about 1~3 times. The medium concentration has a great influence on the corrosion of the valve material, such as the lead is in the sulfuric acid with a small concentration, the corrosion is very small, and when the concentration exceeds 96%, the corrosion rises sharply. On the contrary, carbon steel has the most serious corrosion when the sulfuric acid concentration is about 50%, and when the concentration increases to more than 6%, the corrosion decreases sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but it is seriously corrosive in medium and low concentrations of nitric acid. Although stainless steel is very resistant to dilute nitric acid, it is aggravated by corrosion in more than 95% concentrated nitric acid.
From the above examples, it can be seen 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 manuals.
2. Non-metallic materials are used
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, 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. Plastics such as PTFE and chlorinated polyether, as well as natural rubber, neoprene, nitrile rubber and other rubbers are used for valve lining, while the valve body and bonnet body are made of general cast iron and carbon steel. It not only ensures the strength of the valve, but also ensures that the valve is not corroded. The pinch valve is also designed according to the excellent corrosion resistance and excellent variable properties of rubber. Now more and more right with nylon, PTFE and other plastics, with natural rubber and synthetic rubber to make a variety of sealing surfaces, sealing rings, for all kinds of valves, these non-metallic materials used as sealing surfaces, not only good corrosion resistance, but also good sealing performance, especially suitable for use in media with particles. Of course, they are less strong and heat resistant, and the range of applications is limited. The emergence of flexible graphite has brought non-metals into the field of high temperature, solved the problem of leakage of fillers and gaskets that are 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 means, and it is an indispensable anti-corrosion material and identification mark on valve products. Coating is also a non-metallic material, which is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., covering the metal surface, isolating the medium and the atmosphere, and achieving the purpose of anti-corrosion. Coatings are mainly used in water, salt water, seawater, atmosphere and other environments that are not too corrosive. The inner cavity of the valve is often painted with anticorrosive 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 Faun. Valve spray paint, generally once every six months to one year.
4. Add corrosion inhibitors
The addition of a small amount of other special substances to the corrosive medium and corrosive substance can greatly slow down the rate of metal corrosion, and this special substance is called a corrosion inhibitor.
The mechanism by which corrosion inhibitors control corrosion is that it promotes the polarization of the battery. Corrosion inhibitors are mainly used in media and fillers. The addition of corrosion inhibitors to the medium can slow down the corrosion of equipment and valves, such as chromium-nickel stainless steel in oxygen-free sulfuric acid, a large solubility range into a cremation state, corrosion is more serious, but adding a small amount of copper sulfate or nitric acid and other oxidants, can make the stainless steel turn into a blunt state, the surface of a protective film to prevent the erosion of the medium, in hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced. Valve pressure test is often used as the medium for pressure test, which is easy to cause corrosion of the valve, and adding a small amount of sodium nitrite to the water can prevent the corrosion of the valve by water. Asbestos packing contains chloride, the corrosion of the valve stem is great, if the use of steaming water washing method can reduce the content of chloride, but this method is much more difficult in the implementation, can not be generalized, ester is suitable for special needs.
In order to protect the stem from corrosion of the asbestos packing, in the asbestos packing, the stem is coated with corrosion inhibitor and sacrificial metal. The corrosion inhibitor is made of sodium nitrite and sodium chromate, which can generate a passivation film on the surface of the valve stem and improve the corrosion resistance of the valve stem. The solvent can make the corrosion inhibitor slowly dissolve, and can play a lubricating role; In fact, zinc is also a corrosion inhibitor, which can first combine with the chloride in the asbestos, so that the chance of chloride contact with the stem metal is greatly reduced, so as to achieve the purpose of anti-corrosion. If corrosion inhibitors such as red pill and calcium leadate are added to the coating, spraying on the surface of the valve can prevent atmospheric corrosion.
5. Electrochemical protection
There are two types of electrochemical protection: anodic protection and cathodic protection. The so-called anodic protection is to protect the metal as the anode to introduce the applied straight current, so that the anode potential increases in the positive direction, when it increases to a certain value, the surface of the metal anode generates a dense protective film, that is, the passivation film, then the corrosion of the metal cathode is sharply reduced. Anode protection is suitable for metals that are easily passivated. 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, and when it reaches a certain potential value, the corrosion current velocity is reduced and the metal is protected. In addition, cathodic protection protects the protected metal by using a metal with a more negative electrode potential than the protected metal. If iron is protected with zinc, zinc is corroded, and zinc is called sacrificial metal. In production practice, anode protection is used less, and cathodic protection is used more. This cathodic protection method is an economical, simple and effective method for large and important valves. Zinc is added to the asbestos packing and the protective valve stem is also a 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 of metals and improve the mechanical energy of metals. Surface treated valves are used in a wide range of applications.
Valve connection screw is commonly treated with zinc, chrome plating, and oxidation (bluing) to improve the ability to resist atmospheric and medium corrosion. In addition to the above-mentioned methods, other fasteners are also treated with surface treatments such as phosphating according to the situation.
The sealing surface and the closing parts with small diameter are often made of surface processes such as nitriding and boronizing to improve its corrosion resistance and wear resistance. Disc made of 38CrMoAlA, nitriding layer ≥ 0.4mm.
The anti-corrosion problem of valve stem is a problem that people pay attention to, and has accumulated rich production experience, and often adopts surface treatment processes such as nitriding, boronization, chrome plating, nickel plating, etc., to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatment should be suitable for different stem materials and working environments, in the atmosphere, water vapor medium and asbestos packing contact stem, can use hard chrome plating, gas nitriding process (stainless steel should not use ion nitriding process); In the atmospheric environment of hydrogen sulfide, the valve adopts electroplating and high phosphorus nickel plating, which has good protection performance; 38CrMoAlA can also be corrosion-resistant by ion and gas nitriding, but it is not suitable for hard chrome plating; 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel made of gas nitriding can also resist ammonia corrosion, while all phosphorus-nickel plating layers are not resistant to ammonia corrosion; After gas nitriding, 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and it is mostly used to make valve stems.
The small-bore valve body and handwheel are also often chrome-plated to improve their corrosion resistance and decorate the valve.
7. Thermal spraying
Thermal spraying is a kind of process block for the preparation of coatings, which has become one of the new technologies for material surface protection. It is a national key promotion project. It is a surface strengthening process method that 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 to the pretreated basic surface in the form of atomization to form a spray coating, or heat the basic surface at the same time, so that the coating is melted again on the surface of the substrate to form a surface strengthening process of spray welding layer. Most metals and their alloys, metal-oxide ceramics, cermet compounds, and hard metal compounds can be coated on metallic or non-metallic substrates using one or several 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 spraying special functional coating, with heat insulation, insulation (or abnormal electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding and other special properties; Parts can be repaired with thermal spraying.
8. Control the corrosive environment
The so-called environment has two kinds of broad sense and narrow sense, and the broad sense of environment refers to the environment around the valve installation place and its internal circulation medium; The narrow environment refers to the conditions surrounding the valve installation. Most environments are uncontrollable, and production processes cannot be arbitrarily changed. Only when there is no damage to the product or process, the method of controlling the environment can be adopted, such as deoxygenation of boiler water, adjustment of PH value of alkali in oil refining process, etc. From this point of view, the addition of corrosion inhibitors and electrochemical protection mentioned above is also a way to control the corrosive environment.
The atmosphere is full of dust, water vapor and smoke, especially in the production environment, such as smoke brine, toxic gases and fine powder emitted by equipment, which will cause varying degrees of corrosion to the valve. The operator should regularly clean and purge the valve and refuel regularly according to the provisions of the operating procedures, which is an effective measure to control environmental corrosion. Installing a protective cover on the valve stem, setting a ground well on the ground valve, and spraying paint on the surface of the valve are all ways to prevent corrosive substances from eroding the valve. Rising ambient temperatures and air pollution, especially for equipment and valves in enclosed environments, can accelerate corrosion. Open workshops or ventilation and cooling measures should be used as much as possible to slow down environmental corrosion.
9. Improve the 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, and a valve product with reasonable structural design and correct process method. It is undoubtedly effective in slowing down the corrosion of valves.
Therefore, the design and manufacturing department should improve the parts that are not reasonable in structural design, incorrect in process methods and easy to cause corrosion, so as to adapt them to the requirements of various working conditions.
The gap at the valve connection is a good environment for corrosion of batteries with poor oxygen concentration.
Therefore, the connection between the valve stem and the closing part should not be connected in the form of inner and threaded connections as much as possible; Valve welding should be double-sided welding and continuous welding, spot welding and lap welding are easy to corrode, and the valve threaded connection is made of PTFE raw tape and pad. Not only can it have a good seal, but it can also corrode. In addition to not inverting and paying attention to the discharge of sedimentary medium when using the valve, when manufacturing valve parts, the non-concave structure should be avoided as much as possible, and the valve should be set up with a discharge hole as much as possible.
The contact between different metals will form a point couple, promote the corrosion of anode metals, and when selecting materials, it should be inevitable that the metal contact with a large metal potential gap and cannot produce a passivation film. In the production and processing process, stress corrosion occurs in welding and heat treatment, and attention should be paid to improving the processing method, and corresponding protective measures such as annealing treatment should be used as much as possible after welding. Improve the surface roughness of the 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 fillers, as well as flexible graphite adhesive gaskets and PTFE wrapped gaskets, can improve sealing performance and reduce corrosion of ground valve stems and flange sealing surfaces.
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